Talk:F-number/Archive 1

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I saw F-stop, F/stop and F stop, which is the best one ?

I don't like your edit it's a ratio for example : Diaphragm diameter = 12,5 mm Focal length = 100 mm -> 100/12,5=f/8 Diaphragm diameter = 25 mm Focal length = 50 mm -> 100/25=f/4 Is it logarithmic ? Not really f/4 means twice more light on the film that f/8. and 8=2x4.

You're right, it's misleading: I could have phrased it better. I hope you prefer this version. The Anome
Is there a difference between f-stop and f-number. It's unclear in the article.

I think so: a set of f stops are (by convention) particular f-numbers that form a geometric series with a factor of 2 of exposure as the ratio. The Anome

I think to "half f-stops". F-stop is simply the scale on the lens ?


can you copy-edit the article ?

I think so, although apparently there used to be aperture plates called 'stops' according to one reference. If we ignore the f-number markings, the f-stops make a logarithmic scale of exposure value. Given this interpretation, you can then think of taking a half-step along this scale, to make an exposure difference of "half a stop". The Anome

Suite or serie ? Which one is correct English ? Ericd

I was wondering the same thing. I.e., it says "geometric series" and I wondered if "geometric sequence" should be used instead. A series is a sum, but many persons not schooled in mathematics use "series" when they ought to use "sequence". -- Mike Hardy

Correct word is "suite" in French. Geometric sequence is more correct ? Ericd

French "suite" corresponds to English "sequence", not to English "series". Mathematical illiterates often write "series" where "series" is wrong and "sequence" is right. -- Mike Hardy

This seems OK now. See Geometric progression Ericd

"Geometric progression" will also serve. -- Mike Hardy

I should probably move these f-stop demonstration pics to a separate page? I think it's useful to see the difference, maybe I'm wrong, but I also think that I should take these off for now because many are still on dialup. Opinions. Koyaanis Qatsi

The images are not too big, so that is OK. They belong in the depth of field article, thouch, I'd say. Puut a good referrence to them in f-number. -- Egil 05:15 Jan 29, 2003 (UTC)
I moved them. Koyaanis Qatsi


(this is a translation) User Lochkarte wrote on


Change of September 11. 2003, 02:09PM.

In the article it is written that the f-number (focal number) gets calculated by the diameter of the diapragma and the focal length.


The diaphragma most often is somewhere inside the objective. The lenses before the diaphragma therefore will map it. This map is called the entry pupil. The diameter of this entry pupil is the value crucial for the focal number! (translation end)

I think it does make quite a difference that the mapped (=modified) size of the diapragma is needed for the F-number calculation. This should be reflected in the article. Sorry i am not a native english speaker, and want to pass that task over to someone else. -- 16:08, 10 Jun 2004 (UTC)
I still don't understant the meaning of this comment !
Ericd 22:52, 27 Jan 2005 (UTC)
I'm guessing the original comment pointed out that the actual diameter of the diaphragm cannot be calculated from the effective (nominal) focal length of the overall lens. The diaphragm is usually located inside the lens barrel, between two groups of lenses, so it interatcts with the lens optics and the measured diaphragm opening may be larger or smaller than the calculated diameter. For example, a 50mm lens set to f/8 should have a diaphragm diameter of 6.25mm, but the actual diaphragm opening may be different because it is located inside the lens. --MarkSweep 23:40, 27 Jan 2005 (UTC)
Yes, the page is wrong. The f-number is the ratio of the focal length to the entrance pupil diameter, not the diaphragm aperture diameter. The EP is the image of the aperture stop (presumably the diaphragm) in whatever optical elements come before it.--Srleffler 21:59, 15 November 2005 (UTC)

Where is the F-stop.jpg image gone ? Ericd 22:47, 27 Jan 2005 (UTC)

external links

Perhaps these could be changed to show the site's title, rather than just their URL. 20:28, 15 November 2005 (UTC)

How is f/# read?

How is eg. "f/8" read? Could put this in the second paragraph or so. TomViza 17:15, 22 December 2005 (UTC)

I read it "eff eight".--Srleffler 17:47, 22 December 2005 (UTC)
"Eff eight" was slang shorthand among optical engineering students at the Rochester Institute of Technology in the late 1970s. "The focal length divided by eight" was a rarely-used formal expression among photography students at the same school.Walter Dufresne (talk) 21:38, 16 June 2008 (UTC)

Article is too lens/camera oriented

Focal ratio is a generalized optic term, NOT exclusively associated with cameras or even refracting lenses. It's also used with and applicable to Newtonian and Cassegrain reflecting telescopes, mirror telephoto lenses, and even hand held bathroom mirrors.

That needs to be described, along with a graphic depicting the focal ratio of a parabolic mirror. Something like this, but with arrows showing diameter and focal distance: [3] If nobody else does it, I'll try to add something when I get time. Joema 23:23, 7 January 2006 (UTC)

Another thing for the to-do list: if the article is going to be expanded and refocused on the optics usage, it needs to distinguish between "f-number" and "working f-number". Strictly, f-number is only applicable (IIRC) to the case where a lens is used to image an object that is an infinite distance away. For other geometries, this is typically replaced by the "working f-number". --Srleffler 23:50, 7 January 2006 (UTC)
Forgot to add, focal ratio is not even unique to optics. E.g, there are parabolic microphones and parabolic antennas. They all have focal ratios, and those ratios impact performance and behavior of each. Joema 00:11, 8 January 2006 (UTC)
I have taken a stab at addressing this by putting up a basic page on Focal ratio and removing the REDIRECT there. I think that will be more useful encyclopedically to the reader since Focal ration is more of a basic principle and this article is for the most part about the photographic application of that principle. Halfblue 13:13, 20 October 2006 (UTC)

f-stops and digital cameras

f stop in a digital camera relates to the size of the CCD/CMOS sensor as much as focal length in a 35mm camera relates to the ratio of the projected image versus the diagonal dimansion of the film frame. This is not covered in this article at all adn should be. I'm not the one to describe this in any detail unfortunately.

-- 16:57, 1 February 2006 (UTC)James Laver 1st Feb 2006

I'm not sure I understand what you're referring to, but check if this is already covered in the depth of field article. --MarkSweep (call me collect) 00:22, 2 February 2006 (UTC)
I think he may be just mistaken. AFAIK, f-stop is not directly related to the size of the sensor or film at all. Perhaps he is confusing f-stop and focal length? Or perhaps he is confused because some digital camera manufacturers misrepresent the focal length of their lenses by marking them with the focal length that a 35mm camera lens with similar performance would have, rather than the actual focal length of the lens in the camera. I can't interpret what he is saying.
This does bring to mind though: at some point, the references to "film" in the article are going to have to be replaced.--Srleffler 02:58, 2 February 2006 (UTC)
The size of the sensor differ from 24mmx36mm but it doesn't affect the focal length of the lens nor its aperture. Ericd 22:51, 17 July 2006 (UTC)

The more a DSLR is stopped down the less forgiving it will be of (dreaded) sensor dust. —Preceding unsigned comment added by (talk) 20:28, 7 October 2008 (UTC)


I reverted several edits tonight, for several reasons:

  • I disagree that "&fnof;" (ƒ) is a better symbol for the focal length. The focal length is represented by the letter "f". Like all mathematical variables, it is conventionally set in italics in the same font as the text: f. On Wikipedia, there is a general problem that variables in equations may be typeset differently from variables in the body text of the article, so it may be better to use Latex to display a variable that is used in a Latex equation: <math>f</math> produces . This doesn't always fix it, though, since on some web browsers a simple equation renders in different fonts than a more complex one. This can be fixed by forcing the equation to be slightly more complex by adding a small spacer: <math>f\,</math>, giving . There is also a semantic issue. The character "&fnof;" is intended to represent the symbol for a mathematical function, not the variable . Using <math>f</math> is preferred from this point of view, since the coding then explicitly identifies the symbol as a mathematical variable, not a function or merely italic text.--Srleffler 07:21, 25 April 2006 (UTC)
    • Srleffler, thanks for your careful attention to my various edits last night. I think we have a few points unresolved, so let's discuss them here before I change the page again. My comments indented below each of yours, starting with this one, each one signed:
    • Thanks for pointing out LaTeX math mode as a better alternative. I'm fine with that. Not sure I accept your assertion, however, that the f-number f is "conventionally set in italics in the same font as the text". Do you have a source for that? My impression is that it's always an f-with-hook, as I stated. Dicklyon 14:58, 25 April 2006 (UTC)
      • It sounds like this is a difference in typography standards between science/math and photography. In the most common typographic scheme for mathematics and science publishing, variable names are printed in italics and function names are printed in standard Roman type. The Latex engine in Wikipedia automatically applies this standard: a variable like is automatically italicized, while a function is not, if properly identified: . I think we should use standard math typography for the variables in the equation, including for focal length. As noted in the article, though, "f/#" is actually treated as a single symbol, which doesn't obey the standard math rules for how variables and values are treated. It seems fine to me to use special typography for the f in this combined symbol. I wonder if there is a better way than &fnof;, though. I'm concerned that this special character may not appear in all web browsers' default character sets. It would be better to use a normal f and use css to force a suitable font. I'll look into this.--Srleffler 03:20, 26 April 2006 (UTC)
      • OK, I created a template, {{f/}}, which produces an f/# symbol. Usage:
        • {{f/}}2 produces f/2
        • {{f/|2}} produces f/2
        • {{f/|2|link=yes}} produces f/2
        • {{f/|#}} produces f/#
      • The template uses CSS to request an italic serif font for the f. On Windows browsers it will request the Georgia font, mainly because I don't like Times New Roman. If there is a more suitable font for the f symbol, I can add it to the request list. (CSS allows you to "request" a list of fonts. You get the first one available.)--Srleffler 05:21, 26 April 2006 (UTC)
        • I don't understand. Where is this CSS? What is its scope? Can I edit it to make the f-number look better? What happens on other sites that clone the wikipedia content? Do we care? Dicklyon 01:59, 28 April 2006 (UTC)
        • Never mind, I discovered Template:f/ and hacked it. Looks better now with italic slash. Dicklyon 03:44, 28 April 2006 (UTC)
  • The external site that was removed looks informative. Was its deletion merely a bias against sites lacking fancy graphics? Sometimes content is more important than glamour.--Srleffler 07:21, 25 April 2006 (UTC)
    • I read it and didn't find it informative or correct. It seemed totally lame, full of typos, misspellings, bad grammar, made-up history, and misimpressions, and written by a guy who was admittedly clueless about what he was writing about. In particular, his interpretation of "f" for "fraction" is harmful, and already propagated to some other places that I have trying to fix. It's certainly not authoritative or accurate, so why do we want to reference it? Dicklyon 14:58, 25 April 2006 (UTC)
      • Fine. It's gone.--Srleffler 03:20, 26 April 2006 (UTC)
  • The edit I reverted restored the incorrect assertion that f/16 can be interpreted literally as an aperture diameter equal to focal length divided by 16. While this is commonly believed by photographers, it is not quite true. The focal ratio is not the ratio of the focal length to the physical diameter of the lens' aperture, but rather the ratio of the focal length to the diameter of the lens' entrance pupil. This distinction is discussed above. --Srleffler 07:21, 25 April 2006 (UTC)
    • The aspect that was "incorrect" was your interpretation that by "aperture diameter" that text implied the "diaphragm diameter". It is conventional to speak of the D as an "aperture diameter" for the "effective aperture diameter" (though it is more properly the input pupil diameter, as you note). We should be able to repair my statement to your satisfaction without removing it. The literal interpretation of f/N is quite informative and important, in my opinion, so people can understand why the "f" is there and why it doesn't stand for "fraction". Dicklyon 14:58, 25 April 2006 (UTC)
      • The article defines aperture via a link to aperture, which uses the definition I gave. This is the standard usage in optics. This often happens: terminology usage varies between the science of optics and applied fields that use it, like photography. The article needs to work for both communities. "Effective aperture" is fine with me, but the article needs to be clear about the distinction being made, and should indicate clearly that the effective aperture is not the physical diameter of the diaphragm's opening. I've seen too many photography articles where the author writes that the size of the opening in the iris is given by the ratio of the focal length to the f-number.--Srleffler 03:20, 26 April 2006 (UTC)
  • I'm ambivalent about the change from sunny f/16 rule to sunny 16 rule. If the latter is really more common, the article Sunny f/16 rule should be renamed (moved).--Srleffler 07:21, 25 April 2006 (UTC)
    • I don't have an authoritatiave source, but I did use google to verify that the form I changed to is at least an order of magnitude more commonly used; I had never heard of the rule as stated, which is why I checked. I think we better get this settled about before taking the bigger step of renaming a page. Dicklyon 14:58, 25 April 2006 (UTC)
      • Renaming a page is not a big deal. It usually only takes a few minutes.--Srleffler 03:20, 26 April 2006 (UTC)

All else being equal

The concept of "all else being equal" is often misapplied in depth-of-field reasoning, but clarifying with "For a given image diagonal" (as did, and I reverted) does not make that statement about DOF more correct. The presumption might be that when changing f-number, as many as possible of other variables are kept constant (image format diagonal and hence circle-of-confusion diameter limit, lens focal length, subject distance, absolute aperture diameter). But since they can't ALL be kept constant if f-number is to change, something has to give. Imagining format diagonal changing seems silly, since that won't change the f-number, so why specify that it is constant? The more important thing is that you probably mean to fix the focal length and vary the absolute aperture diameter, as opposed to the other way around. In fact the statement about higher f-number giving higher DOF would be false if the aperture diameter were held fixed and the focal length varied to vary the ratio f/d. So if anything, the clarification should say "for a given focal length". But would anyone really be confused the way it is? Dicklyon 17:41, 15 May 2006 (UTC)

The Kevin Willey page link

Srleffler, thanks for taking out this external link again [4]. To whoever added it, please tell us why. Is there anything informative in it? I found it bizarre. For example, the section entitled "Where do these f-stop numbers come from?" has a long strange discussion of the inverse square law of light, with no suggestion of how that might be relevant -- and nothing else! Dicklyon 00:54, 30 June 2006 (UTC)

I have encountered this before, although explained better. The idea is that the "inverse square law" says that light (magically) decreases in intensity proportional to the inverse square of the distance it travels, so to get the same intensity at the film, when you double the focal length you have to quadruple the area of the aperture to get the same light at the film. This incorrect argument thus reaches the correct conclusion that you must double the aperture diameter to get the same amount of light onto the film. The actual reasons for this are slightly more complex and have nothing to do with the inverse square law. In fact, the light between the camera lens and the film doesn't even obey the inverse square law! That law describes the propagation of light from a point source, not light being focused by a lens. --Srleffler 01:14, 30 June 2006 (UTC)

Well, he does admit that "I have not found the following information presented in any photography books that I have read." Somehow, this whole diatribe about f-numbers managed to not mention focal length. So that can't very well be the distance he was thinking about squaring, can it? Dicklyon 03:03, 30 June 2006 (UTC)

I think he may have read the argument elsewhere (I found it presented on a popular photography website), and failed to understand it, but put a version of it on his web page anyway.--Srleffler 03:10, 30 June 2006 (UTC)

N = ?

"For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or N = 16.": shouldn't N = f/16, and not 16?

No, N stands for the f-number, whose value is 16. If f stands for focal length, then the absolute aperture diameter is f/16, that is, 1/16 of the focal length. Dicklyon 04:02, 9 July 2006 (UTC)
First one defines a relative aperture as . And then the relative aperture is denoted by f/N, which really would be the diameter of the aperture. It looks to me like a mathematically inconsistent convention has been adopted. Anyway, the convention is confusing. Vrangforestillinger 22:15, 15 May 2007 (UTC)
Did the article say that f/N is the relative aperture? It should not, because, as you correctly observe, f/N is the aperture diameter. If that's been done, fix it. Anyway, the relative aperture is called f-number and is denoted N; the aperture has a diameter f/N, and the aperture is often represented by that expression for its diameter, which includes the focal length and f-number as part of it. You're right that it's a bit confusing, but I don't think it's inconsistent. Dicklyon 00:27, 16 May 2007 (UTC)
Oh, I see. The 1961 ASA standard says "f/N" is the "symbol" for the relative aperture. That's not so consistent, but it's just a symbol in that case. I prefer to think of it as a formula for the aperture diameter. Dicklyon 00:30, 16 May 2007 (UTC)


"ISO speed is defined only in one-third stop increments, and shutter speeds of digital cameras are commonly on the same scale in reciprocal seconds. A portion of the ISO range is the sequence

   4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600 "

Well no this not an ISO range its an ASA range. Ericd 20:57, 17 July 2006 (UTC)

Well, no, the film-speed system originally defined by the American Standards Association was later made an international standard by the International Organization for Standardization, and is today known as ISO film speed, not ASA film speed. But it's the same thing, except that ISO also includes the option of using the "degrees" logarithmic speed specification as well. Dicklyon 21:14, 17 July 2006 (UTC)
On further review, I see your point, which is that ISO specifies the inclusion of the DIN degrees number. But, according to Film_speed#ISO_film_speed_scales, the ASA-style number alone is most commonly used, except perhaps in Germany. Do you think that your change has helped the article? It seems to me to obscure the point that was being made. Dicklyon 21:28, 17 July 2006 (UTC)
Yes I think its better, at least it complies with ISO standards ! But I'm still not conviced that we need all that scale. But the question of third stop is clearly an heritage of the DIN degrees. Ericd 21:54, 17 July 2006 (UTC)
Wasn't the ASA scale also specified only in one-third-stop increments? Which came first? Dicklyon 22:09, 17 July 2006 (UTC)
I don't think there was anything limiting the ISO scale to third stops. I don't know wich came first but the German DIN standard was by far the most precise and stable while ASA undervent some revisions see I also recently discovered that the USSR GOST standard is also another flavor of ASA noted as ASA on some camera made for the USSR market. IMO the current ISO standard is mostly based on DIN for the technical procedure used to evaluate the film speed. In adition it seems that there is no standard procedure for evaluating sensor speed on digital camera thus "digital ISO" are IMO ISO equivalents. As a result most test considers that Nikon digital cameras under-expose compared to Canon digital cameras.
Ericd 22:41, 17 July 2006 (UTC)
It says there that the ASA standard was revised in 1961. I know for sure that A.S.PH2.5-1960 "American Standard Method for Determining Speed of Photographic Negative Materials (Monochrome, Continuous Tone)" lists film speeds in 1/3-stop increments per my list. It also has a 1/3-stop scale of "degrees" but it was offset from the DIN standard. I don't know the history before that, but I'll look in some books and see what I can find.
As for digital cameras, there is indeed an ISO standard for digital camera speed ratings, and it's been around long enough that it's been revised recently. See Film_speed#Digital_camera_ISO_speed_and_exposure_index. Dicklyon 22:59, 17 July 2006 (UTC)

The International Organization for Standardization (ISO) has a performance-based ISO speed standard for digital cameras, just as they have for film. ISO Standard 12232:2006 ("Photography -- Digital still cameras -- Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index") defines ISO speed in terms of the amount of light needed to achieve a certain "quality" in the sense of a per-pixel signal-to-noise ratio.

From Film_speed#Digital_camera_ISO_speed_and_exposure_index : "However, this standard ISO speed "rating" for a digital camera is not necessarily very related to the ISO "setting" or "exposure index" used on the camera." Of course they are ISO standard for nearly everything... The dynamic range of a CCD is so narrow compared to film that I don't believe the method has much in common with the method used for film.... Ericd 23:19, 17 July 2006 (UTC)

Thanks for quoting that line I wrote. The point is that the performance-based "rating", which is based on the amount of light needed to make an image of a standardized quality level, is not much related to what setting you choose to use on the camera. Most cameras let you "push" well above the ISO speed rating. The ISO has done what it needs to do. Now it's up to manufacturers to tell you their ISO speed ratings, which they don't presently do. Dicklyon 23:24, 17 July 2006 (UTC)

comparing ISO 16 to 1/15 second

I didn't match it. However I think shutter speed is a bit out of topic here, I don't see any relationship between shutter speed and third stop increment while the relationship between DIN and third stop is obvious IMO. Ericd 23:31, 17 July 2006 (UTC)

The subsection is entitled "Fractional stops" and the sentence you are editing says "shutter speeds of digital cameras are commonly on the same scale". The shutter speed is no further off topic than the film speed is. The one-third stop increments may be obviously related to DIN in your mind, since you are European, but most American and British readers will know nothing of DIN, yet still be interested in understanding one-third-stop increments of f-number, film speed, and shutter speed. If that's not clear enough, let me know. Dicklyon 23:46, 17 July 2006 (UTC)
It's pretty unclear to me because most classic camera have only full shutter speed increments. The standard scale is 1/8, 1/15, 1/30, 1/60... Even my on my old Leica IIIa that has a bunch of intermediate speeds to help to achieve optimal exposure handheld at full aperture the scale is not by third : ....1/20, 1/30, 1/40 and 1/60...
For aperture there Historically there was no one-third stop increments there was either a non-clicked aperture selector allowing continuous aperture variation or a clicked selector allowing one-half increments.
BTW don't overestimate my DIN bias I'm French and we are historically biased against German standards ;-)
Ericd 00:53, 18 July 2006 (UTC)
My classic Kodak had shutter speeds 1/25, 1/50, and 1/100. But digitals these days typically let you have fine control over shutter speed, just as they do with aperture. Having everyone on 1/3 stop boundaries makes life a lot more sensible. Too bad the APEX system was defined with full-stop numbers.
I know you're French, not German, but on the continent people know about DIN, and off they don't, pretty much. But, as I said, ASA had 1/3-stop increments, too. I just don't know the chronology before the 1960 revision; I'll be looking into it. Point remains though, that the DIN numbers aren't going to be very useful to the typical English-language reader of these articles. Dicklyon 03:35, 18 July 2006 (UTC)

Fast lenses

Why not draw the line some place like f/1.2? A long list here pretty much dilutes the article and the point of the section. Dicklyon 00:03, 18 July 2006 (UTC)

Well IMO a fast lens is somewhat relative to the focal length... I've tried to improve this list but I'm still not sure it is at the right place. This is a list of some well-known photographic or TV lenses. But there is a lot of (mostly unknown) faster lenses for special purpose (old photocopiers for instance). I think we could move this to photographic lenses ? Ericd 00:33, 18 July 2006 (UTC)

Yes, I know, a "fast" long lens might have a higher f-number than a "fast" short lens. But the text says fast means low f-number, and if you want to change that meaning you really need to say so. Since someone introduced this section to mention some lenses around f/1, I think we ought to hold to just a few very-low-f-number lenses. Or get rid of it. Dicklyon 03:38, 18 July 2006 (UTC)


I was hunting down the term used with astronomical telescopes f-ratio and found it has no page but that this page covers it. It does have a sub-heading on the page Optical telescope -- > Focal length and f-ratio. I am thinking f-ratio is just short hand for focal ratio and that f-ratio should be directed here. If so it should mention somewhere on this page that "f-ratio" is a variant term. As noted above it looks to me like the intro should be more optically orientated (i.e. it should be more inclusive of other uses and not just "photography" with a photographic illustration). Halfblue 21:33, 5 October 2006 (UTC)

I added f-ratio to the lead where focal ratio already was; it's already generic to optics in general. Dicklyon 22:15, 5 October 2006 (UTC)

Some more stuff I am noticing:

Re: Telescopes: "the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image)" This statement in not 100% true. When imaging self-illuminating point sources at (virtual) infinity (those things we call stars) focal ratio has no effect on brightness. All this would work out better if this article was renamed F-number (photography) and maybe a new article was written that is more inclusive of the optical term "Focal ratio". Halfblue 21:54, 5 October 2006 (UTC)

I don't think that's right. If you "stop down" a telescope of a given focal length, you get fewer photons from the star. Furthermore, due to diffraction, you spread those fewer photons over a larger area. Both effects contribute to the image being "less bright". In what sense did you mean brightness is unaffected by f-ratio? Probably you mean keeping the aperture fixed and changing this focal length. In this case, the number of photons is constant, the one with larger f-ratio spreads them out more due to diffraction, making the image "less bright" in at one sense. It is not fair or sensible to say that the image of a point at infinity is just a point, since diffraction is central to all telescope imaging. Dicklyon 22:15, 5 October 2006 (UTC)
The term we are dealing with is focal ratio i.e. the ratio between the objective's diameter and its focal length (whether actual or controlled by a stop). So we are not talking about changing the diameter (stopping down). What this section of the article seems to be trying to describe is that a f-5 camera lens with a 2" objective and a f-5 camera lens with a 200" objective will give you the same "brightness per unit area of the image", i.e. the same exposure and changing the focal ratio has a proportional effect. This is true when imaging externally illuminated extended objects. This is un-true when imaging "self-illuminating point sources (non-extended) at (virtual) infinity"; focal ratio has absolutly no effect theroreticaly. And changing the focal ratio for a given diameter has no effect. In other words my f-5 binoculars and the f-5 Hale Telescope are very different beasts when used astronomically. There are changes due to Diffraction but that is a different effect. Halfblue 13:50, 6 October 2006 (UTC)
I think you're over-interpreting the comparative statement "the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image)". In the idealisation you're talking about, of point images, it is not applicable, because there is no "brightness per unit area" to measure. So I take your point. But it does NOT say that two systems are the same just due to having the same f-number; as you point out, that would be absurd. But your statement that "focal ratio has absolutly no effect theroreticaly" is equally absurd, unless you specify what kind of measurement in what theory of diffraction-free optics it applies to. I think what you mean is that the total number of photons collected from the source depends only on the aperture diameter, not on the focal length; this is also true to any non-point non-self-luminous object, by the way. Dicklyon 15:27, 6 October 2006 (UTC)

"theoretically" means I am taking into account focal ratio (which this article is about), and not taking into account diffraction (which this article is not about).

Here is the problem. You seem to be going out of your way to find instances where the wording of that whole section can be true. Citing diffraction as the basis will make it sound true... but that is not the same as writing a true article. An Encyclopedia should be at least two things---> factual and useful. When I come across the term f-ratio in an article about astronomical telescopes it’s going to take me here. The article then tells me that each numerical value is "halving of the light intensity from the previous stop". Is this factual? No…. it does not have that effect on astronomical objects. Is it useful? No... by definition something that is not factual is not useful. So there is a problem with the wording "telescopes and binoculars may have a fixed aperture, but the same principle holds". What "principle" are we talking about here? By the way this article is written it’s obvious, the definitions that come after the statement are describing "the principle". If we have one instance where a statement is wrong---> the statement is wrong.

Here is a "to-do". When someone is thinking of buying a telescope this Christmas it would be useful if there was an article what would tell them that an 6" f15 Maksutov telescope does not produce an image that is 10 stops dimmer than a 6" f5 Newtonian. Ripping appart this article about f-numbers in photography may not be the most efficient way to do it. Like I said originally we could probably use a more basic article Focal ratio (optics) that gives the basic theoretical principles and then links out to more relevant useful real world aplications like f-numbers. Halfblue 12:18, 7 October 2006 (UTC)

Why don't you go ahead a write a section on focal ratio as used in astronomy? Personally, it looks to me like you are the one stretching, trying to take a statement that is true in general and find the one condition (point source at infinity, ignoring diffraction, changing f-number by changing focal length) in which it doesn't give the answer you want, because you've made the statement completely inapplicable by removing everything that could give an area to normalize total intensity by. Or maybe it's actually not stated right, and instead of saying "per unit area of the image" it needs to be more explicit in specifying the area the light is spread over. We should be able to clean this up to clearly specify the two applications. Have a go at it. Dicklyon 15:30, 7 October 2006 (UTC)

And your f/15 scope is only about 3.1 stops dimmer than f/5, not 10 stops. But, I take your point, if it's the same diameter, it is "dimmer" only in the sense of less total light in the image due to having a smaller field of view, not in the sense of gettig less light per star. At least it's not totally inconsistent with the photographic use, which says the light per area will be less, which it is if averaged over a given size image field when looking at a uniform star field. Dicklyon 15:30, 7 October 2006 (UTC)

And one more thing; you complain: "The article then tells me that each numerical value is "halving of the light intensity from the previous stop". Is this factual? No…." Actually, with a telescope of a fixed image field size, such as a piece of film, or a given size exit pupil coupled to your retina, this is completely true, whether you "stop down" by decreasing the aperture or you choose a longer focal length with the same aperture, if the object field is "uniform" or has stationary statistics. In the one case, you reduce light from a given object field, and in the other you spread the light out so that only a fraction of the star field gets imaged to your image field. Understand this first if you want to add something that clarifies it. Dicklyon 15:36, 7 October 2006 (UTC)

Finally, there's diffraction. See the section on image quality effects. It makes no sense to have an article on f-number without a discussion of diffraction, since that's the first-order thing that limits your image resolution; this is true especially in telescopes. To say theoretically while ignoring diffraction, where diffraction is the dominant effect as it is in determining the image area of point sources, is just nonsense. Dicklyon 15:40, 7 October 2006 (UTC)

Again you are going off on tangents about the statement and not addressing the statement. If you are trying to say that the statement is true because you can recite many instances where it can be true then your logic is faulty since logic dictates that any instance where a statement is false makes the statement false. If you are saying this is about diffraction then all this is oddly placed (and un-referenced) in an article about f-number (and it also means the statement is vague). I'm going to take a few (encyclopedic) "goes" at it wile trying to keep two points in mind:
  • Statements in an encyclopedia need to be true for "all comers" (this digital encyclopedia has the ability to redirct people to a more relevant topic which may be most helpful here).
  • and Think of the reader.

Halfblue 12:59, 20 October 2006 (UTC)

Just a thought: stars are not point sources. They are, in fact, very large extended objects. If you were not limited by diffraction, the size of the image of a star would scale with focal length the same way any other image does. The point source model works only because you are below the diffraction limit, so the size of the image of the star does not scale as predicted by geometric optics. The statements in the article about the properties of f-number are true whenever diffraction is negligible, and false otherwise. Point taken, though, about the need for this article to be useful to astronomers as well as photographers and optical engineers. Some reworking of the text may be required to accommodate everyone's needs.--Srleffler 23:36, 20 October 2006 (UTC)

Focal ratio article?

I quote latest development from a section above:

I have taken a stab at addressing this by putting up a basic page on Focal ratio and removing the REDIRECT there. I think that will be more useful encyclopedically to the reader since Focal ration is more of a basic principle and this article is for the most part about the photographic application of that principle. Halfblue 13:13, 20 October 2006 (UTC)

I argue that that is not an apppropriate response. Take a look at the page. It is mostly just repetition of this f-number article, with a nice new telescope picture. I see no reason it can be incorporated here, avoiding such splintering of content. I will revert it back to a redirect, doing my best to merge any new content here first, unless somebody else objects. At present, it is just a place for halfblue's misconception or misinformation, in which he implicitly assumes that in varying the focal ratio, the only independent variable is focal length, not diameter. Dicklyon 15:10, 20 October 2006 (UTC)

Since someone beat me to reverting it, so I went back and copied the content, corrected it, and put it in a new section in this article. It's a bit redundant, but that's probably OK. Dicklyon 20:28, 20 October 2006 (UTC)

That was me. I missed the discussion here. I strongly disagree with forking "focal ratio" from "f-number". They are the same thing. Much better to have a section in this article that deals with the differences between photography and astronomy. If a fork were necessary, the appropriate fork would be to keep this article covering the optics and photographic uses of f-number (or focal ratio), and have a more specific page on focal ratio in astronomy. I do not think that that is necessary, however.--Srleffler 23:11, 20 October 2006 (UTC)

I see the article has been moved to f-number. Here is the reason for the article Focal ratio---

  • There is a basic term Focal ratio.
  • That term needs description.
  • That term on its own is referenced in several articles by the term "Focal ratio" or "f-ratio".
  • Those references are mostly about the basic concepts of Focal ratio and its effect on an optical system - not having anything to do with photography.

The problem I see with f-number is not a problem with the article its self (although it does have some fixable errors), it is its place in terms of an encyclopedia. To repeat something I said somewhere else--- There is an old Microsoft joke[5] where a software engineer gives an answer that is 100% correct and relatively useless. I get that feeling when I try to link to f-number.

  • f-number is an article biased towards photography. f-number starts:
In photography and optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture [1]) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens.

and has a diagram of standard photographic stops opposite that statement. In other words this is an article about the photographic application of "focal ratio" more than the article describing the basic concept of "focal ratio". The lead in paragraph is not a good description of "focal ratio" as it applies to mirrors, radar/radio telescopes, and parabolic sound reflectors, ect. That could mostly be fixed by replacing the word "lens" with the word Objective (optics) but it shows the bias in the article.

I don't know if there is a wiki term for this but "f-number" is getting to be a mega-article. It is trying to describe everything under the sun that has to do with the ratio of stop to focal length and doing it poorly. I was noticing this before with Aperture. If you link Optical telescope - Focal length and f-ratio to f-number the article tells you a few things that are flat out wrong re: telescopes, stellar point sources, and focal ratio such as:

  • "The greater the f-number, the less light per unit area reaches the image plane of the system."
  • "Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image)."

So--- when we send people to f-ratio when they click a link with the question "I wonder what "f-ratio" does to these different types of telescopes?". Coming into the top of this article they are not going to get an immediate answer (and they are going to say "wait a minute... this is about photography"), Then they are going to get a bogus answer, then, if they preserver, scanning down what is getting to be a long article they may find the newly added content on telescopes. Its not the readers fault if they are confused. They have been given an answer that is about 95% correct and (from their laymans point of view) almost totally irrelevant to what they were looking for. I think this is what is being missed and what Wikepedia means when they say Think of the reader. Directly linking them to F-number#Focal ratio in telescopes may work for a wile till someone says "wait a minute.. thats redundant.. lemme clean it up and make that sub title "f-ratio"" and kills all the links (unless there is some redundancy in wiki-linking that I am not aware of).

Some more observations:

  • this is not a technical manual... it is an encyclopedia that laymen are reading and should be biased that way.
  • this is an electronic encyclopedia where linking can be used to take a reader directly to what they need to know.
  • I see no reason why a basic concept should not have its own page and just be linked to or have a notation that this is related to, or even another word for, another concept.
  • trying hoard all related information in one mega article that tries to be an all-inclusive everything for everybody is not making good use of the abilities of Wikipedia.
  • I have not gone into the history of the article f-number but looking at it cold I would say it is in fact a subset of the concept of focal ratio and should be a sub heading in a focal ratio article or even be a related article .. uses in photography.

That is more than my two cents.. Halfblue 00:14, 21 October 2006 (UTC)

You make an crucial error right at the top of your comment above, that affects your whole argument: you talk about "a basic term Focal ratio" (emphasis mine). Wikipedia articles are about things, not about the terms that describe them. This is a core principle, that goes right back to Wikipedia is not a dictionary. Each thing gets one article, no matter how many different terms there are that describe it. If there were some difference between focal ratio and f-number, it would be appropriate to have two articles, but this is not the case. They are the same thing. Wikipedia handles multiple terms for the same thing with redirects, not duplicate articles.
Now, I understand that the content of this article is not appropriate for the astronomy audience, and I agree that an amateur astronomer (or photographer or physics student) coming to this article should immediately find a description that is relevant to their needs, and not be told things that are incorrect. The proper response to that is to adjust this article to better incorporate the needs of astronomers without losing its relevance to optics and photography. This article has already been through some of this, in that initially it really was solely focused on photography. A great deal of work was put in by myself and others to make it more general and at least technically correct for optical systems beyond cameras. (caveat: I haven't looked at this page in some time; it somehow got dropped from my watchlist). Apparently more of this is required, to suit the needs of astronomers. If that makes the article too unwieldy, then it would be acceptable to spin off a more specialized article such as focal ratio (astronomy) or f-number (photography). I do not believe that this is necessary, however.
BTW I'm entirely with you on the photography bias. There has been a problem for a long time with optics articles being overly focused on photography, and including distortions and errors that are commonly believed among photographers. It is getting better as the optics content improves, but there is still work to be done. The proper Wiki response to the problem is to fix it, though, not to fork off separate "non-photography" articles.--Srleffler 01:00, 21 October 2006 (UTC)

Besides, the stuff you assert is "flat out wrong" is actually correct, as I've explained in various terms. If there's a better way to explain the effects you care about in astronomy, by all means let's add them, as I tried to do in incorporating your focal ratio article as a new section. I see no inherent difficulty in making this article suit the needs of astronomy, microscopy, and photography. Just add the material as suitable. Dicklyon 06:11, 21 October 2006 (UTC)

Halfblue, I did a quick GBS and found a source for your idea: book page. However, the statement that comparing two 8" scopes the image brightness is not at all dependent on focal ratio is restricted by "if the two scopes are used at the same magnification", which changes it completely, into something that is now true. Without the qualifier, one might naturally assume that brightness referred to equal treatments of the focal plane, e.g. same film or same CCD or same eyepiece, in which case you need some other restriction, such as the total optical power in the spot even as its size changes, to make it true. But for either of these, you also need to make it clear that the diameter is being held fixed, and the focal length is what you are varying. Otherwise, claiming brightness independent of focal ratio is just obviously wrong, in general, for any definition of brightness. Dicklyon 06:20, 21 October 2006 (UTC)

Just curious. How do you vary the magnification of a scope while keeping the focal ratio constant? I haven't thought it through carefully, but I would have thought that the magnification would be fixed if the focal length is fixed.--Srleffler 13:07, 21 October 2006 (UTC)
By the secondary optics; for example, by choosing different eyepieces. If you just stick a piece of film at the prime focus, the focal length alone determines the magnification, and in that case the light per unit area varies as in photography, while the total light per star depends only on the aperture diameter; these are mutually consistent if the per unit area measure is average over a uniform star field or the light per unit area is measured in the Airy disk, I think. Dicklyon 14:20, 21 October 2006 (UTC)
I thought that was it. In that case maybe we do have justification for a separate page for the astronomical use of the term. In optics, the f-number is usually the f/D ratio for the whole system. In astronomical usage, it seems to be conventional to exclude the eyepiece, etc. At the least, this distinction needs to be clearly mentioned in the article, especially since it seems to be the source of the disagreements about whether brightness scales with f-number. As optics uses the term, the brightness of a telescope image does scale with f-number. It's just that the thing the astronomers call focal ratio is not the f-number of the system, and therefore it has different properties.--Srleffler 16:12, 21 October 2006 (UTC)
There may be some distinctions worth mentioning there, but the source of the disagreement is I think more to do with "point sources" versus "extended objects". These distinctions are made in the astronomy books I looked up. For extended objects, a per-unit-area measure works just as in photography, while for point sources they ignore area and count what is essentially the total optical power, and call it the same thing, namely "brightness". The distinction you're talking about may also be useful, but is not unique to astronomy. The focal ratio or f-number is applicable to any part of the system that makes a real image, and you are right that if you include the eyepiece and the eye you'll get a different focal length, and possibly also a different aperture stop and entrance pupil, and therefore a different f-number for that whole system. But it is not conventional, as far as I can tell, to analyze that whole system; rather, the f-number is typical a characterization of the objective lens/mirror subsystem on its own, in photography as in astronomy; in microscopy, they do the same, but use "numerical aperture" instead. Dicklyon 17:48, 21 October 2006 (UTC)

Sorry for slow replies.... the ol' Wikipedia hobby has to take a back seat to life sometimes. Each thing gets one article, no matter how many different terms there are that describe it gets you to the heart of the problem with the f-number article. f-number is not the "thing". It is a number resulting from the thing. The thing here is Focal ratio... f over D. f-number is the results of that equation. Articles get renamed or redirected to their more basic form all the time. In this case we got the redirect going the wrong way. Setting up something like focal ratio (astronomy) is not really a fix because focal ratio is not limited to astronomy... or even the optical spectrum. f-number (with a few minor corrections) is a good article on the applications of focal ratio but it would take a lot to make it inclusive of everything about focal ratio (the intro paragraph needs to be totally rewritten for a start). All of that would be tackling the symptom but not the cause... an article with the wrong name. The simple fix is to set up a page for Focal ratio noting that there are many different applications of the principle (which I have already done) and make some minor edits (and maybe even a name change) to f-ratio to note it is about applications of a more basic principle. Halfblue 12:52, 26 October 2006 (UTC)

There may be a POV issue here between fields. In optics, f-number is the fundamental thing you are calling focal ratio. In absolute terms, both of them are "just a number", or more precisely just a ratio of two numbers. Neither is more fundamental in absolute terms. I feel that the optics usage of the term should take precedence over other fields, because optics is the science that studies these things. The article is then appropriately named from that point of view.--Srleffler 15:09, 26 October 2006 (UTC)
I wasn't previously aware of this divergence of terminology between fields. A quick google books search shows about twice as many books wiht f-number as with focal-ratio, which supports the idea that f-number is the more general term. Certainly it's the one in my optics books. It would be interesting to investigate the history here. I suspect that the term focal ratio was used in astronomy long before f-number in photography and optics more generally, but that doesn't make it "primary" in the current world. Dicklyon 18:18, 26 October 2006 (UTC)
There are a lot of terms like that, where the astronomers have their own usage, distinct from physics. For example, photometry and luminosity. Intensity is another one: for a physicist intensity is synonymous with irradiance. In optics, the term is avoided, but might be taken to mean irradiance or radiant intensity or luminous intensity. The astronomers, however, use none of these. For them, intensity is synonymous with radiance.--Srleffler 20:26, 26 October 2006 (UTC)

Lens speed

This section may end up tying together several threads begun in sections above. I see that someone has created a new article on lens speed. At first glance, that might seem to be redundant with this article, but I think it is actually a good idea. As constructed, that article focuses on the concept of "speed" of a lens, and its role in photography. It eschews the technical details, and uses f-number mainly as a benchmark of "speed". That is a nice complement to this article, which has all the technical detail that is so important to optics. f-number is probably too technical for a reader who is an amateur photographer with little interest in optics. I think we should maintain this separation, and ensure that the links between the articles guide photographers with an interest in more technical detail here, and those with no interest in it there.

We may also want to move some material between the two articles. For example, I moved the section with the list of fast lenses to the other article. It is a much better fit there than here, both conceptually and in terms of who is likely to read which article. One option to consider: we might want to deliberately reduce the focus on photography in this article, and make it more aimed at coverage of f-number in general (i.e. encompassing optics, astronomy, photography, etc.), leaving the lens speed article to cover the aspects of particular interest to photographers. I propose this with some doubt, however: it's not clear to me that that is the way to go.--Srleffler 05:17, 10 November 2006 (UTC)

Help with some comparison


The Barry Lyndon articles states that Kubrick used a NASA f/0.70 focal ratio. Problem is, I don't have a clue what it means out of technical thing. Could someone give comparison examples from real world? Like how far can it see, how dark can it see, what is the animal that comes closest to this, and so on. Thank you.
David Latapie ( | @) 08:00, 29 November 2006 (UTC)

It's bigger in diameter, relative to focal length, than any animal eye, I think. With it one could film in 1/8 as much light as a more typical f/2 lens. Dicklyon 15:48, 29 November 2006 (UTC)
Given that Kubrick used it to allow filming by candlelight, one might argue that the camera+lens combination approaches the capability of the human eye. We see quite well by candlelight, but a conventional movie camera+lens does not: it requires very bright lighting to get good images on the film.--Srleffler 19:31, 29 November 2006 (UTC)
Thank you for these interesting, albeit contradictory, answers. Where could I get some more information regarding the difference between “organic” seeing and “mechanic” seeing?
David Latapie ( | @) 21:03, 30 November 2006 (UTC)
I don't see any contradiction there. We're both right; the movie camera needs to collect more light to make a movie than what we need to see, so the lens is much bigger in light collection area than our pupils. You'll need to study all about vision and photography, including quantum chemical effects of light, to get to the bottom of these things. Dicklyon 21:39, 30 November 2006 (UTC)


I read your definition of F-Number, and I'm sorry to inform you that there is a mistake using the term "square root of 2." The language states "....sequence of the powers of SQRT(2) (or 2^0.5)...." You should correct this to say ...sequence of powers of 2^x with x=0 -> x=n in increments of 1/2 i.e. 2^0, 2^0.5, 2^1.0, 2^1.5, 2^2....Etc. Therefore 2^2 would correspond to an F-Number (or F-Stop) of f/4. 16:49, 29 November 2006 (UTC)

The current wording is just fine. It is the sequence of powers of , i.e. Your proposed definition is mathematically fine, but is more confusing and as worded above is incorrect. The sequence you describe is not a "sequence of powers of 2^x".--Srleffler 19:27, 29 November 2006 (UTC)

I have another mathematical-interpretation question. The article seems to consider that an f-number of f/4 is smaller than f/8. From a mathematical expression however, if f is constant, f/8 is a smaller number than f/4. Unless I am forgetting/misinterpreting something here, I think this is very confusing. It may very well be that in the photographic world this is the way they consider it, but in that case I would mention this confusion in the article. --Patrickdepinguin 21:13, 26 May 2007 (UTC)

That's right, the f-number 4 is smaller than 8, and the aperture f/4 is larger than the aperture f/8. It's not a confusion, but it means that ordering of f-numbers and apertures need to be made clear. Is there some place in the article that is not clear? Dicklyon 21:49, 26 May 2007 (UTC)

I believe that this is naturally confusing. In photography, one often says "increase" or "decrease" regarding aperture or f/stop. In that case, "increase" means increasing f/#, and so decreasing #. I have never known in photography the use of the words "increase" and "decrease" to correspond with the #, but always its reciprocal. Not that the article is wrong, but it is hard to argue with tradition. Gah4 (talk) 08:40, 26 September 2011 (UTC)

New Table

The new table has no generality, and serves only to illustrate that each full stops cuts the area in half. This is easy enough to see without spending a big table on it, in my opinion. I vote we remove it. Srleffler has already tagged it as questionable. Anybody think it is worth keeping? Dicklyon 05:59, 30 December 2006 (UTC)

Actually I don't think it is "easy enough to see without ... a big table". There are a lot of words and some forumalae in the article, but the table illustrates very clearly that what happens to the diameter (therefore radius), and how that affects area. In one location you have the three simple steps of the f/stop: (1) f-number becomes larger, (2) radius shrinks, (3) less area and therefore light gets in.
When I print this article out, there's nine pages of output, and this table would be the clearest indication of the f/stop-area relationship. This gives a concrete example to the general concepts of this article. It may be too big, or be in the wrong section, but I think it best illustrates what goes on in a lens. —The preceding unsigned comment was added by (talk) 18:38, 1 January 2007 (UTC).
I went ahead and added a statement about the area halving in the first figure caption, where it is illustrated; and removed the table and the other link to the "tedious explanation" page. Is that your page by any chance? Dicklyon 19:28, 1 January 2007 (UTC)
No, but it was the only page that I came across that explained how the f/# affected the amount of light getting through a lens in a clear away. Everything else seems to use high falutin' concepts and language (with equations thrown in for fun). If you do a Google search for "f/stop" the 'tediuous' explanation page comes up first (the Wikipedia entry is second). It's a much more down-to-earth explanation that a layman can grasp (heck, I'm an EE and my eyes glaze over trying to grok this article sometimes).
This is sort of where the Cheat Sheet (see below) comes in handy. People with a science and strong mathematics background can work from equations. Other learning styles need examples to 'get it'. Dicklyon, I think this viewpoint is critical to understanding why I see the value in a crib (tabulation) sheet and you don't. If you accept people have multiple learning styles then you may also accept multiple presentation of data (ala tables & crib sheets). --GordonMcKinney 22:40, 7 January 2007 (UTC)

Cheat Sheet again

I propose we remove this talk fragment. Since the sheet doesn't exist on Wiki, the F/stop page doesn't refer to it, and I'm not planning on adding it again. --GordonMcKinney (talk) 01:58, 16 May 2008 (UTC)

Neffk's new lead

I reverted this new lead:

F-number is a technical term used to describe the amount of light allowed through a lense system.

The term was originated when photographers used cameras that allowed the photographer to view the scene before exposing film. In order to make the preview as bright and easy to see as possible, the photographer would open the lense aperture as large as possible. Then, before exposing the film, the photographer would close the aperture to account for film speed, motion, and depth of focus. Eventually, cameras opened and reduced the aperature automatically, and one only set the size to which the reduction would be stopped. The aperture starts open and is "stopped down" to the appropriate diameter just prior to exposing the sensor.

Synonyms of f-number have been introduced over the years, many of which survive today. These include: focal ratio, f-ratio, or relative aperture[1].

The focal length of the lense system is used because the intensity of light is concentrated at higher magnifications. Focal length is a simple way to describe the magnification capability of a lense. Many people have an intuitive grasp of this because it can be demonstrated by burning paper with a magnifying glass in direct sunlight. Covering the lense partially prevents the focused beam from burning the paper because the intensity of light is reduced.

Aperture diameter defines to the amount of light allowed through a lense system, and this is related to the length of time the sensor must be exposed to the in-coming light. The relationship between aperture diameter and exposure time is often quoted as lense speed.

If any of this is correct, please cite a source and we can put it in. Seems mostly not correct to me. Dicklyon 07:12, 26 June 2007 (UTC)
Let's talk specifically about the issues here, instead of just reverting everything. Specifically, what is wrong?
My comments are intercalated below your bullets. Dicklyon 14:38, 26 June 2007 (UTC)
  • The definition, first paragraph, is correct as far as I can tell. It may be more correct to say that the f-number describes the specific (per unit area) intensity of light that falls on the sensor due to an in-focus source. Whatever we decide upon, the current definition is true from a technical point-of-view, but fails to convey the utility and importance of f/D.
It's an indirect definition, and it's wrong; it's not "the amount of light" in any direct sense.
  • My understanding is that the use of "stop" is related to camera mechanics. Perhaps this should be edited and moved out of the opening paragraph.
What does it mean? Where does it come from? What does it have to do with f-number? See aperture stop and diaphragm (optics) for what's true.
  • The synonyms were contributed by others and were on the previous page. I think they should be moved out of the opening paragraph, where they distract from the definition.
"Focal ratio" is an important alternative name, the main name used in astronomy. Relative aperture is also a very common name. Such alternatives are usually listed in bold in the first line with the one chosen as the article name.
  • The bit about magnification should not be left out. I don't see a clear explanation of the relationship between focal length and aperture size anywhere else on the page. 14:32, 26 June 2007 (UTC)
What does it mean? How does "magnification" relate to focal length or f-number? Source?
And what's wrong the rather precise way we had it before? Dicklyon 14:38, 26 June 2007 (UTC)

Why is focal length important? How can that be incorporated into this article? neffk 15:06, 26 June 2007 (UTC)

f-number is defined as the ratio of focal length to the diameter of the entrance pupil. If this isn't clearly enough stated in the article, we can fix it. Focal length also matters to lots of other things including magnification, angle of view, depth of field, focal-plane illuminance, etc. Is some of that what you're thinking or asking about? Dicklyon 18:07, 26 June 2007 (UTC)

I've had time to think and re-read the text of the article and what I tried to contribute. Initially, I was intimidated by Dicklyon, but I think maybe I should have been more bold about discussing the changes I proposed. I see that many of the points were not appropriately addressed. One of the reasons for reverting my lead was an appeal to tradition: "...what's wrong the rather precise way we had it before?" That's against Wikipedia policy, as I understand it; Wikipedia:BRD. Anyway, see the items below below. neffk (talk) 16:34, 25 June 2009 (UTC)

  • The opening sentence is difficult to read. I see no reason to have all alternative phrases right there. I agree that they should be in the introduction. But I think they should be put in a separate paragraph, toward the end of the intro. The objection posted previously seems irrelivent; it implied that some of the terms had been omitted. If that was the case, I didn't mean to omit any terms. I only meant to move them to a more appropriate location in the text. neffk (talk) 16:34, 25 June 2009 (UTC)
  • Every site on the interweb has the trivial definition of f-number. This article defines it directly. Good. Now let's add to that. I think this article should explain how the f-number relates to lens speed. neffk (talk) 16:34, 25 June 2009 (UTC)
  • Camera mechanics may not be the focus of the article. But the whole lead was reverted, not just the part that was related to camera mechanics.
  • magnification, specific intensity (photons/area), and 'lens speed' are related. It doesn't matter if it "seems mostly not correct" to Dicklyon. If it's wrong, explain why it's wrong. Note that lens speed is mentioned in the current intro. And see the bit about t-stops. Clearly, f/D is useful both because it controls the specific intensity of light that gets to the sensor and because it sets the depth of field.
The ball's in your court; you were bold, I reverted, we discussed, and now you can be bold again, but careful in light of the discussion (and that means includes sources for any assertions that I might find confusing or questionable, so we can verify or at least check what your sources say). Dicklyon (talk) 16:40, 25 June 2009 (UTC)
Fair enough. neffk (talk) 18:22, 25 June 2009 (UTC)

Relative aperture is the correct term

The relative aperture of a lens in photography is a ratio of two numbers. What numbers are these?

1) is the diameter across the lens opening (the aperture) (Really the effective aperture - which is what can be seen in actual fact by moving the eye from side to side - multi-element lenses will give a different value than the actual diameter)

2) is the focal length of the lens.

The effective aperture (f stop) is supposed to give the same figure for big cameras and small cameras, for big lenses and small ones.

f/11 means the diameter of the lens will divide into its focal length eleven times.

This can also be expressed as 1:11 where 1 is the focal length of the lens. (Lenses are marked say 1:11) This is known as the maximum relative aperture of the lens.

If we write f/11, f means the focal length of the lens and the slash means "divided by".

The relative aperture gives the amount of light reaching the image plane (eg the film) because the inverse square law says that light increases (or decreases)by the square of the distance. Halving the distance to a light source will not double the light it will quadruple it. And increasing the distance by three times will reduce the light by the square of three = 9. Only one-ninth of the illumination will get there.

In a camera, the relative aperture takes this into account. RPSM 16:22, 11 November 2007 (UTC)

What is your point? "Relative aperture" is already mentioned in the lead as an important alternative term for f-number. Is any part of it not explained as clearly as you would like? Dicklyon 16:26, 11 November 2007 (UTC)

Hi, I was looking at other articles, and some people in other places are getting themselves tied up in knots. Here is from article Camera

Various Cameras: An Agfa Brownie, Polaroid Land Camera, and Yashica 35 mm SLR The size of the aperture and the brightness of the scene control the amount of light that enters the camera during a period of time, and the shutter controls the length of time that the light hits the recording surface.

Well - it's the value of the relative aperture innit?

No, that's a common misconception. The amount of light admitted is proportional to the absolute aperture area. What's determined by the f-number and scene luminance is the focal-plane illuminance, that is, the amount of light per unit area. Dicklyon 17:14, 11 November 2007 (UTC)

I only have simple math, and I was thinking of explaining f/number in the simplest terms possible to make it available to the youngest and least technical readers (like me).

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens.

in terms of is no doubt correct, but will it be understood by people with only + - x and ./. ?

I prefer the focal length divided by the effective aperture.

OK, see if you like the "simpler" definition I added; I omitted "effective" from both aperture and focal length for that one. Dicklyon 17:14, 11 November 2007 (UTC)

What's the effective focal length?

Is that the actual focal length in use, or what. How can you reduce the explanation to be as simple as possible for even young readers?

Well, it's either the actual focal length, or in some cases corrected for lens extension for closeups. I'm not sure if that's what was intended here or not.

You use the term "relative aperture" I think this is the correct term. F stop etc is everyday talk.

"F-number" is a pretty standard term; "F-stop" is more ambiguous and more informally used. Dicklyon 17:14, 11 November 2007 (UTC)

This article is fine. I will try to find the other places in wiki where they go into paroxysms of mathematics without starting from basic principles. Best RPSM 16:47, 11 November 2007 (UTC)

Did you do the article on '''Aperture''' as well?

That looks ok.

I'm just into very simple explanations for beginners without too much math. It doesn't have to be like that on Wiki.

At some point the relationship between basic definitions (focal length, f/stop, etc) and how the math works forces non-mathematicians to say: now I have to get the hang of this in mathematical terms.

Of course light captured by a lens is one thing, and light received at the image plane another.

Are we going to go into foot-lamberts and lux as well?

And what is the math for exposure meters and such? RPSM 17:04, 11 November 2007 (UT

I've worked on lots of photography related articles, trying to improve them. Thanks for the feedback and ideas. Most of this stuff is not hard to understand with a little geometry and algebra; the photometry stuff can get daunting, though (lux, lumens, etc.); there are articles on all that stuff. Dicklyon 17:14, 11 November 2007 (UTC)

f-number is defined as the ratio of focal length to the diameter of the entrance pupil.

This is what you said in chat above: I like that better.

Of course, when you look at a lens it has on it either F/4.5 or else 1:4.5 as well as the focal length expressed as f= 7" or f=5cm.

I just thought that beginners could relate theory to practice by showing that these are mathematical expressions that give a key to the theory. ¨¨¨¨ —Preceding unsigned comment added by RPSM (talkcontribs) 18:09, 11 November 2007 (UTC)

One origin of the word stop is from Waterhouse stops, which, as you know, are slips of metal with a hole in. Let's say you are only using one stop all the time, you stick in the stop and you don't have to read any scale or anything. And you can directly measure the diameter on a rule. They are used with lenses for large field cameras or studio cameras. In that case, you did not have to go round to the front of the camera and peer at the aperture scale.

The other type - normal today - is the iris diaphragm. (diaphragm alone is used for a number of things in other fields) RPSM 18:15, 11 November 2007 (UTC)

I don't think you have the sequence right. I think stops were around before Waterhouse stops. Dicklyon 20:26, 11 November 2007 (UTC)
Just found a 1929 reference (before the invention of photography, stops were well known in other optical instruments). Dicklyon 20:35, 11 November 2007 (UTC)

Focal length, effective focal length, relative aperture, effective aperture

We don't have a simple explanation of focal length. I always say it is the distance from the optical centre of the lens to the image plane when the camera is focused on infinity. Simpler: When the camera (lens) is focused on a distant object - a range of mountains or the sun, the rays of light entering the lens are parallel, or very nearly so for our purposes. Then the distance from the optical centre of the lens to the film (image plane) is the same as the focal length of the lens. is a simple explanation. However I like the full-blown technical explanation as well, and I want relative focal length back somewhere. I did a photography exam once and one question was: What is the difference between relative aperture and effective aperture? and it wasn't in the books we had. Now I get it after seeing it here. Thanks. RPSM 13:58, 12 November 2007 (UTC)

Your term "optical center" remains undefined. The correct point to measure from is actually the rear nodal point, which is defined. And what is "relative focal length"? Dicklyon 15:46, 12 November 2007 (UTC)

Sorry, effective focal length I'm learning all the time. RPSM 14:45, 13 November 2007 (UTC)

optical center

First two references are for opticians (eye doctors), third one is from Slovenia: "So now we come to the second most used formula in optical theory -- Prentice's rule. This formula gives us an approximation of the amount of prism present at any given point on a lens based on the dioptric power of the lens and the distance that the point is from the optical center of the lens." [I think they mean the optical axis (?)] Next reference is better: "The optical center within the camera represents the point in which all the light rays that form the image intersect if the lens is approximated with a single thin lens." It looks like these people were having a hard time trying to find the optical centre - Is that what you mean by saying it is undefined? RPSM 14:50, 13 November 2007 (UTC)

They're just using sloppy language. There is no such point that the second definition implies. Dicklyon 15:28, 13 November 2007 (UTC)

Thanks RPSM 08:01, 14 November 2007 (UTC)

Other f/#s?

I've seen image-space f-number = effective focal length / entrance pupil diameter and paraxial working f/# = 1/ (2 tan U') where U' is the angle between the entering marginal ray and the exiting marginal ray. Also, working f/# = 1/(2 sin U'). Should this article cover these topics or should they get their own pages? 19:21, 3 December 2007 (UTC)

Find us a ref and we can talk about it. I think your first definition is equivalent to what we have, or very nearly so, and it would OK to talk about it if we have a good source. I don't know about the other one. Dicklyon 03:58, 4 December 2007 (UTC)
I am looking at Introduction to Lens Design (With Practical ZEMAX Examples) by Joseph M. Geary. On page 19, it discusses "more on f-number". It says "The traditional f-number is given by the `image space f-number.'" then goes on to define paraxial working f/# as defined in the above equations. "If the object is at some finite distance, then the bend angle U' will be different resulting in a different effective f-number." (talk) 20:40, 26 December 2007 (UTC)
That would be a good construction to illustrate, and show when it's equivalent or not to the usual definition. Certainly it doesn't need a separate article, it's just a different way to write the f-number, in terms of angles and trig instead of as a ratio.
If you imagine an object at infinity, on axis, the incoming rays, including those at the margin of the lens, at at zero angle. The exiting rays converge at the focal length, so their exit angle is tan(r/f) at radius r on the lens. When r is maximized at half the diameter d, you have U' = arctan((d/2)/f). In the paraxial case (radii all small compared to distances, the sin and tan will be equal, so you can also call it U' = arcsin((d/2)/f). Then tan(U') = sin(U') = d/(2f), and the usual definition of f-number = f/d is the same as 1/(2tanU') or 1/(2sinU'). I think the tan is more accurate, or at least more like the conventional definition, but the the sin may give a number that's a better description of how much light you get on the focal plane. You can probably find more about this in other books. For objects closer than infinity, I'm not sure, but perhaps it's equivalent to adjusting to the "effective" focal length including focusing extension? Work it out or look it up. Dicklyon (talk) 22:27, 26 December 2007 (UTC)

What does it measure, really?

The article says f/number measures lens speed. It seems to me that effective lens speed will have more to do with the entrance pupil area times the spherical angle of view divided by the area of the sensor. That is, how much light comes in divided by the area it is projected against. Furthermore, in theory I could imagine a lens with a huge entrance pupil and a narrow angle of view but a tiny exit pupil and a tiny back focal length. Obviously within one lens, changing the f/number changes the amount of light accordingly, but between lenses, what does it really measure? (talk) 17:19, 27 December 2007 (UTC)

That's exactly what it measures. Light collection efficiency per unit area at the sensor. Or focal plane illuminance relative to scene luminance; but you have to square it, throw in the appropriate pi factors, etc. Dicklyon (talk) 18:27, 27 December 2007 (UTC)
But then why is it completely determined by the distance between the exit pupil and the image plane and by the diameter of the exit pupil? (talk) 18:32, 27 December 2007 (UTC)
Just because that's equivalent, at least for distant objects; see this book for a general explanation in terms of numerical aperture, pupils, etc. Maybe you can write it up and improve the article. Dicklyon (talk) 18:37, 27 December 2007 (UTC)
Interesting. I think I read that carefully, but still don't get it. Suppose I take a picture of the sky with one lens with a given focal length and exit-pupil diameter. Suppose I replace it with a different lens with the same focal length, view angle, and exit-pupil diameter but a larger entrance pupil diameter by a factor of sqrt(2). Wouldn't the second lens collect twice as much light and therefor be twice a bright an image? Or is there some constraint that makes this second lens impossible? Thanks. (talk) 19:52, 27 December 2007 (UTC)
For what you said to be possible, the exit pupil on the new lens would have to be sqrt(2) closer to the focal plane, so it all works out. That is, the f-number is constrained to be the same on the exit and entrance sides. Dicklyon (talk) 20:54, 27 December 2007 (UTC)
Interesting. That is exactly what I wanted to know. That is not immediately obvious, though. Do you have any references for that (or was I overlooking something in the above reference)? Thanks. (talk) 14:33, 28 December 2007 (UTC)
I do have lots of optics books, but not time to work on it right now. You can see that it must be true, however, by conservation of energy. Dicklyon (talk) 17:24, 28 December 2007 (UTC)
I'll stare at it more later. The thing I'm thinking is that the view angle is related to the focal length in that bending the chief ray with a lens would also bend its corresponding marginal rays, thereby changing the focal length. This ties into my question at Talk:Photographic_lens#Focal_length_clarification. Thanks again. —Ben FrantzDale (talk) 17:58, 28 December 2007 (UTC)

Here are a couple of good books about all this. Lee speaks of both the entrance-pupil-based and exit-pupil-based approach to f-number, and the fact the pupil magnification may make them not equal; for object at infinity, they will be equal, but otherwise it gets complicated. Also, note that the other book uses positive magnification convention. Dicklyon (talk) 06:56, 25 January 2008 (UTC)

Working f-number and magnification

Now that we've got m negative again, in agreement with the cited source, and "working" f-number being N*(1-m), maybe we should reconsider. When I search books for "working f number", I get pretty much nothing. But with "effective f number" I get quite a lot of hits, and at first glance it appears that more of them use the photography convention of unsigned m, rather than the optics convention of algebraic m. The forumula with 1+m (or 1+M) is common in these books. Should we consider re-wording, and at least give the two conventions equal weight? Dicklyon (talk) 01:25, 25 January 2008 (UTC)

I agree, the photography convention is probably better here. Although deciding which articles should go which way is tricky... should I change the angle of view article back to +ive m, or leave it with -ive m since it references the magnification article's formula. Ugh. ǝɹʎℲxoɯ (contrib) 01:42, 25 January 2008 (UTC)

I vote for the photographically conventional positive / unsigned magnification. This case is explicitly mentioned in the magnification article too. So I would suggest to change all photographically related occurrences of M and especially this odd N*(1-M). Alex.altmaster (talk) 17:05, 29 January 2008 (UTC)


There is a discussion on a change in typography for f/ happening at Template talk:F/. Please join in if you care about f/ vs. f/.--Srleffler (talk) 05:39, 3 August 2008 (UTC)

Effects on image quality: Garbled sentence?

This sentence from "Effects on Image Quality" doesn't make sense to me. Is it missing a phrase, or am I just confused? "Therefore, reduced-depth-of-field effects, like those shown here, will require smaller f-numbers (and thus more complex optics) than do larger format cameras." Billgordon1099 (talk) 16:55, 2 September 2008 (UTC)

I have attempted a fix. See if it makes sense now (in the context of the sentence that precedes it). Dicklyon (talk) 05:13, 3 September 2008 (UTC)

3.3 in the typical third scale

maybe a footnote on 3.3 would be in place? It should really be 3.2, since 2^((10/3)/2)=3.1748, but probably for some historical reasons the 3.3 from the half scale was inherited.

There is even a note on 3.2, but it might be confusing that 3.2 is not to be found on the scale.


Bo —Preceding unsigned comment added by Babbletower (talkcontribs) 22:17, 20 September 2008 (UTC)

I don't know if there is a historical significance to f3.3 but in the half-stop table, 2^[(7/2)(0.5)] ≈ 3.36 = 3.4 by round off rules. —Preceding unsigned comment added by (talk) 16:48, 23 April 2011 (UTC)

Relative aperture

I had some minor issues with the current intro text; eg. the confusing use of "effective" aperture ... and the unintentional suggestion that "f-number" was the preferred term for relative aperture in the scientific field of optics ... minor stuff, but I drafted a "new version" for your approval ;)

  • In optics, the relative aperture is a measure of the light-gathering power of an optical system[2]. In photography it is usually defined by N = f / e, where f is the lens' focal length and e is the entrance pupil diameter[3]. When used to indicate a given 'aperture' setting, eg. "f/2.8, f/4, f/5.6", it is often called the f-number or f-stop. The maximum aperture denotes a lens' widest or largest aperture setting (represented by the smallest f-number) and is often inscribed around the front of the lens barrel as a focal ratio or f-ratio such as "1:2.8".
  1. ^ Smith, Warren Modern Lens Design 2005 McGraw-Hill
  2. ^ Encyclopedia Britannica [1] retrieved Feb 2009.
  3. ^ [2] Ray, Sidney F. Applied Photographic Optics: Lenses and Optical Systems for Photography, Film, Video, Electronic and Digital Imaging Focal Press, 2002, ISBN 0240515404. p.126

It might also be helpful/illustrative to point out the the focal ratio on many zoom lenses includes two values for maximum aperture; eg "1:3.5-5.6", the first referring to max aperture at shortest focal length (wide end), the second at longest (full zoom). AFAIK, the maximum physical aperture itself does not change relative to focal length, but the relative aperture does. --Redbobblehat (talk) 06:31, 18 February 2009 (UTC)

The Britannica article is mixing up several concepts in a very confused article. Why not propose something based on decent sources instead? And don't introduce new terminology like e for what everyone calls d. And don't change the topic of the article. Dicklyon (talk) 06:49, 18 February 2009 (UTC)
IMO unsubstatiated remarks like "The Britannica article is mixing up several concepts in a very confused article" only weaken the credibility of your arguments. I can't claim to speak for "everyone", only the authoritative sources I've referenced. -- Redbobblehat (talk) 16:39, 18 February 2009 (UTC)
It seems to me that the 'topic' of this "f-number" article could be made much clearer if "relative aperture" were a given separate article, ie keep the photography-specific 'jargon' separate from the optical physics used for designing telescopes, binoculars, etc, etc. ? Currently it does seem to wander around quite a lot. --Redbobblehat (talk) 16:54, 18 February 2009 (UTC)

Human eye

There seems to be disagreement whether the focal length of the human eye is 17mm or 22mm. Look, for example .

Is there anyone who can explain the discrepancy in detail?

Well, the diameter of the human eye is apparently roughly 25mm so I have a hard time believing that there's most of a centimeter of wasted space between the pupil and the retina... —Ben FrantzDale (talk) 16:48, 15 June 2009 (UTC)
Apparently the different interpretations do not require so much "wasted space" in the eye but accommodating to different optical properties of air and liquids. What we need to know is the focal length of the (air-filled) lens that best imitates the (liquid-filled) eye. I'm not sure if we can just simply measure the eye while ignoring the liquids inside.

I think it would be appropriate to add (or link to) some information about the variable light sensitivity of the eye. Seems like this kind of information should be available somewhere in Wikipedia, but I didn't find it. neffk (talk) 13:04, 26 June 2009 (UTC)

F-stop of the human eye

This section says the human eye in the dark opens to about f/2.1. Is this true? That would mean that certain lenses would actually be brighter than the human eye (Sigma 50mm f/1.4, Nikon 35mm f/1.8G, Canon 50mm f/1.4) and that doesn't seem right. --The High Fin Sperm Whale (TalkContribs) 05:14, 20 December 2009 (UTC)

Lens quality/precision, design

This article needs some discussion of the manufacturing precision/glass quality required to achieve low f numbers. When I was getting into photography, I got the sense that low f numbers indicated high-quality lenses but wondered why they didn't just allow the aperture to open up further. Similarly, you see old lens designs listed as being an "f/16 landscape lens" and that eventually lens design allowed for f/2.8 lenses and faster. There's obviously potential issues with vignetting, but beyond that, I asked myself "Why can't you take an f/16 lens and make the aperture bigger?"

The answer, as I understand it, is threefold. First is vignetting: Making a faster version of some designs would lead to negative thickness at the edges of elements. Second is the lens design itself, that fundamentally aberrations would start to dominate if you opened a lens up, so while you could make a fast version of an old slow lens, even ideally constructed it would produce very soft images, even at the center of the image. Third is manufacturing quality, that fast lenses are more sensitive to imperfections in surfaces and alignment, since for an entire pencil of rays to come to diffraction-limited focus, the entirety of every surface that pencil crosses must be the right shape to within wavelength tolerances across those areas.

Do those three sound right? If so, let's add some mention of it. —Ben FrantzDale (talk) 12:48, 7 January 2011 (UTC)

You'll need to find a source if you want to talk about that. I believe the second one, aberrations, in the main thing. Dicklyon (talk) 20:05, 7 January 2011 (UTC)

illuminance diagram

Diagram showing why two lenses, with different focal lengths but the same aperture setting, will produce the same illuminance in the focal plane.

The diagram showing how two lenses with different focal lengths but the same aperture setting, and different aperture diameters will produce the same illuminance in the focal plane appears to be incorrect.

I know that the caption states that "Focal lengths and aperture sizes are not to scale", but if it is drawn to scale, it looks a little different. See here for a diagram drawn to scale, where it shows that the lines meeting the aperture diameter actually meet at a point on the focal plane. Martybugs (talk) 12:57, 8 January 2011 (UTC)

A pretty awful diagram, for sure. Not at all clear what its point is or why it's drawn that way. Dicklyon (talk) 18:31, 8 January 2011 (UTC)
I took it out. The cone-like red lines make no sense. A replacement diagram would be a good idea. Dicklyon (talk) 00:15, 6 February 2011 (UTC)

Glockenklang1 changes

In this diff, User:Glockenklang1 prefers this new lead section:

Apertures expressed as focal length divided by the F-number. With photographic one-stop increments each aperture has half the light gathering area of the previous one.

The ratio of focal length and aperture of an imaging system is dimensionless and defines the F-number[1]


F is identical to the inverse of the relative aperture([2]) and is referred to also as focal ratio, f-ratio, f-stop, "f/#".

optical caution

More cautiously the aperture has to be a linear measure of the entrance pupil of rays forming the image, thus the effective aperture. The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.

relative aperture

The inverse of F-number is often used in astronomy.

Iluminance relation

The Iluminance of the image goes inversely with the square of the F-number


A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm focal length lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.

Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).

to this old stable version:

Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments; each aperture has half the light gathering area of the previous one. The actual size of the aperture will depend on the focal length of the lens.

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[3]) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.


The f-number (f/#) is often notated as and is given by

where is the focal length, and is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or . The greater the f-number, the less light per unit area reaches the image plane of the system; the amount of light transmitted to the film (or sensor) decreases with the f-number squared. Doubling the f-number increases the necessary exposure time by a factor of four.

The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.

A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm focal length lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.

Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).

I think this is too much to change without talking about it. Definition, calling it "F", lead image caption, initial section titles, lots of text, etc. Some of these changes may be OK, but it's not clear what problem he's trying to solve. Opinions? Dicklyon (talk) 01:03, 6 February 2011 (UTC)

I am trying to make the F-number text more readable for beginners. For this I had to make consistent changes. I understand that multiple authors have given input and every upcoming author must understand the core of that input and should try to keep that core but is allowed to reformulate it in more consistant manner. So let's wait for more comments. --Glockenklang1 (talk) 13:03, 6 February 2011 (UTC)
So what inconsistency are you trying to fix? And why change from N to F, when N is conventional and F is already overloaded with meanings? Dicklyon (talk) 20:09, 6 February 2011 (UTC)
N is also fine - but any letter f, F or N is overloaded -The article is F-number and could conveniently refer to the definition of F, which is a real-valued number and does not have a physical measure as the focal length f. F is a dimensionless version of the focal length f - that's the idea. --Glockenklang1 (talk) 22:48, 6 February 2011 (UTC)
It's best to stick with what's commonly used in sources, where F or f is focal length, and N is f-number. Dicklyon (talk) 22:53, 6 February 2011 (UTC)
Young people must have the best chance in understanding this world! I think we both have different drives. We need third person's opinions! --Glockenklang1 (talk) 23:00, 6 February 2011 (UTC) also defines relative aperture as the inverse of F. Let's learn from them! --Glockenklang1 (talk) 22:43, 7 February 2011 (UTC)
I’m with Dick on this one—I think the reverted change was far less readable, and somewhat off the wall as well as technically incorrect in a few places. I think we should stick with N as the symbol, for the same reasons as Dick; I don’t understand what Glockenlang1 means by “overloaded”. I also think we should stick with vast preponderance of reliable published sources rather than a self-published web page.
I don’t suggest that the article could not be improved. For example, I’m uncomfortable with the concept of “effective aperture diameter” because the effective f-number derives from an image distance greater than the focal length rather than a change in entrance pupil diameter. JeffConrad (talk) 02:53, 9 February 2011 (UTC)

Proposed new wording


The ratio of focal length and aperture of an imaging system is dimensionless and defines the f-number


N is referred to also as focal ratio, f-ratio, f-stop. Optical Lenses are very often denoted by "f/N". Example: a lens with f=400mm D=100mm the formula gives N=4 and would be denoted by f/4.

optical caution

More cautiously the aperture has to be a linear measure of the entrance pupil of rays forming the image, thus the effective aperture. The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.

Iluminance relation

The Iluminance of the image goes inversely with the square of the f-number


A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. The same lens set to f/8 will have D=12.5mm and supply only 1/4 of the amount of light for exposure. Another lens with f=135 mm setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.

Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image). --Glockenklang1 (talk) 21:24, 11 February 2011 (UTC)

I still much prefer the current version. Among other reasons,
  1. I disagree with removing mention of relative aperture simply because of one personal website that uses a different definition (perhaps others also have the alternative definition, but this was the only source cited). I have no problem also mentioning the alternative definition if we can support it with a reliable source.
  2. I see absolutely no reason for a phrase such as optical caution, and find the proposed wording for that section confusing.
As I previously mentioned, I think the current wording probably could be slightly improved, but I think what’s indicated is some fine tuning rather than the needless wholesale change suggested. JeffConrad (talk) 02:30, 12 February 2011 (UTC)

I agree with Jeff. I still don't see what problem is being addressed by these proposed changes. The lead sentence gets away from normal style, and mentions simple tangents like "dimensionless" before mentioning the topic and the alternative terms for it, which conventionally comes first. The other names shouldn't be indirected through N, and we don't need an example in the lead paragraph. It's not clear where the "caution" thing is coming from, as there's no danger apparent; the entrance pupil is already mentioned, as opposed to the more ambiguous term "aperture" (which is never intended to mean the aperture stop's diameter, I'm pretty sure). Dicklyon (talk) 06:34, 12 February 2011 (UTC)

Working f-number

The current formula for Working f-number, though correct, is uncommon in photography, where inversion of the image is typically ignored, and magnification is always taken as positive (e.g., Ray 2002). I think this section would be more clear if we mentioned the photographic convention and included the formula as it is given in most books on photographic optics, where it’s often called the effective f-number:

Many books on photographic optics, including Ray (2002), distinguish between the effective f-number and the marked f-number (the one given by the formula at the beginning of this article and usually given for camera lenses). Perhaps we should mention this as well. JeffConrad (talk) 10:15, 13 February 2011 (UTC)

Umm . . . wudja believe

JeffConrad (talk) 14:19, 13 February 2011 (UTC)

Typical and vendor specific f-number scales

My ongoing additions and clarifications to this article in regard to the vendor specific usage of f-number scales have been reverted by Dicklyon two times. My lastest in-edit version can be inspected here:

Dick, you wrote that my additions were unsourced and original research and likely untrue. While I cannot claim to be free of errors, the information I gave was carefully compiled and cross-checked from various sources over the years, and in those cases where I found diverting infomation I either omitted the info or added a ? to indicate this. The information on Minolta, Konica Minolta, and Sony A-mount cameras is definitely correct, as I have verified the information with my own large set of cameras and lenses (1985-2011). Even the values outside the range of currently existing lenses have been verified by inspecting the cameras on protocol level and using "fake" adapters to emulate lenses with properties not yet commercially available. Also, some of the camera firmware images have been reverse-engineered and corresponding display values be found in the firmware. I even gave quite a number of references with detailed background information (including proof photos), however, I did not want to clutter the article with references, that's why I gave most of them in the form of HTML source code comments < ! - - - - > not via < ref > tag. Maybe you missed this? The f-numbers displayed by Minolta, Konica Minolta and Sony A-mount cameras have been consistent over all the years, with basically just one exception: While Minolta bodies never reported an aperture of 1.8 (in 1/2EV or 1/3EV steps), Sony cameras started to report both 1.7 and 1.8 depending on the lens used - this is down to internal rounding. Otherwise, all camera bodies report the same values from 1.0 to 64, and the Konica Minolta and Sony cameras also seem to support values downto 90 (and there are some reports of values downto 128, but I did not included this information, because I could not verify it). The Minolta 1/4EV f-number scale is complete and correct as well - there have been in total three Minolta devices supporting the display of apertures in 1/4EV granularity, and I have verified the displayed values on each of them. There seems to be more inconsistency in the f-number scales used by Canon and Pentax, at least older cameras (such as the T90) reported different values than EOS cameras do, and I also found some different information in Canon service manuals regarding some values. Since I do not use Canon and Pentax cameras myself I have to rely on third-party information here (but several user reports have been used to verify the information as much as possible). I am quite sure that even with some inconsistencies here, it is possible to draw a timeline somewhere when these vendors changed their scales. I do think, that this information is encyclopedic and relevant and useful to the article. The article in the current form teaches typical usages of 1/2EV and 1/3EV f-number scales which in fact do not reflect the typical usage of f-numbers of some of the largest camera manufacturers today (and in the past twenty years or so). That's what I would call "likely untrue" (and it is not sourced in any way). I do think that adding this information to the tables (and not to the text) is the best way to represent it, because readers can then easily compare the f-numbers scales this way. Do you have a better idea? Ideally, I would like to see this article list and compare the f-number scales used by *any* camera manufacturer since ca. 1985 (when cameras started to display f-numbers in 1/2 or 1/3EV steps). In some cases, it might be necessary to list more than one scale per vendor, but it wouldn't harm, since the number of vendors is limited. -- (talk) 18:43, 11 July 2011 (UTC)

I have no doubt that the added material was carefully researched, but it constituted WP:OR and was essentially unsourced (a forum post is usually not WP:RS), so we cannot include it. I think the extra material also added needless clutter, making the tables almost impossible to read while providing little added benefit, but that’s just a personal take. I agree that we could have better sourcing for the current scales—we say “typical” without much basis. I’d be OK with a brief mention that actual values vary slightly among manufacturers, without getting into more detail. Such a statement should also have a source, of course, but I assume it’s not one that anyone would seriously challenge. JeffConrad (talk) 00:34, 12 July 2011 (UTC)
Right, what Jeff said. What I said was "likely untrue" was "the idea that a manufacturer is consistent"; it may also be true of some manufacturers, but doesn't seem like a safe assummption, and even if it's true it's unsourced, as are the various scales you reported. Yes, I missed any sources that you may have embedded in source-level comments (those are no-ops, not visible to readers). The existing scales in the article are "typical" only in the sense that they can be found in books; to the extent that different scales are found in books, differences were noted and sourced. I found and added some sources back in 2006, and have since resisted the addition of unsourced stuff, so you're not being discriminated against; but we need motion toward better sourcing, not worse. Dicklyon (talk) 06:01, 12 July 2011 (UTC)

More on Working f-number

In ZEMAX, there are three f/#s listed in the System Data report: Image Space f/#, Paraxial Working f/#, and Working f/#. I'm trying to understand what they mean and the ZEMAX documentation is a bit thin.

  • Image-space f/# appears to be (entrance pupil diameter) / (focal length) -- I'm not sure which of the many focal lengths it is using.
  • Paraxial working f/# appears to be describing the shape of the image-side pencil of light.
  • Working f/# appears to be what this article describes, 1/(2 NAimage).

Could someone comment on these? I haven't found a reference anywhere that gives good optical-engineering definitions. —Ben FrantzDale (talk) 17:05, 4 August 2011 (UTC)

I think I missed the motivation for the "Working f/number" section. Is it that, you focus a conventional lens very close (by moving the lens away from the sensor), the entrance pupil stays the same size but the magnification changes because, while the physical focal length is fixed, the focal length you'd use in the pinhole camera model to describe image projection increases, effectively making the lens slower?
If so, then one could contrast that with a liquid lens where you can actually change the power of the lens. That is, you bring the subject closer (thereby doubling the magnification), and at the same time decrease the focal length accordingly. Then the image-space NA stays the same and the object-space NA goes up, with the entrance-pupil diameter staying the same; correct? I guess the working f/number would still be 1/(2NAimage), but the approximation of (1+m) N would mean less since N would change. Does that all sound right? —Ben FrantzDale (talk) 14:28, 7 November 2011 (UTC)
  1. ^ Born & Wolf Principles of Optics 1999 Cambridge Univ. Press.
  2. ^ Smith, Warren Modern Lens Design 2005 McGraw-Hill
  3. ^ Smith, Warren Modern Lens Design 2005 McGraw-Hill