# Talk:F-number

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## Definition of lens brightness

It took me a while to wrap my head around the definition of lens brightness in this article, but I think what is in the article now is correct. The brightness of the projected image is pretty straightforward: the illuminance on the image sensor, or luminous flux per unit area. The brightness of the scene is the trickier one - I'm pretty sure it is luminance, or luminous intensity per unit area of light travelling in a particular direction and passing through a given area. What area? the front of the lens. What direction? towards the front of the lens and within the lens's field of view. Scene brightness when described this way is largely invariant of the size of the front of the lens, because of the m^-2 in the unit, as it should be: make the front of the lens larger and more light from the scene will fall on the front of the lens, but the scene looks just as bright to the lens, so your description of scene brightness should be normalized against the area of the front of the lens. Illuminance and luminance are measured in different units, but that is o.k. Lens brightness in f-stops or t-stops is really a description of the sensor illuminance to scene luminance quotient, which would have a unit of steradians. Please speak up here if you aren't in agreement. Balazer (talk) 21:05, 15 January 2013 (UTC)

As far as standard units are concerned, we should strive for a more consolidated presentation of the various units and their conversions.
Regarding lens brightness, as I understand it ... Roughly speaking, compared to a smaller lens aperture, the geometry of a larger lens aperture focusing on the same image frame will allow more light to enter the lens and be projected onto the sensor/film/eye-piece. Think of aperture size like the size of a bucket for bailing out a leaking boat. Say you pour water out of a leaking boat at the same rate (pours per second) no matter what sized bucket you have. The larger the bucket, the more water you get each time.
The more light that can enter through the lens and arrive at the sensor/film/eye-piece, as filtered through the lens, the brighter the image and that's what is meant, in generic terms, by lens brightness. Lens material, component geometries, and composite efficiencies all effect light loss and thereby, lens brightness (on this final note, I have to acknowledge that I may not be using the standard terminology). Don't yell at me internet. JimsMaher (talk) 17:34, 17 January 2013 (UTC)
Better yet ... Think of aperture size like a sieve straining rocks and debris. The larger the holes, the looser the sieve, the more stuff will fall through. The smaller the holes, the denser the sieve, the more solid material making up the sieve, the less debris is allowed through. A sieve that stops a lot of material from going through corresponds to a large f number (f/45, f/64). A sieve that's no more than a metal ring, for sake of example, corresponds to a lens with a low f number (f/1.4, f/1). JimsMaher (talk) 17:49, 17 January 2013 (UTC)
Balazer has it right. I haven't looked to see if have good refs to back that up, but they exist (e.g. this page). The resulting ratio (in steradians) is inversely proportional to the square of the f-number. Dicklyon (talk) 18:22, 17 January 2013 (UTC)
Since the light is passing through a circle (for sake of example), with light intensity following the Inverse-square law, every halving of the diameter of the aperture corresponds to a doubling of the distance from a fixed light source, in terms of exposure value. Or atleast that's the parsimonious model. JimsMaher (talk) 20:02, 17 January 2013 (UTC)

I have a question about the fourth paragraph in the "Notation" section. The first three sentences are very clear - explaining how the 200mm lens receives four times as much light. In the next sentence, I believe the focal length numbers need to be reversed, since the 100mm lens is wider than the 200mm lens, and would therefore be the one that covers four times the area. As it's written, it sounds like the 200mm lens produces 16 times the illuminance. BigslyE5 (talk) 15:09, 26 July 2014 (UTC)

## Aperture vs. Entrance Pupil in the definition of f-number

I'm in favor of using entrance pupil instead of aperture in the definition of f-number, because entrance pupil is more precisely defined. There are multiple types of apertures, which are not all the same. The aperture formed by a lens's diaphragm, for example, is usually not the same as the entrance pupil and not what the f-number is defined in terms of. But we must recognize that in common photography speak, people say aperture when they really mean entrance pupil. So I wrote in the first sentence of the Notation section that the entrance pupil is often called the aperture. I'd be fine to move that statement up into the definition also, but I think we should maintain a distinction between aperture and entrance pupil, and not turn them into synonyms. Lens design textbooks are pretty consistent about using entrance pupil or clear aperture, and never just aperture. Balazer (talk) 04:30, 19 January 2013 (UTC)

I suppose, but for the early cameras, when f-number was being defined, with fairly simple lenses and the aperture very close to the lens, it would have been close enough. With modern retrofocus lenses, it is much less obvious, so entrance pupil is better. But unless someone actually takes the lens apart, they won't know the actual size of the aperture! Gah4 (talk) 20:38, 7 February 2013 (UTC)
But there are common real-world examples where the the distinction really matters and the difference is easy to see. Take most any video zoom lens, or any other zoom lens with an f-number that is constant across the zoom range. The physical aperture is unchanged as you zoom, but the entrance pupil gets larger as you zoom in. It's quite obvious if you look into the front of the lens as you increase the focal length: the image of the aperture becomes increasingly magnified. Balazer (talk) 04:59, 10 March 2013 (UTC)

## Higher or Lower?

While as written it seems technically correct, it seems to me that actual photographers do it differently.

Well, maybe I have never heard a photographer say "increase the f-stop" but always "increase the aperture." Now, since as written it is actually a fraction with the numerical part in the denominator, is it wrong to say that, for example, f/8 is larger than f/11? (Is the f/number 8 or 1/8?)

Continuing, photographers usually talk about shutter speed, rarely shutter time, and it is more usual to label the shutter dial with the reciprocal of the time in seconds. That is, 125(/second) and not 1/125(seconds). (In EE terms, the inverse of period is frequency, but that doesn't seem quite right here.) (When the time is longer than 1s, it might be that calling it a time instead of speed is not unusual, but even then speed might be used.)

Similarly, resolution should be an inverse length (spatial frequency) not a distance (dot pitch), such that "high resolution" has the right meaning. Gah4 (talk) 09:12, 4 March 2013 (UTC)

I'm not quite sure if you are asking a question, but f/8 is definitely a larger aperture than f/11; going from f/11 to f/8 would be increasing the aperture. One could say it is also "reducing the f/#", but that's not common and is asking for ambiguity. —Ben FrantzDale (talk) 21:31, 4 March 2013 (UTC)

As I remember it, the aperture terminology was "open up"(f/11 to f/8), "stop down"(f/8 to f/11), "widen the aperture"(f/11 to f/8), "narrow the aperture"(f/8 to f/11) ... but "increase the aperture"(what are you talking about? stop trying to confuse us.)?

Depending on if you think of the aperture as being the hole which allows the light to pass, or the circular structure that restricts variable amounts of light, it could go either way. While the technical definition may go one way or the other, the phrase "increase the aperture" is ambiguous and non-specific as to the many-varied cogency and oft-reversible thinking of the absent-minded photographer. It could mean increase the light being stopped, or increase the light allowed through. I suggest using a different phrase. JimsMaher (talk) 14:35, 7 March 2013 (UTC)

No, it is not ambiguous, and not uncommon to say "increase the aperture" (meaning to open up); see books. The aperture is the hole; increasing it can only mean making it bigger. Dicklyon (talk) 15:49, 7 March 2013 (UTC)
OK, books. But that's not my experience. In verbal communication, there's a different set of vocabulary used. When efficiency of communication is a priority, then any terminology that is known to be easily misunderstood is actively avoided in diverse climates. It is ambiguous for photographic purposes, varying the aperture serves multiple purposes (EV, DOF, compensating for vignetting or CA, etc.) not all of which lead to the same understanding. And I didn't say it was uncommon, I just suggested using a more universally understandable phrasing. But to be clear, that phrase is foreign to my ear. JimsMaher (talk) 19:42, 7 March 2013 (UTC)
It's only confusing if you conflate aperture and f-number. An aperture is an opening. An f-number is a ratio. Increasing the aperture must be taken to mean increasing the size of the aperture. Yes, it has the opposite effect of increasing the f-number. Saying increase the aperture is common in my experience, and is always understood to mean choosing a lens or aperture setting with a smaller f-number. Balazer (talk) 05:16, 10 March 2013 (UTC)
F-number confusion is another matter (referring to the misunderstanding of the fractional representation of f-numbers). I'm referring to the general 'conflation' of two properties, whereby the increasing of one property is positively related to some other property. Compared to the negative correlation of properties. Negative relations are less intuitive, that's all I was addressing. Unless you're suggesting that the phrase "increase the aperture" is the preferred way to tell someone to increase the f-number, i.e. widen the aperture ... but I don't think anyone here is saying that.

I suggest there is potential confusion with either phrase, "increase the F-stop" or "increase the aperture". Which is unfortunate. JimsMaher (talk) 19:02, 11 March 2013 (UTC)

Indeed, it's better to be explicit and say "increase the F-number" or "increase the aperture diameter". Dicklyon (talk) 21:07, 11 March 2013 (UTC)

When teaching this material (which I do for college multimedia production classes), aperture (or the effective term "entrance pupil" used here) and f-stop numbers need to be consistently spoken of in the same breath to avoid the confusion we're talking about. We put our hands in a circle, then make it larger or smaller, saying, "Here's f/2... and here's f/11..." constantly reiterating that the larger aperture has a smaller f-stop number because the f-stop is a ratio. It is my feeling that the same approach should be used in this article as well - and for the most part that is the case. I agree that "increase the aperture" clearly means making the hole larger. During instruction it is common to use redundancy in a single reference - referring to the aperture being larger or smaller, more opened or closed, increased or decreased - to give students a sense that there are multiple ways to say the same thing. BigslyE5 (talk) 14:55, 26 July 2014 (UTC)

## Early mentioning of EV

I would like to have more detail on the importance in the introduction? How about replacement of "It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography" by "The f-number is a quantitative measure of lens speed, and contributes to the Exposure value", which is a very practical photographic property.

Glockenklang1 (talk) 10:06, 14 July 2013 (UTC)

I believe the details of the relationship between f-number, exposure time, and exposure value are better left in the Exposure Value article, and not in the f-number article. Exposure value is a concept quite specific to photography, and not about lenses in general. Exposure value doesn't apply to binoculars, microscopes, telescopes, projectors, and other optical systems where there is no shutter or camera. In the f-number article we already explain the concept of lens brightness, and relate it to image brightness and exposure. I believe that's sufficient. Balazer (talk) 00:41, 15 July 2013 (UTC)
May be I am just stumbling about the wording 'important concept', which is rather a 'starter' but is lacking the impact within the intro - However, there is already enough content lateron. Glockenklang1 (talk) 18:30, 15 July 2013 (UTC)
Should we just remove the "important concept" bit? I always thought it was awkward wording. I usually think the part before the article contents should be a straightforward definition without any editorializing, though I'm not sure what Wikipedia style guides say about the matter. Balazer (talk) 18:44, 15 July 2013 (UTC)
The introduction is quite short for the length of the article, I would like to see one or two sentences about why we have this definition [for the beginning photographer]. The article is hard to digest. Glockenklang1 (talk) 19:49, 15 July 2013 (UTC)

## Two references (google books) after searching for effective aperture and relative aperture (f/ number)

Nanette L. Salvaggio Basic Photographic Materials and Processes'

Sidney F. Ray Applied Photographic Optics: Lenses and Optical Systems for Photography

Nanette Salvaggio makes the mistake of saying "effective aperture" instead of "relative aperture" - a common mistake as Sidney F. Ray points out.

RPSM (talk) 08:44, 12 January 2014 (UTC)

## F-stops rounding error?

This is more of a question that comes out of ignorance. When I calculate the half f-stops series using the formula published in the article the resulting series rounded would yield 3.4 instead of 3.3, 5.7 instead of 5.6, 23 instead of 22 etc. I tried to find on google why the rounding seems to follow a rather arbitrary sometimes up sometimes down rule, but I was unable to shed light on this. My obsessive nature would like to know why. — Preceding unsigned comment added by 50.138.183.186 (talk) 17:33, 9 April 2014 (UTC)

I now see in the article that... "For all practical purposes extreme accuracy is not required (mechanical shutter speeds were notoriously inaccurate as wear and lubrication varied, with no effect on exposure)." - OK, that seems to explain it more or less.

Seems to me that they are powers of two, and powers of two times 1.4 and rounded. More decimal digits is more than has any use, for the reasons noted above. This was all done in the time of slide rules, or, with some luck, mechanical calculators. Once people get used to the values, why change them? I have known lenses with 32 and 45 on them. Gah4 (talk) 09:45, 4 May 2014 (UTC)