# Talk:F-number/Archive 1

## F-stop

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 ?

Ericd

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

## diaphragma/aperture

(this is a translation) User Lochkarte wrote on http://www.heise.de/newsticker/foren/go.shtml?read=1&msg_id=5833355&forum_id=57389

concerning: http://en.wikipedia.org/wiki/F-number

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.

Wrong!

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. -- 217.184.102.13 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)