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Old 12-10-2008, 12:08 AM
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Default The "f" in f-stop

OK, now I now what an "F" stop is & does, but what does the "f" stand for?!?!??!

I've looked & looked and cant find it , I've asked several veteran photog's and they don't know, I am sure someone here can tell me
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Old 12-10-2008, 12:44 AM
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The way I understand it....

(F) Focal Length / Aperture = Actual size of the opening

This is why a 400 f/2.8 is a huge lens while a 200 f/2.8 is smaller (half the size) or 200 f/1.4 is larger (twice the size of 200 f/2.8).
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Old 12-10-2008, 12:50 AM
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Quote:
Originally Posted by Rickr View Post
The way I understand it....

(F) Focal Length / Aperture = Actual size of the opening

This is why a 400 f/2.8 is a huge lens while a 200 f/2.8 is smaller (half the size) or 200 f/1.4 is larger (twice the size of 200 f/2.8).
Thanks but I do know f-stop is & does but what does the F stand for?
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Old 12-10-2008, 12:53 AM
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[quote=Rickr;43864]The way I understand it....


(F) Focal Length

That's why it is expressed as a mathematical function F/A (i.e. F Divided by A)

Unless I'm wrong..anyone else?
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Last edited by Rickr; 12-10-2008 at 12:56 AM..
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Old 12-10-2008, 03:44 AM
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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 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. F IS FOR FOCAL
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Old 01-05-2009, 04:53 AM
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From Wikipedia...

The f-number f/#, often notated as N, is given by
where f is the focal length, and D 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 N = 16. 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 literal interpretation of the f/N notation for f-number N is as an arithmetic expression for the effective aperture diameter (entrance pupil diameter), which is equal to the focal length divided by the f-number: D = f / N. The notation is commonly read aloud as "eff" followed by the number: f/8, for example, is usually pronounced "eff eight".
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 lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm 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 transmit the same amount of light to the film or sensor. 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).
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Old 01-14-2009, 11:48 PM
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http://www.michaelthementor.com/lessons.cfm?lessonID=4
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