Trying To Be a Photographer Part3

About F-Stop

When I was trying to explore more about DSLR I came across the F-STOP from one of the blog . Some one wrote the comment about F-STOP like this
What is meant by f-stop?
A. The focal length of the lens divided by the diameter of the aperture (as seen from the front). It is also called an f-number, and is written like f/8, which means the aperture diameter is 1/8th the focal length.
The term is used both in regard to the maximum aperture of a lens and in regard to the aperture selected in a specific situation. The brightness of the image on the film is inversely proportional to the f-number squared. The depth of field increases but diffraction is worsened when using a large f-number. The effective f-number for all 3 effects changes if the lens is focused extremely close. The term "stops" purportedly comes from old technology in which the aperture was selected by turning a wheel with various sized holes in it, each one of which let in twice the light of the preceding one. Thus the phrase "open up a N stops" means to change to an aperture allowing in 2^N times as much light, and conversely with "stop down N stops".

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Sounds a little more plausible than "fenestra", doesn't it?
Then other guy explained in more interesting way

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After searching on this for hours, I have to go with it being a math equation for Focal Length ("F") over ("/") diameter. A lens with 14 inch "focus" (focal length) which is four inches in diameter he is talking about an f /3.5 lens -- (14 / 4) = 3.5 It's origins appear to go back to the mid 1800's and was used in a letter from Irving Pobboravsky to John William Draper. In it, he gave the above formula with an accompanying detailed explanation of what the F/ on the new lens stood for ("focal length over diamter").
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This is what I read in a magazine recently. The f stands for fraction of the focus length of the lens. In the old days when a 50mm lens really was 50mm physically, an f/2 is an aperture 1/2 the size of the focal length (not sure its diameter or what).So for the same light gathering power, ie same f stop: f/2 of a 50mm lens, the aperture has to be 25mm large f/2 of a 200mm lens, the aperture has to be 100mm large Of course modern optics meant focal lengths no longer need to have a direct relation to the physical size. The f number is now only a indication of light gathering power. However, a longer lens still requires a larger aperture than a shorter lens, which is why it fast telephoto lenses costs so much