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Originally Posted by Adrian
There is a definitive answer  - The f comes from focal, as in focal length. The true way of writing an f stop is, for example, f/5 which expresses the aperture diameter as a fraction of the focal length. In this case 1/5 th the focal length. Hope that helps. |
You're quite right apart from it being the definitive answer... which it isn't.
A lens with a max wide open aperture of 50mm on a 50mm lens would be an f1 lens. It isn't substantiated in fact but it is generally believed to be the origin of the f number to denote aperture size.
The expression 'stops' derives from very early cameras before a mechanical iris was invented. They basically just inserted metal plates into a light trapped slot between the lens and camera. These plates each had a hole or aperture of different sizes and they thought of them as 'stopping' or admitting light depending on the size. Again it's just one theory, but as good as any for why the expression 'f stop' has survived today.