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Originally Posted by CT
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. |
Well, my explanation of the origin of the f number was certainly definitive and substantiated as f/x where x is the focal length is the equation that has been used to standardise the measurement of the amount of light passing through an aperture since the variable aperture was invented. This is, however, just a method of comparing lenses and providing a reference scale, as the light transmission properties of individual lenses of equal maximum aperture vary. This is measured by factoring in the transmission factor to the f number and is expressed as the T-Stop number, although I think you'd be hard pressed to find anyone outside a lens manufacturer's labs using it.
What's not substantiated are the various theories on the "stop", but the explanation that it came from a simple light stop is, IMHO, the most plausible.