f=focal length. F/stop is, say f/16, 1/16 the focal length of the lens. This is the size of the aperture measured from the front of the lens. This mathematical relationship allows for the same amount of light getting through for f/16 on a 1000mm lens or a 50 mm lens.

Should be lower case "f". The formula is f/(divided by) aperture diameter... where "f" represents the distance from the aperture to the focal plane ... wherever the aperture may be. Normally, f/stop markings on the lens are arbitrarily calculated from the position of the aperture when the lens is focused at infinty. They are called "stops" because, originally they were finite holes in a plate that slid through the lens ... and there was a indexing feature ... a "shot bolt pin" that stopped the sliding plate in the proper postion.

The "f" stands for factorial. The ratio of focal length to aperture is a factorial which, as has been stated so well already, gives us a simple measure of the light gathering power of any lens of the same factorial.

Standard notation has "f" (note lower case) representing "focal length". That is why f/stops have the `front' slash. The entire formula is: f/(stop number) = equals diameter of aperture opening. Bell and Howell (Remember them? - They offered a "lifetime guarantee" on their cameras - the "lifetime" was the period of the company's survival) once had a campaign to substitute "T" numbers for "f/stops": the "T" stop (for transmission of light) was based on actual measurements. Unfortunately, the numbers seemed to indicate "slower" lenses in a time when fast lenses were considered superior -

Hello: I think math when I hear 'factorial', and I don't see where that applies here. I have worked out the logarithmic and geometric relationships to my heart's content, and how they relate to EV numbers and luminance. Could you please explain your use of the term "factorial"? Thanks Murray