f-number 
noun Photography
The ratio of the focal length of a camera lens to the diameter of the aperture being used for a particular shot (e.g. f8, indicating that the focal length is eight times the diameter).The f-numbers marked on a lens are only truly accurate when the lens is focused at infinity....
- All these f-numbers, focal lengths, film speeds and apertures are actually really simple to get to grips with once you understand how they relate to each other and to the finished picture.
- The higher f-numbers mean the diameter of the lens can be smaller, so the lens, using only 20% to 25% as much material, will cost less and weigh less.
Origin
Early 20th century: from f (denoting the focal length) and number.