Definitions for f-numberˈɛfˌnʌm bər
Random House Webster's College Dictionary
a number corresponding to the ratio of the focal length to the diameter of a lens system, esp. a camera lens.
Category: Optics, Photography
Ref: Symbol: f/ 9
Origin of f-number:
The ratio of the focal length of a lens to its diameter; a measure of the light-gathering ability of the lens.