F-number definition
The ratio of the focal length of a lens or lens system to the effective diameter of its aperture.
noun
(photog.) A number that represents the relative aperture of a lens, equal to the focal length divided by the effective diameter of the lens aperture: a higher number indicates a smaller opening.
noun
Origin of f-number
- f(ocal length) number
From American Heritage Dictionary of the English Language, 5th Edition