Definitions for "F-NUMBER"
Keywords:  focal, aperture, brighter, iris, lens
A measure of the ability of a lens to gather light. Also called its “speed”. The ratio of the focal length of the lens to its effective aperture.
The number resulting when the focal length of a lens is divided by the diameter of the aperture. A sequence of f-numbers calibrates the aperture in regular steps (know as stops) between the minimum and maximum openings of the lens. The f-numbers generally follow a standard sequence, in such a way that the interval between one full stop and the next represents halving or doubling in the image brightness. Also called f-stop or relative aperture. The f-number becomes progressively higher as the aperture is reduced to allow in less light; i.e. f/5.6, f/8, f/11, f/16, f/22, f/32... Theoretically, all lenses at the same f-number produce images of equal brightness.
a number that equals the focal length of a lens divided by the diameter of the aperture at a given setting.  Theoretically, all lenses at the same f-number produce images of equal brightness.   Also called f-stop or relative aperture.