For F-number we have terms and definitions in 3 topics. The topics are CCTV, Photography and Telescope.

F-number (CCTV)

Indicates the brightness of the image formed by the lens, controlled by the iris. The smaller the F-number the brighter the image.

F-number (Photography)

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.

F-number (Telescope)

Another label for focal ratio.

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