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In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. The number is commonly notated using a hooked f, i.e. f/N, where N is the f-number.
== Notation ==
The f-number N or f# is given by:
where is the focal length, and is the diameter of the entrance pupil (effective aperture). It is customary to write f-numbers preceded by f/, which forms a...