/ef"num'beuhr/ , n. Optics, Photog.
a number corresponding to the ratio of the focal length to the diameter of a lens system, esp. a camera lens. In f / 1.4, 1.4 is the f-number and signifies that the focal length of the lens is 1.4 times as great as the diameter. Abbr.: f/ , f/, f, f: Also, f number . Also called focal ratio, speed, stop number . Cf. relative aperture .
[ 1890-95 ]