Pronunciation: (ef'num"bur),[key] —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.