- the numerical value of the relative aperture. If the relative aperture is f8, 8 is the f-number and indicates that the focal length of the lens is 8 times the size of the lens aperture
See also T-number
WordReference Random House Unabridged Dictionary of American English © 2026
f-number
(ef′num′bər),USA pronunciation n. [Optics, Photog.]
- Optics, Photographya 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
Collins Concise English Dictionary © HarperCollins Publishers::
f-number, f number n
'f-number' also found in these entries (note: many are not synonyms or translations):
exposure meter
- f
- f-stop
- relative aperture
- speed
- stop
- T-number
- F, f
- focal ratio
- f value
- hyperfocal distance
- stop number