f number


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Related to f number: Airy disk, Numerical aperture

f number

Designation for a photographic lens which gives the ratio of the focal length to the diameter of the effective aperture or entrance pupil. Example: f/8 means that the lens has a focal length eight times the diameter of the entrance pupil. Syn. f/stop; f-value; focal ratio; lens speed. See relative aperture.
Table F1 Sequence of f numbers used in photography with the corresponding relative image brightness and exposure time to maintain constant film exposure. The relative image brightness is equal to the square of the f number fraction
f numberfocal length/pupil exact
f number
diameterrelative image brightnessrelative exposure time
f/0.50.5f/0.5001/0.250.12
f/0.70.7f/0.7071/0.50.25
f/11f/1.0001/10.5
f/1.41.4f/1.4141/21
f/22f/2.0001/42
f/2.82.8f/2.8281/84
f/44f/4.0001/168
f/5.65.6f/5.6571/3216
f/88f/8.0001/6432
f/1111f/11.3141/12864
f/1616f/16.0001/256128
f/2222f/22.6271/512256
f/3232f/32.0001/1024512