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 number | focal length/pupil exact f number | diameter | relative image brightness | relative exposure time | ||||
| f/0.5 | 0.5 | f/0.500 | 1/0.25 | 0.12 | ||||
| f/0.7 | 0.7 | f/0.707 | 1/0.5 | 0.25 | ||||
| f/1 | 1 | f/1.000 | 1/1 | 0.5 | ||||
| f/1.4 | 1.4 | f/1.414 | 1/2 | 1 | ||||
| f/2 | 2 | f/2.000 | 1/4 | 2 | ||||
| f/2.8 | 2.8 | f/2.828 | 1/8 | 4 | ||||
| f/4 | 4 | f/4.000 | 1/16 | 8 | ||||
| f/5.6 | 5.6 | f/5.657 | 1/32 | 16 | ||||
| f/8 | 8 | f/8.000 | 1/64 | 32 | ||||
| f/11 | 11 | f/11.314 | 1/128 | 64 | ||||
| f/16 | 16 | f/16.000 | 1/256 | 128 | ||||
| f/22 | 22 | f/22.627 | 1/512 | 256 | ||||
| f/32 | 32 | f/32.000 | 1/1024 | 512 | ||||
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann