f number
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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
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