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
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