f number

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Related to f-number: focal length, 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
References in periodicals archive ?
97) is far less than that of the nocturnally active web building uloborids, whose f-number ranges from 0.
The best theoretical f-number is equal to the focal length (FL) divided by the entrance lens diameter (d).
5-mm aperture has, at best, a theoretical f-number of 3.
The f-number simply is not a valid measure of lens quality.
If the lens elements could pass all the light presented to them, the f-number would reflect the ratio of light energy passing through the lens to the light energy that arrives at the image plane.
The ICM-102Ala employs a lens with a one-piece plastic housing with IR filter, with an infrared filter and an F-number of 2.