# 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 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
References in periodicals archive ?
5-mm aperture has, at best, a theoretical f-number of 3.
A covert lens with an 11-mm FL and a 6-ram aperture has a theoretical f-number of 1.
The most important characteristics of a pinhole lens are its optical speed - the lower the f-number the greater the speed - and its ease of installation and use.
This automatically doubles the f-number of each lens (only one-fourth of the light transmitted).
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.
Generally, the larger the f-number a lens can be set to, the better the picture.
R equals the amount of light that reaches the camera from a known source divided by the amount of light that would have reached the camera through a perfectly transparent lens of the same f-number.
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.
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