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F-number

Setting of diaphragm that determines amount of light transmitted by camera lens.

The f-number of an optical system (such as a camera lens) is the ratio of the system's focal length to the diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. It is also known as the focal ratio, f-ratio, or f-stop. It is the reciprocal of the relative aperture. The f-number is commonly indicated using a hooked f with the format f/N, where N is the f-number.

Key Terms

camera lens
diameter
dimensionless number
entrance pupil
f number
focal length
optical system
quantitative measure lens speed
ratio
system

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F-number
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Last modified on May 14 2019
Content adapted from Wikipedia
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