Definition of F-number

Article ID: ART157200 | Date published: 06/09/2015 | Date last updated: 01/25/2016


Definition of F-number




The inverse number of a relative aperture.
The value obtained by dividing the focal length by the effective aperture.


F-number (F) is expressed by the following formula.

Generally, f-numbers are used to indicate the amount of light that passes through the lens. The smaller the number, the greater the amount of light passing through the lens. For the numbers, a sequence of numbers which use the square root as the common ratio based on 1 (one) is used. The aperture scale consists of approximate values which are two times as great as or half of the basic number of the amount of light that passes the lens by each step (1.0, 1.4, 2, 2.8 and so on). However, the maximum F-number may not be appropriate to this sequence of numbers.

More precisely, the real amount of light passing through a lens cannot be determined by F-number only due to the various losses such as light reflection and light absorption.


(NOTE) Indication of F-number

  • F-numbers of lenses are indicated in various ways in catalogues, photo-magazines, etc. (e.g., 1: 1.4, f/1.4, f1.4, F1.4 for F-number 1.4)
  • The International Standard Organization (ISO) specifies that F-numbers should be indicated on lenses as shown below
  • However, this is not mandatory. Further, in the case of printed matter, usually each country has its own system
  • To standardize photographic terms, it is desirable to take note of the international standards



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