F-number
ratio of the focal length to the diameter of the entrance pupil
An f-number is a measure of the light-gathering ability of an optical system such as a camera lens. It is defined as the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture"). The f-number is also known as the focal ratio, f-ratio, or f-stop, and it is key in determining the depth of field, diffraction, and exposure of a photograph. The f-number is dimensionless and is usually expressed using a lower-case hooked f with the format f/N, where N is the f-number.
The f-number is also known as the inverse relative aperture, because it is the inverse of the relative aperture, defined as the aperture diameter divided by the focal length. A lower f-number means a larger relative aperture and more light entering the system, while a higher f-number means a smaller relative aperture and less light entering the system. The f-number is related to the numerical aperture (NA) of the system, which measures the range of angles over which light can enter or exit the system. The numerical aperture takes into account the refractive index of the medium in which the system is working, while the f-number does not.
The f-number is used as an indication of the light-gathering ability of a lens, i.e. the illuminance it delivers to the film or sensor for a given subject luminance.
This brief starts where responsible research should: with the source description of “F-number” as ratio of the focal length to the diameter of the entrance pupil. Everything that follows is an evidence route, not borrowed authority.
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The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated Sep 7, 2026. The linked authority identifier is Q862169. None of the 0 selected statements returned an explicit reference.
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This entry incorporates text from “F-number” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.