Editing F-number (section)

{{Short description|Measure of lens speed}}
{{other uses}}
{{lowercase title}} 
{{Use dmy dates|date=January 2020||cs1-dates=y}}
[[File:Aperture diagram.svg|right|thumb|320px|Diagram of decreasing [[aperture]]s, that is, increasing f-numbers, in one-stop increments; each aperture has half the light-gathering area of the previous one.]]

An '''f-number''' is a measure of the light-gathering ability of an optical system such as a [[camera lens]]. It is calculated by dividing the system's [[focal length]] by the diameter of the [[entrance pupil]] ("clear [[aperture]]").<ref name="ReferenceA">Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Professional, p. 183.</ref><ref>{{cite book |first=Eugene |last=Hecht |year=1987 |title=Optics |edition=2nd |publisher=Addison Wesley |isbn=0-201-11609-X |page=152}}</ref><ref>{{cite book |first=John E. |last=Greivenkamp |year=2004 |title=Field Guide to Geometrical Optics |publisher= [[SPIE#SPIE Press|SPIE]] |location=Bellingham, Wash|series= SPIE Field Guides vol. FG01 |isbn= 9780819452948 |oclc= 53896720 |page=29}}</ref> 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 (photography)|exposure]] of a photograph.<ref>Smith, Warren ''Modern Lens Design'' 2005 McGraw-Hill.</ref> The f-number is [[dimensionless number|dimensionless]] and is usually expressed using a lower-case [[Ƒ|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 [[Multiplicative inverse|inverse]] of the '''relative aperture''', defined as the aperture diameter divided by focal length.<ref>ISO, Photography—Apertures and related properties pertaining to photographic lenses—Designations and measurements, ISO 517:2008</ref> The relative aperture indicates how much light can pass through the lens at a given 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.