Editing F-number (section)

== Human eye ==
[[File:Pupillary light reflex.jpg|thumb|382x382px|The human pupil in its constricted (3&nbsp;mm) and fully dilated (9&nbsp;mm) states. At 9&nbsp;mm, the effective f-number is approximately {{f/|1.6}}.]]
Computing the f-number of the [[human eye]] involves computing the physical aperture and focal length of the eye. Typically, the pupil can dilate to be as large as 6–7&nbsp;mm in darkness, which translates into the maximal physical aperture. Some individuals' pupils can dilate to over 9&nbsp;mm wide.

The f-number of the human eye varies from about {{f/|8.3}} in a very brightly lit place to about {{f/|2.1}} in the dark.<ref>{{cite book | first=Eugene|last=Hecht|year=1987|title=Optics|edition=2nd|publisher=[[Addison Wesley]]|isbn=0-201-11609-X}} Sect. 5.7.1</ref> Computing the focal length requires that the light-refracting properties of the liquids in the eye be taken into account. Treating the eye as an ordinary air-filled camera and lens results in an incorrect focal length and f-number.