Editing Aperture (section)

== In biology ==
[[File:Voluntary pupil dilation.gif|thumb|Pupil dilation and constriction, controlled consciously]]
{{Main|Pupil|Iris (anatomy)|Pupillary response|l2 = Iris}}

In many living [[Optics|optical systems]], the eye consists of an [[Iris (anatomy)|iris]] which adjusts the size of the [[pupil]], through which light enters. The iris is analogous to the diaphragm, and the pupil (which is the adjustable opening in the iris) the aperture. Refraction in the [[cornea]] causes the effective aperture (the [[entrance pupil]] in optics parlance) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4&nbsp;mm in diameter, although it can range from as narrow as 2&nbsp;mm ({{f/|8.3|link=yes}}) in diameter in a brightly lit place to 8&nbsp;mm ({{f/|2.1}}) in the dark as part of [[Adaptation (eye)|adaptation]]. In rare cases in some individuals are able to dilate their pupils even beyond 8&nbsp;mm (in [[Scotopic vision|scotopic]] lighting, close to the physical limit of the iris. In humans, the average iris diameter is about 11.5&nbsp;mm,<ref>{{Cite journal |last1=Rüfer |first1=Florian |last2=Schröder |first2=Anke |last3=Erb |first3=Carl |date=April 2005 |title=White-to-white corneal diameter: normal values in healthy humans obtained with the Orbscan II topography system |url=https://pubmed.ncbi.nlm.nih.gov/15778595/ |journal=Cornea |volume=24 |issue=3 |pages=259–261 |doi=10.1097/01.ico.0000148312.01805.53 |issn=0277-3740 |pmid=15778595}}</ref> which naturally influences the maximal size of the pupil as well, where larger iris diameters would typically have pupils which are able to dilate to a wider extreme than those with smaller irises. Maximum dilated pupil size also decreases with age.
[[File:Human eye in dim light.jpg|thumb|The pupil dilates in the dark to allow more light in. This pupil is unusually dilated for dark adaptation, as dilation to this extent typically requires the aid of mydriatic agents.]]
The iris controls the size of the pupil via two complementary sets muscles, the [[Iris sphincter muscle|sphincter]] and [[Iris dilator muscle|dilator]] muscles, which are innervated by the [[Parasympathetic nervous system|parasympathetic]] and [[Sympathetic nervous system|sympathetic]] nervous systems respectively, and act to induce [[Miosis|pupillary constriction]] and [[Mydriasis|dilation]] respectively. The state of the pupil is closely influenced by various factors, primarily light (or absence of light), but also by [[emotion]]al state, [[Interest (emotion)|interest]] in the subject of attention, [[arousal]], [[Sexual arousal|sexual stimulation]],<ref>{{cite journal |author1-link=Eckhard Hess |vauthors=Hess EH, Polt JM |date=August 1960 |title=Pupil size as related to interest value of visual stimuli |journal=Science |volume=132 |issue=3423 |pages=349–50 |bibcode=1960Sci...132..349H |doi=10.1126/science.132.3423.349 <!-- | author2-link = James Polt --> |pmid=14401489 |s2cid=12857616}}</ref> physical activity,<ref>{{Cite journal |last1=Kuwamizu |first1=Ryuta |last2=Yamazaki |first2=Yudai |last3=Aoike |first3=Naoki |last4=Ochi |first4=Genta |last5=Suwabe |first5=Kazuya |last6=Soya |first6=Hideaki |date=2022-09-24 |title=Pupil-linked arousal with very light exercise: pattern of pupil dilation during graded exercise |journal=The Journal of Physiological Sciences |volume=72 |issue=1 |pages=23 |doi=10.1186/s12576-022-00849-x |issn=1880-6562 |pmid=36153491 |doi-access=free|pmc=10717467 }}</ref> [[Accommodation (vertebrate eye)|accommodation]] state,<ref name="neuroscience_chapter7">{{cite book |last=Dragoi |first=Valentin |url=http://neuroscience.uth.tmc.edu/index.htm |title=Neuroscience Online: An Electronic Textbook for the Neurosciences |publisher=Department of Neurobiology and Anatomy, The University of Texas Medical School at Houston |chapter=Chapter 7: Ocular Motor System |access-date=24 October 2012 |chapter-url=http://neuroscience.uth.tmc.edu/s3/chapter07.html |archive-url=https://web.archive.org/web/20121102025637/http://neuroscience.uth.tmc.edu/index.htm |archive-date=2 November 2012 |url-status=dead}}</ref> and [[cognitive load]].<ref>{{cite journal |vauthors=Kahneman D, Beatty J |date=December 1966 |title=Pupil diameter and load on memory |journal=Science |volume=154 |issue=3756 |pages=1583–5 |bibcode=1966Sci...154.1583K |doi=10.1126/science.154.3756.1583 |pmid=5924930 |s2cid=22762466}}</ref> The field of view is not affected by the size of the pupil.

Some individuals are also able to directly exert manual and conscious control over their iris muscles and hence are able to voluntarily constrict and dilate their pupils on command.<ref>{{Cite journal |last1=Eberhardt |first1=Lisa V. |last2=Grön |first2=Georg |last3=Ulrich |first3=Martin |last4=Huckauf |first4=Anke |last5=Strauch |first5=Christoph |date=2021-10-01 |title=Direct voluntary control of pupil constriction and dilation: Exploratory evidence from pupillometry, optometry, skin conductance, perception, and functional MRI |url=https://www.sciencedirect.com/science/article/pii/S0167876021008448 |journal=International Journal of Psychophysiology |volume=168 |pages=33–42 |doi=10.1016/j.ijpsycho.2021.08.001 |pmid=34391820 |issn=0167-8760}}</ref> However, this ability is rare and potential use or advantages are unclear.