Editing Eye (section)

==Physiology==

===Visual acuity===
[[File:Hawk eye.jpg|thumb|The eye of a [[red-tailed hawk]]]]

[[Visual acuity]], or resolving power, is "the ability to distinguish fine detail" and is the property of [[cone cells]].<ref name=Ali&Klyne1985p28>{{harvnb|Ali|Klyne|1985|page=28}}</ref> It is often measured in ''cycles per [[degree (angle)|degree]]'' (CPD), which measures an [[angular resolution]], or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white/black stripe cycles. For example, if each pattern is 1.75&nbsp;cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white/black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a grey block, is then the measurement of visual acuity of the eye.

For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD<ref>{{cite book | title=The Image Processing Handbook | author=Russ, John C. | publisher=CRC Press | year=2006 | isbn=978-0-8493-7254-4 | url=https://books.google.com/books?id=Vs2AM2cWl1AC&pg=PT110 | quote=The upper limit (finest detail) visible with the human eye is about 50 cycles per degree,... (Fifth Edition, 2007, Page 94) | oclc=156223054 | access-date=2020-10-19 | archive-date=2023-01-17 | archive-url=https://web.archive.org/web/20230117104217/https://books.google.com/books?id=Vs2AM2cWl1AC&pg=PT110 | url-status=live }}</ref> (1.2 [[arcminute]] per line pair, or a 0.35&nbsp;mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD.<ref>{{cite book | title=Casarett and Doull's Toxicology: The Basic Science of Poisons | author=Klaassen, Curtis D. | publisher=McGraw-Hill Professional | year=2001 | isbn=978-0-07-134721-1 | url=https://books.google.com/books?id=G16riRjvmykC&pg=PA574 | oclc=47965382 | access-date=2020-10-19 | archive-date=2023-01-17 | archive-url=https://web.archive.org/web/20230117104218/https://books.google.com/books?id=G16riRjvmykC&pg=PA574 | url-status=live }}</ref> A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central [[Fovea centralis|fovea]] region.<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/vision/retina.html|title=The Retina of the Human Eye|website=hyperphysics.phy-astr.gsu.edu|access-date=2015-06-03|archive-date=2015-05-04|archive-url=https://web.archive.org/web/20150504053926/http://hyperphysics.phy-astr.gsu.edu/hbase/vision/retina.html|url-status=live}}</ref>

Spherical aberration limits the resolution of a 7&nbsp;mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3&nbsp;mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair.<ref>{{cite book | title=Optical System Design | publisher=McGraw-Hill Professional | year=2000 | isbn=978-0-07-134916-1 | url=https://books.google.com/books?id=byx2Ne9cD1IC&pg=PA164 | author1=Fischer, Robert E. | author2=Tadic-Galeb, Biljana | author3=Plympton, Rick | oclc=247851267 | editor=Steve Chapman | access-date=2020-10-19 | archive-date=2023-01-17 | archive-url=https://web.archive.org/web/20230117104217/https://books.google.com/books?id=byx2Ne9cD1IC&pg=PA164 | url-status=live }}</ref> A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an [[optotype]], corresponds to 20/20 ([[normal vision]]) in humans.

However, in the compound eye, the resolution is related to the size of individual ommatidia and the distance between neighbouring ommatidia. Physically these cannot be reduced in size to achieve the acuity seen with single lensed eyes as in mammals. Compound eyes have a much lower acuity than vertebrate eyes.<ref>{{cite journal|author=Barlow, H.B.|year=1952|url=http://jeb.biologists.org/content/29/4/667.full.pdf+html|title=The size of ommatidia in apposition eyes|journal=J Exp Biol|volume=29|pages=667–674|issue=4|doi=10.1242/jeb.29.4.667|bibcode=1952JExpB..29..667B |access-date=2012-01-01|archive-date=2016-08-31|archive-url=https://web.archive.org/web/20160831132021/http://jeb.biologists.org/content/29/4/667.full.pdf+html|url-status=live}}</ref>

===Colour perception===
{{main|Colour vision}}
"Colour vision is the faculty of the organism to distinguish lights of different spectral qualities."<ref name=Ali&Klyne1985p161>{{harvnb|Ali|Klyne|1985|page=161}}</ref> All organisms are restricted to a small range of electromagnetic spectrum; this varies from creature to creature, but is mainly between wavelengths of 400 and 700&nbsp;nm.<ref name=Fernald1982>{{Cite book
 | year=1982
 | title=The Senses
 | page=[https://archive.org/details/senses0000barl/page/98 98]
 | isbn=978-0-521-24474-9
 | url=https://archive.org/details/senses0000barl
 | url-access=registration
 | publisher=Cambridge University Press
 | location=Cambridge
 |author1=Barlow, Horace Basil  |author2=Mollon, J.D.
}}</ref> This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.<ref name=Fernald1997>{{Cite journal
 | author=Fernald, Russell D.
 | year=1997
 | title=The Evolution of Eyes
 | journal=Brain, Behavior and Evolution
 | volume=50
 | issue=4
 | pages=253–259
 | doi=10.1159/000113339
 | url=http://www.stanford.edu/group/fernaldlab/pubs/1997%20Fernald.pdf
 | pmid=9310200
 | access-date=2008-09-16
 | archive-date=2012-11-20
 | archive-url=https://web.archive.org/web/20121120200307/http://www.stanford.edu/group/fernaldlab/pubs/1997
 | url-status=live
 }}</ref>

The most sensitive pigment, [[rhodopsin]], has a peak response at 500&nbsp;nm.<ref name=Goldsmith1990/> Small changes to the genes coding for this protein can tweak the peak response by a few nm; pigments in the lens can also filter incoming light, changing the peak response.<ref name="Frentiu2008">{{Cite journal |author=Frentiu, Francesca D. |author2=Adriana D. Briscoe |year=2008 |title=A butterfly eye's view of birds |journal=BioEssays |volume=30 |issue=11–12 |pages=1151–1162 |doi=10.1002/bies.20828 |pmid=18937365 |s2cid=34409725}}</ref> Many organisms are unable to discriminate between colours, seeing instead in shades of grey; colour vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of [[cone cell]]s, from which the more sensitive [[rod cell]]s evolved.<ref name=Goldsmith1990>{{Cite journal
 | author=Goldsmith, T.H.
 | year=1990
 | title=Optimization, Constraint, and History in the Evolution of Eyes
 | journal=[[The Quarterly Review of Biology]]
 | volume=65
 | issue=3
 | pages=281–322
 | doi=10.1086/416840
 | jstor=2832368
 | pmid=2146698| s2cid=24535762
 }}</ref> Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioural tests can this be deduced.<ref name=Frentiu2008/>

Most organisms with colour vision can detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light—this precludes the possibility of any UV light being detected, as it does not even reach the retina.<ref name=Goldsmith1990/>

===Rods and cones===
The retina contains two major types of light-sensitive [[photoreceptor cell]]s used for vision: the [[rod cell|rods]] and the [[cone cell|cones]].

Rods cannot distinguish colours, but are responsible for low-light ([[scotopic]]) monochrome ([[black-and-white]]) vision; they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at low light intensity, but saturates at higher ([[photopic]]) intensities. Rods are distributed throughout the retina but there are none at the [[Fovea centralis|fovea]] and none at the [[Blind spot (vision)|blind spot]]. Rod density is greater in the peripheral retina than in the central retina.

Cones are responsible for [[color vision|colour vision]]. They require brighter light to function than rods require. In humans, there are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colours). The colour seen is the combined effect of [[Stimulus (physiology)|stimuli]] to, and [[stimulus–response model|responses]] from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the [[optic nerve]]. When rods and cones are stimulated by light, they connect through adjoining cells within the retina to send an electrical signal to the optic nerve fibres. The optic nerves send off impulses through these fibres to the brain.<ref name=Goldsmith1990/>