Editing Eye (section)

===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>