Editing Presbyopia (section)

==Mechanism==
{{See also|Accommodation (vertebrate eye)}}
[[File:Presbyopia.png|thumb|upright=1.3|Presbyopia]]
The cause of presbyopia is lens stiffening by decreasing levels of [[Crystallin#Alpha-crystallin|{{Script|Grek|α}}-crystallin]], a process which may be sped up by higher temperatures.<ref>{{cite journal|last1=Pathai|first1=S|last2=Shiels|first2=PG|last3=Lawn|first3=SD|last4=Cook|first4=C|last5=Gilbert|first5=C|title=The eye as a model of ageing in translational research--molecular, epigenetic and clinical aspects.|journal=Ageing Research Reviews|date=March 2013|volume=12|issue=2|pages=490–508|pmid=23274270|doi=10.1016/j.arr.2012.11.002|s2cid=26015190}}</ref> It results in a [[near point]] greater than {{val|25|u=cm}}<ref>{{cite book |last1=Katz |first1=Debora M. |title=Physics for Scientists and Engineers: Foundations and Connections, Advance Edition |date=2015 |publisher=Cengage Learning |isbn=978-1-305-53720-0 |url=https://books.google.com/books?id=Z41vCgAAQBAJ&pg=PA1246 |language=en}}</ref> (or equivalently, less than 4 [[Dioptre|diopters]]).

In [[optics]], the closest point at which an object can be brought into focus by the eye is called the eye's near point. A standard near point distance of {{val|25|u=cm}} is typically assumed in the design of optical instruments, and in characterizing optical devices such as [[magnifying glass]]es.{{cn|date=July 2022}}

There is some confusion over how the focusing mechanism of the eye works.{{clarify |date=February 2019 |reason=What is the confusion? Is the example given wrong in some way? }} In the 1977 book, ''Eye and Brain'',<ref>{{cite book|last=Gregory|first=Richard Langton|title=Eye and brain : the psychology of seeing.|year=1977|publisher=McGraw-Hill|location=New York; Toronto|isbn=978-0070246652|edition=3rd ed. rev. and update.|url-access=registration|url=https://archive.org/details/eyebrainpsych00greg}}</ref> for example, the lens is said to be suspended by a membrane, the '[[Zonule of Zinn|zonula]]', which holds it under tension. The tension is released, by contraction of the ciliary muscle, to allow the lens to become more round, for close vision. This implies the ciliary muscle, which is outside the zonula, must be circumferential, contracting like a sphincter, to slacken the tension of the zonula pulling outwards on the lens. This is consistent with the fact that our eyes seem to be in the 'relaxed' state when focusing at infinity, and also explains why no amount of effort seems to enable a [[myopic]] person to see farther away.{{cn|date=March 2023}}

The ability to focus on near objects declines throughout life, from an [[accommodation (eye)|accommodation]] of about 20 [[dioptre]]s (ability to focus at {{val|50|u=mm}} away) in a child, to 10 dioptres at age 25 ({{val|100|u=mm}}), and levels off at 0.5 to 1 dioptre at age 60 (ability to focus down to {{val|1|–|2|u=metres}} only). The expected, maximum, and minimum amplitudes of accommodation in diopters (D) for a corrected patient of a given age can be estimated using Hofstetter's formulas: expected amplitude {{nowrap|1=(D) = 18.5 − 0.3 ×}} (age in years); maximum amplitude {{nowrap|1=(D) = 25 − 0.4 ×}} (age in years); minimum amplitude {{nowrap|1=(D) = 15 − 0.25 ×}} (age in years).<ref>Robert P. Rutstein, Kent M. Daum, ''Anomalies of Binocular Vision: Diagnosis & Management'', Mosby, 1998.</ref>