Editing Refractive error (section)

==Classification==
[[File:Refractive error.jpg|thumb|upright=1.3|Top: farsighted corrected using convex lens Bottom: nearsighted corrected using concave lens.]]
Refractive error – sometimes called "ametropia" – is when the refractive power of an eye does not match the length of the eye, so the image is focused away from the central retina, instead of directly on it.<ref>{{Cite journal |last=Lawrenson |first=John G. |last2=Huntjens |first2=Byki |last3=Virgili |first3=Gianni |last4=Ng |first4=Sueko |last5=Dhakal |first5=Rohit |last6=Downie |first6=Laura E. |last7=Verkicharla |first7=Pavan K. |last8=Kernohan |first8=Ashleigh |last9=Li |first9=Tianjing |last10=Walline |first10=Jeffrey J. |date=2025-02-13 |title=Interventions for myopia control in children: a living systematic review and network meta-analysis |url=https://pubmed.ncbi.nlm.nih.gov/39945354 |journal=The Cochrane Database of Systematic Reviews |volume=2 |issue=2 |pages=CD014758 |doi=10.1002/14651858.CD014758.pub3 |issn=1469-493X |pmc=11822883 |pmid=39945354}}</ref>

Types of refractive error include myopia, hyperopia, presbyopia, and astigmatism.
* '''[[Near-sightedness|Myopia]] or [[Nearsightedness]]''': When the refractive power is too strong for the length of the eyeball, this is called [[myopia]] or nearsightedness. People with myopia typically have blurry vision when viewing distant objects because the eye is refracting more than necessary. Myopia can be corrected with a concave lens, which causes the divergence of light rays before they reach the cornea.<ref>{{Cite journal |last= |date=2016-08-22 |title=Vision and Optical Instruments: Vision Correction |url=https://pressbooks-dev.oer.hawaii.edu/collegephysics/chapter/26-2-vision-correction/ |journal=College Physics: OpenStax |language=en-ca |pages=26-2}}</ref>
* '''[[Far-sightedness|Hyperopia]] or [[Farsightedness]]''': When the refractive power is too weak for the length of the eyeball, one has [[hyperopia]] or farsightedness. People with hyperopia have blurry vision when viewing near objects because the eye is unable to focus the light sufficiently. This can be corrected with convex lenses, which cause light rays to converge prior to hitting the cornea.<ref>{{cite book |last=Grosvenor |first=Theodore |title=Primary care optometry |date=2007 |publisher=Butterworth Heinemann, Elsevier |location=St. Louis (Miss.) |isbn=978-0-7506-7575-8 |page=17 |edition=5|url=https://books.google.com/books?id=uEmQKPAOwccC&pg=PA17}}</ref>
* '''[[Presbyopia]]''': When the flexibility of the lens declines, typically due to age. The individual would experience difficulty in near vision, often relieved by reading glasses, bifocal, or progressive lenses.<ref>{{cite book |last=Khurana |first=AK |title=Theory and practice of optics and refraction |publisher=Elsevier |isbn=978-81-312-1132-8 |pages=100–103 |edition=2nd |chapter=Asthenopia, anomalies of accommodation and convergence |date=September 2008 |chapter-url=https://books.google.com/books?id=qYeD3VHi8OsC&pg=PA100}}</ref>
* '''[[Astigmatism]]''' is when the refractive power of the eye is not uniform across the surface of the cornea because of asymmetry. In other words, the eye focuses light more strongly in one direction than another, leading to distortion of the image.<ref>{{cite book |last=Grosvenor |first=Theodore |title=Primary care optometry |date=2007 |publisher=Butterworth Heinemann, Elsevier |location=St. Louis (Miss.) |isbn=978-0-7506-7575-8 |pages=17–19 |edition=5|url=https://books.google.com/books?id=uEmQKPAOwccC&pg=PA17}}</ref>

Children are typically born hyperopic and shift toward emmetropia or myopia as their eyes lengthen through childhood.<ref>{{Citation |last1=Saluja |first1=Gunjan |title=Childhood Myopia and Ocular Development |date=2023 |url=http://www.ncbi.nlm.nih.gov/books/NBK587350/ |work=StatPearls |access-date=2023-10-31 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=36508524 |last2=Kaur |first2=Kirandeep}}</ref>

Other terminology include [[anisometropia]], when the two eyes have unequal [[refractive power]],<ref>{{cite web |title=Anisometropia - American Association for Pediatric Ophthalmology and Strabismus |url=https://aapos.org/glossary/anisometropia |access-date=10 February 2020 |website=aapos.org |language=en}}</ref> and [[aniseikonia]] which is when the [[magnification]] power between the eyes differ.<ref>{{cite web |title=Aniseikonia - EyeWiki |url=https://eyewiki.aao.org/Aniseikonia |access-date=10 February 2020 |website=eyewiki.aao.org}}</ref>

Refractive errors are typically measured using three numbers: sphere, cylinder, and axis.<ref name=":2">{{Cite web |date=2023-04-18 |title=How to Read an Eyeglasses Prescription |url=https://www.aao.org/eye-health/glasses-contacts/how-to-read-eyeglasses-prescription |access-date=2023-10-31 |website=[[American Academy of Ophthalmology]] |last=Porter |first=Daniel}}</ref>
* '''Sphere:''' This number denotes the strength of the lens needed to correct your vision. A "–" indicates nearsightedness while a "+" indicates farsightedness. Higher numbers indicate more power in either direction.
* '''Cylinder:''' This number denotes the amount of astigmatism, if any.
* '''Axis:''' This number notes the direction of the astigmatism and is written in degrees between 1 and 180.

An eye that has no refractive error when viewing distant objects is said to have ''[[emmetropia]]'' or be ''emmetropic'' meaning the eye is in a state in which it can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object, in this case, is defined as an object located beyond 6 meters, or 20 feet, from the eye, since the light from those objects arrives as essentially parallel rays when considering the limitations of human perception.<ref>{{cite book|last1=Bope|first1=Edward T.|last2=Kellerman|first2=Rick D.|title=Conn's Current Therapy 2016|date=2015|publisher=Elsevier Health Sciences|isbn=978-0-323-35535-3|page=354|url=https://books.google.com/books?id=_x_mCgAAQBAJ&pg=PT390|language=en}}</ref>