Editing Contact lens (section)

====Correction of refractive error====
[[Corrective lens|Corrective]] contact lenses are designed to improve vision, most commonly by correcting [[refractive error]]. This is done by directly focusing light so it enters the eye with the proper [[Dioptre|power]] for clear vision.

A spherical contact lens bends light evenly in every direction (horizontally, vertically, etc.). They are typically used to correct [[myopia]] and [[hypermetropia]].

There are two ways that contact lenses can correct astigmatism. One way is with [[toric lens|toric]] soft lenses that work essentially the same way as eyeglasses with cylindrical correction; a toric lens has a different focusing power horizontally than vertically, and as a result can correct for [[astigmatism]]. Another way is by using a rigid gas permeable lens; since most astigmatism is caused by the shape of the cornea, rigid lenses can improve vision because the front surface of the optical system is the perfectly spherical lens.<ref>{{Cite web |title=Astigmatism |url=http://www.aoa.org/Astigmatism.xml,%20http://www.aoa.org/Astigmatism.xml |access-date=2020-08-06 |website=www.aoa.org |language=en}}{{Dead link|date=December 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Both approaches have advantages and drawbacks. Toric lenses must have the proper orientation to correct for astigmatism, so such lenses must have additional design characteristics to prevent them from rotating out of alignment. This can be done by weighting the bottom of the lens or by using other physical characteristics to rotate the lens back into position, but these mechanisms rarely work perfectly, so some misalignment is common and results in somewhat imperfect correction, and blurring of sight after blinking rotates the lens. Toric soft lenses have all the advantages of soft lenses in general, which are low initial cost, ease of fitting, and minimal adjustment period. Rigid gas permeable lenses usually provide superior optical correction but have become less popular relative to soft lenses due to higher initial costs, longer initial adjustment period, and more involved fitting.<ref>{{Cite web |title=Advantages and Disadvantages of Various Types of Contact Lenses |url=https://www.aoa.org/patients-and-public/caring-for-your-vision/contact-lenses/advantages-and-disadvantages-of-various-types-of-contact-lenses |access-date=2020-08-06 |website=www.aoa.org |language=en}}</ref><ref>{{Cite web |date=2013-01-26 |title=A Complete Guide to Gas Permeable Contact Lenses |url=https://www.eyehealthweb.com/rigid-gas-permeable-lenses/ |access-date=2020-08-06 |website=EyeHealthWeb.com |language=en}}</ref>