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Although technically not a refractive error, presbyopia is included here because it affects all refractive errors and the prescriptions for those conditions. Presbyopia, literally "old eyes", is the inevitable loss of the focusing flexibility of youth. In the young, the crystalline lens of the eye (just behind the iris) easily changes shape to accommodate to different viewing distances. The elasticity of this lens actually starts declining in the 20s, but usually does not become noticeable until the early 40s. By age 55, essentially all elasticity of the lens is gone. The fix for presbyopia is a different correction for near than for far, either in the form of a multifocal (bifocal, trifocal, progressive) or multiple pairs of single vision lenses, or monovision, a technique used in contact lenses and refractive surgery where one eye is corrected for distance, the other for near. |
