Eyeglass Prescriptions - Topic Overview
Prescriptions for glasses have two main components: shape and power.
The shape of a lens determines the type of correction.
- Concave, or minus, spherical lenses are thicker at the sides than in the middle to correct nearsightedness (myopia).
- Convex, or plus, lenses are thicker in the middle than at the sides to correct farsightedness (hyperopia) or presbyopia.
- Cylindrical (toric) lenses are curved more in one direction than another to make up for irregularities in the cornea that cause astigmatism.
- Convex lenses or bifocals refocus the image on the retina when people with presbyopia lose the ability to focus on close objects around age 40.
The power of a lens determines the amount of correction. It is specified in diopters. The higher the number of diopters, the more vision correction the lens provides.
WebMD Medical Reference from Healthwise
September 09, 2014
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Eyeglass Prescriptions Topics