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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 camera.gif 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.

    This information is produced and provided by the National Cancer Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via the Internet web site at http:// cancer .gov or call 1-800-4-CANCER.

    WebMD Medical Reference from Healthwise

    Last Updated: October 16, 2012
    This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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