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 dioptres. The higher the number of dioptres, the more vision correction the lens provides.

Credits

Current as of: December 18, 2019

Author: Healthwise Staff
Medical Review:
Adam Husney MD - Family Medicine
Anne C. Poinier MD - Internal Medicine
Christopher J. Rudnisky MD, MPH, FRCSC - Ophthalmology

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