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.
Current as of: August 31, 2020
Author: Healthwise Staff
Adam Husney MD - Family Medicine
Anne C. Poinier MD - Internal Medicine
Christopher J. Rudnisky MD, MPH, FRCSC - Ophthalmology