Prescriptions for glasses have two main components: shape
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
Convex, or plus,
lenses are thicker in the middle than at the sides to correct
farsightedness (hyperopia) or
Cylindrical (toric) lenses are
curved more in one direction than another to make up for irregularities in the
cornea that cause
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.
In this article
This information is produced and provided by the National
Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National
Institute via the Internet web site at http://
.gov or call 1-800-4-CANCER.
WebMD Medical Reference from Healthwise
September 09, 2014
This information is not intended to replace the advice of a doctor.
Healthwise disclaims any liability for the decisions you make based on this