A diopter is a unit of measurement used to determine the power of a lens needed to correct refractive errors in the eye. This value is written on eyeglasses or contact lens prescriptions and is indicated by the symbol "D."

During an eye exam, an eye doctor will use a phoropter to test a patient's refractive error and determine the correct prescription. The phoropter has lenses with various diopter values that may be compared and changed to provide the sharpest vision.

Nearsighted people require lenses with negative diopter power, while farsighted people need lenses with positive diopter power. The type and amount of curvature of a corrective lens will depend on the type and severity of the refractive error.

In some cases, such as astigmatism, different amounts of power are required in different meridians in the eye, and the diopter value within the lens must vary to correct this type of refractive error.

Almost everyone develops presbyopia by the time they reach their 50s, making it more difficult to see and read up close. A convex lens with a plus diopter power must be placed in front of the eye to help with close-up vision. An eye doctor will note the extra plus diopters as the "add."

A comprehensive eye exam is the best way to determine the diopter power needed in your corrective lenses. During an exam, your eye doctor will also check the health of your eyes so you can maintain clear and healthy vision.
November 09, 2023 — Arnold Trent JR.