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 (Figure 1) 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.

Figure 1 - Cornea, Lens, and Iris

The iris is the coloured part of the eye. A circular muscle in the iris controls the amount of light entering the eye through the pupil, the black area in the centre of the iris.

The cornea is the outer clear, round structure that covers the iris and the pupil. The cornea directs light rays into the eye and helps focus them on the light-sensitive retina at the back of the eye, providing sharp, clear vision.

The lens is located behind the iris and is normally clear. Light passes through the pupil to the lens. Small muscles attached to the lens can make the lens change shape, allowing the eyes to focus on near or far objects.

Current as of: September 27, 2023

Author: Ignite Healthwise, LLC Staff
Clinical Review Board
All Healthwise education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.