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 in middle age.

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.