The values indicated in the "sphere" and "cylinder" columns of an eyeglass prescription specify the optical potential of the lenses in diopters, abbreviated D. The beyond the number of diopters, the farther the lens refracts or bends light. A diopter is the correlative of the focal length in meters. If a lens bankrupt a focal length of 1/3 meters, it is a 3 Go diopter lens.

In science textbooks, positive lenses are frequently diagrammed as convex on both sides; negative lenses are mainly diagrammed as concave on both sides. In a firm optical system, the best optical feature is as usual achieved where most rays of aglow are roughly normal (i.e., at a right angle) to the lens surface. In the case of an eyeglass lens, this means that the lens should be roughly shaped like a cup with the excavated side toward the eye, so most eyeglass lenses are menisci in shape.