The diopter is a unit of measurement used to describe how well a person sees.

Farsightedness is measured in positive diopters, and nearsightedness is measured in negative diopters. Astigmatism is measured in diopters that can be either positive or negative.

With diopters, the farther a measurement is from 0, the more severe the refractive error. For example, if a person has -4 diopters of nearsightedness, it means that the farthest point you can see clearly is one-fourth of a meter (about 10 inches) from your eye.