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.
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.