Kelly Eye Center

Terms Associated with Vision Problems

Diopter
Diopter is the measurement of refractive error: the larger the number, the greater the refractive problem. On a vision prescription there are spaces for four different numbers reading from left to right. The numbers indicate the amount of refractive error (measured in negative diopters for nearsightedness and positive diopters for farsightedness), amount of astigmatism (measured in diopters), the axis of the astigmatism (the place at which the cornea is either the steepest or flattest), and the amount of near vision correction (indicating the need of bifocals for correction of presbyopia). A nearsighted patient with -1 D can see objects clearly at 1 meter, a patient with a -2 D error can see objects clearly at 1/2 meter.

Visual Acuity
Visual acuity refers to the sharpness or clarity of vision. Acuity is measured in the United States in comparison to what a normal eye, without refractive errors, can see at a distance of 20 feet, so that a normal eye is measured at 20/20. A person with 20/100 vision can see at 20 feet what a normal eye can see at 100 feet, a person with 20/40 vision can see at 20 feet what a normal eye can see at 40 feet, and so on. In some cases, people have vision better than 20/20. For example, a person with 20/15 vision can see at 20 feet what a normal eye would have to be 5 feet closer to see.