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

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

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

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