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A diopter is a unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length in meters.
Optical power of a healthy human eye is about 40 diopters, meaning the ability to focus on an object or image 1/40 of a meter from the eye.
The eye of a typical young person can adjust an additional 20 diopters.
Lens power is measured in diopters.