A dioptre (also diopter) is a unit of curvature equal to one per metre; that is, inverse metres, or 1/(metres). A circle with radius 1/2 metre has a curvature of 2 dioptres.
The dioptre is not an SI unit, and is typically used to measure focusing power of lenses and (curved) mirrors; in this case, the power is the inverse of the focal length. Diverging lenses have negative power. Typical over the counterreading glasses will be rated at +1.50 to +2.50 dioptres.
If the curvature of a lens is C and the index of refraction is n, the focusing power is F=(n-1)C. If both surfaces of the lens are curved, consider their curvatures as positive toward the lens and add them. For a mirror the focusing power is F=2C.
Because the formula is based on the reciprocal of the focal length, a 2 diopter lens is not 2 meters but 1/2 meter, a 3 diopter lens is 1/3 meter and so forth.
By age 25 this accommodation is usually reduced to about 10 diopters and by age 50 to a mere 1 diopter.
Generally, the eye care industry classifies reading glasses lenses in diopter strengths that increase by a factor of.25 (e.g., +.75; +1.00: +1.25; +1.50; +1.75; +2.00; +2.25; +2.50; +2.75; +3.00 etc.).
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