Diopters
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Diopter is a measure of the power of a lens (or mirror) and is equal to the reciprocal of focal length in meters. The unit symbol for diopters is dpt, D, or m-1.
In EM we refer to the cm measurement to calculate diopters needed to correct refraction of the eye. If you can see 50cm clearly your diopters will be . Lenses in series add their powers : if you're wearing -2 diopter contacts (adjusted for glasses strength) and put +1 diopter reading glasses over the contacts you're in effect wearing -1 diopters. A lens with a negative diopter sign compensates for nearsightedness while a lens with a positive diopter sign compensates for farsightedness.
0.00 to -0.75 dpt | Probably don't need glasses |
-1.00 to -2.00 dpt | Mild myopia, no differentials needed |
-2.00 to -5.00 dpt | Moderate myopia, glasses always needed |
-5.00 to -10.00 dpt | High myopia |
-10.00+ dpt | Very high myopia. Field of view significantly reduced. |