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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.
Approximate categorizations of myopia by spherical lens power:
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.

Gap and ratio

Comparisons between two diopters is typically expressed using one of these terms:

  • diopter gap (or diopter difference): absolute difference in diopters between the values of the two eyes
  • diopter ratio: ratio of the diopters in one eye over the other one (right eye / left eye)

For example,the following correction:

OD: -1.5 SPH / -1.5 CYL
OS: -1.0 SPH / -2.0 CYL

can be expressed as a 0.5 dpt gap in both SPH and CYL, a 1.5 ratio in SPH and a 0.75 ratio in CYL:

|(-1.5 dpt) - (-1.0 dpt)| = 0.5 dpt
|(-1.5 dpt) - (-2.0 dpt)| = 0.5 dpt
(-1.5 dpt) / (-1.0 dpt) = 1.5
(-1.5 dpt) / (-2.0 dpt) = 0.75

Note that the term diopter ratio is often used interchangeably for diopter gap[1], for example when talking about reducing a correction while keeping the gap the same. This can also be expressed as a percentage difference between the two diopter values[2] (e.g. the 0.5 dpt difference between the right and left eyes here is equivalent to 0.5 dpt / |-1.5 dpt| = 0.33 or 33%).