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. |
Gap and ratio
Comparisons between two diopters is typically expressed using one of these terms:
- diopter gap: 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