- 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 (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, 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 (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%).
- The EndMyopia Blog, https://endmyopia.org/the-diopter-ratio-trap-dont-favor-one-eye/
- The EndMyopia Blog, https://endmyopia.org/reducing-diopter-ratio-diy-patching-solution-pro-topic/