If a person has an -11 Diopter prescription for glasses, what is that in 20/20 vision… numbers?
People often get confused about measuring vision when they try to
compare their glasses prescription with some distance visual acuity measurement
on a Snellen type eye chart. It would be nice if there were a corresponding
Snellen letter acuity for each diopter of nearsighted correction needed. But
that is unfortunately not the case. By the time you have as little as -3 diopters
of nearsightedness you will typically not be able to even read the big "E"
which is 20/400. While it might technically be possible to create a chart with
huge letters (so as to record 20/1,500 for example) this is impractical and in
any case provides little important information. The difference in distance
visual acuity between -1 and -4 is huge. But the difference in distance visual
acuity between a -6 and -11 is negligible.
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic