Re: The Limits of Optometry -- and why.


otisbrown@xxxxxx said:

Otis> There is no exact answer. But on the average
20/40 will required about a -1.25 to -1.5 diopters if PRESCRIBED
for Best-Visual-Acuity (say 20/15 to 20/13). But if 20/40 is
acceptable, the 0.0 diopters would the choice.
Yea that makes sense. However optometrists need to be careful and not
overprescribe. One of the most important factors is BCVA. Two people
may have 20/40 but if one corrects better, he needs more minus. You
have warned of gross overminusing of -2 to -3 for 20/40 UCVA when
rarely is even -1.5 needed.



Otis> But assuming prescription for BVA, the relationship would be
roughly:

20/70 -1.5 D

20/140 -3 D

20/210 -4.5 D

Otis> But it all depends on the subjective judmgent of the OD doing
the perscribing.
Its not linear like that but expotentional. -3 is much worse than -1.5,
well more than twice. No one needs this much minus if they see 20/200,
its probably going to be -2.5 to -3 usually.



Otis> In one case a 3 year-old had 20/50 vision, so they
prescribed a -10 diopter lens. Therefore:

20/50 -10 DIopters

Again for subjective reasons. That is why no relationship
can be established.
Thats because she had a strange myopic distortion in her lens but could
see 20/50 thru the less distorted areas of her lens. Normally, axial
myopia of -10 is so bad youd only be able to count fingers like 2 feet
from your face!

.

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