Re: Diopters to 20/something conversion. The math and science behind this!

As to what Otis said, since most are myopes, the minus lens is tried
first, if this makes things blurrier the person could be a hyperope and
a high one at that if hes young. Test with plus lenses then. Then
theres the cylindar component which is tricky and many optometrists
tend to give spherical equivalent with more minus or less plus. The
pinhole test is a quick diagnosis to check for refractive error or
occular pathalogy.

someone said:

"the real answer depends on the optics of corneal curvature vs axial
length
- an eye with steep cornea and average length can be equally
nearsighted
with an eye with a flat cornea that is very long - but they may not
have
the same uncorrected acuity."


I dont see how this makes a difference be it the cornea, lens, axial
length or even giving someone a plus lens(other than magnificarion) all
refractive error is expressed in units and blur equally. I never heard
of different types of components resulting in refractive error to
affect the final outcome.


"d=log(20/xxx)/.27"

This formula only works somewhat for a narrow range. I havent composed
my own formula, but just a table with the values.


You also have a database with 150 results and said It looks to me like
the only people this works well for have 20/20 uncorrected acuity.

I really, really question all those -.25 guys with 20/40, 20/50, 20/70
or even 20/100 accuracy who "magically" see 20/20 or 20/25 with minus
one quarter diopter! Sorry but this is not possible. I have seen the
differences of quarter diopters with trial lens and when I got my eye
exam thru phororapter. The differences is very small, sometimes I have
to ask the optometrist to try two, three, four times flipping between
quarter diopters to determine which is better. My mom has tried -7.5
and -8 contacts and said there wasnt a difference between the two! She
of course wears -7.5 now since more minus doesnt help.

I also question some of the seemely poor vision with very low dioptric
values(see above) and seemly good accuracy with high diopters.
Squinting perhaps? Overminus? -3 diopters is going to be alot worse
than 20/80 if BCVA is 20/20.


Anyway I already saw those threads from years ago. This thread will be
like a countinuation and add new results. You optometrists get to know
results everyday when doing eye exams. Im sure youve noticed a pattern
by now excluding the odd ones which dont make sense like -.25 and 20/70
or -4 and 20/70 for example.

.

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