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Hello everyone, post your dioptric pescription(how bad are your eyes),
your UCVA(uncorrected) and BSCVA(corrected by glasses) Also if you know
anyone's vision, post that too. I have done much research on the
correlation of diopters and 20/xxx and have compiled several charts,
formulae, tables and comments on the results obtained thus far. To test
for this, do it in room light or dim light. Make sure the eyechart is
well illuminated and your not squinting at all or recalling from memory
or guessing. Blur preception is of course allowed.

I and most optometrists have found that -.25 diopters doesnt cost you a
single line but makes the existing line blurry but still readable. Some
say it costs half a line. Such as if you can see half of 20/15 with a
-.25 lens, youll be a full 20/20 but wont be able to see better than
that uncorrected. Another example is if your seeing all of 20/20, you
may miss half of the 20/20 with -.25 diopters. Others have said it
didnt affect their ability to read a line, just made it harder to do
so.

Minus half diopter(-.5) is generally accepted as resulting in one full
line loss. This means 20/15 becomes 20/20 and 20/20 becomes 20/25. If
you can see half of 20/15 youll be seeing half of 20/20.

Minus one diopter(-1) generally gives you half visual accuracy. My
friends who see 20/20 with glasses see 20/40 without their -1 glasses.
Ditto for 20/15 with -1, 20/30 uncorrected. I was 20/50 corrected to
20/25 with a -1 lens years ago.

Higher dioptric values become harder to calculate and predict,
especially when you get to -6 and up. Someone could be -4.5 another
-5.25 and both see 20/400 UCVA. The -5.25 probably will see a much
blurrier 20/400 than the -4.5 but generally, 20/400 represents a
moderate of myopia around -5 diopters. One website said the range was
-4 to -6 for 20/400 and your best corrected vision played a big factor.
Someone whos 20/15 corrected needs more diopters to see the same blur
as another with 20/30 corrected. For me, my left eye at -5 or so
couldnt see 20/400 while my right eye at -4.5 or so just barely, barely
saw 20/400. I know two people who passed V3 requirement which states
you have to be 20/400 or better uncorrected and both were barely 20/400
with -5.5 pescriptions with 20/15 corrected. Those two guys said they
dont know anyone else -5.5 or more who passed V3 which requires 20/400.
They probably didnt have the great 20/15 corrected vision with glasses
so for them, -5 was the limit(20/20 corrected) I am not correctable to
20/20 so my limit is even less.

Few eyecharts go beyond 20/400 so info is scarce past this. I have a
solid grasp on dioptric values below -6(mild to moderate myopia), but
for -6 and up which is high myopia I am less certain how this converts
to diopters. I do know high myopes are worse than 20/400 but how bad
exactly? Also theres much fewer high myopes than low and moderate
myopes so much of my info is on low(er) myopes and their diopters to
snellen accuracy.

One could stand closer but then accomodation needs to be taken into
account. I can see the 20/200 E from 10 feet less blurry than the
20/400 E from 20 feet. At 5 feet I see the 20/70 line but im certainly
worse than 20/280. I experienced .67 diopter accomodation from the 5
feet mark.

If any of you have charts and formulae, on what expotentional scale
does this equal to? I know that going from -1 to -2 is only twice as
bad(20/40 and 20/80), but going from -3 to -6 is definately more than
twice as bad(20/150 and 20/500?), blurs much more than twice. Does this
gap become even larger still at -6 to -12(20/500? and 20/????) where
the number of times more blurry increases expotentionally?

Me and many others are curious about this and this topic has been
posted before many years ago. Its also useful for some occupations such
as pilot, soldier, law enforcement, etc where they have a requirement
for both corrected(BCVA) and uncorrected(UCVA) vision and people ask
all the time if their UCVA is good enough to make the cut.