by 
LoveZCatZ
ANSWERS: 2
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You can't. You can put your prescription on and test to see if you can see the 20/20 letter size. If the smallest letters you can see are 20/30, then you see 20/30. The prescription your doctor gives you will allow you to see your best. Just how well you see depends on the health of the eye and many other operational functions in your eye and brain.
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You can get an approximation if you are shortsighted. 20/20 plano 20/30 -0.50 20/40 -0.75 20/60 -1.00 20/80 -1.75 20/200 -2.50 If you are long sighted then it all depends on your age eg age 10 +3.00 20/20 age 50 +1.25 20/40 If you want some insight into reading glasses prescriptions then take a look at http://www.insider-guide-to-reading-glasses.com/magnifiers.html
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