Can a glasses prescription be converted to 20/xx ?

Question

My son and I just got new spectacle prescriptions. I now need two pairs of glasses; one for reading/programming (I do this most of the day), and one for when I'm not trying to read. I noticed my son's prescription seems really strong (+6.00 sphere, -2.50 cyl, 001 axis). They didn't give us a number (e.g., 20/100 or 20/60) to describe our vision with and without correction. Is there any way to figure that out based on the prescription, or do I need to call them back and ask? Is it safe to presume they would have told us if either of us is legally blind, or would they be presuming we must already know (since we've both always worn glasses)?

Answer 1

No, the 20/20 value ( visual acuity) can't be determined by seeing the prescription.

You both probably have 20/20 vision with glasses on, or close to it, but what it is without a correction only the Dr. would know, and it would be in his files.

Legally blind is when you can't get better than 20/200 WITH a correction...

Answer 2

No, the glasses prescription cannot be converted to 20/xx as these are separate entities.  The prescription is "how out of focus the eye is" (in this case your son is rather "farsighted with astigmatism") and the Snellen acuity (the 20/xx) is a measure of how well the eye sees.  It is true that if the eye is very out-of-focus, the Snellen acuity will be be reduced i.e. 20/100, 20/200, 20/400, etc. but once the eye is in focus (the correct prescription is in place), the eye will see whatever it is capable of seeing i.e. 20/20 etc.  You can read more about the Snellen acuity (20/20) on my website at http://www.noblur.com/compr.html#20/20 This should help to clarify things for you.

Sincerely,

Dr. Maller