Google Answers Logo
View Question
Q: Vision ( Answered 5 out of 5 stars,   0 Comments )
Subject: Vision
Category: Science > Physics
Asked by: racecar-ga
List Price: $5.00
Posted: 17 Feb 2005 17:17 PST
Expires: 19 Mar 2005 17:17 PST
Question ID: 476275
How do you figure out what a person's uncorrected visual acuity is,
expressed as 20/x, if the person's corrected visual acuity is 20/20,
and you know the person's prescription?  For example, if my eyes are
correctible to 20/20 with -8.0 diopter lenses (that is, lenses with
focal length -12.5 cm), how well do I see without glasses?  20/1000 or
what?  There are online references that say there is no exact
relationship.  There must be at least an approximate
relationship--what is it?
Subject: Re: Vision
Answered By: pinkfreud-ga on 17 Feb 2005 18:14 PST
Rated:5 out of 5 stars
Hello, racecar.

It's true that the 20/something numbers associated with the Snellen
visual acuity chart do not have precise equivalents in diopters. I
squabbled with an ophthalmologist about this many years ago. I asked
him for my exact "20/something" number, and he gave me a bunch of
rigmarole which basically translated to "I dunno." As it turns out, "I
dunno" was the right answer; visual acuity is affected by several
factors in addition to the curvature of the eye's lens. I finally
wrung a guesstimate out of an optometrist, who said that my correction
of -7.75 diopters was in the vicinity of 20/1000. He may have pulled
this out of his navel, of course, but it seems to jibe with what I've
learned elsewhere.

I have found a chart that gives some rough approximations of
"20/something" values for myopic (nearsighted) patients:

"Eyesight is commonly evaluated by reading a Snellen eye chart from a
distance of 20 feet - the distance beyond which the lens does not need
to adjust for distance. (This is the equivalent of the infinity
setting on a camera.) Variances from the 'ideal' 20/20 can be caused
by myopia, astigmatism and other variables including the lighting in
the room. 20/something provides a descriptive approximation of visual
The power of a lens placed in front of the eye to correct for myopia
is measured in terms of diopters. Diopters are a prescriptive measure
of myopia. If myopia is the only variable, the following approximate
relationships exist between measurements in terms of diopters and in
terms of 20/something:

In Diopters     20/something 
  -0.50         20/25 to 20/30 
  -1.00         20/30 to 20/50 
  -3.00         20/300 
  -4.00         20/400 
  -5.00         20/600 
  -6.00         20/800 "

LPF: Diopters and 20/Something

My Google search strategy:

Google Web Search: 20/100 20/200 diopters

I hope this is precisely what you need. If it is not, please request
clarification, and I'll resume the quest.

Best regards,
racecar-ga rated this answer:5 out of 5 stars

There are no comments at this time.

Important Disclaimer: Answers and comments provided on Google Answers are general information, and are not intended to substitute for informed professional medical, psychiatric, psychological, tax, legal, investment, accounting, or other professional advice. Google does not endorse, and expressly disclaims liability for any product, manufacturer, distributor, service or service provider mentioned or any opinion expressed in answers or comments. Please read carefully the Google Answers Terms of Service.

If you feel that you have found inappropriate content, please let us know by emailing us at with the question ID listed above. Thank you.
Search Google Answers for
Google Answers  

Google Home - Answers FAQ - Terms of Service - Privacy Policy