How do I find the Snellen Ratio for my glasses prescription?

Question

My glasses prescription is -3.50 in my right eye and -2.75 in my left eye. Could anyone tell me what that prescription equals in a Snellen ratio (i.e. 20/___ terms)?

I would much appreciate a cited source, but I've looked and had trouble finding one so it isn't absolutely necessary.

Answer 1

That ratio doesn't exist.  Your prescription nummbers are measurements (in diopters) of how far light is focused in front of or behind the retina. In the case of being nearsighted like yourself, it is behind the eye.  The Sneelen chart numbers are just visual acuities.  For example, if you have 20/60 acuities, this means that a person with 20/20 vision can see the same thing at 60 feet, where you would have to be 20 feet away to see the same.

When you read a Snellen chart, it is a test for distance.  Since you are fairly nearsighted, your uncorrected acuities would probably be in the 20/200 to 20/400 range.  The important thing is that, with your prescription, you can see 20/20.  Hope this helps.

Answer 2

The only way to know is to ask the doctor who did your last exam. They generally note in the chart what your vision is expected to be with the new prescription. With normal healthy eyes, it would be reasonable to expect 20/20 vision with your glasses. Some people though, are not able to be fully corrected to 20/20, so going by just the prescription numbers can't tell you your visual acuity. (You and I could wear the same prescription and you be corrected to 20/20, but I could only see 20/30). They can also tell you what you are uncorrected too.