Snellen score and refractive error

What is the correlation (if any) between the visual acuity measured on the Snellen chart and diopter correction required? For example, I assume 20/20 correlates to plano. How many diopters are required to completely correct the refractive error for 20/30, 20/40, 20/50, and on down the line...(-1, -2, -3 etc...)
And a second related question... If the dominant eye is corrected to 20/20 and the non-dominant eye is corrected to a mini-monovision ( and, I know contact lens measurements are completely different), what Snellen value is generally aimed at to give a minimal amount of near vision correction for a completely presbyopic patient (over age 52).

[Sandy Feldman, MD, MS]

Dear Buttercup109,

There is not a one to one correlations between visual acuity and diopter correction.  I would recommend that you speak with your eyeMD about the intended monovision correction.  Each person tolerates a different amount of correction and your result should be individualized for you.

Dr. Feldman

Sandy T. Feldman, M.D., M.S.
ClearView Eye and Laser Medical Center
San Diego, California

[berrywoo]

there is no relationship between refractive error and acuity on the snellen chart!
There are people who require high rx's like - 10 or more diopters and can still read the 20/20 line on the snellen chart. However, there are also, people with - 10 diopters who can only see 20/100.
And just an FYI, not to be mean, this question has been asked many times before.

[berrywoo]

To add to the above, if someone is a -10 diopters and sees 20/30 you can NOT say that increasing the measurement, by a set number of diopters, like -11 diopters will result in 20/20 vision.