Diopters
and the
Snellen
Chart


Myopia is commonly detected by reading a Snellen eye chart from a distance of 20 feet. Notations of 20/20, 20/30, or 20/200 are used to measure visual acuity. The Snellen method is adequate for screening purposes to determine visual acuity, but it does not accurately quantify myopia because there are other variables that affect an interpretation of the chart, such as squinting, guessing at blurred letters, and room lighting.

The "power" of a lens placed in front of an eye to correct for myopia is measured in terms of diopters. The table below gives an approximate idea of the vision to expect with varying degrees of myopia:

Myopic Correction      Snellen Visual Acuity
  In Diopters
 
     -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


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