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20/20 is standard or plano (here are three different ways to say it)
Metric = Snellen = Estimated prescription
6/3 = 20/10 = Plano (zero)
6/4.5 = 20/15 = Plano
6/6 = 20/20 = Plano to -0.25
6/7.5 = 20/25 = -0.25
6/9 = 20/30 = -0.5
6/12 = 20/40 = -0.75
6/15 = 20/50 = -1.00 to -1.25
6/30 = 20/100 = -1.75 to -2.00
6/60 = 20/200 = -2.00 to -2.50
The Snellen fractions, 20/20, 20/30, etc., are measures of sharpness of sight. They relate to the ability to identify a letter of a certain size at a specified distance. They give no information as to whether or not meaning is obtained from visual input, how much effort is needed to see clearly or singly, and whether or not vision is less efficient when using both eyes as opposed to each eye individually. "Normal" vision is 20/20. There are two lines smaller than 20/20, 20/15 and 20/10.
When checking visual acuity, one eye is covered at a time and the vision of each eye is recorded separately, as well as both eyes together. In the Snellen fraction 20/20, the first number represents the test distance, 20 feet. The second number represents the distance that the average eye can see the letters on a certain line of the eye chart. So, 20/20 means that the eye being tested can read a certain size letter when it is 20 feet away. If a person sees 20/40, at 20 feet from the chart that person can read letters that a person with 20/20 vision could read from 40 feet away. The 20/40 letters are twice the size of 20/20 letters; however, it does not mean 50% vision since 20/20 sounds like it is one half of 20/40. If 20/20 is considered 100% visual effiency, 20/40 visual acuity is 85% efficient.
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