If you have perfect visual acuity, you have twenty/twenty vision, as measured by the standard eye chart devised by a Dutch ophthalmologist called Dr Hermann Snellen in eighteen sixteen two. The charts, which are still in use, have a single large letter at the top and lines of progressively smaller letters below. The first figure refers to how far away (in feet) the person whose vision is being measured is sitting or standing from the chart. The second figure refers to how far away a person with good vision would have to be and still be able to read the same line of letters as the person being tested. If you had twenty/thirty vision, a person with perfect vision could read at thirty feet the same letters that you can just make out at twenty feet. These distances are now generally expressed in metres, so that perfect twenty:twenty sight would now be written as six:six
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That's called the "Snellen Fraction." 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 the small size letter that the average person can see from 20 feet away. If a person sees 20/40, at 20 feet from the chart that person can read letters that an average person could read from 40 feet away.
20/20 does not mean perfect vision - it means average vision. There is no "perfect" vision. The top of the scale is 20/10 which is "above average" vision - meaning what you can see at 20 feet away, the average person has to be 10 feet away to see it.