Contrary to popular belief, having 20/20 vision doesn’t mean you have perfect vision. 20/20 is the mathematical average that every human should be able to see. If you have 20/20 vision, that means that your day to day tasks should be visually easy to accomplish. But what does 20/20 mean exactly?
Breaking down the numbers will explain exactly what 20/20 means. The numerator, or first number, is the number of feet you would be standing from a letter that is a particular size. The denominator, or second number, is where the average person would have to stand to see the same sized letter. If you are 20/20, that means when you stand at 20 feet away from a letter, the average person could stand right next to you and see that same letter. If you are 20/40, that would mean that if you stood at 20 feet, the average person could stand at 40 feet and read that same letter. 20/40 is what is required by the government to be able to drive a car without glasses. Most eye surgeries will not be done on your eyes if you are better than 20/40 if you want your insurance to pay for it.
The definition of legally blind is 20/200 in the better seeing eye with correction. If you were legally blind, you would have to stand at 20 feet to see a letter, and the average person could stand 200 feet behind you and see that same sized letter. 300 feet are in a football field. If you are correctable to anything better than 20/200, then you are not legally blind. You cannot be legally blind if you take your glasses off, because the definition is seeing worse than 20/200 with correction.
On the flip side, you can have better than 20/20 vision too! You may have 20/15 vision, which means that you would stand 20 feet away from a letter and other people would have to move up to 15 feet to see that same letter. Most eye charts will go as far as 20/10. In other countries, they do not use 20/20 as a measurement, since it is in feet. Rather, they use the metric system in meters. Their 20/20 is actually 6/6 meters. The 20/20 letter E should be 1/3 inches large if the screen is 20 feet away. Some eye exam rooms are shorter than 20 feet so the letters are actually smaller than 1/3 inches. Another way to make an exam room bigger is to use mirrors to artificially make the letters look 20 feet away.
20/20 is the average that the normal person should be able to see. For this reason, most pairs of glasses are made so that you can see the 20/20 line. Generally, you are likely able to see the 20/15 line (one line better) with both eyes open if each eye is 20/20 individually.