By looking at lots of people, eye doctors have decided what a “normal” human being should be able to see when standing 20 feet away from an eye chart. If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what the “normal” human being can see. In other words, your vision is “normal” — most people can see what you see at 20 feet. So, if you have 20/40 vision you will have to be at 20 feet in order to see what a “normal” person can see at 40 feet.
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