20/20

Have you ever wondered what 20 means when someone says:
"You've got perfect vision: 20/20"

If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can see what a normal human can see when standing 40 feet from the chart. That is, if there is a normal person standing 40 feet away from the chart and you are standing only 20 feet away from it, you and the normal person can see the same detail. 20/100 means that when you stand 20 feet from the chart you can see what a normal person standing 100 feet away can see. 20/200 is the cutoff for legal blindness in the United States, classifying me under extremely, utterly blind person.

Thanks to Professor Callahan for taking note of this. Useful, if i may say.