Explanation of 20/20, 20/40 etc?

Question

Could someone with knowledge on the matter please explain to me the way that visual acuity is classified. For instance what is the difference between 20/40 and 20/400?

Answer 1

General standard. 20/20 is what most people can see at 20 feet you can also see at 20 feet. 20/200 would mean that what most people can see at 200 feet, you have to get closer (20 feet) to be able to see. The larger the second number, the worse your eyesight is.

Answer 2

Someone with perfect eyes can see somthing at 40 feet, you would have to be 20 feet away. Thus you are 20/40. Same thing at 20/400 etc. In Minnesota 20/600 I believe is legally blind.