system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: 20/20 vision. What does it mean, why 20/20, and not 10/10 or 50/50?
gpt
At some point it was decided how clearly a person should be able to see at a distance of 20 feet. If you can see from 20 feet what a person "should" be able to see from 20 feet, you have 20/20 vision. As your vision gets worse, the 2nd number gets larger - e.g., 20/200 (IIRC, considered 'legally blind' in the USA) means that the amount of detail a "normal" person can make out from 200 feet away, you'd have to get to 20 feet away to be able to see.

IDK about why 20 vs. any other possible number.