system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : How is "corrected vision" is tested and why isn't it always 20/20 if it's corrected?

Okay so I am researching a health regulation for a friend of mine and it says your corrected vision needs to be less than 3 diopters. Now considering that a 20/20 vision would be 0 diopters, how can even a corrected vision could be less than perfect? Why is it considered "corrected" even though the patient can't see perfectly?
gpt
It’s tested with those letter charts, to see if you can read the finest lines as well as someone with 20/20.

Some people with horrifically bad 20/200 vision are just too far out from normal to be corrected completely through any traditional means, but bringing them down to 20/40 will improve quality of life considerably.  They’re heavily corrected, but it’s not perfect.