system
Given the following post, respond as if you're commenting on Reddit:
human
Why is good quality vision called '20/20' vision, and not 100% vision?
gpt
20/20 basically just means you can see what a 'normal person' should see at a range of 20 feet, from 20 feet away. It means you have normal eyesight. 

Perfect eyesight can be a lot better than 20/20. You can have 30/20 vision, meaning you can read what a normal person sees from 20 feet away, from 30 feet away, for instance. 

It's not uncommon for people who have had corrective eye surgery to come out of it with better than 20/20 vision.