system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5. Can the definition of 20-20 vision be changed? Since the definition is "what the average person sees at 20 feet" can the goalposts be moved so to speak if peoples' vision gets better or worse enough? Has this ever happened in the past?

I know that the metric for IQ is changed so that average always remains 100 but is it the same for vision testing? If the answer is no could someone who's into statistics maybe link some current vision data so we can see if the average actually is 20-20 or if peoples' vision has actually changed since the tests were invented?
gpt
Having 20/20 vision on the Snellen eyechart does not mean "what the average person sees at 20 feet".  The 20/20 refers to the specific line on the eyechart that the patient can read.  The first number (always 20) is the distance in feet at which the test was conducted.  The second number (and this is a little complicated) is the distance in feet at which the gap in the letter "C" in the font used on the eyechart would cover 1 minute of visual angle (1/60th of a degree).  So, for the 20/20 line on the eyechart, that would mean that 20 feet away from the chart, the gap in the letter C would be 1 minute of visual angle on the retina.  So, if the 20/20 line is the smallest you can read on the chart, then you have 20/20 vision.  If the smallest line you can read is the 20/15 line, then you have 20/15 vision.  20/20 vision is typically considered good (better than average) and the repetition of the numbers is "catchy" so that may be why the phrase caught on.