What is 20/20 vision?

While reading stuff about LASIK surgery I came across term '20/20 vision'. I had known proverb 'Hindsight is 20/20' and knew its meaning - from that I knew that 20/20 is something to do with 'perfect vision'...but did not exactly know meaning of 20/20.

But I found its meaning while browsing the LASIK surgery info.

So what does 20/20 vision mean?

By looking at lots of people, doctors have decided what a "normal" human being should be able to see when standing 20 feet away from an eye chart. If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what the "normal" human being can see. (In metric, the standard is 6 meters and it's called 6/6 vision).

In other words, your vision is "normal" -- most people can see what you see at 20 feet. (From here on, please assume that the word "normal" has quotes around it).

If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can see what a normal human can see when standing 40 feet from the chart. That is, if there is a normal person standing 40 feet away from the chart and you are standing only 20 feet away from it, you and the normal person can see the same detail. 20/100 means that when you stand 20 feet from the chart you can see what a normal person standing 100 feet away can see. 20/200 is the cutoff for legal blindness in the United States.

You can also have vision that is better than the norm. A person with 20/10 vision can see at 20 feet what a normal person can see when standing 10 feet away from the chart.

Hawks, owls and other birds of prey have much more acute vision than humans. A hawk has a much smaller eye than a human being but has lots of sensors (cones) packed into that space. This gives a hawk vision that is eight times more acute than a human's.

A hawk might have 20/2 vision!

~ Kaustubh