A couple of years ago the Blue Mountains Eye Study, a major Australian research project, found that there was a tendency for people to begin eating lots of carrots at the first hint of night-blindness. Many women in the study had a diet that was above average in beta-carotene, but very bland: other sources of beta-carotene, such as green leafy, yellow, orange or red vegies, didn't even get a look-in.

While carrots and other vegies are good for you, they won�t make you see better in the dark unless you are severely deficient � which is highly unlikely in Australia as vitamin A is in a wide variety of foods in our diet.

Night-blindness experienced by the people in the study is more likely to come from cataracts and other age-related eye problems.

Other studies draw mixed conclusions. Some suggest that eating green leafy vegetables and other sources of beta carotene (not just carrots) may help reduce the early signs of age-related problems. However, recent research based on the Australian eye study has found no association between eating more antioxidants (including vitamin A) and reducing age-related eye problems. Source - Choice.com

By looking at lots of people, eye 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, if you have 20/20 vision your vision is "normal" -- a majority of people in the population 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! Source - HowStuffWorks.com