What is the best way to explain the Snellen eye chart results?

When you are told "your vision is 20/20 -or 20/40, or 20/whatever" you are being given a ratio of the chart distance/smallest line able to be read. While the most accurate explanation of this is in terms of arc angle subtended, etc. the easiest way to understand it is to imagine yourself walking down a street with a friend who has "normal" vision, i.e. 20/20. If you have 20/100 vision, that means you will have to walk all the way up to 20 feet to be able to recognize a person or read a sign where your "normal vision" friend would have been able to do so at 100 feet. The bigger the denominator, the worse your distance vision.

You should realize, though, that "20/whatever" tells you nothing about your reading vision, whether you are using both eyes or just one. Furthermore, the letters on the Snellen chart are in 100% contrast to the bright white background, not the way the Real World necessarily appears to us.

_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic