Refractive Surgery Blog

On a regular basis, I am asked: What does 20/20 actually mean? While the technical answer is complicated, the one most people understand is quite straight forward. Your visual acuity is measured on a Snellen eye chart. The chart contains eleven lines of block letters set up in rows that decrease in their size and increase in number as one looks from the top to bottom of the chart. The thickness of the lines equal the thickness of the white spaces between them and the height and width of each letter is five times the thickness of the line. Only ten letters are used in the chart. 20/20 is considered the smallest line that a person with normal visual acuity can read at a distance of twenty feet. If you had 20/40 vision (or roughly 1/2 normal vision), it means that you would need to be only 20 feet away to read what a normally sighted person can read from 40 feet away.

With regard to "Snellen Vision," many patients report that they are "legally blind" without their glasses. This is a contradiction in terms as legal blindness is defined as being able to see no better than 20/200 (the second largest letter on the eye chart) in the best corrected state. That is much different from being able to see no better than 20/200 in the uncorrected state.

Clearly, there too many causes to cause a deviation from normal vision to discuss here. If you are experiencing a problem with your visual acuity, the best thing you can do is find out the cause. And, if you have other questions about your vision, give my office a call and someone should be able to help you.

Labels: 20/20 Vision