Many people wear glasses or contact lenses. In most cases, this is to correct some inherent defect in vision. The three most common vision defects are farsightedness, nearsightedness, and astigmatism.

Detecting Vision Defects

The most common test for detecting vision defects is the visual acuity test. This is the well-known eye chart with a big letter E at the top. The test is conducted by having the patient stand or sit a certain distance from the chart. The person being tested covers one eye at a time and reads the smallest line of text she or he can see clearly. Each line of text is identified by a pair of numbers such as 20/15, 20/40, and so on. The first number is the distance (in feet) from the eye chart. The second number is the distance at which a person with "normal" vision can read that...