Medical Content Created by the Faculty of the
Harvard Medical School
You stand or sit at a specific distance from the eye chart. Usually you are told to cover one eye with a cardboard piece or with your hand while you read letters with the other eye and say them out loud for the doctor.
In an eye clinic, you may have a more sophisticated version of this test in which you look at the chart through different strengths of lenses (a little bit like looking through a telescope) so that your doctor can find the proper strength of glasses or contact lenses for you. Sometimes the Snellen chart you see in an eye clinic is actually a reflection on a mirror from a projector in the back of the room. This enables eye doctors to use a variety of charts without you having to move from your chair. The test takes only a few minutes.