Originally published by Harvard Health.
What is the test?
A Snellen test uses a chart with different sizes of letters or forms to evaluate your visual acuity-that is, the sharpness of your vision. The test shows how accurately you can see from a distance.
How do I prepare for the test?
No preparation is necessary.
What happens when the test is performed?
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.
What risks are there from the test?
There are no risks.
Must I do anything special after the test is over?
How long is it before the result of the test is known?
You can find out immediately whether your vision is normal (“20/20”) or whether you have a vision problem. Glasses do not correct every vision problem, but an eye doctor can tell you if they will help.