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Harvard Health Publications

Snellen Test for Visual Acuity

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.

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