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June 2010

Ask the Doctor

Why does my eye care professional ask me to read the letters on an eye chart?

Catherine Cukras, M.D., Ph.D.
Catherine Cukras, M.D., Ph.D.
Staff Clinician
National Eye Institute

"An eye chart helps your eye care professional assess the sharpness of your vision, or visual acuity," says NEI ophthalmologist Catherine Cukras, M.D., Ph.D.

Visual acuity is a basic measure of visual function. The measurement that is typically associated with normal vision is 20/20. This number means that when you are 20 feet away from an object, you can see details that most people with normal vision can see at that distance. If your vision is 20/40, it means that when you are 20 feet away from an object, you can only see what most people see clearly from 40 feet away.

Snellen Chart
Snellen Chart

There are many different types of eye charts that your eye care professional may use to measure your visual acuity. The eye chart with a large "E" at the top and rows of smaller letters below, which was created in the mid-19th century, is known as a Snellen chart. The chart is named after the Dutch ophthalmologist who developed it.

When clinical research studies became important in vision science beginning in the 1970s, researchers needed a standardized way to measure visual acuity. Therefore, a group of NEI scientists developed a new chart that measures vision consistently, regardless of the exam environment. It is called an ETDRS chart because it was developed as part of the Early Treatment for Diabetic Retinopathy Study. This chart is illuminated from behind in a light box so the lighting is uniform, and it has the same number of letters on each line that decrease in size, based on a mathematical formula.


"The ETDRS chart is an important tool to accurately measure visual acuity, especially for multi-center clinical trials where the participants are located around the country," Dr. Cukras says.

Using either chart, eye care professionals can determine if your eye is functioning properly. If your visual acuity is worse than 20/20, your visual system may not be working properly. You may have a refractive error such as nearsightedness that could be corrected with glasses, or you could have a more serious problem such as a cataract, retinal disease, or glaucoma.

"Based on your eye chart measurement, your eye care professional can conduct additional tests to properly diagnose a more serious visual condition," Dr. Cukras says.

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