Understanding The Eye Chart

The Eye Chart and 20/20 Vision

During an eye test, an eye chart is used by the optometrist or optical assistant to measure how well you see in the distance, compared with other human beings. The classic example is the Snellen chart, developed by Dutch eye doctor Hermann Snellen in the 1860s. There are many variations of the Snellen chart, but in general they show 11 rows of capital letters. The top row contains one letter (usually the “big E,” but other letters can be used). The other rows contain letters that are progressively smaller. During an eye exam, your eye doctor will ask you to find the smallest line of text letters that you can make out, and ask you to read it. If you can read the bottom row of letters, your visual acuity is very good.

What does a 20/20 Vision in an Eye Test mean? In the United States, the standard placement of the eye chart is on a wall that’s 20 feet away from your eyes. Since many optometrist offices don’t have rooms that are 20 feet long, in a smaller room the chart may hang behind the patient chair, using mirrors to make it appear in front of you at a simulated distance of 20 feet. 20/20 vision (or really, 20/20 visual acuity) is considered “normal” vision, meaning you can read at 20 feet a letter that most human beings should be able to read.

Eye charts can be configured in various ways, but generally, if during an eye test you can read the big E at the top but none of the letters lower than that, your vision is considered 20/200. That means you can read at 20 feet a letter that people with “normal” vision can read at 200 feet. So at 20/200, your visual acuity is very poor. In the United States you are considered “legally blind” if your best-corrected visual acuity (meaning, your best distance vision with eyeglasses or contact lenses) is 20/200 or worse. Usually the 20/20 line of letters is fourth from the bottom, with 20/15, 20/10 and 20/5 below that. Not many people have 20/10 or better visual acuity.

These charts measure visual acuity only. They do help your eye doctor figure out whether you need prescription eyeglasses or contact lenses for your distance vision. The Optical/Optometric Assistant program through San Antonio’s Lamson Institute provides a solid ground of knowledge that prepares our students for jobs in this line of work. Graduates of the program will be prepared to test patients for visual acuity, depth perception and color blindness. What are you waiting for? Call or Click today to get your career started in the optometry field.

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