Yearly visits to an eye care professional are a painless and essential part of taking care of your health. This visit may include a visual acuity test using the familiar Snellen chart. The Snellen chart begins at the top with a single large letter (often an E) with rows of increasingly smaller letters beneath. The smallest row of letters that the patient accurately reads is the estimate of visual acuity, with 20/20 considered normal vision. You can check your latest eye test or monitor retinal treatments by using a Snellen chart at home.
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Things you need
- Tape measure
Ask your eye specialist for your Snellen fraction. He may give you a corrected (with glasses) and uncorrected (without glasses) fraction. If your Snellen fraction is 20/20, you can see at 20 feet the same letters a group of people with normal vision could also see at 20 feet. If your fraction is 20/40, you can see the same letters from only 20 feet that the group with normal vision could see from 40 feet. Generally, the higher the bottom fraction, the worse the person's vision.
Print out a Snellen chart; one is available in the Resources section of this article. The better the quality of print the more accurate your chart will be, as the design of the Snellen chart depends upon small but highly contrasted letters. For this same reason, only print the chart out on white paper.
Calculate the distance you will need to stand from the Snellen chart. Normally the height of the first letter is 3.5 inches and you stand 20 feet away. If your first letter is smaller, take that height and divide by 3.5 and then multiply by 20 to get the distance in feet. For example, if the first letter is 2 inches high, divide 2 by 3.5 (which equals 0.57), and then multiply by 20 (which equals 11.43). In this example, if your first letter is two inches high, you would need to stand approximately 11 1/2 feet away from the chart.
Measure the point where you need to stand using your tape measure. Begin at the top of the chart and read down until the letters are too small to see clearly. Have a friend stand beside the chart and watch your accuracy, and compare your Snellen fraction with the one given to you by your eye care professional.
Tips and warnings
- Repeated use of the same Snellen chart will decrease its ability to test your vision as you will begin to memorise the letters. Try using a Snellen chart with different letters if you want to test your vision again.
- Using the Snellen chart at home should not take the place of exams by your eye professional. If you notice changes in your ability to read the Snellen chart, see your eye specialist as soon as possible.
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