Doctors and researchers often use a test called the Snellen eye test to measure how well people can see. In this test, you read a chart with rows of letters. The chart is put 20 feet away from you.
This test shows the doctor the smallest letters you can see clearly. Doctors use this test to measure how good people's eyesight is after a cataract operation and to compare this with normal vision.
The first row is usually just one letter and is the size that a person with normal vision could read 200 feet away. Each row below this has smaller letters than the one before it. The size of the letters in each row matches what a person with normal vision could read at:
- 100 feet (second row)
- 70 feet (third row)
- 50 feet (fourth row)
- 40 feet (fifth row)
- 30 feet (sixth row)
- 25 feet (seventh row)
- 20 feet (eighth row).
If you have normal vision, your score is 20/20. This means that when you stand 20 feet from the chart, you can read the letters that a normal person could read at 20 feet. You might also hear normal vision called 6/6. It means the same. The numbers are just in meters instead of feet.
If you have weaker eyesight, the bottom number is higher. For example, if you stand 20 feet from the chart, you might be able to read the letters only in the fifth row. This score is 20/40, because someone with normal vision could read this row at 40 feet. You might hear this called 6/12 vision. It means the same.
Ask your doctor what your score means. They can tell you whether you need glasses, whether it's safe for you to drive and whether an operation for cataracts is likely to help you.
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This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved. |
