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 exactly 6 metres (about 20 feet) away from you.

This test helps your doctor know which are the smallest letters you can see clearly. Researchers use this test to measure how good people's eyesight is after a cataract operation and to compare it with normal vision.

The first row is usually just one letter. It's the size that a person with normal vision could read 60 metres (about 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:

• 36 metres (second row)

• 18 metres (third row)

• 12 metres (fourth row)

• 9 metres (fifth row)

• 6 metres (sixth row)

• 5 metres (seventh row)

• 4 metres (eighth row).

Doctors use a score to say how well you can see on this test. The score has two numbers.

If you have normal vision, your score is 6/6. This means that when you stand 6 metres from the chart, you can read the letters that a normal person can read at 6 metres away. You may also hear normal vision called 20/20. This means the same as 6/6. It's just that the measurement is in feet instead of metres.

If you have weaker eyesight, the bottom number is higher. For example, if you stand 6 metres from the chart, you might be able to read only the letters in the fourth row. This score is 6/12, because someone with normal vision could read this row at 12 metres. You may also hear this called 20/40 vision. Again, it means the same as 6/12.

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 could help you.

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