What do the numbers mean?
by Bob
(Fort Wort, TX USA)
Eyechart-with-glasses
Question: What is meant when you have an eye exam and your optometrist tells you your vision in the left eye is 20/400?
Answer:
Hi Bob, your question is about the meaning of the technical terms Optometrist's use to record your vision.
There are two parts involved with the numbers. The top of the fraction refers to the distance you are from the eyechart. The standard testing distance is 20 feet (6 metres in metric). The second number refers to the size of the smallest letter you can see from that distance.
So vision of 20/20 indicates that from 20 feet, you can see the smallest letter that an average person with no eye problems can see from that distance. Following that, a person with vision of 20/400 can only see the largest letter on the chart indicating poor vision - such a person would either need glasses to see better, or may have an eye condition/disease preventing good vision. (Why the larger number? This refers to the maximum distance from which the "normal person" can see the same letter eg 400 feet)
In Australia, we measure in metres. So normal vision is 6/6 and poor vision is 6/120.
