What does 20/20 vision mean?
[18 Oct 2010]
Eye chart [source]
Tiger Woods had “severe near-sightendness” and needed to wear contact lenses before he underwent lasik surgery in 1999. He now has 20/15 vision, significantly better than most of us, where “normal” vision is considered 20/20.
What do these eye acuity fractions mean?
An eye chart consists of letters of varying sizes. (Optometrists use a symbol chart if the patient is likely to be illiterate.)
On the chart shown, the height of the 8th line (starting with “D”) is scaled so it is around 1.75 mm. From a distance of 20 feet (6 m), the height of the letters subtends an angle of 1 arc minute.
The angle “1 arc minute”, means 1/60 of one degree. (In turn, 1/60 of an arc minute is one arc second. We get these meaures from the Babylonian base-60 number system). To give an idea what this means, when we look at the full moon from the Earth, it subtends an angle of around 31 minutes, or just over half of one degree.
Someone with “normal” 20/20 vision will be able to read the 8th line at a distance of 20 feet.
In the fraction 20/20, the numerator (the first number) is the distance in feet between the subject and the eye chart. The denominator (the second number) is the distance from which a normal person can read the 8th line (and this gives a visual angle of 1 arc minute).
20/20 vision is the minimum for fighter pilots.
20/40 vision is half the acuity of 20/20. This means the patient can only see clearly at 20 feet what normal people can see at 40 feet. So a patient with 20/40 vision can see clearly only down to about the 6th or 7th line of the chart (text which is 3.5 mm high). This is the legal limit for driving in the United States.
On the other hand, 20/10 vision is twice normal acuity, and this would imply the patient can read down to the lowest line on the chart.
The aim for the optometrist when dealing with someone with vision difficulties is to find the prescription that will provide at least 20/20 vision.
Tiger Woods performed much better in his golf game after his lasik surgery. Some commentators argue this may have given him an unfair advantage (in the same way steroids give athletes an unfair advantage). What do you think? Is eye enhancement cheating?