How do you use a eye exam chart?
Our eyes are designed for distant vision. At 20 feet, the light
rays that enter our eyes are parallel. There is no need for our
eye to bend the light rays to focus them on the retina.
The 20/20 line in the eye chart indicates the size of print a
person with normal vision could read when he stands 20 feet
away.
Although we use foot as the unit of measurement in above
illustration, we can also express it in metre. This is best
illustrated in the table below. The 20/20 vision is equivalent
to 6/6.
A visual acuity score of 20/20 is considered as normal vision.
Visual acuity scores of 20/32 or 20/40 (i.e. 6/9 or 6/12 in
metre terms) indicates the presence of significant but low-power
refractive errors. Visual acuity of between 20/63 and 20/200
(i.e. 6/18 and 6/60 in metre terms) is moderate refractive
errors. Those with severe myopia will not be able to see 20/200
(i.e. 6/60 in metre terms) without glasses. A person with worse
than 6/60 visual acuity despite the best glasses correction, is
considered as legally blind.
On the other hand, if you have 20/15 vision (i.e. you can read
at 20 feet the letters that can only be seen by someone who
meets the standard measure of normal eyesight), you are said to
have good vision. A fortunate 5% of the population have 6/5 or
6/4 vision which is sometimes termed "eagle" vision.