The 20/20 scale is different in different parts of the world. After examining a large number of people, American ophthalmologists decided on the 20/20 scale, saying that “20/20” is the normal visual acuity of the average person. That means standing 20 feet away from something, you can see what the average person can see standing 20 feet away from the same thing. In metric countries doctors measure how well a person can see at 6 meters away (19.69 feet).
The Snellen eye chart is the chart topped with the big E and
consists of 11 rows of capital letters that get progressively
smaller toward the bottom of the chart. A person is placed 20 feet
away from the chart. Since most doctors’ offices are too small,
mirrors are often used to simulate 20 feet. The doctor asks the
person to read out the smallest line of letters that can be seen
at that distance. Most people can read the fourth line up from the
bottom without trouble, so if a person can do this, the vision is
Using the Snellen chart, if a person can only see the big E up top
and none of the other lines of text, he is considered to have
20/200 vision., meaning he sees at 20 feet what an average person
can see at 200 feet away. 20/200 visual acuity and worse is
considered legally blind in the United States, unless it can be
corrected to better with glasses or contacts.
If a person can read the tiny bottom line of text on the chart at
20 feet away, it is considered 20/5 visual acuity. Most humans do
not have the ability to have much better than 20/10 vision.
The 20/20 or 6/6 visual acuity is not a measure of a prescription
as it does not take into account the nature of the problem, only
the result of it.