How the 20/20 Vision Scale Works With more than 150 million people in the United
States (nearly half of the population) requiring some form of corrective eyewear to compensate
for visual impairment, chances are you have had your eyesight graded on the 20/20 scale
before. If you haven’t, you have probably heard other people saying they have “20/20
vision” or even the phrase “hindsight is 20/20.” The vision scale is so prevalent
in American culture that there’s even a TV news show named after it.
Of course, if you’re from, for example, Australia, you might be told you have 6/6
vision. Turns out, the 20/20 scale isn’t universal. So what exactly are eye doctors
measuring, and how can the scale be 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. What
that means is that standing 20 feet away from something, you can see what the average person
can see standing 20 feet away from the same thing.
Take the Snellen eye chart, which is what your eye doctor will usually use to judge
your eyesight. The Snellen chart is the one that’s topped with the big E and consists
of 11 rows of capital letters that get progressively smaller toward the bottom of the chart. You
will be placed 20 feet away from the chart (most doctors’ offices are too small for
this, so mirrors will often be used to simulate 20 feet). The doctor will ask you to read
out the smallest line of letters that you can see from 20 feet away. Most people can
read the fourth line up from the bottom without any trouble, so if you can do this, your vision
is considered 20/20. Now, obviously most places in the world don’t
use the Imperial system to measure distance, they use the Metric system, which is where
the 6/6 scale comes in. In this case, doctors are not measuring how well you can see something
from 20 feet away, they are measuring how well you can see at 6 meters away (that is,
19.69 feet approximately). It’s the same principle, just a slightly different measurement.
Of course, many people will have worse or even better than 20/20 vision. These people
will tip the scales a bit. Back to the Snellen chart, if you can only see the big E up top
and none of the other lines of text, you are considered to have 20/200 vision. That means
you see at 20 feet what the average person can see at 200 feet away. 20/200 visual acuity
and worse is considered legally blind in the United States.
Alternatively, if you can read the tiny bottom line of text on the chart at 20 feet away,
you have 20/5 visual acuity, which means you can see at 20 feet that which most people
can only see at 5 feet away. Most humans don’t have the ability to have
much better than 20/10 vision, with 20/5 vision reserved for animals like birds of prey. These
numbers would obviously be adjusted for the 6/6 scale.
That said, your 20/20 or 6/6 visual acuity is not a measure of your prescription as it
does not take into account the nature of the problem, only the result of it. That’s why
you can’t just pop in to your ophthalmologist, read the Snellen chart, and head out—they
have to measure things like peripheral vision, colour perception, depth perception, and eye
fluid pressure, among other things. Bonus Fact:
• The Snellen eye chart was created by Herman Snellen, a Dutch eye doctor, in the 1860s.
There have been other charts developed, however, which also might be used during an eye exam.
An example is the Tumbling E chart which features capital letter E’s facing in different directions.
This chart comes in handy when young children who don’t know the alphabet are being tested,
or for people who don’t know the English alphabet. Rather than say a letter, they can
pick the smallest line of E’s that they can see, and say or point which way the “arms”
of the E in that line are facing. Numerous studies have shown that this chart and the
Snellen chart come up with nearly the same results.