[Nfbf-l] How the 20/20 and 6/6 vision scales (charts) work.

[Alan Dicey]

Dear Friends,
I imagine many are like me, and have not seen a "Eye Chart" for a long time, 
much less been able to pick out letters on it.
But I found some interesting statistics and other information in this 
article, so I thought some would enjoy it, or even pass it along to family 
members, friends or associates to help "enlighten" them on what exactly 
"Legal Blindness" is.
With Best Regards,
Happy New Year
God Bless,
Alan
Plantation, Florida

How the 20/20 and 6/6 vision scales (charts) work.

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.

So imagine my surprise when I was told during my first Australian eye exam 
that I had 6/6 vision with my corrective lenses. 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. 
So, if you take someone with 20/20 vision and put them 200 feet away from 
the chart, they would still be able to see the big E clearly. 20/200 visual 
acuity and worse is considered legally blind in the United States.
That means that if you have 20/200 vision even with the best correction in 
your better eye, you are considered legally blind.

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. Again, take that 
person with 20/20 vision and put them 5 feet away instead of 20, and they 
would finally be able to see that last line of text. Most humans actually 
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, color perception, depth perception, and eye 
fluid pressure, among other things.

Bonus Fact:
a.. 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.
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