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Of Acuity and Ratio
#1
Along with the 'Better Eyesight Without Glasses' book I purchased, came the Snellen chart, but the problem is, none of the rooms in my house are longer than 12ft; I was wondering if I take a measurement 10ft away from the snellen chart, so the acuity would translate to 10/x, and let's say I can see the line that is meant to be read at 20ft, so it would be 10/20, but say if I wanted to make it 20/x would I be able to convert it by simply saying  20/40 (2x numerator and denominator), or does the ratio not work so simply? [Also if I think about it, I guess there would be a limit to how far a person can see across the Earth, because it is spherical, so you would have to take large distance measurements using Space. DAMN I'm so confused]

Thank-you so much,

-Kaze
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#2
Dear Kaze,

Re: Snellen

<!-- m --><a class="postlink" href="http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html">http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html</a><!-- m -->

The Snellen is INTENDED to be read at 20 feet or 6 meters.

It is possible to read it at 10 feet or 3 meters with
SLIGHT loss of accuracy.

Visual Acuity is specified as the ability to read
0.9 cm letters at 6 meters.

Most DMV tests require that you read 1.8 cm letters
at 6 meters (20/40).

The range of DMV tests run between 20/70 (Florida)
20/60 (Georgia), 20/50 (Texas) to 20/40 for
almost all other states.

Best,

Otis
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#3
Kaze,

10/20 wouldn't necessarily be the same as 20/40 as far as the quality of vision needed to see it, but it's probably close enough that you might as well pretend it is for ease of reference. We're not here to be exact, after all.

There can be a few things to take into account. One thing is myopia isn't an inability to see small letters clearly at a distance; it's the inability to see ANYTHING clearly at a distance. So even the large letters are blurry, and the closer you get, the less blur there is, until when you move up to a foot away you'll find that your x/y ratio is quite a bit better than at 20 feet away. Whereas someone with perfect vision would pretty much maintain the same ratio at any distance, of course until you're talking about great distances where atmospheric effects might come into play.

Another thing is myopia is straining unnecessarily to see at a distance, and yet a greater distance doesn't necessarily mean greater strain. Some people might strain more at 10 feet than 20 feet, for strange reasons, so they might actually get a better ratio at 20 feet. And related to that, there's something to be said for looking at a small object and seeing more clearly versus if you were looking at a large one, because of the principle of central fixation.

Dave
Site Administrator

"Half of our funny, heathen lives, we are bent double to gather things we have tossed away." - George Meredith
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#4
Since the rooms in your house don't go past 12 ft., what you can do is download and use this chart from <!-- m --><a class="postlink" href="ftp://members.aol.com/myopiaprev/10ft-cht.tif">ftp://members.aol.com/myopiaprev/10ft-cht.tif</a><!-- m -->

It's a Snellen chart that is designed to be used at 10 ft.
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#5
It is always possible to use the WRONG chart at
the WRONG distance.

Just as a double check, you will find that the
20/60 letter is 1 inch at 20 feet, and 1/2 inch
at 10 feet.

If that is correct, then the rest of the chart
is correct.

Best,

Otis
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#6
Something to think about --> "The 20/40 letters are twice the size of 20/20 letters; however, it does not mean 50% vision since 20/20 sounds like it is one half of 20/40. If 20/20 is considered 100% visual effiency, 20/40 visual acuity is 85% efficient."

Citation: <!-- m --><a class="postlink" href="http://www.mdsupport.org/library/acuity.html">http://www.mdsupport.org/library/acuity.html</a><!-- m -->
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#7
I would also add this:

1.  A very strong minus will create 20/15 vision -- from
20/50 on the Snellen. 

2. I think that is a very VERY BAD idea.

3.  With 20/70 you would pass the Florida DMV test
naked eye.  Not great but there you have it.
You certainly do not want you VA to STAY
at 20/70.

4. Bates said get rid of the minus (at 20/70 about a
-1.25 diopter.)

5.  I agree with Bates on a scientific level.

6.  You then have the task of clearing your
Snellen to 20/40 by method "X".

7.  If Bates or Prentice is correct, then
go to it -- and pass that 20/40 and avoid
that wretched minus.

Around the world, the DMV test is 20/40 NOT 20/20.

I consider 20/40 to be 100 percent for that
reason.  20/20 then is 150 percent.

Just one man's opinion.

Best,

Otis
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#8
I simply cannot comprehend your reasoning Otis. Your saying that just because some lawmakers in their dusty offices think that 20/40 vision is the absolute minimum vision you should be allowed to operate a vehical without visual augmentation, that should be the standard we measure things by? Even 20/20 isn't the true standard.

<!-- m --><a class="postlink" href="http://en.wikipedia.org/wiki/Visual_acuity">http://en.wikipedia.org/wiki/Visual_acuity</a><!-- m -->

Average vision is 20/16 or 10/12. I have lost the source, but as I recall the average vision for the general population does not drop to 20/20 until 60 years of age.

Though it might be more logical to change the standard to 20/16 or 20/12 or whatever the real average may be, it isn't necessary and would serve no purpose, save to confuse people.
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#9
WOW!  Do you mean the AVERAGE person, or average
college graduate?  Do you mean NAKED EYE?

==============

Average vision is 20/16 or 10/12. I have lost the source, but as I recall the average vision for the general population does not drop to 20/20 until 60 years of age.

Otis> I would truly like to see that statistic -- with NO GLASSES
for distant vision.

Though it might be more logical to change the standard to 20/16 or 20/12 or whatever the real average may be, it isn't necessary and would serve no purpose, save to confuse people.

Otis>  I only report the facts as they are -- not as you may wish
them to be.

Otis> But on the subject of 20/20 vision (naked eye),
about 88 percent of the Hong Kong students are
myopic AT ENTRY, and would not pass the 20/20 line,
let alone the 20/15 line.

Otis> There seems to be a disconnect here.  Could
you provide a reference to the percentage of college
students who have 20/20 or 20/15 vision.  Would
it be 95 percent?

Best,

Otis
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