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| Admittedly,
I
can't
find
any
actual
definition
of
exactly
what
"20/400"
is
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http://en.wikipedia.org/wiki/Snellen_chart
"The biggest letter on an eye chart often represents an acuity of 20/200, the value that is considered "legally blind." Many people with refractive errors have the misconception that they have "bad vision" because they "can't even read the E at the top of the chart without glasses". But in most situations where acuity ratios are mentioned, they refer to best corrected acuity."
Most legitimate pages that show up on google don't even care to elaborate on the differences between being considered 20/400 with or without glasses. Or even mention that detail, unless they're about legal blindness. (Personal, subjective accounts and tabloid-esque stories along the lines of "I went from 20/400 to 20/20" will be disregarded as many laymen and journalists are usually nonchalant about terminology and cannot be considered trustworthy sources. See wikipedia quote.)
Since all actual images of the Snellen Eye Chart that I have found only go up to 20/200, my conclusion is that a popular usage of 20/400 is as a blanket term for anything worse than 20/200 but better than CF10'. (Popular != official medical definition.)
I don't see any reason why the original scale would need to go any higher, as it has nothing to do with determining what prescription you need, but rather whether you do in fact need one.
Meanwhile, if you really want to go beyond 20/200, you would have to use completely different methods. The only useful purpose, aside from curiousity, is to test if you get so low down that glasses wouldn't do you any good anyhow.... which is possibly why it's so incredibly vague - perhaps, if you're in a certain range, it doesn't matter exactly where as any degrees within it are equally hopeless?
http://en.wikipedia.org/wiki/Visual_acuity
"When visual acuity is below the largest optotype on the chart, either the chart is moved closer to the patient or the patient is moved closer to the chart until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, he or she is tested as follows:"
(^ Btw, this would seem to suggest that 20/400 is always myopic, never hyperopic, since you measure it by moving the chart closer.)
http://www.mdsupport.org/library/acuity.html
"It is common to record vision worse than 20/400 as Count Fingers (CF at a certain number of feet), Hand Motion (HM at a certain number of feet), Light Perception (LP), or No Light Perception (NLP)."
This is then converted to an approximate Snellen Acuity equivalent. (Is it part of the scale or not? Who can say?)
However, While a range of prescriptions is given for the 20/30-20/200 categories is given, I can't see any tables on the range of suitable prescriptions for 20/400. Nor have I yet seen any real evidence of where the absolute correctable threshhold is on the Snellen scale.
In short: I will only concede that "worse than 20/200 without glasses" is correctable, but whether or not 20/400 is the only value below 20/200 and higher than CF10' is unknown. None of the google pages directly contradict or makes impossible the interpretation that 20/400 is "worse than 20/200 with glasses". Quote me a source that states this explicitly and I'll be quiet :P
Also, since Robwood was so eager to decrease his peripheral field in exchange for greater acuity, it must be *really* bad. I'm inclined to believe he truly is legally blind. In any case, would you take his word for it that he didn't mix it up with something else or performed a self-diagnose? :P
(Don't worry, I'm just *really* bored and felt like playing Devil's Advocate and try to defend an indefensible viewpoint purely for my own amusement - you don't have to answer, I know bettarr ;)
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AUTHOR OF THIS MESSAGE
Nemi
MESSAGE TIMESTAMP
20 july 2006, 23:43:52
AUTHOR'S IP LOGGED
213.112.164.229
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