h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: What is 20/20 Vision benchmark and how does it work? - A995592
Author: Back_Ache - U197115

Let me present for review my little artical on the 20/20 vision benchmark.

Please be kind to me, I wear glasses.

To be honest I'm not sure this has enough content, there is loads more you can say about 20/20 vision.

6/6 it's equivalent in metres (and used in the UK, and probably more countries)
The use of Snellen Charts for measuring it instead of logMAR charts.
The fact that it is an actual subtend of an angle (ok that didn't make much sense) but the 6/6 line is where the bar of a letter E subtends at angle of 5' (i think, i really can't be bothered at this moment to find out the exact measurement, but if you need it i can find it)
It wouldn't be 10feet away for it to be 20/... it would be 20 feet
You say it's 'old', how old, a bit of history. Why was 20/20 chosen as a 'normal' when most of the population can reach 20/10
It only tests high contrast targets, so it's not an acurate representation of vision.

Sorry for being a little harsh, but with a bit of work this could be a great article. What do you think?

SallyM
Ok can you tell i'm doing my dissertation on visual driving standards?

I agree, this could be a really interesting article but is currently inaccurate & insubstantial.

bach Ache, do you want any help with it, or do you want to go & do a bit of research by yourself?

THis doesn't make any sense to me at all:

20/20 means can read 20pt text at 10 feet
20/10 means can read 10pt text at 20 feet

You'll have to explain in much more detail. What does the first 20 mean? What does the second 20 mean?

Is there any indication of differences between eyes?

http://www.howstuffworks.com/question126.htm says nothing about 20-point text. They say "eye doctors have decided what a "normal" human being should be able to see when standing 20 feet away from an eye chart. If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what the "normal" human being can see.

". . . If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can see what a normal human can see when standing 40 feet from the chart. That is, if there is a normal person standing 40 feet away from the chart and you are standing only 20 feet away from it, you and the normal person can see the same detail. 20/100 means that when you stand 20 feet from the chart you can see what a normal person standing 100 feet away can see. 20/200 is the cutoff for legal blindness in the United States.

"You can also have vision that is better than the norm. A person with 20/10 vision can see at 20 feet what a normal person can see when standing 10 feet away from the chart."

This really could be interesting if some more information were added, along the lines of suggestions already given.

Got to admit I probably have 20/-20 so some insight would be really good as to how vision works and why some of us have problems, how they can be corrected (laser surgery would not work for me) how the optic nerve functions – little informative bits to inform and round the subject off.

Very good idea – look forward to learning more.

Amused0

You see at 20 feet what a normal-sighted person sees from 20 feet beyond the chart? Sounds mighty: not only 60/20 but x-ray as well!

If the point size is not part of the equation, why do metric users bother to say 6/6? The proportion 20/20 will hold whether they are 20 feet, 20 yards, or 20 metres! Looks like they count 20 feet as the standard checking distance; 20mm would favour the short-sighted, and so on. Not so simple after all!

20ft or 6m is the standard distance between the patient & the chart. Many consulting rooms use a mirror or projector for practicality so that the room need only be just over 3m long.

There are limiting factors, such as the separation of retinal receptors, as to how good vision can be. There are also other types of chart available to assess vision, such as contract sensitivity charts, which some consider to be a more discriminating test.

"Contract sensitivity" -- is this the ability to read the small print?

So the description of 20/20 vision in this entry is wrong?

oops that should of course be contrast sensitivity.

Yes Gnomon, that's what we've been trying to point out.

So this entry has only three sentences. Two of them are factually wrong while the third has two grammatical errors in it. Could the author please do a complete re-write, then let us know. In the meantime, we won't discuss this any further.

Actually, I think we'll discuss this a lot further, as I don't think that my asking people not to discuss this will affect you all, oh good people, in any way.

I'd be ineterested to know if the vision benchmark thing takes accounrt of differences between eyes as well.

I know, at least in Florida, the driver's license exams involves looking into a box with the rows of letters. The first set is for the left eye only, a center section must be read by both eyes, and the last section is for the right eye only.

This could become a fascinating entry if somebody put in just a little effort and makes sure of their facts. Hint to Back_Ache - Please do a little more work on this one, you have a great subject that could be quite informative and even help other researchers. There are several of us who would like to see this improved to meet the Writing Guidelines in both fact and style.

F S

I'm used to using the 6/6 benchmark, the way you do it is to test each eye serpratly.

*nods in agreement with the others..*

So, Z, do you get a different rating for each eye then, or are they amalgmated into an overall rating? (I have eyes, so this is of intrest)

Yup you get a different reading for each eye, though when recording funcitional level, such as the level you need to drive a car, you record the level that they can read down the chart using both eyes.

You can also record best corrected vision, that's the best you can get with the patients glasses on..

That last one would probably be the only one I'd be any good at.

Well that's the important one, it determines what you can do in life really..

Another trick worth mentioning is that if you want to quickly determine whether a patients vision problems can be corrected by glasses is to make a pinhole in a piece of paper and ask them to look through the hole at the chart. If the problem in their vision improves by about three or more lines on the snellen chart then it's a refractive error, and can be corrected with glasses.