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

Loads of suggestions here for improving the entry!

This may help towards explainging how vision is measured. (Extract Source: http://www.uveitis.net/miscell.what_is_low_vision.htm


Distance vision

This is measured using the familiar Snellan chart, shown below, our vision being that of the smallest line we can read.

This chart has some limitations as there are so few letters at the top. You may well come across other charts that are more complicated.
Without worrying too much what the measurements literally mean, we can link the figures given for our sight to the number of lines we can manage to see on the chart.

The range of eyesight may go from 6/5 to 6/60 to 3/60 to terms like CF.
6/5 describes very good eyesight, slightly better than most people can manage. It means that at a distance of 6 metres you can read the bottom line of the eye chart.

6/60 would mean that you can only make out the top, largest, letter.
3/60 would mean that you would have to move closer to the chart (3m, instead of 6m away) in order to see the top most letter.

If the topmost letter can not be made out from a metre away then an assessment is carried out as follows:
• ability to count fingers (CF)
• hand movements (HM)
• perception of light (POL)
• no perception of light (NPOL)

The level of eye sight required for driving is approx. 6/10 (it is actually tested by the ability to read a number plate at 25 metres.
It may be worth pointing out here that this is a rather limited way of testing eye sight. Deciding whether you are able to drive is obviously a sensitive subject., and depends on other factors such as how much you see around you, your field of vision. However I can only recommend that if in any doubt, then it would always be best to talk it over with your eye specialist or Optometrist.

Near Vision

This is an important measurement to regularly have monitored. The test is carried out using any reading glasses already used. The smallest print size which is able to be read is noted. Each ‘font’ size is given a number. N5 is a measure of normal close vision. As a guide, newspaper print is usually N8.


Some links for research material:

http://www.academy.org.uk/

On this site there is loads of info and links to optometry etc sites.

http://www.terrymen.demon.co.uk/eyetoey2.htm

A real insight to what the optometrist does – click on the eye to explore further.

http://www.moorfields.co.uk/EyeHealth/Anatomyoftheeye

Moorfields eye hospital – all you need to know about eyes.

A couple of qualifications of earlier points.

Improved vision with a pinhole does not always mean spectacles can achieve the same improvement. Some eye pathology causes poorer best corrected acuity than with a pinhole.

The bottom line of a sight chart is not always 6/5. Some stop at 6/6 some go down to 6/4 or 6/3. So don't worry if you 'read the bottom line' at your last test & can't manage so far somewhere else.

I hope back_ache comes back to this thread. I think the first post was their only contribution, despite the fact that they are still around and subscribe to the convo.

The entry really does need a lot of work to give a reasonable answer to the question the title poses.


Geggs

This is really interesting and would indeed make a good guide entry.
I've never known (or had any idea) til I came across this entry and thread what 20/20 meant - despite the fact that I've worn glasses since I was 11 years old.

Now I wonder how this all relates to the spectacle strength for near/farsightedness. Glasses usually come in strengths like -4.25, -3.00 etc.

2/3

I could easily be wrong on this, but I have the impression that the numbers on your prescription represent the 20/whatever vision rating, so if you wear -4.00 lenses that would mean 20/400 vision.

It might also bear mentioning that eye doctors don't like correcting people's vision to better than 20/20, although I'm not sure why.

I didn't find any mention of how the prescription numbers relate to the vision standard, but the numbers stand for "diopters", which are basically degrees of curve on lenses. For nearsighted people the lenses are concave (curved in )( ) and the prescription is given in negative numbers, going more negative with worse vision. Farsightedness is the opposite. Convex lenses (), positive numbers the increase with worse vision.

I'd also like to know what the vision designation/prescription is for being called legally blind.

Back_Ache, there seems to be great interest in an entry on the 20/20 vision standard, but this entry does not explain it. Firstly, it is far too short and secondly, the explanation give appears to be completely wrong.

Is there any chance you could write us an entry on this yourself?

If not, would you agree to a collaborative entry? We'd provide content and you'd put it into the entry, crediting us as contributors.

If neither of these is to your liking, would you consider moving this to the Flea Market, where someone else could take it over?

Information on blind/partial sight registration

http://www.rnib.org.uk/xpedio/groups/public/documents/PublicWebsite/public_rnib003702.hcsp

Dioptric power is another way of expressing the focal length of a lens. It is related to the curvature, but spectacle wearers may notice that their lenses are not usually () or )( shape but ))- each surface will have a different shape curve.

I agree there is loads that could be written, let me finish my dissertation on visual driving standards, and i'd be happy to contribute.

About the point on if the power of your lens affects your acuity, with minus lenses (short sighted) there is an approximate relationship between the power and line you can read. Ie if you are myopic by 1D theoretically you should only be able to read the 6/18 line not the 6/6 line. If you are hyperopic (long sighted) the power doesn't really affect the lines you can read in the distance, however it will affect the size of the letters you can read close up.

SallyM

Hyperopic -- that used to be hypermetropic, no?

Yep, same thing. Just to clarify what Sally said about correlation between myopia & visual acuity, in her example that's 6/18 without the spectacle correction. Corrected, the vision should be 6/6.

Not sure if we've mentioned it earlier, but because negative lenses minify & positive lenses magnify, with high power negative spectacle lenses, although 6/5 or smaller may be clear, the eye may not be able to resolve such a small image to read the letters.

The author has left the building - Flea market do we think? Or back to entry?

I'm probably not going to have enough time to do any work on this, so if it wants to go back to the entry that's fine with me.

SallyM

Back to entry. The information in the entry is wrong, so it is not a good basis for an entry in the Flea Market.

Thanks for the info Gnomon.

Seconded for a move; and I'll do the e-mail to boot.