[wordup] 20/20 Vision ...

[Adam Shand]

I never did freaking understand what it meant, and I can't be the only
one. :-)  Adam.

From: http://med-aapos.bu.edu/publicinfo/store4/20.20vision8.48PM.html

20/20 Vision
This response submitted by RW Enzenauer, MD on 12/6/00.

There is often a lot of confusion about 20/20 vision. The vast majority
of parents think wrongly that 20/20 is "perfect" vision. 20/20 vision is
a standard that is over 100 years old, and basically means that you can
see at 20 feet what a normal" person is expected to see at 20 feet.
20/20 thus is a measurement of the vision in ONE eye against a given
standard. (a 20/20 measured 9.3 mm at 20 feet/.) You are correct that
the metric equivalent is 6/6, that is you see at six meters what a
normal person is expected to see at six meters. "perfect" vision
attainable by humans is probably about 20/8, that is someone can see at
20 feet what we would need to see at 8 feet, so about a 4.5 mm E on a
chart on the wall.

For near-sightedness, -1.00 or 1 diopter of near-sightedness would
correlate with 20/40 vision or so. About 2 diopters of astigmatism would
also degrade the visual acuity to about 20/40. Fsar-sightedness is
different. It is normal for a child to be far-sighted, and people can
accomodate 'through" normal amounts of far-sightedness with good vision,
so a nroaml child or young adult with +1.00 diopter of far-sightedness
would be expected to see 20/20. it is when far-sightedness is excessive
in kids that visual acuity is affected, say for hyperopia greater than
+3.00 diopters or so. We lose our ability to accomodate as we get older,
which is why older adults need bifocals, and why far-sighted adults are
often so distressed as they get older, since they could see so well in
their youth, and then their visual acuity gets worse, even at distances,
when they lose their ability to accomodate.

Robert W. Enzenauer, MD, MPH
pediatric ophthalmology
chattanooga, TN