better than 20/20

Hi all

Can someone explain the difference between being able to see better
than 20/20 (or 6/6) say for example 20/15, and hyperopia?

For example, an optometrist can correct a myope to 20/15 sometimes.
How does he know that he is not overcorrecting?

Thanks
Richard

 

schreef:

Richard, vision acuity is just an agreement about a standard notation.
It only describes what you can see with or without a correction.

All over the world it is used, some countries with 20/20 (America), 6/6
for the old fashioned Englishmen and for the very modern Europeans it is
written as 1.0

You can have a better vision acuity (i.e. 20/15 or 6/5 or 1.2) or a
worser (20/40 or 6/3 or 0.5)

An emmetropic eye (no correction needed) can also be valued.

Overcorrecting a myope never results in a better vision acuity, even so
does undercorrecting a hyperopic.

Vice versa, undercorrecting a myope results in a worser vision acuity
and overcorrecting in a hyperopic does the same worse thing.

Jan (normally Dutch spoken)

 

acuity of 6/6 is a sort of "normal" clinical benchmark. it is not the
best acuity a human can achieve, which is around 6/4.5.
hyperopia describes a refractive error in which light tends to focus
behind the retina. many hyperopes see 6/6 or better with no glasses/
contacts.

the persons best acuity (6/6, 6/4.5 etc.) would be measured with their
refractive error corrected (glasses, contacts, laser surgery eg.).
Over-correcting will not produce better acuity. if a myope sees 20/15
it does not mean he/she is overcorrected. Some overcorrection can be
tolerated and still show 20/15 though.

as for knowing if someone is overcorrected there are a few ways, one
of which is testing with a red=green filter over the snellen chart
which causes one side of the chart to be seen on a green background
and the other side in red. (overcorrected myopes see letters more
clearly on the green side, opposite for overcorrected hyperopes).

 

michael toulch schreef:

Interesting, I almost never use the R/G test, not reliable when people
still are able to accommodate TMHO.

The vision acuity is my leading issue when refracting.

Jan (normally Dutch spoken)

 

Thanks everyone for your replies. I found it them interesting. The
binocular effect was pretty much what I was thinking of - that if you
were an over corrected myope you would be like a hyperope, therefore
able to see things further away clearly.

I like Mike's idea for checking the final prescription. I have a
history of being given the "wrong" prescription, although it's often
been a case of -0.25 too much sphere but -0.25 too little cylinder
along with an axis 5 degrees out.

Also, when comparing 2 lenses I sometimes can make out the letters
easier on one lens, but the brightness/contrast on the other is much
nicer.

It's interesting to hear about the difference of opinion on the red/
green test.

Thanks again
Richard

 

I think most would agree that 0.25 diopters is within the standard
error of a refraction. Even a widget is manufactured against a set of
tolerances, or a range of values that are acceptable.

DrG