Vision - Can distance to eye chart be compensated for by character size on Snellen Chart?

Author

Can distance to eye chart be compensated for by character size on Snellen Chart?

Bob Peyton

2006-09-25, 9:33 pm

The opthalmology clinic I visit has recently moved into new offices. In the
old office refraction for prescribing eyeglasses was done at a distance of
(I assume) 20'. This was accomplished with the usual
projector/screen/mirror system to achieve the desired eye-to-chart distance,
since the examining room dimension was less than 20'. The new offices use a
computer-driven LCD screen viewed directly from a distance of about 7 feet.
I asked the techician who was actually doing the refraction if this produced
accurate results and she assured me that it did, since the computer adjusted
the size of the letters in the chart to compensate for the shorter viewing
distance. Nevertheless, a recent refraction and a re-refraction have both
produced Rx that yielded blurred distance vision, but which are really good
for TV viewing at ~10'. The optician says the glasses were made to
prescription and the clinic agrees. Refractions done at the old office were
satisfactory.

I have IOL's in both eyes (since 1990), thus no distance accomodation.
IIRC, shortly after the IOL's were inserted my vision was sharpest at about
6 to 7 feet. At that time I needed trifocals for distance vision and
reading/computer work. Over time, my sharpest vision distance has decreased
to about 14".

Is it reasonable to believe that a Rx measured at ~7' will produce good long
distance vision, especially with IOL's? Is it possible that the Rx measured
at 7' could be mathematically adjusted to provide better long distance
results?

Current Rx:

SPH CYL AXIS ADD
OD -1.50 +1.50 173 +2.50
OS -1.75 +1.00 009 +2.50

Lenses are progressive "Transitions".

Thanks to any and all who reply,

Bob Peyton

otisbrown@pa.net

2006-09-25, 9:33 pm

The "official" distanc has always been 20 feet or 6 meters, which
was considered to be optical "infinity".

Ten feet is probably a reasonable compromise. But less
that 10 feet -- well...

If they are prescribing for 7 feet, then the result will
most probably be an under-prescription of 1/4 to 1/2 diopter.

If you wish to check this yourself, I have a nifty IVAC Snellen
on my site at:

www.myopiafree.com

that you can check at 20 feet.

If you read the 20/20 line (1/2 the letters correctly) then you
can confirm that your prescription meets that standard.

Enjoy,

Otis

Bob Peyton wrote:
> The opthalmology clinic I visit has recently moved into new offices. In the
> old office refraction for prescribing eyeglasses was done at a distance of
> (I assume) 20'. This was accomplished with the usual
> projector/screen/mirror system to achieve the desired eye-to-chart distance,
> since the examining room dimension was less than 20'. The new offices use a
> computer-driven LCD screen viewed directly from a distance of about 7 feet.
> I asked the techician who was actually doing the refraction if this produced
> accurate results and she assured me that it did, since the computer adjusted
> the size of the letters in the chart to compensate for the shorter viewing
> distance. Nevertheless, a recent refraction and a re-refraction have both
> produced Rx that yielded blurred distance vision, but which are really good
> for TV viewing at ~10'. The optician says the glasses were made to
> prescription and the clinic agrees. Refractions done at the old office were
> satisfactory.
>
> I have IOL's in both eyes (since 1990), thus no distance accomodation.
> IIRC, shortly after the IOL's were inserted my vision was sharpest at about
> 6 to 7 feet. At that time I needed trifocals for distance vision and
> reading/computer work. Over time, my sharpest vision distance has decreased
> to about 14".
>
> Is it reasonable to believe that a Rx measured at ~7' will produce good long
> distance vision, especially with IOL's? Is it possible that the Rx measured
> at 7' could be mathematically adjusted to provide better long distance
> results?
>
> Current Rx:
>
> SPH CYL AXIS ADD
> OD -1.50 +1.50 173 +2.50
> OS -1.75 +1.00 009 +2.50
>
> Lenses are progressive "Transitions".
>
> Thanks to any and all who reply,
>
> Bob Peyton

Dan Abel

2006-09-25, 9:33 pm

In article <DKYRg.37199$KR1.33099@bignews2.bellsouth.net>,
"Bob Peyton" <pegndob@bellsouth.net> wrote:

> The opthalmology clinic I visit has recently moved into new offices. In the
> old office refraction for prescribing eyeglasses was done at a distance of
> (I assume) 20'. This was accomplished with the usual
> projector/screen/mirror system to achieve the desired eye-to-chart distance,
> since the examining room dimension was less than 20'. The new offices use a
> computer-driven LCD screen viewed directly from a distance of about 7 feet.
> I asked the techician who was actually doing the refraction if this produced
> accurate results and she assured me that it did, since the computer adjusted
> the size of the letters in the chart to compensate for the shorter viewing
> distance.

My old OD had a 20 foot exam room. The layout was a little weird, but I
liked it.

> Is it reasonable to believe that a Rx measured at ~7' will produce good long
> distance vision, especially with IOL's?

No.

> Is it possible that the Rx measured
> at 7' could be mathematically adjusted to provide better long distance
> results?

Jan has replied to this. I am not impressed with the idea of using a
"fudge factor" when testing my distance vision. Perhaps 7 feet is good
enough, but I know that making the letters really tiny at a distance of
18 inches is not a good test of my distance vision.

--
Dan Abel
dabel@sonic.net
Petaluma, California, USA