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

Discussion in 'Eye-Care' started by Bob Peyton, Sep 25, 2006.

  1. Bob Peyton

    Bob Peyton Guest

    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

    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

    Current Rx:

    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
    Bob Peyton, Sep 25, 2006
    1. Advertisements

  2. Bob Peyton

    Jan Guest

    Bob Peyton schreef:
    You can compensate for the shorter distance, in your example you have to
    add about -0,5 diopters since you have no abilities to accommodate.

    7 feet is the same as about 2 meters I think and this results in 1/2=
    0,5dpt which has to be compensated for the far distance.

    The adjustment in size of the characters is only done to assure the
    correct vision acuity can be notated.

    However, refraction on such a short distance is NOT DONE for several
    reasons, i.e. accommodation, binoculair vision related issues etc..

    The only one who is measuring the visus (vision acuity) this way is that
    horrible layman Otis.

    Jan (normally Dutch spoken)
    Jan, Sep 26, 2006
    1. Advertisements

  3. Bob Peyton

    otisbrown Guest

    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:

    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.


    otisbrown, Sep 26, 2006
  4. Bob Peyton

    Dan Abel Guest

    My old OD had a 20 foot exam room. The layout was a little weird, but I
    liked it.
    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, Sep 26, 2006
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.