Formula for computer use bi-focals

Discussion in 'Optometry Archives' started by Rich, Jul 8, 2006.

  1. Rich

    Rich Guest


    What is the typical or ideal difference in diopter power between the
    reading portion of the bi-focal lens & the computer screen (upper)
    portion of the lens? I think I read somewhere that it should be a 1.00
    diopter drop. For instance, I use +2.50 diopters to read with (about 16
    inches distance). Would +1.50 diopters be ideal for the computer screen
    (about 24 to 26 inches distance)?
    Second question: the optometrist determined that +0.5 diopters was
    adequate for distance correction for me(progressive lenses). I later
    realized that in situations like sporting events or church, I see
    distant detail best through the narrow middle of the progression &
    slightly blurry through the top (+0.5) & more blurry through the bottom
    (+2.5). Furthermore, I have found that while lying in bed watching a TV
    about 15 feet away, an old +1.0 diopter pair of reading glasses are
    perfect for seeing the screen. Is this a case of the tendency of a
    patient to over correct when judging the proper strength of lenses?

    All insights are welcome.


    Rich, Jul 8, 2006
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  2. Rich

    Dom Guest

    The difference between computer & near is usually around 0.75D but could
    be anything from 0.50D to 1.00D or easily more, depending on the individual.
    You mean is it a case of the patient giving the wrong answers in the eye
    test? It's more likely a case of the optometrist not doing enough to
    ensure your accommodation was fully relaxed during the testing process -
    or, if the eye test was some time back, your eyes simply may have
    changed since then.

    Dom, Jul 8, 2006
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  3. Rich

    Dick Adams Guest

    Could also depend on where the book is held, and how far out the
    computer display is.

    Say, for a book at 15 inches, and a computer screen at 25 inches,
    taking 40 inches as equivalent to 1 meter:

    1/(15/40) - 1/(25/40) ~ 1.0 diopter

    which is to say that you would need about 1.0 diopter more of
    spherical correction for the book at 15 inches than for the
    computer screen at 25 inches.

    Of course, if you cannot do simple arithmetic, you will need to
    consult a Dr. for a subjective opinion.

    OP, who appears to be ~ 0.5 diopter farsighted, could find a pair
    of Walgreens readers for every non-distant visual task, even if
    totally hyperopic. Or he can get everything rolled together in
    a single pair of $400+ fancy eyeglasses (which he might not even
    Dick Adams, Jul 8, 2006
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