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Thread: Prism for reading glasses

  1. #1
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    Prism for reading glasses

    I received an rx of +3.00-0.50 x 080
    +4.00-0.75 x 100
    with near pd of 58. The doctor added a note saying "Lab to add prism as needed for focal distance of 8 inches". How is this calculated?

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    Rising Star Chad Sobodash's Avatar
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    Someone please feel free to step in, but I think this must be some sort of mistake. If that is a distance Rx, then there's no add power specified to tell you what they need for reading. If that's a reading Rx ... well then is it for 16 inches or 8 inches? And what's this about adding prism?

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    Master OptiBoarder CCGREEN's Avatar
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    Its a mistake or a unfinished Rx. OR whoever wrote the Rx has NO understanding of prism. Sounds like they think they know what they want to accomplish but have no idea how to go about it. Not enough information to work with.

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    First, let me wipe away my tears from laughing. I think this will end up being a phone call to the dr's office to determine what it is that the MD wants. Don't get me wrong I have nothing but respect for Doctors and what they do but OMG that's so funny. Why didn't he just write + oh I dunno something minus banana axis purple OU and I want the patient to probe the visual depths of time AS NEEDED. It may have been easier to fill that RX.

  5. #5
    What's up? drk's Avatar
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    This is sloppy prescribing.

    This is likely an Rx for a low vision patient. (Probably as old as Martellaro!)

    8 inches (2.54 cm/in) = ~20 cm. That infers (implies?) a +5.00 add! (That means that the patient's distance Rx is something like -2.00-0.50x080/-1.00-0.75x100!)

    Demand with a 58 pd @ 20 cm =
    p.d (cm)/d (M) =
    5.8/.20 = 29^

    The guy is supposed to be holding the reading material so close that it would be uncomfortable to converge the eye muscles. How much should he converge his eye muscles?

    Let's say "as much as he would normally do when reading at 40cm".
    At 40 cm, with a 58 pd, his "demand" is 14.5 (pd/d).

    So, let's prescribe the difference: 29^ minus 14.5^ =14.5^. So 7.25 BI OD and OS.
    Last edited by drk; 06-24-2014 at 03:44 PM.

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    Quote Originally Posted by mountainheart View Post
    I received an rx of +3.00-0.50 x 080
    +4.00-0.75 x 100
    with near pd of 58. The doctor added a note saying "Lab to add prism as needed for focal distance of 8 inches". How is this calculated?
    Clinical rule is the add power +2, base in, in each lens. If the the add power above is +5.00, provide 7∆ base in for each lens.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    Thank you drk!

  8. #8
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    Thank you Robert!

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    What's up? drk's Avatar
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    Addendum:

    If he has markedly more horribler acuity in one eye or another (this is probably a macular degeneration case), you could not put the prism in the "good eye" to save decrease in acuity from crappy optics from thick prism (or at least split it unequally). But then you'd have practical edge thickness problems.

    Otherwise, or if unknown, split it.

    You be best-served to put this in a half-eye.

    BTW, you have to measure the pd with him @ 20 cm, too. My pupillometer only goes to 33 cm. You could do the old-fashioned method stationed 20 cm from his face (hope he had a tic-tac), or you could calculate it with the p.d. formula (which is an old trade secret that I've written down somewhere, if you need it. If Chilinguerian is here, he probably has it tattooed on his bicep or something.)

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    What's up? drk's Avatar
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    Robert, I didn't know that rule-of-thumb, but it sure worked here.

  11. #11
    What's up? drk's Avatar
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    An intuitive way to look at this would be this:

    Q: Hey, how much convergence demand does your average Joe get when reading at the average distance?

    A: 15 prism diopters.

    Q: Well, in this case he's holding it twice as close.

    A: Well, then he needs 30 prism diopters, but let's spot him his usual 15.

  12. #12
    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by drk View Post

    <or you could calculate it with the p.d. formula>
    20cm = .881.

    Quote Originally Posted by drk View Post
    Robert, I didn't know that rule-of-thumb, but it sure worked here.
    We posted at the same time. Nice post. I knew the math, but the shortcuts are damn convenient, and close enough most of the time.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  13. #13
    What's up? drk's Avatar
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    Ok,
    1. How did you do the p.d. trick?
    2. Didn't you see my jibe on your age?

  14. #14
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    How does the 20cm=.881 come into play? Do I multiply the 40cm pd by .881?

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    What's up? drk's Avatar
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    Probably the distance p.d.

    I know from memory that distance p.d. x 95% = near pd @ 40 cm., so...

  16. #16
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    Quote Originally Posted by mountainheart View Post
    How does the 20cm=.881 come into play? Do I multiply the 40cm pd by .881?
    The distance PD, yes.

    Quote Originally Posted by drk View Post
    Ok,
    1. How did you do the p.d. trick?
    2. Didn't you see my jibe on your age?
    NPD = DPD - DPD/1 + W(1/s - f/1000)

    W is work distance in mm
    s is the stop distance (average 27mm)
    F is the focal power

    Probably swiped from one of D. Meister's posts.

    Die young, as late as you can!
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  17. #17
    What's up? drk's Avatar
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    You can also measure a Distance PD and calculate the Near PD for any reading distance using:


    Near PD = [Distance PD mm * (10 * Reading Distance cm)] / [(10 * Reading Distance cm) + 27]


    where the Reading Distance is given in centimeters and the PDs are in millimeters.
    From my secret stash. Probably his words verbatim.
    http://www.optiboard.com/forums/show...D-relationship
    Last edited by drk; 06-24-2014 at 06:47 PM.

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