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Thread: Intermediate pd

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    Intermediate pd

    Hello, I am in a fog right now...Iím taking a practice test for my license and itís asking for the intermediate pd measurement....I know the reading is 1 1/2 off of monocular pd and 3 off binocular but what is the rule for intermediate? Iíd appreciate any answers!

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    Quote Originally Posted by Bcandy1029 View Post
    Hello, I am in a fog right now...Iím taking a practice test for my license and itís asking for the intermediate pd measurement....I know the reading is 1 1/2 off of monocular pd and 3 off binocular but what is the rule for intermediate? Iíd appreciate any answers!

    Best thing is use a PD meter, for near and intermediate, the rule of 3 mm off is not always accurate.

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    What's up? drk's Avatar
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    As a practical matter, I never worry about it. If you think about it, people wearing SV glasses never look through a near p.d., ever!

    We only use near p.d. on SVNO because we can. They converge optics on PALs since they're so skinny. We inset segments because we don't like differential prism AND differential power.

    What's more, if you provide it to a lab (instead of the distance p.d.) on a task lens it will goof them up.



    There is a complex formula Darryl once posted, to get the actual number. You won't like it. But for spitballin' how about half the difference between D and N p.d.? But what defines "intermediate" anyway? Is it really "intermediate between infinity and 40 cm?....?

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    I have no idea what the official rule is- but as a general practice, we do a 3mm difference on PDs between 58-70 while adjusting 2mm for anything under and 4mm for any over. That is adjusting for near; I would still cut that for intermediate by half, rounding up.
    Have I told you today how much I hate poly?

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by drk View Post
    As a practical matter, I never worry about it. If you think about it, people wearing SV glasses never look through a near p.d., ever!

    We only use near p.d. on SVNO because we can. They converge optics on PALs since they're so skinny. We inset segments because we don't like differential prism AND differential power.

    What's more, if you provide it to a lab (instead of the distance p.d.) on a task lens it will goof them up.

    There is a complex formula Darryl once posted, to get the actual number. You won't like it. But for spitballin' how about half the difference between D and N p.d.? But what defines "intermediate" anyway? Is it really "intermediate between infinity and 40 cm?....?
    Us spectacle wearing myopes are accustomed to BI prism when we perform near tasks- one might not want to use the near IPD when designing task lenses. These same folks might have unexpected discomfort when performing sustained near tasks after cataract surgery, depending on the final Rx of course. I usually use the distance IPD for myopes and full decentration (or more for segmented multifocals) for hyperopes.

    Trifocals are usually half of the add, although most opticians today won't use this style of multifocal unless they work with low vision or have an elderly clientele.

    Here's the formula for calculating the near IPD.

    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

    Quick reference for shorter work distances

    If stop distance equals 27mm, the near multiplier for a work distance of
    40cm is .937
    35cm is .928
    30cm is .925
    25cm is .903
    20cm is .881

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    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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    Robert,

    You need to add a set of parenthesis to make this look right. NPD = DPD - DPD/(1 + W(1/s - f/1000))

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    Quote Originally Posted by Kwill212 View Post
    Robert,

    You need to add a set of parenthesis to make this look right. NPD = DPD - DPD/(1 + W(1/s - f/1000))
    Sorry guys, I do not understand: DPD-DPD is not 0?

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    That's why the extra parenthesis are needed. It's hard to write nice looking equations on optiboard.

    The second DPD is the numerator and the (1+w.....) is the denominator. This is the inset. Subtract that from the first DPD to get NPD.
    Attached Thumbnails Attached Thumbnails chart.png  
    Last edited by Kwill212; 03-09-2021 at 06:28 PM. Reason: added equation

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by Kwill212 View Post
    It's hard to write nice looking equations on optiboard.
    No kidding. Not to mention my math and notation has been sloppy since the eighties.

    Here it is with values entered to make it easier to follow (and grin and bear my notation).

    Mono PD 33mm and +2.25 add.

    NPD = 33 - 33 / 1 + 400 x (1 / 27 - 2.25 /1000)


    NPD = 33 - 33 / 1 + 400 x (.037 - .002)


    NPD = 33 - 33 / 1 + 400 x .035


    NPD = 33 - 33 / 1 + 14


    NPD = 33 - 2.2


    NPD = 30.8

    And thanks for the hand written formula (you beat me to it), I think from one of the System for Ophthalmic Dispensing editions.

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    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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    I used a TeX editor to make the equation that can than be saved to an image file. http://atomurl.net/math/



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    Crystal clear now thank to all

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