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Thread: Variable near zone inset by monocular p.d. PAL

  1. #1
    What's up? drk's Avatar
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    Variable near zone inset by monocular p.d. PAL

    OK, there are two ideas out there about variable inset.

    Let's clear any confusion and set aside variable inset by distance lens power (i.e. less inset for minus lenses, more inset for plus lenses).

    The one I want to talk about is variable inset based on distance monocular p.d.

    For example: a patient with a 64 distance p.d. will have a near p.d. of about 61 @ 40cm. If the orbits are symmetric, then the split p.d. will be 32/32 for distance and 30.5/30.5 for near. That means that the ideal inset for the near zone would be 1.5 mm each eyeball.

    But look at this freak of nature: 64 distance p.d., but the monocular p.d.s are 34/30. The near p.d. will still be 61, so the midline would still be 30.5/30.5 monocularly. But while the left eye has to make a puny mere 0.5 mm convergence (virtually no inset required) the right eye will have to make a colossal 3.5 mm convergence (more inset required).

    Pair this "unequal split p.d." issue with a high power add with a naturally more narrow zone width ergo less margin for error, and you have potential for trouble. (In fact, I used to gripe about this phenomenon, but could never understand it.)


    So...how many individualized PALs will take into account this factor (and add to it the other variable inset parameter of distance lens power)??

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    Quote Originally Posted by drk View Post
    OK, there are two ideas out there about variable inset.

    Let's clear any confusion and set aside variable inset by distance lens power (i.e. less inset for minus lenses, more inset for plus lenses).

    The one I want to talk about is variable inset based on distance monocular p.d.

    For example: a patient with a 64 distance p.d. will have a near p.d. of about 61 @ 40cm. If the orbits are symmetric, then the split p.d. will be 32/32 for distance and 30.5/30.5 for near. That means that the ideal inset for the near zone would be 1.5 mm each eyeball.

    But look at this freak of nature: 64 distance p.d., but the monocular p.d.s are 34/30. The near p.d. will still be 61, so the midline would still be 30.5/30.5 monocularly. But while the left eye has to make a puny mere 0.5 mm convergence (virtually no inset required) the right eye will have to make a colossal 3.5 mm convergence (more inset required).

    Pair this "unequal split p.d." issue with a high power add with a naturally more narrow zone width ergo less margin for error, and you have potential for trouble. (In fact, I used to gripe about this phenomenon, but could never understand it.)


    So...how many individualized PALs will take into account this factor (and add to it the other variable inset parameter of distance lens power)??
    Feed me the mono DPD and NPD, and I can feed that into my LMS to change the inset for each eye. It can be done.
    I bend light. That is what I do.

  3. #3
    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by drk View Post
    OK, there are two ideas out there about variable inset.

    Let's clear any confusion and set aside variable inset by distance lens power (i.e. less inset for minus lenses, more inset for plus lenses).

    The one I want to talk about is variable inset based on distance monocular p.d.

    For example: a patient with a 64 distance p.d. will have a near p.d. of about 61 @ 40cm. If the orbits are symmetric, then the split p.d. will be 32/32 for distance and 30.5/30.5 for near. That means that the ideal inset for the near zone would be 1.5 mm each eyeball.

    But look at this freak of nature: 64 distance p.d., but the monocular p.d.s are 34/30. The near p.d. will still be 61, so the midline would still be 30.5/30.5 monocularly. But while the left eye has to make a puny mere 0.5 mm convergence (virtually no inset required) the right eye will have to make a colossal 3.5 mm convergence (more inset required).

    Pair this "unequal split p.d." issue with a high power add with a naturally more narrow zone width ergo less margin for error, and you have potential for trouble. (In fact, I used to gripe about this phenomenon, but could never understand it.)


    So...how many individualized PALs will take into account this factor (and add to it the other variable inset parameter of distance lens power)??
    Drk,

    If the mono distance pd is 34/30, then the near inset would be 1.5mm in per eye for 32.5/28.5, and would be true for even "dumb" PALs. Moreover, some archaic semifinished PALs would vary the inset by base curve, more for steeper BCs, and less for flatter BCs, assuming hyperopia and myopia respectively, and would do so within their lowbrow status. This has been true for many decades, well before freeform platforms and their associated software instructions became common place.

    Best regards,

    Robert Martellaro
    Last edited by Robert Martellaro; 08-15-2020 at 06:22 PM.
    Roberts Optical Ltd.
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    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    It is easier to fool a man, than to convince a man he has been fooled. -Mark Twain

  4. #4
    What's up? drk's Avatar
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    If the mono distance pd is 34/30, then the near inset would be 1.5mm in per eye for 32.5/28.5
    Hmm...need paper and pencil...

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    How can that be true, Robert?

    If someone has a distance pd of 70 , their near pd is 66.5, meaning each eye has to converge 1.75 mm.

    If someone has a distance pd of 55, their near pd is 52.25, meaning each eye has to converge 1.375 mm.

    Point being, larger pds have to converge more than smaller pds.

    Why would that not apply per eye?
    Last edited by drk; 08-17-2020 at 10:50 AM.

  5. #5
    One eye sees, the other feels. OptiBoard Gold Supporter
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    Drk,

    I see what you're getting at, that wider PDs converge more than narrower PDs, where a 75mm PD will converge 2.25mm @40cm/eye, and a 55mm PD will converge 1.65mm/eye, and can't be accounted for entirely with old school PALs (we could turn the blank before surfacing). I believe that many but not all freeform PALs will correct for this parameter (contact the manufacturer).

    However, the difference between the inset required for a 60mm PD and a 64mm PD @40cm, 1.8mm and 1.92mm respectively, for a total of 0.12mm, can probably be ignored.

    I wish there were more designs that could provide for a zero or custom inset, primarily for those with monocular vision, where an error of 2.0mm is truly consequential, and is more common in my office than 4.0mm differences in mono PDs.

    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

    It is easier to fool a man, than to convince a man he has been fooled. -Mark Twain

  6. #6
    What's up? drk's Avatar
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    Well, frankly when I did the little calculation I was surprised at the relative insignificance of the difference, even in an extreme case.

    So, maybe it's just fluff.

    Or, maybe the effect is more amplified at the spectacle plane, which seems intuitive but I can't think of how to hash it out. Sort of like looking through a keyhole at a closer distance or farther. Maybe the term "entrance pupil" is germane?

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    It seems as though a client might benefit from a custom inset if, for example, she has a 3.00 add and a 55mm PD. With a standard inset, she might find the reading area to be noticeably wider if the inset were 1.2 - 1.5mm as opposed to 2.5mm/lens.

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