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Thread: pd compensation for prism

  1. #1
    OptiBoardaholic Randle Tibbs, ABOM's Avatar
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    pd compensation for prism

    I have a patient with 13dp of prism BI ou. he is wearing PALS (demanded them). does anyone have a maximum amount they would compensate the PD for this much prism?

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    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
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    Quote Originally Posted by Randle Tibbs, ABOM View Post
    I have a patient with 13dp of prism BI ou. he is wearing PALS (demanded them). does anyone have a maximum amount they would compensate the PD for this much prism?
    We would grind the required prism without compensating for PD.
    And yes, no FF, just a traditional molded pal.
    I bend light. That is what I do.

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    What's up? drk's Avatar
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    Hey, I don't know this. Why would we compensate p.d. for any prism, whatsoever?

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    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
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    Quote Originally Posted by drk View Post
    Hey, I don't know this. Why would we compensate p.d. for any prism, whatsoever?
    Common practice w/compensated FF wraps. Not so common with traditional toric grinds.
    I bend light. That is what I do.

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    What's up? drk's Avatar
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    Is the job in question a wrap frame?

    Edit: OK, now I get it. Never mind.
    Last edited by drk; 04-20-2022 at 02:13 PM.

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    Quote Originally Posted by drk View Post
    Hey, I don't know this. Why would we compensate p.d. for any prism, whatsoever?
    https://www.optiboard.com/forums/sho...sm+Progressive

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    13 Diopters of base in prism in each eye would mean compensating the PD 4mm out per eye. Depending on the lens power, the prism amount may also need to be compensated.

    *edit. I would also take a good long look at his current glasses and how he thinks they work for him. If he says they work well, forget what the book says and give him what he has always had. If he is wishy washy about how they work at all, make them up as they should be and tell them there will be an adjustment period.

  8. #8
    What's up? drk's Avatar
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    OK, let me summarize the grand unified theory of induced prism and how to modify placement of things on lenses.

    FACTORS
    1. Convergence occurs with a near object of regard, and all segments/PAL zones need inset to be near-portion convergence-induced prism-neutral so as to view the near object of regard through zero lateral prism (as well as maximizing field of view).

    2. If converging through "higher-power" lenses , additional (plus power) or reduced (minus power) inset for segment/PAL zones is need to compensate for distance-portion contribution to convergence-induced prism.

    3. If "significant" prescribed lateral prism is desired, segment/near zone horizontal locations need likewise compensated.

    These factors are additive.

    What's more, there can be vertical prism issues as well.

    1. On downgaze (which is where reading occurs most of the time), asymmetric downgaze-induced prism occurs with "significantly" dissimilar lens powers. In theory, if it were possible, offsetting graduated correcting prism should be ground in one lens in order to keep vertical disparity from occurring. (Currently, as far as I know, unilateral variable vertical prism can only be delivered by a rare free-form program, but it is possible to a degree.) This is the basis of slab off prism, but it is unfortunately a fixed--not variable--amount.

    2. When "significant" unilateral vertical prism is desired, the vertical location of any segment/PAL zone must be adjusted to maintain equal apparent location. If a BD prism is desired, that lens should have the segment raised a proportionate amount to compensate for the upward image displacement (which would also be a net result with distance portion power vertical prism at the location of the segment/PAL zone).

    3. When choosing segment near zone vertical location, in a "significantly" high-power minus-lens situation, the vertical median of the near zone should be placed higher in order to coincide with the apparently higher object of regard due to BD prism on downgaze, and vice-versa with a high-power plus lens situation. This is only practicable with differing segment types (D-segments for minus lenses, round segments for plus lenses) or variable corridor lengths (shorter corridors for minus lenses, longer corridors for plus lenses).



    IS THAT ALL?

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