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Thread: Slab off and prism

  1. #1
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    Slab off and prism

    Happy Monday everyone. My knowledge on slab off is severely lacking, so hopefully some of you can help me out on this one...

    OD -2.50 -0.25 110 6 BU
    OS -1.75 -0.50 100 4.5 BD Add 2.25

    Doctor has recommended PAL w/ slab off. Is there any chance of this working? I know slab off typically goes on the most minus lens but that's about it. I don't believe the pt has worn a PAL before.

    He also ordered a digital PAL but both of the labs we normally use say they can only do slab on a conventional PAL. Those of you who have encountered this before, where do you get digital PALs with slab off?

  2. #2
    Master OptiBoarder optical24/7's Avatar
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    Unusual to need a slab off with only .75 power imbalance at 90.

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    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
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    10.5D of phoria/tropia is a big red flag for PALs. I would insist on STs w/o slab, unless you want to drive your remakes and COGs through the roof.

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    Master OptiBoarder optical24/7's Avatar
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    Slabs and PALs are flags too. Seldom successfull. Contact the prescriber too get their thoughts on the need for a slab. Make sure to adjust the fitting height 1/3rd mm toward the apex for each Diopter of prism to compensate for prismatic induced pupillary shift.

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    OptiBoard Apprentice JGor's Avatar
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    My 2cents worth? A slab off is generally used to minimise prismatic imbalance between lenses with significantly different powers thus causing said imbalance. Throw in prescribed prism then add a PAL design and I'm surprised your contact at your lab didn't dissolve into a whimpering mess at the mention of this beast.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    The patient is going to expect these glasses to perform well as they'll cost more than a washer and dryer which generally work for 10+ years. Chances are they have no clue as to the optical gymnastics their brain is going to need to perform to make them work.

    If successful it will be because Ryser's Rule* played into the equation.

    It's a big, big, if. Maybe if it was a +1.25 add I might, reluctantly give it a try but not at +2.25. You need to make very clear the monetary risks involved in a non-adapt unless your doc doesn't mind the taste of expensive high index lenses sauced with AR.

    *Ryser's rule- The success of a progressive is directly proportional to the desire of the wearer to wear progressives.

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    Thanks for the help everyone. Sounds like exploring other options will be the best route.

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    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
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    Quote Originally Posted by Manual View Post
    Thanks for the help everyone. Sounds like exploring other options will be the best route.
    We are here to help, even if we don’t always agree. Keep us posted, please. I’m sure that I am not the only one that’s interested in the final outcome.

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    Personally I would contact the doctor and ask their reasoning for requesting slab off.

    i have thought about using slab off and reverse slab off at the same time to cancel out the prism in a lens in the reading portion when prism is needed in distance only, so far I have done Franklin bifocals for these people. Which are rare to begin with. I would definitely call the doctor.

  10. #10
    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by Manual View Post
    Happy Monday everyone. My knowledge on slab off is severely lacking, so hopefully some of you can help me out on this one...

    OD -2.50 -0.25 110 6 BU
    OS -1.75 -0.50 100 4.5 BD Add 2.25

    Doctor has recommended PAL w/ slab off. Is there any chance of this working? I know slab off typically goes on the most minus lens but that's about it. I don't believe the pt has worn a PAL before.

    He also ordered a digital PAL but both of the labs we normally use say they can only do slab on a conventional PAL. Those of you who have encountered this before, where do you get digital PALs with slab off?
    A freeform generated lens can be slabbed, but the slab line is wide and diffuse.

    Is this client an adult? Justify the prescribers recommendation of a PAL instead of segmented bifocals, considering your understanding that the client has not worn PALs heretofore.

    The degree of prescribed prism in the above Rx, in my experience, is not compatible with freeform generated lenses. You might find a lab that will do it, but it will probably be unwearable. Instead, use a good quality semifinished, high Abbe PAL for the best chance of success.

    Quote Originally Posted by Tallboy View Post
    Personally I would contact the doctor and ask their reasoning for requesting slab off.

    i have thought about using slab off and reverse slab off at the same time to cancel out the prism in a lens in the reading portion when prism is needed in distance only, so far I have done Franklin bifocals for these people. Which are rare to begin with. I would definitely call the doctor.
    The the only reason I can think of that would require a slab is for an incomitant deviation, that is, a deviation that varies with the angle of gaze. If true, the prescriber needs to specify the prism needed at near as well as the primary gaze, with the optician charged with the task of finding a spectacle lens based solution.

    Best regards,

    Robert Martellaro
    Last edited by Robert Martellaro; 08-07-2018 at 06:04 PM.
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

  11. #11
    Master OptiBoarder optical24/7's Avatar
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    Quote Originally Posted by Robert Martellaro View Post


    The the only reason I can think of that would require a slab is for an incomitant deviation, that is, a deviation that varies with the angle of gaze. If true, the prescriber needs to specify the prism needed at near as well as the primary gaze, with the optician charged with the task of finding a spectacle lens based solution.

    Best regards,

    Robert Martellaro
    I thought about this too, but a near deviation of .75D? And for a dude with this much vertical to begin with? Plus, the prism directions (up OD). Is already the direction you use for a slab.

    Specialized, complex Rx’s are more expensive that 2nd opinions.

  12. #12
    What's up? drk's Avatar
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    The only potential purpose for slab off in this non-anisometropic Rx is if the vertical deviation changed in downgaze. All the above advice is good.

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