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Thread: A Slab off question

  1. #1
    OptiBoard Professional Robert Wagner's Avatar
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    Angry A Slab off question

    Here is the question:

    Rx

    O.D. -2.25 -1.50 x90
    O.S. +0.25 -1.50 x90

    Add +2.00

    Natural PAL

    Slab off O.D.

    As a result of the aniso, when the presciption is ordered with no prism, there is a small amount of induced vertical prism at the fitting cross. The induced prism measures 1^ base up O.D. as the distance from prism dot to fitting cross in 4 mm. In an effort to eliminate this vertical imbalance at the fitting cross that the patient doesn't need, we're going to order the prescription with 1^ base up O.S. We chose this route instead of ordering 1^ base down O.D. to keep from increasing the edge thickness O.D. And.. a little yoked vertical never hurt anyone. :)

    The problem is with the slab-off and our understanding of standard lab protocol. The goal is to eliminate all binocular imbalance of vertical prism at near. In order to do so, the lab should leave 1^ base up O.D. under the slab. If all prism is eliminated under the slab, the patient will be left with an unwanted 1^ base up O.S. What do labs usually do...eliminate all vertical prism or maintain any vertical ordered in the distance?

    Also, can someone tell us (with a PAL) if we can order the slab at any height we wish? Thanks in advance for any help/advice!

    Robert Wagner (chief optician and fireworks master)
    Beaverton, OR

  2. #2
    Master OptiBoarder Texas Ranger's Avatar
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    Robert, first , has the patient worn a slab off before? I'd usually try do ing this job, without the slab-off, and flipping the normal base curves, and do a normal amount of "prism thinning" on the O.S., likely about 0.75 base down, with no prism in the O.D., I'd advise the pt. that there is the slight possibility that they "may" experience some diplopia at near and that it may be necessary to do a slab-off, but it's "borderline" on this rx, and it's likely to save them a considerable amount of money to try it without it at first. this is a borderline slab-off problem, and you can always redo it later, and just charge the pt the extra slab-off charge. but tell them how much, up front. very rarely on this rx would I have someone back for the slab-off; if the dr. rx'd it that way, call him/her up and discuss it with them....

  3. #3
    One eye sees, the other feels OptiBoard Silver Supporter
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    Robert

    If you do not want VI at some specified point at near just subtract one prism diopter from the slab calculation. If you instruct the lab to use, let's say, a 3^ slab in the rt eye that's what you will get regardless of the distance prism. The lab won't know that the prism wasn't prescribed unless you tell them. Just to keep confusion to a minimum don't tell them what your up to with the distance prism and they will assume it has been Rx'd. The slab can be placed anywhere you wish.

    Robert

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    Master OptiBoarder Jeff Trail's Avatar
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    I think you guys might be a little "confused" on how we actually grind a slab, cutting the degree of amount of prism in the slab has ZERO effect on the distant portion of the lens.. when we cut a slab we actually cut both area's above and below the slab line as we would TWO separate lens.. that is why you end up with that distinct "line"... you block the lens and cut the correct curves to give you your sph/cyl. than you rerun that same lens with a degree of neutralizing prism amount to get the "slab"... now if you want to really make it easy on every one including yourself and the lab just request that the lens be blocked on center 3 mm ABOVE the fitting cross.. this will move the induced imbalance further away from your distant target area and still make sure that the imbalance is not increased as you travel downwards as well (at least not as much as increase as when you start tinkering by adding prism in the one eye in the distant portion, which I would think is a very big mistake.. because you have diametrically opposing effects due to the power, in a minus the OC goes the opposite way you want it to travel and with the movement in a plus power..
    If you wanted to really take advantage of design and optics I would request a true aspherical design and NOT a semi-aspherical. Than have them block it instead of on the 180 say two or three mm above ..moving the 180 (fitting cross) down below the layout 0-180 ..;) BTW Robert is correct on having any choice you may want to fit the slab location.. if you have never fit a PAL slab before I would recommend using Fresnel to double check before ordering.. save you time and money and the lab rats from screaming at you and using your face on the "dart board" as a bullseye :) .. another thing to always consider when fitting the slab is that we tend to overlook that a PAL is technically a trifocal type design and the majority of new and existing presbyops do NOT need an intermediate power per se' (yet) and so this does come into play when locating the slab location and getting them to adapt to the slab...to high and the tend to go crazy and blame you for it not working correctly.

    Think about it for a while it that little light bulb will go off above your head:cheers:

    Jeff "maybe I have ground one to many slabs in my time" Trail

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    Wink

    One other problem - you are trying to read prism at the fitting cross instead of the prism reference (2 to 6 mm below the cross, depending on the type of progessive). With this much disparity in powers it is impossible to eliminate "all" of the prism. The patients main problem will be down in the seg since the prism increases according to the prentice rule. If you specify zero prism at the fitting cross, the lab should be able to calculate and cut it
    accordingly (even though this is not recommended)

    shutterbug

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    Confused Dazed and Confused

    Guys,

    I'm a little lost here. Slabs are ground to neutralize vertical imbalance at near due to aniso or antiso at a distance.

    Grinding a slab for this lens will do nothing for vertical imbalance at the fitting cross. Imbalance here is more likely due to a blocking mistake or uncalibrated cylinder machines. Prism should be equal R to L at the PRP and may be very slightly different at the cross.

    When looking straight thought the optical centers of a pair of lenses (or distance region for a PAL), the patient should experience no vertical imbalance even if the powers between right and left differ considerably. However, as the eyes track down the lens to the near, effective prism is always naturally induced as the patient is no longer looking through the OCs (hey, that is the very definition of prism, isn't it?!). If the R to L Rxs at 90 are different, a different amount of prism is induced for each eye, resulting in double vision at near.

    If the total powers of the lenses are close at 90, the effect will be minimal. If they differ, a dramatic amount of prism imbalance can occur. This can be very simply proven using a derivative of Prentice's Rule. Assuming 10 mm difference between the visual axis at far and near, and 4 diopters of difference between right and left Rxs at 90, the wearer would experience about 4^ imbalance from right to left, where ^ = 10 * 4 / 10.

    Note that a patient can be a candidate for a slab-off with as little as 1.5^ vertical imbalance. Values over 6^ generally indicate a slab will not work and some other solution should be explored.

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    OOOOOOOOOOOOPS!

    This was actually supposed to be a reply to "A Slab Off Question." Perhaps I should pay more attention to which buttons I'm pushing. ;)

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    Forever Liz's Dad Steve Machol's Avatar
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    I merged it for you. :)


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    Master OptiBoarder Jeff Trail's Avatar
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    I don't think it was a cylinder machine boo-boo or blocking as much as the "where" they were checking the lens for verification, it was originally posted as vertical imbalance at the fitting cross.. of course with that power I'll bet you if it was checked are the MRP you would not have had any imbalance in the distant :).. taking into consideration lens design (semi-aspherical natural) and the mix of power (even though the total power OU was myopic in nature) you would see a slight spread of power position as you got away from the MRP, but than again it would be such in any S-A design checking up by the fitting cross..
    I do think the mixing "slab" and thinking it had an effect on the location of the distant OC was a slight "brain freeze" some of the posters parts..:) ..happens to the best of us at times..
    Sometimes it does get confusing I guess if you have not tinkered in a lab (official "lab rat" badge) and not think of how bicentric grinding is actually done and what you are doing to the surface of the lens and it might be easy to think, "well if I'm pushing prism here...why it should be over the whole lens surface" ...I have seen a LOT worse, had an OD ask me one time to grind a "slab" on the 90 degree merridian.. I think he was trying to be a little "over analytical" myself (nice way of saying he had no clue what he was asking for)... I asked him "why don't you just ask for prism" of course by than he would never admitt he made a mistake... so I did just what he asked... of course it amounted to just base out prism :)
    Is it me or is optics getting more loony every day

    Jeff "wish I had a penny for every crazy request" Trail

  10. #10
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    Jeff Trail said: I think you guys might be a little "confused" on how we actually grind a slab, cutting the degree of amount of prism in the slab has ZERO effect on the distant portion of the lens..
    Jeff,

    I believe the concern was how the lab will handle the slab calculation if there is non-prescibed prism in the distance i.e. prism added by the optician to neutralize the prism induced on the distance gaze. If the lab treats this like prescribed prism the slab prism will be incorrect. To eliminate the chance of a misunderstanding between parties I suggested that the optician should do the slab calc themselves so that the distance prism can be subtracted from the slab prism calculation. For example if the reading depth is 15mm the slab would be 3.75 prism diopters base up OD. With 1^ UP aded to the OS (non-prescribed) the slab needs to be reduced to 2.75 if there is to be no VI at the 15mm reading depth.

    Robert

  11. #11
    Master OptiBoarder Jeff Trail's Avatar
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    Robert,


    If the lab would screw this up they need to change labs :) .. es we can use prism thinning even on a slab job... so that the imbalance would not be that great or to the pont where you apply prism. (distant) Another thing to consider is the actual imbalance and the person and what they see naturally.. when it is distance we are tinker with and a naturally induced imbalance I'm still amazed at times what people can tolerate.. that 1^ at the cross is not really all that much and really it is not a ^ of of prism per se' as as much a side effect of design vs. power.
    Now if we start tinkering with prism the optician better keep in mind a lot of things, such as the use of prism thinning, even if we (the lab) are slabbing.. the power and what way the OC travels (with or against) and how moving the OC would effect it, the amount of slab and the slab location...
    If it were me, I would tend to let the lab do it, especially if you trusted your lab..which you should. :)
    Oh and if you did add the 1^ up as you mentioned.. it would NOT reduce any of the slab prism, what we do in the distant portion of the lens has nothing to do with the slab... you base the slab on the amount of power and reading location.. So no matter what we still would need the same degree of slab. So if you did write this up and you tinkered with the distant OC (which is what you are technically doing) and cut teh degree of slab you would have induced a trouble spot in the near.. with a distant gaze you can tolerate more "little" problems than in the near point with total powers and imbalance.
    The only thing that would change the amount of slab prism would be moving the "slab" line in reference to the hght. .. in your calculations.
    I still think you are trying to mix apples and oranges.. looks good on paper but won't work.. oh you could order it.. it could be done as ordered and could it be tolerated? .. probably :)

    Have a great 4th!!!....

    Jeff "isn't tinkering fun?" Trail

  12. #12
    OptiBoard Professional Robert Wagner's Avatar
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    Thumbs up The "Slab"

    Thank you all for your input,

    This patient went to brand x to get her glasses and has been having trouble ever since. Brand x has made her glasses 3 times with and without slab, without slab of course double vision in the near, with slab and putting the line where we would like it, the patient then has trouble with her distance vision, worse at night. So brand x is having "fun" with this job!;)

    I have talked to the lab manager many times about this job with all of your ideas and I haven't heard from the patient in about 2 weeks. Again thank you all for your help.

    In many cases like this "no news is often good news!"

    Thank you all,
    Robert:D

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    Master OptiBoarder Jeff Trail's Avatar
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    Robert,

    Just out of curosity this person didn't happen to get an IOL did they :)

    Jeff "doubling in low light..heard this one before" Trail

  14. #14
    OptiBoard Professional Robert Wagner's Avatar
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    Prism

    Hi Jeff,

    As per your question, no this patient has not had an IOL and I still have not heard anything from the patient, I will give her a call but this will most likely open another can of worms, but I have a need to know the outcome.

    Robert

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