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Thread: Prism in an exec trifocal...

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Idea Prism in an exec trifocal...

    My co worker took in the following Rx and it's come back with the vertical prism negated in the reading area. Is there a way to maintain it?

    R -2.00-1.00x153
    L -4.50-0.50x5 2^Up
    Add +1.75

    Is it as simple as 2^up OS slab-off? Can't remember the last time I tried to get slab off on an exec. It is possible isn't it?:hammer:

  2. #2
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    use the total power, differance of a 2.50 bd in the distance power. Prism will read about nil in the the seg. Actually about a half down. I'm not much of a formula guy, just off the top of my head.

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    Opps, misread the post. Are you trying to get 2D up in the near? Prism in distance may in part be due to the imbalance in the rx. It negates imbalance at near point.

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    Doh! braheem24's Avatar
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    Quote Originally Posted by Uncle Fester View Post
    Is it as simple as 2^up OS slab-off? Can't remember the last time I tried to get slab off on an exec. It is possible isn't it?:hammer:
    You're right they are made, an EXEC slab is probably the ony good looking slab too :)


    Good luck, you dont need us for this one:cheers:

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Dawn has risen over Marblehead...

    I was kind of hoping someone might show me with Prentice's Rule the formula how the prism was negated 15mm below the OC.

    Well the lazy guy that I am brought out the book and saw the Rule written another way than I learned it Prism=cF.

    I rounded off the vertical power difference to 2.38 and viola. Negated prism!

    Or did I just stumble onto coincidence? :hammer:

    It's funny because in school our Ophthalmic Optics courses had us determining powers using thickness and curves and the ever popular Gauss Equations to 5 decimal points. Our teacher would look over our shoulders sometimes and after you traced the light ray through 5 or 6 substances point to your second focal length and say "Too bad!" and you knew that was wrong so it was back to those formulas again. Every now and then I actually got one right but God help me if I had to do one now:)
    Last edited by Uncle Fester; 12-17-2007 at 12:20 PM. Reason: Keeping the Chipster happy...

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    Did the patient have problems?
    Last edited by gemstone; 12-31-2007 at 09:25 AM.

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    Capitalize God!

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    OptiBoard Professional jrumbaug's Avatar
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    Fester

    I would of calculated the need for slab like this

    R -2.00-1.00x153 ...........vert power = -3.00 or a hair less
    L -4.50-0.50x5 2^Up ......vert power = -5.00 or a hair less

    about 2 diop vert difference

    I calc near oc at 10mm below dist oc ,therefore, 2 x 10 x .1 = 2

    therefore left would have about 2.0 more base down than the right and that 2 down would cancel out the 2.0 base up.

    So..... yes, a slab would put the vert prism back in the near part.
    but as Gemstone asked... did the patient have problems?? If not, don't worry. Sometimes the prism to correct the imbalance in a straight ahead gaze is different than the imbalance in a lower gaze.

    Jim Rumbaugh

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