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Thread: Regarding to ABO exam practice test question

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    Confused Regarding to ABO exam practice test question

    Hello, I am very new to Optiborad. I am taking the ABO exam on this November and I studying with ABO study guide by Gary McArrel, but there was a difficult question that I really don'y understand. The question was " The lab used prism thinning to improve the cosmetics of a pair of progressive addition lenses. The lab ticket calls for OD 1 ^ base up and OS 1 ^ BD. When you are verifying the finished glasses at the prism reference point which of the following prism combination would be? and the answer from this book is OD 1 ^ BD / OS 3^ BD.

    I do not know how he got 3 base down on OS. This question is driving me crazy!!!

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    Depending on the other answers, it sounds right. The Rx called for 2D vertical displacement, down OS. The answer shows 2D base down OS (Imbalance). He's assuming a +3.00 add which would have 2/3rds of the add power (2D prism) ground base down OU for prism thinning.

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    Prism thinning is used for plus distance powered progressives by grinding base down prism calculated by addition power (.66 x add power). Since addition power is not giving the next clue to the answer is the difference in prism through the distance. So OD 1 up and OS 1dn gives you 2 diopters opposing prism through the distance. Now move down to the PRP or prism reference point. Now the OD 1up combined with 2 down (for thinning) yields 1 bd @ PRP and OS 1dn combined with 2 bd yields 3 BD.

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    Oh, my Gosh! Thank you so much for your help! Your explanation is very neat and easy to understand.

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    Thank you!!!

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