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Thread: reverse slab off

  1. #1
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    reverse slab off

    I got a job in today from a lab I am forced to use by insurance company.
    rx plano-2.25x91
    +4.50-2.25x95
    +2.25 add ou

    ordered with a slab off.
    what I got has a reverse slab-its on the left lens but it does not have what I think should be the correct amount of prism. In that a reverse slab is pre molded I would think you can't screw it up, maybe I am not reading it right, if I read it just under the seg line I can almost convince myself its 2 base down. Am I mis remembering how to figure this out? or did they use a blank with too little prism. shouldn't it be closer to 4 base down?
    any and all help would be appreciated.
    thanks

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    Master OptiBoarder optical24/7's Avatar
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    How much prism are you seeing in the reading area of the R & L lenses?

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Forget about the distance --- Measuring 5mm below the seg line how much prism do you read when you switch between the right and left lens?

    You may want to put the convex side of the seg against the aperture, again, don't worry about axis or power. Just any prism difference. It should be minimal.

    Is a welcome back in order?

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    none in the right barely 2 down in the left

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    it should be 4.50...you should read the left lens first
    Last edited by CME4SPECS; 03-05-2013 at 03:22 PM.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    The difference is excessive.

    When you measure at the distance oc between right and left are you reading any vertical prism?

    Is it a FT?

    How far below the distance oc is the seg line? Same seg height ou?

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    Master OptiBoarder optical24/7's Avatar
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    You are correct, they used the wrong reverse slab blank.

    Quote Originally Posted by CME4SPECS View Post
    it should be 4.50
    4.5 if you are using a standard 10 mm difference between Dist OC and near OC. More slab if the OC's are further apart, less if they are closer.

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    true, 10mm is std. and quite simple

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    you can tell what blank they used by measuring the dist bc, then place the center pin of lens clock on the slab line, the other pins one goes in the dist the other in lower portion of lens (not on the seg) the diff. in the 2 readings is the amount of prism in the blank

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    none in the right barely 2 down in the left

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    seems like someone taught me that a couple of decades ago, but was long forgotten.
    dist bc is 7.75 turning clock and reading with middle pin on the slab line, is 4.75 so that would be 3 diopters of prism.
    I still think it sould be more. unless I have forgotten everything I learned.
    the cyl is not effective in the vertical meridian- so is it not +4.50 imbalance there about?

  12. #12
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    as 24/7 noted, using the std 10mm... yes...less if oc's are closer

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    To verify scroll down to Opticampus on the home page and click "Optical calculators" then "vertical imbalance".

    Plug in the rx for an exact amount.

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    Quote Originally Posted by Uncle Fester View Post
    To verify scroll down to Opticampus on the home page and click "Optical calculators" then "vertical imbalance".

    Plug in the rx for an exact amount.
    thanks, had no idea that was there, how handy- I learn something new everyday.

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    Prism should be the same (or very close)about 5 mm below the seg line. That's the easiest way to look at it. After all, that's the point.

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    We can also use the mirror method to determine the reading depth. (site:optiboard.com "mirror method") or

    http://www.optiboard.com/forums/show...g-progressives

    Draw horizontal reference lines on the lenses along with or instead of vertical lines. Beats getting down on the floor, and is more accurate than looking from behind the client.

    Don't ignore VI on the distance gaze.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    Quote Originally Posted by ih8wlmrt View Post
    none in the right barely 2 down in the left
    If you are checking distance vert prism, you need to align and check the left first. You should still see none in the right. Then check the near as instructed above.

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