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Thread: Slab-off (need formula) or Tips

  1. #1
    Bad address email on file
    Join Date
    May 2002
    Jacksonville FL

    Smilie Slab-off (need formula) or Tips

    Preparing to sit for Florida state boards in oct. need to find either formula or easiest way to figure slaboff. I hope this is the correct forum. I haven't posted that much to know. Any help would be great.

  2. #2
    Master OptiBoarder BobV's Avatar
    Join Date
    Apr 2000
    Blue Springs, MO USA
    Dispensing Optician

    try the ophthalmics forum, that's where you'll get an answer. Your post may be routed to that forum.

    Bob V.

  3. #3
    Bad address email on file Michael Disanto's Avatar
    Join Date
    Feb 2003
    Hudson / Ohio
    Other Optical Manufacturer or Vendor

    Slab Off

    The formula you use to determine slab off is the prentice rule.

    Step 1 --- Identify an Rx with different amounts of power in the 90 degree meridian
    Step 2 ---- Right Lens determine the power at 90 ( If a reading depth is not specified use 10 MM)
    Apply Prentice Rule --- Power at 90 x Reading Depth Divided by 10
    Step 3 --- This answer is the prism encountered at the reading depth. If the power at 90 is minus
    the prism is base down. If the power at 90 is plus the prism is base up.

    Repeat steps 1-3 for the left eye

    At this point you have 2 prism values (1 for each eye) If they are up and down they compound so add
    them together. If they are up and up or down and down they cancel ( subtract the smaller from the
    larger ) the answer to these scenarios is the total imbalance.

    Rule of thumb 1.5 diopters of prism or more -- Slab off /// less do not -- And a few other things.

    If you or anyone else wants this info in more depth drop me an e mail at

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