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Thread: Slab-off (need formula) or Tips

  1. #1
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    Wink Slab-off (need formula) or Tips

    Looking for formula or easiest way to figure slaboff. Preparing to sit for FL state boards next month. Any help would be great I'm a little rusty with slaboff.
    Thanks:hammer:

  2. #2
    Master OptiBoarder Jeff Trail's Avatar
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    The first step is to find the power in the 90 degree meridian for each lens....you know the formula for this I take it? .. well once you found the power than you multiply by the reading level.. most often everyone uses 10 mm as a refrence point since it makes the math so much easier :) .. but you can fill in any number into the equation...this works as long as you have the SAME add power..if not it gets a little more complicated..

    using the formula above and here is an example..

    the RX is OD -1.00 -2.00x90 add 2.50 OU
    OS+1.00 -1.00x60

    10mm drop

    OD P= 10x1.00/10 OS P=10x0.75/10
    P=1^ bd p=0.8^bu

    so now you just add them together, in the OD you would have 1.8^ bd or in the OS it would be 1.8^ bu .. so if you were slabbing you would order a 1.75^ slab for the OD, if you wanted a "reverse slab" than it would be 1.75^ OS

    the rule of thumb to help you track the slab is

    LENS SLAB REVERSE
    two - highest minus lowest minus
    two + lowest plus highest plus
    one +/one - minus plus

    You might also want to memorize the seg oc in the most common lens to help you know what number to plug into the equation for seg location oc from distant oc.. in case they want to be tricky :) .. they might say something like give you the distant location but only give you a seg type.. so you would have to know the seg drop oc from design to design to fill in the blank.. but I don't remeber it ever being that complicated on the State boards.. They (as best I can remember were all fairly simple, I think the majority of them I did in my head, didn't even have to write it down to figure it out..

    Hope this made some sense to you, feel free to aask away or if anyone seen a mistake in my post or knows a more simple way feel free to jump on the bandwagon.. BTW this thread should have been posted in the ophthalmics section.. its where we technical people love to tinker :) .. maybe a moderator will move it over there..

    Jeff "grind em if ya got'em"Trail

  3. #3
    Forever Liz's Dad Steve Machol's Avatar
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    Here's a link to an Excel spreadsheet written by JRS for figuring slabs:

    http://www.optiboard.com/pafiledb3/p...ion=file&id=14


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  4. #4
    Master OptiBoarder Jeff Trail's Avatar
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    Steve Machol said:
    Here's a link to an Excel spreadsheet written by JRS for figuring slabs:

    Steve,

    Gee I WISH they would let you use a program to take into the testing but...:) .. on the State board we have to figure out how to slab and they give you the questions and you have to figure the slab and amount and which eye to slab, now if they let us take a laptop to the test..that might be hard to sneak in...it might come in handy to give yourself some questions to practice with..enter in an RX and you get the answer and try to do a reverse formula, what numbers plugged in where to come up with that answer...
    One thing for those guys taking the test, it is not as hard as it might look, if you remember the formula for which eye to slab..I listed it in my post, you can remove a couple of the choices from the list of possible answer's..if you know which eye needs slabbed and can figure the formula fairly quickly in your head (power in the 90) and the imbalance than the choices get eliminated pretty quickly.. of course this will get you through this part of the exam.. BUT you should know how to figure slabs :) ..in the real world the person getting the glasses does NOT come in with a sheet of multiple choices..

    Jeff "educated guessing might work for State boards, but the real world?" Trail

  5. #5
    Forever Liz's Dad Steve Machol's Avatar
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    Jeff,

    The point of viewing the Excel spreadsheet is that the formulas are there. ;)


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    RETIRED JRS's Avatar
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    That is why it is an un-protected worksheet w/o any hidden fields. People can copy off the portion of interest.
    J. R. Smith


  7. #7
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    One note - for slab-off calculations you need base UP prism on the weakest plus (or strongest minus) lens. If ordering a reverse slab it will be base DOWN on the opposite eye. This was not made clear in the other post.

    shutterbug

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