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  1. #1
    OptiBoard Apprentice
    Join Date
    Nov 2019
    North America
    Dispensing Optician

    MBS Formula issue


    I use the minimum blank size formula every day, I add 2 at the end of the formula for margin of error but when tracing the demo lens sometimes the lens won’t cut out and the pd would be off by 3 or 4.

    Is this formula an exact science because it doesn’t seem to work consistently especially if the numbers are off by more than 4 or 5mm.

  2. #2
    Rising Star
    Join Date
    Jul 2016
    Optical Wholesale Lab (other positions)
    if your input numbers are being taken with a mm scale you will be wrong as often as you are correct. To get the E.D. Of a frame you must box the shape or get from the frame specs or get from your tracer.

  3. #3
    Join Date
    Feb 2010
    Optical Wholesale Lab (other positions)
    As you know MBS calculations require decentration + ED + 2. If decentration is not calculated correctly then MBS will be off.

    Boxing method requires method along Datum Line aka longest point of measure for the frame. Check your actual measure vs frame markings. Usually there should only be a marginal difference.

    Are you having lenses cribbed to exact MBS?

    Here is a reference

    Click image for larger version. 

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