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Thread: Calculating minimum lens blank size

  1. #26
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    can you just tell me what OS and OD stand for ? and any chance u can just explain the calculations and steps so i can do it myself in the future

  2. #27
    What's up? drk's Avatar
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    Dude, you cannot be an optometry student.

  3. #28
    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    Quote Originally Posted by AM View Post
    can you just tell me what OS and OD stand for ? and any chance u can just explain the calculations and steps so i can do it myself in the future
    No, I think that's why you're in school.

    Unless, of course, you're willing to forward your tuition to me.
    Last edited by Judy Canty; 04-30-2010 at 01:57 PM.

  4. #29
    Independent Owner kcount's Avatar
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    if memory serves OD means Optometric Doctor and OS means Optician-Sinner
    • Optician
    • Frame Maker/Designer
    • Teacher of the art of crafting handmade eyewear.

  5. #30
    Master OptiBoarder Darryl Meister's Avatar
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    You cannot calculate the minimum blank size "very accurately" with the data provided. Chances are, they want you to calculate the minimum blank size using the traditional rule-of-thumb method in conjunction with a calculation for the decentration required to induce the specified prism (3.0 Base Out and 2.0 Base Up) with a finished stock lens.

    Begin by calculating the normal horizontal decentration of the lens required to align the prism reference point (or optical center, in the absence of prism) with the pupil center (PD). As Judy indicated, it is 2 mm in for the right eye. You can find this by subtracting the monocular PD from one-half the BCD/DBC measurement, or 66 / 2 - 31 = 2 mm in.

    Then you must calculate the additional decentration from this point required to induce the specified prism. They have simplified the problem considerably by keeping the cylinder axis at 180. The power of this lens through the horizontal meridian is +2.50 D and the power through the vertical meridian is +2.50 + 2.50 = +5.00 D. The decentration required to induce 3.0 pism diopters Base Out is given by:





    Note that pllus powers require decentration in the direction of the prism base, so this represents 12 mm of decentration out.

    Now, since this decentration is out, the original decentration of 2 mm in, should be subtracted from this value, since the prism decentration is in the opposite direction. The new horizontal decentration is 12 mm Out - 2 mm In = 10 mm out. The decentration required to induce 2.0 pism diopters Base Up is given by:





    Again, the decentration is up for Base Up prism. Finally, we need to determine the total horizontal and vertical decentration using:





    The rule of thumb for calculating minimum blank size (MBS) is:



    The rule of thumb for calculating minimum blank size typically relies on the effective diameter (ED) of the frame. Since one was not provided in this particular case, you should just use the eyesize or A measurement of 46 mm, although this would result in an error in practice:





    The smallest possible minimum blank size is therefore 67.5 mm. Again, in reality, the minimum blank size would be larger than this value. In fact, the minimum blank size could reach up to 81.6 mm in this case, for a perfectly rectangular frame (not that these are made). Typically, a 1 mm edging allowance is also added to the final result.

    I have attached some diagrams to illustrate these calculations.
    Attached Thumbnails Attached Thumbnails mbs_prism.pdf  
    Darryl J. Meister, ABOM

  6. #31
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    My head was spinning for a while there, but you put things right back into perspective, Darryl (pardon the pun!) My students would have been amazed!
    BTW, I always thought that OS stood for Optician Sinister! Imagine my embarrassment!

  7. #32
    Master OptiBoarder Darryl Meister's Avatar
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    My head was spinning for a while there, but you put things right back into perspective, Darryl (pardon the pun!) My students would have been amazed!
    I'm glad problems like this appear on OptiBoard occasionally; it keeps me from forgetting some of this stuff after 20 years.
    Darryl J. Meister, ABOM

  8. #33
    OptiBoard Apprentice Lab King's Avatar
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    Wink very simple formula

    PD-NB/2
    ANSWER-FRAME SIZE X 2
    ADD 1 MM FITTING ALLOWANCE

    Example
    frame size = 52
    nose bridge = 19
    PD = 62
    then
    62-19/2
    ans - frame size x 2

    21.5-frame size x 2
    30.5 x 2 = 61

    add 1mm Fitting allowance
    61+1 = 62mm

  9. #34
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  10. #35
    OptiBoard Professional OptiBoard Silver Supporter elaneo's Avatar
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    Quote Originally Posted by kcount View Post
    if memory serves OD means Optometric Doctor and OS means Optician-Sinner
    This site is hilarious.

  11. #36
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    Question min diameter

    the formula i have used for many years in measuring for single vision cout out is this

    eyesize of frame is 56mm you would then add the dbl which is 19mm this gives you frame pd of 75mm then you take away the patients pd say 58mm that will then give you 17mm you add that to the eyesize of 56mm this gives you 73mm then i add a 4mm buffer due to coating ring around the lens edge whether it be hard coated or ar coated this also includes the normal 1mm buffer for fitting into frame.giving you an effective diameter of 77mm therefore you would need to grind,unless it was such a low power you could move pd`s to within tolerance something i am not at all keen on.

    56+19+75
    75-58=17
    17+56=73
    73+4=77mm

  12. #37
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    Very helpful information.
    Thank you

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