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Thread: Lens Calculation?

  1. #1
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    Confused Lens Calculation?

    Hallo

    I am using "Sagital 2009" to calculate CT and ET of a progressive lens.
    This program is very helpfull but i would like to calculate the Ct and ET if i have prism in my lens how can i do it?

    thanx

  2. #2
    Doh! braheem24's Avatar
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    Salam or Shalom,

    I dont see where you can on the screenshots, You can however input prism as a form of decentration or OC placement.

    +3.00 OU 3BI 60PD

    would be a

    +3.00 with a 50PD

    Good luck, :cheers:

    Ibrahim

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    thanx braheem

    but can u tell me how do u know that 3prism in with 60 PD is the same like no prism with 50 pd?

    is there a calculation? is this the same like decentration?

    can u tell me the calc if i have prism in or prism out or decentration?

    thank u very much

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    Prism and de-centration are exactly the same. It's just that in many cases the blank isn't thick or big enough to achieve the desired prism by decentration, hense it must be ground in. Or in other circumstances where a bifocal or other feature is involved once obviously cannot de-center the segment to achive much prism, so it must be ground in.

    Actually when view in a certian way all lenses are nothing but pie shaped prism pressed together and curved for convienence or cosmetics.

    Chip

  5. #5
    ATO Member HarryChiling's Avatar
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    Decentration = Prism * 10 / Power

    This gives you the decentration in mm.

    If (power<0) {
    Base In Prism, decentration = +
    Base Out Prism, decentration = -
    Base Up Prism, decentration = +
    Base Down Prism, decentration = -}

    If (power>0) {
    Base In Prism, decentration = -
    Base Out Prism, decentration = +
    Base Up Prism, decentration = -
    Base Down Prism, decentration = +}

    Theses decentration values should be added to your decentration values before computeing the minimum blank size, then this minimum blank size should be used for determining thickness. You will notice that the lower the dioptric power the less accurate this method is, I am workin on a calculator right now and again will post it when I am ready, the initial version is here (along with the source code):

    http://www.optiboard.com/forums/showthread.php?t=35604
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    thanx harrychilling

    could u give me an example?

    if i have

    SPH: +3.00 and PD 60 and i would like to make it in 50mm lens with 5 dec.

    so is this the same like i would use +3.00 with 60 mm dia and 70 PD??

    thanx

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