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Thread: High plus spectacle/contact lens combination

  1. #1
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    High plus spectacle/contact lens combination

    This is a continuation of the goggles I'm trying to make that I addressed int eh vertex distance thread. I figured that we've gone past the whole vertex distance thing, so I'll start a new thread.

    I'm trying to make up some sort of spectacle lens/contact lens combination that will add up to about +44D at the corneal plane. Basically I'm trying to make a pair of lenses that you can see underwater with (without having air in them)

    I have no idea even how to start, and unfortunately my ophthalmic optics is pretty rusty

    Any help would be gratefully received

    steff
    Last edited by steff; 04-29-2007 at 11:05 PM. Reason: can't spell

  2. #2
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    Have you tried perscription goggles??

    Hi Steff,

    Have you tried perscription swimming goggles? If not click here to find out more info on them and where you can buy them.

    Sean

  3. #3
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    Hi Sean,
    I don't think they'll fit a super high plus (like a +33D at 12mm) into a prescription swimming goggle.

    steff

  4. #4
    Master OptiBoarder lensgrinder's Avatar
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    Well I think I will give this a try, but this is just an acedemic exercise, because this lens could not be made.

    First you need to find the power that you need to equal +33.00 D(The power you used above) in water.

    I am using the highest available index so the power in air will be as low as possible

    Pwater=((nr-ni)/(nr-1)) X Pair

    33 = ((1.74 - 1.33)/(1.74 - 1)) X Pair
    Pair = 33/0.554
    Pair = +59.57 or 59.62

    For obvious reasons this will be a bi-convex lens, so let us use a nominal front +25.00.

    Our refractive front curve will be:

    Dnominal/Drefractive = 0.53/(n-1)
    25/Drefractive = 0.53/0.74
    Drefractive = 34.91

    Now that we have our front curve we need a back curve, but first we need to estimate out thickness. Keep in mind that due to the steep curve your diameter will be limited to about 20mm

    Sagfront = r - sqrt(r2 - (1/2d)2)
    Sagfront = 21.1 - sqrt(449.33 - 100)
    Sagfront = 21.1 - 18.71 = 2.39

    Now we will estimate our back power so we can get an estimated thickness

    Dn=D1+D2
    59.62 - 34.91 = D2
    D2 = 24.71

    Now find the sag of the back curve
    Sagback = r - sqrt(r2 - (1/2d)2)
    Sagback = 29.94 - sqrt(896.85 - 100)
    Sagback = 29.94 - 28.23= 1.71

    Now add the two sags together to get thickness

    Thickness = 1.71 + 2.39
    Thickness = 4.1

    Now use this thickness to find the real value of your back curve
    D2 = (Dv - D1)/(1 - ((tM/n) X D1)

    D2 = (59.62 - 34.91)/(1 - (0.0023 X 34.91)
    D2 = 24.71 / 0.919707
    D2 = 26.86

    So to make a lens equal +33.00D in water you need to make it a +59.62 in air with:
    1. Diameter of 20mm
    2. CT of approximately 4.1mm
    3. D1 refractive +34.91
    4. D2 refractive +26.86

  5. #5
    ATO Member HarryChiling's Avatar
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    Cool point on that one Lensgrinder.:cheers:
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    *Dave at OptiVision has a web based tracer integration package that's awesome.

  6. #6
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    Aagh! Now I'm really confused. Lensgrinder, could you have a quick squizz at the thread entitled 'vertex distance question' for me? I think that I"ve already figured out what the power in air that I need for this is, and I think it's about the 33D mark (in air), to achieve pl in water.

    Now can I just divide that up, and put +20 in a cl, and +13 in a lens (and then allow for vertex distance in the lens), or do I have to do something funky?

    Many thanks for all the help.

    steff

  7. #7
    Master OptiBoarder lensgrinder's Avatar
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    OK, so you want a +33.00D total power. You can do a +15.00 CL and a +18.00DS or whatever combination, you just need to compensate for vertex.

    Find focal length:

    1/18 = 0.0556M

    Add this to your vertex of say 14mm

    0.0556M + 0.014 = 0.0696

    Find out what power you need

    1/0.0696 = +14.37D

    Now what power do you need in air so the lens will equal a +14.37D in water.

    14.37 = ((1.74-1.33))/(0.74) X Pair
    Pair=25.94

    The lens needs to be made to a +25.94 DS to equal a +14.37 in water.

  8. #8
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    OK. I'll get our lab to make something up, and keep you updated!

    Thanks

    steff

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