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Thread: I need a formula to calc. add powers.

  1. #1
    Master OptiBoarder OptiBoard Silver Supporter Craig's Avatar
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    Confused I need a formula to calc. add powers.

    I have a +4.25 with a 2.50 add that I promised thin lenses; she ordered transitions ft drill-mount and I find out no aspheric blank is made in a drillable material. 1.56 does not count in my book as a worthwhile option.
    I asked my lab to surface on a digital machine as a blend, but they are not set up to run on an apsheric blank either. They ran a set and got a low add power after bumping it up, but we are wondering what the formula might be that would get us closer than we are. They are expensive blanks to play with for demo's.

    The problem is that each curve on an Aspheric lens has a different drop causing the add power to come out differently on any given RX based on the add power and curve being used. If we had a formula to calculate the add power, we might be in good shape.

    Thanks for any help and this is the next generation of free form when they can utilize and aspheric blank when it makes sense.


    Craig

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    Quote Originally Posted by Craig View Post
    I have a +4.25 with a 2.50 add that I promised thin lenses; she ordered transitions ft drill-mount and I find out no aspheric blank is made in a drillable material. 1.56 does not count in my book as a worthwhile option.
    I asked my lab to surface on a digital machine as a blend, but they are not set up to run on an apsheric blank either. They ran a set and got a low add power after bumping it up, but we are wondering what the formula might be that would get us closer than we are. They are expensive blanks to play with for demo's.

    The problem is that each curve on an Aspheric lens has a different drop causing the add power to come out differently on any given RX based on the add power and curve being used. If we had a formula to calculate the add power, we might be in good shape.

    Thanks for any help and this is the next generation of free form when they can utilize and aspheric blank when it makes sense.


    Craig
    Here's a thought, if you take the marked base curve and the clocked base curve of the lens, their sag equations would be equal to each other. Since the only variable that would be missing would be the aspheric coefficient then you could solve for that. With that aspheric coefficient you could guestimate a front curve power at the point you are trying to surface the add segment into and use that in your further power calculations as your front curve. Hope that helps.

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    Carl Zeiss Vision OptiBoard Gold Supporter Darryl Meister's Avatar
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    Craig, Can you specify the specific type of lens that you using? Is it a progressive lens? And, if so, is it a back-surface free-form lens (which generally wouldn't use an aspheric lens blank)?

    Or by surfacing a "blend" with a "digital machine," do you mean the round bifocal seg option that some free-form software packages offer?

    And is 1.60 or 1.67 high-index not available in the lens type that you are using? Both are excellent lens materials for drill-mounts.

    In any case, for a back-surface free-form lens, the "add power" measured by a focimeter is very nearly equal to the difference in surface power readings between the distance and near zones of the lens, as you might measure with a lens clock (compensating for refractive index, of course).

    For such lenses, you will probably not be able to simply "calculate" these surface powers in advance though, since these calculations are rather complex and typically handled by the free-form surfacing software.
    Darryl J. Meister, ABOM

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    Master OptiBoarder OptiBoard Silver Supporter Craig's Avatar
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    Thanks for the reply.

    Quote Originally Posted by Darryl Meister View Post
    Craig, Can you specify the specific type of lens that you using? Is it a progressive lens? And, if so, is it a back-surface free-form lens (which generally wouldn't use an aspheric lens blank)?

    Or by surfacing a "blend" with a "digital machine," do you mean the round bifocal seg option that some free-form software packages offer?

    And is 1.60 or 1.67 high-index not available in the lens type that you are using? Both are excellent lens materials for drill-mounts.

    In any case, for a back-surface free-form lens, the "add power" measured by a focimeter is very nearly equal to the difference in surface power readings between the distance and near zones of the lens, as you might measure with a lens clock (compensating for refractive index, of course).

    For such lenses, you will probably not be able to simply "calculate" these surface powers in advance though, since these calculations are rather complex and typically handled by the free-form surfacing software.
    This is a transitions ft that needs a very thin edge for to ensure they are thin and the trivex is the only one to hold up over time. We find that high index cracks over time as it ages. The women is also 82 and I need to ensure they can handle some abuse due to her age and the fact that she can't see with out her glasses to grab them.

    The lenses were surfaced on an aspheric blank and I will get the results today as to how they came out.

    I will keep you posted and does Zeiss plan on using aspheric blanks for + progressives?

    CRaig

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    Carl Zeiss Vision OptiBoard Gold Supporter Darryl Meister's Avatar
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    I will keep you posted and does Zeiss plan on using aspheric blanks for + progressives?
    For our free-form lenses, we generally use spherical lens blanks and directly surface the "asphericity" onto the back of the lens blank along with the prescription curves and progressive optics.
    Darryl J. Meister, ABOM

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    Doh! OptiBoard Gold Supporter braheem24's Avatar
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    1.60 or 1.67 FT-28 Transitions VI is a lens I'm waiting to find,

    I generally have to talk FT Drill mount customers out of Transitions because my only options are thick trivex lenses or poly (which I will never sell for a drill)

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    Rising Star jimrask's Avatar
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    Quote Originally Posted by braheem24 View Post
    1.60 or 1.67 FT-28 Transitions VI is a lens I'm waiting to find,

    I generally have to talk FT Drill mount customers out of Transitions because my only options are thick trivex lenses or poly (which I will never sell for a drill)
    In Belgium, these kind of lenses are available from any big lens manufacturer (Zeiss, Essilor, De Ceuninck, Tokai,...) I wouldn't understand why these aren't available in the US?

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    Craig,
    I think your best choice is Trilogy D28 Transitions. Even ground at knife edge it will be OK in a rimless mount. The fact that it can be ground so thinly may well offset its lack of asphericity and higher index. For strength, there is nothing that compares no matter what the account managers of big labs that don't have trivex will tell you.

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