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Thread: Decentering plano prism lenses?

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    Decentering plano prism lenses?

    Is there a limit in how much I can decenter the PD and height in plano prism lenses in order to reduce thickness? Since the prism value is the same in all parts of the lens, it shouldn't matter, right?
    Last edited by Airegin; 06-26-2020 at 09:48 PM.

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    You can move the lens along the base apex line to reduce thickness. Moving up or down will have no effect on the thickness as it the same 90 degrees away from the base apex line.

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    Why would it need to be decentered for thickness and not just surfaced to the correct thickness to start with?

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    We are assuming it was surfaced to the correct thickness but when edging you can position the lens so the minimum edge (apex) is not at the edge of the lens. This will give you a finished product that is not as thin as possible. To put it in a number format. If the base of the uncut is 6 mm and the apex is 1 mm and you place the 1 mm edge at the edge of the frame you will have the thinnest result. If you position the lens where the apex is 2 mm you will have a lens that is both thicker on the apex by 1 mm and thicker on the base by the same amount 1 mm.

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    Quote Originally Posted by Lensman11 View Post
    but when edging you can position the lens so the minimum edge (apex) is not at the edge of the lens. This will give you a finished product that is not as thin as possible.

    Yes you could, but why would you? Send a tracing, specify a lens thickness and block accordingly.

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    Go back to the original question. Can you move the lens to optimize thickness. Very few opticians can calculate the center thickness on a Plano prism. Remember in this case the center thickness is neither the thinnest point or thickest point so how you position the lens will surely effect the end result.

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    Quote Originally Posted by Lensman11 View Post
    Can you move the lens to optimize thickness.
    Move it from where? Moving it(decentering) assumes there is a correct place to move it from. Why layout the lens in a suboptimal position then move it to a more optimal position?

    I guess the premise of the question doesn't make sense to me. Obviously I understand the answer to what the question implies.

    Remember in this case the center thickness is neither the thinnest point or thickest point so how you position the lens will surely effect the end result.
    Obviously.

    Very few opticians can calculate the center thickness on a Plano prism
    Why would one need to for this?

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    This is my last reply to this thread. You stated simply order the lens with a stated center thickness send a tracing and block accordingly. I believe that most could not calculate the center thickness on a Plano lens with prism which is not part of the answer to the question. It seems the question was will moving the lens by an extreme amount have an effect on the resulting thickness of the final product. The answer is that it definitely will.

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    Quote Originally Posted by Lensman11 View Post
    You stated simply order the lens with a stated center thickness send a tracing and block accordingly.
    I certainly did not say anything about ordering center thickness.

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    You absolutely can decenter a low power at 180 *spherical* lens for either cutout reasons or thickness considerations. To answer the OP’s question, as Kwill mentioned, if the surfacing lab fabrication was correct, there would be a point that the lens cuts out with minimal thickness, say 1.mm. You can certainly adjust your cutout to minimize the thickness.

    But don’t over/under decenter aspheric/atoric for cutout or to induce prism. By deviating the lens OC on these designs, you are moving the optical off axis corrections that these lenses are designed for. Basic rule: only use spherical lenses for over/under decentration.

    I hope this helps.

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    Quote Originally Posted by Airegin View Post
    Is there a limit in how much I can decenter the PD and height in plano prism lenses in order to reduce thickness? Since the prism value is the same in all parts of the lens, it shouldn't matter, right?
    In a plano prism lens, p.d. and height is irrelevant. You could tell the lab anything and it will come out the same.

    Why?

    Because: p.d. and vertical height is the desired X-Y location of the optical center. And in a plano prism there is no optical center.

    I hope you were never sent out by older lab workers to find a box of "optical centers"...

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    Quote Originally Posted by drk View Post
    In a plano prism lens, p.d. and height is irrelevant. You could tell the lab anything and it will come out the same.

    Why?

    Because: p.d. and vertical height is the desired X-Y location of the optical center. And in a plano prism there is no optical center.

    I hope you were never sent out by older lab workers to find a box of "optical centers"...
    Well suppose you order a couple plano lenses (not blanks) with 8 prism base out, each with different PD values. I would expect a lower thickness with larger PD values, even though there's technically no optical center. That just seems like common sense to me?

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    Quote Originally Posted by Airegin View Post
    Well suppose you order a couple plano lenses (not blanks)
    I assume this means ordering a job cut and edged by a remote lab, and not uncut lenses, right?

    Quote Originally Posted by Airegin View Post
    with 8 prism base out, each with different PD values. I would expect a lower thickness with larger PD values, even though there's technically no optical center. That just seems like common sense to me?
    As Drk stated the PD for this job is irrelevant. The lab should surface and edge the thin edge of the lens to match the frame type. There is no OC, so there is no way to verify any kind of PD. Grab a prism trial lens and put in the your lensometer. Orient the prism horizontally, then move the lens left and right. Nothing will happen.

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    Quote Originally Posted by Kwill212 View Post
    I assume this means ordering a job cut and edged by a remote lab, and not uncut lenses, right?
    Exactly. That's how all orders are done where I work. So even though the PD doesn't matter I'd still expect a different cut depending on the PD I input.

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    Quote Originally Posted by Airegin View Post
    Exactly. That's how all orders are done where I work. So even though the PD doesn't matter I'd still expect a different cut depending on the PD I input.
    Ok. Let's see if we can get you to answer you own question. How would you verify the PD of the finished job you proposed, before dispensing?

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    Quote Originally Posted by Kwill212 View Post
    Ok. Let's see if we can get you to answer you own question. How would you verify the PD of the finished job you proposed, before dispensing?
    I can't. But I'm pretty sure the lab just uses the center of the lens blank and that physical point will be located at the OC I put in.

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    Quote Originally Posted by Airegin View Post
    I can't. But I'm pretty sure the lab just uses the center of the lens blank and that physical point will be located at the PD I ordered.
    That is not how a lab makes lenses. If you can't verify the PD the lab can't layout a PD.

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    Quote Originally Posted by Kwill212 View Post
    That is not how a lab makes lenses. If you can't verify the PD the lab can't layout a PD.
    Well it was just a guess. Our software does predetermine the blank size based on the PD and height I order. That would change how the lenses are cut.

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    Quote Originally Posted by Airegin View Post
    Well it was just a guess. Our software does predetermine the blank size based on the PD and height I order. That would change how the lenses are cut.

    Yes for jobs that are not plano. The minimum blanks size for a plano is equal to the ED+2mm. Your software might not include this exception. This is why we should work to understand the underlying optics and not just rely on our fancy instruments, devices, software, etc. I would recommend contacting your lab for a better understanding of how lenses are made. Ask for an in person tour if possible. Short of that, pick of some books on surfacing and dispensing.

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    Quote Originally Posted by Kwill212 View Post
    Yes for jobs that are not plano. The minimum blanks size for a plano is equal to the ED+2mm. Your software might not include this exception. This is why we should work to understand the underlying optics and not just rely on our fancy instruments, devices, software, etc. I would recommend contacting your lab for a better understanding of how lenses are made. Ask for an in person tour if possible. Short of that, pick of some books on surfacing and dispensing.
    Thank you, I will do just that!

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