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Thread: the backside revisited

  1. #1
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    the backside revisited

    If we compare a front side conventional (what is conventional these days) progressive lens with a full backside lens, same rx, lets say plano add 2.50......will the magnification be greater in the conventional? Given the add is on the front wouldnt that be the case? Same blank size same base curve same material....

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    Great question, im going to throw a dart and say the backside progressive sits closer to the eye giving you wider field of vision with less distortion in the periphery. I hope im not just drinking the cool-aid on this. Ive actually experienced the difference between a conventional or traditional front side progressive lens like the comfort, and see a big difference in the comfort enhanced. Im also going to say that ive started taking some new meds this week and im a little dizzy so I might just be talking out of the wrong end. Thanks for listening.

    Kindest regards,

    Chaoticneutral

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    What's up? drk's Avatar
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    Darryl used to say there was an effect but that it was weak.

    Ditto on the magnifcation question.

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    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by drk View Post
    Darryl used to say there was an effect but that it was weak.

    Ditto on the magnifcation question.
    About as weak as the drinks at a Pentecostal wedding!

    Quote Originally Posted by optimensch View Post
    If we compare a front side conventional (what is conventional these days) progressive lens with a full backside lens, same rx, lets say plano add 2.50......will the magnification be greater in the conventional? Given the add is on the front wouldnt that be the case? Same blank size same base curve same material....
    6 BC, 2mm thick, back vertex distance 13mm, magnification = 4.2%. Reduce the vertex to 12mm and the magnification is reduced to 3.9%.

    Quote Originally Posted by chaoticneutral View Post
    Great question, im going to throw a dart and say the backside progressive sits closer to the eye giving you wider field of vision with less distortion in the periphery. I hope im not just drinking the cool-aid on this. Ive actually experienced the difference between a conventional or traditional front side progressive lens like the comfort, and see a big difference in the comfort enhanced.
    The New Comfort has a shorter and wider corridor than the original Comfort- you're probably seeing the difference in the fundamental PAL design, not the placement of the progressive optics.

    A full backside side PAL, in and of itself, does not explain any significant reduction in zone width, skew distortion, magnification, and aberrations for a given PAL design.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  5. #5
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    "About as weak as the drinks at a Pentecostal wedding!"

    LOL.



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    Quote Originally Posted by optimensch View Post
    "About as weak as the drinks at a Pentecostal wedding!"

    LOL.


    Drinks are a must to get through one of those.

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    ATO Member HarryChiling's Avatar
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    Yes magnification is reduced.

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    What's up? drk's Avatar
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    Si, senor, but order of magnitude, por favor.

    You are going to know more than the rest of us. (No formulas, please.)

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    Master OptiBoarder CCGREEN's Avatar
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    Quote Originally Posted by chaoticneutral View Post
    backside progressive sits closer to the eye giving you wider field of vision with less distortion in the periphery. I hope im not just drinking the cool-aid on this. Ive actually experienced the difference between a conventional or traditional front side progressive lens like the comfort, and see a big difference in the comfort enhanced.
    Chaoticneutral
    In theory there is a lot of truth spoken here but in reality it is really splitting hairs. 99.9% of the people will never know the difference.

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    What's up? drk's Avatar
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    That's what I believe, too, CC. Now yer talkin' my language.

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by drk View Post
    Si, senor, but order of magnitude, por favor.

    You are going to know more than the rest of us. (No formulas, please.)
    In the plano 2.50 add example with a 6 base curve the lower portion would be a roughly 8.5 base or 2.50 change assuming all else remains the same as given in the example. I am going to assume 2.0 center.

    Change in mag percentage = 2.5 * 2 / 15 = 0.33

    That's 33% Change in percentage mag in the case of a plano lens we're talking about 0.8% to 1.1% roughly negligible but start plugging in real powers and it starts to get significant.

  12. #12
    ATO Member HarryChiling's Avatar
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    I wish opticians were taught discretization, many of these formulas have bounds were a certain range and domain have one answer and another range and domain have a completely different answer. That is why PALs incorporate merit functions to allow the designer to discretize a set of formulas for different areas and then further refine on a global scale.

    In the scenario given you would want to say "no" their is no significant difference, but the scenario is not a real common scenario. If it was me I think to generalize I would use worst case scenarios in which case the answer is unequivocally "yes".

  13. #13
    What's up? drk's Avatar
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    I wish opticians were taught discretization, many of these formulas have bounds were a certain range and domain have one answer and another range and domain have a completely different answer. That is why PALs incorporate merit functions to allow the designer to discretize a set of formulas for different areas and then further refine on a global scale.
    You know, I was just thinking the same thing.




    (Not.)

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    Master OptiBoarder DanLiv's Avatar
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    Quote Originally Posted by Robert Martellaro View Post
    Originally Posted by chaoticneutralGreat question, im going to throw a dart and say the backside progressive sits closer to the eye giving you wider field of vision with less distortion in the periphery. I hope im not just drinking the cool-aid on this. Ive actually experienced the difference between a conventional or traditional front side progressive lens like the comfort, and see a big difference in the comfort enhanced.

    The New Comfort has a shorter and wider corridor than the original Comfort- you're probably seeing the difference in the fundamental PAL design, not the placement of the progressive optics.

    A full backside side PAL, in and of itself, does not explain any significant reduction in zone width, skew distortion, magnification, and aberrations for a given PAL design.
    The best evidence that, as Robert said, it's the improved design and not freeform backside design that you are experiencing is that the Comfort Enhanced is not a (completely) backside design, still frontside molded. The DRx line is backside design.

  15. #15
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    Quote Originally Posted by DanLiv View Post
    The best evidence that, as Robert said, it's the improved design and not freeform backside design that you are experiencing is that the Comfort Enhanced is not a (completely) backside design, still frontside molded. The DRx line is backside design.
    Nice catch! That is the best evidence (the progressive optics are not on the back surface of the lens).
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    ATO Member HarryChiling's Avatar
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    As an aside lens optimization via free form processing is done via asphericity. Since best form generally keeps the back curve between 4 and 7 the back side generally has more curve to aspherize. Aka more meat on the bones. This does not hold true for powers greater than 0 where the back curve gets flatter and the front steeper.

    That is the reasoning behing front side designs or dual working better for hyperopes and back side better for myopes.

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    "That is the reasoning behing front side designs or dual working better for hyperopes and back side better for myopes."

    I may not understand the theory here but in my experience that seems to be true. I find hyperopes do have more adaptation issues with full backside progressives than myopes. I thought it had to do with magnification effects but obviously there must be more to it than that. Thanks for the input.

  18. #18
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by optimensch View Post
    "That is the reasoning behing front side designs or dual working better for hyperopes and back side better for myopes."

    I may not understand the theory here but in my experience that seems to be true. I find hyperopes do have more adaptation issues with full backside progressives than myopes. I thought it had to do with magnification effects but obviously there must be more to it than that. Thanks for the input.
    There is still a lot of assumptions in my post. I am assuming that best form is created via a sphericity and not spherical. Magnification and distortion are intrinsically tied together so your not off in your though that mag has and effect keep in mind all these aberrations are tied together so one change that improves may have consequences elsewhere. The goal is to find this magical sweet spot where you have equalized all the positives and negatives towards a specific goal.

  19. #19
    One eye sees, the other feels OptiBoard Silver Supporter
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    Younger's solution.

    http://camberlens.com

    Now you can have a full backside progressive high plus without being forced to use low Abbe materials and/or +10 BCs.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  20. #20
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    Quote Originally Posted by Robert Martellaro View Post
    Younger's solution.

    http://camberlens.com

    Now you can have a full backside progressive high plus without being forced to use low Abbe materials and/or +10 BCs.
    I have to admit, I'm intrigued. Are these on the market yet? Redirecting material options back down from High-Index to Trivex would certainly be attractive!

    Any white papers that let us scrutinize their customer satisfaction data? I'm also sketchy on the verbiage about ' “using up” their design tools for “compensation correction” rather than incorporating the best enhancements for the patient. ' (I'm no digital surfacing info processessing guru, so maybe there's a sound argument there. Just not yet sure what it would be.)

    I wonder if the lenses look 'pregnant' or "buddha belly"ish from the front after being cut?

  21. #21
    Master OptiBoarder DanLiv's Avatar
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    From the Camber website:

    Traditional Front Side Progressive

    When the progressive design was on the front, there was a “varying” base curve “built in”– a higher base curve in the reading area vs. in the distance area. This is actually the correct optics for visual acuity.
    Digital Back Side Progressive

    Digital surfacing brings the progressive design to the back surface of the lens and no longer has a “varying” base curve “built in” since a single vision front base curve is used. This creates optical problems that must be corrected.
    So "freeform" created new problems that didn't exist in conventional front side progressives, and camber adds back in an aspheric front curve to compensate for that.

    Isn't that Essilor's whole long-standing (and vehemently derided) argument for maintaining molded progressives in conjunction with backside digital design?

  22. #22
    What's up? drk's Avatar
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    I've done two or three. They are very nice. They look normal. It's a very good lens.

    If there are general categories--bad, decent, very good, excellent--it would rate excellent.

    N = 3, though.

  23. #23
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by DanLiv View Post
    From the Camber website:

    Traditional Front Side Progressive

    When the progressive design was on the front, there was a “varying” base curve “built in”– a higher base curve in the reading area vs. in the distance area. This is actually the correct optics for visual acuity.
    Digital Back Side Progressive

    Digital surfacing brings the progressive design to the back surface of the lens and no longer has a “varying” base curve “built in” since a single vision front base curve is used. This creates optical problems that must be corrected.
    So "freeform" created new problems that didn't exist in conventional front side progressives, and camber adds back in an aspheric front curve to compensate for that.

    Isn't that Essilor's whole long-standing (and vehemently derided) argument for maintaining molded progressives in conjunction with backside digital design?
    By aspherizing the front surface change in base the back surface asphericity can be computed to focus on prescription correction.

    Dual Optics is done on a PAL front surface so if the blank is off base the a sphericity in top distance will vary from the asphericity in the reading requiring a more complex back surface. The camber is already compensated on the front so the asphericity needed on the back to compensate for the off base effects are going to be consistent from the distance to the reading.

    Now let's bring into the fold atoricity and as worn comp and the dual surface Various starts to get complex where the camber blank provides a simpler solution to the same problem. This becomes important for no optical reason as ideally both should perform the same, however manufacturing more complex curves with polishing often leads to errors so a simpler solution means the opportunity to deliver a more consistent result.

  24. #24
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by drk View Post
    N = 3, though.
    But EV = excellent.

  25. #25
    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by Hayde View Post
    I'm also sketchy on the verbiage about ' “using up” their design tools for “compensation correction” rather than incorporating the best enhancements for the patient.
    Marketing. As Harry said, it's a lot harder and potentially more prone to errors when producing two complex surfaces than one complex surface. If we really wanted the best for the high plus, high add wearer, we'd work the front curve with the PAL optics and optimizations, then add the power curve on the back.

    I wonder if the lenses look 'pregnant' or "buddha belly"ish from the front after being cut?
    Probably no more than a semifinished PAL.

    Quote Originally Posted by DanLiv View Post
    Isn't that Essilor's whole long-standing (and vehemently derided) argument for maintaining molded progressives in conjunction with backside digital design?
    1) Moderate to high hyperopes are a very small percentage of RXs.
    2) One of Zeiss's patent covers rotationally symmetrical (spherical) front surfaces.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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