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Thread: I think Zeiss is now in a very good position for a change.

  1. #101
    What's up? drk's Avatar
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    Thank you.

  2. #102
    What's up? drk's Avatar
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    OK, more help needed.

    People talk about a "6-drop" lens or a "4-drop" lens, and it always makes me wonder what they're talking about.

    So here's what I'm chewing on...

    First of all, Zeiss Labs give this really, really cool print out with jobs (fanboying again, I know) that has a lot of good information on it. Of course it has a mini-layout diagram.

    So...with Zeiss they've chosen to put the prism reference point (the prescribed prism--which is usually goose egg) six millimeters below the fitting cross (that is, where the pupil center is).

    So...that means that our Zeiss PAL wearers will be getting a certain amount of prism in the distance zone, depending on the lens power in the vertical meridian, of course.

    Why do they choose this approach? I get that a lower optical center (typically) can help make sure the lens thickness is minimized, and Zeiss' website does list thinness as a design goal.

    Question: is this a problem for anisometropic patients? Sure, it would help minimize prismatic imbalance on downgaze, but at the expense of prismatic imbalance on straight-ahead gaze. True?

    Question: will this give patients a weird feeling to be looking through "more" prism (than, say, a 4-drop lens) in straight-ahead gaze?
    Last edited by drk; 10-01-2020 at 10:52 AM.

  3. #103
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    Irrespective of the technicalities, most people prefer Zeiss progressives for distance vision. That's been the case for a long time.

  4. #104
    What's up? drk's Avatar
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    It's probably the "harder" design philosophy, I'm thinking.

  5. #105
    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by drk View Post

    Question: is this a problem for anisometropic patients? Sure, it would help minimize prismatic imbalance on downgaze, but at the expense of prismatic imbalance on straight-ahead gaze. True?
    Yes.

    Question: will this give patients a weird feeling to be looking through "more" prism (than, say, a 4-drop lens) in straight-ahead gaze?
    Probably not, due to habitualization. CL wearers might have difficulties, along with changes from magnification and convergence effects.

    Most of your concerns are addressed in this thread:

    https://www.optiboard.com/forums/sho...l=1#post172064

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

  6. #106
    What's up? drk's Avatar
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    Hoo-hoo, another "collectible" Hall of Fame thread.

    (Heck, anything with Darryl on it is HOF-worthy).

    Thank you!

  7. #107
    What's up? drk's Avatar
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    BTW, you were pretty smart back in 2007.

    What happened to you? hahahaha

  8. #108
    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by drk View Post
    BTW, you were pretty smart back in 2007.

    What happened to you? hahahaha
    My wife noticed I've been referring to RBG as RGB when discussing Ginsburg lately. Does that mean I'm slipping away?

    Robert
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

  9. #109
    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Quote Originally Posted by Robert Martellaro View Post
    My wife noticed I've been referring to RBG as RGB when discussing Ginsburg lately. Does that mean I'm slipping away?

    Robert
    Robert- You, drk and several other Guru's here could fall from the peak of Everest, losing knowledge all the way down, to equal mine in the valley below.

  10. #110
    What's up? drk's Avatar
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    I'm not at guru status.

    I'm still at gu.

  11. #111
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    OK Devil's advocate here, but knowing I'm not smart enough I'll lobb a softball to those who are. Zeiss is mostly a 6 drop (Darryl actually told me why) Seiko is a zero drop. What does that do to vertical imbalance in anisometropes, antimetropes - wrestling with this as I hate to introduce prism to people who can cope without it. Please help the unknowing.

  12. #112
    What's up? drk's Avatar
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    If you remember, what did Darryl say?

  13. #113
    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by Jason H View Post
    OK Devil's advocate here, but knowing I'm not smart enough I'll lobb a softball to those who are. Zeiss is mostly a 6 drop (Darryl actually told me why) Seiko is a zero drop. What does that do to vertical imbalance in anisometropes, antimetropes - wrestling with this as I hate to introduce prism to people who can cope without it. Please help the unknowing.

    Since blank size is no longer a concern due to free form production the PRP location ultimately is not relevant. One could say increased distance between the fitting cross and the PRP reduces prism at near, but it will increase prism at the fitting cross compared to a decreased distance.
    If we assume that the surface of a progressive is spherical(obviously it is not) then we can calculate the prism at the fitting cross(assuming no prism thinning) and if you have a 6 mm difference there will be more prism than that of a 4 mm difference, however, we cannot calculate it this way. What power would you use for the calculation? The power at the fitting cross is different than that of the ordered power(power verified at the DRP) and the power at the PRP.
    A progressive lens is a non symmetrical surface, which lacks a center of curvature, therefore, it lacks an OC.
    For this reason prism at any point in a progressive lens is calculated using ray tracing, that takes the wrap and tilt along with the thickness and distance from the center of rotation, among other criteria) to the point on the back surface and front surface to calculate the prism at any given point.
    The location of the PRP will not, in most cases, have an impact.

  14. #114
    What's up? drk's Avatar
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    A progressive lens is a non symmetrical surface, which lacks a center of curvature, therefore, it lacks an OC.
    Mind. Blown.

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