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Thread: ..again on aspheric lenses

  1. #1
    Rising Star mauroventura's Avatar
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    ..again on aspheric lenses

    Some weeks ago ,
    an optician told me that normally when it must edge an aspheric lens on frame it needs to introduce a vertical decentration of about 2 mm , even if the prescription doesn't require it.
    Other opticians told me the same thing, but i don't know to explain by myself the reason.
    Can i have help ??

    BYe

    Mauro

  2. #2
    Master OptiBoarder lensgrinder's Avatar
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    What you need to do is lower the OC of a lens 1mm for every 2o of pantoscopic tilt. So if a frame has 10o of pantoscopic tilt then you would lower the OC 5mm. The reason is that you want the ray entering through the OC to go through the eye's center of rotation. See the attachment where the eye is looking straight, but the ray is above the center of rotation.
    Attached Thumbnails Attached Thumbnails Bad Panto.jpg  

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    Master OptiBoarder Darryl Meister's Avatar
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    I, personally, would leave the optical center at the datum (180 line), and then tilt the lens as needed to ensure that the appropriate relationship is at least nearly maintained. (It won't need to be exact, but you don't want to have the optical center sitting directly in front of the pupil center if the lens has 15 degrees of tilt.) Once you start vertically decentering the optical center, you will generally increase the maximum thickness of the lens, which somewhat defeats the purpose of using an aspheric lens in the first place. Also keep in mind that the relationship between tilt and vertical centration is more critical in higher powers.
    Darryl J. Meister, ABOM

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Pantoscopic tilt, like so many other parameters, needs to have an agreed upon convention to measure and validate. Darryl, I personally prefer the Carl Zeiss "plumb" gauge.

    What do others use to make this increasingly critical measurement?

    Barry

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    What's up? drk's Avatar
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    I am going to infer a few things from your post, Darryl. Please comment as to the accuracy:

    1.) You would NOT specify level MRP when fitting aspheric SV lenses, generally.
    2.) You would instead, specify the MRP to be placed at 1/2 B.
    3.) When dispensing, you would evaluate the outcome: if MRP ended up rather high, then you'd induce some degree of panto to compensate. If the MRP ended up rather low, then you'd minimize panto.
    4.) You seem to imply that the whole purpose of aspheric SV lenses should be cosmetics (weight not mentioned, here) and that decentering the level MRP for maximum optics is less important than the reduction in cosmetic effect.

    Is that true? If so, I love the practical thinking involved.

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    fitting aspherics

    When Rodenstock introduced the Cosmolit lens the recommended method was to lower the optical center 2mm from each degree of tilt from the Pupil Center reference point.S an 8 degree tilt resulted in 4mm below pupil center.

    This was one of 2 methods recommended. The other was to have the patient tilt their head back until the frame had no pano tilt ( frame is now perpendicular to the floor) and the mark the pupil center.

    This point it was explained was where a patients viewing position would be 62% of the time and thus was using the aspheric optical center for a majority of viewing.

    Alan

  7. #7
    What's up? drk's Avatar
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    Quote Originally Posted by ayoco View Post
    This point it was explained was where a patients viewing position would be 62% of the time and thus was using the aspheric optical center for a majority of viewing.

    Alan
    Yeah, straight-ahead gaze!

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    Master OptiBoarder Darryl Meister's Avatar
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    Is that true? If so, I love the practical thinking involved.
    You pretty much summed it up. Aspheric lenses, below +8.00 D anyway, really do not improve optical performance compared to conventional "best form" lenses (there are several threads on here that discuss why in more detail). Nevertheless, while I think cosmetics are very important with aspheric lenses, optics shouldn't totally be disregarded, so I would recommend maximizing the optics as much as possible without compromising the cosmetics.

    This point it was explained was where a patients viewing position would be 62% of the time and thus was using the aspheric optical center for a majority of viewing.
    It has more to do with ensuring that the optical axis of the lens passes through the center of rotation of the eye, as Lensgrinder illustrated in his earlier post. If you dropped the optical center 4 mm for 8 degrees of tilt, for instance, the wearer would generally be looking above the optical center, not through it.
    Darryl J. Meister, ABOM

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Quote Originally Posted by ayoco View Post
    The other was to have the patient tilt their head back until the frame had no pano tilt ( frame is now perpendicular to the floor) and the mark the pupil center.
    Alan
    Correctly, we should tilt the client's head back until the "lens-plane" is perpendicular to the floor...yes?

    Barry

  10. #10
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Darryl Meister View Post
    I, personally, would leave the optical center at the datum (180 line), and then tilt the lens as needed to ensure that the appropriate relationship is at least nearly maintained. (It won't need to be exact, but you don't want to have the optical center sitting directly in front of the pupil center if the lens has 15 degrees of tilt.) Once you start vertically decentering the optical center, you will generally increase the maximum thickness of the lens, which somewhat defeats the purpose of using an aspheric lens in the first place. Also keep in mind that the relationship between tilt and vertical centration is more critical in higher powers.

    That's the way I prefer to make the lenses, unless the frame sits grossly low or high in which case I will move the OC some and tilt the frame some to try and keep the cosmetics in check.
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