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Thread: Variable Seg Inset

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    Variable Seg Inset

    How important is set inset to the average patient's comfort and perceived performance of the glasses? It seems to me that the ability to adjust the set inset correctly should make a big difference to a good percentage of people. If that is true, how are Seiko lenses not dominating the market? Is it just a branding/marketing issue or is it just not that important?

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    Quote Originally Posted by Sylter View Post
    How important is set inset to the average patient's comfort and perceived performance of the glasses? It seems to me that the ability to adjust the set inset correctly should make a big difference to a good percentage of people. If that is true, how are Seiko lenses not dominating the market? Is it just a branding/marketing issue or is it just not that important?
    Any advanced free-form lens design (branded or private label) offers a variable inset option, however most dispensers choose not to perform the additional task of measuring the monocular near PD also and including it on the order.

    How important is it? Not overly really, although it does provide an extra level of precision for those patients that do not converge equally when shifting from distance to near.

    All companies besides Seiko offer this technology, so is available across the industry and not patented to just one company per se.

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    To my knowledge only a handful of designs actually adjust and align the corridor based on NPD, Seiko and Zeiss IIRC.

    cs

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    So if I supply mono near PD with any free form lens order the lab will be able to modify the seg inset accordingly? This also would modify the corridor/eye path as well then correct? I haven't been able to verify this with my Hoya or Essilor rep...

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    From what I understand unless it's a Seiko Supernal, Surmount, Superior or a Zeiss Individual 2 the eye path is NOT adjusted even when giving the NPD.
    Search the forum for post from sharpstick777 ... He has more detailed info and what actually happens with those lenses I mentioned and other that claim they do. PM him would be a more direct method.

    cs

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    Quote Originally Posted by Sylter View Post
    How important is set inset to the average patient's comfort and perceived performance of the glasses? It seems to me that the ability to adjust the set inset correctly should make a big difference to a good percentage of people.
    What do we need to know to correctly position the near reference point?

    1. Monocular distance IPD.
    2. Object distance.
    3. Distance power on the horizontal meridian.

    A very wide PD; i.e. 77mm, will need a total inset of about 4.5mm. A very narrow PD of 55 needs a total inset of only 3.3mm. Semi-finished PALs can not know this and will be slightly mispositioned at the NRP.

    Most PALs, even those made on a free-form platform, do not calculate for #2, although I'm reasonably sure that most PALs are designed so that Adds over +2.50 will account for a shorter object distance than 40cm.

    Moderate to high myopes and hyperopes require less and more inset respectively. Some premium semi-finished lenses can take this into account by using more or less inset depending on the base curve. Most free-form software will do this automatically.

    How does a mispositioned NRP influence the vision?

    I found this nice summary from Darryl Meister in my optical gold folder.

    I was recently asked by a colleague to explore more rigorously a related subject (effects of playing with near inset on progressive near zone performance), so I thought I'd share my conclusions with the group since we've been deliberating this issue.

    Reduction in viewing zone. Every 1 mm of error in PD will result in a similar reduction in the viewing zone size to one side as well as an equal reduction in the effective binocular viewing zone size (the region of the viewing zones seen simultaneously with both eyes). The size of the near zone of a progressive lens varies significantly between designs, and will also depend on the distance from the fitting point, but the width of the 1.00 cyl boundary at 85% of the Add for a typical +2.00 Add progressive is around 10 mm. This point is close to the upper limit of the functional near zone, and is roughly equal to the minimum width of the near zone. Consequently, you can assume that 1 mm of variation in inset will reduce the viewing zone size by up to 10%. Errors in inset will have an even greater impact on the intermediate zone; every 1 mm of error will reduce the intermediate zone by up to 15% or more, depending upon the design.

    Induced horizontal prism. The horizontal prism induced by varying the inset is generally equal to the product of the inset error (in cm) and the Add power (i.e., Prentice's rule applied to the seg). Excessive inset will create base in prism at near while insufficient inset will create base out prism. For a +2.00 Add, "flipping" the lens will result in roughly 2.0 prism diopters of base out prism (which may relieve some convergence excess or esophoria). This assumes that the optical center of each distance zone is at the correct distance PD. (As a side note, my colleague suggested that this induced horizontal prism may be why some wearers prefer abnormal insets.)

    Unwanted astigmatism. This value, in particular, will vary widely between progresive lenses, particularly since it is not uncommon for progressive lenses to have a little unwanted astigmatism at the center of the near zone. For relatively small changes in inset (less than 2.5 mm), you can expect up to 0.25 D or more of unwanted cyl around the reading point. I would say 0.10 D per 1 mm would be a good approximation for a +2.00 Add, which is pretty consistent with the Minkwitz theorem. This error will have a much greater impact on the intermediate zone though, which is not only more narrow but also has more rapidly changing astigmatism.

    There were some other, less significant effects (like slightly reduced add power), but these are the issues worth considering. Keep in mind that these approximations will depend on the individual lens design, Add power, and the wearer's tolerance of blur. I have assumed a "worst case scenario" for some of the rules in order to keep the math simple and the issues in perspective.

    Best regards,
    Darryl
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    An expert is a man who has made all the mistakes which can be made, in a narrow field. -Niels Bohr

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    I found a few diagrams that show how prism induced behind the distance power as the eyes converge at near influences the inset or NRP.
    Attached Thumbnails Attached Thumbnails Convergence with Emmetropia.jpg   Convergence with Minus Lenses.jpg   Convergence with Plus Lenses.jpg  
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    An expert is a man who has made all the mistakes which can be made, in a narrow field. -Niels Bohr

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    Great info Robert! Thank you

    cs

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    Quote Originally Posted by Sylter View Post
    How important is set inset to the average patient's comfort and perceived performance of the glasses? It seems to me that the ability to adjust the set inset correctly should make a big difference to a good percentage of people. If that is true, how are Seiko lenses not dominating the market? Is it just a branding/marketing issue or is it just not that important?
    I believe the marketing department of Seiko (pre-Hoya) just wasn't all that good, and the Essilor marketing department is much better.

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