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Thread: Object Displacement

  1. #1
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    Object Displacement

    I am putting together a ppt. presentation, and can't recall if Object Displacemet is only noticeable in SV lenses, or if it can be detected in Multi-focals as well. Brooks and Borish doesn't mention it--just Image Jump, and I can't find my old Stimpson textbook.

    Thanks.
    J.B.

  2. #2
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by johnnyboy View Post
    I am putting together a ppt. presentation, and can't recall if Object Displacemet is only noticeable in SV lenses, or if it can be detected in Multi-focals as well. Brooks and Borish doesn't mention it--just Image Jump, and I can't find my old Stimpson textbook.

    Thanks.
    J.B.
    Don't think of object displacement as a result of a lens, it's a result of prism. Bifocals, SV, Progressives any lens can have prism especially when the visual axis and the optical center don't coincide.

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    Thank you for your help!

    J.B.

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    A quick exercise in image displacement, as well as a hands-on exercise for students.
    hold a lens over a horizontal line, such as the corner edge of a counter-top. displace the line up and down by moving the lens up and down. Note the difference between "with the motion" and "against the motion" between plus and minus lenses. Now find the point on the lens where the line is unbroken, or where the image is not displaced, and mark it with a felt pen, rotate the lens 90 degees, repeat and mark. The optical center will be where the marks are, and can be verified with a lensometer. Its a good demonstation of the prismatic effect, Prentice's rule, image displacment, ect. Note higher powers will, as per Prentice's rule, be easier to find the OC. Hope this helps.

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    Master OptiBoarder Darryl Meister's Avatar
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    In addition to the comments provided by Dave and Harry, you should keep in mind that the total object displacement during near vision in a multifocal lens is due both to prism induced by the distance correction and to prism induced by the segment (or "image jump") at the same position.

    Many years ago, bifocal segments were sometimes chosen in an attempt to minimize the total object displacement during near vision. The base up prism produced during near vision by a plus lens could be offset, for instance, by choosing a round bifocal with the segment center located below the intended reading level, inducing base down prism through the segment. Conversely, for a minus lens, you would choose a bifocal with the segment center located higher up than the reading level, such as a flat-top or Executive-style bifocal.
    Last edited by Darryl Meister; 09-17-2008 at 09:21 AM.
    Darryl J. Meister, ABOM

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    So maybe somebody can help me out with this problem. It seems like every time we get a job that has vertical prism ,lets say greater than 4.00 diopters prescribed I get 2 different seg heights one low and one high. Or if it has horizontal prism the PD comes out narrow or wide depending on whether its BI or BO. Is there a workable formula that can cut down on my bkgs?

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Kevin Howtopat View Post
    So maybe somebody can help me out with this problem. It seems like every time we get a job that has vertical prism ,lets say greater than 4.00 diopters prescribed I get 2 different seg heights one low and one high. Or if it has horizontal prism the PD comes out narrow or wide depending on whether its BI or BO. Is there a workable formula that can cut down on my bkgs?
    When the lens sits on the blocker you have light entering one side be it the back or the front of the lens and then projected onto a grid of some sort that allows you to align the PD's and the Seg Hgt's if the lens and the grid are any distance away from each other guess what? Displacement, now the amount is going to differ depending on distance and amount of prism.

  8. #8
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    Thanks Harry. Ive always had a good idea as to why this happens. What I was looking for was maybe a response from someone else who has recognized this problem besides me and has a workable solution. As it is now, I've got to count on my first inspector to cull these jobs out so that adjustments can be made. If he happens to overlook them they end up as a breakage, thus my frustration. Maybe something can be done in the optical programs

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