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Thread: Induced unwanted optical effect when line of sight isn't perpendicular

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    Induced unwanted optical effect when line of sight isn't perpendicular

    Hello,

    How can I calculate prizmatic and induced unwanted power effect if we have next situation:
    line of sight isn't perpendicular in respect to the back side of lens, it is about 76 deg. and power of lens is +1.00 D. What kind of calculation is needed to find the answer?

    Thanks in advance,
    Wlada

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    The rule of thumb is roughly .5mm at the spectacle lens plane per (one) degree of ocular rotation. Use Prentice's rule to determine the induced prism. Power error will be inconsequential at +1 D.
    Last edited by Robert Martellaro; 03-03-2016 at 04:44 PM.
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    There are many things you'd need to assume to answer this question. What is the vertex distance? If your talking about the angle at which the eye is gazing, VD will change where in the lens the eye is looking. You can calculate the (more or less) exact point at which the eye is looking in the lens with trigonometric functions and then use the prentice rule or ray tracing to calculate somewhat precise induced prism. Or you can use the approximation method Robert just gave, which is easier and less time consuming.

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    Quote Originally Posted by Robert Martellaro View Post
    The rule of thumb is roughly .5mm at the spectacle lens plane per (one) degree of ocular rotation. Use Prentice's rule to determine the induced prism. Power error will be inconsequential at +1 D.
    To determine the induced prism is not a problem. But, I want to figure is it problem induced unwanted power effect.
    For example, person had old eyeglassess OU: +1,00 Dsph, correct PD (59 mm) and pantoscopic tilt 15 deg.
    With new eyeglassess OU: +1,00 Dsph, uncorrect PD (65 mm) and pantoscopic tilt 2 deg. she has a symptoms: it appears that walls are not in stable position, and when she walks the floor is moving away from her. J found that this decentration induce 0,3 prism base out per eye. But this calculation does not take into calculation pantoscopic tilt. So, does someone know how to include pantoscopic tilt into calculation?

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    Quote Originally Posted by Wlada View Post
    To determine the induced prism is not a problem. But, I want to figure is it problem induced unwanted power effect.
    For example, person had old eyeglassess OU: +1,00 Dsph, correct PD (59 mm) and pantoscopic tilt 15 deg.
    If the OC is vertically center pupil then the Rx to grind would be +1.00 -.12 x 180. If the vertical OC is 7.5mm below center pupil, there is no need to compensate. See Martin's Rule for Tilt, and use a Rx compensation program to adjust the lens powers.

    With new eyeglassess OU: +1,00 Dsph, uncorrect PD (65 mm) and pantoscopic tilt 2 deg. she has a symptoms: it appears that walls are not in stable position, and when she walks the floor is moving away from her.
    The base out prism is causing spatial distortion. Neutralize or eliminate the prism.

    J found that this decentration induce 0,3 prism base out per eye. But this calculation does not take into calculation pantoscopic tilt. So, does someone know how to include pantoscopic tilt into calculation?
    Tilting the lens around the horizontal axis (pantoscopic tilt) shouldn't induce prism on the horizontal meridian.
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    Quote Originally Posted by Robert Martellaro View Post
    Tilting the lens around the horizontal axis (pantoscopic tilt) shouldn't induce prism on the horizontal meridian.
    Your yolking right? (Sorry couldn't help it)
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    I found on OptiCampus calculator for Panto and face form, also found MakeOptic's calculator, but I want to have in my spredsheet that kind of calculator too. So, I need to figure all process of calculation... I also found paper by Keating and Ralph. If I can not figure something I will ask for help...

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    Quote Originally Posted by Wlada View Post
    If I can not figure something I will ask for help...
    I have a problem with final axis... It does not match...

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    Quote Originally Posted by Wlada View Post
    I have a problem with final axis... It does not match...
    What parameters are you using (i.e. Face form, pantoscopic angle, material, power)? Are you asking for Effective or compensated? What axis do you get?

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    Now is OK. I found my mistake.
    One question. This calculation will give a) effective power of tilted lens, and b) power of compensated lense for that tilt.
    It is assumed that front vertex of lens match position of center of pupil (looking both x and y axis), but tilt position of lens produce that z axis of lens doesn't match z axis of center of pupil, right?
    Last edited by Wlada; 03-13-2016 at 06:52 PM.

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