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Thread: Toric Vertex Compensation

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    OptiBoard Professional skirk1975's Avatar
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    Toric Vertex Compensation

    Spectacle RX -4.25 -.75 X 090
    Contact Lens RX -4.00 -.75 X 090

    If X is the spherical and Y is the Cylinder what powers are they?

    No cyl at 90 correct? Y would be -4.25
    all in effect at 180. X would be -5.00 no?

    Also, do you round up or down when simply splitting -1.75 at 45 or 135?

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    Quote Originally Posted by skirk1975 View Post
    Spectacle RX -4.25 -.75 X 090
    Contact Lens RX -4.00 -.75 X 090

    If X is the spherical and Y is the Cylinder what powers are they?

    No cyl at 90 correct? Y would be -4.25
    all in effect at 180. X would be -5.00 no?

    Also, do you round up or down when simply splitting -1.75 at 45 or 135?
    Your question isn't exactly clear since you have already both the Spectacle Rx and Contact lens vertex compensated Rx.

    Prescribed RX - SPH power at 180 = -4.25D, total power at 090 = -5.00D

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    Quote Originally Posted by skirk1975 View Post

    Also, do you round up or down when simply splitting -1.75 at 45 or 135?
    DRK would say (correctly) use the 1/8 diopter answer (0.87) in the vertex formula.

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    Master OptiBoarder optical24/7's Avatar
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    Quote Originally Posted by Lab Insight View Post
    Prescribed RX - SPH power at 180 = -4.25D, total power at 090 = -5.00D
    Maybe I'm missing something, but I believe it's just the opposite of this. (sph @90 -4.25, power at 180 -5.00)

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    What's up? drk's Avatar
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    If I read the question "If X is the spherical and Y is the Cylinder what powers are they?", then "X" (which is the sphere component AT the 90 degree meridian) = -4.25D which vertexes to -4.00 (assuming ~14mm spectacle vertex distance) at the ocular surface.

    "Y" (which is the sphere component plus cylinder component AT the 180 degree meridian) = -5.00D which vertexes to about -4.62 at the ocular surface.

    So, if I'm getting the question right, the exact contact lens would be: -4.00-0.62 x 090.

    But the closest actually available contact lens will be : -4.00-0.75 x 090.



    As to the second question, I'm not getting it. Can someone explain it to me?

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    Lab Insight- you done goofed.

    24/7 - I don't think you are missing anything. Power at 180 is -5.00.

    Drk - I believe they want to know of they should round the resultant cylinder power at 45/135 degrees of -0.875 up to -1.00 or down to -0.75. I think there is a typo and they mean -0.75 at 45/135 not -1.75, so rounding from -0.375 up to -.50 or down to -0.25. It all seem very strange for a contact lens question.

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    What's up? drk's Avatar
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    OK, I trust you.

    As a general rule (almost an absolute law of the known universe) one always rounds down for astigmatic correction.

    Otherwise, the orbits of the outer gas giants will be slightly altered.

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    OptiBoard Professional skirk1975's Avatar
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    Quote Originally Posted by optical24/7 View Post
    Maybe I'm missing something, but I believe it's just the opposite of this. (sph @90 -4.25, power at 180 -5.00)

    That’s what I thought too!!!

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