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Thread: Help Needed With Sph Cyl Forms

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    Help Needed With Sph Cyl Forms

    I am revising for my smc exam next week, and having trouble understanding
    this question

    Am astigmatic prescription requires a vertical power of +4.25 and a horizontal power of +6.75 ...

    Write out this rx in each of the following forms ...

    A) both sph and cyl
    B) toric form -3.50 base curve
    C) toric form +8.00 sphere curve

    I know is means

    +4.25 x 180 / +6.75 x 90

    so to work sph and cyl form out

    isit what gets me from +4.25 to +6.75 = +2.50

    then simply 4.25 x 180 / +2.50 x 90? & 6.75 x 90 / -2.50 x 180
    for a ?

    Many thanks lee, and revising tips or advice would be great :) x

  2. #2
    ATO Member OptiBoard Bronze Supporter HarryChiling's Avatar
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    Quote Originally Posted by leeberbillion View Post
    Am astigmatic prescription requires a vertical power of +4.25 and a horizontal power of +6.75 ...

    Write out this rx in each of the following forms ...

    A) both sph and cyl
    B) toric form -3.50 base curve
    C) toric form +8.00 sphere curve
    A) Sphero-cylindrical form: +6.75DS -2.50DC x 180 or +4.25DS +2.50DC x 090
    Crossed Cylinder form: +4.25DC x 180 / +6.75DC x 90 (since it asked for sph and cyl the cross cylinder form which you had written properly would not be the correct answer, however it does help in the next few questions as the sum of the sphere+base / sphere+cross should be equal to the crossed cylinder form.)
    B)
    C)

    Example:


    If the prescription in sphero-cylindrical form has the cylinder in the same sign with the base curve, then the base and sphere curves will sum up to the spherical powers and the difference between the cross curve and base curve sum up to the cylinder power with the axis the same as the cross curve.

    When you are starting out place the notations DS and DC after every measure, this will help you to simplify the questions a bit and gain a bit more intuition. I hope I helped.

    PS - Post these types of questions in the Ophthalmic Optics forum for quicker answers and more responses, they get lost in the Just Conversations forum.
    Last edited by HarryChiling; 06-11-2010 at 04:18 AM.

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