RGP vs glasses Rx - Large difference in power

Discussion in 'Glasses' started by Victor Poriazov, Jan 6, 2005.

  1. Because of increasing astigmatism my doctor switched me to RGP lenses.
    Could you explain why there is such a great difference in power b/w my
    glasses Rx and RGP rx?

    sph cyl ax
    OD -4.00 -1.00 15
    OS -3.00 -1.00 5

    power diam. b.c.
    OD -3.00 9.0 7.75
    OS -1.50 9.0 7.75
    (the doctor did not give me the print-out with my 'k' readings)

    Thank you,

    Victor Poriazov, Jan 6, 2005
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  2. Victor Poriazov

    Neil Brooks Guest

    I'm not a doctor, but . . . to my understanding . . . this is because of
    "vertex distances."

    The vertex distance is the distance between the front surface of /your/
    cornea and the back surface of the corrective lens. When a lens for
    /nearsightedness/ (what you have) is moved /closer/ to the eye, it becomes
    "stronger;" therefore, when you fit a contact lens, instead of glasses, you
    need a lower power lens.

    If you were farsighted, the power of the contact lenses would be /higher/ by
    roughly the same amount.

    Does that make sense?

    Neil Brooks, Jan 6, 2005
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  3. Victor Poriazov

    poriazov Guest

    Thanks all for your replies.

    Robert Martellaro wrote:
    No, the lenses are regular daily wear. (no monovision)

    I'm just curious if it is the normal practice to fit rgps with -1.0 or
    -2.0 lower power than the glasses rx.
    poriazov, Jan 7, 2005
  4. Victor Poriazov

    drfrank21 Guest

    The final power of the cl is determined by the fitting
    relationship of the k readings and the bc of the lens
    (as M.T. alluded to).

    After vertex is taken into account, we use SAMFAP (steeper add minus,
    flatter add plus)- basically means if the base curve of the cl
    is flatter than the flatter k, plus pwr (or less minus)is
    added and if the bc is steeper, more minus is added to the final

    drfrank21, Jan 7, 2005
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