Sphere power of an RGP for astigmatism

Discussion in 'Optometry Archives' started by Pipino Il Breve, Jul 27, 2003.

  1. If one has -2.00 myopia and -1.00 astigmatism (this is the prescription
    for glasses), what would be the needed power for a spherical RGP lens
    which corrects the myopia and masks the astigmatism?

    a. -2.00 (only the sphere part of the glasses prescription)
    b. -2.50 (the sphere equivalent)
    c. -3.00 (direct sum of sphere + cylinder)


    Thanks in advance
    Pipino Il Breve, Jul 27, 2003
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  2. Pipino Il Breve

    Mike Tyner Guest

    There are two missing varialbles.

    The tear film between the lens and the eye must be considered a "lens" also,
    with two surfaces. Its front surface is defined by curvature of the back surface
    of the contact lens (base curve). The back surface of the tear lens is defined
    by the cornea.

    So you must specify base curve and keratometry readings before an answer is

    If the cornea's flattest K reading is 45.00 and if the contact lens has a base
    curve radius of 7.5 mm, you would predict -2.00 as the first power to try, then
    overrefract to get a final answer.

    Mike Tyner, Jul 27, 2003
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