• Part B modified by C. H. Wooley.
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    Refractive Mgmt/Intervention

    A, The reference sphere (in red) is represented in 2 dimensions by a circular arc centered on point B and drawn through the center of the exit pupil of the lens. If the image is stigmatic, all light from point A crosses the reference sphere simultaneously. B, When the image is astigmatic, light rays from the object point simultaneously cross the wavefront (in blue), not the reference sphere.