Fig. 6. A. When radial astigmatism is corrected, the two focal lines coincide to form a point image of a point object, but the image point is not on the far-point sphere, creating a power error. B. When the tangential and sagittal foci are equidistant from the far-point sphere, there is no power error, but radial astigmatism is present. (Modified from Atchison DA: The clinical importance of spectacle lens base curves. Clin Exp Optom 69:31, 1986.)