Blurred imagery and the cylinder sine-squared law

The sine-squared law is shown to describe changes in spherical power which either minimize the blur or maximize the visibility of gratings oriented at various angles to the principal meridians of an astigmatic eye. These results help justify the use of the sine-squared law in applications such as meridional refraction. This derivation and its interpretation is quite different from the usual approach which emphasizes the meridional changes in curvature of a toric surface.