A computer receives image data from a star-shaped optical target in the object plane and calculates angle-dependent boundary sharpness. The horizontal, x-direction, amplitude derivative and the vertical, y-direction, amplitude derivative are computed over a portion of each star pattern image from a Z panning sequence. A microscope slide stage, carrying the target, is moved vertically from a level just below where the target is in focus to a level just above where the target is in focus. For each small increment of vertical motion, Z panning, an image of the star pattern is captured for analysis. Computations are performed on the differentiated images to search for evidence of elongation of the point spread function and variation with stage Z position of the angle of long axis of such an out-of-round point spread function. The presence of a distorted point spread function that varies along the optical axis indicates astigmatism.