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One method for improving the vision system of a robot is to allow the robot to automatically focus its cameras and to use focus as a depth cue. In this paper, the problem of automatic focusing is reduced, by a theoretical analysis of defocus as a wave aberration which attenuates high spatial-frequencies, to the problem of measuring the high spatial-frequency content of an image. The problem of recovering range from focus is treated using a focus criterion function along with knowledge of the lens parameters to solve the lens law for range. The accuracy of the range measurements is approximately 10% of object distances between 1 and 2m.