We describe a novel formulation of the range recovery problem based on computation of the differential variation in image intensities with respect to changes in camera position. This method uses a single stationary camera and a pair of calibrated optical masks to directly measure this differential quantity. We also describe a variant based on changes in aperture size. The subsequent computation of the range image involves simple arithmetic combinations, and is suitable for real-time implementation. We present the theory of this technique and results from a prototype camera which we have constructed.