A method including acquiring images of medical slides at a plurality of different focus positions, determining a position which produces a maximum value of pixel values relative to a pixel value mean, wherein said determining comprises using a pixel value mean as a coarse estimate of coarse focus position, and subsequently refining said coarse focus position to find a fine focus position, and wherein said refining comprises fitting to a polynomial, and using a specified portion of the polynomial as a fine estimate of focus position, and producing a focus control signal that is related to said maximum value to control a focus position.