Full aperture focus detection apparatus produces an amplified differential signal having a magnitude and a polarity indicative of the extent and direction, relative to the image plane of a projection lens, that a selected portion of the upper surface of a semiconductive wafer is out of focus with respect to that image plane. Lens positioning apparatus moves the projection lens by an amount and in a direction corresponding to the magnitude and polarity of this signal to position the image plane of the projection lens at the selected portion of the upper surface of the semiconductive wafer. A chuck on which the semiconductive wafer is held may be differentially leveled at three selected off-center locations by leveling apparatus so that the chuck may be raised or lowered at any of those locations to position the selected portion of the upper surface of the semiconductive wafer in a plane parallel to the image plane of the projection lens.