Abstract: A focusing apparatus for use in a step and repeat photo-exposure system is provided in which air jets are located immediately adjacent the exposure area of the exposure system. Back pressure of the air jets is sensed to determine the distance of the optical unit of the photo-exposure system to the surface of a semiconductor wafer being exposed. The average back-pressure is utilized to control movement of the optical system in order to achieve precise focusing. A map of the surface aberration of the wafer may be obtained by measuring the position of the camera at each die site after focusing has been achieved.