Abstract: An apparatus for detecting and compensating for focus errors in a photolithography tool which holds a reticle and a substrate substantially parallel is described. The apparatus includes at least one proximity gauge for determining a first distance between the reticle and the substrate, thereby determining if a focus error condition exists. The apparatus also includes an actuator for adjusting the position of the reticle with respect to the substrate, thereby compensating for the focus error condition. According to another embodiment, a lens system actuator is employed for adjusting a lens system parameter, thereby compensating for the focus error condition detected by the at least one proximity gauge.