Abstract: An automatic focus correction system for E-beam lithography uses optical light from a narrow angle light source to illuminate a horizontal slit, the image of which is projected onto a target surface. Prior to and after reflection of the slit image from the target surface the light beam is projected parallel to the target surface to minimize vertical space requirements in the E-beam column. Variations in height, z-position, cause the slit image to move vertically and the focus of reflection to shift laterally and this image is redeflected in the horizontal plane by a prism to a linear diode array used to produce a video-type output signal. Autofocus electronics are used to convert the video output signal into an analog correction signal to the E-beam fine focus coil. The video-type signal is also converted to a digital height value to be used for corrections in beam magnification and rotation.