The Adjustable Optical Slit utilizes a precision micrometer drive for fine beam shaping and aperture control in an optical system. The adjustable mechanical slit opens evenly from fully closed (center) to 6.0mm (maximum clear opening). The adjustable mechanical slit mechanism is mounted in a black anodized metal housing 55.5mm long x 38.1mm high x 22.2mm thick with a 28mm long x 23mm diameter tube protruding from the face. A removable tube is threaded into the main body of the mechanical slit housing for use as a light baffle, and a ¼-20 mounting hole on the bottom of the housing is centered on the base. Slit length is 6.3mm at the center of the aperture.