The present invention provides an optical analyzer having illumination optics that include a light source, such as a laser or other source, adapted to emit a collimated, or approximately collimated, light beam, and a focusing lens that focuses the beam onto a focus spot within a detection region, wherein the focusing lens is mounted in a lens positioning apparatus that allows for precise positioning of the focus spot within the detection region. The lens positioning apparatus comprises a lens holder adapted to rotate through a small angle around a pivot axis parallel to the optical path, such that the lens holder rotates in a plane perpendicular to the optical path, and an actuator adapted to provide an angular displacement of the lens holder around the pivot axis. The lens holder holds the focusing lens at a first distance from the pivot axis, and is coupled to the actuator at a second distance from the pivot axis, wherein the second distance is larger than the first distance.