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, a focusing lens that focuses the beam onto a focus spot within a detection region, and beam-adjusting optics positioned in the light path between the light beam source and the focusing lens, which allow for precise positioning of the focus spot within the detection region. The beam-adjusting optics of the present invention comprise at least one movable focusing lens, mounted in a positioning device that allows repositioning of the lens in a plane perpendicular to the light path.