A beam brush includes one lens fixed for receiving a laser beam propagating along a first beam axis, a retroreflector positioned for redirecting the laser beam onto a second beam axis through a change in beam path length, and a second lens transmitting the redirected beam. The retroreflector is rotatable about an offset axis and has its angle of rotation controlled for affecting a change in beam path length. The angle of rotation resulting in the path length change is selected for providing a focus or divergence of the reflected beam transmitted through the second lens. Such a system is useful as a Z-axis focusing device operable an X-Y scanner located downstream the modified beam.