The APO-Telyt-R Module lenses are manual focus tele-photo lenses for Leica-R single lens reflex cameras that have been corrected for chromatic and spherical aberration using apochromatic lens elements. The lens system is modular, which makes it highly unusual. There are three focusing units and two heads available that can be combined to obtain six different lenses with four focal lengths of 280mm, 400mm, 560mm, and 800mm. The 400mm and 560mm focal lengths can be achieved in a faster (but heavier) and a slower (but lighter) configuration. The different heads and focusing units are attached to each other through a locking bayonet. All lenses have a slot for Series-6 filter holders, so that Neutral Density filters or Circular Polarizers can be used.