This document discusses different types of lenses that can be used with digital single-lens reflex (DSLR) cameras. It explains that DSLR cameras use a mirror system to direct light from the lens to an optical viewfinder. It then provides details on wide-angle lenses, standard lenses, telephoto lenses, and zoom lenses. Specifically, it discusses lens mounts, apertures, focal lengths and specific lenses like Canon's L-series lenses.