The sigma 50 500mm f45 63 dg os hsm lens is an upgrade to sigmas popular 50 500mm ultra telephoto zoom and now includes an optical stabilizer which offers the use of shutter speeds approximately 4 stops slower than would otherwise be possible a particularly useful addition on a lens of this size the sigma 50 500mm os lens incorporates hsm hyper sonic motor to ensure quick and quiet auto focus as well as full time manual focus capability four sld special low dispersion glass element