Sigma 120-300mm f/2.8 DG OS APO HSM Lens for SigmaThe Sigma 120 300mm f 2. 8 EX DG OS APO HSM is delivers high speed superior optical performance for Full Frame or APS C sized sensors. It features a Hyper Sonic Motor (HSM), an Optical Stabilizer, Inner Focusing and Zooming System, as well as SLD and FLD Glass Elements (FLD has a performance equal to fluorite). The HSM ensures quiet, high speed, accurate autofocusing while the OS allows for use of shutter speeds 4 stops slower than would otherwise be