Sigma 150-500mm f/5-6.3 DG APO OS (Optical Stabilizer) HSM AutoFocus Telephoto Zoom Lens for Pentax AF Cameras Incorporating Sigma's original OS (Optical Stabilizer) function. An ultra-telephoto zoom lens covers a telephoto range up to 500mm and allows short perspective. Three SLD glass elements and a rear focus system provide optimum image quality throughout the entire zoom range. Super multi-layer coating reduces flare and ghosting. HSM ensures quiet and high-speed autofocus.