Sigma's new Hyper Sonic Motors (H.S.M.) provide silent, responsive autofocus action with Canon, Sigma, and now Nikon AF cameras. H.S.M. also permits

Sigma's new Hyper Sonic Motors (H.S.M.) provide silent, responsive autofocus action with Canon, Sigma, and now Nikon AF cameras. H.S.M. also permits manual adjustment of focus without the necessity of switching off the autofocus function. Focus Free mechanism keeps the manual focus ring from spinning when the camera is autofocusing.