Sigma's 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. This lens features apochromatic optical design and four elements composed of Special Low Dispersion (SLD) glass for high image contrast, resolution and color saturation. The front barrel does not rotate during focusing, due to internal focusing system. Internal zooming mechanism allows the lens length to remain constant during zooming.