This telephoto zoom lens has a large maximum aperture of f/2.8 that is constant throughout the zoom range. To ensure high-quality images, it uses two pieces of Special Low-Dispersion glass (SLD) in the front lens group and another two in the rear lens group. Sigma's Hyper Sonic Motors (H.S.M.) provide silent, responsive autofocus action with Canon, Sigma, and Nikon AF cameras. H.S.M. also permits manual adjustment of focus without the necessity of switching off the autofocus function. Focusing does not change the lens' length and, therefore, allows the photographer to hold the camera easily during shooting. Use of a "Petal-type hood" and a tripod to prevent unintentional movement, will help optimize the imaging quality. High-speed focusing is also possible when the dedicated 1.4x or 2x APO TELE converter (optional) is used.