The Sigma 50-200mm f4-5.6 DC OS HSM sports many excellent features, including Sigma's own Optical Stabilizer function and SLD (Special Low Dispersion). The Optical Stabilizer (OS) function helps reduce the effects of camera shake so subjects can be seen clearly seen in the viewfinder and images can be captured blur free in the camera. The OS allows the use of shutter speeds up to 4 stops slower than normal while maintaining steady, handheld photography. The inner focusing system provides high image quality throughout the entire zoom range, while the HSM (Hyper-Sonic Motor) ensures quiet and high-speed auto focus. A Super Multi-Layer Coating reduces flare and ghosting and provides high contrast images throughout the entire zoom range. Used on digital cameras with an APS-C sized sensor the Sigma 50-200mm offers equivalent focal lengths of approximately 50-320mm (in 35mm terms).