Sigma's 17-50mm F2.8 EX DC OS HSM large aperture standard zoom lens covers a focal length from 17mm wide angle and offers a large aperture of F2.8 throughout the entire zoom range, making it ideal for many types of photography especially portraiture and landscapes. The OS, Sigma's anti shake feature offers the use of shutter speeds approximately 4 stops slower than would otherwise be possible. The OS feature allows Sony and Pentax users the option of viewing a stabilized image in the viewfinder, a feature offered only by Sigma. HSM provides fast and quiet AF. Sigma's own FLD glass elements, which have performance equal to fluorite along with two glass mold and one hybrid aspherical lens, provide excellent correction for all types of aberrations. The super multi layer coating reduces flare and ghosting. This lens boasts superior peripheral brightness and provides sharp, high contrast images even at the maximum apertures. With a minimum focusing distance of 11 inches throughout the entire zoom range and a magnification ratio of 1:5. When you need to stand ready for any photo opportunity you need a fast aperture so you can capture in low existing light or at maximum range with a flash. This Sigma lens is perfect.