Sigma s new 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.