The Sigma 17-50mm f/2.8  OS function offers the use of shutter speeds approximately 4 stops slower than would otherwise be possible. The lens has a compact construction with an overall length of 91.8mm making it ideal for travel photography. Two FLD (��F�� Low Dispersion) glass elements, two glass mold and one hybrid aspherical lens provide excellent correction for all types of aberrations. This lens has superior peripheral brightness and provides sharp and high contrast images even at the maximum apertures. The incorporation of HSM (Hyper Sonic Motor) provides fast and quiet AF.