The Sigma 17-50mm f/2.8 EX DC OS HSM Zoom Lens is an incredibly versatile, lightweight, and compact lens designed specifically for digital SLR cameras with APS-C size sensors. The lens has a fast f/2.8 maximum aperture throughout the entire zoom range. It features a construction of 17 elements in 13 groups with glass that corrects for flare, ghosting and all types of aberrations. It produces high contrast and sharp images throughout the zoom range even at its maximum aperture.