The Sigma 17-50mm f/2.8 EX DC HSM Zoom Lens for Sony DSLRs with APS-C Sensors is a versatile, lightweight, and compact lens designed specifically for Sony A-mount and Minolta digital SLR cameras with APS-C size sensors. The lens has a fast f/2.8 maximum aperture throughout the entire zoom range and its 35mm focal length equivalency is 25.5 - 75mm. It features a construction of 17 elements in 13 groups with glass that corrects for aberrations and multi-layered lens coating to minimize flare and ghosting. It produces high contrast, sharp images throughout the zoom range even at its maximum aperture. This lens is equipped with Sigma's Hyper-Sonic Motor (HSM) which provides fast and quiet auto focusing, which is ideal when shooting video. This Sigma 17-50mm f/2.8 lens does not contain the OS (Optical Stabilization) designation because Sony Alpha DSLR's equip their camera bodies with image stabilization technology. Note: Autofocus does not work properly with the Sony Alpha SLT-A65 and SLT-A77 DSLR digital cameras. A free update is available from Sigma to correct this phenomenon and future production of this lens will be fully compatible with these cameras. This lens is for use with Sony digital cameras with APS-C size sensors only. Its images will contain substantial vignetting when used with full frame cameras. The DC designation on the barrel means that this is a special lens designed so that the image circle matches the smaller size (APS-C) of the image sensor of your digital SLR. Its specialized design gives this lens the ideal properties for APS-C digital cameras, including a compact size and lightweight construction. Constructed with 17 elements in 13 groups, it features FLD ("F" Low Dispersion) glass which is the highest level low dispersion glass available with extremely high light transmission. FLD glass along with aspherical lens elements offer reduced aberrations and superior optical performance. An f/2.8 maximum aperture throughout the zoom range provides shallow depth of field focus and offers sharp imaging at fast shutter speeds even in low light. Its HSM (Hyper-Sonic Motor) ensures a quiet & high-speed auto focus.