17 to 50mm Di II high-performance zoom lens. The Tamron SP designation is reserved for lenses developed with special emphasis on extraordinary photographic performance. Engineers are free to innovate in an uncommon atmosphere where optical performance comes before price, resulting in lenses that satisfy the demands of discerning photographers. One of two Tamron 17 to 50mm F/2.8 offerings, the image integrity and compositional options afforded by the SP 17 to 50mm F/2.8 Di-II's wide-to-moderate-telephoto zoom characteristics are prized by experienced photographers. Both versions of the SP 17-50mm F/2.8 stem from the long-successful SP 28-75mm F/2.8 originally designed for 35mm film SLRs. Later refined to meet the needs of high-resolution digital SLRs with APS-C-format sensors and like its further refined variant (the SP 17-50mm F/2.8 XR Di-II VC, model B005), the 17-50mm renders a picture angle range approximating a 26-78mm lens on a full frame DSLR or a 35mm SLR. A fast and constant F/2.8 maximum aperture benefits photographers in several ways, including the ability to render cluttered backgrounds well out of focus, expanded low-light shooting freedom and a brighter viewfinder image. With its mildly less complex optical formula and without tri-axial VC (vibration compensation), the SP 17-50mm F/2.8 Di-II (model A16) stands out as a remarkable value and a favorite of passionate photographers.