Manufacturer description: 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. The compositional options afforded by the SP 17-50mm F/2.8 Di-II VC's wide-to-moderate-telephoto zoom characteristics are prized by experienced photographers, rendering 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 allows cluttered backgrounds to be softly obscured, additional freedom in lowlight conditions and added viewfinder brightness. State-of-the art properties, including XR (Extra Refractive Index) and LD (Low Dispersion) glass in an optical formula employing 19 elements and 14 groups—assures sharpness, optimal contrast and breathtaking image fidelity. Its form factor and handling characteristics can be traced to the use of aspherical element design and internal focus. Tamron's proprietary tri-axial VC (Vibration Compensation) image stabilization adds to image sharpness and further expands low-light shooting freedom by combating image blur resulting from camera shake. Tri-axial VC addresses the adverse effects of camera shake in three planes making it possible to shoot handheld at as many as four shutter speeds slower than would otherwise be possible.