Tamron SP AF 17-50mm f/2.8 XR Di II LD Aspherical [IF] Autofocus Lens for Canon with Manufacturer Accessories + 3 Piece Filter Kit (UV+CPL+FLD) + MORE - International Version (No Warranty) The use of XR (Extra Refractive Index) glass optimizes the overall distribution of optical power, and also reduces various aberrations to the absolute minimum while achieving remarkable compactness. In addition, suitable positioning of two hybrid aspherical elements serves to maintain imaging performance and to shorten and compress the entire optical system. An LD (Low Dispersion) glass element is used to accomplish effective compensation of on-axis and lateral chromatic aberrations, a critical factor for enhancing optical quality in digital photography, in order to achieve outstanding optical performance over the entire zoom range. Peripheral light fall-off is minimized when compared with that of conventional lenses for film cameras, so images are uniformly bright from the center to the periphery. By employing "Internal Surface Coatings" (i.e., multiple-layer coatings on cemented surfaces of plural elements) and multiple-layer coating technology on ordinary elements, ghosting and flare due to the reflections that occur when light enters through the front element and reflections caused by the imagers themselves are reduced to the absolute minimum. The Zoom Lock prevents unwanted barrel extension when carrying the lens/camera combination.