From the ManufacturerTamron's original development of proprietary core technologies such as Aspherical element production, Multiple Cam Mechanism and Integrated Focus Cam design lend to the creation of the world's lightest, most compact 11x zoom lens made for digital SLR cameras.AF18-200mm F/3.5-6.3 XR Di II HighlightsDi IILenses are designed for exclusive use on digital cameras with smaller-size imagers and inherit all of the benefits of our Di products. These lenses are not designed for conventional cameras and digital cameras with image sensors larger than 24mm x 16mm.Extra Refractive Index Glass (XR)Extra Refractive Index (XR) glass can bend light rays at steeper angles, thereby decreasing the physical length of the lens while enhancing imaging performance by minimizing optical aberrations. This has allowed Tamron to develop a line of shorter, smaller-diameter, lighter lenses without sacrificing lens speed, and actually upgrading image quality compared to older designs. XR glass is costlier than conventional glass but it yields enhanced optical power distribution, making possible many of the outstanding and innovative lens designs that bear the XR designation. XR glass, with its superior light-bending power, makes it possible to design a short-barrel lens with the same light-gathering ability (aperture value) as a long-barrel lens-even with a smaller lens diameter. By using this principle Tamron has been able to shorten the length of the entire optical system and produce lighter, more compact lenses of the same speed, and also to provide greater zoom ranges in lenses that are much more convenient to carry by hand.Low Dispersion (LD)Low dispersion (LD) glass elements in a lens help reduce chromatic aberration; the tendency of light of different colors to come to different points of focus at the image plane. Chromatic aberration reduces the sharpness of an image, but glass with an extremely lowdispersion index, has less of a tendency to separate (defract) a ray of light into a rainbow of colors. This characteristic allows the lens designer to effectively compensate for chromatic aberration at the center of the field (on axis), a particular problem at long focal lengths (the telephoto end of the zoom range), and for lateral chromatic aberration (towards the edges of the field) that often occurs at short focal lengths (the wide-angle end of the zoom range.)Internal Focus (IF)Internal focusing (IF) provides numerous practical benefits to photographers including a non-rotating front filter ring that facilitates the positioning of polarizing and graduated filters, and more predictable handling because the lens length does not change during focusing. Even more important, Tamron’s Internal Focusing (IF) system provides a much closer minimum focusing distance (MFD) throughout its entire focusing range. In addition, IF improves optical performance by minimizing illumination loss at the corners of the image field, and helps to suppress other aberrations that become more troublesome at different focusing positions.Zoom LockAnother original Tamron mechanical engineering concept is the Zoom Lock (ZL), a simple convenience feature that prevents undesired extension of the lens barrel when carrying the camera/lens unit on a neck strap.Product DescriptionTamron AF 18-200mm f/3.5-6.3 XR Di II LD Aspherical (IF) Macro Zoom Lens for Canon Digital SLR Cameras