The Tamron SP 17-50mm f/2.8 Di II LD Aspherical [IF] Lens made for the DX-format Nikon F-mount DSLRs incorporates a motor built into the lens itself, which gives it the capability to autofocus with all Nikon DSLR cameras (including the D40 and D40x). In addition, with the motor built into the lens, focusing will be quieter and more responsive. The Tamron SP 17-50mm f/2.8 Di II LD lens is a lightweight, compact, fast standard zoom lens designed exclusively for digital SLR cameras. The focal length of this lens is equivalent to approx. 26-78mm in 35mm format. This lens offers an f/2.8 maximum aperture throughout the entire zoom range, to ensure stress-free photographic shots at all focal lengths and distances. Additionally, a broader photographic expression through the use of faster shutter speeds as a result of the maximum aperture offers enhanced photographic pleasure.