Since the size of full-size format is different from the APS-C size imager, scopes captured are different even when focal lengths are the same (i.e., angles of view differ).
The AF18-270mm Di-II is designed to have shorter focal lengths to achieve optimum angles of view in its wide-angle side.
Attempts to design shorter focal lengths inevitably result in a larger lens diameter. Tamron has solved this problem by designing the size of the lensí image circle to match that of APS-C size imagers. In the compact design, it is almost comparable to a lens offering the same angles of view in full-size format.
* When pictures are taken with a Di-II lens on a full-size format, image corners become dark (ie., vignetting becomes noticeable).