Just had to comment on this comparison. I have the Pentax . I read many reviews about the lens, many commenting about how expensive it is. However, in comparing this lens to the Cannon, one fact seems clear: it compares quite favourably. But two factors clearly favor the Pentax: firstly, the pentax barrel is constructed of high quality aluminium, while the Cannon is plastic. Good plastic, but plastic. Secondly, the Pentax can be purchased for around $650.00; the Cannon cost nearly $1000.00. Much of the difference in price can be explained by the fact that the Cannon has the USM nearly silent focus motor, while the Pentax is screw drive. Furthermore, the Cannon lens has image stabilization built in, while the Pentax IS is built into the camera, essentially making IS available for every lens attached to the Pentax body. But the bottom line is that the performance of both lenses is quite similar, but the Pentax cost about $350.00 less. Just an observation.