Autofocus
This lens features a built-in motor for autofocus on the Nikon and Canon mounts, while the Pentax and Sony variants will couple with the camera body's screw-drive system. The AF motor is unfortunately not of the ultrasonic type, but a simple DC micro motor, and is distinctly the weakest link in the package (on our Canon mount sample at least). Autofocusing on a static object is generally just fine in decent light, but in even moderately low light AF becomes decidedly slow, especially in comparison to the Canon 70-200mm F2.8 IS USM on the same body. Continuous AF performance for moving subjects is also disappointing; it's OK for relatively undemanding situations, such as distant subjects moving at a relatively constant speed, but the moment the subject starts moving randomly or at any great pace, the focus motor really struggles to keep up. This lens wouldn't therefore be my first choice for shooting sports and action work (or even simply children running around in the garden).
