Great All-in-One Fisheye. The Canon 8-15mm f/4L works with both full-frame and crop-frame cameras, but there are a couple of limitations: at the wide end, it fills nearly the entire frame on a crop-sensor camera, but on a full-frame camera it becomes a circular fisheye (a circular image inside the rectangular image with black edges). As you zoom to longer focal lengths, it becomes a standard fisheye filling a full-frame image (see the images below for examples).

Fast, Accurate Autofocus. Now that we’ve covered that, the Canon 8-15mm f/4 L has a USM focusing motor for fast, silent focus, as well as full-time manual focus override. Optically, it has a floating element (for sharp, close-up photos) and a fluorine coating that helps keep the front element clean.