The Sigma 4.5mm f/2.8 EX DC HSM is a circular fisheye lens designed for crop-frame Canon cameras. Key features include:

Circular Fisheye. The Sigma 4.5mm f/2.8 EX DC HSM Fisheye provides the completely circular image with a 180º angle of view within a black surrounding frame (see Roger’s Take for example images). If you’re looking for an ultra-wide image that fills the image frame (this is called a rectilinear fisheye), this is not the lens for you.

Exaggerated Perspective. The Sigma can focus as close as 5.3 inches, providing some interesting close-up effects, especially given its huge depth of field. It allows full-time manual focus, but autofocuses quickly and accurately.

Specific Use. The Sigma 4.5mm gives interesting (bizarre, some say) effects and is sometimes used for scientific applications because it provides a consistent angle throughout its 180° field of view. OK, well it could be used for scientific purposes but mostly it’s used to give weird “cow’s-eye view” scenics: think about it—you take a picture, turn around and take another picture, and now you’ve got a 360° panorama. This is not simply an ultra-wide angle lens. Circular fisheye means the image is a circle in the center of your frame, it does not cover the entire frame. If you’re not looking for a fully circular image, please don’t rent this lens or you’ll be unhappy.