Sigma 4.5mm F2.8 EX DC HSM Circular Fisheye Lens
You can't get much wider than 4.5mmthis excellent lens has no problem snapping an 180 degree view. Wider lenses tend to be more prone to that fisheye lookbut that's what fisheye lenses are for. This lens gives images a circular look and exaggerated perspective. The f2.8 maximum aperture is excellent for letting in light and creating a softer depth of field. Built with coatings to reduce ghosting and flare, this Sigma also uses a quiet autofocus, with full time manual focus available just by turning the focus ring. The lens weighs just over a pound.