Currently Canon's premiere Ultra Wide Angle zoom lens. This is the Mark II version of this lens and it produces excellent results. Put this one on top of a full frame sensor to get a taste of what ultra wide really looks like. Very cool, very cool.

Broaden your perspective with the Canon EF 16-35mm ultra wide angle zoom lens. Specifically designed for improved edge to edge image quality that meets the strict requirements of professional and high-end amateur photographers alike, the lens lets you bring more area into focus while providing greater depth of field. The lens features three high precision aspherical lens elements ground, replica, and GMo that produce even better image quality than the original Canon EF 16-35mm USM lens. The circular aperture, meanwhile, produces a beautiful and natural background blur when shooting at wider apertures. Other details include internal focusing, a ring type ultra-sonic monitor (USM), and new AF algorithms for fast and quiet autofocusing. The lens carries a one year warranty.