The widest true low light lens Canon makes, it is great for Wedding Ceremonies and Receptions, but I have also used it for portraits, architecture, night shooting, painting with light, video & steadicam type shots.

If you are a wedding photographer, this is a lens that you should strongly consider after the Canon 24-70 2.8L , Canon 70-200 2.8 IS L , Canon 16-35mm 2.8 L II , and Canon 50mm 1.2 L, as it will allow you to get some absolutely incredible, clean, low ISO noise type shots at dark receptions.