Olympus WCON-08B 0.8x Wide Angle Extension and the Canon 24-85 USM. Even with a large adapter ring there are no dark corners (as long as you dont use a lot of filters before it). You can use it on a variety of lenses. Flare is a definite problem, I'm still looking for a very large mm hood, for now using my hand to shield (hard to hold camera and all with one hand, the W/A is pretty heavy). Got the idea from a guy on DPRevue Nikon Forum. His name is Ching-Kuang Shene, and the article is "Poor Man's Wide Angle Lens" if you do a search. He goes into much detail. Use at own risk...I have heard CA is a problem with some lenses more than others.