Here is a different camera shot of the same lightning discharge as the one below. I'm not sure why, but this camera caught more of the side-branched lightning than the other camera.
It might have something to do with a difference in shutter lag in the camera (top = 20Da, bottom = 30D. Perhaps the lens (top = 70-200 EF 2.8L), bottom = 24-70 EF f2.8L).
Both were on manual exposure. I don't know the reason (except that I like the top image better than the one below)....



More lightning captures at: http://www.pbase.com/pegsam/lightning