About the Canon 24-105 F4L... I own the 24-70 but thought I'd prefer the weight and range of the 24-105. Did a two-week swap with a fellow photog to try it out. Found that the distortion (both close in and wide) was loads worse than what I was used to from the 24-70 (which only has some distortion zoomed out wide). Also, in the end I decided I preferred the wider aperture to the IS. So no final swap. Now I'm thinking I'll end up with the 17-40 F4L and the 100 F2.8 IS Macro. And possibly the 50mm F1.8II because it's dead cheap, light and nice to have indoors in poor light.
