For my next lens I am tossing up between the two. I love the 500mm as it's razor sharp and very portable. The 600mm is a bit longer and magnification is about 44% more, so the reach is useful. However, it is dang heavy. I've used a Nikon 600 recently and handholding it is a pain. Also tried Canon's 800, but the price is just a bit too steep. I know IQ should be par for both really (500 vs. 600)...
So do you reckon go 1D MkIV/500 or 1D MkIII/600 (as I cannot have a MkIV and 600)? I am leaning in favor of the 500 as many wonderful images are captured with it and it is far more portable IMHO. Any tips or suggestions?


