Ok, right now I have a 1ds mark III with a 1ds mark II as a backup (so far I have not needed it as my 1ds mark III takes a lickin and keeps on tickin) I am considering a 5D mark 111 to replace my 1ds mark II as a back up. Obviously there are mucho differences between the two cameras regarding the weather sealing ergonomics etc but what I am curious about (perhaps from a purely academic point of view) is the difference in file quality, a side by side comparison, same situation, same lighting, same lens. Has anyone done this?