I thought this studio comparison was quite interesting. In my view the IQ between the 5DII and 1DsmkIII are very similar even at high isos. Being a studio controlled comparison, I wonder whether there is going to be any real life differences. I suspect that AF accuracy may in the end be what affects IQ in the field
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http://luminous-landscape.com/forum/index.php?showtopic=29896
http://luminous-landscape.com/forum/index.php?showtopic=29821