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](http://blog.vincentlaforet.com/2009/10/20/update-notes-and-explanations/)
Link: Update, Notes, and answers to technical questions: « Vincent Laforet’s Blog:

I haven’t done scientific testing.  But basically I’d wager that you can expect a 2 stop improvement in terms of low light performance/image quality with the 1D MKIV relative to the 5D MKII.