1. You need to downsize the files to the same size. Downsizing reduces noise. It is not fair to compare original photos of difference MPs.
2. Regardless the above, 5D2 and D700 is within 1 stop of each other.
3. AFTER downsizing the D800 photos, it is comparable to 5D2's (one may argue one way or another, but it should be within 1 stop of each other). In previous generations, no one would even bother to ask the question: "Is Nikon D3 within one stop of Canon 1D2" because it should be so obvious. But now, we are arguing about this new D800 within one stop of 5D2.
