Craig, someone already mentioned on another thread that taking a D3X exposure and downsampliing to the resolution of the D700 gives sharper prints than prints from the D700 itself. So it's likely that something similar will be true of the D800.
The only "something similar" that will likely be true is that the same person may make that same claim for the D800.
I have captured identical images with the D3X and D700 with the same lens, in that particular case it was a flower macro. I used the 200mm/f4 AF-D macro lens mounted on a tripod and merely swap bodies. Even the D700 has sufficient pixels to cover an 8.5x11 print at 300 dpi. I printed both images to 8.5x11 and at least I cannot tell any difference in the prints.
If you make huge prints like 24x16 inches with a lot of fine details, and you stick with the D3X's base ISO 100 and middle apertures such as f5.6 on high-quality lenses shooting from a tripod, it'll be obvious that the D3X has better resolution, but for such huge prints, we rarely look at them from up close to notice the difference.
The D3X is great at ISO 100 and 200. But when you get up to ISO 800 and 1600 under indoor, dim-light situations, pixel quality from the D3X will degrade enough that it would more than wipe out any advantages from 24MP. Under such conditions, you actually get worse results compared to what you could have gotten from the D3/D700 under low light, let alone the D3S.
This is like the third thread on the D800 and lens performance. Without an actual D800 to test with, it is all hypothetical, but in order to get the most out of 24MP, 36MP, you need to:
- Use the base ISO
- Start with a high-quality lens and stop down 2, 3 stops. A 50mm/f1.4 wide open might not be able to out-resolve the D800E, but when you stop down to f5.6, things can change
- Always shoot from a tripod. Use a cable release, or in my case, I prefer the 1-second shutter delay option so that I don't need to carry another item that occupies the 10-pin connection.
In other words, if you are a landscape photographer or always shoot from inside a studio, the D800 can be a great camera for you. If you are a wedding photographer, 36MP can be useful when you shoot formal portraits inside a studio, but if you shoot PJ style during the ceremony and reception, at least I would prefer 12MP from the D3S or 16MP from the D4 to give me good ISO 3200 results inside a dim church where no flash is allowed. Even for weddings, I prefer to have faster than 4 frames/sec in some situations.
That is partly why I wish this mega-pixel race would have ended sooner. I could buy a D800 just for certain applications, but for a more practical, general-purpose DSLR, I would much rather have 16MP with an ISO range to 12800. The D700 has served me well for a few years. For still-image capture, I need a true successor to the D700 with the kind of improvements you can get from the D3S and D4, not a big jump to 36MP.