This following image is just 30.9 megapixels from a stitch. Shot it yesterday. Did the rest of the frames then waited for a bird to land for the final frame (there were of course millions of birds while I was shooting the other frames but had to wait 20 minutes for this one to land). I'd kill to be able to get this level of detail and tonality with a single shot. However I was using a very sharp prime lens for each frame. All the goodness of that lens was being used for every part of that final image. Even though I had to stop down to f20 for the DOF (just enough) the diffraction based on a 12 megapixel sensor wasn't that bad, especially when you had so many frames making up the final image.
If I was to take the same image with a higher megapixel camera the diffraction would be far worse although I'd not have to stop down the same amount due to using a shorter lens but all the resolving power of the lens would be used up in one go and I'm not sure that I would be getting the same resolution at all. Infact when I shot with a 1Ds3 I certainly didn't get the same resolution from a single shot at 21 megapixels as I did from stitching my 5D to the same 21 megapixel final image.
On the other hand and although I'm about to buy one for studio use I haven't done much landscape testing, a Leaf 40 megapixel back seems to come much closer in a stitch vs straight image comparison. In both cases I believe the lens is the limiting factor. When stitching you are using the full power of the lens X6 or X10 (or whatever) rather than X1. No doubt the gap would close rapidly when using a digitar or the like on a well calibrated Tech Cam.
However. Few factors. I never believed that the tonality would be better on the 1Ds3 than my 5D. Not for a second. Impossible, the pixels were tiny in comparison. Boy was I wrong. The 1Ds3 has the best tonality of any DSLR I've ever seen or played with. Including the 5DII by a large margin. Tonality counts, a lot, it counts even when you're shooting a 1962 Takumar 50mm f1.4 wide open like I'm doing at the moment
, when resolution matters *bleep* all but tonality is the name of the game. Secondly, you can stitch the higher megapixel camera with just 2 or 3 frames and close any gap that there might have been due to the lens. Lot faster, lot easier, both when shooting and processing. Thirdly and most importantly. The images below were single shots which for various reasons such as time, logistics or location, I was unable to stitch. There is a limit to what size I can print these because I only had 12.7 megapixels and with all the very best technique, lenses and stability, at some point you just run out of printability for fine art. With the higher megapixels you have the choice, no it might not be ideal but it will be far closer to what you would have got with a 1/3 or 2/3 of the megapixel count. You have the choice. I'd love a 1Ds3 for my work but a camera with over 20 more megapixels and significantly smaller and lighter, oh and cheaper than even the current 2nd hand price of a 1Ds3, certainly makes you think!
That's not even beginning to talk about studio stuff...