Sorry to extend the conversation - but lets try. But try as we might, it depends on your standards, what you are shooting, and how you wish to judge. Some personal observation on this : I've been given the opportunity to try a Leica Monochrome. It has an 18mp CCD sensor, which is generally understood to be very much at the top of the smaller cameras, equal more or less to the top DSLRs (see Sean Reid's reviews for more on this). Its CCD sensor is used to only give BW information. I have not compared it to a D800 tho, but rather used it as a small camera and tried shooting very demanding architectural and landscape scenes. The local Leica dealer calls these scenes torture tests, so the gear gets a workout. The lenses used were the 21 SE, the 28 2.8 Asph, and a 50 'cron.
Up to about 13x19 inches, the MM results looks as good as MFDB (Leaf 33 mp). Everything is there and super crisp. The standard used is looking at a print for absolute detail at about 10" (25 cm) viewing distance. The prints are gorgeous.
When taken up to 17x22, things starts to change. Here's where its tricky: for the MM, if the lens is a really good one, the shooting technique is good, and full frame, the MM holds up reasonably well. If any of those factors slip, well, you can see it. If all is good, then the results are good, very good, but… you can see a difference between the MM and MFDB at this size. At first, its not so apparent, but put the prints side by side, and one is very very nice pops and the other pops. When shooting landscape (leaves, very fine texture), for example, the difference is there and it is visible, not just to me, but to others. The MFDB simply renders at 17x22 with more detail. However, if the subject is more dramatic, then the difference between the two is less obvious - that is, it is still there, but it is not so apparent. So for me, 17x22 is the upper limit for the smaller camera to make really really, blow you away, crisp results. Again, this is with prints viewed at very close distances. If you made bigger prints, the viewing distance should increase as well, and its likely larger prints would still captivate, as many others have reported.
These results were generally confirmed in observation with friends. I think if you could see the prints you'd agree. Whether you think its a critical difference - well, now we are back to art….However, these results don't necessarily match what others have reported, who say they take MM prints up to larger sizes with great success. I believe the print is the ultimate way to judge, and haven't seen their work in person. Its possible they get better results. Perhaps their trade craft is better, or have better up-rezzing techniques (which I don't use). I use only modest sharpening (LR for the MM, C1 for the Leaf). Maybe C1 is better for the MM, I don't know.
These are my observations as well. I call 13x19 "A3" and 17x22 "A2" and my experience with an A900 is that the resolution is sufficient for A2, but that I am limited in my choices of lenses and apertures if I want lens defects to be negligible. The A900 has roughly the same resolution as your Leica.
I'll take the example of landscape pictures. Basically, a camera of about 20-25 mpix
can produce perfect A3 prints (13x19) and very nice A2 prints (17x22). For the next size up, A1 (which is twice as big… 22x34), I suppose I would need double the resolution.
The emphasis is on "can". And this is the main difference with medium format: with the A900 or the D800, I am very limited in my choices of lenses and apertures if I want to print that big. I need to play a bit with various software pieces to correct lenses defects (e.g. chromatic aberrations), get the sharpening right, etc… I need a heavy tripod, because the focal plane shutter induces vibrations at certain speeds. I need to calibrate AF precisely. etc…
With the H3D-50, I take any lens, any aperture, a light tripod and shoot. I enter the files into Phocus and only need to adjust the sharpness slider to taste, everything else is more or less automatic. Even the AF, with its single point, is well calibrated most of the times (it is still an occasional problem, but I am planning a new laptop to be able to shoot tethered). Not that I am saying that the H is perfect or better than any competition. I suppose that Phase One allows the same workflow, and I am quite aware that the camera is big, heavy, slow and more dependent on light quality.
The difference is not that one cannot produce a very nice picture with a 24x36 camera. The difference is that, with the highest resolution 24x36 camera, you will need to select your lenses amongst a very limited choice of, maybe, 4 to 8, and when everything works perfectly, you may be able, with the help of various software to produce a landscape picture which is sharp corner to corner when printed to A2. With the H3D-50, it is much easier and you get enough pixels to print to A1. And this is on a single criteria: sharpness. With the H, I also get cleaner bokeh, more pleasant skin tones and flash sync at any speed.
I also find the camera a joy to use, but that is just me…