Sorry, but it is not actually a fact. A larger format simply has larger pixels spread over a greater area, that's all. Assuming you frame the same subject the same way, with the same physical aperture, the 35mm sensor would perform the same as the MFD sensor. It's called etendue. The MFD sensor might have a potentially higher maximum saturation point (FWC), however what actually matters is how much light actually reaches the sensor. With the same physical aperture, both the 35mm and MFD sensors would gather the same amount of light...since it is the lens that captures light, it's front element doing the gathering and the aperture controls how much of what was gathered actually reaches the sensor.
Put the same number of pixels on subject (i.e. fill the frame and compose the same), and for the same aperture and exposure time, S/N of the resulting image will be identical. The only difference would be ISO, which doesn't do anything other than change what number of electrons constitute "maximum saturation". You would need to use a higher ISO in the MFD than with the 35mm sensor, but both would produce identical pictures with the same noise characteristics. The MFD lens would have to bend light more, so you'll have a greater problem with optical aberrations, which ironically would actually give the 35mm setup the advantage.
Please, read this article before you continue insisting that MFD is just plain and simply better without question: http://www.clarkvision.com/articles/telephoto.system.performance/index.html
Roger Clark argues that the 7D is actually better from a "gathering detail at the same S/N" standpoint than a 35mm FF camera, but the same argument holds true regarding FF and MFD, as the two have about the same difference in terms of ratio of area as an APS-C and FF do.
Thats all fine an dandy but your excluding what make's Medium format what it is.
Its much larger than 35mm, thus has more area to stuff more pixels than 35mm!
That fact alone means that any thing 35mm can resolve can be done better on MF. This fact hasn't changed and will never change from film to digital.
Your changing your argument now, though. Which is fine, and yes, if you stuffed more pixels into the MFD sensor and framed your subjects the same, the MFD would put more pixels on subject, and thus have more detail overall. But that wasn't the original argument...your original argument was that no matter what, MFD is and will always be better. That is not necessarily true. There are a few ways where 35mm can outperform MFD, especially with a sensor like Exmor.
Thats Right, MF will always be better than 35mm for resolving detail. Even if the MP counts are the same, MF will still be better.
IE: If you have a 12mp FF sensor, That would be around a 30mp Medium format sensor. Be fair with the pixel density. There is no replacement for displacement. There is no replacement for a larger sensor.
You are still assuming everything else is equal, such as subject size in frame.
A simple example here would be in a reach-limited scenario. If you are physically incapable of getting closer to your subject, and cannot user a longer lens, then a sensor with smaller pixels is the solution. MFD sensors are large, and have large pixels. Increasingly, their pixels are 2x the size of a FF sensor, and even larger than the pixels found in APS-C sensors. That means you can't put as many pixels on subject with the MFD, so either a FF or an APS-C DSLR sensor is going to be better...it'll put more pixels on subject, and assuming you use a lens with a similar entrance pupil, you'll achieve the same S/N. Focal length doesn't even necessarily matter in this scenario....entrance pupil size, exposure time, and # of pixels on subject are all that really matter to normalize S/N across sensors regardless of sensor dimensions or pixel pitch.
So your assertion that MFD is and always will be better
is ignoring some of the facts. I'm trying to point out that you CAN get results just as good, if not better, with a physically smaller sensor with physically smaller pixels. The reason for this is Etendue. Roger Clark clearly, mathematically and visually, demonstrates how and why the 7D can and is better in many situations than a 1D IV (APS-H) or 5D II (FF). Just because a sensor is 44x33mm in size doesn't mean it is magically excluded from the facts.
Your contorting your argument to make it sound as though MFD sensors always have more pixels, which is not actually the case. Hassy has a 30mp MFD, which is actually FEWER pixels than the D800's 36.3mp. In just about every scenario, the D800 will outperform the 30mp Hassy MFD, and not necessarily because of the improved DR. Because it is easier to bet pixels on subject, and because there are lenses that exist for DSLR that have immense physical apertures (such as a 300mm f/2.8 or 500mm f/4). The best thing Hasselblad has to offer in their H-system is a 300mm f/4.5. Lets even assume you use a H4D-50 (btw, H4D-200ms is still actually a 50mp sensor, it just integrates their "multi-shot" technology...you could achieve the same results with a pano rig or a Tilt/Shift lens and a normal DSLR) with the 300mm f/4.5, and compare it to a D800 with a 300mm f/2.8. The hassy has 6 micron pixels, where as the D800 has 4.6 micron pixels. If you shoot the same scene from the same distance, the D800 is going to take the upper hand here every time (even ignoring its better DR). Smaller pixels, larger physical aperture, more light on the sensor, higher S/N for that particular exposure for a greater number of pixels on subject. If you stop the D800 down to f/4.5, it will still win out because it puts more pixels on subject, even though the two exposures will have the same S/N.
I'm not saying medium-format cameras are not awesome cameras. They most certainly are, and when used for the things they were designed to be used for, such as studio photography where there is nothing to limit you from putting as many pixels on subject as possible (fill the frame), they can and will produce stellar IQ. Assuming you use a MFD with more pixels than a comparable DSLR (i.e. you use a 40, 50, 60, or 80 megapixel MFD), and fill the frame, then yes, MFD will also outperform 35mm. On the flip side, you need to acknowledge that there are situations where 35mm FF or even APS-C DSLR's are the FAR better tool for the job, and they can produce better IQ
for those kinds of work than a medium-format digital camera could ever hope to aspire to.