Quote Originally Posted by Benjamin
why does the upcoming camera that replaces the current 1D III better to remain in APS-H?
We had a very similar discussion a few weeks ago:

APS-H vs FF, which makes more sense to be on the 1D Mark IV?

Quote Originally Posted by peety3
The 1D Mark IV, to me, doesn't make sense as a 16mp FF camera...The 60D, to me, doesn't make sense at APS-H.
Agreed on both counts. I would be very surprised if Canon released a low-end 1.3X body or stopped releasing 10FPS 1.3X bodies. Things that would be nice, though: a high-end, high-speed 1.6X body and a 10FPS 1.0X body. I think the idea of using sensor size to differentiate market segments is too ingrained into the Canon marketing M.O., and even Nikon's switch will not be enough to get them to change.

Quote Originally Posted by peety3
The 1D Mark IV, to me, doesn't make sense as a 16mp FF camera. Although the pixel density matches its predecessor, leading to great noise performance and diffraction threshold
For the benefit of the reader, I would like to point out that peety3 and I are in a state of disagreement on this matter. I think the idea that larger pixels have superior noise performance and suffer less negative effects from diffraction is a common misconception caused by erroneous image analysis. I detailed the position in the following thread with experimental evidence:

Myth busted: smaller pixels have more noise, less dynamic range, worse diffraction, etc.

However, I don't think I ever actually discussed diffraction in that thread. (I meant to, as indicated in the title, but I must have forgot.) So here's a quick description: There are many things that can affect the resolution of an image, including diffraction, aberrations, motion blur (from camera shake or subject movement), and mechanical issues such as collimation, back focus, tilt, and manufacturing tolerances.

In the face of these issues, some will claim that small pixels are actually worse than large pixels. This is easily proven false. The reality is that all of these factors may cause diminishing returns, but returns never diminish below 0%.

The most frequently misunderstood factor in diminishing returns is diffraction. As pixel size decreases there are two points: one at which diffraction is just barely beginning to noticably diminish returns (from 100% of the expected improvement, to, say, 90%); and another where the resolution improvement is so small that it's immeasurable (0-1%). One common mistake is to think both are the same point, but in reality they are often very far apart.

Another diffraction-related mistake is to think that diffraction will ever cause a small pixel sensor to have lower performance. In fact, the worst that can ever happen is for smaller pixels to have a 0% improvement. That is, for performance to be the same.

For example, anyone shooting 5 micron pixels at f/32 because they really need DOF (e.g. macro) is not going to get any benefit from smaller pixels: the returns will be close to 0%. At f/11, the returns will be diminished slightly, but an improvement can still be had from smaller pixels.

Lens aberrations can be an issue too. Usually even the cheapest lenses will have pretty good performance in the center, stopped down. But their corners wide open will sometimes not benefit very much from smaller pixels, so the returns in those mushy corners may be 0-5% due to aberrations.

And there's the mechanical issues. If the collimation is not perfect, but it's good enough for large pixels, then it will have to be better to get the full return of even smaller pixels. This relates to manufacturing tolerances of everything in the image chain: the higher the resolution, the more difficult it is to get full return from that additional resolution. Even things like tripods have to be more steady to prevent diminishing returns.

So essentially the diminishing returns depend on the circumstances, but the higher the resolution, the more often the returns will be diminished.