Still quite a lot of misinformation in this thread, but before addressing any of that - some thread-title relevant material quite decoupled from the current discussions.About pixel density, resolution and diffraction
Here's an image series that shows pretty graphically what jrista probably intended with this thread. Diffraction is NOT a problem. Lens sharpness is NOT a problem. And they won't be - for yet quite a long time... We have to double the number of MP's, and then double twice more before having trouble (with "better than mediocre" lenses of course...!)
Well, the Canon 400/5.6 is no slouch, but on the other hand it's no sharpness monster either. In the following image series, it was used wide open from a sturdy tripod - at 1/60s shutter speed for both cameras, on A) the 5Dmk2 and then on B) the Pentax Q.
The Q has a pixel pitch of ~1.55µm - giving an APS sensor of about 150MP!
I'll begin with explaining what the images are, and they show. both original images developed from raw with CaptureOne, sharpened individually. No noise reduction applied.
1 - 5D2, full frame scaled down.
2 - 5D2, 1:1 pixel scale, crop to about 6.2x4.55mm center of the 5D2 sensor - size as the Q sensor
3 - Pentax Q full frame, downsampled to same size
4 - Pentax Q, 1:1 pixel scale. This is like a 100% crop from a ~360MP FF camera
5 - What the 5D2 looks like when upsampled to the same presentation size.
So, at F5.6, you can see that the amount of red longitudinal CA in the 400/5.6 is a much bigger problem than diffraction - on a 150MP APS sensor.
And, in the last two images (4+5) you can clearly see that the 5Dmk2 isn't even close to scratching the surface of what the 400/5.6 is capable of. Note the difference in the feather pins, lower right of the image. Also note that the small-pixel image is a LOT less noisy than the 5D2 when scaled down to the same presentation size.
A.L., from whom I borrowed these images (with permission) is on assignment in South Korea at the moment, so I can't get at the raw files unfortunately.
I've done similar comparisons with the small Nikon 1-series and the FF D800. Same result there. The smaller pixels have a lot less noise at low ISOs, and the D800 isn't even close to resolving the same amount of detail as the smaller camera. Not even with cheap lenses like the 50/1.8 and the 85/1.8. But I thought a 5Dmk2 comparison would be more acceptable here...