Originally posted by TrueFocus First, this is just about the technical part of a camera. I am not saying anything about any brands nor am I saying that FF is the only way to go.
Sensor size doesn't affect the size of the airy disk (aperture and lens design do). That means that an APS-C sized sensor needs an aperture of f4.0 to resolve the same amount of detail as a FF sensor at f5.6.
Given our current state of technology the APS-C sized sensor will expose flaws in production and lens design more than a FF sensor. (we have limited precision)
I'm not sure if I'm reading you right.. but I don't think that's exactly accurate. Saying we have limited precision... you might want to give us some evidence something from somewhere, there certainly isn't much evidence in the lens charts comparing different sensors on IR. The difference between a 24 Mp D600 and a 24 MP K-3 is about a 100 lw/ph when looking at a total lw/ph of about 2800, so well less than a 5% difference.
The diffraction is the same with the same lens no matter what format it's used on, but a 24 Mp APS-c will have much smaller Pixels sites. As with CA, the most efficient and useful way to describe diffraction would be as a fraction of pixel width. Once your diffraction (or CA) get's over 1 pixel, you have micro-contrast problems, once it gets over 2 pixels you have image degeneration problems. The smaller APS-c pixels will start to display diffraction issues sooner because the pixels are smaller for cameras with he same number of MP. Surprisingly then, both FF and APS-c appear, for the most part start to be diffraction limited after ƒ5.6, so while the point between 5.6 and 8 where the lens becomes limited is probably different, it makes no practical difference for the shooter. At first, the distances just aren't that noticeable. I know there is a measurable difference between ƒ5.6 and ƒ8 on my Sigma 70 macro, but the change in DoF is far more noticeable than the change in sharpness.
But if you check the IR images, you see, a K-3 resolves what a D600 resolves at ƒ5.6, and in many test images the K-3 images appear to be sharper because they have more DoF. You have to look in the image for the places where the D600 images are sharp, where as often the K-3 images are sharp pretty much through out the whole image. If you're shooting for more DOF, the FF noise advantage in terms of total light etc. is lost because you have to stop down to ƒ8 and shoot at a higher ISO to achieve the same DOF. That brings in both diffraction and more noise, making the FF and APS-c images roughly equivalent.
You have to be really careful when making overall conclusions based on a very narrow set of shooting circumstances. And be especially wary of the constant assumption made by FF propagandists, that narrow DoF is the preferred method of shooting and that APS-c "must" emulate it. In the real world narrow DoF is often the problem, not the solution cancelling any FF advantage.
If trying to keep everything in focus I'm shooting ƒ16 APS-c, I'd have to shoot ƒ22 FF, and the diffraction wold rather seriously affect the FF camera, going from ƒ16 to ƒ22. The APS-c camera is probably going to produce the best image.