One has to be careful about defining diffraction limits on resolution based on mathematical calculations of Airy disc size in relation to pixel size.
I agree , it is only PART of the story of good image making, and as with all things, one needs to use these various technichal limitations with a serious dose of common sense.
In practice, a camera such as the 50D will continue to provide more resolution than the 40D all the way down to F16, when images are compared at 200%, representing a 6ftx9ft print. However, at F22 the increase in detail is so small, one could consider it as non-existent.
Having played with this reasonably extensivley lately, I can't actually agree with you Ray. I can see noticable difference in large prints betwen an F10 shot and an F16 or an F22 Shot. The F10 ones contain far more micro detail than the F22 ones. (For reference, I'm shooting real world things, not test charts. My favourite candidate is the Opera House, with its tiny tiles. at F10 they are just visible, at F16 they are mushy, and F22 all gone). Like anything though YMMV. (And I'm not using either a 40D or a 50D, but a 1dsIII so yes, I'm not comparing the same thing).
The fact is, diffraction is a property of the lens, not the sensor.
Diffraction is a physics property of Light encountering an obstacle such as a small hole (the Apeture in this case). It only has passing association with the Lens in the fact that the apeture in inside the lens. It dosn't have anything to do with quality of the glass etc. HOWEVER that is ONLY Diffraction
I'm referring to. Captured resolution definatly has a lot to do with the Glass, and only in the 'sharpest' lenses does all this matter anyway, as other properties in the Lens cause loss of resolution and sharpness. (In this case, I belive Refraction is the biggest culprit, along with , Dispersion, Reflection(Flaring), etc. All of which then cause loss of sharpness, loss of effective resolution, and loss of contrast)
It is only related to the sensor in that, the sensor defines an absolutle limit on capture resolution, when the size of the effect of the diffraction is greater than the size of the capture medium's resolution, then detail will be lost due to diffraction. (i.e. the effect on our photo's is a combination of the two). This is one of the reason bigger pixels (and bigger sensors for those pixels) = good.
I guess the reason we all talk about Diffraction so much is it is the only one we as the shooter can control in the process of taking a shot. Refraction, Dispersion, Reflection etc are soley in the hands of the lens manufacturers, and we can only control this by our purchases of different lenses (Prime vs Zoom, good quality vs poor etc)
The recorded image is always something less than the lens can provide. When a good prime lens is used at its sharpest aperture, say F5.6, the recorded detail is likely to be significantly less than what the lens actually transmits, at least in the centre of the image. As one stops down, all lenses lose contrast in fine detail to the point where the detail becomes too faint to be recorded. At this point there is no advantage to the sensor with the higher pixel count.
True. And all this dosn't matter for many smaller prints when you head into issues of viewing distance, etc etc. Just to be clear I'm refering to maximising the Captured resolution, and not reducing that in the field. It is also important to realise, that (IMO) even when a shot is diffraction limited, more pixels DO help. for exampe they provide a greater 'blend' of tones between one value and the next. Just the amount they help is reduced.
Huge caveat with all of this. It is in a way, a significant form of 'pixel peeping'. My Personal intent when finding out and testing it, was to ensure I was maximising my CAPTURE resolution, having just spent a small fortune on the 21MP camera. This came about when I was not happy with the initial results over my 5D or Velvia on the 4x5. (shots at f22 for example).
Having now changed my shooting to accomodate these factors, I find the technical
quality of the shots to be significantly better.
Asthetics is a whole 'nother ball game entirely
Merry Christmass and a Happy new Year to All.