Regarding diffraction, to those who are interested in continuing the discussion

I understand Eruditass' point about diffraction always being the same physical size for any given aperture...an effect of the lens

For any given sensor of the same physical dimensions, higher resolution should be better, only revealing the diffraction that was unrealizable at a lower MP

However, given a larger sensor of lower MP, vs a smaller sensor of higher MP...does the same fact hold true?

Is the 16mp sensor of a 1DsMkII better or worse at, say, f/16, than the 18mp sensor of the 60D?

hey

My analysis of sensor pixel densities assumes all else equal and is not meant to compare sensors of different sizes and technologies. I was careful in not saying an 18mp sensor will always be better than a 12mp sensor. I'm only asserting that diffraction does not say that a higher resolution sensor is inherently worse. P does not imply Q.

When comparing a 1DsMkII vs 60D with regards to resolution only, it's more complicated because of equivalent framing. I think a correct model would basically take the pixel spacing of one sensor and transform it using the relationship between sensor sizes.

While one could do this, things like dynamic range, color depth, and noise performance are more important to me (and are usually not equal as my aforementioned model assumes). These qualities usually go up with a larger sensor as it will be capturing more light, but technology improves as well.

But yes, all things being equal, an 18mp camera will produce better 4x6, 20x30, 4mp, 8mp, and 12mp, etc. photos than a 12mp camera.

As pointed out by the graph in my comments with Karel in the other answer, resolution is most often limited by lens quality. In the end, resolution all of todays DSLR sensors is excellent for large prints when paired with a good lens.

Also, with regards to dynamic range, noise, etc. I'd conjecture that lower sensor pixel density mattered more when the electronics used to minimize noise on sensors were larger, years ago. Electronics has shrunk, efficiency of these circuits and components have gone up, and they can get away with higher density. Of course, when you get down to really high density, like in P&S sensors, you've probably sacrificed better electronics for pixels.