It's just the matter of diffraction.
In perfect, diffraction-limited lens, using ideal values for the Rayleigh limit, and applying the contrast theory of Kühler, resolution for the average wavelength of light (0,555 micron) is:
1,4 550 lp/mm
2,0 385 lp/mm
2,8 263 lp/mm
4,0 185 lp/mm
5,6 135 lp/mm
8,0 94 lp/mm
11 69 lp/mm
16 48 lp/mm
22 30 lp/mm
32 21 lp/mm
So theoretically, in case of 1Ds3 sensor (which resolution is 78 lp/mm), diffraction should affect image quality for apretures smaller than f8.
Just for anyone unaware of this topic, you can read about it here:
Given the above quote, the "practical" diffraction is dependent upon print size or monitor size and distance viewed from image. So you could use f16 and get an image where your eyes cannot see the diffraction, such as shooting a 1ds3 at f16 and printing it in a magazine at 4x6 inches (or whatever size cancels the eye perceiving the diffraction). From my experience, and from reading, the sweet spot is virtually always between around 5.6 and 11, and most full frame cameras, and maybe even MF (If I recall correctly) is around f8--please correct if wrong. It's just what I remember and what I go by.
Since most people here are interested in landscapes, this would mean figuring out from the above link what your camera's best aperture is, and using it when you can, where "when you can" is the operative phrase. For instance, if you're aiming at the ground 3 feet in front of you and you want the ground and background mountains crisp, you won't be using f8 at 30mm (It may not happen whatever you do, but you get my point).
Diffraction simply means, for practical application, that you will be getting a softer image, all things being equal, which they are not.
The reason I started this thread is because I am interested in what people had to say about diffraction issues and the comparison between the 20D, 5D, and 1DS MKIII. The reason I am interested in this topic is because I do a little product photography, and the higher the f stop the better when rendering an entire product in focus. If the above is true, then I'm wondering if the 5D would be a better camera for product photography than the 1DS3 in the studio, where DoF and sharpness are most important, in respect to print size and detail quality?
I just wanted to be clear about what I was thinking on this topic and what my interest is for posting it.