I'll digg up some aperture bracketing images to demonstrate the principle at close to infinity focus.
As promised, an aperture bracketing series (crops at 100% zoom) taken with a TS-E 90mm f/2.8 lens, on the same camera body as the macro series:
The result is consistent with what was more clearly shown with the macro. There are unfortunately some fluctuations due to changing light, but I suggest to focus on low contrast detail, like the thatched roof, or even the bricks, or one of the ropes that is actually a chain. The moment you go from f/8 to f/11 into diffraction affected territory for this sensel pitch, the loss is visible. Mind you, I wouldn't hesitate to use f/11, or even reluctantly use f/16 if the situation calls for it, but I'll also know that subsequent deconvolution sharpening is needed to recover some of the losses.
The images were not sharpened beyond the Capture One Raw conversion process with all sharpening turned off. Capture One is the Raw converter that extracts the most detail from the 1Ds3 files.
The important thing to realise is that there is more to diffraction than a loss of sharpness. The MTF will suffer, and that will start to affect the overall cripness of the image in addition to things near the limiting resolution. Of course the MTF is the result of all (optical) components in the chain, so individual lenses may react a bit differently on stopping down.
Of course there is more to an image than its technical quality alone. I'm just trying to make sure that the implications of our choices are know, so we can make informed choices when we
can choose different settings. Again, the technical quality mostly plays a role with significant enlargements when we want the best that our gear can provide. That also includes the use of tripods if possible.
Bart