I was browsing through Zuiko lenses at Cyberphoto, one of Swedens largest camera-retailers. I noticed that they have tested and graded some of the lenses on the site, but to my amazement (and horror), they claim that all the zuiko-lenses loose sharpness at high apertures due to the size of the 4/3 format:
https://www.cyberphoto.se/info.php?article=EM-P5020

Here's a quote (translated from swedish) from the website:
"Use it preferably from full opening to aperture f8. At f11 the sharpness is diminished and by f22 it's significantly worse. An effect (diffraction) that's probably enhanced by a smaller image-sensor."

To illustrate this claim, they have put up some images taken at all the apertures for every lens they've tested. I have compared the images and while it seems that some of them indeed look worse at f22, the result varies between the different lenses they've tested (some actually looked better at f22 than at f11). I looked at one of the Canon-lenses they tested on the same website and I found the same "problem" there, even though they used a 35mm camera for that test...

I'm interested to read your opinions on this matter... do they (cyberphoto) have any real basis of truth to their claim, or is their test-person just biased toward Canon?