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It might look out of focus, but it isn't. The centre of the frame is in sharp focus and as the shot was taken parallel to the building it should also be in focus, I know that it is because I have the whole image to look at. What this is showing is the loss of resolution at the extreme edges of the frame, albeit in a rather false situation, where the aperture is wide open. If you compare the performance of this lens on a crop frame camera it is very much better than on a full frame one which is why the OP believes the L lenses out perform the APS-C ones.
It should be bourn in mind though that this lens being one of Canons top flight L series should perform better than this, you may not be too bothered about 100% crops, but there are others who are, and it's important for me to know what parts of a lens are not really useable. What this goes to show is that in the real world the findings of Photozone are in fact correct.
"The problem is, again, the 17mm setting. The center and even the border performance is very high here but there isn't really anything left deserving the word "resolution" in the extreme corners at f/4 and f/5.6. If you require at least a decent corner resolution you should stick to f/11 at this focal length. "
"it seems as if Canon has overstretched things a little with respect to the 17mm setting. The lens is able to deliver a very high resolution for most of the image field but the corner performance is poor thus spoiling the game here - this is like cappuccino without the cream for a ultra-wide lens where corner to corner sharpness is simply more critical than for a tele lens for instance. High vignetting and heavy distortions on top don't make things any better here. "
Please note that all lenses do show a loss in resolving power when used on a full frame camera, and the higher the MP count the more noticeable it becomes. This particular lens is not unique here, but does provide a good example. There is also a test on a crop frame body where it performs very much better, it's worth reading the two to gain an understanding of the fall in performance at the edges when used FF
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