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Just a note about the Digital picture, it's not what I'd call a neutral site reviewing only Canon kit and some third party ones, it can be rather partisan in its approach which does occasionally fly in the face of accepted opinion.
What I was trying to illustrate is the way which an image degrades at the extreme edges regardless of the particular lens, some of them are worse than others but they all do show it.
The graphs which anglefire has posted show a centre resolution of something like 38 line pairs in the centre at f/4, but at the extreme edge this has fallen to somewhere around 18 which illustrates the problem.
The other problem is resolution, the higher it is the more noticeable the problem becomes, putting a FF lens on a crop frame camera means that the resolution will be quite high across the frame, so it won'r be as noticeable, but put the same lens on a FF and it will perform very differently, and this is the problem when you use an APS-c lens on a camera, because you have the same issue with border resolution that the FF cameras have. If that's something you don't like then stick to FF lenses where you can't see it as much.
I'll try to post some 100% crops tomorrow to show what happens. I've seen this effect on virtually every lens I've used, (some more than others) I thought the camera might have been faulty, but it's just the effect described above.
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