Canon's L lenses are no good. Locked

They give excellent resolution as long as your technique is perfect, but as soon as it slips, you don't get good results. A tiny bit of camera shake or misfocus, or diffraction and they are rubbish. So, if you buy L lenses, accept the fact that you have to use a tripod, shoot static subjects only, and use wafer thin DOF. In fact, for the vast majority of photographic situations an 18-55 kit zoom is as good, if not better. Shame on Canon for producing such useless products as the L lens range.
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Bob