I'm going to bet against a problem lens. I have the 24-70mm f/2.8 L and it can be quite sharp and clean. However, I also use a 70-200MM F/2.8 L IS USM and I forget how much shake the IS is fixing for me. I'll be shooting along at 1/60 or even 1/30 and the IS will keep things looking good, then I swap lens and BANG, my pictures suck. I'm just not that good below 1/100 off hand.


Iwould suggest a tripod test with stellar lights and about 1/1000. If the sharpness comes back, blame camerashake. If it is still soft, start looking at the lens and body.