I find that my Nikon D7000 is not producing sharp images with a Sigma 28-105mm f/3.8-5.6 lens. It is very soft from f/3.8 - f/11, but from f/13- f/16 it is sharp. But I cannot shoot with these settings in all situations. Would really appreciate any advice or suggestions.
I used to have this kind of problem. Previously I owned Nikon D5000 and it worked superbly with Sigma 17-50 OS. Later, I used the same lens with a Nikon D7100, and most of the time it was out of focus and blurred images resulted. I raised the issue to Sigma and they asked to bring the camera and lens along. They concluded that the lens needed to be re-chipped and calibrated. Now, I have a very good camera and superb lens.