are you using the Standard or Physical renderer? It looks like standard or the sampling quality is up a bunch in physical but either way you should not get grey blur in the DOF unless there is something else going on. What version of C4D? Can you post the scene file?
Are you applying the DOF effect in post? If so the problem is that Cinema's depth pass is aliased, which confuses DOF plugins. The solution is to use the Position Pass instead of the depth pass. I did a video on this a while back: