[KeithT]

Why shoot 5.6 with your lens? You said you were after pin sharp images and 5.6 being your optimum setting will deliver this more consistently than any of the other settings. If you shoot wide open you need to be aware of narrow dof and how easy it is to get images out of focus at the point of focus unless you are well practiced at doing it. The point of having fast lenses is to give you a brighter viewfinder in dark conditions. You can shoot at whatever aperture you want, but beyond 5.6 diffraction will begin to slowly appear, so not always best for pin sharp images. My suggestions would be to use centre point focus so that your subject is always pin sharp. Let the rest of the frame take care of itself.