I need at least F3.2 and onward to get a good sharp photo, F4 is very sharp, couple of challenges: 1. AF is always not prefer for my DSLR camera, 2. I mainly use the 50mm to take object within 2.5 meters, so the DOF will be short and easily out of focus, 3. my object is always moving, 4. 100% no flash, only natural lighing in door. 5. ISO at max is 400, prefer 100 or 200 if allowed, I think if you allow to flash and longer distance to your object, the F2.8 or even F2 should be sharp.