I did some test shots yesterday with my 150mm lens wide open. Examining the negatives with a loupe shows that either the lens is simply not very sharp at F4, or the focus was off. The focus point was about 6-7 feet away. I took some later shots at F32 and the object is very focused. I'm using a split-prism screen, using the focusing magnifier on the waist level finder. Is this just not accurate enough for such precise focusing, is the focus off? Is the lens not sharp? Is there a way to adjust the focus?