Quote Originally Posted by newcan1 View Post
Understood. But the phenomenon I was referring to is this: I focus on an object using the split screen, then I move the camera slightly so I can see the same object on the ground glass, and then I find that the sharpest image of the same object on the ground glass is at a slightly different focusing point.
It will be, you moved the camera. When you tilt it up or down you are changing the spot from where you focused to somewhere else.

If however you are seeing a change in focus in your results rather than as implied on the screen, so you accurately focus one way or another, and then your neg shows the focus point is a fraction nearer or further away, you may be getting aperture related focus shift. In which case the lens is (say) calibrated for accurate focus wide open, but when you shoot the picture your lens automatically stops down and in many lenses as you stop down the focus point changes. This is something rangefinder users can get obsessed about because it is more difficult for them to know if it's their rangefinder out of adjustment or the lens has aperture related focus shift. A lot of lenses are calibrated to be accurate wide open, when DOF is most critical, and of course that is how you see things with an auto-diaphragm SLR, but focus may wander as you stop down, until you get to f/8 when DOF takes over and compensates for any focus shift. Because focus shift is a flaw in the design of the lens (given that no lens can be perfectly corrected), the only option may be to stop down after you focused and see if with the smaller aperture the focus point still looks crisp, but it can be hard or impossible to do. The other option is to know your lens inside out and compensate each time you think it may happen.

Steve