Quote Originally Posted by BobD View Post
Your camera can only focus to one particular distance at a time. Different areas of the scene may appear somewhat out of focus because they are at difference distances from the film plane than the point you've focused on.
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.