Consider a photographer with an SLR camera and a PC lens attached in its default, unshifted position, standing on the ground at the base of a columned building whose height is X times that of the person and is located Y feet away from him.
The photographer composes a frame with the center of the building in the center of the frame with some empty space above it, to do so they have to pitch the camera slightly upwards which creates a keystone effect with the columns appearing to converge vertically towards a point above the frame.
Next the shooter
maintains the camera orientation levels the camera and adjusts the shift of the PC lens until the columns appear parallel in the image.
Is there any change in focus at the bottom of the columns and the focus at the top of the columns before and after the shift ?