I'm sure this is a really ignorant question, but it's something I've never understood. Suppose I set up a scene with the camera framing an object at, let's say, 50mm focal. If I now switch from 4x3 picture proportions to 3x4 (i.e. landscape to portrait) the object seems to leap away from the camera as if the focal length had been changed. This doesn't happen with a real camera if I turn it sideways. What is the explanation?