Motion parallax is one of those perceptual things that you notice the most when looking out the window of a fast moving vehicle, like a train. It refers to the fact that objects moving at a constant speed across the frame will appear to move a greater amount if they are nearer to an observer (or camera) than they would if they were at a great distance (parallax = change in position). This phenomenon is true whether (i) the observer/camera is moving relative to the object, or (ii) object itself that is moving relative to the observer/camera. The rationale for this effect has to do with the distance the object moves with respect to the percentage of the camera’s field of view that it moves across. This helps provide perceptual cues about difference in distance and motion, and is associated with depth perception.

Consider the example below simulating taking a photograph out of a moving vehicle. The tree that is 300m away will move 20m in a particular direction (opposite the direction of the vehicle), but only traverse 25% of the field-of-view. The closer tree, which is only 100m away will move out of the frame completely with the same 20m displacement.

Motion parallax is an attribute of perception, so it exists in real scenes, but not when one views a photograph. Can a photograph contain artifacts of motion parallax? Yes, and it is easy – just take a photograph from a moving vehicle (trains are best), using a relatively slow shutter speed. The picture below was taken on the VIA train to Montreal, using my iPhone pressed up against the glass, with the focus plane approximately in the middle of the window.