Two objects that move the same distance...

have different apparent motions. As we have seen, the field of view widens at a distance. Apparent motion is the ratio of movement distance to field of view: The greater the distance, the wider the field of view, and the smaller the apparent movement. In this figure, we see the angle of distance is greater for the green spot than the blue spot, even though their motion is the same.

Although our animated graphic shows the points in motion, it works out just the same if the observer moves and the points are fixed. As Einstein pointed out, its all relative.

But why does it matter?

Everything we have spoken about is a property of optical imaging. Any imaging system will detect the same phenomenon. But people use motion parallax as a cue to depth: when we observe a starfield that has different relative motions, our minds interpret this as a cue to depth.

But distant stars are just as bright as close ones, which leads to a conflict of cues. For this reason, many people choose flatland. Next we consider the final cue to depth as we...