Summary

As we move into bigger space there are several perceptual principles that come into play. Skill at illusionism depends on being able to provide stimulus on a retinal level that can be processed by the brain in the same way as reality. We need the knowledge because what we actually perceive in consciousness is so different from what we have to draw. We work at a pre-conscious level.

As the distance increases the size on the retina gets smaller, but this is immediately processed into distance, keeping the internal sense of size of the object constant. The lamppost in the distance is the same size as the lamppost nearby. But this makes it hard to draw the size change because the consciously known and accurate information interferes with seeing what happens on the retina. The camera obscura showed just how small the retinal image of something was as the distance increased, and artists, ever-fascinated by optical instruments would sometimes take advantage of them. Vermeer is said to have used a version of the camera obscura to break down the complicated interior spaces in his compositions.

Your tool for creating the illusion of space is your knowledge of what has to happen. The overarching rule in this case is that everything diminishes with increasing distance. Size gets smaller. Brightness gets dimmer. Contrast diminishes. The intensity of color diminishes. The amount of detail diminishes. In the principle we tackle now, space between parallel lines moving away from you in space also diminishes. Just like the tonal change keeps pace with the angle of the plane in relation to our eye, how fast the lines converge is processed into the angle of the plane.