Ames Room Illusion

A giant and a dwarf in an ordinary room

Look through the peephole of an Ames room and you see an ordinary rectangular room, with two people standing in its back corners. One looks like a giant, the other like a child, and if they swap corners, they swap sizes, as if walking across the room had made one shrink and the other grow. The demo above recreates it: the two figures are exactly the same height.

The room's real shape

The room isn't rectangular at all. One back corner is much further from the peephole than the other, the back wall runs at a slant, and the floor, ceiling and windows are all shaped to compensate. The demo builds its room the way Ames rooms are designed: it starts from a normal rectangular room and slides every point along its line of sight from the peephole, further away on the left and nearer on the right. From the peephole, each point still lands exactly where it would in a normal room, so nothing looks wrong. Move the slider away from the peephole and the real shape appears, including a floor that slopes down towards the far corner.

Why the people change size, not the room

The visual system judges a room's shape from converging lines, the sizes of windows and floor tiles, and the assumption that rooms are rectangular, all learned from a lifetime of standing in ordinary rooms. From the peephole, every one of those cues says "normal rectangular room", so the brain has no reason to doubt it. It isn't being careless; it's reasoning soundly from evidence that has been rigged.

Having settled that the room is normal, the brain treats both back corners as equally far away. But they aren't: the person in the far corner really is much further off, so their image on the retina is much smaller. Normally the brain would read that as distance. Here it has already decided the distance is the same, so it reads the difference as height instead. The size constancy that usually stops people seeming to shrink as they walk away (Gregory, 1997) is turned against itself. The room's geometry is persuasive enough to override even the strong expectation that adults are roughly the same size.

Best seen in person, with one eye

The Ames room was designed as a real space to be viewed through a peephole with one eye. With two eyes, or if you move your head, the slight difference between the views gives away the true depths and weakens the effect. Photos and films get round that because a camera, like a single eye, has one fixed viewpoint, which is why film-makers have used Ames-style sets to make actors look different sizes.

Ames's demonstrations

The American scientist Adelbert Ames Jr. built the room in 1946 as one of a series of demonstrations of how much ordinary seeing depends on assumptions rather than direct measurement, an idea that, in his view, applied to all perception, not just illusions. William Ittelson and Franklin Kilpatrick described the demonstrations and the experiments based on them (Ittelson & Kilpatrick, 1951; Ittelson, 1952), and Ames rooms have since been built in science museums around the world. The Ponzo illusion and forced perspective photography exploit the same size constancy.