Forced Perspective (Photography Technique)

Holding up the Leaning Tower

Forced perspective is a technique rather than an accident. A photographer or film-maker places objects at very different distances from the camera so that, from one carefully chosen viewpoint, they seem to be side by side and directly comparable in size. Someone far in the background appears to stand in a friend's palm; a model building near the lens looks like a skyscraper in the distance. Move the camera slightly and the effect collapses, which is why forced perspective shots need a fixed camera and careful rehearsal.

Why the brain accepts it

The technique relies on size constancy. As an object moves away, its image on your retina shrinks, and the visual system compensates using distance cues, so people walking away don't seem to shrink (Holway & Boring, 1941). That compensation only works if the distance is judged correctly. Forced perspective controls the distance cues in the frame, hiding the ground between the two objects, matching their focus and lighting, and lining up their edges, so that objects at very different distances look as if they're at the same distance. With no reliable depth information, the visual system assumes they are, and reads their different image sizes as different real sizes. Richard Gregory argued that many size illusions come from this kind of constancy scaling applied in the wrong place (Gregory, 1963).

A photographed Ponzo illusion

The Ponzo illusion makes one of two identical bars look bigger because converging lines suggest it's further away. Forced perspective often uses real converging lines, such as a road, a corridor or a row of trees, in the same way, and places objects along them so that their true sizes and distances cancel out. One works on paper, the other on a whole staged space, but the trick underneath is the same.

In film

Film-makers have used forced perspective since the early days of cinema, building sets with shrinking doorways and furniture to make rooms look deeper, or placing actors at different distances along a carefully matched line of sight to make one look much smaller than another. The Lord of the Rings films famously used it to make hobbits look half the size of the humans and wizards beside them, combining it with scale doubles and digital effects.

Where else it turns up

Forced perspective is a staple of holiday photos: holding up the Leaning Tower of Pisa, or pinching a distant friend's head between finger and thumb, needs nothing more than careful positioning. Architects and garden designers have used a related trick for centuries, narrowing a path or shrinking features towards the far end to make a space look longer. The Ames room turns the idea around: instead of placing objects carefully for a camera, it builds a whole distorted room around one fixed viewpoint.