Puppet-theater effect
If people and objects look smaller than they are, as if they were inside a scale model or on a puppet stage, this is called the puppet-theater effect. Woods, Docherty and Koch (1993) associate this effect with the combination of a wide interaxial and a long lens.
Geometric explanation
Two scales can be compared in perceived space:
- Height scale: the ratio of an object's perceived height to its real height.
- Depth scale: the ratio of the perceived distance between two objects to the real distance.
In orthostereoscopic display the two are equal and the scene appears in its natural proportions. If the depth scale is larger than the height scale, the objects remain small relative to the space between them; the scene is perceived like a puppet stage. If it is smaller, the opposite happens: the layers move closer together and a flattening similar to cardboarding occurs.
Examples
At a ratio of 1 the scene appears in its natural proportions; above 1 the objects remain small relative to the depth between them (scale model); below 1 the layers move closer together (flattening).
| Shot | Depth scale / height scale | Result |
|---|---|---|
| 35 mm, b 65 mm, person at 3.5 m | ×0.63 | layers move closer, flattening |
| 85 mm, b 300 mm, person at 15 m | ×0.83 | close to natural, slight flattening |
| 85 mm, b 500 mm, person at 15 m | ×1.79 | objects remain small, scale-model effect |
| 200 mm, b 1000 mm, person at 40 m | ×3.11 | pronounced puppet-theater effect |
When a distant object is shot with a long lens, the interaxial is increased to recover depth; as the interaxial grows, the depth between objects grows faster than the size of the objects.
Limits
This ratio is the geometric part of the effect. Size perception also relies on other cues such as familiar size, perspective and texture. In hyperstereo shots, where the interaxial is much larger than the interocular distance, the effect is also used deliberately, to make a landscape look like a scale model.