Perspective is described as "forced" when an object appears larger or smaller, or nearer or more distant, than its actual size or position. The technique, based upon optical illusion, has been used architecture and landscaping for centuries, and is now used in photography and film-making. It is most commonly used to create an enhanced perception of space.
Human visual perception is manipulated by careful use of scaled objects and the correlation of their apparent sizes and positions relative to the camera or viewer. Ancient Greek architects designed columns for their grand buildings that tapered as they rose from the ground towards the top to create an illusion of a greater height. A long flight of rising steps appears even longer when the steps narrow towards the top. In still-life photography, a similar technique can be used to enhance the viewer's perception of space in an image.
Forced perspective might, for example, be used to make more distant parts of a scene seem further away than they really are. This could be achieved by making them progressively smaller than they would normally be as they recede. The viewer's eye consequently concludes, given the apparent size of the objects, that they must be more distant. This increases perceived depth in the scene.
Lighting can also be used to produce forced perspective. The principle is simple. Since the intensity of light from a point source falls off with distance from the source, in accordance with the inverse square law, the eye expects distant elements of a still-life scene to be less well lit. Consequently, if a distant object is well lit and appropriately scaled it may be perceived as being closer to nearer parts of the scene. This can be achieved only by lighting more distant objects with a separate source and maintaining adequate depth of field. The intensity of light required may, in some instances, result in undesirable heating of the objects by the powerful light source.