Although cinema creates a very strong impression of reality by reproducing movement, two main characteristics distinguish the cinematographic image from the images of the world acquired through natural perception:
1) the image is delimited by a frame (cf. Session 1)
2) the image is flat, two dimensional, even if it gives some impression of depth.
Representing depth in the filmic image relies on two crucial elements: perspective and depth of field.
Perspective is a geometric transformation whereby a three-dimensional space is represented on a flat surface, thus making it possible to represent depth.
There are different types of perspective. The movie camera, like the photo camera, produces images constructed according to the rules of perspectiva artificialis.
This perspective can be emphasised by the framing and mise-en-scène. The vanishing point thus constitutes a strategic position in the composition.
The choice of camera lens modifies the representation of the filmed space, particularly its depth.
The onscreen space covered by the camera is wide when a short-focal-length lens is used and narrow with a long-focal-length lens.
The distances between the characters or objects staged in depth seem larger with a short focal length lens, and smaller with a long focal length lens.
Depth of field is the portion of the film image that is clearly in focus.
Depth of field depends on several factors:
- the shorter the focal length, the greater the depth of field,
- the narrower the diaphragm, the greater the depth of field.
The depth of field plays an important aesthetic and expressive role.