Special effects in cinema can be divided into physical and optical effects (in the industry often referred to as "effects" and "special effects," respectively), the former done in front of the camera, the latter after the negative has been exposed. Unfortunately, this neat distinction breaks down over some optical effects that are produced by double exposures of the film strip or rear projection during shooting, and increasingly in the use of physical ("practical") elements as resources in digital postproduction. Effects are most commonly associated with creating images of scenes, events, and characters that do not exist in the real world or that cannot be photographed, but they are also used for economic reasons. Cost is both a stimulus to and a major constraint on the use of special effects. Closely related to the cost factor are time constraints, and increasingly the physical capacity of computer processors. Many effects techniques have been designed expressly to increase the temporal and computing efficiency of complex sequences. Despite much recent press criticism of Hollywood blockbuster films, it is relatively rare for a film to be promoted exclusively for its special effects; nevertheless, many films depend on effects for their appeal.
The crucial qualities sought by most effects professionals are believability and innovation: the phrases "special effects" and "cutting edge" are difficult to disassociate, providing the profession with its greatest single challenge. At the same time, while taking pride in their craft, effects professionals commonly refer to the subordination of special effects to the narrative demands of the project, and are particularly sensitive to the possibilities of creating creatures, objects, and locations with distinctive personalities.