Special effects are traditionally divided into the categories of optical effects and mechanical effects. With the emergence of digital film-making tools a greater distinction between special effects and visual effects has been recognized, with "visual effects" referring to digital post-production and "special effects" referring to on-set mechanical effects and in-camera optical effects.
Optical effects (also called photographic effects), are techniques in which images or film frames are created photographically, either "in-camera" using multiple exposure, mattes, or the Schüfftan process, or in post-production processes using an optical printer. An optical effect might be used to place actors or sets against a different background.
Mechanical effects (also called practical or physical effects), are usually accomplished during the live-action shooting. This includes the use of mechanized props, scenery, scale models, pyrotechnics and Atmospheric Effects: creating physical wind, rain, fog, snow, clouds etc. Making a car appear to drive by itself, or blowing up a building are examples of mechanical effects. Mechanical effects are often incorporated into set design and makeup. For example, a set may be built with break-away doors or walls to enhance a fight scene, or prosthetic makeup can be used to make an actor look like a monster.
Since the 1990s, computer generated imagery (CGI) has come to the forefront of special effects technologies. CGI gives film-makers greater control, and allows many effects to be accomplished more safely and convincingly – and even, as technology marches on, at lower costs. As a result, many optical and mechanical effects techniques have been superseded by CGI.