The illusions used in the film, television, theater, or entertainment industries to simulate the imagined events in a story are traditionally called special effects (a.k.a. SFX, SPFX, or simply FX).

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, 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 As a result, many optical and mechanical effects techniques have been superseded by CGI.

Early development

In 1857, Oscar Gustave Rejlander created the world's first "trick photograph" by combining different regions of 32 other photographs into a single image. In 1895, Alfred Clark created what is commonly accepted as the first-ever special effect on film. While filming a reenactment of the beheading of Mary, Queen of Scots, Clarke instructed an actor to step up to the block in Mary's costume. As the executioner brought the axe above his head, Clarke stopped the camera, had all of the actors freeze, and had the person playing Mary step off the set. He placed a Mary dummy in the actor's place, restarted filming, and allowed the executioner to bring the axe down, severing the dummy's head. “Such… techniques would remain at the heart of special effects production for the next century” (Rickitt, 10).

This was not only the first use of trickery in the cinema, it was the first type of photographic trickery that could only be done in a motion picture, i.e. the "stop trick".