The technique of cinematography includes the knowledge of a whole range of special effects produced not only by variations from the normal procedure but by tricks of one kind or another made possible by special apparatus. Many effects are by this time so well established that, by professionals anyway, they are almost taken for granted.
The professional is fortunate in that his special effects can be produced for him in the laboratory by means of the optical printer after his negative has been normally exposed and processed.
The technique of masking can be employed in a number of subtle and often more complex ways. Instead of double exposure combined with the use of black masks, only one "mask" is used and it is not black it is a painted representation on glass of what will seem like part of the real scene, supported in front of the camera so that it will be photographed at the same time as the real scene, without double exposure.
Scale models of foreground objects are mounted relatively close in front of the lens to convey the impression of real objects at a greater distance. The expense of building the upper storeys of tall buildings is often saved by suspending elaborate scale models in front of the camera in this way.
In the Schiifftan process, a mirror set at 45 degrees is used to reflect the image of a scale model at one side while the main subject is seen through areas scraped from the reflecting surface of the mirror.
Not only may one add details to a scene to make it complete but there are devices that enable the action in a studio to be portrayed in any desired surroundings.
These systems normally employ two films-one of the background, and one for the complete action.
In principle most of these devices date from the earliest days of cinematography. They involve such things as back projection, in which the action is filmed against a screen on to which a projector throws the required background from behind; traveling mattes, whereby a complicated system of masking is used during the film printing to combine separate films as if they were one; and numerous other effects based on similar methods.