Optical printer

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An optical printer is a device consisting of one or more film projectors mechanically linked to a movie camera. It allows filmmakers to re-photograph one or more strips of film. The optical printer is used for making special effects for motion pictures, or for copying and restoring old film material.
Common optical effects include fade outs and fade ins, dissolves, slow motion, fast motion, and matte work. More complicated work can involve dozens of elements, all combined into a single scene.

Hystory

The first, simple optical printers were constructed early in the 1920s. Linwood G. Dunn expanded the concept in the 1930s, and the development continued well into the 1980s, when the printers were controlled with minicomputers. Prime examples of optical printing work include the matte work in 2001: A Space Odyssey and Star Wars.
In the late 1980s, digital compositing began to supplant optical effects. By the mid-nineties, computer graphics have evolved to rival and surpass what was possible with optical printers, and optical printing has all but gone. Improvements in film scanners and recorders allow for a complete feature film to be ingested into computers, have any effects applied to it, and output back to film.
Today, optical printing is mostly used as an artistic tool by experimental filmmakers, or for education purposes. As a technique, it proves particularly useful for making copies of hand painted or physically manipulated film.

Artifacts

As in any analog process, every optical "pass" degraded the picture, just like a photocopy of a photocopy. Also, since a new, different piece of negative was exposed and printed, matching the exact colors of the original was an issue. For economical reasons, especially in the 1950s and later in TV series produced on film, printer work was limited to only the parts of a scene needing the effect. The original footage was spliced mid-shot with the optically-printed portion, resulting in an obvious change in image quality when the transition occurs. Other problematic artifacts depend on the effect attempted, most often alignment inaccuracies in matte work.