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One of the advantages of creating special effects during postproduction is that they can often be more carefully controlled at this stage than during the production stage. Mistakes made during production are often costly if a scene must be reconstructed and actors reengaged. Postproduction special effects are often “added on” to the initial recordings and rarely require the initial scene to be reshot.
A variety of optical film effects are still widely used today, including step printing, traveling matters, and aerial- image printing. An optical printer is needed to create many special effects on film. A basic optical printer consists of a camera and a projector. The two machines face each other, and the lens of the camera is focused on the image from the projector. The camera and projector can be moved toward or away from each other to increase the size of the image. An optical blowup can be created by using a larger – format and a smaller – format projector. Using a smaller – format camera and a larger – format projector creates an optical reduction. The camera and the projector must be precisely positioned so that the full frame of picture in the projector fills the full frame of picture in the camera.
Optical flips can be achieved by simply rotating elements within special optical printer lenses. Freeze frames are made by exposing many frames in the camera while holding the same frame in the projector. Stretch printing slows down or retards the perceived action by printing each frame more than once. Skip printing is often used to speed up a slow- moving sequence by recording every other frame of the original film.
Wipes, split screens and optical combinations of animation and live action involve the creation of special travelling mattes. Mattes consist high – contrast, black and – white images that are made from artwork. For example, suppose a color title must be inserted into a background scene. The two images cannot simply be superimposed on one another, because the colors will bleed together rather than producing solid lettering. A black – and – white, high – contrast copy of the titles can be made, so that the black letters will block out the portion of the back- ground image where the colored letters are to be inserted.
The optical printer must have three projectors to do this: one for the background scene, one for the matte (unless the matte and the background scene are bi- packed, or run physically in contact with each other in one projector), and one for the color titles. The combination of the three images is then recorded by the camera.
Wipes and spilt screens can be made from similar travelling mattes, which block out a portion of the screen into which a second image is then inserted. It is possible to combine live action and animation by using travelling mattes in this manner. One sequence can also be recorded against a blue or black screen so that another sequence can be inserted into the blue screen area. Many special effects in science fiction and horror films are achieved by using a blue screen process. Spaceships are often recorded as they move in front of a blue screen. This blue screen portion of the frame is then used to create a matte that blocks out the area of the frame where the spaceship should appear in a highly detailed background scene with stars in outer space. The spaceship is then inserted as a foreground object into this area.
Aerial image photography combines optical printing and animation by using a film projector with an animation stand. Live – action images can be projected from beneath predrawn cels, so that color titles or animated figures can be combined with live action. The opaque portions of the cel block out the background scene, which is projected underneath it so that the titles are superimposed over the background scene. The film camera suspended overhead records the combined image. Aerial image photography eliminates the need for special intermediate matters, such as those that are used during film printing to block out or blacken areas of the frame into which titles and other images are to be inserted. However, aerial imaging requires bright projection illumination.
The choice between doing special effects on film, videotape, or through a digital medium is often a difficult one to make unless it has already been decided to use film or videotape for an entire production. The obvious advantage of digital video is the savings in overall production time. Effects can be set up and viewed immediately, without waiting for laboratory processing. Electronic effects facilities normally have sophisticated computerized editing and switching equipment, so that several images can be run simultaneously. Keys and mattes can be created instantaneously. Careful preplanning must go into the creation of a special electronic effect before entering the studio. Optical film effects are time – consuming to produce, but a very high degree of control and precision can be achieved through multiple passes of the same artwork with film. It is also possible to make sophisticated special effects in films with very low cost equipment. A basic optical printer, consisting of a simple projector and camera on adjustable platforms, can be purchased for a modest sum, allowing freeze frames, step printing, superimpositions, dissolves, and many other optical effects to be created.
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