Reverse motion (a.k.a. reverse motion photography or reverse action) is a special effect in cinematography whereby the action that is filmed ends up being shown backwards (i.e. time-reversed) on screen. It can either be an in-camera effect or an effect produced with the use of an optical printer.
There are several uses for reverse action. Some are artistic in nature. For example, reverse action can be used for comedic effect. Or it can be used to bring things "back to life" on screen, by filming a process of destruction or decay in reverse. Sometimes it is necessary, for the intended effect to be achieved, for all actions other than the one that it is intended to reverse to be performed, during shooting, backwards—actors have to perform their actions and dialogue backwards, for example. This also enhances the visual impact of the effect.
The artistic use of reverse action is pervasive in the films of Jean Cocteau. In his Beauty and the Beast (1946), for example, an actor placing a piece of paper in a fire and then walking backwards away was filmed in reverse motion, causing it to appear as though the character walked up to a fire and pulled the paper out of it. Similar instances are where the petals are peeled off a flower. Cocteau filmed this in reverse motion, making it appear on screen as if the flower comes back to life, with petals rejoining the stem. Yet other examples include Cocteau sketching drawings with a rag and unsmashing pottery, with fragments flying up into his hand and joining together. By the time of Le Testament d'Orphée, use of reverse action was endemic in Cocteau's work, with more than one critic declaring it so overused as to be an embarrassing personal tic.
Other uses of reverse motion photography are technical in nature. For example, it is difficult to target helicopter shots precisely. Having the point of view swoop down from the sky into a close-up on a particular object or scene is almost impossible to achieve with a helicopter, since it is almost impossible to end up with a perfectly framed and focused final image. So such shots are filmed in reverse motion, starting with the helicopter close to the target, and then drawing back and up into the sky.
There are two techniques for achieving reverse motion. The first is not an in-camera effect, but is achieved by printing the film backwards in an optical printer, starting from the final frame and working to the initial one. (This requires a true optical effect, since simply playing the film in reverse when exposing it onto a new negative causes it to come out upside down.)
The second is an in-camera effect, achieved either by running the camera itself backwards or by turning the camera upside down. Most cameras are directly capable of running the film backwards (spooling from bottom to top rather than from top to bottom) and those that cannot can mostly be adapted into doing so by the simple expedient of rewiring the electric motor, switching its polarity (for DC motors) or changing over any two of its phases (for synchronous three-phase motors).
Turning the camera upside down and running it forwards as normal, so that the film spools from bottom to top (Projecting the resultant print in reverse will cause the image to come out the right way up.), has several disadvantages. First, it places the soundtrack on the wrong side. Second, the film is required to be perforated on both sides, otherwise the negative cannot be cut into the rest of the film. Third, it requires that the camera be lined up for shooting in the opposite way, with the guidelines in the viewfinder that indicate the Academy area needing to be reversed. Fourth, it can cause subsequent processing difficulties for the negative, because the registration pins will be engaging the film perforations on their opposite sides to normal.
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