Motion Pictures

Motion pictures, also called film, cinema or movies, are a series of images, recorded on strips of film, that create the illusion of continuous motion. A series of rapidly shown images appear to move because of a phenomena called the persistence of vision. The eye does not react instantly to changes in its field of vision. In fact, the eye continues to "see" an image for 1/10-1/20 of a second after it changes. So if a sequence of images can be shown at a rate of approximately 20 per second, it will appear as continuous motion to the eye. That is just what happens in a motion picture.

Camera

In photography the exposure of film can be controlled by changing the amount of light entering the lens, or the amount of time the shutter remains open. The shutter speed in a motion picture camera is controlled by the fact that 24 frames must be shot per second. No exposure can be longer than 1/24th of a second. Motion picture cameras use a shutter that looks like a rotating propeller with two blades. The propeller can be made wider to decrease the percentage of time the lens is open, and thereby shorten exposures.

The pull-down mechanism, invented at the end of the nineteenth century, moves the film through the camera, holds it still in position for 1/24th of a second while the exposure is made, then moves the film to the next frame. It does this in perfect synchronization with the revolving shutter that exposes the film.

Early motion picture cameras were large and heavy. But by the mid-1950s, technology developed during World War II lead to smaller, lighter cameras that even allowed cinematographers to hold the smallest cameras. This freed the cameras from a tripod, allowing for more innovative camera work. For moving camera effects, cameras can be put on platforms that are attached to rubber wheels or steel rails like railroad tracks. They can also be raised and lowered on cranes.