How Film Projectors Work

Projectors and cameras transport the film in the same way. The camera simply uses a slightly larger shutter aperture than the projector. Many alternative mechanisms were tried and discarded before the sprocket-driven serial frame design became dominant. A visit to the National Museum of Film and Television in Bradford, UK or the Science Museum in London will reward you with some insights into the way that film technology has evolved. Both of these are worth visiting if you have the time. There are similar museums in the United States (such as the California Museum of Photography) and in other countries where there is any history of filmmaking. Some URLs for Web sites of other museums are listed at the bottom of the page. Museums featuring scientific or engineering subjects may also have sections devoted to filmmaking.

Gate and Film Transport Mechanism

The film is pulled through the gate of the camera and projector by a claw mechanism that is driven by a cam or eccentric motion of some kind. This is easier to understand if it is shown visually.
As the motor drives the eccentric cam around, the claw is moved up and down and in and out. This is a vastly simplified example and the actual mechanism is a bit more complicated than this, but the principle is the same. The claw moves up, the teeth come out through an aperture in the gate, and they penetrate the film via the sprocket holes and pull the film down through the gate. This movement has to be very precise. The film must be moved by exactly one frame distance. The sprockets must be aligned perfectly each time or the film will be torn by the claw mechanism.

The shutter allows the light through in order to expose the film in the camera or project it onto a screen when viewing. The shutter is mounted on a spindle and rotates once per frame. The projector contrives to flash the image on the screen twice. This effectively presents the image at 48 fps (frames per second), although the film is only moving at 24 fps through the gate. Figure 4-2 shows shutter designs for a camera and projector. Some projectors allow the shutter timing synchronization to be adjusted so as to open or close the aperture with respect to the gate movement. Even more sophisticated projectors allow adjustments to how much time the aperture stays open.