How Do Movie Cameras Work?

Optical Illusion

The principles behind a movie camera lie in an optical illusion known as persistence of vision. When individual images--each just a little different from the last--are flashed past us at high speeds, our eyes lose the ability to differentiate between them, and they appear to move. Numerous devices such as flip books and zoetropes made use of this phenomenon before the coming of the movie camera, but it wasn't until the advent of photography that the concept could be explored in serious depth.

Image

Every movie camera keeps its film hidden from the light until the cameraman is ready to expose it. The film strip then passes in front of a shutter, which is placed behind the camera lens. The shutter opens very briefly, exposing the image in front of the lens which is then captured on the film. This process wouldn't work without a stop-motion device, which freezes the film for a fraction of a second just as it is exposed.

The shutter and film strip need to be synchronized for the resulting image to be clear. Otherwise, it just looks like a big blur. Sprocket holes on the side of the film itself allow it to be pulled across the shutter at a uniform speed. Each exposure captures a single image--slightly different than the one before as the object being filmed moves in front of the camera. Standard speed for most cameras is 24 frames per second; at that rate, the completed film appears to move as a normal speed when it is projected onto a wall.

Sound

Sound can be achieved through two different sorts of cameras. A double-system camera records the sound separately from the film; advanced ones have motors to synchronize the sound with the image, while less advanced ones lack such a feature (the sound must often be dubbed later to properly sync up with the image).

Single-system sound cameras were designed to be small and lightweight: news organizations would take them to far-flung locales, and home movie aficionados could use them easily without purchasing a lot of equipment. The sound was recorded optically by placing a waveform image onto a thin strip of film alongside the image. When played back, the image would translate into soundwaves matching up with the picture.

As a side note, the famous clapper board used by movie production companies was intended to help synchronize the sound and the image in the camera. By lining the image of the clapboard coming together with the clap sound on the soundtrack, the filmmakers could ensure the sound played back at the proper time.

References

• A History of Narrative Film; David A. Cook; 1990
• Film Art: An Introduction; David Bordwell and Kristen Thompson; 1990