How Movie Projectors Work

Spooling the Film

The platter sits beside the projector.
The platter sits beside the projector.

It takes an amazing amount of film to make a movie. Most movies are shot on 35mm film stock. You can get 16 frames (individual pictures) on 1 foot (30.5 cm) of film. Movie projectors move the film at a speed of 24 frames per second, so it takes 1.5 feet (45.7 cm) of film to create every single second of a movie.

At this rate, you end up needing a lot of film pretty quickly. Consider these calculations:

  • One second = 1.5 feet (24 frames per second divided by 16 frames per foot)
  • One minute = 90 feet (1.5 feet per second multiplied by 60 seconds)
  • One hour = 5,400 feet (90 feet per minute multiplied by 60 minutes)
  • Typical two-hour movie plus five minutes of previews = 2.13 miles (11,250 feet divided by 5,280)

You can use this formula to figure out just how much film it took to show the next movie you go see. Just multiply the number of minutes in the movie by 90 to get the number of feet of film.

Because a feature length film is so long, distributors divide it into segments that are rolled onto reels. A typical two-hour movie will probably be divided into five or six reels. In the early days, films were shown with two projectors. One projector was threaded with the first reel and the other projector with the second reel of the movie. The projectionist would start the film on the first projector, and when it was 11 seconds from the end of the reel, a small circle flashed briefly in the corner of the screen. This alerted the projectionist to get ready to change to the other projector. Another small circle flashed when one second was left and the projectionist pressed a changeover pedal to start the second projector and stop the first one. While the second reel was rolling, the projectionist removed the first reel on the other projector and threaded the third reel. This swapping continued throughout the movie.

In the 1960s, a device called a platter began to show up in theaters. The platter consists of two to four large discs, about 4 or 5 feet in diameter, stacked vertically 1 to 2 feet apart. A payout assembly on one side of the platter feeds film from one disc to the projector and takes the film back from the projector to spool onto a second disc. The discs are large enough to hold one large spool of the entire film, which the projectionist assembles by splicing together all of the lengths of film from the different reels. Splicing is the process of cutting the end of one strip of film so that it carefully matches up to the beginning of the next strip of film, and then taping the strips together.

Once projectionists could put all of the film for a movie on a single spool, a couple of things happened:

  • One projector could show the entire film.
  • One projectionist could easily run movies in several auditoriums at the same time.

These two factors made it less expensive to show movies because you needed less manpower and fewer projectors. This led to the birth of the multiplex, a group of several auditoriums in one theater. Since their introduction, multiplexes have grown from two or four auditoriums to 15 to 20. These super-sized theaters are often referred to as megaplexes.