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Movie theater projection technology

Curious about how movies actually work? This here is a first attempt at a very basic primer on the technology of motion picture projection. For more details, visit the outside links page and see what else is out there.

A 35mm movie is made of essentially the same film you use in your 35mm still camera at home, but it's used vertically, and there are four perforations per frame instead of still's eight perfs per frame. The film moves at 24 frames per second through the camera, and in the end the negative is cut up and glued together to make the edited film, and prints are made from that. (That is of course a gross oversimplification, but you get the idea.) There are 16 frames per foot; a 112 minute movie is over 160,000 frames and is over 10,000 feet in length! A film print is usually shipped on six to eight reels in two cans and weighs fifty to eighty pounds.

Movie theaters used to show films right from those reels, and a few still do. It's a process called "changeover" and it requires two projectors and a person to be paying attention for the starts, changes and rewinds (done with a pair of handles and spindles on a table). Most theaters now show films after building them up on something called a platter system.

A platter looks like a pizza pan, maybe five feet across. With the aid of motors for the platter and an external unit, the trailers and the theater logo and the sound system logo and then all the reels of the film are wound around a core sitting on the platter that's roughly a foot and a half in diameter, with the ends of the film attached to either other with a special tape. The film lays on its side, with the head in the middle and the tail near the outside edge. After the film is built up, the core is removed and the head of the film is fed into a "brain", which is a unit with pulleys and tensioners that pulls the film away from the center, up and out from the platter, into more pulleys and eventually up into the projector, and then more pulleys take the film back to another platter where it is wrapped around a core again. With this system whole movies are shown without needing to change reels, and the film never needs to be rewound - it's simply pulled from the middle for the next showing and it goes back to the empty platter it was on before. Someday I'll try to get pictures of this.

In the projector, a very bright light shines through the film, and a shutter makes sure the light only goes through while the film frame is very briefly stopped in front of the lens gate. A lens is used to focus the light on the screen. That's pretty much it.

Typical film frame
This is a four-times-size drawing of a typical film frame. The frame's aspect ratio (height to width) is about 1:1.33, roughly the same as your TV. This aspect ratio (or the very similar 1:1.37) is called "Academy". Nearly all films before the 1950's were made in this size; when you watch "Casablanca" or "The Third Man" on video you're getting the whole picture, for once.
Film frame w/box for 1.85
Most US movies you see aren't in the shape of your TV, though; they're shown at about 1:1.85. This "wide screen" is achieve through the simple process of cropping the top and bottom out of the frame. The red box shows the approximate area you see on the screen. The rest of the space is sometimes matted black, or has picture that isn't meant to be shown - frequently the top of the frame will be littered with boom microphones.
Film frame with squeezed CinemaScope image
The true "wide screen" process is "CinemaScope", which has an aspect ratio of 1:2.35, which is how I think most movies should be, darn it. The CinemaScope process uses special lenses on both the camera and the projector to squeeze that wide image down into the 1.33 frame and then unsqueeze it when projected. The image ends up looking very narrow on the film, but fine when projected. The entire film frame is used.

70mm film is twice as wide as 35mm, and uses 5 perfs per frame, and the entire frame is used. The approximate aspect ratio is 1:2.2 and there is no squeezing of the image required.

Sound is covered on the next page.

Otto Kitsinger 13 Jan 97

I can remember a time when where we went to the movies was just as important as the movies we went to see .... From the moment moviegoers arrived to buy their tickets, there was a sense of something special, a feeling that to step inside was to enter another time and place. - Gene Kelly