Film Basics

There are a number of things that differentiate the various film standards. The most common are:

The following sections discuss these points as they relate to Cinema Tools.

Frame Size

Cinema Tools supports two common film frame sizes: 16mm and 35mm. The aspect ratio (height versus width) of each size is different.

Because of its lower costs, 16mm film is typically used for productions with smaller budgets. If you intend to shoot 16mm but release your project as 4-perf 35mm, you should use Super 16mm film. It has perforations along only one edge and a larger frame that more closely matches the 4-perf 35mm aspect ratio.

Figure. Illustration showing examples of 16mm and Super 16mm film.

35mm film is most commonly used for theatrical releases, with 4 perforations per frame (4-perf) being the most prevalent version. Another common version uses 3 perforations per frame (3-perf). There are other versions of 35mm, such as 8-perf, but they are not currently supported by Cinema Tools.

Figure. Illustrations showing examples of 4-perf and 3-perf 35mm film.


Camera and projection equipment uses the perforations, also known as sprocket holes, along one or both edges of film to pull it past the shutter.

16mm Film

16mm film has a single perforation for each frame, which comes out to 40 perforations per foot. 16mm is available as single perforated (perforations along one edge only, allowing space for an optical track, or in the case of Super 16mm film, for a larger frame) and double perforated (perforations along both edges). Cinema Tools supports single and double perforated 16mm film as long as it has a key number every 20 frames. See Key Numbers for more information.

35mm Film

35mm film has 64 perforations per foot. Cinema Tools supports the 4-perf 35mm and 3-perf 35mm formats for all types of film lists and change lists. These are by far the most common 35mm formats.

The 4-perf 35mm film format has 16 frames per foot. The 3-perf 35mm format does not have a whole number of film frames in a foot (there are 21 and one-third per foot). To avoid tracking fractions of frames, the 3-perf 35mm format is considered to have a pattern of two 21-frame feet followed by a 22-frame foot. See 3-Perf 35mm Offsets for more information.

Film Edge Code

To aid in locating specific film frames, film manufacturers place numbers along the edge of the film. These key numbers (also known as latent edge code) appear when the film is developed. For workprints, film labs can add numbers called ink numbers (also known as Acmade numbers).

Edge code is essential to your Cinema Tools database because it makes it possible for you to export cut lists or change lists that specify exactly where your negatives or workprints need to be cut in order to match your digital edits.

Key Numbers

Key numbers provide both an identification number for each roll of film and an incremental footage count number used to identify specific film frames. They often appear as both regular text and as a bar code.

Figure. Illustration showing a key number on the edge of a piece of film.

Each film standard uses key numbers differently:

  • 16mm film can have a key number every 20 frames (most common) or 40 frames, depending on the film stock. Cinema Tools supports the 16mm-20 format.

  • 35mm film has a key number every 64 perforations (which works out to every 16 frames with the 4-perf format, or 21 and one-third frames with the 3-perf format).

Unlike video timecode, which provides a unique number for each video frame, key numbers do not appear on every frame of film. For this reason, when identifying a specific frame in a log book or in Cinema Tools, key numbers have a frame count extension added specifying the actual frame. A “+08” at the end of a key number indicates it is the eighth frame from that key number’s first frame.

In the previous illustration, the actual key number for the center frame is KJ 29 1234 5678+00. The “•” following the number indicates frame 00 for that key number. (With 4-perf film—the kind shown in the illustration—there are 16 frames per key number, with the first one starting at “00.”) The frame to the right would be KJ 29 1234 5678+01. The frame to the left would be the last frame of the previous key number, KJ 29 1234 5677+15. (16mm film places the “•” at the beginning of the key number.)

35mm film also has mid-foot markers halfway between the zero frame markers. These help to identify a midpoint (the “+08” frame in the previous example) and reduce the chance of a miscount. These markers use the same key number with a “+32” appended (indicating the perforation number, not the frame number) in a smaller font.

3-Perf 35mm Offsets

Because the 3-perf 35mm format does not have a whole number of frames between each key number, an additional “perforation offset” number is added to the end of the key number. This number indicates the relationship of the perforation marked with a “•” and the frame at that position.

Figure. Illustration showing offsets on a piece of 3-perf 35mm film.

Ink Numbers

Ink numbers, frequently used for workprints, are another method of encoding the edge of film in order to track feet and frames. Ink numbers are added to workprints and corresponding magnetic-stripe film soundtracks (called mag tracks) after the workprint and the mag track have been synchronized. On transferred workprints, ink numbers are easier to read than key numbers, and they provide a counting mechanism that is synchronized for both the soundtrack and the workprint. Ink numbers are sometimes called Acmade numbers because Acmade makes a machine that is used to print ink numbers. Machines that print ink numbers are commonly rented or owned and run by film crews.

The typical style of ink numbering is a three-digit prefix followed by a character or space, followed by four digits representing the footage number, followed by digits representing the frame offset. For example, in ink number 123 4567+08, “123” is the prefix and “4567+08” is the frame number, indicating that the frame occurs at 4567 feet and 8 frames. The ink numbers encoded on the film do not actually include the last part (the frame offset number). Rather, the frame offset is calculated by the telecine and appears in the telecine log.

The prefix may contain fewer or more than three digits, and the numbering technique for the prefix is usually determined by an editing assistant. For example, the numbering could be associated with the scene number, as in “042” for the footage in scene 42. Or, the prefixes might represent daily roll numbers.

Window Burn

As part of the telecine transfer process (described in Transferring Film to Video), the key number is typically burned in to the video (along with the video and audio timecode), helping to identify specific frames. The burned-in numbers are called window burn.

Figure. Window showing an example of window burn on a frame of video.

It is much easier to use Cinema Tools if you can see the key numbers. You can use Cinema Tools without the window burn, but it requires more effort on your part to ensure that edits are being tracked properly.

Note: After you have captured your video but before you start editing, check the burned-in key numbers and timecode to make sure they match the actual ones on the film and videotape. There are a variety of reasons why the window burn values might not be correct, ranging from incorrectly entered values to faulty automatic detection. Any errors at this point will result in serious problems when the negative is conformed. The most common way to verify these numbers is to have the lab or transfer facility physically punch a hole or otherwise mark a film frame, note its key number, and compare it to the burned-in key number when viewing the transferred video. Make sure you verify this at least once for each camera roll (preferably for each take). Compare the timecode in the window burn with the value the videotape deck displays.


Film normally has a frame rate of 24 frames per second (fps). This means a new image is exposed or projected 24 times a second. To ease conversion to video frame rates, it is common to run the film at rates other than 24 fps during the telecine transfer. Cinema Tools supports film transferred to video with the telecine running at the rates of 23.98 fps, 24 fps, 25 fps, and 29.97 fps (often referred to as 30 fps). See Frame Rate Basics for more information about frame rates.