Apple Cinema Tools 4 - Manual (Page 232)

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.
Offset 1
Offset 2
Offset 3
232 Appendix A Film Background Basics