Film Preservation 101: What are the holes on the sides of the film for?
Have you ever thought to yourself: “Those holes on the side of the film, I wonder what they’re for?” Maybe you were afraid to ask? If you think it sounds like a dumb question, let me assure you that some super-smart people have asked us this before. Today we’ll tell you not only what those holes do, but why they’re often a problem for those of us who work to preserve film.
First, some terminology: “those holes on the side of the film” are commonly referred to as “perforations”, or sometimes “sprocket holes”. Here in the National Archives Motion Picture Lab, you’ll most likely hear us abbreviating the term to just “perf.”
The perforations are what sprocket rollers grab onto in order to pull film through a camera (for creation) or a projector (to present a finished product). In the case of the lab, the film is transported through a printer or scanner for duplication. If none of that means anything to you, picture a bike: The mechanics of film transport are the same as a bike chain traveling over a chainring.
The teeth of a sprocket roller engage a piece of film.
In an ideal world, perforations would always be able to perform as needed. Two major issues arise as a film ages that keep this from being the case. One is simply that the perforations are torn, or sometimes ripped completely from the film. Running a film with torn perfs over sprocket rollers frequently leads to multi-frame tears. Once a film is torn in the image area, the only thing we can do to physically repair it is carefully tape the pieces together with archivally-safe film splicing tape. The tear will always be visible in the physical copy (we can digitally restore it, but that does not change the film itself). To prevent further damage, we repair broken and torn perforations before we run a film on any kind of machine.
Can anybody see what’s wrong here?
Secondly, as a film ages, it tends to shrink. The metal sprockets on machines needed to play back film are set a specified distance apart, and do not shrink accordingly, of course. Even a seemingly small amount of shrinkage (1-1.5%) can cause problems for a film. Running an old film on equipment without first checking shrinkage can cause serious edge damage that could take hours or days to repair. In the lab, we frequently encounter old projection prints of educational films that were threaded incorrectly when projected. Usually, after about ten or twenty feet of perf damage, someone either realized something was really wrong and re-threaded, or we find a tear sloppily repaired with a piece of scotch tape because the film snapped (ask us how much we hate scotch tape).
Happily, our newer equipment was produced with shrunken film in mind. We usually have two sets of sprocket rollers to accommodate higher amounts of shrinkage. As long as we inspect and repair all of our film properly, we shouldn’t be damaging it.
I think I can safely say that extensive perf damage is the greatest single source of tedious labor in the Motion Picture Preservation Lab. We are glad to do it, though, because it means we will be able to duplicate the film to new film stock for preservation, or to digitize it for wider access.
Film Preservation 101 is an occasional series in which preservation specialists answer frequently asked questions about film. We welcome your questions in the comments. You can also tweet questions to @NARAMediaLabs.