Film perforations

Film perforations, also known as perfs and sprocket holes, are the holes placed in the film stock during manufacturing and used for transporting (by sprockets and claws) and steadying (by pin registration) the film. Films may have different types of perforations depending on film gauge, film format, and intended usage. Perforations are also used as a standard measuring reference within certain camera systems to refer to the size of the frame.

Some formats are referred to in terms of the ratio "perforations per frame/gauge size" to provide an easy way of denoting size. For instance, 35mm Academy is also known as 4 perf-35mm; VistaVision is 8 perf-35mm; the long-time standard Todd-AO 70 mm film is 5 perf-70mm; and IMAX is 15 perf-70mm. This description does not indicate whether the film transport is horizontal or vertical, but uncertainty is precluded because there are currently no horizontal systems using the same number of perforations on the same gauge as a vertical one.

One of the characteristics of perforations is their "pitch". This is the measurement of the distance between the tops of two sequential perforations. For 35mm and 16mm motion picture film, there are two different pitches—short pitch (camera stocks intended for duplication or printing, and for most intermediate applications) and long pitch (camera stocks intended for direct projection, print stocks, and special intermediate applications, as well as 135 still camera film). For 35 mm film these are 0.1866" and 0.1870" (4.740 mm and 4.750 mm); for 16 mm film they are 0.2994" and 0.3000" (7.605 mm and 7.620 mm).

This distinction arose because early nitrocellulose film base naturally shrank about 0.3% in processing due to heat, so film printing equipment was designed to account for a size difference between its (processed) input and (unprocessed) output. When cellulose acetate film was developed, which does not shrink, two forms were produced for compatibility with existing equipment.

Additionally, for 35 mm film only, there are several different shapes for these perforations.

BH (Bell and Howell) perforations are used on camera negative film and have straight tops and bottoms with outward curving sides; they have been in use since the beginning of the 20th century. The BH perforation is a circle of approximately diameter 0.110" (2.79 mm), with flattened sides giving a height of approximately 0.073" (1.85 mm). The corners used to be sharp, but were slightly rounded in 1989 by 0.005" (0.127 mm) to give them greater strength. The BH1866 perforation, or BH perforation with a pitch of 0.1866", is the modern standard for negative and intermediate (interpositive / internegative) lab film. The BH1870 perforation, or BH perforation with a pitch of 0.1870", was the original standard for positive prints intended for direct projection (release prints).

KS (Kodak Standard) perforations were introduced in the 1920s to improve the life of projected film stock by eliminating the sharp corners which were prone to tearing. and thus are occasionally used for high-speed filming, but failed to displace BH perforations for filming operations prior to projection. KS perfs are rectangular with rounded corners, and measure 0.0780" (1.981 mm) in height by 0.1100" (2.794 mm) in width.

KS perforations were once recommended for negative and intermediate films, too, but only the Eastern Bloc countries (the Soviet Union and its satellites) adopted KS for these uses. The Western Bloc countries maintained BH perforations for negative and intermediate films, but adopted KS perforations for positive print films and for amateur films which were on a 35 mm wide base.