35 Mm Film - Perforations

Perforations

BH perfs

Film perforations were originally round holes cut into the side of the film, but as these perforations were subject to wear and deformation, the shape was changed to what is now called the Bell & Howell (BH) perforation, which has straight top and bottom edges and outward curving sides. The BH perforation's dimensions are 0.110 inches (2.79 mm) from the middle of the side curve to opposite top corner by 0.073 inches (1.85 mm) in height. The BH1866 perforation, or BH perforation with a pitch of 0.1866 inches (4.74 mm), is the modern standard for negative and internegative films.

KS perfs

Because BH perfs have sharp corners, the repeated use of the film through intermittent movement projectors creates strain that can easily tear the perforations. Furthermore, they tended to shrink as the print slowly decayed. Therefore, larger perforations with a rectangular base and rounded corners were introduced by Kodak in 1924 to improve steadiness, registration, durability, and longevity. Known as "Kodak Standard" (KS), they are 0.0780 inches (1.981 mm) high by 0.1100 inches (2.794 mm) wide. Their durability makes KS perfs the ideal choice for some (but not all) intermediate and all release prints, and original camera negatives which require special use, such as high-speed filming, but not for bluescreen, front projection, rear projection, or matte work as these specific applications demand the more accurate registration which is only possible with BH or DH perforations. The increased height also means that the image registration was considerably less accurate than BH perfs, which remains the standard for negatives. The KS1870 perforation, or KS perforation with a pitch of 0.1870 inches (4.75 mm), is the modern standard for release prints.

These two perforations have remained by far the most commonly used ones. BH perforations are also known as N (negative) and KS as P (positive). The Bell & Howell perf remains the standard for camera negative films because of its perforation dimensions in comparison to most printers, thus it can keep a steady image compared to other perforations.

DH perfs

The Dubray-Howell (DH) perforation was first proposed in 1932 to replace the two perfs with a single hybrid. The proposed standard was, like KS, rectangular with rounded corners and a width of 0.1100 inches (2.79 mm), and, like BH, was 0.073 inches (1.85 mm) tall. This gave it longer projection life but also improved registration. One of its primary applications was usage in Technicolor's dye imbibition printing (dye transfer). The DH perf never had broad uptake, and Kodak's introduction of monopack Eastmancolor film in the 1950s reduced the demand for dye transfer, although the DH perf persists in special application intermediate films to this day.

CS perfs

In 1953, the introduction of CinemaScope by Fox Studios required the creation of a different shape of perforation which was nearly square and smaller to provide space for four magnetic sound stripes for stereophonic and surround sound. These perfs are commonly referred to as CinemaScope (CS) or "Fox hole" perfs. Their dimensions are 0.0780 in (1.85 mm) in width by 0.0730" (1.98 mm) in height. Due to the size difference, CS perfed film cannot be run through a projector with standard KS sprocket teeth, but KS prints can be run on sprockets with CS teeth. Shrunken film with KS prints that would normally be damaged in a projector with KS sprockets may sometimes be run far more gently through a projector with CS sprockets because of the smaller size of the teeth. Though CS perfs have not been widely used since the late 1950s, Kodak still retains CS perfs as a special-order option on at least one type of print stock.

During continuous contact printing, the raw stock and the negative are placed next to one another around the sprocket wheel of the printer. The negative, which is the closer of the two to the sprocket wheel (thus creating a slightly shorter path), must have a marginally shorter pitch between perforations (0.1866 in pitch); the raw stock has a long pitch (0.1870 in). While cellulose nitrate and cellulose diacetate stocks used to shrink during processing slightly enough to have this difference naturally occur, modern safety stocks do not shrink at the same rate, and therefore negative (and some intermediate) stocks are perforated at a pitch of 0.2% shorter than print stock.