In the early days of 35 mm motion pictures, film perforations were round. Because these perforations were more subject to wear, the shape was changed to that now known as the Bell & Howell (BH) or 'negative' perforation. See Figure 43. This modification improved positioning accuracy and was the standard for many years. During this time, 35 mm professional motion picture cameras and optical printers were designed with registration pins that conformed to negative (BH) perforation and are still so designed to this day. Thus, camera films and many laboratory films use the negative (BH) perforations. The high shrinkage of older films on nitrate base made the negative perforation a problem on projection films because of the excessive wear and noise during projection as the sprocket teeth ticked the hold-back side of the perforations as they left the sprocket. The sharp corners also were weak points and projection life of the film was shortened. To compensate for this, a new perforation was designed with increased height and rounded corners to provide added strength. This perforation, commonly known as the KS or "positive" perforation, has since become the world standard for 35 mm projection print films.

During the period when the production of color prints involved the multiple printing of separation negatives onto a common print film, a third design, known as the Dubray-Howell perforation, was introduced. It had the same height as the negative (BH) perforation to maintain the necessary registration but had rounded corners to improve projection life. This perforation is still available for special applications and on certain films (Eastman Color Intermediate II Film 5243, for example). Because shrinkage in current films is low, the shorter perforation height poses no projection wear problems. In 1953, the introduction of CinemaScope produced a fourth type of perforation. This wide-screen projection system incorporated 35 mm film with perforations that were nearly square and smaller than the positive (KS) perforation. The design provided space on the film to carry four magnetic-sound stripes for stereophonic and surround sound. Although not widely used now, this perforation is still available on 35 mm Eastman Color Print Film.

Except for early experimentation, perforation dimensions on 16 mm and 8 mm films have remained unchanged since their introduction.

Each type of perforation is referred to by a letter identifying its shape and by a number indicating the perforation pitch dimension. Perforation pitch is the distance from the bottom edge of one perforation to the bottom edge of the next perforation. The letters BH indicate negative perforations, which are generally used on camera films, on intermediate films, and on films used in special-effect processes. The letters KS indicate positive perforations, which are used on most positive sound recording films and color print films The letters CS designate the smaller perforations used for projection prints on which additional space must be provided for multiple sound tracks in the CinemaScope process.

The designation BH 1866, for example, indicates a film having negative- type perforations with a pitch dimension of O.1866 inch (4.740 mm).

Camera films may be perforated along both edges (double perforated) or along only one edge (single perforated). All 35 mm camera films are double perforated. Films for single-pass 16 mm and 8 mm camera use may be single or double perforated. Single-perforated 16 mm films are often magnetically striped for single-system sound or post process sound addition. Double-perforated super 8 and regular 8 film is always suppled in 16 mm width to allow two-pass camera operation. Films used in laboratories for intermediate and release prints are supplied in a variety of perforation formats. The letter R preceded by a number designates the number of rows of perforations in a strip (1R-one row, 2R-two rows, etc.).

Some flexibility is possible in selecting double- or single-perforated film. You can use double-perforated film in cameras having a single pull-down claw. Also, you can duplicate or print footage exposed on double-perforated film on single-perforation stock if a photographic (optical) or magnetic sound track is to be added to the film. (NOTE: Do not use single- perforated film in equipment designed for double-perforated film.)

Figure 43

Figure 44

Figure 45