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In the early days of motion pictures, film perforations were a variety of shapes and sizes. Perforations, or sprocket holes, were round, square, oblong or slots. They ran along one edge of the film, both edges, down the centre, or a combination of these. Because these perforations were more subject to wear and other mechanical problems, the shape was changed to that now known as the Bell & Howell (BH) or "negative" perforation.
In 1908, Bell & Howell of Chicago, introduced film perforating equipment which produced accurate, standardized sprocket holes for the first time. In 1912 came the Bell & Howell register pin movement, used both in cameras and optical printers, which made it possible to double expose both original photography and prints without the separate images moving in relation to each another. Now perfect frame registration was at least theoretically possible, except that the available cameras still allowed the film to weave and jitter.
Bell & Howell began making cameras. The expensive, all-metal, Model 2709 of 1912 had most of the features cinematographers wanted, especially a system of pilot pin registration which eliminated the unsteadiness that had compromized in-camera effects. Because of its high cost, the camera was slow to catch on, but within a few years it became the most popular studio camera, and remained so during the rest of the silent film era.
The Bell & Howell (BH) or "negative" perforation improved positioning accuracy and was the standard for many years. During this time, 35mm professional motion picture cameras and optical printers were designed with registration pins that conformed to negative (BH) perforations. To this day, newly designed professional equipment incorporates registration pins conforming to the negative (BH) perforations. Thus camera stocks, and most laboratory intermediate print stocks 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 were also 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 subsequently became the world standard for 35mm projection print films. KS stands for Kodak Standard.
When you compare the BH and KS sprocket hole, there are a number of significant differences. They differ in shape. The BH perforation is smaller. While they have similar widths, the height of the BH perforation (0.073 inches or 1.854 mm) is less then the KS perforation (0.078 inches or 2.794 mm). The difference in height is 0.005 inches (5 thousandths of an inch) or 0.94 mm (or almost a tenth of a millimeter).
During the period when the production of colour prints involved the multiple printing of separate negatives onto a common print film, a third design, know 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 stocks like 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 35mm film with perforations that were nearly square and smaller that the positive (KS) perforations. The design provided enough 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 35mm Eastman Color Print Film.
Except for some early experimentation, perforation dimensions on 16mm and 8mm films have remained unchanged since their introduction.
Perforation pitch is the distance from the leading edge of one perforation to the leading edge of the next, and is expressed in decimal inches. It is the same as the distance from the bottom edge of one perforation to the bottom edge of the next perforation, or the centre of one perforation to the centre of the next, etc. Motion picture film perforations are commonly referred to as having either "long" or "short" pitch. When the camera original is contact printed, it is bi-packed with raw print stock and the two films, pressed together emulsion-to-emulsion, are threaded around a large sprocket wheel where the actual exposure takes place. During this process the camera original is on the inside and the print stock is on the outside. With most printers, the diameter of the print sprocket is such that the print stock has about a 0.2% to 0.4% greater distance to travel because it is on the outside. If both the original and the print stock were the same pitch, it should be obvious that something would have to give, and what would happen would be a constant slipping between original and print. This would undoubtedly blur the printed image and cause a loss in sharpness and definition. To prevent this from occurring, the print stock is designed with a pitch that is approximately 0.2% longer than that of the camera original. 35mm camera negative usually has a "short" pitch of 0.1866 inches, while the print stock has a "long" pitch of 0.1870 inches.
In Kodak terminology, each type of perforation is referred to by a letter identifying its shape and by a number indicating the perforation pitch dimension. 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 in most positive sound recording films and on colour print films. The letters CS designate the smaller perforation 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 a negative-type perforations with a pitch dimension of 0.1866 inches (4.740 mm). KS 1870 indicates a film having a positive-type perforations with a pitch dimension of 0.18706 inches.
Camera films may be perforated along both edges (double perforated) or along only one edge (single perforated). Super 8 films are single perforated. 16mm films can be single or double perforated. All 35mm and 65mm camera films are double perforated.
65mm camera negative and duplicating film is perforated KS-0.1866, in other words, a negative film with positive "short" pitch perforations. When first introduced, this film was just supplied with a "long" perforation pitch because only step-printing was available at the time. When continuous contact printing became available, this changed to a "short" pitch like most camera negative films. 70mm release prints are "long" pitch or KS-0.1870. Two other types of perforations and pitch exist for 70mm prints used for special venue applications.
Copyright © Peter Gray