Sizes and Shapes
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

Perforation Types
35 mm and 65 mm End Use

1. BH- 1870-35 mm Bell-Howell negative perforations with a pitch measurement of 0.1870" (long pitch), ANSI PH22.93-1980
2. BH-1866-35 mm Bell-Howell negative perforations with a pitch measurement of 0.1866" (short pitch), ANSI PH22.93-1980
3. KS-1870-35 mm and 65 mm KODAK Standard Positive perforations with a pitch measurement of 0.l870" (long pitch), ANSI PH22.139- 1980; PH22.145-1981
4. KS-1866-35 mm and 65 mm KODAK Standard Positive perforations with a pitch measurement of 0.1866" (short pitch), ANSI PH22.139-1980; PH22.145-1981
5. DH-1870-35mm Dubray-Howell perforations with a pitch measurement of 0.1870" (long pitch), ANSI PH22.102-1980
6. CS-1870-35 mm CinemaScope perforations with a pitch measurement of 0.1870" (long pitch), ANSI PH22.102-1980
7. KS-1870-70 mm film perforated 65 mm KODAK Standard Positive perforations with a pitch measurement of 0.1870" (long pitch), ANSI PH22.119-1981
   
   16 mm End Use
8. 2R-2994-16 mm film perforated two edges with a perforation pitch of 0.2994" (short pitch), ANSI PH22.110-1980
9. 2R-3000-16 mm film perforated two edges with a perforation pitch of 0.3000" (long pitch), ANSI PH22.110-1980
10. IR-2994-Same as No. 8 except perforated one edge, ANSI PH22.109-1980
11. 3R-2994-35mm film perforated 16 mm with perforation pitch of 0.2994" (short pitch), ANSI PH22.171-1980
12. IR-3000-Same as No. 11 except with a perforation pitch of 0.3000" (long pitch), ANSI PH22.171-1980
13. 3R-3000-Same as No.11 except with a perforation pitch of 0.3000" (long pitch) ANSI PH22.171-1980


Optimum Pitch for Printing
Confinuous printers used for motion-picture film are designed so that the original film and the print raw stock are in contact (emulsion-to-emulsion) with each other as they pass around the printing sprocket, with the raw stock on the outside. To prevent slippage between the two films during printing (which would produce an unsharp or unsteady image on the screen), the original film must be slightly shorter in pitch than the print stock. In most continuous printers, the diameter of the printing sprocket, Figure 46, is such that the pitch of the original must be 0.2 to 0.4 percent (theoretically, 0.3 percent) shorter than that of the print stock. With nitrate film and early safety film, this condition was achieved by natural shrinkage of the original during processing and early aging. However, the substantially lower shrinkage of present safety films makes such a natural adjustment impossible; therefore, film used as printing originals is now manufactured with the pitch slightly shorter than the pitch of the print film. For 35 mm film, the pitch dimensions are 0.1870 inch (4.750 mm) on print film and 0.1866 inch (4.740 mm) on original film; for 16 mm film, they are 0.3000 inch (7.620 mm) on print film, 0.2994 inch (7.605 mm) on original film. For intermediate and print films used to make super 8 prints, the pitch dimensions are 0.1667 inch (4.234 mm) on print film, 0.1664 inch (4.227 mm) on intermediate film. This difference in pitch accounts for about 0.2 percent of the theoretical 0.3 percent; processing and aging shrinkage of the original film before printing usually provides the balance. See the first perforation type reference for additional information.



Figure 46

SPROCKET

Projection Print Aspect Ratios
The aspect ratio is the relationship between the width and height of an image. While the image dimensions may vary in size according to projection requirements, the aspect ratio should comply with the cinematographic intent. The industry standard for theatrical motion pictures remained a constant 1.37:1 between the introduction of sound and the introduction of CinemaScope in 1953 when wide screen presentations were developed. While the original stereophonic (four-track magnetic) CinemaScope presentation had an aspect ratio of 2.55: 1, the flat, or nonanamorphic  systems, designed to simulate wide screen images, provided several aspect ratios from 1.66:1 all the way up to and including 2:1. During this uncertain period, release prints were often printed with wider frame lines to emphasize that increased ratios were intended. During printing, the frame lines could be varied by printing the lines in to cover some of the original film image. At the same time, television's demands for feature films increased. However, because the typical television display provides a fixed ratio of 1.33:1, many of the films shown on television, after adjustment to fill the video screen height, lost a substantial part of the image at the edges. See Figure 47. Several approaches to rectifying this incompatibility were tried with various levels of success until the industry came to the current "consensus" that 1.85:1 would be the "normal" theatrical projection ratio but that the print would have an image of greater height so that it could fill a television screen without creating borders. Today, the usual procedure when filming productions for theatrical release and eventual TV showing is to "matte" the camera viewfinder to clearly indicate 1:85:1 and to keep all pertinent action within this area. Nevertheless, the entire 1.37:1 frame is exposed. The cinematographer must make certain no scene rigging, mike books, cables, or lighls are included in the expanded area. Subsequent release prints, therefore, contain a sufficient frame height to provide normal telecine transmission. In the theater, the projectionist must use a 1:85:1 aperture plate and exercise some judgment in adjusting the projector framing. This can be done conveniently during the showing of the titles.

CAMERA 1.37:1	TV 1.33:1
1.37:1	1.85:1

Potential image lossed when changing
aspect ratios

Figure 47





---

Table of Contents