|Why Do Sound Negative Films Use Kodak Standard Perforations?|
A question was asked, "Why do sound negative films use KS (Kodak Standard) perforations rather than BH (Bell & Howell) perforations since they are used as printing negatives?" Here's the answer:
The BH perforation originated with the Bell & Howell perforator introduced in 1910. It was used for both camera negative and print film. The perforation pitch was the same for both films, as nitrate film shrunk about 0.3 percent in processing, the negative naturally became "short pitch" after processing. The diameter of the B&H contact printer sprocket was designed based on this assumed shrinkage of nitrate film.
The BH perf worked well for cameras. As registration pins were introduced for cameras and step printers, full-fitting pins were used that tightly fit the BH perforations, assuring precise registration. But the sharp radius corner concentrated stresses, resulting in poor projection life of prints, where the print had to be projected hundreds of times. The Journal of the Society of Motion Picture Engineers (JSMPE), Volume 16, May 1922, details the discussion around the adoption of the Kodak Standard (KS) perforation for prints. The KS perf worked well for contact printing and greatly improved projection life. It was not originally intended for registration pin use.
Prior to World War II, there was discussion of adopting the KS perforation for camera negatives. But by that time, the industry had pretty much standardized on BH registration pins for cameras and step printers. The Dubray-Howell (DH) perforation was developed as a compromise (same shape as a KS, but 0.073 high, so a BH pin provides some registration), but it never really caught on. Ironically, the USSR adopted the KS perforation for camera negative film based on the SMPTE discussions.
With the advent of lower shrinkage acetate film, pre-print films had to be perforated with a shorter pitch to work well on contact printers designed for the 0.3 percent shrinkage of nitrate.
The most recent change is the introduction of a slightly rounded radius on the BH perf, which does improve wear characteristics.
The 65mm and 70mm motion picture films all have KS perforations, as they were developed after the benefits of the KS perf were established, so cameras and printers were designed for the KS perfs.
As far as sound negative, it normally uses KS perfs because it is not used with registration pins and the KS perf gives better wear than the BH perf. One catalog number does specify BH perfs: CAT 199-9630 for 5373. The pitch is 0.1866 inch, because it is used as a printing original.