Film perforations

This distinction arose because early nitrocellulose film base naturally shrank about 0.3% in processing due to heat, so film printing equipment was designed to account for a size difference between its (processed) input and (unprocessed) output.

When cellulose acetate film was developed, which does not shrink, two forms were produced for compatibility with existing equipment.

BH (Bell and Howell) perforations are used on camera negative film and have straight tops and bottoms with outward curving sides; they have been in use since the beginning of the 20th century.

KS (Kodak Standard) perforations were introduced in the 1920s to improve the life of projected film stock by eliminating the sharp corners which were prone to tearing.

[citation needed] To this day, all Western Bloc professional cameras employ BH perforations, and so also do the intermediate applications (interpositives and internegatives, also known as the IP/IN process).

The increased height also means that the image registration was considerably less accurate than with BH perfs, which remain the standard for negatives.

The Dubray Howell (DH) perforation was first suggested in 1931 to replace both the BH and KS perfs with a single standard perforation which was a hybrid of the two in shape and size, being like KS a rectangle with rounded corners and a width of 0.1100" (2.79 mm), but with BH's height of 0.073" (1.85 mm).

[7] The DH perf never caught on, and Kodak's introduction of monopack Eastmancolor film in the 1950s reduced the demand for dye transfer,[5] although the DH perf persists in intermediate films to this day,[8] such as long-pitch interpositives contact-printed from short-pitch negatives.

17.5 mm magnetic film was used as a secondary "shop standard" at Paramount and Universal for location dialogue recording ; it was most often run at 45 feet/minute, one-half of the usual 35 mm magnetic film speed, thereby achieving a 4-to-1 increase in economy although at a significant sacrifice in sound fidelity, but adequate for monophonic dialogue.

Because most cameras can handle both, and because of the increased popularity of Super 16 film, most 16 mm stock manufactured today is single perf unless requested otherwise.

This is however considered a liability, since any sprocket or claw error will likely damage the center of the frame itself rather than the outer edges.

Mechanical devices exist for this purpose, but the classic method is to place the finger and thumb of a gloved hand on the edges of the film, which is mounted on a winding bench, and to slowly run the film through the fingers, feeling for snags.