, also known as perfs
, are the holes placed in the film stock
during manufacturing and used for transporting (via sprockets
and claws) and steadying (via pin registration
) the film
. Films may have different types of perforations depending on film gauge
, film format
, and the intended usage. Perforations are also used as a standard measuring reference within certain camera systems to refer to the size of the frame.
Some formats are in fact referred to as (perforations per frame/gauge size) to give an easy way of denoting this. For instance, VistaVision is also known as (8/35), while standard 70 mm film is (5/70) as compared to IMAX, which is (15/70). This system does not indicate whether or not the film transport is horizontal or vertical, but there are currently no horizontal systems using the same number of perforations on the same gauge as a vertical one.
One of the characteristics of perforations is their "pitch". This is the measurement of the distance between the top of two perforations in sequence. For motion picture 35 mm film
and 16 mm film
, there are two different pitches - short pitch (camera negative film) and long pitch (print stock, optical printers, bi-pack). For 35 mm these are 0.1866" and 0.1870"; for 16 mm they are 0.2994" and 0.3000".
Additionally, in 35 mm and 65/70 mm there are several different shapes for these perforations. BH
(Bell and Howell) or N
perforations are used in camera negative film and have straight top and bottom with outward curving sides and have been in use since the very beginning of the 20th century. The BH perforation's dimensions are 0.110" (2.79 mm) from the middle of the side curve to opposite top corner by 0.073" (1.85 mm) in height. The corners used to be sharp, but were slightly rounded in 1989 by 0.005" (.13 mm) to give them greater strength. The BH1866 perforation, or BH perforation with a pitch
of 0.1866", is the modern standard for negative and internegative films.
Standard) or P
perforations were introduced in the 1920s to improve steadiness, registration, durability, and longevity and thus are used for intermediate and release prints as well as camera negatives which require it, such as high-speed filming, bluescreen
, front projection
, rear projection
, or matte
work. KS perfs are rectangular with rounded corners, and measure 0.0780" (1.981 mm) high by 0.1100" (2.794 mm) wide.
The increased height also means that the image registration was considerably less accurate than BH perfs, which remains the standard for negatives. The KS1870 perforation, or KS perforation with a pitch of 0.1870", is the modern standard for release prints, as well as for 135 still camera film.
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). This gave it longer projection life but also improved registration. One of its primary applications was usage in Technicolor
's dye imbibition printing (dye transfer). The DH perf never caught on, and Kodak's introduction of monopack Eastmancolor film in the 1950s reduced the demand for dye transfer, although the DH perf persists in intermediate films to this day, such as long-pitch interpositives contact-printed from short-pitch negatives.
In 1953, the introduction of CinemaScope required the creation of a different shape of perforation which was nearly square and smaller to provide space for four magnetic sound stripes for stereophonic and surround sound. These perfs are commonly referred to as CinemaScope (CS) or "fox hole" perforations, or simply "Foxholes" (because all Cinemascope
films were made by 20th Century Fox
). Their dimensions are 0.0780" (1.85 mm) in width by 0.0730" (1.98 mm) in height.
Due to the size difference, CS perfed film cannot be run through a projector with standard KS sprocket teeth, but KS prints can
be run on sprockets with CS teeth. Shrunken film with KS prints that would normally be damaged in a projector with KS sprockets may sometimes be run far more gently through a projector with CS sprockets because of the smaller size of the teeth. Though CS perfs have not been widely used since the late 1950s, Kodak still retains CS perfs as a special-order option on at least one type of print stock.
All 16 mm perforations are rectangles with rounded corners and are 0.0500" high by 0.078" wide. The tolerance for these perforation dimensions was reduced to 0.01 mm in 1989, which allowed 16 mm camera manufacturers to slightly enlarge their registration pins and thus improve image registration and steadiness tolerances to less than 1/750th of the image height of the 16 mm frame.
Standard 8 mm film
uses 16 mm film that is perforated twice as frequently (half the pitch of normal 16 mm) and then split down the middle after development. Super 8
uses slightly smaller perfs on film which is already 8 mm wide. Super 8 pitch is 0.1667" and perfs are 0.045" high by 0.036" wide.
All of the systems described above place the perforations down either one side (Standard and Super 8, Super 16) or both sides (35 mm and 65/70 mm). Standard 16 mm can be either (single or double perf); some older cameras require double perf, but most can handle either. Because most cameras can handle both and increased popularity of Super 16, most 16 mm stock manufactured today is single perf unless requested otherwise.
Some obsolete formats such as 9.5 mm film and some variants of 17.5 mm film used a single perforation in the middle of the frame line between each image. This is considered more of a liability however, since any sprocket or claw error will likely damage the center of the frame itself rather than the outer edges.
Damage and inspection
Damaged or broken perforations can lead to a tear in the film as it runs through the projector. Film is commonly checked for broken sprocket holes before presentation, a process known as "spooling". Mechanical devices exist for this, but the classic method is to place finger and thumb of a gloved hand on the edges of the film mounted on a winding bench and slowly run the film through the fingers, feeling for snags.