The process of producing a motion film production usually requires many different stages: from original photography and editing to the final release version. The film stock used at each stage is optimised for one particular characteristic, e.g. a release print is designed for projection and is much less suited for duplication.

In managing a collection the purpose of each item needs to be considered from the technical characteristics point of view. For example, it may not be suitable to offer access to the original negative for duplication as the various effects or final colour grading information may not be included, let alone any preservation risk concerns.

Gauge

The gauge is the width and other physical characteristics of the film itself. The perforation shape and the actual image size can also be indicative of the gauge, e.g. Standard 8mm and Super 8mm.

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Emulsion

Films can be categorised by the emulsion type. The emulsion type describes the purpose that the film has been designed for, e.g. a camera negative for original photography where multiple copies will be made.

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Negative materials generally have just the base density around the perforations, whereas reversal materials are black in this area. Colour negatives have an orange base density. This is a masking layer used to improve the performance of the colour dyes.

Intermediate positives/duplicating positives, used for creating copies of negatives, are usually very dark and low in contrast. In colour materials these have the typical orange base colour of the masking layer found in colour negatives.

Perforation types

The perforations are used to transport the film through the cameras, printers and other equipment. Over the years many different types of perforation have been tried and for one reason or another discarded. For example, the early Lumiere films had single round perforations, one pair per frame.

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On 35mm motion picture film there are currently two types of perforation commonly used. These are the positive or KS (Kodak Standard) and negative or Bell & Howell. 16mm film has only one type of perforation shape but may be perforated on only one or both sides of the film.

Perforations not only differ in shape but also the distance that separates them, known as 'perforation pitch’. Continuous contact printers operate with the original film and raw stock in contact passing around a large sprocket. To allow for the increase in the circumference, caused by the thickness of the inner layer of film, the outer layer of film requires a larger perforation pitch. This prevents slippage on the sprocket and results in a steadier image when projected. The difference in the pitch is between 0.2-0.4 per cent. The pitch is also adjusted on 16mm film by a similar amount. All films designed to be used in a camera will have the shorter pitch dimensions and generally duplicating materials will have the longer pitch as will release prints.

Film Duplication and generations

Professional motion picture film production goes through many stages from the camera to the screen. The production chain for image material is usually from negative to positive to negative etc. There are materials that were used for a short time, Colour Reversal Intermediates (CRIs), that could make a direct copy of a negative saving a duplication stage. However, these materials were awkward to control and are no longer used.

In identifying whether a film is an original negative or a duplicate some clues that may help are:

• 'print through’ of edge markings
• camera gate marks
• black framelines (may indicate a CRI)
• additional frame edges

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