There are two main printing transport systems used in motion picture film duplication:
- continuous – the film is constantly moving
- step – the film moves intermittently and is exposed one frame at a time
There are also two image transfer methods:
- optical – a lens is used to project an image onto the receiving film
- contact – the two pieces of film are in emulsion to emulsion contact, no lens is used
As the name implies, continuous printers move the film continuously, including during the moment of exposure. The original film passes over a narrow slit, through which the exposing light passes. Mostly continuous printers employ emulsion-to-emulsion contact, contact printing. This allows a very high speed process but does permit a degree of slippage between the films that will degrade the image. If the film being duplicated is at all shrunken, the slippage factor becomes very significant. Usually contact printing is used for high production runs, such as.
One form of duplication that requires continuous printing is that of optical sound tracks.
Step printing is a generally slower process. The film is held stationary during the moment of exposure, which removes all slippage induced artefacts. The whole frame is exposed at once with a shutter controlling the exposure duration, making the process more like a cine camera. Step printers can use either an optical or a contact image transfer.
Step printers are used in commercial laboratories for the production of duplicating materials, dupe positives and dupe negatives where maximum image quality is required and since the production run is low, time is less important.
Due to the gate used to hold the image still during exposure step printers are entirely unsuitable for optical sound track printing.
may allow compensation for shrinkage of the image by optically enlarging the image frame.
|Step printers||no image slippage during exposure||cannot print optical sound tracks|
|can generally deal with shrinkage better||slower printing times|
|Continuous printers||can print sound tracks||slippage during printing|
|faster printing times||loss of resolution|
|less able to cope with shrinkage|
|Optical||resizing shrunken frames||optical losses|
|correcting register problems||potential for dust on optics|
|(shifting frames in respect to perforations)|
|Contact||no optical losses||cannot resize frames|
|cannot correct register|
Wet gate film printing is used to cover physical scratches and abrasions on the base and emulsion surfaces of the film. It involves the negative being temporarily coated with a fluid with the same, or 'light bending power’, as the film. This reduces scratches by not diffracting the light as it passes from the film since both phases now have the same refractive index, whereas film and air are quite different. Wet gate devices can be used on both types of printers.
As with rewashing, wet gate printing will reduce the effect of the scratch but will not restore lost image information, as can be seen in Fig 13.15ii).