National Film and Sound Archive > Preservation

Photographic Duplication

Printers

Wet Gates

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 recieving 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 release prints.

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, this 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.

Optical printers may allow compensation for shrinkage of the image by optically enlarging the image frame.

	Pro's	Con's
Step printers	• no image slippage during exposure • can generally deal with shrinkage better	• cannot print optical sound tracks • slower printing times
Continuous printers	• can print sound tracks • faster printing times	• slippage during printing • loss of resolution • less able to cope with shrinkage
Optical	• resizing shrunken frames • correcting register problems (shifting frames in respect to perforations)	• optical losses • potential for dust on optics
Contact	• no optical losses	• cannot resize frames • cannot correct register

Table 13.5 Pro's and con's of printing systems

Fig 13.14 Wet gate light path

Wet Gates

Wet gate devices can be used on both types of printers. A wet gate uses a fluid with the same refractive index, or "light bending power", as the film to reduce the effect of scratching. It does this 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.

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).

i) Straight contact printed

ii) Wet gate printed

Fig 13.15 Wet gate printing and the effect on scratching

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