When two identical films are driven by the same sprocket over a curved
surface as with a rotary printer, the outer layer
moves on a circumference of a slightly larger radius than the inner and some slip
between the two is bound to occur. If this slip or relative movement of the two
occurs during exposure, there is a loss of definition of the printed image.
Slip occurs more obviously the smaller the radius of the curve and slip is
less obvious if the film is flat in the gate,
thus flat bed printers made up until the 1950s, are still used by some
laboratories. Flat bed printers are particularly useful, but slow, for badly
damaged and shrunken film. To avoid slip and consequent loss of image definition,
it became the practice to manufacture negative raw stock (the inner film on a
rotary gate) with a short perforation pitch (which became "Bell &
Howell" or B&H) and the print raw stock (the outer film) with the
Kodak Standard pitch (KS, or "long pitch"). The dimension of the
exposure aperture of the continuous contact printer is made as small as
possible so that the distance over which the two films must stay in contact is
at a minimum. The gate aperture need only be a narrow slit.
Serious problems are encountered when printing shrunken material. The shrinkage of the original makes it even smaller than the design aim and causes inevitable slippage between the original and the duplicating stock. This is seen as a loss in definition and is sometimes mistakenly assumed to be poor definition in the original image. A further problem in handling shrunken film is that the sprocket drive, designed for longer pitch film can damage the perforation eventually, in severe cases ripping the perforations and the film. Flatbed printers can handle shrunken film more easily than rotary printers as the curvature of the gate is much less, especially if the film is driven by only one sprocketed roller instead of the normal two.