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PROCESSING MOTION PICTURE FILM 139
The difference between contact and optical printing has been mentioned to draw attention to the difference between specular and diffuse light transmission. As would be expected from these differences, a further error must be allowed for, dependent upon the optical system used in the printing machines employed for this work. If curves are produced to show the relationship between the negative exposure and the print transmission, it is seen that a curve of slightly different shape will be produced by printing machines having different optical constructions. The value of these variations, as compared with the ideal condition, is termed The Printer Factor' and, since it is constant for any one machine, may be decided upon after initial tests on a new machine and should always be automatically allowed for in all subsequent calculations.
Although mention has only been made of three possible sources of error which may lead to the wrong interpretation of sensitometric results when controlling Variable Density sound recordings, these are most generally met with in practice and may be briefly summarised as follows :
(1) The Projection Factor or the contrast ratio between what is put into the sound reproducing optical system and what arrives at the photo-cell.
(2) Failure of the Reciprocity Law or the difference in density between exposures of high intensity over short periods of time and exposures of low intensity over long periods of time although, in both cases, the product of intensity and time may be equal.
(3) The Printer Factor or difference in density produced on the positive copies by various printers although the inherent brightness at all the negative printing apertures may be constant.
The Processing of Variable-Width Recordings When VariableWidth recordings are being processed it is more necessary to faithfully reproduce the form or outline of the negative rather than the degree of density or tonal values. The transmission of a variablewidth track is proportional to the width of the clear area, and varies with this area. The variations in the width of the positive track should obviously be equal to the corresponding variations in the negative and, apart from this, the transmission over the clear area should be very high, the transmission over the opaque areas very low, and the outline or 'envelope' of the wave-form should be very sharply defined.