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162 PRINCIPLES OF CINEMATOGRAPHY
light-change mechanism becomes automatic once the card hat been punched and threaded into the machine. However, in the case of the notched film, it is only possible to make notches of one depth and these will always operate a relay system in one manner. It is therefore necessary to use a manual control which loads a mechanism to the required extent to bring the diaphragm or resistance to the necessary setting, but which does not come into operation until the notch in the film operates the relay mechanism which, in turn, brings the diaphragm or resistance to the prearranged position. Such mechanisms are known as 'Semi-automatic light-change devices' and require an operator in constant attendance while the machine is printing from an unmatched negative.
With this type of light-change a small card is fitted into a holder attached to the printer in a prominent position so that it may be easily read by the operator. This card carries a list of numbers from one to twenty-one in any order, dependent upon the grading of the film, and repeated as often as necessary. For example, if the card reads as follows: 6, 4, 7, 9, etc., this would indicate that the first scene should be printed at 'light 6', the second scene at 'light 4', the third at 'light T and the fourth at 'light 9'. The operator is therefore required to set the selector lever firstly to a position marked '6' and immediately the first notch in the negative passes the edge roller, the relay system will come into operation and set the exposure control to the sixth intensity level.
A distinct warning 'buzz' is created by the operation of solenoids in the circuit and, by such noise, the operator is informed that the light-change has taken place. He then immediately sets the selector lever to position '4' (the exposure required for the next scene). However, the printer will continue to operate at exposure level '6* until the second notch passes through the relay control, when the exposure will then be changed to level '4'. In this manner the operator is continually pre-setting the selector lever to the exposure next required, and the relay system is arranged to perform the necessary operations to give such an exposure immediately the circuit is closed by a notch in the negative and as this notch passes the relay head.
One other light-change system should be mentioned; this is a type in which a large bank of resistances are previously adjusted to the required setting for each scene in the roll of film which is to be printed. If, for example, there are forty light-changes to be made throughout one roll of film, it is necessary to set forty resistances to the correct position in which to obtain the required exposure for