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64 PRINCIPLES OF CINEMATOGRAPHY
T which is to be rotated with respect to the main shutter *S\ Both gear 'B' and shutter blade "T are free to rotate with respect to shaft 'J' and only use this shaft as a supporting bearing.
Referring now to the gear train seen in 'position 1', Figure 21, gear 'A' is again seen at the top of the diagram. Gear 'B', is of course, directly behind gear 'A' and, since both gears are equal in size, will not be seen in the figure. The shafts supporting gear 4 A' and gear 'C rotate about fixed centres. Gear 'A' meshes with gear 'C ; gear 'C meshes with gear 'D'; gear 'D' meshes with gear 'E' and, finally, gear 'E' meshes with gear 'B'. It is important to realise that gear 'E' does not mesh with gear 'A'. Gears 'D' and 4E' are mounted on bearings housed in the link system *F\ 'G' and 'H\ These links, together with line 'L-M', which passes through the centre of gears 'A' and 'C, form a parallelogram which may be adjusted by setting the control lever 'P'. The mechanism is shown again at 'position 2' after lever 'P' has been raised to the upper position.
To fully understand the operation of this mechanism let it be assumed that gear wheel 'A' is stationary. Since gear 'C is mounted on a fixed centre, and is in mesh with 'A', it also must remain stationary. If lever 'P' is then moved from position 1 to position 2, gear 4D' will be forced to rotate about gear 'C\ This rotation will then be transmitted to gear 'E' and also, through 'E\ to gear 'B\ Thus, moving lever 'P' causes gear 'B' to rotate with respect to gear 'A'. Obviously this operation may be carried out with equal success when the camera is running.
Shutter 'CutOff' Efficiency
Before leaving the shutter mechanism it is worth noting the effect shutter size has upon its efficiency. By efficiency is meant the time taken to completely cover the camera aperture. Obviously, the change-over period, when the aperture is neither completely open or thoroughly closed, must be considered as lost time. A shutter which completes this change-over quickly is said to have a high efficiency.
This variable is illustrated in Figure 22. Shutters 'A' and 'B' are both of equal size but, in the first case, the centre of shutter 'A' has been placed at the maximum possible distance from the exposing aperture. The shutter will therefore only need to rotate through angle 'C in order to completely cover the aperture. In the second case, the centre of shutter 'B' has been placed close to the exposing aperture and, consequently, the shutter blade must rotate through the much greater angle shown at 'D' in order to