The rotation of the handle also ensures, through gearing, the revolution of the shutter P, whereby the lens is eclipsed intermittently. The shutter is a ring fitted with an opaque sector which comes before the lens and shuts out the light during the movement of the film through the gate H by the claw N. In the Williamson camera this shutter is recessed into the case.
Although the lens may be of the fixed focus class--the focussing distance varying with the stop used--focussing can be carried out independently if very critical work is required. In the case of the Williamson camera and others of this type, focussing is accomplished by opening the shutter and the side of the camera and looking through the gate. In some cameras a focussing tube is provided. This passes from the gate to the rear of the instrument, through a space provided between the superimposed film dark-boxes. It is telescopic at the forward end. Thus, when focussing is being carried out, it can be extended so as to come flush with the gate, and pushed back out of the way when all is ready for working, so that the free movement of the film is not obstructed in any way. The rear end of the tube, which extends through the rear face of the camera box, is fitted with a cap to save the film from being fogged by light entering from behind.
One conspicuous advantage of the Williamson machine is that the whole of the mechanism is mounted upon a skeleton casting fixed to the interior of the mahogany case by means of four screws. By withdrawing these the whole of the internal mechanism may be removed intact, and much trouble is saved when inspection or repairs are necessary.
In some cameras the intermittent movement of the film is effected by a single claw which engages with the perforations upon one side of the film only. But this movement is not perfect. All the pulling strain is thrown upon one side of the film. This gives it a tendency to move unevenly into the gate and also increases the risk of tearing.
The driving gear of the camera is so adjusted that one complete revolution of the handle completes eight exposures. Consequently two revolutions have to be made per second to maintain the necessary speed of sixteen pictures per second. In the Williamson camera this is emphasised as a fixed speed in ordinary working, and any compensations demanded by the varying intensity of the light are made by altering the aperture of the lens. This is a logical method, for if the operator is required to make such compensations by varying the speed of his handle he is apt to obtain an indifferent result. It requires a very skilled operator indeed to vary the speed of the handle with judgment between the narrow limits possible. In some cameras this compensation for light is effected by varying the area of the opaque section of the shutter, but this is not so simple or effective a method as the variation of the stop. The latter can be accomplished while the camera is being driven, but in the former it is necessary that the work should be stopped while the front panel camera is opened and the shutter adjusted.