Practical Cinematography and Its Applications

by Frederick Arthur Ambrose Talbot

The film is brought into position before the lens by passing through what is known as the "gate." This device H consists of two parts of which the first is fixed irremovably while the second is hinged to the first at one side and kept flat against it by means of a spring. Both of the parts are provided with an aperture or window, the exact size of a cinematograph picture—1 inch wide by ¾-inch deep—through which the light passes, after admission through the lens, to strike upon the sensitized surface of the film. There is just sufficient space between the two parts of the gate to permit the film to move easily, and its object is to hold the film perfectly flat and steady during the period of exposure. Each picture is thus kept in absolute focus.

The feed through the gate is accomplished by the claw N, which is mounted upon the eccentric O as already described. At the instant of exposure the claw is free of the film, or in the "out position," as it is termed, so that the sensitized ribbon is absolutely still. When an exposure has been made, the claw, having risen to the highest point of its travel, re-engages with the film and jerks it down ¾ inch, so as to bring a fresh unexposed surface before the lens. As the film emerges from the gate it is picked up once more by the sprocket A, the engagement of the perforations in the film with the sprocket teeth being assured by the two rollers J and K. The film then passes under the guide roller L, and is wound up on the bobbin M in the exposed film box C, the bobbin being worked through the handle that drives the mechanism.

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.

As for the tripod, one cannot be too careful in choosing it. This apparently insignificant detail has a far-reaching effect upon the picture results. Any ordinary tripod used in photographic work may suffice, but its absolute rigidity is essential. A tight head, too, is most necessary, without which the operator will get a side-to-side sway upon the picture. It must be borne in mind that in turning the handle there is a tendency, especially at first, to exert an unequal pressure upon the handle side of the camera, and, unless the support and its head are kept absolutely rigid, the pictures will betray evidences of the defect. The telescopic ladder tripod is very handy for topical work. This, when it is extended and when the camera is fixed, brings the lens some 7 feet above the ground. A cross-rail placed from 12 to 25 inches above the ground, and attached to the rear legs, offers a platform upon which the operator can stand to work his instrument. In this way both camera and operator are brought above the heads of the crowd, and an uninterrupted view can be obtained.

For moving-picture work a special type of tripod head has been evolved, which allows the camera to be moved bodily through both the horizontal and the vertical planes while exposures are being made. Thus it can follow a subject travelling in either of these directions. The movements are upon the rack and pinion principle, a small handle being fitted to each motion so that either can be operated independently of the other. By means of this panoramic attachment the main object in the picture, such as a carriage or an aeroplane, may be followed in either direction. But if both movements are to be completed simultaneously, the operator will require help. One person must sight the object and keep it in the picture by the manipulation of the two handles governing the respective movements of the tripod head, while the other confines his energies to turning the camera handle.

Although the moving-picture camera is built substantially and strongly, its mechanism is comparatively delicate. While it will withstand the hardest descriptions of legitimate work, it succumbs readily to brutal treatment. Although operated by means of a handle, it is neither a coffee mill nor a barrel organ, but a sensitive scientific instrument, and it must be treated as such if the finest results are to be obtained. Rough usage will throw out the registration and alignment. If handled carefully a camera should perform its task for years without needing repair. The effects of wear and tear can be mitigated very appreciably by keeping the moving parts well lubricated with good oil, such as is used for clocks, which neither gums nor clogs the bearings, nor injures the mechanism in any way.