The film is taken from the upper unexposed film-box, passed over the sprocket, through the gate, under the sprocket and wound into the lower exposed film-box.
The Lens of the Williamson Camera.
(For explanation see p. [43].)
The Adjustable Shutter of the Jury Camera.
In cinematography, as in ordinary photography, the judgment of the brilliance of the light and of the right stop to use on each occasion, is the one important factor for which mechanical provision is impossible. This is because of the extreme variation of the light conditions. But, while no hard and fast rules concerning exposure can be laid down, it is possible to give the beginner a little guidance to keep him on the safe side. Practice alone can make perfect, and experience is the more necessary because the cinematograph is an all-the-year-round machine. In topical work the operator is compelled to make the most of the existing weather conditions, no matter how deplorable they may be.
Under these circumstances it is well to have what might be termed a very flexible lens. The lens, that is to say, should be fitted with the means of varying the size of the aperture, and varying it within wide limits, according to the light conditions. The simplest way of achieving this is by means of what is called an iris diaphragm.
To illustrate the functions of the iris diaphragm we will take the Williamson instrument. This is fitted with a Zeiss-Tessar lens of 2-inch focus. By the aid of the iris diaphragm the diameter of the aperture may be varied from approximately ⅗ to ⅛ of an inch. Now it is obvious that more light can be passed through the lens with the larger, than with the smaller, aperture. While the larger aperture would do excellently for filming a football match on a dull day in mid-winter, it would be useless for a seascape on a cloudless day in July. For the latter the smallest aperture would suffice. But the requirements between these two extremes must be met: in other words the aperture must be adapted to intermediate demands. By turning the milled ring in which the iris diaphragm is mounted the size of the aperture can be varied even to a minute degree and thus adjusted to any sort of light conditions. For the guidance of the operator the total rotary travel of the ring is graduated to six different definite points or as many different sized apertures. These are as follows:—