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BRITISH KTNEMATOGRAPHY
Vol. 18, No. 3
Timing Methods
It is interesting to look back at the historical and original work of Marey, as he was probably the first person to embody accurate methods of timing in his photographic records. Accurate timing was, to Marey, one of the most important aspects of his work which he called " Chronophotography."
Method A. Time marker within the field of view of the camera. This is the oldest method and is illustrated and described in
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I . Early Timing Apparatus of Marey showing the Acceleration of a Falling Body due to Gravity.
Marey' s book " Movement " of 1895. He used a rotating pointer driven at a known rate by a carefully regulated clockwork mechanism, and the pointer was included in the field of view of the camera.
The method is simple and is still a useful one today in certain cases, and can be used either with a suitable clock or watch or a small synchronous motor,
Method B. Motion picture camera driven at a known rate. This method is also simple to arrange, but the accuracy is usually rather low, depending upon the type of camera drive. Synchronous motor drive is the most accurate and reliable, but under present-day conditions in this country, the frequency of the public A.C. mains cannot be relied upon and often falls considerably below the standard 50 cycles per second.
For 16 mm. cameras a new synchronous motor has recently been marketed in this country and can be had either for 24 or 16 frames per second.
Method C. Timing marks on the actual film. Timing marks on the actual film are to be preferred in all cases where high accuracy is desirable, but this method entails some modification of the camera.
Time-marking on Film
There are several methods of making the time marks, such as the battery operated tuning-fork time-basa for the Kodak HighSpeed Camera, and simpler time markers Lising larger tuning-forks which can be arranged to generate small sparks which are photographed on to the film. The accuracv of the tuning-fork method is high and is particularly suitable for high-speed cameras.
A simple and relatively crude method of time-marking on the film is to use a small gas discharge tube, as is now embodied in the latest Eastman high-speed camera.
If the discharge tube is operated from the supply mains, as it is intended to be, the remarks made above concerning the accuracy of the supply frequency also apply here. Operation from A.C. mains may also be a disadvantage if work has to be done in the field. This method of time-marking for a high-speed camera is much inferior to thai given by a tuning-fork, and also much less convenient in the subsequent analysis of the films.
In the Kodak time-base, by using a 500 c/s tuning-fork, the marks appearing on the film are spaced at intervals of one milli second. Each mark is onlv a few millimetres length and at 3,000 pictures per second there will be approximated three frames between two marks. With the East