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Fig. 2. A rock site installation of camera and power source.
upon the time of the year, one may reasonably expect reflected-light readings on average countryside to be between 800 and 1600 Weston at these hours. With this lighting, quite satisfactory pictures were obtained at frame rates as high as 4000 per second when the background consisted of either a lightcolored sand or a light-colored clay. The chief problem that arose was that of obtaining sufficient contrast between the particular part of the ground in which we were interested and the background (see the second picture in the sequence of Fig. 3). In some desperation we finally resorted to the expedient of spreading powdered coal over the surface of the ground immediately behind the object we were photographing. Since the object seen by the camera was of a light color in general this provided a reasonable degree of contrast in the final photograph. Without such precautions it was sometimes extremely difficult to distinguish between the rise of the ground, in the area of interest,
and the terrain behind it. When the field to be photographed is rather dark, as, for example, when the ground is covered with sagebrush, it is necessary to reduce the frame rate of the camera to about 2000 frames/sec when the light reading is of the order of 800 Weston if satisfactory pictures are to be obtained. Due to the requirements of the program, it was necessary to set up the high-speed motion picture cameras a few hours in advance of the expected time of detonation of the charge. This was done to allow the photographic crew time to set up and man other equipment at stations located a mile or more distant from the charge site. It was thus generally impossible to readjust the high-speed motion picture cameras for the light conditions existing at the time of the detonation. Even in the event of considerable changes in the prevailing light, as were occasioned by storms which frequently arose in the late afternoon, we were quite surprised at the quality of the pictures that were
R. M. Blunt: Photography of Underground Explosions
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