International photographer (Jan-Dec 1935)

August, 1935 T h INTERNATIONAL PHOTOGRAPHER Fifteen of an inch or less, the thickness of a piece of paper, during this exposure time. Thus, although the object photographed maybe moving at a high rate of speed, and although the film itself is traveling rapidly all the while, the time of exposure is so short that neither object nor film can move an appreciable distance during the exposure. As a result, good defnition of highspeed objects can be obtained since, at maximum camera speed, the film moves only 0.004 inch during the full time of each exposure. In the second place, during this extremely short exposure time, the intensity of the illumination is very great, for a perfectly normal exposure may be made by a single flash using an f/2.5 lens with ordinary panchromatic film. All our readers know by experience how well an object must be illuminated to secure a normal exposure with an f/2.5 lens and a fast 0.005 second shutter. They can, perhaps, realize how intense an illumination must be required to secure the same exposure in 0.000005 second, the equivalent of a shutter speed 10000 times as fast. Finally, the exact instant at which each flash occurs can be determined very accurately by means of a commutator mounted directly on the shaft carrying the film sprocket so that, whatever the actual camera speed, the successive exposures on the film will be framed for subsequent projection, accurately and automatically. It is indeed hard to imagine any type of mechanical shutter capable of giving a normal exposure in 0.000005 second with a constantly illuminated object and, at the same time, being capable of repeating such a procedure 1200 or more times per second. The accompanying illustration, Figure 1, shows a form of camera designed to take short runs of superspeed pictures. The upper magazine reel holds 100 feet of 35-millimeter negative which is fed down over a portion of the circumference of a 5-inch diameter driving sprocket and thence onto the take-up reel below. The exposure is made while the film is moving on the driving sprocket and forced to lie flat against the rim of the sprocket by upper and lower guide rollers. The optical system of the camera is such that the slight curvature of the sprocket over the %-inch length of each frame does not appreciably disturb the focus. By observing the back side of the film through two holes in the driving sprocket by means of a telescope in the rear of the camera, one may focus the adjustable lens mounting and align the camera on the region to be photographed. While the mechanism of this camera is quite simple compared to that of the standard intermittent-action machine, all parts must be made and fitted with considerable accuracy so that they shall function smoothly at the high speeds employed. A single slide gives access to the whole interior of the camera and, by employing black paper leaders for the film on the special lighttight magazine and take-up reels, loading and unloading may be accomplished in the daylight. The other view, Figure 2, shows that two independent electric motors are used to drive this superspeed camera. One of these motors, mounted on the body of the camera, is connected directly to the shaft of the take-up reel. The other motor, mounted on the camera base, drives the shaft carrying both the external framing commutator and the internal film sprocket by means of a belt. The use of these two motors and the proper design of the film path with the absence of any loops or film slack are vital to secure the quick initial acceleration and uniform travel at high film speeds obtained in this camera. The entire film magazine must be emptied with each shot, since it is impracticable to attempt to stop the film in transit at such speeds. The actual speed of the camera may be controlled by adjusting the voltages on the driving motors. At speeds below 500 frames per second the film may be stopped in transit, if desired. Figure 3 shows the portable, table-type electrical power unit, which is energized from 60-cycle power mains, together with two large mercury-arc stroboscope lamps used for obtaining pictures at the rate of 1200 per second. A much smaller model of the same type of stroboscope, which is designed primarily for visual stroboscopic work, may, nevertheless, be used to take high-speed movies up to the comparatively "slow" speed of about 120 frames per second. When superspeed speed movies taken at the rate of 1200 frames per second are subsequently projected 16 frames per second, the apparent speed of motion is slowed down to 75:1 and an event actually occurring in one second takes one and onequarter minutes in showing. It should be borne in mind that these results are reflected light photographs and not merely the silhouettes obtained in most of the earlier forms of high-speed movies. Equipment of this kind is extremely valuable in scientific and engineering studies of the motion and vibrations of highspeed machinery such as is found in the automotive, aeronautical, electrical, textile, woodworking and other industries. From the point of view of a movie camera, the necessary stroboscopic light source limits the field of action to close-ups of a somewhat limited area. Nevertheless, many surprising shots of general interest have been made at these superspeeds, such as the kicking of the football, the stroke of a golf drive, the fall of an animal, Ihe flight of birds and insects, the rapid movements of a snake's tongue, the interesting motions in a jet of water, the beautiful phenomena exhibited by a spatter of liquid drops in a pool or against a smooth hard surface, the shattering of objects, the impact of bullets, the wink of the eye and other physical motions, and so forth. In addition to the regular practice of taking 1200 full 3/4-inch frames per second, it is possible, by reducing the effective frame length on the negative and driving the sprocket at somewhat greater speeds, to attain three or four times this pictorial speed under certain conditions. Such "compressed" negatives must, of course, be separated to 3A of an inch frame spacing when printing the positive film for projection. There is an old saying that the hand is quicker than the eye. While this may be true for the unaided, credulous eye of the victim of the old-time shell game, nevertheless the eye, aided by the superspeed movie camera, can convert the sudden motions of a magician or the fastest efforts of a great athlete into something far slower than the hesitant plodding of a reluctant schoolboy past the old swimming hole on a June morning. ROY DAVIDGE FILM LABORATORIES ••• An Exclusive "Daily" Laboratory ••• Quality and Service 6701-6715 SANTA MONICA BOULEVARD GRanite 3108 Please mention The International Photographer when corresponding .with advertisers.