Interrupted light type of shutter tester. For the study of uniformity of shutter action alone the apparatus shown in Fig. [23] may be employed. A is a high intensity light source, such as an arc or a gas filled tungsten lamp. L is a convex lens, focussing an image of the light source on a small aperture in the screen E. D is a sector disc which, driven by the motor M, interrupts the transmitted light with a frequency determined by the number of openings of the sector and by the speed of rotation, which must be measured by a tachometer. The light diverging from the aperture in E falls upon the shutter S, which for this test is reduced to a narrow slit of one millimeter or less. Passing through the shutter opening the light falls upon the photographic plate P. The principle is simple: If the light is uninterrupted, the plate P is exposed at all points; due to the interruptions, a series of parallel lines of photographic action result, and their distance apart gives a measure of the speed of the shutter at any chosen point in its travel. A performance curve of the French Klopcic shutter is shown in Fig. [24]. The variation in speed lies over a range of two to one. So serious is this defect in these shutters that diaframs are sometimes inserted in the French cameras to cut off part of the light from the lens on the most exposed end of the plate. This expedient produces uniformity of photographic action, but does not overcome the movement of the image, which is one of the chief faults of excessive exposure.
Fig. 24.—Performance of Klopcic shutter.
Fig. 25.—Optical system of shutter tester for Air Service, U. S. Army.
A more complete apparatus, adapted both to absolute speed determinations and to the study of uniformity of action, is that worked out and used in the United States Air Service (Fig. [25]). At A is a high intensity light source, an image of which is focussed by the lens L1 upon a slit E, in front of which stands a tuning fork T, of period 1024 or 2048 per second. The light diverging from the slit is received by a second lens, L2 which is arranged either to focus the slit image upon the shutter curtain or to render the rays parallel, so that an entire camera may be inserted. In the latter case the camera lens L3 serves to focus the slit image on the curtain C. After passing through the curtain aperture the light is focussed by the lens L4 on the rotatable drum D, which carries a strip of sensitive film.
The operation of testing a shutter consists in focussing the slit image on the portion of the shutter whose performance is required, striking the tuning fork to set it vibrating, rotating the drum rapidly and setting off the shutter. There is thus obtained on the sensitive film an exposed strip resembling in appearance the edge of a saw, the number of teeth showing the time interval in vibrations of the tuning fork. Three exposures usually give all the points necessary for a practical knowledge of the shutter's uniformity of action. A point of some importance, learned from numerous shutter tests, is that a focal-plane shutter should be tested in the position in which it is to be used. Aerial camera shutters should be tested in the horizontal position.
Types of Focal-plane Shutters.—A variety of means have been utilized for securing the necessary variation in speed in focal-plane shutters. Their success is to be measured by the actual speed range and by the uniformity of speed attained. In aerial cameras at present in use we find variable tension of the curtain spring, the aperture being fixed; variable opening with fixed tension; multiple curtain openings with fixed spring tension; and combinations of two or all of these methods of speed control. The problem of covering the aperture during the operation of winding up or setting the shutter has led to further elaborations of shutter mechanism. These take the form of lens or shutter flaps, auxiliary curtains, and shutters of the self-capping type. Shutters embodying all these features are briefly described below.
Representative Shutters.—The Folmer variable tension shutter is used on the United States Air Service hand-held and hand-operated plate camera and on some of the film cameras. It consists of a fixed aperture curtain wound on a curtain roller in which the spring can be set to various tensions, numbered 1 to 10. The range of speeds attainable is at best about three to one, or from 1
100 to 1
300 second, considerably shorter than the range indicated as desirable. Its uniformity of travel is variable with the tension, as shown by representative performance curves in Fig. [30]. Lacking any self-capping feature the shutter is provided either with an auxiliary curtain, or in the hand-held camera with flaps in front of the lens, opened by the exposing lever before the curtain is released (Fig. [39]). This shutter is made a removable unit in the 18 × 24 centimeter hand-operated camera, but is built into the hand-held and film cameras.
