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March 20, 1891.] THE PHOTOGRAPHIC NEWS. 223 PHOTOGRAPHIC SHUTTERS * BY PRANCIS BLAKE. 19. Prosch (Special) Duplex.—Made to order for euryscope wide-angle lenses. Time exposure, squeezing the bulb as quickly as possible, 0'211 s. Instantaneous exposure, direct compression of bulb, 0'031 s. Instan taneous exposure, released compression of bulb—1st test, 0'020 s.; 2nd test, 0'021 s. This is a most conveniently arranged shutter, as it is attached to the front-board of the camera ; and carries on its front face a flange which receives any one of a set of wide-angle lenses. Now, the results which have just been given hurriedly have probably made but little impression on the minds of most of those who are present. A few of you may have had an intelligent interest in the statement of results obtained with some particular shutters with which you are familiar; but, in general, you probably have but an indistinct memory of a lot of unintelligible figures. Nevertheless, if you had had time to carefully digest the results obtained, I feel sure that you would have reached the conclusion that the average working speed of com mercial shutters is about three hundredths of a second, and that while a few of these shutters may be forced to come well within this average speed for test purposes, they are made to do so at the expense of so great a loss of light that they are of but little practical use when so forced. Three hundredths of a second is too slow a speed for photographing the quickest motions of animate or inani mate nature, and the successful pictures obtained with shutters of that speed are due to the fortunate coincidence between the moment of exposure and a slower phase of motion. Thus, with such a shutter, the most rapid phases of motion produce blurred plates, which are thrown into the waste box ; and science receives but a few special— and, therefore, misleading—data in place of the many which the art of photography should be made to yield. This generalisation forced itself upon Mr. Hubbard and myself after two years of patient experimentation with every conceivable form of shutter applied before, between, or immediately behind the lens; and, at his suggestion, we then abandoned all work in that direction, and devoted ourselves to the perfection of the focal plane shutter as described in the quotation from Sutton at the beginning of this paper. I have known of no commercial shutter which passes more than 50 per cent, of the light which falls on the lens during its action ; while the focal plane shutter has been lying idle for twenty-five years, in spite of its ability to pass close on to 100 per cent. What may be called the “ light advantage ” of the focal plane over an ordinary shutter is well indicated by the following considerations. With any form of shutter in which a slot one inch in width passes at a uniform velocity behind a lens one inch in diameter, the sensitive plate will receive but 50 per cent, of the light which falls on the lens during the action of the shutter. Supposing the lens to be five inches in focal length; if the slot is merely moved backward to within one-quarter of an inch of the plate, 95 per cent, of the light falling on the lens during the action of the shutter will reach the plate. Farther consideration of the subject shows that in any single shutter placed immediately before, between, or behind a lens of one-inch opening, the percentage of light passed is equal to the number of inches of slot travels less * Continued from page 188, one times the units of light per inch. Thus, a one-inch slot travels two inches in uncovering and covering the lens. Calling the total light falling on the lens during the action of the shutter equal to 100 units, we have 50 units to each inch of the slot travel. Slot travel two inches minus one=1 inch X 50=50, which is the percentage of light passed. A two-inch slot would travel three inches, and there would be 33} units of light to each inch : — 3—1 = 2 X 33}= 663, the percentage of light passed. Continuing the computation, it will be seen that a 3-inch slot would pass 75 per cent. ; a 4-inch slot 80 per cent. ; and so on until, in order to pass 90 per cent, of the light, it would be necessary to have a slot nineteen inches long pass by the lens in the same time that the focal-plane slot passes over five hundredths of an inch, which is the diameter of the cone of rays from a one-inch lens of five inches focus at a point one quarter of an inch in front of the sensitive plate. Opposed to this tremend ous advantage, there is only one theoretical objection to the principle of the focal-plane shutter; and that is, that all parts of the sensitive plate are not exposed at the same time. But practically this objection does not hold good, since the velocity of slot motion may be made so great that there is no sensible distension of the phase of motion of the moving object. Moreover, the possibility of any distension may be eliminated by setting up the camera at such a distance from the moving object that the angular value of its image on the sensitive plate shall be equal to, or slightly less than, the width of the shutter slot. The focal-plane shutter which I now show you is the outcome of the experimental work carried on by Mr. Hubbard and myself during the last four years. It con sists of a mahogany case attached to the back-board of a 6} by 8} camera. The case is 18} inches long, 9} inches high, and Z inch thick outside measurements. At its centre is an opening in which' may be placed a focussing screen on a 4 by 5 plate-holder. Within the case are two screens, 5} by 54, and 4} by 53 inches in size. They are made of a very light frame-work of bamboo covered with thin tissue paper rendered thoroughly light-proof by the application of a mixture of lamp black and shellac. These screens run freely on two brass wires strained lengthwise across the wooden case above and below the plate-holder opening. Attached to the base of the larger screen is a piece of thin sheet steel pivoted to the corner of the base at one end, and divided on its lower edge into ten notches one tenth of an inch apart. Attached to the opposite corner of the base of the smaller screen is a screw stud, over which the notched piece may be slipped. By this simple bit of mechanism the two screens may be, at will, attached to each other with a slot between them varying from one tenth of an inch to one inch in width by tenths. The exposure is made while this slot is passing over the sen sitive plate, motion being imparted to the screens by means of a steel pin connecting them with a wooden piston, which, in turn, is driven through a brass tube by com pressed air. With a one-tenth inch slot, in connection with a 2B Dallmeyer lens, three tests for speed of this shutter have given the following results:— 1st test ... ... ... ... ... 0'0006 2nd 0'0006 3rd 0-0005 Mean 0'0006
