A certain number of the readers of this journal are occupied with photography, and all assuredly are interested in this marvelous art, whose progress is so remarkable. So it has seemed to us that it would be of interest to treat of a question that is the order of the day. We desire to speak of those photographic apparatus called instantaneous shutters.
Numerous apparatus of this kind have been proposed to the public, and several even have been described in this journal, but we have to state that, despite the success in certain cases, none of them has proved remarkable for its qualities and superiority. This is due, we believe, to the fact that inventors, while showing arrangements that were often ingenious, have not always taken into account the end that the shutter is to subserve, and the qualities that it must possess in order to attain such end.
In face of the progress made by extra rapid dry processes, the question of shutters has become the most important, since cabinet-making, optics, and photographic chemistry give us apparatus, objectives, and products which, although they will doubtless be improved upon, satisfy for the present all our needs.
What is understood by instantaneousness? To our knowledge, no definition thereof has as yet been given. For our part, we propose to style "instantaneous" any photograph that is taken in a fraction of a second that our senses will not permit us to estimate. The shutter is the apparatus which allows the light to enter the photographic chamber during this very short time.
In order to examine the different rules that govern the question of shutters, we shall take as an example the type styled the "Guillotine."
This apparatus, as every one knows, is a stiff plate containing an aperture and passing over the line of the rays of light. Some place it in front and others behind, while others again place it within the objective. Let us examine and discuss what occurs in the three cases. Suppose a rectilinear objective of the kind most usually employed in instantaneous photography, and an object, A B, that we wish to reproduce (Fig. 1), the objective being provided with any sort of diaphragm. The point, A, sends a bundle of rays, a"b", to the first lens. Here they are slightly refracted, and then go on parallel lines to the second lens, where they are again refracted and form at A' an image of A. It is this image that we see upon the ground glass, and which makes an impression upon the sensitive film. The point, B, behaves in the same way and gives an image at B', but, as will be at once seen, the image will be reversed. In our figure, A corresponds to the sky and B to the earth. If, then, the shutter passes in front of the objective, it will first allow of the passage of the rays which come from the sky, then, on continuing its travel, it will unveil the landscape, and lastly the ground.
As it is submitted to the law of the fall of bodies and has a uniformly increasing velocity, it follows that the time of exposure will uniformly decrease between A' and B', and that the sky will pose longer than the foreground. Such a result is contrary to all photographic rules, which require that objects shall pose so much the longer the less they are lighted. This position of the "guillotine" shutter is absolutely false, and must be altogether discarded. If the shutter be placed behind the objective, it will follow, as a consequence of the same demonstration, that the time of exposure will go diminishing from B' to A', and that the foreground will be exposed longer than the sky. The solution is logical, then, and will permit of obtaining excellent negatives.
Let us now examine how the image, A'B', is formed. The point, A, appears first, and becomes lighter and lighter up to the moment at which all the rays that emanate from the point, A, are unveiled. The point, B', is not yet visible. As the shutter continues its travel the point, B', appears in its turn and becomes illuminated like the point, A'. At this moment the objective is completely uncovered; the image, A'B', is perfect, and possesses its maximum intensity. Then the point, A', gradually becomes obscured and disappears; and the same is the case with all parts of A'B'. The image is developed progressively from A' to B', and makes its impression upon the sensitive plate successively--a fact which, as may be conceived, may have its importance. If, for example, we are photographing a ship that is being tossed about by the sea (and we borrow this example from our colleague, Mr. Davanne), the image of the top of the mast will not be formed at the same instant as that of the base, and if the motion of the mast has sufficient extent it may take on a curved form, due to the fact that it has effected a movement between the moments during which its apex and base were being photographed.
Upon placing the guillotine shutter in the optical center of the objective, what will occur? The shutter will permit the passage of an equal fraction of the rays derived from A and B, that is to say, the image will be complete from the first instant of the exposure. The points, A' and B', will be illuminated precisely at the same moment. As the shutter continues its travel, a fresh quantity of rays coming from A and B will be admitted, and the image will be illuminated more and more up to the moment at which all the rays can pass. It will then possess its maximum intensity. Then a portion of the rays from A and B being intercepted, the image will become darker and darker until complete extinction. The image here, then, is not produced successively as in the former case, but is entire from the beginning. In this case the image of our mast cannot be misshapen, since it has been accurately photographed at the same moment.
The true place for the guillotine shutter, then, from a theoretical standpoint, is in the interior of the objective. Are there any other advantages to be gained by so placing it? Yes; it is easy to understand that for the same time of exposure, and consequently for the same result, the aperture may be so much the smaller in proportion as the optical center is approached.