With the shutter at the back of the lens the attitude of the man would be reversed. It might show him as in the act of falling forward, for the image of his feet would be first exposed, and the hat last. Of course, these are exaggerated examples of the want of truth that might be exhibited, but they are possible.
With the shutter close to the plate the distortion is much more likely to be marked than in either of the above cases, for even with a guillotine shutter on the lens in which the slit is mod erately narrow, the image seen through it at one instant includes more than that width, for, were this not so, the image would not be of bigger dimensions than the lens itself. With the shutter close to the plate, on the other hand, it is the width of the slit which determines the amount of image exposed. The accompanying silhouettes show what happens when a narrow slit passes across the plate. A is the image of a man actually photographed with a shutter at the diaphragm. At the proper distance from the lens this image was made to move at the rate a man would move, and exposure given to a plate, with a guillotine shutter next the plate, the slit being narrow. B is the photograph obtained of A when the figure was moved in the di rection he faced ; and C when the motion was in the reverse direction. B looks tum bling forward, while C looks tumbling back ward.
When the shutter is placed near the diaphragm the conditions are totally changed. The exposures for all parts of the image commence and finish together, and are of precisely the same duration. If it be necessary to give a certain exposure to an image, for satisfactory results, it is manifest that it does not matter theoretically whether the whole image is exposed for such time, or whether the different parts are exposed successively for the same time; but it matters a good deal practically, for the total time occupied in giving the successive exposures must necessarily be longer than that in which the exposures are given at the same time; and if the camera be held in the hand, the probability is that during the longer total of successive exposures a greater movement in the hands of the operator will be found than during the shorter and simultaneous exposure.
The accompanying cut shows the movement of a hand camera in a quarter of a second. The top line is the movement when held in both hands without any support; the second line shows the movement when held under the chin and above the chest. The third line shows prac tically no movement. In this case the camera was held in two hands, and supported on an umbrella handle. The lowest line shows the movement when held against the bottom of the chest, so that the beat ing of the heart and the breathing interfered with the steadiness. The writer cannot help thinking that the
diagram is very instructive, and shows that practically perfect steadiness can be attained by the use of a sup port.
These records were obtained by fixing a vertical card, in which a pinhole was pierced, to the camera, and allowing the light from a stationary lamp to send a beam through the hole, the image of which was focused on a plate moving horizontally.
On the ground, then, that distortion, and also total time of exposure, is minimized when the shutter is placed at the diaphragm, the writer has come to the conclusion that practically that is the best position for it. Another point is this : that the movement necessary in the shutter itself is less in this than when in any other position, as the diameter of the diaphragm must or dinarily be smaller than that of the surface of the lens or plate itself.
Let us now consider as to the circumstances which favor the movement of a camera when held by the hand, owing to the release of the shutter. In the first place, it must be borne in mind that the camera and shutter are practically one structure, and that when the shutter is placed ready for exposure, the combination has its center of gravity in some one position. If, when the shutter is moving, the center of gravity shifts to another position, a movement, slight though it may be, must take place unless the camera be rigidly supported. There are two ways of meeting this difficulty; one is to make the motion of the shutter symmetrical on each side of the lens (in other words, to close to the center); and the other is to make the movable parts of the shutter as light as possible compared with the camera. Now, most shutters are light, but there are many which do not close centrally, and, in fact, as we shall see shortly, such a method of closing is detrimental, except when it is placed at the diaphragm. Another point is, that the release of the shutter should be as light as possible, and should not be by a push or pull, but by a pressure. All rifle shots know that the sights of a rifle are invariably moved from the mark if the trigger is pulled, whereas when a pressure is applied between the thumb and finger this does not obtain. The release, then, should be by a pressure, and all other methods should be avoided. With the pneumatic arrangement, of course, this does not apply, for the release is then given by an appar atus which is not rigidly attached to the camera. For hand-cameras, however, the pneumatic re lease is inconvenient, though it can be effected by inflating the tube by the mouth instead of by pressure with the hand.