The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Shutters, Instantaneous
One of the best ways of studying the effects of the focal plane shutter is to get some one to roll a thin iron hoop, painted white, rapidly along a level path, and then to photograph it broadside on, the motion of the slit being sometimes vertical and sometimes horizontal. If the camera is held in the hand and is moved a little during the exposure the image may be quite sharp, but new factors, as regards distortion, will step in, a matter fully dis-cussed by Abney in the place mentioned above.
The speed of the shutter\ or rather, the duration of the exposure given by it may be determined by experiment, and this with great exactness, when an ordinary shutter close to the place of the diaphragm is used, but, as above explained, this is not the case when a focal plane shutter is employed. The following, by the Reviser of this Dictionary, appeared recently in The Amateur Photographer (March 8th, 1901), and it touches on some of the chief questions of distortion and speed.
There are two distinct usages in stating the speed of a focal plane shutter: one being to specify the period during which any assignable point on the plate is exposed, while the other is to specify the whole time of exposure, that is to say, the time which it takes the slit to travel from one edge of the plate to the other. In illustration let us suppose that a plate is ten inches square, and that the slit of the focal plane shutter is one-tenth of an inch wide ; further, let us suppose that this slit rises at a uniform speed from the bottom of the plate to the top in a period of one second; assumptions rather extreme by the light of ordinary practice, but convenient as illustrations. In this case the ex-posure of any assignable point on the plate will be one-hundredth of a second, while the total exposure will be one second. We can make the case clear by studying the movements of a bar or pole stretching across the scene to be photographed ; this pole moving at such a rate as to cross the field in one second. If the bar descends in the field of view, and the image formed by the lens happens to hit the shutter slit, the image of the bar will travel with the slit, and will be diffused over the whole plate, and so will be lost; while if the image does not happen to hit the slit, or correspond with it, the moving bar will have absolutely no influence on the plate, or in other words, the camera would in this case miss the bar altogether. If instead of descending in the field of view the bar ascends, the shutter will catch its image