The Dictionary Of Photography

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

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Shutters, Instantaneous
the lenses it is in the very best position, and acts for part of the time as a small stop, and therefore should give better definition with a large aperture than any other kind. The last type of shutter is the focal plane, and this is a flexible blind working close to the surface of the plate. This is the best position as to light, for no matter how brief the exposure, the full intensity of the light transmitted by the lens reaches the plate. The action of such a shutter is shown in fig. 120. Such a shutter may depict a moving object with microscopic sharpness, owing to the nar-rowness of the slit, yet deform the image taken as a whole.
Distortion by the use of the focal plane shutter. Strictly speaking, no object of finite magnitude and in motion can be reproduced in true drawing if a focal plane shutter is used, and wonderful as some of the focal plane photographs may appear on the score of sharpness, such production very frequently have so weird and unnatural an appearance, as to show at a glance that something must be wrong. Writing in 1893, Captain Sir William Abney {Photographic Work, September 8th, 1893, p. 425) traced distortion, as caused by shutters, through all its various phases, and he said almost everything that is to be Said on the subject. Sir William Abney explained and illustrated that distortion to which moving bodies must be necessarily subject when the shutter is a slit moving across the focal plane, and he showed that a certain amount of distortion must take place, even when the slit moves before or behind the lens, and near to the lens, the distortion being one way or the other, according as the slit is before or behind the lens. For photo-graphy which is to be an accurate record there is but one place for the shutter, and that is at or near the position of the diaphragm. Sir William Abney says: " 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." Thus it is only possible to accurately define the time of exposure for any particular moving objects when the shutter is close to or associated with the lens; the reason of this being obvious when it is remembered that when a focal plane shutter is used, different moving objects in a scene may receive various exposures ranging from the whole time the slit is travelling across the plate to nothing.