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CAMERA APPLIANCES
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rapid exposures the focal plane shutter is perhaps the favourite.
The principle of this shutter is simply that by the movement of a dark blind or blinds immediately in front of the sensitized surface of the plate, the whole plate is left exposed to the light passing through the lens usually for a very small fraction of a second, although the blinds can be arranged for time exposures if necessary. In the focal plane shutter supplied with the Ross Panros Camera there are two short blinds winding from one roller on to another without any strain beyond the tension of the springs. One blind begins to move first, and after an interval sufficient to attain the desired aperture the second blind, which is automatically released, follows. The mechanism connected with the two blinds then becomes so locked that the distance between the blinds, i.e. the aperture, remains constant during the whole time the blinds are in motion. The increase or decrease of the width of the aperture is controlled by a milled head, and the actual speed of the blinds increases automatically as the aperture is decreased.
Another important feature of these blinds is that they are self-closing, one blind overlapping the other and remaining in this position during the next setting of the shutter, so that should the slide of the plate -holder have been withdrawn, no light will reach the film or sensitized plate.
This camera being supplied with a focussing hood, it is also arranged that the focal plane blind can be opened for focussing purposes.
Fig. 57.