FOCAL PLANE SHUTTER A shutter that works immediately in front of the plate, and now commonly fitted to the highest class cameras. It is believed to owe its practical form to B. J. Edwards, who in 188 published a description of his apparatus ; but some eighteen or twenty years previously the principles were known to William England, who used a crude device working on the same principle a long time before Edwards's ideas were published. England's device was ki board containing a horizontal slit which travelled in front of the plate in the same manner as the drop shutter of the present day travels in front of the lens, and it was caught in a kind of bag suspended from the camera. A shutter of a similar nature had previously been experimented with by Dr. Mann, who recognised its power to utilise the whole of the light admitted by the lens. Prom 188z, in which year Edwards lectured upon his invention before the old South London Photo graphic Society, until 1892, the focal plane shutter seems to have been lost sight of, but in the latter year the Thornton-Pickard Company placed upon the market their now well-known shutter of this type, and, simultaneously, Stolze and Ottomar Anschutz, quite unknown to each other, were both working to the same end—the simplification and perfection of the shutter, more particularly, perhaps, in the means of adjusting the slit.
The principle of the shutter is as follows : A roller blind, containing a slit or aperture the whole length of the plate, is made to travel immediately in front of the sensitive surface. Assuming that this gives an exposure of A-th of a second, by reducing the slit to one-fifth of its original width the exposure is reduced to of a second ; again, by increasing the tension of the spring by five or ten times an exposure of or nth of a second respectively is obtainable. Most subjects come within the range of nth to of a second. The efficiency of the focal plane shutter is greater than that of the lens shutter ; some types of lens shutter pass for only a very small proportion of the total exposure the whole of the light that the lens is capable of transmitting, much of the time of exposure being taken up by the shutter in uncovering and then covering the lens, there being only a brief period when the lens is quite uncovered. With the focal plane shutter the whole of the light admitted by the lens is avail able for action upon any particular portion of the plate uncovered by the slit ; and another advantage is the high speed at which the shutter can work, rendering it indispensable for " in stantaneous " work of any kind.
It is sometimes affirmed that the focal plane shutter gives distorted results, but its advocates affirm that the distortion (if any) is practically negligible. In practice, distortion may be divided into two classes: (a) that in which the outline of any one body is rendered untruth fully, and (b) that in which the relative position of a group of figures or objects is incorrectly delineated ; this latter may, of course, include the former, and of the two is the more serious. While it matters but little if an image of, say, a rapidly moving railway train is slightly longer or shorter than would be the case were the train photographed at rest, it is a serious matter if the result of a closely contested cycle race is rendered incorrectly. To minimise any possible error, the following points on the correct use of the shutter should be remembered. If the image (on the focusing screen) of the subject being photographed is rapidly moving in the same direction as the slit in the shutter, the slit must be made to move at a much greater speed than if the image and slit were travelling in opposite directions. The use of a shutter travelling at great speed in an opposite direction to that of the image has a tendency to shorten the object, while, on the other hand, the use of a shutter travelling in the same direction as the image may lengthen it. But such distortion is so trifling that it would be practically impossible to discover it. When engaged in high-speed work a good distance at which to work is about seven or eight yards from the subject. Using a 6-in, lens at a distance of eight yards, for objects moving at right angles to the camera these exposures would be about correct :— Trains, horses galloping, cycle rac ing, etc. . . . . sec.
Men racing, jumping, etc. . sec.
Diving . . . . . sec.
If the object is taken end on—that is, coming towards, or receding from the camera—exposures three times as long as the foregoing may be given. By doubling the distance from the object, the length of exposure may be doubled.
To develop plates which have received a minimum of exposure, a very energetic developer must be employed. The following formula is recommended, using equal parts of A and B :— A. Pyrogallic acid . 4o grs. 4'4 g.
Metol • . 35 ,, p, Potassium meta bisuiphite - pp to „ Potassium bromide 15 „ 1-6 „Water to . . 20 oz. t,000 ccs.
B. Sodium carbonate. 3 oz. z64 g.
Water to . . zo „ z,000 ccs.