Since the size of the uncovering aperture should be considerably greater than the dia phragm, especially when the angle of view is large and the shutter is some distance from the lens, the exposure time for equal velocity of the moving parts of the shutter will be necessarily greater, and particularly the initial and final periods, during which the shutter is not com pletely open, become of considerable importance in regard to the efficiency of the shutter (§ 129), and the sharpness of the images. A shutter placed elsewhere than at the diaphragm should not open from the centre, since, whilst it is partly open it intercepts the oblique beams 129. Efficiency of a Shutter. Except in the case of a shutter working in the plane of the image, a shutter always takes a certain time to reach its maximum aperture, after which it remains fully open until it begins, in a similar way, to close gradually. This characteristic will be understood by studying Fig. 96, which is traced from a cinematograph film and repre sents the complete action of a diaphragm shutter (P. G. Nutting, 1916). The time of exposure of each of the images is irno,000th second, the interval between two successive images corresponding to I/L000th second. When the shutter (quite a good one of its type) is set for an exposure of i/Iooth second it takes about iii,000th second to open ; it remains at its maximum aperture during another 4/1,000th second and takes a further 3/1,000th second to whilst allowing all or part of those only slightly inclined to the axis to pass through, thus exaggerating the differences of illumination between the centre and the edges of the image (§ 54). If, however, the shutter begins to open from one edge and closes when at the opposite edge (e.g. by the movement of an opaque screen containing a rectangular aperture), at the beginning and the end of exposure, the image will be formed solely by the portions of each beam traversing the diaphragm in the peripheral region. Since the rays which pass through the outer zones of a lens are always those which have the greatest aberrations, the image will necessarily lose a certain amount of its sharpness if the time during which the shutter is incompletely open represents an appreciable fraction of the total exposure time.
Very fortunately these different drawbacks only occur to any great extent when the shutter is at a considerable distance from the lens. Certain of these shutters working quite near the lens give satisfactory results when they are not required for shorter times than 1/25 second ; they are quite suitable for portraits or landscape photography.
close, making a total exposure time of about IT/L000th second.
A moving object, brightly illuminated, at least over part of its visible surface, can act on the photographic emulsion during almost the total time that the shutter is working, say during i/xooth second. But the illumination received by the sensitive film will be very far from representing x/iooth of the exposure that it would receive during one second, supposing that the times taken for the actual opening and closing are negligible. Thus, in photographing a moving object, the disadvantage of an ex posure of itiooth second in giving blurring of the image is not entirely compensated by the full benefit of an exposure of this duration for the shadow details.
By a simple method we can determine with a sufficiently close approximation the quantity of light transmitted by a shutter during its complete working as represented above, relative to that which would be transmitted during the same time if the shutter were fully open. We
trace on a card of uniform thickness on any suitable magnified scale the successive limits to the beam transmitted by the shutter, and after cutting out each of the shapes so obtained such as those in Fig. 96, we ,can weigh the eleven pieces together and also the one piece repre senting the full aperture. Suppose it were found that the eleven separate pieces together weighed 4-66 grm, and that the single piece representing full aperture weighed o-71 grm., then, since the weights may be considered as proportional to the surfaces, the ratio 4-66/(o-71 x o-6 mea sures the quantity of light passing through the shutter, taking as unity the quantity which would traverse it in the same time if the shutter were fully open the whole time.
The efficiency of a shutter is the ratio of the quantity of light effectively transmitted by the shutter during the local time of exposure to the quantity of light which would have passed through the full aperture of the lens in the same section of each beam, in any plane whatever, is reduced proportionally, and, even supposing that the shutter does begin to close as soon as it has finished opening, it will still have trans mitted the complete reduced beam during a certain definite time. Thus it can be seen that, except in the case of a shutter working in the plane of the image (for which the efficiency is unity), the efficiency of a shutter increases when the exposure is increased and when the working aperture of the lens is reduced. Thus it is obviously the minimum efficiency of a shutter which characterizes it best, since the question of efficiency is of special importance when the exposures are very short and are thus being made with the full aperture of the objec tive.
Under conditions of working of a shutter of time. Or another way of expressing it is to say, that the efficiency is the ratio of the time which would be necessary to transmit through the full aperture the same quantity of light as is effec tive in the local exposure to the time of the local exposure itself.
As the local exposure time is generally limited to a maximum depending on the speed of the images of the moving objects included in the field, and is only just sufficient to obtain a photograph in correct tone values, a shutter should be considered as more nearly perfect the more nearly that its efficiency approaches unity, all other conditions being the same.
It should be noted that as far as we have considered, the efficiency of a shutter varies both with the exposure time and with the dimensions of the effective aperture. At the shortest exposures which can be given by some shutters, the section of the transmitted beam starts to decrease as soon as it has ceased increasing, being only momentarily at full aperture. On the other hand, at relatively long exposures the times taken by the shutter in opening and closing are very much the same as in very short exposures, the difference of time in the exposure being due almost entirely to the increased period at full aperture. When the diameter of the diaphragm is reduced, the which the movement is described by Fig. 96, the efficiency would be called o'6o, or, more generally, 6o per cent.