Fast plates and large diaphragms are the order of the day, and the range of work will be circumscribed indeed if exposures have always to be given by means of the lens cap. Such exposures must be haphazard if they are timed with a watch, and are of less period than, say, two or three seconds; whereas the average exposure required for landscape in good light is a small fraction of a second. The forms of shutter available may be divided into four classes according to their position, and the manner in which they work. (1) The focal plane close to the plate. (2) In front of the lens. (3) Behind the lens. (4) The diaphragm shutter between the components of the doublet,
The focal plane shutter is attached to the camera immediately in front of the focussing screen and dark slide. Some ingenuity is required in fixing it on field cameras as it must form part of the swing back. Theoretically it is the very best kind of shutter for photographing very rapidly moving objects. Its construction has been brought to the very highest perfection, and for very short exposures it works with much less loss of light than others. In principle it consists of a roller blind with an adjustable slit which is caused to travel with great speed across the surface of the plate. For exposures varying between 1/60 and 1/700 sec. no shutter can compare for efficiency with the focal plane. In practice the operator will more frequently give 1/50 to 1/10, as full exposure of the plate is the first consideration. At such moderate speeds (and still more, of course, with 1/2 sec. or 1 sec.) focal plane shutters are not recommended, owing to their tendency to jerk and cause vibration in the camera.
The Thornton Pickard type of roller-blind shutter is still often supplied with field cameras fixed on the sliding front, and the lens flange screwed on to it. Though sometimes regarded as old-fashioned this is a very reliable piece of apparatus, very durable, and in practice we have found the speeds from 1/15 to 1/90 sec. fairly accurate. It is not, however, noiseless; and no provision is made for lower speeds such as 1/2 sec.
The shutter just mentioned is often employed as a movable cap to be placed on the lens just before exposure. The blind will not always exclude bright light for any length of time, and care should be taken not to open the slide until the last moment. There are also one or two good pneumatic shutters made to rise and fall in a manner giving fuller exposure to the foreground than to the sky - often a great desideratum in landscape work: but the range of speed is necessarily limited.
Of these there are many approved types: the "Koilos," "Sector," and "Automat" - to mention only a few; and the moderately priced "Unicum," which is probably more widely used than any other by amateur photographers throughout the world. There is no doubt that the consensus of opinion would be in favour of this kind of shutter for nearly all classes of work. It is equally serviceable for high speeds, half-seconds, and time exposures. It works noiselessly, and without the least danger of vibration, whether released by pneumatic or hand pressure. Many patterns are "self-setting," and the higher qualities give a range varying from 1/300 to 1 sec. If any fault is to be found it is that the speed markings are sometimes quite arbitrary, and need to be tested.
Messrs. Ross & Co.'s "Multispeed" shutter, introduced last year, is a much more complicated instrument, and marks an entirely new departure in diaphragm shutters. It seems to combine all the advantages of the best between-lens shutters when working at useful low speeds as 1/2, 1/4 , and 1/5 sec., and is at the same time capable of the highest speeds attainable by the focal-plane shutter; and this without the danger of lateral distortion, which will unexpectedly manifest itself at times in focal-plane exposures.
For a small fee many of the leading firms undertake the testing of shutter speeds.
Sir W. Abney has devised a rough-and-ready method, which may commend itself to enterprising readers. A small silvered bead is attached to the rim of a bicycle front wheel on which there is a cyclometer. The wheel is caused to rotate, say about three times a second when the machine is upturned. The speed may be judged by the click of the cyclometer, the number of clicks in each second being counted. If the sun shines on the bead, and an exposure be given in the camera, taking in the revolving wheel, an opaque circular line will give the track of the bead. For example, suppose the image on the plate shows a segment of 780, and the wheel was rotating 37 times a second. The time of exposure was therefore 78.
(3.7 x 360) or 1/17 sec.
A more elaborate method for testing the efficiency and speed of a shutter was communicated to the Royal Photographic Society last year by Mr. E. A. Salt, and is illustrated in the accompanying figure. The apparatus records durations of exposure down to about 1/100 sec. On the right is a light-tight box or camera fitted with a lens, L, in sliding box for focussing. On the left is an incandescent burner illuminating a slit, S, in the board, CC', shown in section, with front view immediately below. This illuminated slit is focussed on a dry plate, P, by means of a silvered mirror, M, set at an angle of 450. The dry plate is supported on a carrier capable of rotation at definite speeds. The shutter is placed in front of the slit. On the release of the shutter a point of light is first recorded on the rotating plate, F, broadening into a circular band representing full aperture, and tailing off again into a point on the completion of the exposure. By applying the developed plate to a home-made protractor on glass (secured by copying a drawing in the camera) the number of degrees covered can be read off, and, deducting the width of the slit image, the duration of exposure is ascertained. The second figure shows the form of protractor adopted. In order to avoid crowding, it is divided into 125 divisions only, each division in practice being read as representing 40. With the motor revolving twice a second each degree will therefore indicate 1/1000 sec. A special spring motor, sold by Messrs. George Adams & Co., for driving gramaphones, is used to rotate the plate, which is rigidly held in a carrier of simple design. At two revolutions per second this motor runs with great accuracy.
All photographic apparatus is now so readily accessible and so easily obtained that it is not worth while to include in this volume working drawings for those few who wish to make their own cameras. Some of the firms in the Midlands publish catalogues containing all the necessary information, with sheets showing the brasswork, bellows, and other parts supplied ready for the purpose. Broken parts may generally be replaced in this way in case of mishap. The home-made camera need not be a very elaborate instrument; the only absolute necessity for taking photographs is a light-tight chamber with some means of focussing the lens. A box, with a groove at one end for the dark slide, and of the length just sufficient for the lens when focussed at infinity, will do a lot of good work; the lens may slide out in a "focussing jacket" or a brass tube, when objects nearer the camera than, say, 20 ft. have to be considered. Of course, many views cannot be attempted without the rising or swing front.
Now that the rising and swing front has been adapted to do the work formerly accomplished by means of the swing back, it would seem that the old-fashioned field camera will gradually drop out of use. A new type of universal camera finds favour from its portability, variety of movements, and ready adaptability, either for instantaneous snapshots or for well-considered compositions focussed upon the ground-glass screen. We should advise the reader to turn to the chapter on Hand Cameras, in order to see whether any of these types will meet his requirements.
Fig. 6. The Protractor.