To allow of two subjects being taken on the two halves of a single plate, a groove is sometimes provided in the camera back. A piece of black card can be slipped in to mask the half of the plate to be shielded whilst the other half is being exposed. To permit of stereoscopic pic tures being taken on a single plate, some cameras are fitted with a partition, made pleated like a camera bellows, and guided between elastic cords. It is fastened to strips of wood, which in turn fit into recesses in the camera front and back.
The sizes in which cameras of the above pattern are most generally used for commercial photography and groups are whole-plate (8i x 61 in.), ro x 8 in., and 12 X ro in.
All the above cameras have square bellows and a reversing back.
For the sake of reduced weight and bulk, the stand camera for amateur use is almost always made with taper bellows, as shown in Fig. 130. While less rigid than the square-bellows pattern. and less satisfactory in affording proper parallel ism of front and back, it embodies all the movements of the heavier model, and, in the case of some of them, to a greater degree. The size of this light-model camera which is by far the most largely sold and used is 61 x 41 in. Larger sizes, e.g. 81 x 6i- in., ro X 8 in., 12 X ro in., etc., are made, but the smaller, 4 x 31 in., and 3 X 2 in., are now seldom used.
In the use of cameras for technical or archi tectural photography, it is advantageous to focus on a ground glass ruled in squares, which allow the operator to judge the verticality of upright lines and the size of particular parts of the subject. For landscape work it may suffice to rule on the screen two vertical and two horizontal lines dividing the sides into three equal parts, this plan serving to set out the strong and weak points of composition. The frame of the focussing screen is usually hinged to the camera." i56. The dark slides employed with portable stand cameras are generally of the double pattern, with rigid shutters. There are also, at least in the 61- x 4 in. size, single metal holders, as commonly used for hand cameras. By virtue of their construction, double slides with curtain shutters (as popular in France) afford a better protection of the plate against entrance of light whilst the slide is being carried or is in use, for both plates are completely enclosed by he two curtains, and when one of the plates is uncovered, the curtain that is drawn back slides over the other, thus giving the plate not being exposed a double protection. 1 In slides with rigid shutters (solid or hinged) the of the sliding shutter is ensured by an elastic packing of thick velvet pressed against the slide by springs. After long use, wear of the velvet may cause a falling-off in the efficiency of this light-trap, for which reason it is customary to protect the slides by carrying them under the focussing cloth and by keeping the cloth over the slide when drawing the shutter and making the exposure.
Double slides with rigid shutters are of two patterns. In current French makes a plate is introduced after drawing the corresponding shutter, the plate being kept within the rabbets by turn-buttons ; in book-form slides, as com monly used in Great Britain and the United States, the frame is made in two parts hinged together. The plates are loaded without drawing the shutters or touching the film side. After one of the plates has been put in, a partition of blackened metal is turned down over it and secured by turn-buttons, and the whole is then folded over the other plate in its bed. This great convenience in loading is unfortunately sometimes offset by a drawback ; in the event of any warping of the wood used in the construc tion of the hinged frame, the light-tightness of the latter is pretty certain to suffer.
Numerous devices are employed for attaching the slides to the camera. The best are those in which any sliding movement is avoided, since there is the risk of the camera being shifted when the slide is being put in or taken 157. Tripod Stands. For heavy apparatus it is unfortunately difficult to obtain stands which are sufficiently rigid for the long exposures that are sometimes needed and that are also com paratively light in weight and easy in manipu lation. The legs are often too short and too flexible, and the top is often too small for adequate support of the camera.
In our opinion, the most practical model (despite its weight) is that known as the " double box," in which each leg consists of two wooden channel pieces sliding in each other, without any hinge or joint other than the one connecting each leg to the top. The bottom limb is solid, and slides in the inner channel or " box." Set-screws enable the amount of slide to be fixed at any intermediate point desired. The best length of leg at full extension is 64-72 in. This height enables the lens to be at eye-level after the legs have been splayed out as desired. Tripods of this pattern are occasionally made for a much greater height.' The tripod with detachable folding legs and metal top (which latter is sometimes incor porated in the camera itself as a turntable) takes longer to set up, and allows no variation of the height except by splaying the legs out more or less, which obviously gives little latitude. Light tripods with folding legs always tend to give at the joints and are therefore suitable only for cameras not larger than half-plate for outdoor use only, that is to say, usually with short exposures.