At various times the suggestion has been made (Disderi, 1864; Melcy, 1905) to use for studio portraiture two cameras, placed one on the other, and fitted with lenses of identical focal length (but which may be of different optical quality). The upper camera is used only for focussing and composing the picture, while the lower one is the camera proper.
154. The studio camera must be capable of being fixed at various heights and at various angles. When a triple-body camera is to be used for copying by transmitted light, it must even be capable of being pointed to the sky at an angle of about in order to illuminate as strongly and as uniformly as possible the nega tive or transparency placed in the front body beneath a sheet of ground glass.
The studio stand has long been in the form of a small table, or platform supported on pil lars sliding within the framework. Racks and pinions enable the front and the back of the top to be raised or lowered either simultaneously or separately. On this system the maximum height to which the top can be raised is only thus be lowered to about 16 in. from the floor, and can be raised to a height limited only by the height of the pillars.
In either case the stand must be mounted on castors, allowing it to be moved easily and rapidly. It is preferable, especially when the stand is to move on carpets, that the castors be of large diameter with rubber tyres. Once the stand has been brought into position it can be fixed there by means of four movable rods which project a little under the stand and lift it slightly, usually by operation of a pedal.
Various patterns of stands, of simple construc tion, similar to the former of the two types described, can be used with studio cameras of small size.
The camera is usually secured to the top of the stand by means of screw clamps.
It has been suggested (H. Hill, 191o) that a large mirror be stood in front of the camera, with the silvering removed in places to form apertures for the lens and the operator, both the camera and the operator being thus bidden from the sitter.
155. Portable Stand Cameras. Cameras for technical, architectural, and landscape photo graphy and for outdoor groups may be divided into two principal types : Square bellows (Fig.
129A) and taper bellows (Fig. 129B). Rigidity is more easily obtainable with the former. The swing front of the taper bellows type must be used with extreme caution, for any inclination of the camera front tends to cause the lens to work at the edges of the field over which it covers sharply, i.e. under the most unfavourable conditions. As a matter of fact, the front swing should never be used by itself ; its purpose is to permit a considerable rise or fall of front by tilting the baseboard of the camera and then bringing back the front and back into the vertical.
The essential conditions that must be fulfilled by portable cameras are comparatively small bulk and weight, great rigidity even at full extension, and a satisfactory parallelism between front and back. The lens should be capable of being used well above or below the central position.' The vertical swing, which is of little advantage when photographing groups, becomes useful in landscape work, and necessary in architectural or technical photography. It must allow of an inclination of at least The side swing can occasionally be used with advantage, e.g. in reducing the convergence of horizontal lines in architectural photographs taken with a wide-angle lens, or in avoiding the use of a very small stop in taking views of a street with one row of houses much nearer than the other. The maximum and minimum extensions avail able will be decided by the focal lengths of the lenses which it is intended to use, bearing in mind that in technical photography, the photo graphing of small objects " same size " (exten sion of twice the focal length) often needs to be done with lenses of comparatively long focus in order to avoid an unpleasant perspective. In extreme cases, the available extensions can be varied by using extension or recessed tubes or boxes, enabling the lens flange to be placed at some distance in front of the camera front or within the bellows at some distance behind the front.
The baseboard must be fitted with a circular spirit level or with two tubular levels fixed at right angles to each other (T-level). When the camera has a swing back it is advisable to fix a plumb level to the back.