In smaller sizes it is usual to employ a photo graphic camera of the normal type, but of heavy build in the wooden parts, strongly reinforced with metal, and thus capable of withstanding daily wear and tear. The fine finish and orna mentation often lavished on studio cameras are out of place here. Various movements, such as swing and rising or cross front, which are of great value for other purposes, are useless or a positive drawback in this particular case.
The rigid baseboard, made in one piece, must ensure the strict parallelism of the bodies. Its length is usually sufficient to permit not only of copying to same size, but also of some slight enlargement. For this, the extension must be somewhat longer than twice the focal length of the lens employed. In order that the same lens used for copying same size or on a reduced scale may be used for making copies larger than the original, the baseboard of the camera would have to be of such excessive length as to be unmanageable in ordinary work. When a copying camera is used to make an occasional enlarge ment, the usual lens is replaced by one of much shorter focal length.
To a greater degree than in the case of any other camera, the maker must assure theabsolute coincidence of the focussing plane and the plane of the sensitive plate. This absolute register cannot be obtained if the ordinary kind of carrier be used when employing plates smaller in size than those for which the camera is made. A universal adapter must be used. It is formed of two horizontal bars movable up and down between the vertical sides of the dark slide, the arrangement being similar to that used for supporting at various heights the lugs bearing the shelves of a cupboard or bookcase.
Owing to the frequent use of wet-collodion plates in reproduction work, the dark slides are loaded from behind, and the lower bar, of aluminium, is so shaped that the drippings coming from the plate during exposure may be caught on strips of filter paper.
To check register, it is advisable that focussing be done in the dark slide itself after placing a matt glass screen in the rebate provided for the sensitive plate. For this reason the curtain of the slide should be completely removable. This focussing in the dark slide should be done, not on a sheet of ordinary glass, the surface of which is more or less uneven, but on plate glass, on which some transparent patches have been left, e.g. along the diagonals, when matting the
surface. The attachment of the dark slide to the back of the camera must be very smooth, but without any play, which can be done only by making the attaching parts entirely of metal.' 151. Originals to be photographed are some times secured by their corners by drawing-pins to a board built up of several layers of wood in order to lessen the risk of warping. Access of stray light into the camera is avoided by blackening the face of the board to which originals are fixed. Blackening the front body of the camera likewise reduces the risk of reflections from glossy prints or such as are mounted under glass. Valuable originals cannot, however, be pinned ; they may be secured between the copying easel and the heads of drawing pins inserted around their edges, or under clips sliding in grooves in the easel, or they may be held flat under glass, using, say, a printing frame fitted with springs (§ 504) ; or, if the whole or part of a page of a book that cannot be taken apart has to be copied, a special frame can be used Framed documents and mounted paintings are usually held between the jaws of an easel similar to an artist's easel. The copying board may also be held in this manner.
152. As strict parallelism must be automatic ally assured between the copying easel, the lens board, and the sensitive surface, it is necessary that the easel and the camera be mounted on one and the same rigid base, at least one of the two being movable on rails parallel to the optical axis. The orientation of the object holder must be adjustable according to need.
In all cases where the floor of the work-room is subject to vibration, the rigid base mentioned above must be arranged so that it is free to move as a whole. This is done either by cords hanging from the ceiling or by springs fastened to another framework resting freely on the floor. The worst that can then occur is that the whole apparatus swings, without, however, any dis placement of the camera relatively to the original. This enables perfect sharpness to be obtained even if the exposures are of consider able length.