Camera extension is the term used to denote the distance between the front surface of the lens panel and the surface of the sensitive plate or film.' In the case of cameras especially designed for portraiture, commercial and landscape photography, it is advisable for the rear body not to be a fixture in a plane perpendicular to the normal position of the optical axis, but capable of inclination to ro° to 15° on this axis. To do this the rear body is pivoted on horizontal pins fixed to a frame which can be moved along the baseboard, and the camera is then said to have a vertical A similar mounting on vertical pins is sometimes employed and is termed horizontal swing or side swing.
When it is necessary to copy transparent objects, with avoidance of the light reflected from the surface of the object which faces the lens (enlargement and reduction of negatives, etc.), a camera, consisting of three bodies, is generally used. In this the lens is mounted on the midway body, while the transparent object is carried by the front body. If a very long extension should be necessary, the lens can be fixed to this front body, but as a rule it is pre ferred to fix it to the middle one, in which case the front body and the bellows connecting it to the middle body form a most efficient lens hood (§ 124).
The baseboard of a triple-body camera is generally formed of two portions sliding one within the other to avoid unnecessary bulk when the maximum extension is not being used. The front body is then fixed to the sliding extension, and the movement of this extension from the fixed portion of the baseboard is regu lated by a winch-screw operated from the rear of the camera. The baseboard of a camera which extends in this way is never as rigid as a one-piece baseboard, and the extension must therefore be supported on a table.
In the case of studio cameras and some field cameras, bellows of square section are used. The photographic plates, which are usually oblong in shape, can then be placed one way or the other as desired, the long side being either vertical or horizontal. For this purpose, in studio and copying cameras, square dark slides are often used, fitted with rebates to take the plate in either position. In other cameras, chiefly those of portable square bellows pattern, the rear body is made square and is fitted with a loose frame (reversing back), which carries the oblong dark slide and allows of it being placed either upright or horizontally.
When, as often happens, plates are used of smaller size than the maximum size for which the camera is built, adapting frames or carriers are placed in the dark slides. It is possible to have a set of these carriers (called kits in the United States) fitting in each other and each corresponding with a given plate size.
When, however, a camera is regularly used with small plates, it is generally preferred to employ dark slides of that size, fitting these to the camera back by means of an adapter.
The dark slides used with studio cameras are nearly always single slides, taking only one plate or film. Field cameras, on the other hand, have double slides containing two plates, one on each side of the slide.
The slides of studio and copying cameras are usually closed by a flexible curtain shutter, formed of thin strips of wood glued on an opaque fabric. To uncover the plate, this cur tain or blind is drawn aside by means of a tag at the back of the slide after the latter has been fitted to the camera (roller-blind or curtain slide). Most dark slides of smaller size are closed by a rigid shutter or by a shutter hinged in one or more places.
If the image seen on the ground-glass or focussing screen is to be recorded with equal sharpness on the sensitive emulsion, it is essen tial that the plane of the front ground surface of the focussing screen shall coincide exactly with the plane of the sensitive surface as deter mined by the rebates or turn-buttons.
15o. Cameras for Commercial Copying. In making large-size copies of originals, as is done in cartographic and photo-mechanical work, the following arrangement may be employed with advantage, provided the building is not subject to vibration from machinery or heavy traffic.' The lens is mounted on a plate fixed to a partition separating the dark room from the studio. Rails, sunk in the floor parallel with the optical axis, carry in the studio a travelling easel and in the dark room a travelling support for the plate, the rails being fitted with carriages and mechanism for moving the latter slowly and holding them fixed. One of the advantages of this arrangement is that a dark slide becomes unnecessary ; such a slide would be exceedingly bulky and heavy when the plate is as large as 5 square feet in area, or larger.