Cameras and Enlargers

Construction.

In some portable cameras the application of the "fixed focus" principle renders any focussing adjustment su perfluous, but otherwise the latter is a primary constructional re quirement. Today most focussing is accomplished automatically. In the earlier cameras the extension—almost invariably by means of leather bellows pleated accordion-wise—was regulated by an endless screw operated from the front or back. The method is occasionally employed in big copying apparatus, but has been largely supplanted by a rack-and-pinion action, on the back of the camera if the bellows are parallel and on the front where conical bellows are racked out from the back. The front, some times the back also, is kept rigid by struts or other supports affixed to the baseboard, and the lens, usually mounted on a sliding panel, can be lowered or raised to modify the amount of foreground included. Convergence of vertical lines caused by tilting the camera can be corrected by a swinging back or front, which enables the base of the sensitive plate to be kept hori zontal, and its sides vertical, irrespectively of the tilt. A "swing back" or a "side-swing" is also occasionally useful for bringing into focus a foreground, or a near object on one side or the other of the subject, without "stopping down." Focussing is ordinarily carried out with stand cameras by viewing the image formed by the lens on the piece of finely ground glass consti tuting the focussing screen, a cloth being thrown over the camera to exclude extraneous light. In some cameras, notably those of the reflex type, a focussing chamber enables the focussing cloth to be dispensed with, and in most hand cameras the focussing screen is replaced by the finder (q.v.). For certain scientific purposes it is preferable to focus on a piece of plain glass with a focussing magnifier which thus virtually becomes the eyepiece of a telescopic system; but "aerial focussing," as it is called, is unsuited to ordinary photography in which compo sition of the subject as a whole must be studied.

The focussing screen, and, later, the dark slide or other recep tacle for the sensitive plate or film is now usually held at the back of stand cameras in a frame which is reversible for pictures taken with the plate lengthwise or upright.

"Dark slides" to hold sensitive plates or cut films in sheaths are made to interchange with the focussing screen, the sensitive surface of the plate or film being in accurate register with the ground surface of the screen. Slides are commonly made to hold two plates or films back to back, with a metal partition to prevent the passage of light. There are two patterns, "book-form," hinged to open like a book, and solid, the latter generally known as plate-holders. With some hand cameras single metal slides are supplied. In all slides light is temporarily excluded from the front of the plate or film by a sliding or roller shutter which is removed or opened, of course, for exposure.

In the case of studio cameras (q.v.) a "repeating slide" enables two plates to be quickly exposed in succession without removing the slide itself. In field cameras other methods are adopted, both to facilitate speedy consecutive exposures and to permit the carriage of a number of plates or cut films in less space than is possible with separate slides or plate-holders. Some patterns of changing-box are very compact and convenient. The usual con struction is in the form of a box with a sliding door at the back, to the open top of which a small bag of light-tight material is affixed. The sliding door at the back having been removed the plates or cut films in sheaths are packed into the box, and a spring at the back ensures constant and even pressure upon all the sheaths in the pack, with the result that the front plate or film is always in register with the focussing screen. In the

front of the box there is, as in the case of the ordinary dark slide, a shutter which is withdrawn for, and replaced after, exposure. The front plate after exposure is either lifted or levered up into the bag at the top of the box, and transferred to the back.

Slides, plate-holders and changing-boxes have largely been superseded for amateur use by roll-film cameras based on the Eastman Kodak system. In the earlier applications of the latter the roll of film was carried on a spool in a separate holder with removable back and shuttered front, which thus became a sort of changing-box. But the roll-holder is now incorporated in the camera itself, with appreciable increase of portability and con venience and no loss of efficiency. The film itself in a long strip sufficient for several exposures is backed by a longer strip of black paper on which at suitable intervals the number of the exposure is boldly stamped. The excess of black paper at either end of the film provides a safety covering during the process of loading the spool into the camera, and thus enables the latter operation to be performed in daylight. In practice, the lens being normally capped by a central shutter, the back of the camera is removed, and the spool inserted at one side of the roll-holding arrangement. At the other side an empty slotted spool should be in position. The seal of the full spool having been broken a portion of the safety covering is unwound and threaded into the slot of the empty spool. The back, which is provided with a small red "window," is now replaced. By means of a winder more of the black paper is wound off until the number (I) appears in the little red window. The section of film for the first picture is now in position.

An alternative method of daylight loading is provided by the film-pack system whereby cut films are interleaved with pieces of black paper provided with numbered tags, the whole pack being backed by a sheet of metal provided with springs. By pulling out one of the tags a sensitive film is brought into position for ex posure, the preceding film having been simultaneously displaced. Much progress has been made in packing film, methods of loading and transfer, with the result that backings may be entirely elimi nated or specially treated to protect against unusual exposure, such as X-rays. Nearly all loading to-day is integral with the camera and actuated by automatic or semi-automatic means.

With all hand cameras some form of finder is required. The earliest finder was a second camera, and this idea survives in twin-lens and binocular cameras although a combined "single" unit has been developed. It is an attachment either of the "direct vision" kind, with or without a lens or lenses, or in the form of an angled reflector in which an image produced by a miniature lens, and which represents on a small scale the image trans mitted by the lens in actual use, is viewed by the photographer, either on the mirror itself, or on a tiny ground glass screen. The simplest form of direct-vision finder is a wire frame which, held at a correct distance, represents the proportions of the plate, the view included being identical with that photographed. In other direct-vision finders the subject is collected in a small rectangular negative lens, and this miniature image, accurately proportioned to the size of the plate, is viewed through an eye-hole or eye lens.