The advantages of a reflex camera are obvious. The image seen on the screen at the actual instant that the exposure is made is of the same size and has the same limits as the picture taken, whatever the camera extension and degree of use of the rising front. There is no finder which affords this facility.' The range in extension is sufficient to allow of some latitude in the choice of a lens, an advantage that is offered by scarcely any other hand camera. This latitude is increased by the fitting of an extension piece to the camera front (some cameras are specially built for lenses of very great focal length), but the use of the reflex is incompatible with the use of wide-angle lenses on account of the necessity of leaving a clear space for the passage of the mirror. In the case of a 31 X 41 in. square reflex, the shortest focal length which can be used is 6 in., or a little less if the camera is built for oblong pictures only. The chief objection to the reflex camera is the practical impossibility of using it at eye-level. To meet this difficulty, some makers fit a sighting hood arranged horizontally above the ground glass and fitted with a mirror at 45° to the latter. The picture on the ground glass may thus be seen in the mirror.
It is curious to note that in the minds of many amateur and professional photographers the reflex is regarded as specially intended for Press photographers and, generally, for the photography of rapidly-moving objects. Yet specialists in these two branches of work con sider it unsuitable, and prefer to use cameras of the folding focal-plane type. It is difficult, on the one hand, to work at waist-level in a crowd, and not all reflex cameras can be held upside down for use above the head. On the other hand, the space of time during which a figure subject appears well placed, or during which a rapidly-moving subject passes at the right distance, is often too short to allow of any re-adjustment of the focussing, so that the reflex thus loses its principal advantage.
Nevertheless, the reflex is the best hand camera for the great majority of photographic work, artistic and scientific and especially for the photography of animal subjects. Its only disadvantages are its price, necessarily high in view of the complexity of its mechanism, and —in the case of box models such as the one in Fig. 13r—its bulk and weight.
177. Twin-lens Cameras. The method of focussing a rigid or folding camera by means of an auxiliary lens of the same focal length as the main lens, but less fully corrected, was used in the past, but was abandoned almost at once as being very cumbersome at a period when plates of size under 7 X 5 in. were scarcely used. This idea has been successfully revived in some film cameras of miniature size, in order to give the user the same facilities that a reflex would give, a reflex being difficult to build with such short focal lengths as are usually employed. The auxiliary camera is then of the reflex type, but with a fixed mirror.' The differences in the framing of the two images are then compensated for by decentring or swinging the auxiliary lens, or by moving a.
mask under the ground glass, all such adjust ments being actuated by the focussing movement so as to ensure the coincidence of the fields included in the plane of space-object conjugate to the plane of the sensitive emulsion.
To facilitate the examination of the image the customary ground glass is sometimes replaced. by a plano-convex lens of which the plane sur face is finely matted, the lens then acting as a collecting lens (the marginal rays being thus turned towards the observer), and as a magnifier. A focussing glass is sometimes carried by a movable arm inside the hood.
To reduce the depth of field of the auxiliary lens and thus permit of more accurate focussing, this lens is sometimes of a relative aperture greater than that of the main lens, or it may even be of greater focal length in spite of the mechanical difficulties presented by the linkage of the focussing movements of the two lenses.
178. Sensitive Material. Hand cameras are used with plates (glass support), films (trans parent flexible support), or sensitized paper (§ 230). The films may be in long strips (roll film) or in cut sizes.
Various attempts, attended with scant success, have been made to supply plates packed in such a manner that the packet can be placed in the camera in full daylight. In order not to restrict the user to the plates carried inside the camera, the number of which cannot be large without increasing the weight of the apparatus, plates are usually carried in dark slides for one or two plates, or in changing boxes for 6, 12, or 18 plates, which slides and boxes are detachable and interchangeable, and may be purchased in any number required.
Films cut in sizes are supplied either for use like glass plates, in the same slides or boxes, or in special packings called film packs. A film pack is itself a changing box for 12 films, and only needs placing in a special adapter (film-pack adapter) interchangeable with the slides or boxes.
Roll film, formerly used in roll-holders of somewhat complicated construction, was soon issued with a str p of black paper at each end of the strip of film. These bands of black paper, in conjunction with the flanges of the spool on which the film is wound, afford full protection against light, thus permitting of loading and unloading the holder in daylight. The mechan ism intended to ensure the supply of the same length of film for each exposure was soon dis carded by backing the strip of film with a strip of opaque black paper bearing numbered marks visible from outside through a window of red celluloid.' To reduce the price of the cameras, and also to compel to a certain extent their pur chasers to use roll film exclusively, the detach able roll-holder was no longer supplied, and the film magazine became an integral part of the camera. Later on, there was a reaction against the excessive specialization of these instruments, and many of them can now be supplied with ad apters allowing of the use of dark slides for plates.