On the one hand, progress in the manufacture of sensitive emulsions has made it possible to obtain without any great loss of speed images of fine grain (§ 196) lending themselves to much greater degrees of enlargement than previously admissible.' Further, an appreciable saving results in the cost of sensitive material, the only negatives that are enlarged being, of course, those of some value. On the other hand, it is known that at one and the same relative lens aperture the depth of field increases as the focal length of the lens becomes shorter. Finally, it is thus possible to use lenses of ultra luminosity, designed chiefly for the cinema industry, and obtainable only in short focal lengths.
The use on several of these cameras of inter changeable lenses of very diverse focal lengths (ranging from 2-8 to 50 cm. on a 24 X 36 mm. camera, for instance) raised difficult problems in ensuring in all cases precision in the indica tions given by the finder and the range-finder. While it is possible to accept a fairly rough con struction in the case of cameras giving pictures usable without enlargement or with only a moderate degree of enlargement, miniature cameras must be of an extreme optical and mechanical precision. They are therefore neces sarily high in price. Such precision can only be obtained in a rigid, non-folding instrument (see also § 166 and 176).
As a rule no provision is made for a rising front in these cameras. It may therefore be necessary to tilt the camera when taking the photograph, the perspective being then cor rected (§ 743) when enlarging.
165. The Chief Types of Hand Camera. Hand cameras may best be classified according to their general shape. We shall therefore divide hand cameras into rigid and folding patterns, without considering for the moment whether they are for use with plates or with films, and specially for the reason that many can be used with all kinds of sensitive material.
Rigid cameras comprise box cameras of very simple construction intended for beginners: the French type known as jitmelles (of square pyra mid shape) which about 1900 were the most perfect form of hand camera, miniature cameras of high precision, and also the majority of reflex cam eras (§ 176) and instruments derived from them.
Folding cameras are ol three main types : (a) folding cameras that differ from the hand-stand cameras (§ 159) already described merely by their smaller dimensions and lighter weight, the hinged baseboard forming a protective cover when the camera is closed ; (b) folding cameras with self-erecting front in which the lens board is automatically placed in position and returned into the camera body by means of jointed levers with spring catches, when the baseboard is opened and closed ; (c) cameras usually known as Press cameras, without a protective cover, the opening and closing of which resemble the movements in an opera hat with automatic locking in the extreme positions.
Al! these instruments are made for monocular photography (with one lens), or for stereoscopic photography. For details of this latter use of them the reader should refer to the chapter (XLVIII) on stereoscopy.
166. Rigid Cameras. Rigid cameras, bulky and heavy, especially when fitted with changing boxes for 12 or 18 plates, are no longer used in medium sizes such as 31X 2 and quarter-plate except for some special purposes.
Owing to the reflection of light which may always take place from the walls of box cameras, and especially those of the jumelle type, due to their angular shape, it is essential that the walls be a perfectly dead black. This is usually done by lining them with black cloth.' Miniature cameras using roll-film or cine film arc usually fitted with a focal plane shutter. This facilitates the interchange of various lenses as well as the linkage of the mechanism setting the shutter and that winding the exposed film. As the slit can travel very close to the sensitive surface the efficiency is excellent, and the use of a narrow slit gives the fast exposures demanded by the lenses of great luminosity.
The very slight depth of field of the large aperture lenses usually fitted to these cameras and of the long focus lenses sometimes employed on them would render illusory any focussing by judgment of distances, so that these cameras must be equipped with range-finders (§ 173), and in order to allow of photographing subjects in rapid motion it is necessary that the adjustment of the range-finder be coupled with the focussing adjustment, so as to avoid having to read off the distance in the range-finder and then regulate the focussing accordingly.
167. Folding Cameras. The essential advan tages of folding cameras are their small bulk and their extreme lightness—features which are sometimes obtained at the expense of conveni ence in handling and of rigidity after a period of use.