Diaphragms.—In the portrait-objective, as in most other combinations, the diaphragm is placed mid way between the front and back lenses, as in Fig. 1080. The object of the diaphragm at A is to impart distinctness to every part of the picture. By employing t4 very small aperture in the dia phragm, a portrait or landscape may be rendered so distinct all over its surface as to closely resemble o map. Judgment is therefore required in the use of diaphragms, so as to secure the necessary degree of distinctness without losing the effect of relief and distance. The use of small diaphragms is restricted in another way ; the smaller the aperture employed, the longer will be the time required to produce a picture.
Waterhouse diaphragms are those most commonly used. They are made of thin plates of blackened brass, pierced with an aperture (Fig. 1081), and calculated to slip into a slit in the inner lens-tube (Fig. 1082).
When great rapidity is required, as in photographing children, the diaphragm is dispensed with, and the objective is worked with full aperture.
In copying a map or plan full of minute detail, the smallest diaphragm yields the most perfect definition.
Modern objectives for landscape and architectural photography are so perfectly corrected as to produce faultless images. With what are known as "sym metrical" and " rectilinear " lenses, the straight lines of a building are reproduced as straight lines. With the old miniscus, or plano-convex landscape lenses, there was always a degree of curvature and distor tion, which greatly marred the photographs taken by them.
Another improvement effected in the construction of landscape-objectives is the larger field which some of them are made to cover, embracing an angle of over 90°. These render it possible to photograph, without distortion, objects near at hand and of great elevation. Special objectives are also made to cover a large field, and to be used with an aperture wide enough to fit them for taking groups, and instantaneous street views. The London opticians, Ross and Dallmeyer, have indeed brought photographic optics to a degree of perfection which leaves little or nothing to be desired.
Cameras.— The camera obscura or dark chamber of photography, to which the objective is attached, is presented in its rudimentary form in Fig. 1083. The mahogany box a is made perfectly lightproof; the objective b has a movable inner brass tube with rack and pinion for focussing ; c is the front cap for timing exposure ; and d is a ground-glass screen used in focussing, upon which the image falls. By throwing a dark cloth olter the head, and the end of camera, the
operator is enabled to see and adjust the inverted image on the ground glass. When the image is centred and focussed, the screen is withdrawn, and replaced by a sensitive plate in a dark closed carrier (Fig. 1084). The dark slide is composed of a strong outer frame a, while d represents the open space, with corners upon which the prepared plate rests. Exposure is effected by the front draw-up shutter c; and the hack shutter b Opens to receive the plate.
The plate, prepared in a chemically dark room, is then consigned to the slide, which is closed, and carried to the camera, where it is exposed ; it is finally taken back to the dark room, and developed.
There are an infinite variety of cameras, each designed to serve some special end, and all of them framed on the principle of the instrument described. But the camera in its most advanced state is a complex instrument, in general form represented by Fig. 1085. The rigid rudimentary body is replaced by a bellows a, fixed to the front and back of the instrument. This bellows is expanded and contracted by rack and pinion at b, to suit objectives of different foci. The back swings vertically and horizontally, and may be fixed at any angle by pinching-screws. The swing back is useful in obtaining more uniform focus, where a near and distant object have to be photo graphed together on one plate. Another advantage in this instrument is its perfect portability. The baseboard is so framed as to admit of the camera being folded up into very small compass. The slides or plate-carriers attached to this instrument are both single and double. The single elide is used for the "wet collodion" process, and the double, which carries two plates, for the " dry " process.
Stereoscopic camora.—The stereoscopic camera, in its most complete form, carries two objectives about 4 in. apart. By this means, two pictures of the same object are taken at once. These, when reversed, and combined by the stereoscope, pro duce a mimicry of objects in relief. (Figs. 1086-7.) Enlarging camera.—When the object to be photographed is focussed on the screen, and the distance of the objective from the focussing screen is greater than the distance of the objective from the object, an enlarged image of the object is obtained. The enlarging camera is thus capable of considerable expansion, and herein lies the main difference of the enlarging camera from the instrument required for small photographs.