175. Levels and Plumbs. It is absolutely necessary to be able to level the camera when holding it in the hand and sighting the subject, in order to avoid the distortion which is produced if the sensitive surface is not in a vertical plane when the photograph is being taken and the reduction in the size resulting from the trimming which becomes necessary when the camera has been slanted to the right or left.
When the camera is held at waist-height, its level can be indicated by a circular spirit level' or by two tubular air-bubble levels placed at right angles to each other ; the levels must be placed near the finder or over it.
, For sighting at eye-level, the use has been suggested of a transparent mirror inclined at showing the image of a level to the eye placed behind the finder, but the separation of the image of the level and of the image of the subject and the difference in their luminosity make it difficult to observe both at the same time. For this reason recourse is usually had to plumbs, which are needles or plummets freely suspended in the frame of the finder or along the exterior sides of this frame. These needles or plummets are pendulums of extreme sensitive ness, the oscillations of which are limited by a ring of small diameter. So long as the camera is not held perfectly level the plumb rests motionless against the ring, but immediately the camera is level the plumb hangs without touching the ring and oscillates owing to the unconscious movements of the body.
Any camera that can be used for both vertical and horizontal views should be fitted with two levels and two plumbs.
176. Reflex Cameras. The reflex camera, first suggested by Sutton in 1861 and made in practical form by C. R. Smith in 1884, only became popular about 1900, and for a long time was constructed only in a form similar to that of box cameras. At the present time there are folding models, made to meet the objection of bulk often advanced against this type of camera, unless of small size. The principle of these cameras is shown in the sectional diagram (Fig. 131). When the camera is not in use, the plate P is covered by a focal plane shutter. The rays from the lens 0 fall on a mirror arranged at 45° in the plane bisecting the angle between the sensitive surface and the ground glass D, which latter is shielded from surrounding light by the hood C. In these conditions, if the lens is in such a position that a sharp image is formed on the sensitive surface ab, the reflected image a' b' will also be sharp on the ground glass and vice versa the image on the screen is the right way up, although the wrong way round.
As reflex cameras cannot be used conveniently except in their normal upright position, they are usually fitted with a square reversing back, which carries the dark slide in one or other position. In some of the more highly priced models the rotation of the back automatically operates a mask, which thus shows an upright or oblong picture on the ground glass. Otherwise the latter is clearly marked with the edges of the upright or oblong picture. There is usually only a vertical rise and fall of front. The usual sizes of reflex cameras range from 31 X 21 in. to 61. X 41 in. As the camera is hardly ever carried in a case, the lens is protected by a rigid flap of wood or metal, which can be set at various angles and serves as a sky shade.
The camera, which is often hung from the neck by a strap, is held in both hands, one actuating the focussing knob while the subject is being followed on the screen. When it is judged that the favourable moment has arrived for taking the photograph, the release which frees the mirror is pressed. Actuated by springs, the mirror rises to the position 111' against stops, thus effecting a light-tight closure of the upper portion of the space within the camera. At the moment that the mirror reaches the position 1k/' it releases in turn the focal-plane shutter, which uncovers the sensitive surface for a time fixed by the speed setting.
To set the camera for taking another photo graph the mirror is allowed to return into position, by releasing a lever, which also resets the shutter.
An ordinary mirror, silvered beneath the glass, would produce a double image on the focussing screen, and cannot therefore be em ployed in a good instrument, as the finest focussing is not possible. The silvering requires to be on the surface of the glass presented to the lens, and is usually protected by a very thin coat of varnish, which does not interfere with its optical performance. To avoid occasion for the re-silvering of the mirror, care must be taken not to finger or rub the surface. Any dust that may settle on it should be removed very gently by means of a clean and perfectly dry brush. On some cameras built recently a mirror of stainless steel has been fitted.