CAMERA OBSCURA. (Latin, "Camera," a chamber). A dark box, in which the real image formed by a convex lens is received on a focussing screen. A photographic camera is one in which a sensitive photographic tablet can be inserted to receive the image.
As might have been expected, there are many forms of photo graphic camera, and most of them fulfil to a certain extent the required conditions, but none appear to satisfy completely all the conditions of a really good instrument. We shall therefore, in the present article, first describe briefly some of the common forms of the camera, and point out their defects ; and then suggest such an improvement upon them as would be free from the same objections.
The Folding aonera.—The Csimplest form of photographic camera, is that called the "Folding Camera." In this instrument, the sides are made of two longitudinal pieces hinged together, and also to the top and bottom, so as to fold between the top and bottom when the front of the instrument is removed. This is a very convenient arrangement as regards portability. The front of the folding camera has a double slider for carrying the lens, so that the lens can be either raised or lowered when the camera is in its usual position, or on its side for taking upright views. The length of this camera is invariable, and the picture is focussed by means of a sliding adjust ment of the brass work which carries the lens, the lens being mounted in a tube which slides in a jacket. This form of camera answers tolerably well for views, when it is not required to work within a few degrees of the sun; but when it is required to take skies, or work in the face of a strong light, the folding camera allows too much diffused light to fall on the picture. This defect cannot be remedied without completely altering its form.
It would be a great improvement to the folding camera, if pro vision were made for inserting a diaphragm at about one third of the distance between the lens and the picture, for the purpose of cutting off the light which falls on the sides of the camera, and is thence reflected on the picture. The picture formed by the lens is circular and not oblong, so that four segments of what would be an illuminated circular disc, are thrown on the sides of the camera, and a great deal of light is thus reflected on the picture, the whole of which would be intercepted by the proposed diaphragm. These
segments of light will be perceived on looking into the camera, with the head and shoulders covered with the focussing cloth. The light reflected in this way has no doubt caused many failures, for which the chemicals have had to bear the blame.
The common folding camera is therefore a useful and portable but not a comprehensive instrument, and it is faulty in a particular which it is very important to remedy.
Vie Expanding Camera.—This camera may be used either for portraits or views. It is composed of two parts ; the front part rigid and carrying the lens, the after part sliding in the front part, and carrying the dark slide and focussing screen. This camera may be brought to within a comparatively short distance of the object to be copied, and the focus of the lens proportionally lengthened. It should be made to take square pictures of the largest dimensions which the lens will cover; there will then be no necessity for placing it on its side when it is required to take an upright picture. The same kind of diaphragm should be employed as was recommended in the folding camera.
In every expanding camera, the end which carries the slide sh-oxikt be provided with adjustments for the purpose of placing the plane of the picture at various angles of inclination with respect to the axis of the lens. These adjustments are so extremely useful in enabling the operator to overcome difficulties which are continually occurring in practice, that no camera can be considered perfect without them.
No provision is made in the common expanding camera for intercepting a vast quantity of diffused light, which is likely to enter when a lens of large aperture is used, or when the instrument is brought to face a strong light. An arrangement which will be described presently, will be found to remedy this defect. The expanding camera can be focussed either by means of the sliding body, or the sliding tube which carries the lens.