To begin with, we must note that except where the axis of the finder coincides with the axis of the lens, a condition that is fulfilled only in " reflex " cameras (§ 176), a finder which has been adjusted to indicate for distant objects the same field as that given by the lens, indicates, at very close distances, a field which is not in agreement with that of the camera, the differ ence increasing as the object is nearer and the axes of finder lens and camera lens are more widely separated.' Another difficulty results from the rising and falling movements of the lens, which movements lose much of their value if agreement between the finder image and the camera image is not attained, at least approximately, for all degrees of rise or fall of lens.
170. Ground-glass Finders. The simplest form of ground-glass finder is used only on cheap box cameras (without rising front) and consists of a miniature box camera fitted with a bi-convex lens of in. to i in. and provided with a tiny mirror at by which the image is reflected on to a horizontal piece of ground glass, partially shielded against surrounding light by a hood which can be folded down over the ground glass when the finder is not in use. The linage seen in the ground glass is very small and very dim owing to the small aperture necessary with an uncorrected lens.
A very well made pattern of this type of finder constitutes one of the two chambers of the twin-lens cameras described below ( 177).
171. The Brilliant Erecting Finder. In many cameras of the folding type a finder similar to that just described is used, but the ground glass is replaced by a convergent lens with a mask cut so as to show the image obtained with the camera held vertically or horizontally. If the image is not formed exactly in the front nodal point of the convergent lens, it does not remain stationary in the mask of the finder when the eye (the correct position of which is not indicated in any way) is moved. The limits of the field are therefore very badly defined ; all the more so as the image usually measures not more than about x in. It is true that the image is very bright, so bright, in fact, that the beginner is tempted to under-expose. To allow of use when the camera is used either way of the plate, these finders are mounted on a pivot to allow of their being turned over. As the camera can be closed only when the finder is in one position, closing may result in the bending of the finder bracket, and thus destroy the agreement between the field of the finder and that of the lens, assuming such agreement to have been present in the camera as sold.
A finder of this kind is only admissible in cameras which have no rising front. 1 Much better results are obtained in the case of a camera without a rising front used at the level of the chest, at least as concerns showing the line of sight, by using a finder formed by a concave-surfaced metal mirror of double curvature, placed at an average angle of 45° (E. Busch, 1907) with a sighting pin and sighting notch. This gives an upright and unreversed image of quite decent size (about X I in.), and folds into quite a small space.
172. Direct-vision Brilliant Finders. Perfect finders, at least when they are correctly mounted, consist of a frame of sheet metal or wire of the same size as the picture to be formed in the camera. These are fixed to the camera front (on which they can be turned down when not in use) so as to follow the lens in all its move ments). This frame finder is completed by a sight, fixed at a distance from the frame, which is always equal to the extension of the camera and in a position such that a line passing through the sight to the centre of the frame is parallel with the optical axis of the lens when the latter is not decentred (Huillard, 1900). The lack of coincidence in the foreground can, moreover, be corrected to a satisfactory degree in practice by mounting the sight on a sliding stem with a scale corresponding with the various distances of subject.' Instead of fixing the position of the eye relatively to the frame by means of a sight (which is always troublesome to a wearer of glasses) a second frame may be used. This is similar to the first, but of smaller dimensions, and so placed that the straight lines joining corresponding corners of the two rectangles join together exactly at the sighting point. In this form, however, the finder no longer automatic ally follows the decentring of the lens. If the distance between the two frames is one-fifth of the focal distance (L. Benoist, 1897), the decentring of the larger frame must be one-fifth that of the lens. There is then no advantage in using a frame of the same size as the picture.