In order to retain the benefit of the automatic concordance of the field in the case of decentring, while reducing the size of the finder, the plan has been adopted of filling the space of the frame with a divergent It is that the divergent lens used should have a focal length equal to that of the objective, that its frame should have linear dimensions half those of the image recorded in the photograph, and that it should be placed at a distance from the sight equal to the focal length, the divergent lens being fixed on the lens front and the sight being fixed on the camera back (Gillon, 190o). This finder, unlike the one described above, does not show the same image as the lens when the extension differs greatly from the focal length.
Let us add that all the conditions set forth here, which ensure the perfection (for practical purposes) of the direct-vision brilliant finder, are by no means invariably fulfilled in all cameras fitted with finders of one or another of these patterns. It often happens that a given finder is fitted to very different cameras with out any arrangement for compensating for the decentring of the lens.' In some cameras the divergent lens is fixed in a convenient position, and the eyepiece is replaced by a pin which follows the movements of the lens. The centre of the field is then indicated fairly correctly, whatever the amount of rise or fall of lens (except, of course, when the subject is quite close), but the margins of the field are not exactly shown. 2 One distinct advantage of the direct-vision brilliant finders is that, in the case of moving objects, it is possible to watch with the naked eye the oncoming moving object in the direction which at the proper moment becomes the normal line of sight.
173. Various Other Finders. Among the other finders used on some cameras may be mentioned the telescopes of the Galilean type (opera-glass) or of the astronomical type, either straight for use at eye-level or in an elbow form for use at waist-level, fitted with a graticule for indicating the centre and, in some cases, the margins of the field. The focussing of these telescopes is sometimes done by sliding a tube graduated in distances, the finder then acting as a telemeter. There is also another type of finder, copied from the collimator, used in artillery, but not generally popular.
The collimator finder is formed of a convergent lens, at the anterior focus of which there is a graticule, the lines of which will appear as rectangles in the extreme distance. The whole
is housed in a little metal tube in one of the corners of the camera back. If one eye be placed at the collimator and the other looks directly at the subject, the rectangles of the graticule will appear on the subject itself and show the amount of view included. The lines of the graticule can indicate the limits of the field at near distances, and additional lines can show the various degrees of decentring and even a stadilnetric scale, indicating—according to the height of a standing figure—the approximate distance of the latter from the camera.
174. Telemetric Finders. As far back as 1890 Dallinger proposed for this purpose two finders, one fixed and the other pivoting by the action of a cam linked with the focussing mechanism ; the camera was correctly focussed when the subject appeared in the centres of both finders. This arrangement has been simplified by using two optical systems, suitably separated from each other, the images of which are intermingled after one has been reflected on a mirror or prism. which is turned through an angle by the focuss ing movement. In Fig. 130, the eye 0 sees directly an image of the subject aimed at, and another image of the same subject after two reflections in the mirrors M and The mirror M is pivoted on an axis A and linked to a lever L held by a spring R in constant contact with a cam C cut in an extension of the inter mediate tube of the helical focussing mount. The mirror can therefore pass from the orienta tion M to the orientation M' when the lens, first focussed on a very distant object P, is focussed on a near object Q. The camera is correctly focussed on a given subject when the two intermingled images coincide in the portion covering that subject.
The accuracy of focussing is all the greater as the base is greater and as the subject is seen of apparently greater diameter. The range finder, therefore, sometimes includes magnifying lenses. It can on occasion constitute a view finder as well.
There are also on the market various types of coincidence range-finders, separate from the camera. In these, the orientation of the movable optical system is governed by a cam joined to an external knob bearing a distance scale. The distance of the subject being thus known, focussing is effected by means of the camera focussing scale.