Patent GB-728371-A (GB)
Title: Improvements in or relating to combined view-finding and range-finding apparatus
IPC: G03B13/20
Filed: 1952-09-19
Abstract: 728,371. Photographic view- and rangefinders. COOK, G. H., and TAYLOR, TAYLOR & HOBSON, Ltd. Sept. 11, 1953 [Sept. 19, 1952], No. 23586/52. Classes 97 (1) and 98 (1) A combined view-finder and range-finder, particularly for a photographic camera, comprises a range-finder in which two beams from the scene being viewed are brought together by a beam-splitting prism, and a view-finder having an objective member and an eyepiece and giving a real external entrance pupil which is located approximately at the beam-splitting prism of the range-finder. As shown in Fig. 6, a beam-splitting prism A combines a beam passing through it without deflection with a beam reflected by a small reflector B, which may be rotated to combine the beams in the manner usual in range-finders. The combined beams pass to the objective D of the view-finder, which forms an image at E. In the simplest form, this image is viewed by an eyepiece, and a field lens G may be placed near the image plane E to form an image of the entrance pupil C near the observer's eye. Such an arrangement gives an inverted image. Preferably, therefore, an erecting lens forms a second image of the image in the plane E, and the second image is viewed by an eyepiece F. A field-defining mask J in the plane of the second image, and a second field lens G<SP>1</SP> forms the image of the entrance pupil C near the observer's eye. To save space, the optical axis is deflected as by reflectors K, K<SP>1</SP>. The angular field of view may be varied to suit photographic objectives of different focal lengths by varying the effective size of mask J. E.g. a number of interchangeable masks may be used, and may be carried by a rotatable disc. Alternatively the apparent size of mask J may be varied by making the eyepiece F of two relatively movable components so that its focal length may be varied. Again, the erecting lens may be moved axially to vary the magnification of the second image, the mask J and eyepiece F being moved to suit the movements of the erecting lens. In the arrangement shown, however, the erecting lens comprises two convergent components L, M which are moved relatively to each other and to the rest of the system so as to vary the magnification between the first and second images without altering their positions. For this purpose, as shown in Fig. 7, the parts are mounted in a tubular casing N with a branch N<SP>1</SP> at one end housing the beam-splitting prism A, the objective D and the first field lens G, and a branch N<SP>2</SP> at the other end housing the eyepiece F. The reflectors K, K<SP>1</SP> are mounted in tube N opposite the branches. The components L, M of the erecting lens are mounted in cells L<SP>1</SP>, M<SP>1</SP> sliding in tube N, which also houses the mask J and the second field lens G<SP>1</SP>. Cell L<SP>1</SP> has a pin L<SP>2</SP> slidable along a longitudinal slot in tube N and engaging a spiral slot in a sleeve O rotatable round the tube N. A pin on cell M<SP>1</SP> likewise engages a second longitudinal slot in tube N and a second spiral slot in sleeve O. The spiral slots are so proportioned that by rotating sleeve O the components L, M are moved so as to form an image at the plane of mask J of variable magnification.