Patent GB-704381-A (GB)
Title: Improvements in and relating to coupled optical viewfinders for television and othercameras
IPC: H04N5/232
Filed: 1949-11-30
Abstract: 704,381. Cameras with coupled viewfinders. BRITISH BROADCASTING CORPORATION, BLISS, J. L., and WORSWICK, T. Nov. 29, 1950 [Nov. 30, 1949], No. 30785/49. Class 98(1) [Also in Group XXXVIII] In apparatus of the kind comprising an optical viewfinder including a lens for forming an image of a scene on, for example, a ground-glass screen, and a camera including a lens for imaging at least part of said screen on a light sensitive surface and of a focal length substantially different from that of the viewfinder lens and wherein the focusing mechanism of the viewfinder lens is coupled to that of the camera lens, the viewfinder and camera lenses are so mounted in the apparatus, that an object distance u for the viewfinder lens minus the object distance U for the camera lens focused on the same object is substantially equal to the focal length f of the viewfinder lens minus the focal length F of the camera lens, and the coupling ratio between the two focusing mechanisms is adjustable to be equal to the ratio of f<SP>2</SP> to F<SP>2</SP>. The apparatus shown comprises an optical viewfinder in a housing 10 on the top of a television camera 11. The viewfinder includes a lens 12 and a mirror system including mirrors 13, 14 and 16 arranged so that the viewfinder axis has a part lying along three sides of a rectangular plane figure located perpendicularly to the axis A-A of the camera body. The image produced by lens 12 is viewed on a groundglass screen 18 by a magnifying lens 19 and focusing is effected by rotating a handwheel which actuates a flexible cable 21 entrained around pulleys and fastened to a carrier 20 in which the screen 18 and lens 19 are mounted. Rotation of the handwheel also changes the setting of a potentiometer 24. Parallax is reduced by pivoting the mirror 13 about a horizontal axis disposed in its reflecting surface and normal to the axis A-A by means (not shown) actuated by the handwheel. The camera lens 26 is displaced with its first nodal plane in front of the first nodal plane of the lens 12 and at a distance therefrom measured along the optical axis equal to the difference of their focal lengths. Focusing of the camera is effected by movement of a pick-up tube 27 and its cradle 28 by means of an electric motor coupled by gears to a lead screw 30 engaging in a nut formed on the cradle 28. Rotation of the motor causes rotation of an induction generator 35 which prevents hunting of said motor; and rotation of the lead screw effects through gearing rotation of the arm of a potentiometer 36. This potentiometer and the potentiometer 24 in the viewfinder housing 10 form two arms of an A.C. bridge (see Group XXXVIII) energized through a transformer, two windings of which constitute the other two arms of the bridge. Rotation of the handwheel to focus the viewfinder actuates potentiometer 24 to change the impedance of the respective bridge arm and the unbalance voltage so produced is fed to an amplifier and thence to the electric motor so that the pick-up tube is simultaneously moved and the arms of potentiometer 36 is rotated until the bridge is again balanced, the camera being then focused on the same object as its viewfinder. The coupling ratio between the two focusing mechanisms is selected by means of a switch connecting different resistors in series with one or other of the two potentiometers. The screen 18 and pick-up tube 27 may be stationary and the lenses 12 and 26 moved for focusing purposes. Specification 672,895, [Group XXXVIII], is referred to.