Patent GB-652043-A (GB)
Title: Improvements in photographic reflex cameras
IPC: G03B13/20
Filed: 1947-12-23
Abstract: 652,043. Photographic reflex and twin-lens cameras. DODIN, L. J. E. A. Dec. 23, 1947, No. 34083. Convention date, Dec. 27, 1946. [A Specification was laid open to inspection under Sect. 91 of the Acts, April 28, 1948.] [Class 98 (i)] A viewfinder for a photographic reflex or twin-lens camera of the type wherein the image is viewed through an eyepiece at the back of the camera in a direction parallel with the optical axis of the objective, this image being erect and unreversed laterally, the viewfinder comprising a combination of the reflex or viewing mirror with three additional plane mirrors interposed between the first mirror and the observer's eye, these four mirrors constituting, two by two, right-angled dihedrons, the edges of which lie at right-angles to one another and the bisector planes of which are parallel to said optical axis is characterized in that in a reflex camera the axis of rotation of the reflex mirror is vertical when the camera is in its normal position with the axes of its film spools vertical, and that in a reflex or twin-lens camera the image reflected by the reflex or viewing mirror is formed through a field lens disposed in the path of the rays between the reflex or viewing mirror and the next following mirror. One or more of the auxiliary mirrors may be replaced by a prism. The lens may have its lightreceiving face finely ground, and may be divided at a meridian plane into two half-lenses which may be relatively displaced to form a coincidence range-finder. As shown, Fig. 3, a reflex camera has a mirror 3 which swings on a vertical axis 4 between a viewing position and a position 3<SP>1</SP> clear of rays passing from the objective 1 to the film 5. The principal ray 2 entering the objective is reflected by mirror 3 in viewing position at 45‹ to the incident ray. The reflected ray is reflected a second and third time by a pair of mirrors 11, which are mutually perpendicular and form a dihedral with its edge parallel to the optical axis. The emergent ray falls on a mirror 13, above and at right-angles to mirror 3, and is thereby reflected through an eyepiece 16 mounted in the camera back to the user's eye. In an alternative arrangement, the second mirror is perpendicular to mirror 3 and reflects light forward to a pair of mutually perpendicular mirrors forming a dihedral with its edge perpendicular to the optical axis, the ray being thereby reflected back to the eyepiece. The image is, in either case, unreversed laterally and vertically. A converging lens 18 may be placed between mirrors 3, and the pair of mirrors 11, at the point where the first image is formed to intensify the image viewed, and may be finely ground to a satin finish when objectives of different focal lengths are to be used. Lens 18 may be divided on a meridian plane and the half-lenses displaced, giving a split-field effect. Coincidence is obtained with objects at a distance depending on the objective aperture and the displacement of the halflenses. Focusing may be effected at a given aperture of the objective, or the objective diaphragm may be connected by cams and levers to the means for displacing the halflenses so that a correction is automatically made when the diaphragm is adjusted. In a twin-lens camera, the view-finder may occupy a space above the normal position of the ground - glass screen. U.S.A. Specification 886,739 is referred to.