Patent GB-634511-A (GB)
Title: Improvements in and relating to focussing range-finders for photographic cameras
IPC: G03B13/20
Filed: 1947-04-14
Abstract: 634,511. Range-finders for photographic cameras. BARNET ENSIGN, Ltd., and NORTH, J. C. April 14, 1947, No. 9917. [Classes 97 (i) and 98 (i)] In means for coupling the movable reflector of a short home base coincidence range-finder to the lens-focusing mechanism comprising a straight edge 18 rigidly secured to the movable reflector 14 and maintained in contact with a pin 12 movable in a straight line and operatively connected to the lens-focusing mechanism so that its displacements are directly proportional to changes of the distance between the lens and the.focal plane, and in which the pivot axis O-O<SP>1</SP> of the reflector 14 lies in or close to the reflecting surface and on or close to the rangefinder baseline and the straight-edge 18 intersects or is slightly offset from the pivot axis, the angle α, enclosed by the displacement line of the pin 12 and the straight-edge 18 when the lens is focused at infinity, and the perpendicular distance q of the reflector pivot axis O-O<SP>1</SP> from the displacement line of the pin 12, are determined by the formulµ tan α=B/2m ........... (1) q=2kf<SP>2</SP>B/(B<SP>2</SP>+4m<SP>2</SP>) .... (2) wherein B is the length of the rangefinder base, f is the focal length of the camera lens, k is the ratio of pin displacement to the change of distance from the lens to the focal plane, and m=2fn - Z, where n is the distance between the nodal points of the lens and Z is the distance by which the rangefinder base line is in front of the focal plane for ray from infinity. Either the focal plane or the lens may be moved relative to the rangefinder base-line. to effect focusing, but when the lens is movable the values of tan a and q are more accurately obtained from formulae (3) and (4) below, where focusing of objects at two chosen distances D 1 , D 2  is considered. The corresponding displacements x 1 , x 2  of the lens from its infinity position may be calculated from the lens characteristics, and the angles # 1 , # 2  through which the pivoted mirror is rotated from its infinity position for coincidence at distances D 1 , D 2  respectively are determined trigonometrically. The formulae are tan α=(x 1  - x 2 )/(x 1  cot # 1 - x 2  cot # 2 ) .. (3) q=kx 1  sin α (sin α cot # 1 - cos α) .. (4) The above formulae hold strictly only when the pivot axis of the movable reflector intersects the rangefinder base-line, the reflecting surface, and the line of the straight-edge, but departures from these conditions, if small compared with the rangefinder base-length, introduce only negligible errors. As shown, the focusing is effected by a knob 5 on a shaft 4 which varies the distance from the lens to the focal plane in proportion to the angle through which the knob is turned. Shaft 4 carries a toothed sector 6 meshing with a rack 7 on a plate 8 sliding on the camera wall, by means of pins 10 and slots 9, parallel to the rangefinder base-line. Pin 12 slides in a slot 13 on the camera wall, preferably parallel to the optical axis of the lens, and is maintained in contact with an inclined straight edge 11 on the plate 8. The straight-edge 18 is formed on one side of a slot 17 in a disc 15 carrying the reflector 14 and pivoted at 16, and is spring-urged into contact with pin 12, which may be formed with a knife-edge in order that the locus of its point of contact with straight-edge 18 may conform more closely to a straight line.