Patent GB-548098-A (GB)
Title: Improvements in photographic focal plane shutters
IPC: G03B9/28
Filed: 1941-06-13
Abstract: 548,098. Photographic focal-plane shutters. HOUGHTON-BUTCHER MANUFACTURING CO., Ltd., and HUNTER, D. G. June 13, 1941, No. 7514. [Class 98 (i)] A photographic focal-plane shutter comprises a pair of rigid plates pivoted substantially centrally to pivoted carriers, the plates being so guided by gearing that as the carriers are turned one edge of each plate moves in a path substantially parallel to the emulsion surface, the gap between these two edges forming a slit for the passage of light to the emulsion. As shown in Fig. 1, the shutter mechanism is carried by a light-tight casing 1 extending between the objective and the exposure aperture of the camera. Parallel shafts 5, 6 rotatably mounted in the casing carry pairs of arms 51, 52 and 61, 62 respectively, to which are pivoted semi-circular sectors 53, 54 and 63, 64. The sectors 53 and 63 are toothed to engage fixed quadrantal racks 55, 65 respectively. The racks 55, 65 are concentric with the shafts 5, 6 respectively, and their pitch circles are of a radius twice that of the toothed sectors 53, 63, which latter radius is equal to the length between pivots of the arms 51, 52 and 61, 62. As a result, when the shafts are rotated, causing the sectors to roll on the racks, any point on the pitch circle of the sectors describes a straight line. Between the bases of the sectors extend shutter plates 9, 10, which are substantially perpendicular to one another, each having a vertical edge adjacent the emulsion surface lying on the pitch circle of its sector, so that when the shafts 5, 6 are rotated at the same speed these two edges move in a straight path parallel to the emulsion, whilst the plates remain perpendicular to one another. During setting of the shutter, the edges of the plates may abut in a lighttight manner, but preferably a narrow plate 11 connected by a light-tight hinge to the plate 10 and having extensions 12 moving in straight guides 13 is provided to overlap the end of the plate 9 during the setting movement. The operating mechanism shown in Fig. 5 comprises discs 20, 24 secured to the shafts 6, 5 respectively, driving springs 29, 35 extending between studs secured to them and to a setting knob 31. To set the shutter, the knob is first turned clockwise, relaxing the springs 29, 35 and causing a rod 38, which is anchored to the stud 36 on the knob and extends within the spring 35, to engage the stud 34 on the disc 24 and rotate the disc through 30 degrees. Simultaneously, a lighter spring 33 connecting the stud 34 to one on the disc 20 rotates the disc 20 to the same extent. The discs are retained in this position by a notched release lever 25 and a double-arm lever 27, which engage notches in them, so that on turning the knob 31 back the springs 29, 38 are tensioned. The exposure time is selected by angular adjustment of a member 21 in relation to the disc 20, which is then locked to the disc by mean of a pawl 22 engaging teeth on it. On release of the disc 20 by the lever 25 the plate 10, Fig. 1, first moves to establish a slit of a width predetermined by the setting of the member 21, this member then engaging the end of the lever 27 to cause it to release the disc 24 so that the plate 9 begins its movement. Fig. 8 shows a modification in which an escapement mechanism allows longer exposures to be obtained. The mechanism comprises a gear train and escapement mounted in a frame which is pivoted at 47 and has an extension 45 urged into contact with a cam 44 which is adjustable in relation to the disc 20 to vary the exposure time similarly to the member 21 in Fig. 5. For the shorter exposures, when a cut-away in the cam reaches the extension 45 the gear train pivots on the axis 47 and it send 48 engages and trips a pawl 41 to release the disc 24. For longer exposure a lever 50 is adjusted to engage an extension 53 of the first member of the gear train during the movement, further movement being then retarded owing to the train being driven by the extension 53.