Patent GB-953865-A (GB)
Title: Improvements in or relating to optical objectives of variable focal length
Inventor: COOK GORDON HENRY
IPC: G02B15/14
Filed: 1959-09-03
Abstract: 953,865. Vari-focal length objectives; lens mounts. RANK PRECISION INDUSTRIES Ltd., [trading as TAYLOR, TAYLOR & HOBSON]. Aug. 31, 1960 [Sept. 3, 1959], No. 30046/59. Heading G2J. An optical objective of variable focal length, has a normally stationary rear assembly and a front assembly including members relatively movable under the control of a zoom control element for effecting variation of the equivalent focal length of the objective whilst maintaining constant position of the final image plane of the complete objective throughout the range of relative movement, such front assembly being substantially a focal throughout the range of movement and comprising three members of which the front and rear members are convergent and are mechanically interconnected to perform approximately equal and opposite axial movements under the control of a cam mechanism actuated by the zoom control element, whilst the middle member is divergent and is caused to perform an axial movement which bears an approximately linear relationship to the movement of the zoom control element, the movement of the middle member and the movements of the front and rear members being so interrelated as to cause the equivalent focal length of the objective to vary in accordance with an approximately logarithmic law relatively to the movement of the zoom control element. The objective is illustrated with reference to the objectives described in Specification 953,864 and the objective which is described in said application and also in the Specification 953,863 these objectives being shown in Figs. 1 and 2 and optical data therefor being given. A mechanism for actuating the objective is shown in Fig. 4 where A, B and C indicate the front, middle and rear members of the front assembly and E indicates the rear assembly which is provided with a diaphragm actuating ring E<SP>1</SP> and a second actuating ring E<SP>2</SP> for moving the middle member of the rear assembly to either of two preset positions. The middle member B is moved by a nut F<SP>1</SP> held against rotation on a screw thread F driven through gear-wheels F<SP>2</SP>, F<SP>3</SP> from a zoom control element G. This zoom control element also acts through gearwheels H<SP>1</SP>, H<SP>2</SP> to drive a hollow drum H having a cam groove H<SP>3</SP> engaged by a follower J<SP>1</SP>, carried by a rack J, which directly operates the rear member C and is coupled through a pinion K<SP>1</SP> to a rack K operating the front member A. The cam groove H<SP>3</SP> is so shaped that, while the member B moves from one end to the other of its range, the rear member C and the front member A first move outwardly away from each other and then move inwardly again back to their original starting positions. The groove H<SP>3</SP> is also so shaped in relation to the screw thread F that the equivalent focal length of the objective varies in accordance with an approximately logarithmic law relatively to the movement of the zoom control element G. Focusing for short object distances is effected by rotation of a control element L which drives a screw thread M and a nut M<SP>1</SP> connected to the pinion K<SP>1</SP> and held against rotation. Specifications 689,206, 898,745, and 899,635 also are referred to.