Patent GB-899635-A (GB)
Title: Improvements in or relating to control devices for optical objectives of variable focal length
Inventor: COOK GORDON HENRY
IPC: G02B7/10
Filed: 1958-06-06
Abstract: 899,635. Zoom lenses. TAYLOR, TAYLOR & HOBSON Ltd. Oct. 28, 1958 [June 6, 1958], No. 18134/58. Class 97(1). A control device for effecting "Zoom" control of an optical objective of variable equivalent fecal length having a stationary rear member-such as C-and two relatively movable members-such as A and B-of opposite powers, is arranged to cause the equivalent focal length of the objective to vary in accordance with an approximately logarithmic law relatively to the movement of a control element-such as F<SP>2</SP>. As shown (Fig. 2), the member B is housed in a mount B<SP>1</SP> bearing an external screw thread of quick pitch which engages with the internal screw thread F<SP>6</SP> (Fig. 6) in a driving element F. The mount B<SP>1</SP> is keyed against rotation by means of a roller B<SP>2</SP> engaging in a longitudinal slot D<SP>3</SP> in a fixed part D<SP>2</SP> carried by the main casing D. To prevent fouling of the driving element F by the connecting part B<SP>3</SP> the driving element is provided with a helical slot F<SP>1</SP>, through which the part B<SP>3</SP> passes. The control element F<SP>2</SP> is mounted coaxially on the main casing D and carries an internally projecting key which engages in the longitudinal slot F<SP>4</SP> (Fig. 6) causing-on rotation of the element F<SP>2</SP>-the driving element F to rotate and the mount B<SP>1</SP> to move axially at a constant rate relative to F. The driving element F is also mounted to slide axially in the fixed part D<SP>2</SP> carried by the casing D, and for this purpose is provided with two similar cam grooves F<SP>5</SP> in diametrically opposite positions, each engaged by a roller carried by the fixed part D<SP>2</SP>. Thus, when the control ring F<SP>2</SP> is rotated the driving element F rotates in synchronism therewith, and at the same time moves axially in the manner determined by the shape of the cam grooves F<SP>5</SP>. At the same time however, the member B moves at a constant rate relative to the driving element F and its resultant motion relative to the stationary member C is, therefore, due to the superimposing of thess two axial movements. Axial movement of the driving element F is transmitted to the member A of the objective through a transmission mechanism comprising a floating lever, which has a 2:1 ratio. The floating lever consists of a gimbal ring G having two externally mounted rollers G<SP>2</SP> and G<SP>3</SP> arranged at opposite ends of a diameter lying at right angles to the diameter determined by the two internally mounted rollers G<SP>1</SP> which engage in a circumferential groove F<SP>7</SP> in the driving element F. Roller G<SP>2</SP> fits into a recess in the slidable mount A<SP>1</SP> of the front member A, and roller G<SP>3</SP> fits into a recess in a focusing bar H, which is moved axially when focusing control element H<SP>3</SP> is rotated and which focuses the objective by giving the front element A an independent axial movement. A diaphragm E is provided coupled to a setting element E<SP>2</SP>. In a modification (Fig. 8), a differential gear is employed instead of a floating lever. Roller J<SP>3</SP> engages in the circumferential groove F' of the driving element F and conveys axial movements of this element to the pinion J<SP>1</SP>, which is engaged between two toothed racks K<SP>1</SP> and K<SP>2</SP>, one of which is attached to the mount A<SP>1</SP> and the other to the focusing bar H. Various other modifications are referred to, such as inversions of the drives. Specifications 689,206 and 898,745 are referred to.