Patent GB-685945-A (GB)
Title: Improvements in or relating to variable magnification optical systems
IPC: G02B15/14
Filed: 1950-07-18
Abstract: 685,945. Variable magnification optical systems. WATSON & SONS, Ltd., W., and HOPKINS, H. H. July 17, 1951 [July 18, 1950], No. 17981/50. Class 97 (i). In a variable magnification optical system of the kind comprising two normally stationary lenses of like sign and two axially movable lenses, having like sign which is opposite to the sign of the powers of the normally stationary lenses, all of which lenses are co-axial with the movable lenses between and spaced from the two normally stationary lenses, the movable lenses are compound, are optically identical except, it may be, in respect of their apertures, are arranged with their refracting surfaces symmetrically positioned about an axial point midway between the movable lenses and are corrected for coma and spherical aberration at one limit of the range of movement of the movable lenses, whereby the change in coma and spherical aberration of each movable lens during variation of the magnification is substantially compensated continuously by a change of coma and spherical aberration of the same magnitude, but of opposite sign, in the other movable lens, over at least part of the range from that limit. The system shown in Fig. 1, comprises three positive achromatic lenses 10, 11, 12 and two negative compound achromats 13, 14. Optical data are given for this system. Lenses 11, 12, 13 and 14 form a variable magnification system which is converted by lens 10 into a system having variable focal length. The lenses 10, 12 are mounted in a cell 52, Fig. 2 and lens 11 in a cell 51. The lenses 13, 14 are mounted in cells 63, 64 attached to sleeves 65 mounted on parallel rods 61 in a carrier 57 which is slidable along rods 56 under the control of a lead screw 74 which threads into a nut 73 on the carrier. This lead screw is geared at 76, 82 to a shaft 78 carrying a worm 72 that is movable with the carrier and meshes with a pinion 71, Fig. 3, attached to a cam 68, diametrically opposite points of whose periphery are contacted by rollers 67 depending from the sleeves 65 so that rotation of the cam produces differential movement of the lenses 13, 14. The lead screw 74 is rotated by a. chain 92 entrained around a gear 91 on an extension shaft 77 and a gear 93 keyed to a shaft 94 that is axially movable and rotatable by a handle 101. Connected by a pin and slot arrangement to the shaft 94 is an arm 106, the other end of which is secured to a shaft 105. The other end of this shaft is linked at 104 to an arm 100 connected to a member 102 which is lidable along parallel rods 103 and secured to the lens cell 51. An axial focusing adjustment of lens 11 can thus be produced by moving shaft 94 axially. A diaphragm housing 111 is rigidly mounted on the carrier 57 and a diaphragm operating lever 114 extends therefrom into a slot 116 in an arm 117, pivoted at one end to a base-plate at 118 and at the other end sliding along a quadrant guide plate 122. As the carrier 57 moves, the diaphragm is therefore adjusted to maintain the system at a substantially constant relative aperture. The value at which the relative aperture is maintained may be varied by rotating a knob on a shaft 126 which carries an arm 125 provided with a pin 124 engaging another slot 123 in the pivoted arm 117. Specifications 639,610, 639,611, 639,612 and 646,409 are referred to.