Patent GB-953863-A (GB)
Title: Improvements in or relating to optical objectives of variable focal length
Inventor: COOK GORDON HENRY
IPC: G02B15/14
Filed: 1959-07-14
Abstract: 953,863. Vari-focal objectives. RANK PRE - CISION INDUSTRIES Ltd., [trading as TAYLOR, TAYLOR & HOBSON]. July 13, 1960 [July 14, 1959], No.24156/59. Heading G2J. A variable focal length optical objective has front and rear assemblies, of which the front assembly includes members relatively movable for effecting variation of the equivalent focal length of the objective whilst maintaining constant position of the resultant image plane of the complete objective throughout the range of relative movement, and the rear assembly includes a convergent stationary rear member, and a divergent member located in front of such rear member and adjustable from one to the other of two preset positions in which such divergent member has the same conjugates, whereby in such two preset positions of the divergent member the relative movement of the members of the front assembly will produce two different ranges of variation of equivalent foca length of the complete objective with the same position of the resultant image plane, the magnifications of the divergent member of the rear assembly in its two preset positions respectively being #M and 1/#M, where M is the ratio between the equivalent focal lengths of the objective in the two ranges. In the objective shown in Fig. 1, the front assembly comprises a convergent doublet and a divergent doublet, both these members being movable in such a way that, as the rear member moves backwardly, the front member first moves forwardly and then back again to its initial position. The rear assembly comprises a stationary front member in the form of a convergent doublet, a stationary rear member made up of a convergent doublet and a convergent element and a middle divergent member which is adjustable from one to the other of two present positions in which the magnifications due to this member are respectively #M and 1/#M. Optical data are given for one example of the objective of Fig. 1. Optical data are also given for two objectives of the kind shown in Fig. 2 in which the front assembly is substantially a focal throughout the range of movement and comprises three members of which the front and rear are convergent and are mechanically interconnected to perform 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 approximate 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 inter-related as to cause the equivalent focal length of the objective to vary in accordance with an approximate logarithmic law relatively to the movement of the zoom control element. The objectives shown in Fig. 2 are described also in Specifications 953,864 and 953,865. Focusing for rear objects may be effected by movement of the front member. In modifications, the front assembly as a whole may be convergent instead of divergent and may form its image at a constant position behind the front surface of the rear assembly. In such a case, the front member of the rear assembly may consist merely of a single element or may even be omitted.