Patent GB-953866-A (GB)
Title: Improvements in or relating to optical systems of variable equivalent focal length
Inventor: COOK GORDON HENRY
IPC: G02B15/14
Filed: 1960-01-01
Abstract: 953,866. Variable focal length systems; wide angle attachments. RANK PRECISION INDUSTRIES Ltd., [trading as TAYLOR, TAYLOR & HOBSON]. Dec. 22, 1960 [Jan. 1, 1960], No. 61/60. Heading G2J. A variable focal length optical system for projecting an image of an object on to a generally rectangular image-receiving area in a stationary image plane, comprises an objective of variable equivalent focal length having 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 the resultant image position always in the stationary image plane throughout the range of relative movement, whilst the rear assembly has an inverted telephoto characteristic with effective axial beam diameter greater at its rear surface than at its front surface and includes a stationary convergent rear member having equivalent focal length f r  and in front thereof at least one member which co-operates with the front assembly to form a divergent group whose equivalent focal length bears to the equivalent focal length of the complete objective a constant ratio lying between f r  /F 0  and f r  /3F 0 , (where F 0  is the minimum focal length of the objective), such divergent group acting to produce a virtual image of the object in a constant position lying between f r  and 4f r  in front of the front surface of the convergent rear member, where by such rear member always receives a divergent beam and forms from such virtual image a resultant image of the object on the stationary image plane, and a wide angle attachment which can be placed at will in front of the front assembly of the objective for increasing the angular field and decreasing the values of the equivalent focal length of the objective in accordance with the telescopic power of the attachment without reducing the back focal distance of the objective. The system may include, as the variable focal length objective, an objective as disclosed in Specifications 953,863, 953,864 and 953,865. Such objective has, as shown in Fig. 2, a front assembly of which the front and rear members are convergent doublets connected to perform equal and opposite movements under control of a cam actuated by a zoom control element, while the middle member is formed by a doublet and a simple component and has an axial movement related to the movement of the zoom control element so as to vary the equivalent focal length of the objective in accordance with a logarithmic law. The front member may have an additional forward movement for focusing purposes. The rear assembly comprises stationar front and rear convergent members and a middle member which is divergent and movable to one or other of two pre-set positions in which it has the same conjugates, whereby the objective is provided with two ranges of variation of its equivalent focal length. By using the wide angle attachment having a telescopic power of two, four ranges of variation of equivalent focal length of the whole system is possible. This attachment is preferably afocal and may consist of a divergent doublet in front of a convergent doublet, or of a simple divergent front component, a middle divergent doublet, and a convergent rear doublet. Optical data are given in the specification for three examples, one of which is as shown in Fig. 1.