Photographic objective having a focusing control with prevention of change in angular field of view

#This invention relates to a photographic objective provided with a mechanism for preventing changes in angular field of view during focusing, and more particularly to such mechanism for a zoom lens.

#As one form of provision for focusing, it is known to axially move the front part of a lens system which is arranged on the object side. Particularly in the case of the zoom lens, since the image shift is prevented from occurring with zooming by always giving an object point (an image point of the focusing lens) at a fixed position relative to the zooming lens groups such as variator independently of the object distance, the provision for focusing is generally made not in the entire lens system but at a front lens group (focusing lens). Also for long focal length objectives, it has been proposed to focus by moving of the front lens group for the purpose of avoiding an increase in size and weight of the lens mounting which would be otherwise caused when the entire system is moved to effect focusing.

#In this front lens group dependent focusing method, for purposes of minimizing the diameter of the front lens member and of shortening the object distance, it is known to divide the front lens group into two parts which are moved relative to each other during focusing. This type focusing mechanism is disclosed in U.S. Pat. No. 3,632,188 where negative and positive lens subgroups are moved in the same direction by different amounts during focusing, Japanese patent application Laid-Open No. Sho 49-7752 where the focusing lens group is comprised either of two lens subgroups having negative and positive powers, or of a negative, positive and a further negative subgroup, and the front two members are made movable in opposite directions to each other. The technique is also disclosed in U.S. Pat. No. 4,124,274 where the focusing lens group is comprised of a negative, positive and a further positive subgroup, and the front two members are made movable in opposite directions to each other.

#FIGS. 1(A) and 1(B) show diagramatically the principles of the focusing method of the invention.

#FIG. 2 is a block diagram of one embodiment of a lens system employing the focusing method according to the present invention.

#FIG. 3 is a graph showing a relationship in which the first and second lens groups of FIG. 2 are moved for focusing.

#FIGS. 1(A) and (B) show diagrammatically and graphically illustrate the effect on angular field of view in the image space when a lens system is focused on an object at infinity and on an object at a distance, S.sub.1, respectively. This lens system comprises first and second lens groups 11 and 12 movable for focusing and a third lens group 13 stationary during focusing, and forms an image of an object at a focal plane 15. The rear principal plane of the entire system is indicated at 14. The reference, y, designates a maximum image height. Where the lens system is focused on the infinitely distant object, e'i.infin. designates the interval between the principal points of the i-th and (i+1)th lens groups, .DELTA.'.infin. the interval between the rear principal plane of the third lens group 13 and the rear principal plane 14 of the entire system, f' the focal length, S'F the back focal length, and .omega..infin. the angular field of view. Where the lens system is focused on the object at the finite distance S.sub.1, e'i designates the interval between the principal points of the i-th and (i+1)th lens groups, .DELTA.' the interval between the rear principal plane of the third lens group 13 and the rear principal plane 14 of the entire lens system, g'k the interval between the aforesaid principal plane 14 and the image plane 15, S'k the lens back, and .omega. the angular field of view. When the lens system is focused down from the reference position of FIG. 1(A) on the infinitely distant object to the finite object distance S.sub.1, the first lens group 11 is moved toward the image plane 15 by a distance, x.sub.1, and the second lens group 12 is concurrently moved toward the image plane 15 by a distance, x.sub.2. The angular fields of view .omega..infin. for the reference focusing position is given by

#.omega..infin.=y'/f'

#The angular field of view .omega. for a focusing position for a finite object distance is given by