Telephoto type zoom lens

The present invention relates to a focusing lens group of a telephoto zoom lens having a long range of focal length. The focusing lens group consists of four sub-groups, wherein first, second, and fourth sub-groups are fixed while a third sub-group can be shifted for focusing. And the first sub-group has a positive refractive power and the second sub-group has a negative refractive power. The third sub-group has a positive refractive power and the fourth sub-group has a negative refractive power. As a result, the variation in the image quality at a time when the telephoto zoom lens is focused will be eliminated and the diameters of the lenses in the second, third and fourth sub-groups will be smaller than that of the first sub-group.

BACKGROUND OF THE INVENTION 
The present invention relates to a zoom lens and particularly to an optical 
system suitable to a so-called telephoto type zoom lens having an angle of 
view at the wide angle end which is smaller than 40.degree.. 
If the focal length at the telescopic end is quite long and the diameter is 
large, a lens diameter (front lens diameter) at the front end of the 
object side will be considerably large. Therefore, when a conventionally 
known focusing system in which an entire focusing group is shifted is 
employed, the weight of the total focusing lens will become very heavy, 
and a warp will be generated in the lens barrel while the lens is shifted. 
Thus, deterioration of properties will occur by eccentricity of optic 
axis, and the barrel wall thickness of the lens barrel must be thick for 
preventing the same. Accordingly, the total system becomes large and 
heavy, and its handling characteristics become poor. 
On the other hand, a focusing lens having a number of lenses reduced is 
known in U.S. Pat. Nos. 3,594,066 and 3,598,476. Such zoom lens is 
suitable to a wide angle photographing. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to reduce lens diameters in a 
movable lens group and a lens group behind the group within the front lens 
group and at a same time to reduce the weight of the movable lens group. 
This will prevent the front lens group from being large and heavy weight. 
This occurs when a zoom lens is made suitable for telescopic purpose as 
such conventionally known focusing system that a portion of a convergent 
front lens group which is placed at a position closer to an object than a 
lens group being shifted for zooming is shifted, is employed. As a result, 
an entire lens system-containing lens barrel is made light and compact and 
the handling characteristic is also improved. 
A particular arrangement of the present invention shall be explained. That 
is, in a zoom lens in which a convergent front lens group having a 
focusing function is provided at a position closer to an object than that 
of a movable lens group (a variator, a compensator) for zooming, the 
convergent front lens group (I) has a convergent lens group (II), a 
divergent lens group (III), a convergent movable lens group (IV), and a 
divergent rear lens group (V) in said order from an object side, and the 
ratio between the focal length of the convergent lens group (II) and the 
absolute value of the focal length of the divergent lens group (III) lies 
between 1 : 0.65 and 1 : 0.9, while the absolute value of the focal length 
of the divergent rear lens group (V) is greater than the focal length of 
the convergent movable lens group (IV). Focusing is done against different 
object distances by shifting the convergent movable lens group (IV) on an 
optic axis. Also it is desired that an almost afocal system is made of the 
convergent lens group (II) and the divergent lens group (III), that is to 
be specific, it is desired that a combined focal length of the convergent 
lens group (II) and the diverging lens group (III) is greater than ten 
times the focal length of the convergent lens group (I) in a state where 
focusing is done at an infinite object distance. 
Further in an embodiment of this arrangement, the convergent lens group 
(II) consists of positive lenses only, at least one of which is made of 
fluorosilicate crown glass or fluorospar, while the divergent lens group 
(III) is made of negative lenses only, at least one of which is made of 
dense lanthanum flint glass or lanthanum flint glass. The convergent 
movable lens group (IV) consists of positive lenses only, at least one of 
which is made of fluorosilicate crown glass or fluorspar. 
To explain the same in detail, it is desired that each of the convergent 
lens group (II) and the convergent movable lens group (IV) uses a biconvex 
lens and a meniscus positive lens having its convex plane facing to an 
object side, being positioned from an object side in said order and using 
fluorosilicate crown glass or fluorspar, and the divergent lens group 
(III) has two meniscus negative lenses having its convex plane facing the 
object side, using dense lanthanum flint glass or lanthanum flint glass, 
while the divergent rear lens group (V) uses a meniscus negative lens 
having its convex plane facing the object side wherein the absolute value 
of the focal length of the divergent rear lens group (V) is greater than 
five times of the focal length of the convergent movable lens group (IV) 
and is smaller than seven times the same. 
As a result of the above arrangement, a lens group having an effective 
diameter considerably smaller than the minimum effective diameter H.sub.A 
(the focal length at a telescopic end .div. F number) needed by the 
diameter of a lens at front end at an object side (front lens diameter) 
becomes a focusing lens group, within a convergent front lens group having 
a focusing function, even if the focal length at the telescopic end is 
very long and the lens diameter is great.