Telephoto zoom lens

DETAILED DESCRIPTION Accordingly, it is an object of the invention to provide a telephoto zoom lens which is compact in structure and has good image forming performance throughout the variation range of magnification.
It is another object of the invention to provide a telephoto zoom lens in which the variation of F-number, that is, the brightness of the lens system with magnification change is very small as compared with the prior art ones and which needs no complicated mechanism for changing the opening of aperture stop.
In order to attain the above objects, the present invention provides a zoom lens consisting of a first lens group of positive refractive power, a second lens group of negative refractive power, a third lens group of positive refractive power and a fourth lens group of positive refractive power arranged in the named order from the object side, which is characterized in that, as the second lens group, there is used a lens group comprising a first cemented lens component composed of a positive and negative lens elements cemented together and having a negative composite refractive power and a second cemented lens component composed of negative and positive lens elements cemented together and having a negative composite refractive power arranged in the named order from the object side, and in that in the first cemented lens component of the second lens group there is provided a cemented surface having a negative refractive power thereby preventing the increase of coma at the wide angle side while reducing the variation of coma with magnification change and also increasing the negative refractive power of the second lens group which enables to realize a compact construction of the zoom lens.
According to another feature of the invention to attain the above objects, the first and fourth lens groups are moved toward the object side for the magnification change from wide angle end to telephoto end and the third lens group is moved also toward the object side but in a non-linear fashion whilst keeping the second lens stationary relative to the image plane in such manner as to satisfy the conditions: x