1. Field of the Invention
This invention relates to five-component zoom lenses suited for still cameras, cine cameras and video cameras, and more particularly to such zoom lenses which have a high relative aperture and a greatly increased zoom ratio in a compact form while enabling good stability of aberration correction throughout the extended zooming range.
2. Description of the Prior Art
Prior art five-component zoom lenses with a power distribution of positive, negative, positive, negative and positive lens groups, from front to back are described in Japanese Patent Publication No. SHO 39-6127 (hereinafter called prior art example 1), F.P. No. 1336300 (prior art example 2), and British Pat. No. 2,078,997 (prior art example 3). In prior art example 1, as illustrated in FIG. 1 beginning with the first lens group 1, positive, negative, positive, negative and positive refractive powers are distributed in this order. The second lens group 2, third lens group 3 and fourth lens group 4 are axially movable for zooming, contribute respective ranges of magnification powers each across -1 times. Since the first lens group is held stationary during zooming, for the range of magnification powers of the second lens group to be increased, the total movement of the second lens group must be largely increased. Since the movement of the third lens group is directed forward to increase its magnification power, and that of the fourth lens group is rearwardly directed to increase its magnification power, the space the third and fourth lens groups occupy to perform the zooming operation must be created long enough. Therefore, the physical length of the lens system tends to increase in the longitudinal direction. The prior art example 2 represents a zoom lens of the optical compensation type with positive, negative, positive, negative and positive refractive powers in this order from the front. During zooming from the shortest focal length to the longest focal length, the second and fourth lens groups move rearward as a unit, while the third lens group moves forward. Since the first lens group is fixed against zooming, for the second lens group to contribute to an increase of the zoom ratio, it must be moved a far larger distance. As the second lens group is united with the fourth lens group, the physical length of the entire system tends to increse. The prior art example 3, though having the same power distribution as the above, furnishes another type of zoom lens of which all the five lens groups are made axially movable. Since the movement of the fifth lens group is made to be directed forward while zooming to the longest focal length, a sufficient movement cannot be imparted into the fourth lens group. Therefore, despite the fourth lens group being made to move during zooming, its contribution to a varifocal power of the entire system is lowered. Further, since a much-desired increase in the zoom ratio can be achieved solely by elongating the total movements of the positive or third and fifth lens groups, a zoom lens of very long total length will result. Though such increase in the total movement of the positive lens groups heightens the varifocal effect, it has a drawback of increasing variation of aberrations. Also when the shortening of the total length of the lens system is otherwise achieved by strengthening the positive refractive powers of the third and fifth lens groups, it has a tendency to make the variation of aberrations during zooming become very prominent.