HIGH ZOOM-RATIO ZOOM LENS SYSTEM
A high-ratio zoom lens system includes a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group. Upon zooming from the short to long focal length extremities, the lens groups move while increasing the distance between the first and second lens groups, and decreasing the distance between the second and third lens groups. The first lens group includes a negative lens element having a concave surface on the image side, a positive lens element having a convex surface on the object side, and a positive lens element having a convex surface on the object side, in that order from the object side. The following conditions (1) and (2) are satisfied: 1.7< f 1/ f 4<2.02 (1), and 1.82<( f 1× f 4)/( f 3) 2 <2.3 (2), wherein f 1, f 3 and f 4 designate the focal length of the first, third and fourth lens groups, respectively.
1 . A high-ratio zoom lens system comprises a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, in that order from an object,
wherein upon zooming from the short focal length extremity to the long focal length extremity, said first through fourth lens groups move along the optical axis thereof, while the distance between said first lens group and said second lens group increases, and the distance between said second lens group and said third lens group decreases;
wherein said first lens group comprises a negative lens element having a concave surface on the image side, a positive lens element having a convex surface on the object side, and a positive lens element having a convex surface on the object side, in that order from the object; and
wherein said high-ratio zoom lens system satisfies the following conditions:
1.7 <f 1 /f 4<2.02
1.82<( f 1 ×f 4)/( f 3) 2 <2.3
wherein
f 1 designates the focal length of said first lens group,
f 3 designates the focal length of said third lens group, and
f 4 designates the focal length of said fourth lens group.
2 . The high-ratio zoom lens system according to claim 1 , further satisfying the following condition:
|Δ P 11-12 /Δνd 11-12 |<0.0015
wherein
Δ P 11-12 =(Δ P g-F 11 −ΔP g-F 12), and
Δν d 11-12 =νd 11 −νd 12,
wherein
ΔP g-F 11 designates the partial dispersion ratio of said negative lens element in said first lens group,
ΔP g-F 12 designates the partial dispersion ratio of the object-side positive lens element in said first lens group;
νd 11 designates the Abbe number with respect to the d-line of said negative lens element in said first lens group; and
νd 12 designates the Abbe number with respect to the d-line of said object-side positive lens element in said first lens group.
3 . The high-ratio zoom lens system according to claim 1 , wherein said fourth lens group comprises a positive lens element, and a cemented lens formed from a negative lens element and a positive lens element, in that order from the object.
4 . The high-ratio zoom lens system according to claim 3 , further satisfying the following condition:
νd41>80
wherein
νd 41 designates the Abbe number with respect to the d-line of said object-side positive lens element in said fourth lens group.
5 . The high-ratio zoom lens system according to claim 1 , wherein said third lens group comprises a positive lens element, a positive lens element, and a negative lens element having a concave surface on the object side, in that order from the object.