Zoom lens system

1. A 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 this order from an object, the positive third lens group having positive power in an entire zooming range of zoom lens system,

wherein upon zooming from the short focal length extremity to the long focal length extremity, a distance between said positive first lens group and said negative second lens group increases, and a distance between said negative second lens group and said positive third lens group decreases;

wherein said positive third lens group comprises a positive first sub-lens group and a negative second sub-lens group, in this order from the object;

wherein said positive first sub-lens group comprises a positive lens element provided at the most object-side thereof and a cemented lens provided at the most image-side thereof;

wherein said cemented lens comprises a positive lens element and a negative lens element; and

wherein said zoom lens system satisfies the following conditions:

0.3 <f 3 a/f 31<0.6

−0.15 <f 3 a/f 3 PN ≦0.06

wherein

f 3 a designates a focal length of said positive first sub-lens group;

f 31 designates a focal length of said positive lens element provided at the most object-side of said positive first sub-lens group; and

f 3 PN designates a focal length of said cemented lens provided at the most image-side of said positive first sub-lens group.

2. The zoom lens system according to claim 1 , wherein said negative second lens group comprises a negative lens element, a negative lens element, a positive lens element, a negative lens element and a negative lens element, in this order from the object.

3. The zoom lens system according to claim 1 , wherein said negative second lens group comprises a negative lens element, a negative lens element and a positive lens element, in this order from the image; and

wherein each of said two negative lens elements has a concave surface facing toward the object.

4. The zoom lens system according to claim 2 , wherein said negative second lens group comprises a negative meniscus lens element having a convex surface facing toward the object, a biconcave negative lens element, a cemented lens including a biconvex positive lens element and a negative lens element having a concave surface facing toward the object, and a negative lens element having a concave surface facing toward the object, in this order from the object.

5. The zoom lens system according to claim 1 , further satisfying the following condition:

2.5 <f 1/ R 2 F <5.0

wherein

f 1 designates a focal length of said positive first lens group; and

R 2 F designates a radius of curvature of the most object-side surface of said negative second lens group.

6. The zoom lens system according to claim 1 , wherein a positive lens element is incorporated into said negative second lens group.

7. The zoom lens system according to claim 1 , wherein said positive first sub-lens group and the negative second sub-lens group are arranged to maintain a fixed distance therebetween when zooming is performed.

8. The zoom lens system according to claim 1 , wherein said positive first sub-lens group and said negative second sub-lens group are arranged to vary a distance therebetween when zooming is performed.

9. A 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 this order from an object, the positive third lens group having positive power in an entire zooming range of the zoom lens system,

wherein upon zooming from the short focal length extremity to the long focal length extremity, a distance between said positive first lens group and said negative second lens group increases, and a distance between said negative second lens group and said positive third lens group decreases;

wherein said positive third lens group comprises a positive first sub-lens group and a negative second sub-lens group, in this order from the object;

wherein said positive first sub-lens group comprises a cemented lens provided at the most image-side thereof and at least two positive lens elements on the object side of said cemented lens;

wherein said cemented lens comprises a positive lens element and a negative lens element; and

wherein said zoom lens system satisfies the following conditions:

0.3 <f 3 a/f 3 p <0.7

−0.15 <f 3 a/f 3 PN ≦0.06

wherein

f 3 a designates a focal length of said positive first sub-lens group;

f 3 p designates a focal length of a positive lens element having the strongest refractive power among said at least two positive lens elements on the object side of said cemented lens; and

f 3 PN designates a focal length of said cemented lens provided at the most image-side of said positive first sub-lens group.

10. The zoom lens system according to claim 9 , wherein said negative second lens group comprises a negative lens element, a negative lens element, a positive lens element, a negative lens element and a negative lens element, in this order from the object.

11. The zoom lens system according to claim 9 , wherein said negative second lens group comprises a negative lens element, a negative lens element and a positive lens element, in this order from the image; and wherein each of said two negative lens elements has a concave surface facing toward the object.

12. The zoom lens system according to claim 10 , wherein said negative second lens group comprises a negative meniscus lens element having the convex surface facing toward the object, a biconcave negative lens element, a cemented lens including a biconvex positive lens element and a negative lens element having a concave surface facing toward the object, and a negative lens element having a concave surface facing toward the object, in this order from the object.

13. The zoom lens system according to claim 9 , further satisfying the following condition:

2.5 <f 1 /R 2 F <5.0

wherein

fl designates the focal length of said positive first lens group; and

R 2 F designates the radius of curvature of the most object-side surface of said negative second lens group.

14. The zoom lens system according to claim 9 , wherein a positive lens element is incorporated into said negative second lens group.

15. The zoom lens system according to claim 9 , wherein said positive first sub-lens group and the negative second sub-lens group are arranged to maintain a fixed distance therebetween when zooming is performed.

16. The zoom lens system according to claim 9 , wherein said positive first sub-lens group and said negative second sub-lens group are arranged to vary the distance therebetween when zooming is performed.

17. The zoom lens system according to claim 3 , wherein said negative second lens group comprises a negative meniscus lens element having a convex surface facing toward the object, a biconcave negative lens element, a cemented lens including a biconvex positive lens element and a negative meniscus lens element having the concave surface facing toward the object, and a negative lens element having a concave surface facing toward the object, in this order from the object.

18. The zoom lens system according to claim 11 , wherein said negative second lens group comprises a negative meniscus lens element having a convex surface facing toward the object, a biconcave negative lens element, a cemented lens including a biconvex positive lens element and a negative meniscus lens element having the concave surface facing toward the object, and a negative lens element having a concave surface facing toward the object, in this order from the object.