Zoom lens system

1. A zoom lens system comprises a negative first lens group, a positive second lens group, a diaphragm, a negative third lens group, and a positive fourth lens group, in this order from an object,

wherein said negative third lens group comprises cemented lens elements having a positive lens element and a negative lens element;

wherein said positive fourth lens group comprises a positive single lens element, and cemented lens elements having a positive lens element and a negative lens element, in this order from the object;

wherein said negative first lens group is a focusing lens group; and

wherein said zoom lens system satisfies the following conditions:

0.4 <|fi ( i= 1)|/ fBw< 0.8

1.0 <fi ( i= 4)/ fi ( i= 2)/<1.8

wherein

fi designates the focal length of the i th lens group(1≦i≦4); and

fBw designates the back focal distance at the short focal length extremity.

2. The zoom lens system according to claim 1 , wherein said cemented lens elements of said positive fourth lens group have a positive refractive power and are arranged to position said positive lens element on the object side, and said negative lens element is on the image side.

3. The zoom lens system according to claim 1 , satisfying the following conditions:

1.0 <|fi ( i= 3)|/ fw< 2.5

1.3 <fi ( i= 4)/ fw< 2.5

wherein

fi designates the focal length of the i th lens group (1≦i≦4); and

fw designates the focal length of said zoom lens system at the short focal length extremity.

4. The zoom lens system according to claim 1 , satisfying following condition:

0.5 <|fi ( i= 1)|/( fw×ft ) 1/2 <1.2

wherein

fi designates the focal length of the i th lens group (1≦i≦4);

fw designates the focal length of said zoom lens system at the short focal length extremity; and

ft designates the focal length of said zoom lens system at the long focal length extremity.

5. The zoom lens system according to claim 1 , wherein a fixed-aperture diaphragm is provided in the close vicinity of the most object-side lens element of said positive second lens group to limit an off-axis bundle of light rays at the short focal length extremity.

6. The zoom lens system according to claim 1 , wherein a variable-aperture diaphragm is provided between said positive second lens group and said negative third lens group.

7. A zoom lens system comprises a negative first lens group, a positive second lens group, a diaphragm, a negative third lens group, and a positive fourth lens group, in this order from an object,

wherein said negative first lens group is arranged to be a focusing lens group;

wherein said zoom lens system satisfies the following conditions:

0.4 <|fi ( i= 1)|/ fBw< 0.8

1.0 <fi ( i= 4)/ fi ( i= 2)<1.8

1.3 <|fi ( i= 3)|/ fw <2.0

wherein

fi designates the focal length of the i th lens group (1≦i≦4);

fBw designates the back focal distance at the short focal length extremity; and

fw designates the focal length of the zoom lens system at the short focal length extremity.

8. The zoom lens system according to claim 7 , satisfying the following condition:

0.5 <|fi ( i= 1)|/( fw×ft ) 1/2 <1.2

wherein

fi designates the focal length of the i th lens group (1≦i≦4);

fw designates the focal length of said zoom lens system at the short focal length extremity; and

ft designates the focal length of said zoom lens system at the long focal length extremity.

9. The zoom lens system according to claim 7 , wherein a fixed-aperture diaphragm is provided in the close vicinity of the most object-side lens element of said positive second lens group to limit an off-axis bundle of light rays at the short focal length extremity.

10. The zoom lens system according to claim 7 , wherein a variable-aperture diaphragm is provided between said positive second lens group and said negative third lens group.

11. The zoom lens system according to claim 7 , satisfying the following conditions:

1.0 <|fi ( i= 1)|/ fw< 1.8

1.2 <fi ( i= 2)/ fw< 1.8

1.5 <fi ( i= 4)/ fw< 2.2

wherein

fi designates the focal length of the i th lens group(1≦i≦4); and

fw designates the focal length of the zoom lens system at the short focal length extremity.

12. A zoom lens system comprises a negative first lens group, a fixed-aperture diaphragm, a positive second lens group, a diaphragm, a negative third lens group, and a positive fourth lens group, in this order from an object, wherein:

said negative third lens group comprises cemented lens elements having a positive lens element and a negative lens element; and

said positive fourth lens group comprises a positive single lens element, and cemented lens elements having a positive lens element and a negative lens element, in this order from the object

said fixed-aperture diaphragm is provided proximate the most object-side lens element of said positive second lens group and is configured to limit an off-axis bundle of light rays at a short focal length extremity.

13. The zoom lens system according to claim 12 , wherein said cemented lens elements of said positive fourth lens group have a positive refractive power and are arranged to position said positive lens element on the object side, and said negative lens element is on the image side.

14. The zoom lens system according to claim 12 , satisfying the following conditions:

1.0 <|fi ( i= 3)|/ fw< 2.5

1.3 <fi ( i= 4)/ fw< 2.5

wherein

fi designates the focal length of the i th lens group(1≦i≦4); and

fw designates the focal length of said zoom lens system at the short focal length extremity.

15. The zoom lens system according to claim 12 , satisfying following condition:

0.5 <|fi ( i= 1)|/( fw×ft ) 1/2 <1.2

wherein

fi designates the focal length of the i th lens group(1≦i≦4);

fw designates the focal length of said zoom lens system at the short focal length extremity; and

ft designates the focal length of said zoom lens system at the long focal length extremity.

16. The zoom lens system according to claim 12 , wherein said diaphragm is a variable-aperture diaphragm provided between said positive second lens group and said negative third lens group.

17. The zoom lens system according to claim 12 , wherein at least one of said fixed-aperture diaphragm and said diaphragm are movable along an optical axis during a zooming operation.

18. A zoom lens system comprising a negative first lens group, a fixed-aperture diaphragm, a positive second lens group, a diaphragm, a negative third lens group, and a positive fourth lens group, in this order from an object, wherein:

said negative first lens group is arranged to be a focusing lens group;

said fixed-aperture diaphragm is provided proximate the most object-side lens element of said positive second lens group and is configured to limit an off-axis bundle of light rays at a short focal length extremity; and

said zoom lens system satisfies the following conditions:

0.4 <|fi ( i= 1)|/ fBw< 0.8

1.0 <fi ( i= 4)/ fi ( i= 2)<1.8

wherein

fi designates the focal length of the i th lens group(1≦i≦4); and

fBw designates the back focal distance at the short focal length extremity.

19. The zoom lens system according to claim 18 , satisfying the following condition:

0.5 <|fi ( i= 1)|/( fw×ft ) 1/2 <1.2

wherein

fi designates the focal length of the i th lens group(1≦i≦4);

fw designates the focal length of said zoom lens system at the short focal length extremity; and

ft designates the focal length of said zoom lens system at the long focal length extremity.

20. The zoom lens system according to claim 18 , wherein said diaphragm is a variable-aperture diaphragm provided between said positive second lens group and said negative third lens group.

21. The zoom lens system according to claim 18 , satisfying the following conditions:

1.0 <|fi ( i= 1)|/ fw< 1.8

1.2 <fi ( i= 2)/ fw< 1.8

1.5 <fi ( i= 4)/ fw< 2.2

wherein

fi designates the focal length of the i th lens group(1≦i≦4); and

fw designates the focal length of the zoom lens system at the short focal length extremity.

22. The zoom lens system according to claim 18 , wherein at least one of said fixed-aperture diaphragm and said diaphragm are movable along an optical axis during a zooming operation.