Zoom lens system, and image capture device and interchangeable lens unit including the zoom lens system

1 . A zoom lens system comprising multiple lens groups and an aperture stop,

the multiple lens groups consisting of:

a first lens group having negative power;

a second lens group having positive power;

a third lens group having negative power; and

a fourth lens group having negative power,

the first, second, third, and fourth lens groups being arranged in this order such that the first lens group is located closer to an object than any of the second, third, or fourth lens group is, and that the fourth lens group is located closer to an image plane than any of the first, second, or third lens group is,

the aperture stop being disposed between the first lens group and the second lens group,

an interval between each pair of lens groups that are adjacent to each other among the multiple lens groups changing as the first, second, third, and fourth lens groups move along an optical axis of the zoom lens system while the zoom lens system is zooming,

the third lens group being configured to move along the optical axis while the zoom lens system is focusing to make a transition from an infinity in-focus state toward a close-object in-focus state, and

the zoom lens system satisfying the following inequality (1):

0.5

<

fw

/

BFw

<

1.5

(

1

)

where fw is a focal length of the zoom lens system at a wide-angle end,

BFw is a back focus of the zoom lens system at the wide-angle end,

a lens located closest to the object among the second lens group being a negative lens, and

the second lens group further including a positive lens located closer to the image plane than the negative lens, the positive lens and the negative lens being bonded together to form a bonded lens.

2 . The zoom lens system of claim 1 , wherein

the aperture stop is configured to move along the optical axis together with the second lens group while the zoom lens system is zooming.

3 . The zoom lens system of claim 1 , wherein

the first lens group is configured to move to draw a locus that is convex toward the image plane while the zoom lens system is zooming.

4 . The zoom lens system of claim 1 , wherein

the first lens group includes a plurality of lenses, the plurality of lenses comprising:

a negative lens G 1 L 1 located closest to the object among the plurality of lenses;

a negative lens G 1 L 2 located second closest to the object among the plurality of lenses; and

a negative lens G 1 L 3 located third closest to the object among the plurality of lenses, and

the zoom lens system satisfies the following inequality (2):

65

<

vd

⁢

1

<

100

(

2

)

where vd 1 is an abbe number of at least one negative lens, selected from the group consisting of the negative lens G 1 L 1 , the negative lens G 1 L 2 , and the negative lens GIL 3 , in response to a d-line.

5 . The zoom lens system of claim 4 , wherein

the zoom lens system satisfies the following inequality (3):

0.1

<

DL

⁢

1

/

DL

⁢

2

<

0.9

(

3

)

where DL 1 is a thickness of the negative lens G 1 L 1 as measured along the optical axis, and

DL 2 is a thickness of the negative lens G 1 L 2 as measured along the optical axis.

6 . The zoom lens system of claim 1 , wherein

the zoom lens system satisfies the following inequality (4):

0.5

<

❘

"\[LeftBracketingBar]"

fG

⁢

3

/

fG

⁢

2

❘

"\[RightBracketingBar]"

<

2.5

(

4

)

where fG 2 is a focal length of the second lens group, and

fG 3 is a focal length of the third lens group.

7 . The zoom lens system of claim 1 , wherein

the third lens group includes at least one positive lens LG 3 p that satisfies the following inequality (5):

14

<

vG

⁢

3

⁢

p

<

35

(

5

)

where vG 3 p is an abbe number of the at least one positive lens LG 3 p in response to a d-line.

8 . The zoom lens system of claim 1 , wherein

the fourth lens group includes at least one positive lens LG 4 p that satisfies the following inequality (6):

65

<

vG

⁢

4

⁢

p

<

100

(

6

)

where vG 4 p is an abbe number of the at least one positive lens LG 4 p in response to a d-line.

9 . An image capture device configured to transform an optical image of an object into an electrical image signal and output the electrical image signal thus transformed, the image capture device comprising:

a zoom lens system configured to form the optical image of the object; and

an image sensor configured to transform the optical image formed by the zoom lens system into the electrical image signal,

the zoom lens system comprising multiple lens groups that consist of:

a first lens group having negative power;

a second lens group having positive power;

a third lens group having negative power; and

a fourth lens group having negative power,

the first, second, third, and fourth lens groups being arranged in this order such that the first lens group is located closer to the object than any of the second, third, or fourth lens group is, and that the fourth lens group is located closer to an image plane than any of the first, second, or third lens group is,

an interval between each pair of lens groups that are adjacent to each other among the multiple lens groups changing as the first, second, third, and fourth lens groups move along an optical axis of the zoom lens system while the zoom lens system is zooming,

the third lens group being configured to move along the optical axis while the zoom lens system is focusing to make a transition from an infinity in-focus state toward a close-object in-focus state, and

the zoom lens system satisfying the following inequality (1):

0.5

<

fw

/

BFw

<

1.5

(

1

)

where fw is a focal length of the zoom lens system at a wide-angle end,

BFw is a back focus of the zoom lens system at the wide-angle end,

a lens located closest to the object among the second lens group being a negative lens, and

the second lens group further including a positive lens located closer to the image plane than the negative lens, the positive lens and the negative lens being bonded together to form a bonded lens.

10 . An interchangeable lens unit configured to be removably connected, via a mount, to a camera body, the camera body including: an image sensor configured to receive an optical image and transform the optical image into an electrical image signal; and the mount,

the interchangeable lens unit forming an optical image of an object on the image sensor,

the interchangeable lens unit including a zoom lens system,

the zoom lens system comprising multiple lens groups that consist of:

a first lens group having negative power;

a second lens group having positive power;

a third lens group having negative power; and

a fourth lens group having negative power,

the first, second, third, and fourth lens groups being arranged in this order such that the first lens group is located closer to the object than any of the second, third, or fourth lens group is, and that the fourth lens group is located closer to an image plane than any of the first, second, or third lens group is,

an interval between each pair of lens groups that are adjacent to each other among the multiple lens groups changing as the first, second, third, and fourth lens groups move along an optical axis of the zoom lens system while the zoom lens system is zooming,

the third lens group being configured to move along the optical axis while the zoom lens system is focusing to make a transition from an infinity in-focus state toward a close-object in-focus state, and

the zoom lens system satisfying the following inequality (1):

0.5

<

fw

/

BFw

<

1.5

(

1

)

where fw is a focal length of the zoom lens system at a wide-angle end,

BFw is a back focus of the zoom lens system at the wide-angle end,

a lens located closest to the object among the second lens group being a negative lens, and

the second lens group further including a positive lens located closer to the image plane than the negative lens, the positive lens and the negative lens being bonded together to form a bonded lens.