Zoom lens system, interchangeable lens device, and camera system

1. A zoom lens system including a plurality of lens groups each of which includes at least one lens element, the zoom lens system comprising, in order from an object side to an image side:

a front group including a first focusing lens group having a negative power; and

a rear group including a second focusing lens group having a positive power,

wherein when zooming is performed from a wide end to a telephoto end, the first focusing lens group and the second focusing lens group move along an optical axis,

when focusing is performed from an infinity focusing state to a proximity object focusing state, the first focusing lens group and the second focusing lens group move to perform the focusing, and

conditional expressions (1), (2), and (3) below are satisfied:

−20<β1 t/β 1 w×β 2 t/β 2 w< 20  (1)

0.25<| f 1/ f 2|<1.30  (2)

ν p 2<30  (3)

where:

β 1 t is a lateral magnification of the first focusing lens group at the telephoto end;

β 1 w is a lateral magnification of the first focusing lens group at the wide end;

β 2 t is a lateral magnification of the second focusing lens group at the telephoto end;

β 2 w is a lateral magnification of the second focusing lens group at the wide end;

f 1 is a focal length of the first focusing lens group;

f 2 is a focal length of the second focusing lens group; and

νp 2 is the Abbe number of a lens element which has the strongest positive power in the at least one lens element constituting the second focusing lens group.

2. The zoom lens system according to claim 1 , further comprising

an aperture diaphragm,

wherein the first focusing lens group and the second focusing lens group are disposed on the image side of the aperture diaphragm.

3. The zoom lens system according to claim 1 , wherein, when focusing is performed from the infinity focusing state to the proximity object focusing state, the first focusing lens group moves toward the image side along the optical axis and the second focusing lens group moves toward the object side along the optical axis.

4. The zoom lens system according to claim 1 , wherein, when zooming is performed from the wide end to the telephoto end, the first focusing lens group moves along a locus convex toward the image side.

5. The zoom lens system according to claim 1 , wherein, when zooming is performed from the wide end to the telephoto end, the second focusing lens group moves getting closer to an adjacent lens group on the image side.

6. The zoom lens system according to claim 1 , wherein the second focusing lens group consists of one lens element having a positive power.

7. The zoom lens system according to claim 1 , wherein the first focusing lens group has a single lens element having a positive power and a single lens element having a negative power, and satisfies conditional expression (4) below:

ν p 1<30  (4)

where:

νp 1 is the Abbe number of a lens element which has the strongest positive power in the lens elements constituting the first focusing lens group.

8. The zoom lens system according to claim 1 , wherein the first focusing lens group includes a cemented lens made up of, in order from the object side to the image side:

a single lens element having a positive power; and

a single lens element having a negative power.

9. The zoom lens system according to claim 1 , wherein conditional expression (5) below is satisfied:

mf 2 w/mf 1 w>mf 2 t/mf 1 t   (5)

where:

mf 2 w is an amount of movement of the second focusing lens group at the wide end when focusing is performed from the infinity focusing state to an object at any certain distance;

mf 1 w is an amount of movement of the first focusing lens group at the wide end when focusing is performed from the infinity focusing state to an object at the certain distance;

mf 2 t is an amount of movement of the second focusing lens group at the telephoto end when focusing is performed from the infinity focusing state to an object at the certain distance; and

mf 1 t is an amount of movement of the first focusing lens group at the telephoto end when focusing is performed from the infinity focusing state to an object at the certain distance.

10. The zoom lens system according to claim 1 , wherein the front group includes a first lens group on the most object side, and the first lens group has a positive power.

11. The zoom lens system according to claim 1 , wherein, when zooming is performed from the wide end to the telephoto end, the first lens group is fixed with respect to an image plane.

12. The zoom lens system according to claim 1 , further comprising:

an aperture diaphragm, and

two lens groups between the first focusing lens group and the aperture diaphragm, the two lens groups moving along the optical axis when zooming is performed from the wide end to the telephoto end.

13. The zoom lens system according to claim 1 , wherein the front group includes on the object side of the first focusing lens group, in order from the object side to the image side:

a first lens group having a positive power;

a second lens group having a negative power; and

a third lens group having a positive power.

14. The zoom lens system according to claim 13 , wherein the third lens group consists of a single lens element having a positive power.

15. The zoom lens system according to claim 13 , wherein a lens group which is adjacent to and on the object side of the first focusing lens group includes, in order from the object side to the image side:

a first sub-lens group having a positive power;

an aperture diaphragm;

a second sub-lens group having a negative power;

a third sub-lens group which has a positive power and performs image stabilization by moving in a direction perpendicular to the optical axis; and

a fourth sub-lens group having a positive power.

16. The zoom lens system according to claim 15 , comprising, between the third sub-lens group and the fourth sub-lens group, a variable light aperture diaphragm which changes a diameter of the variable light aperture when zooming is performed from the wide end to the telephoto end.

17. The zoom lens system according to claim 15 , wherein the third sub-lens group includes, in order from the object side to the image side:

a cemented lens made up of a lens element having a negative power and a lens element having a positive power; and

a single lens element having a positive power.

18. An interchangeable lens device comprising:

a zoom lens system including a plurality of lens groups each of which includes at least one lens element, the zoom lens system comprising, in order from an object side to an image side:

a front group including a first focusing lens group having a negative power; and

a rear group including a second focusing lens group having a positive power, wherein when zooming is performed from a wide end to a telephoto end, the first focusing lens group and the second focusing lens group move along an optical axis, and when focusing is performed from an infinity focusing state to a proximity object focusing state, the first focusing lens group and the second focusing lens group move to perform the focusing; and

a lens mount which is attachable to a camera body including an imaging element which receives an optical image formed by the zoom lens system and converts the optical image into an electric image signal,

wherein conditional expressions (1), (2), and (3) below are satisfied:

−20<β1 t/β 1 w×β 2 t/β 2 w< 20  (1)

0.25<| f 1/ f 2|<1.30  (2)

ν p 2<30  (3)

where:

β 1 t is a lateral magnification of the first focusing lens group at the telephoto end;

β 1 w is a lateral magnification of the first focusing lens group at the wide end;

β 2 t is a lateral magnification of the second focusing lens group at the telephoto end;

β 2 w is a lateral magnification of the second focusing lens group at the wide end;

f 1 is a focal length of the first focusing lens group;

f 2 is a focal length of the second focusing lens group; and

νp 2 is the Abbe number of a lens element which has the strongest positive power in the at least one lens element constituting the second focusing lens group.

19. A camera system comprising:

an interchangeable lens device; and

a camera body,

wherein the interchangeable lens device includes:

a zoom lens system including a plurality of lens groups each of which includes at least one lens element, the zoom lens system comprising, in order from an object side to an image side:

a front group including a first focusing lens group having a negative power; and

a rear group including a second focusing lens group having a positive power, wherein when zooming is performed from a wide end to a telephoto end, the first focusing lens group and the second focusing lens group move along an optical axis, and when focusing is performed from an infinity focusing state to a proximity object focusing state, the first focusing lens group and the second focusing lens group move to perform the focusing; and

a lens mount which is attachable to the camera body, and

wherein the camera body has a camera mount, and is detachably coupled to the interchangeable lens device via the camera mount and which includes an imaging element which receives an optical image formed by the zoom lens system and converts the optical image into an electric image signal,

wherein conditional expressions (1), (2), and (3) below are satisfied:

−20<β1 t/β 1 w×β 2 t/β 2 w< 20  (1)

0.25<| f 1/ f 2|<1.30  (2)

ν p 2<30  (3)

where:

β 1 t is a lateral magnification of the first focusing lens group at the telephoto end;

β 1 w is a lateral magnification of the first focusing lens group at the wide end;

β 2 t is a lateral magnification of the second focusing lens group at the telephoto end;

β 2 w is a lateral magnification of the second focusing lens group at the wide end;

f 1 is a focal length of the first focusing lens group;

f 2 is a focal length of the second focusing lens group; and

νp 2 is the Abbe number of a lens element which has the strongest positive power in the at least one lens element constituting the second focusing lens group.