Zoom lens and imaging apparatus

1. A zoom lens consisting of, in order from the object side to the image side:

a first lens group having a negative refractive power;

a stop;

a second lens group having a positive refractive power;

a third lens group having a negative refractive power; and

a fourth lens group having a positive refractive power,

all of the lens groups moving along the optical axis when changing magnification from the wide angle end to the telephoto end such that the distance between the first lens group and the second lens group decreases and the distance between the second lens group and the third lens group increases;

the first lens group having only three lenses, said three lenses of said first lens group consisting of, in order from the object side to the image side, a first lens of the first lens group having a negative refractive power, a second lens of the first lens group having a negative refractive power, and a third lens of the first lens group having a positive refractive power;

the second lens group having only four lenses, said four lenses of said second lens group consisting of, in order from the object side to the image side, a first lens of the second lens group having a positive refractive power, a second lens of the second lens group having a positive refractive power, a third lens of the second lens group having a negative refractive power, and a fourth lens of the second lens group having a positive refractive power; and

the conditional formula below being satisfied:

1.75<ndave<1.92  (1)

wherein ndave is the average refractive index of the first lens of the first lens group and the second lens of the first lens group with respect to the d line (wavelength: 587. 6 nm).

2. The zoom lens as defined in claim 1 , wherein:

the second lens of the first lens group has at least one aspherical surface, and the conditional formula below is satisfied:

0.80<( r 12 f+r 12 r )/( r 12 f−r 12 r )<1.70  (2)

wherein r12f is the paraxial radius of curvature of the second lens of the first lens group toward the object side, and r12r is the paraxial radius of curvature of the second lens of the first lens group toward the image side.

3. The zoom lens as defined in claim 1 , wherein:

the conditional formula below is satisfied:

9.0< TLw/fw< 12.5  (3)

wherein TLw is the distance from the apex of the lens surface most toward the object side to an image formation plane at the wide angle end, and fw is the focal length of the entire zoom lens at the wide angle end.

4. The zoom lens as defined in claim 1 , wherein:

the conditional formula below is satisfied:

2.5< X 2/ fw< 6.1  (4)

wherein X 2 is the amount of movement of the second lens group from the wide angle end to the telephoto end, and fw is the focal length of the entire zoom lens at the wide angle end.

5. The zoom lens as defined in claim 1 , wherein:

the third lens group consists of only one lens, the one lens of the third lens group having a negative refractive power, the one lens of the third lens group having at least one aspherical surface, and the conditional formula below is satisfied:

0.40<( r 31 f+r 31 r )/( r 31 f−r 31 r )<1.60  (5)

wherein r31f is the paraxial radius of curvature of the one lens of the third lens group toward the object side, and r31r is the paraxial radius of curvature of the one lens of the third lens group toward the image side.

6. The zoom lens as defined in claim 1 , wherein:

focusing operations are performed by moving the fourth lens group in the direction of the optical axis.

7. The zoom lens as defined in claim 1 , wherein:

the fourth lens group consists of only one lens, the one lens of the fourth lens group having a positive refractive power.

8. The zoom lens as defined in claim 1 , wherein the conditional formula below is satisfied:

1.78< ndave< 1.88  (1-1).

9. The zoom lens as defined in claim 1 , wherein:

the second lens of the first lens group has at least one aspherical surface, and the conditional formula below is satisfied:

1.00<( r 12 f+r 12 r )/( r 12 f−r 12 r )<1.50  (2-1)

wherein r12f is the paraxial radius of curvature of the second lens of the first lens group toward the object side, and r12r is the paraxial radius of curvature of the second lens of the first lens group toward the image side.

10. The zoom lens as defined in claim 1 , wherein the conditional formula below is satisfied:

10.0< TLw/fw< 11.8  (3-1)

wherein TLw is the distance from the apex of the lens surface most toward the object side to an image formation plane at the wide angle end, and fw is the focal length of the entire zoom lens at the wide angle end.

11. The zoom lens as defined in claim 1 , wherein the conditional formula below is satisfied:

2.9< X 2/ fw< 5.7  (4-1)

wherein X 2 is the amount of movement of the second lens group from the wide angle end to the telephoto end, and fw is the focal length of the entire zoom lens at the wide angle end.

12. The zoom lens as defined in claim 1 , wherein:

the third lens group consists of only one lens, the one lens of the third lens group having a negative refractive power, the one lens of the third lens group having at least one aspherical surface, and the conditional formula below is satisfied:

0.70<( r 31 f+r 31 r )/( r 31 f−r 31 r )<1.40  (5-1)

wherein r31f is the paraxial radius of curvature of the one lens of the third lens group toward the object side, and r31r is the paraxial radius of curvature of the one lens of the third lens group toward the image side.

13. An imaging apparatus equipped with the zoom lens defined in claim 1 .