Zoom lens and imaging apparatus

1. A zoom lens comprising:

a first lens group having positive refractive power;

a second lens group having negative refractive power;

a third lens group having positive refractive power;

a fourth lens group which has negative refractive power and is configured with only a single lens; and

a fifth lens group having positive refractive power,

wherein the first lens group, the second lens group, the third lens group, the fourth lens group, and the fifth lens group are arranged in order from an object side to an image side,

wherein during zooming, at least the first lens group, the second lens group, the third lens group, and the fourth lens group are movable in an optical axis direction,

wherein focusing from infinity to a close range is performed by the fourth lens group being moved in the optical axis direction, and

wherein at least one of following conditional expressions (1) and (2) is satisfied:

−11.0<Fno — w×Y /{(1−β4 — w 2 )×β5 — w 2 }<−7.0 and  (1)

−11.0<Fno — t×Y /{(1−β4 — t 2 )×β5 — t 2 }<−7.0  (2)

where

Fno_w: an open f-number at a wide-angle end,

Fno_t: an open f-number at a telephoto end,

Y: a paraxial image height at the wide-angle end represented by fw×tan ω, when fw and ω respectively represent a focal length and a half angle of view of an entire system at the wide-angle end,

β4_w: a lateral magnification of the fourth lens group at the wide-angle end,

β4_t: a lateral magnification of the fourth lens group at the telephoto end,

β5_w: a lateral magnification of the fifth lens group at the wide-angle end, and

β5_t: a lateral magnification of the fifth lens group at the telephoto end.

2. The zoom lens according to claim 1 ,

wherein during zooming from the wide-angle end to the telephoto end, the first lens group is moved so as to widen a distance to the second lens group and the third lens group is moved so as to narrow a distance to the second lens group, and

wherein the first lens group and the third lens group are respectively positioned on the object side at the telephoto end rather than at the wide-angle end.

3. The zoom lens according to claim 1 ,

wherein an aspherical shape of at least one surface is formed in each of the second lens group, the third lens group and the fourth lens group.

4. The zoom lens according to claim 1 ,

wherein a following conditional expression (3) is satisfied:

−1.3< f 4/ f 5<−0.9  (3)

where

f4: a focal length of the fourth lens group, and

f5: a focal length of the fifth lens group.

5. The zoom lens according to claim 1 ,

wherein a following conditional expression (4) is satisfied:

0.01< t 4/( f — w×f — t ) 1/2 <0.045  (4)

where

t4: a center thickness of the fourth lens group,

f_w: a focal length of an entire system at the wide-angle end, and

f_t: a focal length of an entire system at the telephoto end.

6. The zoom lens according to claim 1 ,

wherein the third lens group is configured with a single lens having positive refractive power, a cemented lens which is configured with a positive lens and a negative lens and has negative refractive power, and a single lens having positive refractive power, which are arranged in order from an object side to an image side.

7. The zoom lens according to claim 1 ,

wherein a following conditional expression (5) is satisfied:

(Fno — w +Fno — t )/2<3.0.  (5)

8. The zoom lens according to claim 6 ,

wherein the zoom lens has a vibration reduction function by the single lens positioned closest to the image side of the third lens group being displaced in a direction perpendicular to the optical axis, and

wherein at least one surface of the single lens positioned closest to the image side is formed as an aspherical surface.

9. The zoom lens according to claim 1 ,

wherein the fifth lens group is fixed in the optical axis direction and configured with two lenses of at least a positive lens and a negative lens.

10. The zoom lens according to claim 1 ,

wherein the fifth lens group is movable in the optical axis direction.

11. The zoom lens according to claim 1 ,

wherein the fourth lens group is moved in the optical axis direction by a driving force of a piezo element.

12. An imaging apparatus comprising:

a zoom lens; and

an imaging device that converts an optical image formed by the zoom lens into an electrical signal,

wherein the zoom lens includes a first lens group having positive refractive power; a second lens group having negative refractive power; a third lens group having positive refractive power; a fourth lens group which has negative refractive power and is configured with only a single lens; and a fifth lens group having positive refractive power, wherein the first lens group, the second lens group, the third lens group, the fourth lens group, and the fifth lens group are arranged in order from an object side to an image side,

wherein during zooming, at least the first lens group, the second lens group, the third lens group, and the fourth lens group are movable in an optical axis direction,

wherein focusing from infinity to a close range is performed by the fourth lens group being moved in the optical axis direction, and

wherein at least one of following conditional expressions (1) and (2) is satisfied:

−11.0<Fno — w×Y /{(1−β4 — w 2 )×β5 — w 2 }<−7.0 and  (1)

−11.0<Fno — t×Y /{(1−β4 — t 2 )×β5 — t 2 }<−7.0  (2)

where

Fno_w: an open f-number at a wide-angle end,

Fno_t: an open f-number at a telephoto end,

Y: a paraxial image height at the wide-angle end represented by fw×tan ω, when fw and ω respectively represent a focal length and a half angle of view of an entire system at the wide-angle end,

β4_w: a lateral magnification of the fourth lens group at the wide-angle end,

β4_t: a lateral magnification of the fourth lens group at the telephoto end,

β5_w: a lateral magnification of the fifth lens group at the wide-angle end, and

β5_t: a lateral magnification of the fifth lens group at the telephoto end.