Zoom lens system, imaging device and camera

1. A zoom lens system having a plurality of lens units defining object and image sides of the lens system, each lens unit being composed of at least one lens element, the zoom lens system, in order from the object side to the image side, comprising:

a first lens unit having negative optical power and consisting of, in order from the object side to the image side, a lens element having negative optical power and a lens element having positive optical power; and

a second lens unit, arranged at an interval from the first lens unit, having positive optical power; wherein

zooming the lens system from its wide-angle limit to its telephoto limit to vary the lens system's magnification moves the lens units individually along the lens system's optical axis such that the interval between the first lens unit and the second lens unit decreases, and

the following conditions (1), (2), and (a) are satisfied:

f W /D 1 >7.5  (1)

f T /D 1 ≧44.3  (2)

Z=f T /f W >4.0  (a)

where,

f W is a focal length of the entire system at the wide-angle limit,

f T is a focal length of the entire system at the telephoto limit, and

D 1 is a center thickness of the negative optical power lens element of the first lens unit.

2. The zoom lens system as claimed in claim 1 , further comprising:

a third lens unit having positive optical power, arranged on the image side of the second lens unit at an interval therefrom; wherein

zooming the lens system from its wide-angle limit to its telephoto limit to vary the lens system's magnification moves the lens units individually along the optical axis such that the interval between the second lens unit and the third lens unit increases.

3. The zoom lens system as claimed in claim 2 , wherein the following condition (4) is satisfied:

0.60<( D G1 +D G3 )/ Ir< 1.40  (4)

where,

D G1 is a center thickness of the first lens unit,

D G3 is a center thickness of the third lens unit,

Ir is a value represented by the following equation:

Ir=f T ×tan(ω T ),

f T is the focal length of the entire system at the telephoto limit, and

ω T is a half value (°) of a maximum view angle at a telephoto limit.

4. The zoom lens system as claimed in claim 2 , wherein

in the zoom lens system being zoomed from the system's minimum-focal-length state at its wide-angle limit to the system's maximum-focal-length state at its telephoto limit, the optical-axial position of the third lens unit in an intermediate-focal-length state of the lens system is object-side-ward relative to the optical-axial position of the third lens unit in the minimum-focal-length state, and object-side-ward relative to the optical-axial position of the third lens unit in the maximum-focal-length state.

5. The zoom lens system as claimed in claim 2 , wherein the third lens unit is composed of one lens element.

6. The zoom lens system as claimed in claim 5 , wherein the following condition (8) is satisfied:

0.5<((2× D 7 ×R 7F ×R 7R )/( R 7R −R 7F )) 0.5 /Ir< 1.5  (8)

where,

D 7 is a center thickness of the lens element in the third lens unit,

R 7F is a radius of curvature of an object side surface of the lens element in the third lens unit,

R 7R is a radius of curvature of an image side surface of the lens element in the third lens unit,

Ir is a value represented by the following equation:

Ir=f T ×tan(ω T ),

f T is the focal length of the entire system at the telephoto limit, and

ω T is a half value (°) of a maximum view angle at a telephoto limit.

7. The zoom lens system as claimed in claim 5 , wherein the lens element in the third lens unit includes an aspheric surface.

8. The zoom lens system as claimed in claim 5 , wherein the lens element in the third lens unit is made of a resin material.

9. The zoom lens system as claimed in claim 1 , wherein the following condition (3) is satisfied:

0.50< D G1 /Ir< 1.06  (3)

where,

D G1 is a center thickness of the first lens unit,

Ir is a value represented by the following equation:

Ir=f T ×tan(ω T ),

f T is the focal length of the entire system at the telephoto limit, and

ω T is a half value (°) of a maximum view angle at a telephoto limit.

10. The zoom lens system as claimed in claim 1 , wherein the following condition (5) is satisfied:

0.01< D 1 /D 12 <0.40  (5)

where,

D 1 is the center thickness of the negative optical power lens element of the first lens unit, and

D 12 is an air space between the negative optical power lens element and the positive optical power lens element of the first lens unit.

11. The zoom lens system as claimed in claim 1 , wherein the following condition (6) is satisfied:

1.2<((2× D 2 ×R 2F ×R 2R )/( R 2R −R 2F )) 0.5 /Ir< 2.0  (6)

where,

D 2 is a center thickness of the positive optical power lens element of the first lens unit,

R 2F is a radius of curvature of an object side surface of the positive optical power lens element of the first lens unit,

R 2R is a radius of curvature of an image side surface of the positive optical power lens element of the first lens unit,

Ir is a value represented by the following equation:

Ir=f T ×tan(ω T ),

f T is the focal length of the entire system at the telephoto limit, and

ω T is a half value (°) of a maximum view angle at a telephoto limit.

12. The zoom lens system as claimed in claim 1 , wherein the second lens element is a meniscus lens element with a convex surface facing the object side.

13. The zoom lens system as claimed in claim 1 , wherein the first lens unit includes at least one lens element having an aspheric surface.

14. The zoom lens system as claimed in claim 1 , wherein the first lens unit includes at least two aspheric surfaces.

15. The zoom lens system as claimed in claim 1 , wherein

the second lens unit includes at least one cemented lens element,

a cement surface of the cemented lens element is a convex surface facing the object side, and

the following condition (7) is satisfied:

1.03< Nd 2 /Nd 1 <2.00  (7)

where,

Nd 1 is a refractive index to the d-line of a lens element located on the most object side in the cemented lens element, and

Nd 2 is a refractive index to the d-line of a lens element which is cemented with the lens element located on the most object side in the cemented lens element.

16. The zoom lens system as claimed in claim 1 , wherein the second lens unit moves in a direction perpendicular to the optical axis.

17. The zoom lens system as claimed in claim 16 , wherein the entire system satisfies the following conditions (9) and (10):

Y T >Y   (9)

1.5<( Y/Y T )/( f/f T )<3.0  (10)

where,

f is a focal length of the entire system,

f T is the focal length of the entire system at the telephoto limit,

Y is an amount of movement in the direction perpendicular to the optical axis for maximum blur compensation by the second lens unit, with respect to the focal length f of the entire system, and

Y T is an amount of movement in the direction perpendicular to the optical axis for maximum blur compensation by the second lens unit, with respect to the focal length f T of the entire system at a telephoto limit.

18. An imaging device capable of outputting an optical image of an object as an electric image signal, comprising:

a zoom lens system that forms an optical image of the object; and

an image sensor that converts the optical image formed by the zoom lens system into the electric image signal, wherein

the zoom lens system is a zoom lens system as claimed in claim 1 .

19. A camera for converting an optical image of an object into an electric image signal and then performing at least one of displaying and storing of the converted image signal, comprising:

an imaging device including a zoom lens system that forms an optical image of the object, and an image sensor that converts the optical image formed by the zoom lens system into the electric image signal, wherein

the zoom lens system is a zoom lens system as claimed in claim 1 .