Zoom lens system, imaging device and camera

1. A zoom lens system for forming an optical image of an object with a variable magnification, in order from the object side to the image side, comprising:

a first lens unit having negative optical power; a second lens unit having positive optical power; and a third lens unit having positive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens units move respectively along an optical axis in such a manner that an interval between the first lens unit and the second lens unit decreases while an interval between the second lens unit and the third lens unit changes so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens element with the convex surface facing the object side; and one image side positive lens element with the convex surface facing the object side that is arranged with an air space on the image side of the object side negative lens element,

each of the two lens elements constituting the first lens unit has an aspheric surface, and

the following conditions (1), (2) and (3) are satisfied:

n12>1.88  (1)

v12<26  (2)

5.0<αi W <20.0  (3)

(where, ω W ≧33.97)

in which,

n12 is a refractive index of the image side positive lens element of the first lens unit,

v12 is an Abbe number of the image side positive lens element of the first lens unit,

αi W is an incident angle of the principal ray onto the image sensor at the maximum image height at a wide-angle limit (here, the incident angle is defined as positive when the principal ray is incident on the light receiving surface of the image sensor with departing from the optical axis), and

ω W is a half view angle at a wide-angle limit.

2. The zoom lens system as claimed in claim 1 , satisfying the following conditions (4), (5), (6) and (7):

n11>1.50  (4)

v11>35  (5)

n 12 −n 11>0.10  (6)

v11−v12>15.0  (7)

wherein,

n11 is a refractive index of the object side negative lens element of the first lens unit,

v11 is an Abbe number of the object side negative lens element of the first lens unit,

n12 is the refractive index of the image side positive lens element of the first lens unit, and

v12 is the Abbe number of the image side positive lens element of the first lens unit.

3. The zoom lens system as claimed in claim 1 , satisfying the following condition (8):

T 1 /Y <1.5  (8)

wherein,

T1 is a center thickness of the first lens unit, and

Y is the maximum image height.

4. The zoom lens system as claimed in claim 1 , satisfying the following condition (9):

( T 1 +T 2 +T 3)/ Y <3.5  (9)

wherein,

T1 is a center thickness of the first lens unit,

T2 is a center thickness of the second lens unit,

T3 is a center thickness of the third lens unit, and

Y is the maximum image height.

5. The zoom lens system as claimed in claim 1 , wherein the third lens unit is composed of one lens element.

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

an object side surface of the most object side lens element that constitutes the second lens unit is aspheric, and wherein

the second lens unit, in order from the object side to the image side, comprises one set of positive cemented lens element and one positive lens element, or alternatively comprises one positive lens element, one set of cemented lens element and one positive lens element, or alternatively comprises two sets of positive cemented lens elements.

7. An imaging device capable of converting an optical image of a photographic object into an electric image signal and then outputting the signal, comprising:

a zoom lens system that forms the optical image of the photographic object with a variable magnification; and

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

the zoom lens system, in order from the object side serving as the photographic object side to the image side, comprises: a first lens unit having negative optical power; a second lens unit having positive optical power; and a third lens unit having positive optical power,

in zooming from a wide-angle limit to a telephoto limit, the lens units move respectively along an optical axis in such a manner that an interval between the first lens unit and the second lens unit decreases while an interval between the second lens unit and the third lens unit changes so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens element with the convex surface facing the object side; and one image side positive lens element with the convex surface facing the object side that is arranged with an air space on the image side of the object side negative lens element,

each of the two lens elements constituting the first lens unit has an aspheric surface, and

the following conditions (1), (2) and (3) are satisfied:

n12>1.88  (1)

v12<26  (2)

5.0<αi W <20.0  (3)

(where, ω W ≧33.97)

in which,

n12 is a refractive index of the image side positive lens element of the first lens unit,

v12 is an Abbe number of the image side positive lens element of the first lens unit,

αi W is an incident angle of the principal ray onto the image sensor at the maximum image height at a wide-angle limit (here, the incident angle is defined as positive when the principal ray is incident on the light receiving surface of the image sensor with departing from the optical axis), and

ω W is a half view angle at a wide-angle limit.

8. The imaging device as claimed in claim 7 , wherein the zoom lens system satisfies the following conditions (4), (5), (6) and (7):

n11>1.50  (4)

v11>35  (5)

n 12 −n 11>0.10  (6)

v 11 −v 12>15.0  (7)

wherein,

n11 is a refractive index of the object side negative lens element of the first lens unit,

v11 is an Abbe number of the object side negative lens element of the first lens unit,

n12 is the refractive index of the image side positive lens element of the first lens unit, and

v12 is the Abbe number of the image side positive lens element of the first lens unit.

9. The imaging device as claimed in claim 7 , wherein the zoom lens system satisfies the following condition (8):

T 1/ Y <1.5  (8)

wherein,

T1 is a center thickness of the first lens unit, and

Y is the maximum image height.

10. The imaging device as claimed in claim 7 , wherein the zoom lens system satisfies the following condition (9):

( T 1+ T 2+ T 3)/ Y< 3.5  (9)

wherein,

T1 is a center thickness of the first lens unit,

T2 is a center thickness of the second lens unit,

T3 is a center thickness of the third lens unit, and

Y is the maximum image height.

11. The imaging device as claimed in claim 7 , wherein the third lens unit of the zoom lens system is composed of one lens element.

12. The imaging device as claimed in claim 7 , wherein

an object side surface of the most object side lens element that constitutes the second lens unit of the zoom lens system is aspheric, and wherein

the second lens unit, in order from the object side to the image side, comprises one set of positive cemented lens element and one positive lens element, or alternatively comprises one positive lens element, one set of cemented lens element and one positive lens element, or alternatively comprises two sets of positive cemented lens elements.

13. A camera capable of shooting a photographic object and then outputting its image as an electric image signal, comprising:

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

the zoom lens system, in order from the object side serving as the photographic object side to the image side, comprises: a first lens unit having negative optical power; a second lens unit having positive optical power; and a third lens unit having positive optical power,

in zooming from a wide-angle limit to a telephoto limit, the lens units move respectively along an optical axis in such a manner that an interval between the first lens unit and the second lens unit decreases while an interval between the second lens unit and the third lens unit changes so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens element with the convex surface facing the object side; and one image side positive lens element with the convex surface facing the object side that is arranged with an air space on the image side of the object side negative lens element,

each of the two lens elements constituting the first lens unit has an aspheric surface, and

the following conditions (1), (2) and (3) are satisfied:

n12>1.88  (1)

v12<26  (2)

5.0<αi W <20.0  (3)

(where, ω W ≧33.97)

in which,

n12 is a refractive index of the image side positive lens element of the first lens unit,

v12 is an Abbe number of the image side positive lens element of the first lens unit,

αi W is an incident angle of the principal ray onto the image sensor at the maximum image height at a wide-angle limit (here, the incident angle is defined as positive when the principal ray is incident on the light receiving surface of the image sensor with departing from the optical axis), and

ω W is a half view angle at a wide-angle limit.

14. The camera as claimed in claim 13 , wherein the camera is a digital still camera capable of acquiring a still image of the photographic object.

15. The camera as claimed in claim 13 , wherein the camera is a digital video camera capable of acquiring a moving image of the photographic object.