Zoom lens and image pickup apparatus using the same

1. A zoom lens comprising in order from an object side:

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

the second lens unit comprises in order from the object side

a positive lens element,

a negative lens element, and

a positive lens element, and

at the time of zooming from a wide angle end to a telephoto end

the third lens unit moves toward the object side, and the fourth lens unit moves, and

a distance between the first lens unit and the second lens unit decreases monotonically, and

a distance between the second lens unit and the third lens unit increases monotonically, and

a distance between the third lens unit and the fourth lens unit increases monotonically.

2. The zoom lens according to claim 1 , wherein the zoom lens satisfies the following conditional expression (2)

0.33≦Δ2 G ( w−t )/ dt≦ 0.6  (2)

where,

Δ2G(w−t) denotes an amount of movement of the second lens unit from the wide angle end to the telephoto end, and

dt denotes an overall length of the zoom lens at the telephoto end.

3. The zoom lens according to claim 2 , wherein the zoom lens satisfies the following conditional expression (3)

0<Δ3 G ( w−t )/Δ2 G ( w−t )≦0.7  (3)

where,

Δ3G(w−t) denotes an amount of movement of the third lens unit from the wide angle end to the telephoto end, and

Δ2G(w−t) denotes the amount of movement of the second lens unit from the wide angle end to the telephoto end.

4. The zoom lens according to claim 1 , wherein the zoom lens satisfies the following conditional expression (4)

0.1≦|φ 3 /φ 4 |≦1.5  (4)

where,

φ 3 denotes a refractive power of the third lens unit, and

φ 4 denotes a refractive power of the fourth lens unit.

5. The zoom lens according to claim 1 , wherein

the third lens unit consists one lens element, and

the zoom lens satisfies the following conditional expression (5)

3.0 ≦|f 3 |/fw≦ 13.5  (5)

where,

f3 denotes a focal length of the third lens unit, and

fw denotes a focal length of the overall zoom lens at the wide angle end.

6. The zoom lens according to claim 1 , wherein the zoom lens satisfies the following conditional expression (6)

Σ d 2 G/y≦ 1.5  (6)

where,

Σd2G denotes a distance between a lens surface nearest to the object side in the second lens unit and a lens surface nearest to an image side in the second lens unit, which is also a distance of the longest line segment from among line segments which are drawn in a direction parallel to an optical axis of each of a position on the optical axis and a position on a virtual line, and

y denotes the maximum image height on an image forming surface in the zoom lens.

7. The zoom lens according to claim 1 , wherein the zoom lens satisfies the following conditional expression (6)

Σ d 2 G/y≦ 1.5  (6)

where,

Σd2G denotes a distance between a lens surface nearest to the object side in the second lens unit and a lens surface nearest to an image side in the second lens unit, which is also a distance of the longest line segments from among line segments which are drawn in a direction parallel to an optical axis of each of a position on the optical axis and a position on a virtual line, and

y denotes the maximum image height on an image forming surface in the zoom lens.

8. The zoom lens according to claim 1 , wherein the zoom lens satisfies the following conditional expression (7)

8.0 ≦dt/y≦ 12  (7)

where,

dt denotes an overall length of the zoom lens at the telephoto end, and

y denotes the maximum image height on an image forming surface in the zoom lens.

9. The zoom lens according to claim 1 , wherein

the second lens unit comprises a lens nearest to the object side and a lens nearest to an image side, and

a surface on the object side of the lens nearest to the object side has a convex shape, and

a surface on the image side of the lens nearest to the image side has a convex shape.

10. The zoom lens according to claim 1 , wherein

the first lens unit consist two lens elements,

the third lens unit consist one lens element, and

the fourth lens unit consist one lens element.

11. The zoom lens according to claim 1 , wherein the zoom lens satisfies the following conditional expression (8)

fw/y≦ 1.3  (8)

where,

y denotes the maximum image height on an image forming lens in the zoom lens, and

fw denotes a focal length of the overall zoom lens at the wide angle end.

12. An image pickup apparatus comprising:

a zoom lens according to claim 1 ; and

an image pickup element having an image pickup surface which converts an optical image to an electric signal.

13. A zoom lens comprising in order from an object side:

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

the second lens unit comprises in order from the object side

a positive lens element,

a negative lens element, and

a positive lens element, and

the zoom lens satisfies the following conditional expression (1)

ft/fw≧ 5  (1)

where,

ft denotes a focal length of the overall zoom lens system at a telephoto end, and

fw denotes a focal length of the overall zoom lens system at a wide angle end.

14. The zoom lens according to claim 13 , wherein the zoom lens satisfies the following conditional expression (2)

0.33≦Δ2 G ( w−t )/ dt≦ 0.6  (2)

where,

Δ2G(w−t) denotes an amount of movement of the second lens unit from the wide angle end to the telephoto end, and

dt denotes an overall length of the zoom lens at the telephoto end.

15. The zoom lens according to claim 14 , wherein the zoom lens satisfies the following conditional expression (3)

0<Δ3 G ( w−t )/Δ2 G ( w−t )≦0.7  (3)

where,

Δ3G(w−t) denotes an amount of movement of the third lens unit from the wide angle end to the telephoto end, and

Δ2G(w−t) denotes an amount of movement of the second lens unit from the wide angle end to the telephoto end.

16. The zoom lens according to claim 13 , wherein the zoom lens satisfies the following conditional expression (4)

0.1≦|φ 3 /φ 4 |≦1.5  (4)

where,

φ 3 denotes a refractive power of the third lens unit, and

φ 4 denotes a refractive power of the fourth lens unit.

17. The zoom lens according to claim 13 , wherein

the third lens unit consist one lens, and

the zoom lens satisfies the following conditional expression (5)

3.0 ≦|f 3 |/fw≦ 13.5  (5)

where,

f3 denotes a focal length of the third lens unit, and

fw denotes a focal length of the overall zoom lens at the wide angle end.

18. The zoom lens according to claim 13 , wherein the zoom lens satisfies the following conditional expression (7)

8.0 ≦dt/y≦ 12  (7)

where,

dt denotes an overall length of the zoom lens at the telephoto end, and

y denotes the maximum image height on an image forming surface in the zoom lens.

19. The zoom lens according to claim 13 , wherein

the second lens unit comprises a lens nearest to the object side and a lens nearest to an image side, and

a surface on the object side of the lens nearest to the object side has a convex shape, and

a surface on the image side of the lens nearest to the image side has a convex shape.

20. The zoom lens according to claim 13 , wherein

the first lens unit consist two lens elements,

the third lens unit consist one lens element, and

the fourth lens unit consist one lens element.

21. The zoom lens according to claim 13 , wherein the zoom lens satisfies the following conditional expression (8)

fw/y≦ 1.3  (8)

where,

y denotes the maximum image height on an image forming lens in the zoom lens, and

fw denotes the focal length of the overall zoom lens at the wide angle end.

22. An image pickup apparatus comprising:

a zoom lens according to claim 13 ; and

an image pickup element having an image pickup surface which converts an optical image to an electric signal.