Imaging lens and imaging apparatus

1. An imaging lens comprising: in order from an object side of the imaging lens,

a first lens group having a positive power as a whole;

a second lens group, wherein an object-side surface of a lens closest to the object side is concave toward the object side;

a third lens group of a single lens having a positive power; and

a fourth lens group having a negative power as a whole,

wherein at least five lenses form said first, second, third and fourth lens groups,

said imaging lens satisfying conditional expression:

3.5 mm<D 2 <6.0 mm  (8)

wherein D 2 represents a distance on an optical axis of the imaging lens from a surface closest to the object side to a surface closest to the image side in the second lens group.

2. The imaging lens according to claim 1 , satisfying conditional expressions (1) and (2):

0.8< L/f< 1.7  (1)

0.2< Bf/f< 0.4  (2)

wherein L represents a distance on an optical axis of the imaging lens from a surface closest to the object side in the first lens group to an image plane, Bf represents a distance on the optical axis from a surface closest to an image side in a lens group that is closest to the image side of the imaging lens, to the image plane, and f represents a focal length of the imaging lens.

3. The imaging lens according to claim 1 , wherein an image-side surface of a lens closest to the image side in the second lens group is convex toward the image side.

4. The imaging lens according to claim 1 , wherein the second lens group is a cemented lens including, in order from the object side, a lens having a concave surface on the object side and a lens having a convex surface on the image side.

5. The imaging lens according to claim 1 , wherein

the first lens group is a single lens having a convex surface on the object side, and

the fourth lens group is a cemented lens including, in order from the object side, a lens having a concave surface on the object side and a lens having a convex surface on the image side.

6. The imaging lens according to claim 1 , wherein

the first lens group is a single lens having a convex surface on the object side, and

the fourth lens group is a single meniscus lens having a concave surface on the object side.

7. The imaging lens according to claim 1 , satisfying conditional expression:

−1.0< f/f 2 <1.0  (3)

wherein f 2 represents a focal length of the second lens group, and f represents a focal length of the imaging lens.

8. The imaging lens according to claim 1 , satisfying conditional expression:

0.3< R 2A /R 2B <1.5  (4)

wherein R 2A represents a radius of curvature of a surface closest to the object side in the second lens group, and R 2B represents a radius of curvature of a surface closest to the image side in the second lens group.

9. The imaging lens according to claim 1 , wherein the second lens group includes a lens having a negative power, and an Abbe number of at least one negative lens included in the second lens group is 40 or less at the d-line.

10. The imaging lens according to claim 1 , wherein a stop is disposed closer to the object side than a lens closest to the object side in the second lens group.

11. The imaging lens according to claim 1 , satisfying conditional expression:

ENP/L< 0.5  (5)

wherein ENP represents a distance from a vertex of a surface closest to the object side in the first lens group to an entrance pupil, and L represents a distance on an optical axis of the imaging lens from a surface closest to the object side in the first lens group to an image plane.

12. The imaging lens according to claim 1 , wherein the fourth group includes a cemented lens and satisfies conditional expression:

ν F4 /ν R4 >1.0  (6)

wherein ν F4 represents an Abbe number of a lens closest to the object side in the cemented lens at the d-line, and ν R4 represents an Abbe number of a lens closest to the image side in the cemented lens at the d-line.

13. The imaging lens according to claim 1 , satisfying conditional expression:

−3.0< f 4 /f<− 0.5  (7)

wherein f 4 represents a focal length of the fourth lens group, and f represents a focal length of the imaging lens.

14. The imaging lens according to claim 1 , having an effective diameter of 15 mm or less.

15. The imaging lens according to claim 1 , satisfying conditional expression (1):

0.8 <L/f< 1.7  (1)

wherein L represents a distance on an optical axis of the imaging lens from a surface closest to the object side in the first lens group to an image plane, and f represents a focal length of the imaging lens.

16. The imaging lens according to claim 1 , satisfying conditional expression (2):

0.2 <Bf/f< 0.4  (2)

wherein Bf represents a distance on the optical axis from a surface closest to an image side in a lens group that is closest to the image side of the imaging lens, to the image plane, and f represents a focal length of the imaging lens.

17. An imaging apparatus comprising:

an imaging lens according to claim 1 ; and

an imaging device that converts an optical image formed by the imaging lens into an electric signal.

18. An imaging lens comprising: in order from an object side of the imaging lens,

a first lens group having a positive power as a whole;

a second lens group, wherein an object-side surface of a lens closest to the object side is concave toward the object side;

a third lens group of a single lens having a positive power; and

a fourth lens group having a negative power as a whole,

wherein at least five lenses form said first, second, third and fourth lens groups,

said imaging lens satisfying conditional expression:

1.39 ≦f 3 /f< 3.5  (11′)

wherein f 3 is a focal length of the third lens group and f is the focal length of the imaging lens.

19. The imaging lens according to claim 18 , satisfying conditional expression:

−1.0 <f/f 2 <1.0  (3)

wherein f 2 represents a focal length of the second lens group, and f represents a focal length of the imaging lens.

20. The imaging lens according to claim 18 , satisfying conditional expression:

0.3 <R 2A /R 2B <1.5  (4)

wherein R 2A represents a radius of curvature of a surface closest to the object side in the second lens group, and R 2B represents a radius of curvature of a surface closest to the image side in the second lens group.

21. The imaging lens according to claim 18 , wherein the fourth group includes a cemented lens and satisfies conditional expression:

ν F4 /ν R4 >1.0  (6)

wherein ν F4 represents an Abbe number of a lens closest to the object side in the cemented lens at the d-line, and ν R4 represents an Abbe number of a lens closest to the image side in the cemented lens at the d-line.

22. An imaging lens comprising: in order from an object side of the imaging lens,

a first lens group having a positive power as a whole;

a second lens group, wherein an object-side surface of a lens closest to the object side is concave toward the object side;

a third lens group of a single lens having a positive power; and

a fourth lens group having a negative power as a whole,

wherein at least five lenses form said first, second, third and fourth lens groups,

said imaging lens satisfying conditional expression:

0.8 <L/f< 1.7  (1)

wherein L represents a distance on an optical axis of the imaging lens from a surface closest to the object side in the first lens group to an image plane, and f represents a focal length of the imaging lens.

23. The imaging lens according to claim 22 , satisfying conditional expression:

−1.0 <f/f 2 <1.0  (3)

wherein f 2 represents a focal length of the second lens group, and f represents a focal length of the imaging lens.

24. The imaging lens according to claim 22 , satisfying conditional expression:

0.3 <R 2A /R 2B <1.5  (4)

wherein R 2A represents a radius of curvature of a surface closest to the object side in the second lens group, and R 2B represents a radius of curvature of a surface closest to the image side in the second lens group.

25. The imaging lens according to claim 22 , wherein the fourth group includes a cemented lens and satisfies conditional expression:

ν F4 /ν R4 >1.0  (6)

wherein ν F4 represents an Abbe number of a lens closest to the object side in the cemented lens at the d-line, and ν R4 represents an Abbe number of a lens closest to the image side in the cemented lens at the d-line.

26. An imaging lens comprising: in order from an object side of the imaging lens,

a first lens group having a positive power as a whole;

a second lens group, wherein an object-side surface of a lens closest to the object side is concave toward the object side;

a third lens group of a single lens having a positive power; and

a fourth lens group having a negative power as a whole,

wherein at least six lenses form said first, second, third and fourth lens groups,

said imaging lens satisfying conditional expression:

−3.0 <f 4 /f<− 0.81  (7′)

wherein f 4 represents a focal length of the fourth lens group, and f represents a focal length of the imaging lens.

27. The imaging lens according to claim 26 , satisfying conditional expression:

−1.0 <f/f 2 <1.0  (3)

wherein f 2 represents a focal length of the second lens group, and f represents a focal length of the imaging lens.

28. The imaging lens according to claim 26 , satisfying conditional expression:

0.3 <R 2A /R 2B <1.5  (4)

wherein R 2A represents a radius of curvature of a surface closest to the object side in the second lens group, and R 2B represents a radius of curvature of a surface closest to the image side in the second lens group.

29. The imaging lens according to claim 26 , wherein the fourth group includes a cemented lens and satisfies conditional expression:

ν F4 /ν R4 >1.0  (6)

wherein ν F4 represents an Abbe number of a lens closest to the object side in the cemented lens at the d-line, and ν R4 represents an Abbe number of a lens closest to the image side in the cemented lens at the d-line.