Imaging lens

1. An imaging lens comprising:

a first lens having positive refractive power;

a second lens having positive refractive power;

a third lens;

a fourth lens;

a fifth lens; and

a sixth lens having negative refractive power, arranged in this order from an object side to an image plane side with spaces in between each of the lenses,

wherein said imaging lens has a total of six lenses,

said sixth lens is formed in a shape so that a curvature radius of a surface thereof on the object side and a curvature radius of a surface thereof on the image plane side are negative, and

said sixth lens has the surface on the image plane side having the curvature radius R 6 r , said second lens has Abbe's number νd 2 , said fourth lens has Abbe's number νd 4 , and said sixth lens has Abbe's number νd 6 so that the following conditional expressions are satisfied:

−10< R 6 r/f<− 1,

35<ν d 2<75,

15<ν d 4<35,

35<ν d 6<75,

where f is a focal length of a whole lens system.

2. The imaging lens according to claim 1 , wherein said fourth lens is formed in a shape so that a surface thereof on the object side has a curvature radius R 4 f and a surface thereof on the image plane side has a curvature radius R 4 r so that the following conditional expression is satisfied:

0.5<| R 4 f/R 4 r|< 2.0.

3. The imaging lens according to claim 1 , wherein said second lens has a focal length f 2 and said third lens has a focal length f 3 so that the following conditional expression is satisfied:

0.2<| f 3/ f 2|<1.2.

4. The imaging lens according to claim 1 , wherein said third lens has a focal length f 3 so that the following conditional expression is satisfied:

−2.0< f 3/ f<− 0.5.

5. The imaging lens according to claim 1 , wherein said third lens has a focal length f 3 and said sixth lens has a focal length f 6 so that the following conditional expression is satisfied:

0.5< f 3/ f 6<1.5.

6. The imaging lens according to claim 1 , wherein said sixth lens has a focal length f 6 so that the following conditional expression is satisfied:

−2.0< f 6/ f<− 0.5.

7. The imaging lens according to claim 1 , wherein said third lens is arranged to be away from the fourth lens by a distance D 34 on an optical axis thereof so that the following conditional expression is satisfied:

0.05< D 34/ f< 0.2.

8. The imaging lens according to claim 1 , wherein said fourth lens is arranged to be away from the fifth lens by a distance D 45 on an optical axis thereof so that the following conditional expression is satisfied:

0.02< D 45/ f< 0.2.

9. The imaging lens according to claim 1 , wherein said fifth lens is arranged to be away from the sixth lens by a distance D 56 on an optical axis thereof so that the following conditional expression is satisfied:

0.05< D 56/ f< 0.2.

10. The imaging lens according to claim 1 , wherein said fifth lens has a thickness T 5 along an optical axis thereof and said sixth lens has a thickness T 6 along an optical axis thereof so that the following conditional expression is satisfied:

0.5< T 5/ T 6<3.0.

11. The imaging lens according to claim 1 , wherein said fifth lens and said sixth lens have a composite focal length f 56 so that the following conditional expression is satisfied:

−10< f 56/ f<− 0.5, and

there are no intervening lenses between the fifth lens and the sixth lens.

12. The imaging lens according to claim 1 , wherein said first lens has Abbe's number νd 1 and said third lens has Abbe's number νd 3 , so that the following conditional expressions are satisfied:

35<ν d 1<75,

15<ν d 3<35.

13. The imaging lens according to claim 1 , wherein said fifth lens has Abbe's number νd 5 so that the following conditional expression is satisfied:

35<ν d 5<75,

14. The imaging lens according to claim 1 , wherein said sixth lens has the surface on the image plane side formed in an aspheric shape so that a curvature thereof increases monotonously away from an optical axis thereof toward a periphery thereof in a direction perpendicular to the optical axis.