Imaging lens

1. An imaging lens comprising:

a first lens having negative refractive power;

a second lens having negative refractive power;

a third lens having positive refractive power;

a fourth lens having negative refractive power; and

a fifth lens having positive refractive power, arranged in this order from an object side to an image plane side,

wherein said first lens is formed in a shape so that a curvature radius of a surface thereof on the image plane side is positive,

said second lens is formed in a shape so that a surface thereof on an object side has a sharp convex surface facing the object side from an optical axis to an periphery, and a curvature radius of a surface thereof on the image plane side is positive,

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

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

said fifth lens is formed in a shape so that a curvature radius of a surface thereof on the object side is positive and a curvature radius of a surface thereof on the image plane side is negative, and the surface thereof on the image plane side is formed in an aspheric surface having an inflection point.

2. The imaging lens according to claim 1 , wherein said second lens is formed to have refractive power weaker than that of the first lens, the third lens, the fourth lens, and the fifth lens.

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

−40 <f 2 /f<− 5.

4. The imaging lens according to claim 1 , wherein said first lens has the surface on the image plane side having the curvature radius R 2 and the second lens has the surface on the object side having the curvature radius R 3 so that the following expression is satisfied:

0.01< R 2 /R 3<0.4.

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

0.02 <f 1 /f 2<0.8.

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

0.5 <f 3 /f< 1.2.

7. The imaging lens according to claim 1 , wherein said second lens, said third lens, and said fourth lens are arranged so that a distance df on an optical axis from the surface of the second lens on the image plane side to the surface of the third lens on the object side and a distance dr on the optical axis from the surface of the third lens on the image plane side to the surface of the fourth lens on the object side satisfy the following expression:

0.8 <df/dr< 2.5.

8. The imaging lens according to claim 1 , wherein said fourth lens and said fifth lens have a composite focal length f 45 so that the following expression is satisfied when a whole lens system has a focal length f:

5 <f 45 /f< 15.

9. The imaging lens according to claim 1 , wherein said fourth lens has an Abbe's number νd 4 and said fifth lens has an Abbe's number νd 5 so that the following expressions are satisfied:

ν d 4<35;

45 <νd 5<80.

10. The imaging lens according to claim 9 , wherein said first lens has an Abbe's number νd 1 , said second lens has an Abbe's number νd 2 , and said third lens has an Abbe's number νd 3 so that the following expressions are satisfied:

45 <νd 1<80;

45 <νd 2<80;

45 <νd 3<80.