Imaging lens with positive-negative-negative-positive refractive power lenses on object side

1. An imaging lens comprising:

a first lens having positive refractive power;

a second lens having negative refractive power;

a third lens having negative refractive power;

a fourth lens having positive refractive power;

a fifth lens;

a sixth lens;

a seventh lens;

an eighth lens; and

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

wherein said ninth lens is formed in a shape so that a surface thereof on the image plane side has an aspherical shape having an inflection point,

the first lens to the ninth lens are arranged with air intervals between adjacent ones of the first lens to the ninth lens, and

all of the nine lenses that compose the imaging lens are made of plastic materials.

2. The imaging lens according to claim 1 , wherein said second lens has a focal length f2 and said third lens has a focal length f3 so that the following conditional expression is satisfied:

0.1< f 2/ f 3<1.2.

3. The imaging lens according to claim 1 , wherein said eighth lens is disposed away from the ninth lens by a distance D89 so that the following conditional expression is satisfied:

0.05< D 89/ f< 0.15,

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

4. The imaging lens according to claim 1 , wherein said ninth lens is formed in the shape so that the surface thereof on the image plane side has a paraxial curvature radius R9r so that the following conditional expression is satisfied:

0.2< R 9 r/f< 0.6,

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

5. The imaging lens according to claim 1 , wherein said ninth lens has a focal length f9 so that the following conditional expression is satisfied:

−2< f 9/ f<− 0.2,

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