Imaging lens comprising a diffractive optical surface

1. An imaging lens for a solid-state image sensor in which lenses are arranged in order from an object side to an image side, comprising:

an aperture stop;

a first lens as a meniscus lens with positive refractive power having a convex surface on the object side near an optical axis;

a second lens as a meniscus lens with positive refractive power having a convex surface on the image side near the optical axis;

a third lens as a meniscus lens with positive refractive power having a convex surface on the image side near the optical axis; and

a fourth lens as a meniscus lens with negative refractive power having a concave surface on the image side near the optical axis,

wherein all lens surfaces are aspheric;

wherein all the lenses are made of plastic material;

wherein a diffractive optical surface is formed on at least one of lens surfaces from an image-side surface of the first lens to an image-side surface of the second lens;

wherein at least one of the three lenses with positive refractive power satisfies a conditional expression (1) below:

1.58< Ndi   (1)

where

Ndi: refractive index of the at least one of the three positive lenses at d-ray;

wherein a conditional expression (2) below is satisfied:

0.7<TTL/(2IH)<0.9  (2)

where

TTL: distance on the optical axis from the object-side surface of the first lens to an image plane (a total track length), and

IH: maximum image height;

wherein a conditional expression (3) below is satisfied:

0.7< f 1/ f< 1.1  (3)

where

f1: focal length of the first lens, and

f: focal length of an overall optical system and

wherein an F-value of the imaging lens falls in the range from 2.31 to 2.41.

2. The imaging lens according to claim 1 , wherein a conditional expression (4) below is satisfied:

0.40< r 1/ f< 1.0  (4)

where

r1: curvature radius of the object-side surface of the first lens, and

f: focal length of the overall optical system.

3. The imaging lens according to claim 1 , wherein a conditional expression (5) below is satisfied:

0.6< f 123/ f< 0.9  (5)

where

f123: composite focal length of the first, second, and third lenses, and

f: focal length of an overall optical system.

4. The imaging lens according to claim 1 , wherein the object-side surface and image-side surface of the fourth lens have a pole-change point off the optical axis.

5. The imaging lens according to claim 1 , wherein a conditional expression (6) below is satisfied:

0.6<Σ d /TTL<0.8  (6)

where

Σd: distance on the optical axis from the object-side surface of the first lens to the image-side surface of the fourth lens, and

TTL: distance on the optical axis from the object-side surface of the first lens to the image plane (a total track length).

6. The imaging lens according to claim 1 , wherein a conditional expression (7) below is satisfied:

0.02< T 12/Σ d< 0.20  (7)

where

T12: distance on the optical axis from the image-side surface of the first lens to the object-side surface of the second lens, and

Σd: distance on the optical axis from the object-side surface of the first lens to the image-side surface of the fourth lens.

7. The imaging lens according to claim 1 , wherein the first lens, the second lens, the third lens, and the fourth lens satisfy a conditional expression (8) below:

0.6< Tie/Ti< 1.3  (8)

where

Ti: center thickness of an i-th lens, and

Tie: edge thickness of the i-th lens.

8. The imaging lens according to claim 1 , wherein a conditional expression (9) below is satisfied:

1≦ Dn≦ 20

where

Dn: a number of a diffraction grating of the diffractive optical surface (integer).