Imaging lens
An imaging lens which can be very compact and thin, corrects various aberrations properly and provides a small F-value and a wide view angle at low cost. In the imaging lens, designed for a solid-state image sensor, arranged in the following order from an object side to an image side are: a first positive (refractive power) lens with a convex object-side surface; a second positive lens; a third positive lens; a fourth positive lens; and a fifth negative lens with a concave image-side surface. None of these lenses is joined to each other and all the lens surfaces are aspheric. The object-side and image-side aspheric surfaces of the fifth lens have a pole-change point in a position other than a point of intersection with an optical axis. A diffractive optical surface is formed on one of three surfaces from the first lens image-side surface to the second lens image-side 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:
a first lens with positive refractive power having a convex surface on the object side;
a second lens with positive refractive power;
a third lens with positive refractive power;
a fourth lens with positive refractive power; and
a fifth lens with negative refractive power having a concave surface on the image side,
wherein none of the constituent lenses is joined to each other;
wherein all the lens surfaces are aspheric;
wherein an object-side aspheric surface and an image-side aspheric surface of the fifth lens have a pole-change point in a position other than a point of intersection with an optical axis; and
wherein a diffractive optical surface is formed on one of three surfaces from the image-side surface of the first lens to the image-side surface of the second lens.
2. The imaging lens according to claim 1 , wherein the first lens is a biconvex lens.
3. The imaging lens according to claim 2 , wherein the first lens has an aspheric shape in which a more distant portion from the optical axis has weaker positive refractive power.
4. The imaging lens according to claim 2 , wherein the second lens is a meniscus lens having a convex surface on the object side.
5. The imaging lens according to claim 1 , wherein the first lens has an aspheric shape in which a more distant portion from the optical axis has weaker positive refractive power.
6. The imaging lens according to claim 1 , wherein the second lens is a meniscus lens having a convex surface on the object side.
7. The imaging lens according to claim 1 , wherein in the third lens a more distant portion from the optical axis has weaker positive refractive power and the refractive power changes to negative refractive power in a peripheral portion.
8. The imaging lens according to claim 1 , wherein the fourth lens has a meniscus shape with a concave surface on the object side near the optical axis and in the fourth lens, a more distant portion from the optical axis has weaker positive refractive power and the refractive power changes to negative refractive power in a peripheral portion.
9. The imaging lens according to claim 1 , wherein conditional expressions (1) to (3) below are satisfied:
1.0< f 1/ f (1)
1.0< f 2/ f (2)
0.8< f 4/ f (3)
where
f: focal length of the overall optical system of the imaging lens
f 1 : focal length of the first lens
f 2 : focal length of the second lens
f 4 : focal length of the fourth lens.
10. The imaging lens according to claim 1 , where a conditional expression (4) below is satisfied:
f 123/ f< 1.5 (4)
where
f: focal length of the overall optical system of the imaging lens
f 123 : composite focal length of the first lens, the second lens, and the third lens.
11. The imaging lens according to claim 1 , where a conditional expression (5) below is satisfied:
3.4< f 45/ f< 9.1 (5)
where
f: focal length of the overall optical system of the imaging lens
f 45 : composite focal length of the fourth lens and the fifth lens.
12. The imaging lens according to claim 1 , wherein all the constituent lenses are made of plastic material and satisfy conditional expressions (6) and (7) below:
1.50 <Nd< 1.65 (6)
20 <νd< 70 (7)
where
Nd: refraction index of each lens at d-ray
νd: Abbe number of each lens at d-ray.