Imaging lens and camera system including the same lens

1. An imaging lens comprising, in order from an object side:

a first lens having a negative refractive power with a concave surface directed to an image side thereof;

a second lens having a positive refractive power with a convex surface directed to an image side thereof and having an absolute value of a radius of curvature on the image side is equal to or smaller than that on an object side thereof;

a stop;

a third lens which is a planoconvex lens having a plane on an object side thereof or a double-convex lens having on an object side thereof a surface of which an absolute value of a radius of curvature is larger than that on an image side thereof; and

a compound lens of a fourth lens and a fifth lens, the compound lens having a positive composite power,

wherein the second lens is a double-convex lens, and

the number of lenses of the imaging lense is five.

2. The imaging lens as set forth in claim 1 ,

wherein the fourth lens is a double-convex lens.

3. The imaging lens as set forth in claim 2 ,

wherein letting a focal length of the imaging lens be F, a focal length of the third lens be F 3 , and an airspace between the first lens and the second lens be D 2 , the following conditional expressions are satisfied:

1< F 3 /F< 4  (1)

0.1< D 2 /F< 2  (2)

4. The imaging lens as set forth in claim 3 ,

wherein letting a distance from the image side surface of the second lens to the stop be D 4 , a distance from the stop to the object side surface of the third lens be D 5 , and refractive indexes relative to a d-line of the fourth lens and the fifth lens be N 4 , N 5 , respectively, the following conditional expressions are satisfied:

1< D 4 /D 5  (3)

0.05< |N 4− N 5|<0.40  (4)

5. The imaging lens as set forth in claim 4 ,

wherein a linear expansion coefficient of a material for the first lens to the fifth lens is 12×10 −6 [1/K] or less.

6. The camera system comprising:

an imaging lens as set forth in claim 5 ; and

a solid state imaging device that converts an optical image picked up by the imaging lens into an electric signal.

7. An imaging lens comprising, in order from an object side:

a first lens having a negative refractive power with a concave surface directed to an image side thereof;

a second lens having a positive refractive power with a convex surface directed to an image side thereof and having an absolute value of a radius of curvature on the image side is equal to or smaller than that on an object side thereof;

stop;

a third lens which is a planoconvex lens having a plane on an object side thereof or a double-convex lens having on an object side thereof a surface of which an absolute value of a radius of curvature is larger than that on an image side thereof; and

a compound lens of a fourth lens and a fifth lens, the compound lens having a positive composite power,

wherein letting a distance from the image side surface of the second lens to the stop be D 4 , a distance from the stop to the object side surface of the third lens be D 5 , and refractive indexes relative to a d-line of the fourth lens and the fifth lens be N 4 , N 5 , respectively, the following conditional expressions are satisfied:

1< D 4 /D 5  (3)

0.05< |N 4 −N 5|<0.40   (4).

8. The imaging lens as set forth in claim 7 ,

wherein the second lens is a double-convex lens.

9. The imaging lens as set forth in claim 7 ,

wherein the fourth lens is a double-convex lens.

10. The imaging lens as set forth in claim 7 ,

wherein a linear expansion coefficient of a material for the first lens to the fifth lens is 12×10 −6 [1/K] or less.

11. The camera system comprising:

an imaging lens as set forth in claim 7 ; and

a solid state imaging device that converts an optical image picked up by the imaging lens into an electric signal.