Imaging optical system, camera device and mobile information terminal device

1. An imaging optical system, comprising: in the order from an object side to an image plane side,

a first lens group having negative refracting power that includes a negative lens having a concave surface on the image plane side;

a second lens group having positive refracting power that includes, in the order from the object side to the image plane side, a negative lens having a concave surface on the object side, and a cemented lens of a negative lens and a positive lens, said negative lens of the cemented lens being on the object side and said positive lens of the cemented lens being on the image plane side and having a convex surface on the object side;

an aperture stop;

a third lens group having positive refracting power that consists of a cemented lens of a positive lens and a negative lens; and

a fourth lens group having negative refracting power that consists of a negative lens having a concave surface on the object side,

wherein the imaging optical system does not have any other lenses other than the lenses of the first to fourth lens groups between the first lens group and the fourth lens group, and

wherein the imaging optical system is configured to satisfy the following conditional expression (1):

0.60< IY/AP< 0.85  (1)

where a maximum image height of the imaging optical system is IY, and a distance on an optical axis from an image plane to an exit pupil position of the imaging optical system is AP.

2. The imaging optical system according to claim 1 , wherein the imaging optical system is configured to satisfy the following conditional expression (2):

1.00< AP/TL< 1.40  (2)

where the distance on the optical axis from the image plane to the exit pupil position of the imaging optical system is AP, and a distance on the optical axis from a forefront surface of the first lens group to a backmost surface of the fourth lens group when focusing on infinity is TL.

3. The imaging optical system according to claim 1 , wherein an air lens that is formed between a backmost surface of the second lens group and a forefront surface of the third lens group has positive refracting power.

4. The imaging optical system according to claim 1 , wherein the imaging optical system is configured to satisfy the following conditional expression (3):

0.50< f 12/ f 34<1.60  (3)

where a combined focal distance of the first lens group and the second lens group is f12, and a combined focal distance of the third lens group and the fourth lens group is f34.

5. The imaging optical system according to claim 1 , wherein the imaging optical system includes at least one or more aspheric surfaces in the fourth lens group, and is configured to satisfy the following conditional expression (4):

0< f/|f 4|<0.30  (4)

where a focal distance of an entire system is f, and a focal distance of the fourth lens group is f4.

6. The imaging optical system according to claim 1 , wherein the imaging optical system is configured to satisfy the following conditional expression (5):

0.70< TL/f< 1.10  (5)

where a distance on the optical axis from a forefront surface of the first lens group to a backmost surface of the fourth lens group when focusing on infinity is TL, and a focal distance of an entire system is f.

7. The imaging optical system according to claim 1 , wherein the imaging optical system is configured to satisfy the conditional expression (6):

1.00<| AP/Rg 41|<2.50  (6)

where the distance on the optical axis from the image plane to the exit pupil position in the imaging optical system is AP, and a curvature radius of a forefront surface of the fourth lens group is Rg41.

8. A camera device comprising the imaging optical system according to claim 1 .

9. The camera device according to claim 8 comprising a function for converting a photographic image into digital information.

10. A mobile information terminal device comprising the imaging optical system according to claim 1 .