Image pickup optical system and image pickup apparatus using the same

1. An image pickup optical system comprising, in order from an object side,

a first lens having a positive refractive power,

a second lens having a negative refractive power,

a third lens having a positive refractive power,

a fourth lens having a positive refractive power,

a fifth lens having a negative refractive power, which is formed into a meniscus shape on an optical axis and having its concave surface facing toward the object side,

and an aperture located on the most object side,

wherein the following conditional expression (5) is satisfied:

1<( r 8 +r 9)/( r 8 −r 9)<2.7  (5)

where

r8 is a paraxial radius of curvature of an object side surface of the fourth lens, and

r9 is a paraxial radius of curvature of an image side surface of the fourth lens.

2. The image pickup optical system according to claim 1 , wherein the following conditional expression (1) is satisfied:

−4.2 <f/f 5<−0.8  (1)

where

f5 is a focal length of the fifth lens, and

f is a focal length of the entire image pickup optical system.

3. The image pickup optical system according to claim 1 , wherein the following conditional expression (2) is satisfied:

−2.7<( r 10 +r 11)/( r 10 −r 11)<−0.6  (2)

where

r10 is a paraxial radius of curvature of an object side surface of the fifth lens, and

r11 is a paraxial radius of curvature of an image side surface of the fifth lens.

4. The image pickup optical system according to claim 1 , wherein the first lens is a biconvex lens.

5. The image pickup optical system according to claim 1 , wherein the following conditional expression (3) is satisfied:

−1<( r 2 +r 3)/( r 2 −r 3)<−0.59  (3)

where

r2 is a paraxial radius of curvature of an object side surface of the first lens, and

r3 is a paraxial radius of curvature of an image side surface of the first lens.

6. The image pickup optical system according to claim 1 , wherein the second lens is a meniscus lens facing its convex surface toward the object side.

7. The image pickup optical system according to claim 1 , wherein the following conditional expression (4) is satisfied:

1<( r 4 +r 5)/( r 4 −r 5)<2  (4)

where

r4 is a paraxial radius of curvature of an object side surface of the second lens, and

r5 is a paraxial radius of curvature of an image side surface of the second lens.

8. The image pickup optical system according to claim 1 , wherein the fourth lens is a meniscus lens facing its concave surface toward the object side.

9. The image pickup optical system according to claim 1 , wherein the following conditional expression (6) is satisfied:

1.3 <f 3 /f 4<7.8  (6)

where

f3 is a focal length of the third lens, and

f4 is a focal length of the fourth lens.

10. The image pickup optical system according to claim 1 , wherein the first lens, the second lens, the third lens, the fourth lens, and the fifth lens in the image pickup optical system are all made of resin.

11. An image pickup apparatus comprising

the image pickup optical system according to claim 1 , and

an electronic image pickup device having an image pickup plane,

wherein the following conditional expression (7) is satisfied:

15°<αi<30°  (7)

where αi is an angle of incidence of a principle ray onto the image pickup plane in the case of the maximum image height.

12. The image pickup apparatus according to claim 11 , further comprising an autofocus mechanism integrated into the image pickup optical system.

13. The image pickup apparatus according to claim 11 , wherein the image pickup optical system and the electronic image pickup device are integrated as a unit.