Image pickup optical system and image pickup apparatus using the same

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

an aperture stop;

a first lens having a positive refracting power;

a second lens having a negative refracting power;

a third lens having a positive refracting power;

a fourth lens having a positive refracting power; and

a fifth lens having a negative refracting power.

2. The image pickup optical system according to claim 1 , wherein the image pickup optical system satisfies the following conditional expression

0.28< f 1/ f< 1.23  (3)

where,

f1 denotes a focal length of the first lens, and

f denotes a focal length of the overall image pickup optical system.

3. The image pickup optical system according to claim 1 , wherein a surface on an image surface side of the second lens is concave shaped toward the image surface side.

4. The image pickup optical system according to claim 1 , wherein the image pickup optical system satisfies the following conditional expression

0.57< f 3/ f< 6.31  (4)

where,

f3 denotes a focal length of the third lens, and

f denotes a focal length of the overall image pickup optical system.

5. The image pickup optical system according to claim 1 , wherein the image pickup optical system satisfies the following conditional expression

0<|(SAG4 AS −SAG4 AA )/4 AR|< 0.147  (5)

0<|(SAG4 BS −SAG4 BA )/4 BR|< 0.395  (6)

where,

SAG4AS denotes a sag amount at a position which is 60% of an effective diameter when a surface on the object side of the fourth lens is let to be a spherical surface,

SAG4AA denotes a sag amount at a position which is 60% of an effective diameter of the surface on the object side of the fourth lens,

SAG4BS denotes a sag amount at a position which is 60% of an effective diameter when a surface on an image pickup surface side of the fourth lens is let to be a spherical surface,

SAG4BA denotes a sag amount at a position which is 60% of an effective diameter of the surface on the image pickup surface side of the fourth lens,

4AR denotes a paraxial radius of curvature of the surface on the object side of the fourth lens, and

4BR denotes a paraxial radius of curvature of the surface on the image pickup surface side of the fourth lens.

6. The image pickup optical system according to claim 1 , wherein a surface on the object side of the fifth lens is a concave shaped lens.

7. 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 are formed of a resin.

8. An image pickup apparatus comprising:

an image pickup optical system according to claim 1 ; and

an electronic image pickup element having an image pickup surface, wherein

the image pickup apparatus satisfies the following conditional expression

15°<α i< 30°  (7)

where,

αi denotes an angle of incidence of principal light rays on an image pickup surface at the maximum image height.

9. The image pickup apparatus according to claim 8 , further comprising:

a shutter mechanism which is provided at the endmost position on to the object side of the image pickup optical system.

10. The image pickup apparatus according to claim 8 , wherein the image pickup optical system includes an auto-focus mechanism.

11. The image pickup apparatus according to claim 8 , wherein the image pickup optical system and the electronic image pickup element are integrated.

12. The image pickup optical system according to claim 1 , wherein the image pickup optical system satisfies the following conditional expression

0.36 <f 4 /f< 3.88  (1)

where,

f4 denotes a focal length of the fourth lens, and

f denotes a focal length of the overall image pickup optical system.

13. The image pickup optical system according to one of claims 1 and 12 , wherein the image pickup optical system satisfies the following conditional expression

0.81 <f 3/ f 4<3.25  (2)

where,

f3 denotes a focal length of the third lens, and

f4 denotes the focal length of the fourth lens.