Image forming optical system and image pick up apparatus using the same

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

a first lens group having a negative refractive power;

a second lens group having a positive refractive power, which comprises an aperture stop;

a third lens group having a negative refractive power; and

a fourth lens group having a positive refractive power, wherein

the first lens group, in order from the object side, comprises a negative sub-group, and a positive sub-group, and

the third lens group is a focusing group which comprises only a cemented lens having a convex surface directed toward the object side, of a negative lens and a positive meniscus lens, in order from the object side.

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

N d3 ≦−0.03×ν d3 +2.37  (1)

where,

N d3 denotes a refractive index for a d-line of a glass material of the positive meniscus lens in the third lens group of the image forming optical system,

ν d3 denotes Abbe's number (nd 1 −1)/(nF 1 −nC 1 ) with reference to the d-line, for the glass material of the positive meniscus lens in the third lens group of the image forming optical system, and

nd 1 , nC 1 , and nF 1 denote refractive indices of the glass material of the positive meniscus lens in the third lens group for the d-line, a C-line, and an F-line respectively.

3. The image forming optical system according to claim 1 , wherein the image forming optical system satisfies the following conditional expression (2)

3.3≦ f 1T /F W ≦5.8  (2)

where,

f 1T denotes a focal length of the positive sub-group in the first lens group of the image forming optical system, and

F W denotes a focal length of the overall optical system at a wide angle end, of the image forming optical system.

4. The image forming optical system according to claim 1 , wherein the image forming optical system satisfies the following conditional expression (3)

0.375 ≦d 1T /d G1 ≦0.52  (3)

where,

d 1T denotes a distance on an optical axis, between the negative sub-group and the positive sub-group in the first lens group of the image forming optical system, and

d G1 denotes a distance on the optical axis from a first surface up to a last surface of the first lens group of the image forming optical system.

5. The image forming optical system according to claim 1 , wherein the image forming optical system satisfies the following conditional expression (4)

0.91≦( MG W ×MG t ) 1/2 ≦1.15  (4)

where,

MG W denotes a combined magnification of the image forming optical system from the second lens group onward, at the wide angle end, and

MG t denotes a combined magnification of the image forming optical system from the second lens group onward, at a telephoto end.

6. The image forming optical system according to claim 1 , wherein the image forming optical system satisfies the following conditional expression (5)

−2.5≦( R G3b +R G3a )/( R G3b −R G3a )<−1  (5)

where,

R G3a denotes a radius of curvature of a surface on the object side, of the cemented lens in the third lens group of the image forming optical system, and

R G3b denotes a radius of curvature of a surface on an image side, of the cemented lens in the third lens group of the image forming optical system.

7. The image forming optical system according to claim 1 , wherein the image forming optical system satisfies the following conditional expression (6)

−1.95≦( R G3ob +R G3oa )/( R G3ob −R G3oa )<−1  (6)

where,

R G3oa denotes a radius of curvature of a surface on the object side, of the negative lens in the third lens group of the image forming optical system, and

R G3ob denotes a radius of curvature of a surface on an image side, of the negative lens in the third lens group of the image forming optical system.

8. The image forming optical system according to claim 1 , wherein in the fourth lens group, there is at least one lens component which satisfies the following conditional expression (7)

−4.5≦( R G4b +R G4a )/( R G4b −R G4a )≦−1.2  (7)

where,

R G4a denotes a radius of curvature of a surface on the object side, of a lens in the fourth lens group of the image forming optical system, and

R G4b denotes a radius of curvature of a surface on an image side, of a lens in the fourth lens group of the image forming optical system.

9. The image forming optical system according to claim 1 , wherein in the second lens group of the image forming optical system, there are at least two air lenses having a meniscus shape.

10. The image forming optical system according to claim 9 , wherein in the second lens group of the image forming optical system, there is only one lens component between the air lenses having the meniscus shape.

11. The image forming optical system according to claim 9 , wherein in the second lens group, the air lens having the meniscus shape has a convex surface directed toward the object side, and satisfies the following conditional expression (8)

1.3≦|( R G2mb +R G2ma )/( R G2mb −R G2ma )|  (8)

where,

R G2ma denotes a radius of curvature of a surface on the object side, of the air lens having the meniscus shape in the second lens group of the image forming optical system, and

R G2mb denotes a radius of curvature of a surface on an image side, of the air lens having the meniscus shape in the second lens group of the image forming optical system.

12. The image forming optical system according to claim 1 , wherein the aperture stop is farther on the object side of the last surface of the second lens group.

13. The image forming optical system according to claim 1 , wherein a position of the fourth lens group is fixed at the time of zooming change.

14. The image forming optical system according to claim 1 , wherein a position of the first lens group is fixed at the time of zooming change.

15. An image pickup apparatus comprising:

an image forming optical system according to claim 1 ; and

an image pickup element.