Image forming optical system and electronic image pickup apparatus using the same

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

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

at the time of zooming, distances between adjacent lens units change, and

at the time of zooming from a wide angle end to a telephoto end, the first lens unit, after moving toward an image side, moves toward the object side, and a second lens unit moves only toward the object side, and

a lens component is one of a single lens and a cemented lens, and

the first lens unit consists of a first negative lens, a second negative lens, and a positive lens in order from the object side, and

the second lens unit comprises in order from the object side, a first lens component and a second lens component, and

a total number of lens components included in the second lens unit is two, and

the first lens component either consists of a positive single lens, or consists of a cemented lens in which, a positive lens and a negative lens are cemented in order of the positive lens and the negative lens, and

the second lens component consists of a cemented lens, and

the third lens unit consists of either one lens or two lenses as a whole, and comprises a lens having an absolute value of a curvature of a surface on the image side larger than an absolute value of a curvature of a surface on the object side, which is disposed nearest to the object side, and

the fourth lens unit consists of one positive lens component, wherein the following conditional expression (2) is satisfied:

−10<( R 22F +R 22R )/( R 22F −R 22R )<10  (2)

where,

R 22F denotes a paraxial radius of curvature of a surface nearest to an object of the second lens component, and

R 22R denotes a paraxial radius of curvature of a surface nearest to an image of the second lens component.

2. The image forming optical system according to claim 1 , wherein

an aperture stop is disposed between the first lens component and the second lens component.

3. The image forming optical system according to claim 1 , wherein

the cemented lens in the second lens component comprises a negative lens, and a positive lens which is cemented to the negative lens, and

the following conditional expressions (6) and (7) are satisfied:

0 <v 22p −v 22n <80  (6)

−0.7 <N 22p −N 22n <0.2  (7)

where,

N 22p denotes a refractive index for d-line of one of the positive lenses cemented to the negative lens in the second lens component,

V 22p denotes Abbe's number with reference to d-line of one of the positive lenses in the second lens component,

N 22n denotes a refractive index for d-line of one of the negative lenses cemented to one of the positive lenses in the second lens component, and

V 22n denotes Abbe's number with reference to d-line of one of the negative lenses in the second lens component.

4. The image forming optical system according to claim 1 , wherein

the following conditional expression (8) is satisfied:

0.0 <M 3 /M 2 <0.85  (8)

where,

M 2 =|M 2w −M 2t |, M 3 =|M 3w −M 3t |,

M 2w denotes a position of the second lens unit at a wide angle end,

M 2t denotes a position of the second lens unit at a telephoto end,

M 3w denotes a position of the third lens unit at the wide angle end, and

M 3t denotes a position of the third lens unit at a telephoto end, and

M 2w , M 2t , M 3w , and M 3t are positions at the time of infinite object point focusing.

5. The image forming optical system according to claim 1 , wherein

the fourth lens unit is fixed at the time of zooming and at the time of focusing, and

the following conditional expression (24) is satisfied:

0.8< f b /f w <1.5  (24)

where,

f b denotes a distance on an optical axis from the last surface of the fourth lens unit up to a Gauss image point, and

f w denotes the focal length of the overall image forming optical system at the wide angle end.

6. The image forming optical system according to claim 1 , wherein

the image forming optical system is a zoom lens which consists of four lens unit.

7. An electronic image pickup apparatus comprising:

an image forming optical system according to claim 1 ; and

an image pickup element which has an image pickup surface.

8. The image forming optical system according to claim 1 , wherein

the following conditional expression (2′) is satisfied:

−10<( R 22F +R 22R )/( R 22F −R 22R )<6  (2′).

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

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

at the time of zooming, distances between adjacent lens units change, and

at the time of zooming from a wide angle end to a telephoto end, the first lens unit, after moving toward an image side, moves toward the object side, and the second lens unit moves only toward the object side, and

a lens component is one of a single lens and a cemented lens, and

the first unit comprises in order from the object side, a negative lens and a positive lens, and

a total number of lenses included in the first lens unit is two, and

the second lens unit consists of a first lens component, a second lens component, and a third lens component in order from the object side, and

the first lens component either consists of a positive single lens, or consists of a cemented lens in which, a positive lens and a negative lens are cemented in order of the positive lens and the negative lens, and

the second lens component consists of a cemented lens, and

the third lens unit consists of either one lens or two lenses as a whole, and comprises a lens having an absolute value of a curvature of a surface on the image side larger than an absolute value of a curvature of a surface on the object side, which is disposed nearest to the object side, and

the fourth lens unit consists of one positive lens component.

10. An electronic image pickup apparatus comprising:

an image forming optical system according to claim 9 ; and

an image pickup element which has an image pickup surface.

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

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

at the time of zooming, distances between adjacent lens units change, and

at the time of zooming from a wide angle end to a telephoto end, a first lens unit, after moving toward an image side, moves toward the object side, and the second lens unit moves only toward the object side, and

a lens component is one of a single lens and a cemented lens, and

the first lens unit consists of a first negative lens, a second negative lens, and a positive lens in order from the object side, and

the second lens unit comprises in order from the object side, a first lens component and a second lens component, or further consists of a third lens component on the image side of the second lens component, and

the first lens component either consists of a positive single lens, or consists of a cemented lens in which, a positive lens and a negative lens are cemented in order of the positive lens and the negative lens, and

the second lens component consists of a cemented lens, and

the third lens unit consists of either one lens or two lenses as a whole, and comprises a lens having an absolute value of a curvature of a surface on the image side larger than an absolute value of a curvature of a surface on the object side, which is disposed nearest to the object side, and

the fourth lens unit consists of one positive lens component, and

the following conditional expressions (1) and (13′) are satisfied:

−100<( R 21F +R 21R )/( R 21F −R 21R )<−0.2  (1)

1 <SF 11n −SF 12n <3.35  (13′)

where,

R 21F denotes a paraxial radius of curvature of a surface nearest to an object of the first lens component,

R 21R denotes a paraxial radius of curvature of a surface nearest to an image of the first lens component,

SF 11n =( R 11nF +R 11nR )/( R 11nF −R 11nR )  (13a),

SF 12n =( R 12nF +R 12nR )/( R 12nF −R 12nR )  (13b),

R 11nF denotes a paraxial radius of curvature of an object-side surface of the first negative lens in the first lens unit,

R 11nR denotes a paraxial radius of curvature of an image-side surface of the first negative lens in the first lens unit,

R 12nF denotes a paraxial radius of curvature of an object-side surface of the second negative lens in the first lens unit, and

R 12nR denotes a paraxial radius of curvature of an image-side surface of the second negative lens in the first lens unit.

12. An electronic image pickup apparatus comprising:

an image forming optical system according to claim 11 ; and

an image pickup element which has an image pickup surface.

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

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

at the time of zooming, distances between adjacent lens units change, and

at the time of zooming from a wide angle end to a telephoto end, the first lens unit, after moving toward an image side, moves toward the object side, and a second lens unit moves only toward the object side, and

a lens component is one of a single lens and a cemented lens, and

the first lens unit consists of a first negative lens, a second negative lens, and a positive lens in order from the object side, and

the second lens unit comprises in order from the object side, a first lens component and a second lens component, and

a total number of lens components included in the second lens unit is two, and

the first lens component either consists of a positive single lens, or consists of a cemented lens in which, a positive lens and a negative lens are cemented in order of the positive lens and the negative lens, and

the second lens component consists of a cemented lens, and

the third lens unit consists of either one lens or two lenses as a whole, and comprises a lens having an absolute value of a curvature of a surface on the image side larger than an absolute value of a curvature of a surface on the object side, which is disposed nearest to the object side, and

the fourth lens unit consists of one positive lens component, wherein

the following conditional expression (13) is satisfied:

1 <SF 11n −SF 12n <4.0  (13)

where,

SF 11n =( R 11nF +R 11nR )/( R 11nF −R 11nR )  (13a),

SF 12n =( R 12nF +R 12nR )/( R 12nF −R 12nR )  (13b),

R 11nF denotes a paraxial radius of curvature of an object-side surface of the first negative lens in the first lens unit,

R 11nR denotes a paraxial radius of curvature of an image-side surface of the first negative lens in the first lens unit,

R 12nF denotes a paraxial radius of curvature of an object-side surface of the second negative lens in the first lens unit, and

R 12nR denotes a paraxial radius of curvature of an image-side surface of the second negative lens in the first lens unit.

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

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

at the time of zooming, distances between adjacent lens units change, and

at the time of zooming from a wide angle end to a telephoto end, the first lens unit, after moving toward an image side, moves toward the object side, and a second lens unit moves only toward the object side, and

a lens component is one of a single lens and a cemented lens, and

the first lens unit consists of a first negative lens, a second negative lens, and a positive lens in order from the object side, and

the second lens unit comprises in order from the object side, a first lens component and a second lens component, and

a total number of lens components included in the second lens unit is two, and

the first lens component either consists of a positive single lens, or consists of a cemented lens in which, a positive lens and a negative lens are cemented in order of the positive lens and the negative lens, and

the second lens component consists of a cemented lens, and

the third lens unit consists of either one lens or two lenses as a whole, and comprises a lens having an absolute value of a curvature of a surface on the image side larger than an absolute value of a curvature of a surface on the object side, which is disposed nearest to the object side, and

the fourth lens unit consists of one positive lens component, wherein

the following conditional expression (14) is satisfied:

−0.1 <R 1pF /R 12nR <1  (14)

where,

R 1pF denotes a paraxial radius of curvature of an object-side surface of the positive lens in the first lens unit, and

R 12nR denotes the paraxial radius of curvature of the image-side surface of the second negative lens in the first lens unit.

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

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

at the time of zooming, distances between adjacent lens units change, and

at the time of zooming from a wide angle end to a telephoto end, the first lens unit, after moving toward an image side, moves toward the object side, and a second lens unit moves only toward the object side, and

a lens component is one of a single lens and a cemented lens, and

the first lens unit consists of a first negative lens, a second negative lens, and a positive lens in order from the object side, and

the second lens unit comprises in order from the object side, a first lens component and a second lens component, and

a total number of lens components included in the second lens unit is two, and

the first lens component either consists of a positive single lens, or consists of a cemented lens in which, a positive lens and a negative lens are cemented in order of the positive lens and the negative lens, and

the second lens component consists of a cemented lens, and

the third lens unit consists of either one lens or two lenses as a whole, and comprises a lens having an absolute value of a curvature of a surface on the image side larger than an absolute value of a curvature of a surface on the object side, which is disposed nearest to the object side, and

the fourth lens unit consists of one positive lens component, wherein

the following conditional expression (15) is satisfied:

0.1 <d 112 /f w <0.7  (15)

where,

d 112 denotes an air space on an optical axis between the first negative lens and the second negative lens in the first lens unit, and

f w denotes a focal length of the overall image forming optical system at a wide angle end.

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

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power;

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power, wherein

at the time of zooming, distances between adjacent lens units change, and

at the time of zooming from a wide angle end to a telephoto end, the first lens unit, after moving toward an image side, moves toward the object side, and the second lens unit moves only toward the object side, and

a lens component is one of a single lens and a cemented lens, and

the first unit comprises in order from the object side, a negative lens and a positive lens, and

a total number of lenses included in the first lens unit is two, and

the second lens unit comprises in order from the object side, a first lens component and a second lens component, and

a total number of lens components included in the second lens unit is two, and

the first lens component either consists of a positive single lens, or consists of a cemented lens in which, a positive lens and a negative lens are cemented in order of the positive lens and the negative lens, and

the second lens component consists of a cemented lens, and

the third lens unit consists of either one lens or two lenses as a whole, and comprises a lens having an absolute value of a curvature of a surface on the image side larger than an absolute value of a curvature of a surface on the object side, which is disposed nearest to the object side, and

the fourth lens unit consists of one positive lens component, and

the following conditional expressions (1′) is satisfied:

−100<( R 21F +R 21R )/( R 21F −R 21R )<−0.75  (1′)

where,

R 21F denotes a paraxial radius of curvature of a surface nearest to an object of the first lens component,

R 21R denotes a paraxial radius of curvature of a surface nearest to an image of the first lens component.

17. An electronic image pickup apparatus comprising:

an image forming optical system according to claim 16 ; and

an image pickup element which has an image pickup surface.