Objective optical system for endoscope

1. An objective optical system for endoscope, comprising in order from an object side:

a first group having a negative refractive power;

an aperture stop; and

a second group having a positive refractive power, wherein

the first group includes in order from the object side, a negative meniscus lens having a convex surface directed toward the object side, and a positive lens having a convex surface directed toward the object side, and

the second group includes in order from the object side, a predetermined lens, a positive cemented lens, and a positive single lens, or includes in order from the object side, the predetermined lens, the positive single lens, and the positive cemented lens, and

the negative meniscus lens has an aspheric surface, and

the predetermined lens is a meniscus lens having a convex surface directed toward an image side, and

the positive cemented lens includes an object-side lens positioned on the object side and an image-side lens positioned on the image side, and

the following conditional expressions (1′), (2), and (4′) are satisfied:

0.7<| R 1/ R 2|<1.1  (1′),

0.6<| R 1/ FL|< 3  (2), and

5≤| Fc/FL|≤ 7  (4′)

where,

R 1 denotes a paraxial radius of curvature of a surface on the object side of the predetermined lens,

R 2 denotes a paraxial radius of curvature of a surface on an image side of the predetermined lens,

FL denotes a focal length of the overall objective optical system for endoscope,

Fc denotes a focal length at a cemented surface of the positive cemented lens,

here,

the focal length Fc at the cemented surface is expressed by the following expression (b)

Fc=Rc /( nd′−nd )  (b)

Rc denotes a paraxial radius of curvature of the cemented surface,

nd denotes a refractive index of the object-side lens for a d-line, and

nd′ denotes a refractive index of the image-side lens for the d-line.

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

0.005≤Δ ASP /FL≤ 0.08  (3)

where,

Δ ASP denotes an aspheric surface amount at a point of intersection of a principal light ray at the maximum image height and an aspheric surface,

FL denotes the focal length of the overall objective optical system for endoscope, and

here,

the aspheric surface amount Δ ASP is expressed by the following expression (a)

Δ

ASP

=

c

⁢

⁢

ρ

2

1

+

1

-

(

K

+

1

)

⁢

c

2

⁢

ρ

2

+

∑

i

⁢

A

i

⁢

ρ

i

-

c

⁢

⁢

ρ

2

1

+

1

-

c

2

⁢

ρ

2

(

a

)

where,

c denotes a reciprocal of a paraxial radius of curvature at the aspheric surface,

ρ denotes a distance from an optical axis up to the point of intersection, which is a distance in a plane orthogonal to the optical axis including the point of intersection,

K denotes a conical coefficient, and

A i denotes an aspherical coefficient.

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

1.5≤| Fr/Ff|≤ 5  (5)

where,

Ff denotes a focal length of a negative meniscus lens having the convex surface directed toward the object side, and

Fr denotes a focal length of the positive single lens.