Objective lens for endoscopes and endoscope

1. An objective lens for endoscopes consisting of, in order from the object side to the image side:

a front group;

an aperture stop; and

a rear group having a positive refractive power as a whole;

the front group consisting of; in order from the object side to the image side, a first lens having a negative refractive power, in which the absolute value of the radius of curvature of the surface toward the image side thereof is less than the absolute value of the radius of curvature of the surface toward the object side thereof, and at least one plane parallel plate;

the rear group consisting of, in order from the object side to the image side, a second lens having a positive refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power;

the third lens and the fourth lens being cemented together to constitute a cemented lens;

the coupling surface of the cemented lens being convex toward the image side; and

all of Conditional Formulae (1) through (3) below being satisfied:

0.045

<

1

f

⁢

∑

i

=

1

k

⁢

⁢

ti

×

(

Ndi

-

1

)

vdi

<

0.15

(

1

)

-

0.9

<

f

⁢

/

⁢

fl

<

-

0.5

(

2

)

-

2.9

<

Dsc

⁢

/

⁢

RC

<

-

1

(

3

)

wherein f is the focal length of the entire lens system, k is the total number of plane parallel plates within the front group, i is a natural number within a range from 1 to k, ti is the thickness of an ith plane parallel plate from the object side within the front group, Ndi is the refractive index with respect to the d line of an ith plane parallel plate from the object side within the front group, vdi is the Abbe's number with respect to the d line of an ith plane parallel plate from the object side within the front group, fl is the focal length of the first lens, Dsc is the distance along the optical axis from the aperture stop to the coupling surface of the cemented lens, and RC is the radius of curvature of the coupling surface of the cemented lens.

2. An objective lens for endoscopes as defined in claim 1 , in which Conditional Formula (1-1) below is satisfied:

0.05

<

1

f

⁢

∑

i

=

1

k

⁢

⁢

ti

×

(

Ndi

-

1

)

vdi

<

0.12

.

(

1

⁢

-

⁢

1

)

3. An objective lens for endoscopes as defined in claim 1 , in which Conditional Formula (2-1) below is satisfied:

−0.8 <f /fl<−0.6  (2-1).

4. An objective lens for endoscopes as defined in claim 1 , in which Conditional Formula (3-1) below is satisfied:

−2.7 <Dsc /RC<−1.2  (3-1).

5. An endoscope equipped with an objective lens for endoscopes as defined in claim 1 .

6. An objective lens for endoscopes as defined in claim 1 , in which Conditional Formula (4) below is satisfied:

2.5

⁢

⁢

mm

<

∑

i

=

1

k

⁢

⁢

Ndi

×

ti

×

6

⁢

⁢

mm

(

4

)

wherein mm is millimeters as a unit of length.

7. An objective lens for endoscopes as defined in claim 6 , in which Conditional Formula (4-1) below is satisfied:

2.8

⁢

⁢

mm

<

∑

i

=

1

k

⁢

⁢

Ndi

×

ti

×

5.5

⁢

⁢

mm

.

(

4

⁢

-

⁢

1

)

8. An objective lens for endoscopes as defined in claim 1 , in which Conditional Formula (5) below is satisfied:

0.8

<

1

f

⁢

∑

i

=

1

k

⁢

⁢

ti

Ndi

<

3.

(

5

)

9. An objective lens for endoscopes as defined in claim 8 , in which Conditional Formula (5-1) below is satisfied:

1

<

1

f

⁢

∑

i

=

1

k

⁢

⁢

ti

Ndi

<

2.

(

5

⁢

-

⁢

1

)

10. An objective lens for endoscopes as defined in claim 1 , wherein:

the first lens and the plane parallel plate provided most toward the object side is configured to be separable from optical members toward the image side of the plane parallel plate provided most toward the object side.

11. An objective lens for endoscopes as defined in claim 10 , wherein:

the first lens has a planar portion;

the planar portion abuts the plane parallel plate provided most toward the object side in the direction of the optical axis.

12. An objective lens for endoscopes as defined in claim 10 , wherein:

only two plane parallel plates are included in the front group.

13. An objective lens for endoscopes as defined in claim 11 , in which Conditional Formula (6) below is satisfied:

0.4<( R 2− D 2)/ f< 1  (6)

wherein R 2 is the radius of curvature of the lens surface of the first lens toward the image side, and D 2 is the air distance along the optical axis between the first lens and the plane parallel plate provided most toward the object side.

14. An objective lens for endoscopes as defined in claim 13 , in which Conditional Formula (6-1) below is satisfied:

0.5<( R 2− D 2)/ f< 0.85  (6-1)