Objective optical system for endoscope and endoscope having same
An objective optical system for an endoscope comprising, in order from the object side, a first lens unit G 1 having a negative refractive power, a second lens unit G 2 having a positive refractive power, and a third lens unit G 3 having a positive refractive power. The first lens unit G 1 includes a negative lens L 1 . The second lens unit G 2 includes a positive meniscus lens L 2 having a convex surface facing the image side. The third lens unit G 3 includes, in order from the object side, an object side positive lens L 3 and a cemented lens CL made up of a positive lens L 4 and a negative lens L 5 . The second lens unit G 2 is moved for focusing. The objective optical system satisfies the following conditional expressions (1) and (2): - 0 . 0 7 < f 1 / f 2 < - 0 .015 ( 1 ) - 0.4 < f 1 / f 3 < - 0 . 1 5 ( 2 ) where f1 is the focal length of the first lens unit, f2 is the focal length of the second lens unit, and f3 is the focal length of the third lens unit.
1 . An objective optical system for an endoscope comprising, in order from the object side: a first lens unit having a negative refractive power; a second lens unit having a positive refractive power; and a third lens unit having a positive refractive power, wherein the first lens unit includes a negative lens, the second lens unit includes a positive meniscus lens having a convex surface facing the image side, the third lens unit includes, in order from the object side, an object side positive lens and a cemented lens made up of a positive lens and a negative lens, the second lens unit is moved for focusing, and the objective optical system for an endoscope satisfies the following conditional expressions (1) and (2): −0.07<f1/f2<−0.015 (1), −0.4<f1/f3<−0.15 (2), where f1 is the focal length of the first lens unit, f2 is the focal length of the second lens unit, and f3 is the focal length of the third lens unit.
2 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (3):
-
1
.
1
3
<
f
1
/
d
23
e
<
-
0.4
,
(
3
)
where f1 is the focal length of the first lens unit, and d23e is the distance between the second lens unit and the third lens unit in the state in which the optical system is focused on an object at a far distance.
3 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (4):
-
0
.
5
9
<
f
12
e
/
f
3
′
<
-
0.35
,
(
4
)
where f12e is the combined focal length of the first lens unit and the second lens unit in the state in which the optical system is focused on an object at a far distance, and f3′ is the combined focal length of the object side positive lens and the cemented lens.
4 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (5):
-
5
0
<
r
2
mf
/
fe
<
-
2.5
,
(
5
)
where r2mf is the radius of curvature of the object side surface of the positive meniscus lens, and fe is the focal length of the objective optical system in the state in which it is focused on an object at a far distance.
5 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (6):
0.01
<
f
3
p
/
f
3
ce
<
0.3
,
(
6
)
where f3p is the focal length of the object side positive lens, and f3ce is the focal length of the cemented lens.
6 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (7):
0.5
<
d
3
p
/
d
3
pce
<
3
,
(
7
)
where d3p is the thickness of the object side positive lens, and d3pce is the distance between the object side positive lens and the cemented lens.
7 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (8):
-
3
<
f
23
′
k
/
f
1
<
-
2
,
(
8
)
where f23′k is the focal length of a group of lenses including the second lens unit, the object side positive lens, and the cemented lens in the state in which the optical system is focused on an object at a near distance, and f1 is the focal length of the first lens unit.
8 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (9):
0.38
<
Δ2
/
fe
<
0.8
,
(
9
)
where Δ2 is the amount of movement of the second lens unit, and fe is the focal length of the objective optical system for an endoscope in the state in which it is focused on an object at a far distance.
9 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (10):
-
1
8
<
(
r
3
pf
-
r
3
pr
)
/
(
r
3
pf
+
r
3
pr
)
<
40
,
(
10
)
where r3pf is the radius of curvature of the object side surface of the object side positive lens, and r3pr is the radius of curvature of the image side surface of the object side positive lens.
10 . An objective optical system for an endoscope according to claim 1 , satisfying the following conditional expression (11):
-
2
.
1
5
<
(
r
3
cpf
-
r
3
cpr
)
/
(
r
3
cpf
+
r
3
cpr
)
<
-
1
,
(
11
)
where r3cpf is the radius of curvature of the object side surface of the cemented lens, and r3cpr is the radius of curvature of the image side surface of the cemented lens.
11 . An objective optical system for an endoscope according to claim 1 , wherein the third lens unit further includes an image side positive lens disposed on the image side of the cemented lens.
12 . An objective optical system for an endoscope according to claim 1 further comprising an aperture stop disposed on the object side of the third lens unit.
13 . An endoscope comprising an objective optical system for an endoscope according to claim 1 .