Objective optical system for endoscope and endoscope
An objective optical system for an endoscope consists of a front group, an aperture stop, and a positive rear group in order from an object side. The front group consists of a negative first lens having a smaller absolute value of a curvature radius of a lens surface on an image side than that on an object side, and one or more parallel planar members of which an incidence surface and an emission surface are perpendicular to an optical axis, in order from the object side. The rear group consists of a positive second lens and a cemented lens in which a negative third lens, and a positive fourth lens are joined together in order from the object side and a cemented surface has a concave surface toward the image side. A predetermined conditional expression is satisfied.
1. An objective optical system for an endoscope consisting of:
a front group having negative refractive power as a whole, an aperture stop, and a rear group having positive refractive power as a whole, in order from an object side,
wherein the front group consists of a first lens, having negative refractive power, in which an absolute value of a curvature radius of a lens surface on an image side is smaller than an absolute value of a curvature radius of a lens surface on an object side, and at least one parallel planar member of which an incidence surface and an emission surface are perpendicular to an optical axis, in order from the object side,
wherein the rear group consists of a second lens having positive refractive power, a third lens having negative refractive power, and a fourth lens having positive refractive power, in order from the object side,
wherein the third lens and the fourth lens are joined together to constitute a cemented lens, and a cemented surface of the cemented lens has a concave surface directed to the image side, and
wherein the following Conditional Expression (1) is satisfied,
0.02< f/fc< 0.10 (1)
where
f: Focal distance of entire system, and
fc: Focal distance of the cemented lens.
2. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (2) is satisfied,
0.51< f/f 2<0.75 (2)
where
f2: Focal distance of the second lens.
3. The objective optical system for an endoscope according to claim 2 ,
wherein the following Conditional Expression (2-1) is satisfied,
0.53< f/f 2<0.73 (2-1).
4. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (3) is satisfied,
−0.83< f/f 1<−0.61 (3)
where
f1: Focal distance of the first lens.
5. The objective optical system for an endoscope according to claim 4 ,
wherein the following Conditional Expression (3-1) is satisfied,
−0.80< f/f 1<−0.64 (3-1).
6. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (4) is satisfied,
0.49< f/fr< 0.74 (4)
where
fr: Focal distance of the rear group.
7. The objective optical system for an endoscope according to claim 6 ,
wherein the following Conditional Expression (4-1) is satisfied,
0.53< f/fr< 0.70 (4-1).
8. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (5) is satisfied,
0.55<max| IH/LRi|< 0.90 (5)
where
IH: Maximum image height,
LRi: Curvature radius of i-th lens surfaces from object side in case where i is an integer of 1 to 7, and
max|IH/LRi|: Maximum value of |IH/LRi| of entire system.
9. The objective optical system for an endoscope according to claim 8 ,
wherein the following Conditional Expression (5-1) is satisfied,
0.60<max| IH/LRi|< 0.85 (5-1).
10. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (6) is satisfied,
0.19< ave|IH/LRi|< 0.32 (6)
where
IH: Maximum image height,
LRi: Curvature radius of i-th lens surfaces from object side in case where i is an integer of 1 to 7, and
ave|IH/LRi|: Average value of |IH/LRi| of entire system.
11. The objective optical system for an endoscope according to claim 10 ,
wherein the following Conditional Expression (6-1) is satisfied,
0.20< ave|IH/LRi|< 0.31 (6-1).
12. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (7) is satisfied,
1.79<min Ndp< 1.98 (7)
where
minNdp: Minimum value of refractive index with respect to the d line of all positive lenses included in the entire system.
13. The objective optical system for an endoscope according to claim 12 ,
wherein the following Conditional Expression (7-1) is satisfied,
1.81<min Ndp< 1.95 (7-1).
14. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (8) is satisfied,
1.82< aveNdp< 1.98 (8)
where
aveNdp: Average value of refractive index with respect to the d line of all positive lenses included in the entire system.
15. The objective optical system for an endoscope according to claim 14 ,
wherein the following Conditional Expression (8-1) is satisfied,
1.84< aveNdp< 1.96 (8-1).
16. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (9) is satisfied,
0.30< f/Rc< 0.60 (9)
where
Rc: Curvature radius of cemented surface of the cemented lens.
17. The objective optical system for an endoscope according to claim 16 ,
wherein the following Conditional Expression (9-1) is satisfied,
0.33< f/Rc< 0.57 (9-1).
18. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (10) is satisfied,
0.10< Nd 3− Nd 4<0.15 (10)
where
Nd3: Refractive index of the third lens with respect to the d line, and
Nd4: Refractive index of the fourth lens with respect to the d line.
19. The objective optical system for an endoscope according to claim 1 ,
wherein the following Conditional Expression (1-1) is satisfied,
0.04< f/fc< 0.08 (1-1).
20. An endoscope comprising:
the objective optical system for an endoscope according to claim 1 .