JP2011034106A - Objective lens for endoscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens for an endoscope for a retro-focus type that further decreases its diameter and, at the same time, copes with peripheral blur without increasing the power of the first lens. <P>SOLUTION: The objective lens includes, in order from an object side, a front group G1 of a negative refractive power as a whole, an aperture stop S, and a rear group G2 of a positive refractive power as a whole. The front group G1 includes, in order from the object side, a first lens L1 of negative refractive power, and a second group L2G of negative refractive power as a whole, whose face closest to the image is a concave face oriented on the image side. The second group L2G includes a single or cemented lens. The rear group G2 includes, in order from the object side, a positive lens L3 and a cemented lens composed of a positive lens L4 and a negative lens L5. In addition, the following conditional expressions are satisfied: -10≤Q1≤-2 and 0.719≤¾f<SB>0</SB>/f<SB>1</SB>¾<0.81, wherein Q1 represents the shaping factor (R<SB>2</SB>+R<SB>1</SB>)/(R<SB>2</SB>-R<SB>1</SB>) of the second group, R<SB>1</SB>and R<SB>2</SB>represent the curvature radii of the faces of the second group, the faces being closest to the object and image respectively, f<SB>0</SB>is the composite focal length of the front group, and f<SB>1</SB>is the focal length of the first group. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an objective lens used in an endoscope.

  2. Description of the Related Art Conventionally, an endoscope is used for observing a site that is difficult to observe from the outside, such as treatment and diagnosis in a patient's body in the medical field. In recent years, along with the reduction in the diameter of endoscopes represented by transnasal endoscopes, small-sized imaging devices for endoscopes (eg, CCD and CMOS) have been developed, and the pixel pitch has been reduced year by year. ing. Accordingly, it is necessary to satisfy the optical performance such as widening of the angle, correction of aberration, and prevention of reduction of the light amount while achieving miniaturization of the objective lens for endoscope.

Conventionally, as an objective lens for an endoscope, for example, a lens described in the following Patent Document 1 has been proposed.

Japanese Patent Laid-Open No. 10-20189

The objective lens for an endoscope described in Patent Document 1 suppresses the curvature radius of the front lens group from becoming too small, and widens the angle while correcting the distortion aberration to a small extent without reducing the peripheral light amount so much. It is aimed.
In order to achieve the object, in order from the object side, the first lens group having a negative refractive power as a whole, an aperture stop, and a second lens group having a positive refractive power as a whole, The first lens unit is composed of two single lenses, a first lens on the object side having a negative refractive power and a second lens on the image side, and satisfies the following conditional expressions (2 1 ) and (2 2 ): It is characterized by.
−5.0 <q ₁ <−0.9 (2 1 )
−0.5 <q ₂ <9.0 (2 2 )
Where q ₁ is the shaping factor of the first lens (= (r2 + r1) / (r2−r1)), q ₂ is the shaping factor of the second lens (= (r4 + r3) / (r4−r3)), and r1 is the first. The radius of curvature of the object side surface of the lens, r2 is the radius of curvature of the image side surface of the first lens, r3 is the radius of curvature of the object side surface of the second lens, and r4 is the curvature of the image side surface of the second lens. Radius.

Thus, the endoscope objective lens described in Patent Document 1, the front lens group of the object side of the lens or aperture of the most object side has a negative refractive power, the image side of the aperture In a so-called retrofocus type lens configuration in which the rear lens group has a positive refractive power, a combination of predetermined conditions represented by the conditional expressions (21) and (2 2 ) is satisfied.

In a retrofocus type lens configuration, in order to obtain a certain wide angle of view, it is necessary to form the most object side lens with a concave lens.
However, in the retrofocus type objective lens adopted for the endoscope objective lens, the lens outer diameter is further reduced in order to meet the demand for further reducing the diameter of the endoscope as compared with the conventional endoscope. On the other hand, in order to achieve the wide angle required for the endoscope, it is conceivable to increase the power of the concave lens in the front lens group, that is, to increase the negative power of the concave lens closest to the object side.

However, when the negative power of the most object-side concave lens in the endoscope objective lens is increased along with further reduction in the diameter of the endoscope, the curvature of the concave surface formed in the most object-side concave lens increases. This deteriorates the processability of the concave lens and has a large negative power as the lens, so that the effect on the image when the lens is decentered in the frame is increased, and the peripheral portion of the image obtained by the endoscope It causes a phenomenon called blurring that causes blurring.
However, there are some conventional endoscope objective lenses such as the endoscope objective lens described in Patent Document 1 that pay attention to the problem of one-sided blur that occurs when the diameter of the endoscope is further reduced. However, in order to eliminate the one-sided blurring, an optical system that has been considered in terms of design has not been proposed.

  The present invention has been made in view of the above-described conventional problems. The power of the first lens is achieved while further reducing the diameter of the endoscope, that is, further reducing the diameter of the endoscope objective lens. An object of the present invention is to provide a retrofocus type endoscope objective lens that is resistant to one blur without increasing the size.

In order to achieve the above object, an endoscope objective lens according to the present invention has, in order from the object side, a front lens group having a negative refractive power as a whole, an aperture stop, and a positive refractive power as a whole. It consists of a rear group lens group. The front group lens group consists of a first lens having negative refractive power in order from the object side , and a negative refraction as a whole, with the most image side surface facing the concave surface on the image side. The second lens group is a single lens or a cemented lens, and the rear lens group is a positive lens, and a cemented lens of a positive lens and a negative lens in order from the object side. And the following conditional expressions (1) and (2) are satisfied.
−10 ≦ Q1 ≦ −2 ( 1 )
0.719 ≦ | f ₀ / f ₁ | <0.81 (2)
Where Q1 is the shaping factor (R ₂ + R ₁ ) / (R ₂ −R ₁ ) of the second lens group, R ₁ is the radius of curvature of the surface closest to the object in the second lens group, and R ₂ is The radius of curvature of the surface closest to the image side in the second lens group , f _o is the combined focal length of the front lens group, and f ₁ is the focal length of the first lens .

It is sectional drawing which follows the optical axis which shows the structure of the objective lens for endoscopes concerning Example 1 of this invention. 6 is a graph showing spherical aberration, coma aberration (meridional ray), and coma aberration (sagittal ray) field curvature in the optical system of Example 1 ; It is sectional drawing which follows the optical axis which shows the structure of the objective lens for endoscopes concerning Example 2 of this invention. 6 is a graph showing spherical aberration, coma aberration (meridional ray), and coma aberration (sagittal ray) field curvature in the optical system of Example 2 . It is sectional drawing which follows the optical axis which shows the structure of the objective lens for endoscopes concerning Example 3 of this invention. 6 is a graph showing spherical aberration, coma aberration (meridional ray), and coma aberration (sagittal ray) field curvature in the optical system of Example 3 . It is sectional drawing which follows the optical axis which shows the structure of the objective lens for endoscopes concerning Example 4 of this invention. 10 is a graph showing spherical aberration, coma aberration (meridional ray), and coma aberration (sagittal ray) field curvature in the optical system of Example 4 . It is sectional drawing which follows the optical axis which shows the structure of the objective lens for endoscopes concerning Example 5 of this invention. 10 is a graph showing spherical aberration, coma aberration (meridional ray), and coma aberration (sagittal ray) field curvature in the optical system of Example 5 .

  According to the present invention, it is possible to further reduce the diameter of an endoscope, that is, to further reduce the diameter of an objective lens for an endoscope, and to improve the resistance against a single blur without increasing the power of the first lens. A focus-type endoscope objective lens is obtained.

Prior to the description of the embodiments, the effects of the present invention will be described.
This onset Ming endoscope objective lens includes, in order from the object side, from the front lens group having negative refractive power as a whole, and an aperture stop, and a rear group lens group having positive refractive power as a whole The first lens group having negative refractive power in order from the object side, and the second lens having negative refractive power as a whole, with the most image side surface facing the concave surface toward the image side. The second lens group is composed of a single lens or a cemented lens, and satisfies the following conditional expression ( 1 ).
−10 ≦ Q1 ≦ −2 ( 1 )
Where Q1 is the shaping factor (R ₂ + R ₁ ) / (R ₂ −R ₁ ) of the second lens group, R ₁ is the radius of curvature of the surface closest to the object in the second lens group, and R ₂ is This is the radius of curvature of the surface closest to the image side in the second lens group.

Conditional expression ( 1 ) relates to the lens shape of the second lens unit having a negative refractive power as a whole, with the most image side surface facing the concave surface on the image side.
Conditional expression ( 1 ) is also an expression that prescribes the lens power distribution in the front lens group having negative refractive power as a whole. In other words, this is an equation that defines conditions for dispersing negative power necessary for the front lens group to the second lens group without concentrating it on the first lens on the most object side. By giving negative refractive power as a whole, the degree of dispersion of the negative power to the second lens group can be increased, the decentering effect of each lens can be reduced, and single blurring can be suppressed. it can.

However, if the upper limit of conditional expression ( 1 ) is exceeded, there is an effect on power dispersion, but on the other hand, the power of the concave surface near the aperture stop becomes too large, and the field curvature becomes overcorrected, Aberration correction becomes difficult.
On the other hand, if the lower limit of conditional expression ( 1 ) is not reached, the effect of power dispersion on the second lens group in the front lens group will be reduced.
Those described in Patent Document 1, as shown in conditional expression (22), shaping factor q ₂ of the second lens is -0.5 <q ₂ <9.0, the upper limit of condition (1) It is far above the value. If the upper limit value of conditional expression ( 1 ) is exceeded, the power of the concave surface existing in the vicinity of the aperture stop becomes too large, the field curvature becomes overcorrected, and aberration correction becomes difficult. When the shaping factor q ₂ of the second lens becomes a positive value, the direction of the concave surface of the meniscus lens in the second lens group is reversed as described in Patent Document 1 . In this case, it is difficult to give a strong power to the front lens group having negative refractive power as a whole, and when power is given, the power is concentrated on the first lens closest to the object side and is weak against one blur. Become. If the lower limit of conditional expression ( 1 ) is not reached, the power dispersion to the second lens group will be reduced, and the effect of making the second lens group a lens having negative refractive power will be reduced.

Thus, the thing of the said patent document 1 does not have the idea of eliminating the one-side blur of the objective lens for endoscopes. In the field of endoscope objective lenses, there is a background that has been manufactured with the problem of narrowing the tolerance of lenses and frames in parallel with the miniaturization of endoscope objective lenses. However, in recent years, downsizing of image pickup devices has been remarkable, and it has been desired to obtain an optical system that is resistant to one-sided blur by designing the objective lens for endoscope itself. In addition, when the present invention is applied to a conventional endoscope objective lens, not only can one-side blur in a conventional endoscope optical system be improved, but also the processability of the lens on the most object side and the manufacturability of the objective lens are easier. become. Therefore, the problem to be solved differs between the conditional expression described in Patent Document 1 and the conditional expression in the present invention.

In addition, the present onset Akira of the endoscope objective lens is, you satisfy the following condition (2).
0.719 ≦ | f ₀ / f ₁ | <0.81 ( 2 )
Where f _o is the combined focal length of the front lens group, and f ₁ is the focal length of the first lens.

Condition (2) is in the onset bright, an equation that defines the power distribution of the lens in the front lens group having negative refractive power as a whole.
If the upper limit of conditional expression ( 2 ) is exceeded, the degree of dispersion of the negative power to the second lens group in the front lens group will be poor, and the decentration effect of the front lens group cannot be reduced.
On the other hand, if the lower limit value of conditional expression (2) is not reached, negative power will be excessively distributed to the second lens group in the front lens group, and aberration correction will be difficult.

Here, the conditional expression ( 2 ) will be described in more detail. The left side | f ₀ / f ₁ | of the conditional expression ( 2 ) can be expressed as the following expression ( 2 ) -1 as described above.
| F ₀ / f ₁ | = | (f ₀ / f) / (f ₁ / f) | ( 2 ) -1
Where f is the total focal length of the entire system.
Here, some examples described in Patent Document 1 satisfy the conditional expression ( 2 ) of the present invention. However, the conditional expression (2) -1 of the structure factors in a denominator of | f ₁ / f | and molecular | f ₀ / f | for analyzing, Looking at Table 2, the Patent Document 1 Example 3 except, | f ₀ / f | is large either, the onset bright is different from the configuration which gave a strong power front lens premised. That is, the present onset Ming, by having an increased power dispersion, the front lens group of | those proposed good configuration without while small to have a large power in the first lens | f ₀ / f Yes, the configuration is significantly different from that of the above-described embodiment described in Patent Document 1 .

In Example 3 of Patent Document 1 , the front lens group has a large power of | f ₀ /f|=0.544. However, on the other hand, | f ₁ /f|=0.582. That is, in Example 3 described in Patent Document 1 , a large power is given to the first lens in order to give a large power to the front lens group. This is not Soguwa for the purpose of preventing, yet sided blur small and wide-angle objective lens, the configuration different from the present onset bright.
For the same reason also in Example 8, 9 and 10 described in JP 61-162021 JP cited as prior examples of Patent Document 1, | f ₀ / f | is large both, the inventions and purpose, structure, effect is significantly different.

Further, in this onset light of the objective lens, and dispersing the negative power of the first lens closest to the object in the front lens group having negative refractive power as a whole to the second lens group. However, like the wide-angle lens, as represented by a fisheye lens, by dividing into most two negative lens on the object side and three lenses, the objective lens to perform power dispersion, the onset Ming objective Lenses vary greatly in how they distribute power.
That is, in this onset light of the objective lens, are dispersed into the vicinity of the aperture stop in the front lens group of negative power of the first lens on the most object side. That is, unlike the negative lens power dispersion by increasing the number of lenses typified by a fisheye lens, the negative power of the first lens necessary for the entire front lens group is distributed to the entire front lens group. .

The distribution of the present onset light of the objective lens, the entire front lens group of negative power of the first lens required overall front lens group satisfies the following condition (3) It is desirable that
d / f ≦ 0.3 ( 3 )
Here, d is the surface distance between the most image side surface of the second lens unit in the front lens group and the aperture stop, and f is the total focal length of the entire system.
This specifies the position of the surface that disperses the negative power of the first lens closest to the object side to the second lens group in the front lens group having a negative refractive power as a whole. Conditional expression ( 3 ) It is preferable to satisfy

The present onset Ming endoscope objective lens, the rear group lens group having positive refractive power as a whole, in order from the object side, ing from a positive lens, a positive lens and a negative lens of the cemented lens.
In contrast to the front lens group having negative refractive power as a whole, the rear lens group requires positive refractive power as a whole in order to determine the focal length. However, if this positive power is distributed to the single lens and the cemented lens, the lateral chromatic aberration and the coma aberration can be corrected.

Embodiments of the present invention will be described below with reference to the drawings.
Example 1
Figure 1 is a sectional view taken along the optical axis, of a endoscope objective lens according to Example 1 of the present invention, the spherical aberration in the optical system of Figure 2 Example 1, coma (meridional rays), coma (Sagittal ray) It is a graph which shows curvature of field.
The endoscope objective lens of Example 1 includes a front lens group G1 and a rear lens group G2 with an aperture stop S interposed therebetween. In FIG. 1 , CG is a cover glass, and IM is an image plane.

The front lens group G1 includes, in order from the object side, a plano-concave first lens L1 having a plane on the object side and a concave surface on the image side, and a second lens group L2G, and has a negative refractive power as a whole. ing.
The second lens unit L2G is composed of a cemented lens of a biconvex lens L2 1 and a biconcave lens L2 2 in order from the object side, and has a negative refractive power as a whole.
The rear lens group G2 includes a planoconvex third lens L3 having a plane on the object side and a convex surface on the image side, a fourth lens L4 having a biconvex shape, and a meniscus shape having a negative refractive power with a concave surface facing the object side. The fifth lens L5 has a positive refractive power as a whole. A fourth lens L4 fifth lens L5 is that are joined.

As described above, the endoscope objective lens of Example 1 includes the front lens group having negative refractive power as a whole, the aperture stop, and the rear lens group having positive refractive power as a whole. The front lens group includes, in order from the object side, a first lens having negative refractive power, and a second lens group having negative refractive power as a whole, with the most image-side surface facing the concave surface on the image side. The second lens group is composed of a cemented lens. In the endoscope objective lens of Example 1 , since the second lens group is a cemented lens, aberration correction can be performed more easily. The shaping factor at this time is (R ₂ + R ₁ ) / (R ₂ ) where R _{1 is} the radius of curvature of the most object-side surface of the cemented lens and R ₂ is the radius of curvature of the most image-side surface of the cemented lens. -R ₁₎ is calculated from.

Next, numerical data of optical members constituting the endoscope objective lens of Example 1 are shown.
The numbers r ₁ , r ₂ ,... And d ₁ , d ₂ ,... In the lens cross-sectional view shown in FIG. . In the following numerical data, the refractive index and the Abbe number are values at the e-line. These are common in other embodiments.
Numerical example 1
Unit: mm
Surface data surface number Curvature radius Surface spacing Refractive index Abbe number Object surface ∞ 17.0000
1 ∞ 0.4668 1.88815 40.76
2 0.8050 0.6629
3 3.7346 0.6536 1.93429 18.90
4 -11.2039 0.4668 1.80922 39.59
5 2.2408 0.0861
6 (Aperture) ∞ 0.0560
7 ∞ 1.1994 1.88815 40.76
8 -1.6246 0.1027
9 2.7767 1.1401 1.51825 64.14
10 -1.8905 0.5137 1.93429 18.90
11 -3.6810 1.5200
11 ∞ 1.4005 1.51825 64.14
12 ∞ 0
13 (image plane) ∞ 0
Various data Focal length 1.00000
F number 4.958
Half angle of view 69.06718 °
Statue height 0.969
Total lens length 8.2686
Back focus -0.06614

Example 2
FIG. 3 is a cross-sectional view along the optical axis showing the configuration of an endoscope objective lens according to Example 2 of the present invention. FIG. 4 shows spherical aberration, coma aberration (meridional ray), and coma aberration in the optical system of Example 2 . (Sagittal ray) It is a graph which shows curvature of field.
The endoscope objective lens of Example 2 includes a front lens group G1 and a rear lens group G2 with an aperture stop S interposed therebetween. In FIG. 3 , CG is a cover glass, and IM is an image plane.
The front lens group G1 includes, in order from the object side, a meniscus first lens L1 ′ having a negative refractive power with a concave surface facing the image side, and a meniscus shape having a negative refractive power with a concave surface facing the image side. the second lens L 2 and is made up of a, it has a negative refractive power as a whole.
The basic lens configuration of the rear lens group G2 is substantially the same as that of the first embodiment.

Next, numerical data of optical members constituting the endoscope objective lens of Example 2 are shown.
Numerical example 2
Unit: mm
Surface data surface number Curvature radius Surface spacing Refractive index Abbe number Object surface ∞ 13.0000
1 5.6171 0.4681 1.88815 40.76
2 0.8202 0.5336
3 3.3320 1.0298 1.93429 18.90
4 2.5277 0.0843
5 (Aperture) ∞ 0.0562
6 ∞ 1.0531 1.88815 40.76
7 -1.3336 0.1123
8 3.9736 1.0631 1.59143 61.14
9 -1.4788 0.3722 1.85504 23.78
10 -3.3480 0.8313
11 ∞ 1.4043 1.51825 64.14
12 ∞ 0
13 (image plane) ∞ 0
Various data Focal length 1.00000
F number 6.042
Half angle of view 59.69241 °
Image height 0.972
Total lens length 7.0084
Back focus -0.06583

Example 3
FIG. 5 is a cross-sectional view along the optical axis showing the configuration of an endoscope objective lens according to Example 3 of the present invention. FIG. 6 shows spherical aberration, coma aberration (meridional ray), and coma aberration in the optical system of Example 3 . (Sagittal ray) It is a graph which shows curvature of field.
The endoscope objective lens of Example 3 includes a front lens group G1 and a rear lens group G2 with an aperture stop S interposed therebetween. In FIG. 5 , CG is a cover glass and IM is an image plane.
In order from the object side, the front lens group G1 includes a plano-concave first lens L1 having a flat object side and a concave surface on the image side, and a meniscus second lens L having a negative refractive power with the concave surface facing the image side. 2 and has a negative refractive power as a whole.
The basic lens configuration of the rear lens group G2 is substantially the same as that of the first embodiment.

Next, numerical data of optical members constituting the endoscope objective lens of Example 3 are shown.
Numerical Example 3
Unit: mm
Surface data surface number Curvature radius Surface spacing Refractive index Abbe number Object surface ∞ 16.0000
1 ∞ 0.4492 1.88815 40.76
2 0.7746 0.6379
3 2.9670 1.0475 1.93429 18.90
4 2.3355 0.0828
5 (Aperture) ∞ 0.0539
6 ∞ 1.1140 1.88815 40.76
7 -1.5434 0.0809
8 2.6719 1.0971 1.51825 64.14
9 -1.8004 0.4943 1.93429 18.90
10 -3.5420 1.4500
11 ∞ 1.3476 1.51825 64.14
12 ∞ 0
13 (image plane) ∞ 0
Various data Focal length 1.00001
F number 5.525
Half angle of view 64.22018 °
Image height 0.933
Total lens length 7.8551
Back focus -0.06218

Example 4
FIG. 7 is a cross-sectional view along the optical axis showing the configuration of an endoscope objective lens according to Example 4 of the present invention. FIG. 8 shows spherical aberration, coma aberration (meridional ray), and coma aberration in the optical system of Example 4 . (Sagittal ray) It is a graph which shows curvature of field.
The endoscope objective lens of Example 4 includes a front lens group G1 and a rear lens group G2 with an aperture stop S interposed therebetween. In FIG. 7 , CG is a cover glass, and IM is an image plane.
The basic lens configurations of the front group lens group G1 and the rear group lens group G2 are substantially the same as those in the third embodiment.

Next, numerical data of optical members constituting the endoscope objective lens of Example 4 are shown.
Numerical Example 4
Unit: mm
Surface data surface number Curvature radius Surface spacing Refractive index Abbe number Object surface ∞ 16.0000
1 ∞ 0.4634 1.88815 40.76
2 0.8006 0.6593
3 3.1571 1.1980 1.93429 18.90
4 2.2285 0.0856
5 (Aperture) ∞ 0.0557
6 ∞ 1.2443 1.88815 40.76
7 -1.6101 0.0836
8 2.7616 1.1339 1.51825 64.14
9 -1.8608 0.5109 1.93429 18.90
10 -3.6609 1.6109
11 ∞ 1.3815 1.51825 64.14
12 ∞ 0
13 (image plane) ∞ 0
Various data Focal length 0.99999
F number 6.636
Half angle of view 68.62833 °
Statue height 0.964
Total lens length 8.4279
Back focus -0.06914

Example 5
FIG. 9 is a cross-sectional view along the optical axis showing the configuration of an endoscope objective lens according to Example 5 of the present invention. FIG. 10 shows spherical aberration, coma aberration (meridional ray), and coma aberration in the optical system of Example 5 . (Sagittal ray) It is a graph which shows curvature of field.
The endoscope objective lens of Example 5 includes a front group lens group G1 and a rear group lens group G2 with an aperture stop S interposed therebetween. In FIG. 9 , CG is a cover glass and IM is an image plane.
The basic lens configurations of the front group lens group G1 and the rear group lens group G2 are substantially the same as those in the third embodiment.

Next, numerical data of optical members constituting the endoscope objective lens of Example 5 are shown.
Numerical Example 5
Unit: mm
Surface data Surface number Curvature radius Surface spacing Refractive index Abbe number Object surface ∞ 16.5000
1 ∞ 0.4634 1.88815 40.76
2 0.7991 0.6210
3 3.1514 1.1122 1.93429 18.90
4 2.0391 0.0855
5 (Aperture) ∞ 0.0556
6 ∞ 1.2420 1.88815 40.76
7 -1.5479 0.0834
8 2.7566 1.1318 1.51825 64.14
9 -1.8574 0.5100 1.93429 18.90
10 -3.6543 1.1122
11 ∞ 2.1318 1.51825 64.14
12 ∞ 0
13 (image plane) ∞ 0
Various data Focal length 1.00000
F number 6.47
Half angle of view 63.68250 °
Statue height 0.927
Total lens length 8.5490
Back focus -0.07165

Next, Table 1 shows the values of the conditional expression parameters in each example.
Table 1

Further, as a comparative example of the present invention shows the corresponding value of the conditional expression parameter of the present invention which definitive in Patent Document 1 in the following Table 2. Examples in the table 2, comparative example, Example in Patent Document 1 is a comparative example.
Table 2

  The objective lens for an endoscope of the present invention is useful in the medical and industrial fields in which a narrow hole such as a narrow lumen in a patient's body is required to be observed using an endoscope.

G1 Front lens group G2 Rear lens group L1 Plano-concave first lens having a flat object side and a concave surface on the image side L1 ′ Meniscus first lens L2 having a negative refractive power with the concave surface facing the image side L2 Image side Meniscus-shaped second lens L2G having a negative refractive power with a concave surface facing to the second lens group L21G having a negative refractive power as a whole with the most image-side surface facing the concave surface toward the image side L21 Biconvex lens L22 Biconcave lens L3 Plano-convex third lens having a plane on the object side and convex on the image side L4 Fourth lens having a biconvex shape L5 Meniscus fifth lens having a negative refractive power with the concave surface facing the object side CG Cover glass IM Image surface S Brightness stop

Claims (1)

In order from the object side, it consists of a front lens group having negative refractive power as a whole, an aperture stop, and a rear lens group having positive refractive power as a whole,
The front lens group includes, in order from the object side, a first lens having a negative refractive power, and a second lens group having a negative refractive power as a whole, with the most image-side surface facing a concave surface on the image side. Consists of
The second lens group comprises a single lens or a cemented lens;
The rear group lens group includes, in order from the object side, a positive lens, a cemented lens of a positive lens and a negative lens, and
An endoscope objective lens characterized by satisfying the following conditional expressions (1) and (2):
−10 ≦ Q1 ≦ −2 ( 1 )
0.719 ≦ | f ₀ / f ₁ | <0.81 (2)
Where Q1 is the shaping factor (R ₂ + R ₁ ) / (R ₂ −R ₁ ) of the second lens group, R ₁ is the radius of curvature of the surface closest to the object in the second lens group, and R ₂ is The radius of curvature of the surface closest to the image side in the second lens group , f _o is the combined focal length of the front lens group, and f ₁ is the focal length of the first lens .

Images