阅读说明：本技术 内窥镜用物镜光学系统 (Objective optical system for endoscope ) 是由 中村稔 于 2018-08-28 设计创作，主要内容包括：提供一种具有足够的后焦距、并且光学系统的全长短、良好地校正各像差的内窥镜用物镜光学系统。内窥镜用物镜光学系统具备从物体侧向像侧依次配置的具有负折射力的第一透镜组(G1)、第二透镜组(G2)以及具有正折射力的第三透镜组(G3),在聚焦时,第二透镜组(G2)沿光轴方向移动,第三透镜组(G3)具有从物体侧向像侧配置的前组和后组,前组由正折射力的接合透镜组成,或者由正折射力的单透镜组成,后组由正折射力的接合透镜组成,满足下面的条件式(1)。0.45<d3t/f32<0.8(1)在此,d3t为从前组的位于最靠物体侧的位置的透镜面到后组的位于最靠物体侧的位置的透镜面的距离,f32为后组的焦距。(Provided is an objective optical system for an endoscope, which has a sufficient back focus, has an optical system with a short total length, and can correct various aberrations well. An objective optical system for an endoscope includes a first lens group (G1) having negative refractive power, a second lens group (G2), and a third lens group (G3) having positive refractive power, which are arranged in this order from an object side to an image side, wherein the second lens group (G2) moves in an optical axis direction during focusing, the third lens group (G3) includes a front group and a rear group arranged from the object side to the image side, the front group is composed of cemented lenses having positive refractive power or is composed of a single lens having positive refractive power, the rear group is composed of cemented lenses having positive refractive power, and the following conditional expression (1) is satisfied. 0.45< d3t/f32<0.8(1), where d3t is the distance from the lens surface of the front group located most toward the object side to the lens surface of the rear group located most toward the object side, and f32 is the focal length of the rear group.)

1. An objective optical system for an endoscope, characterized in that,

comprises a first lens group having negative refractive power, a second lens group having negative refractive power, and a third lens group having positive refractive power, which are arranged in this order from an object side to an image side,

at the time of focusing, the second lens group moves in the optical axis direction,

the third lens group has a front group and a rear group arranged from the object side to the image side,

the front group consists of positive refractive power cemented lenses, or positive refractive power singlet lenses,

the rear group consists of positive refractive cemented lenses,

the objective optical system for an endoscope satisfies the following conditional expression (1),

0.45＜d3t/f32＜0.8 (1)

in this case, the amount of the solvent to be used,

d3t is the distance from the lens surface of the front group located at the most object side to the lens surface of the rear group located at the most object side,

f32 is the focal length of the back group.

2. The endoscopic objective optical system according to claim 1,

the objective optical system for an endoscope satisfies the following conditional expression (2),

1.2＜f31/f32＜5.0 (2)

in this case, the amount of the solvent to be used,

f31 is the focal length of the front set,

f32 is the focal length of the back group.

3. The objective optical system for an endoscope according to claim 1 or 2,

the objective optical system for an endoscope satisfies the following conditional expression (3),

0.28＜d3p/f32＜0.5 (3)

in this case, the amount of the solvent to be used,

d3p is the distance from the lens surface of the front group located at the most image side to the lens surface of the rear group located at the most object side,

f32 is the focal length of the back group.

4. The objective optical system for an endoscope according to claim 2 or 3,

the objective optical system for an endoscope satisfies the following conditional expression (4),

2.3＜f3/f＜3.2 (4)

in this case, the amount of the solvent to be used,

f3 is the focal length of the third lens group,

f is a focal length of the objective optical system for an endoscope in normal observation.

Technical Field

The present invention relates to an objective optical system for an endoscope.

Background

In a medical endoscope, a magnified observation is used for observation of a lesion. In order to observe the lesion under magnification, the lesion needs to be found. In the magnifying observation, the observation range is narrower than that in the conventional observation (hereinafter referred to as "normal observation"). Therefore, finding a lesion by magnifying observation is not easy. From such a situation, it is desirable to be able to perform normal observation and magnification observation by one objective optical system.

In the magnified observation, the distance from the objective optical system to the object position (hereinafter referred to as "object distance") is, for example, about 3 mm. On the other hand, in normal observation, the object distance is much longer than 3 mm.

When the optical system is configured such that the object position during normal observation coincides with the focal position of the objective optical system, the object image during normal observation (hereinafter referred to as "normal image") is a focused image.

On the other hand, the object position in the magnified view is far from the object position in the normal view. The object position during the magnified observation is not included in the depth of field of the objective optical system during the normal observation. Therefore, in an optical system in a state of focusing on a normal image, an object image under magnification observation (hereinafter referred to as a "magnified image") is not a focused image.

In order to form a focused object image even in the magnified observation, the objective optical system may have a focusing function. By providing the objective optical system with a focusing function, both a normal image and an enlarged image can be formed in a focused state.

Patent document 1 and patent document 2 disclose objective optical systems having a focusing function. The objective optical system includes a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power. Upon focusing, only the second lens group moves.

Disclosure of Invention

Problems to be solved by the invention

In normal observation and enlarged observation, it is preferable that the lesion part can be observed in more detail. For this purpose, for example, the aperture value of the objective optical system may be reduced. However, when the aperture value of the objective optical system is reduced, the depth of field becomes narrow. Therefore, the focusing range in the optical axis direction is narrowed in both the normal observation and the magnification observation.

As a method of expanding the focus range in the optical axis direction, for example, there is a method of generating an image having a deep depth of field by combining two images having different focal positions. In order to acquire two images having different focal positions, it is necessary to form two optical images having different focal positions.

For example, two optical images can be formed by arranging prisms that form two optical paths in the optical path of the objective optical system. By making the length of one optical path different from that of the other, two optical images having different focal positions can be formed. The prism is disposed between the lens located closest to the image side and the imaging element. Therefore, it is desirable that the objective optical system have a sufficient back focal length.

However, in the objective optical system disclosed in patent document 1 and the objective optical system disclosed in patent document 2, a sufficient back focus is not secured.

The present invention has been made in view of the above problems, and an object thereof is to provide an objective optical system for an endoscope, which has a sufficient back focal length, has a short total length of the optical system, and corrects each aberration well.

Means for solving the problems

In order to solve the above-described problems and achieve the object, an objective optical system for an endoscope according to at least some embodiments of the present invention includes a first lens group having negative refractive power, a second lens group, and a third lens group having positive refractive power, which are arranged in order from an object side to an image side,

at the time of focusing, the second lens group moves in the optical axis direction,

the third lens group has a front group and a rear group arranged from the object side to the image side,

the front group consists of positive refractive power cemented lenses, or positive refractive power singlet lenses,

the rear group consists of positive refractive cemented lenses,

the objective optical system for an endoscope satisfies the following conditional expression (1),

0.45＜d 3t/f 32＜0.8 (1)

in this case, the amount of the solvent to be used,

d3t is the distance from the lens surface of the front group located at the most object side to the lens surface of the rear group located at the most object side,

f32 is the focal length of the back group.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide an endoscope objective optical system having a sufficient back focus, having an overall length of the optical system, and correcting various aberrations satisfactorily.

Drawings

Fig. 1 is a sectional view showing a specific configuration of an endoscopic objective optical system and a specific configuration of a prism according to the present embodiment.

Fig. 2 is a sectional view of the endoscopic objective optical system according to example 1.

Fig. 3 is an aberration diagram of the endoscopic objective optical system according to example 1.

Fig. 4 is a sectional view of the endoscopic objective optical system according to example 2.

Fig. 5 is an aberration diagram of the endoscopic objective optical system according to example 2.

Fig. 6 is a sectional view of the endoscopic objective optical system according to example 3.

Fig. 7 is an aberration diagram of the endoscopic objective optical system according to example 3.

FIG. 8 is a sectional view of the endoscopic objective optical system according to example 4.

Fig. 9 is an aberration diagram of the endoscopic objective optical system according to example 4.

FIG. 10 is a sectional view of the endoscopic objective optical system according to example 5.

Fig. 11 is an aberration diagram of the endoscopic objective optical system according to example 5.

FIG. 12 is a sectional view of the endoscopic objective optical system according to example 6.

Fig. 13 is an aberration diagram of the endoscopic objective optical system according to example 6.

FIG. 14 is a sectional view of the endoscopic objective optical system according to example 7.

Fig. 15 is an aberration diagram of the endoscopic objective optical system according to example 7.

FIG. 16 is a sectional view of the endoscopic objective optical system according to example 8.

Fig. 17 is an aberration diagram of the endoscopic objective optical system according to example 8.

Detailed Description

The reason and operation of adopting such a configuration will be described below with reference to the drawings with respect to the endoscopic objective optical system according to the present embodiment. The present invention is not limited to the following embodiments.

The endoscopic objective optical system according to the present embodiment can perform normal observation and enlarged observation by one optical system in endoscopic observation. Therefore, in the objective optical system for an endoscope according to the present embodiment, the optical system is configured by a plurality of lens groups, and one lens group is moved on the optical axis.

This enables normal observation when focusing on a distant object and magnified observation when focusing on a close object. The object distance of a distant object is longer than that of a close object. In the magnification observation, observation can be performed at a higher magnification than in the normal observation.

A basic configuration of the endoscopic objective optical system according to the present embodiment will be described. In a basic configuration, an objective optical system for an endoscope includes a first lens group having negative refractive power, a second lens group, and a third lens group having positive refractive power, which are arranged in order from an object side to an image side, the second lens group moves in an optical axis direction at the time of focusing, the third lens group has a front group and a rear group arranged from the object side to the image side, the front group is composed of cemented lenses having positive refractive power or is composed of single lenses having positive refractive power, and the rear group is composed of cemented lenses having positive refractive power.

In a basic configuration, an objective optical system for an endoscope is configured by a first lens group having negative refractive power, a second lens group, and a third lens group having positive refractive power, which are arranged in order from an object side to an image side. Thus, the entire length of the optical system can be shortened while securing a wide angle of view.

The object distance is different between the normal observation and the magnified observation. In the observation of an object, it is preferable that a clear image be formed even if the object distance changes. For this reason, at least one lens group needs to be moved.

The number of lens groups to be moved is preferably small. If the number of lens groups to be moved is one, the driving mechanism can be simplified. As described above, in the basic structure, the second lens group moves upon focusing. Since the number of lens groups to be moved is one, the focusing mechanism can be simplified. The first lens group and the third lens group are preferably fixed all the time.

When the object distance changes, the astigmatic difference fluctuates in the first lens group. In order to form a clear image, it is necessary to suppress the variation of the astigmatic difference. As described above, the second lens group moves when the object distance changes. The second lens group moves, and thus the astigmatic difference also fluctuates in the second lens group.

In the basic structure, the astigmatic difference in the second lens group is generated in a direction opposite to the direction of variation of the astigmatic difference in the first lens group. The amount of variation of the astigmatic difference in the second lens group is substantially the same as the amount of variation of the astigmatic difference in the first lens group. Therefore, the variation of the astigmatic difference in the first lens group can be cancelled out by the astigmatic difference in the second lens group.

In this manner, variation in astigmatic difference can be suppressed by the first lens group and the second lens group. However, in order to form a clear image, it is preferable to suppress variations in aberrations other than astigmatic aberration, for example, variations in spherical aberration, variations in coma aberration, and variations in chromatic aberration.

In addition, variation in spherical aberration, variation in coma aberration, and variation in chromatic aberration easily occur in the first lens group and the second lens group. In the basic configuration, variations of these aberrations can be reduced individually. As a result, variations in aberrations can be suppressed in the entire optical system.

In order to form a clear image, it is important to reduce the aberration amount while suppressing the fluctuation of aberration. In order to reduce the amount of aberration, it is sufficient to suppress the amount of aberration generated or to correct the aberration well in the entire optical system.

Spherical aberration, coma aberration, and chromatic aberration cannot be corrected well only by the first lens group and the second lens group. These aberrations can be corrected well by the third lens group.

The third lens group has a front group and a rear group arranged from the object side to the image side. The front group is composed of positive refractive power cemented lenses or positive refractive power einzel lenses, and the rear group is composed of positive refractive power cemented lenses.

At the position of the front group, the height of the light on the axis is high. Therefore, spherical aberration can be corrected well by the front group. At the rear group position, the height of the off-axis ray is high. Therefore, coma aberration and astigmatism can be corrected well by the rear group. Further, since the rear group includes the cemented lens, chromatic aberration of magnification can be corrected satisfactorily by the cemented lens.

The front group consists of cemented lenses, or of singlet lenses. Spherical aberration can be corrected well regardless of which of the cemented lens and the einzel lens is used. When the cemented lens is used for the front group, not only spherical aberration but also axial chromatic aberration can be corrected well.

In this manner, in the third lens group, the on-axis aberration can be corrected mainly well by the front group, and the off-axis aberration can be corrected mainly well by the rear group. In addition, the entire length of the optical system can be shortened while securing a wide focusing range and a sufficient back focus.

A cemented lens is arranged for the anterior group or a single lens. A cemented lens is arranged for the posterior group. This can reduce the number of lenses constituting the third lens group.

In the optical system at the time of normal observation, coma aberration is largely generated in the second lens group. On the other hand, coma aberration is also generated in the third lens group. Coma aberration in the third lens group is generated in a direction opposite to a generation direction of coma aberration in the second lens group.

The amount of generation of coma aberration in the second lens group is larger than the amount of generation of coma aberration in the third lens group. Therefore, coma aberration generated by the second lens group cannot be corrected only by the third lens group.

Coma aberration is also generated in the first lens group. Coma aberration in the first lens group is generated in a direction opposite to a generation direction of coma aberration in the second lens group. That is, coma aberration in the first lens group is generated in the same direction as the generation direction of coma aberration in the third lens group. Accordingly, coma aberration in the second lens group can be corrected by the first lens group and the third lens group.

A specific configuration example of the basic configuration will be described. Fig. 1 is a sectional view showing a specific configuration of an endoscopic objective optical system according to the present embodiment, (a) is a sectional view of configuration example 1, (b) is a sectional view of configuration example 2, and (c) is a sectional view of a prism.

In configuration example 1, the objective optical system for an endoscope is composed of the first lens group G1 having negative refractive power, the second lens group G2 having positive refractive power, and the third lens group G3 having positive refractive power, which are arranged in this order from the object side to the image side. An aperture stop S is disposed between the second lens group G2 and the third lens group G3.

The first lens group G1 includes a negative first lens L1, a negative second lens L2, and a positive third lens L3, which are arranged in this order from the object side to the image side. The second lens L2 and the third lens L3 are cemented to constitute a cemented lens CL 1.

The second lens group G2 has a positive fourth lens L4.

The third lens group G3 includes a positive fifth lens L5, a negative sixth lens L6, a positive seventh lens L7, and a negative eighth lens L8, which are arranged in order from the object side to the image side. The fifth lens L5 and the sixth lens L6 are cemented to constitute a cemented lens CL 2. The seventh lens L7 and the eighth lens L8 are cemented to constitute a cemented lens CL 3.

In configuration example 1, focusing is performed by moving the second lens group G2. Fig. 1 (a) shows a state of focusing on a distant object. In focusing on a close-distance object, the second lens group G2 moves toward the image side.

A first parallel flat plate F1 is disposed between the first lens L1 and the second lens L2. The first parallel flat plate F1 can be disposed at any position in the optical system. The second parallel flat plate F2 and the third parallel flat plate F3 are disposed on the image side of the eighth lens L8. The second parallel flat plate F2 is joined to the third parallel flat plate F3.

A prism P is disposed on the image side of the third parallel flat plate F3. A parallel plate CG is disposed on the image side of the prism P. The parallel plate CG is cover glass of the image pickup device. An imaging element (not shown) is disposed on the image side of the parallel plate CG. The image side surface of the parallel flat plate CG is an image surface I. The image pickup surface of the image pickup device coincides with the image side surface of the parallel plate CG.

As shown in fig. 1 (c), the prism P is formed of a first prism P1 and a second prism P2. The first prism P1 and the second prism P2 are bonded by, for example, a bonding agent.

A joint is formed at the junction of the first prism P1 and the second prism P2. The coupling surface is a polarization beam splitting surface. On the polarization beam splitting surface, for example, P-polarized light is reflected and S-polarized light is transmitted. The polarization direction of the P-polarized light is orthogonal to the polarization direction of the S-polarized light.

The light emitted from the objective optical system for an endoscope enters the first prism P1 along the optical axis AX. Of the light incident on the first prism P1, P-polarized light is reflected by the joining surface and travels along the optical path a. The S-polarized light passes through the joint surface and then travels along the optical path B. In this manner, two optical paths are formed by the prism P.

The light traveling on the optical path a passes through 1/4 wave plate WL and reaches the reflective element REF. The light having reached the reflecting element REF is reflected by the reflecting surface Rs 1. The reflected light passes through the 1/4 wave plate WL, the first prism P1, the junction surface, the second prism P2, and the parallel plate GC and reaches the image plane I.

The light traveling on the optical path B reaches the reflection surface Rs2 of the second prism P2. The light having reached the reflecting surface Rs2 is reflected by the reflecting surface Rs 2. The reflected light passes through the second prism P2 and the parallel plate GC and reaches the image plane I.

The image pickup surface of the image pickup device IM is located at the image plane I. The position of the optical path a and the position of the optical path B are formed in parallel on the imaging surface. Therefore, two optical images are formed in parallel on the imaging surface. In addition, the length of the optical path a is different from the length of the optical path B. Therefore, two optical images having different focal positions are formed on the imaging surface. The two optical images are captured by the image pickup device IM.

In configuration example 2, the objective optical system for an endoscope is composed of the first lens group G1 having negative refractive power, the second lens group G2 having positive refractive power, and the third lens group G3 having positive refractive power, which are arranged in this order from the object side to the image side. An aperture stop S is disposed between the second lens group G2 and the third lens group G3.

The first lens group G1 includes a negative first lens L1, a negative second lens L2, and a positive third lens L3, which are arranged in this order from the object side to the image side. The second lens L2 and the third lens L3 are cemented to constitute a cemented lens CL 1.

The second lens group G2 has a positive fourth lens L4.

The third lens group G3 has a positive fifth lens L5, a positive sixth lens L6, and a negative seventh lens L7 arranged in order from the object side to the image side. The sixth lens L6 and the seventh lens L7 are cemented to constitute a cemented lens CL 2.

In configuration example 2, focusing is performed by moving the second lens group G2. Fig. 1 (b) shows a state of focusing on a distant object. In focusing on a close-distance object, the second lens group G2 moves toward the image side.

In configuration examples 1 and 2, an aperture stop S is disposed between the second lens group G2 and the third lens group G3. By disposing the aperture stop S at this position, the height of the light beam passing through the second lens group G2 can be reduced. As a result, the outer diameter of the second lens group G2 can be reduced.

The second lens group G2 moves upon focusing. The second lens group G2 is moved so as to be able to focus regardless of the position of the object between the far distance and the near distance. Normal observation can be performed when focusing on a distant object, and magnified observation can be performed when focusing on a close object.

In order to move the second lens group G2 in the optical axis direction, a moving mechanism is required. As described above, since the second lens group G2 can be downsized, the moving mechanism can be easily disposed around the second lens group G2.

In addition, since the number of lens groups moved is one, the weight of the lens groups moved can also be small. Therefore, the load applied to the moving mechanism can be reduced. Further, the moving mechanism can be simplified.

The moving mechanism is, for example, an actuator. An actuator is connected to the lens frame for holding the second lens group G2, and a driving force is supplied to the lens frame by the actuator.

In configuration examples 1 and 2, the first parallel plate F1 is disposed in the first lens group G1. The first parallel plate F1 is an optical filter for cutting off a specific wavelength, for example, laser light of a YAG laser (light having a wavelength of 1060 nm), laser light of a semiconductor laser (light having a wavelength of 810 nm), or light having a wavelength in the near-infrared region.

Configuration examples 1 and 2 are, for example, objective optical systems for endoscopes having the following specifications.

Viewing angle: 140 to 165 degrees.

Back focal length: less than 4 times the focal length.

And (3) overall length: less than 12 times the focal length.

Aperture value: 4.2.

in configuration example 1, the objective optical system for an endoscope was constituted by 8 single lenses. In configuration example 2, the objective optical system for an endoscope was constituted by 7 single lenses. As described above, in configuration examples 1 and 2, since the optical system is configured by a small number of lenses, a compact objective optical system for an endoscope can be realized.

The endoscopic objective optical system according to the present embodiment has the basic configuration described above, and satisfies the following conditional expression (1).

0.45＜d 3t/f 32＜0.8 (1)

In this case, the amount of the solvent to be used,

d3t is the distance from the lens surface of the front group located at the most object side to the lens surface of the rear group located at the most object side,

f32 is the focal length of the back group.

By satisfying the conditional expression (1), the rear group can be appropriately separated from the front group. In this case, at the position of the rear group, the on-axis light flux can be separated from the off-axis light flux. Therefore, the on-axis aberration can be corrected mainly well by the front group, and the off-axis aberration can be corrected mainly well by the rear group. As a result, good imaging performance can be maintained by a small number of lenses.

If the value is lower than the lower limit value of the conditional expression (1), the rear group cannot be sufficiently separated from the front group. Therefore, off-axis aberrations cannot be corrected well by the rear group. In the case where the value exceeds the upper limit value of the conditional expression (1), the rear group is caused to be too far from the front group. Therefore, the total length of the third lens group becomes long, and the lens diameter of the third lens group becomes large.

The endoscopic objective optical system according to the present embodiment preferably satisfies the following conditional expression (2).

1.2＜f 31/f 32＜5.0 (2)

In this case, the amount of the solvent to be used,

f31 is the focal length of the front group,

f32 is the focal length of the back group.

By satisfying the conditional expression (2), the refractive power of the rear group can be made larger than that of the front group. When the refractive power of the rear group becomes large, the principal point position of the third lens group is located on the image side. As a result, the back focus can be made longer.

If the value is lower than the lower limit value of conditional expression (2), the refractive power of the front group becomes large. Therefore, the on-axis aberration cannot be completely corrected. In addition, when the refractive power of the front group becomes large, the principal point position of the third lens group is located on the object side. Therefore, a back focus of a sufficient length cannot be ensured.

When the value exceeds the upper limit value of the conditional expression (2), the refractive power of the rear group becomes large. Therefore, the off-axis aberration cannot be completely corrected. In particular, coma aberration generated by the third lens group becomes small. Therefore, even if the coma aberration generated with the first lens group is added together, the coma aberration generated with the second lens group cannot be cancelled out. Further, the total length of the third lens group becomes long.

It is preferable that the following conditional formula (2') is satisfied in place of the conditional formula (2).

1.23＜f 31/f 32＜5.0 (2’)

The endoscopic objective optical system according to the present embodiment preferably satisfies the following conditional expression (3).

0.28＜d 3p/f 32＜0.5 (3)

In this case, the amount of the solvent to be used,

d3p is the distance from the lens surface of the front group located at the most image side to the lens surface of the rear group located at the most object side,

f32 is the focal length of the back group.

By satisfying the conditional expression (3), the rear group can be appropriately separated from the front group. In this case, at the position of the rear group, the on-axis light flux can be separated from the off-axis light flux. Therefore, the on-axis aberration can be corrected mainly well by the front group, and the off-axis aberration can be corrected mainly well by the rear group. As a result, good imaging performance can be maintained by a small number of lenses.

If the value is lower than the lower limit value of conditional expression (3), the rear group cannot be sufficiently separated from the front group. Therefore, off-axis aberrations cannot be corrected well by the rear group. If the value exceeds the upper limit value of the conditional expression (3), the rear group is too far from the front group. Therefore, the total length of the third lens group becomes long, and the lens diameter of the third lens group becomes large.

Preferably, the following conditional formula (3') is satisfied in place of the conditional formula (3).

0.282＜d 3p/f 32＜0.5 (3’)

It is preferable that the following conditional expression (3 ") is satisfied in place of the conditional expression (3).

0.295＜d 3p/f 32＜0.5 (3”)

The endoscopic objective optical system according to the present embodiment preferably satisfies the following conditional expression (4).

2.3＜f 3/f＜3.2 (4)

In this case, the amount of the solvent to be used,

f3 is the focal length of the third lens group,

f is the focal length of the endoscopic objective optical system in normal observation.

In order to secure a sufficiently long back focus, the refractive power of the third lens group may be reduced or the power of the third lens group may be increased.

When the value is lower than the lower limit value of the conditional expression (4), the refractive power of the third lens group becomes large. Therefore, a sufficient back focus cannot be secured, and aberrations cannot be corrected well.

When the value exceeds the upper limit value of the conditional expression (4), the entire length of the optical system becomes long, and the outer diameter of the optical system becomes large. Therefore, when the objective optical system for an endoscope is mounted on the endoscope, the diameter of the insertion portion becomes large. As a result, the operability of the endoscope is deteriorated. Further, the resistance increases when the catheter is inserted into the body cavity.

In the endoscope objective optical system according to the present embodiment, it is preferable that the first lens group includes a negative lens and a cemented lens arranged from the object side, an image side surface of the negative lens is a surface recessed on the image side, an object side surface of the cemented lens is a surface recessed on the object side, the second lens group includes a positive lens, an image side surface of the positive lens is a surface recessed on the image side, and the second lens group moves from the object side to the image side when focusing from a distant object to a near object.

In the first lens group, the object side negative lens is disposed closest to the object side. The image-side surface of the object-side negative lens is preferably a surface that is concave on the image side. By doing so, coma aberration generated by the second lens group can be corrected well by the first lens group and the third lens group.

In the first lens group, an image-side cemented lens is disposed on the image side of the object-side negative lens. The object-side surface of the image-side cemented lens is preferably a surface recessed on the object side. In this way, the orientation of the surface is set to be opposite to the orientation of the surface in which the aperture stop is concentric. As a result, an astigmatic difference generated in the first lens group can be corrected. Further, the chromatic aberration of magnification can be corrected satisfactorily without increasing the outer diameter of the object side surface of the object side negative lens.

In addition, the second lens group is composed of positive lenses. The image side surface of the positive lens is preferably concave on the image side. When focusing from a distant object to a close object, the second lens group is moved from the object side to the image side.

In this way, even in the case where the object distance is a distance close to the optical system, the astigmatic difference generated in the first lens group can be corrected by the second lens group, and the variation amount of the coma aberration in the first lens group and the variation amount of the coma aberration in the second lens group can each be made small.

Further, not only the fluctuation amount of each coma aberration can be reduced, but also the coma aberrations can be offset from each other. As a result, coma aberration can be suppressed to be small in the entire optical system. In addition, the number of lenses used in the second lens group can be reduced.

The aperture stop is preferably disposed between the second lens group and the third lens group. More preferably, an aperture stop is disposed in the third lens group in the vicinity of the lens located most to the object side.

Hereinafter, embodiments of the objective optical system are described in detail based on the drawings. The present invention is not limited to the embodiment.

The lens cross-sectional views of the respective examples will be explained. (a) Is a sectional view in a normal observation state, and (b) is a sectional view in an enlarged observation state.

The first lens group is denoted by G1, the second lens group is denoted by G2, the third lens group is denoted by G3, the aperture stop is denoted by S, the prism is denoted by P, and the image plane (imaging plane) is denoted by I. Parallel plates are denoted F1, F2, F3, CG. The parallel plate CG is cover glass and is disposed between the prism P and the image plane I.

The parallel plate F1 is a filter for cutting a specific wavelength, for example, laser light of a YAG laser (light having a wavelength of 1060 nm), laser light of a semiconductor laser (light having a wavelength of 810 nm), or light having a wavelength in the near-infrared region. The parallel plate F2 and the parallel plate F3 are filters having a depolarization function.

Aberration diagrams of the respective examples will be explained. (a) The terms (b), (c), (d) and (e) are aberration diagrams in a normal observation state, respectively. (a) Spherical Aberration (SA), (b) Astigmatism (AS), (c) magnification chromatic aberration (CC), (d) distortion aberration (DT), and (e) coma aberration (CM).

(f) The aberration diagrams (g), (h), (i) and (j) are respectively aberration diagrams in an enlarged observation state. (f) Spherical Aberration (SA), (g) Astigmatism (AS), (h) magnification chromatic aberration (CC), (i) distortion aberration (DT), and (j) coma aberration (CM).

Coma is represented by lateral aberrations. The image height position is a position 0.8 times the maximum image height. For example, in the case where the maximum image height is 1.141mm, the image height position is 0.913 mm.

In each aberration diagram, the abscissa axis represents the aberration amount. Regarding spherical aberration, astigmatism, magnification aberration, and coma aberration, the unit of the aberration amount is mm. In addition, regarding distortion aberration, the unit of the aberration amount is%. Further, FNO is an aperture value, and FIY is an image height in mm (millimeters). The unit of the wavelength of the aberration curve is nm.

(example 1)

An endoscopic objective optical system according to example 1 will be described. The endoscopic objective optical system according to example 1 includes, in order from the object side to the image side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 whose object side is a plane, a biconcave negative lens L2, and a biconvex positive lens L3. Here, the cemented lens CL1 is formed by the double concave negative lens L2 and the double convex positive lens L3.

The second lens group G2 has a positive meniscus lens L4 with the convex surface facing the object side.

The third lens group G3 has a double convex positive lens L5, a negative meniscus lens L6 with the convex surface facing the image side, a double convex positive lens L7, and a negative meniscus lens L8 with the convex surface facing the image side. Here, the cemented lens CL2 is formed by the double convex positive lens L5 and the negative meniscus lens L6. The cemented lens CL3 is formed by a biconvex positive lens L7 and a negative meniscus lens L8.

The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

Upon focusing, the second lens group G2 moves. When focusing on a near object from a state of focusing on a far object, the second lens group G2 moves to the image side.

A parallel flat plate F1 is disposed on the image side of the plano-concave negative lens L1. On the image side of the third lens group G3, a parallel plate F2, a parallel plate F3, a prism P, and a cover glass CG are disposed.

The aspherical surface is provided on the object side surface of the biconvex positive lens L7.

(example 2)

An endoscopic objective optical system according to example 2 will be described. The endoscopic objective optical system according to example 2 includes, in order from the object side to the image side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 whose object side is a plane, a biconcave negative lens L2, and a biconvex positive lens L3. Here, the cemented lens CL1 is formed by the double concave negative lens L2 and the double convex positive lens L3.

The second lens group G2 has a positive meniscus lens L4 with the convex surface facing the object side.

The third lens group G3 has a double convex positive lens L5, a negative meniscus lens L6 with the convex surface facing the image side, a double convex positive lens L7, and a negative meniscus lens L8 with the convex surface facing the image side. Here, the cemented lens CL2 is formed by the double convex positive lens L5 and the negative meniscus lens L6. The cemented lens CL3 is formed by a biconvex positive lens L7 and a negative meniscus lens L8.

The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

Upon focusing, the second lens group G2 moves. When focusing on a near object from a state of focusing on a far object, the second lens group G2 moves to the image side.

A parallel flat plate F1 is disposed on the image side of the plano-concave negative lens L1. On the image side of the third lens group G3, a parallel plate F2, a parallel plate F3, a prism P, and a cover glass CG are disposed.

The aspherical surface is provided on the object side surface of the biconvex positive lens L7.

(example 3)

An endoscopic objective optical system according to example 3 will be described. The endoscopic objective optical system according to example 3 includes, in order from the object side to the image side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 whose object side is a plane, a biconcave negative lens L2, and a biconvex positive lens L3. Here, the cemented lens CL1 is formed by the double concave negative lens L2 and the double convex positive lens L3.

The second lens group G2 has a positive meniscus lens L4 with the convex surface facing the object side.

The third lens group G3 has a double convex positive lens L5, a negative meniscus lens L6 with the convex surface facing the image side, a positive meniscus lens L7 with the convex surface facing the image side, and a negative meniscus lens L8 with the convex surface facing the image side. Here, the cemented lens CL2 is formed by the double convex positive lens L5 and the negative meniscus lens L6. The cemented lens CL3 is formed by the positive meniscus lens L7 and the negative meniscus lens L8.

The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

Upon focusing, the second lens group G2 moves. When focusing on a near object from a state of focusing on a far object, the second lens group G2 moves to the image side.

A parallel flat plate F1 is disposed on the image side of the plano-concave negative lens L1. On the image side of the third lens group G3, a parallel plate F2, a parallel plate F3, a prism P, and a cover glass CG are disposed.

The aspheric surface is disposed on the object side of the positive meniscus lens L7.

(example 4)

An endoscopic objective optical system according to example 4 will be described. The endoscopic objective optical system according to example 4 includes, in order from the object side to the image side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 whose object side is a plane, a biconcave negative lens L2, and a biconvex positive lens L3. Here, the cemented lens CL1 is formed by the double concave negative lens L2 and the double convex positive lens L3.

The second lens group G2 has a positive meniscus lens L4 with the convex surface facing the object side.

The third lens group G3 has a double convex positive lens L5, a negative meniscus lens L6 with the convex surface facing the image side, a positive meniscus lens L7 with the convex surface facing the image side, and a negative meniscus lens L8 with the convex surface facing the image side. Here, the cemented lens CL2 is formed by the double convex positive lens L5 and the negative meniscus lens L6. The cemented lens CL3 is formed by the positive meniscus lens L7 and the negative meniscus lens L8.

The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

Upon focusing, the second lens group G2 moves. When focusing on a near object from a state of focusing on a far object, the second lens group G2 moves to the image side.

A parallel flat plate F1 is disposed on the image side of the plano-concave negative lens L1. On the image side of the third lens group G3, a parallel plate F2, a parallel plate F3, a prism P, and a cover glass CG are disposed.

The aspheric surface is disposed on the object side of the positive meniscus lens L7.

(example 5)

An endoscopic objective optical system according to example 5 will be described. The endoscopic objective optical system according to example 5 includes, in order from the object side to the image side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 whose object side is a plane, a biconcave negative lens L2, and a biconvex positive lens L3. Here, the cemented lens CL1 is formed by the double concave negative lens L2 and the double convex positive lens L3.

The second lens group G2 has a positive meniscus lens L4 with the convex surface facing the object side.

The third lens group G3 has a double convex positive lens L5, a negative meniscus lens L6 with the convex surface facing the image side, a positive meniscus lens L7 with the convex surface facing the image side, and a negative meniscus lens L8 with the convex surface facing the image side. Here, the cemented lens CL2 is formed by the double convex positive lens L5 and the negative meniscus lens L6. The cemented lens CL3 is formed by the positive meniscus lens L7 and the negative meniscus lens L8.

The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

Upon focusing, the second lens group G2 moves. When focusing on a near object from a state of focusing on a far object, the second lens group G2 moves to the image side.

A parallel flat plate F1 is disposed on the image side of the plano-concave negative lens L1. On the image side of the third lens group G3, a parallel plate F2, a parallel plate F3, a prism P, and a cover glass CG are disposed.

The aspheric surface is disposed on the object side of the positive meniscus lens L7.

(example 6)

An endoscopic objective optical system according to example 6 will be described. The endoscopic objective optical system according to example 6 includes, in order from the object side to the image side, the first lens group G1 having negative refractive power, the second lens group G2 having positive refractive power, and the third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 whose object side is a plane, a biconcave negative lens L2, and a biconvex positive lens L3. Here, the cemented lens CL1 is formed by the double concave negative lens L2 and the double convex positive lens L3.

The second lens group G2 has a positive meniscus lens L4 with the convex surface facing the object side.

The third lens group G3 has a biconvex positive lens L5, a biconvex positive lens L6, and a negative meniscus lens L7 with the convex surface facing the image side. Here, the cemented lens CL2 is formed by the double convex positive lens L6 and the negative meniscus lens L7.

The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

Upon focusing, the second lens group G2 moves. When focusing on a near object from a state of focusing on a far object, the second lens group G2 moves to the image side.

A parallel flat plate F1 is disposed on the image side of the plano-concave negative lens L1. On the image side of the third lens group G3, a parallel plate F2, a parallel plate F3, a prism P, and a cover glass CG are disposed.

The aspherical surfaces were provided on the object side surface of the biconvex positive lens L5 and the object side surface of the biconvex positive lens L6.

(example 7)

An endoscopic objective optical system according to example 7 will be described. The endoscopic objective optical system according to example 7 includes, in order from the object side to the image side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 whose object side is a plane, a biconcave negative lens L2, and a biconvex positive lens L3. Here, the cemented lens CL1 is formed by the double concave negative lens L2 and the double convex positive lens L3.

The second lens group G2 has a positive meniscus lens L4 with the convex surface facing the object side.

The third lens group G3 has a double convex positive lens L5, a negative meniscus lens L6 with the convex surface facing the image side, a positive meniscus lens L7 with the convex surface facing the image side, and a negative meniscus lens L8 with the convex surface facing the image side. Here, the cemented lens CL2 is formed by the double convex positive lens L5 and the negative meniscus lens L6. The cemented lens CL3 is formed by the positive meniscus lens L7 and the negative meniscus lens L8.

The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

Upon focusing, the second lens group G2 moves. When focusing on a near object from a state of focusing on a far object, the second lens group G2 moves to the image side.

A parallel flat plate F1 is disposed on the image side of the plano-concave negative lens L1. On the image side of the third lens group G3, a parallel plate F2, a parallel plate F3, a prism P, and a cover glass CG are disposed.

The aspheric surface is disposed on the object side of the positive meniscus lens L7.

(example 8)

An endoscopic objective optical system according to example 8 will be described. The endoscopic objective optical system according to example 8 includes, in order from the object side to the image side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 whose object side is a plane, a biconcave negative lens L2, and a biconvex positive lens L3. Here, the cemented lens CL1 is formed by the double concave negative lens L2 and the double convex positive lens L3.

The second lens group G2 has a positive meniscus lens L4 with the convex surface facing the object side.

The third lens group G3 has a biconvex positive lens L5, a biconvex positive lens L6, and a negative meniscus lens L7 with the convex surface facing the image side. Here, the cemented lens CL2 is formed by the double convex positive lens L6 and the negative meniscus lens L7.

The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

Upon focusing, the second lens group G2 moves. When focusing on a near object from a state of focusing on a far object, the second lens group G2 moves to the image side.

A parallel flat plate F1 is disposed on the image side of the plano-concave negative lens L1. On the image side of the third lens group G3, a parallel plate F2, a parallel plate F3, a prism P, and a cover glass CG are disposed.

The aspherical surfaces were provided on the object side surface of the biconvex positive lens L5 and the object side surface of the biconvex positive lens L6.

Next, numerical data of the above-described embodiments is shown. In the surface data, r is a curvature radius of each lens surface, d is an interval between each lens surface, ne is a refractive index of each lens with respect to e-line, vd is an abbe number of each lens, and the symbol is an aspherical surface. The aperture is an aperture stop.

In various data, OBJ is the object distance, f is the focal length for e-line, Fno is the aperture value, ω is the half view angle, and IH is the image height. The focal length and the aperture value are a normally observed focal length and a normally observed aperture value, respectively. In the close-up observation state, the magnified observation can be performed.

When the optical axis direction is z, the direction orthogonal to the optical axis is y, the conic coefficient is k, and the aspherical coefficients are a4, a6, A8, a10, and a12 …, the aspherical shape is expressed by the following expression.

z＝(y²/r)/[1+{1-(1+k)(y/r)²}^1/2]

+A 4y⁴+A 6y⁶+A 8y⁸+A 10y¹⁰+A 12y¹²+…

In the aspherical surface coefficient, "e-n" (n is an integer) means "10^-n”。

Numerical example 1

Unit mm

Surface data

Aspheric data

The 14 th side

k＝-34.9348

A4＝-1.6411e-02，A6＝1.8958e-03，A8＝-5.5499e-04

Various data

Numerical example 2

Unit mm

Surface data

Aspheric data

The 14 th side

k＝363.9601

A4＝-1.7143e-02，A6＝-2.7983e-04，A8＝6.2807e-04

Various data

Numerical example 3

Unit mm

Surface data

Aspheric data

The 14 th side

k＝129.6954

A4＝-1.3827e-02，A6＝3.0751e-04，A8＝1.4007e-04

Various data

Numerical example 4

Unit mm

Surface data

Aspheric data

The 14 th side

k＝303.7869

A4＝-1.3218e-02，A6＝2.5417e-05，A8＝2.5201e-04

Various data

Numerical value example 5

Unit mm

Surface data

Aspheric data

The 14 th side

k＝237.0071

A4＝-1.5143e-02，A6＝1.2981e-03，A8＝-3.8382e-04

Various data

Numerical value example 6

Unit mm

Surface data

Aspheric data

The 11 th plane

k＝-5.9414

The 13 th side

k＝341.6896

A4＝-1.4297e-02，A6＝7.3030e-04，A8＝7.3026e-04

Various data

Numerical value example 7

Unit mm

Surface data

Aspheric data

The 14 th side

k＝237.0218

A4＝-1.4900e-02，A6＝1.4311e-03，A8＝-6.0966e-04

Various data

Numerical example 8

Unit mm

Surface data

Aspheric data

The 11 th plane

k＝-58413

The 13 th side

k＝341.6894

A4＝-1.4541e-02，A6＝8.1408e-04，A8＝9.0620e-04

Various data

Next, the values of the conditional expressions in the examples are listed below.

While various embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and embodiments configured by appropriately combining the structures of these embodiments without departing from the scope of the present invention also belong to the scope of the present invention.

(attached note)

Further, an invention of the following structure was derived based on these embodiments.

(attaching note item)

The first lens group is composed of a negative lens and a cemented lens arranged from the object side,

the image side surface of the negative lens is a concave surface at the image side,

the object side that engages the lens is the side that is concave under the object side,

the second lens group is composed of a positive lens,

the image side surface of the positive lens is a concave surface on the image side,

upon focusing from a distant object to a close object, the second lens group is moved from the object side to the image side.

Industrial applicability

The present invention is useful for an endoscope objective optical system having a sufficient back focus, having an optical system with a short overall length, and correcting various aberrations satisfactorily.

Description of the reference numerals

G1: a first lens group; g2: a second lens group; g3: a third lens group; L1-L8: a lens; CL1, CL2, CL 3: bonding a lens; s: an aperture stop; i: an image plane; f1, F2, F3, CG: parallel plates; AX: an optical axis; p, P1, P2: a prism; A. b: an optical path; WL: 1/4 a wave plate; REF: a reflective element; rs1, Rs 2: a reflective surface; IM: an image pickup element.