JP2019148675A - Endoscope objective optical system - Google Patents

Abstract

To provide an endoscope objective optical system which is less susceptible to a manufacturing error, has a wide angle and small size and is well corrected for aberrations.SOLUTION: The endoscope objective optical system comprises, in order from the object side, a first lens group having positive refractive power, an aperture stop and a second lens group having positive refractive power. The first lens group includes a negative first lens and a positive second lens each having a flat surface on the object side. The second lens group includes a cemented lens composed of a positive third lens and a negative fourth lens and satisfies the following conditional expression (1). -6.0<f1×f2/f<-2.7 (1) where f1 is focal distance of the first lens, f2 is focal distance of the second lens, and f is focal distance of the entire endoscope objective optical system.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an endoscope objective optical system.

  In the field of medical endoscopes, there is a demand for ensuring minimally invasiveness and improving diagnostic accuracy. In order to realize these demands, it is necessary to reduce the diameter of the insertion part at the distal end of the endoscope, observe at a wide angle, and acquire a high-quality image. For this purpose, it is necessary to mount a small and multi-pixel imaging device on the insertion portion. For example, a CCD or CMOS is used as the image sensor.

  A small and multi-pixel image sensor has a smaller pixel pitch than a conventional image sensor. Therefore, when a small and multi-pixel image sensor is used, resolution is degraded due to the influence of diffraction. In order to avoid resolution degradation, a bright optical system, that is, an optical system having a small F number is required. In an optical system with a small F number, it is particularly necessary to correct field curvature well.

  In the production of the optical system, adjustment is performed after the optical system is assembled. In the adjustment, the lens or the lens group is moved to improve the imaging performance (hereinafter referred to as “peripheral performance”) in the peripheral portion of the field of view. Even if the peripheral performance is improved by adjustment, if the adjusted manufacturing error (hereinafter referred to as “manufacturing error”) is large, the peripheral performance deteriorates. An example of the manufacturing error is shrinkage of the adhesive layer. The shrinkage of the adhesive layer occurs when the adhesive is cured.

  In addition, it is desirable that a lesioned part can be easily found by observation with an endoscope. Therefore, a wide-angle optical system is used as the objective optical system for the endoscope. In general, in an objective optical system for an endoscope, barrel distortion is generated in order to obtain a wide angle of view.

  In an optical system having a barrel-shaped distortion, the magnification at the center of the field of view is made larger than the magnification at the periphery of the field of view. As a result, an image in the peripheral part of the visual field (hereinafter referred to as “peripheral image”) is distorted.

  Therefore, an optical system having a barrel-shaped distortion is easily affected by manufacturing errors. For example, when the lens position or the lens group position shifts in the optical axis direction due to a manufacturing error, the optical magnification changes. In this case, since the distortion of the peripheral image changes greatly, the angle of view changes greatly. Thus, in an optical system having a barrel-shaped distortion, the angle of view is likely to change. In particular, in a wide-angle objective optical system having an angle of view of about 160 °, the distortion of the peripheral image is further increased. Therefore, the change in the angle of view due to manufacturing errors tends to be large.

  For this reason, in an endoscope objective optical system that is small and has a high resolution, it is necessary to suppress a change in the angle of view due to a manufacturing error and a deterioration in peripheral performance due to the manufacturing error.

  For example, Patent Documents 1 to 10 disclose general objective optical systems. These objective optical systems are optical systems with a small number of lenses. In these objective optical systems, 4 to 6 lenses are used.

JP 2017-26897 A JP 2009-258659 A JP 2001-154100 A JP 2004-258611 A JP 2006-276777 A WO2016 / 190184 JP 2004-344230 A WO2016 / 208367 Publication WO2013 / 077139 JP-A-4-275514

  Patent Document 1 discloses an objective optical system having an F number of 1.6 or less and an angle of view of about 70 °. This objective optical system is compatible with a multi-pixel imaging device. However, since the angle of view is narrow, it is not suitable as an objective optical system for an endoscope.

  Patent Document 2 discloses an endoscope objective optical system having an angle of view of about 100 °. Since this endoscope objective optical system has a narrow angle of view, it is not suitable for easy detection of a lesion.

  Patent Documents 3 and 4 disclose an endoscope objective optical system having an angle of view of about 130 to 140 °. In these endoscope objective optical systems, an aperture stop and a positive lens are arranged on the image side of the negative lens. For this reason, the height of the light beam tends to be higher at a position closer to the image side than the aperture stop. Therefore, these endoscope objective optical systems are not suitable for miniaturization.

  Patent Documents 5, 6, and 7 disclose an endoscope objective optical system having an angle of view of about 130 to 140 °. These endoscope objective optical systems have a negative front group, an aperture stop, and a positive rear group. In these endoscope objective optical systems, the refractive power of the front group and the refractive power of the rear group are asymmetric with respect to the aperture stop. For this reason, it cannot be said that various aberrations are sufficiently corrected.

  Patent Document 6 discloses an endoscope objective optical system in which a field angle adjustment width is ensured. Using this angle of view adjustment range, variations in the angle of view due to manufacturing errors are corrected. However, the refractive power of the negative first lens and the second lens is not specified. Furthermore, no device for reducing the influence of manufacturing errors is disclosed.

  In this endoscope objective optical system, when the relative position of the first lens and the second lens changes due to a manufacturing error, the angle of view changes and the peripheral performance deteriorates. However, the suppression of the change in the angle of view due to the manufacturing error and the suppression of the deterioration of the peripheral performance due to the manufacturing error are not considered.

  Patent Document 8 discloses an endoscope objective optical system having an angle of view of about 130 to 140 °. In this endoscope objective optical system, the refractive power of the front group and the refractive power of the rear group are symmetric with respect to the aperture stop. However, in order to reduce the diameter of the first lens, the refractive power of the first lens is relatively increased. Therefore, it is easily affected by manufacturing errors. However, the reduction of the influence of manufacturing errors is not fully considered. Therefore, it cannot be applied as it is to a wide-angle objective optical system, for example, an objective optical system having an angle of view of 140 ° or more.

  Patent Documents 9 and 10 disclose an endoscope objective optical system having an angle of view of about 160 °. Also in these endoscope objective optical systems, in order to reduce the diameter of the first lens, the refractive power of the first lens is relatively increased. Therefore, it is easily affected by manufacturing errors. However, the reduction of the influence of manufacturing errors is not fully considered.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide an endoscope objective optical system that is not easily affected by manufacturing errors, has a wide angle, is small, and has various aberrations corrected satisfactorily. And

In order to solve the above-described problems and achieve the object, an endoscope objective optical system according to at least some embodiments of the present invention includes:
In order from the object side, the first lens group having a positive refractive power, an aperture stop, and a second lens group having a positive refractive power,
The first lens group includes a negative first lens having a flat object side and a positive second lens,
The second lens group includes a cemented lens of a positive third lens and a negative fourth lens,
The following conditional expression (1) is satisfied.
−6.0 <f1 × f2 / f ² <−2.7 (1)
However,
f1 is the focal length of the first lens,
f2 is the focal length of the second lens,
f is the focal length of the entire endoscope objective optical system,
It is.

  According to the present invention, it is possible to provide an endoscope objective optical system that is not easily affected by manufacturing errors, has a wide angle, is small, and various aberrations are well corrected.

It is a lens sectional view of the objective optical system for endoscopes of this embodiment. 1 is a lens cross-sectional view of an endoscope objective optical system according to Example 1. FIG. FIG. 4 is an aberration diagram of the endoscope objective optical system according to Example 1. 6 is a lens cross-sectional view of an endoscope objective optical system according to Example 2. FIG. FIG. 6 is an aberration diagram of the endoscope objective optical system according to Example 2. 10 is a lens cross-sectional view of an endoscope objective optical system according to Example 3. FIG. FIG. 10 is an aberration diagram of the endoscope objective optical system according to Example 3. 6 is a lens cross-sectional view of an endoscope objective optical system according to Example 4. FIG. FIG. 10 is an aberration diagram of the endoscope objective optical system according to Example 4. 10 is a lens cross-sectional view of an endoscope objective optical system according to Example 5. FIG. FIG. 10 is an aberration diagram of the endoscope objective optical system according to Example 5.

  Hereinafter, the reason and effect | action which took such a structure are demonstrated using drawing about the objective optical system for endoscopes which concerns on this embodiment. In addition, this invention is not limited by the following embodiment.

The endoscope objective optical system according to the present embodiment includes, in order from the object side, a first lens group having a positive refractive power, an aperture stop, and a second lens group having a positive refractive power. The first lens group includes a negative first lens having a plane on the object side and a positive second lens. The second lens group includes a cemented lens of a positive third lens and a negative fourth lens. The following conditional expression (1) is satisfied.
−6.0 <f1 × f2 / f ² <−2.7 (1)
However,
f1 is the focal length of the first lens,
f2 is the focal length of the second lens,
f is the focal length of the entire endoscope objective optical system,
It is.

  In general, a retro-focus type optical system is used for a wide-angle objective optical system. In a retrofocus type optical system, a lens group having a negative refractive power is disposed on the object side, and a lens group having a positive refractive power is disposed on the image side. In contrast, the endoscope objective optical system according to the present embodiment includes, in order from the object side, a first lens group having a positive refractive power, an aperture stop, and a second lens group having a positive refractive power. It consists of.

  In the endoscope objective optical system according to the present embodiment, the first lens group includes a negative first lens and a positive second lens, and the second lens group includes a positive third lens and a negative lens. And a fourth lens. Therefore, the first lens is regarded as a lens group located on the object side, and the second lens, the third lens, and the fourth lens are regarded as a lens group located on the image side.

  In this case, the refractive power of the lens group located on the object side is negative. On the other hand, the refractive power of the lens group located on the image side is positive. Therefore, also in the endoscope objective optical system of the present embodiment, the arrangement of refractive power is the same as that of the retrofocus type optical system. Thus, in the endoscope objective optical system of the present embodiment, a retrofocus type optical system is employed. Therefore, a wide angle of view can be secured.

  Further, from the object side, lenses are arranged in the order of a negative lens, a positive lens, an aperture stop, a positive lens, and a negative lens. Therefore, the arrangement of refractive power on the object side with respect to the aperture stop and the arrangement of refractive power on the image side with respect to the aperture stop are symmetrical with respect to the aperture stop. As a result, various aberrations can be corrected satisfactorily.

  In the first lens, the object side surface is a flat surface. In observation with an endoscope, dirt, blood, and the like adhere to the lens surface on the object side of the first lens. In this case, the lens surface is cleaned by ejecting water from a nozzle provided at the distal end of the endoscope.

  When the shape of the lens surface on the object side is a convex shape, dirt is difficult to remove. In addition, when the lens surface on the object side is concave, water drainage such as accumulation of water becomes worse. Further, when the lens surface on the object side of the first lens has a convex shape, scratches and cracks due to impact are likely to occur.

  Therefore, the first lens is a plano-concave lens, and the first lens is disposed so that the plane faces the object side. By doing so, water breakage during observation can be improved, and lens cracking due to impact can be reduced.

  The third lens and the fourth lens are cemented. Since the cemented lens includes a negative lens and a positive lens, chromatic aberration can be corrected well. The cemented lens is disposed on the most image side. At this position, the on-axis ray and the off-axis ray are separated. Moreover, at this position, the height of the off-axis ray is high. Therefore, in particular, the lateral chromatic aberration can be corrected satisfactorily.

  The endoscope objective optical system of the present embodiment satisfies the conditional expression (1). Conditional expression (1) is a conditional expression for reducing the influence of manufacturing errors.

  By satisfying conditional expression (1), it is possible to suppress changes in the angle of view due to manufacturing errors and deterioration of peripheral performance due to manufacturing errors. Furthermore, the optical system can be reduced in size.

  When the value exceeds the upper limit value of conditional expression (1), the refractive power of the first lens is increased or the refractive power of the second lens is increased. In this case, the angle of view changes due to manufacturing errors, and the deterioration of peripheral performance due to manufacturing errors increases. Therefore, high assembly accuracy is required. Therefore, it is not preferable that the value exceeds the upper limit value of conditional expression (1).

  When the value is lower than the lower limit value of conditional expression (1), the refractive power of the first lens becomes small. In this case, the light ray height at the first lens is increased. This makes it difficult to reduce the size of the optical system.

The lower limit value of the conditional expression (1) may be the lower limit value of the conditional expression (1 ′) or the lower limit value of the conditional expression (1 ″). The upper limit value of the conditional expression (1) is the conditional expression (1 ′ ) Or an upper limit value of conditional expression (1 ″).
−5.0 <f1 × f2 / f ² <−2.8 (1 ′)
−4.0 <f1 × f2 / f ² <−3.0 (1 ″)

  By satisfying the conditional expression (1 ′) or satisfying the conditional expression (1 ″), it is possible to better suppress the change in the angle of view due to the manufacturing error and the deterioration of the peripheral performance due to the manufacturing error. Can be further reduced in size.

  A specific configuration example of the endoscope objective optical system according to the present embodiment will be described. FIG. 1 is a cross-sectional view showing an endoscope objective optical system according to this embodiment.

  The endoscope objective optical system includes, in order from the object side to the image side, a first lens group G1 having a positive refractive power and a second lens group G2 having a positive refractive power. An aperture stop S is disposed between the first lens group G1 and the second lens group G2.

  The first lens group G1 includes a negative first lens L1 and a positive second lens L2 in order from the object side. The second lens group G2 includes a positive third lens L3 and a negative fourth lens L4. The third lens L3 and the fourth lens L4 are cemented to form a cemented lens CL1.

  A plane parallel plate F1 is disposed between the first lens L1 and the second lens L2. A plane parallel plate F2 is disposed between the second lens L2 and the cemented lens CL1. The plane parallel plate F1 and the plane parallel plate F2 are optical filters.

  The parallel flat plate F1 and the parallel flat plate F2 can cut light of a specific wavelength. The light with a specific wavelength is, for example, a YAG laser having a wavelength of 1060 nm or a semiconductor laser having a wavelength of 810 nm.

  The plane parallel plate F1 or the plane parallel plate F2 may be disposed on the image side of the cemented lens CL1.

  A cover glass CG1 and a cover glass CG2 are disposed on the image side of the cemented lens CL1. The cover glass CG1 and the cover glass CG2 are joined. An image sensor (not shown) is disposed on the image side of the cover glass CG2.

  The cover glass CG2 is provided on the imaging surface of the imaging element. The cover glass CG2 can prevent scratches and the like from entering the imaging surface.

It is desirable that the endoscope objective optical system of the present embodiment satisfies the following conditional expression (2).
1.0 <f34 / f2 <2.0 (2)
However,
f34 is the focal length of the cemented lens,
f2 is the focal length of the second lens,
It is.

  In a retrofocus type optical system, a lens group having a negative refractive power is disposed on the object side, and a lens group having a positive refractive power is disposed on the image side. Conditional expression (2) is a conditional expression regarding a lens group having a positive refractive power.

  In the endoscope objective optical system according to this embodiment, the second lens and the cemented lens constitute a lens group having a positive refractive power. Therefore, the conditional expression (2) is a conditional expression regarding the second lens and the cemented lens.

  When the value exceeds the upper limit value of the conditional expression (2), it becomes difficult to correct coma and astigmatism. Furthermore, the change in the angle of view due to manufacturing errors in the second lens becomes large. Therefore, high assembly accuracy is required. Therefore, it is not preferable that the value exceeds the upper limit value of conditional expression (2).

  When the value is lower than the lower limit value of the conditional expression (2), the curvature of field becomes under, and the deterioration of the peripheral performance due to the manufacturing error becomes large. Therefore, it is not preferable that the value falls below the lower limit value of conditional expression (2).

The lower limit value of the conditional expression (2) may be the lower limit value of the following conditional expression (2 ′) or the lower limit value of the conditional expression (2 ″). The upper limit value of the conditional expression (2) is the conditional expression ( The upper limit value of 2 ′) or the upper limit value of conditional expression (2 ″) may be used.
1.4 <f34 / f2 <1.6 (2 ′)
1.43 <f34 / f2 <1.5 (2 ")

  Satisfying the conditional expression (2 ′) or satisfying the expression (2 ″) not only enhances the coma correction effect, the astigmatism correction effect, and the field curvature correction effect, It is possible to more effectively suppress the change in the angle of view due to the manufacturing error in the two lenses.

It is desirable that the endoscope objective optical system of the present embodiment satisfies the following conditional expression (3).
1.0 <f24 / f <5.0 (3)
However,
f24 is a focal length of a predetermined optical system,
f is the focal length of the entire endoscope objective optical system,
The predetermined optical system includes an optical system including a second lens, a third lens, and a fourth lens,
It is.

  As described above, when the second lens, the third lens, and the fourth lens are regarded as lens groups located on the image side, these lens groups correspond to positive lens groups of a retrofocus type optical system. Conditional expression (3) is a conditional expression regarding the refractive power of the positive lens group of the retrofocus type optical system.

  When the value exceeds the upper limit value of the conditional expression (3), the positive refractive power in the predetermined optical system becomes small. In this case, the curvature of field and coma generated by the first lens cannot be corrected satisfactorily by a predetermined optical system. As a result, the deterioration of the peripheral performance due to manufacturing errors becomes large.

  When the value is below the lower limit of the conditional expression (3), correction with a predetermined optical system is excessive for field curvature and coma aberration generated in the first lens. As a result, the degradation of peripheral performance due to manufacturing errors increases.

The lower limit value of the conditional expression (3) may be the lower limit value of the following conditional expression (3 ′) or the lower limit value of the conditional expression (3 ″). The upper limit value of the conditional expression (3) is the conditional expression ( The upper limit value of 3 ′) or the upper limit value of conditional expression (3 ″) may be used.
1.5 <f24 / f <4.0 (3 ′)
2.0 <f24 / f <3.2 (3 ")

  When the conditional expression (3 ′) is satisfied or the conditional expression (3 ″) is satisfied, correction of curvature of field and correction of coma are further facilitated.

It is desirable that the endoscope objective optical system of the present embodiment satisfies the following conditional expression (4).
0.2 <(R3 + R4) / (R3-R4) <0.8 (4)
However,
R3 is the radius of curvature of the object side surface of the second lens,
R4 is the radius of curvature of the image side surface of the second lens,
It is.

  Conditional expression (4) is a conditional expression related to the shape of the second lens.

  When the value exceeds the upper limit value of the conditional expression (4), the spherical aberration falls to the under side and the coma aberration is inclined. Therefore, it is not preferable that the value exceeds the upper limit value of conditional expression (4).

  When the value is below the lower limit value of conditional expression (4), the spherical aberration falls to the over side and the coma and astigmatism cannot be corrected sufficiently. Therefore, it is not preferable that the value falls below the lower limit value of conditional expression (4).

The lower limit value of the conditional expression (4) may be the lower limit value of the following conditional expression (4 ′) or the lower limit value of the conditional expression (4 ″). The upper limit value of the conditional expression (4) is the conditional expression ( The upper limit value of 4 ′) or the upper limit value of conditional expression (4 ″) may be used.
0.50 <(R3 + R4) / (R3-R4) <0.75 (4 ′)
0.56 <(R3 + R4) / (R3-R4) <0.60 (4 ")

  When the conditional expression (4 ′) is satisfied or the conditional expression (4 ″) is satisfied, correction of spherical aberration, correction of coma aberration, and correction of astigmatism are further facilitated.

It is desirable that the endoscope objective optical system of the present embodiment satisfies the following conditional expression (5).
| PS × f | <0.05 (5)
However,
PS is the Petzval sum of the entire endoscope objective optical system,
f is the focal length of the entire endoscope objective optical system,
It is.

  Conditional expression (5) is a conditional expression related to the Petzval sum of the endoscope objective optical system.

  When the value exceeds the upper limit value of conditional expression (5), correction of field curvature is insufficient. For this reason, even if no manufacturing error occurs, peripheral performance deteriorates. Further, when a manufacturing error occurs, the deterioration of the peripheral performance due to the manufacturing error is likely to increase. Therefore, it is not preferable that the value exceeds the upper limit value of conditional expression (5).

  In particular, when the value is on the plus side, the image plane falls to the under side. On the other hand, when the value becomes negative, the image plane is inclined to the over side, and the difference between the meridional image plane and the sagittal image plane, that is, the astigmatic difference is increased.

The lower limit value of the conditional expression (5) may be the lower limit value of the following conditional expression (5 ′) or the lower limit value of the conditional expression (5 ″). The upper limit value of the conditional expression (5) is the conditional expression ( The upper limit value of 5 ′) or the upper limit value of conditional expression (5 ″) may be used.
| PS × f | <0.04 (5 ′)
| PS × f | <0.03 (5 ")

  Satisfying the conditional expression (5 ′) or satisfying the conditional expression (5 ″) not only further enhances the effect of correcting the curvature of field, but also more effectively degrades the peripheral performance due to manufacturing errors. Can be suppressed.

It is desirable that the endoscope objective optical system of the present embodiment satisfies the following conditional expression (6).
0.2 <Σda / df <0.7 (6)
However,
Σda is the sum of air intervals at a predetermined interval,
df is a predetermined interval,
The predetermined interval is an interval from the object side surface of the first lens to the aperture stop,
It is.

  Conditional expression (6) is a conditional expression that prescribes the ratio of the air interval to the predetermined interval. The predetermined interval is an interval from the object side surface of the first lens to the aperture stop. When a plane parallel plate is disposed between the object side surface of the first lens and the aperture stop, the thickness of the plane parallel plate is replaced with an air conversion length. This replacement is performed in both the calculation of Σda and the calculation of df.

  In general, in an objective optical system having an angle of view of about 160 °, the angle of view changes greatly due to lens dimensional errors, lens frame dimensional errors, and assembly errors. For this reason, in the production of an optical system, a step of adjusting the angle of view may be provided after assembly. In the step of adjusting the angle of view, the angle of view is set within an allowable range.

  In the step of adjusting the angle of view, the lens is moved in the optical axis direction. Therefore, a lens having a large change in the angle of view with respect to the movement amount is selected as a lens to be adjusted. The target of adjustment need not be a lens. For example, a lens group may be an adjustment target.

  In adjusting the angle of view, it is necessary to secure a sufficient space in which the lens can move. In the endoscope objective optical system of the present embodiment, the first lens and the second lens are suitable for the lens to be adjusted. Therefore, it is desirable to satisfy conditional expression (6).

  When the value exceeds the upper limit value of conditional expression (6), the ratio of the air interval to the predetermined interval increases. In this case, the thickness of the first lens is reduced, or the thickness of the second lens is reduced. Therefore, for example, the risk of lens cracking due to impact from the outside increases. Furthermore, since the lens border cannot be secured sufficiently, workability in lens processing and optical system assembly deteriorates. Therefore, it is not preferable that the value exceeds the upper limit value of conditional expression (6).

  When the value is lower than the lower limit value of the conditional expression (6), the ratio of the air interval to the predetermined interval becomes low. In this case, the distance between the first lens and the second lens or the distance between the second lens and the aperture stop becomes narrow. Therefore, a sufficient space for adjusting the angle of view cannot be secured. Therefore, it is not preferable that the value falls below the lower limit value of conditional expression (6).

The lower limit value of conditional expression (6) may be the lower limit value of conditional expression (6 ′) or the lower limit value of conditional expression (6 ″). The upper limit value of conditional expression (6) is the conditional expression (6 ′). ) Or an upper limit value of conditional expression (6 ″).
0.30 <Σda / df <0.60 (6 ′)
0.40 <Σda / df <0.55 (6 ″)

  By satisfying conditional expression (6 ′) or satisfying conditional expression (6 ″), it is possible to prevent deterioration of workability in lens processing and deterioration of workability in assembly of an optical system. It is easier to secure the space necessary for adjusting the corners.

It is desirable that the endoscope objective optical system of the present embodiment satisfies the following conditional expression (7).
3.6 <R3 / f <20 (7)
However,
R3 is the radius of curvature of the object side surface of the second lens,
f is the focal length of the entire endoscope objective optical system,
It is.

  Conditional expression (7) is a conditional expression that defines the shape of the object side surface of the second lens.

  When the value exceeds the upper limit value of the conditional expression (7), field curvature occurs and the image plane falls to the under side. Furthermore, coma and astigmatism cannot be corrected sufficiently. Therefore, it is not preferable that the value exceeds the upper limit value of conditional expression (7).

  When the value is lower than the lower limit value of the conditional expression (7), field curvature occurs, and in particular, the meridional image plane falls over. Therefore, the astigmatic difference is increased. Furthermore, axial chromatic aberration and lateral chromatic aberration are deteriorated. In this case, the g-line image is largely inclined toward the under side, and the C-line image is largely inclined toward the over side. Therefore, it is not preferable that the value falls below the lower limit value of conditional expression (7).

The lower limit value of the conditional expression (7) may be the lower limit value of the following conditional expression (7 ′) or the lower limit value of the conditional expression (7 ″). The upper limit value of the conditional expression (7) is the conditional expression ( The upper limit value of 7 ′) or the upper limit value of conditional expression (7 ″) may be used.
6.3 <R3 / f <15 (7 ′)
9.0 <R3 / f <12 (7 ")

  By satisfying conditional expression (7 ′) or satisfying conditional expression (7 ″), not only the curvature of field correction effect but also the axial chromatic aberration correction effect and the lateral chromatic aberration correction effect are further enhanced. It is done.

It is desirable that the endoscope objective optical system of the present embodiment satisfies the following conditional expression (8).
−5.0 <R4 / f <−2.0 (8)
However,
R4 is the radius of curvature of the image side surface of the second lens,
f is the focal length of the entire endoscope objective optical system,
Is

  Conditional expression (8) is a conditional expression that defines the shape of the image side surface of the second lens.

  When the value exceeds the upper limit value of conditional expression (8), the image surface falls to the under side and coma aberration cannot be corrected sufficiently. Furthermore, axial chromatic aberration and lateral chromatic aberration are deteriorated. In this case, the g-line image is largely inclined toward the under side, and the C-line image is largely inclined toward the over side. Therefore, it is not preferable that the value exceeds the upper limit value of conditional expression (8).

  When the value falls below the lower limit value of conditional expression (8), the spherical aberration and the curvature of field are overcorrected to the over side. Further, the g-line image is greatly inclined toward the over side. Therefore, it is not preferable that the value falls below the lower limit value of conditional expression (8).

The lower limit value of the conditional expression (8) may be the lower limit value of the following conditional expression (8 ′) or the lower limit value of the conditional expression (8 ″). The upper limit value of the conditional expression (8) is the conditional expression ( The upper limit value of 8 ′) or the upper limit value of conditional expression (8 ″) may be used.
−4.0 <R4 / f <−2.2 (8 ′)
−3.0 <R4 / f <−2.3 (8 ″)

  By satisfying conditional expression (8 ′) or satisfying conditional expression (8 ″), not only correction of curvature of field and correction of coma, but also correction of axial chromatic aberration and correction of lateral chromatic aberration can be performed. It becomes easier.

It is desirable that the endoscope objective optical system of the present embodiment satisfies the following conditional expression (9).
1.0 <f2 / f3 <2.0 (9)
However,
f2 is the focal length of the second lens,
f3 is the focal length of the third lens,
It is.

  Conditional expression (9) is a conditional expression regarding the refractive power of the second lens and the refractive power of the third lens.

  When the value exceeds the upper limit value of the conditional expression (9), the image plane is greatly inclined to the under side. Therefore, it is not preferable that the value exceeds the upper limit value of conditional expression (9).

  When the value is lower than the lower limit value of the conditional expression (9), field curvature occurs, and in particular, the meridional image plane is inclined to the over side. Therefore, the astigmatic difference is increased. Therefore, it is not preferable that the value falls below the lower limit value of conditional expression (9).

The lower limit value of the conditional expression (9) may be the lower limit value of the following conditional expression (9 ′) or the lower limit value of the conditional expression (9 ″). The upper limit value of the conditional expression (9) is the conditional expression ( 9 ′) or the upper limit value of conditional expression (9 ″).
1.4 <f2 / f3 <1.9 (9 ′)
1.59 <f2 / f3 <1.8 (9 ")

  By satisfying conditional expression (9 ′) or satisfying conditional expression (9 ″), it is possible to more effectively suppress the difference between the meridional image plane and the sagittal image plane, that is, the generation of astigmatism.

  Note that the endoscope objective optical system described above may simultaneously satisfy a plurality of configurations. This makes it possible to obtain a more preferable configuration in terms of performance or manufacturing. Moreover, the combination of a preferable structure is arbitrary. For each conditional expression, only the upper limit value or lower limit value of the numerical range of the more limited conditional expression may be limited.

  Hereinafter, embodiments of an endoscope objective optical system will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

  2, FIG. 4, FIG. 6, FIG. 8, and FIG. 10 are lens cross-sectional views of the objective optical system for endoscopes of Examples 1 to 5, respectively. 3, 5, 7, 9, and 11 are aberration diagrams of the endoscope objective optical system according to Examples 1 to 5, respectively.

  A lens cross-sectional view of each example will be described. The first lens group is indicated by G1, the second lens group is indicated by G2, the aperture stop is indicated by S, and the image plane (imaging plane) is indicated by I. The plane parallel plates are indicated by F1 and F2, and the cover glass is indicated by CG1 and CG2.

  An aberration diagram of each example will be described. (A) shows spherical aberration (SA), (b) shows astigmatism (AS), (c) shows distortion (DT), and (d) shows lateral chromatic aberration (CC).

  In each aberration diagram, the horizontal axis represents the amount of aberration. For spherical aberration, astigmatism, and magnification aberration, the unit of aberration is mm. For distortion, the unit of aberration is%. Fno is the F number, IH is the image height, and the unit is mm (millimeter). The unit of the wavelength of the aberration curve is nm.

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

  The first lens group G1 includes a plano-concave negative lens L1 having a plane on the object side and a biconvex positive lens L2.

  The second lens group G1 includes a biconvex positive lens L3 and a negative meniscus lens L4 having a convex surface facing the image side. Here, the biconvex positive lens L3 and the negative meniscus lens L4 form a cemented lens.

  The aperture stop S is disposed between the first lens group G1 and the second lens group G2.

  A plane-parallel plate F1 is disposed between the plano-concave negative lens L1 and the biconvex positive lens L2. A plane parallel plate F2 is disposed between the biconvex positive lens L2 and the biconvex positive lens L3. A cover glass CG1 and a cover glass CG2 are disposed on the image side of the negative meniscus lens L4.

(Example 2)
An endoscope objective optical system according to Example 2 will be described. The endoscope objective optical system of Example 2 includes, in order from the object side to the image side, a first lens group G1 having a positive refractive power and a second lens group G2 having a positive refractive power.

  The first lens group G1 includes a plano-concave negative lens L1 having a plane on the object side and a biconvex positive lens L2.

  The second lens group G1 includes a biconvex positive lens L3 and a negative meniscus lens L4 having a convex surface facing the image side. Here, the biconvex positive lens L3 and the negative meniscus lens L4 form a cemented lens.

  The aperture stop S is disposed between the first lens group G1 and the second lens group G2.

  A plane-parallel plate F1 is disposed between the biconvex positive lens L2 and the biconvex positive lens L3. A cover glass CG1 and a cover glass CG2 are disposed on the image side of the negative meniscus lens L4.

(Example 3)
An endoscope objective optical system according to Example 3 will be described. The endoscope objective optical system of Example 3 includes, in order from the object side to the image side, a first lens group G1 having a positive refractive power and a second lens group G2 having a positive refractive power.

  The first lens group G1 includes a plano-concave negative lens L1 having a plane on the object side and a biconvex positive lens L2.

  The second lens group G1 includes a biconvex positive lens L3 and a negative meniscus lens L4 having a convex surface facing the image side. Here, the biconvex positive lens L3 and the negative meniscus lens L4 form a cemented lens.

  The aperture stop S is disposed between the first lens group G1 and the second lens group G2.

  A plane-parallel plate F1 is disposed between the plano-concave negative lens L1 and the biconvex positive lens L2. A cover glass CG1 and a cover glass CG2 are disposed on the image side of the negative meniscus lens L4.

(Example 4)
An endoscope objective optical system according to Example 4 will be described. The endoscope objective optical system according to Example 4 includes, in order from the object side to the image side, a first lens group G1 having a positive refractive power and a second lens group G2 having a positive refractive power.

  The first lens group G1 includes a plano-concave negative lens L1 having a plane on the object side and a biconvex positive lens L2.

  The second lens group G1 includes a biconvex positive lens L3 and a negative meniscus lens L4 having a convex surface facing the image side. Here, the biconvex positive lens L3 and the negative meniscus lens L4 form a cemented lens.

  The aperture stop S is disposed between the first lens group G1 and the second lens group G2.

  A plane-parallel plate F1 is disposed between the biconvex positive lens L2 and the biconvex positive lens L3. A cover glass CG1 and a cover glass CG2 are disposed on the image side of the negative meniscus lens L4.

(Example 5)
An endoscope objective optical system according to Example 5 will be described. The endoscope objective optical system of Example 5 includes, in order from the object side to the image side, a first lens group G1 having a positive refractive power and a second lens group G2 having a positive refractive power.

  The first lens group G1 includes a plano-concave negative lens L1 having a plane on the object side and a biconvex positive lens L2.

  The second lens group G1 includes a biconvex positive lens L3 and a negative meniscus lens L4 having a convex surface facing the image side. Here, the biconvex positive lens L3 and the negative meniscus lens L4 form a cemented lens.

  The aperture stop S is disposed between the first lens group G1 and the second lens group G2.

  A plane parallel plate F1, a cover glass CG1, and a cover glass CG2 are arranged on the image side of the negative meniscus lens L4.

  Below, the numerical data of each said Example are shown. In the surface data, r is the radius of curvature of each lens surface, d is the distance between the lens surfaces, ne is the refractive index of the e-line of each lens, and νd is the Abbe number of each lens. The stop is an aperture stop.

  In various data, f is a focal length at the e line, Fno is an F number, ω is a half angle of view, OBJ is an object point distance, and IH is an image height.

Numerical example 1
Unit mm

Surface data surface number rd ne νd
1 ∞ 0.3163 1.88815 40.76
2 1.1703 1.0280
3 ∞ 0.6326 1.51825 64.14
4 ∞ 0.1898
5 9.4274 2.1034 1.88815 40.76
6 -2.5953 0.0474
7 (Aperture) ∞ 0.0474
8 ∞ 1.2652 1.52300 65.13
9 ∞ 0.5377
10 3.4967 1.1703 1.75844 52.32
11 -1.5198 0.4745 1.97189 17.47
12 -4.4583 0.5029
13 ∞ 0.7908 1.51825 64.14
14 ∞ 0.0158 1.51500 64.00
15 ∞ 0.7908 1.61350 50.49
16 ∞ 0
Image plane ∞

Various data f 1.00
Fno 5.673
ω 80.6
OBJ 11.9
IH 1.031

Numerical example 2
Unit mm

Surface data surface number rd ne νd
1 ∞ 0.3163 1.88815 40.76
2 1.1703 1.5140
3 11.7142 2.3936 1.88815 40.76
4 -2.5035 0.0474
5 (Aperture) ∞ 0.0474
6 ∞ 1.2652 1.52300 65.13
7 ∞ 0.4342
8 3.4940 1.2652 1.75844 52.32
9 -1.4708 0.4744 1.97189 17.47
10 -4.7474 0.5232
11 ∞ 0.7907 1.51825 64.14
12 ∞ 0.0158 1.51500 64.00
13 ∞ 0.7907 1.61350 50.49
14 ∞ 0
Image plane ∞

Various data f 1.000
Fno 5.614
ω 80.5
OBJ 11.5
IH 1.031

Numerical Example 3
Unit mm

Surface data surface number rd ne νd
1 ∞ 0.3174 1.88815 40.76
2 1.2028 1.0033
3 ∞ 1.2694 1.52300 65.13
4 ∞ 0.2380
5 14.2474 1.6288 1.88815 40.76
6 -2.7887 0.0476
7 (Aperture) ∞ 1.3 170
8 4.4801 1.2694 1.75844 52.32
9 -1.4757 0.4760 1.97189 17.47
10 -3.7235 0.8341
11 ∞ 0.7934 1.51825 64.14
12 ∞ 0.0159 1.51500 64.00
13 ∞ 0.7934 1.61350 50.49
14 ∞ 0
Image plane ∞

f 1.000
Fno 4.585
ω 80.0
OBJ 12.1
IH 1.035

Numerical Example 4
Unit mm

Surface data surface number rd ne νd
1 ∞ 0.3894 1.88815 40.76
2 1.1058 0.8607
3 5.8392 2.9592 1.88815 40.76
4 -2.7292 0.0467
5 (Aperture) ∞ 0.0467
6 ∞ 0.9345 1.52300 65.13
7 ∞ 0.3426
8 3.1052 1.2897 1.82017 46.62
9 -1.4952 0.4672 1.97189 17.47
10 -4.9248 0.5221
11 ∞ 0.7787 1.51825 64.14
12 ∞ 0.0156 1.51500 64.00
13 ∞ 0.7787 1.61350 50.49
14 ∞ 0
(Image plane) ∞

Various data f 1.000
Fno 4.871
ω 80.5
OBJ 12.9
IH 1.015

Numerical Example 5
Unit mm

Surface data surface number rd ne νd
1 ∞ 0.3960 1.88815 40.76
2 1.1245 1.1475
3 7.2899 2.8509 1.88815 40.76
4 -2.6112 0.0475
5 (Aperture) ∞ 0.8394
6 3.6960 1.3116 1.82017 46.62
7 -1.5205 0.4752 1.97189 17.47
8 -5.8447 0.1267
9 ∞ 0.3168 1.52300 65.13
10 ∞ 0.2894
11 ∞ 0.7919 1.51825 64.14
12 ∞ 0.0158 1.51500 64.00
13 ∞ 0.7919 1.61350 50.49
14 ∞ 0
(Image plane) ∞

Various data f 1.000
Fno 4.885
ω 80.5
OBJ 12.5
IH 1.033

Next, the values of the conditional expressions in each example are listed below.
Conditional Example 1 Example 2 Example 3 Example 4 Example 5
(1) f1 × f2 / f ² -3.29 -3.32 -3.72 -3.11 -3.17
(2) f34 / f2 1.44 1.51 1.35 1.20 1.47
(3) f24 / f 2.26 2.20 2.33 2.14 2.05
(4) (R3 + R4) / (R3-R4) 0.57 0.65 0.67 0.36 0.47
(5) | PS × f | 0.023 0.012 -0.002 0.044 0.005
(6) Σda / df 0.41 0.37 0.52 0.21 0.27
(7) R3 / f 9.43 11.71 14.25 5.84 7.29
(8) R4 / f -2.60 -2.50 -2.79 2.73 -2.61
(9) f2 / f3 1.61 1.64 1.71 1.78 1.69

  In the endoscope objective optical system of each embodiment, a plane-parallel plate is disposed in the optical path. However, the plane parallel plate may not be disposed in the optical path. When the plane parallel plate is not arranged, the thickness of the plane parallel plate may be replaced with the air conversion length. Further, there may be a plurality of plane parallel plates arranged in the optical path.

  Although various embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and may be implemented by appropriately combining the configurations of these embodiments without departing from the spirit of the present invention. The form is also within the scope of the present invention.

  As described above, the present invention is useful for an endoscope objective optical system that is not easily affected by manufacturing errors, has a wide angle, is small, and has various aberrations corrected satisfactorily.

G1 First lens group G2 Second lens group L1 to L4 Lens CL1 Joint lens S Aperture stop I Image plane F1, F2 Parallel plane plate CG, CG2 Cover glass

Claims (9)

In order from the object side, the first lens group having a positive refractive power, an aperture stop, and a second lens group having a positive refractive power,
The first lens group includes a negative first lens having a plane on the object side and a positive second lens,
The second lens group includes a cemented lens of a positive third lens and a negative fourth lens,
An objective optical system for an endoscope which satisfies the following conditional expression (1).
−6.0 <f1 × f2 / f ² <−2.7 (1)
However,
f1 is the focal length of the first lens,
f2 is the focal length of the second lens,
f is a focal length of the entire endoscope objective optical system;
It is. The objective optical system for an endoscope according to claim 1, wherein the following conditional expression (2) is satisfied.
1.0 <f34 / f2 <2.0 (2)
However,
f34 is the focal length of the cemented lens,
f2 is the focal length of the second lens,
It is. The objective optical system for an endoscope according to claim 1, wherein the following conditional expression (3) is satisfied.
1.0 <f24 / f <5.0 (3)
However,
f24 is a focal length of a predetermined optical system,
f is a focal length of the entire endoscope objective optical system;
The predetermined optical system is an optical system including the second lens, the third lens, and the fourth lens,
It is. The objective optical system for an endoscope according to claim 1, wherein the following conditional expression (4) is satisfied.
0.2 <(R3 + R4) / (R3-R4) <0.8 (4)
However,
R3 is the radius of curvature of the object side surface of the second lens,
R4 is a radius of curvature of the image side surface of the second lens,
It is. The objective optical system for an endoscope according to claim 1, wherein the following conditional expression (5) is satisfied.
| PS × f | <0.05 (5)
However,
PS is the Petzval sum of the entire endoscope objective optical system,
f is a focal length of the entire endoscope objective optical system;
It is. The objective optical system for an endoscope according to claim 1, wherein the following conditional expression (6) is satisfied.
0.2 <Σda / df <0.7 (6)
However,
Σda is the sum of air intervals at a predetermined interval,
df is the predetermined interval,
The predetermined interval is an interval from the object side surface of the first lens to the aperture stop,
It is. The objective optical system for an endoscope according to claim 1, wherein the following conditional expression (7) is satisfied.
3.6 <R3 / f <20 (7)
However,
R3 is the radius of curvature of the object side surface of the second lens,
f is a focal length of the entire endoscope objective optical system;
It is. The objective optical system for an endoscope according to claim 1, wherein the following conditional expression (8) is satisfied.
−5.0 <R4 / f <−2.0 (8)
However,
R4 is a radius of curvature of the image side surface of the second lens,
f is a focal length of the entire endoscope objective optical system;
Is The objective optical system for an endoscope according to claim 1, wherein the following conditional expression (9) is satisfied.
1.0 <f2 / f3 <2.0 (9)
However,
f2 is the focal length of the second lens,
f3 is the focal length of the third lens,
It is.

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