Objective optical system, endoscope, and imaging device

Abstract

This objective optical system comprises, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. Focusing is performed as a result of the second lens group moving. The first lens group comprises two lenses, namely a first lens, which is a negative lens, and a second lens, which is a negative lens with a concave surface facing the image side. The second lens group comprises one positive meniscus lens with a convex surface facing the object side. The third lens group includes, in order from the object side, a single lens having positive refractive power, and a cemented lens formed from a positive lens and a negative lens. Provided that L1_Rr is the radius of curvature of the image-side surface of the first lens and L2_Rr is the radius of curvature of the image-side surface of the second lens, 0.01 < L1_Rr / L2_Rr < 0.95.

Description

Objective optical system, endoscope and imaging device

[0001] The present invention relates to an objective optical system, an endoscope, and an imaging device.

[0002] Endoscopes are widely used as medical devices that allow users, such as technicians and doctors, to perform examinations, treatments, and procedures while directly viewing images of lesions in a subject. Because the insertion section of an endoscope is inserted into the body from outside the subject's body, it is preferable that the overall length of the objective optical system provided in the insertion section be short and the diameter of the objective optical system be small. Furthermore, in order to improve the accuracy of examinations of lesions and the like, it is preferable that the objective optical system be capable of capturing images of a narrow range with high resolution. Previously, objective optical systems that are short in overall length, small in diameter, and high in resolution and can be installed in endoscopes have been proposed, and efforts have been made to improve the quality of the images acquired (see, for example, Patent Documents 1 to 5).

[0003] Japanese Patent No. 4819969 Japanese Patent No. 5930257 Japanese Patent No. 4819969 Japanese Patent Publication No. 2017-219783 International Publication No. 2020 / 217443

[0004] The objective optical systems disclosed in Patent Documents 1 to 5 are capable of observing an object at a predetermined magnification by focusing on a long-distance object point, and capable of observing the object at a magnified size by focusing on a close-distance object point. For example, in the objective optical systems disclosed in Patent Documents 1, 3, and 4, the first to third lens groups are arranged in order from the object side to the image side, and only the second lens group moves during focusing. In the objective optical system disclosed in Patent Document 2, a first lens with negative refractive power, a second meniscus lens, a third meniscus lens, a lens group with positive refractive power, and a cemented lens are arranged in order from the object side, and the third meniscus lens moves during focusing.

[0005] In conventional objective optical systems, including those disclosed in Patent Documents 1 to 5, the depth of field has become shallower as resolution has increased and image quality has improved when the optical system is incorporated into an imaging device, making it impossible to obtain a sufficient depth of field. Furthermore, in the objective optical systems disclosed in Patent Documents 1 to 4, for example, the overall length and diameter of the optical system have not been sufficiently reduced. Furthermore, in the objective optical system disclosed in Patent Document 5, for example, there has been a lack of space for the lens groups to move when autofocusing is implemented.

[0006] The present invention has been made in view of the above-mentioned problems, and has an object to provide a small-diameter, high-performance objective optical system that has a focusing function, is capable of autofocusing, ensures a sufficient depth of field, and has a sufficient range of movement along the optical axis of the lens group. Another object of the present invention is to provide an endoscope and an imaging device that include the above-mentioned objective optical system.

[0007] The objective optical system of the present invention comprises, in order from the object side, a first lens group with negative refractive power, a second lens group with positive refractive power, and a third lens group with positive refractive power. Focusing from a far-distance object point to a near-distance object point is performed by moving the second lens group from the object side to the image side. The first lens group comprises two lenses: a first lens which is a negative lens, and a second lens which is a negative lens with a concave surface facing the image side. The second lens group comprises a positive meniscus lens with a convex surface facing the object side. The third lens group comprises, in order from the object side, a single lens with positive refractive power and a cemented lens of a positive lens and a negative lens. The objective optical system of the present invention satisfies the following conditional expression (1): 0.01<L1_Rr / L2_Rr<0.95 (1), where L1_Rr is the radius of curvature of the image-side surface of the first lens, and L2_Rr is the radius of curvature of the image-side surface of the second lens.

[0008] The endoscope of the present invention comprises a tip portion in which the above-mentioned objective optical system is housed, a bendable extension portion connected to the base end of the tip portion, and an operating unit connected to the base end of the extension portion opposite the tip connected to the tip portion and having a handle for freely changing the axial shape of the extension portion.

[0009] An imaging device of the present invention includes the above-described endoscope and an imaging element that converts an image acquired by the objective optical system into an electrical signal.

[0010] According to the objective optical system of the present invention, the second lens group having a positive refractive power moves when focusing from a long-distance object point to a short-distance object point, so that the objective optical system has a focusing function, is capable of autofocusing, and ensures a sufficient depth of field and a sufficient range of movement of the lens groups, thereby enabling miniaturization. Furthermore, according to the objective optical system of the present invention, the first lens of the first lens group satisfies the above-mentioned conditional expression (1), so that a sufficient depth of field and a small diameter can be achieved, and various aberrations that occur as a whole can be balanced and well corrected, thereby achieving high performance. Furthermore, according to the present invention, an endoscope and an imaging device including the above-mentioned objective optical system can be provided.

[0011] 1 is a cross-sectional view of an objective optical system of Example 1. FIG. 2 is a cross-sectional view of an objective optical system of Example 2. FIG. 3 is a cross-sectional view of an objective optical system of Example 3. FIG. 4 is a cross-sectional view of an objective optical system of Example 4. FIG. 5 is a cross-sectional view of an objective optical system of Example 5. FIG. 6 is a cross-sectional view of an objective optical system of Example 6. FIG. 7 is a cross-sectional view of an objective optical system of Example 7. FIG. 8 is a schematic diagram of an endoscope and an imaging device according to an embodiment of the present invention.

[0012] Hereinafter, embodiments of an objective optical system, an endoscope, and an imaging device according to aspects of the present invention will be described with reference to the drawings. When specifically explaining the effects of the present embodiments, specific examples will be shown and described. However, as with the examples described below, the illustrated embodiments are only a portion of the aspects included in the present invention. There are multiple variations of the illustrated embodiments. Therefore, the present invention is not limited to the illustrated embodiments.

[0013] The objective optical system of this embodiment is incorporated into, for example, an endoscope and used to observe a lesion in a subject through the endoscope. The objective optical system of this embodiment is capable of autofocusing on a close object point that is relatively close to the optical system and a far object point that is farther away than the close object point. By focusing on the close object point, the object to be observed can be observed at a magnification greater than a predetermined magnification. Furthermore, by focusing on the far object point, the object to be observed can be observed at a predetermined magnification.

[0014] The objective optical system of this embodiment comprises a first lens group with negative refractive power, a second lens group with positive refractive power, and a third lens group with positive refractive power. The first lens group, the second lens group, and the third lens group are arranged in this order from the object side to the image side. Focusing from a long-distance object point to a close-distance object point is performed by moving the second lens group from the object side to the image side. When focusing the objective optical system of this embodiment, only the second lens group moves, and the first lens group and the third lens group remain fixed.

[0015] In the objective optical system of this embodiment, the first lens group includes a first lens that is a negative lens and a second lens that is a negative lens with a concave surface facing the image side. The second lens group includes a positive meniscus lens with a convex surface facing the object side. The third lens group includes a single lens with positive refractive power and a cemented lens of a positive lens and a negative lens. In the third lens group, the single lens and the cemented lens are arranged in order from the object side.

[0016] The objective optical system of this embodiment includes multiple lens groups, from the first lens group to the third lens group, thereby reducing the diameter of the entire system in a plane perpendicular to the optical axis and configuring the second lens group as a movable group. Furthermore, in the objective optical system of this embodiment, the first lens group includes the first and second lenses, which are negative lenses, thereby shortening the focal length of the entire system and ensuring a sufficient depth of field. Furthermore, the first lens group includes two negative lenses, which allows for correction of chromatic aberration and coma compared to when the focal length of the entire system is shortened using only one negative lens. In other words, when the first lens group includes only one negative lens, attempting to shorten the focal length of the entire system results in significant chromatic aberration and coma, making it impossible to obtain a high-performance objective optical system with good aberration performance.

[0017] In order to reduce the diameter of the objective optical system and ensure the movable range of the second lens group during focusing, a movable range on the object side of the second lens group is required, and it is necessary to increase the maximum distance between the image-side surface of the lens in the first lens group located closest to the image and the object-side surface of the lens in the second lens group located closest to the object. The lenses and optical elements other than lenses in each lens group are held by a holding member such as a lens barrel from the radially outer side in a plane perpendicular to the optical axis. Therefore, it is necessary to ensure an air space on the object side of the second lens group, i.e., the maximum distance between the image-side surface of the first lens group and the object-side surface of the second lens group, in a relatively radially outer portion in a plane perpendicular to the optical axis. It is also necessary to ensure a space for disposing the third lens group and a back focus for adjusting the focal position.

[0018] In this specification, the term "spacing" refers to an air gap, i.e., the distance in air between one surface and another surface in a direction parallel to the optical axis. Unless otherwise specified, the term "spacing" refers to the distance on the optical axis between one surface and another surface in the objective optical system.

[0019] In the objective optical system of this embodiment, since the second lens group is composed of a positive meniscus lens with a convex surface facing the object side, the principal points of the lenses constituting the second lens group can be positioned on the object side of the second lens group, which makes it possible to ensure a movable range of the second lens group in the objective optical system of this embodiment and a space on the object side of the second lens group, i.e., a space between the first lens group and the second lens group.

[0020] In order to shorten the overall length of the objective optical system, it is preferable to have a small number of lenses constituting the objective optical system. In the objective optical system of this embodiment, the second lens group is composed of a single positive meniscus lens, so the length of the second lens group can be shortened, and as a result, the overall length can be shortened. In this specification, a lens refers to an optical element other than a parallel plate whose object-side surface and image-side surface are parallel to a flat surface perpendicular to the optical axis, and is, for example, a single lens or a cemented lens. Either the object-side surface or the image-side surface of the lens includes a curved surface. The object-side surface and the image-side surface of the single lens and the cemented lens are in contact with air.

[0021] Furthermore, in an objective optical system, it is preferable to ensure a wider movable range on the image side of the second lens group. However, for example, in an objective optical system having three lens groups, simply shortening the space on the image side of the second lens group may result in aberrations occurring in the first and second lens groups not being corrected and remaining as they are on the imaging plane. Reducing the space on the image side of the second lens group means increasing the maximum distance between the image-side surface of the lens located closest to the image in the second lens group and the object-side surface of the lens located closest to the object in the third lens group.

[0022] In the objective optical system of this embodiment, the third lens group includes at least a single lens with positive refractive power and a cemented lens of a positive lens and a negative lens. Therefore, even if the diameter and total length of the entire system of the objective optical system of this embodiment are restricted and need to be kept within a predetermined range, aberrations can be well corrected, and spherical aberration and chromatic aberration in particular can be well maintained. As a result, an objective optical system with good aberration performance can be obtained.

[0023] In the objective optical system of this embodiment, it is preferable that a lens with positive refractive power be arranged on the image side of the cemented lens at a distance from the cemented lens in the third lens group. This lens serves as a cover glass that contacts the imaging surface from the object side. In other words, a lens with positive refractive power may be arranged on the imaging surface of the image sensor, i.e., the object side surface, instead of a parallel plate cover glass. With this configuration, the image plane of the objective optical system is aligned with the imaging surface by adjusting the distance between the cemented lens and the lens with positive refractive power arranged on the imaging surface of the image sensor. Furthermore, since positive refractive power is exerted in the image-side region of the objective optical system of this embodiment, i.e., the region close to the imaging surface, error sensitivity in image plane alignment is reduced. As a result, image plane positional deviation can be suppressed during image plane alignment in the objective optical system of this embodiment.

[0024] In the objective optical system of this embodiment, an aperture diaphragm may be located in a space closer to the object than the single lens located closest to the object in the third lens group. However, regardless of the position of the aperture diaphragm in the space between the second lens group and the third lens group in a direction parallel to the optical axis, the brightness of the image and the optical performance of the objective optical system do not change significantly. Therefore, the position of the aperture diaphragm between the second lens group and the third lens group is determined according to the shapes of the lenses in the objective optical system and the lens barrel for holding the aperture diaphragm. Note that the aperture diaphragm may be located in a space closer to the image than the positive meniscus lens in the second lens group, and configured to move in conjunction with the second lens group during focusing and to move integrally with the second lens group.

[0025] The objective optical system of this embodiment satisfies the following conditional expression (1): 0.01<L1_Rr / L2_Rr<0.95 (1) where, in expression (1), L1_Rr is the radius of curvature of the image-side surface of the first lens, which is a negative lens in the first lens group, and L2_Rr is the radius of curvature of the image-side surface of the second lens, which is a negative lens in the first lens group.

[0026] Conditional formula (1) is a conditional formula regarding an appropriate ratio between the radius of curvature of the image-side surface of the first lens and the radius of curvature of the image-side surface of the second lens in the first lens group. The first lens is a single lens that is arranged closest to the object among the multiple single lenses in the first lens group. The second lens is arranged closer to the image than the first lens among the multiple lenses in the first lens group, for example, in a space closer to the image than the first lens.

[0027] To achieve a retrofocus type configuration using the objective optical system of this embodiment, the first lens in the first lens group needs to have a relatively strong negative refractive power. However, if the negative refractive power of the first lens is too strong, aberrations such as chromatic aberration and coma may worsen. Therefore, in order to appropriately set the negative refractive power of the first lens, it is preferable to appropriately set the radius of curvature. Furthermore, adjusting only the negative refractive power of the first lens may not be able to effectively correct aberrations, making it difficult to achieve an objective optical system with a deep depth of field. Therefore, it is necessary to impart negative refractive power to the second lens. Therefore, in order to appropriately set the negative refractive power of the second lens, it is preferable to appropriately set the radius of curvature of each side surface of the first lens and the second lens.

[0028] By satisfying conditional expression (1), in an objective optical system having a relatively small F-number such as the objective optical system of this embodiment, it is possible to achieve a good balance of aberrations overall and a compact objective optical system with a deep depth of field.

[0029] If the lower limit of conditional expression (1) is not satisfied, the negative refractive power of the first lens in the first lens group becomes too strong, making it easier for chromatic aberration, coma, and the like to occur, which is undesirable. Furthermore, if the lower limit of conditional expression (1) is not satisfied, the negative refractive power of the second lens in the first lens group cannot be ensured, making the depth of field of the objective optical system shallow. As a result, the diameters of the first lens and the entire system become large, which undesirably leads to an increase in the size of the objective optical system of this embodiment.

[0030] If the upper limit of conditional expression (1) is exceeded, it is not possible to ensure the negative refractive power of the first lens in the first lens group and to deepen the depth of field of the objective optical system, and the diameter of the first lens becomes large, which is undesirable. Also, if the upper limit of conditional expression (1) is exceeded, the amount of aberration generated by the second lens in the first lens group becomes too large, particularly, coma aberration and chromatic aberration of magnification become worse, and it becomes difficult to correct the aberrations worsened in the first lens group as described above in the second and third lens groups arranged after the first lens group.

[0031] It is preferable that the objective optical system of this embodiment satisfy the following conditional expression (2): 0.136<L1 / L2<0.95 (2) where L1 is the focal length of the first lens in the first lens group, and L2 is the focal length of the second lens.

[0032] Conditional expression (2) is a conditional expression relating to an appropriate ratio between the negative refractive power of the first lens and the negative refractive power of the second lens in the first lens group. By satisfying conditional expression (2), various aberrations in the objective optical system of this embodiment can be corrected well, and a small-diameter objective optical system with a deep depth of field can be realized.

[0033] If the lower limit of conditional expression (2) is not satisfied, the negative refractive power of the first lens in the first lens group becomes too strong, the Petzval sum becomes large, and the curvature of field is overcorrected, which is undesirable. Also, if the lower limit of conditional expression (2) is not satisfied, the negative refractive power of the second lens in the first lens group becomes too weak, making it impossible to shorten the focal length of the entire system and making it difficult to ensure the depth of field of the objective optical system of this embodiment.

[0034] If the upper limit of conditional expression (2) is exceeded, the negative refractive power of the first lens in the first lens group becomes weak, and the height of the ray of light incident on the first lens becomes large, which undesirably increases the diameter of the first lens. Also, if the upper limit of conditional expression (2) is exceeded, the negative refractive power of the second lens in the first lens group becomes too strong, which moves the position of the principal point toward the image side, and as a result, the overall length of the objective optical system becomes long, making it difficult to achieve a compact objective optical system of this embodiment.

[0035] It is preferable that the objective optical system of this embodiment satisfies the following conditional expression (3): 0.2<L2_SF<1.85 (3) where L2_SF is the shaping factor of the second lens in the first lens group.

[0036] The shaping factor of the second lens in the first lens group is expressed by the following equation (4): L2_SF=(L2_Lr-L2_Rr) / (L2_Lr+L2_Rr) (4) In equation (4), L2_Lr is the radius of curvature of the object-side surface of the second lens in the first lens group, and L2_Rr is the radius of curvature of the image-side surface of the second lens.

[0037] Conditional expression (3) is a conditional expression relating to the shape of the second lens in the first lens group. By satisfying conditional expression (3), it is possible to maintain the small diameter of the objective optical system of this embodiment, to effectively correct astigmatism, and to realize an objective optical system with a deep depth of field.

[0038] If the lower limit of conditional expression (3) is not reached, the radius of curvature of the second lens in the first lens group becomes too large, the negative refractive power of the second lens cannot be maintained, and the depth of field becomes shallow, which is undesirable.

[0039] If the upper limit of conditional expression (3) is exceeded, the radius of curvature of the second lens in the first lens group becomes too small, which deteriorates astigmatism and makes it difficult to ensure the aberration performance of the objective optical system of this embodiment.

[0040] It is preferable that the objective optical system of this embodiment satisfy the following conditional expression (5): −0.4<L1 / f2<−0.145 (5) where L1 is the focal length of the first lens in the first lens group, and f2 is the focal length of the second lens group.

[0041] Condition (5) relates to an appropriate ratio between the negative refractive power of the first lens in the first lens group and the positive refractive power of the second lens group. By satisfying condition (5), various aberrations in the objective optical system can be corrected well, and a compact objective optical system with a deep depth of field can be realized.

[0042] If the lower limit of conditional expression (5) is not reached, the negative refractive power of the first lens in the first lens group will be too weak, making it impossible to shorten the focal length and making it difficult to ensure the depth of field. Also, if the lower limit of conditional expression (5) is not reached, the positive refractive power of the second lens group will be too strong, causing significant degradation in performance in accordance with the decentering of the frame member that holds the positive meniscus lens in the second lens group relative to the frame member that holds the first lens, making it difficult to ensure the optical performance of the objective optical system during focusing.

[0043] If the upper limit of conditional expression (5) is exceeded, the negative refractive power of the first lens in the first lens group will be weak, and the Petzval sum will be large, which makes it easier for curvature of field to be overcorrected, which is undesirable. Also, if the upper limit of conditional expression (5) is exceeded, the positive refractive power of the second lens group will be too weak, which makes it possible to reduce the error sensitivity corresponding to the decentering of the frame member that holds the positive meniscus lens relative to the frame member that holds the first lens, but it also makes the amount of movement of the second lens group large, which makes the objective optical system of this embodiment large, which is undesirable.

[0044] It is preferable that the objective optical system of this embodiment satisfy the following conditional expression (6): −3<L1 / fw<−0.955 (6) where, in expression (6), L1 is the focal length of the first lens in the first lens group, and fw is the focal length of the entire objective optical system of this embodiment when focusing on a long-distance object point.

[0045] Condition (6) relates to an appropriate ratio between the negative refractive power of the first lens in the first lens group and the refractive power of the entire objective optical system. By satisfying condition (6), various aberrations in the objective optical system can be corrected well, and a compact objective optical system with a deep depth of field can be realized.

[0046] If the lower limit of conditional expression (6) is not reached, the negative refractive power of the first lens in the first lens group becomes too weak, making it difficult to reduce the overall length of the objective optical system of this embodiment.

[0047] If the upper limit of conditional expression (6) is exceeded, the negative refractive power of the first lens in the first lens group becomes too strong, which tends to cause coma and astigmatism to occur and become large, which is undesirable. Also, if the upper limit of conditional expression (6) is exceeded, the radius of curvature of the image-side surface of the first lens becomes too small, which tends to cause the error sensitivity corresponding to the decentering of the first lens relative to the optical axis to become large, which is undesirable.

[0048] It is preferable that the objective optical system of this embodiment satisfy the following conditional expression (7): −3<f1 / fw<−1.06 (7) where, in expression (7), f1 is the focal length of the first lens group, and fw is the focal length of the entire objective optical system of this embodiment when focusing on a long-distance object point.

[0049] Condition (7) relates to an appropriate ratio between the refractive power of the first lens group and the refractive power of the entire objective optical system. By satisfying condition (7), various aberrations can be corrected well, and the objective optical system of this embodiment can be made compact.

[0050] If the lower limit of conditional expression (7) is not reached, the negative refractive power of the first lens group becomes too weak, and the height of the ray of light incident on the first lens of the first lens group becomes large, which undesirably increases the diameter of the first lens.

[0051] If the upper limit of conditional expression (7) is exceeded, the negative refractive power of the first lens group becomes too strong, and the focal point of the first lens group moves closer to itself, i.e., the object side, resulting in an increase in the overall length of the objective optical system of this embodiment, which is undesirable.

[0052] It is preferable that the objective optical system of this embodiment satisfy the following conditional expression (8): 0.25<thi_3g_L1 / thi_3g_air<1.5 (8) In expression (8), thi_3g_L1 is the thickness of the single lens in the third lens group on the optical axis, and thi_3g_air is the air gap between the single lens and the cemented lens in the third lens group on the optical axis.

[0053] Conditional expression (8) is a conditional expression relating to the ratio of the axial thickness of the first single lens from the object side in the third lens group to the axial distance between the single lens and the cemented lens in the third lens group. By satisfying conditional expression (8), it is possible to reduce the size of the third lens group and thereby reduce the size of the objective optical system of this embodiment.

[0054] If the lower limit of conditional expression (8) is not reached, the distance between the single lens and the cemented lens in the third lens group becomes large, which is undesirable as it increases the overall length of the objective optical system of this embodiment.

[0055] If the upper limit of conditional expression (8) is exceeded, the single lens in the third lens group becomes large, and the overall length of the objective optical system of this embodiment becomes long, which is undesirable.

[0056] It is preferable that the objective optical system of this embodiment satisfy the following conditional expression (9): 0.325<v / fw<0.6 (9) where v is the amount of movement of the second lens group from when focusing on a long-distance object point to when focusing on a close-distance object point, and fw is the focal length of the entire objective optical system of this embodiment when focusing on a long-distance object point.

[0057] Conditional expression (9) is a conditional expression relating to the amount of movement of the positive meniscus lens in the second lens group on the optical axis. In order to achieve a compact and high-performance objective optical system having a movable group, such as the objective optical system of this embodiment, it is important to appropriately suppress the amount of movement of the movable group. By satisfying conditional expression (9), the amount of movement of the second lens group, which is the movable group in the objective optical system of this embodiment, on the optical axis can be appropriately set in accordance with the focal length of the entire system of the objective optical system of this embodiment when focusing on a long-distance object point, thereby achieving a compact and high-performance objective optical system of this embodiment.

[0058] If the lower limit of conditional expression (9) is not reached, the error sensitivity of the image plane position in the objective optical system of this embodiment to the amount of movement of the positive meniscus lens in the second lens group becomes high, which is undesirable.

[0059] If the upper limit value of conditional expression (9) is exceeded, the distance between the first lens group and the second lens group becomes large, and although the amount of movement of the second lens group can be secured, the overall length of the objective optical system of this embodiment becomes long, making it difficult to reduce the size of the objective optical system of this embodiment.

[0060] It is preferable that the objective optical system of this embodiment satisfies the following conditional expression (10): −0.4<G3_L1_SF<0.4 (10) where G3_L1_SF is the shaping factor of the single lens in the third lens group.

[0061] The shaping factor of the single lens in the third lens group is expressed by the following equation (11): G3_L1_SF=(G3_L1_Lr+G3_L1_Rr) / (G3_L1_Lr-G3_L1_Rr) (11) In equation (11), G3_L1_Lr is the radius of curvature of the object-side surface of the single lens in the third lens group, and G3_L1_Rr is the radius of curvature of the image-side surface of the single lens.

[0062] Conditional expression (10) is a conditional expression relating to the shape of the single lens in the third lens group. By satisfying conditional expression (10), it is possible to reduce the size of the objective optical system of this embodiment and to perform favorable correction of spherical aberration and coma.

[0063] If the lower limit of condition (10) is not reached, the radius of curvature of either the object-side or image-side surface of the single lens in the third lens group becomes too small, making it difficult to correct spherical aberration and coma.

[0064] If the upper limit of condition (10) is exceeded, the radius of curvature of either the object-side or image-side surface of the single lens in the third lens group becomes too small, making it difficult to correct spherical aberration and coma.

[0065] It is preferable that the objective optical system of this embodiment satisfies the following conditional expression (12): −1.5<G3_Lce_SF<−0.2 (12) where G3_Lce_SF is the shaping factor of the cemented lens in the third lens group.

[0066] The shaping factor of the cemented lens in the third lens group is expressed by the following equation (13): G3_Lce_SF=(G3_Lce_Lr+G3_Lce_Rr) / (G3_Lce_Lr-G3_Lce_Rr) (13) In equation (13), G3_Lce_Lr is the radius of curvature of the object-side surface of the cemented lens in the third lens group, i.e., the radius of curvature of the object-side surface of the lens that is arranged on the object side of the cemented lens and has positive refractive power. In equation (13), G3_Lce_Rr is the radius of curvature of the image-side surface of the cemented lens in the third lens group, i.e., the radius of curvature of the image-side surface of the lens that is arranged on the image side of the cemented lens and has negative refractive power.

[0067] Conditional expression (12) is a conditional expression relating to the shape of the cemented lens in the third lens group. By satisfying conditional expression (12), astigmatism and coma in the objective optical system of this embodiment can be corrected satisfactorily.

[0068] If the lower limit of conditional expression (12) is not reached, the radius of curvature of the object-side surface of the cemented lens in the third lens group becomes too small, which is undesirable, and coma aberration is likely to occur.Furthermore, if the lower limit of conditional expression (12) is not reached, correction of coma aberration and astigmatism caused by the image-side surface of the cemented lens becomes insufficient, which is undesirable.

[0069] If the upper limit of conditional expression (12) is exceeded, the radius of curvature of the object-side surface of the cemented lens in the third lens group becomes too large, the refractive power of the cemented lens cannot be maintained, and the overall length of the objective optical system of this embodiment becomes large, which is undesirable. Also, if the upper limit of conditional expression (12) is exceeded, the radius of curvature of the image-side surface of the cemented lens becomes excessively small, which makes it easy for coma and astigmatism to be over-corrected, which is undesirable.

[0070] The endoscope of this embodiment is characterized by comprising a tip portion in which the objective optical system of this embodiment is housed, a bendable extension portion connected to the base end of the tip portion, and an operating unit connected to the base end of the extension portion opposite the tip connected to the tip portion and having a handle for freely changing the axial shape of the extension portion.

[0071] The imaging device of this embodiment is characterized by including the endoscope of this embodiment and an imaging element that converts an image acquired by the objective optical system of this embodiment into an electrical signal.

[0072] An endoscope and an imaging device are provided that are compatible with autofocusing, can ensure sufficient depth of field and movable range, and are equipped with a small-diameter, high-performance objective optical system, thereby enabling high-precision observation of an observation object with simple operation and enabling diagnosis of a lesion or the like in the observation object.

[0073] For the above-described conditional expressions (1) to (3), (5) to (10), and (12), at least one of the lower limit and upper limit may be changed as follows: By changing them in this way, the effect of satisfying each conditional expression is further enhanced.

[0074] Condition (1) is as follows: The lower limit is more preferably set to 0.1, and even more preferably to 0.335. The upper limit is more preferably set to 0.9, and even more preferably to 0.795.

[0075] Condition (2) is as follows: The lower limit is more preferably set to 0.2, and even more preferably to 0.25. The upper limit is more preferably set to 0.8, and even more preferably to 0.625.

[0076] Condition (3) is as follows: The lower limit is more preferably set to 0.3, and even more preferably to 0.55. The upper limit is more preferably set to 1.7, and even more preferably to 1.5.

[0077] Condition (5) is as follows: It is more preferable to set the lower limit to −0.35, and even more preferable to set it to −0.25. It is more preferable to set the upper limit to −0.15, and even more preferable to set it to −0.16.

[0078] Condition (6) is as follows: It is more preferable to set the lower limit to −2.8, and even more preferable to set it to −2.5. It is more preferable to set the upper limit to −1.0, and even more preferable to set it to −1.5.

[0079] Condition (7) is as follows: It is more preferable to set the lower limit to −2.0, and even more preferable to set it to −1.6. It is more preferable to set the upper limit to −1.1, and even more preferable to set it to −1.2.

[0080] Condition (8) is as follows: The lower limit is more preferably set to 0.3, and even more preferably to 0.4 The upper limit is more preferably set to 1.4, and even more preferably to 1.25.

[0081] Condition (9) is as follows: It is more preferable to set the lower limit to 0.35, and it is more preferable to set the upper limit to 0.55.

[0082] Condition (10) is as follows: It is more preferable to set the lower limit to −0.35, and even more preferable to set it to −0.3. It is more preferable to set the upper limit to 0.3, and even more preferable to set it to 0.25.

[0083] Condition (12) is as follows: It is more preferable to set the lower limit to −1.4, and even more preferable to set it to −1.3. It is more preferable to set the upper limit to −0.3, and even more preferable to set it to −0.35.

[0084] Next, examples of the objective optical system of this embodiment will be described, but the present invention is not limited to the following examples.

[0085] 1 to 7 are cross-sectional views of the objective optical systems of Examples 1 to 7. In each of Figures 1 to 7, (a) is a cross-sectional view when focusing at a long-distance object point, and (b) is a cross-sectional view when focusing at a short-distance object point. In (b) of each of Figures 1 to 7, the position of the meniscus lens of the second lens group when focusing at a long-distance object point is indicated by a two-dot chain line for reference.

[0086] 8 to 14 are aberration diagrams of the objective optical systems of Examples 1 to 7. In each of FIGS. 1 to 7, (a), (b), (c), and (d) are aberration diagrams when focusing at a far object point, and (e), (f), (g), and (h) are aberration diagrams when focusing at a close object point. In each of FIGS. 1 to 7, (a) and (e) are diagrams of spherical aberration (SA). (b) and (f) are diagrams of astigmatism (AS). (c) and (g) are diagrams of distortion (DT). (d) and (h) are diagrams of lateral chromatic aberration (CC). In each of (a), (d), (e), and (h), the g-line represents the aberrations at a wavelength of 435.84 nm, and the C-line represents the aberrations at a wavelength of 656.27 nm. In (a) and (e), the d-line represents the aberrations at a wavelength of 587.56 nm. In (b) and (f), ΔM represents the aberration at the d-line relative to the meridional image plane, and ΔS represents the aberration at the d-line relative to the sagittal image plane.

[0087] In FIGS. 1 to 7, the first lens group of the objective optical system of each embodiment is indicated by G1, the second lens group is indicated by G2, the third lens group is indicated by G3, the infrared filter is indicated by CF, the aperture stop is indicated by AS, the cover glass is indicated by CG, and the image plane, i.e., the imaging plane, is indicated by I.

[0088] As shown in FIGS. 1 to 7, the objective optical system of each of Examples 1 to 7 includes, in order from the object 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.

[0089] In each of the objective optical systems of Examples 1 to 7, when focusing from a long-distance object point to a close-distance object point, the second lens group G2 moves from the object side to the image side, while the first lens group G1 and the third lens group G3 are fixed. At this time, the infrared filter F is fixed in the same manner as the first lens group G1. The aperture stop P moves on the optical axis in conjunction with the second lens group G2. The cover glass C is fixed in the same manner as the third lens group G3.

[0090] In each of the objective optical systems of Examples 1 to 7, the aspheric surface is the meniscus lens L of the second lens group G2. ３ The lens is provided on the object side of the lens.

[0091] The detailed configuration of each of the objective optical systems of Examples 1 to 7 will be described below.

[0092] Example 1 As shown in FIG. 1, in the objective optical system of Example 1, the first lens group G1 comprises, in order from the object side, a plano-concave lens L having a negative refractive power. １ and a negative refractive power plano-concave lens L ２ The plano-concave lens L １ corresponds to the "first lens" described in the claims below. ２ corresponds to the "second lens" described in the claims below. １ , L ２ The concave surface of the plano-concave lens L faces the image side. １ , L ２ The object side surface of the plano-concave lens L is a flat surface perpendicular to the optical axis. １ , L ２ The image side surface of the lens is a concave surface that is recessed toward the object side. The infrared filter F is disposed in the first lens group G1, specifically, the plano-concave lens L ２ The object-side and image-side surfaces of the infrared filter F are flat surfaces that are perpendicular to the optical axis.

[0093] The second lens group G2 includes a single positive meniscus lens L ３ The meniscus lens L ３ corresponds to the "positive meniscus lens" described in the claims below. ３ The meniscus lens L has a convex surface facing the object side. ３ The object-side and image-side surfaces are concave surfaces recessed toward the object side.

[0094] The third lens group G3 is, in order from the object side, a biconvex lens L ４ and a positive biconvex lens L ５ and a meniscus lens L which is a negative meniscus lens. ６and a plano-convex lens L with positive refractive power. ７ The aperture stop P is disposed in the third lens group G3, specifically, the biconvex lens L ４ The aperture stop P is located in the space closer to the object than the aperture stop P. ... ４ The biconvex lens L has an opening with a smaller diameter than the ４ corresponds to the "single lens" described in the claims below. ４ , L ５ The object side surface of the biconvex lens L is a convex surface that protrudes toward the object side. ４ , L ５ The image side surface of the meniscus lens L is a convex surface that protrudes toward the image side. ６ The object side surface of the meniscus lens L is a concave surface that is recessed toward the image side. ６ The image side surface of the biconvex lens L is a convex surface that protrudes toward the image side. ５ and meniscus lens L ６ are cemented together to form one cemented lens L Ｃ That is, the biconvex lens L ５ The image side of the meniscus lens L ６ The object-side surfaces of the cemented lens L are in contact with each other. Ｃ corresponds to the "cemented lens" described in the claims below.

[0095] Plano-convex lens L ７ The object side surface of the plano-convex lens L is a convex surface that protrudes toward the object side. ７ The image side surface of the plano-convex lens L is a flat surface perpendicular to the optical axis. The cover glass C is disposed in a space closer to the image side than the third lens group G3. The object side surface and the image side surface of the cover glass C are flat surfaces perpendicular to the optical axis. The image side surface of the cover glass C is the image plane I of the objective optical system, i.e., the imaging surface. ７ The image side surface of the lens element 1 and the object side surface of the cover glass C are in contact with each other.

[0096] Tables 1 to 5 show the numerical data of Example 1. Note that, common to all the numerical data of Examples 1 to 7, in the surface data, r is the radius of curvature of each surface, d is the distance between each surface, nd is the refractive index of each lens at a wavelength of 587.56 nm, i.e., at the d-line, and νd is the Abbe number of each lens. The unit of each numerical value is millimeters (mm). * indicates an aspherical surface. AS indicates an aperture stop.

[0097] In addition, common to the numerical data of each of Examples 1 to 7, the aspherical shape is expressed by the following formula, where z is the direction parallel to the optical axis of the objective optical system, y is the direction perpendicular to the optical axis, k is the conic coefficient, and A4, A6, A8, and A10 are aspherical coefficients. 2 / r) / [1+{1-(1+k)(y / r) 2} 1 / 2 ]+A4y 4 +A6y 6 +A8y 8 +A10y 10

[0098] [Numerical Example 1]

[0099] As can be seen from FIG. 8, in the objective optical system of Example 1, among various aberrations, for example, spherical aberration, astigmatism, distortion, and chromatic aberration of magnification are corrected, and good aberration characteristics are obtained in the visible wavelength range.

[0100] Example 2 As shown in FIG. 2, in Example 2, the first lens group G1 includes, in order from the object side, a plano-concave lens L having a negative refractive power. １ and a negative refractive power plano-concave lens L ２ In each of the second and subsequent embodiments, the same lenses as those in the previously described embodiments are designated by the same reference numerals, and a description of the object side surface and image surface of the lenses will be omitted. ２ It is located in the space closer to the image than the

[0101] The second lens group G2 includes one meniscus lens L, which is a positive meniscus lens. ３It consists of:

[0102] The third lens group G3 is, in order from the object side, a biconvex lens L ４ and a positive biconvex lens L ５ and a meniscus lens L which is a negative meniscus lens. ６ and a plano-convex lens L with positive refractive power. ７ The aperture stop P is made up of a biconvex lens L ４ The biconvex lens L is located closer to the object than the ５ and meniscus lens L ６ is the cemented lens L Ｃ The cover glass C is disposed in a space closer to the image side than the third lens group G3.

[0103] Tables 6 to 10 show the numerical data of Example 2.

[0104] [Numerical Example 2]

[0105] As can be seen from FIG. 9, in the objective optical system of Example 2, among various aberrations, for example, spherical aberration, astigmatism, distortion, and chromatic aberration of magnification are corrected, and good aberration characteristics are obtained in the visible wavelength range.

[0106] Third Embodiment As shown in FIG. 3, in the third embodiment, the first lens group G1 includes, in order from the object side, a plano-concave lens L having a negative refractive power. １ and a negative refractive power plano-concave lens L ２ The infrared filter F is made up of a plano-concave lens L ２ It is located in the space closer to the image than the

[0107] The second lens group G2 includes one meniscus lens L, which is a positive meniscus lens. ３ It consists of:

[0108] The third lens group G3 is, in order from the object side, a biconvex lens L ４ and a positive biconvex lens L ５ and a meniscus lens L which is a negative meniscus lens. ６ and a plano-convex lens L with positive refractive power.７ The aperture stop P is made up of a biconvex lens L ４ The biconvex lens L is located closer to the object than the ５ and meniscus lens L ６ is the cemented lens L Ｃ The cover glass C is disposed in a space closer to the image side than the third lens group G3.

[0109] Tables 11 to 15 show the numerical data of Example 3.

[0110] [Numerical Example 3]

[0111] As can be seen from FIG. 10, the objective optical system of Example 3 also corrects various aberrations, such as spherical aberration, astigmatism, distortion, and chromatic aberration of magnification, and provides good aberration characteristics in the visible wavelength range.

[0112] Example 4 As shown in FIG. 4, in Example 4, the first lens group G1 includes, in order from the object side, a plano-concave lens L having a negative refractive power. １ and a negative refractive power plano-concave lens L ２ The infrared filter F is made up of a plano-concave lens L ２ It is located in the space closer to the image than the

[0113] The second lens group G2 includes one meniscus lens L, which is a positive meniscus lens. ３ It consists of:

[0114] The third lens group G3 is, in order from the object side, a biconvex lens L ４ and a positive biconvex lens L ５ and a meniscus lens L which is a negative meniscus lens. ６ and a plano-convex lens L with positive refractive power. ７ The aperture stop P is made up of a biconvex lens L ４ The biconvex lens L is located closer to the object than the ５ and meniscus lens L ６ is the cemented lens L ＣThe cover glass C is disposed in a space closer to the image side than the third lens group G3.

[0115] Tables 16 to 20 show the numerical data of Example 4.

[0116] [Numerical Example 4]

[0117] As can be seen from FIG. 11, in the objective optical system of Example 4, among various aberrations, for example, spherical aberration, astigmatism, distortion, and chromatic aberration of magnification are corrected, and good aberration characteristics are obtained in the visible wavelength range.

[0118] Fifth Embodiment As shown in FIG. 5, in the fifth embodiment, the first lens group G1 includes, in order from the object side, a plano-concave lens L having a negative refractive power. １ and a negative refractive power plano-concave lens L ２ The infrared filter F is made up of a plano-concave lens L ２ It is located in the space closer to the image than the

[0119] The second lens group G2 includes one meniscus lens L, which is a positive meniscus lens. ３ It consists of:

[0120] The third lens group G3 is, in order from the object side, a biconvex lens L ４ and a positive biconvex lens L ５ and a meniscus lens L which is a negative meniscus lens. ６ and parallel plate PP １ The aperture stop P is made up of a biconvex lens L ４ The biconvex lens L is located closer to the object than the ５ and meniscus lens L ６ is the cemented lens L Ｃ The object side and image side surfaces of the parallel plate PP1 are flat surfaces perpendicular to the optical axis. The cover glass C is disposed in a space closer to the image side than the third lens group G3. １ The image side surface of the lens element 1 and the object side surface of the cover glass C are in contact with each other.

[0121] Tables 21 to 25 show the numerical data for Example 5.

[0122] [Numerical Example 5]

[0123] As can be seen from FIG. 12, the objective optical system of Example 5 also corrects various aberrations, such as spherical aberration, astigmatism, distortion, and chromatic aberration of magnification, and provides good aberration characteristics in the visible wavelength range.

[0124] Sixth Embodiment As shown in FIG. 6, in the sixth embodiment, the first lens group G1 includes, in order from the object side, a plano-concave lens L having a negative refractive power. １ and a meniscus lens L having a negative refractive power. ８ and the meniscus lens L ８ is a negative meniscus lens, which corresponds to the "second lens" described in the claims below. ８ The object side surface of the meniscus lens L is a convex surface that protrudes toward the object side. ８ The image side of the infrared filter F is a concave surface that is recessed toward the object side. ２ It is located in the space closer to the image than the

[0125] The second lens group G2 includes one meniscus lens L, which is a positive meniscus lens. ３ It consists of:

[0126] The third lens group G3 is, in order from the object side, a biconvex lens L ４ and a positive biconvex lens L ５ and a biconcave lens L which is a negative meniscus lens. ９ and a plano-convex lens L with positive refractive power. ７ The aperture stop P is made up of a biconvex lens L ４ The biconcave lens L is located closer to the object than the ９ The object side surface of the biconcave lens L is a concave surface that is concave toward the image side. ９ The image side surface of the biconvex lens L is a concave surface that is concave toward the object side. ５ and biconcave lens L ９ is the cemented lens L ＣThat is, the biconvex lens L ５ The image side of the lens and the biconcave lens L ９ The object-side surfaces of the third lens group G3 and the second lens group G4 are in contact with each other. The cover glass C is disposed in a space closer to the image side than the third lens group G3.

[0127] Tables 26 to 30 show the numerical data for Example 6.

[0128] [Numerical Example 6]

[0129] As can be seen from FIG. 13, in the objective optical system of Example 6, among various aberrations, for example, spherical aberration, astigmatism, distortion, and chromatic aberration of magnification are corrected, and good aberration characteristics are obtained in the visible wavelength range.

[0130] Example 7 As shown in FIG. 7, in Example 7, the first lens group G1 includes, in order from the object side, a plano-concave lens L having a negative refractive power. １ and a biconcave lens L having negative refractive power. １０ and the biconcave lens L １０ corresponds to the "second lens" described in the claims below. １０ The object side surface of the biconcave lens L is a concave surface that is concave toward the image side. １０ The image side of the lens is concave toward the object side. １０ It is located in the space closer to the image than the

[0131] The second lens group G2 includes one meniscus lens L, which is a positive meniscus lens. ３ It consists of:

[0132] The third lens group G3 is, in order from the object side, a biconvex lens L ４ and a positive biconvex lens L ５ and a biconcave lens L which is a negative meniscus lens. ９ and a plano-convex lens L with positive refractive power. ７ The aperture stop P is made up of a biconvex lens L ４ The biconcave lens L is located closer to the object than the ９The object side surface of the biconcave lens L is a concave surface that is concave toward the image side. ９ The image side surface of the biconvex lens L is a concave surface that is concave toward the object side. ５ and biconcave lens L ９ is the cemented lens L Ｃ That is, the biconvex lens L ５ The image side of the lens and the biconcave lens L ９ The object-side surfaces of the third lens group G3 and the second lens group G4 are in contact with each other. The cover glass C is disposed in a space closer to the image side than the third lens group G3.

[0133] Tables 31 to 35 show the numerical data for Example 7.

[0134] [Numerical Example 7]

[0135] As can be seen from FIG. 14, in the objective optical system of Example 7, among various aberrations, for example, spherical aberration, astigmatism, distortion, and chromatic aberration of magnification are corrected, and good aberration characteristics are obtained in the visible wavelength range.

[0136] Next, the endoscope and the imaging device of this embodiment will be described. Fig. 15 is a schematic diagram of an endoscope 100 and an imaging device 200 of this embodiment.

[0137] As shown in FIG. 15 , the endoscope 100 includes an insertion section 110 and an operation section 120. The insertion section 110 is elongated and configured to be insertable into a body cavity (not shown) of a patient. The insertion section 110 includes an extension section 112 and a tip section 114. The extension section 112 can be freely bent along an axis JX by a user (not shown) operating the operation section 120. That is, the axial shape of the extension section 112 along the axis JX can be freely changed along an anatomical passageway into which the extension section 112 is inserted, such as the stomach, duodenum, kidney, or ureter. The extension section 112 is formed of a flexible material. The tip section 114 is disposed at a tip 112a of the extension section 112, has approximately the same diameter as the extension section 112, and is inserted into the anatomical passageway together with the extension section 112. That is, the tip 112 a of the extension portion 112 is connected to the base end 114 b of the tip portion 114 .

[0138] Although not shown, the insertion section 110 includes a plurality of extremely elongated functional members, such as treatment instruments such as a cholangioscope, a light guide cable, an electric cable, a fluid passage, a guide wire, and a pull wire, as well as a covering member that covers these functional members from the outer periphery in the radial direction of the axis JX. The objective optical system of this embodiment is housed in the distal end portion 114 of the insertion section 110.

[0139] The operating unit 120 is connected to the base end 112b of the extension section 112 of the insertion section 110. That is, the operating unit 120 is connected to the base end 112b of the extension section 112, which is opposite the tip end 112a connected to the tip end section 114. The operating unit 120 has a control knob 122 and a port 130. The control knob 122 is used by the user to manually move the insertion section 110 forward and backward, change the axial shape of the extension section 112 to bend it, or change the direction in which the tip end section 114 faces. The control knob 122 corresponds to a "handle" as described in the claims below. The port 130 is configured to allow various functional components, such as electrical cables, guidewires, auxiliary scopes, and fluid tubes, to be attached to the operating unit 120 for connection to the insertion section 110.

[0140] The imaging device 200 includes an endoscope 100 and a control device 150. The control device 150 includes a controller 152, an output device 154, an input device 156, a light source 160, a fluid source 170, and a suction pump 172. The controller 152 receives data related to an observation object from the endoscope 100 and transmits the data to the endoscope 100, and includes an imaging element 180. The operation unit 120 of the endoscope 100 is connected to the controller 152 via a connection unit 190 such as a universal cord. The imaging element 180 receives, via the connection unit 190, an image acquired by the objective optical system of this embodiment, i.e., an image formed on the image plane I of the objective optical system. The imaging element 180 processes the received image, converts it into an electrical signal, and transmits it to the output device 154. The imaging element 180 is an image sensor such as a complementary metal-oxide semiconductor (CMOS) or a charge coupled device (CCD).

[0141] The output device 154 outputs a plurality of pieces of information including an image of the observation object and information related to the observation object transmitted from the image sensor 180, information transmitted from the controller 152, and information related to the operation of the endoscope 100. The output device 154 is, for example, a display capable of displaying the plurality of pieces of information transmitted to the output device 154 as described above. The input device 156 inputs a plurality of pieces of information including information related to the operation of the endoscope 100 and information related to the subject, mainly to the controller 152. The output device 154 is, for example, a keyboard, but may also be a mouse or the like.

[0142] The light source 160 emits light for acquiring an image of an observation target. The light emitted from the light source 160 is irradiated from the distal end 114 toward the observation target via a fiber link and a light guide cable inserted through the connection section 190, the control section 120, and the insertion section 110 of the endoscope 100. The fluid source 170 is configured to be able to communicate with the controller 152 and supplies a liquid such as air or treatment water to the endoscope 100 via the port 130. The suction pump 172 evacuates fluid from the anatomical region into which the insertion section 110 of the endoscope 100 is inserted and has a port for generating vacuum suction, for example.

[0143] The endoscope 100 and the imaging device 200 of the present embodiment described above are equipped with the objective optical system of the present embodiment. Therefore, the endoscope 100 and the imaging device 200 of the present embodiment enable the miniaturization of the tip portion 114 of the endoscope 100 and the reduction in the diameter of the extension portion 112, and also enable the high-performance objective optical system to be used to observe an observation object such as a lesion with high resolution, and the imaging element 180 to be used to acquire a high-definition image of the observation object.

[0144] The endoscope 100 and imaging device 200 described above are examples of the endoscope and imaging device of this embodiment. Therefore, the configurations of the endoscope and imaging device of this embodiment may be changed as appropriate from the configurations of the endoscope 100 and imaging device 200. For example, the operation unit 120 of the endoscope 100 may house a power supply, a light source, an imaging element, and various supply devices (not shown). Furthermore, for example, in the imaging device 200, the fluid source 170 and the suction pump 172 may be omitted, a videoscope (not shown) may be provided, and a storage device and a communication terminal (not shown) may be connected via wire or wirelessly.

[0145] REFERENCE SIGNS LIST 100 Endoscope 110 Insertion section 112 Extension section 114 Tip section 120 Operation section 180 Image pickup element 200 Image pickup device G1 First lens group G2 Second lens group G3 Third lens group L １ Plano-concave lens (first lens) L ２ Plano-concave lens (second lens) L ３ Meniscus Lens L ４ Biconvex lens (single lens) L ８ Meniscus lens (second lens) L １０ Biconcave lens (second lens) L Ｃ cemented lens

Claims

1. The lens comprises, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power, the second lens group moves from the object side to the image side to perform focusing from a far-distance object point to a near-distance object point; the first lens group is composed of two lenses: a first lens which is a negative lens; and a second lens which is a negative lens with a concave surface facing the image side; the second lens group is composed of one positive meniscus lens with a convex surface facing the object side, the third lens group includes, in order from the object side, a single lens having positive refractive power and a cemented lens of a positive lens and a negative lens, An objective optical system characterized by satisfying the following conditional expressions (1) and (8): 0.01<L1_Rr / L2_Rr<0.95...(1) 0.25<thi_3g_L1 / thi_3g_air<1.5...(8) where: L1_Rr: radius of curvature of the image side surface of the first lens, L2_Rr: radius of curvature of the image side surface of the second lens, thi_3g_L1: the thickness of the single lens on the optical axis, thi_3g_air: an air gap between the single lens and the cemented lens on the optical axis, is.

2. 2. The objective optical system according to claim 1, wherein the following conditional expression (2) is satisfied: 0.136<L1 / L2<0.95...(2) where: L1: focal length of the first lens, L2: the focal length of the second lens, is.

3. 2. The objective optical system according to claim 1, wherein the following conditional expression (3) is satisfied: 0.2<L2_SF<1.85...(3) where: L2_SF: the shaping factor of the second lens, The shaping factor of the second lens is expressed by the following equation (4). L2_SF=(L2_Lr-L2_Rr) / (L2_Lr+L2_Rr)...(4) Also, L2_Lr: the radius of curvature of the object side surface of the second lens, L2_Rr: radius of curvature of the image side surface of the second lens, is.

4. 2. The objective optical system according to claim 1, wherein the following conditional expression (5) is satisfied: -0.4<L1 / f2<-0.145...(5) where: L1: focal length of the first lens, f2: the focal length of the second lens group, is.

5. 2. The objective optical system according to claim 1, wherein the following conditional expression (6) is satisfied: -3<L1 / fw<-0.955...(6) where: L1: focal length of the first lens, fw: focal length of the entire objective optical system when focusing on a far-distance object point, is.

6. 2. The objective optical system according to claim 1, wherein the following conditional expression (7) is satisfied: -3<f1 / fw<-1.06...(7) where: f1: focal length of the first lens group, fw: focal length of the entire objective optical system when focusing on a far-distance object point, is.

7. 2. The objective optical system according to claim 1, wherein the following conditional expression (9) is satisfied: 0.325<v / fw<0.6...(9) where: v: the amount of movement of the second lens group from when focusing on a far-distance object point to when focusing on a near-distance object point, fw: focal length of the entire objective optical system when focusing on a far-distance object point, is.

8. 2. The objective optical system according to claim 1, wherein the following conditional expression (10) is satisfied: -0.4<G3_L1_SF<0.4...(10) where: G3_L1_SF: Shaping factor of the single lens, The shaping factor of the single lens is expressed by the following equation (11). G3_L1_SF=(G3_L1_Lr+G3_L1_Rr) / (G3_L1_Lr−G3_L1_Rr)...(11) Also, G3_L1_Lr: radius of curvature of the object side surface of the single lens, G3_L1_Rr: radius of curvature of the image side surface of the single lens, is.

9. 2. The objective optical system according to claim 1, wherein the following conditional expression (12) is satisfied: -1.5<G3_Lce_SF<-0.2 (12) where: G3_Lce_SF: shaping factor of the cemented lens, The shaping factor of the cemented lens is expressed by the following equation (13): G3_Lce_SF=(G3_Lce_Lr+G3_Lce_Rr) / (G3_Lce_Lr−G3_Lce_Rr)...(13) Also, G3_Lce_Lr: radius of curvature of the object side surface of the cemented lens, G3_Lce_Rr: radius of curvature of the image side surface of the cemented lens, is.

10. a tip portion that accommodates the objective optical system according to claim 1; a bendable extension portion connected to a base end of the tip portion; an operating unit connected to the base end of the extension portion opposite the tip end connected to the tip portion, and having a handle for changing the axial shape of the extension portion.

11. The endoscope according to claim 10; an image sensor that converts an image acquired by the objective optical system into an electrical signal.

12. A lens system comprising, in order from the object side, 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, the second lens group moves from the object side to the image side to perform focusing from a far-distance object point to a near-distance object point; the first lens group is composed of two lenses: a first lens which is a negative lens; and a second lens which is a negative lens with a concave surface facing the image side; the second lens group is composed of one positive meniscus lens with a convex surface facing the object side, the third lens group includes, in order from the object side, a single lens having positive refractive power and a cemented lens of a positive lens and a negative lens, An objective optical system characterized by satisfying the following conditional expressions (1) and (2): 0.01<L1_Rr / L2_Rr<0.95...(1) 0.136<L1 / L2<0.95...(2) where: L1_Rr: radius of curvature of the image side surface of the first lens, L2_Rr: radius of curvature of the image side surface of the second lens, L1: focal length of the first lens, L2: the focal length of the second lens, is.

13. A lens system comprising, in order from the object side, 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; the second lens group moves from the object side to the image side to perform focusing from a far-distance object point to a near-distance object point; the first lens group is composed of two lenses: a first lens which is a negative lens; and a second lens which is a negative lens with a concave surface facing the image side; the second lens group is composed of one positive meniscus lens with a convex surface facing the object side, the third lens group includes, in order from the object side, a single lens having positive refractive power, a cemented lens of a positive lens and a negative lens, and a single lens having positive refractive power; An objective optical system characterized by satisfying the following conditional expressions (1) and (2): 0.01<L1_Rr / L2_Rr<0.95...(1) 0.136<L1 / L2<0.95...(2) where: L1_Rr: radius of curvature of the image side surface of the first lens, L2_Rr: radius of curvature of the image side surface of the second lens, L1: focal length of the first lens, L2: the focal length of the second lens, is.

14. The lens comprises, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power, the second lens group moves from the object side to the image side to perform focusing from a far-distance object point to a near-distance object point; the first lens group is composed of two lenses: a first lens which is a negative lens; and a second lens which is a negative lens with a concave surface facing the image side; the second lens group is composed of one positive meniscus lens with a convex surface facing the object side, the third lens group includes, in order from the object side, a single lens having positive refractive power, a cemented lens of a positive lens and a negative lens, and a plane-parallel plate; An objective optical system characterized by satisfying the following conditional expressions (1) and (2): 0.01<L1_Rr / L2_Rr<0.95...(1) 0.136<L1 / L2<0.95...(2) where: L1_Rr: radius of curvature of the image side surface of the first lens, L2_Rr: radius of curvature of the image side surface of the second lens, L1: focal length of the first lens, L2: the focal length of the second lens, is.