Objective lens for endoscope and endoscope

Abstract

Provided is an endoscope objective lens that has good performance while taking into account chromatic aberration correction and miniaturization, and an endoscope provided with the endoscope objective lens. The endoscope objective lens is composed in order from the object side of: a first lens group composed of one negative lens; a second lens group composed of two lenses joined to each other; an aperture; a third lens group composed of two lenses joined to each other and having different signs of refractive power from each other; and a fourth lens group composed of two lenses joined to each other and having different signs of refractive power from each other. The endoscope objective lens satisfies predetermined conditional expressions related to the Abbe number and the length of the lens system.

Description

Technical Field

[0001] This invention relates to an objective lens for endoscopy and an endoscope. Background Technology

[0002] Previously, lens systems described in Patent Documents 1, 2, 3 and 4 have been known as optical systems that can be used in endoscopes.

[0003] Patent Document 1: Japanese Patent No. 5855793

[0004] Patent Document 2: Japanese Patent No. 5478421

[0005] Patent Document 3: Japanese Patent No. 2556984

[0006] Patent Document 4: Japanese Patent No. 2639963

[0007] In recent years, there has been a demand for an endoscope objective that combines chromatic aberration correction, miniaturization, and good performance. Summary of the Invention

[0008] The present invention was made in view of the above circumstances, and its object is to provide an endoscope objective lens that combines chromatic aberration correction and miniaturization with good performance, and an endoscope having the endoscope objective lens.

[0009] The endoscope objective lens according to the first aspect of the present invention is composed of the following parts in sequence from the object side to the image side: a first lens group consisting of a negative lens; a second lens group consisting of two lenses joined together; an aperture; a third lens group consisting of two lenses with different signs of refractive power joined together; and a fourth lens group consisting of two lenses with different signs of refractive power joined together. When the Abbe number of the d-line reference of the object-side lens in the second lens group is set to ν2f, the Abbe number of the d-line reference of the image-side lens in the second lens group is set to ν2r, the distance on the optical axis from the object-side lens surface of the first lens group to the closest image-side lens surface of the fourth lens group is set to AL, and the paraxial radius of curvature of the joint surface of the second lens group is set to R2c, the following conditional expression (1) is satisfied.

[0010] -300<(ν2f-ν2r)×|AL / R2c|<30 (1)

[0011] The endoscope objective lens involved in the first method preferably satisfies the following condition (1-1).

[0012] -250<(ν2f-ν2r)×|AL / R2c|<20 (1-1)

[0013] The endoscope objective lens according to the second aspect of the present invention comprises, from the object side to the image side, the following parts in sequence: a first lens group consisting of a negative lens; a second lens group consisting of two lenses joined together; an aperture; a third lens group consisting of two lenses with different refractive powers joined together; and a fourth lens group consisting of two lenses with different refractive powers joined together. The Abbe number of the d-line reference of the object-side lens in the third lens group is set to ν3f, the Abbe number of the d-line reference of the image-side lens in the third lens group is set to ν3r, and the... When the Abbe number of the d-line reference of the object-side lens is set to ν4f, the Abbe number of the d-line reference of the image-side lens in the fourth lens group is set to ν4r, the larger of the values ​​of |ν3f-ν3r| and |ν4f-ν4r| is set to |νf-νr|, the focal length of the entire system is set to f, the distance on the optical axis from the object-side lens surface of the first lens group to the lens surface closest to the image side of the fourth lens group is set to AL, and the air conversion distance on the optical axis from the lens surface closest to the image side of the fourth lens group to the focal position on the image side of the entire system is set to Bf, the following condition (2) is satisfied.

[0014] 3.9<|νf-νr|×f / (AL+Bf)<20 (2)

[0015] The endoscope objective involved in the second method preferably satisfies the following condition (2-1).

[0016] 4<|νf-νr|×f / (AL+Bf)<10 (2-1)

[0017] When the Abbe number of the d-line reference of the object-side lens in the second lens group is set to ν2f, the Abbe number of the d-line reference of the image-side lens in the second lens group is set to ν2r, and the paraxial radius of curvature of the joint surface of the second lens group is set to R2c, the endoscope objective lens involved in the second method preferably satisfies the following condition (1), and more preferably satisfies the following condition (1-1).

[0018] -300<(ν2f-ν2r)×|AL / R2c|<30 (1)

[0019] -250<(ν2f-ν2r)×|AL / R2c|<20 (1-1)

[0020] Hereinafter, the endoscopic objectives according to the first and second methods will be collectively referred to as endoscopic objectives according to the above methods. In the endoscopic objectives according to the above methods, it is preferable that the second lens group as a whole has positive refractive power. In the endoscopic objectives according to the above methods, it is preferable that the third lens group as a whole has positive refractive power. In the endoscopic objectives according to the above methods, it is preferable that the fourth lens group as a whole has positive refractive power.

[0021] When the focal length of the first lens group is set to f1, the focal length of the second lens group is set to f2, the focal length of the third lens group is set to f3, and the focal length of the fourth lens group is set to f4, the endoscope objective involved in the above method preferably satisfies the following condition (3), and more preferably satisfies the following condition (3-1).

[0022] -30<(f3×f4) / (f1×f2)<-0.2 (3)

[0023] -20 < (f3 × f4) / (f1 × f2) < -0.4 (3-1)

[0024] When the focal length of the entire system is set to f, the focal length of the second lens group is set to f2, the focal length of the third lens group is set to f3, and the focal length of the fourth lens group is set to f4, the endoscope objective involved in the above method preferably satisfies the following condition (4), and more preferably satisfies the following condition (4-1).

[0025] 10 < (f2 × f3 × f4) / (f × f × f) < 300 (4)

[0026] 30 < (f2 × f3 × f4) / (f × f × f) < 200 (4-1)

[0027] When the air conversion distance on the optical axis from the closest image-side lens surface of the fourth lens group to the focal position on the image side of the entire system is set to Bf, the endoscope objective involved in the above method preferably satisfies the following condition (5), and more preferably satisfies the following condition (5-1).

[0028] 1 < AL / Bf < 4 (5)

[0029] 2.2 < AL / Bf < 3 (5-1)

[0030] When the distance on the optical axis from the lens surface on the object side of the first lens group to the aperture is set to Df, and the distance on the optical axis from the aperture to the lens surface closest to the image side of the fourth lens group is set to Dr, the endoscope objective involved in the above method preferably satisfies the following condition (6), and more preferably satisfies the following condition (6-1).

[0031] |Df-Dr| / AL<1 (6)

[0032] 0.01<|Df-Dr| / AL<0.15 (6-1)

[0033] When the focal length of the third lens group is set to f3 and the focal length of the fourth lens group is set to f4, the endoscope objective involved in the above method preferably satisfies the following condition (7), and more preferably satisfies the following condition (7-1).

[0034] f4 / f3<5 (7)

[0035] 0.3 < f4 / f3 < 3 (7-1)

[0036] When the thickness of the second lens group on the optical axis is set to Dc2 and the thickness of the fourth lens group on the optical axis is set to Dc4, the endoscope objective lens involved in the above method preferably satisfies the following condition (8).

[0037] Dc4 / Dc2 < 1.2 (8)

[0038] The endoscope of the present invention comprises at least one of the endoscope objectives described above.

[0039] In addition, the terms "composed of" and "composed of" in this specification mean that, in addition to the constituent elements listed, it may also include: a lens that does not substantially have refractive power; optical elements other than the lens, such as aperture, filter and cover glass; and lens flange, lens barrel and imaging element, etc.

[0040] Compound aspherical lenses (i.e., lenses in which a spherical lens and an aspherical film formed on the spherical lens are integrated and function as a single aspherical lens) are not considered as combined lenses, but rather as a single lens. Regarding lenses including aspherical surfaces, unless otherwise specified, the sign of the refractive power and the radius of curvature are defined in the paraxial region.

[0041] In this specification, "the entire system" refers to the endoscope objective. The "focal length" used in the conditional formulas is the paraxial focal length. The values ​​used in the conditional formulas are based on the d-line. The "d-line," "C-line," and "F-line" described in this specification are bright lines; the wavelength of the d-line is 587.56 nm, the wavelength of the C-line is 656.27 nm, and the wavelength of the F-line is 486.13 nm.

[0042] Invention Effects

[0043] According to the present invention, it is possible to provide an endoscope objective that combines chromatic aberration correction and miniaturization with good performance, and an endoscope having the endoscope objective. Attached Figure Description

[0044] Figure 1 This is a cross-sectional view showing the structure of an endoscopic objective lens according to one embodiment.

[0045] Figure 2 This is a cross-sectional view showing the structure and beam of the endoscope objective lens of Embodiment 1.

[0046] Figure 3 These are aberration diagrams of the endoscope objective lens in Example 1.

[0047] Figure 4 This is a cross-sectional view showing the structure and beam of the endoscope objective lens of Embodiment 2.

[0048] Figure 5 This is an aberration diagram of the objective lens for endoscopy in Example 2.

[0049] Figure 6 This is a cross-sectional view showing the structure and beam of the endoscope objective lens of Example 3.

[0050] Figure 7 These are aberration diagrams of the objective lens for the endoscope in Example 3.

[0051] Figure 8 This is a cross-sectional view showing the structure and beam of the endoscope objective lens of Example 4.

[0052] Figure 9 This is an aberration diagram of the objective lens for endoscopy in Example 4.

[0053] Figure 10 This is a cross-sectional view showing the structure and beam of the objective lens for the endoscope in Example 5.

[0054] Figure 11 These are aberration diagrams of the objective lens for the endoscope in Example 5.

[0055] Figure 12 This is a cross-sectional view showing the structure and beam of the endoscope objective lens of Embodiment 6.

[0056] Figure 13 These are aberration diagrams of the objective lens for the endoscope in Example 6.

[0057] Figure 14 This is a cross-sectional view showing the structure and beam of the endoscope objective lens of Embodiment 7.

[0058] Figure 15 These are aberration diagrams of the objective lens for the endoscope in Example 7.

[0059] Figure 16 This is a cross-sectional view showing the structure and beam of the endoscope objective lens of Example 8.

[0060] Figure 17 This is an aberration diagram of the objective lens for endoscopy in Example 8.

[0061] Figure 18This is a schematic structural diagram of an endoscope according to one embodiment of the present invention.

[0062] Symbol Explanation

[0063] 1-Endoscope objective lens, 2-On-axis beam, 3-Brightest beam with maximum viewing angle, 5-Imaging element, 100-Endoscope, 102-Operating part, 104-Insertion part, 106-Universal lanyard, 107-Flexible part, 108-Bending part, 109-Bending operating knob, 110-Front end, G1-First lens group, G2-Second lens group, G3-Third lens group, G4-Fourth lens group, L1~L7-Lens, PP-Optical components, Pr-Prism, Sim-Image plane, St-Aperture ring, Z-Optical axis. Detailed Implementation

[0064] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Figure 1 The diagram shows the structure in a cross-section of the optical axis Z of an endoscope objective lens 1 according to an embodiment of the present invention. Figure 1 The example corresponds to Embodiment 1 described later. Figure 1 In the diagram, the left side of the endoscope objective lens 1 is the object side, and the optical axis Z is represented by a single-dotted line. Light from the object is incident on the image side of the prism Pr, which is positioned after the transmission endoscope objective lens 1. After being reflected on a reflecting surface that is inclined relative to the incident surface, the light passes through an exit surface that is perpendicular to the incident surface and then enters the imaging element 5. The exit surface of the prism Pr is engaged with the imaging element 5. Figure 1 The prism Pr functions as a light path deflection component that bends the light path perpendicularly.

[0065] The endoscope objective lens 1, along the optical axis Z from the object side to the image side, consists of lens group 1 G1, lens group 2 G2, aperture diaphragm St, lens group 3 G3, and lens group 4 G4. Additionally, Figure 1 The aperture St does not represent the shape, but rather the position along the Z-axis of the optical axis.

[0066] The first lens group G1 consists of a single negative lens. By designing the first lens group G1 as a single lens with negative refractive power, miniaturization is achieved while ensuring a wide field of view for observing the endoscope. Furthermore, in this specification, "single lens" refers to a single lens that is not joined together.

[0067] The second lens group G2 consists of two lenses joined together. That is, the second lens group G2 is a joined lens formed by the joining of two lenses. The two lenses constituting the second lens group G2 can be two lenses with refractive powers of different signs, or two lenses with refractive powers of the same sign. Two lenses with refractive powers of different signs refer to a positive lens and a negative lens. In this case, the order of the positive and negative lenses is irrelevant. When the second lens group G2 consists of two lenses with refractive powers of different signs, it is beneficial for correcting on-axis chromatic aberration and magnification chromatic aberration. When the second lens group G2 consists of two positive lenses, it is beneficial for correcting astigmatism associated with miniaturization.

[0068] The second lens group G2 preferably has positive refractive power as a whole. Because the second lens group G2 has positive refractive power as a whole, it is beneficial to suppress astigmatism and image plane curvature.

[0069] The third lens group G3 consists of two lenses with refractive powers of different signs that are joined together. That is, the third lens group G3 can be a combined lens in which a positive lens and a negative lens are joined sequentially from the object side, or it can be a combined lens in which a negative lens and a positive lens are joined sequentially from the object side. The above structure of the third lens group G3 is beneficial for the correction of axial chromatic aberration and magnification chromatic aberration.

[0070] The third lens group G3 preferably has positive refractive power as a whole. Because the third lens group G3 has positive refractive power as a whole, it is beneficial to suppress astigmatism and image plane curvature.

[0071] The fourth lens group G4 consists of two lenses with refractive powers of different signs that are joined together. That is, the fourth lens group G4 can be a combined lens in which a positive lens and a negative lens are joined sequentially from the object side, or it can be a combined lens in which a negative lens and a positive lens are joined sequentially from the object side. The above structure of the fourth lens group G4 is beneficial for the correction of chromatic aberration.

[0072] The fourth lens group G4 preferably has positive refractive power as a whole. Since the fourth lens group G4 has positive refractive power as a whole, it helps to suppress the increase of chromatic aberration at magnification while also suppressing the large-diameter of the lens that is closer to the image side than the aperture St.

[0073] As described above, the endoscope objective 1 consists of seven lenses and an aperture st. Figure 1 In the endoscope objective lens 1 shown, the first lens group G1 consists of one lens L1; the second lens group G2 consists of lenses L2 and L3 sequentially from the object side to the image side; the third lens group G3 consists of lenses L4 and L5 sequentially from the object side to the image side; and the fourth lens group G4 consists of lenses L6 and L7 sequentially from the object side to the image side. As an example, in... Figure 1In the example, lenses L1, L3, L4, and L7 are negative lenses, while lenses L2, L5, and L6 are positive lenses.

[0074] When the Abbe number of the d-line reference of the object-side lens in the second lens group G2 is set to ν2f, the Abbe number of the d-line reference of the image-side lens in the second lens group G2 is set to ν2r, the distance on the optical axis Z from the object-side lens surface of the first lens group G1 to the closest image-side lens surface of the fourth lens group G4 is set to AL, and the paraxial radius of curvature of the joint surface of the second lens group G2 is set to R2c, the endoscope objective lens 1 preferably satisfies the following conditional expression (1). By satisfying conditional expression (1), excessive chromatic aberration correction in the second lens group G2 is suppressed to maintain good chromatic aberration correction, which is beneficial for miniaturization. By setting it to not be below the lower limit of conditional expression (1), the absolute value of the radius of curvature of the joint surface of the second lens group G2 can be increased, thus reducing the thickness related to the surface curvature of the joint surface closer to the object side than the aperture St. This is beneficial for miniaturization. Furthermore, "thickness related to the curvature of the mating surface" is the distance in the Z-direction between the point where the mating surface intersects the optical axis Z and the point at the effective diameter end of the mating surface. By setting it to not exceed the upper limit of condition (1), it is beneficial to suppress the increase in the total length of the lens system. To obtain better characteristics, the endoscope objective 1 more preferably satisfies the following condition (1-1).

[0075] -300<(ν2f-ν2r)×|AL / R2c|<30 (1)

[0076] -250<(ν2f-ν2r)×|AL / R2c|<20 (1-1)

[0077] When the Abbe number of the d-line reference of the object-side lens in the third lens group G3 is set to ν3f, the Abbe number of the d-line reference of the image-side lens in the third lens group G3 is set to ν3r, the Abbe number of the d-line reference of the object-side lens in the fourth lens group G4 is set to ν4f, the Abbe number of the d-line reference of the image-side lens in the fourth lens group G4 is set to ν4r, the larger of the values ​​of |ν3f-ν3r| and |ν4f-ν4r| is set to |νf-νr|, the focal length of the entire system is set to f, the distance from the object-side lens surface of the first lens group G1 to the closest image-side lens surface of the fourth lens group G4 on the optical axis Z is set to AL, and the air conversion distance from the closest image-side lens surface of the fourth lens group G4 to the focal position of the image side of the entire system on the optical axis Z is set to Bf, the endoscope objective lens 1 preferably satisfies the following condition (2). By setting it below the lower limit of condition (2), chromatic aberration correction is facilitated while suppressing the increase in the total length of the lens system. By setting it above the upper limit of condition (2), miniaturization is facilitated while ensuring the various performance characteristics of the endoscope objective 1. The various performance characteristics referred to here include not only performance related to various aberrations, including chromatic aberration, but also performance related to specifications such as field of view and focal length. In order to obtain better characteristics, the endoscope objective 1 more preferably satisfies the following condition (2-1).

[0078] 3.9<|νf-νr|×f / (AL+Bf)<20 (2)

[0079] 4<|νf-νr|×f / (AL+Bf)<10 (2-1)

[0080] When the focal length of the first lens group G1 is set to f1, the focal length of the second lens group G2 is set to f2, the focal length of the third lens group G3 is set to f3, and the focal length of the fourth lens group G4 is set to f4, the endoscope objective lens 1 preferably satisfies the following conditional expression (3). By setting it to be below the lower limit of conditional expression (3), it is easy to suppress the deviation of the refractive power of the third lens group G3 and the fourth lens group G4, thus helping to suppress the large-diameter of the lens that is closer to the image side than the aperture St. Here, the so-called deviation of the refractive power of the third lens group G3 and the fourth lens group G4 means that the refractive power of one lens group of the third lens group G3 and the fourth lens group G4 becomes too strong, and the refractive power of the other lens group becomes too weak. Furthermore, by setting it to be below the lower limit of conditional expression (3), while setting the endoscope objective lens 1 to a focal-shifted structure, the refractive power of the lens group that is closer to the image side than the aperture St is adjusted, and it is beneficial to ensure an appropriate back focal length. By setting the value to be above the upper limit of conditional expression (3), it is easier to suppress the deviation of the refractive power of the first lens group G1 and the second lens group G2, thus helping to suppress the large-diameter increase of the lens closer to the object side than the aperture St. Here, the deviation of the refractive power of the first lens group G1 and the second lens group G2 refers to the situation where the refractive power of one lens group becomes too strong and the refractive power of the other lens group becomes too weak. Furthermore, by setting the value to be above the upper limit of conditional expression (3), it is beneficial to ensure a wide field of view. To obtain better characteristics, the endoscope objective lens 1 more preferably satisfies the following conditional expression (3-1).

[0081] -30<(f3×f4) / (f1×f2)<-0.2 (3)

[0082] -20 < (f3 × f4) / (f1 × f2) < -0.4 (3-1)

[0083] When the focal length of the entire system is set to f, and the focal length of the second lens group G2 is set to f2, the focal length of the third lens group G3 is set to f3, and the focal length of the fourth lens group G4 is set to f4, the endoscope objective 1 preferably satisfies the following conditional expression (4). By setting it to be below the lower limit of conditional expression (4), it is beneficial to maintain the refractive power that contributes to optical performance such as wide field of view and high resolution. By setting it to be above the upper limit of conditional expression (4), it is easy to suppress the deviation of the refractive power of the second lens group G2, the third lens group G3, and the fourth lens group G4, and thus it is beneficial to suppress the increase in the total length of the lens system. In order to obtain better characteristics, the endoscope objective 1 more preferably satisfies the following conditional expression (4-1).

[0084] 10 < (f2 × f3 × f4) / (f × f × f) < 300 (4)

[0085] 30 < (f2 × f3 × f4) / (f × f × f) < 200 (4-1)

[0086] When the distance along the optical axis Z from the object-side lens surface of the first lens group G1 to the closest image-side lens surface of the fourth lens group G4 is set to AL, and the air-converted distance along the optical axis Z from the closest image-side lens surface of the fourth lens group G4 to the image-side focal position of the entire system is set to Bf, the endoscope objective lens 1 preferably satisfies the following conditional expression (5). By setting it to be below the lower limit of conditional expression (5), it is beneficial to obtain good resolving performance. In addition, as Figure 1 As shown, when the endoscope objective 1 and imaging element 5 are used in combination, various filters are usually arranged between the endoscope objective 1 and imaging element 5, thus requiring a relatively long back focal length. Furthermore, as... Figure 1 In the type where the imaging surface of the imaging element 5 is arranged parallel to the long axis of the insertion part of the endoscope, a light path deflection component such as a prism Pr is usually inserted between the lens system and the imaging element 5 to deflect the light path, thus requiring a sufficiently long back focal length. By setting it to not exceed the upper limit of condition (5), it is advantageous to ensure a sufficiently long back focal length required in the above configuration. To obtain better characteristics, the endoscope objective lens 1 more preferably satisfies the following condition (5-1).

[0087] 1 < AL / Bf < 4 (5)

[0088] 2.2 < AL / Bf < 3 (5-1)

[0089] When the distance from the object-side lens surface of the first lens group G1 to the aperture St on the optical axis Z is set to Df, the distance from the aperture St to the closest image-side lens surface of the fourth lens group G4 on the optical axis Z is set to Dr, and the distance from the object-side lens surface of the first lens group G1 to the closest image-side lens surface of the fourth lens group G4 on the optical axis Z is set to AL, the endoscope objective 1 preferably satisfies the following conditional expression (6). By satisfying conditional expression (6), it is easy to maintain the balance between the length of the object-side lens system of the aperture St and the length of the image-side lens system of the aperture St. As a result, it is possible to reduce the deviation of the object-side and image-side lens systems of the aperture St in the optical axis Z direction and radial dimension, thus contributing to miniaturization. To obtain better characteristics, the endoscope objective 1 more preferably satisfies the following conditional expression (6-1). By setting it to not be below the lower limit of conditional expression (6-1), it is beneficial to suppress the increase in the overall length of the lens system while maintaining optical performance such as wide field of view and high resolution.

[0090] |Df-Dr| / AL<1 (6)

[0091] 0.01<|Df-Dr| / AL<0.15 (6-1)

[0092] When the focal length of the third lens group G3 is set to f3 and the focal length of the fourth lens group G4 is set to f4, the endoscope objective 1 preferably satisfies the following conditional expression (7). By satisfying conditional expression (7), it is easy to suppress the deviation of the refractive power of the third lens group G3 and the fourth lens group G4, thus helping to suppress the large-diameter of the lens that is closer to the image side than the aperture St. To obtain better characteristics, the endoscope objective 1 more preferably satisfies the following conditional expression (7-1). By setting it to not be below the lower limit of conditional expression (7-1), the refractive power of the third lens group G3 and the fourth lens group G4 can be well adjusted, thus helping to suppress the generation of astigmatism and image plane curvature.

[0093] f4 / f3<5 (7)

[0094] 0.3 < f4 / f3 < 3 (7-1)

[0095] When the thickness of the second lens group G2 on the optical axis Z is set to Dc2 and the thickness of the fourth lens group G4 on the optical axis Z is set to Dc4, the endoscope objective 1 preferably satisfies the following conditional expression (8). Furthermore, the thickness of the second lens group G2 on the optical axis Z is the distance on the optical axis Z from the lens surface of the second lens group G2 closest to the object side to the lens surface of the second lens group G2 closest to the image side. Similarly, the thickness of the fourth lens group G4 on the optical axis Z is the distance on the optical axis Z from the lens surface of the fourth lens group G4 closest to the object side to the lens surface of the fourth lens group G4 closest to the image side. By satisfying conditional expression (8), it is beneficial to suppress the increase in length of the lens system closer to the image side than the aperture St in the optical axis Z direction. To obtain better characteristics, the endoscope objective 1 more preferably satisfies the following conditional expression (8-1). By setting it to not be a lower limit of conditional expression (8-1), it is beneficial to suppress the increase in length of the lens system closer to the object side than the aperture St in the optical axis Z direction.

[0096] Dc4 / Dc2 < 1.2 (8)

[0097] 0.5 < Dc4 / Dc2 < 1 (8-1)

[0098] In addition, Figure 1 The prism Pr is used as a light path deflection component, but the light path deflection component is not limited to the prism Pr; it can also be a mirror or a diffractive optical element, etc. Furthermore, the angle at which the light path is bent can be any angle other than perpendicular.

[0099] Structures related to conditional expressions are also included. The preferred and implementable structures described above can be any combination, and are preferably selectively adopted appropriately according to the required specifications. Furthermore, the scope of implementable conditional expressions is not limited to conditional expressions written in formula form, but includes the range obtained by arbitrarily combining the lower and upper limits from the preferred and more preferred conditional expressions.

[0100] The following describes two preferred configurations of the endoscope objective lens 1. The first configuration consists of the following parts in sequence from the object side to the image side: a first lens group G1, consisting of a negative lens; a second lens group G2, consisting of two lenses joined together; an aperture diaphragm St; a third lens group G3, consisting of two lenses joined together with refractive powers of different signs; and a fourth lens group G4, consisting of two lenses joined together with refractive powers of different signs, and satisfying the above condition (1).

[0101] The second method consists of the following parts in sequence from the object side to the image side: a first lens group G1, consisting of a negative lens; a second lens group G2, consisting of two lenses joined together; an aperture St; a third lens group G3, consisting of two lenses joined together with refractive powers of different signs; and a fourth lens group G4, consisting of two lenses joined together with refractive powers of different signs, and satisfying the above condition (2).

[0102] Next, an embodiment of the objective lens for endoscopy of the present invention will be described.

[0103] [Example 1]

[0104] A cross-sectional view showing the structure and beam of the endoscopic objective lens of Example 1 is shown below. Figure 2 In. Figure 2 In the image, the left side is the object side, and the right side is the image side. The on-axis beam 2 and the beam 3 with the maximum viewing angle are shown as light beams. Figure 2 For ease of understanding, a diagram is shown in which the optical system is deployed in such a way that the optical path from the endoscope objective to the image plane Sim is a straight line. Figure 2 The optical component PP includes at least one of an optical path deflection component, various filters, and a cover glass. In the Z-axis direction, the position of the image-side surface of the optical component PP is the position of the image plane Sim. Figure 2 The aperture St does not represent the shape, but rather the position on the optical axis Z.

[0105] The objective lens of the endoscope in Example 1 consists, from the object side to the image side, a first lens group G1 with negative refractive power, a second lens group G2 with positive refractive power, an aperture St, a third lens group G3 with positive refractive power, and a fourth lens group G4 with positive refractive power. The first lens group G1 consists of a lens L1, which is a negative lens. Lens L1 is a single lens. The second lens group G2 consists of a conjoined lens formed by sequentially joining a lens L2 (positive lens) and a lens L3 (negative lens) from the object side to the image side. The third lens group G3 consists of a conjoined lens formed by sequentially joining a lens L4 (negative lens) and a lens L5 (positive lens) from the object side to the image side. The fourth lens group G4 consists of a conjoined lens formed by sequentially joining a lens L6 (positive lens) and a lens L7 (negative lens) from the object side to the image side. Furthermore, the phrase "the first lens group G1 with negative refractive power" refers to the fact that the first lens group G1 as a whole has negative refractive power. Similarly, "the second lens group G2 with positive refractive power" means that the second lens group G2 as a whole has positive refractive power. The same applies to "the third lens group G3 with positive refractive power" and "the fourth lens group G4 with positive refractive power".

[0106] Regarding the endoscopic objective lens of Example 1, basic lens data are shown in Table 1, and specifications are shown in Table 2. In Table 1, the Sn column shows the surface number when the surface closest to the object side is designated as surface 1 and the numbering increases sequentially towards the image side; the R column shows the radius of curvature of each surface; and the D column shows the surface spacing along the optical axis Z between each surface and its image-side adjacent surface. The Nd column shows the refractive index of each component relative to the d-line, and the νd column shows the Abbe number of each component based on the d-line reference.

[0107] In Table 1, the radius of curvature of surfaces with a convex shape facing the object side is marked positive, and the radius of curvature of surfaces with a convex shape facing the image side is marked negative. Table 1 also shows the aperture st and optical components PP. In Table 1, the surface number and (St) are recorded in the surface number column corresponding to the aperture st. The value in the bottom column of D in Table 1 is the distance between the surface closest to the image side and the image plane Sim.

[0108] Table 2 shows the focal length f, back focal length in air equivalent distance, F-number FNo., and maximum full field of view 2ω for endoscopic objectives. Additionally, in the description of the embodiments, the field of view is expressed as a visual angle. The (°) in the 2ω column indicates that the unit is degrees. Table 2 shows the values ​​based on the d-line reference.

[0109] The tables shown below contain values ​​rounded to a predetermined number of decimal places. Furthermore, all data in the embodiments described below are based on the focal length of the endoscopic objective lens normalized to 1.00. The optical system can also be used with magnification or reduction.

[0110] [Table 1]

[0111] Example 1

[0112] 1 ∞ 0.4151 1.88300 40.95 2 0.8610 0.3459 3 ∞ 0.9685 1.80518 25.46 4 -1.2578 0.8185 1.59522 67.73 5 -1.5718 0.2591 6 (St) ∞ 0.0484 7 ∞ 0.4151 1.95375 32.32 8 1.4564 1.2867 1.49700 81.61 9 -1.4564 0.1384 10 ∞ 0.9685 2.00100 29.13 11 -1.6670 0.3597 1.94595 17.98 12 -4.0092 0.4987 13 ∞ 3.0093 1.55920 53.92 14 ∞ 0.0000

[0113] [Table 2]

[0114] Example 1

[0115] Bf 2.34 FNo. 5.42 2ω(°) 135.4

[0116] Figure 3 The diagram shows the aberrations of the endoscope objective lens of Example 1 when the object distance is 10.1. The object distance is the distance from the object to the object-side surface of lens L1 along the optical axis Z. Figure 3 The diagram, from left to right, shows the spherical aberration diagram, astigmatism diagram, distortion aberration diagram, and magnification chromatic aberration diagram. In the spherical aberration diagram, solid lines, long dashed lines, and short dashed lines represent aberrations along the d-line, C-line, and F-line, respectively. In the astigmatism diagram, solid lines represent aberrations along the d-line in the sagittal direction, and short dashed lines represent aberrations along the d-line in the meridional direction. In the distortion aberration diagram, solid lines represent aberrations along the d-line. In the magnification chromatic aberration diagram, long dashed lines and short dashed lines represent aberrations along the C-line and F-line, respectively. In the spherical aberration diagram, FNo. refers to the F-value; in the other aberration diagrams, ω refers to the half-angle of view. Figure 3 The data contains the FNo. and ω value corresponding to the upper end of the vertical axis of each figure.

[0117] Unless otherwise stated, the symbols, meanings, recording methods, and illustrations of the data related to Embodiment 1 above are the same in the following embodiments, and therefore repeated descriptions are omitted below.

[0118] [Example 2]

[0119] A cross-sectional view showing the structure and beam of the endoscopic objective lens of Example 2 is shown below. Figure 4 In Example 2, the endoscopic objective lens, from the object side to the image side, consists of a first lens group G1 with negative refractive power, a second lens group G2 with positive refractive power, an aperture St, a third lens group G3 with positive refractive power, and a fourth lens group G4 with positive refractive power. The first lens group G1 consists of a lens L1, which is a negative lens. Lens L1 is a single lens. The second lens group G2 consists of a conjoined lens formed by sequentially joining lenses L2 and L3, which are positive lenses, from the object side to the image side. The third lens group G3 consists of a conjoined lens formed by sequentially joining lenses L4 and L5, which are negative lenses, from the object side to the image side. The fourth lens group G4 consists of a conjoined lens formed by sequentially joining lenses L6, which are negative lenses, and L7, which are positive lenses, from the object side to the image side.

[0120] Regarding the endoscopic objective lens of Example 2, the basic lens data is shown in Table 3, the specifications are shown in Table 4, and the aberrations at an object distance of 10.1 are illustrated in Table 5. Figure 5 middle.

[0121] [Table 3]

[0122] Practical example 2

[0123] 1 ∞ 0.4150 1.88300 40.95 2 0.8911 0.3458 3 ∞ 1.0097 1.80518 25.46 4 -1.6782 0.7746 1.95375 32.32 5 -2.0321 0.2213 6 (St) ∞ 0.0484 7 ∞ 0.4841 2.00100 29.13 8 1.6782 1.1204 1.64769 33.79 9 -1.6782 0.1660 10 ∞ 0.3596 1.92286 20.88 11 2.2972 0.8714 1.69680 55.53 12 -2.2972 0.6128 13 ∞ 3.0085 1.55920 53.92 14 ∞ 0.0000

[0124] [Table 4]

[0125] Example 2

[0126] Bf 2.45 FNo. 5.64 2ω(°) 136.4

[0127] [Example 3]

[0128] A cross-sectional view showing the structure and beam of the endoscopic objective lens of Example 3 is shown below. Figure 6 In Example 3, the endoscopic objective lens, from the object side to the image side, consists of a first lens group G1 with negative refractive power, a second lens group G2 with positive refractive power, an aperture St, a third lens group G3 with positive refractive power, and a fourth lens group G4 with positive refractive power. The first lens group G1 consists of a lens L1, which is a negative lens. Lens L1 is a single lens. The second lens group G2 consists of a conjoined lens formed by sequentially joining a lens L2 (positive lens) and a lens L3 (negative lens) from the object side to the image side. The third lens group G3 consists of a conjoined lens formed by sequentially joining a lens L4 (positive lens) and a lens L5 (negative lens) from the object side to the image side. The fourth lens group G4 consists of a conjoined lens formed by sequentially joining a lens L6 (positive lens) and a lens L7 (negative lens) from the object side to the image side.

[0129] Regarding the endoscopic objective lens of Example 3, basic lens data are shown in Table 5, specifications are shown in Table 6, and aberrations at an object distance of 9.6 are illustrated in Table 6. Figure 7 middle.

[0130] [Table 5]

[0131] Practical example 3

[0132] 1 ∞ 0.3276 1.83006 44.99 2 0.8651 0.3670 3 27.0835 0.9961 1.66701 32.36 4 -1.1915 0.8519 1.91038 36.96 5 -2.6212 0.2987 6 (St) ∞ 0.0459 7 26.2128 0.9961 1.69474 48.91 8 -1.5419 0.3932 1.41999 70.00 9 -2.6213 0.1311 10 3.1420 0.7995 1.54471 73.12 11 -1.6383 0.3014 1.99999 18.55 12 -3.8808 0.4047 13 ∞ 2.8506 1.55920 53.92 14 ∞ 0.0000

[0133] [Table 6]

[0134] Example 3

[0135] Bf 2.13 FNo. 5.79 2ω(°) 133.4

[0136] [Example 4]

[0137] A cross-sectional view showing the structure and beam of the endoscopic objective lens of Example 4 is shown below. Figure 8 In Example 4, the endoscopic objective lens, from the object side to the image side, consists of a first lens group G1 with negative refractive power, a second lens group G2 with positive refractive power, an aperture St, a third lens group G3 with positive refractive power, and a fourth lens group G4 with positive refractive power. The first lens group G1 consists of a lens L1, which is a negative lens. Lens L1 is a single lens. The second lens group G2 consists of a conjoined lens formed by sequentially joining a lens L2 (positive lens) and a lens L3 (negative lens) from the object side to the image side. The third lens group G3 consists of a conjoined lens formed by sequentially joining a lens L4 (positive lens) and a lens L5 (negative lens) from the object side to the image side. The fourth lens group G4 consists of a conjoined lens formed by sequentially joining a lens L6 (negative lens) and a lens L7 (positive lens) from the object side to the image side.

[0138] Regarding the endoscopic objective lens of Example 4, basic lens data are shown in Table 7, specifications are shown in Table 8, and aberrations at an object distance of 10.1 are illustrated in Table 8. Figure 9 middle.

[0139] [Table 7]

[0140] Example 4

[0141] 1 96.9417 0.4293 1.87627 40.37 2 0.8527 0.3670 3 -8.8396 0.9832 1.83769 23.12 4 -1.3849 0.9971 1.92326 35.67 5 -2.0269 0.2770 6 (St) ∞ 0.0485 7 -51.6758 0.8447 1.64553 59.22 8 -1.7311 0.5539 2.01162 26.37 9 -2.3964 0.1385 10 4.6969 0.3462 2.02196 25.69 11 1.5388 1.2325 1.60940 63.17 12 -2.5645 0.4270 13 ∞ 3.0121 1.55920 53.92 14 ∞ 0.0000

[0142] [Table 8]

[0143] Example 4

[0144] Bf 2.27 FNo. 5.73 2ω(°) 135.4

[0145] [Example 5]

[0146] A cross-sectional view showing the structure and beam of the endoscopic objective lens of Example 5 is shown in [the figure]. Figure 10 In Example 5, the endoscopic objective lens, from the object side to the image side, consists of a first lens group G1 with negative refractive power, a second lens group G2 with positive refractive power, an aperture St, a third lens group G3 with positive refractive power, and a fourth lens group G4 with positive refractive power. The first lens group G1 consists of a lens L1, which is a negative lens. Lens L1 is a single lens. The second lens group G2 consists of a conjoined lens formed by sequentially joining a negative lens L2 and a positive lens L3 from the object side to the image side. The third lens group G3 consists of a conjoined lens formed by sequentially joining a negative lens L4 and a positive lens L5 from the object side to the image side. The fourth lens group G4 consists of a conjoined lens formed by sequentially joining a positive lens L6 and a negative lens L7 from the object side to the image side.

[0147] Regarding the endoscopic objective lens of Example 5, basic lens data are shown in Table 9, specifications are shown in Table 10, and aberrations at an object distance of 10.1 are illustrated in Table 10. Figure 11 middle.

[0148] [Table 9]

[0149] Example 5

[0150] 1 ∞ 0.4292 2.00100 29.13 2 0.8666 0.3461 3 10.1771 0.4646 1.99999 21.50 4 2.0746 0.8445 1.94779 17.61 5 -1.7349 0.2769 6 (St) ∞ 0.0762 7 -13.8444 0.4154 2.05001 20.90 8 1.7307 1.0522 1.50688 78.94 9 -1.2452 0.1384 10 16.3244 0.8445 1.92967 35.03 11 -1.7307 0.3461 2.05001 20.90 12 -3.4853 0.4462 13 ∞ 3.0114 1.55920 53.92 14 ∞ 0.0000

[0151] [Table 10]

[0152] Example 5

[0153] Bf 2.28 FNo. 5.71 2ω(°) 134.0

[0154] [Example 6]

[0155] A cross-sectional view showing the structure and beam of the endoscopic objective lens of Example 6 is shown in [the figure]. Figure 12 In Example 6, the endoscopic objective lens, from the object side to the image side, consists of a first lens group G1 with negative refractive power, a second lens group G2 with positive refractive power, an aperture St, a third lens group G3 with positive refractive power, and a fourth lens group G4 with positive refractive power. The first lens group G1 consists of a lens L1, which is a negative lens. Lens L1 is a single lens. The second lens group G2 consists of a conjoined lens formed by sequentially joining a negative lens L2 and a positive lens L3 from the object side to the image side. The third lens group G3 consists of a conjoined lens formed by sequentially joining a negative lens L4 and a positive lens L5 from the object side to the image side. The fourth lens group G4 consists of a conjoined lens formed by sequentially joining a negative lens L6 and a positive lens L7 from the object side to the image side.

[0156] Regarding the endoscopic objective lens of Example 6, basic lens data are shown in Table 11, specifications are shown in Table 12, and aberrations at an object distance of 10.1 are illustrated in Table 12. Figure 13 middle.

[0157] [Table 11]

[0158] Example 6

[0159] 1 ∞ 0.4293 2.05090 26.94 2 0.8651 0.3462 3 -63.6381 0.4847 2.00001 27.35 4 3.1984 1.0108 2.00000 20.69 5 -1.7738 0.2770 6 (St) ∞ 0.0485 7 -13.8470 0.4154 1.83613 33.79 8 1.6626 0.9831 1.57676 68.19 9 -1.6291 0.1385 10 12.6805 0.3462 2.02973 25.03 11 1.7169 0.8446 1.71702 55.65 12 -2.2287 0.4402 13 ∞ 3.0118 1.55920 53.92 14 ∞ 0.0000

[0160] [Table 12]

[0161] Example 6

[0162] Bf 2.28 FNo. 5.73 2ω(°) 130.8

[0163] [Example 7]

[0164] A cross-sectional view showing the structure and beam of the endoscopic objective lens of Example 7 is shown below. Figure 14 In Example 7, the endoscopic objective lens, from the object side to the image side, consists of a first lens group G1 with negative refractive power, a second lens group G2 with positive refractive power, an aperture St, a third lens group G3 with positive refractive power, and a fourth lens group G4 with positive refractive power. The first lens group G1 consists of a lens L1, which is a negative lens. Lens L1 is a single lens. The second lens group G2 consists of a conjoined lens formed by sequentially joining a negative lens L2 and a positive lens L3 from the object side to the image side. The third lens group G3 consists of a conjoined lens formed by sequentially joining a positive lens L4 and a negative lens L5 from the object side to the image side. The fourth lens group G4 consists of a conjoined lens formed by sequentially joining a positive lens L6 and a negative lens L7 from the object side to the image side.

[0165] Regarding the endoscopic objective lens of Example 7, basic lens data are shown in Table 13, specifications are shown in Table 14, and aberrations at an object distance of 9.7 are illustrated in Table 15. Figure 15 middle.

[0166] [Table 13]

[0167] Example 7

[0168] 1 ∞ 0.3324 1.72574 55.21 2 0.8693 0.3524 3 6.9422 0.4255 2.00000 28.00 4 1.2088 0.9839 1.79711 25.14 5 -10.4940 0.2792 6 (St) ∞ 0.1130 7 -10.2233 0.9573 1.77521 50.48 8 -1.3297 0.3058 1.42000 70.00 9 -1.8995 0.1330 10 3.5391 0.8244 1.62840 60.25 11 -1.6416 0.3058 1.99999 19.62 12 -3.5769 0.4243 13 ∞ 2.8920 1.55920 53.92 14 ∞ 0.0000

[0169] [Table 14]

[0170] Example 7

[0171] Bf 2.18 FNo. 5.76 2ω(°) 134.8

[0172] [Example 8]

[0173] A cross-sectional view showing the structure and beam of the endoscopic objective lens of Example 8 is shown in [the diagram]. Figure 16 In Example 8, the endoscopic objective lens, from the object side to the image side, consists of a first lens group G1 with negative refractive power, a second lens group G2 with positive refractive power, an aperture St, a third lens group G3 with positive refractive power, and a fourth lens group G4 with positive refractive power. The first lens group G1 consists of a lens L1, which is a negative lens. Lens L1 is a single lens. The second lens group G2 consists of a conjoined lens formed by sequentially joining a negative lens L2 and a positive lens L3 from the object side to the image side. The third lens group G3 consists of a conjoined lens formed by sequentially joining a positive lens L4 and a negative lens L5 from the object side to the image side. The fourth lens group G4 consists of a conjoined lens formed by sequentially joining a negative lens L6 and a positive lens L7 from the object side to the image side.

[0174] Regarding the endoscopic objective lens of Example 8, basic lens data are shown in Table 15, specifications are shown in Table 16, and aberrations at an object distance of 10.0 are illustrated in Table 16. Figure 17 middle.

[0175] [Table 15]

[0176] Example 8

[0177] 1 96.2950 0.4952 1.73921 54.08 2 0.7512 0.4815 3 17.3603 0.6190 2.00001 23.92 4 1.7195 1.1143 1.63000 35.00 5 -1.3756 0.2063 6 (St) ∞ 0.0481 7 -9.3206 0.7016 2.00001 15.00 8 -2.3136 0.8254 2.05001 20.90 9 -3.2882 0.1376 10 3.9395 0.3439 2.05000 12.50 11 1.6879 1.0455 1.59250 65.77 12 -3.0623 0.4300 13 ∞ 2.9920 1.55920 53.92 14 ∞ 0.0000

[0178] [Table 16]

[0179] Example 8

[0180] Bf 2.26 FNo. 5.77 2ω(°) 136.2

[0181] Table 17 shows the corresponding values ​​of conditional formulas (1) to (8) for the endoscopic objectives of Examples 1 to 8. In Examples 1 to 8, the d-line was set as the reference wavelength. Table 17 shows the values ​​under the d-line reference.

[0182] [Table 17]

[0183] (1) (ν2f-ν2r)×|AL / R2c| -202.48 -23.78 -21.29 -56.39 (2) |νf-νr|×f / (AL+Bf) 5.89 4.19 7.13 4.41 (3) (f3×f4) / (f1×f2) -16.45 -7.75 -3.34 -8.21 (4) (f2×f3×f4) / (f×f×f) 60.91 49.47 93.99 63.79 (5) AL / Bf 2.58 2.37 2.58 2.74 (6) |Df-Dr| / AL 0.07 0.05 0.03 0.02 (7) f4 / f3 0.45 1.05 2.22 1.15 (8) Dc4 / Dc2 0.74 0.69 0.60 0.80

[0184] (1) (ν2f-ν2r)×|AL / R2c| 9.81 11.08 11.84 -38.76 (2) |νf-νr|×f / (AL+Bf) 7.74 4.54 5.67 6.43 (3) (f3×f4) / (f1×f2) -18.36 -15.24 -0.48 -8.02 (4) (f2×f3×f4) / (f×f×f) 45.35 40.41 184.78 69.71 (5) AL / Bf 2.29 2.34 2.30 2.67 (6) |Df-Dr| / AL 0.10 0.04 0.05 0.03 (7) f4 / f3 0.46 0.80 1.86 1.14 (8) Dc4 / Dc2 0.91 0.80 0.80 0.80

[0185] Although the endoscopic objectives of Examples 1-8 are compact, all aberrations, including chromatic aberration, are well corrected to maintain high optical performance. Furthermore, the endoscopic objectives of Examples 1-8 have a maximum full field of view of 130 degrees or more, ensuring a wide field of view.

[0186] Next, the endoscope according to the embodiments of the present invention will be described. Figure 18 The diagram shows a schematic overall structure of an endoscope according to an embodiment of the present invention. Figure 18The endoscope 100 shown mainly includes an operating section 102, an insertion section 104, and a universal plug 106 connected to a connector section (not shown). The insertion section 104 is mostly a flexible section 107 that bends in any direction along the insertion path. A bending section 108 is connected to the front end of the flexible section 107, and a front end section 110 is connected to the front end of the bending section 108. The bending section 108 is provided to orient the front end section 110 in a desired direction, and bending can be performed by rotating a bending operation knob 109 provided on the operating section 102. An endoscope objective lens 1 and an imaging element 5 according to an embodiment of the present invention are disposed at the inner front end of the front end section 110. The imaging element 5 is a CCD (Charge-Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor), etc. The imaging element 5 is arranged such that its imaging plane coincides with the image plane of the endoscope objective lens 1. Furthermore, Figure 18 The endoscope objective lens 1 and imaging element 5 are shown conceptually.

[0187] The above description, through examples and embodiments, illustrates the technology of the present invention. However, the technology of the present invention is not limited to the above examples and embodiments, and various modifications are possible. For example, the radius of curvature, interplanar spacing, refractive index, and Abbe number of each lens are not limited to the values ​​shown in the above embodiments, and other values ​​may be used.

Claims

1. An objective lens for an endoscope, comprising, from the object side to the image side, the following parts in sequence: The first lens group consists of a negative lens, the image plane side of which is concave; the second lens group consists of two lenses joined together, which have positive refractive power, the image plane side of the lens closest to the image plane side is convex. aperture; The third lens group consists of two lenses with different signs of refractive power that are joined together. It has positive refractive power, and the image plane of the lens closest to the image side is convex. The fourth lens group consists of two lenses with different signs of refractive power that are joined together. It has positive refractive power, and the image plane of the lens closest to the image side is convex. The Abbe number of the d-line reference of the object-side lens in the second lens group is set as ν2f. The Abbe number of the d-line reference of the image-side lens in the second lens group is set as ν2r. The distance along the optical axis from the lens surface on the object side of the first lens group to the lens surface closest to the image side of the fourth lens group is denoted as AL. The paraxial radius of curvature of the joint surface of the second lens group is set as R2c. Set the focal length of the first lens group to f1. Set the focal length of the second lens group to f2. Set the focal length of the third lens group to f3. When the focal length of the fourth lens group is set to f4... The objective lens for the endoscope satisfies the following conditions (1) and (3-2): -300<(ν2f-ν2r)×|AL / R2c|<30 (1) -20 < (f3 × f4) / (f1 × f2) < -7.75 (3-2).

2. An objective lens for an endoscope, comprising, from the object side to the image side, the following parts in sequence: The first lens group consists of a negative lens, the image plane side of which is concave; the second lens group consists of two lenses joined together, which have positive refractive power, the image plane side of the lens closest to the image plane side is convex. aperture; The third lens group consists of two lenses with different signs of refractive power that are joined together. It has positive refractive power, and the image plane of the lens closest to the image side is convex. The fourth lens group consists of two lenses with different signs of refractive power that are joined together. It has positive refractive power, and the image plane of the lens closest to the image side is convex. The Abbe number of the d-line reference of the object-side lens in the third lens group is set to ν3f. The Abbe number of the d-line reference of the image-side lens in the third lens group is set to ν3r. The Abbe number of the d-line reference of the object-side lens in the fourth lens group is set to ν4f. The Abbe number of the d-line reference of the image-side lens in the fourth lens group is set to ν4r. Let the larger of |ν3f-ν3r| and |ν4f-ν4r| be |νf-νr|. Set the focal length of the entire system to f. The distance along the optical axis from the lens surface on the object side of the first lens group to the lens surface closest to the image side of the fourth lens group is denoted as AL. Let Bf be the air-equivalent distance along the optical axis from the lens surface closest to the image side of the fourth lens group to the focal position on the image side of the entire system. Set the focal length of the first lens group to f1. Set the focal length of the second lens group to f2. Set the focal length of the third lens group to f3. When the focal length of the fourth lens group is set to f4... The objective lens for the endoscope satisfies the following conditions (2) and (3-2): 。 3. The endoscope objective lens according to claim 2, wherein, The Abbe number of the d-line reference of the object-side lens in the second lens group is set as ν2f. The Abbe number of the d-line reference of the image-side lens in the second lens group is set as ν2r. When the paraxial radius of curvature of the joint surface of the second lens group is set to R2c, The objective lens for the endoscope satisfies the following conditional expression (1): -300<(ν2f-ν2r)×|AL / R2c|<30 (1).

4. The endoscope objective lens according to any one of claims 1 to 3, wherein, If the focal length of the entire system is set to f... The objective lens for the endoscope satisfies the following condition (4): 10 < (f2 × f3 × f4) / (f × f × f) < 300 (4).

5. The endoscope objective according to any one of claims 1 to 3, wherein, If the air-equivalent distance along the optical axis from the lens surface closest to the image side of the fourth lens group to the focal position on the image side of the entire system is set to Bf, The objective lens used in the endoscope satisfies the following conditional expression (5): 1 < AL / Bf < 4 (5).

6. The endoscope objective according to any one of claims 1 to 3, wherein, Let Df be the distance along the optical axis from the lens surface on the object side of the first lens group to the aperture. If the distance along the optical axis from the aperture to the lens surface closest to the image side of the fourth lens group is defined as Dr, The objective lens for the endoscope satisfies the following conditional expression (6): |Df-Dr| / AL<1 (6).

7. The endoscope objective lens according to any one of claims 1 to 3, wherein, The objective lens for the endoscope satisfies the following conditional expression (7): f4 / f3 < 5 (7).

8. The endoscope objective according to any one of claims 1 to 3, wherein, The thickness of the second lens group on the optical axis is set to Dc2. When the thickness of the fourth lens group on the optical axis is set to Dc4. The objective lens for the endoscope satisfies the following conditional expression (8): Dc4 / Dc2<1.2 (8).

9. The endoscopic objective lens according to claim 1 or 3, wherein, The objective lens for the endoscope satisfies the following conditional expression (1-1): -250<(ν2f-ν2r)×|AL / R2c|<20 (1-1).

10. The endoscopic objective lens according to claim 2, wherein, The objective lens for the endoscope satisfies the following conditional expression (2-1): 4<|νf-νr|×f / (AL+Bf)<10 (2-1).

11. The endoscopic objective lens according to claim 4, wherein, The objective lens for the endoscope satisfies the following conditional expression (4-1): 30 < (f2 × f3 × f4) / (f × f × f) < 200 (4-1).

12. The endoscopic objective lens according to claim 5, wherein, The objective lens for the endoscope satisfies the following conditional expression (5-1): 。 13. The endoscopic objective lens according to claim 6, wherein, The objective lens for the endoscope satisfies the following conditional expression (6-1): 。 14. The endoscope objective lens according to claim 7, wherein, The objective lens for the endoscope satisfies the following conditional expression (7-1): 。 15. An endoscope comprising an endoscope objective lens according to any one of claims 1 to 14.

Images