Objective lens

Abstract

To provide an objective lens with a long working distance, a high numerical aperture, and good aberration performance. [Solution] The objective lens 1 consists of, in order from the object side, a first lens group G1 having positive refractive power and containing three or more lens components, a second lens group G2 moving along the optical axis where the ray height is highest, a third lens group G3 containing a lens component with a concave surface facing the image side at the image side of the third lens group G3, and a fourth lens group G4 containing a lens component with a concave surface facing the object side at the object side of the fourth lens group G4. The first lens group G1 includes a cemented lens on the image side of the lens component on the object side of the first lens group G1.

Description

[Technical Field]

[0001] The disclosures herein relate to objective lenses. [Background technology]

[0002] In recent years, research using cell aggregates such as spheroids and organoids, which are formed by collecting and culturing a large number of cells in three dimensions, has attracted attention. Since such samples are typically 100 μm to 500 μm in size, immersion-type objective lenses are generally used for observing deep cells within them.

[0003] In immersion objective lenses, a higher numerical aperture is achieved compared to dry objective lenses by filling the space between the objective lens and the sample (more precisely, the holder that holds the sample) with an immersion liquid. This allows for high-resolution and bright observation. Furthermore, by using an immersion liquid with a refractive index close to that of the sample, spherical aberration caused by refractive index mismatch at the interface between the sample and the immersion liquid can be suppressed. The effect of spherical aberration due to refractive index mismatch becomes more pronounced as the observation position deepens; therefore, suppressing spherical aberration makes it possible to observe at greater depths. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] U.S. Patent No. 10732395 [Overview of the project] [Problems that the invention aims to solve]

[0005] To observe samples like those described above to their depths, a long working distance is necessary. Patent Document 1 describes an immersion-type objective lens for microscopes with a long working distance. However, its numerical aperture is somewhat small, and there is room for improvement in its chromatic aberration performance. Therefore, there is a need for an objective lens with a long working distance, a higher numerical aperture, and good aberration performance.

[0006] Based on the circumstances described above, one aspect of the present invention is to provide an objective lens with a long working distance, a high numerical aperture, and good aberration performance. [Means for solving the problem]

[0007] An objective lens according to one aspect of the present invention comprises, in order from the object side, a first lens group having positive refractive power and containing three or more lens components; a second lens group moving along the optical axis where the ray height is highest; a third lens group, the third lens group having a lens component with a concave surface facing the image side at the image side of the third lens group; and a fourth lens group, the fourth lens group having a lens component with a concave surface facing the object side at the object side of the fourth lens group, wherein the first lens group includes a cemented lens on the image side of the lens component of the first lens group that is closest to the object. [Effects of the Invention]

[0008] According to the above embodiment, it is possible to provide an objective lens with a long working distance, a high numerical aperture, and good aberration performance. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional view of the objective lens 1 according to Embodiment 1 of the present invention. [Figure 2] This is a cross-sectional view of the imaging lens 10. [Figure 3] This is an aberration diagram of the optical system in its first state, consisting of an objective lens 1 and an imaging lens 10. [Figure 4] This is an aberration diagram of the second state of an optical system consisting of an objective lens 1 and an imaging lens 10. [Figure 5] This is a cross-sectional view of the objective lens 2 according to Embodiment 2 of the present invention. [Figure 6] This is an aberration diagram of the optical system in its first state, consisting of an objective lens 2 and an imaging lens 10. [Figure 7]This is an aberration diagram in the second state of an optical system composed of an objective lens 2 and an imaging lens 10.

Embodiments for Carrying out the Invention

[0010] An objective lens according to an embodiment of the present application will be described. The objective lens according to this embodiment (hereinafter simply referred to as the objective lens) is an infinity-corrected microscope objective lens used in combination with an imaging lens. In this specification, a lens component refers to a single block of lenses where only two surfaces, the object-side surface and the image-side surface through which light rays from an object point pass, are in contact with air, regardless of whether it is a single lens or a cemented lens. That is, one single lens is one lens component, and one cemented lens is also one lens component. On the other hand, a plurality of single lenses or a plurality of cemented lenses arranged through air are not referred to as one lens component. Also, a meniscus lens component refers to a lens component having a meniscus lens shape, where one of the object-side surface and the image-side surface of the lens component is concave and the other is convex.

[0011] The objective lens is composed of a first lens group having a positive refractive power, a second lens group, a third lens group, and a fourth lens group, arranged in order from the object side. The first lens group includes three or more lens components and includes a cemented lens on the image side of the lens component closest to the object side The second lens group is a moving group that is arranged at a position where the ray height is the highest and moves along the optical axis. The third lens group includes a lens component with a concave surface facing the image side on the image side of the third lens group. The fourth lens group includes a lens component with a concave surface facing the object side on the object side of the fourth lens group. That is, the third lens group and the fourth lens group are arranged with their concave surfaces facing each other.

[0012] In the objective lens configured as described above, since the first lens group is composed of three or more lens components, it is possible to gently converge the divergent light from the object point with the first lens group. Therefore, the occurrence of spherical aberration and coma aberration can be suppressed to a small extent. Further, since the first lens group includes a cemented lens in a region on the image side rather than the lens component on the object side where both the off-axis chief ray and the marginal ray are high, it is possible to correct mainly both axial chromatic aberration and magnification chromatic aberration with that cemented lens.

[0013] Also, in the above-described objective lens, by arranging the second lens group behind the first lens group having a positive refractive power, light from the object point enters the second lens group in a state where the divergence of the light is alleviated. Further, by configuring the second lens group, where the marginal ray height is the highest, as a moving group that moves in the optical axis direction, the marginal ray height passing through the second lens group can be greatly varied, and the amount of spherical aberration generated can be sufficiently changed. Therefore, by moving the second lens group, the spherical aberration caused by a change in the depth of observation or the like can be corrected well.

[0014] Also, in the objective lens, the light passing through the second lens group is converted into parallel light while correcting various aberrations well with the third lens group and the fourth lens group. Here, by making the form in which the most image-side surface of the third lens group and the most object-side surface of the fourth lens group face each other with concave surfaces (an optical system so-called Gauss group), the height of the marginal ray can be lowered at the facing concave surfaces. Therefore, it becomes possible to effectively correct the Petzval sum, and the field curvature can be made sufficiently small.

[0015] According to the objective lens configured as described above, it is possible to realize good aberration performance while realizing a long working distance and a high NA. Hereinafter, a desirable configuration of the objective lens will be described. <0​​The first lens group preferably includes a cemented lens at the object side, consisting of a positive lens with its convex surface facing the image side and a meniscus lens with its concave surface facing the object side. This configuration allows for good correction of field curvature. More specifically, by applying the refractive effect of the concave surface of the meniscus lens to the cemented surface of the lens component at the object side, which is incident before the marginal ray height increases after emitting from the object point, the Petzval sum can be effectively corrected.

[0017] The first lens group preferably includes a three-element cemented lens, and in particular, the cemented lens positioned in the image-side region, rather than the object-side lens component, is preferably a three-element cemented lens. In the image-side region of the first lens group, as described above, both the off-axis principal rays and marginal rays are high, so by placing a three-element cemented lens in that region, it becomes possible to correct both axial chromatic aberration and lateral chromatic aberration.

[0018] The second lens group preferably consists of two cemented lenses. By using two cemented lenses in the second lens group, where the marginal rays are highest, it becomes possible to correct not only spherical aberration but also axial chromatic aberration to a greater extent.

[0019] The third lens group should preferably consist of a single lens component. This avoids the increase in manufacturing costs that would result from composing the third lens group with multiple lens components.

[0020] The fourth lens group preferably includes a meniscus lens component with a concave surface facing the object on the image side of the lens component with a concave surface facing the object, which is the lens component located closest to the object in the fourth lens group. This makes it possible to correct chromatic aberration, astigmatism, and coma aberration more effectively over a wide wavelength range. A more detailed explanation is as follows: In the fourth lens group located closest to the image, it is preferable to arrange lens pairs with different dispersion characteristics to correct chromatic aberration. However, if these are joined together, the coma and astigmatism generated at the joining surface will differ across wavelengths, resulting in differences in these aberrations. Therefore, the fourth lens group is composed of a lens component that plays the role of the image side of the Gaussian group and one or more lens components arranged across an air surface. This allows for the provision of one or more air interfaces in addition to the corrective effect of the Petzval sum of the opposing concave surfaces mentioned above, making it possible to create differences in the refraction angle and ray height of off-axis rays at these surfaces and thus effectively correct astigmatism and coma aberration. In particular, because one or more lens components include a meniscus lens component with a concave surface facing the object, the light can be refracted more gently before the ray height becomes high, compared to plano-convex or biconvex lenses. This suppresses the occurrence of aberrations such as coma and astigmatism, especially those caused by off-axis performance. As a result, it becomes possible to correct chromatic aberration, astigmatism, and coma more effectively across a wide wavelength range.

[0021] The objective lens is preferably configured to satisfy the following condition (1). Here, δ is the distance along the optical axis between the lens component in the fourth lens group with its concave surface facing the object and the meniscus lens component with its concave surface facing the object. D is the distance along the optical axis from the sample plane to the lens surface closest to the image in the fourth lens group (hereinafter also referred to as the total optical length). The sample plane is the plane that includes the focal point when an infinity beam of light is incident on the objective lens from the image side, that is, the front focal point. 0.002 ≦ δ / D ≦ 0.02 (1)

[0022] Conditional equation (1) primarily defines the air gap between the lens component located closest to the object in the fourth lens group and the meniscus lens component with its concave surface facing the object. By satisfying conditional equation (1), the Petzval sum can be corrected effectively. More specifically, by setting the air gap between the lens component with its concave surface facing the object and the meniscus lens component in the fourth lens group so that δ / D is greater than or equal to the lower limit, interference between lenses can be avoided without narrowing the tolerances of each component. Furthermore, by narrowing the air gap between the lens component with its concave surface facing the object and the meniscus lens component in the fourth lens group so that δ / D is less than or equal to the upper limit, it becomes possible to make the lens component with its concave surface facing the object, which constitutes the Gaussian group, sufficiently thick while satisfying the predetermined objective lens length. As a result, a sufficient Petzval correction effect can be achieved.

[0023] It is desirable for the fourth lens group to have negative refractive power. By making the fourth lens group negative refractive power, the marginal rays entering the fourth lens group from the third lens group can be designed to be low. Therefore, the Petzval sum can be effectively corrected at the opposing concave surfaces of the third and fourth lens groups, making it possible to sufficiently reduce field curvature.

[0024] It is desirable that the third and fourth lens groups each include a cemented lens. In the third lens group, the marginal ray height is high. Therefore, including a cemented lens in the third lens group allows for effective correction of axial chromatic aberration. In the fourth lens group, in addition to the marginal ray height, the off-axis principal ray height is also high. Therefore, including a cemented lens in the fourth lens group allows for effective correction of both axial and lateral chromatic aberration. By configuring both lens groups to include cemented lenses, both axial and off-axis chromatic aberration can be corrected more effectively.

[0025] In the objective lens configured as described above, it is desirable that spherical aberration can be corrected by moving the second lens group to a position corresponding to the immersion liquid with a refractive index of 1.33 to 1.40. By being able to correct spherical aberration using immersion liquids with refractive indices ranging from 1.33 (equivalent to water) to 1.40 (equivalent to silicone oil), the amount of spherical aberration, which can vary greatly depending on the refractive index of the immersion liquid, can be well corrected.

[0026] Furthermore, it is desirable that the objective lens be configured such that spherical aberration can be corrected by the movement of the second lens group to a position corresponding to an immersion liquid with a refractive index of 1.33 to 1.40, and that it satisfies the following conditions (2) and (3). Here, f is the focal length of the objective lens. NA is the numerical aperture on the object side of the objective lens. WD is the working distance of the objective lens. D / f ≤ 7 ···(2) D / (NA 2 WD) ≤ 35 ···(3)

[0027] Conditional equation (2) defines the relationship between focal length and total optical length. Conditional equation (3) defines the relationship between total optical length, numerical aperture, and working distance. By satisfying conditional equation (2), which substantially limits the total length of the objective lens, and further satisfying conditional equation (3), it is possible to provide an objective lens that has the required working distance and numerical aperture while keeping the total length of the objective lens within a predetermined range.

[0028] The following describes in detail the embodiment of the objective lens mentioned above. (Example 1) Figure 1 is a cross-sectional view of the objective lens 1 according to this embodiment. The objective lens 1 is an immersion-type objective lens for a microscope. The objective lens 1 consists of, in order from the object side, a first lens group G1 having positive refractive power, a second lens group G2, a third lens group G3, and a fourth lens group G4. The space between the first lens group G1 and the cover glass C is filled with immersion liquid IM.

[0029] The first lens group G1 consists of, in order from the object side, a cemented lens CL1, a meniscus lens L3 with its concave surface facing the object side, and a cemented lens CL2. The cemented lens CL1 is a two-element cemented lens formed by joining a positive lens L1 with its convex surface facing the image side and a meniscus lens L2 with its concave surface facing the object side. The cemented lens CL2 is a three-element cemented lens formed by joining a biconvex lens L4, a biconcave lens L5, and a biconvex lens L6.

[0030] The second lens group G2 consists of a cemented lens CL3 that moves along the optical axis. The cemented lens CL3 is a two-element cemented lens formed by joining a biconvex lens L7 and a meniscus lens L8 with its concave surface facing the object.

[0031] The third lens group G3 consists of a cemented lens CL4. The cemented lens CL4 is a two-element cemented lens formed by joining a biconvex lens L9 and a biconcave lens L10.

[0032] The fourth lens group G4 consists of, in order from the object side, a cemented lens CL5 and a lens L13 which is a meniscus lens (meniscus lens component) with its concave surface facing the object side. The cemented lens CL5 is a two-element cemented lens formed by bonding a meniscus lens L11 with its concave surface facing the object side and a meniscus lens L12 with its concave surface facing the object side.

[0033] The various data for objective lens 1 are as follows. Note that f, f G1 ,f G2 ,f G3 ,f G4 n0 and ν0 are the focal length of objective lens 1, the focal length of the first lens group G1, the focal length of the second lens group G2, the focal length of the third lens group G3, the focal length of the fourth lens group G4, the refractive index of the immersion liquid IM, and the Abbe number of the immersion liquid IM, respectively. The reference wavelength is the d line. In this embodiment, silicone oil and water are used as the immersion liquid IM. Note that there is a difference between using silicone oil and using water as the immersion liquid IM. G1 ,f G2 ,f G3, f G4 , δ does not change. On the other hand, since fWD and D can change depending on the immersion liquid IM, they are disclosed for each immersion liquid IM. f G1 = 9.797 mm, f G2 = 22.926 mm, f G3 = -22.135 mm, f G4 = -139.849 mm, δ = 0.504 mm (When the immersion liquid IM is silicone oil (n0 = 1.40410, ν0 = 51.90)) f = 7.212 mm, NA = 0.85, WD = 2.100 mm, D = 50.026 mm (When the immersion liquid IM is water (n0 = 1.33304, ν0 = 55.79)) f = 7.407 mm, NA = 0.85, WD = 2.030 mm, D = 49.957 mm

[0034] The lens data of the objective lens 1 is as follows. Note that INF in the lens data indicates infinity (∞). Objective lens 1 s r d nd νd 1 INF 0 n0ν0 2 INF 0.17 1.52344 54.40 3 INF WD n0ν0 4 INF 1.464 1.51633 64.14 5 -2.1846 5.129 1.88300 40.76 6 -6.4173 0.200 7 -47.6119 3.189 1.80400 46.53 8 -11.5195 0.200 9 124.3531 4.487 1.56907 71.30 10 -10.2048 0.636 1.63775 42.41 11 14.4760 4.017 1.43875 94.66 12 -38.7728 D1 13 14.8086 6.611 1.43875 94.66 14 -13.0671 2.232 1.63775 42.41 15 -19.7676 D2 16 8.1172 5.269 1.43875 94.66 17 -25.0635 1.629 1.63775 42.41 18 5.1825 4.353 19 -4.9512 0.652 1.73800 32.33 20 -54.8305 2.839 1.60300 65.44 21 -10.8096 0.504 22 -13.3289 3.067 1.85478 24.80 23 -9.0513 110.000

[0035] Here, s represents the surface number, r represents the radius of curvature (mm), d represents the interplanar spacing (mm), nd represents the refractive index, and νd represents the Abbe number. The reference wavelength is the d line (587.56 nm). These symbols are the same in subsequent examples. The surfaces indicated by surface numbers s1 and s2 are both the object-side surfaces of the cover glass CG, and the surface indicated by surface number s3 is the image-side surface of the cover glass CG. The surfaces indicated by surface numbers s4 and s23 are the lens surfaces closest to the object and closest to the image, respectively, of the objective lens 1. For example, the interplanar spacing d1 indicates the distance along the optical axis from the surface indicated by surface number s1 to the surface indicated by surface number s2. The interplanar spacing d23 indicates the distance along the optical axis from the surface indicated by surface number s23 to the imaging lens, and although omitted in the lens data, it is 110 mm.

[0036] If we define the first state as the state in which the second lens group is moved in response to silicone oil as the immersion liquid IM, and the second state as the state in which the second lens group is moved in response to water as the immersion liquid IM, then the values ​​of the intervals d12 and d15 in the lens data in each state, D1 and D2 (in mm), and the various parameters are as follows. First state Second state n01.40410 1.33304 ν051.90 55.79 WD 2.100 2.030 f 7.212 7.407 D1 0.350 0.933 D2 0.931 0.348

[0037] Objective lens 1 satisfies conditions (1) to (3) in both the first and second states, as shown below. (1) (First state) δ / D = 0.010 (1) (Second state) δ / D = 0.010 (2) (First state) D / f = 6.937 (2) (Second state) D / f = 6.745 (3) (First state) D / (NA 2 WD) = 32.972 (3) (Second state) D / (NA 2 WD) = 34.063

[0038] Figure 2 is a cross-sectional view of the imaging lens 10 used in combination with the objective lens 1. The imaging lens 10 is a microscope imaging lens that forms a magnified image of an object in combination with an infinity-corrected objective lens. The imaging lens 10 consists of a cemented lens CTL1 and a cemented lens CTL2, arranged in order from the object side. The cemented lens CTL1 is a two-element cemented lens consisting of a biconvex lens TL1 and a meniscus lens TL2 with its concave surface facing the object side. The cemented lens CTL2 is a two-element cemented lens consisting of a biconvex lens TL3 and a biconcave lens TL4. The focal length ft of the imaging lens 10 is 180 mm.

[0039] The lens data for the imaging lens 10 is as follows: Imaging lens 10 srd nd νd 1 68.7541 7.7321 1.48749 70.21 2 -37.5679 3.4742 1.80610 40.95 3 -102.8477 0.6973 4 84.3099 6.024 1.83400 37.17 5 -50.7100 3.030 1.64450 40.82 6 40.6619 9.038

[0040] Figures 3 and 4 are aberration diagrams of an optical system consisting of an objective lens 1 and an imaging lens 10, showing the aberrations in the image plane formed by the objective lens 1 and the imaging lens 10 in the first and second states, respectively. Figures 3(a) and 4(a) are spherical aberration diagrams. Figures 3(b) and 4(b) show the amount of sinusoidal deviation. Figures 3(c) and 4(c) are astigmatism diagrams. Figures 3(d) and 4(d) are coma aberration diagrams at an image height ratio of 0.6 (image height 7.95 mm). In the figures, "M" represents the meridional component and "S" represents the sagittal component. As shown in Figures 3 and 4, in this embodiment, aberrations are well corrected without the use of immersion solution.

[0041] (Example 2) Figure 5 is a cross-sectional view of the objective lens 2 according to this embodiment. The objective lens 2 is an immersion-type objective lens for a microscope. The objective lens 2 consists of, in order from the object side, a first lens group G1 having positive refractive power, a second lens group G2, a third lens group G3, and a fourth lens group G4. The space between the first lens group G1 and the cover glass C is filled with immersion liquid IM.

[0042] The first lens group G1 consists of, in order from the object side, a cemented lens CL1, a meniscus lens L3 with its concave surface facing the object side, and a cemented lens CL2. The cemented lens CL1 is a two-element cemented lens formed by joining a positive lens L1 with its convex surface facing the image side and a meniscus lens L2 with its concave surface facing the object side. The cemented lens CL2 is a three-element cemented lens formed by joining a biconvex lens L4, a biconcave lens L5, and a biconvex lens L6.

[0043] The second lens group G2 consists of a cemented lens CL3 that moves along the optical axis. The cemented lens CL3 is a two-element cemented lens formed by joining a biconvex lens L7 and a meniscus lens L8 with its concave surface facing the object.

[0044] The third lens group G3 consists of a cemented lens CL4. The cemented lens CL4 is a two-element cemented lens formed by joining a biconvex lens L9 and a biconcave lens L10.

[0045] The fourth lens group G4 consists of, in order from the object side, a cemented lens CL5 and a lens L13 which is a meniscus lens (meniscus lens component) with its concave surface facing the object side. The cemented lens CL5 is a two-element cemented lens formed by bonding a meniscus lens L11 with its concave surface facing the object side and a meniscus lens L12 with its concave surface facing the object side.

[0046] The various data for objective lens 2 are as follows: f G1 =10.002mm, f G2 =23.436mm, f G3 = -21.584 mm, f G4 = -139.219 mm, δ = 0.931 mm (When the immersion liquid IM is silicone oil (n0=1.40410, ν0=51.90)) f=7.214mm, NA=0.85, WD=2.100mm, D=50.074mm (When the immersion liquid IM is water (n0=1.33304, ν0=55.79)) f=7.399mm, NA=0.85, WD=2.032mm, D=50.007mm

[0047] The lens data for objective lens 2 is as follows: Objective lens 2 srd nd νd 1 INF 0 n0ν0 2 INF 0.17 1.52344 54.40 3 INF WD n0ν0 4 INF 1.366 1.51633 64.14 5 -2.2639 5.540 1.88300 40.76 6 -6.7611 0.200 7 -28.0949 2.798 1.80400 46.53 8 -10.9119 0.200 9 36.1770 3.699 1.43875 94.66 10 -14.9013 0.500 1.63775 42.41 11 17.2064 3.737 1.43875 94.66 12 -31.8767 D1 13 16.0358 6.288 1.43875 94.66 14 -12.3571 3.374 1.63775 42.41 15 -18.4540 D2 16 8.4646 4.896 1.43875 94.66 17 -26.2350 2.679 1.63775 42.41 18 5.1972 4.267 19 -4.9189 0.505 1.73800 32.33 20 -66.2573 2.811 1.60300 65.44 21 -9.6525 0.931 22 -12.3167 3.001 1.85478 24.80 23 -9.0209 110.000

[0048] The values ​​of the intervals d12 and d15 in the lens data for the first and second states, D1 and D2 (in mm), and the various parameters are as follows: First state Second state n01.40410 1.33304 ν051.90 55.79 WD 2.100 2.032 f 7.214 7.399 D1 0.214 0.772 D2 0.800 0.242

[0049] The objective lens 2 satisfies conditions (1) to (3) in both the first and second states, as shown below. (1) (First state) δ / D = 0.019 (1) (Second state) δ / D = 0.019 (2) (First state) D / f = 6.942 (2) (Second state) D / f = 6.759 (3) (First state) D / (NA 2 WD) = 33.003 (3) (Second state) D / (NA 2 WD) = 34.059

[0050] Figures 6 and 7 are aberration diagrams of an optical system consisting of an objective lens 2 and an imaging lens 10, showing the aberrations in the image plane formed by the objective lens 2 and the imaging lens 10 in the first and second states, respectively. Figures 6(a) and 7(a) are spherical aberration diagrams. Figures 6(b) and 7(b) show the amount of sinusoidal condition violation. Figures 6(c) and 7(c) are astigmatism diagrams. Figures 6(d) and 7(d) are coma aberration diagrams at an image height ratio of 0.6 (image height 7.95 mm). As shown in Figures 6 and 7, in this embodiment, aberrations are well corrected without the use of immersion solution. [Explanation of Symbols]

[0051] 1, 2: Objective lens, 10: Imaging lens, CG: Cover glass, CL1~CL5, CTL1, CTL2: Cemented lens, G1: First lens group, G2: Second lens group, G3: Third lens group, G4: Fourth lens group, L1~L13, TL1~TL4: Lens

Claims

1. Starting from the object side, A first lens group having positive refractive power and containing three or more lens components, The second lens group moves along the optical axis, where the light ray height is highest, A third lens group, wherein the third lens group includes a lens component with a concave surface facing the image side, located closest to the image side. A fourth lens group comprising a lens component having a concave surface facing the object, located furthest towards the object, and the fourth lens group, The first lens group includes a cemented lens on the image side of the lens component closest to the object in the first lens group. An objective lens characterized by the following features.

2. In the objective lens according to claim 1, The lens component of the first lens group that is closest to the object is a cemented lens consisting of a positive lens with a convex surface facing the image side and a meniscus lens with a concave surface facing the object side. An objective lens characterized by the following features.

3. In the objective lens according to claim 1 or claim 2, The fourth lens group further includes a meniscus lens component with a concave surface facing the object side on the image side of the lens component in the fourth lens group with a concave surface facing the object side. An objective lens characterized by the following features.

4. In the objective lens according to claim 3, The following conditional expression 0.002 ≦ δ / D ≦ 0.02 (1) An objective lens characterized by satisfying the following conditions. However, δ is the distance along the optical axis between the lens component and the meniscus lens component included in the fourth lens group. D is the distance along the optical axis from the specimen surface to the image-side lens surface of the fourth lens group.

5. In the objective lens according to claim 1 or claim 2, The third lens group and the fourth lens group each include a cemented lens. An objective lens characterized by the following features.

6. In the objective lens according to claim 1 or claim 2, Spherical aberration is corrected by moving the second lens group to a position corresponding to an immersion liquid with a refractive index of 1.33 to 1.

40. An objective lens characterized by the following features.

7. In the objective lens according to claim 1 or claim 2, The fourth lens group has negative refractive power An objective lens characterized by the following features.

8. In the objective lens according to claim 1 or claim 2, The first lens group includes a three-element cemented lens. An objective lens characterized by the following features.

9. In the objective lens according to claim 1 or claim 2, The second lens group consists of two cemented lenses. An objective lens characterized by the following features.

10. In the objective lens according to claim 1 or claim 2, The third lens group consists of a single lens component. An objective lens characterized by the following features.

11. In the objective lens according to claim 6, The following conditional expression D / f≦7...(2) D / (NA) ２ ・WD) ≦ 35 ・・・(3) An objective lens characterized by satisfying the following conditions. However, D is the distance along the optical axis from the specimen surface to the image-side lens surface of the fourth lens group. f is the focal length of the objective lens. NA is the numerical aperture of the objective lens on the object side. WD is the working distance of the objective lens.

Images