Objective lens

Abstract

To provide an objective lens having a high NA in which chromatic aberration and off-axis performance are satisfactorily corrected.SOLUTION: The objective 1 includes, in order from the object side, a positive first lens group G1, a positive second lens group G2 that moves along the optical axis, a third lens group G3 having two or more cemented surfaces, and a fourth lens group L14 including, in order from the object side, a lens L12 having a convex surface facing the object side, a lens L13 having a concave surface facing the image side, a lens G4 having a concave surface facing the object side, and a lens having a convex surface facing the image side. L11. The first lens group G1 includes at least three or more lens components including a first lens component (cemented lens L1) in which a lens L2 that is a positive lens having a convex surface facing the image side and a lens CL1 that is a meniscus-shaped lens having a concave surface facing the object side are cemented to each other at a position closest to the object side.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The disclosure herein relates to an objective lens. [Background technology]

[0002] In recent years, the number of pixels in image sensors has increased significantly, and in the field of microscopy, there is a demand for microscope devices that can observe and acquire images with both a wide field of view and high resolution, and objective lenses with a high numerical aperture (hereinafter referred to as NA) are desired.

[0003] Objective lenses with high NA exhibit significant variations in spherical aberration with respect to changes in the thickness of the cover glass, the refractive index of the specimen, and the refractive index of the immersion liquid. Therefore, to correct for the variations in spherical aberration, it is desirable to have a mechanism for moving the lens components within the objective lens along the optical axis. Such an objective lens is described, for example, in Patent Document 1. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-029067 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in conventional objective lenses, aberrations other than spherical aberration, such as axial chromatic aberration and coma, change significantly with the movement of the lens components, making it difficult to achieve high performance over a wide wavelength range and a wide field of view.

[0006] In view of the above circumstances, an object of one aspect of the present invention is to provide an objective lens having a high NA and in which chromatic aberration and off-axis performance are well corrected. [Means for solving the problem]

[0007] An objective lens according to one aspect of the present invention includes, in order from the object side, a first lens group having positive refractive power, a second lens group moving along an optical axis and having positive refractive power, a third lens group having two or more cemented surfaces, and a fourth lens group including, in order from the object side, a lens having a convex surface facing the object side, a lens having a concave surface facing the image side, a lens having a concave surface facing the object side, and a lens having a convex surface facing the image side, wherein the first lens group has at least three or more lens components closest to the object side, including a first lens component having a positive lens having a convex surface facing the image side cemented with a meniscus lens having a concave surface facing the object side, and none of the at least three or more lens components is a cemented lens in which a negative lens is located closer to the object than a positive lens. [Effects of the Invention]

[0008] According to the above aspect, it is possible to provide an objective lens having a high NA and in which chromatic aberration and off-axis performance are well corrected. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view of an objective lens 1 according to Example 1 of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of the imaging lens 10. [Figure 3] 2 is an aberration diagram of an optical system consisting of an objective lens 1 and an imaging lens 10 in a first state. [Figure 4] 10 is an aberration diagram of the optical system consisting of the objective lens 1 and the imaging lens 10 in a second state. [Figure 5] 10 is an aberration diagram of the optical system consisting of the objective lens 1 and the imaging lens 10 in a third state. [Figure 6] FIG. 3 is a cross-sectional view of an objective lens 2 according to Example 2 of the present invention. [Figure 7] 2 is an aberration diagram of an optical system consisting of an objective lens 2 and an imaging lens 10 in a first state. [Figure 8] 10 is an aberration diagram of the optical system consisting of the objective lens 2 and the imaging lens 10 in a second state. [Figure 9]10 is an aberration diagram of the optical system consisting of the objective lens 2 and the imaging lens 10 in a third state. [Figure 10] FIG. 10 is a cross-sectional view of an objective lens 3 according to Example 3 of the present invention. [Figure 11] 2 is an aberration diagram of an optical system consisting of an objective lens 3 and an imaging lens 10 in a first state. [Figure 12] 10 is an aberration diagram of the optical system consisting of the objective lens 3 and the imaging lens 10 in a second state. [Figure 13] 10 is an aberration diagram of the optical system consisting of the objective lens 3 and the imaging lens 10 in a third state. [Figure 14] FIG. 10 is a cross-sectional view of an objective lens 4 according to Example 4 of the present invention. [Figure 15] 2 is an aberration diagram of an optical system consisting of an objective lens 4 and an imaging lens 10 in a first state. [Figure 16] 10 is an aberration diagram of the optical system consisting of the objective lens 4 and the imaging lens 10 in a second state. [Figure 17] 10 is an aberration diagram of the optical system consisting of the objective lens 4 and the imaging lens 10 in a third state. DETAILED DESCRIPTION OF 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 lens or a cemented lens, a lens block in which 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. In other words, one single lens is one lens component, and one cemented lens is also one lens component. On the other hand, multiple single lenses or multiple cemented lenses arranged with air between them are not called one lens component.

[0011] The objective lens consists of, arranged in order from the object side, a first lens group having a positive refractive power, a second lens group having a positive refractive index, a third lens group, and a fourth lens group. The second lens group is a movable group that moves along the optical axis. The third lens group has two or more cemented surfaces. The fourth lens group includes, in order from the object side, a lens with a convex surface facing the object side, a lens with a concave surface facing the image side, a lens with a concave surface facing the object side, and a lens with a convex surface facing the image side.

[0012] In the objective lens configured as described above, by disposing the second lens group, which is a movable group having positive refractive power, after the first lens group having positive refractive power, light is incident on the second lens group with the divergence of light from an object point sufficiently alleviated. This prevents the amount of spherical aberration (correction amount) generated in the second lens group from becoming excessively large. Furthermore, by moving the second lens group, the height of the marginal ray passing through the first lens group can be greatly changed, thereby sufficiently changing the amount of spherical aberration generated in the first lens group. Therefore, by moving the second lens group, spherical aberration generated due to changes in the thickness of the cover glass, etc. can be effectively corrected.

[0013] By having two or more cemented surfaces in the third lens group, axial chromatic aberration can be effectively corrected in the third lens group, through which light rays with high marginal ray heights pass. Furthermore, by providing the third lens group with a large axial chromatic aberration correction effect, the axial chromatic aberration correction effect required of the first and second lens groups can be made relatively small. By providing the first and second lens groups, which are located closer to the object than the third lens group and whose marginal ray heights fluctuate greatly with movement of the second lens group, with a relatively small axial chromatic aberration correction effect, it is possible to minimize fluctuations in axial chromatic aberration even when spherical aberration is corrected by moving the second lens group in response to changes in cover glass thickness, etc., and as a result, stable and effective chromatic aberration correction can be achieved.

[0014] The two or more cemented surfaces in the third lens group may be, for example, two cemented surfaces included in two cemented doublet lenses, i.e., they may be included in two or more cemented lenses.Alternatively, the two or more cemented surfaces in the third lens group may be, for example, two cemented surfaces included in one cemented triplet lens, i.e., they may be included in one cemented lens.

[0015] The third lens group refers to the lens components located closer to the image than the second lens group, which is a moving group, up to and including the second cemented surface, counting from the lens component closest to the second lens group (closest to the object). Therefore, for example, in an objective lens including three doublet cemented lenses on the image side of the second lens group, the third lens group refers to the lens components located closer to the image than the second lens group, up to and including the second doublet cemented lens that is closer to the image than the second lens group, and the subsequent lens components constitute the fourth lens group. Furthermore, for example, in an objective lens including a triplet cemented lens immediately adjacent to the image side of the second lens group, the third lens group consists only of that triplet cemented lens, and the subsequent lens components constitute the fourth lens group.

[0016] The fourth lens group includes an optical system known as a Gaussian group, which has a pair of concave surfaces facing each other and a pair of convex surfaces facing outward, located outside the pair of concave surfaces. This makes it possible to reduce the height of marginal rays at the pair of facing concave surfaces, thereby effectively correcting the Petzval sum and sufficiently reducing the curvature of field.

[0017] In the objective lens described above, the first lens group has at least three lens components. This configuration allows the first lens group to gently converge light diverging from an object point. As a result, it is possible to reduce the occurrence of spherical aberration and coma.

[0018] In the above-described objective lens, the first lens group includes a first lens component, closest to the object, which is formed by cementing a positive lens having a convex surface facing the image side and a meniscus lens having a concave surface facing the object side. This configuration enables excellent correction of field curvature. More specifically, by providing a refractive effect due to the concave surface of the meniscus lens at the cemented surface of the first lens component, where a marginal ray emerges from an object point and enters before its height increases, it becomes possible to effectively correct Petzval sum.

[0019] Furthermore, because the lens cemented to the object-side concave surface of the meniscus lens is a positive lens, the first surface of the objective lens (the lens surface closest to the object) does not become strongly concave. This makes it difficult for air bubbles to get in between the objective lens and the immersion liquid when the objective lens is immersed in immersion liquid. This makes it possible to avoid aberrations caused by air bubbles.

[0020] Furthermore, the objective lenses described above share a common feature in that none of the at least three lens components, including the first lens component, in the first lens group are cemented lenses in which a negative lens is located closer to the object than a positive lens. This configuration enables excellent correction of axial chromatic aberration and spherical aberration between different wavelengths. More specifically, if the first lens group includes a cemented lens in which a negative lens is located closer to the object, divergent light will be incident on the cemented surface with achromatic properties located on the image side of the negative lens, increasing the angle of incidence of marginal rays on the cemented surface. Because the refraction angle is proportional to the sine of the angle of incidence, the larger the angle of incidence, the larger the difference in refraction angle between wavelengths. In other words, the presence of a cemented lens in which a negative lens is located closer to the object in the first lens group causes a large difference in refraction angle between wavelengths. When such a cemented lens is included in the first lens group, if the height of the marginal ray at the first lens group changes due to the movement of the second lens group, the large difference in refraction angle between different wavelengths at the cemented surface will fluctuate significantly, making it difficult to properly correct axial chromatic aberration and spherical aberration between different wavelengths. In contrast, the objective lens described above does not include a cemented lens in which a negative lens is located closer to the object than a positive lens in the first lens group, so such difficulties do not arise and the objective lens can properly correct axial chromatic aberration and spherical aberration between different wavelengths. For the same reason, if the first lens group includes a cemented lens other than the first lens component, it is desirable that the cemented surface thereof also has a concave surface facing the object side.

[0021] The objective lens configured as described above can achieve a high NA while satisfactorily correcting chromatic aberration and off-axis performance. More specifically, in an objective lens having a high NA, even when the amount of spherical aberration correction is adjusted by moving the second lens group, which is a movable group, the chromatic aberration and off-axis performance can be stably and satisfactorily corrected.

[0022] A desirable configuration of the objective lens will be described below. It is desirable that the third lens group include two cemented lenses. By including two cemented lenses, the third lens group has two or more cemented surfaces and two or more sets of air interfaces. This makes it possible to differentiate the refraction angles and ray heights of marginal rays at each refractive surface (lens surface). As a result, it becomes possible for the third lens group to have the function of correcting not only axial chromatic aberration but also mainly coma aberration, making it possible to better correct peripheral (off-axis) aberration performance.

[0023] Each of the two cemented lens components included in the third lens group desirably has a cemented surface facing concave toward the image side. To minimize the difference in coma aberration between wavelengths, it is desirable for the cemented lens component to be composed of a negative lens on the object side and a positive lens on the image side. In other words, it is desirable for the cemented lens component to have a concave surface facing toward the image side. Divergence of light entering the third lens group is reduced in the first and second lens groups, and the light passes through the third lens group while converging. Therefore, by having the cemented surface inside the third lens group facing concave toward the image side, the refraction angle of off-axis marginal rays at the cemented surface does not become too large, minimizing the occurrence of coma aberration at the cemented surface. As a result, it is possible to further minimize the difference in coma aberration between wavelengths, known as chromatic coma.

[0024] It is desirable that the fourth lens group be composed of three or more lens components. By having three or more lens components in the fourth lens group, it is possible to simultaneously correct lateral chromatic aberration and differences in astigmatism and coma occurring between wavelengths. More detailed explanations are provided below. In the fourth lens group, which is located closest to the image, it is preferable to arrange a pair of lenses with different dispersion characteristics to correct lateral chromatic aberration. However, if the pair of lenses is cemented, the coma and astigmatism occurring at the cemented surface will differ between wavelengths, resulting in differences in these aberrations. Therefore, the fourth lens group is composed of three or more lens components. This allows for the provision of four or more air interfaces in addition to the opposing concave surfaces that correct the Petzval sum described above. This makes it possible to provide differences in the refraction angles and ray heights of off-axial rays at these air interfaces, thereby enabling effective correction of astigmatism and coma. As a result, it is possible to more effectively correct lateral chromatic aberration, astigmatism, and coma over a wide wavelength range.

[0025] The second lens group desirably consists of a single lens element having a meniscus shape with its concave surface facing the object side. By having the lens element that moves in the optical axis direction have a meniscus shape with its concave surface facing the object side, the occurrence of coma aberration can be minimized. Therefore, the fluctuation of coma aberration due to the movement of the lens element can also be minimized.

[0026] The second lens group is preferably made up of a single lens element, which does not have a significant effect on correcting chromatic aberration and moves in the optical axis direction. This makes it possible to minimize fluctuations in axial chromatic aberration caused by the movement of the lens element.

[0027] The first lens group desirably includes a cemented lens component, arranged in order from the object side, cemented together with a positive lens and a negative lens, located closer to the image than the first lens component. Having another cemented lens in addition to the first lens component in the first lens group, which is located closest to the object, enables excellent correction of lateral chromatic aberration. However, as described above, if a cemented lens with a negative lens located closer to the object side is located in the first lens group, movement of the second lens group undesirably increases axial chromatic aberration and fluctuations in spherical aberration between different wavelengths. Therefore, the first lens group includes, in addition to the first lens component, a cemented lens component, arranged from the object side, with a positive lens and a negative lens. This makes it possible to minimize axial chromatic aberration and fluctuations in spherical aberration between different wavelengths by moving the second lens group, while also providing excellent correction of lateral chromatic aberration. For the reasons described above, it is even more preferable that the cemented lens component other than the first lens component has a cemented lens component with its cemented surface facing concave toward the object side.

[0028] It is desirable that the objective lens be configured so as to satisfy at least one of the following conditional expressions (1) and (2). 0.25≦|d 1GR / TTL|≦0.5 (1) 0.3≦|d 3GF / d FB |≦1 (2)

[0029] However, d 1GR is the distance on the optical axis from the object plane to the lens surface of the first lens group closest to the image. TTL is the distance on the optical axis from the object plane to the lens surface of the objective lens closest to the image. d 3GF is the distance on the optical axis from the object surface to the lens surface in the third lens group closest to the object. FB is the distance on the optical axis from the object plane to the back focal position of the objective lens. Note that the object plane is the plane that includes the light-focusing position when an infinitely far beam is incident on the objective lens from the image side, i.e., the front focal position. More specifically, it is the plane that includes the front focal position of the objective lens when the position of the second lens group is adjusted to effectively correct spherical aberration under standard conditions (specifically, a cover glass thickness of 0.17 mm).

[0030] Conditional formula (1) defines the space provided to the first lens group. Satisfying conditional formula (1) is desirable for effectively correcting primarily spherical aberration and coma simultaneously. 1GR By ensuring that / TTL| does not fall below the lower limit, there is sufficient space for the first lens group, which has the function of gradually reducing the divergence of light emitted from the object. This makes it possible to reduce the divergence of incident light while minimizing the occurrence of spherical aberration. 1GR By ensuring that / TTL| does not exceed its upper limit, it is possible to ensure sufficient space for the third lens group and beyond while also ensuring sufficient space for the second lens group to move. This makes it possible to gently converge off-axis light beams in the third lens group, and to minimize the occurrence of coma in the third lens group.

[0031] Conditional formula (2) defines the positional relationship between the exit pupil position and the second lens group. Satisfying conditional formula (2) is desirable mainly for minimizing fluctuations in coma caused by movement of the second lens group. When the objective lens is configured as an object-side telecentric optical system, the rear focal position becomes the exit pupil position, and coma occurs more significantly on the image side of this exit pupil position. |d 3GF / d FB By ensuring that | does not exceed the upper limit, the second lens group is positioned farther to the object side than the exit pupil position, and as a result, the occurrence of coma in the second lens group can be kept small. Therefore, it is also possible to keep small the fluctuation of coma due to the movement of the second lens group. 3GF / d FB By ensuring that | is not below the lower limit, it is possible to ensure sufficient space for the second lens group to move while also ensuring that the first lens group has sufficient space. This makes it possible to reduce the divergence of incident light while minimizing the occurrence of spherical aberration.

[0032] Examples of the above-mentioned objective lens will now be described in detail. Example 1 1 is a cross-sectional view of an objective lens 1 according to this embodiment. The objective lens 1 is a water immersion objective lens for a microscope. The objective lens 1 is composed of, in order from the object side, a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, a third lens group G3, and a fourth lens group G4.

[0033] The first lens group G1 consists of, in order from the object side, a cemented lens CL1, a lens L3 which is a meniscus lens 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 cementing together lens L1 which is a positive lens with its convex surface facing the image side and lens L2 which is a meniscus lens with its concave surface facing the object side, and is the first lens component. The cemented lens CL2 is a two-element cemented lens formed by cementing together lens L4 which is a biconvex lens and lens L5 which is a meniscus lens with its concave surface facing the object side. Note that neither the cemented lens CL1 nor the cemented lens CL2 is a cemented lens in which the negative lens is positioned closer to the object than the positive lens.

[0034] The second lens group G2 is made up of a lens L6 that moves along the optical axis. The lens L6 is a single lens that has a meniscus shape with its concave surface facing the object side.

[0035] The third lens group G3 consists, in order from the object side, of a cemented lens CL3 and a cemented lens CL4. The cemented lens CL3 is a two-element cemented lens formed by cementing together lens L7, a meniscus lens with its concave surface facing the image side, and lens L8, a biconvex lens. The cemented lens CL4 is a two-element cemented lens formed by cementing together lens L9, a meniscus lens with its concave surface facing the image side, and lens L10, a biconvex lens. The cemented lenses CL3 and CL4 each have a cemented surface with its concave surface facing the image side.

[0036] The fourth lens group G4 consists of, in order from the object side, a cemented lens CL5, a biconcave lens L13, and a meniscus lens L14 with its concave surface facing the object side. The cemented lens CL5 is a two-piece cemented lens formed by cementing together a biconvex lens L11 and a biconcave lens L12.

[0037] The various data of the objective lens 1 are as follows. NA is the numerical aperture of the objective lens 1 on the object side. f, f G1 , f G2 , f G3 , f G4 are the focal lengths of the objective lens 1, the first lens group G1, the second lens group G2, the third lens group G3, and the fourth lens group G4, respectively. The reference wavelength is the d-line.

[0038] NA=1.2,f=3.001mm,f G1 =4.499mm,f G2 =60.488mm,f G3 =-48.804mm,f G4 =-139.527mm,TTL=49.537mm,d 1GR =15.074mm,d 3GF =19.609mm,d FB =21.766mm

[0039] The lens data of the objective lens 1 is as follows: Note that INF in the lens data represents infinity (∞). Objective lens 1 srd nd νd 1 INF 0 1.33304 55.79 2 INF t 1.52397 54.41 3 INF D0 1.33304 55.79 4 INF 0.395 1.45858 67.84 5 -1.2180 4.192 1.80400 46.53 6 -3.5667 0.150 7 -71.0697 2.533 1.56907 71.30 8 -7.8008 0.250 9 20.0965 5.182 1.43875 94.66 10 -6.5846 1.904 1.63775 42.41 11 -12.6961 D1 12 -14.4620 1.900 1.43875 94.66 13 -9.7360 D2 14 131.1700 1.350 1.63775 42.41 15 5.9861 4.131 1.43875 94.66 16 -21.5087 0.260 17 40.1015 1.150 1.67790 55.35 18 7.2757 2.410 1.43875 94.66 19 -177.6884 2.794 20 6.3135 4.437 1.56907 71.30 21 -13.6241 6.304 1.80400 46.53 22 4.2213 2.185 23 -3.8809 0.941 1.51823 58.90 24 19.5035 1.312 25 -192.6448 2.655 1.67300 38.26 26 -6.2054 110.000

[0040] Here, s represents the surface number, r represents the radius of curvature (mm), d represents the surface spacing (mm), nd represents the refractive index, and vd represents the Abbe number. The reference wavelength is the d-line (587.56 nm). These symbols are the same in the following examples. The surfaces designated by surface numbers s1 and s2 are both the object-side surfaces of the cover glass CG, and the surface designated by surface number s3 is the image-side surface of the cover glass CG. The surfaces designated by surface numbers s4 and s26 are the lens surfaces of the objective lens 1 closest to the object and the image, respectively. For example, the surface spacing d1 represents the distance on the optical axis from the surface designated by surface number s1 to the surface designated by surface number s2. The surface spacing d26 represents the distance on the optical axis from the surface designated by surface number s26 to the imaging lens, which is 110 mm.

[0041] The state in which the second lens group is moved to align with a cover glass CG with a thickness of 0.17 mm is called the first state (standard state), the state in which the second lens group is moved to align with a cover glass CG with a thickness of 0.11 mm is called the second state, and the state in which the second lens group is moved to align with a cover glass CG with a thickness of 0.23 mm is called the third state. The values ​​D0, D1, and D2 (unit: mm) of the distances d3, d11, and d13 in the lens data for each state are as follows: 1st state 2nd state 3rd state t 0.17 0.11 0.23 D0 0.298 0.335 0.261 D1 1.961 2.108 1.808 D2 0.674 0.527 0.827

[0042] The objective lens 1 satisfies the following conditional expressions (1) and (2). (1)|d 1GR / TTL|=0.30 (2)|d 3GF / d FB |=0.90

[0043] FIG. 2 is a cross-sectional view of an imaging lens 10 used in combination with 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 cemented lenses CTL1 and 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.

[0044] The lens data of 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

[0045] Figures 3 to 5 are aberration diagrams of an optical system consisting of the objective lens 1 and the imaging lens 10, showing the aberrations at the image plane formed by the objective lens 1 and the imaging lens 10 in the first, second, and third states, respectively. Figures 3(a), 4(a), and 5(a) are spherical aberration diagrams. Figures 3(b), 4(b), and 5(b) are diagrams showing the amount of violation of the sine condition. Figures 3(c), 4(c), and 5(c) are astigmatism diagrams. Figures 3(d), 4(d), and 5(d) are coma aberration diagrams at an image height ratio of 0.6 (image height 7.95 mm). Note that "M" in the diagrams indicates the meridional component, and "S" indicates the sagittal component. As shown in Figures 3 to 5, in this embodiment, aberrations are well corrected regardless of the cover glass thickness.

[0046] Example 2 6 is a cross-sectional view of an objective lens 2 according to this embodiment. The objective lens 2 is a water-immersion objective lens for a microscope. The objective lens 2 is composed of, in order from the object side, a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, a third lens group G3, and a fourth lens group G4.

[0047] The first lens group G1 consists of, in order from the object side, a cemented lens CL1, a lens L3 which is a meniscus lens 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 cementing together lens L1 which is a positive lens with its convex surface facing the image side and lens L2 which is a meniscus lens with its concave surface facing the object side, and is the first lens component. The cemented lens CL2 is a two-element cemented lens formed by cementing together lens L4 which is a biconvex lens and lens L5 which is a meniscus lens with its concave surface facing the object side. Note that neither the cemented lens CL1 nor the cemented lens CL2 is a cemented lens in which the negative lens is positioned closer to the object than the positive lens.

[0048] The second lens group G2 is made up of a lens L6 that moves along the optical axis. The lens L6 is a single lens that has a meniscus shape with its concave surface facing the object side.

[0049] The third lens group G3 consists, in order from the object side, of a cemented lens CL3 and a cemented lens CL4. The cemented lens CL3 is a two-element cemented lens formed by cementing together lens L7, a meniscus lens with its concave surface facing the image side, and lens L8, a biconvex lens. The cemented lens CL4 is a two-element cemented lens formed by cementing together lens L9, a meniscus lens with its concave surface facing the image side, and lens L10, a plano-convex lens with its flat surface facing the image side. The cemented lenses CL3 and CL4 each have a cemented surface with its concave surface facing the image side.

[0050] The fourth lens group G4 consists of, in order from the object side, a cemented lens CL5, a biconcave lens L13, and a meniscus lens L14 with its concave surface facing the object side. The cemented lens CL5 is a two-piece cemented lens formed by cementing together a biconvex lens L11 and a biconcave lens L12.

[0051] The various data of the objective lens 2 are as follows: NA=1.17,f=3.001mm,f G1 =4.503mm,f G2 =54.399mm,f G3 =-48.23mm,f G4 =-126.867mm,TTL=49.56mm,d 1GR =14.443mm,d 3GF =20.167mm,d FB =26.119mm

[0052] The lens data of the objective lens 2 is as follows: Objective Lens 2 srd nd νd 1 INF 0 1.33304 55.79 2 INF t 1.52397 54.41 3 INF D0 1.33304 55.79 4 INF 0.395 1.45858 67.84 5 -1.2180 3.881 1.80400 46.53 6 -3.4453 0.188 7 -27.0426 2.932 1.56907 71.30 8 -6.7830 0.250 9 18.7649 4.878 1.43875 94.66 10 -6.5896 1.450 1.63775 42.41 11 -12.6961 D1 12 -15.4644 2.227 1.43875 94.66 13 -9.7962 D2 14 98.6743 1.350 1.63775 42.41 15 5.8453 4.745 1.43875 94.66 16 -25.4378 0.260 17 45.9780 1.150 1.75500 52.32 18 6.7232 4.268 1.56907 71.30 19 INF 0.381 20 6.0546 5.163 1.56907 71.30 21 -11.5090 4.808 1.80400 46.53 22 4.1733 2.324 23 -3.6776 0.944 1.51823 58.90 24 19.1150 1.270 25 -115.0525 2.731 1.67300 38.26 26 -6.0445 110.000

[0053] The values ​​D0, D1, and D2 (unit: mm) of the distances d3, d11, and d13 in the lens data in each of the first to third states are as follows: 1st state 2nd state 3rd state t 0.17 0.11 0.23 D0 0.300 0.337 0.263 D1 2.682 2.830 2.530 D2 0.816 0.668 0.968

[0054] The objective lens 2 satisfies the following conditional expressions (1) and (2). (1)|d 1GR / TTL|=0.29 (2)|d 3GF / d FB |=0.77

[0055] FIGS. 7 to 9 are aberration diagrams of an optical system consisting of the objective lens 2 and the tube lens 10, showing aberrations on the image plane formed by the objective lens 2 and the tube lens 10 in the first, second, and third states, respectively. FIGS. 7(a), 8(a), and 9(a) are spherical aberration diagrams. FIGS. 7(b), 8(b), and 9(b) are diagrams showing the amount of violation of the sine condition. FIGS. 7(c), 8(c), and 9(c) are astigmatism diagrams. FIGS. 7(d), 8(d), and 9(d) are coma aberration diagrams at an image height ratio of 0.6 (image height of 7.95 mm). As shown in FIGS. 7 to 9, in this embodiment, aberrations are well corrected regardless of the cover glass thickness.

[0056] Example 3 10 is a cross-sectional view of the objective lens 3 according to this embodiment. The objective lens 3 is a water immersion objective lens for a microscope. The objective lens 3 is composed of, in order from the object side, a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, a third lens group G3, and a fourth lens group G4.

[0057] The first lens group G1 consists of, in order from the object side, a cemented lens CL1, a lens L3 which is a meniscus lens 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 cementing together lens L1 which is a positive lens with its convex surface facing the image side and lens L2 which is a meniscus lens with its concave surface facing the object side, and is the first lens component. The cemented lens CL2 is a two-element cemented lens formed by cementing together lens L4 which is a biconvex lens and lens L5 which is a meniscus lens with its concave surface facing the object side. Note that neither the cemented lens CL1 nor the cemented lens CL2 is a cemented lens in which the negative lens is positioned closer to the object than the positive lens.

[0058] The second lens group G2 is made up of a lens L6 that moves along the optical axis. The lens L6 is a single lens that has a meniscus shape with its concave surface facing the object side.

[0059] The third lens group G3 consists, in order from the object side, of a cemented lens CL3 and a cemented lens CL4. The cemented lens CL3 is a two-element cemented lens formed by cementing together a biconcave lens L7 and a biconvex lens L8. The cemented lens CL4 is a two-element cemented lens formed by cementing together a meniscus lens L9 with its concave surface facing the image side and a biconvex lens L10. The cemented lenses CL3 and CL4 each have a cemented surface with its concave surface facing the image side.

[0060] The fourth lens group G4 consists of, in order from the object side, a cemented lens CL5, a biconcave lens L13, and a meniscus lens L14 with its concave surface facing the object side. The cemented lens CL5 is a two-piece cemented lens formed by cementing together a biconvex lens L11 and a biconcave lens L12.

[0061] The various data of the objective lens 3 are as follows: NA=1.15,f=3.002mm,f G1 =4.245mm,f G2 =29.855mm,f G3 =-28.805mm,f G4 =-104.167mm,TTL=49.561mm,d 1GR =14.284mm,d 3GF =19.173mm,d FB =19.601mm

[0062] The lens data of the objective lens 3 is as follows: Objective Lens 3 srd nd νd 1 INF 0 1.33304 55.79 2 INF t 1.52397 54.41 3 INF D0 1.33304 55.79 4 INF 0.395 1.45858 67.84 5 -1.2180 3.935 1.80400 46.53 6 -3.3786 0.150 7 -85.1683 2.799 1.56907 71.30 8 -7.7283 0.250 9 18.1315 5.186 1.43875 94.66 10 -6.9596 1.100 1.63775 42.41 11 -12.6961 D1 12 -75.7240 2.188 1.43875 94.66 13 -11.2656 D2 14 -12.4588 2.127 1.63775 42.41 15 9.0479 4.278 1.43875 94.66 16 -11.2853 0.260 17 29.7244 1.100 1.67790 55.35 18 5.9211 4.147 1.43875 94.66 19 -50.2190 0.492 20 6.1176 5.107 1.56907 71.30 21 -13.9035 4.733 1.80400 46.53 22 4.4559 2.222 23 -3.4162 2.023 1.51823 58.90 24 23.6893 1.548 25 -49.3212 2.351 1.67300 38.26 26 -6.3462 110.000

[0063] The values ​​D0, D1, and D2 (unit: mm) of the distances d3, d11, and d13 in the lens data in each of the first to third states are as follows: 1st state 2nd state 3rd state t 0.17 0.11 0.23 D0 0.298 0.337 0.259 D1 1.673 1.774 1.574 D2 1.028 0.927 1.127

[0064] The objective lens 3 satisfies the following conditional expressions (1) and (2). (1)|d 1GR / TTL|=0.29 (2)|d 3GF / d FB |=0.98

[0065] FIGS. 11 to 13 are aberration diagrams of an optical system consisting of the objective lens 3 and the tube lens 10, showing the aberrations on the image plane formed by the objective lens 3 and the tube lens 10 in the first, second, and third states, respectively. FIGS. 11(a), 12(a), and 13(a) are spherical aberration diagrams. FIGS. 11(b), 12(b), and 13(b) are diagrams showing the amount of violation of the sine condition. FIGS. 11(c), 12(c), and 13(c) are astigmatism diagrams. FIGS. 11(d), 12(d), and 13(d) are coma aberration diagrams at an image height ratio of 0.6 (image height of 7.95 mm). As shown in FIGS. 11 to 13, in this embodiment, aberrations are well corrected regardless of the cover glass thickness.

[0066] Example 4 14 is a cross-sectional view of the objective lens 4 according to this example. The objective lens 4 is a water immersion objective lens for a microscope. The objective lens 4 is composed of, in order from the object side, a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, a third lens group G3, and a fourth lens group G4.

[0067] The first lens group G1 consists of, in order from the object side, a cemented lens CL1, a biconvex lens L3, and a cemented lens CL2. The cemented lens CL1 is a first lens component, and is a two-element cemented lens formed by cementing together lens L1, a positive lens with its convex surface facing the image side, and lens L2, a meniscus lens with its concave surface facing the object side. The cemented lens CL2 is a two-element cemented lens formed by cementing together lens L4, a biconvex lens, and lens L5, a meniscus lens with its concave surface facing the object side. Note that neither the cemented lens CL1 nor the cemented lens CL2 is a cemented lens in which the negative lens is positioned closer to the object than the positive lens.

[0068] The second lens group G2 is made up of a lens L6 that moves along the optical axis. The lens L6 is a biconvex single lens.

[0069] The third lens group G3 consists, in order from the object side, of a cemented lens CL3 and a cemented lens CL4. The cemented lens CL3 is a two-element cemented lens formed by cementing together a biconcave lens L7 and a biconvex lens L8. The cemented lens CL4 is a two-element cemented lens formed by cementing together a biconvex lens L9 and a meniscus lens L10 with its concave surface facing the object side. The cemented lens CL3 has a cemented surface with its concave surface facing the image side, and the cemented lens CL4 has a cemented surface with its concave surface facing the object side.

[0070] The fourth lens group G4 consists of, in order from the object side, a cemented lens CL5, a biconcave lens L13, and a meniscus lens L14 with its concave surface facing the object side. The cemented lens CL5 is a two-piece cemented lens formed by cementing together a biconvex lens L11 and a biconcave lens L12.

[0071] The various data of the objective lens 4 are as follows: NA=1.12,f=3.002mm,f G1 =4.31mm,f G2 =22.205mm,f G3 =-19.463mm,f G4 =-54.391mm,TTL=49.561mm,d 1GR =16.092mm,d 3GF =19.624mm,d FB =29.765mm

[0072] The lens data of the objective lens 4 is as follows: Objective Lens 4 srd nd νd 1 INF 0 1.33304 55.79 2 INF t 1.52397 54.41 3 INF D0 1.33304 55.79 4 INF 0.395 1.45858 67.84 5 -1.2180 4.001 1.80400 46.53 6 -3.3879 0.100 7 69.7154 2.836 1.56907 71.30 8 -8.9959 0.100 9 16.7994 5.419 1.43875 94.66 10 -6.7030 2.773 1.63775 42.41 11 -12.6961 D1 12 53.8449 2.743 1.43875 94.66 13 -11.7097 D2 14 -13.4266 3.474 1.63775 42.41 15 6.2754 4.738 1.43875 94.66 16 -10.8675 0.100 17 25.2227 4.930 1.43875 94.66 18 -6.4493 1.350 1.67790 55.35 19 -313.6822 0.100 20 4.9709 4.898 1.49700 81.54 21 -9.2306 1.881 1.75500 52.32 22 3.9528 2.048 1.00000 23 -3.9088 1.100 1.51823 58.90 24 16.5633 1.296 1.00000 25 -170.3558 4.020 1.67300 38.26 26 -6.7821 110.000

[0073] The values ​​D0, D1, and D2 (unit: mm) of the distances d3, d11, and d13 in the lens data in each of the first to third states are as follows: 1st state 2nd state 3rd state t 0.17 0.11 0.23 D0 0.298 0.337 0.259 D1 0.325 0.372 0.276 D2 0.465 0.418 0.514

[0074] The objective lens 4 satisfies the following conditional expressions (1) and (2). (1)|d 1GR / TTL|=0.32 (2)|d 3GF / d FB |=0.66

[0075] FIGS. 15 to 17 are aberration diagrams of an optical system consisting of the objective lens 4 and the tube lens 10, showing aberrations on the image plane formed by the objective lens 4 and the tube lens 10 in the first, second, and third states, respectively. FIGS. 15(a), 16(a), and 17(a) are spherical aberration diagrams. FIGS. 15(b), 16(b), and 17(b) are diagrams showing the amount of violation of the sine condition. FIGS. 15(c), 16(c), and 17(c) are astigmatism diagrams. FIGS. 15(d), 16(d), and 17(d) are coma aberration diagrams at an image height ratio of 0.6 (image height of 7.95 mm). As shown in FIGS. 15 to 17, in this embodiment, aberrations are well corrected regardless of the cover glass thickness.

[0076] The above-described embodiments are illustrative examples provided to facilitate understanding of the invention. The present invention is not limited to the above-described embodiments, and should be understood to encompass various modifications and alternative forms of the above-described embodiments. For example, it will be understood that the above-described embodiments can be embodied by modifying the components without departing from the spirit of the invention. It will also be understood that various embodiments can be implemented by appropriately combining multiple components disclosed in the above-described embodiments. Furthermore, it will be understood by those skilled in the art that various embodiments can be implemented by deleting some components from all of the components shown in the embodiments, or by adding some components to the components shown in the embodiments. [Explanation of symbols]

[0077] 1, 2, 3, 4: Objective lenses, 10: Tube lens, CG: Cover glass, CL1 to CL5, CTL1, CTL2: Cemented lenses, G1: First lens group, G2: Second lens group, G3: Third lens group, G4: Fourth lens group, L1 to L14, TL1 to TL4: Lenses

Claims

1. From the object side, a first lens group having positive refractive power; a second lens group having positive refractive power, moving along the optical axis; a third lens group having two or more cemented surfaces; a fourth lens group including, in order from the object side, a lens having a convex surface facing the object side, a lens having a concave surface facing the image side, a lens having a concave surface facing the object side, and a lens having a convex surface facing the image side, The first lens group is the optical system has at least three or more lens components, including a first lens component closest to the object, in which a positive lens having a convex surface facing the image side and a meniscus lens having a concave surface facing the object side are cemented together; None of the at least three lens components is a cemented lens in which a negative lens is located closer to the object than a positive lens. An objective lens characterized by:

2. 2. The objective lens according to claim 1, The third lens group includes two cemented lens components. An objective lens characterized by:

3. 3. The objective lens according to claim 2, Each of the two cemented lens components has a cemented surface with a concave surface facing the image side. An objective lens characterized by:

4. The objective lens according to any one of claims 1 to 3, The fourth lens group is composed of three or more lens components. An objective lens characterized by:

5. The objective lens according to any one of claims 1 to 3, The second lens group is composed of one lens element having a meniscus shape with a concave surface facing the object side. An objective lens characterized by:

6. The objective lens according to any one of claims 1 to 3, The second lens group is composed of a single lens. An objective lens characterized by:

7. The objective lens according to any one of claims 1 to 3, The first lens group includes a cemented lens component, which is located closer to the image than the first lens component and is formed by cementing together a positive lens and a negative lens, which are arranged in this order from the object side. An objective lens characterized by:

8. The objective lens according to any one of claims 1 to 3, The following conditional expression 0.25≦|d １ＧＲ / TTL|≦0.5 ・・・(1) An objective lens characterized by satisfying the above. However, d １ＧＲ is the distance on the optical axis from the object surface to the lens surface of the first lens group that is closest to the image. TTL is the distance on the optical axis from the object surface to the lens surface of the objective lens that is closest to the image.

9. The objective lens according to any one of claims 1 to 3, The following conditional expression 0. 3≦|d ３ＧＦ / d ＦＢ |≦1 ・・・(2) An objective lens characterized by satisfying the above. However, d ３ＧＦ is the distance on the optical axis from the object surface to the lens surface of the third lens group closest to the object. ＦＢ is the distance on the optical axis from the object plane to the back focal position of the objective lens.

Images