Observation optical system and optical apparatus

Abstract

To provide a compact observation optical system having excellent optical performance, and an optical device equipped with the observation optical system.SOLUTION: The observation optical system includes, in order from the object side to the eye point side, two lenses of an objective lens having a negative refractive power and an eyepiece lens having a positive refractive power. The observation optical system satisfies the following condition: 1.7 <(R4 - R2) / (R4 + R2) <4, where R4 is a curvature radius near the optical axis of the eyepoint-side surface of the ocular lens, and is a curvature radius near the optical axis of the eyepoint-side surface of the objective lens. R2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The technology of the present disclosure relates to an observation optical system and an optical device. [Background technology]

[0002] Conventionally, optical systems described in Patent Documents 1 to 7 below have been known as finders for cameras and the like. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-214542 [Patent Document 2] Japanese Patent Application Publication No. 10-282408 [Patent Document 3] Japanese Patent Application Publication No. 7-092387 [Patent Document 4] Japanese Patent Application Publication No. 6-230276 [Patent Document 5] Japanese Patent Application Publication No. 63-071822 [Patent Document 6] Japanese Patent Publication No. 61-091618 [Patent Document 7] Japanese Patent Application Publication No. 52-062023 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been a demand for observation optical systems that are compact and have good optical performance.

[0005] The present disclosure provides a compact observation optical system with good optical performance, and an optical device including this observation optical system. [Means for solving the problem]

[0006] An observation optical system according to one embodiment of the present disclosure is an observation optical system consisting of two lenses, in order from the object side to the eyepoint side, an objective lens having negative refractive power and an eyepiece lens having positive refractive power, where R4 is the paraxial radius of curvature of the eyepoint side surface of the eyepiece lens, R2 is the paraxial radius of curvature of the eyepoint side surface of the objective lens, Dsum is the distance from the intersection of the object side surface of the objective lens and the optical axis to the intersection of the eyepoint side surface of the eyepiece lens and the optical axis, f1 is the focal length of the objective lens, f2 is the focal length of the eyepiece lens, Nave is the average value of the refractive index at the d-line of all lenses included in the observation optical system, and D2 is the central thickness of the eyepiece lens. 1.7<(R4-R2) / (R4+R2)<4 (1) 50 <Dsum / (|f1| / f2)<80 (2) 1.45 <Nave<1.65 (3) 0.1 <D2 / Dsum<0.3 (4) The conditions (1), (2), (3), and (4) are satisfied.

[0007] The surface of the objective lens on the eyepoint side preferably has a concave surface facing the eyepoint side in the paraxial region and includes a region in which the negative refractive power weakens with increasing distance from the optical axis.

[0008] The surface of the eyepiece on the eyepoint side preferably has a convex surface facing the eyepoint side in the paraxial region and includes a region in which the positive refractive power increases with increasing distance from the optical axis.

[0009] The viewing optical system preferably satisfies the following conditional formula (1-1), more preferably satisfies the following conditional formula (1-2), and even more preferably satisfies the following conditional formula (1-3). 1.8<(R4-R2) / (R4+R2)<3.5 (1-1) 1.85<(R4-R2) / (R4+R2)<3 (1-2) 1.9<(R4-R2) / (R4+R2)<2.7 (1-3)

[0010] The viewing optical system preferably satisfies the following conditional formula (2-1), more preferably satisfies the following conditional formula (2-2), and even more preferably satisfies the following conditional formula (2-3). 55 <Dsum / (|f1| / f2)<78 (2-1) 60 <Dsum / (|f1| / f2)<75 (2-2) 61.5 <Dsum / (|f1| / f2)<74 (2-3)

[0011] The observation optical system preferably satisfies the following conditional formula (3-1), more preferably satisfies the following conditional formula (3-2), and even more preferably satisfies the following conditional formula (3-3). 1.46 <Nave<1.6 (3-1) 1.47 <Nave<1.58 (3-2) 1.475 <Nave<1.56 (3-3)

[0012] The observation optical system preferably satisfies the following conditional formula (4-1), more preferably satisfies the following conditional formula (4-2), and even more preferably satisfies the following conditional formula (4-3). 0.105 <D2 / Dsum<0.26 (4-1) 0.11 <D2 / Dsum<0.24 (4-2) 0.115 <D2 / Dsum<0.22 (4-3)

[0013] If the refractive index of the objective lens at the d-line is N1 and the Abbe number of the objective lens based on the d-line is ν1, the observation optical system is as follows: 1.8 <N1+0.01×ν1<2.14 (5) It is preferable to satisfy conditional expression (5) below.

[0014] If the refractive index of the eyepiece at the d-line is N2 and the Abbe number of the eyepiece at the d-line is ν2, the observation optical system is 1.8 <N2+0.01×ν2<2.14 (6) It is preferable to satisfy conditional expression (6) below.

[0015] The observation optical system is 1 <f2 / Dsum<2 (7) It is preferable to satisfy conditional expression (7) below.

[0016] If the air-equivalent length on the optical axis from the surface of the objective lens on the eyepoint side to the surface of the eyepiece lens on the object side is D12, the observation optical system is as follows: 0.05 <R2 / D12<1 (8) It is preferable to satisfy conditional expression (8) below.

[0017] An optical device according to another aspect of the present disclosure includes the observation optical system of the above aspect.

[0018] In this specification, the terms "consisting of" and "consisting of" are intended to mean that, in addition to the listed components, the components may also include lenses that have substantially no refractive power, as well as optical elements other than lenses such as apertures, filters, cover glasses, and prisms, as well as lens flanges, lens barrels, and image sensors.

[0019] In this specification, the terms "lens having positive refractive power" and "positive lens" are synonymous. The terms "lens having negative refractive power" and "negative lens" are synonymous. A compound aspherical lens (i.e., a lens in which a spherical lens and an aspherical film formed on the spherical lens are integrally configured to function as a single aspherical lens as a whole) is not considered a cemented lens but is treated as a single lens. Unless otherwise specified, the sign of the refractive power, radius of curvature, and surface shape of a lens including an aspherical surface are considered in the paraxial region. Regarding the sign of the radius of curvature, the sign of the radius of curvature of a surface with a convex surface facing the object side is positive, and the sign of the radius of curvature of a surface with a convex surface facing the eyepoint side is negative.

[0020] The "focal length" used in the conditional expressions is the paraxial focal length. The values ​​used in the conditional expressions are values ​​based on the d-line when the diopter is -1 diopter. The "d-line," "C-line," and "F-line" used in this specification are emission lines. The wavelength of the d-line is 587.56 nm (nanometers), the wavelength of the C-line is 656.27 nm (nanometers), and the wavelength of the F-line is 486.13 nm (nanometers). [Effects of the Invention]

[0021] According to the present disclosure, it is possible to provide a compact observation optical system with good optical performance, and an optical device including this observation optical system. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a cross-sectional view showing the configuration and light beams of an observation optical system according to an embodiment of the present disclosure, which corresponds to the observation optical system of Example 1. FIG. [Figure 2] 3A to 3C are diagrams showing various aberrations of the observation optical system of Example 1. [Figure 3] 10 is a cross-sectional view showing the configuration of an observation optical system and a light beam according to a second embodiment. [Figure 4] 10A to 10C are diagrams showing various aberrations of the observation optical system of Example 2. [Figure 5] 10 is a cross-sectional view showing the configuration of an observation optical system and a light beam according to a third embodiment. [Figure 6] 10A to 10C are diagrams showing various aberrations of the observation optical system of Example 3. [Figure 7] FIG. 2 is a perspective view of the rear side of the optical device according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0024] FIG. 1 shows a cross-sectional view of the configuration of an observation optical system according to an embodiment of the present disclosure and light beams. In FIG. 1, the light beams shown are an on-axis light beam 2 and an off-axis light beam 3 corresponding to the maximum apparent field of view. In FIG. 1, the left side is the object side and the right side is the eyepoint side. The eyepoint EP shown in FIG. 1 does not indicate the shape, but indicates the position in the optical axis direction. The example shown in FIG. 1 corresponds to Example 1, which will be described later.

[0025] This observation optical system consists of two lenses, in order from the object side to the eyepoint side along the optical axis Z: an objective lens L1 with negative refractive power and an eyepiece lens L2 with positive refractive power. This configuration, in order from the object side to the eyepoint EP, of a negative lens and a positive lens, is advantageous for shortening the overall optical length and facilitating miniaturization.

[0026] It is preferable that the surface of the objective lens L1 on the eyepoint side has a concave surface facing the eyepoint side in the paraxial region and includes a region in which the negative refractive power weakens with increasing distance from the optical axis Z. In this case, an increase in distortion can be suppressed, and good optical performance can be maintained in the peripheral area.

[0027] It is preferable that the object-side surface of the objective lens L1 is flat, which can improve the ease of assembly into the mechanical frame.

[0028] It is preferable that the surface of the eyepiece L2 on the eyepoint side be convex toward the eyepoint in the paraxial region and include a region in which the positive refractive power increases with increasing distance from the optical axis Z. In this case, the increase in distortion can be suppressed, and good optical performance can be maintained in the peripheral area.

[0029] It is preferable that the object-side surface of the eyepiece L2 is flat, which improves the ease of assembly into the mechanical frame.

[0030] Next, preferred and possible configurations for the conditional expressions of the viewing optical system of the present disclosure will be described. In the following description of the conditional expressions, to avoid redundancy, the same symbols will be used for elements with the same definitions, and duplicate explanations of the symbols will be omitted.

[0031] If the paraxial radius of curvature of the surface of the eyepiece lens L2 facing the eyepoint is R4 and the paraxial radius of curvature of the surface of the objective lens L1 facing the eyepoint is R2, it is preferable that the observation optical system satisfy the following conditional formula (1). By ensuring that the corresponding value of conditional formula (1) is not below the lower limit, the refractive power of the objective lens L1 does not become too weak, which is advantageous for radial compactness. By ensuring that the corresponding value of conditional formula (1) is not above the upper limit, the refractive power of the eyepiece lens L2 does not become too weak, which is advantageous for correcting distortion. 1.7<(R4-R2) / (R4+R2)<4 (1)

[0032] To obtain better characteristics, the lower limit of conditional expression (1) is preferably set to any one of 1.8, 1.85, 1.9, and 1.95. The upper limit of conditional expression (1) is preferably set to any one of 3.5, 3, 2.7, and 2.5. For example, the observation optical system preferably satisfies the following conditional expression (1-1), more preferably satisfies the following conditional expression (1-2), even more preferably satisfies the following conditional expression (1-3), and even more preferably satisfies the following conditional expression (1-4). 1.8<(R4-R2) / (R4+R2)<3.5 (1-1) 1.85<(R4-R2) / (R4+R2)<3 (1-2) 1.9<(R4-R2) / (R4+R2)<2.7 (1-3) 1.95<(R4-R2) / (R4+R2)<2.5 (1-4)

[0033] If Dsum is the distance from the intersection of the object-side surface of objective lens L1 and the optical axis Z to the intersection of the eyepoint-side surface of eyepiece lens L2 and the optical axis Z, f1 is the focal length of objective lens L1, and f2 is the focal length of eyepiece lens L2, it is preferable that the observation optical system satisfy the following conditional formula (2). Here, Dsum is expressed in millimeters (mm). Ensuring that the corresponding value of conditional formula (2) is not less than the lower limit is advantageous for expanding the apparent field of view while maintaining good aberration control. Ensuring that the corresponding value of conditional formula (2) is not more than the upper limit is advantageous for shortening the overall optical length while maintaining the apparent field of view, facilitating compactness. As an example, Figure 1 shows Dsum for the observation optical system of Figure 1 and D2 related to the conditional formula described below. 50 <Dsum / (|f1| / f2)<80 (2)

[0034] To obtain better characteristics, the lower limit of conditional expression (2) is preferably set to one of 55, 60, 61.5, and 62. The upper limit of conditional expression (2) is preferably set to one of 78, 75, 74, and 73. For example, the observation optical system preferably satisfies the following conditional expression (2-1), more preferably satisfies the following conditional expression (2-2), even more preferably satisfies the following conditional expression (2-3), and even more preferably satisfies the following conditional expression (2-4). 55 <Dsum / (|f1| / f2)<78 (2-1) 60 <Dsum / (|f1| / f2)<75 (2-2) 61.5 <Dsum / (|f1| / f2)<74 (2-3) 62 <Dsum / (|f1| / f2)<73 (2-4)

[0035] If the average value of the refractive index at the d-line of all lenses included in the observation optical system is Nave, it is preferable that the observation optical system satisfy the following conditional expression (3): By ensuring that the corresponding value of conditional expression (3) is not equal to or less than the lower limit, it is possible to prevent the Petzval sum from becoming large, which is advantageous for correcting field curvature. By ensuring that the corresponding value of conditional expression (2) is not equal to or greater than the upper limit, it is possible to prevent the materials that can be selected as lens materials from being limited to those with small Abbe numbers, which is advantageous for correcting chromatic aberration. 1.45 <Nave<1.65 (3)

[0036] To obtain better characteristics, the lower limit of conditional expression (3) is preferably set to one of 1.46, 1.47, 1.475, and 1.48. The upper limit of conditional expression (3) is preferably set to one of 1.6, 1.58, 1.56, and 1.54. For example, the observation optical system preferably satisfies the following conditional expression (3-1), more preferably satisfies the following conditional expression (3-2), even more preferably satisfies the following conditional expression (3-3), and even more preferably satisfies the following conditional expression (3-4). 1.46 <Nave<1.6 (3-1) 1.47 <Nave<1.58 (3-2) 1.475 <Nave<1.56 (3-3) 1.48 <Nave<1.54 (3-4)

[0037] If the central thickness of eyepiece lens L2 is D2, it is preferable that the observation optical system satisfy the following conditional formula (4): By ensuring that the corresponding value of conditional formula (4) is not below the lower limit, it becomes easier to ensure the edge thickness of eyepiece lens L2 (the thickness of the outermost periphery of the lens), which is advantageous in terms of lens workability. By ensuring that the corresponding value of conditional formula (4) is not above the upper limit, Dsum does not become too small, which prevents the angle of the light ray incident on eyepiece lens L2 from objective lens L1 with respect to optical axis Z from becoming too steep, which is advantageous in suppressing aberrations. 0.1 <D2 / Dsum<0.3 (4)

[0038] To obtain better characteristics, the lower limit of conditional expression (4) is preferably set to any one of 0.105, 0.11, 0.115, and 0.118. The upper limit of conditional expression (4) is preferably set to any one of 0.26, 0.24, 0.22, and 0.2. For example, the observation optical system preferably satisfies the following conditional expression (4-1), more preferably satisfies the following conditional expression (4-2), even more preferably satisfies the following conditional expression (4-3), and even more preferably satisfies the following conditional expression (4-4): 0.105 <D2 / Dsum<0.26 (4-1) 0.11 <D2 / Dsum<0.24 (4-2) 0.115 <D2 / Dsum<0.22 (4-3) 0.118 <D2 / Dsum<0.2 (4-4)

[0039] When the refractive index of objective lens L1 at the d-line is N1 and the Abbe number of objective lens L1 based on the d-line is v1, it is preferable that the observation optical system satisfy the following conditional formula (5): By ensuring that the corresponding value of conditional formula (5) is not equal to or less than the lower limit, it is possible to select materials other than those with low refractive indexes and low Abbe numbers, making it easier to correct chromatic aberration of magnification. By ensuring that the corresponding value of conditional formula (5) is not equal to or greater than the upper limit, it is possible to select materials other than those with high refractive indexes and high Abbe numbers, making it possible to select materials with a low specific gravity, making it easier to reduce the weight. 1.8 <N1+0.01×ν1<2.14 (5)

[0040] To obtain better characteristics, the lower limit of conditional expression (5) is preferably set to one of 1.85, 1.9, and 1.95. The upper limit of conditional expression (5) is preferably set to one of 2.13, 2.12, and 2.11. For example, the observation optical system preferably satisfies the following conditional expression (5-1), more preferably satisfies the following conditional expression (5-2), and even more preferably satisfies the following conditional expression (5-3): 1.85 <N1+0.01×ν1<2.13 (5-1) 1.9 <N1+0.01×ν1<2.12 (5-2) 1.95 <N1+0.01×ν1<2.11 (5-3)

[0041] When the refractive index of eyepiece lens L2 at the d-line is N2 and the Abbe number of eyepiece lens L2 based on the d-line is ν2, it is preferable that the observation optical system satisfy the following conditional formula (6): By ensuring that the corresponding value of conditional formula (6) is not equal to or less than the lower limit, it is possible to select materials other than those with low refractive indexes and low Abbe numbers, making it easier to correct chromatic aberration of magnification. By ensuring that the corresponding value of conditional formula (6) is not equal to or greater than the upper limit, it is possible to select materials other than those with high refractive indexes and high Abbe numbers, making it possible to select materials with a lower specific gravity, making it easier to reduce the weight. 1.8 <N2+0.01×ν2<2.14 (6)

[0042] To obtain better characteristics, the lower limit of conditional expression (6) is preferably set to one of 1.85, 1.9, and 1.95. The upper limit of conditional expression (6) is preferably set to one of 2.13, 2.12, and 2.11. For example, the observation optical system preferably satisfies the following conditional expression (6-1), more preferably satisfies the following conditional expression (6-2), and even more preferably satisfies the following conditional expression (6-3): 1.85 <N2+0.01×ν2<2.13 (6-1) 1.9 <N2+0.01×ν2<2.12 (6-2) 1.95 <N2+0.01×ν2<2.11 (6-3)

[0043] It is preferable that the observation optical system satisfy the following conditional expression (7): By ensuring that the corresponding value of conditional expression (7) is not below the lower limit, it is possible to prevent the finder magnification from decreasing too much. By ensuring that the corresponding value of conditional expression (7) is not above the upper limit, Dsum does not become too small, making it easier to correct various aberrations. 1 <f2 / Dsum<2 (7)

[0044] To obtain better characteristics, the lower limit of conditional expression (7) is preferably set to one of 1.1, 1.2, and 1.3. The upper limit of conditional expression (7) is preferably set to one of 1.8, 1.7, and 1.6. For example, the observation optical system preferably satisfies the following conditional expression (7-1), more preferably satisfies the following conditional expression (7-2), and even more preferably satisfies the following conditional expression (7-3): 1.1 <f2 / Dsum<1.8 (7-1) 1.2 <f2 / Dsum<1.7 (7-2) 1.3 <f2 / Dsum<1.6 (7-3)

[0045] If the air-equivalent length on the optical axis Z from the eyepoint-side surface of objective lens L1 to the object-side surface of eyepiece lens L2 is D12, it is preferable that the observation optical system satisfy the following conditional expression (8). By ensuring that the corresponding value of conditional expression (8) is not equal to or smaller than the lower limit, it is possible to prevent the overall optical length from becoming too long. By ensuring that the corresponding value of conditional expression (8) is not equal to or larger than the upper limit, it is possible to prevent negative distortion from becoming too large. 0.05 <R2 / D12<1 (8)

[0046] To obtain better characteristics, the lower limit of conditional expression (8) is preferably set to any one of 0.15, 0.2, and 0.25. The upper limit of conditional expression (8) is preferably set to any one of 0.8, 0.6, and 0.5. For example, the observation optical system preferably satisfies the following conditional expression (8-1), more preferably satisfies the following conditional expression (8-2), and even more preferably satisfies the following conditional expression (8-3): 0.15 <R2 / D12<0.8 (8-1) 0.2 <R2 / D12<0.6 (8-2) 0.25 <R2 / D12<0.5 (8-3)

[0047] The above-described preferred and possible configurations, including those related to the conditional expressions, can be combined in any desired manner, and are preferably selectively adopted as appropriate according to the required specifications. The example in Fig. 1 is merely an example, and various modifications are possible within the scope of the gist of the technology of the present disclosure.

[0048] As an example, a preferred embodiment of the observation optical system of the present disclosure is an observation optical system consisting of two lenses, in order from the object side to the eyepoint side, an objective lens L1 having negative refractive power and an eyepiece lens L2 having positive refractive power, and satisfies the above conditional expressions (1), (2), (3), and (4).

[0049] Next, examples of the observation optical system according to the present disclosure will be described with reference to the drawings. The reference symbols assigned to the components of the observation optical system in the cross-sectional views of each example are used independently for each example to avoid complication of the explanation and drawings due to an increase in the number of digits in the reference symbols. Therefore, even if common reference symbols are assigned in drawings of different examples, this does not necessarily mean that the components have the same configuration.

[0050] [Example 1] The configuration of the observation optical system of Example 1 and a cross-sectional view of the light beam are shown in FIG. 1. The illustration method and configuration are as described above, so some overlapping explanations will be omitted here. The observation optical system of Example 1 consists of two lenses, in order from the object side to the eyepoint side: an objective lens L1 with negative refractive power and an eyepiece lens L2 with positive refractive power. The eyepoint-side surface of the objective lens L1 is aspherical, with a concave surface facing the eyepoint side in the paraxial region and including a region in which the negative refractive power decreases with increasing distance from the optical axis Z. The object-side surface of the objective lens L1 is flat. The eyepoint-side surface of the eyepiece lens L2 is aspherical, with a convex surface facing the eyepoint side in the paraxial region and including a region in which the positive refractive power increases with increasing distance from the optical axis Z. The object-side surface of the eyepiece lens L2 is flat.

[0051] For the viewing optical system of Example 1, basic lens data is shown in Table 1, and aspherical coefficients are shown in Table 2.

[0052] The table of basic lens data is written as follows. The Sn column indicates the surface number, with the surface closest to the object being surface 1, and the numbers increasing by one as you move towards the eyepoint. The R column indicates the radius of curvature of each surface. The D column indicates the surface spacing on the optical axis between each surface and its adjacent surface on the eyepoint side. The Nd column indicates the refractive index for the d-line of each component element. The νd column indicates the Abbe number of each component element based on the d-line. The ER column indicates the effective radius of each lens surface.

[0053] In the basic lens data table, the sign of the radius of curvature of a surface with a convex surface facing the object side is positive, and the sign of the radius of curvature of a surface with a convex surface facing the eyepoint side is negative. The value in the bottom row of the table's column D is the distance between the surface closest to the eyepoint and the eyepoint EP. Also, outside the basic lens data table, the value of the field of view at full angle of view when the diopter is -1 diopter is shown. This field of view value corresponds to the value of the apparent field of view.

[0054] In the basic lens data, the aspherical surface numbers are marked with an *, and the paraxial radius of curvature value is listed in the aspherical radius of curvature column. In Table 2, the Sn row shows the aspherical surface numbers, and the KA and Am (m = 4, 6, 8, 10) rows show the numerical values ​​of the aspherical coefficients for each aspherical surface. The numerical values ​​of the aspherical coefficients, "E±n" (n: integer), are expressed as "×10 ±n " KA and Am are aspherical coefficients in the aspherical formula given below. Zd=C×h 2 / {1+(1-KA×C 2 ×h 2 ) 1 / 2}+ΣAm×h m however, Zd: Aspheric depth (length of the perpendicular line drawn from a point on the aspheric surface at height h to a plane perpendicular to the optical axis Z where the vertex of the aspheric surface is in contact) h: Height (distance from optical axis Z to lens surface) C: Reciprocal of paraxial curvature radius KA, Am: aspherical coefficients In the aspherical formula, Σ means the summation over m.

[0055] In the data in each table, degrees are used as the unit of angle and mm (millimeters) as the unit of length, but since the optical system can be used with proportional enlargement or reduction, other appropriate units can also be used. Also, in each table below, values ​​are listed rounded to a certain number of decimal places.

[0056] [Table 1]

[0057] [Table 2]

[0058] FIG. 2 shows aberration diagrams of the observation optical system of Example 1 when the diopter is -1 diopter. From left to right, FIG. 2 shows spherical aberration, astigmatism, distortion, and lateral chromatic aberration. In the spherical aberration diagram, aberrations at the d-line, C-line, and F-line are shown by solid lines, long-dashed lines, and short-dashed lines, respectively. In the astigmatism diagram, aberrations at the d-line in the sagittal direction are shown by solid lines, and aberrations at the d-line in the tangential direction are shown by short-dashed lines. In the distortion diagram, aberrations at the d-line are shown by solid lines. In the lateral chromatic aberration diagram, aberrations at the C-line and F-line are shown by long-dashed lines and short-dashed lines, respectively. The unit dpt on the horizontal axis of the spherical aberration diagram and astigmatism diagram represents diopters. The unit min on the horizontal axis of the lateral chromatic aberration diagram represents angular minutes. In spherical aberration diagrams, the diameter of the eyepoint EP in millimeters (mm) is shown after "Φ=". In other aberration diagrams, the apparent field of view at half angle of view is shown after "ω=".

[0059] The symbols, meanings, notation methods, and illustration methods of each data item related to the above-mentioned Example 1 are the same in the following Examples unless otherwise specified, and therefore, redundant explanations will be omitted below.

[0060] [Example 2] Figure 3 shows the configuration of the observation optical system of Example 2 and a cross-sectional view of the light beam. The observation optical system of Example 2 consists of two lenses, from the object side to the eyepoint side: an objective lens L1 with negative refractive power and an eyepiece lens L2 with positive refractive power. The surface of the objective lens L1 on the eyepoint side is aspherical, with a concave surface facing the eyepoint side in the paraxial region, and including a region in which the negative refractive power decreases with increasing distance from the optical axis Z. The surface of the objective lens L1 on the object side is flat. The surface of the eyepiece lens L2 on the eyepoint side is aspherical, with a convex surface facing the eyepoint side in the paraxial region, and including a region in which the positive refractive power increases with increasing distance from the optical axis Z. The surface of the eyepiece lens L2 on the object side is flat.

[0061] For the viewing optical system of Example 2, basic lens data is shown in Table 3, aspherical coefficients are shown in Table 4, and aberration diagrams at a diopter of -1 diopter are shown in FIG.

[0062] [Table 3]

[0063] [Table 4]

[0064] [Example 3] Figure 5 shows the configuration of the observation optical system of Example 3 and a cross-sectional view of the light beam. The observation optical system of Example 3 consists of two lenses, from the object side to the eyepoint side: an objective lens L1 with negative refractive power and an eyepiece lens L2 with positive refractive power. The surface of the objective lens L1 on the eyepoint side is aspherical, with a concave surface facing the eyepoint side in the paraxial region, and including a region in which the negative refractive power decreases with increasing distance from the optical axis Z. The surface of the objective lens L1 on the object side is flat. The surface of the eyepiece lens L2 on the eyepoint side is aspherical, with a convex surface facing the eyepoint side in the paraxial region, and including a region in which the positive refractive power increases with increasing distance from the optical axis Z. The surface of the eyepiece lens L2 on the object side is flat.

[0065] For the viewing optical system of Example 3, basic lens data is shown in Table 5, aspherical coefficients are shown in Table 6, and aberration diagrams at a diopter of -1 diopter are shown in FIG.

[0066] [Table 5]

[0067] [Table 6]

[0068] Table 7 shows the corresponding values ​​of conditional expressions (1) to (8) for the observation optical systems of Examples 1 to 3. The values ​​shown in Table 7 are based on the d-line.

[0069] [Table 7]

[0070] The observation optical systems of Examples 1 to 3 are constructed to be compact, yet achieve high optical performance with excellent correction of various aberrations.

[0071] Next, an optical device equipped with an observation optical system according to an embodiment of the present disclosure will be described. Fig. 7 is a perspective view showing a schematic configuration of the rear side of a camera 100, which is an optical device according to an embodiment of the present disclosure. The camera 100 is a digital camera, as an example. The camera 100 is equipped with a finder 101 according to an embodiment of the present disclosure, located on top of a camera body 102. The finder 101 is an example of an observation optical device, and is equipped with an observation optical system according to an embodiment of the present disclosure.

[0072] Camera 100 is provided with operation buttons 103 for making various settings, a zoom lever 104 for changing magnification, and a monitor 106 for displaying images and various setting screens on the back of camera body 102, and a shutter button 105 on the top of camera body 102. Camera 100 also has an imaging lens (not shown) on the front of camera body 102, and an imaging element (not shown) inside camera body 102 that captures a subject image formed by the imaging lens. A user looks through viewfinder 101 from the back side to observe the subject image.

[0073] Although the technology of the present disclosure has been described above using embodiments and examples, the technology of the present disclosure is not limited to the above embodiments and examples and can be modified in various ways. For example, the radius of curvature, surface spacing, refractive index, Abbe number, aspherical coefficient, etc. of each lens are not limited to the values ​​shown in the above examples and can take other values.

[0074] Furthermore, the optical device according to the embodiment of the present disclosure is not limited to the above configuration, and can also be applied to, for example, a film camera, a video camera, a head-mounted display, and the like.

[0075] The following additional notes are provided regarding the above embodiments and examples. [Appendix 1] An observation optical system consisting of two lenses, in order from the object side to the eyepoint side, an objective lens having negative refractive power and an eyepiece lens having positive refractive power, The paraxial radius of curvature of the surface of the eyepiece on the eyepoint side is R4, The paraxial curvature radius of the surface of the objective lens on the eyepoint side is R2, The distance from the intersection of the object side surface of the objective lens and the optical axis to the intersection of the eye point side surface of the eyepiece lens and the optical axis is Dsum, The focal length of the objective lens is f1, The focal length of the eyepiece is f2, The average value of the refractive index at the d line of all lenses included in the observation optical system is Nave, If the center thickness of the eyepiece is D2, 1.7<(R4-R2) / (R4+R2)<4 (1) 50 <Dsum / (|f1| / f2)<80 (2) 1.45 <Nave<1.65 (3) 0.1 <D2 / Dsum<0.3 (4) An observation optical system that satisfies conditional expressions (1), (2), (3), and (4) expressed by the following formulas: [Appendix 2] The surface of the objective lens on the eye point side is a concave surface facing the eye point side in a paraxial region, and includes a region in which the negative refractive power weakens with increasing distance from the optical axis. Attachment 1: The observation optical system. [Appendix 3] The surface of the eyepiece on the eyepoint side is convex toward the eyepoint in the paraxial region, and includes a region in which the positive refractive power increases with increasing distance from the optical axis. 10. The observation optical system according to claim 1 or 2. [Appendix 4] 1.8<(R4-R2) / (R4+R2)<3.5 (1-1) 4. The viewing optical system according to claim 1, which satisfies the conditional expression (1-1) shown below. [Appendix 5] 1.85<(R4-R2) / (R4+R2)<3 (1-2) 4. The viewing optical system according to claim 1, which satisfies conditional expression (1-2) shown below. [Appendix 6] 1.9<(R4-R2) / (R4+R2)<2.7 (1-3) 4. The viewing optical system according to claim 1, which satisfies conditional expression (1-3) shown below. [Appendix 7] 55 <Dsum / (|f1| / f2)<78 (2-1) 7. The viewing optical system according to claim 1, which satisfies conditional expression (2-1) shown below. [Appendix 8] 60 <Dsum / (|f1| / f2)<75 (2-2) 7. The viewing optical system according to claim 1, which satisfies conditional expression (2-2) shown below. [Appendix 9] 61.5 <Dsum / (|f1| / f2)<74 (2-3) 7. The viewing optical system according to claim 1, which satisfies conditional expression (2-3) shown below. [Appendix 10] 1.46 <Nave<1.6 (3-1) 10. The viewing optical system according to claim 1, which satisfies conditional expression (3-1) shown below. [Appendix 11] 1.47 <Nave<1.58 (3-2) 10. The viewing optical system according to any one of claims 1 to 9, which satisfies conditional expression (3-2) shown below. [Appendix 12] 1.475 <Nave<1.56 (3-3) 10. The viewing optical system according to any one of claims 1 to 9, which satisfies conditional expression (3-3) shown below. [Appendix 13] 0.105 <D2 / Dsum<0.26 (4-1) 13. The viewing optical system according to claim 1, which satisfies conditional expression (4-1) shown below. [Appendix 14] 0.11 <D2 / Dsum<0.24 (4-2) 13. The viewing optical system according to claim 1, which satisfies conditional expression (4-2) shown below. [Appendix 15] 0.115 <D2 / Dsum<0.22 (4-3) 13. The viewing optical system according to claim 1, which satisfies conditional expression (4-3) shown below. [Appendix 16] The refractive index of the objective lens at the d line is N1, When the Abbe number of the objective lens based on the d-line is ν1, 1.8 <N1+0.01×ν1<2.14 (5) 16. The viewing optical system according to claim 1, which satisfies conditional expression (5) shown below. [Appendix 17] The refractive index of the eyepiece at the d line is N2, When the Abbe number of the eyepiece based on the d-line is ν2, 1.8 <N2+0.01×ν2<2.14 (6) 17. The viewing optical system according to claim 1, which satisfies conditional expression (6) shown below. [Appendix 18] 1 <f2 / Dsum<2 (7) 18. The viewing optical system according to claim 1, which satisfies conditional expression (7) shown below. [Appendix 19] If the air-equivalent length on the optical axis from the surface of the objective lens on the eyepoint side to the surface of the eyepiece lens on the object side is D12, 0.05 <R2 / D12<1 (8) 19. The viewing optical system according to any one of claims 1 to 18, which satisfies conditional expression (8) shown below. [Appendix 20] An optical device comprising the observation optical system according to any one of Supplementary Note 1 to Supplementary Note 19. [Explanation of symbols]

[0076] 2 On-axis luminous flux 3 Off-axis luminous flux corresponding to maximum apparent field of view 100 cameras 101 Finder 102 Camera Body 103 Operation Button 104 Zoom Lever 105 Shutter button 106 monitors D2 eyepiece center thickness Dsum: The distance from the intersection of the object-side surface of the objective lens and the optical axis to the intersection of the eyepiece-side surface and the optical axis. EP Eyepoint L1 objective lens L2 eyepiece Z optical axis

Claims

1. An observation optical system comprising two lenses, in order from the object side to the eyepoint side, an objective lens having negative refractive power and an eyepiece lens having positive refractive power, The paraxial radius of curvature of the surface of the eyepiece on the eyepoint side is R4, The paraxial radius of curvature of the surface of the objective lens on the eyepoint side is R2, Dsum is the distance from the intersection of the object-side surface of the objective lens and the optical axis to the intersection of the eyepoint-side surface of the eyepiece lens and the optical axis, The focal length of the objective lens is f1, The focal length of the eyepiece is f2, Nave is the average value of the refractive index at the d line of all lenses included in the observation optical system, When the center thickness of the eyepiece is D2, 1.7<(R4-R2) / (R4+R2)<4 (1) 50<Dsum / (|f1| / f2)<80 (2) 1.45<Nave<1.65 (3) 0.1<D2 / Dsum<0.3 (4) An observation optical system that satisfies conditional expressions (1), (2), (3), and (4) expressed by the following formulas.

2. The surface of the objective lens on the eye point side is a concave surface facing the eye point side in a paraxial region, and includes a region in which the negative refractive power weakens with increasing distance from the optical axis. The observation optical system according to claim 1 .

3. The surface of the eyepiece on the eyepoint side is convex toward the eyepoint in the paraxial region, and includes a region in which the positive refractive power increases with increasing distance from the optical axis. The observation optical system according to claim 1 .

4. 1.8<(R4-R2) / (R4+R2)<3.5 (1-1) 2. The viewing optical system according to claim 1, which satisfies the conditional expression (1-1) expressed as follows:

5. 1.85<(R4-R2) / (R4+R2)<3 (1-2) 2. The viewing optical system according to claim 1, which satisfies conditional expression (1-2) expressed by:

6. 1.9<(R4-R2) / (R4+R2)<2.7 (1-3) 2. The viewing optical system according to claim 1, which satisfies conditional expression (1-3) expressed by:

7. 55<Dsum / (|f1| / f2)<78 (2-1) 2. The viewing optical system according to claim 1, which satisfies conditional expression (2-1) expressed as follows:

8. 60<Dsum / (|f1| / f2)<75 (2-2) 2. The viewing optical system according to claim 1, which satisfies conditional expression (2-2) expressed as follows:

9. 61.5<Dsum / (|f1| / f2)<74 (2-3) 2. The viewing optical system according to claim 1, which satisfies conditional expression (2-3) expressed by:

10. 1.46<Nave<1.6 (3-1) 2. The viewing optical system according to claim 1, which satisfies conditional expression (3-1) expressed as follows:

11. 1.47<Nave<1.58 (3-2) 2. The viewing optical system according to claim 1, which satisfies conditional expression (3-2) expressed as follows:

12. 1.475<Nave<1.56 (3-3) 2. The viewing optical system according to claim 1, which satisfies conditional expression (3-3) expressed as follows:

13. 0.105<D2 / Dsum<0.26 (4-1) 2. The viewing optical system according to claim 1, which satisfies conditional expression (4-1) expressed as follows:

14. 0.11<D2 / Dsum<0.24 (4-2) 2. The viewing optical system according to claim 1, which satisfies conditional expression (4-2) expressed as follows:

15. 0.115<D2 / Dsum<0.22 (4-3) 2. The viewing optical system according to claim 1, which satisfies conditional expression (4-3) expressed by:

16. The refractive index of the objective lens at the d line is N1, When the Abbe number of the objective lens based on the d-line is ν1, 1.8<N1+0.01×ν1<2.14 (5) 2. The viewing optical system according to claim 1, which satisfies conditional expression (5) expressed as follows:

17. The refractive index of the eyepiece at the d line is N2, When the Abbe number of the eyepiece based on the d-line is ν2, 1.8<N2+0.01×ν2<2.14 (6) 2. The viewing optical system according to claim 1, which satisfies conditional expression (6) expressed as follows:

18. 1<f2 / Dsum<2 (7) 2. The viewing optical system according to claim 1, which satisfies conditional expression (7) expressed as follows:

19. When the air-equivalent length on the optical axis from the surface of the objective lens on the eyepoint side to the surface of the eyepiece lens on the object side is D12, 0.05<R2 / D12<1 (8) 2. The viewing optical system according to claim 1, which satisfies conditional expression (8) expressed as follows:

20. An optical device comprising the observation optical system according to any one of claims 1 to 19.

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