Observation optical system and imaging device equipped with the same

Abstract

To provide an observation optical system with which it is possible to sufficiently correct field curvature and astigmatism while securing a sufficient space for installing optical path branching means, a wide viewing angle, and a sufficient eye point at normal use time.SOLUTION: Provided is an observation optical system comprising a first lens L1 of positive refractive power, a second lens L2 of negative refractive power, and a third lens L3 of positive refractive power, which are arranged in order from a display panel LCD side to an observation side. The absolute value of curvature radius of the lens surface on the observation side of the first lens L1 is smaller than the absolute value of curvature radius of the lens surface on the display panel LCD side of the first lens L1. Each of a distance from the display surface of the display panel LCD to the lens surface on the display panel LCD side of the first lens L1, a focal distance of the observation optical system, a distance on an optical axis from the lens surface on the observation side of the second lens L2 to the lens surface on the display panel LCD side of the third lens L3, and a distance on the optical axis from the lens surface on the observation side of the first lens L1 to the lens surface on the display panel LCD side of the second lens L2 is appropriately set.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an observation optical system, and more particularly to an observation optical system suitable for an electronic viewfinder. [Background technology]

[0002] Conventionally, there has been known an observation optical system for observing a display panel having a diagonal length of about 10 mm, which has a first lens with positive refractive power, a second lens with negative refractive power, and a third lens with positive refractive power, arranged in that order from the display panel side to the observation side (see Patent Document 1).

[0003] In recent years, it has been proposed to add a gaze detection function to the observation optical system, which detects the gaze of the observer who is observing the display panel and detects which position on the panel the observer is viewing. When the gaze detection function is added, it becomes possible to select the main subject and perform various operations based on gaze direction information. In order to detect the observer's gaze, it is necessary to place an optical path branching means on the viewfinder optical path that branches and extracts a part of the light beam. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-109496 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the optical system of Patent Document 1, even if an optical path branching means is arranged and a gaze detection function can be added, various aberrations of the optical system cannot be suppressed because the optical characteristics and arrangement position of the optical path branching means are not clear. Also, in the optical system of Patent Document 1, a strong concave negative lens is arranged on the observation side, making it difficult to ensure a sufficient eye point length.

[0006] The present invention aims to provide an observation optical system that can sufficiently correct field curvature and astigmatism while ensuring sufficient space for arranging an optical path branching means, a wide field of view, and a sufficient eyepoint length during normal use. [Means for solving the problem]

[0007] The observation optical system according to one aspect of the present invention includes an optical path branching means for branching an optical path, a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive refractive power, which are arranged in this order from a display panel side to an observation side. Alternatively, the optical path splitting means is arranged in order from the display panel side to the observation side, and ... An observation optical system, the absolute value of the radius of curvature of a lens surface on the observation side of a first lens being smaller than the absolute value of the radius of curvature of a lens surface on the display panel side of the first lens, and a diopter of -1m -1 When the distance from the display surface of the display panel to the lens surface of the first lens on the display panel side is d1, the focal length of the observation optical system is f, the distance on the optical axis from the lens surface of the second lens on the observation side to the lens surface of the third lens on the display panel side is d23, the distance on the optical axis from the lens surface of the first lens on the observation side to the lens surface of the second lens on the display panel side is d12, the refractive index of the optical path branching means at the d line is Ndg0, and the refractive index of the second lens at the d line is Ndg2, 0.5 <d1 / f<2.0 0.0 <d23 / d12<0.5 1.55 <Ndg0<2.00 1.6 <Ndg2<1.8 1.5 <Ndg1<2.0 The present invention is characterized in that the following conditional expression is satisfied: Effect of the Invention

[0008] According to the present invention, it is possible to provide an observation optical system that can sufficiently correct field curvature and astigmatism while ensuring sufficient space for arranging an optical path branching means, a wide field of view, and a sufficient eyepoint length during normal use. [Brief description of the drawings]

[0009] [Figure 1]1 is a cross-sectional view showing a lens configuration of an observation optical system according to a first embodiment. [Diagram 2] 4A to 4C are aberration diagrams of the observation optical system of Example 1. [Diagram 3] FIG. 11 is a cross-sectional view showing the lens configuration of the observation optical system of Example 2. [Figure 4] 11A to 11C are aberration diagrams of the observation optical system of Example 2. [Diagram 5] FIG. 11 is a cross-sectional view showing the lens configuration of the observation optical system of Example 3. [Figure 6] 11A to 11C are aberration diagrams of the observation optical system of Example 3. [Figure 7] FIG. 11 is a cross-sectional view showing the lens configuration of the observation optical system of Example 4. [Figure 8] 11A to 11C are aberration diagrams of the observation optical system of Example 4. [Figure 9] FIG. 13 is a cross-sectional view showing the lens configuration of the observation optical system of Example 5. [Figure 10] 13A to 13C are aberration diagrams of the observation optical system of Example 5. [Figure 11] FIG. 13 is a cross-sectional view showing the lens configuration of the observation optical system of Example 6. [Figure 12] 13A to 13C are aberration diagrams of the observation optical system of Example 6. [Figure 13] FIG. 13 is a cross-sectional view showing the lens configuration of the observation optical system of Example 7. [Figure 14] 13A to 13C are aberration diagrams of the observation optical system of Example 7. [Figure 15] FIG. 13 is a cross-sectional view showing the lens configuration of the observation optical system of Example 8. [Figure 16] 13A to 13C are aberration diagrams of the observation optical system of Example 8. [Figure 17] FIG. 13 is a cross-sectional view showing the lens configuration of the observation optical system of Example 9. [Figure 18] 13A to 13C are aberration diagrams of the observation optical system of Example 9. [Figure 19] FIG. 23 is a cross-sectional view showing the lens configuration of the observation optical system of Example 10. [Figure 20] 13A to 13C are aberration diagrams of the observation optical system of Example 10. [Figure 21] FIG. 21 is a cross-sectional view showing the lens configuration of the observation optical system of Example 11. [Figure 22] 13A to 13C are aberration diagrams of the observation optical system of Example 11. [Diagram 23] FIG. 1 is a schematic diagram of an imaging device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In each drawing, the same reference numerals are given to the same members, and duplicated explanations will be omitted.

[0011] 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, and 21 are cross-sectional views showing the lens configurations of the observation optical systems of Examples 1 to 11. In each cross-sectional view, the left side is the display panel side (object side) and the right side is the observation side (exit side).

[0012] The observation optical system of each embodiment is used, for example, as an electronic viewfinder of an imaging device.

[0013] In order to observe a small display panel with a diagonal length of about 10 mm at a magnified viewing angle of 35 to 45 degrees, the observation optical system needs to have a strong positive refractive power. In the peripheral area of ​​the subject, as the height from the center increases, curvature of field and astigmatism occur, and the optical performance deteriorates. In order to improve the curvature of field and astigmatism, the observation optical system of each embodiment has a first lens L1 with a positive refractive power, a second lens L2 with a negative refractive power, and a third lens L3 with a positive refractive power, which are arranged in this order from the display panel side to the observation side. By using such a lens configuration, it becomes easier to improve the performance of the observation optical system while providing a wide viewing angle.

[0014] In addition, in the observation optical system of each embodiment, EP is the eye point (exit surface). LCD is a display panel. The display panel LCD is composed of a liquid crystal element, an organic EL, or the like. The display panel LCD is arranged with its image display surface (display surface) facing the observation optical system. Between the display panel LCD and the first lens L1, an optical path branching means DP is arranged, which is composed of a dichroic prism and branches the optical path. As the optical path branching means DP, an optical member other than a dichroic prism may be used. For example, a half mirror may be used as the optical path branching means DP. The optical path branching means DP functions to transmit light from the display panel LCD and guide it to the user's pupil, while reflecting light from the user's pupil (gaze). This makes it possible to detect the user's gaze.

[0015] In the observation optical system of each embodiment, among various aberrations, distortion and lateral chromatic aberration may be corrected by electrical image processing, which makes it possible to reduce the overall lens diameter, increase the imaging magnification, and effectively correct chromatic aberration, field curvature, and the like.

[0016] 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, and 22 are aberration diagrams of the observation optical systems of Examples 1 to 11, respectively. -1 (standard diopter).

[0017] The spherical aberration diagram shows the amount of spherical aberration for the d-line (wavelength 587.6 nm) and F-line (wavelength 486.1 nm). The astigmatism diagram shows M the amount of astigmatism on the meridional image plane, and S the amount of astigmatism on the sagittal image plane. The distortion diagram shows the amount of distortion for the d-line. The chromatic aberration diagram shows the amount of chromatic aberration for the F-line.

[0018] Next, the characteristic configuration of the optical system of each embodiment will be described.

[0019] In the viewing optical system of each embodiment, the absolute value of the radius of curvature of the lens surface of the first lens L1 on the viewing side is smaller than the absolute value of the radius of curvature of the lens surface of the first lens on the display panel side.

[0020] The observation optical system of each embodiment satisfies the following conditional expressions (1) and (2). Here, d1 is the distance from the display surface of the display panel LCD to the lens surface of the first lens L1 on the display panel side. f is the focal length of the observation optical system at standard diopter. d23 is the distance on the optical axis from the lens surface of the second lens L2 on the observation side to the lens surface of the third lens L3 on the display panel side. d12 is the distance on the optical axis from the lens surface of the first lens L1 on the observation side to the lens surface of the second lens L2 on the display panel side.

[0021] 0.5 <d1 / f<2.0 (1) 0.0 <d23 / d12<0.5 (2) Conditional formula (1) specifies the relationship between the distance from the display panel LCD to the lens surface of the first lens L1 on the display panel side and the focal length of the observation optical system at standard diopter. By satisfying conditional formula (1), it is possible to correct various aberrations such as spherical aberration, curvature of field, astigmatism, etc. while ensuring sufficient space for arranging the optical path branching means DP and a wide viewing angle. If the upper limit of conditional formula (1) is exceeded and the focal length of the observation optical system becomes short, various aberrations such as spherical aberration, curvature of field, astigmatism, etc. cannot be corrected well. If the lower limit of conditional formula (1) is exceeded, it becomes difficult to ensure sufficient space distance for arranging the optical path branching means DP.

[0022] Conditional formula (2) defines the relationship between the axial distance from the observation side lens surface of the second lens L2 to the display panel side lens surface of the third lens L3 and the axial distance from the observation side lens surface of the first lens L1 to the display panel side lens surface of the second lens L2. By satisfying conditional formula (2), it is possible to correct various aberrations such as spherical aberration, curvature of field, and astigmatism while ensuring a wide viewing angle. If the axial distance from the observation side lens surface of the first lens L1 to the display panel side lens surface of the second lens L2 exceeds the upper limit of conditional formula (2) and becomes short, it becomes difficult to suppress the curvature of field and spherical aberration. If the axial distance from the observation side lens surface of the first lens L1 to the display panel side lens surface of the second lens L2 falls below the lower limit of conditional formula (2) and becomes long, it becomes difficult to suppress the lens diameter when the viewing angle is widened.

[0023] It is preferable that the numerical ranges of the conditional expressions (1) and (2) be the numerical ranges of the following conditional expressions (1a) and (2a).

[0024] 0.51 <d1 / f<1.70 (1a) 0.01 <d23 / d12<0.40 (2a) It is further preferable that the numerical ranges of the conditional expressions (1) and (2) be the numerical ranges of the following conditional expressions (1b) and (2b).

[0025] 0.51 <d1 / f<1.40 (1b) 0.02 <d23 / d12<0.30 (2b) Next, the conditions that the observation optical system of each embodiment preferably satisfies are described. The observation optical system of each embodiment preferably satisfies one or more of the following conditional expressions (3) to (17). Here, f1 is the focal length of the first lens L1. f3 is the focal length of the third lens L3. G1R1 is the radius of curvature of the lens surface of the first lens L1 on the display panel side. G1R2 is the radius of curvature of the lens surface of the first lens L1 on the observation side. G2R1 is the radius of curvature of the lens surface of the second lens L2 on the display panel side. G2R2 is the radius of curvature of the lens surface of the second lens L2 on the observation side. G3R1 is the radius of curvature of the lens surface of the third lens L3 on the display panel side. G3R2 is the radius of curvature of the lens surface of the third lens L3 on the observation side. Ndg1 is the refractive index of the first lens L1 at the d line. Ndg2 is the refractive index of the second lens L2 at the d line. Ndg3 is the refractive index of the third lens L3 at the d-line. νdg1 is the Abbe number of the first lens L1 at the d-line. νdg2 is the Abbe number of the second lens at the d-line. νdg3 is the Abbe number of the third lens L3 at the d-line. gt2 is the distance on the optical axis from the lens surface of the second lens L2 on the display panel side to the lens surface of the second lens L2 on the observation side. Ndg0 is the refractive index of the optical path branching means DP at the d-line. νdg0 is the Abbe number of the optical path branching means DP at the d-line.

[0026] 0.55 <f1 / f<2.00 (3) 0.8 <f3 / f1<2.0 (4) 1.3<(G1R2+G2R1) / (G1R2-G2R1)<15.0 (5) 0.05<(G1R1+G1R2) / (G1R1-G1R2)<2.00 (6) -10.0<(G2R1+G2R2) / (G2R1-G2R2)<-0.3 (7) 0.1<(G3R1+G3R2) / (G3R1-G3R2)<3.0 (8) 1.5 <Ndg1<2.0 (9) 1.6 <Ndg2<1.8 (10) 1.5 <Ndg3<2.0 (11) 30<νdg1<70 (12) 18<νdg2<30 (13) 30<νdg3<70 (14) 0.02 <gt2 / f<0.15 (15) 1.55 <Ndg0<2.00 (16) 33<νdg0<75 (17) Conditional formula (3) specifies the relationship between the focal length of the first lens and the focal length of the observation optical system. By satisfying conditional formula (3), it is possible to suppress the curvature of field and axial chromatic aberration of the observation optical system while ensuring high magnification and a sufficient distance from the display panel to the lens surface of the first lens L1 on the display panel side. If the upper limit of conditional formula (3) is exceeded and the power of the first lens L1 is weakened, this is disadvantageous for high magnification. In addition, since the principal point position of the observation optical system moves to the observation side, it becomes difficult to ensure the distance from the display panel LCD to the lens surface of the first lens L1 on the display panel side. If the lower limit of conditional formula (3) is not satisfied, it becomes difficult to suppress the curvature of field and axial chromatic aberration of the observation optical system.

[0027] Conditional formula (4) specifies the relationship between the focal length of the third lens L3 and the focal length of the first lens L1. By satisfying conditional formula (4), it is possible to achieve high magnification and to ensure a sufficient distance from the display panel to the lens surface of the first lens L1 on the display panel side while satisfying telecentricity on the entrance side (panel display surface side) of the observation optical system. If the upper limit of conditional formula (4) is exceeded and the power of the first lens L1 is weakened, this is disadvantageous for achieving high magnification. In addition, since the principal point position of the observation optical system moves to the observation side, it becomes difficult to ensure the distance from the display panel LCD to the lens surface of the first lens L1 on the display panel side. If the lower limit of conditional formula (4) is exceeded, the light emitted from the display panel LCD is bent significantly by the first lens L1. In order to pass the light to the observation side, it is necessary to increase the exit angle from the display panel LCD, which weakens the telecentricity. Since the display panel LCD can cause color unevenness and a decrease in the amount of light depending on the exit angle of the light ray, it is preferable for the observation optical system to be telecentric and have a small exit angle from the display panel LCD.

[0028] Conditional formula (5) specifies the shape factor of the lens surface of the first lens L1 on the observation side and the lens surface of the second lens L2 on the display panel side. By satisfying conditional formula (5), it is possible to suppress the curvature of field and axial chromatic aberration of the observation optical system while ensuring high magnification and a sufficient distance from the display panel LCD to the lens surface of the first lens L1 on the display panel side. If the upper limit of conditional formula (5) is exceeded and the power of the first lens L1 becomes weak, it is disadvantageous to high magnification. In addition, since the principal point position of the observation optical system moves to the observation side, it becomes difficult to ensure the distance from the display panel LCD to the lens surface of the first lens L1 on the display panel side. If the lower limit of conditional formula (5) is not satisfied, it becomes difficult to suppress the curvature of field and axial chromatic aberration of the observation optical system.

[0029] Conditional formula (6) specifies the shape factor of the lens surface of the first lens L1 on the display panel side and the lens surface of the first lens L1 on the observation side. By satisfying conditional formula (6), it is possible to suppress the curvature of field and coma aberration of the observation optical system while ensuring high magnification and a sufficient distance from the display panel LCD to the lens surface of the first lens L1 on the display panel side. If the upper limit of conditional formula (6) is exceeded and the power of the lens surface of the first lens L1 on the observation side becomes strong, it becomes difficult to suppress spherical aberration and curvature of field. If the lower limit of conditional formula (6) is exceeded and the power of the lens surface of the first lens L1 on the observation side becomes weak, the light emitted from the display panel LCD is bent significantly by the first lens L1. In order to pass the light to the observation side, it is necessary to increase the exit angle from the display panel LCD, which weakens the telecentricity. Since the display panel LCD can cause color unevenness and a decrease in the amount of light depending on the exit angle of the light ray, it is preferable for the observation optical system to be telecentric and have a small exit angle from the display panel LCD.

[0030] Conditional formula (7) defines the shape factor of the lens surface of the second lens L2 on the display panel side and the lens surface of the second lens L2 on the observation side. If the upper limit of conditional formula (7) is exceeded and the power of the lens surface of the second lens L2 on the display panel side becomes strong, it becomes difficult to suppress distortion and astigmatism. If the lower limit of conditional formula (7) is exceeded and the power of the lens surface of the second lens on the display panel side becomes weak, it becomes disadvantageous for achieving high magnification. In addition, the principal point position of the observation optical system moves to the observation side, making it difficult to ensure the distance from the display panel LCD to the lens surface of the first lens L1 on the display panel side.

[0031] Conditional formula (8) specifies the shape factor of the lens surface of the third lens L3 on the display panel side and the lens surface of the third lens L3 on the observation side. By satisfying conditional formula (8), it is possible to sufficiently correct various aberrations such as spherical aberration, curvature of field, and astigmatism while ensuring a wide viewing angle. If the power of the lens surface of the third lens L3 on the observation side becomes strong by exceeding the upper limit of conditional formula (8), it becomes difficult to suppress curvature of field and coma aberration. If the power of the lens surface of the third lens L3 on the observation side becomes weak by falling below the lower limit of conditional formula (8), the observation magnification of the observation optical system decreases.

[0032] Conditional expression (9) defines the refractive index at the d-line of the first lens L1. By satisfying conditional expression (9), it is possible to suppress aberrations in the observation optical system. If conditional expression (9) is not satisfied, the curvature shape of the first lens L1 becomes too sharp, making it difficult to suppress spherical aberration and curvature of field.

[0033] Conditional expression (10) defines the refractive index at the d-line of the second lens L2. By satisfying conditional expression (10), it is possible to suppress aberrations in the observation optical system. If conditional expression (10) is not satisfied, the curvature shape of the lens surface of the second lens L2 on the display panel side becomes too sharp, making it difficult to suppress spherical aberration and curvature of field.

[0034] Conditional expression (11) defines the refractive index of the third lens L3 at the d-line. By satisfying conditional expression (11), it is possible to suppress aberrations in the observation optical system. If conditional expression (11) is not satisfied, the curvature shape of the third lens L3 becomes too sharp, making it difficult to suppress spherical aberration and curvature of field.

[0035] Conditional expression (12) defines the Abbe number for the d-line of the first lens L1. By satisfying conditional expression (12), various aberrations such as lateral chromatic aberration and axial chromatic aberration can be corrected. If conditional expression (12) is not satisfied, it becomes difficult to suppress lateral chromatic aberration and axial chromatic aberration.

[0036] Conditional expression (13) defines the Abbe number for the d-line of the second lens L2. By satisfying conditional expression (13), various aberrations such as lateral chromatic aberration and axial chromatic aberration can be corrected. If conditional expression (13) is not satisfied, it becomes difficult to suppress lateral chromatic aberration and axial chromatic aberration.

[0037] Conditional expression (14) defines the Abbe number for the d-line of the third lens L3. By satisfying conditional expression (14), various aberrations such as lateral chromatic aberration and axial chromatic aberration can be corrected. If conditional expression (14) is not satisfied, it becomes difficult to suppress lateral chromatic aberration and axial chromatic aberration.

[0038] Conditional formula (15) defines the relationship between the axial distance from the lens surface of the second lens L2 on the display panel side to the lens surface of the second lens L2 on the observation side and the focal length of the observation optical system. If the axial distance from the lens surface of the second lens L2 on the display panel side to the lens surface of the second lens L2 on the observation side becomes long by exceeding the upper limit of conditional formula (15), it becomes difficult to suppress the lens diameter of the observation optical system. If the axial distance from the lens surface of the second lens L2 on the display panel side to the lens surface of the second lens L2 on the observation side becomes short by falling below the lower limit of conditional formula (15), it becomes difficult to ensure the edge and center thickness of the lens.

[0039] Conditional formula (16) specifies the refractive index at the d-line of the optical path branching means DP. Conditional formula (16) is an equation. If the upper limit of conditional formula (16) is exceeded, it becomes difficult to process the prism arranged in the observation optical system. If the lower limit of conditional formula (16) is not reached and the power of the observation optical system becomes weak, the observation magnification of the observation optical system decreases.

[0040] Conditional expression (17) defines the Abbe number of the optical path branching means for the d-line. By satisfying conditional expression (17), it is possible to correct various aberrations such as lateral chromatic aberration and axial chromatic aberration. If conditional expression (17) is not satisfied, it becomes difficult to suppress lateral chromatic aberration and axial chromatic aberration.

[0041] It is preferable that the numerical ranges of the conditional expressions (3) to (17) be the numerical ranges of the following conditional expressions (3a) to (17a).

[0042] 0.57 <f1 / f<1.70 (3a) 0.85 <f3 / f1<1.90 (4a) 1.4<(G1R2+G2R1) / (G1R2-G2R1)<13.0 (5a) 0.08<(G1R1+G1R2) / (G1R1-G1R2)<1.60 (6a) -8.0<(G2R1+G2R2) / (G2R1-G2R2)<-0.5 (7a) 0.15<(G3R1+G3R2) / (G3R1-G3R2)<2.40 (8a) 1.51 <Ndg1<1.95 (9a) 1.61 <Ndg2<1.76 (10a) 1.51 <Ndg3<1.95 (11a) 32<νdg1<66 (12a) 19<νdg2<28 (13a) 33<νdg3<66 (14a) 0.03 <gt2 / f<0.14 (15a) 1.57 <Ndg0<1.95 (16a) 35<νdg0<71 (17a) It is further preferable that the numerical ranges of the conditional expressions (3) to (17) be the numerical ranges of the following conditional expressions (3b) to (17b).

[0043] 0.59 <f1 / f<1.40 (3b) 0.90 <f3 / f1<1.80 (4b) 1.5<(G1R2+G2R1) / (G1R2-G2R1)<11.0 (5b) 0.11<(G1R1+G1R2) / (G1R1-G1R2)<1.20 (6b) -6.0<(G2R1+G2R2) / (G2R1-G2R2)<-0.7 (7b) 0.2<(G3R1+G3R2) / (G3R1-G3R2)<1.8 (8b) 1.52 <Ndg1<1.89 (9b) 1.62 <Ndg2<1.72 (10b) 1.52 <Ndg3<1.89 (11b) 34<νdg1<62 (12b) 20<νdg2<26 (13b) 36<νdg3<62 (14b) 0.04 <gt2 / f<0.13 (15b) 1.59 <Ndg0<1.90 (16b) 37<νdg0<67 (17b) Next, the optical system of each embodiment will be described in detail.

[0044] The viewing optical system of Examples 1 to 6, 10, and 11 includes a first lens L1 with positive refractive power, a second lens L2 with negative refractive power, and a third lens L3 with positive refractive power, arranged in this order from the display panel side to the viewing side.

[0045] The observation optical system of Examples 7 to 9 has, arranged in order from the display panel side to the observation side, a first lens L1 with positive refractive power, a second lens L2 with negative refractive power, a third lens L3 with positive refractive power, and a fourth lens L4 with positive refractive power.

[0046] Numerical examples 1 to 11 corresponding to the first to eleventh embodiments, respectively, are given below.

[0047] In each numerical example, the diagonal length of the screen display means the diagonal length of the display panel, and is twice the maximum image height of the object surface (display panel).

[0048] In the surface data of each numerical example, r represents the radius of curvature of each optical surface, and d (mm) represents the axial distance (distance on the optical axis) between the mth surface and the (m+1)th surface. Here, m is the surface number counted from the display panel side. Additionally, nd represents the refractive index of each optical member with respect to the d-line, and νd represents the Abbe number of the optical member. Note that the Abbe number νd of a certain material is given by the following when the refractive indices at the d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) of the Fraunhofer lines are Nd, NF, and NC, respectively: νd=(Nd-1) / (NF-NC) It is expressed as:

[0049] In addition, in each numerical example, the unit of length described is [mm] unless otherwise specified. However, since the observation optical system can obtain the same optical performance even if proportional enlargement or reduction is performed, the unit is not limited to [mm] and other appropriate units may be used.

[0050] If the optical surface is aspheric, a * symbol is added to the right of the surface number. The aspheric shape is expressed as follows, where X is the displacement from the apex of the surface in the optical axis direction, h is the height from the optical axis in the direction perpendicular to the optical axis, R is the paraxial radius of curvature, K is the conic constant, and A2, A4, A6, A8, and A10 are the aspheric coefficients of each order: X = (h 2 / R) / [1+{1-(1+K)(h / R) 2} 1 / 2 +A2×h 2 +A4×h 4 +A6×h 6 +A8×h 8 +A10×h 10 In addition, "e±XX" in each aspheric coefficient is "×10± XX " It means.

[0051] [Numerical Example 1] Standard diopter focal length f=18.7 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.40 3 ∞ 7.50 1.83400 37.2 4 ∞ (variable) 5 45.231 4.65 1.76802 49.2 6* -15.489 3.49 7* -4.014 1.20 1.63550 23.9 8* -7.629 0.55 9 151.752 6.87 1.76802 49.2 10* -14.490 (variable) 11 ∞ 1.00 1.49171 57.4 12∞23.00 13 (eye point) Aspheric Data 6th side K =-6.66748e-001 A 4=-2.85417e-005 A 6= 1.39887e-006 A 8=-1.10652e-008 A10= 2.81105e-011 7th side K =-2.18343e+000 A 4= 1.54046e-004 A 6=-1.65834e-006 A 8=-4.30312e-008 A10= 3.85331e-010 8th side K =-5.65588e+000 A 4= 7.37605e-004 A 6=-1.02719e-005 A 8= 6.14376e-008 A10=-1.34191e-010 Surface 10 K =-7.64153e-001 A 4= 3.04449e-005 A 6=-4.75055e-007 A 8= 4.20704e-009 A10=-1.22627e-011 Variable Interval 0m-1 -1 -5 +2 d4 2.55 2.19 1.07 3.23 d10 1.30 1.65 2.78 0.62 [Numerical Example 2] At standard diopter focal length f=17.3 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.00 3 ∞ 8.50 1.83400 37.2 4 ∞ (variable) 5 32.488 6.22 1.85135 40.1 6* -14.808 3.68 7* -6.818 1.42 1.65100 21.5 8* -112.547 0.15 9 131.272 6.37 1.85135 40.1 10* -13.950 (variable) 11 ∞ 1.00 1.49171 57.4 12∞23.00 13 (eye point) Aspheric Data 6th side K =-5.17963e-001 A4= 1.31147e-004 A6=-9.49493e-007 A8= 7.11126e-009 A10=-2.06714e-011 7th side K =-2.30355e+000 A4=-4.79553e-005 A6=-4.11730e-007 A8= 4.18329e-009 A10=-9.10282e-012 8th side K =-2.29196e+002 A4= 9.77074e-005 A6=-6.23395e-007 A8=-5.31801e-011 A10= 6.30220e-012 Surface 10 K =-2.73927e+000 A4=-4.98063e-006 A6=-4.94289e-007 A8= 4.69850e-009 A10=-1.20091e-011 Variable Interval 0m-1 -1 -5 +2 d4 2.68 2.43 1.07 3.61 d10 2.61 3.01 4.29 1.44 [Numerical Example 3] Standard diopter focal length f=20.9 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.40 3 ∞ 11.00 1.51633 64.1 4 ∞ (variable) 5 70.050 5.00 1.53500 55.7 6* -10.453 2.24 7* -8.090 1.47 1.63550 23.9 8* -30.489 0.20 9 565.772 5.30 1.53500 55.7 10* -11.482 (variable) 11 ∞ 1.00 1.49171 57.4 12∞23.00 13 (eye point) Aspheric Data 6th side K =-1.14236e+000 A4= 7.88210e-005 A6=-6.98908e-007 A8= 6.72781e-009 A10= 3.25984e-011 7th side K =-7.73767e-001 A4=-1.65804e-004 A6= 3.40985e-006 A8=-5.33102e-008 A10= 4.56325e-010 8th side K = 1.10047e+000 A4=-1.61563e-004 A6= 1.59586e-006 A8=-2.23675e-008 A10= 1.59401e-010 10th side K =-5.64372e-001 A4= 2.41444e-005 A6= 9.78734e-007 A8=-3.91803e-009 A10=-1.07217e-011 Variable Interval 0m-1 -1 -5 +2 d4 2.80 2.44 1.07 3.97 d10 2.61 3.01 4.29 1.44 [Numerical Example 4] Standard diopter focal length f=19.4 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.40 3∞5.50 1.83400 37.2 4 ∞ (variable) 5 53.212 4.23 1.76802 49.2 6* -17.471 3.64 7* -6.264 1.47 1.63550 23.9 8* -17.286 0.20 9 -978.513 6.33 1.76802 49.2 10* -13.131 (variable) 11 ∞ 1.00 1.49171 57.4 12∞23.00 13 (eye point) Aspheric Data 6th side K =-1.77022e-001 A4= 1.46611e-005 A6=-2.33559e-007 A8= 8.98377e-009 A10=-4.50295e-011 7th side K =-1.48356e+000 A4= 3.08907e-005 A6=-2.68254e-006 A8= 2.86950e-008 A10=-1.19054e-010 8th side K =-1.00098e+001 A4= 1.17321e-004 A6=-1.03184e-006 A8= 1.13551e-009 A10= 2.52839e-011 10th side K =-8.57586e-001 A4= 2.88196e-005 A6=-4.48100e-007 A8= 4.84736e-009 A10=-2.05834e-011 Variable Interval 0m-1 -1 -5 +2 d4 4.60 4.24 2.92 5.72 d10 2.61 3.01 4.29 1.44 [Numerical Example 5] At standard diopter focal length f=17.3 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞1.40 3 ∞ 7.50 1.83400 37.2 4 ∞ (variable) 5 38.725 6.19 1.76802 49.2 6* -12.483 3.78 7* -6.314 1.47 1.63550 23.9 8* -36.874 0.20 9 142.426 6.67 1.76802 49.2 10* -12.969 (variable) 11 ∞ 1.00 1.49171 57.4 12∞23.00 13 (eye point) Aspheric Data 6th side K =-6.29498e-001 A4= 1.19468e-004 A6=-4.49758e-007 A8= 8.06285e-010 A10= 4.56492e-012 7th side K =-1.47340e+000 A4= 1.85691e-004 A6=-3.51354e-006 A8= 1.54064e-008 A10=-3.68192e-012 8th side K =-9.42027e+000 A4= 1.27913e-004 A6=-9.21440e-007 A8=-2.65348e-010 A10= 7.48724e-012 Surface 10 K =-1.26617e+000 A4= 6.28629e-005 A6=-9.49756e-007 A8= 6.89539e-009 A10=-1.46285e-011 Variable Interval 0m-1 -1 -5 +2 d4 2.68 2.43 1.07 3.61 d10 2.61 3.01 4.29 1.44 [Numerical Example 6] Standard diopter focal length f=18.7 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.40 3 ∞ 7.50 1.83400 37.2 4 ∞ (variable) 5 57.475 4.45 1.76802 49.2 6* -14.399 3.52 7* -5.527 2.20 1.63550 23.9 8* -16.114 0.15 9 189.354 6.00 1.76802 49.2 10* -13.731 (variable) 11 ∞ 1.00 1.49171 57.4 12∞23.00 13 (eye point) Aspheric Data 6th side K =-6.90825e-001 A4= 8.38523e-005 A6=-6.11664e-007 A8= 7.67392e-009 A10=-4.11453e-011 7th side K =-1.90508e+000 A4= 3.96382e-005 A6=-1.65188e-006 A8=-8.19135e-009 A10= 9.04048e-011 8th side K =-7.66349e+000 A4= 2.88263e-004 A6=-3.08349e-006 A8= 1.31547e-008 A10=-1.74846e-011 Surface 10 K =-1.83793e+000 A4=-1.51567e-005 A6=-4.27702e-007 A8= 4.02270e-009 A10=-1.15184e-011 Variable Interval 0m-1 -1 -5 +2 d4 2.72 2.37 1.07 3.55 d10 1.45 1.80 3.09 0.62 [Numerical Example 7] At standard diopter focal length f=16.7 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.40 3 ∞ 8.00 1.83400 37.2 4 ∞ (variable) 5 26.560 9.25 1.85135 40.1 6* -11.463 2.78 7* -5.419 1.48 1.65100 21.5 8* -559.787 0.30 9 64.096 4.96 1.85135 40.1 10* -16.116 0.38 11* -330.547 1.50 1.85135 40.1 12 -87.784 (variable) 13 ∞ 1.00 1.49171 57.4 14∞23.00 15 (eye point) Aspheric Data 6th side K =-8.96739e-001 A4= 1.40579e-004 A6=-1.40304e-007 A8=-1.20332e-009 A10= 4.37568e-012 7th side K =-1.72726e+000 A4= 2.86825e-004 A6=-2.54542e-006 A8= 1.07202e-008 A10=-1.64392e-011 8th side K =-2.00703e+001 A4= 7.06128e-005 A6=-2.04709e-007 A8= 1.03566e-010 A10= 7.04374e-013 Surface 10 K =-4.88826e+000 A4= 1.02869e-004 A6=-1.00942e-006 A8= 3.92627e-009 A10=-3.22191e-012 Side 11 K = 0.00000e+000 A4= 1.40836e-009 A6=-1.47238e-007 A8= 8.72409e-010 Variable Interval 0m-1 -1 -5 +2 d4 2.60 2.36 1.07 3.46 d12 2.61 3.01 4.29 1.44 [Numerical Example 8] At standard diopter focal length f=19.6 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.00 3 ∞ 15.00 1.83400 37.2 4 ∞ (variable) 5 26.960 6.81 1.88202 37.2 6* -20.302 4.41 7* -6.551 1.50 1.65100 21.5 8* -49.760 0.30 9 31.260 4.87 1.53100 56.0 10* -13.846 0.40 11* -316.353 1.50 1.53100 56.0 12 -60.253 (variable) 13 ∞ 1.00 1.49171 57.4 14∞23.00 15 (eye point) Aspheric Data 6th side K =-3.30456e-001 A4= 6.13001e-005 A6=-7.08532e-008 A8=-8.15267e-010 A10= 5.59645e-012 7th side K =-1.31789e+000 A4= 3.64324e-004 A6=-3.19790e-006 A8= 1.52536e-008 A10=-2.50693e-011 8th side K = 7.74508e+000 A4= 8.92913e-005 A6=-5.69190e-007 A8=-4.29620e-010 A10= 1.40568e-011 10th side K =-4.27201e+000 A4= 1.60683e-004 A6=-8.25502e-007 A8= 4.85238e-009 A10=-2.05050e-011 Side 11 K = 0.00000e+000 A4= 1.26889e-010 A6=-3.88095e-008 A8=-1.50250e-009 Variable Interval 0m-1 -1 -5 +2 d4 2.76 2.39 1.07 3.88 d12 2.61 3.01 4.29 1.44 [Numerical Example 9] Standard diopter focal length f=19.4 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.40 3∞5.50 1.83400 37.2 4 ∞ (variable) 5 80.942 4.31 1.76802 49.2 6* -17.118 3.84 7* -7.014 1.50 1.63550 23.9 8* -29.619 0.30 9 33.202 6.80 1.53500 55.7 10* -12.491 0.40 11* -231.468 2.20 1.53500 55.7 12 -38.773 (variable) 13 ∞ 1.00 1.49171 57.4 14∞23.00 15 (eye point) Aspheric Data 6th side K =-3.40902e-001 A4=-8.45938e-006 A6=-7.30356e-007 A8= 1.19537e-008 A10=-4.78498e-011 7th side K =-8.34256e-001 A4=-3.07734e-005 A6=-5.02200e-007 A8= 1.44453e-008 A10=-2.23447e-011 8th side K = 1.61326e+000 A4= 9.82052e-005 A6=-8.14780e-007 A8=-4.37799e-009 A10= 4.12233e-011 10th side K =-9.02152e-001 A4= 4.56397e-005 A6= 1.40184e-007 A8= 4.52440e-009 A10=-2.34725e-011 Side 11 K = 0.00000e+000 A4= 3.05411e-010 A6= 8.29368e-011 A8=-1.89591e-010 Variable Interval 0m-1 -1 -5 +2 d4 2.76 2.40 1.07 3.87 d12 2.61 3.01 4.29 1.44 [Numerical Example 10] Standard diopter focal length f=18.7 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞1.811 3∞7.50 1.60311 37.2 4 ∞ (variable) 5 57.475 4.45 1.76802 49.2 6* -14.399 3.52 7* -5.527 2.20 1.63550 23.9 8* -16.114 0.15 9 189.354 6.00 1.76802 49.2 10* -13.731 (variable) 11 ∞ 1.00 1.49171 57.4 12∞23.00 13 (eye point) Aspheric Data 6th side K =-6.90825e-001 A4= 8.38523e-005 A6=-6.11664e-007 A8= 7.67392e-009 A10=-4.11453e-011 7th side K =-1.90508e+000 A4= 3.96382e-005 A6=-1.65188e-006 A8=-8.19135e-009 A10= 9.04048e-011 8th side K =-7.66349e+000 A4= 2.88263e-004 A6=-3.08349e-006 A8= 1.31547e-008 A10=-1.74846e-011 Surface 10 K =-1.83793e+000 A4=-1.51567e-005 A6=-4.27702e-007 A8= 4.02270e-009 A10=-1.15184e-011 Variable Interval 0m-1 -1 -5 +2 d4 2.72 2.37 1.07 3.55 d10 1.45 1.80 3.09 0.62 [Numerical Example 11] Standard diopter focal length f=18.7 Pupil diameter 10 Screen diagonal length 6.434 Surface Data Surface number rd nd νd 1 ∞ 0.70 1.52100 65.1 2∞2.13 3 ∞ 7.50 1.71995 37.2 4 ∞ (variable) 5 57.475 4.45 1.76802 49.2 6* -14.399 3.52 7* -5.527 2.20 1.63550 23.9 8* -16.114 0.15 9 189.354 6.00 1.76802 49.2 10* -13.731 (variable) 11 ∞ 1.00 1.49171 57.4 12∞23.00 13 (eye point) Aspheric Data 6th side K =-6.90825e-001 A4= 8.38523e-005 A6=-6.11664e-007 A8= 7.67392e-009 A10=-4.11453e-011 7th side K =-1.90508e+000 A4= 3.96382e-005 A6=-1.65188e-006 A8=-8.19135e-009 A10= 9.04048e-011 8th side K =-7.66349e+000 A4= 2.88263e-004 A6=-3.08349e-006 A8= 1.31547e-008 A10=-1.74846e-011 Surface 10 K =-1.83793e+000 A4=-1.51567e-005 A6=-4.27702e-007 A8= 4.02270e-009 A10=-1.15184e-011 Variable Interval 0m-1 -1 -5 +2 d4 2.72 2.37 1.07 3.55 d10 1.45 1.80 3.09 0.62 Various values ​​in each numerical example are summarized in the following Tables 1 to 3. In each table, the values ​​of conditional expressions (5) to (8) are abbreviated as "sfa1", "sfg1", "sfa2", and "sfa3", respectively.

[0052] [Table 1]

[0053] [Table 2]

[0054] [Table 3]

[0055] [Imaging device] Next, an image pickup apparatus including the observation optical system according to any one of the above-mentioned embodiments will be described.

[0056] FIG. 23 shows an image pickup device 200 such as a digital still camera or video camera in which the viewing optical system of each of the above-mentioned embodiments is provided as an electronic viewfinder 100.

[0057] The imaging device 200 captures a subject image formed by an imaging lens 201 with an imaging sensor 202 such as a CCD sensor or a CMOS sensor. An imaging signal output from the imaging sensor 202 is input to an arithmetic processing circuit (processing means) 203. The arithmetic processing circuit 203 performs various image processes on the imaging signal to generate captured image data. The captured image data is output to the electronic viewfinder 100 and displayed on a display panel in the electronic viewfinder 100. A user can view the image displayed on the display panel by looking into the observation optical system in the electronic viewfinder 100 with his / her eyeball 10.

[0058] At this time, the arithmetic processing circuit 203 calculates (acquires) the user's line of sight direction using data branched and detected by an optical path branching means included in the electronic viewfinder 100. Then, the arithmetic processing circuit 203 calculates the user's gaze position within the captured image screen from this line of sight direction. The arithmetic processing circuit 203 can select an area including the gaze position from within the captured image screen, and perform processes such as automatic exposure and autofocus using captured image data of the selected area.

[0059] By using such a small observation device with high line-of-sight detection accuracy, it is possible to realize an imaging device that is small yet capable of performing processes such as auto-exposure and auto-focus well.

[0060] Although the preferred embodiments and examples of the present invention have been described above, the present invention is not limited to these embodiments and examples, and various combinations, modifications, and changes are possible within the scope of the gist of the present invention. [Explanation of symbols]

[0061] L1 First lens L2 Second lens L3 Third lens LCD display panel

Claims

1. An observation optical system that is composed of an optical path branching means for branching an optical path, a first lens with positive refractive power, a second lens with negative refractive power, and a third lens with positive refractive power, which are arranged in this order from a display panel side to an observation side, or that is composed of an optical path branching means for branching an optical path, a first lens with positive refractive power, a second lens with negative refractive power, a third lens with positive refractive power, and a fourth lens with positive refractive power, which are arranged in this order from a display panel side to an observation side, an absolute value of a radius of curvature of a lens surface of the first lens on an observation side is smaller than an absolute value of a radius of curvature of a lens surface of the first lens on a display panel side; Visibility is -1m －１ When the distance from the display surface of the display panel to the lens surface of the first lens on the display panel side is d1, the focal length of the observation optical system is f, the distance on the optical axis from the lens surface of the second lens on the observation side to the lens surface of the third lens on the display panel side is d23, the distance on the optical axis from the lens surface of the first lens on the observation side to the lens surface of the second lens on the display panel side is d12, the refractive index of the optical path branching means at the d line is Ndg0, the refractive index of the second lens at the d line is Ndg2, and the refractive index of the first lens at the d line is Ndg1, 0.5<d1 / f<2.0 0.0<d23 / d12<0.5 1.55<Ndg0<2.00 1.6<Ndg2<1.8 1.5<Ndg1<2.0 1. An observation optical system characterized in that the following condition is satisfied:

2. When the distance on the optical axis from the lens surface of the second lens on the display panel side to the lens surface of the second lens on the observation side is gt2, 0.02<gt2 / f<0.15 2. The viewing optical system according to claim 1, wherein the following condition is satisfied:

3. When the Abbe number of the optical path branching means with respect to the d line is νdg0, 37<νdg0<75 3. The viewing optical system according to claim 1, wherein the following condition is satisfied:

4. When the focal length of the first lens is f1, 0.55<f1 / f<2.00 4. The viewing optical system according to claim 1, wherein the following condition is satisfied:

5. When the focal length of the first lens is f1 and the focal length of the third lens is f3, 0.8<f3 / f1<2.0 5. The viewing optical system according to claim 1, wherein the following condition is satisfied:

6. When the radius of curvature of the lens surface of the first lens on the observation side is G1R2 and the radius of curvature of the lens surface of the second lens on the display panel side is G2R1, 1.3<(G1R2+G2R1) / (G1R2-G2R1)<15.0 6. The viewing optical system according to claim 1, wherein the following condition is satisfied:

7. When the radius of curvature of the lens surface of the first lens on the display panel side is G1R1 and the radius of curvature of the lens surface of the first lens on the observation side is G1R2, 0.05<(G1R1+G1R2) / (G1R1-G1R2)<2.00 7. The viewing optical system according to claim 1, wherein the following condition is satisfied:

8. When the radius of curvature of the lens surface of the second lens on the display panel side is G2R1 and the radius of curvature of the lens surface of the second lens on the observation side is G2R2, -10.0<(G2R1+G2R2) / (G2R1-G2R2)<-0.3 8. The viewing optical system according to claim 1, wherein the following condition is satisfied:

9. When the radius of curvature of the lens surface of the third lens on the display panel side is G3R1 and the radius of curvature of the lens surface of the third lens on the observation side is G3R2, 0.1<(G3R1+G3R2) / (G3R1-G3R2)<3.0 9. The viewing optical system according to claim 1, wherein the following condition is satisfied:

10. When the refractive index of the third lens at the d line is Ndg3, 1.5<Ndg3<2.0 10. The viewing optical system according to claim 1, wherein the following condition is satisfied:

11. When the Abbe number of the first lens with respect to the d line is νdg1, 30<νdg1<70 11. The viewing optical system according to claim 1, wherein the following condition is satisfied:

12. When the Abbe number of the second lens with respect to the d line is νdg2, 18<νdg2<30 12. The viewing optical system according to claim 1, wherein the following condition is satisfied:

13. An observation optical system comprising: an optical path branching means for branching an optical path, a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive refractive power, which are arranged in this order from a display panel side to an observation side; or an observation optical system comprising: an optical path branching means for branching an optical path, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a positive refractive power, which are arranged in this order from a display panel side to an observation side; an absolute value of a radius of curvature of a lens surface of the first lens on an observation side is smaller than an absolute value of a radius of curvature of a lens surface of the first lens on a display panel side; Visibility is -1m －１ is a distance from the display surface of the display panel to the lens surface of the first lens on the display panel side, f is a focal length of the observation optical system, d23 is a distance on the optical axis from the lens surface of the second lens on the observation side to the lens surface of the third lens on the display panel side, d12 is a distance on the optical axis from the lens surface of the first lens on the observation side to the lens surface of the second lens on the display panel side, G1R2 is a radius of curvature of the lens surface of the first lens on the observation side, G2R1 is a radius of curvature of the lens surface of the second lens on the display panel side, G2R2 is a radius of curvature of the lens surface of the second lens on the observation side, and νdg2 is an Abbe number for the d-line of the second lens, 0.5<d1 / f<2.0 0.0<d23 / d12<0.5 1.3<(G1R2+G2R1) / (G1R2-G2R1)<15.0 -10.0<(G2R1+G2R2) / (G2R1-G2R2)<-0.3 18<νdg2<30 1. An observation optical system characterized in that the following condition is satisfied:

14. When the Abbe number of the third lens with respect to the d line is νdg3, 30<νdg3<70 14. The viewing optical system according to claim 1, wherein the following condition is satisfied:

15. The observation optical system described in any one of claims 1 to 14, characterized in that the observation optical system is composed of an optical path branching means for branching an optical path, a first lens with positive refractive power, a second lens with negative refractive power, a third lens with positive refractive power, and a fourth lens with positive refractive power, arranged in order from the display panel side to the observation side.

16. An observation optical system comprising: an optical path branching means for branching an optical path, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a positive refractive power, which are arranged in this order from a display panel side to an observation side, an absolute value of a radius of curvature of a lens surface of the first lens on an observation side is smaller than an absolute value of a radius of curvature of a lens surface of the first lens on a display panel side; When the diopter is -1 m -1 , the distance from the display surface of the display panel to the lens surface of the first lens on the display panel side is d1, the focal length of the observation optical system is f, the distance on the optical axis from the lens surface of the second lens on the observation side to the lens surface of the third lens on the display panel side is d23, the distance on the optical axis from the lens surface of the first lens on the observation side to the lens surface of the second lens on the display panel side is d12, the refractive index of the optical path branching means at the d line is Ndg0, and the refractive index of the second lens at the d line is Ndg2, 0.5<d1 / f<2.0 0.0<d23 / d12<0.5 1.55<Ndg0<2.00 1.6<Ndg2<1.8 1. An observation optical system characterized in that the following condition is satisfied:

17. When the refractive index of the optical path branching means at the d line is Ndg0, 1.60311≦Ndg0<2.00 17. The viewing optical system according to claim 1, wherein the following condition is satisfied:

18. An observation optical system comprising an optical path branching means for branching an optical path, a first lens with positive refractive power, a second lens with negative refractive power, and a third lens with positive refractive power, arranged in that order from the display panel side to the observation side, or comprising an optical path branching means for branching an optical path, a first lens with positive refractive power, a second lens with negative refractive power, a third lens with positive refractive power, and a fourth lens with positive refractive power, arranged in that order from the display panel side to the observation side, an absolute value of a radius of curvature of a lens surface of the first lens on an observation side is smaller than an absolute value of a radius of curvature of a lens surface of the first lens on a display panel side; When the diopter is -1 m -1 , the distance from the display surface of the display panel to the lens surface of the first lens on the display panel side is d1, the focal length of the observation optical system is f, the distance on the optical axis from the lens surface of the second lens on the observation side to the lens surface of the third lens on the display panel side is d23, the distance on the optical axis from the lens surface of the first lens on the observation side to the lens surface of the second lens on the display panel side is d12, the refractive index of the optical path branching means at the d line is Ndg0, the refractive index of the second lens at the d line is Ndg2, and the refractive index of the optical path branching means at the d line is Ndg0, 0.5<d1 / f<2.0 0.0<d23 / d12<0.5 1.60311≦Ndg0<2.00 1.6<Ndg2<1.8 1. An observation optical system characterized in that the following condition is satisfied:

19. 19. The observation optical system according to claim 1, further comprising a display panel for displaying an image.

20. 20. An imaging apparatus comprising: an observation optical system according to claim 1; and an imaging element.

Images