Observation optical system and optical device

Abstract

Provided is an observation optical system (LS1) that has, in the stated order from the object side, an objective optical system (OBL1), an erecting prism (PR1) for erecting an image formed by the objective optical system (OBL1), and an eyepiece optical system (EPL1) for observing the image formed by the objective optical system (OBL1) erected by the erecting prism (PR1), and that performs image correction by rotating the erecting prism (PR1) from a prescribed position, the center of rotation about which the erecting prism (PR1) is rotated being positioned in a distance range that is no more than one fourth the length of the erecting prism (PR1) in a direction along the optical axis (Z12) of the objective optical system (OBL1) from the position of the intersection of the plane of the erecting prism (PR1) closest to the objective optical system and the optical axis (Z11) of the objective optical system (OBL1) when the erecting prism (PR1) is in the aforementioned prescribed position.

Description

Observation optical systems and optical instruments

[0001] The present invention relates to an observation optical system and an optical instrument.

[0002] Conventionally, various optical systems have been proposed that include means for correcting image blur caused by vibrations such as camera shake by rotating a prism within the optical system (see, for example, Japanese Patent Application Laid-Open No. 2003-222294).

[0003] However, conventional optical systems have a problem in that the rotation angle of the prism required for the image blur correction angle is large, and decentering aberrations caused by the rotation of the prism deteriorate the optical performance.

[0004] Japanese Patent Application Laid-Open No. 2021-131429

[0005] A first observation optical system according to the present invention has, in order from the object side, an objective optical system, an erecting prism for erecting an image formed by the objective optical system, and an eyepiece optical system for observing the image formed by the objective optical system erected by the erecting prism, and corrects the image by rotating the erecting prism from a predetermined position, wherein the center of rotation about which the erecting prism is rotated is located within a distance range of not more than one-fourth of the length of the erecting prism in a direction along the optical axis of the objective optical system from the position of the intersection between the surface of the erecting prism closest to the objective optical system and the optical axis of the objective optical system when the erecting prism is located at the predetermined position.

[0006] A second observation optical system according to the present invention has, in order from the object side, an objective optical system, an erecting prism for erecting an image formed by the objective optical system, and an eyepiece optical system for observing the image formed by the objective optical system erected by the erecting prism, and is an observation optical system that corrects the image by rotating the erecting prism from a predetermined position, and the center of rotation about which the erecting prism is rotated is located between the surface of the erecting prism that is closest to the objective optical system and the surface of the objective lens that is closest to the erecting prism when the erecting prism is located at the predetermined position.

[0007] An optical instrument according to the present invention is configured to include the above-described observation optical system.

[0008] 1 is a side view showing the lens configuration of the observation optical system according to Example 1. FIG. 2 is a cross-sectional view showing the lens configuration of the observation optical system according to Example 1. FIG. 3 is an optical path diagram of the observation optical system according to Example 1. FIG. 4 is a perspective view showing the configuration of an erecting prism of the observation optical system according to Example 1. FIG. 5 is a diagram showing the position of the prism rotation center of the observation optical system according to Example 1. FIG. 6 is a diagram showing various aberrations before prism rotation of the observation optical system according to Example 1. FIG. 7 is a diagram showing lateral aberrations before prism rotation of the observation optical system according to Example 1. FIG. 8 is a diagram showing lateral aberrations after prism rotation of the observation optical system according to Example 1. FIG. 9 is a diagram showing a spot diagram before prism rotation of the observation optical system according to Example 1. FIG. 10 is a diagram showing a spot diagram after prism rotation of the observation optical system according to Example 1. FIG. 11 is a diagram showing lateral aberrations after prism rotation of the observation optical system according to Example 2. FIG. 12 is a diagram showing a spot diagram after prism rotation of the observation optical system according to Example 2. FIG. 13 is a diagram showing lateral aberrations after prism rotation of the observation optical system according to Example 3. FIG. 14 is a diagram showing a spot diagram after prism rotation of the observation optical system according to Example 3. FIG. 15 is a diagram showing lateral aberrations after prism rotation of the observation optical system according to Example 4. 10 is a side view showing the lens configuration of the observation optical system according to Example 5. FIG. 11 is a cross-sectional view showing the lens configuration of the observation optical system according to Example 5. FIG. 12 is a diagram showing the position of the prism rotation center of the observation optical system according to Example 5. FIG. 13 is a diagram showing various aberrations before prism rotation of the observation optical system according to Example 5. FIG. 14 is a diagram showing lateral aberrations before prism rotation of the observation optical system according to Example 5. FIG. 15 is a diagram showing lateral aberrations after prism rotation of the observation optical system according to Example 5. FIG. 16 is a diagram showing a spot diagram before prism rotation of the observation optical system according to Example 5. FIG. 17 is a diagram showing a spot diagram after prism rotation of the observation optical system according to Example 5. FIG. 18 is a diagram showing lateral aberrations after prism rotation of the observation optical system according to Example 6. FIG. 19 is a diagram showing a spot diagram after prism rotation of the observation optical system according to Example 6. FIG. 19 is a diagram showing lateral aberrations after prism rotation of the observation optical system according to Example 7. FIG. 19 is a diagram showing a spot diagram after prism rotation of the observation optical system according to Example 7. FIG. 19 is a diagram showing lateral aberrations after prism rotation of the observation optical system according to Example 8. FIG. 19 is a diagram showing a spot diagram after prism rotation of the observation optical system according to Example 8.10 is a diagram showing lateral aberration of the observation optical system according to the first reference example after prism rotation. FIG. 11 is a diagram showing a spot diagram of the observation optical system according to the first reference example after prism rotation. FIG. 12 is a diagram showing lateral aberration of the observation optical system according to the second reference example after prism rotation. FIG. 13 is a diagram showing a spot diagram of the observation optical system according to the second reference example after prism rotation. FIG. 14 is a diagram showing lateral aberration of the observation optical system according to the third reference example after prism rotation. FIG. 15 is a diagram showing a spot diagram of the observation optical system according to the third reference example after prism rotation. FIG. 16 is a diagram showing a spot diagram of the observation optical system according to the fourth reference example after prism rotation. FIG. 17 is a diagram showing a spot diagram of the observation optical system according to the fourth reference example after prism rotation. FIG. 18 is a diagram showing a spot diagram of the observation optical system according to the fifth reference example after prism rotation. FIG. 19 is a diagram showing a spot diagram of the observation optical system according to the fifth reference example after prism rotation. FIG. 19 is a diagram showing a spot diagram of the observation optical system according to the sixth ... table showing specification data of the observation optical systems according to the first to fourth examples and the first to third reference examples. 1 is a graph showing the relationship between the image blur correction angle and the rotation angle of the prism at each rotation center position P0 to P6. FIG. 2 is a graph showing the relationship between the image blur correction angle and the angular deviation at each rotation center position P0 to P6. FIG. 3 is a table showing specification data of the observation optical systems according to the fifth to eighth examples and the fourth to sixth reference examples. FIG. 4 is a graph showing the relationship between the image blur correction angle and the rotation angle of the prism at each rotation center position Q0 to Q6. FIG. 5 is a graph showing the relationship between the image blur correction angle and the angular deviation at each rotation center position Q0 to Q6. FIG. 6 is a diagram illustrating the rotation angle of the prism and the image blur correction angle in the observation optical system. FIG. 7 is a diagram explaining the coordinates of each point on the erecting prism before prism rotation in the observation optical system. FIG. 8 is a diagram explaining the coordinates of each point on the erecting prism after prism rotation in the observation optical system. FIG. 9 is a diagram showing an unfolded view of the erecting prism in the observation optical system. FIG. 10 is a diagram showing an example of the arrangement of the objective optical system and the erecting prism after prism rotation in the observation optical system. FIG. 11 is a diagram showing the back focus of the objective optical system in the observation optical system. FIG. 12 is a cross-sectional view of binoculars.

[0009] Preferred embodiments of the present invention will now be described. First, binoculars will be described with reference to FIG. 55 as an example of an optical instrument equipped with an observation optical system according to one embodiment of the present invention. As shown in FIG. 55, these binoculars BFG are primarily composed of two observation optical systems LS1, LS1 for observing an object and a mirror body BD that holds the two observation optical systems LS1, LS1 arranged in parallel on the left and right. Each of the two observation optical systems LS1, LS1 includes an objective optical system OBL1 that collects incident light beams to form an image, an erecting prism PR1 that erects the image formed by the objective optical system OBL1, and an eyepiece optical system EPL1 for observing the image formed by the objective optical system OBL1. In these binoculars BFG, light from an object (not shown) is collected by the objective optical system OBL1 and reaches the erecting optical system PR1. The light that reaches the erecting optical system PR1 is reflected multiple times by the erecting optical system PR1 and directed to the eyepiece optical system EPL1. This allows the viewer to view the image of the object as an erect image through the eyepiece optical system EPL1.

[0010] The erecting prism PR1 in the two observation optical systems LS1, LS1 rotates about a predetermined rotation center position (point P0 in the figure) when camera shake or the like occurs, thereby correcting the image formed by the objective optical system OBL1 (image blur correction). For example, the erecting prism PR1 is rotationally driven by a rotating device (not shown) that is composed of a stepping motor, a rotary actuator, a voice coil motor, or the like. This makes it possible to obtain binoculars BFG that have an image stabilization function that can obtain a sufficient image blur correction angle for camera shake or the like, while also having little decentering aberration that occurs when the prism is rotated for image blur correction.

[0011] Next, an observation optical system according to a first embodiment will be described. When directions are referred to below, the directions of arrows X, Y, and Z shown in FIG. 1 and elsewhere will be referred to as the X-axis direction, the Y-axis direction, and the Z-axis direction, respectively. The observation optical system LS1 shown in FIG. 1 will be used as the observation optical system according to the first embodiment. This observation optical system LS1 has, in order from the object side, an objective optical system OBL1, an erecting prism PR1 for erecting an image formed by the objective optical system OBL1, and an eyepiece optical system EPL1 for observing the image formed by the objective optical system OBL1. The observation optical system corrects the image by rotating the erecting prism PR1 from a predetermined position. The center of rotation for rotating the erecting prism PR1 is located within a distance range of less than one-fourth of the length of the erecting prism PR1 in the direction along the optical axis Z11 of the objective optical system OBL1 from the position of the intersection between the surface of the erecting prism PR1 closest to the objective optical system OBL1 (referred to as the ``front prism surface PR11a'') and the optical axis Z11 of the objective optical system OBL1 when the erecting prism PR1 is located at a predetermined position.

[0012] With this observation optical system LS1 (the observation optical system according to the first embodiment), it is possible to obtain a sufficient image blur correction angle for camera shake and the like, and to reduce decentering aberrations during image blur correction. The "predetermined position of the erecting prism PR1" refers to the position of the erecting prism PR1 when the front prism surface PR11a of the erecting prism PR1 is perpendicular to the optical axis Z11 of the objective optical system OBL1, and is hereinafter also referred to as the "prism reference position." The rotation angle of the erecting prism PR1 when in the prism reference position is set to 0°.

[0013] It is preferable that the center of rotation of the erecting prism PR1 be located on a straight line passing through the intersection of the surface of the erecting prism PR1 closest to the objective optical system OBL1 (front prism surface PR11a) and the optical axis Z11 of the objective optical system OBL1, and the intersection of the surface of the erecting prism PR1 closest to the ocular optical system EPL1 (referred to as the ``rear prism surface PR12a'') and the optical axis Z12 of the ocular optical system EPL1 when the erecting prism PR1 is located at a predetermined position (prism reference position).

[0014] Furthermore, it is preferable that the rotation center of the erecting prism PR1 is located within the erecting prism PR1. That is, when an imaginary plane perpendicular to the optical axis Z11 is set at a position away from the intersection of the front prism surface PR11a and the optical axis Z11 of the objective optical system OBL1 by a quarter of the length in the direction along the optical axis Z11 of the erecting prism PR1, it is preferable that the rotation center of the erecting prism PR1 be located within the erecting prism PR1 sandwiched between the imaginary plane and the front prism surface PR11a.

[0015] It is also preferable that the observation optical system LS1 satisfy the following conditional expression (A1).

[0016] 0.25<γ<0.50 (A1) where γ is the angular magnification of the objective optical system.

[0017] Conditional formula (A1) is a conditional formula related to the angular magnification of the observation optical system LS1. If the lower limit of conditional formula (A1) is not met, a sufficient image blur correction angle cannot be obtained relative to the rotation angle of the erecting prism PR1. If the upper limit of conditional formula (A1) is met, the image blur correction angle relative to the rotation angle of the erecting prism PR1 becomes too large, causing large fluctuations in the image accompanying the rotation of the erecting prism PR1, making it impossible to observe a good image. Therefore, if the observation optical system LS1 satisfies conditional formula (A1), a sufficient image blur correction angle can be obtained relative to the rotation angle of the erecting prism PR1, and a good image can be observed during image blur correction.

[0018] Next, an observation optical system according to a second embodiment will be described. As an example of the observation optical system according to the second embodiment, reference will be made to the observation optical system LS1 shown in FIG. 1 . This observation optical system LS1 includes, in order from the object side, an objective optical system OBL1, an erecting prism PR1 for erecting an image formed by the objective optical system OBL1, and an eyepiece optical system EPL1 for observing the image formed by the objective optical system OBL1. The observation optical system corrects the image by rotating the erecting prism PR1 from a predetermined position. The center of rotation for rotating the erecting prism PR1 is located between the surface of the erecting prism PR1 closest to the objective optical system OBL1 (front prism surface PR11a) and the surface of the objective optical system OBL1 closest to the objective optical system OBL1 when the erecting prism PR1 is located at a predetermined position (prism reference position).

[0019] With this observation optical system LS1 (observation optical system according to the second embodiment), it is possible to obtain a sufficient image blur correction angle for camera shake and the like, and to reduce decentering aberrations during image blur correction.

[0020] It is preferable that the center of rotation of the erecting prism PR1 be located on a straight line passing through the intersection between the surface of the erecting prism PR1 closest to the objective optical system OBL1 (front prism surface PR11a) and the optical axis Z11 of the objective optical system OBL1, and the intersection between the surface of the erecting prism PR1 closest to the eyepiece optical system EPL1 (rear prism surface PR12a) and the optical axis Z12 of the eyepiece optical system EPL1, when the erecting prism PR1 is located at a predetermined position (prism reference position), or on the optical axis Z11 of the objective optical system OBL1.

[0021] It is also preferable that the observation optical system LS1 satisfies the above conditional expression (A1).

[0022] In the second embodiment as well, by satisfying conditional expression (A1), a sufficient image blur correction angle can be obtained with respect to the rotation angle of the erecting prism PR1, and a good image can be observed during image blur correction.

[0023] The observation optical system LS1 is an anti-vibration optical system with an anti-vibration function, and is used as an observation optical system in, for example, a telescope, binoculars, laser range finder, etc. In the case of a telescope, a telescope optical system is formed by providing a single observation optical system LS1, while in the case of binoculars, a binocular optical system is formed by providing a pair of observation optical systems LS1 on the left and right (in the X-axis direction).

[0024] The observation optical system LS1 has, in order from the object side, an objective optical system OBL1, an erecting prism PR1, and an eyepiece optical system EPL1. Subject light incident on the observation optical system LS1 passes through the objective optical system OBL1 and the erecting prism PR1 and forms an intermediate subject image (erect image) on an image forming plane IM. The subject image formed on the image forming plane IM is enlarged by the eyepiece optical system EPL1 and is observed by an observer whose eye is placed at the eyepoint EP.

[0025] As an example, the objective optical system OBL1 is composed of, in order from the object side, a first objective lens OL11 consisting of a cemented lens and having positive refractive power, a second objective lens OL12 consisting of a single lens and having positive refractive power, and a third objective lens OL13 consisting of a single lens and having negative refractive power.

[0026] The erecting prism PR1 is composed of an auxiliary prism PR11 and a roof prism PR12 disposed with a predetermined gap (air gap) from the auxiliary prism PR11. The auxiliary prism PR11 and the roof prism PR12 are disposed so that the front prism surface PR11a of the auxiliary prism PR11 and the rear prism surface PR12a of the roof prism PR12 are parallel to each other. As shown in FIG. 4 , light emitted from the objective optical system OBL enters the auxiliary prism PR11 from the front prism surface PR11a, is reflected multiple times within the auxiliary prism PR11, and then enters the roof prism PR12 via the gap between the roof prism PR12 and the auxiliary prism PR11. The light incident on the roof prism PR12 is reflected multiple times within the roof prism PR12 before exiting from the rear prism surface PR12a and proceeding toward the eyepiece optical system EPL1. For ease of explanation, the erecting prism PR1 shown in FIG. 4 is different from the actual shape, and the gap between the auxiliary prism PR11 and the roof prism PR12 is also shown larger than it actually is.

[0027] The rotation center of the erecting prism PR1 is set, for example, as follows. Fig. 5 shows the erecting prism PR1 located at a reference position and seven rotation centers P0 to P6. In Fig. 5, the rotation center P1 is located at the intersection of the front prism surface PR11a of the erecting prism PR1 and the optical axis Z11 of the objective optical system OBL1, and the rotation center P5 is located at the intersection of the rear prism surface PR12a of the erecting prism PR1 and the optical axis Z12 of the eyepiece optical system EPL1. In addition, the seven rotation centers P0 to P6 are each located on an imaginary line (referred to as a "prism-penetrating imaginary line") that passes through the position of the rotation center P1 (the intersection of the front prism surface PR11a and the optical axis Z11) and the position of the rotation center P5 (the intersection of the rear prism surface PR12a and the optical axis Z12).

[0028] The rotation center P0 is located outside the erecting prism PR1, between the front prism surface PR11a and the surface of the objective optical system OBL1 closest to the erecting prism PR1. The rotation center P0 is located at a distance (L1 / 4) that is one-fourth the length L1 of the erecting prism PR1 in the direction along the optical axis Z11 of the objective optical system OBL1 from the position of the rotation center P1 (front prism surface PR11a). Specifically, when an imaginary plane perpendicular to the optical axis Z11 is set between the erecting prism PR1 and the objective optical system OBL1 at a position L1 / 4 away from the position of the rotation center P1, the rotation center P0 is located at the intersection of the imaginary plane and a virtual line that passes through the prism. The other four rotation centers P2, P3, P4, and P6 are located inside the erecting prism PR1.

[0029] The rotation center P2 is located at a distance (L1 / 4) that is one-quarter of the length L1 of the objective optical system OBL1 of the erecting prism PR1 in the direction along the optical axis Z11 from the position of the rotation center P1 (front prism surface PR11a). Specifically, when an imaginary plane perpendicular to the optical axis Z11 is set at a position L1 / 4 away from the position of the rotation center P1 inside the erecting prism PR1, the rotation center P2 is located at the intersection of the imaginary plane and an imaginary line that passes through the prism. Similarly, the rotation center P3 is located at a distance of 2×L1 / 4 (=L1 / 2) away from the position of the rotation center P1, and the rotation center P4 is located at a distance of 3×L1 / 4 away from the position of the rotation center P1. Furthermore, the rotation center P6 is located at a distance L1 / 8 away from the position of the rotation center P1.

[0030] Thus, four of the seven rotation centers P0 to P6, P0, P1, P2, and P6, are located within a distance range of one-fourth or less of the length of the erecting prism PR1 in the direction along the optical axis Z11 of the objective optical system OBL1 from the position of the intersection between the front prism surface PR11a of the erecting prism PR1 and the optical axis Z11 of the objective optical system OBL1. The rotation center of the erecting prism PR1 does not have to be set on the imaginary straight line that passes through the prism. Furthermore, when the rotation center is set between the erecting prism PR1 and the objective optical system OBL1, it may be set on the optical axis Z11 of the objective optical system OBL1. Furthermore, the length L1 may be the length between the positions of the rotation centers P1 and P5 on the imaginary straight line that passes through the prism.

[0031] It is desirable that the erecting prism PR1 rotate in all directions around the rotation center position. When the observation optical system LS1 is used in a handheld telescope, binoculars, or the like, runout in the pitch direction (the direction of rotation around an axis parallel to the X axis) is important, so the erecting prism PR1 may be configured to be rotatable only around an axis parallel to the X axis. Furthermore, when configured to be rotatable in the yaw direction (the direction of rotation around an axis parallel to the Y axis) in addition to the pitch direction, the rotation axis in the yaw direction and the rotation axis in the pitch direction do not necessarily need to intersect at a single point.

[0032] As an example, the eyepiece optical system EPL1 is composed of, in order from the object side, a first eyepiece lens EL11 made of a cemented lens and having negative refractive power, a second eyepiece lens EL12 made of a cemented lens and having positive refractive power, a third eyepiece lens EL13 made of a cemented lens and having positive refractive power, and a fourth eyepiece lens EL14 made of a single lens and having positive refractive power.

[0033] 49 to 51 show an observation optical system LS having the same lens configuration and prism configuration as the observation optical system LS1. As shown in Fig. 49, when the erecting prism PR1 in the observation optical system LS is rotated from the prism reference position around a rotation center position (for example, rotation center P1) (here, rotation about an axis parallel to the X axis passing through the rotation center position, denoted by the rotation angle θ), an image blur correction angle u is obtained. Note that the rotation angle θ and the image blur correction angle u of the erecting prism PR1 are positive values ​​when rotated clockwise in Fig. 49 etc., and negative values ​​when rotated counterclockwise.

[0034] The image blur correction angle u can be calculated as follows. The calculation procedure will be explained below with reference to Figs. 50 to 54. As shown in Figs. 50 and 51, the image blur correction angle relative to the rotation angle of the erecting prism PR1 is calculated by tracing back the light rays from the eyepiece optical system EPL1 side. Fig. 50 shows the coordinates of points A to D before the rotation of the erecting prism PR1, and Fig. 51 shows the coordinates of points E, G, and I after the rotation of the erecting prism PR1. Note that here, the rotation center point of the erecting prism PR1 is set to the position of rotation center O, and its coordinates are set to (Zo, Yo) = (0, 0).

[0035] Point A shown in Fig. 50 is the incident point (referred to as the "pre-prism rotation object-side incident point") of a ray (referred to as the "pre-prism rotation back-tracing incident ray") that enters the rear prism surface PR12a of the erecting prism PR1 located at the prism reference position along its optical axis Z12 from the eyepiece optical system EPL1 side, and its coordinates are (Za, Ya). Point B is a point corresponding to the position of a corner of the roof prism PR12 before the prism rotation, and its coordinates are (Zb, Yb). Point C is the exit point (referred to as the "pre-prism rotation object-side exit point") of a ray (referred to as the "pre-prism rotation back-tracing exit ray") that emerges from the front prism surface PR1 of the erecting prism PR1 when the pre-prism rotation back-tracing incident ray that entered the pre-prism rotation eye-side incident point (point A) is further back-traced, and its coordinates are (Zc, Yc). Point D corresponds to the position of the corner of the auxiliary prism PR11 before the prism is rotated, and its coordinates are (Zd, Yd).

[0036] Point E shown in Fig. 51 is the incident point (referred to as the "eye-side incident point after prism rotation") of a ray that enters the rear prism surface PR12a of the erecting prism PR1 after rotation by angle θ along its optical axis Z12 from the eyepiece optical system EPL1 side (referred to as the "back-tracing incident ray after prism rotation"), and its coordinates are (Ze, Ye). Point G is the exit point (referred to as the "object-side exit point after prism rotation") of a ray that emerges from the front prism surface PR1 of the erecting prism PR1 when the back-tracing incident ray after prism rotation that entered the eye-side incident point after prism rotation (point E) is further back-traced, and its coordinates are (Zg, Yg). Point I corresponds to the position of the intersection between the front prism surface PR11a of the auxiliary prism PR11 after prism rotation and the optical axis Z11 of the objective optical system OBL1, and its coordinates are (Zi, Yi).

[0037] In the following explanation, points and coordinates that have moved after the erecting prism PR1 has rotated are marked with a dash ('). When the erecting prism PR1 is rotated by θ [°], points A, B, C, and D move to points A', B', C', and D', respectively. The coordinates of the respective moved points (Za', Ya'), (Zb', Yb'), (Zc', Yc'), and (Zd', Yd') can be calculated using the following equations (1) to (4), respectively.

[0038]

[0039] A straight line passing through points A' and B' is determined. The straight line is expressed by the following equation (5).

[0040]

[0041] Furthermore, the distance dh3 between points A and B after the erecting prism PR1 has been rotated (see FIG. 52) can be calculated by the following equation (6).

[0042]

[0043] Furthermore, the distance dh3' between the point E and the point B' after the erecting prism PR1 has been rotated (see FIG. 52) can be calculated by the following equation (7).

[0044]

[0045] Therefore, the positional deviation amount Δh (see Figure 52) on the rear prism surface PR12a between the entrance point on the anterior eye side and the entrance point on the posterior eye side before and after rotation of the erecting prism PR1 can be calculated using the following equation (8):

[0046]

[0047] Figure 52 shows a development of the erecting prism PR1, which is considered to be a plane-parallel plate. The imaginary line LN1 shown in solid line in Figure 52 indicates the optical path of a ray that enters the rear prism surface PR12a at the eye-side incident point (point E) after prism rotation as a back-tracing incident ray and emerges from the front prism surface PR11a at the object-side exit point (point G) after prism rotation as a back-tracing exit ray. The ray represented by the imaginary line LN1 is refracted at the rear prism surface PR12a, the boundary between the roof prism PR12 and the auxiliary prism PR11, and the front prism surface PR11a.

[0048] When a light ray is incident on a parallel plate, the angle of the incident ray and the angle of the exiting ray do not change, but the positions of the incident ray and the exiting ray shift. Let ΔP1 be the amount of shift of the light ray at the roof prism PR12, and ΔP2 be the amount of shift of the light ray at the auxiliary prism PR11. Let d1 and d2 be the path lengths of the roof prism PR12 and the auxiliary prism PR11, respectively. Let n1 and n2 be the refractive indices of the roof prism PR12 and the auxiliary prism PR11, respectively. Let θ1' and θ2' be the refraction angles at the roof prism PR12 and the auxiliary prism PR11, respectively. Since the relationship in the following equation (9) holds, the shift amount ΔP1 can be calculated using the following equation (10). Similarly, since the relationship in the following equation (11) holds, the shift amount ΔP2 can be calculated using the following equation (12).

[0049]

[0050] When the total shift amount of the incident light beam and the emerging light beam at the roof prism PR12 and the auxiliary prism PR11 is ΔPs (see FIG. 52), this total ΔPs can be calculated by the following equation (13).

[0051]

[0052] The glass path length dt of the entire erecting prism PR1, including the gap (air gap) between the roof prism PR12 and the auxiliary prism PR12, is calculated by the following formula (14): Furthermore, the deviation amount ΔS (see FIG. 52) when tilted by θ [°] with this glass path length dt is calculated by the following formula (15):

[0053]

[0054] Therefore, the amount of deviation Δ (see FIG. 52) of the exit position of the light ray on the front prism surface PR1 of the erecting prism PR1 before and after the rotation of the erecting prism PR1 can be calculated by the following formula (16).

[0055]

[0056] If the distance between points C and D is dh1 (see FIG. 52), this distance dh1 can be calculated by the following equation (17).

[0057]

[0058] If the distance between point D' and point G is Δj (see FIG. 52), this distance Δj can be calculated by the following equation (18).

[0059]

[0060] The coordinate values ​​Zg and Yg of point G are calculated by the following equations (19) and (20), respectively.

[0061]

[0062] The deviation amounts ΔZ in the Z axis direction and ΔY in the Y axis direction between the object side exit point (point C) of the erecting prism PR1 before prism rotation and the object side exit point (point G) after prism rotation are calculated by the following equations (21) and (22), respectively.

[0063]

[0064] When the erecting prism PR1 is rotated by θ [°], a light ray emerges from point G on the front prism surface PR1 of the erecting prism PR1 at an angle of 2θ [°]. When this light ray is extended in a direction intersecting with the optical axis Z11 of the objective optical system OBL1, the point of intersection is the object point Ot (see FIG. 53). If the distance between point G and object point Ot in the direction of the optical axis Z11 is df, this distance df can be calculated by the following equation (23):

[0065]

[0066] Fig. 53 shows the distance dop between the final surface (the surface closest to the erecting prism PR1) of the objective optical system OBL1 and the front prism surface PR11a of the erecting prism PR1 before rotation. Fig. 54 also shows the back focus bf1 of the objective optical system OBL1. The distance Zot (see Fig. 54) between the focal position F' of the objective optical system OBL1 and the object point Ot can be calculated by the following equation (24):

[0067]

[0068] When the focal length of the objective optical system OBL1 is f, the lateral magnification β of the objective optical system OBL1 is calculated by the following formula (25), and the angular magnification γ is calculated by the following formula (26).

[0069]

[0070] The finally obtained image blur correction angle u is calculated by the following formula (27), since the angle changes by twice the tilt angle (θ) in the erecting prism PR1 with respect to the rotation angle θ of the erecting prism PR1, and the angle also changes in accordance with the angular magnification γ in the objective optical system OBL1.

[0071]

[0072] The larger the angular magnification γ, the larger the image blur correction angle obtained for the rotation angle θ of PR1.

[0073] (First Example) An example of an observation optical system according to an embodiment of the present invention will be described below with reference to the drawings. First, the first example will be described using Figs. 1 to 10 and Table 1. Figs. 1 and 2 show an observation optical system LS1 according to the first example, which is used in telescopes and binoculars.

[0074] The observation optical system LS1 according to the first example has, in order from the object side, an objective optical system OBL1, an erecting prism PR1 for erecting an image formed by the objective optical system OBL1, and an eyepiece optical system EPL1 for observing the image formed by the objective optical system OBL1, and corrects the image by rotating the erecting prism PR1 from a predetermined position (prism reference position). The center of rotation for rotating the erecting prism PR1 is located at a rotation center P0 (see FIG. 5 ), which is a position a quarter of the length of the erecting prism PR1 in the direction along the optical axis Z11 of the objective optical system OBL1 from the position of the intersection between the surface of the erecting prism PR1 closest to the objective optical system OBL1 (front prism surface PR11 a) and the optical axis Z11 of the objective optical system OBL1 when the erecting prism PR1 is located at the predetermined position.

[0075] The rotation center P0 is located between the surface of the erecting prism PR1 that is closest to the objective optical system OBL1 (front prism surface PR11a) and the surface of the objective optical system OBL1 that is closest to the erecting prism PR1 when the erecting prism PR1 is located at a predetermined position (prism reference position). Furthermore, the rotation center P0 is located on an imaginary line (a through-prism imaginary line) that passes through the intersection of the front prism surface PR11a of the erecting prism PR1 with the optical axis Z11 of the objective optical system OBL1 and the intersection of the rear prism surface PR12a with the optical axis Z12 of the eyepiece optical system EPL1.

[0076] 2, the objective optical system OBL1 includes, in order from the object side, a first objective lens OL11 made of a cemented lens and having positive refractive power, a second objective lens OL12 made of a single lens and having positive refractive power, and a third objective lens OL13 made of a single lens and having negative refractive power. The first objective lens OL11 is a cemented lens made of a negative meniscus lens with a convex surface facing the object side and a biconvex lens. The second objective lens OL12 is a positive meniscus lens with a convex surface facing the object side, and the third objective lens OL13 is a negative meniscus lens with a convex surface facing the object side.

[0077] As shown in Figure 3, the observation optical system LS1 is an afocal system that receives a parallel light beam incident on the objective optical system OBL1 from the object side and emits it as a parallel light beam toward the eyepoint EP via the erecting prism PR1 and the eyepiece optical system EPL1. The subject light incident on the observation optical system LS1 passes through the objective optical system OBL1 and the erecting prism PR1 and forms an erect subject image as an intermediate image on the image plane IM. The subject image formed on the image plane IM is enlarged by the eyepiece optical system EPL1 and is observed by an observer whose eye is positioned at the eyepoint EP.

[0078] The erecting prism PR1 is composed of an auxiliary prism PR11 and a roof prism PR12 arranged with a predetermined gap (air gap) from the auxiliary prism PR11. The auxiliary prism PR11 and the roof prism PR12 are arranged so that the front prism surface PR11a of the auxiliary prism PR11 and the rear prism surface PR12a of the roof prism PR12 are parallel to each other. As shown in Figure 4, light emitted from the objective optical system OBL1 enters the auxiliary prism PR11 from the front prism surface PR11a, is reflected multiple times within the auxiliary prism PR11, and then enters the roof prism PR12 via the gap between the roof prism PR12 and the auxiliary prism PR11. The light incident on the roof prism PR12 is reflected multiple times within the roof prism PR12 before exiting from the rear prism surface PR12a and heading toward the eyepiece optical system EPL1.

[0079] The eyepiece optical system EPL1, for example, comprises, in order from the object side, a first eyepiece lens EL11 consisting of a cemented lens with negative refractive power, a second eyepiece lens EL12 consisting of a cemented lens with positive refractive power, a third eyepiece lens EL13 consisting of a cemented lens with positive refractive power, and a fourth eyepiece lens EL14 consisting of a single lens with positive refractive power. The first eyepiece lens EL11 is a cemented lens of a biconcave lens and a positive meniscus lens with its convex surface facing the object side. The second eyepiece lens EL12 is a cemented lens of a biconcave lens and a biconvex lens. The third eyepiece lens EL13 is a cemented lens of a biconvex lens and a negative meniscus lens with its concave surface facing the object side. The fourth eyepiece lens EL14 is a positive meniscus lens with its convex surface facing the object side.

[0080] Tables 1 to 14 are shown below, listing the specifications of the observation optical systems of Examples 1 to 8 and Reference Examples 1 to 6, respectively. In the "Specification Data" section of each table, the prism rotation angle is the rotation angle of the erecting prism required to obtain an image blur correction angle of 1.5°. The smaller this value, the smaller the prism rotation angle required to obtain a larger image blur correction angle. The angular magnification γ is the angular magnification of the objective optical system, and the larger this value, the larger the image blur correction angle obtained. Furthermore, the angular deviation |ΔC-F| is the angular deviation (unit: minutes) between the C-line (wavelength λ=656.3 nm) and the F-line (wavelength λ=486.1 nm) at the eyepoint of the chief ray from an object point at infinity at the center of the field of view when the erecting prism is rotated around the rotation center to obtain an image blur correction angle of 1.5°. As this value increases, colors appear separated, resulting in poor image visibility.

[0081] In [Lens Data], the surface number indicates the number of each lens surface (including prism surfaces and virtual surfaces) counted from the object side, R indicates the radius of curvature of each lens surface, D indicates the spacing between each lens surface (distance on the optical axis), nd indicates the refractive index for the d-line (wavelength λ=587.6 nm), and vd indicates the Abbe number for the d-line. Note that a radius of curvature of "∞" indicates a flat surface, and the refractive index of air, nd=1.0000, is omitted.

[0082] In addition, the radius of curvature R, surface spacing D, and other length units listed in all of the following specifications are generally in "mm," but this is not limited to this because the optical system can achieve the same optical performance even when proportionally enlarged or reduced. Furthermore, the same symbols as in this embodiment are used for the specifications of the second to eighth embodiments and first to sixth reference examples described below.

[0083] Table 1 below shows the specifications of the first embodiment. Note that the radii of curvature R of surfaces 1 to 24 in Table 1 correspond to the reference symbols R1 to R24 assigned to surfaces 1 to 24 in FIG. 2. Surface 15 is a virtual surface corresponding to image plane IM where an intermediate image (subject image) is formed, and surface 24 is a virtual surface corresponding to eyepoint EP. The surface spacing between surfaces 23 and 24 is the distance (eye relief) from the final lens surface (surface 23) to eyepoint EP.

[0084] (Table 1) [Specification data] Magnification: 10 Diameter (unit: mm): 42 Actual field of view 2ω (unit: °): 6.9 Center of rotation: P0 Prism rotation angle (unit: °): 1.82 Angular magnification γ: 0.41 Angular deviation |ΔC-F| (unit: '): 1.5 [Lens data] Surface number R D nd νd 1 110.543 2.0 1.80400 46.60 2 46.940 8.5 1.49782 82.57 3 353.823 0.5 4 44.380 6.8 1.51680 64.13 5 196.20 14.5 6 113.000 2.5 1.51680 64.13 7 49.008 36.5 8 ∞ 38.8 1.56883 56.00 9 ∞ 0.4 10 ∞ 54.5 1.51680 64.13 11 ∞ 4.2 12 -25.500 1.0 1.61266 44.46 13 17.162 3.7 1.80610 33.35 14 40.676 2.3 15 ∞ 5.8 16 -52.350 1.2 1.84666 23.80 17 108.616 9.8 1.69680 55.52 18 -19.800 0.2 19 44.270 11.4 1.72916 54.61 20 -25.588 1.5 1.84666 23.80 21 -203.924 0.2 22 21.500 6.0 1.69680 55.52 23 126.136 16.2 24 (Eyepoint)

[0085] FIG. 6 shows various aberrations (spherical aberration, astigmatism, and distortion) when the erecting prism is not rotated (when the image blur correction angle is 0.0°) in the observation optical system of Example 1. FIG. 7 shows lateral aberrations when the erecting prism is not rotated (when the image blur correction angle is 0.0°) in the observation optical system of Example 1. FIG. 8 shows lateral aberrations when the erecting prism is rotated about the rotation center P0 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.82°) in the observation optical system of Example 1. In each of the aberration diagrams in FIGS. 6 to 8 , d indicates aberrations for the d-line (wavelength λ=587.6 nm), C indicates aberrations for the C-line (wavelength λ=656.3 nm), F indicates aberrations for the F-line (wavelength λ=486.1 nm), and g indicates aberrations for the g-line (wavelength λ=435.8 nm). In the spherical aberration diagram, the vertical axis shows the value normalized with the maximum value of the entrance pupil radius set to 1, and the horizontal axis shows the aberration for each light ray in diopters. In the astigmatism diagram, the solid line shows the sagittal plane for each wavelength, and the dashed line shows the meridional plane for each wavelength. In the astigmatism diagram, the vertical axis shows the angle of view [°], and the horizontal axis shows the aberration for each light ray in diopters. In the distortion diagram, the vertical axis shows the angle of view [°], and the horizontal axis shows the proportion of aberration as a percentage (% value). Each lateral aberration diagram shows the aberration values ​​(unit: ['] (minutes)) when the image height ratio RFH (Relative Field Height) is 1.00, 0.70, 0.50, 0.00, or 1.00, 0.70, 0.50, 0.00, -0.50, -0.70, -1.00. Note that the same symbols as in this embodiment are used in the aberration diagrams of each embodiment and reference example shown below, and duplicate explanations will be omitted.

[0086] FIG. 9 shows spot diagrams when the erecting prism is not rotated (when the image blur correction angle is 0.0°) in the observation optical system according to Example 1. FIG. 10 shows spot diagrams when the erecting prism is rotated around the rotation center P0 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.82°) in the observation optical system according to Example 1. Each spot diagram shows the angular deviation (unit: [′] (minutes)) of light rays of the d-line, C-line, F-line, and g-line wavelengths at the eyepoint position relative to the principal ray. Each spot diagram also shows the value [′] calculated as the root mean square (RMS) of the degree of angular deviation. This also applies to the spot diagrams shown below, and redundant explanations will be omitted.

[0087] The aberration diagrams shown in Figures 6 and 7 indicate that Example 1 provides excellent optical performance by effectively correcting various aberrations when the erecting prism is not rotated. Furthermore, the lateral aberration diagram shown in Figure 8 indicates that even when the erecting prism is rotated about the rotation center P0, aberrations are effectively corrected and excellent optical performance is maintained. Furthermore, Example 1 provides excellent optical performance with minimal color separation, as the angular deviation |ΔC-F| is suppressed to 1.5 ['] when the erecting prism is rotated about the rotation center P0. Furthermore, the spot diagram shown in Figure 10 indicates that the RMS value is suppressed to 12.3 ['] when the erecting prism is rotated about the rotation center P0, also demonstrating excellent optical performance. As a result, by incorporating the observation optical system of Example 1, excellent optical performance can be ensured in telescopes and binoculars.

[0088] Second Example A second example according to an embodiment of the present invention will be described below with reference to Table 2 and FIGS. 11 and 12. The observation optical system according to the second example has the same lens and prism configuration as the observation optical system LS1 according to the first example, and differs from the first example only in that the center of rotation of the erecting prism is P1. Therefore, FIGS. 1 to 5, which show the observation optical system LS1 according to the first example, will be used to explain the observation optical system according to the second example, and "Lens Data" will be omitted from Table 2. As shown in FIG. 5, the center of rotation P1 is located at the intersection of the front prism surface PR11a of the erecting prism PR1 and the optical axis Z11 of the objective optical system OBL1.

[0089] (Table 2) [Specification data] Magnification: 10 Diameter (unit: mm): 42 Actual field of view 2ω (unit: °): 6.9 Rotation center: P1 Prism rotation angle (unit: °): 2.05 Angular magnification γ: 0.37 Angle deviation |ΔC-F| (unit: '): 1.9

[0090] Fig. 11 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center P1 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.05°) in the observation optical system of Example 2. Fig. 12 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center P1 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.05°) in the observation optical system of Example 2.

[0091] The lateral aberration diagram shown in Figure 11 shows that aberrations are well corrected and excellent optical performance is maintained even when the erecting prism is rotated about the rotation center P1. Furthermore, in the second embodiment, the angular deviation |ΔC-F| when the erecting prism is rotated about the rotation center P1 is suppressed to 1.9 ['], demonstrating excellent optical performance with little color separation. Furthermore, the spot diagram shown in Figure 12 shows that the RMS value when the erecting prism is rotated about the rotation center P1 is suppressed to 12.1 ['], demonstrating excellent optical performance in this respect as well. As a result, by incorporating the observation optical system of the second embodiment, excellent optical performance can be ensured even in telescopes and binoculars.

[0092] Third Example A third example according to an embodiment of the present invention will be described below with reference to Table 3 and FIGS. 13 and 14. The observation optical system according to the third example has the same lens and prism configuration as the observation optical system LS1 according to the first example, and differs from the first example only in that the center of rotation of the erecting prism is P6. Therefore, FIGS. 1 to 5, which show the observation optical system LS1 according to the first example, will be used to explain the observation optical system according to the third example, and [Lens Data] will be omitted from Table 3. As shown in FIG. 5, the center of rotation P6 is located on the imaginary straight line that passes through the prism in the erecting prism PR1, at a distance of L1 / 8 from the intersection of the front prism surface PR11a of the erecting prism PR1 and the optical axis Z11 of the objective optical system OBL1.

[0093] (Table 3) [Specification data] Magnification: 10 Diameter (unit: mm): 42 Actual field of view 2ω (unit: °): 6.9 Rotation center: P6 Prism rotation angle (unit: °): 2.19 Angular magnification γ: 0.34 Angle deviation |ΔC-F| (unit: '): 2.2

[0094] Fig. 13 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center P6 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.19°) in the observation optical system of Example 3. Fig. 14 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center P6 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.19°) in the observation optical system of Example 3.

[0095] The lateral aberration diagram shown in Figure 13 shows that aberrations are well corrected and excellent optical performance is maintained even when the erecting prism is rotated about the rotation center P6. Furthermore, in the third embodiment, the angular deviation |ΔC-F| when the erecting prism is rotated about the rotation center P6 is suppressed to 2.2 ['], demonstrating excellent optical performance with little color separation. Furthermore, the spot diagram shown in Figure 14 shows that the RMS value when the erecting prism is rotated about the rotation center P6 is suppressed to 12.3 ['], demonstrating excellent optical performance in this respect as well. As a result, by incorporating the observation optical system of the third embodiment, excellent optical performance can be ensured even in telescopes and binoculars.

[0096] Fourth Example A fourth example according to an embodiment of the present invention will be described below with reference to Table 4 and FIGS. 15 and 16. The observation optical system according to the fourth example has the same lens and prism configuration as the observation optical system LS1 according to the first example, and differs from the first example only in that the center of rotation of the erecting prism is P2. Therefore, FIGS. 1 to 5, which show the observation optical system LS1 according to the first example, will be used to explain the observation optical system according to the fourth example, and [Lens Data] will be omitted from Table 4. As shown in FIG. 5, the center of rotation P2 is located on the imaginary straight line passing through the erecting prism PR1, at a distance of L1 / 4 from the intersection of the front prism surface PR11a of the erecting prism PR1 and the optical axis Z11 of the objective optical system OBL1.

[0097] (Table 4) [Specification data] Magnification: 10 Diameter (unit: mm): 42 Actual field of view 2ω (unit: °): 6.9 Rotation center: P2 Prism rotation angle (unit: °): 2.35 Angular magnification γ: 0.32 Angle deviation |ΔC-F| (unit: '): 2.5

[0098] Fig. 15 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center P2 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.35°) in the observation optical system of Example 4. Fig. 16 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center P2 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.35°) in the observation optical system of Example 4.

[0099] The lateral aberration diagram shown in Figure 15 shows that aberrations are well corrected and excellent optical performance is maintained even when the erecting prism is rotated about the rotation center P2. Furthermore, in Example 4, the angular deviation |ΔC-F| when the erecting prism is rotated about the rotation center P2 is suppressed to 2.5 ['], demonstrating excellent optical performance with little color separation. Furthermore, the spot diagram shown in Figure 16 shows that the RMS value when the erecting prism is rotated about the rotation center P2 is suppressed to 12.4 ['], demonstrating excellent optical performance in this respect as well. As a result, by incorporating the observation optical system of Example 4, excellent optical performance can be ensured even in telescopes and binoculars.

[0100] Fifth Example A fifth example according to an embodiment of the present invention will be described below with reference to Figs. 17 to 24 and Table 5. Figs. 17 and 18 show an observation optical system LS2 according to the fifth example, which is used in telescopes or binoculars.

[0101] The observation optical system LS2 in Example 5 has, in order from the object side, an objective optical system OBL2, an erecting prism PR2 for erecting an image formed by the objective optical system OBL2, and an eyepiece optical system EPL2 for observing the image formed by the objective optical system OBL2, and corrects the image by rotating the erecting prism PR2 from a predetermined position (prism reference position). The center of rotation about which the erecting prism PR2 is rotated is located at a rotation center Q0 (see FIG. 19 ), which is a quarter of the length of the erecting prism PR2 in the direction along the optical axis Z21 of the objective optical system OBL2 from the position of the intersection between the surface of the erecting prism PR2 closest to the objective optical system OBL2 (front prism surface PR21 a) and the optical axis Z21 of the objective optical system OBL2 when the erecting prism PR2 is located at the predetermined position.

[0102] The rotation center Q0 is located between the surface of the erecting prism PR2 that is closest to the objective optical system OBL2 (front prism surface PR21a) and the surface of the objective optical system OBL2 that is closest to the erecting prism PR2 when the erecting prism PR2 is located at a predetermined position (prism reference position). Furthermore, the rotation center Q0 is located on an imaginary line (a through-prism imaginary line) that passes through the intersection of the front prism surface PR21a of the erecting prism PR2 with the optical axis Z21 of the objective optical system OBL2 and the intersection of the rear prism surface PR22a with the optical axis Z22 of the eyepiece optical system EPL2.

[0103] As shown in FIG. 18 , the objective optical system OBL2 includes, in order from the object side, a first objective lens OL21 consisting of a cemented lens with positive refractive power, a second objective lens OL22 consisting of a single lens with positive refractive power, and a third objective lens OL23 consisting of a single lens with positive refractive power. The first objective lens OL21 is a cemented lens consisting of a negative meniscus lens with a convex surface facing the object side and a biconvex lens. The second objective lens OL22 is a positive meniscus lens with a convex surface facing the object side, and the third objective lens OL23 is a negative meniscus lens with a convex surface facing the object side. The observation optical system LS2 is an afocal system that receives a parallel beam of light incident on the objective optical system OBL2 from the object side and emits it as a parallel beam of light toward the eyepoint EP via the erecting prism PR2 and the eyepiece optical system EPL2.

[0104] Subject light incident on the observation optical system LS1 passes through the objective optical system OBL2 and the erecting prism PR2, and forms an erect subject image as an intermediate image on the image forming plane IM. The subject image formed on the image forming plane IM is enlarged by the eyepiece optical system EPL2 and is observed by an observer whose eye is placed at the eyepoint EP.

[0105] The erecting prism PR2 is composed of an auxiliary prism PR21 and a roof prism PR22 arranged with a predetermined gap (air gap) from the auxiliary prism PR21. The auxiliary prism PR21 and the roof prism PR22 are arranged so that the front prism surface PR21a of the auxiliary prism PR21 and the rear prism surface PR22a of the roof prism PR22 are parallel to each other. Light emitted from the objective optical system OBL2 enters the auxiliary prism PR21 from the front prism surface PR21a, is reflected multiple times within the auxiliary prism PR21, and then enters the roof prism PR22 via the gap between the roof prism PR22 and the auxiliary prism PR21. The light entering the roof prism PR22 is reflected multiple times within the roof prism PR22, then exits from the rear prism surface PR22a and heads toward the eyepiece optical system EPL2.

[0106] The rotation center of the erecting prism PR2 is set, for example, as follows. Fig. 19 shows the erecting prism PR2 located at a reference position and seven rotation centers Q0 to Q6. In Fig. 19, the rotation center Q1 is located at the intersection position of the front prism surface PR21a of the erecting prism PR2 and the optical axis Z21 of the objective optical system OBL2, and the rotation center Q5 is located at the intersection position of the rear prism surface PR22a of the erecting prism PR2 and the optical axis Z22 of the eyepiece optical system EPL2. In addition, the seven rotation centers Q0 to Q6 are each located on an imaginary line (an imaginary line that passes through the prism) that passes through the position of the rotation center Q1 (the intersection position of the front prism surface PR21a and the optical axis Z21) and the position of the rotation center Q5 (the intersection position of the rear prism surface PR22a and the optical axis Z22).

[0107] The rotation center Q0 is located outside the erecting prism PR2, between the front prism surface PR21a and the surface of the objective optical system OBL2 closest to the objective optical system OBL2. The rotation center Q0 is located at a distance (L2 / 4) that is one-fourth the length L2 of the erecting prism PR2 in the direction along the optical axis Z21 of the objective optical system OBL2 from the position of the rotation center Q1 (front prism surface PR21a). Specifically, when an imaginary plane perpendicular to the optical axis Z21 is set between the erecting prism PR2 and the objective optical system OBL2 at a position L2 / 4 away from the position of the rotation center Q1, the rotation center Q0 is located at the intersection of the imaginary plane and a virtual line that passes through the prism. The other four rotation centers Q2, Q3, Q4, and Q6 are located inside the erecting prism PR2.

[0108] The rotation center Q2 is located at a distance (L2 / 4) that is one-fourth the length L2 of the objective optical system OBL2 of the erecting prism PR2 in the direction along the optical axis Z21 from the position of the rotation center Q1 (front prism surface PR21a). Specifically, when an imaginary plane perpendicular to the optical axis Z21 is set at a position L2 / 4 away from the position of the rotation center Q1 inside the erecting prism PR2, the rotation center Q2 is located at the intersection of the imaginary plane and a virtual line that passes through the prism. Similarly, the rotation center Q3 is located at a distance of 2×L2 / 4 (=L2 / 2) away from the position of the rotation center Q1, and the rotation center Q4 is located at a distance of 3×L2 / 4 away from the position of the rotation center Q1. Furthermore, the rotation center Q6 is located at a distance L2 / 8 away from the position of the rotation center P1.

[0109] In this way, four of the seven rotation centers Q0 to Q6, Q0, Q1, Q2, and Q6, are each located within a distance range of less than one-fourth of the length of the erecting prism PR2 in the direction along the optical axis Z21 of the objective optical system OBL2 from the position of the intersection between the front prism surface PR21a of the erecting prism PR2 and the optical axis Z21 of the objective optical system OBL2.

[0110] The eyepiece optical system EPL2 is, for example, composed of, from the object side, a first eyepiece lens EL21 consisting of a single lens with negative refractive power, a second eyepiece lens EL22 consisting of a single lens with positive refractive power, a third eyepiece lens EL23 consisting of a cemented lens with positive refractive power, and a fourth eyepiece lens EL24 consisting of a single lens with positive refractive power. The first eyepiece lens EL21 is a biconcave lens, the second eyepiece lens EL22 is a positive meniscus lens with its concave surface facing the object side. The third eyepiece lens EL23 is a cemented lens consisting of a biconcave lens and a biconvex lens, and the fourth eyepiece lens EL24 is a positive meniscus lens with its convex surface facing the object side.

[0111] Table 5 below shows the specifications of the fifth embodiment. Note that the radii of curvature R of surfaces 1 to 22 in Table 5 correspond to the symbols R1 to R22 assigned to surfaces 1 to 22 in FIG. 18. Surface 14 is a virtual surface corresponding to image plane IM where an intermediate image (subject image) is formed, and surface 22 is a virtual surface corresponding to eyepoint EP. The surface spacing between surfaces 21 and 22 is the distance (eye relief) from the final lens surface (surface 21) to eyepoint EP.

[0112] (Table 5) [Specification data] Magnification: 12 Diameter (unit: mm): 30 Actual field of view 2ω (unit: °): 5.5 Rotation center: Q0 Prism rotation angle (unit: °): 1.52 Angular magnification γ: 0.49 Angular deviation |ΔC-F| (unit: '): 1.5 [Lens data] Surface number R D nd νd 1 59.051 1.6 1.80400 46.60 2 35.168 6.1 1.49782 82.57 3 -287.516 0.5 4 32.883 3.7 1.48749 70.32 5 63.395 14.6 6 55.142 2.0 1.51742 52.20 7 29.733 13.5 8 ∞ 36.6 1.56883 56.00 9 ∞ 0.4 10 ∞ 46.5 1.51680 64.13 11 ∞ 4.3 12 -17.653 3.1 1.48749 70.32 13 16.157 4.8 14 ∞ 3.6 15 -51.413 7.9 1.75500 52.34 16 -15.278 0.2 17 -370.476 1.0 1.94595 17.98 18 20.281 7.8 1.60300 65.44 19 -27.271 0.2 20 18.697 5.2 1.80400 46.60 21 500.000 15.0 22 (Eyepoint)

[0113] Fig. 20 is a diagram showing various aberrations (spherical aberration, astigmatism, and distortion) when the erecting prism is not rotated (when the image blur correction angle is 0.0°) in the observation optical system of Example 5. Fig. 21 is a diagram showing lateral aberrations when the erecting prism is not rotated (when the image blur correction angle is 0.0°) in the observation optical system of Example 5. Fig. 22 is a diagram showing lateral aberrations when the erecting prism is rotated about the rotation center Q0 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.52°) in the observation optical system of Example 5.

[0114] Fig. 23 is a spot diagram when the erecting prism is not rotated (when the image blur correction angle is 0.0°) in the observation optical system of Example 5. Fig. 24 is a spot diagram when the erecting prism is rotated about the rotation center Q0 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.52°) in the observation optical system of Example 5.

[0115] The aberration diagrams shown in Figures 20 and 21 indicate that Example 5 provides excellent optical performance, with excellent correction of aberrations when the erecting prism is not rotated. Furthermore, the lateral aberration diagram shown in Figure 22 indicates that even when the erecting prism is rotated about the rotation center Q0, aberrations are excellently corrected, and excellent optical performance is maintained. Furthermore, Example 5 provides excellent optical performance with minimal color separation, with the angular deviation |ΔC-F| being suppressed to 1.5 ['] when the erecting prism is rotated about the rotation center Q0. Furthermore, the spot diagram shown in Figure 24 indicates that the RMS value is suppressed to 10.9 ['] when the erecting prism is rotated about the rotation center Q0, also demonstrating excellent optical performance. As a result, by incorporating the observation optical system of Example 5, excellent optical performance can be ensured in telescopes and binoculars.

[0116] Sixth Example A sixth example according to an embodiment of the present invention will be described below with reference to Table 6 and FIGS. 25 to 26. The observation optical system according to the sixth example has the same lens and prism configuration as the observation optical system LS2 according to the fifth example, and differs from the fifth example only in that the center of rotation of the erecting prism is Q1. Therefore, FIGS. 17 to 19, which show the observation optical system LS2 according to the fifth example, will be used to explain the observation optical system according to the sixth example, and the "Lens Data" section will be omitted from Table 6. As shown in FIG. 19, the center of rotation Q1 is located at the intersection of the front prism surface PR21a of the erecting prism PR2 and the optical axis Z21 of the objective optical system OBL1.

[0117] (Table 6) [Specification data] Magnification: 12 Diameter (unit: mm): 30 Actual field of view 2ω (unit: °): 5.5 Rotation center: Q1 Prism rotation angle (unit: °): 1.72 Angular magnification γ: 0.44 Angle deviation |ΔC-F| (unit: '): 2.0

[0118] Fig. 25 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center Q1 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.72°) in the observation optical system of Example 6. Fig. 26 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center Q1 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.72°) in the observation optical system of Example 6.

[0119] The lateral aberration diagram shown in Figure 25 shows that aberrations are well corrected and excellent optical performance is maintained even when the erecting prism is rotated about the rotation center Q1. Furthermore, in Example 6, the angular deviation |ΔC-F| when the erecting prism is rotated about the rotation center Q1 is suppressed to 2.0 ['], demonstrating excellent optical performance with little color separation. Furthermore, the spot diagram shown in Figure 26 shows that the RMS value when the erecting prism is rotated about the rotation center Q1 is suppressed to 10.7 ['], demonstrating excellent optical performance in this respect as well. As a result, by incorporating the observation optical system of Example 6, excellent optical performance can be ensured even in telescopes and binoculars.

[0120] Seventh Example A seventh example according to an embodiment of the present invention will be described below with reference to Table 7 and FIGS. 27 to 28. The observation optical system according to the seventh example has the same lens configuration and prism configuration as the observation optical system LS2 according to the fifth example, and differs from the fifth example only in that the center of rotation of the erecting prism is Q6. Therefore, FIGS. 17 to 19, which show the observation optical system LS2 according to the fifth example, will be used to explain the observation optical system according to the seventh example, and [Lens Data] will be omitted from Table 7. As shown in FIG. 19, the center of rotation Q6 is located on the imaginary straight line that passes through the prism in the erecting prism PR2, at a position that is a distance of L2 / 8 from the intersection of the front prism surface PR21a of the erecting prism PR2 and the optical axis Z21 of the objective optical system OBL2.

[0121] (Table 7) [Specification data] Magnification: 12 Diameter (unit: mm): 30 Actual field of view 2ω (unit: °): 5.5 Rotation center: Q6 Prism rotation angle (unit: °): 1.84 Angular magnification γ: 0.41 Angular deviation |ΔC-F| (unit: '): 2.3

[0122] Fig. 27 is a diagram showing lateral aberration when the erecting prism is rotated about the rotation center Q6 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.84°) in the observation optical system of Example 7. Fig. 28 is a diagram showing spot diagrams when the erecting prism is rotated about the rotation center Q6 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.84°) in the observation optical system of Example 7.

[0123] The lateral aberration diagram shown in Figure 27 shows that aberrations are well corrected and excellent optical performance is maintained even when the erecting prism is rotated about the rotation center Q6. Furthermore, in Example 7, the angular deviation |ΔC-F| when the erecting prism is rotated about the rotation center Q6 is suppressed to 2.3 ['], demonstrating excellent optical performance with little color separation. Furthermore, the spot diagram shown in Figure 28 shows that the RMS value when the erecting prism is rotated about the rotation center Q6 is suppressed to 10.6 ['], demonstrating excellent optical performance in this respect as well. As a result, by incorporating the observation optical system of Example 7, excellent optical performance can be ensured even in telescopes and binoculars.

[0124] Eighth Example An eighth example according to an embodiment of the present invention will be described below with reference to Table 8 and FIGS. 29 to 30. The observation optical system according to the eighth example has the same lens and prism configuration as the observation optical system LS2 according to the fifth example, and differs from the fifth example only in that the center of rotation of the erecting prism is Q2. Therefore, FIGS. 17 to 19, which show the observation optical system LS2 according to the fifth example, will be used to explain the observation optical system according to the eighth example, and [Lens Data] will be omitted from Table 8. As shown in FIG. 19, the center of rotation Q2 is located on the imaginary straight line passing through the erecting prism PR2, at a distance of L2 / 4 from the intersection of the front prism surface PR21a of the erecting prism PR2 and the optical axis Z21 of the objective optical system OBL2.

[0125] (Table 8) [Specification data] Magnification: 12 Diameter (unit: mm): 30 Actual field of view 2ω (unit: °): 5.5 Rotation center: Q2 Prism rotation angle (unit: °): 1.97 Angular magnification γ: 0.38 Angle deviation |ΔC-F| (unit: '): 2.3

[0126] Fig. 29 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center Q2 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.97°) in the observation optical system of Example 8. Fig. 30 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center Q2 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 1.97°) in the observation optical system of Example 8.

[0127] The lateral aberration diagram shown in Figure 29 shows that aberrations are well corrected and excellent optical performance is maintained even when the erecting prism is rotated about the rotation center Q2. Furthermore, in Example 8, the angular deviation |ΔC-F| when the erecting prism is rotated about the rotation center Q2 is suppressed to 2.6 ['], demonstrating excellent optical performance with little color separation. Furthermore, the spot diagram shown in Figure 30 shows that the RMS value when the erecting prism is rotated about the rotation center Q2 is suppressed to 10.5 ['], demonstrating excellent optical performance in this respect as well. As a result, by incorporating the observation optical system of Example 8, excellent optical performance can be ensured even in telescopes and binoculars.

[0128] First Reference Example A first reference example, which is a reference example of the first to fourth embodiments of the present invention, will be described below with reference to Table 9 and FIGS. 31 and 32. The observation optical system according to the first reference example has the same lens and prism configuration as the observation optical system LS1 according to the first example, and differs from the first example only in that the center of rotation of the erecting prism is P3. Therefore, FIGS. 1 to 5, which show the observation optical system LS1 according to the first example, will be used to explain the observation optical system according to the first reference example, and [Lens Data] will be omitted from Table 9. As shown in FIG. 5, the center of rotation P3 is located on the imaginary straight line that passes through the prism in the erecting prism PR1, at a distance of L1 / 2 from the intersection of the front prism surface PR11a of the erecting prism PR1 and the optical axis Z11 of the objective optical system OBL1.

[0129] (Table 9) [Specification data] Magnification: 10 Diameter (unit: mm): 42 Actual field of view 2ω (unit: °): 6.9 Rotation center: P3 Prism rotation angle (unit: °): 2.75 Angular magnification γ: 0.27 Angular deviation |ΔC-F| (unit: '): 3.2

[0130] Fig. 31 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center P3 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.75°) in the observation optical system according to Reference Example 1. Fig. 32 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center P3 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.75°) in the observation optical system according to Reference Example 1.

[0131] 31, it can be seen that when the erect prism is rotated about the rotation center P3, the aberrations are not sufficiently corrected, and the optical performance is inferior to that of the first to fourth embodiments. Moreover, in the first reference example, the angle deviation |ΔC−F| when the erect prism is rotated about the rotation center P3 is large at 3.2 ['], and it can be seen that the color separation is large and the optical performance is inferior to that of the first to fourth embodiments.

[0132] Second Reference Example A second reference example, which is a reference example of the first to fourth embodiments of the present invention, will be described below with reference to Table 10 and FIGS. 33 to 34. The observation optical system according to the second reference example has the same lens and prism configuration as the observation optical system LS1 according to the first example, and differs from the first example only in that the center of rotation of the erecting prism is P4. Therefore, FIGS. 1 to 5, which show the observation optical system LS1 according to the first example, will be used to explain the observation optical system according to the second reference example, and the "Lens Data" section will be omitted from Table 10. As shown in FIG. 5, the center of rotation P4 is located on the imaginary straight line that passes through the prism in the erecting prism PR1, at a distance of 3×L1 / 4 from the intersection of the front prism surface PR11a of the erecting prism PR1 and the optical axis Z11 of the objective optical system OBL1.

[0133] (Table 10) [Specification data] Magnification: 10 Diameter (unit: mm): 42 Actual field of view 2ω (unit: °): 6.9 Rotation center: P4 Prism rotation angle (unit: °): 3.32 Angular magnification γ: 0.23 Angle deviation |ΔC-F| (unit: '): 4.3

[0134] Fig. 33 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center P4 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 3.32°) in the observation optical system according to Reference Example 2. Fig. 34 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center P4 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 3.32°) in the observation optical system according to Reference Example 2.

[0135] 33, it can be seen that when the erect prism is rotated about the rotation center P4, the aberrations are not sufficiently corrected, and the optical performance is inferior to that of the first to fourth embodiments. Also, in the second reference example, the angle deviation |ΔC−F| when the erect prism is rotated about the rotation center P4 is large at 4.3 ['], and it can be seen that the color separation is large and the optical performance is inferior to that of the first to fourth embodiments.

[0136] Third Reference Example A third reference example, which is a reference example of the first to fourth embodiments of the present invention, will be described below with reference to Table 11 and FIGS. 35 to 36. The observation optical system according to the third reference example has the same lens and prism configuration as the observation optical system LS1 according to the first example, and differs from the first example only in that the center of rotation of the erecting prism is P5. Therefore, FIGS. 1 to 5, which show the observation optical system LS1 according to the first example, will be used to explain the observation optical system according to the third reference example, and the "Lens Data" section will be omitted from Table 11. As shown in FIG. 5, the center of rotation P5 is located at the intersection of the rear prism surface PR12a of the erecting prism PR1 and the optical axis Z12 of the eyepiece optical system EPL1.

[0137] (Table 11) [Specification data] Magnification: 10 Diameter (unit: mm): 42 Actual field of view 2ω (unit: °): 6.9 Rotation center: P5 Prism rotation angle (unit: °): 4.19 Angular magnification γ: 0.18 Angle deviation |ΔC-F| (unit: '): 5.9

[0138] Fig. 35 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center P5 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 4.19°) in the observation optical system according to Reference Example 3. Fig. 36 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center P5 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 4.19°) in the observation optical system according to Reference Example 3.

[0139] 35, it can be seen that when the erect prism is rotated about the rotation center P5, the aberrations are not sufficiently corrected, and the optical performance is inferior to that of the first to fourth embodiments. Also, in the third reference example, the angle deviation |ΔC−F| when the erect prism is rotated about the rotation center P5 is large at 5.9 ['], and it can be seen that the color separation is large and the optical performance is inferior to that of the first to fourth embodiments.

[0140] Fourth Reference Example A fourth reference example, which is a reference example of the fifth to eighth embodiments of the present invention, will be described below with reference to Table 12 and FIGS. 37 to 38. The observation optical system according to the fourth reference example has the same lens and prism configuration as the observation optical system LS2 according to the fifth example, and differs from the fifth example only in that the center of rotation of the erecting prism is Q3. Therefore, FIGS. 17 to 19, which show the observation optical system LS2 according to the fifth example, will be used to explain the observation optical system according to the fourth reference example, and the [Lens Data] section will be omitted from Table 12. As shown in FIG. 19, the center of rotation Q3 is located on the imaginary straight line that passes through the prism in the erecting prism PR2, at a distance of L2 / 2 from the intersection of the front prism surface PR21a of the erecting prism PR2 and the optical axis Z21 of the objective optical system OBL2.

[0141] (Table 12) [Specification data] Magnification: 12 Diameter (unit: mm): 30 Actual field of view 2ω (unit: °): 5.5 Rotation center: Q3 Prism rotation angle (unit: °): 2.32 Angular magnification γ: 0.32 Angle deviation |ΔC-F| (unit: '): 3.4

[0142] Fig. 37 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center Q3 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.32°) in the observation optical system according to Reference Example 4. Fig. 36 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center Q3 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.32°) in the observation optical system according to Reference Example 4.

[0143] 37, it can be seen that when the erect prism is rotated about the rotation center Q3, the aberrations are not sufficiently corrected, and the optical performance is inferior to that of the fifth to eighth embodiments. Also, in the fourth reference example, the angle deviation |ΔC−F| when the erect prism is rotated about the rotation center Q3 is large at 3.4 ['], and it can be seen that the color separation is large and the optical performance is inferior to that of the fifth to eighth embodiments.

[0144] Fifth Reference Example A fifth reference example, which serves as a reference example of the fifth to eighth embodiments of the present invention, will be described below with reference to Table 13 and FIGS. 39 to 40. The observation optical system according to the fifth reference example has the same lens and prism configuration as the observation optical system LS2 according to the fifth example, and differs from the fifth example only in that the center of rotation of the erecting prism is Q4. Therefore, FIGS. 17 to 19, which show the observation optical system LS2 according to the fifth example, will be used to explain the observation optical system according to the fifth reference example, and [Lens Data] will be omitted from Table 13. As shown in FIG. 19, the center of rotation Q4 is located on the imaginary straight line that passes through the prism in the erecting prism PR2, at a distance of 3×L2 / 4 from the intersection of the front prism surface PR21a of the erecting prism PR2 and the optical axis Z21 of the objective optical system OBL2.

[0145] (Table 13) [Specification data] Magnification: 12 Diameter (unit: mm): 30 Actual field of view 2ω (unit: °): 5.5 Rotation center: Q4 Prism rotation angle (unit: °): 2.82 Angular magnification γ: 0.27 Angle deviation |ΔC-F| (unit: '): 4.4

[0146] Fig. 39 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center Q4 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.82°) in the observation optical system according to Reference Example 5. Fig. 40 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center Q4 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 2.82°) in the observation optical system according to Reference Example 5.

[0147] 39, it can be seen that when the erect prism is rotated about the rotation center Q4, the aberrations are not sufficiently corrected, and the optical performance is inferior to that of the fifth to eighth embodiments. Also, in the fifth reference example, the angular deviation |ΔC−F| when the erect prism is rotated about the rotation center Q4 is large at 4.4 ['], and it can be seen that the color separation is large and the optical performance is inferior to that of the fifth to eighth embodiments.

[0148] Sixth Reference Example A sixth reference example, which serves as a reference example of the fifth to eighth embodiments of the present invention, will be described below with reference to Table 14 and FIGS. 41 and 42. The observation optical system according to the sixth reference example has the same lens and prism configuration as the observation optical system LS2 according to the fifth example, and differs from the fifth example only in that the center of rotation of the erecting prism is Q5. Therefore, FIGS. 17 to 19, which show the observation optical system LS2 according to the fifth example, will be used to explain the observation optical system according to the fifth reference example, and the "Lens Data" section will be omitted from Table 14. As shown in FIG. 19, the center of rotation Q5 is located at the intersection of the rear prism surface PR22a of the erecting prism PR2 and the optical axis Z22 of the eyepiece optical system EPL2.

[0149] (Table 14) [Specification data] Magnification: 12 Diameter (unit: mm): 30 Actual field of view 2ω (unit: °): 5.5 Rotation center: Q5 Prism rotation angle (unit: °): 3.58 Angular magnification γ: 0.21 Angle deviation |ΔC-F| (unit: '): 6.1

[0150] Fig. 41 is a diagram showing lateral aberration when the erect prism is rotated about the rotation center Q5 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 3.58°) in the observation optical system according to Reference Example 6. Fig. 42 is a diagram showing spot diagrams when the erect prism is rotated about the rotation center Q5 to obtain an image blur correction angle of 1.5° (when the prism rotation angle is 3.58°) in the observation optical system according to Reference Example 6.

[0151] 41, it can be seen that when the erect prism is rotated about the rotation center Q5, the aberrations are not sufficiently corrected, and the optical performance is inferior to that of the fifth to eighth embodiments. Also, in the sixth reference example, the angle deviation |ΔC−F| when the erect prism is rotated about the rotation center Q5 is large at 6.1 ['], and it can be seen that the color separation is large and the optical performance is inferior to that of the fifth to eighth embodiments.

[0152] Fig. 43 shows the prism rotation centers, prism rotation angles required to obtain an image blur correction angle of 1.5°, angular magnifications, and angular deviation |ΔC-F| during prism rotation for the first to fourth examples and the first to third reference examples. Fig. 44 is a graph showing the relationship between the image blur correction angle and the prism rotation angle for each of the rotation centers P0 to P6. Fig. 45 is a graph showing the angular deviation |ΔC-F| for each of the rotation centers P0 to P6.

[0153] As shown in Fig. 43, the prism rotation angle required to obtain an image blur correction angle of 1.5° is smaller and the angular magnification γ is larger in Examples 1 to 4 compared to Examples 1 to 3. Therefore, it can be seen that Examples 1 to 4 can obtain a larger image blur correction angle with a smaller prism rotation angle than Examples 1 to 3. Also, as shown in Fig. 43, Examples 1 to 4 have a smaller value of angular deviation |ΔC-F| than Examples 1 to 3. Therefore, it can be seen that Examples 1 to 4 have less color separation and provide better image visibility during image blur correction than Examples 1 to 3.

[0154] In the graphs of Fig. 44 and Fig. 45, P0, P1, P6, and P2 are rotation centers located within a distance range of less than one-fourth of the length L1 of the erect prism PR1 in the direction along the optical axis Z11 of the objective optical system OBL1 from the position of the intersection between the front prism surface PR11a of the erect prism PR1 and the optical axis Z11 of the objective optical system OBL1, as shown in Fig. 5. In contrast, P3, P4, and P5 are rotation centers located within a range where the distance from the intersection exceeds one-fourth of the length L1, as shown in Fig. 5. It can be seen from Fig. 44 that, compared to the rotation centers P3, P4, and P5, the rotation centers P0, P1, P6, and P2 can obtain a larger image blur correction angle with a smaller prism rotation angle. Furthermore, from FIG. 45, it can be seen that the values ​​of the angle deviation |ΔC−F| when the erecting prism PR1 rotates (during image blur correction) are smaller for the rotation centers P0, P1, P6, and P2 than for the rotation centers P3, P4, and P5.

[0155] Furthermore, the rotation center P0 is located outside the erecting prism PR1 and between the erecting prism PR1 and the objective optical system OBL1. Even when the rotation center is P0, a large image blur correction angle can be obtained by a small prism rotation angle, and the value of the angle deviation |ΔC-F| during image blur correction is also small. However, when the rotation center is set between the erecting prism PR1 and the objective optical system OBL1, the farther the position of the rotation center is from the erecting prism PR1, the larger the movement amount of the erecting prism PR1 when it rotates. For these reasons, it is preferable that the rotation center of the erecting prism PR1 be located within a distance range from the intersection point that is equal to or less than one-fourth of the length L1.

[0156] Fig. 46 shows the prism rotation centers, prism rotation angles required to obtain an image blur correction angle of 1.5°, angular magnifications, and angular deviation |ΔC-F| during prism rotation for the fifth to eighth examples and the fourth to sixth reference examples. Fig. 47 is a graph showing the relationship between the image blur correction angle and the prism rotation angle for each of the rotation centers Q0 to Q6. Fig. 48 is a graph showing the angular deviation |ΔC-F| for each of the rotation centers Q0 to Q6.

[0157] As shown in Fig. 47, the prism rotation angle required to obtain an image blur correction angle of 1.5° is smaller and the angular magnification γ is larger in the fifth to eighth embodiments than in the fourth to sixth embodiments. Therefore, it can be seen that the fifth to eighth embodiments can obtain a larger image blur correction angle with a smaller prism rotation angle than the fourth to sixth embodiments. Furthermore, as shown in Fig. 48, the values ​​of angular deviation |ΔC-F| are smaller in the fifth to eighth embodiments than in the fourth to sixth embodiments. Therefore, it can be seen that the fifth to eighth embodiments have smaller color separation and provide better image visibility during image blur correction than the fourth to sixth embodiments.

[0158] In the graphs of Fig. 47 and Fig. 48, Q0, Q1, Q6, and Q2 are rotation centers located within a distance range of less than one-fourth of the length L2 of the erect prism PR2 in the direction along the optical axis Z21 of the objective optical system OBL2 from the position of the intersection between the front prism surface PR21a of the erect prism PR2 and the optical axis Z21 of the objective optical system OBL2, as shown in Fig. 19. In contrast, Q3, Q4, and Q5 are rotation centers located within a range where the distance from the intersection exceeds one-fourth of the length L2, as shown in Fig. 19. It can be seen from Fig. 47 that, compared to the rotation centers Q3, Q4, and Q5, the rotation centers Q0, Q1, Q6, and Q2 can obtain a larger image blur correction angle with a smaller prism rotation angle. Furthermore, from Figure 45, it can be seen that the value of the angle deviation |ΔC-F| when the erecting prism PR1 rotates (during image blur correction) is smaller for the rotation centers Q0, Q1, Q6, and Q2 than for the rotation centers Q3, Q4, and Q5.

[0159] Furthermore, the rotation center Q0 is located outside the erecting prism PR2, between the erecting prism PR2 and the objective optical system OBL2. Even when the rotation center is Q0, a large image blur correction angle can be obtained with a small prism rotation angle, and the value of the angle deviation |ΔC-F| during image blur correction is also small. However, when the rotation center is set between the erecting prism PR2 and the objective optical system OBL2, the farther the position of the rotation center is from the erecting prism PR2, the larger the movement amount of the erecting prism PR2 when it rotates. For these reasons, it is preferable that the rotation center of the erecting prism PR2 be located within a distance range from the intersection point that is equal to or less than one-fourth of the length L2.

[0160] Next, the table of [Values ​​Corresponding to Conditional Expressions] is shown below. This table summarizes the values ​​of angular magnification γ corresponding to conditional expression (A1) for all examples (first to eighth examples). As shown in the table below, all of the first to eighth examples satisfy conditional expression (A1). Conditional expression (A1) 0.25<γ<0.50

[0161] [Conditional expression corresponding values] 1st Example 0.41 2nd Example 0.37 3rd Example 0.34 4th Example 0.32 5th Example 0.49 6th Example 0.44 7th Example 0.41 8th Example 0.38

[0162] According to each of the above embodiments, it is possible to realize an observation optical system that can obtain a sufficient image blur correction angle and reduce decentering aberrations during image blur correction.

[0163] It should be noted that the above examples show specific examples of embodiments of the present invention, and the present invention is not limited to these. For example, the above embodiments may include a zoom lens (variable magnification optical system). For example, an eyepiece zoom optical system may be provided instead of the above-described eyepiece optical system.

[0164] LS1, LS2 Observation optical system OBL1, OBL2 Objective optical system PR1, PR2 Erecting prism EPL1, EPL2 Eyepiece optical system BFG Binoculars

Claims

1. An observation optical system having, in order from an object side, an objective optical system, an erecting prism for erecting an image formed by the objective optical system, and an eyepiece optical system for observing the image formed by the objective optical system erected by the erecting prism, and correcting the image by rotating the erecting prism from a predetermined position, An observation optical system in which the center of rotation for rotating the erecting prism is located within a distance range of less than one-fourth of the length of the erecting prism in a direction along the optical axis of the objective optical system from the position of the intersection between the surface of the erecting prism that is closest to the objective optical system and the optical axis of the objective optical system when the erecting prism is located at the specified position.

2. 2. The observation optical system according to claim 1, wherein the rotation center is located on a straight line passing through an intersection between a surface of the erecting prism closest to the objective optical system and the optical axis of the objective optical system and a intersection between a surface of the erecting prism closest to the eyepiece optical system and the optical axis of the eyepiece optical system when the erecting prism is located at the specified position, or on the optical axis of the objective optical system.

3. 2. The observation optical system according to claim 1, wherein the center of rotation is located within the erecting prism.

4. 2. The observation optical system according to claim 1, wherein the rotation center is located on a surface of the erecting prism that is closest to the objective optical system.

5. An observation optical system having, in order from an object side, an objective optical system, an erecting prism for erecting an image formed by the objective optical system, and an eyepiece optical system for observing the image formed by the objective optical system erected by the erecting prism, and correcting the image by rotating the erecting prism from a predetermined position, an observation optical system in which the center of rotation for rotating the erecting prism is located between a surface of the erecting prism that is closest to the objective optical system and a surface of the objective optical system that is closest to the erecting prism when the erecting prism is located at the specified position.

6. 6. The observation optical system according to claim 5, wherein the rotation center is located on a straight line passing through an intersection between a surface of the erecting prism closest to the objective optical system and the optical axis of the objective optical system and a point of intersection between a surface of the erecting prism closest to the eyepiece optical system and the optical axis of the eyepiece optical system when the erecting prism is located at the specified position, or on the optical axis of the objective optical system.

7. 6. The viewing optical system according to claim 1, which satisfies the following condition: 1.times. ... 0.25<γ<0.50 where γ is the angular magnification of the objective optical system.

8. An optical instrument comprising the observation optical system according to claim 1 or 5.