Image-erecting viewing optical system

An image-erecting viewing optical system includes a first prism having an incident surface and at least two reflection surfaces, a front lens group of an objective optical system, a second prism having an incident surface and at least two reflection surfaces, a rear lens group of the objective optical system, a field stop, and an eyepiece optical system, in this order from the object. One reflection surface of the first or second prism includes a roof-mirror surface. The following conditions are satisfied:sin(θ−ω′)>1/n  (1)6°<ω<16°  (2)

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image-erecting viewing optical system having a large apparent visual angle and a low magnification.

2. Description of the Prior Art

An image-erecting viewing optical system used in, for example, a binocular or a monocular, generally includes an objective optical system, an image-erecting optical system, and an eyepiece optical system, in this order from the object. In an image-erecting optical system having a wide real field-of-view at a low magnification, there is a need to make the focal length of the objective optical system shorter; however, in an optical arrangement in which the objective optical system is provided in front of the image-erecting optical system, the focal length of the objective optical system has to be made longer by the optical path length of the image-erecting optical system. Consequently, the focal length of the eyepiece optical system has also to be made longer, so that miniaturization of the entire optical system becomes difficult.

SUMMARY OF THE INVENTION

The present invention provides a miniaturized image-erecting optical system having a particularly wide real field-of-view and low magnification.

As an aspect of the present invention, there is provided an image-erecting viewing optical system including a first prism having an incident surface and at least two reflection surfaces, a front lens group of an objective optical system, a second prism having an incident surface and at least two reflection surfaces, a rear lens group of the objective optical system, a field stop, and an eyepiece optical system, in this order from the object. One reflection surface of one of the first prism and second prism includes a roof-mirror surface, and satisfies the following conditions:
sin(θ−ω′)>1/n(1)
6°<ω<16°  (2)whereinθ designates an angle between the incident surface and the first reflection surface of the first prism;ω designates a real field-of-view ω (half amount)ω′ designates an angle between a light ray of a real field-of-view ω (half amount) and the optical axis in the first prism; andn designates the refractive index of the first prism.

On the other hand, in the case where the incident surface and the reflection surface of the first prism are the same surface, the first prism satisfies the following conditions:
sin(2α−ω′)>1/n(3)
6°<ω<16°  (2)whereinα designates an angle between the incident surface and the first reflection surface of the first prism;ω designates a real field-of-view ω (half amount);ω′ designates an angle between a light ray of a real field-of-view ω (half amount) and the optical axis in the first prism; andn designates the refractive index of the first prism.

In the above optical arrangement, focusing is performed by moving at least one of the rear lens group of the objective optical system and the eyepiece optical system.

As another aspect of the present invention, there is provided an image-erecting viewing optical system including a first prism, a front lens group of an objective optical system, a second prism, a rear lens group of the objective optical system, a field stop, and an eyepiece optical system, in this order from the object. The image-erecting viewing optical system satisfies the following condition in the case where the rear lens group of the objective optical system is a focusing lens group:
0<fO/f2<0.5  (4)whereinf2designates the focal length of the rear lens group of the objective optical system; andfO designates the focal length of the entire the objective optical system.

The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-370584 (filed on Dec. 4, 2001) which is expressly incorporated herein in its entirety.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 through 7show the optical structure of the image-erecting optical system according to the first through sixth embodiments. More specifically, in each of the first through sixth embodiments, the image-erecting optical system includes a first prism10(110,210,310,410,510), a front lens group21of an objective optical system20(120,220,320,420,520), a second prism30(130,230,330,430,530), a rear lens group (focusing lens group)22of the objective optical system20(120,220,320,420,520), a field stop40, and an eyepiece optical system50, in this order from the object.

In the first through sixth embodiments, a reflection surface of either first prism10(110,210,310,410,510) or the second prism30(130,230,330,430,530) is a roof-mirror surface, i.e., either of the first prism10or the second prism30is a roof prism. Furthermore, in each of the first through sixth embodiments, there are five reflection surfaces, in total, in the first prism10(110,210,310,410,510) and the second prism30(130,230,330,430,530); and one of the five reflection surfaces is a roof-mirror surface. The field stop40is provided at a position where an image of an object at infinity is formed through the objective optical system20(120,220,320,420,520).

Specifically, in the first embodiment shown inFIG. 1, the first prism10includes an incident surface10i, a first reflection surface10-1r, a second reflection surface10-2r, and an exit surface10e. The second prism30includes an incident surface30i, a first reflection surface30-1r, a second reflection surface (roof-mirror surface)30-2r(D), a third reflection surface30-3r, and an exit surface30e.

The exit surface10eand the first reflection surface10-1rare the same surface. The incident surface30iand the third reflection surface30-3rare the same surface; and likewise, the first reflection surface30-1rand the exit surface30eare the same surface.

In the second embodiment shown inFIG. 2, a first prism110includes an incident surface110i, a first reflection surface110-1r, a second reflection surface (roof-mirror surface)110-2r(D), a third reflection surface110-3r, and an exit surface110e. The second prism130includes a incident surface130i, a first reflection surface130-1r, a second reflection surface130-2r, and an exit surface130e.

The incident surface110iand the third reflection surface110-3rare the same surface; and the exit surface110eand the first reflection surface110-1rare the same surface. Likewise, the incident surface130iand the second reflection surface130-2rare the same surface.

In the third embodiment shown inFIG. 3, a first prism210includes an incident surface210i, a first reflection surface210-1r, a second reflection surface (roof-mirror surface)210-2r(D), and an exit surface210e. A second prism230includes an incident surface230i, a first reflection surface230-1r, a second reflection surface230-2r, a third reflection surface230-3r, and an exit surface230e.

The first reflection surface210-1rand the exit surface210eare the same surface; and the incident surface230iand the second reflection surface230-2rare the same surface.

In the fourth embodiment shown inFIG. 4, a first prism310includes an incident surface310i, a first reflection surface310-1r, a second reflection surface (roof-mirror surface)310-2r(D), a third reflection surface310-3r, and an exit surface310e. A second prism330includes an incident surface330i, a first reflection surface330-1r, a second reflection surface330-2r, and an exit surface330e.

The incident surface310iand the third reflection surface310-3rare the same surface; and the first reflection surface310-1rand the exit surface310eare the same surface.

In the fifth embodiment shown inFIG. 5, a first prism410includes an incident surface410i, a first reflection surface410-1r, a second reflection surface410-2r, and an exit surface410e. A second prism430includes an incident surface430i, a first reflection surface430-1r, a second reflection surface (roof-mirror surface)430-2r(D), a third reflection surface430-3r, and an exit surface430e.

The incident surface410iand the second reflection surface410-2rare the same surface. The incident surface430iand the third reflection surface430-3rare the same surface. The first reflection surface430-1rand the exit surface430eare the same surface.

In the sixth embodiment shown inFIG. 6, a first prism510includes an incident surface510i, a first reflection surface (roof-mirror surface)510-1r(D), a second reflection surface510-2r, and an exit surface510e. A second prism530includes an incident surface530i, a first reflection surface530-1r, a second reflection surface530-2r, a third reflection surface530-3r, and an exit surface530e.

The incident surface510iand the second reflection surface510-2rare the same surface. The incident surface530iand the second reflection surface530-2rare the same surface.

In each of the first through sixth embodiments, the incident surfaces and the exit surfaces extend perpendicularly to the page on whichFIGS. 1 through 6are shown; and likewise, the reflection surfaces other than the roof-mirror surfaces extend perpendicularly to the page on whichFIGS. 1 through 6are shown.

The first through sixth embodiments can be divided into two groups, i.e., (i) a first group: the first through fourth embodiments shown inFIGS. 1 through 4; and (ii) a second group: the fifth and sixth embodiments shown inFIGS. 5 and 6.

The first group, i.e., the first through fourth embodiments, satisfies the following conditions:
sin(θ−ω′)>1/n(1)
6°<ω<16°  (2)whereinθ designates an angle between the incident surface and the first reflection surface of the first prism;ω designates a real field-of-view ω (half amount);ω′ designates an angle between a light ray of the real field-of-view ω (half amount) and the optical axis in the first prism; andn designates the refractive index of the first prism.

By satisfying conditions (1) and (2), light rays do not pass through the reflection surfaces in the first prism, so that reduction of peripheral illumination at the peripheral area of the view field, and ghosting caused by transmitting light rays can be avoided.

In the second group, i.e., the fifth and sixth embodiments, the incident surface410i(510i) and the second reflections surface410-2r(510-2r) of the first prism410(510) are the same surface, and the first prism410(510) satisfies the following conditions:
sin(2α−ω′)>1/n(3)
6°<ω<16°  (2)whereinα designates an angle between the incident surface and the first reflection surface of the first prism;ω designates a real field-of-view ω (half amount);ω′ designates an angle between a light ray of a real field-of-view ω (half amount) and the optical axis in the first prism; andn designates the refractive index of the first prism.

By satisfying conditions (3) and (2), light rays do not pass through the reflection surfaces in the first prism, so that reduction of peripheral illumination at the peripheral area of the view field, and ghosting caused by transmitting light rays can be avoided.

In the above-described embodiments, focusing is carried out by moving the rear lens group22of the objective optical system20(120,220,320,420,520) in a forward or rearward direction along the optical axis. Alternatively, the eyepiece optical system can be used as a focusing lens group.

According to such an internal-focusing arrangement, no movable members are exposed to outside, so that strength of a lens-frame structure can be increased with ease; and the volume of a member in which the image-erecting viewing optical system is contained does not vary, so that a water-proof structure can be easily achieved.

Furthermore, in the objective optical system20(120,220,320,420,520) including the front lens group21and the rear lens group22, it is preferable that the rear lens group22have a weak optical power for the purpose of reducing fluctuations of aberrations.

Condition (4) specifies the power of both the front lens group21and the rear lens group22.

If fO/f2exceeds the lower limit of condition (4), the traveling distance of the rear lens group22upon focusing becomes longer, so that the size of the optical system increases in order to secure enough space to cover the traveling distance of the rear lens group22.

If fO/f2exceeds the upper limit of condition (4), it becomes difficult to correct the change in aberrations upon focusing, and especially difficult to correct field curvature.

Specific numerical data of the embodiments will be described hereinafter. In the diagrams of chromatic aberration (on-axis chromatic aberration) represented by spherical aberration, the solid line and the two types of dotted lines respectively indicate spherical aberrations with respect to the d, g and C lines. Also, in the diagrams of lateral chromatic aberration, the two types of dotted lines respectively indicate magnification with respect Lo the g and C lines; however, the a line as the base line coincides with the ordinate. Furthermore, S designates the sagittal image, and M designates the meridional image; ER designates the diameter of the exit pupil, and B designates the apparent visual angle (°). In the tables, r designates the radius of curvature, d designates the lens-element thickness or distance between lens elements, Nd designates the refractive index of the d-line, and ν designates the Abbe number.

In addition to the above, an aspherical surface which is symmetrical with respect to the optical axis is defined as follows:
x=cy2/(1+[1−{1+K}c2y2]1/2)+A4y4+A6y6+A8y8+A10y10wherein:c designates a curvature of the aspherical vertex (1/r);y designates a distance from the optical axis;x designates an amount of change in the optical axis direction with respect to the distance “y” from the optical axis;K designates the conic coefficient; andA4 designates a fourth-order aspherical coefficient;A6 designates a sixth-order aspherical coefficient;A8 designates a eighth-order aspherical coefficient; andA10 designates a tenth-order aspherical coefficient.
[Numerical Embodiment 1]

FIGS. 8 through 10Dshow the image-erecting optical system according to the first numerical embodiment of the present invention.FIG. 8is a lens arrangement of the first numerical embodiment.FIGS. 9A through 9Dshow aberrations occurred in the lens arrangement shown inFIG. 8, when an object at an infinite distance is in an in-focus state.FIGS. 10A through 10Dshow aberrations occurred in the lens arrangement shown inFIG. 8, when an object at a distance of 1.2 m is in an in-focus state. Table 1 shows the numerical data of the first numerical embodiment. D7 (14.94;26.89) designates the distance d7 (FIG. 8) between the most object-side surface of the rear lens group22and the exit surface of the second prism30, when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.2 m is in an in-focus state. D9 (24.80; 12.85) designates the distance d9 (FIG. 8) between the most image-side surface of the rear lens group22and the most object-side surface of the eyepiece optical system50, when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.2 m is in an in-focus state.

TABLE 1Surface No.rdNdν1∞31.281.5688356.32∞1.00——359.2643.871.5163364.14−26.4781.601.6200436.35−67.6143.00——6∞54.211.5163364.17∞D7——8−14.3008.001.7847225.79−16.720D9——10*−250.0005.501.4917657.411−27.3630.50——12116.3721.501.8466623.81324.61110.961.5891361.214−24.6110.50——1523.8395.611.5891361.216−150.000———*designates the aspherical surface which is rotationally symmetrical with respect to the optical axis.

Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):

FIGS. 11 through 13Dshow the image-erecting optical system according to the second numerical embodiment of the present invention.FIG. 11is a lens arrangement of the second numerical embodiment.FIGS. 12A through 12Dshow aberrations occurred in the lens arrangement shown inFIG. 11, when an object at an infinite distance is in an in-focus state.FIGS. 13A through 13Dshow aberrations occurred in the lens arrangement shown inFIG. 11, when an object at a distance of 1.5 m is in an in-focus state. Table 2 shows the numerical data of the second numerical embodiment. D7 (14.18; 27.10) designates the distance d7 (FIG. 8) between the most object-side surface of the rear lens group22and the exit surface of the second prism30, when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.5 m is in an in-focus state. D9 (20.95; 8.03) designates the distance d9 (FIG. 8) between the most image-side surface of the rear lens group22and the most object-side surface of the eyepiece optical system50, when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.5 m is in an in-focus state.

Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):

FIGS. 14 through 16D show a third numerical embodiment of an image-erecting optical system according to the present inventionFIG. 14is a lens arrangement of the third numerical embodiment.FIGS. 15A through 15Dshow aberrations occurred in the lens arrangement shown inFIG. 14, when an object at an infinite distance is in an in-focus state.FIGS. 16A through 16Dshow aberrations occurred in the lens arrangement shown inFIG. 14, when an object at a distance of 1.5 m is in an in-focus state.

Table 3 shows the numerical data of the third numerical embodiment. D7 (13.89; 27.21) designates the distance d7 (FIG. 8) between the most object-side surface of the rear lens group22and the exit surface of the second prism30, when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.5 m is in an in-focus state. D9 (19.48; 6.16) designates the distance d9 (FIG. 8) between the most image-side surface of the rear lens group22and the most object-side surface of the eyepiece optical system50, when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.5 m is in an in-focus state.

TABLE 3Surface No.rdNdν1∞31.281.5688356.32∞3.00——348.7963.871.5163364.14−28.6151.601.6200436.35−84.8424.00——6∞54.211.5163364.17∞D7——8−17.1938.001.7847225.79−18.658D9——10*−120.0006.001.4917657.411−19.2120.96——12∞1.501.8466623.81325.3078.701.5891361.214−25.3071.04——1520.4145.801.5891361.216−100.000———*designates the aspherical surface which is rotationally symmetrical with respect to the optical axis.

Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):

Table 4 shows the numerical values of each condition for each numerical embodiment.

According to the above description, a miniaturized image-erecting optical system having a wide field-of-view and a low magnification can be achieved.