Patent Publication Number: US-2021181491-A1

Title: Optical system and optical apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of International Application No. PCT/JP2019/010087 filed on Mar. 12, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an optical system and an optical apparatus including the same. 
     Description of the Related Art 
     The numerical aperture is used as an indicator representing the brightness of an optical system and the resolving power of an optical system. The larger the value of numerical aperture is, the higher the brightness is and the higher the resolving power is. Furthermore, the F-number is also used as an indicator representing the brightness of an optical system and the resolving power of an optical system. The smaller the value of F-number is, the higher the brightness is and the higher the resolving power is. 
     Japanese Patent Application Laid-open No. 2012-48774 discloses an objective lens having a numerical aperture greater than 1. This objective lens includes a solid immersion lens having a super-hemispherical shape or a hemispherical shape. 
     Japanese Patent Application Laid-open No. 2017-207772 discloses an immersion objective lens having a numerical aperture of 1.3. Japanese Patent Application Laid-open No. 2018-66913 discloses a microscope immersion objective having a numerical aperture of 1.5. 
     For example, when an object is a luminous body, light coming toward the optical system and light going away from the optical system are emerged from the object. The brightness and the resolving power of the optical system are determined by the amount of light incident on the optical system. As light that can be incident on the optical system increases, the brightness of the optical system becomes high and the resolving power becomes high. 
     SUMMARY 
     An optical system according to at least some embodiments of the present disclosure is an optical system in which 
     an enlargement-side conjugate point positioned on an enlargement side and a reduction-side conjugate point positioned on a reduction side are conjugate, 
     a distance from the optical system to the enlargement-side conjugate point being longer than a distance from the optical system to the reduction-side conjugate point, 
     the optical system including a super-hemispherical meniscus lens, wherein 
     the super-hemispherical meniscus lens has a reduction-side surface positioned on the reduction side and an enlargement-side surface positioned on the enlargement side, 
     the reduction-side surface is a concave curved surface on the reduction side, 
     the enlargement-side surface is a convex curved surface on the enlargement side, having a positive refractive power, 
     the curved surface of the enlargement-side surface is a curved surface extending beyond a hemisphere, 
     an intersection of the reduction-side surface and an optical axis of the optical system is positioned closer to the enlargement side than the reduction-side conjugate point, and 
     a reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point, 
     where 
     the reduction-side intersection is an intersection of a reduction-side virtual plane and the optical axis of the optical system, 
     the reduction-side virtual plane is a plane including an intersection of an outside ray and the reduction-side surface and being orthogonal to the optical axis of the optical system, and 
     the outside ray is a light ray passing through a position farthest from a center of the reduction-side surface, among light rays contributing to image formation. 
     An optical apparatus according to at least some embodiments of the present disclosure includes: 
     the optical system described above and 
     an image pickup apparatus disposed at the reduction-side conjugate point. 
     Another optical apparatus according to at least some embodiments of the present disclosure includes: 
     the optical system described above and 
     a light source disposed at the reduction-side conjugate point. 
     Another optical apparatus according to at least some embodiments of the present disclosure includes: 
     the optical system described above and 
     a holding mechanism configured to position an object at the reduction-side conjugate point. 
     Another optical apparatus according to at least some embodiments of the present disclosure includes: 
     the optical system described above and a display device disposed at the enlargement-side conjugate point. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a first example of an optical system of the present embodiment; 
         FIG. 2  is a diagram illustrating a second example of the optical system of the present embodiment; 
         FIG. 3  is a diagram illustrating an outside ray in the optical system of the first example; 
         FIG. 4  is a diagram illustrating an outside ray in the optical system of the second example; 
         FIG. 5  is a lens sectional view of an optical system of Example 1; 
         FIG. 6  is a lens sectional view of an optical system of Example 2; 
         FIG. 7  is a partial sectional view of the optical system of Example 2; 
         FIG. 8  is a lens sectional view of an optical system of Example 3; 
         FIG. 9  is a partial sectional view of the optical system of Example 3; 
         FIG. 10  is a lens sectional view of an optical system of Example 4; 
         FIG. 11  is a partial sectional view of the optical system of Example 4; 
         FIG. 12  is a lens sectional view of an optical system of Example 5; 
         FIG. 13  is a partial sectional view of the optical system of Example 5; 
         FIG. 14  is a lens sectional view of an optical system of Example 6; 
         FIG. 15  is an aberration diagram of the optical system of Example 1; 
         FIG. 16  is an aberration diagram of the optical system of Example 3; 
         FIG. 17  is an aberration diagram of the optical system of Example 4; 
         FIG. 18  is an aberration diagram of the optical system of Example 5; 
         FIG. 19A  and  FIG. 19B  are diagrams illustrating a first example of an optical apparatus of the present embodiment; 
         FIG. 20A  and  FIG. 20B  are diagrams illustrating a second example of the optical apparatus of the present embodiment; 
         FIG. 21A  and  FIG. 21B  are diagrams illustrating a third example of the optical apparatus of the present embodiment; 
         FIG. 22A  and  FIG. 22B  are diagrams illustrating a holding member; and 
         FIG. 23  is a diagram illustrating a fourth example of the optical apparatus of the present embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Prior to the explanation of examples, action and effect of embodiments according to certain aspects of the present disclosure will be described below. In the explanation of the action and effect of the embodiments concretely, the explanation will be made by citing concrete examples. However, similar to a case of the examples to be described later, aspects exemplified thereof are only some of the aspects included in the present disclosure, and there exists a large number of variations in these aspects. Consequently, the present disclosure is not restricted to the aspects that will be exemplified. 
     An optical system of the present embodiment is an optical system in which an enlargement-side conjugate point positioned on an enlargement side and a reduction-side conjugate point positioned on a reduction side are conjugate. A distance from the optical system to the enlargement-side conjugate point is longer than a distance from the optical system to the reduction-side conjugate point. The optical system includes a super-hemispherical meniscus lens. The super-hemispherical meniscus lens has a reduction-side surface positioned on the reduction side and an enlargement-side surface positioned on the enlargement side. The reduction-side surface is a concave curved surface on the reduction side, and the enlargement-side surface is a convex curved surface on the enlargement side, having a positive refractive power. The curved surface of the enlargement-side surface is a curved surface extending beyond a hemisphere. An intersection of the reduction-side surface and an optical axis of the optical system is positioned closer to the enlargement side than the reduction-side conjugate point, and a reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point, 
     where 
     the reduction-side intersection is an intersection of a reduction-side virtual plane and the optical axis of the optical system, 
     the reduction-side virtual plane is a plane including an intersection of an outside ray and the reduction-side surface and being orthogonal to the optical axis of the optical system, and 
     the outside ray is a light ray passing through a position farthest from a center of the reduction-side surface, among light rays contributing to image formation. 
       FIG. 1  is a diagram illustrating a first example of the optical system of the present embodiment.  FIG. 2  is a diagram illustrating a second example of the optical system of the present embodiment. In the first example, the optical system includes one meniscus lens. In the second example, the optical system includes three meniscus lenses. 
     The optical system of the present embodiment is an optical system in which an enlargement-side conjugate point Po and a reduction-side conjugate point Pi are conjugate. The enlargement-side conjugate point Po and the reduction-side conjugate point Pi are positioned on an optical axis AX of the optical system. 
     The enlargement-side conjugate point Po is positioned on the enlargement side, and the reduction-side conjugate point Pi is positioned on the reduction side. The distance from the optical system to the enlargement-side conjugate point Po is longer than the distance from the optical system to the reduction-side conjugate point Pi. 
     In the optical system of the present embodiment, it is possible to position an object at the enlargement-side conjugate point Po. In this case, an optical image of the object is formed at the reduction-side conjugate point Pi. Thus, it is possible to use the optical system of the present embodiment, for example, as a photographic lens of a camera. The object is not necessarily positioned at the enlargement-side conjugate point Po. For example, the object may be positioned at infinity. 
     Furthermore, in the optical system of the present embodiment, it is possible to position an object at the reduction-side conjugate point Pi. In this case, an optical image of the object is formed at the enlargement-side conjugate point Po. Thus, it is possible to use the optical system of the present embodiment, for example, as an objective lens of a microscope, an eyepiece optical system of VR goggles, or a projection lens of a projector. The optical image is not necessarily formed at the enlargement-side conjugate point Po. For example, the optical image may be formed at infinity. 
     When a spherical surface is used for a lens surface, the lens surface is represented by an arc in the sectional view of the lens. It is possible to represent the range of the lens surface by the length of the arc. It is possible to represent the length of the arc by the angle (hereinafter referred to as “arc angle”) formed between both ends of the arc and the center of the circle. 
     Here, a curved surface having an arc angle greater than 180° is referred to as a curved surface extending beyond a hemisphere. A meniscus lens having a curved surface extending beyond a hemisphere is referred to as a super-hemispherical meniscus lens. 
     As illustrated in  FIG. 1 , the optical system of the first example has a meniscus lens LE. The meniscus lens LE has a reduction-side surface R 1  positioned on the reduction side and an enlargement-side surface R 2  positioned on the enlargement side. The reduction-side surface R 1  is a concave curved surface on the reduction side. The enlargement-side surface R 2  is a convex curved surface on the enlargement side, having a positive refractive power. 
     In the meniscus lens LE, the enlargement-side surface R 2  is a curved surface extending beyond a hemisphere. Thus, the meniscus lens LE is a super-hemispherical meniscus lens. 
     As illustrated in  FIG. 2 , the optical system of the second example includes a meniscus lens LE 1 , a meniscus lens LE 2 , and a meniscus lens LE 3 . 
     The meniscus lens LE 1  has a reduction-side surface R 11  positioned on the reduction side and an enlargement-side surface R 12  positioned on the enlargement side. The reduction-side surface R 11  is a concave curved surface on the reduction side. The enlargement-side surface R 12  is a convex curved surface on the enlargement side, having a positive refractive power. 
     In the meniscus lens LE 1 , the enlargement-side surface R 12  is a curved surface extending beyond a hemisphere. Thus, the meniscus lens LE 1  is a super-hemispherical meniscus lens. 
     The meniscus lens LE 2  has a reduction-side surface R 21  positioned on the reduction side and an enlargement-side surface R 22  positioned on the enlargement side. The reduction-side surface R 21  is a concave curved surface on the reduction side. The enlargement-side surface R 22  is a convex curved surface on the enlargement side, having a positive refractive power. 
     In the meniscus lens LE 2 , the enlargement-side surface R 22  is a curved surface extending beyond a hemisphere. Thus, the meniscus lens LE 2  is a super-hemispherical meniscus lens. 
     The meniscus lens LE 3  has a reduction-side surface R 31  positioned on the reduction side and an enlargement-side surface R 32  positioned on the enlargement side. The reduction-side surface R 31  is a concave curved surface on the reduction side. The enlargement-side surface R 32  is a convex curved surface on the enlargement side, having a positive refractive power. 
     In the meniscus lens LE 3 , the enlargement-side surface R 32  is a curved surface extending beyond a hemisphere. Thus, the meniscus lens LE 3  is a super-hemispherical meniscus lens. 
     As described above, the enlargement-side conjugate point Po and the reduction-side conjugate point Pi are conjugate. When a luminous body is disposed at the enlargement-side conjugate point Po, light is incident on the enlargement-side surface and thereafter emerged from the reduction-side surface. The light emerged from the reduction-side surface reaches the reduction-side conjugate point Pi. 
     On the other hand, when a luminous body is disposed at the reduction-side conjugate point Pi, light is incident on the reduction-side surface and thereafter emerged from the enlargement-side surface. The light emerged from the enlargement-side surface reaches the enlargement-side conjugate point Po. In the following description, it is assumed that a luminous body is disposed at the reduction-side conjugate point Pi. 
       FIG. 3  is a diagram illustrating an outside ray in the optical system of the first example. When a luminous body is disposed at the reduction-side conjugate point Pi, light coming toward the meniscus lens LE and light going away from the meniscus lens LE are emitted from the reduction-side conjugate point Pi. The light coming toward the meniscus lens LE is light traveling toward the enlargement side, and the light going away from the meniscus lens LE is light traveling toward the reduction side. 
     If it is possible to make the light going away from the meniscus lens LE as well as the light coming toward the meniscus lens LE to contribute to image formation, it is possible to implement an optical system that is bright and has a high resolving power. 
     As illustrated in  FIG. 3 , an intersection Pc 1  of the reduction-side surface R 1  and the optical axis AX is positioned closer to the enlargement side than the reduction-side conjugate point Pi. In this case, a part of the reduction-side surface R 1  is positioned in the traveling direction of light coming toward the meniscus lens LE. Therefore, in the optical system of the first example, it is possible to make light coming toward the meniscus lens LE to contribute to image formation. 
     A reduction-side intersection P 1  is the intersection of a reduction-side virtual plane IP 1  and the optical axis AX. The reduction-side virtual plane IP 1  is a plane including an intersection PR 1  of an outside ray LBm and the reduction-side surface R 1  and being orthogonal to the optical axis AX. The outside ray LBm is a light ray passing through a position farthest from the center of the reduction-side surface R 1 , among light rays contributing to image formation. As just described, the reduction-side intersection P 1  is determined by the outside ray LBm. The center of the reduction-side surface R 1  is the intersection Pc 1 . 
     When the reduction-side intersection P 1  is positioned closer to the enlargement side than the reduction-side conjugate point Pi, the outside ray LBm comes toward the meniscus lens LE. When the reduction-side intersection P 1  is positioned closer to the reduction side than the reduction-side conjugate point Pi, the outside ray LBm goes away from the meniscus lens LE. As just described, the position of the reduction-side intersection P 1  and the position of the reduction-side conjugate point Pi are relevant to the direction of the outside ray LBm. 
     As the reduction-side intersection P 1  goes away from the reduction-side conjugate point Pi toward the reduction side, an angle θ increases. The angle θ is the angle formed between a reduction-side conjugate plane PLi and the outside ray LBm. As the angle θ increases, the proportion of light rays reaching the reduction-side surface of the meniscus lens LE increases. As a result, the quantity of light increases. 
     In the optical system of the first example, the reduction-side intersection P 1  is positioned closer to the reduction side than the reduction-side conjugate point Pi is. Thus, the outside ray LBm is a light ray going away from the meniscus lens LE. Since the outside ray LBm reaches the reduction-side surface R 1 , the light ray going away from the meniscus lens LE reaches the reduction-side surface R 1 . Since the outside ray LBm is a light ray contributing to image formation, it is possible to make the light ray going away from the meniscus lens LE to contribute to image formation in the optical system of the first example. 
     As just described, in the optical system of the first example, it is possible to make light going away from the optical system as well as light coming toward the optical system to contribute to image formation. Thus, it is possible to implement an optical system that is bright and has a high resolving power. 
       FIG. 4  is a diagram illustrating an outside ray in the optical system of the second example. When a luminous body is disposed at the reduction-side conjugate point Pi, light coming toward the meniscus lens LE 1  and light going away from the meniscus lens LE 1  are emitted from the reduction-side conjugate point Pi. If it is possible to make the light going away from the meniscus lens LE 1  as well as the light coming toward the meniscus lens LE 1  to contribute to image formation, it is possible to implement an optical system that is bright and has a high resolving power. 
     The meniscus lens LE 1  is described. The intersection of the reduction-side surface of the meniscus lens LE 1  and the optical axis is positioned closer to the enlargement side than the reduction-side conjugate point Pi. In this case, a part of the reduction-side surface of the meniscus lens LE 1  is positioned in the traveling direction of light coming toward the meniscus lens LE 1 . Therefore, in the optical system of the second example, it is possible to make light coming toward the meniscus lens LE 1  to contribute to image formation. 
     A reduction-side intersection of the meniscus lens LE 1  is the intersection of a reduction-side virtual plane of the meniscus lens LE 1  and the optical axis. The reduction-side virtual plane of the meniscus lens LE 1  is a plane including the intersection of an outside ray LBm 1  and the reduction-side surface of the meniscus lens LE 1  and being orthogonal to the optical axis. The outside ray LBm 1  is a light ray passing through a position farthest from the center of the reduction-side surface of the meniscus lens LE 1 , among light rays contributing to image formation. The reduction-side intersection of the meniscus lens LE 1  is determined by the outside ray LBm 1 . 
     When the reduction-side intersection is positioned closer to the enlargement side than the reduction-side conjugate point Pi, the outside ray LBm 1  comes toward the meniscus lens LE 1 . When the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi, the outside ray LBm 1  goes away from the meniscus lens LE 1 . As just described, the position of the reduction-side intersection and the position of the reduction-side conjugate point Pi are relevant to the direction of the outside ray LBm 1 . 
     As the reduction-side intersection goes away from the reduction-side conjugate point Pi toward the reduction side, an angle θ 1  increases. The angle θ 1  is the angle formed between the reduction-side conjugate plane and the outside ray LBm 1 . As the angle θ 1  increases, the proportion of light rays reaching the reduction-side surface of the meniscus lens LE 1  increases. As a result, the quantity of light increases. 
     In the meniscus lens LE 1 , the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi is. Thus, the outside ray LBm 1  is a light ray going away from the meniscus lens LE 1 . Since the outside ray LBm 1  reaches the reduction-side surface of the meniscus lens LE 1 , it follows that the light ray going away from the meniscus lens LE 1  reaches the reduction-side surface of the meniscus lens LE 1 . Since the outside ray LBm 1  is a light ray contributing to image formation, it is possible that the light ray going away from the meniscus lens LE 1  contributes to image formation in the optical system of the second example. 
     The meniscus lens LE 2  is described. A reduction-side intersection of the meniscus lens LE 2  is the intersection of a reduction-side virtual plane of the meniscus lens LE 2  and the optical axis. The reduction-side virtual plane of the meniscus lens LE 2  is a plane including the intersection of an outside ray LBm 2  and the reduction-side surface of the meniscus lens LE 2  and being orthogonal to the optical axis. The outside ray LBm 2  is a light ray passing through a position farthest from the center of the reduction-side surface of the meniscus lens LE 2 , among light rays contributing to image formation. The reduction-side intersection of the meniscus lens LE 2  is determined by the outside ray LBm 2 . 
     The reduction-side surface of the meniscus lens LE 2  is not positioned closest to the reduction-side conjugate point Pi. Thus, in the meniscus lens LE 2 , the position of the reduction-side intersection and the position of the reduction-side conjugate point Pi are relevant to the direction of the outside ray LBm 2 . 
     In the meniscus lens LE 2 , the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Therefore, compared with when the reduction-side intersection is positioned closer to the enlargement side than the reduction-side conjugate point Pi, it is possible to reduce change in direction of the outside ray LBm 2  relative to the direction of the outside ray LBm 1 . 
     If the change in direction of the outside ray LBm 2  relative to the direction of the outside ray LBm 1  is large, a large aberration occurs. In order to reduce the change in direction, for example, the direction of the outside ray LBm 1  may be changed such that the angle θ 1  is reduced. However, in this case, the proportion of light rays reaching the reduction-side surface of the meniscus lens LE 1  is reduced. As a result, the quantity of light is reduced. 
     In the meniscus lens LE 2 , it is possible to reduce the change in direction of the outside ray LBm 2  relative to the direction of the outside ray LBm 1  without reducing the angle θ 1 . Thus, it is possible to suppress reduction of the quantity of light and to suppress occurrence of aberration. 
     The meniscus lens LE 3  is described. A reduction-side intersection of the meniscus lens LE 3  is the intersection of a reduction-side virtual plane of the meniscus lens LE 3  and the optical axis. The reduction-side virtual plane of the meniscus lens LE 3  is a plane including the intersection of an outside ray LBm 3  and the reduction-side surface of the meniscus lens LE 3  and being orthogonal to the optical axis. The outside ray LBm 3  is a light ray passing through a position farthest from the center of the reduction-side surface of the meniscus lens LE 3 , among light rays contributing to image formation. The reduction-side intersection of the meniscus lens LE 3  is determined by the outside ray LBm 3 . 
     The reduction-side surface of the meniscus lens LE 3  is not positioned closest to the reduction-side conjugate point Pi. Thus, in the meniscus lens LE 3 , the position of the reduction-side intersection and the position of the reduction-side conjugate point Pi are relevant to the direction of the outside ray LBm 3 . 
     In the meniscus lens LE 3 , the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Therefore, compared with when the reduction-side intersection is positioned closer to the enlargement side than the reduction-side conjugate point Pi, it is possible to reduce change in direction of the outside ray LBm 3  relative to the direction of the outside ray LBm 2 . 
     If the change in direction of the outside ray LBm 3  relative to the direction of the outside ray LBm 2  is large, a large aberration occurs. In order to reduce the change in direction, for example, the direction of the outside ray LBm 1  may be changed such that the angle θ 1  is reduced. However, in this case, the proportion of light rays reaching the reduction-side surface of the meniscus lens LE 1  is reduced. As a result, the quantity of light is reduced. 
     In the meniscus lens LE 3 , it is possible to reduce the change in direction of the outside ray LBm 3  relative to the direction of the outside ray LBm 2  without reducing the angle θ 1 . Thus, it is possible to suppress reduction of the quantity of light and to suppress occurrence of aberration. 
     As just described, in the optical system of the second example, it is possible to make light going away from the optical system as well as light coming toward the optical system to contribute to image formation. Thus, it is possible to implement an optical system that is bright and has a high resolving power. In addition, it is possible to implement an optical system in which occurrence of aberration is suppressed. 
     In the optical system of the present embodiment, it is preferable that the reduction-side conjugate point be positioned inside a spherical segment of the reduction-side surface. 
     A spherical segment surface is a surface of a spherical surface partially cut off. In the optical system of the first example, as illustrated in  FIG. 3 , the sectional profile of the reduction-side surface R 1  is defined by an arc connecting the intersection Pc 1 , the intersection PR 1 , and a tip end TP. The surface of a spherical surface partially cut off is formed by rotating this arc around the optical axis AX. Thus, it is possible to say that the reduction-side surface R 1  is a spherical segment surface. 
     In the optical system of the first example, the reduction-side conjugate point Pi is positioned closer to the enlargement side than the tip end TP. The enlargement side relative to the tip end TP is inside the spherical segment surface. Thus, it is possible to say that the reduction-side conjugate point Pi is positioned inside the spherical segment surface. 
     When the reduction-side conjugate point Pi is positioned inside the spherical segment surface, the intersection Pc 1  is positioned closer to the enlargement side than the reduction-side conjugate point Pi, and the reduction-side intersection P 1  is positioned closer to the reduction side than the reduction-side conjugate point Pi. In this case, it is possible to make light going away from the optical system as well as light coming toward the optical system to contribute to image formation. Thus, it is possible to implement an optical system that is bright and has a high resolving power. 
     It is possible to make the reduction-side surface R 1  an aspheric surface. In this case, the spherical segment surface is represented based on a sphere represented by a paraxial radius of curvature. 
     In the optical system of the present embodiment, it is preferable that an enlargement-side intersection be positioned closer to the reduction side than the reduction-side conjugate point, 
     where 
     the enlargement-side intersection is an intersection of an enlargement-side virtual plane and the optical axis of the optical system, 
     the enlargement-side virtual plane is a plane including an intersection of a predetermined outside ray and the enlargement-side surface and being orthogonal to the optical axis of the optical system, and 
     the predetermined outside ray is a light ray after the outside ray passes through the reduction-side surface. 
     The optical system of the first example is described. As illustrated in  FIG. 3 , an enlargement-side intersection P 2  is the intersection of an enlargement-side virtual plane IP 2  and the optical axis AX. The enlargement-side virtual plane IP 2  is a plane including an intersection PR 2  of an outside ray LBm′ and the enlargement-side surface R 2  and being orthogonal to the optical axis AX. As just described, the enlargement-side intersection P 2  is determined by the outside ray LBm′. The outside ray LBm′ is a light ray after the outside ray LBm passes through the reduction-side surface R 1 . 
     As described above, the outside ray LBm is a light ray passing through the position farthest from the center of the reduction-side surface, among light rays contributing to image formation. Thus, it is possible to say that the outside ray LBm′ is a light ray passing through the position farthest from the center of the enlargement-side surface, among light rays contributing to image formation. 
     The enlargement-side surface R 2  is not positioned closest to the reduction-side conjugate point Pi. Thus, the position of the enlargement-side intersection P 2  and the position of the reduction-side conjugate point Pi are relevant to the outside ray LBm′. 
     In the meniscus lens LE, the enlargement-side intersection P 2  is positioned closer to the reduction side than the reduction-side conjugate point Pi is. Therefore, compared with when the enlargement-side intersection P 2  is positioned closer to the enlargement side than the reduction-side conjugate point Pi, it is possible to reduce change in direction of the outside ray LBm′ relative to the direction of the outside ray LBm. 
     If the change in direction of the outside ray LBm′ relative to the direction of the outside ray LBm is large, a large aberration occurs. In order to reduce the change in direction, for example, the direction of the outside ray LBm may be changed such that the angle θ is reduced. However, in this case, the proportion of light rays reaching the reduction-side surface R 1  is reduced. As a result, the quantity of light is reduced. 
     In the meniscus lens LE, it is possible to reduce the change in direction of the outside ray LBm′ relative to the direction of the outside ray LBm without reducing the angle θ. Thus, it is possible to suppress reduction of the quantity of light and to suppress occurrence of aberration. 
     As just described, in the optical system of the first example, it is possible to make light going away from the optical system as well as light coming toward the optical system to contribute to image formation. Thus, it is possible to implement an optical system that is bright and has a high resolving power. 
     The enlargement-side surface R 2  is not positioned closest to the reduction-side conjugate point Pi, similarly to the reduction-side surface of the meniscus lens LE 1 , the reduction-side surface of the meniscus lens LE 2 , and the reduction-side surface of the meniscus lens LE 3 . Thus, in the enlargement-side surface R 2 , an operation effect similar to that of these reduction surfaces is produced. 
     The optical system of the second example is described. However, a detailed description is omitted. The meniscus lens LE 1  is described, and a description of the meniscus lens LE 2  and the meniscus lens LE 3  is omitted. 
     In the meniscus lens LE 1 , the enlargement-side intersection is the intersection of the enlargement-side virtual plane and the optical axis AX. The enlargement-side virtual plane is a plane including the intersection of a predetermined outside ray and the enlargement-side surface and being orthogonal to the optical axis AX. As just described, the enlargement-side intersection is determined by the predetermined outside ray. The predetermined outside ray is a light ray after the outside ray LBm 1  passes through the reduction-side surface. 
     As described above, the outside ray LBm 1  is a light ray contributing to image formation. Since the predetermined outside ray is a light ray after the outside ray LBm 1  passes through the reduction-side surface, the predetermined outside ray is a light ray contributing to image formation. The outside ray LBm 1  is a light ray passing through the position farthest from the center of the reduction-side surface. Thus, the predetermined outside ray passes through the position farthest from the center of the enlargement-side surface. 
     The enlargement-side surface of the meniscus lens LE 1  is not positioned closest to the reduction-side conjugate point Pi. Thus, the position of the enlargement-side intersection and the position of the reduction-side conjugate point Pi are relevant to the direction of the predetermined outside ray. 
     In the meniscus lens LE 1 , the enlargement-side intersection is positioned closer to the reduction side than the reduction-side conjugate point. Therefore, compared with when the enlargement-side intersection is positioned closer to the enlargement side than the reduction-side conjugate point, it is possible to reduce change in direction of the predetermined outside ray relative to the direction of the outside ray LBm 1 . 
     If the change in direction of the predetermined outside ray relative to the direction of the outside ray LBm 1  is large, a large aberration occurs. In order to reduce the change in direction, for example, the direction of the outside ray LBm 1  may be changed such that the angle θ 1  is reduced. However, in this case, the proportion of light rays reaching the reduction-side surface is reduced. As a result, the quantity of light is reduced. 
     In the meniscus lens LE, it is possible to reduce the change in direction of the outside ray LBm′ relative to the direction of the outside ray LBm without reducing the angle θ. Thus, it is possible to suppress reduction of the quantity of light and to suppress occurrence of aberration. 
     The enlargement-side surface of the meniscus lens LE 1  is not positioned closest to the reduction-side conjugate point Pi, similarly to the reduction-side surface of the meniscus lens LE 1 , the reduction-side surface of the meniscus lens LE 2 , and the reduction-side surface of the meniscus lens LE 3 . Thus, in the enlargement-side surface of the meniscus lens LE 1 , an operation effect similar to that of these reduction surfaces is produced. 
     The enlargement-side surface of the meniscus lens LE 2  and the enlargement-side surface of the meniscus lens LE 3  are also not positioned closest to the reduction-side conjugate point Pi. Thus, in the enlargement-side surface of the meniscus lens LE 2  and the enlargement-side surface of the meniscus lens LE 3 , an operation effect similar to that of the enlargement-side surface of the meniscus lens LE 1  is produced. 
     As just described, in the optical system of the second example, it is possible to make light going away from the optical system as well as light coming toward the optical system to contribute to image formation. Thus, it is possible to implement an optical system that is bright and has a high resolving power. 
     In the optical system of the present embodiment, it is preferable that the following Conditional Expression (1) be satisfied: 
       0 (mm)&lt; D 1sag  (1)
 
     where 
     D1sag is a distance between the reduction-side conjugate point and the reduction-side intersection. 
     Conditional Expression (1) is a conditional expression for the amount of protrusion of the reduction-side surface toward the reduction side. The amount of protrusion is the distance between the reduction-side conjugate point Pi and the reduction-side intersection and determined by the reduction-side conjugate point Pi as a reference. When the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi, the amount of protrusion has a positive value. 
     When Conditional Expression (1) is satisfied, it is possible to make light going away from the optical system to contribute to image formation. Thus, it is possible to implement an optical system that is bright and has a high resolving power. 
       FIG. 3  illustrates the amount of protrusion of the reduction-side surface in the optical system of the first example. The amount of protrusion D1sag is the distance between the reduction-side conjugate point Pi and the reduction-side intersection P 1 . Since the reduction-side intersection P 1  is positioned closer to the reduction side than the reduction-side conjugate point Pi, the amount of protrusion D1sag has a positive value. 
       FIG. 4  illustrates the amount of protrusion of the reduction-side surface in the optical system of the second example. The amount of protrusion D1sag in the meniscus lens LE 1  is the distance between the reduction-side conjugate point Pi and the reduction-side intersection of the meniscus lens LE 1 . In the meniscus lens LE 1 , the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Thus, the amount of protrusion D1sag has a positive value. 
     The amount of protrusion D1sag in the meniscus lens LE 2  is the distance between the reduction-side conjugate point Pi and the reduction-side intersection of the meniscus lens LE 2 . In the meniscus lens LE 2 , the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Thus, the amount of protrusion D1sag has a positive value. 
     The amount of protrusion D1sag in the meniscus lens LE 3  is the distance between the reduction-side conjugate point Pi and the reduction-side intersection of the meniscus lens LE 3 . In the meniscus lens LE 3 , the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Thus, the amount of protrusion D1sag has a positive value. 
     In the optical system of the present embodiment, it is preferable that the following Conditional Expression (2) be satisfied: 
       0 (mm)&lt; D 2sag  (2)
 
     where 
     D2sag is a distance between the reduction-side conjugate point and the enlargement-side intersection. 
     Conditional Expression (2) is a conditional expression for the amount of protrusion of the enlargement-side surface toward the reduction side. The amount of protrusion is the distance between the reduction-side conjugate point Pi and the enlargement-side intersection and determined by the reduction-side conjugate point Pi as a reference. When the enlargement-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi, the amount of protrusion has a positive value. 
     When Conditional Expression (2) is satisfied, it is possible to make light going away from the optical system to contribute to image formation. Thus, it is possible to implement an optical system that is bright and has a high resolving power. 
       FIG. 3  illustrates the amount of protrusion of the enlargement-side surface in the optical system of the first example. The amount of protrusion D2sag is the distance between the reduction-side conjugate point Pi and the enlargement-side intersection P 2 . Since the enlargement-side intersection P 2  is positioned closer to the reduction side than the reduction-side conjugate point Pi, the amount of protrusion D2sag has a positive value. 
       FIG. 4  illustrates the amount of protrusion of the enlargement-side surface in the optical system of the second example. The amount of protrusion D2sag in the meniscus lens LE 1  is the distance between the reduction-side conjugate point Pi and the enlargement-side intersection of the meniscus lens LE 1 . In the meniscus lens LE 1 , the enlargement-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Thus, the amount of protrusion D2sag has a positive value. 
     The amount of protrusion D2sag in the meniscus lens LE 2  is the distance between the reduction-side conjugate point Pi and the enlargement-side intersection of the meniscus lens LE 2 . In the meniscus lens LE 2 , the enlargement-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Thus, the amount of protrusion D2sag has a positive value. 
     The amount of protrusion D2sag in the meniscus lens LE 3  is the distance between the reduction-side conjugate point Pi and the enlargement-side intersection of the meniscus lens LE 3 . In the meniscus lens LE 3 , the enlargement-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Thus, the amount of protrusion D2sag has a positive value. 
     The technical significance of Conditional Expression (1) and the technical significance of Conditional Expression (2) are on the premise that a luminous body is disposed at the reduction-side conjugate point Pi. It is also possible to dispose a luminous body at the enlargement-side conjugate point Po, as described above. In this case, light reaches the reduction-side conjugate point Pi from the enlargement side. Thus, it follows that “light going away from the optical system” is “light reaching the optical system from a position closer to the reduction side than the reduction-side conjugate point Pi”. 
     It is preferable that the optical system of the present embodiment include at least one super-hemispherical meniscus lens that satisfies both of Conditional Expression (1) and Conditional Expression (2). 
     The technical significance of Conditional Expression (1) and the technical significance of Conditional Expression (2) are as described above. 
     When at least two super-hemispherical meniscus lenses are disposed in the optical system, it is possible to refract light using at least four refraction surfaces. In this case, since it is possible to bend light little by little, it is possible to suppress occurrence of spherical aberration. Furthermore, it is possible to use glass materials having different optical characteristics for the glass materials of the super-hemispherical meniscus lenses. Therefore, it is possible to favorably correct chromatic aberration. 
     In the optical system of the present embodiment, it is preferable that a predetermined lens be the super-hemispherical meniscus lens positioned closest to the reduction-side conjugate point, and the following Conditional Expression (3) be satisfied: 
       0&lt; R 1/ F&lt; 0.6  (3)
 
     where 
     R1 is a radius of curvature of the reduction-side surface of the predetermined lens, and 
     F is a focal length of the optical system. 
     Conditional Expression (3) is a conditional expression for the radius of curvature of the reduction-side surface. Light going away from the optical system reaches the reduction-side surface. Thus, it is possible to say that Conditional Expression (3) is a conditional expression for making light going away from the optical system to reach the reduction-side surface. 
     In a case in which a value falls below a lower limit value of Conditional Expression (3), the reduction-side surface becomes a convex curved surface on the reduction side. In the optical system of the present embodiment, the reduction-side surface must be a concave curved surface on the reduction side. Thus, the value does not fall below the lower limit value of Conditional Expression (3). 
     In a case in which the value exceeds an upper limit value of Conditional Expression (3), the radius of curvature of the reduction-side surface becomes too large. In this case, even when the reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point, it is not possible to increase the distance between the reduction-side intersection and the reduction-side conjugate point. Therefore, light rays that can reach the reduction-side surface among light rays going away from the optical system become fewer. 
     In the optical system of the present embodiment, it is preferable that a predetermined lens be the super-hemispherical meniscus lens positioned closest to the reduction-side conjugate point, and the following Conditional Expression (4) be satisfied: 
       0&lt; R 2/ F&lt; 0.6  (4)
 
     where 
     R2 is a radius of curvature of the enlargement-side surface of the predetermined lens, and 
     F is a focal length of the optical system. 
     Conditional Expression (4) is a conditional expression for the radius of curvature of the enlargement-side surface. Light reaching the reduction-side surface is directed toward the enlargement-side surface. Conditional Expression (4) is a conditional expression for making light to reach the enlargement-side surface from the reduction-side surface. 
     In a case in which a value falls below a lower limit value of Conditional Expression (4), the enlargement-side surface becomes a concave curved surface on the enlargement side. In the optical system of the present embodiment, the enlargement-side surface must be a convex curved surface on the enlargement side. Thus, the value does not fall below the lower limit value of Conditional Expression (4). 
     In a case in which the value exceeds an upper limit value of Conditional Expression (4), the radius of curvature of the enlargement-side surface becomes too large. As described above, the enlargement-side surface is a curved surface extending beyond a hemisphere. Therefore, when the radius of curvature of the enlargement-side surface is too large, the portion extending beyond a hemisphere becomes small. 
     When light travels from the enlargement side to the reduction-side conjugate point, it is difficult to refract light reaching the enlargement-side surface greatly on the enlargement-side surface, if the portion extending beyond a hemisphere is small. Therefore, total reflection occurs on the reduction-side surface. As a result, it is not possible to make light to travel from the reduction-side surface toward the reduction-side conjugate point. 
     Embodiments and examples of an optical system will be described below in detail by referring to the accompanying diagrams. However, the present disclosure is not restricted to the embodiments and the examples described below. 
     An optical system of Example 1 is illustrated in  FIG. 5 .  FIG. 5  is a lens sectional view of the optical system of Example 1. In  FIG. 5 , the left side in the drawing sheet is the enlargement side and the right side is the reduction side.  FIG. 15  is an aberration diagram of spherical aberration of the optical system of Example 1. 
     The optical system of Example 1 is an image pickup optical system. In the optical system of Example 1, an object is positioned on the enlargement side. Thus, an optical image I of the object is formed at a reduction-side conjugate point.  FIG. 5  illustrates a state in which light from the object positioned at infinity is collected at the reduction-side conjugate point. 
     The image pickup optical system includes, in order from the enlargement side, a negative meniscus lens L 1  having a convex surface facing the enlargement side, a negative meniscus lens L 2  having a convex surface facing the enlargement side, a negative meniscus lens L 3  having a convex surface facing the enlargement side, and a negative meniscus lens L 4  having a convex surface facing the enlargement side. An aperture stop S is positioned closer to the reduction side than a surface apex of an enlargement-side surface of the negative meniscus lens L 1 . 
     An enlargement-side surface of the negative meniscus lens L 2 , an enlargement-side surface of the negative meniscus lens L 3 , and an enlargement-side surface of the negative meniscus lens L 4  are curved surfaces extending beyond a hemisphere. Thus, the negative meniscus lens L 2 , the negative meniscus lens L 3 , and the negative meniscus lens L 4  are super-hemispherical meniscus lenses. In the optical system of Example 1, three super-hemispherical meniscus lenses are disposed on the reduction side. 
     An aspheric surface is provided at the enlargement-side surface of the negative meniscus lens L 1 . 
     An optical system of Example 2 is illustrated in  FIG. 6 .  FIG. 6  is a lens sectional view of the optical system of Example 2. Furthermore, a partial sectional view of the optical system of Example 2 is illustrated in  FIG. 7 .  FIG. 7  illustrates lenses positioned near a reduction-side conjugate point Pi. In  FIG. 6  and  FIG. 7 , the left side in the drawing sheet is the enlargement side and the right side is the reduction side. 
     The optical system of Example 2 is an illumination optical system. In the optical system of Example 2, an object is positioned on the reduction side, for example, at the reduction-side conjugate point Pi. Thus, an optical image of the object is formed on the enlargement side.  FIG. 6  illustrates a state in which light from the object, for example, light emitted from a light source becomes parallel light on the enlargement side. Furthermore, an optical image I in the optical system of Example 1 is illustrated. In the optical system of Example 2, an object is disposed at the position of the optical image I. 
     The illumination optical system includes, in order from the enlargement side, a negative meniscus lens L 1  having a convex surface facing the enlargement side, a negative meniscus lens L 2  having a convex surface facing the enlargement side, a negative meniscus lens L 3  having a convex surface facing the enlargement side, and a negative meniscus lens L 4  having a convex surface facing the enlargement side. An aperture stop S is positioned at a surface apex of an enlargement-side surface of the negative meniscus lens L 1 . 
     An enlargement-side surface of the negative meniscus lens L 3  and an enlargement-side surface of the negative meniscus lens L 4  are curved surfaces extending beyond a hemisphere. Thus, the negative meniscus lens L 3  and the negative meniscus lens L 4  are super-hemispherical meniscus lenses. In the optical system of Example 2, two super-hemispherical meniscus lenses are disposed on the reduction side. 
     An aspheric surface is provided at the enlargement-side surface of the negative meniscus lens L 1 . 
       FIG. 7  illustrates the amount of protrusion in the negative meniscus lens L 3  and the amount of protrusion in the negative meniscus lens L 4 . 
     An optical system of Example 3 is illustrated in  FIG. 8 .  FIG. 8  is a lens sectional view of the optical system of Example 3. Furthermore, a partial sectional view of the optical system of Example 3 is illustrated in  FIG. 9 .  FIG. 9  illustrates lenses positioned near a reduction-side conjugate point Pi. In  FIG. 8  and  FIG. 9 , the left side in the drawing sheet is the reduction side and the right side is the enlargement side.  FIG. 16  is an aberration diagram of spherical aberration of the optical system of Example 3. 
     The optical system of Example 3 is a dry-type microscope objective lens. In the optical system of Example 3, an object is positioned on the reduction side, for example, at the reduction-side conjugate point Pi. Thus, an optical image of the object is formed on the enlargement side. The optical system of Example 3 is an infinity-corrected objective lens.  FIG. 8  illustrates a state in which light from the object becomes parallel light on the enlargement side. 
     The microscope objective lens includes, in order from the reduction side, a negative meniscus lens L 1  having a convex surface facing the enlargement side, a negative meniscus lens L 2  having a convex surface facing the enlargement side, a biconvex positive lens L 3 , and a biconvex positive lens L 4 . 
     The microscope objective lens further includes a biconvex positive lens L 5 , a biconcave negative lens L 6 , a biconvex positive lens L 7 , a biconvex positive lens L 8 , a negative meniscus lens L 9  having a convex surface facing the enlargement side, a negative meniscus lens L 10  having a convex surface facing the reduction side, and a positive meniscus lens L 11  having a convex surface facing the reduction side. 
     The biconvex positive lens L 5 , the biconcave negative lens L 6 , and the biconvex positive lens L 7  are cemented. The biconvex positive lens L 8  and the negative meniscus lens L 9  are cemented. The negative meniscus lens L 10  and the positive meniscus lens L 11  are cemented. 
     The microscope objective lens further includes a negative meniscus lens L 12  having a convex surface facing the reduction side, a biconvex positive lens L 13 , a negative meniscus lens L 14  having a convex surface facing the enlargement side, a biconcave negative lens L 15 , a biconvex positive lens L 16 , a biconcave negative lens L 17 , and a biconvex positive lens L 18 . 
     The negative meniscus lens L 12 , the biconvex positive lens L 13 , and the negative meniscus lens L 14  are cemented. The biconcave negative lens L 15  and the biconvex positive lens L 16  are cemented. The biconcave negative lens L 17  and the biconvex positive lens L 18  are cemented. 
     As illustrated in  FIG. 9 , an enlargement-side surface of the negative meniscus lens L 1  is a curved surface extending beyond a hemisphere. Thus, the negative meniscus lens L 1  is a super-hemispherical meniscus lens. In the optical system of Example 3, one super-hemispherical meniscus lens is disposed on the reduction side. 
     A method of calculating a resolving power of the optical system includes a method of calculating a resolving power of the optical system using a numerical aperture and a method of calculating a resolving power of the optical system using a pupil diameter. Both methods are methods by approximation. 
     In the method of calculating a resolving power of the optical system using a numerical aperture, the resolving power δ is given by the following Expression (A): 
       δ=(0.61×λ)/ NA   (A)
 
     where 
     λ is a wavelength, and 
     NA is a numerical aperture. 
     In the method of calculating a resolving power of the optical system using a pupil diameter, the resolving power δ is given by the following Expression (B): 
       δ=1.22λ/ D   (B)
 
     where 
     λ is a wavelength, and 
     D is a pupil diameter. 
     In the optical system of Example 3, since the angular aperture exceeds 90°, the resolving power δ is calculated using a pupil diameter. The larger the pupil diameter is, the smaller the resolving power δ is. In the optical system of Example 3, the resolving power δ is calculated using the pupil diameter on the enlargement side. 
     An optical system of Example 4 is illustrated in  FIG. 10 .  FIG. 10  is a lens sectional view of the optical system of Example 4. Furthermore, a partial sectional view of the optical system of Example 4 is illustrated in  FIG. 11 .  FIG. 11  illustrates lenses positioned near a reduction-side conjugate point Pi. In  FIG. 10  and  FIG. 11 , the left side in the drawing sheet is the reduction side and the right side is the enlargement side.  FIG. 17  is an aberration diagram of spherical aberration of the optical system of Example 4. 
     The optical system of Example 4 is a water immersion-type microscope objective lens. In the optical system of Example 4, an object is positioned on the reduction side, for example, at the reduction-side conjugate point Pi. Thus, an optical image of the object is formed on the enlargement side. The optical system of Example 4 is an infinity-corrected objective lens.  FIG. 10  illustrates a state in which light from the object becomes parallel light on the enlargement side. 
     The microscope objective lens includes, in order from the reduction side, a negative meniscus lens L 1  having a convex surface facing the enlargement side, a negative meniscus lens L 2  having a convex surface facing the enlargement side, and a biconvex positive lens L 3 . 
     The microscope objective lens further includes a negative meniscus lens L 4  having a convex surface facing the reduction side, a biconvex positive lens L 5 , a biconcave negative lens L 6 , a biconvex positive lens L 7 , a negative meniscus lens L 8  having a convex surface facing the reduction side, and a biconvex positive lens L 9 . 
     The negative meniscus lens L 4  and the biconvex positive lens L 5  are cemented. The biconcave negative lens L 6  and the biconvex positive lens L 7  are cemented. The negative meniscus lens L 8  and the biconvex positive lens L 9  are cemented. 
     The microscope objective lens further includes a positive meniscus lens L 10  having a convex surface facing the enlargement side, a biconcave negative lens L 11 , a biconcave negative lens L 12 , a biconvex positive lens L 13 , a negative meniscus lens L 14  having a convex surface facing the enlargement side, and a positive meniscus lens L 15  having a convex surface facing the enlargement side. 
     The positive meniscus lens L 10  and the biconcave negative lens L 11  are cemented. The biconcave negative lens L 12  and the biconvex positive lens L 13  are cemented. The negative meniscus lens L 14  and the positive meniscus lens L 15  are cemented. 
     As illustrated in  FIG. 11 , an enlargement-side surface of the negative meniscus lens L 1  is a curved surface extending beyond a hemisphere. Thus, the negative meniscus lens L 1  is a super-hemispherical meniscus lens. In the optical system of Example 4, one super-hemispherical meniscus lens is disposed on the reduction side. 
     An optical system of Example 5 is illustrated in  FIG. 12 .  FIG. 13  is a lens sectional view of the optical system of Example 5. Furthermore, a partial sectional view of the optical system of Example 5 is illustrated in  FIG. 13 .  FIG. 13  illustrates lenses positioned near a reduction-side conjugate point Pi. In  FIG. 12  and  FIG. 13 , the left side in the drawing sheet is the reduction side and the right side is the enlargement side.  FIG. 18  is an aberration diagram of spherical aberration of the optical system of Example 5. 
     The optical system of Example 5 is a water immersion-type microscope objective lens. In the optical system of Example 5, an object is positioned on the reduction side, for example, at the reduction-side conjugate point Pi. Thus, an optical image of the object is formed on the enlargement side. The optical system of Example 5 is an infinity-corrected objective lens.  FIG. 12  illustrates a state in which light from the object becomes parallel light on the enlargement side. 
     The microscope objective lens includes, in order from the reduction side, a negative meniscus lens L 1  having a convex surface facing the enlargement side, a biconvex positive lens L 2 , a negative meniscus lens L 3  having a convex surface facing the enlargement side, and a positive meniscus lens L 4  having a convex surface facing the enlargement side. The biconvex positive lens L 2  and the negative meniscus lens L 3  are cemented. 
     The microscope objective lens further includes a negative meniscus lens L 5  having a convex surface facing the reduction side, a biconvex positive lens L 6 , a positive meniscus lens L 7  having a convex surface facing the enlargement side, a negative meniscus lens L 8  having a convex surface facing the enlargement side, a negative meniscus lens L 9  having a convex surface facing the reduction side, and a biconvex positive lens L 10 . 
     The negative meniscus lens L 5  and the biconvex positive lens L 6  are cemented. The positive meniscus lens L 7  and the negative meniscus lens L 8  are cemented. The negative meniscus lens L 9  and the biconvex positive lens L 10  are cemented. 
     The microscope objective lens further includes a biconvex positive lens L 11 , a negative meniscus lens L 12  having a convex surface facing the enlargement side, a biconcave negative lens L 13 , a positive meniscus lens L 14  having a convex surface facing the reduction side, a negative meniscus lens L 15  having a convex surface facing the reduction side, and a positive meniscus lens L 16  having a convex surface facing the reduction side. 
     The biconvex positive lens L 11  and the negative meniscus lens L 12  are cemented. The biconcave negative lens L 13  and the positive meniscus lens L 14  are cemented. The negative meniscus lens L 15  and the positive meniscus lens L 16  are cemented. 
     As illustrated in  FIG. 13 , an enlargement-side surface of the negative meniscus lens L 1  is a curved surface extending beyond a hemisphere. Thus, the negative meniscus lens L 1  is a super-hemispherical meniscus lens. In the optical system of Example 5, one super-hemispherical meniscus lens is disposed on the reduction side. 
     An optical system of Example 6 is illustrated in  FIG. 14 .  FIG. 14  is a lens sectional view of the optical system of Example 6. In  FIG. 14 , the left side in the drawing sheet is the reduction side and the right side is the enlargement side. 
     The optical system of Example 6 is an eyepiece optical system. In the optical system of Example 6, an object is positioned at an enlargement-side conjugate point Po. Thus, an optical image of the object is formed on the reduction side.  FIG. 14  illustrates a state in which light from the object positioned at the enlargement-side conjugate point Po, for example, light from a display element, reaches the reduction-side conjugate point Pi. 
     The eyepiece optical system includes, in order from the reduction side, a negative meniscus lens L 1  having a convex surface facing the enlargement side and a biconvex positive lens L 2 . An aperture stop S is positioned on the reduction side of the negative meniscus lens L 1 . 
     An enlargement-side surface of the negative meniscus lens L 1  is a curved surface extending beyond a hemisphere. Thus, the negative meniscus lens L 1  is a super-hemispherical meniscus lens. In the optical system of Example 6, one super-hemispherical meniscus lens is disposed on the reduction side. 
     Numerical data of each example described above is shown below. In Surface data, r denotes radius of curvature of each lens surface, d denotes a distance between respective lens surfaces, ne denotes a refractive index of each lens for a e-line, νd denotes an Abbe number for each lens and * denotes an aspherical surface. 
     Moreover, in Various data, f denotes a focal length of the optical system, FNO. denotes an F number, ω denotes a half angle of view, EDP denotes a diameter of a pupil, θout denotes a half angle of emergence, φEXP denotes a diameter of an exit pupil, ONA denote an angular aperture, OBH denote an object height, ωmax denotes a maximum angle of view, ωeye denotes an angle of view of both eyes, IH denote an image height. Unit of length is mm and unit of angle is degree. 
     A shape of an aspherical surface is defined by the following expression where the direction of the optical axis is represented by z, the direction orthogonal to the optical axis is represented by y, a conical coefficient is represented by K, aspherical surface coefficients are represented by A4, A6, A8, A10, A12 . . . . 
         Z =( y   2   /r )/[1+{1−(1+ k )( y/r ) 2 } 1/2 ]+ A 4  y   4   +A 6  y   6   +A 8  y   8   +A 10  y   10   +A 12  y   12 + . . . .
 
     Further, in the aspherical surface coefficients, ‘e−n’ (where, n is an integral number) indicates ‘10 −n ’. Moreover, these symbols are commonly used in the following numerical data for each example. 
     Example 1 
       
     
       
         
           
               
             
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
            
               
                 Surface data 
               
            
           
           
               
               
               
               
               
            
               
                 Surface no. 
                 r 
                 d 
                 ne 
                 νd 
               
               
                   
               
               
                 Object plane 
                 ∞ 
                 ∞ 
               
               
                 1(Stop) 
                 ∞ 
                 −36.000 
               
               
                  2* 
                 46.733 
                 24.117 
                 1.8830 
                 40.7 
               
               
                 3 
                 67.843 
                 1.000 
               
               
                 4 
                 30.711 
                 17.649 
                 1.8830 
                 40.7 
               
               
                 5 
                 27.274 
                 1.198 
               
               
                 6 
                 13.709 
                 9.313 
                 1.8830 
                 40.7 
               
               
                 7 
                 11.550 
                 1.000 
               
               
                 8 
                 3.889 
                 3.076 
                 1.8830 
                 40.7 
               
               
                 9 
                 2.138 
                 2.000 
               
               
                 Image plane 
                 ∞ 
               
               
                   
               
            
           
           
               
            
               
                 Aspherical surface data 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 2nd surface 
               
               
                   
                 k = −1.4700e−001 
               
               
                   
                   
               
            
           
           
               
            
               
                 Various data 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 f 
                 25.88 
               
               
                   
                 Fno 
                 0.261 
               
               
                   
                 ω 
                 ±0.1 
               
               
                   
                 EPD 
                 99.0 
               
               
                   
                 θout 
                 125 
               
               
                   
                   
               
            
           
         
       
     
     Example 2 
       
     
       
         
           
               
             
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
            
               
                 Surface data 
               
            
           
           
               
               
               
               
               
            
               
                 Surface no. 
                 r 
                 d 
                 ne 
                 νd 
               
               
                   
               
               
                 Object plane 
                 ∞ 
                 ∞ 
               
               
                 1(Stop) 
                 ∞ 
                 0.000 
               
               
                  2* 
                 328.349 
                 8.265 
                 1.4879 
                 70.4 
               
               
                 3 
                 759.429 
                 1.000 
               
               
                 4 
                 40.598 
                 20.949 
                 2.0033 
                 28.3 
               
               
                 5 
                 43.306 
                 1.000 
               
               
                 6 
                 19.983 
                 17.480 
                 2.0033 
                 28.3 
               
               
                 7 
                 14.189 
                 1.000 
               
               
                 8 
                 4.391 
                 5.287 
                 2.0033 
                 28.3 
               
               
                 9 
                 1.500 
                 0.905 
               
               
                 Image plane 
                 ∞ 
               
               
                   
               
            
           
           
               
            
               
                 Aspherical surface data 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 2nd surface 
               
               
                   
                 k = 0.0 
               
               
                   
                 A4 = 7.0674e−007, A6 = −9.6354e−012 
               
               
                   
                   
               
            
           
           
               
            
               
                 Various data 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 f 
                 20.39 
               
               
                   
                 Fno 
                 0.206 
               
               
                   
                 ω 
                 ±0.1 
               
               
                   
                 EPD 
                 99.0 
               
               
                   
                 θout 
                 150 
               
               
                   
                   
               
            
           
         
       
     
     Example 3 
       
     
       
         
           
               
             
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
            
               
                 Surface data 
               
            
           
           
               
               
               
               
               
            
               
                 Surface no. 
                 r 
                 d 
                 ne 
                 νd 
               
               
                   
               
               
                 Object plane 
                 ∞ 
                 0.198 
               
               
                 1 
                 −0.441 
                 0.920 
                 1.5163 
                 64.1 
               
               
                 2 
                 −0.728 
                 0.013 
               
               
                 3 
                 −2.583 
                 1.265 
                 1.8830 
                 40.7 
               
               
                 4 
                 −2.953 
                 0.100 
               
               
                 5 
                 24.297 
                 1.370 
                 1.5831 
                 59.4 
               
               
                 6 
                 −69.936 
                 0.100 
               
               
                 7 
                 12.754 
                 2.102 
                 1.5174 
                 52.4 
               
               
                 8 
                 −43.793 
                 0.100 
               
               
                 9 
                 8.748 
                 4.889 
                 1.4387 
                 94.9 
               
               
                 10 
                 −8.452 
                 0.767 
                 1.6377 
                 42.4 
               
               
                 11 
                 9.685 
                 3.798 
                 1.4387 
                 94.9 
               
               
                 12 
                 −11.646 
                 0.239 
               
               
                 13 
                 10.154 
                 4.480 
                 1.4387 
                 94.9 
               
               
                 14 
                 −6.874 
                 0.700 
                 1.6377 
                 42.4 
               
               
                 15 
                 −32.003 
                 0.284 
               
               
                 16 
                 11.532 
                 0.859 
                 1.6541 
                 39.7 
               
               
                 17 
                 4.848 
                 3.818 
                 1.4387 
                 94.9 
               
               
                 18 
                 13.502 
                 1.551 
               
               
                 19 
                 24.553 
                 0.731 
                 1.7582 
                 34.8 
               
               
                 20 
                 3.575 
                 6.900 
                 1.7412 
                 32.2 
               
               
                 21 
                 −3.421 
                 0.700 
                 1.7437 
                 32.4 
               
               
                 22 
                 −11.348 
                 1.816 
               
               
                 23 
                 −13.678 
                 0.700 
                 1.7638 
                 42.9 
               
               
                 24 
                 7.897 
                 1.688 
                 1.5286 
                 47.7 
               
               
                 25 
                 −21.077 
                 1.316 
               
               
                 26 
                 −4.261 
                 1.778 
                 1.6150 
                 44.0 
               
               
                 27 
                 4.784 
                 4.817 
                 1.6205 
                 46.8 
               
               
                 28 
                 −7.743 
                 −3.000 
               
               
                 29 
                 ∞ 
               
               
                   
               
            
           
           
               
            
               
                 Various data 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 f 
                 4.236 
               
               
                   
                 φEχP 
                 8.45 
               
               
                   
                 θNA 
                 106.64 
               
               
                   
                 OBH 
                 0.01 
               
               
                   
                   
               
            
           
         
       
     
     Example 4 
       
     
       
         
           
               
             
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
            
               
                 Surface data 
               
            
           
           
               
               
               
               
               
            
               
                 Surface no. 
                 r 
                 d 
                 ne 
                 νd 
               
               
                   
               
               
                 Object plane 
                 ∞ 
                 0.055 
                 1.3330 
                 55.7 
               
               
                 1 
                 −0.500 
                 1.095 
                 1.5163 
                 64.1 
               
               
                 2 
                 −0.740 
                 0.014 
               
               
                 3 
                 −3.598 
                 1.793 
                 1.8830 
                 40.7 
               
               
                 4 
                 −3.676 
                 0.100 
               
               
                 5 
                 171.081 
                 1.725 
                 1.6385 
                 55.4 
               
               
                 6 
                 −14.814 
                 0.100 
               
               
                 7 
                 17.417 
                 0.700 
                 1.6377 
                 42.4 
               
               
                 8 
                 8.469 
                 5.921 
                 1.4387 
                 94.9 
               
               
                 9 
                 −7.089 
                 0.100 
               
               
                 10 
                 −285.010 
                 0.701 
                 1.6377 
                 42.4 
               
               
                 11 
                 5.896 
                 4.919 
                 1.4387 
                 94.9 
               
               
                 12 
                 −18.905 
                 0.105 
               
               
                 13 
                 12.207 
                 4.692 
                 1.6377 
                 42.4 
               
               
                 14 
                 6.717 
                 6.334 
                 1.4387 
                 94.9 
               
               
                 15 
                 −7.611 
                 2.454 
               
               
                 16 
                 −8.993 
                 3.041 
                 1.5481 
                 45.8 
               
               
                 17 
                 −3.527 
                 0.700 
                 1.5638 
                 60.6 
               
               
                 18 
                 11.532 
                 1.190 
               
               
                 19 
                 −47.136 
                 0.703 
                 1.6223 
                 53.1 
               
               
                 20 
                 3.958 
                 5.260 
                 1.5827 
                 46.4 
               
               
                 21 
                 −6.000 
                 1.007 
               
               
                 22 
                 −4.547 
                 1.444 
                 1.7859 
                 44.2 
               
               
                 23 
                 −13.421 
                 3.851 
                 1.4387 
                 94.9 
               
               
                 24 
                 −6.174 
                 −3.000 
               
               
                 25 
                 ∞ 
               
               
                   
               
            
           
           
               
            
               
                 Various data 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 f 
                 4.218 
               
               
                   
                 φEχP 
                 11.19 
               
               
                   
                 θNA 
                 104.5 
               
               
                   
                 OBH 
                 0.01 
               
               
                   
                   
               
            
           
         
       
     
     Example 5 
       
     
       
         
           
               
             
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
            
               
                 Surface data 
               
            
           
           
               
               
               
               
               
            
               
                 Surface no. 
                 r 
                 d 
                 ne 
                 νd 
               
               
                   
               
               
                 Object plane 
                 ∞ 
                 0.241 
                 1.3330 
                 55.7 
               
               
                 1 
                 −0.810 
                 1.086 
                 1.8830 
                 40.7 
               
               
                 2 
                 −1.015 
                 0.120 
               
               
                 3 
                 71.412 
                 3.194 
                 1.4387 
                 94.9 
               
               
                 4 
                 −6.117 
                 1.070 
                 1.5578 
                 53.8 
               
               
                 5 
                 −13.831 
                 0.346 
               
               
                 6 
                 −14.879 
                 3.698 
                 1.4387 
                 94.9 
               
               
                 7 
                 −7.506 
                 1.179 
               
               
                 8 
                 27.083 
                 1.000 
                 1.7205 
                 34.7 
               
               
                 9 
                 13.411 
                 10.351 
                 1.4387 
                 94.9 
               
               
                 10 
                 −11.859 
                 4.913 
               
               
                 11 
                 −27.342 
                 10.000 
                 1.4387 
                 94.9 
               
               
                 12 
                 −9.748 
                 1.000 
                 1.6377 
                 42.4 
               
               
                 13 
                 −28.985 
                 4.331 
               
               
                 14 
                 16.809 
                 1.000 
                 1.7205 
                 34.7 
               
               
                 15 
                 11.884 
                 11.410 
                 1.4387 
                 94.9 
               
               
                 16 
                 −21.149 
                 0.100 
               
               
                 17 
                 55.604 
                 7.156 
                 1.8340 
                 37.1 
               
               
                 18 
                 −11.014 
                 1.000 
                 1.7995 
                 42.2 
               
               
                 19 
                 −365.106 
                 2.803 
               
               
                 20 
                 −15.025 
                 1.000 
                 1.8830 
                 40.7 
               
               
                 21 
                 6.250 
                 3.934 
                 1.7847 
                 25.7 
               
               
                 22 
                 15.779 
                 1.296 
               
               
                 23 
                 17.255 
                 1.000 
                 1.8929 
                 20.4 
               
               
                 24 
                 8.567 
                 4.772 
                 2.0033 
                 28.3 
               
               
                 25 
                 82.326 
                 −3.000 
               
               
                 26 
                 ∞ 
               
               
                   
               
            
           
           
               
            
               
                 Various data 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 f 
                 4.307 
               
               
                   
                 φEχP 
                 14.36 
               
               
                   
                 θNA 
                 108.00 
               
               
                   
                 OBH 
                 0.01 
               
               
                   
                   
               
            
           
         
       
     
     The optical system of Example 3, the optical system of Example 4, and the optical system of Example 5 are an infinity-corrected objective lenses. The infinity-corrected objective lens is used with a tube lens. Examples of the tube lens are given below. 
     
       
         
           
               
            
               
                   
               
               
                 Surface data 
               
            
           
           
               
               
               
               
               
            
               
                 Surface no. 
                 r 
                 d 
                 ne 
                 νd 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 68.754 
                 7.732 
                 1.4875 
                 70.2 
               
               
                 2 
                 −37.568 
                 3.474 
                 1.8061 
                 40.9 
               
               
                 3 
                 −102.848 
                 0.697 
               
               
                 4 
                 84.310 
                 6.024 
                 1.8340 
                 37.1 
               
               
                 5 
                 −50.710 
                 3.030 
                 1.6445 
                 40.8 
               
               
                 6 
                 40.662 
                 150.000 
               
               
                 Image plane 
                 ∞ 
               
               
                   
               
            
           
         
       
     
     Example 6 
       
     
       
         
           
               
             
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
            
               
                 Surface data 
               
            
           
           
               
               
               
               
               
            
               
                 Surface no. 
                 r 
                 d 
                 ne 
                 νd 
               
               
                   
               
               
                 Object plane 
                 ∞ 
                 ∞ 
               
               
                 1(Stop) 
                 ∞ 
                 5.083 
               
               
                 2 
                 −15.631 
                 12.725 
                 1.9590 
                 17.5 
               
               
                 3 
                 −16.793 
                 1.000 
               
               
                 4 
                 221.773 
                 26.090 
                 1.9590 
                 17.5 
               
               
                 5 
                 −111.714 
                 34.663 
               
               
                 Image plane 
                 ∞ 
               
               
                   
               
            
           
           
               
            
               
                 Various data 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 f 
                 29.30 
               
               
                   
                 φEχP 
                 5.00 
               
               
                   
                 ωmax 
                 100.00 
               
               
                   
                 ωeye 
                 200.00 
               
               
                   
                 IH 
                 53.74 
               
               
                   
                   
               
            
           
         
       
     
     Values of conditional expressions in each example are given below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                   
                 Example1 
                 Example2 
                 Example3 
               
               
                   
                   
               
               
                   
                 (1)D1sag 
                 1.264 
                 1.420 
                 0.127 
               
               
                   
                   
                 4.775 
                 8.137 
               
               
                   
                   
                 1.744 
               
               
                   
                 (2)D2sag 
                 1.748 
                 2.137 
                 0.134 
               
               
                   
                   
                 4.849 
                 8.397 
               
               
                   
                   
                 0.420 
               
               
                   
                 (3)R1/F 
                 −0.083 
                 −0.074 
                 −0.104 
               
               
                   
                 (4)R2/F 
                 −0.150 
                 −0.215 
                 −0.172 
               
               
                   
                   
               
               
                   
                   
                 Example4 
                 Example5 
                 Example6 
               
               
                   
                   
               
               
                   
                 (1)D1sag 
                 0.091 
                 0.241 
                 5.123 
               
               
                   
                 (2)D2sag 
                 0.105 
                 0.249 
                 5.027 
               
               
                   
                 (3)R1/F 
                 −0.119 
                 −0.188 
                 −0.533 
               
               
                   
                 (4)R2/F 
                 −0.175 
                 −0.236 
                 −0.573 
               
               
                   
                   
               
            
           
         
       
     
     Values of parameters are given below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                   
                 Example1 
                 Example2 
                 Example3 
               
               
                   
                   
               
               
                   
                 R1 
                 −2.138 
                 −1.500 
                 −0.441 
               
               
                   
                 R2 
                 −3.889 
                 −4.391 
                 −0.728 
               
               
                   
                 F 
                 25.881 
                 20.390 
                 4.236 
               
               
                   
                   
               
               
                   
                   
                 Example4 
                 Example5 
                 Example6 
               
               
                   
                   
               
               
                   
                 R1 
                 −0.500 
                 −0.81 
                 −15.631 
               
               
                   
                 R2 
                 −0.740 
                 −1.015 
                 −16.793 
               
               
                   
                 F 
                 4.218 
                 4.307 
                 29.303 
               
               
                   
                   
               
            
           
         
       
     
     An optical apparatus of the present embodiment includes the optical system described above and an image sensor disposed at the reduction-side conjugate point. 
       FIG. 19A  and  FIG. 19B  are diagrams illustrating a first example of the optical apparatus of the present embodiment. The optical apparatus of the first example is an image pickup apparatus.  FIG. 19A  is a diagram illustrating a first example of the image pickup apparatus.  FIG. 19B  is a diagram illustrating a second example of the image pickup apparatus. 
     The image pickup apparatus of the first example is described. An image pickup apparatus  1  includes an image pickup optical system  2  and an image sensor  3 . The image pickup optical system  2  includes a lens  4 . For example, it is possible to use a thin-film image sensor as the image sensor  3 . The image sensor  3  is held by a holding member  5 . 
     In the lens  4 , a reduction-side surface  4   a  is a concave curved surface on the reduction side, and an enlargement-side surface  4   b  is a convex curved surface on the enlargement side. The enlargement-side surface  4   b  is a curved surface extending beyond a hemisphere. Thus, the lens  4  is a super-hemispherical meniscus lens. In the image pickup optical system  2 , one super-hemispherical meniscus lens is disposed on the reduction side. 
     In the lens  4 , the intersection of the reduction-side surface  4   a  and the optical axis of the image pickup optical system  2  is positioned closer to the enlargement side than the reduction-side conjugate point Pi. The reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Thus, it is possible to make the light ray LBm going away from the lens  4  to contribute to image formation. 
     When an object is positioned at the reduction-side conjugate point Pi, the light ray LBm represents a light ray going away from the lens  4 . However, in the image pickup apparatus  1 , the object is positioned on the enlargement side. In this case, light travels from the enlargement side to the reduction side. Thus, in the image pickup apparatus  1 , the light ray LBm is a light ray coming toward the lens  4 . 
     As illustrated in  FIG. 19A , the light ray LBm reaches the reduction-side conjugate point Pi from a position closer to the reduction side than the reduction-side conjugate point Pi. In the image pickup apparatus  1 , it is possible to make such a light ray to contribute to image formation. 
     The image pickup apparatus of the second example is described. An image pickup apparatus  10  includes an image pickup optical system  2  and an image sensor  11 . The image pickup optical system  2  includes a lens  4 . For example, it is possible to use a back-illuminated image sensor as the image sensor  11 . The image sensor  11  is held by a holding member  12  and a holding member  13 . 
     The holding member  13  is held by the holding member  12 . A front end portion of the holding member  13  protrudes from the holding member  12 . The image sensor  11  is disposed at the front end portion of the holding member  13 . 
     The front end portion of the holding member  13  is formed of a material that allows transmission of light. Thus, it is possible to make the light ray LBm illustrated in  FIG. 19A  to reach the image sensor  11 . At the front end portion of the holding member  13 , a though hole may be provided at a place through which the light ray LBm passes. 
     The image pickup optical system  2  is used also in the image pickup apparatus  10 . Thus, it is possible to make the light ray LBm to contribute to image formation even in the image pickup apparatus  10 , although a detailed description thereof is omitted. 
     The optical apparatus of the present embodiment includes the optical system described above and a light source disposed at the reduction-side conjugate point. 
       FIG. 20  is a diagram illustrating a second example of the optical apparatus of the present embodiment. The optical apparatus of the second example is a microscope.  FIG. 20A  is a diagram illustrating a part of an illumination optical system.  FIG. 20B  is a diagram illustrating a microscope. 
     An illumination optical system  30  includes a lens  31 , a lens  32 , and a lens  33 . The illumination optical system  30  further includes a plurality of lenses. These lenses are not illustrated in  FIG. 20A . A light source  34  is disposed at a reduction-side conjugate point of the illumination optical system  30 . 
     It is possible to dispose a luminous body or a light-emerging surface at the position of the light source  34 . For example, it is possible to use a halogen lamp or an LED as the luminous body. For example, it is possible to use a light-emerging surface of an optical fiber bundle as the light-emerging surface. 
     In the lens  31  and the lens  32 , a reduction-side surface is a concave curved surface on the reduction side, and an enlargement-side surface is a convex curved surface on the enlargement side. The enlargement-side surface is a curved surface extending beyond a hemisphere. Thus, the lens  31  and the lens  32  are super-hemispherical meniscus lenses. In the illumination optical system  30 , two super-hemispherical meniscus lenses are disposed on the reduction side. 
     In the lens  31  and the lens  32 , an intersection of the reduction-side surface and the optical axis of the illumination optical system  30  is positioned closer to the enlargement side than the reduction-side conjugate point. A reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point. Thus, it is possible to use a light ray going away from the lens  31  as illumination light. As a result, it is possible to implement bright illumination. 
     A microscope  40  includes an epi-illumination device  41  and a transmitted illumination device  42 . The illumination optical system  30  is used for each of the epi-illumination device  41  and the transmitted illumination device  42 . 
     The optical apparatus of the present embodiment includes the optical system described above and a holding mechanism configured to position an object at the reduction-side conjugate point. 
       FIG. 21  is a diagram illustrating a third example of the optical system of the present embodiment. The optical apparatus of the third example is a microscope.  FIG. 21A  is a diagram illustrating a part of a microscope objective lens.  FIG. 21B  is a diagram illustrating a microscope. 
     A microscope objective lens  50  includes a lens  51  and a lens  52 . The microscope objective lens  50  further includes a plurality of lenses. These lenses are not illustrated in  FIG. 21A . A specimen  53  is positioned at a reduction-side conjugate point Pi. 
     In the lens  51 , a reduction-side surface  51   a  is a concave curved surface on the reduction side, and an enlargement-side surface  51   b  is a convex curved surface on the enlargement side. The enlargement-side surface  51   b  is a curved surface extending beyond a hemisphere. Thus, the lens  51  is a super-hemispherical meniscus lens. In the microscope objective lens  50 , one super-hemispherical meniscus lens is disposed on the reduction side. 
     In the lens  51 , an intersection of the reduction-side surface  51   a  and the optical axis of the microscope objective lens  50  is positioned closer to the enlargement side than the reduction-side conjugate point Pi. A reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point Pi. Thus, it is possible to make a light ray going away from the lens  51  to contribute to image formation. 
     A microscope  60  includes a microscope objective lens  61  and a stage  62 . The microscope objective lens  61  is disposed under the stage  62 . The stage  62  is the holding mechanism configured to position an object at the reduction-side conjugate point. A specimen  63  is placed on the stage  62 . The specimen  63  is held by a holding member. 
       FIG. 22  and  FIG. 22B  are diagrams illustrating the holding member.  FIG. 22A  is a diagram illustrating a first example of the holding member.  FIG. 22B  is a diagram illustrating a second example of the holding member. 
     The holding member of the first example is described. A holding member  70  includes a plate-like portion  71  and concave portions  72 . The plate-like portion  71  is formed of a material that allows transmission of light. In each concave portion  72 , a specimen  74  is held together with liquid  73 . 
     In the microscope  60 , the microscope objective lens  50  described above is used as the microscope objective lens  61 . Thus, the lens  51  is positioned under the concave portion  72 . Both of a front surface  72   a  and a back surface  72   b  of the concave portion  72  are spherical surfaces. A radius of curvature of the back surface  72   b  is identical to a radius of curvature of the reduction-side surface  51   a . Thus, it is possible to bring the lens  51  in proximity to the back surface  72   b.    
     When the specimen  74  is a thick specimen, one surface is positioned on the back surface  72   b  side and the other surface is positioned on the liquid  73  side. In the microscope objective lens  50 , it is possible to make a light ray going away from the lens  51  to contribute to image formation. The light ray going away from the lens  51  can be considered as light from the other surface. Thus, in the microscope  60 , it is possible to form an optical image of the other surface. 
     In the holding member  70 , a specimen is held in each of the concave portions  72  two-dimensionally arranged. Thus, in observation using the holding member  70 , it is necessary to move the microscope objective lens  50  and the holding member  70  relative to each other in a direction orthogonal to the optical axis. 
     At a time of observation of the concave portion  72 , the reduction-side surface  51   a  is in proximity to the back surface  72   b . Therefore, it is not possible to move the microscope objective lens  50  and the holding member  70  relative to each other in the direction orthogonal to the optical axis in the proximate state. 
     In observation using the microscope objective lens  50 , when the adjacent concave portion  72  is to be observed, the microscope objective lens  50  and the holding member  70  are moved relative to each other in the optical axis direction. With relative movement in the optical axis direction, it is possible to move the microscope objective lens  50  away from the holding member  70 . Thus, it is possible to move the microscope objective lens  50  to below the concave portion  72  to be observed next. 
     In the concave portion  72 , the specimen  74  is held together with the liquid  73 . Therefore, there is a possibility that the specimen  74  moves if the holding member  70  is moved. Furthermore, there is a possibility that the liquid  73  spills out of the concave portion  72 . Thus, it is preferable that the holding member  70  be fixed. 
       FIG. 22A  illustrates a state in which the lens  51  is moved in a state in which the holding member  70  is fixed. As illustrated in  FIG. 22A , the lens  51  moves in the optical axis direction and the direction orthogonal to the optical axis. 
     In the microscope  60 , the microscope objective lens  61  is fixed to a revolver. Thus, the microscope  60  may be provided with a mechanism configured to move the revolver. 
     When the concave portion  72  need not be filled with liquid, it is possible to move the holding member  70 . In this case, the stage  62  may be provided with a movement mechanism. 
     The holding member of the second example is described. A holding member  80  includes a plate-like portion  81 , concave portions  82 , and spherical portions  83 . The plate-like portion  81  and the spherical portions  83  are formed of a material that allows transmission of light. Each spherical portion  83  has the same shape as the lens  51 . Thus, the holding member  80  has a lens action. 
     The optical apparatus of the present embodiment includes the optical system described above and a display device disposed at the enlargement-side conjugate point. 
       FIG. 23  is a diagram illustrating a fourth example of the optical system of the present embodiment. The optical apparatus of the fourth example is VR goggles. VR goggles  90  include an eyepiece optical system  91  and a display element  92 . The eyepiece optical system  91  includes a lens  93  and lens  94 . The optical system of Example 6 is used for the eyepiece optical system  91 . A straight line  96  is a straight line passing through the reduction-side conjugate point. 
     In the lens  94 , a reduction-side surface is a concave curved surface on the reduction side, and an enlargement-side surface is a convex curved surface on the enlargement side. The enlargement-side surface is a curved surface extending beyond a hemisphere. Thus, the lens  94  is a super-hemispherical meniscus lens. In the eyepiece optical system  91 , one super-hemispherical meniscus lens is disposed on the reduction side. 
     In the lens  94 , an intersection of the reduction-side surface and the optical axis of the eyepiece optical system  91  is positioned closer to the enlargement side than the reduction-side conjugate point. An reduction-side intersection is positioned closer to the reduction side than the reduction-side conjugate point. Thus, it is possible to make a light ray going away from the lens  51  to contribute to image formation. 
     In the VR goggles  90 , the display element  92  is positioned on the enlargement side. Thus, light reaches the reduction-side conjugate point from a position closer to the reduction side than the reduction-side conjugate point, in the same manner as in the image pickup apparatus  1 . In the VR goggles  90 , it is possible to make such a light ray to contribute to image formation. 
     The display element  92  is disposed at the enlargement-side conjugate point of the eyepiece optical system  91 . When the VR goggles  90  are mounted on a user&#39;s head  95 , the pupils of the user&#39;s eyes are positioned on the straight line  96 . Then, the user can view an image appearing on the display element  92 . 
     In the eyepiece optical system  91 , two lenses are used. Therefore, it is difficult to sufficiently correct chromatic aberration and distortion. In this case, it is preferable to process an image appearing on the display element  92 . The processed image is processed such that the user is unable to recognize chromatic aberration and distortion when the user views the image. By viewing the processed image, the user can view a sharp image with no distortion. 
     According to the present disclosure, it is possible to provide an optical system that is bright and has a high resolving power, and an optical apparatus including the same. 
     As described above, the embodiments according to the present disclosure are suitable for an optical system that is bright and has a high resolving power, and an optical apparatus including the same.