Abstract:
A light interception structure for an automatic focus detection surveying instrument includes a telescopic optical system having an objective optical system and an ocular optical system, a beam splitting optical element which splits object light transmitted through the objective optical system from the telescopic optical system, and a focus detector having a light receiver which receives the object light split by the beam splitting optical element. One surface of the beam splitting optical element upon which external light entering the telescopic optical system through the ocular optical system is first made incident after the external light passes through or is reflected by a beam splitting surface of the beam splitting optical element is subject to an anti-reflection treatment.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light interception structure for an AF sensor of an automatic focus detection surveying instrument. 
     2. Description of the Related Art 
     In a, conventional auto-focus surveying instrument having a collimating telescope, such as a total station, an optical system is composed of a collimating optical system including an objective lens, a focusing lens, a Porro prism, a focusing plate and an eyepiece in this order from the object side, and a beam splitting optical system including a beam splitter provided in the vicinity of the Porro prism to split a light path of a focus detection optical system from a light path of the collimating optical system. In the focusing system, the focus state of a plane which is optically equivalent to a focal plane of the collimating optical system located within the focus detection optical system is detected by a phase difference detection type auto-focus sensor having a pair of CCD sensors to calculate the amount of defocus (defocus amount), so that a focusing lens is moved to a focal position in accordance with the defocus amount to carry out the automatic focusing (AF) operation. 
     However, in the conventional structure, since the beam splitting system and the Porro prism are located on the optical axis of the collimating optical system, if external light enters the collimating optical system through the eyepiece, the external light reaches the AF sensor due to internal reflections, so that the external light overlaps the light emitted from the objective lens, thus resulting in an incorrect focusing operation. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide an automatic focus detection surveying instrument in which light incident through the eyepiece is intercepted so as not to reach the AF sensor, so that a precise focusing operation can be carried out. 
     To achieve the object mentioned above, according to an aspect of the present invention, there is provided a light interception structure for an automatic focus detection surveying instrument including a telescopic optical system having an objective optical system and an ocular optical system, a beam splitting optical element which splits object light transmitted through the objective optical system from the telescopic optical system, and a focus detection device having a light receiving device for receiving the object light split by the beam splitting optical element, wherein one surface of the beam splitting optical element upon which external light entering the telescopic optical system through the ocular optical system is first made incident after the external light passes through or is reflected by a beam splitting surface of the beam splitting optical element is subject to an anti-reflection treatment. 
     Preferably, the anti-reflection treatment is for light diffusion. 
     Preferably, the anti-reflection treatment for light diffusion is surface polished. 
     Preferably, the surface of the beam splitting optical element which is subject to the anti-reflection treatment is coated with an anti-reflection paint. 
     According to another aspect of the present invention there is provided a light interception structure for an automatic focus detection surveying instrument including a telescopic optical system having an objective optical system and an ocular optical system, a beam splitting optical element which splits object light transmitted through the objective optical system from the telescopic optical system, and a focus detection device having a light receiving device for receiving the object light split by the beam splitting optical element, wherein an inner surface of the automatic focus detection surveying instrument receives emitted external light which is first made incident on and passes through the surface of the beam splitting optical element after passing through a beam splitting surface of the beam splitting optical element which allows the external light to pass through or reflect therefrom. 
     Preferably, one surface of the beam splitting optical element upon which external light entering the telescopic optical system through the ocular optical system is first made incident after the external light passes through or is reflected by a beam splitting surface of the beam splitting optical element is coated with an anti-reflection layer. 
     Preferably, the anti-reflection treatment includes an application of a fabric or cloth to the surface. 
     The anti-reflection treatment can be an application of a molt plane to the surface. 
     The anti-reflection treatment can be a coating of an anti-reflection paint. 
     According to another aspect of the present invention, there is provided a light interception structure for an automatic focus detection surveying instrument including a telescopic optical system having an objective optical system and an ocular optical system, a beam splitting optical element which splits object light transmitted through the objective optical system from the telescopic optical system, and a focus detection device having a light receiving device for receiving the object light split by the beam splitting optical element, wherein a reflection surface is provided in a position opposed to a surface upon which external light entering the telescopic optical system through the ocular optical system and emitted from the beam splitting optical element is first made incident after the external light is transmitted through or reflected by the beam splitting surface of the beam splitting optical element so as to reflect the external light emitted from the beam splitting optical element in a direction whereby the light does not reach the beam splitting optical element. 
     Preferably, the reflection surface is formed on an inner surface of a barrel of the automatic focus detection surveying instrument. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No.10-176348 (filed on Jun. 23, 1998) which is expressly incorporated herein by reference in its entirety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be discussed below in detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a partially cut-out side elevational view of a total station to which the present invention is applied; 
     FIG. 2 is a perspective view of a Porro prism which is provided on its second reflection surface with a prism cemented thereto; 
     FIG. 3 is a perspective view of a Porro prism which is provided on its third reflection surface with a prism cemented thereto; 
     FIG. 4 is a perspective view of a Porro prism which is provided n its fourth reflection surface with a prism cemented thereto; 
     FIG. 5 is a perspective view of a Porro prism which is provided n its first reflection surface with a prism cemented thereto; 
     FIG. 6 is a perspective view of a Porro prism and a beam splitter located on the front side (objective lens side) thereof; 
     FIGS. 7A and 7B are a side elevational view and a front elevational view, of an AF sensor which receives light incident through an eyepiece, respectively; 
     FIG. 8 is a view of an example 1 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a first embodiment of the present invention; 
     FIG. 9 is a view of an example 2 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a first embodiment of the present invention; 
     FIG. 10 is a view of an example 3 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a first embodiment of the present invention; 
     FIG. 11 is a view of an example 4 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a first embodiment of the present invention; 
     FIG. 12 is a view of an example 5 of an optical arrangement a Porro prism, a prism, and an AF sensor according to a first embodiment of the present invention; 
     FIG. 13 is a view of an example 1 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a second embodiment of the present invention; 
     FIG. 14 is a view of an example 2 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a second embodiment of the present invention; 
     FIG. 15 is a view of an example 3 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a second embodiment of the present invention; 
     FIG. 16 is a view of an example 4 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a second embodiment of the present invention; 
     FIG. 17 is a view of an example 5 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a second embodiment of the present invention; 
     FIG. 18 is a view of an example 1 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a third embodiment of the present invention; 
     FIG. 19 is a view of an example 2 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a third embodiment of the present invention; 
     FIG. 20 is a view of an example 3 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to a third embodiment of the present invention; 
     FIG. 21 is a view of an example 4 of an optical arrangement a Porro prism, a prism, and an AF sensor according to a third embodiment of the present invention; and 
     FIG. 22 is a view of an example 5 of an optical arrangement of a Porro prism, a prism, and an AF sensor according to third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Several embodiments of the present invention will be discussed below, referring to the drawings. FIG. 1 shows a partially cut-out side view of a total station  1  to which the present invention is applied. Light from an object is transmitted through an objective lens  2  and a focusing lens  3 ; is reflected by four reflection surfaces of a Porro prism  4 ; and is converged onto a focusing plate  5  to form an erect image thereon. A user can view the erect image through an eyepiece  6 . As shown in FIG. 2, a prism  9  is cemented to a second reflection surface  8   b  of the Porro prism  4 , so that the boundary therebetween defines a beam splitting surface (coated surface)  11   a . The prism  9  and the beam splitting surface  11   a  form a beam splitting optical element. A portion of the light incident upon the beam splitting surface  11   a  is transmitted therethrough and is made incident upon the AF sensor  7  through the prism  9 . 
     In this structure, as shown in FIGS. 7A and 7B, the external light  10  which enters the collimating optical system through the eyepiece  6  is received by the AF sensor  7  (focus detection device having a light receiving device) and overlaps the light from the objective lens  2 , thus resulting in an incorrect focus detection. Namely, the external light  10  from the eyepiece  6  is reflected by the fourth reflection surface  8   d  and the third reflection surface  8   c  of the Porro prism  4  and the external light  10  transmitted through the beam splitting surface  11   a  is reflected by the side surface  12  of the prism  9  or is transmitted through the side surface  12  and is reflected by the barrel inner surface  13 . The light is then transmitted through the side surface  12  and is reflected by the beam splitting surface  11   a  and is received by the AF sensor  7 . 
     The above discussion has been directed to an arrangement in which the second reflection surface  8   b  of the Porro prism  4  defines the beam splitting surface. The same problem is raised if the beam splitting surface is defined by the reflection surface on any other reflection surface of the Porro prism  4  having four reflection surfaces, other than the second reflection surface. The different arrangements of the Porro prism  4  and the prism  9 , in which the beam splitting surface is defined by the reflection surface of the Porro prism  4  other than the second reflection surface  8   b  are shown in FIGS. 3 through 5. Furthermore, the same problem occurs if the beam splitter  17  is provided at the front (on the objective lens side) or rear of the Porro prism  4  (FIG.  6 ). 
     Embodiments of the invention to solve the problem will be discussed below. 
     In a first embodiment, at least one specific surface of the prism  9  is a polished surface which can be subject to an anti-reflection treatment, such as an anti-reflection paint to prevent the external light  10  incident through the eyepiece  6  from reaching the AF sensor  7  due to the internal reflections shown in FIGS. 8 through 12. The external light  10  transmitted through the beam splitting surface  11   a  is diffused or absorbed by the polished surface with or without the anti-reflection treatment, so that the quantity of the external light  10  which can be received by the AF sensor  7  can be reduced. Examples of the specific surface of the prism  9  to be polished with or without the anti-reflection treatment in the first embodiment will be discussed below. 
     EXAMPLE 1 
     In the case that the beam splitting surface  11   a  is defined by the connection surface between the second reflection surface  8   b  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  2 ), as shown in FIG. 8, the surface of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   a  is the specific surface to be polished with or without the anti-reflection treatment, i.e., the side surface  12 . Thus, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d  and the third reflection surface  8   c ; passes through the beam splitting surface  11   a ; and is diffused or absorbed by the side surface  12 . Consequently, the quantity of the external light  10  which is reflected by the side surface  12 , is reflected by the beam splitting surface  11   a , and is received by the AF sensor  7 ; can be markedly reduced. 
     EXAMPLE 2 
     In the arrangement in which the beam splitting surface  11   b  is defined by the connection surface between the third reflection surface  8   c  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  3 ), as shown in FIG. 9, the surface of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   b  is the specific surface to be polished with or without the anti-reflection treatment, i.e., the side surface  14 . Thus, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d  and passes through the beam splitting surface  11   a ; and is diffused or absorbed by the side surface  14 . Consequently, the quantity of the external light  10  which is reflected by the side surface  14 , is reflected by the beam splitting surface  11   b , and is received by the AF sensor  7 ; can be markedly reduced. 
     EXAMPLE 3 
     In the arrangement in which the beam splitting surface  11   c  is defined by the connection surface between the fourth reflection surface  8   d  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  4 ), as shown in FIG. 10, the surface of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   c  is the specific surface to be polished with or without the anti-reflection treatment, i.e., the side surface  15 . In this arrangement, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is made incident upon the prism  9 ; passes through the beam splitting surface  11   c ; and is diffused or absorbed by the side surface  15 . Thus, the quantity of the external light  10  which is reflected by the side surface  15 ; is reflected by the beam splitting surface  11   c ; and is received by the AF sensor  7  can be markedly reduced. 
     EXAMPLE 4 
     In the arrangement in which the beam splitting surface  11   d  is defined by the connection surface between the first reflection surface  8   a  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  5 ), as shown in FIG. 11, the surface  16  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   d  is the specific surface to be polished with or without the anti-reflection treatment. In this example, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d , the third reflection surface  8   c  and the second reflection surface  8   b ; passes through the beam splitting surface  11   d ; and is diffused or absorbed by the side surface  16 . Consequently, the quantity of the external light  10  which is reflected by the side surface  16 , is reflected by the beam splitting surface  11   d , and is received by the AF sensor  7 , can be markedly reduced. 
     EXAMPLE 5 
     In the arrangement in which the beam splitter  17  is provided on the front side (objective lens side) of the Porro prism (FIG.  6 ), as shown in FIG. 12, the surface  18  of the beam splitter  17  upon which the external light  10  from the eyepiece  6  is first made incident after the light is reflected by the beam splitting surface  11   e  is the specific surface to be polished with or without the anti-reflection  10  treatment. In this example, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d , the third reflection surface  8   c , the second reflection surface  8   b  and the first reflection surface  8   a ; is made incident upon the beam splitter  17 , is reflected by the beam splitting surface  11   e , and is diffused or absorbed by the side surface  18 . Consequently, the quantity of the external light  10  which is reflected by the beam splitting surface  11   e  or is transmitted through the beam splitting surface  11   e  and is returned to the beam splitting surface  11   e  to pass therethrough or to be reflected thereby, and is received by the AF sensor  7 , can be markedly reduced. This example can be equally applied to an arrangement in which the beam splitter  17  is located on the rear side of the Porro prism  4 . 
     In a second embodiment, one specific surface of the prism  9  is coated with an anti-reflection layer  19  to pass the external light  10  therethrough and the inner surface of the barrel opposed to the specific surface of the prism is provided with an anti-reflection surface  20 , as shown in FIGS. 13 through 17. The anti-reflection surface can be made of, for example, (light absorbent) artificial fur member, sponge member, molt plane, fabric, cloth, or felt applied to the surface or anti-reflection paint, etc. The external light  10  transmitted through the beam splitting surface  11  passes through the anti-reflection surface (coat)  19  of the prism  9  and is emitted outward therefrom, and is absorbed by the barrel inner surface which is provided with the anti-reflection surface  20 , so that the quantity of the external light  10  reflected by the anti-reflection surface  19  and  20  and received by the AF sensor  7 , can be markedly reduced. Examples of the specific surface which is subject to the above-mentioned surface treatment in the second embodiment will be discussed below. 
     EXAMPLE 1 
     In the arrangement in which the beam splitting surface  11   a  is defined by the connection surface between the second reflection surface  8   b  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  2 ), as shown in FIG. 13, the side surface  12  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   a  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   a  opposed thereto and outside the prism  9  is provided with the anti-reflection surface  20 . Thus, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d  and the third reflection surface  8   c , passes through the beam splitting surface  11   a , passes through the side surface  12 ; and is absorbed by the anti-reflection surface  20  of the barrel inner surface  13   a . Consequently, the quantity of the external light  10  received by the AF sensor  7  can be markedly reduced. 
     EXAMPLE 2 
     In the arrangement in which the beam splitting surface  11   b  is defined by the connection surface between the third reflection surface  8   c  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  3 ), as shown in FIG. 14, the side surface  14  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   b  is coated with the anti-reflection layer  19 ; and the barrel inner surface  13   b  opposed thereto is provided with the anti-reflection surface  20 . Thus, since a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d , passes through the beam splitting surface  11   b , passes through the side surface  14 ; and is absorbed by the anti-reflection surface  20  of the barrel inner surface  13   b , the quantity of the external light  10  reaching the AF sensor  7  can be markedly reduced. 
     EXAMPLE 3 
     In the arrangement in which the beam splitting surface  11   c  is defined by the connection surface between the fourth reflection surface  8   d  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  4 ), as shown in FIG. 15, the side surface  15  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   c  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   c  opposed thereto is provided with the anti-reflection surface  20 . A substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is transmitted through the beam splitting surface  11   c ; passes through the side surface  15 ; and is absorbed by the anti-reflection surface  20  of the barrel inner surface  13   c . Consequently, the quantity of the external light  10  received by the AF sensor  7  can be markedly reduced. 
     EXAMPLE 4 
     In the arrangement in which the beam splitting surface  11   d  is defined by the connection surface between the first reflection surface  8   a  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  5 ), as shown in FIG. 16, the side surface  16  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   d  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   d  opposed thereto is provided with the anti-reflection surface  20 . A substantial part of the external light  10   10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d , the third reflection surface  8   c  and the second reflection surface  8   b , passes through the beam splitting surface  11   d , passes through the side surface  16 , and is absorbed by the anti-reflection surface  20  of the barrel inner surface  13   d . Consequently, the quantity of the external light  10  received by the AF sensor  7  can be markedly reduced. 
     EXAMPLE 5 
     In the arrangement in which the beam splitter  17  is provided on the front side (objective lens side) of the Porro prism (FIG.  6 ), as shown in FIG. 17, the side surface  18  of the beam splitter  17  upon which the external light  10  from the eyepiece  6  is first made incident after the light is reflected by the beam splitting surface  11   e  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   e  opposed thereto is provided with the anti-reflection surface  20 . A substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d , the third reflection surface  8   c , the second reflection surface  8   b  and the first reflection surface  8   a ; is made incident upon the beam splitter  17 , is reflected by the beam splitting surface  11   e , passes through the side surface  18 , and is absorbed by the barrel inner surface  13   e . Consequently, the quantity of the external light  10  reflected by the beam splitting surface  11   e , or transmitted through the beam splitting surface  11   e  and returned again to the beam splitting surface  11   e  to pass therethrough or to be reflected thereby, and received by the AF sensor  7  can be markedly reduced. The above discussion can be equally applied if the beam splitter  17  is provided on the rear side of the Porro prism  4 . 
     In a third embodiment which is a modification of the second embodiment, one specific surface of the prism  9  is coated with an anti-reflection layer  19 , and the inner surface of the barrel opposed to the specific surface of the prism  9  and outside the prism  9  forms a predetermined angle with respect to the specific surface of the prism  9 , as shown in FIGS. 18 through 22. The quantity of the external light  10  transmitted through the beam splitting surface  11  and reflected by the anti-reflection surface (coat)  19  of the prism  9  can be reduced. Moreover, the external light  10  transmitted through the anti-reflection surface  19  and reflected by the barrel inner surface travels in a direction deviated from the light path toward the anti-reflection coat  19  or the prism  9 , due to the angle of the barrel inner surface with respect to the anti-reflection coat  19 . The predetermined angle of the barrel inner surface is such that no external light  10  reflected by the barrel inner surface is made incident again upon the prism  9 . Examples of the specific surface which is subject to the above-mentioned surface treatment in the third embodiment will be discussed below. 
     EXAMPLE 1 
     In the arrangement in which the beam splitting surface  11   a  is defined by the connection surface between the second reflection surface  8   b  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  2 ), as shown in FIG. 18, the side surface  12  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   a  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   a  opposed thereto forms an angle α1 with respect to the side surface  12 . Thus, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8 d and the third reflection surface  8   c , passes through the beam splitting surface  11   a , passes through the side surface  12 , and is reflected by the barrel inner surface  13   a  in a direction away from the prism  9 . Consequently, the quantity of the external light  10  received by the AF sensor  7  can be markedly reduced. 
     EXAMPLE 2 
     In the arrangement in which the beam splitting surface  11   b  is defined by the connection surface between the third reflection surface  8   c  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  3 ), as shown in FIG. 19, the side surface  14  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   b  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   b  opposed thereto forms an angle α2 with respect to the side surface  14 . Thus, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d , passes through the beam splitting surface  11   b , passes through the side surface  14 , and is reflected by the barrel inner surface  13   b  in a direction away from the prism  9 . Consequently, the quantity of the external light  10  received by the AF sensor  7  can be markedly reduced. 
     EXAMPLE 3 
     In the arrangement in which the beam splitting surface  11   b  is defined by the connection surface between the fourth reflection surface  8   d  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  4 ), as shown in FIG. 20, the side surface  15  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   c  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   c  opposed thereto forms an angle α3 with respect to the side surface  15 . Thus, a substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  passes through the beam splitting surface  11   c , passes through the side surface  15 , and is reflected by the barrel inner surface  13   c  in a direction away from the prism  9 . Consequently, the quantity of the external light  10  received by the AF sensor  7  can be markedly reduced. 
     EXAMPLE 4 
     In the arrangement in which the beam splitting surface  11   b  is defined by the connection surface between the first reflection surface  8   a  of the Porro prism  4  and the prism  9  cemented thereto (FIG.  5 ), as shown in FIG. 21, the side surface  16  of the prism  9  upon which the external light  10  from the eyepiece  6  is first made incident after the light passes through the beam splitting surface  11   d  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   d  opposed thereto forms an angle α4 with respect to the side surface  16 . A substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d , the third reflection surface  8   c  and the second reflection surface  8   b ; passes through the beam splitting surface  11   d , passes through the side surface  16 , and is reflected by the barrel inner surface  13   d  in a direction away from the prism  9 . Consequently, the quantity of the external light  10  received by the AF sensor  7  can be markedly reduced. 
     EXAMPLE 5 
     In the arrangement in which the beam splitter  17  is provided on the front side (objective lens side) of the Porro prism (FIG.  6 ), as shown in FIG. 22, the side surface  18  of the beam splitter  17  upon which the external light  10  from the eyepiece  6  is first made incident after the light is reflected by the beam splitting surface  11   e  is coated with the anti-reflection layer  19 , and the barrel inner surface  13   e  opposed thereto forms an angle α5 with respect to the side surface  18 . A substantial part of the external light  10  incident upon the Porro prism  4  through the eyepiece  6  is reflected by the fourth reflection surface  8   d , the third reflection surface  8   c , the second reflection surface  8   b  and the first reflection surface  8   a ; is made incident upon the beam splitter  17 , is reflected by the beam splitting surface  11   e , passes through the side surface  18 , and is reflected by the barrel inner surface  13   e  in a direction away from the beam splitter  17 . Consequently, the quantity of the external light  10  reflected by the beam splitting surface  11   e  or transmitted through the beam splitting surface  11   e  and returned again to the beam splitting surface  11   e  to pass therethrough or to be reflected thereby, and received by the AF sensor  7 , can be markedly reduced. The above discussion can be equally applied if the beam splitter  17  is provided on the rear side of the Porro prism  4 . 
     As can be understood from the above discussion, according to the present invention, since no external light entering the telescopic optical system through the ocular optical system reaches the focus detection device due to the internal reflections, the external light does not interfere with the object light from the objective optical system, thus resulting in a precise AF operation. 
     Obvious changes may be made in the specific embodiments of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.