1.  Field of the Invention
The present invention relates to a binocular telescope with a photographing function, which has a pair of observation optical systems and a photographing optical system, and is constructed in such a manner that a focusing mechanism for the pair of observation optical systems and a focusing mechanism for the photographing optical system are operated in association with each other so that the pair of observation optical systems is utilized as a focusing device for the photographing optical system. 
2.  Description of the Related Art
As is well known, binocular telescopes are used for watching sports, wild birds, and so on.  When using a binocular telescope, it is often the case that the user sees something that he or she would like to photograph.  Typically, he or she will fail to photograph the desired scene because he or she must change a camera for the binocular telescope and during this time the chance is lost.  For this reason, a binocular telescope containing a camera is proposed, whereby a photograph can be taken immediately by using the camera contained in the binocular telescope while continuing the observation through the binocular telescope. 
For example, Japanese Unexamined Utility Model Publication (JUUMP) (KOKAI) No.  6-2330 discloses a binocular telescope with a photographing function, i.e., a combination of a binocular telescope and a camera, in which the camera is simply mounted in the binocular telescope.  The binocular telescope is provided with a pair of telescopic optical systems for observing an observed object in an enlarged state, and a photographing optical system for photographing the observed image.  Namely, in the binocular telescope with a photographing function, the pair of telescopic optical systems functions not only as a viewfinder optical system for the photographing optical system, but also as a telescopic binocular system. 
Generally, in an observation optical system such as a binocular telescope, when the rear focal point of the objective lens system and the front focal point of the ocular lens system roughly coincide with each other, an observed object at infinity (i.e., distant view) can be observed in an in-focus state through the observation optical system.  Accordingly, for observing an observed object at a shorter distance than infinity (i.e., close-range view) in an in-focus state, a focusing operation is needed for focusing on the close-range view.  In such a focusing operation, the objective lens system and the ocular lens system are separated from the in-focus state of the distant view.  Therefore, in the observation optical system, a focusing mechanism is mounted, which moves the objective lens system and the ocular lens system to adjust the distance therebetween.  Concretely, the focusing mechanism comprises a rotary wheel, disposed adjacent to the observation optical system, and a movement conversion mechanism for converting a rotational movement of the rotary wheel into a relative back-and-forth movement of the objective lens system and the ocular lens system. 
In the binocular telescope with a photographing function disclosed in the above-described JUUMP '330, however, there is no description of the focusing operation of the pair of observation optical systems.  Further, as described above, the pair of observation optical systems functions as a viewfinder optical system for indicating an observed range, and '330 does not indicate how the photographing optical system focuses on an object to be photographed. 
U.S. Pat.  No.  4,067,027 discloses another type of binocular telescope with a photographing function, which is provided with a pair of observation optical systems and a photographing optical system.  In this binocular telescope with a photographing function, a focusing mechanism for the pair of observation optical systems is provided with a mechanism for performing a focusing operation of the photographing optical system.  Namely, by rotating the rotary wheel of the focusing mechanism manually, the objective lens system and the ocular lens system are moved relative to each other in each of the observation optical systems, which causes the photographing optical system to move relative to a surface of a silver halide film, and thus, the focusing operations are performed for the pair of observation optical systems and the photographing optical system.  Thus, when an observed object is observed in an in-focus state through the pair of observation optical systems, the object is also in an in-focus state in the photographing optical system.  Therefore, if a photographing operation is carried out when the observed object is observed in an in-focus state through the pair of observation optical systems, the object image is focused on a surface of the silver halide film. 
When different users observe an observed object in an in-focus state through an observation optical device such as a binocular telescope, the observation optical system is not necessarily observed with the same dioptric power for each user.  This is because, generally, human eyes have the ability to adjust, so that an object in a range from 15 cm to infinity ahead of the eyes can be focused.  This ability to adjust depends upon the age of the observer, so that the range in which the eyes can focus on an object is different depending upon the observer.  Thus, even if the dioptric power of the observation optical system is offset from a usual value, a human can still observe the observed object image through the observation optical system as a focused image.  Therefore, in the binocular telescope with the photographing function described in U.S. Pat.  No. ' 027, even if the observed object image is observed through the pair of observation optical systems in an in-focus state after manual operation of the rotary wheel, the observed object image is not necessarily focused by the photographing optical system.  Thus, it may happen that, although the observed object image is observed through the pair of observation optical systems in an in-focus state, the photographed image is not in an in-focus state. 
To solve the problem described above, it is proposed in Japanese Examined Patent Publication (KOKOKU) No.  36-12387 that a reticle element (or focusing index element) be movably provided at an in-focus position of the ocular optical system of the observation optical system so that the observation optical system is always focused with a constant dioptric power.  The reticle element is a transparent glass plate, for example, on which an index having a proper shape (e.g., a cross) is formed.  If the index element is positioned at an in-focus position of the ocular optical system of the observation optical system, the user observes the observed object in an in-focus state at the position of the index.  Namely, the observed object is always observed with a constant dioptric power.  Therefore, when the observation optical system reaches an in-focus state, the photographing optical system is adjusted in an in-focus state in association with the observation optical system.  Thus, in the binocular telescope with a photographing function, the observation optical system can be utilized as a focusing device for the photographing optical system. 
Generally, the reticle element is not assembled in each optical system of a pair of observation optical systems, but provided only in one of the observation optical systems.  This is because, when the interpupillary distance of the pair of observation optical systems is adjusted so as to fuse the right and left observed images, it is difficult to completely superpose the right and left reticle images on each other.  Namely, although a binocular telescope is designed in such a manner that, when the interpupillary distance of the pair of observation optical systems is perfectly adjusted, the optical axes of the pair of observation optical systems are completely coincident with each other, it is very difficult to make the optical axes completely coincident.  Therefore, when the reticle element is assembled in each of the observation optical systems, the pair of reticles are observed slightly offset from each other, when fusing the right and left observed images, and the offset reticle images are distracting to the user.  Accordingly, the reticle element is assembled in one of the observation optical systems In practice, the reticle element is often provided in the right observation optical system, since, in most people (around 80%), the right eye is more skillful than the left eye. 
In the binocular telescope with a photographing function, in which a reticle element is provided, it is necessary to perform a dioptric power adjustment depending upon the eyesight of the user.  Therefore, the ocular optical systems of a pair of observation optical systems are constructed in such a manner that their positions relative to the in-focus positions of the objective optical systems can be adjusted.  Namely, first, the user adjusts the positions of the ocular lens systems so as to be able to observe the reticle image in an in-focus state.  In this state, when performing a focusing operation of the pair of observation optical systems to focus on the observed image, the observed image is formed in an in-focus state in the photographing optical system as a photographed image, and thus, a focused photographed image can be obtained. 
In a binocular telescope in which a reticle element is assembled in the right observation optical system, although the position of the ocular lens system can be adjusted relative to the in-focus position of the objective optical system to adjust the dioptric power in the left observation optical system, a reticle element is not assembled in the left observation optical system, and therefore, it is difficult to perform a dioptric power adjustment properly in the left observation optical system.  Thus, in the left observation optical system, the observed image may be observed in a state which is offset from a proper dioptric power with which the left eye, in a relaxed state, of the user observes the observed image.  If an observation using the binocular telescope is carried out for a long time in the offset state, the eyes will become tired since the adjusting performance of the eyes is continually carried out. 
On the other hand, for a user whose left eye is more skillful than the right eye, since the focusing operation for the observed image is forcedly performed using the right observation optical system, the eyes will become more strained.  Further, when a user whose left eye is more skillful than the right eye is not consciously focusing an observed image using the right observation optical system, the focusing operation for the observed image tends to be performed using the left observation optical system.  In this focusing operation, since there is no reticle element in the left observation optical system, the adjusting operation of the left eye occurs, so that the dioptric powers of the right and left observation optical systems are offset.  Thus, in the photographing optical system, the observed image cannot be formed in an in-focus state.