BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an optical arrangement of a camera for observing therethrough an object to be photographed, and more particularly to a viewfinder optical arrangement for taking out part of a picture-taking light beam from within the optical path of a picture-taking lens and confirming therethrough the picture-taking field of view by this light beam, and especially an optical arrangement suitable for a video camera or a cinecamera. 
     2. Description of the Prior Art 
     An ordinary video camera, as shown in FIG. 1 of the accompanying drawings, generally forms an image on an image pick-up tube 15 with the aid of a picture-taking lens comprising a forward lens group or unit 11 and a rearward lens group or unit 12. A TTL viewfinder is designed such that part of a picture-taking light beam is reflected by a beam splitter such as a half-transmitting prism 10 provided in the optical path between the forward lens group 11 and the rearward lens group 12, an inverted image is formed on a focus screen 4 by a viewfinder master lens 2 via a reflecting mirror 3, an erect image is formed on another focus screen 6 by a relay lens 5 and the erect image on the focus screen 6 is observed by the observer&#39;s eye E through an eyepiece 7. The fact that such a relay lens is required has led to a very great length of the optical path of the viewfinder optical arrangement, and this has been disadvantageous to making the entire camera compact. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to realize compactness of the arrangement and to provide a viewfinder optical arrangement of a camera which may form a clear image. 
     The present invention includes a beam splitter disposed in the optical path of a picture-taking lens for extracting a light beam from light passed through the picture-taking lens, and a viewfinder optical system for directing the light beam to a viewfinder eyepiece. The optical system has a focus screen, a master lens for imaging the light beam on the focus screen, and a pentaroof prism disposed between the master lens and the focus screen, and an erect image is formed on the focus screen by the pentaroof prism. 
     The invention will become more fully apparent from the following detailed description thereof taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view showing the optical arrangement of a video camera according to the prior art. 
     FIG. 2 is a schematic cross-sectional view showing the optical arrangement of a video camera according to an embodiment of the present invention. 
     FIG. 3 is a developed optical path view of the viewfinder optical system according to an embodiment of the present invention. 
     FIGS. 4 and 5 show the cross-sectional shapes of an effective light beam reaching a focus screen on the entrance surface and the exit surface of a pentaroof prism. 
     FIG. 6 is a schematic cross-sectional view showing the optical arrangement of a camera according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will hereinafter be described with respect to some embodiments thereof. 
     A video camera 100 shown in FIG. 2 has a picture-taking lens comprising a forward lens group or unit 21 and a rearward lens group or unit 22, and a beam splitter 20 for dividing a viewfinder optical path and a rectangular prism 24 for reflecting a picture-taking light beam perpendicularly are provided between the two lens groups. An image pick-up tube 25 is vertically disposed along the optical axis bent by the rectangular prism 24. The forward lens group 21 comprises a plurality of lenses, not shown, and serves as a magnification changing lens group and a focusing lens group, and the rearward lens group 22 serves as a master lens, and a stop 23 is provided between the two lens groups. The light beam for the viewfinder reflected by the beam splitter 20 passes through a viewfinder master lens 32 and is reflected by a first reflecting surface 30b of a pentaroof prism 30 and a second reflecting surface 30c thereof which is a roof surface, whereafter it forms an erect image on a focus screen 34. This erect image may be observed by the observer&#39;s eye E through an eyepiece 37. 
     In such a viewfinder optical system, the optical path from the beam splitter 20 to the focus screen 34 can be prolonged by bending the optical path in the pentaroof prism 30 and therefore, the focal length of the viewfinder master lens 32 can be made longer than that of the master lens 22 of the picture-taking lens system to thereby make the image on the focus screen 34 larger than the image formed on the image pick-up tube 25. Accordingly, to obtain the same degree of viewfinder magnification as the conventional viewfinder optical system, the magnification by the eyepiece 37 can be made smaller than the conventional one and therefore, the refractive power of the eyepiece can become small to make the construction simple. Moreover, the enlargement magnification by the eyepiece 37 is smaller than the conventional one and therefore, even if dust or the like adheres to the focus plate 34, it cannot be conspicuous and a clearer viewfinder field of view can be obtained. 
     In the viewfinder optical system of the above-described construction, the pentaroof prism can be formed more compactly by the roof surface of the pentaroof prism 30 being provided in the second reflecting surface 30c, namely, the surface facing the object side, rather than in the first reflecting surface 30b with respect to the incident light. This will hereinafter be described. 
     Referring to FIG. 3, there is shown a developed optical path from the beam splitter 20 of the optical system of FIG. 2 to the focus screen 34. In FIG. 3, a light beam reaching the central point P of the field of view is indicated by solid lines and a light beam reaching the marginal point Q of the field of view is indicated by dotted lines. An auxiliary positive lens 32&#39; is provided between the pentaroof prism 30 and the focus screen 34. This auxiliary lens serves as a field lens and also performs correction of lateral chromatic aberration, and an object image is formed on the focus screen 34 by a combined system comprising the auxiliary lens 32&#39; and the viewfinder master lens 32. The focus screen 34 is provided with a square view field frame, not shown, for prescribing the picture-taking range, and the cross-sectional shape of the effective light beam reaching the focus screen 34 becomes as shown in FIGS. 4 and 5 on the entrance surface 30a and the exit surface 30d of the pentaroof prism. That is, the area of the effective light beam on the exit surface 30d of the pentaroof prism 30 is the sum of the areas through which passes the light beam condensed at every point on the focus screen 34, and is substantially rectangular as indicated by hatching in FIG. 5 reflecting the fact that the effective area of the focus screen 34 is square. On the other hand, the effective light beam area on the entrance surface 30a of the pentaroof prism 30 is substantially circular as indicated by hatching in FIG. 4. The reason for this is that the effective image height on the focus screen 34 differs depending on the bearing that the area through which the effective light beam passes becomes non-isotropic as shown in FIG. 5, and observing the light beam reaching the central point P of the field of view and the light beam reaching the marginal point Q of the field of view, their path areas are spaced apart from each other on the exit surface 30d and are close to each other on the entrance surface 30a. That is, the variation which the effective image height imparts to the position of the area through which the light beam passes is strong on the exit surface and weak on the entrance surface. Therefore, the effective light beam area is approximate to a circle on the entrance surface and approximate to a rectangle on the exit surface. 
     Accordingly, if it is taken into consideration that where one of the two reflecting surfaces of the pentaroof prism is formed into a roof surface, one of two transmitting surfaces which function as the entrance surface or the exit surface of the pentaroof prism necessarily becomes pentagonal, forming into a pentagonal shape the surface in which the shape of the effective area is circular rather than square can make that surface smaller and also can make the shape of the entire prism smaller. That is, in the present invention, as already shown in FIG. 4, if the second reflecting surface 30c as a pentaroof prism is formed into a roof surface so that the entrance surface 30a of the pentaroof prism is pentagonal, the size of this pentagonal entrance surface 30a may sufficiently be a pentagon which circumscribes the approximately circular effective light beam area on this surface, and may be smaller than the pentagon circumscribing the rectangle and therefore, the pentaroof prism of the present invention can be made smaller than the conventional pentaroof prism in which the first reflecting surface is formed into a roof surface. 
     Such a compact optical arrangement can further lead to compactness of an entire camera by being applied to a camera whose dimensions have been reduced by disposing the image pick-up tube vertically as shown in FIG. 2. The optical arrangement of the present invention, as shown in FIG. 6, can suitably be combined with a photographic apparatus using a solid state image sensing element such as CCD (charge coupled device). The image sensing element 45 in this case is much smaller than the conventional image pick-up tube and therefore the photographic apparatus itself also is compact and thus, the compact viewfinder optical system according to the present invention becomes more effective and the configuration of the entire camera can be formed compactly as indicated by the dot-dash line in FIG. 6.
A beam splitter extracts a light beam from light passing through a picture-taking lens system, and a viewfinder optical system directs the light beam to a viewfinder eyepiece. The optical system includes a focus screen, a master lens for imaging the light beam on the focus screen, a pentaroof prism disposed between the master lens and the focus screen, and an eyepiece for observing therethrough the image on the focus screen.