Abstract:
The invention relates to an electronic imaging system wherein an image pickup device is horizontally located while ensuring the space for a reflective member thereby achieving height, length and cost reductions. The imaging system comprises a taking lens and an image pickup device. The taking lens comprises a front lens group G 1  consisting of a negative lens component and a positive lens component and having positive refracting power and a rear lens group G 2  consisting of a negative lens element and a positive lens element and having positive refracting power, with an aperture stop S interposed between the front lens group G 1  and the rear lens group G 2 . The taking lens satisfies conditions (1) and (2). 
     2.8&lt; f   B   /IH &lt;4.5  (1) 
     2&lt;SF&lt;3  (2) 
     Here f B  is back focus of the taking lens, IH is a length that is half the diagonal length of the effective plane of the electronic image pickup device, and SF is a shape factor for the negative lens component in the front lens group.

Description:
[0001]    This application claims benefit of Japanese Application No. 2002-117777 filed in Japan on Apr. 19, 2002, the contents of which are incorporated herein by this reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates generally to an electronic imaging system represented by a video camera or still video camera, and more particularly to a wide-angle image pickup lens designed to reduce the size of such an electronic imaging system by use of a reflective member.  
           [0003]    With the recent advent of miniature electronic image pickup devices such as CCDs, imaging units become smaller and smaller. Typically, JP-A 10-39207 shows an imaging unit in which a taking lens is made up of as few as four lens elements with a reduced length, and so only a lens portion is made compact.  
           [0004]    JP-A 11-95096 discloses a taking lens made up of as few as four lens elements; however, this taking lens uses an aspheric lens and so costs much. The size of the taking lens itself is reduced by virtue of an image plane size reduction; as shown in FIG. 17, however, the presence of wires and terminals on the outer region of the light-receptive plane of an image pickup device  15  and the package size of the image pickup device  15  make it impossible to reduce size relative to the height direction. It is noted that FIG. 17 is illustrative in section of an imaging unit inclusive of its optical axis. Reference numeral  10  indicates a frame (lens barrel),  11  an image pickup lens,  12  an aperture stop,  13  an optical filter,  14  a cover glass and  15  an image pickup device. Regarding an objective lens for endoscopes, another approach to size reductions in the height direction has been proposed, wherein an optical path is bent to position an image pickup device laterally.  
           [0005]    JP-A 9-122070 shows that an optical filter and a triangular prism are located at a back focus portion of a taking lens. However, this is not favorable for the total length of an imaging unit because of the need of a long bending space. Although constraints on the length of an endoscopic objective lens are loose, application of a long lens unit to a camera is not preferable because it means that camera thickness increases.  
           [0006]    JP-A 11-109223 shows that a long back focus is ensured; however, the location of an aperture stop between a negative front lens group and a positive rear lens group causes rim rays at the rear lens group to become high, resulting in an increased lens diameter. Accordingly, when a reflective member is located at a back focus portion of this type, rays at the entrance surface of the reflective member become high, leading to the need of increasing the volume of the location where the reflective member is located.  
         SUMMARY OF THE INVENTION  
         [0007]    In view of such problems with the prior art as described above, the primary object of the present invention is to provide an electronic imaging system and an image pickup optical system in which an electronic image pickup device is horizontally located to ensure the space needed for the location of a reflective member while height reductions, length reductions and cost reductions are all achieved.  
           [0008]    According to the present invention, this object is attained by the provision of an electronic imaging system comprising a taking lens and an electronic image pickup device located on an image side thereof, characterized in that:  
           [0009]    the taking lens comprises a front lens group consisting of a negative lens component and a positive lens component and having positive refracting power and a rear lens group consisting of, in order from an object side thereof, a negative lens element and a positive lens element and having positive refracting power, with an aperture stop interposed between the front lens group and the rear lens group, and  
           [0010]    conditions (1) and (2) are satisfied. 
           2.8 &lt;f   B   /IH &lt;4.5  (1) 
           2 &lt;SF &lt;3  (2) 
           [0011]    Here f B  is the length, as calculated on an air basis, of the back focus of the taking lens, IH is a length that is half the diagonal length of the effective plane of the electronic image pickup device, and SF is a shape factor (R F +R R )/(R F −R R ) for the negative lens component in the front lens group where R F  and R R  are the radii of curvature of the object side and image side of the negative lens component in the front lens group, respectively.  
           [0012]    The advantages of, and the requirements for, the aforesaid arrangement are now explained.  
           [0013]    To ensure a long back focus (f B ) for a taking lens, a retrofocus type comprising a front lens group of negative power and a rear lens group of positive power is commonly used. As shown in FIG. 14( a ), however, a problem with the retrofocus type is that its overall length becomes too long with respect to its focal length. In FIGS.  14 ( a ) to  14 ( e ), the front and rear lens groups are indicated by G 1  and G 2 , respectively. If the front lens group G 1  is allowed to have positive refracting power as shown in FIG. 14( b ), the overall length can then be cut down although f B  becomes short. Here, if the rear lens group G 2  is of the retrofocus type of −+ as shown in FIG. 14( c ), f B  can also be ensured. Thus, the taking lens of the present invention is made up of a front lens group consisting of a negative lens component and a positive lens component and a rear lens group consisting of a negative lens element and a positive lens element.  
           [0014]    With rim rays in mind, the aperture stop must be interposed between the front lens group and the rear lens group, because the location of the aperture stop at the center of the optical system allows the diameters of the front and rear lens groups to be evenly cut down.  
           [0015]    It is then necessary to satisfy conditions (1) and (2).  
           [0016]    As the lower limit of 2.8 to condition (1) is not reached, any space for the location of the reflective member cannot be ensured and any bending layout cannot be achieved either. As the upper limit of 4.5 is exceeded, the front lens group cannot be constructed with positive power, and takes on a retrofocus type, resulting in an increase in the length of the optical system.  
           [0017]    As the lower limit of 2 to condition (2) is not reached, large negative distortion occurs at the negative lens in the front lens group, and correction of that distortion at the positive lens in the front lens group becomes insufficient. As the upper limit of 3 is exceeded, the diverging effect of that negative lens becomes slender and the height of a rim light beam becomes high, resulting in an increase in lens diameter.  
           [0018]    For the front lens group, it is desired that the negative lens and the positive lens be located in this order from its object side. With the negative lens located nearest to the object side of the front lens group, the desired wide-angle arrangement can be achieved while the diameter of the first lens is kept small.  
           [0019]    It is noted that the electronic imaging system of the present invention should preferably satisfy the following condition (a) with respect to the maximum half taking angle of view ω. 
           20°&lt;ω&lt;35°  (a) 
           [0020]    A maximum half taking angle of view that is lower than the lower limit of 20 ° to this condition (a) is not sufficient for the taking angle view used with imaging systems such as generally available cameras. As the upper limit of 35° is exceeded, it is difficult to make correction with a limited number of lenses for aberrations throughout a taking lens.  
           [0021]    As the positive power of the front lens group is weak, the total length of the taking lens becomes long as shown in FIG. 14( d ). Conversely, too strong power causes f B  to become too short as shown in FIG. 14( e ). It is thus desired to optimize the power of the front lens group; it is preferable to satisfy the following condition (3). 
           0.02 &lt;f/f   1 &lt;0.50  (3) 
           [0022]    Here f is the focal length of the taking lens, and f 1  is the focal length of the front lens group.  
           [0023]    As the lower limit of 0.02 to condition (3) is not reached, the taking lens becomes long and the power of the rear lens group becomes too strong; any good performance cannot be obtained with an arrangement comprising two spherical lenses. As the upper limit of 0.50 is exceeded, any long f B  cannot be taken and the space for the reflective member cannot be ensured either.  
           [0024]    When the reflective member (reflecting mirror) is located in the rear of the rear lens group as shown in FIGS.  15 ( a ) and  15 ( b ), the space for the reflective member may be saved by keeping the height of rays emerging the rear lens group G 2  low (see FIG. 15( b )). Since the height of rays emerging from the rear lens group G 2  is largely dependent on the position of the stop  12 , the distance from the stop  12  to the exit surface of the rear lens group G 2  should preferably be as short as possible. However, the positive lens in the rear lens group has strong power and so has a small radius of curvature. To add some edge to the positive lens, it is impossible to make the positive lens extremely thin. It is thus preferable to satisfy the following condition (4). 
           1.5 &lt;f   B   /d   S-R &lt;3  (4) 
           [0025]    Here d S-R  is the axial length from the aperture stop to the final surface of the rear lens group.  
           [0026]    As the lower limit of 1.5 to condition (4) is not reached, the diameter of the rear lens group becomes too large, leading to an increase in the volume of the location where the reflective member is placed. As the upper limit of 3 is exceeded, the length of the rear lens group arrangement becomes too long for the positive lens to have processible shape.  
           [0027]    By the way, an electronic image pickup device requires an optical filter such as an infrared cut filter, and it is ordinarily interposed between a lens system and the image pickup device. As shown in FIG. 16( a ), however, the location of a reflective member (reflecting mirror)  16  in the rear of a filter  13  causes the system to become long, and a long back focus must be provided to a lens  11 . If, as shown in FIG. 16( b ), the optical filter  13  is located on the object side with respect to an aperture stop  12 , only the reflective member (reflecting mirror)  16  is located at a back focus portion so that the volume occupied by the lens  11  and an image pickup device  15  can be minimized. In FIG. 16( b ), a flare stop  17  is located on the object side of the reflective member (reflecting mirror)  16 .  
           [0028]    It is noted that the optical filter  13 , if interposed between the front lens group and the aperture stop as shown in FIG. 16( b ), can be reduced in diameter and set up at a low cost.  
           [0029]    For the reflective member, a prism and a mirror may be utilized; however, the prism is more expensive and heavier than the mirror. When the prism is used, a cover glass for the image pickup device comes close to the exit surface of the prism, resulting in the likelihood of ghosts between both planes. The use of the mirror as the reflective member makes cost reductions, weight reductions and prevention of ghosts possible. For the mirror, it is acceptable to use either a front surface mirror or a back-surface mirror defined by a plane-parallel plate.  
           [0030]    In this connection, when the effective image pickup area of the electronic image pickup device is in a rectangular (oblong) form, the reflective member may be located in such a way that the entrance axis of the taking lens is substantially parallel with the long-side direction of that rectangular form. The taking lens having a long back focus according to the present invention enables such a layout to be easily achieved, leading to an increase in the degree of freedom in an imaging system layout.  
           [0031]    Conversely, the reflective member may be located in such a way that the entrance axis of the taking lens is substantially parallel with the short-side direction of the rectangular form, thereby reducing the space where the reflective member is located.  
           [0032]    The thickness of the electronic imaging system in the entrance axis direction may be increased with respect to the height or width direction of the electronic imaging system. With this arrangement applied to an electronic imaging system having a large thickness in the thickness direction, e.g., a camera, the size of the system in the height or width direction can be more reduced thereby slimming down the overall size of the system.  
           [0033]    The reflective member should also preferably be located in such a way that the optical axis is bent in either one of the height and width directions, the length of which is shorter than that in the thickness direction, thereby making a contribution to reductions in the overall size of the system.  
           [0034]    The electronic imaging system of the present invention may be designed such that the position of the taking portion for receiving the taking lens relative to its main body is variable, thereby slimming down the whole system and, hence, improving the portability of the system. With this arrangement, it is further possible to improve the capability of keeping hold of the system during taking.  
           [0035]    For conditions (1) to (4), the upper and/or lower limits should preferably be set as follows, because the advantages described in connection therewith can be more enhanced.  
           [0036]    In view of securing the space for the reflective member, the lower limit to condition (1) should be set at preferably 3.1 or more preferably either one of 3.22 and 3.31.  
           [0037]    In view of reducing the length of the optical system, the upper limit to condition (1) should be set at preferably 4.0 or more preferably either one of 3.85 and 3.32.  
           [0038]    In view of correction of aberrations, the lower limit to condition (2) should be set at preferably 2.2 or more preferably either one of 2.23 and 2.27.  
           [0039]    In view of making lens diameter small, the upper limit to condition (2) should be set at preferably 2.5 or more preferably either one of 2.32 and 2.28.  
           [0040]    In view of reducing the length of the optical system, the lower limit to condition (3) should be set at preferably 0.03 or more preferably either one of 0.04 and 0.13.  
           [0041]    In view of securing the space for the reflective member, the upper limit to condition (3) should be set at preferably 0.4 or more preferably either one of 0.28 and 0.22.  
           [0042]    In view of reducing the volume of the location for the reflective member, the lower limit to condition (4) should be set at preferably 1.8 or more preferably either one of 1.85 and 2.49.  
           [0043]    In view of improvements in the processability of the rear lens group, the upper limit to condition (4) should be set at preferably 2.93 or more preferably either one of 2.68 and 2.49.  
           [0044]    Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.  
           [0045]    The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts that will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]    [0046]FIG. 1( a ) is illustrative in section of Example 1 of the taking lens used with the electronic imaging system according to the present invention, and FIG. 1( b ) is an aberration diagram for Example 1.  
         [0047]    [0047]FIG. 2( a ) is illustrative in section of Example 2 of the taking lens used with the electronic imaging system according to the present invention, and FIG. 2( b ) is an aberration diagram for Example 2.  
         [0048]    [0048]FIG. 3( a ) is illustrative in section of Example 3 of the taking lens used with the electronic imaging system according to the present invention, and FIG. 3( b ) is an aberration diagram for Example 3.  
         [0049]    [0049]FIG. 4( a ) is illustrative in section of Example 4 of the taking lens used with the electronic imaging system according to the present invention, and FIG. 4( b ) is an aberration diagram for Example 4.  
         [0050]    [0050]FIG. 5( a ) is illustrative in section of Example 5 of the taking lens used with the electronic imaging system according to the present invention, and FIG. 5( b ) is an aberration diagram for Example 5.  
         [0051]    [0051]FIG. 6 is illustrative of the diagonal length of the effective image pickup plane of an electronic image pickup device.  
         [0052]    [0052]FIG. 7 is a front perspective view of the outside shape of a digital camera in which the taking lens of the present invention is built.  
         [0053]    [0053]FIG. 8 is a rear perspective view of the FIG. 7 digital camera.  
         [0054]    [0054]FIG. 9 is a partly cut-away side view of the FIG. 7 digital camera.  
         [0055]    [0055]FIG. 10 is a front perspective view of the outside shape of a video camera in which the taking lens of the present invention is built.  
         [0056]    [0056]FIG. 11 is a partly cut-away top view of the FIG. 10 video camera.  
         [0057]    FIGS.  12 ( a ) and  12 ( b ) are a front and a side view of a cellular phone in which the taking lens of the present invention is built, and FIG. 12( c ) is a sectional view of a phototaking optical system for the same.  
         [0058]    FIGS.  13 ( a ),  13 ( b ) and  13 ( c ) are a front, a side and a sectional view of a modification to the cellular phone in which the taking lens of the present invention is built.  
         [0059]    FIGS.  14 ( a ),  14 ( b ),  14 ( c ),  14 ( d ) and  14 ( e ) are diagrams for the paraxial arrangements of the taking lens according to the present invention.  
         [0060]    FIGS.  15 ( a ) and  15 ( b ) are diagrams for showing the reflective member space in terms of rays emerging from the taking lens.  
         [0061]    FIGS.  16 ( a ) and  16 ( b ) are diagrams for how an optical filter is located according to the present invention.  
         [0062]    [0062]FIG. 17 is a diagram for showing that simple size reductions of the image plane impose a certain limitation on size reductions of an imaging unit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0063]    Examples 1 to 5 of the taking lens used with the electronic imaging system of the present invention are now explained. FIGS.  1 ( a ),  2 ( a ),  3 ( a ),  4 ( a ) and  5 ( a ) are illustrative in section of Examples 1 to 5 upon focused on an infinite object point. In these figures, G 1  stands for a front lens group, S an aperture stop, G 2  a rear lens group, F an optical filter, and I an image plane. FIGS.  1 ( b ),  2 ( b ),  3 ( b ),  4 ( b ) and  5 ( b ) are aberration diagrams for Examples 1 to 5. In these figures, “SA”, “AS” and “DT” are representative of spherical aberrations, astigmatisms and distortions, respectively.  
       Example 1  
       [0064]    As shown in FIG. 1( a ), this example is directed to a taking lens made up of a first lens group G 1  composed of a negative meniscus lens deeply concave in its image-plane side and a positive meniscus lens steeply convex on its image side, an optical filter F for cutting infrared, an aperture stop S and a rear lens group G 2  composed of a cemented positive lens consisting of a negative meniscus lens element deeply concave on its image side and a double-convex positive lens element.  
         [0065]    In the instant example, the negative meniscus lens is used as the first lens, thereby keeping the diameter of the front lens small while achieving a wide-angle lens arrangement.  
       Example 2  
       [0066]    As shown in FIG. 2( a ), this example is directed to a taking lens made up of a front lens group G 1  composed of a negative meniscus lens deeply concave on its image-plane side and a positive meniscus lens steeply convex on its image side, an optical filter F for cutting infrared, an aperture stop S and a rear lens group G 2  composed of a cemented positive lens consisting of a negative meniscus lens element deeply concave on its image side and a double-convex positive lens element.  
         [0067]    In the instant example, the thin negative meniscus lens is used as the first lens, thereby achieving weight reductions.  
       Example 3  
       [0068]    As shown in FIG. 3( a ), this example is directed to a taking lens made up of a front lens group G 1  composed of a negative meniscus lens deeply concave on its image-plane side and a double-convex positive lens steeply convex on its image side, an optical filter F for cutting infrared, an aperture stop S and a rear lens group G 2  composed of a negative meniscus lens deeply concave on its image side and a double-convex positive lens.  
       Example 4  
       [0069]    As shown in FIG. 4( a ), this example is directed to a taking lens made up of a front lens group G 1  composed of a negative meniscus lens deeply concave on its image-plane side and a positive meniscus lens steeply convex on its image side, an optical filter F for cutting infrared, an aperture stop S and a rear lens group G 2  composed of a negative meniscus lens deeply concave on its image side and a double-convex positive lens.  
       Example 5  
       [0070]    As shown in FIG. 5( a ), this example is directed to a taking lens made up of a front lens group G 1  composed of a negative meniscus lens deeply concave on its image-plane side and a thick planoconvex positive lens steeply convex on its image side, an optical filter F for cutting infrared, an aperture stop S and a rear lens group G 2  composed of a cemented positive lens consisting of a planoconcave negative lens element deeply concave on its image side and a double-convex positive lens element.  
         [0071]    In the instant example, the two lenses in the front lens group G 1  are engaged at their planar portions with each other, thereby achieving improvements in assembly precision as well as efficiency of fabrication work.  
         [0072]    Numerical data on the respective examples are given below. Symbols used hereinafter but not hereinbefore have the following meanings.  
         [0073]    f: focal length of the taking lens,  
         [0074]    F NO : F-number,  
         [0075]    ω: half angle of view,  
         [0076]    IH: image height,  
         [0077]    r 1 , r 2 , . . . : radius of curvature of each lens surface,  
         [0078]    d 1 , d 2 , . . . : spacing between adjacent lens surfaces,  
         [0079]    n d1 , n d2 , . . . : d-line refractive index of each lens, and  
         [0080]    ν d1 , ν d2 , . . . : d-line based Abbe number of each lens.  
       Example 1  
       [0081]    [0081]                                                               f   5.32                               F NO     2.80       ω   24.2°       IH   2.30       f B     7.167       r 1  =   5.591   d 1  =   2.314   n d1  =   1.77250   υ d1  =   49.60       r 2  =   2.128   d 2  =   2.645       r 3  =   −31.816   d 3  =   1.405   n d2  =   1.62004   υ d2  =   36.26       r 4  =   −4.551   d 4  =   0.141       r 5  =   ∞   d 5  =   1.000   n d3  =   1.49782   υ d3  =   66.83       r 6  =   ∞   d 6  =   0.300       r 7  =   ∞ (Stop)   d 7  =   0.300       r 8  =   21.773   d 8  =   1.868   n d4  =   1.84666   υ d4  =   23.78       r 9  =   2.928   d 9  =   1.706   n d5  =   1.67003   υ d5  =   47.23       r 10  =   −4.366       f B /IH   3.116       SF   2.229       f/f 1     0.039       f B /d S-R     1.850                    
       Example 2  
       [0082]    [0082]                                                               f   5.24                               F NO     2.80       ω   24.5°       IH   2.30       f B     7.410       R 1  =   5.636   d 1  =   0.713   n d1  =   1.77250   υ d1  =   49.60       r 2  =   2.237   d 2  =   3.173       r 3  =   −101.818   d 3  =   1.729   n d2  =   1.62004   υ d2  =   36.26       r 4  =   −4.003   d 4  =   1.332       r 5  =   ∞   d 5  =   1.000   n d3  =   1.49782   υ d3  =   66.83       r 6  =   ∞   d 6  =   0.300       r 7  =   ∞ (Stop)   d 7  =   0.300       r 8  =   13.366   d 8  =   0.600   n d4  =   1.84666   υ d4  =   23.78       r 9  =   2.78   d 9  =   2.069   n d5  =   1.67003   υ d5  =   47.23       r 10  =   −8.307       f B /IH   3.222       SF   2.316       f/f 1     0.385       f B /d S-R     2.496                    
       Example 3  
       [0083]    [0083]                                                               f   5.54                               F NO     2.82       ω   23.3°       IH   2.30       f B     7.614       r 1  =   6.559   d 1  =   2.177   n d1  =   1.77250   υ d1  =   49.60       r 2  =   2.542   d 2  =   2.698       r 3  =   328.8   d 3  =   1.516   n d2  =   1.62004   υ d2  =   36.26       r 4  =   −5.193   d 4  =   1.954       r 5  =   ∞   d 5  =   1.000   n d3  =   1.49782   υ d3  =   66.83       r 6  =   ∞   d 6  =   0.300       r 7  =   ∞ (Stop)   d 7  =   0.300       r 8  =   15.038   d 8  =   0.600   n d4  =   1.84666   υ d4  =   23.78       r 9  =   3.494   d 9  =   0.400       r 10  =   5.095   d 10  =   1.540   n d5  =   1.67003   υ d5  =   47.23       r 11  =   −4.329       f B  /IH   3.311       SF   2.266       f/f 1     0.131       f B /d S-R     2.681                    
       Example 4  
       [0084]    [0084]                                                               f   4.75                               F NO     2.78       ω   21.5°       IH   1.80       f B     6.873       r 1  =   4.556   d 1  =   1.276   n d1  =   1.77250   υ d1  =   49.60       r 2  =   1.809   d 2  =   3.257       r 3  =   −30.16   d 3  =   0.914   n d2  =   1.62004   υ d2  =   36.26       r 4  =   −3.519   d 4  =   0.695       r 5  =   ∞   d 5  =   0.800   n d3  =   1.49782   υ d3  =   66.83       r 6  =   ∞   d 6  =   0.240       r 7  =   ∞ (Stop)   d 7  =   0.240       r 8  =   6.463   d 8  =   0.480   n d4  =   1.84666   υ d4  =   23.78       r 9  =   2.458   d 9  =   0.057       r 10  =   2.379   d 10  =   1.576   n d5  =   1.51633   υ d5  =   64.14       r 11  =   −5.579       f B /IH   3.818       SF   2.317       f/f 1     0.280       f B /d S-R     2.921                    
       Example 5  
       [0085]    [0085]                                                               f   5.03                               F NO     2.81       ω   25.2°       IH   2.30       f B     7.643       r 1  =   5.836   d 1  =   0.900   n d1  =   1.77250   υ d1  =   49.60       r 2  =   2.273   d 2  =   1.000       r 3  =   ∞   d 3  =   3.800   n d2  =   1.62004   υ d2  =   36.26       r 4  =   −4.201   d 4  =   0.940       r 5  =   ∞   d 5  =   1.000   n d3  =   1.49782   υ d3  =   66.83       r 6  =   ∞   d 6  =   0.300       r 7  =   ∞ (Stop)   d 7  =   0.300       r 8  =   ∞   d 8  =   0.900   n d4  =   1.84666   υ d4  =   23.78       r 9  =   2.645   d 9  =   2.900   n d5  =   1.67003   υ d5  =   47.23       r 10  =   −4.07       f B /IH   3.323       SF   2.276       f/f 1     0.222       f B /d S-R     1.864                    
         [0086]    Here the “IH” used herein is now explained. The “IH” represents a length that is half the diagonal length of an effective plane of the electronic image pickup device. FIG. 6 is illustrative of the diagonal length of the effective image pickup plane of an electronic image pickup device. The “effective image pickup plane” used herein is understood to mean a certain area in the photoelectric conversion surface on an image pickup device used for the reproduction of a phototaken image (on a personal computer or by a printer). The effective image pickup plane shown in FIG. 6 is set at an area narrower than the total photoelectric conversion surface on the image pickup device (CCD or CMOS), depending on the performance of the optical system used (an image circle that can be ensured by the performance of the optical system). The diagonal length L of an effective image pickup plane, i.e., the diagonal length L of the effective plane is thus defined by that of the effective image pickup plane. Although the image pickup range used for image reproduction may be variable, it is noted that when the taking lens of the present invention is used on an imaging system having such functions, the diagonal length L of its effective image pickup plane varies. In that case, the diagonal length L of the effective image pickup plane according to the present invention is defined by the maximum value in the possible widest range for L. FIG. 6 is illustrative of one exemplary pixel array for the electronic image pickup device, wherein R (red), G (green) and B (blue) pixels or four pixels, i.e., cyan, magenta, yellow and green (G) pixels are mosaically arranged at a pixel spacing  a .  
         [0087]    The present image pickup lens constructed as described above may be applied to electronic phototaking systems where object images formed through image pickup lenses are received at image pickup devices such as CCDs for photo-taking purposes, inter alia, digital cameras or video cameras as well as portable telephones. Given below are some such embodiments.  
         [0088]    [0088]FIGS. 7, 8 and  9  are conceptual illustrations of a digital camera, in which the image pickup lens of the present invention is built. FIG. 7 is a front perspective view of the outside shape of a digital camera  20 , and FIG. 8 is a rear perspective view of the same. FIG. 9 is a partly cut-away side view of the construction of the digital camera  20 . In this embodiment, the digital camera  20  is built up of a digital camera body  21  and a taking portion  22  wherein, indicated by a double arrow, the taking portion  22  is mounted on the camera body  21  in a vari-angle fashion. At the taking portion  22 , a taking lens  24  of the present invention including an entrance axis  23  is located together with a reflective member (reflecting mirror)  16  and an image pickup device (CCD)  15 . At the camera body  21 , a shutter  25 , a flash  26 , a liquid crystal display monitor  27 , etc. are located. As the shutter  25  mounted on the upper portion of the camera body  21  is pressed down while a subject indicated on the liquid crystal display monitor  27  is viewed, phototaking takes place through the taking lens  24  at the taking portion  22 , for instance, the taking lens according to Example 1. In this case, the angle of the camera body  21  with the taking portion  22  is freely settable. Although not shown, a finder may or may not be located at the taking portion  22 .  
         [0089]    In this embodiment, the reflective member  16  is located in such a way that the short-side direction of the image pickup device  15  positioned at the image pickup plane of the taking lens  24  at the taking portion  22  is substantially parallel with the entrance axis  23 , so that the space for receiving the reflecting member  16  can be cut down.  
         [0090]    The thickness of the taking portion  22  in the direction of the entrance axis  23  is so large relative to the height direction that the size of the system in the height direction can be more reduced to slim down the system.  
         [0091]    In this embodiment, a shaft and other parts for the vari-angle mechanism may be located in a space on the back of the opposite surface, which is created by holding the image pickup device  15  back with respect to the entrance axis  23 , thereby increasing the degree of freedom in the layout of the whole electronic imaging system.  
         [0092]    [0092]FIGS. 10 and 11 are conceptual illustrations of a video camera in which the taking lens of the present invention is built. FIG. 10 is a front perspective view of the outside shape of a video camera  30 , and FIG. 11 is a partly cut-away top view of that video camera  30 . In this embodiment, the video camera  30  is built up of a video camera body  31 , and a liquid crystal display monitor  32  that is collapsible with respect to the camera body  31  during carrying and mounted at a controllable angle (in a vari-angle fashion). Within the camera body  31 , a taking lens  34  having an entrance axis  33  according to the present invention is located together with a reflective member (reflecting mirror)  16  and an image pickup device (CCD)  15 , and processing means  37  for processing signals of a phototaken image and recording means  38  for recording such signals are incorporated. On the camera body  31 , a manipulation button  36  for manipulating the video camera  30 , a stereo-microphone  35  for capturing sounds, etc. are provided. As the manipulation button  36  is manipulated while a subject indicated on the liquid crystal display monitor  32  is viewed, phototaking occurs through the taking lens  34 , e.g., the taking lens according to Example 1, and signals of a phototaken image are recorded in the recording means  38  via the processing means  37 . In this case, the angle of the liquid crystal display monitor  32  with the camera  31  is freely settable. Although not shown, a finder may or may not be located.  
         [0093]    In this embodiment, the reflective member  16  is located in such a way that the long-side direction of the image pickup device  15  positioned at the image pickup plane of the taking lens  34  is substantially parallel with an entrance axis  33 , so that the degree of freedom in the layout of the imaging system can be increased.  
         [0094]    The thickness of the system in the direction of the entrance axis  33  is so large relative to the width direction that the size of the system in the width direction can be more reduced to slim down the system.  
         [0095]    FIGS.  12 ( a ),  12 ( b ) and  12 ( c ) are illustrative of a telephone set that is one example of the information processor in which the taking lens of the present invention is built in the form of a phototaking optical system, especially a convenient-to-carry cellular phone. FIG. 12( a ) and FIG. 12( b ) are a front and a side view of a cellular phone  40 , respectively, and FIG. 12( c ) is a sectional view of a phototaking optical system  45 . As shown in FIGS.  12 ( a ),  12 ( b ) and  12 ( c ), the cellular phone  40  comprises a microphone  41  for entering the voice of an operator therein as information, a speaker  42  for producing the voice of the person on the other end, an input dial  43  via which the operator enters information therein, a monitor  44  for displaying an image taken of the operator or the person on the other end and indicating information such as telephone numbers, a phototaking optical system  45 , an antenna  46  for transmitting and receiving communication waves, and processing means (not shown) for processing image information, communication information, input signals, etc. Here the monitor  44  is a liquid crystal display device. It is noted that the components are not necessarily arranged as shown. The phototaking optical system  45  comprises on a phototaking optical path  47  the taking lens according to the present invention, e.g., the taking lens according to Example 1 as well as a reflective member (reflecting mirror)  16  and an image pickup device (CCD or C-MOS)  15 . These components are built in the cellular phone  40 . The phototaking optical system  45  is provided at its end with a cover glass  49  for protecting the taking lens.  
         [0096]    An object image received at the image pickup device  15  is entered via a terminal of the image pickup device  15  in processing means (not shown), so that the object image can be displayed as an electronic image on the monitor  44  and/or a monitor at the other end. The processing means also include a signal processing function for converting information about the object image received at the image pickup device  15  into transmittable signals, thereby sending the image to the person at the other end.  
         [0097]    In accordance with the arrangement shown in FIGS.  12 ( a ),  12 ( b ) and  12 ( c ), the size of the cellular phone in the height direction can be reduced. A specific layout for reducing the size of the cellular phone in the thickness direction, on the other hand, is shown in FIGS.  13 ( a ),  13 ( b ) and  13 ( c ) that are a front, a side and a sectional view of a modification to the cellular phone  40 , respectively. This cellular phone works as shown in FIGS.  12 ( a ),  12 ( b ) and  12 ( c ) excepting what is described below.  
         [0098]    In this embodiment, a light beam is incident on the side of the cellular phone  40 , and an optical path is bent by a reflective member (reflecting mirror)  16  in the thickness direction of the cellular phone  40 . In this way, the cellular phone  40  having a phototaking function can be slimmed down. This embodiment provides an additional favorable layout because an antenna  46  is located at a space on the back of the reflecting surface  16 .  
         [0099]    As can be understood from the foregoing, the present invention can provide an electronic imaging system and an image pickup optical system in which an electronic image pickup device is horizontally located to ensure the space needed for the location of a reflective member while height reductions, length reductions and cost reductions are all achieved.