Abstract:
The invention relates to a focus detection optical system used with the so-called autofocus (AF) system mounted on single-lens reflex cameras (SLRs) or the like, and an imaging apparatus incorporating it. The focus detection optical system comprises at least n focus detection areas that are adjacent to or intersect each other on a predetermined imaging plane, where n≧2. A re-imaging lens group comprises n+1 re-imaging lenses, A (n−1)th re-imaging lens and an nth re-imaging lens are a pair of re-imaging lenses that correspond to a (n−1)th focus detection area and are adjacent to each other. An nth re-imaging lens and a (n+1)th re-imaging lens are a pair of re-imaging lenses that correspond to the nth focus detection area and are adjacent to each other. The (n−1)th re-imaging lens and (n+1)th re-imaging lens are located at different positions.

Description:
[0001]    This application claims benefit of Japanese Application No. 2007-128782 filed in Japan on May, 15, 2007, the contents of which are incorporated by this reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to a focus detection system used with the so-called phase difference type autofocus (AF) system mounted on single-lens reflex cameras (SLRs) or the like, and an imaging apparatus incorporating it. More particularly, the invention is concerned with a focus detection optical system that, with a taking lens defocused theoretically, enables a defocus direction and a defocus quantity to be so figured out that the taking lens can be more quickly placed in an in-focus state than achieved with the so-called contrast method, and an imaging apparatus incorporating it. 
         [0003]    So far, there has been a system comprising a taking lens for projecting a subject onto an imaging plane, a means for splitting or selecting an optical path from the taking lens to the imaging plane (a quick return mirror or half-mirror), a primary imaging plane (predetermined imaging plane) set at a position roughly equivalent to said imaging plane on an optical path passing through them, a field stop located near that primary imaging plane to form a focus detection area, a condenser lens located near the primary imaging plane, aperture stops for implementing pupil division by a plurality of openings, and a plurality of re-imaging lenses and a light receptor element array located corresponding to the respective aperture stops, wherein the condenser lens is adapted to project different areas within the taking lens onto the respective aperture stops, and the re-imaging lenses are adapted to project an aerial image at the primary imaging position onto the light receptor element array through the corresponding aperture stops. 
         [0004]    With this system, positions of images projected onto the light receptor element array corresponding to the focus detection area are compared to implement range finding, but constant areas at both ends of the focus detection area are where to obtain defocus information: the focus of the subject at that position cannot be detected. 
         [0005]    Known as a typical prior art relying upon such a method is a focus detection system wherein, to implement detection for each of a plurality of focus detection areas, there are a pair of re-imaging lenses provided, and the respective re-imaging lenses are shared upon re-imaging of a plurality of focus detection areas (Patent Publication 1). 
         [0006]    As shown in  FIG. 20 , the prior art comprises a re-imaging lens group  504  comprising a pair of re-imaging lenses corresponding to each of three focus detection areas lined up in one direction on a predetermined imaging plane. The re-imaging lens group  504  is made up of three re-imaging lenses  541 ,  542  and  543  lined up in one direction; both the outer re-imaging lenses  541  and  543  cooperate to implement focus detection for the center focus detection area, and one outer re-imaging lens  541  and the center re-imaging lens  542  cooperate to implement focus detection for the outer focus detection areas. The prior art also says that the focus detection system may just as well be located in the direction coming out of the paper. 
         [0007]    Patent Publication 1 
         [0008]    JP (A) 1-266503 
         [0009]    However, this focus detection system is designed to implement focus detection using mutually spaced re-imaging lenses for the intermediate focus detection area. Such an optical layout is likely to have an increased angle of incidence to the re-imaging lenses and so be affected by aberrations. An attempt to decrease the angle of incidence to the re-imaging lenses would work against size reductions, because the distance between the focus detection areas and the re-imaging lens group grows long. 
         [0010]    Further, the prior art comes up with a plurality of combinations, each comprising at least three re-imaging lenses, wherein a light receptor element array corresponding to the respective re-imaging lenses is independently set up. And focus detection at one range-finding area is implemented by detecting a phase difference in one direction: phase difference information obtained by only one set of re-imaging lenses. 
         [0011]    With such an arrangement, an attempt to increase focus detection area size would increase the angle of incidence on each re-imaging lens in the re-imaging lens group, and so tend to produce aberrations. An effort to keep the aberrations in check would bring the re-imaging lenses away from the focus detection area, again working against size reductions. 
         [0012]    In view of such problems with the prior art as mentioned above, the invention has for its object to provide a focus detection optical system that comprises a plurality of closely located focus detection areas and is easily capable of making sure focus detection capability while a re-imaging lens group gets close to the focus detection areas, and an imaging apparatus incorporating it. Another object of the invention is to provide a focus detection optical system that has a wide focus detection area and is less affected by aberrations, and an imaging apparatus incorporating it. 
       SUMMARY OF THE INVENTION 
       [0013]    To accomplish the aforesaid objects, the invention provides a focus detection optical system, comprising a condenser lens located near a focus detection area on a predetermined imaging plane for a taking lens, a pupil division stop having a pair of openings arranged at a spacing wide enough to make sure focusing precision corresponding to said focus detection area, a re-imaging lens group having a plurality of re-imaging lenses located corresponding to said openings, and a light receptor element array located at imaging positions set by said re-imaging lenses, wherein said light receptor element array is adapted to receive a light intensity distribution of double beams passing through different areas of said taking lens and then through said focus detection area to detect a phase difference of an output signal indicative of a light intensity distribution obtained from said light receptor element array, thereby implementing focus detection for said focus detection area, characterized in that said focus detection optical system comprises at least n such focus detection areas that are adjacent to or intersect each other on said predetermined imaging plane, where n≧2; said re-imaging lens group comprises n+1 such re-imaging lenses; a (n−1)th re-imaging lens and an nth re-imaging lens are a pair of re-imaging lenses that correspond to said (n−1)th focus detection area and are adjacent to each other; an nth re-imaging lens and a (n+1)th re-imaging lens are a pair of re-imaging lenses that correspond to said nth focus detection area and are adjacent to each other; and said (n−1)th re-imaging lens and said (n+1)th re-imaging lens are located at different positions. 
         [0014]    Thus, each of the n adjoining or intersecting focus detection areas may be set up with a pair of adjoining openings and a pair of adjoining re-imaging lenses. And the respective re-imaging lenses being adjacent to each other renders it easy to make the angle of incidence of light rays on them small and keep aberrations in check, even when the distance between the focus detection area and the re-imaging lens is shorter. 
         [0015]    Especially if the aforesaid nth and (n+1)th re-imaging lenses are located corresponding to the aforesaid nth focus detection area, it is easy to achieve more simplified arrangement and increase opening size, because (n+1) re-imaging lenses can be used to implement focus detection for the adjoining or crossing n focus detection areas. 
         [0016]    It is also easy to bring the respective focus detection areas close to one another because of no need of making the focus detection systems independent from the respective focus detection areas. 
         [0017]    A pair of re-imaging lenses corresponding to one focus detection area being adjacent to each other contributes to slimming down the focus detection system, because it is easy to prevent light rays from entering the same light receptor element array from a plurality of focus detection area. 
         [0018]    Consequently, there can be a focus detection system provided, which is simplified in the construction of pupil division stops and re-imaging lens group, comprises a plurality of focus detection areas that can be easily close to one another and facilitates making sure focus detection capability while the re-imaging lens group is kept close to the focus detection areas. 
         [0019]    For instance, when, to make focus detection precision high, two pairs of pupil division stops and re-imaging lenses differing in the direction that they are lined up are used in correspondence to vertical and horizontal lines on a subject, too, it is possible to make openings in the pupil division stops large. 
         [0020]    By definition, that the focus detection areas are “adjacent to one another” here means that between the centers of the mutual focus detection areas, there is no focus detection area capable of detecting a phase difference in the direction of connecting the centers of the mutual focus detection areas. 
         [0021]    Also, that the re-imaging lenses in pair form are “adjacent to each other” here means that on a line of connecting the centers of the effective surfaces of a pair of re-imaging lenses, there is none of the effective surface of other re-imaging lens. 
         [0022]    According to the invention, for instance, three focus detection areas may be set up with four pupil division stops and four re-imaging lenses; typically, (n+1) pupil division stops and (n+1) re-imaging lenses may be used for n focus detection areas. 
         [0023]    The focus detection optical system of the invention is further characterized in that said openings corresponding to said first to (n+1)th re-imaging lenses are lined up in a row, and said first to nth focus detection areas are lined up in a row as well. 
         [0024]    This arrangement enables the focus detection system to comprise n focus detection areas that can be close to one another in the direction that they are lined up, and makes it easy to bring re-imaging lenses corresponding to the respective focus detection areas close to one another. 
         [0025]    The focus detection optical system of the invention is further characterized in that the light receptor element arrays corresponding to said first to (n+1)th re-imaging lenses are located on the same light receptor member. 
         [0026]    The provision of the respective light receptor element array on a single light receptor member goes in favor of making sure focus detection precision because of keeping light receiving performance from varying from light receptor member to light receptor member. 
         [0027]    The focus detection optical system of the invention is further characterized in that the pair of openings corresponding to the first to nth focus detection areas are lined up in the same direction, and one piece of phase difference information is detected of light intensity distributions across a plurality of focus detection areas out of said first to nth focus detection areas. 
         [0028]    By making the individual focus detection areas small thereby reducing a drop of detection precision due to aberration and drawing phase difference information out of the bulk of a plurality of focus detection areas, it is possible to set up a substantially wide focus detection area. It is also possible to increase a defocus quantity capable of focus detection. 
         [0029]    The focus detection optical system of the invention is further characterized in that said plurality of focus detection areas are adjacent to each other, and said phase difference information is detected using a discontinuous intensity distribution separated for each focus detection area on the light receptor element array. 
         [0030]    Thus, if there is a phase difference detected from a phase based on the adjoining focus detection areas, then it is possible to implement focus detection for a continuity of focus detection areas with improved focusing precision. It is also possible to implement a spot form of focus detection by a single focus detection area, an area form of focus detection changeover with a continuity of multiple focus detection areas, and selection of areas where focus detection is implemented, and so on. 
         [0031]    The focus detection optical system of the invention is further characterized in that each one pair of openings corresponding to each of said plurality of adjoining focus detection areas have the same spacing. 
         [0032]    This arrangement allows the defocus quantity and phase difference quantity for each focus detection area to have the same relations, facilitating focus detection. 
         [0033]    The focus detection optical system of the invention is further characterized in that a conjugate position to said condenser lens of each one pair of openings corresponding to each of said plurality of adjoining focus detection areas is a pair of the same areas. 
         [0034]    This arrangement enables an optical path through the taking lens to be efficiently laid out: by use of phase difference information about a light beam passing through the same area in the taking lens, focus detection precision can be improved. 
         [0035]    The focus detection optical system of the invention is further characterized in that there are at least two rows of said openings lined up in a row, two such rows of said openings are arranged parallel with each other, there are at least two rows of said focus detection areas lined up in a row, and two such rows of said focus detection areas are arranged parallel with each other as well. It is thus possible to broaden an area having a focus detectable on the predetermined imaging plane. 
         [0036]    The focus detection optical system of the invention is further characterized in that there are at least two rows of said openings lined up in a row, two such rows of said openings intersect each other, there are at least two rows of said focus detection areas lined up in a row, and two such rows of said focus detection areas intersect each other as well. It is thus possible to facilitate focus detection because an area having a focus detectable on the predetermined imaging plane can be widened, and there is a phase difference detectable in a different direction. 
         [0037]    The focus detection optical system of the invention is further characterized in that there are a plurality of rows of said openings lined up in a row, said plurality of rows of said openings comprise a plurality of rows of mutually parallel openings and a plurality of rows of openings that intersect said plurality of rows of said parallel openings, there are a plurality of rows of said focus detection areas lined up in a row as well, and said plurality of rows of said focus detections areas comprise a plurality of rows of mutually parallel focus detection areas and a plurality of rows of focus detection areas that intersect said plurality of rows of mutually parallel focus detection areas. It is thus possible to facilitate focus detection because an area having a focus detectable on the predetermined imaging plane can be widened, and there is a phase difference detectable in a different direction. 
         [0038]    The focus detection optical system of the invention is further characterized in that a direction that the openings corresponding to said (n−1)th and nth re-imaging lenses are lined up is different from a direction that said nth and (n+1)th re-imaging lenses are lined up. It is thus possible to facilitate focus detection because the phase difference information of the subject may be obtained not only from one direction but also from other directions. 
         [0039]    The focus detection optical system of the invention is further characterized in that between said (n−1)th and nth focus detection areas there is none of other focus detection area, and one of said focus detection areas is positioned in a longitudinal direction of another. 
         [0040]    It is thus preferable that the direction of the phase difference to be subjected to phase detection is placed close to a plurality of different focus detection areas, because the focus detection precision for a main subject can be increased. 
         [0041]    Further, when one focus detection area is positioned on an extension of the longitudinal direction of another focus detection area, it is possible to make the whole focus detection area large. 
         [0042]    On the other hand, as two pair of pupil division stops and re-imaging lenses lined up in different directions are used to allow two pairs of pupil division stops and re-imaging lenses to intersect, it is preferable because a spot form of focus detection can be implemented in correspondence to vertical and horizontal lines on a subject with improved focus detection precision. 
         [0043]    Further, the intersecting focus detection areas may be taken as one focus detection area; this is preferable because of a decrease in the total number of focus detection areas. This leads to an increase in the integration density of focus detection areas corresponding to vertical and horizontal lines. 
         [0044]    The focus detection optical system of the invention is further characterized in that said condenser lens comprises a plurality of optical axes corresponding to a plurality of said focus detection areas. Thus, by setting the optical axes of the condenser lens for each focus detection area, light beams passing through the taking lens can be efficiently laid out. 
         [0045]    The focus detection optical system of the invention is further characterized by comprising field apertures located near said predetermined imaging plane and corresponding to the respective focus detection areas. It is thus possible to prevent crosstalk with other focus detection areas on the light receptor element array, thereby bringing the focus detection areas even closer to one another. 
         [0046]    The focus detection optical system of the invention is further characterized by comprising a light block wall located just before said light receptor element array for each imaging area of the light receptor element array. It is thus possible to prevent crosstalk with other focus detection areas on the light receptor element array, thereby bringing the focus detection areas even closer to one another. 
         [0047]    The focus detection optical system of the invention is further characterized in that said pupil division stop has at least two pairs of openings lined up in different directions at a spacing wide enough to make sure focusing precision corresponding to any one of said focus detection areas, the re-imaging lens group has a re-imaging lens located corresponding to each of said at least two pairs of openings, and said light receptor element array has a light receptor element array located at an imaging position by said re-imaging lens. 
         [0048]    By use of two pairs of aperture stops and re-imaging lenses located corresponding to the same focus detection area and lined up in different direction, it is possible to increase focus detection precision in correspondence to vertical and horizontal lines on a subject. This is also preferable not only because the total number of focus detection areas can be decreased but also because the integration density of focus detection areas corresponding to the vertical and horizontal lines can be increased. 
         [0049]    The focus detection optical system of the invention is further characterized in that said re-imaging lens group comprises a plurality of re-imaging lenses that are located regularly in a planar fashion, and said light receptor element array comprises a plurality of light receptor elements that are located regularly in a planar fashion. 
         [0050]    Consequently, there is an increase in the degree of flexibility in combinations of the adjoining re-imaging lenses in pairs with the corresponding light receptor element array. Also, by a choice of the light receptor element array used, it is possible to increase the degree of flexibility in the selection of focus detection areas for which focus detection is to be implemented. 
         [0051]    Further, the invention provides an imaging apparatus, comprising an imaging device adapted to take an image formed by a taking lens, a focus detection optical system, and a reflective member adapted to reflect a light beam from said taking lens and guide said light beam to a predetermined imaging plane, characterized in that said reflective member retracts out of a taking optical path when the image is taken by said imaging device. By the retraction of the reflective member when an image is taken by the imaging device, it is thus possible to provided an imaging apparatus that prevents a decrease in the quantity of light of an image taken by the imaging device. 
         [0052]    Yet further, the invention provides an imaging apparatus, comprising an imaging device adapted to take an image formed by a taking lens, a focus detection optical system, and a reflective member adapted to reflect or transmit a light beam from said taking lens, characterized in that said imaging device is located on one of a reflective or transmissive side of said reflective member, and said focus detection optical system is located on another. It is thus possible to provide an imaging apparatus capable of implementing focus detection while images are being taken by the imaging device. 
         [0053]    Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
         [0054]    The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which 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 
         [0055]      FIG. 1  is illustrative of the first embodiment of the invention of this application. 
           [0056]      FIG. 2  is illustrative of comparisons of the first embodiment with a prior art. 
           [0057]      FIG. 3  is illustrative of a modification to the first embodiment wherein the field of view is extended. 
           [0058]      FIG. 4  is illustrative of an embodiment of the invention wherein light block walls are located. 
           [0059]      FIG. 5  is illustrative of the second embodiment of the invention of this application. 
           [0060]      FIG. 6  is illustrative of a modification to the second embodiment wherein the field of view is extended. 
           [0061]      FIG. 7  is illustrative of the third embodiment of the invention of this application. 
           [0062]      FIG. 8  is illustrative of a numerical example of the invention. 
           [0063]      FIG. 9  is a layout view of the photoelectric transformation member as viewed from the optical axis direction. 
           [0064]      FIG. 10  is illustrative of another embodiment of the invention of this application. 
           [0065]      FIG. 11  is a layout view for pupil division stop apertures and re-imaging lenses. 
           [0066]      FIG. 12  is illustrative of an example of the focus detection optical system mounted on an imaging apparatus. 
           [0067]      FIG. 13  is illustrative of an example of the focus detection optical system mounted on an imaging apparatus. 
           [0068]      FIG. 14  is illustrative of an example of the focus detection optical system mounted on an imaging apparatus. 
           [0069]      FIG. 15  is illustrative of a general phase difference type focus detection optical system. 
           [0070]      FIG. 16  is illustrative of a reference example. 
           [0071]      FIG. 17  is illustrative of a comparative example. 
           [0072]      FIG. 18  is illustrative of a comparative example. 
           [0073]      FIG. 19  is illustrative of a comparative example. 
           [0074]      FIG. 20  is illustrative of one prior art. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0075]    Exemplary embodiments of the invention are now explained inclusive of a general focus detection optical system and references examples. 
         [0076]    A basics configuration of, and problems with, a general phase difference type focus detection optical system are now explained.  FIG. 15  is a basic configuration of the phase difference type focus detection optical system. Located from a subject side are a taking optical system  101 , a predetermined imaging plane  102  lying at a position equivalent to an imaging plane, a field stop aperture  103  (herein also called a field aperture) for a focus detection area set near the predetermined imaging plane  102 , a condenser lens  104  located near the predetermined imaging plane  102 , a pair of openings  105   a  and  105   b  (hereinafter also called a pair of pupil division apertures) in a pair of aperture stops (pupil division stops) for implementing pupil division, re-imaging lenses  106   a  and  106   b  located near the respective pupil division stop apertures, and photoelectric transformation portions  107   a  and  107   b  corresponding to the respective re-imaging lenses  106   a  and  106   b  (here, an array of light receptor elements that is an array of photoelectric transformation elements lined up in the direction that the aperture stop openings are lined up). Of these, what is defined by the field aperture  103 , condenser lens  104 , pupil division stop apertures  105   a  and  105   b , re-imaging lenses  106   a  and  106   b  and photoelectric transformation planes of the photoelectric transformation portions  107   a  and  107   b  provided on a light receptor member  107 A is generally called a focal detection optical system. 
         [0077]    The photoelectric transformation portion  107   a ,  107   b  is a line sensor or the like, and it is often constructed in such a way as to produce the light intensity distribution of an image re-imaged on the sensor. 
         [0078]    Suppose here that the taking lens  101  has therein a pair of virtual apertures  108   a  and  108   b  through which light beams are not shaded and which have a space enough for focus detection. Those apertures are preferably supposed to lie on the exit pupil of the taking lens  101 ; indeed, however, they are often set in consideration of the whole optical system, because there is much difficulty in keeping the exit pupil at a constant position as focusing, zooming, lens replacement or the like is implemented. The virtual aperture  108   a  is conjugated by the condenser lens  104  to the opening  105   a  in the pupil division stop, and the field aperture  103  is projected by the condenser lens  104  and re-imaging lens  106   a  onto the light receiving plane of the photoelectric transformation portion  107   a . Likewise, the virtual aperture  108   b  is conjugated by the condenser lens  104  to the pupil division stop aperture  105   b , and the field aperture  103  is projected by the condenser lens  104  and re-imaging lens  106   b  onto the light receiving plane of the photoelectric transformation portion  107   b . 
         [0079]    The requirement here is that the images on the field aperture  103  projected onto the respective photoelectric transformation portions  107   a  and  107   b  do not overlap each other on the light receiving planes of the photoelectric transformation portions  107   a  and  107   b . The total length of the focal detection optical system is substantially determined by such specifications as the size of the field aperture  103 , the magnification of the focal detection optical system, and the spacing between a pair of pupil division stop apertures  105   a  and  105   b . 
         [0080]      FIG. 16  is illustrative in schematic of one of error factors due to aberrations of the focal detection optical system. In  FIG. 16 , ΔU a  is a distance between a center point C and a point U on the field aperture  103  as they are projected onto the light receiving plane of the photoelectric transformation portion  107   a , ΔL a  is a distance between the center point C and a point L on the field aperture  103  as they are projected onto the light receiving plane of the photoelectric transformation portion  107   a , ΔU b  is a distance between the center point C and the point U on the field aperture  103  as they are projected onto the light receiving plane of the photoelectric transformation portion  107   b , and ΔL b  is a distance between the center point C and the point L on the field aperture  103  as they are projected onto the light receiving plane of the photoelectric transformation portion  107   b . Note here that the distance between the center point C and the point L on the field aperture  103  is supposed to be the same as the distance between the center point C and the point U on the field aperture  103 . 
         [0081]    The phase difference type focus detection method involves estimation of the quantity of a misalignment between the light intensity distributions of two subject images projected onto the light receiving planes of the photoelectric transformation portions  107   a  and  107   b  via the field aperture  3 , etc. 
         [0082]    The shapes of the light intensity distributions of the two images themselves here must be the same. As there is a shape difference between the two images, it becomes an error factor upon estimation of a misalignment between the two images. A significant factor ascribable to such an optical intensity deformation is distortion of the focus detection optical system (an image position-depending magnification error). Basically, distortion occurs symmetrically about the optical axis of the re-imaging lens  106   a ,  106   b . Generally, the condenser lens  104  and the re-imaging lenses  106   a ,  106   b  are each made up of a single lens; in  FIG. 16 , it is difficult to make ΔU a  the same as ΔL a  by means of distortion. And the longer the distance between the points U and L, the more likely the difference between ΔU a  and ΔL a  is to grow large. In the absence of any fabrication error, ΔU a  and ΔL b  would have the same value; however, it is difficult to get rid of the difference between ΔU a  and ΔU b  that are compared to obtain phase difference information. In particular, this tends to grow as the power of the re-imaging lens  106   a ,  106   b  is increased to curtail the total length of the focus detection optical system. 
         [0083]    For simplification of explanation, the photoelectric transformation plane of each of the photoelectric transformation portions  107   a  and  107   b  is divided into two; in actual applications, however, it is divided into a lot more, and there are other error factors occurring as well by reason of chromatic aberrations and field curvature in addition to distortion. Widening the field aperture  103  may result in a wider range-finding area, but it is difficult to maintain range-finding precision. 
         [0084]      FIG. 17  is illustrative of an arrangement wherein, to slim down the focus detection optical system and widen a range-finding field, a field aperture  203  at a focus detection area is divided into sub-apertures, with the focus detection optical system located at each. In  FIG. 17 , a taking lens  201 , virtual areas  208   a ,  208   b  and predetermined imaging plane  202  similar to those of  FIG. 15  are or are supposed to be located, a field aperture  203  divided into  203   1 ,  203   2  and  203   3  is located near the predetermined imaging plane  202 , and pupil division stop apertures  205   a1  to  205   b3  corresponding to the respective field apertures  203   1 ,  203   2  and  203   3 , re-imaging lenses  206   a1  to  206   b3  and a photoelectric transformation portion having photoelectric transformation planes  207   a1  to  207   b3  provided on a light receptor member are located. 
         [0085]    And a condenser lens  204   1  is located near the field aperture  203   1 , the pupil division stop aperture  205   a1  is located at a conjugate position of a virtual area  208   a  to the condenser lens  204   1 , and the re-imaging lens  206   a1  is located near that pupil division stop aperture  205   a1 . 
         [0086]    That re-imaging lens  206   a1  projects an image on the field aperture  203   1  onto the photoelectric transformation plane  207   a1  via the condenser lens  204   1  and pupil division stop aperture  205   a1 . 
         [0087]    Provided in similar relations, there are a combination of virtual area  208   b , field aperture  203   1 , condenser lens  204   1 , pupil division stop aperture  205   b1 , re-imaging lens  206   b1  and photoelectric transformation plane  207   b1 , a combination of virtual area  208   a , field aperture  203   2 , condenser lens  204   2 , pupil division stop aperture  205   a2 , re-imaging lens  206   a2  and photoelectric transformation plane  207   a2 , a combination of virtual area  208   b , field aperture  203   2 , condenser lens  204   2 , pupil division stop aperture  205   b2 , re-imaging lens  206   b2  and photoelectric transformation plane  207   b2 , a combination of virtual area  208   a , field aperture  203   3 , condenser lens  204   3 , pupil division stop aperture  205   a3 , re-imaging lens  206   a3  and photoelectric transformation plane  207   a3 , and a combination of virtual area  208   b , field aperture  203   3 , condenser lens  204   3 , pupil division stop aperture  205   b3 , re-imaging lens  206   b3  and photoelectric transformation plane  207   b3 . 
         [0088]    This arrangement may work favorably for slimming-down and making sure range-finding precision, but it is not preferable because the field apertures  203   1 ,  203   2  and  203   3  are discretely located on the predetermined imaging plane  202 . 
         [0089]      FIG. 18  is illustrative of an example of the focus detection optical system wherein by reflection of an optical path corresponding to the outermost of three adjoining field apertures, the discreteness of each field aperture is diminished. 
         [0090]    In Comparative Example 2 of  FIG. 18 , a taking lens  301 , virtual areas  308   a  and  308   b  and predetermined imaging plane  302  similar to those in Comparative Example 1 of  FIG. 17  are, or are supposed to be, located, and a field aperture  303  divided into three focus detection areas  303   1 ,  303   2  and  303   3  is located near the predetermined imaging plane  302 . And are located pupil division stop apertures  305   a1  to  305   b3  corresponding to the respective field apertures  303   1 ,  303   2  and  303   3 , re-imaging lenses  306   a1  to  306   b3 , and photoelectric transformation planes  307   a1  to  307   b3  provided on the light receptor member. 
         [0091]    And there is a condenser lens  304   1  located near the field aperture  303   1 , with the pupil division stop aperture  305   a1  located at a conjugate position of the virtual area  308   a  to the condenser lens  304   1  and the re-imaging lens  306   a1  located near that pupil division stop aperture  305   a1 . 
         [0092]    The re-imaging lens  306   a1  projects an image on a field aperture  303   a  onto a photoelectric transformation plane  307   a1  via the pupil division stop aperture  305   a1 . 
         [0093]    Provided in similar relations, there are a combination of virtual area  308   b , field aperture  303   1 , condenser lens  304   1 , pupil division stop aperture  305   b1 , re-imaging lens  306   b1  and photoelectric transformation surface  307   b1 , a combination of virtual area  308   a , field aperture  303   2 , condenser lens  304   2 , pupil division stop aperture  305   a2 , re-imaging lens  306   a2  and photoelectric transformation surface  307   a2 , a combination of virtual area  308   b , field aperture  303   2 , condenser lens  304   2 , pupil division stop aperture  305   b2 , re-imaging lens  306   b2  and photoelectric transformation surface  307   b2 , a combination of virtual area  308   a , field aperture  303   3 , condenser lens  304   3 , pupil division stop aperture  305   a3 , re-imaging lens  306   a3  and photoelectric transformation surface  307   a3 , and a combination of virtual area  308   b , field aperture  303   3 , condenser lens  304   3 , pupil division stop aperture  305   b3 , re-imaging lens  306   b3  and photoelectric transformation surface  307   b3 . 
         [0094]    On the optical path between the condenser lens  304   1  and the re-imaging lens  306   a1 ,  306   b1  there is a reflective member  309   1  located as shown, and there is a reflective member  309   3  located between the condenser lens  304   3  and the re-imaging lens  306   a3 ,  306   b3  as shown. 
         [0095]    Such location of the reflective members makes it possible to overcome the problems of interferences of the photoelectric transformation planes  307   b1  and  307   a2  as well as  307   b2  and  307   a3 , whereby the problem of the discreteness of the field aperture  303  can be obviated to some extents. 
         [0096]    In this case, however, there must be some space to receive the reflective members for reflecting light beams about the field apertures  303   1  and  303   3 . To prevent interference of light beams that spread out following the size of the field apertures  303 , it is necessary to make sure the field apertures  303  have a space between them. Otherwise, the re-imaging lens  306  can never be flush with the photoelectric transformation plane  307 , not only resulting in an increased parts size and an increased parts count but also leading to a drop of assembly precision and an increased cost. 
         [0097]      FIG. 19  is illustrative in schematic of variants of the direction of bending light by reflective members. Here, a condenser lens group  304 , a re-imaging lens group  306 , a photoelectric transformation plane  307 , etc. are lined up in the direction coming out of the paper. 
         [0098]    Even with such a layout, it is needed to have some space to receive reflective members for reflecting light beams. To prevent interferences of light beams that spread out following the size of the field apertures  303 , they must have a space between them. 
         [0099]    The invention of this application is now explained.  FIGS. 1(   a ),  1 ( b ) and  2 ( a ) are illustrative of the first embodiment of the invention, and a basic arrangement common to other embodiments as well.  FIG. 2(   b ) is illustrative of a comparative example showing one exemplary prior art. 
         [0100]      FIG. 1(   a ) illustrates chief rays passing through the center of each focus detection area and the center of each aperture stop openings.  FIG. 1(   b ) illustrates light rays passing through the upper or lower end of one focus detection area and the upper end, center and lower end of an aperture stop opening adjacent to it. 
         [0101]    In  FIGS. 1(   a ) and  1 ( b ), a taking lens  1 , virtual areas  8   a  and  8   b  and a predetermined imaging plane  2  are, or are supposed to be, located as shown in  FIG. 15 . And a field aperture is supposed as an opening in the field stop for a focus detection area lined up near the predetermined imaging plane  2  in a row and marked off as  3   1 ,  3   2  and  3   3 . A condenser lens  4 , a plurality of pupil division stop apertures  5  that define openings in a pupil division stop, a re-imaging lens group  6  and a photoelectric transformation plane  7  that is an array of light receptor elements provided on a light receptor member  7 A, whose optical axes differ corresponding to the respective apertures  3   1 ,  3   2  and  3   3 , are lined up and located in the same direction. 
         [0102]    And a condenser lens  4   1  is located near the field aperture  3   1 . A pupil division stop aperture  5   1  is located at a conjugate position of a virtual area  8   a  to the condenser lens  4   1 , and a re-imaging lens  6   1  is located near it. The re-imaging lens  6   1  substantially projects an image on a field aperture  3   1  onto a photoelectric transformation plane  7   a1  through the condenser lens  4   1  and pupil division stop aperture  5   1 . 
         [0103]    A pupil division stop aperture  5   2  is located at a conjugate position of a virtual area  8   b  to the condenser lens  4   1 . A re-imaging lens  6   2  is located near that aperture, and the re-imaging lens  6   2  substantially projects an image on the field aperture  3   1  onto a photoelectric transformation surface  7   b1  through the condenser lens  4   1  and pupil division stop  5   2 . 
         [0104]    Likewise, an optical system is set up for other field aperture  3 , too. The pupil division stop aperture  5  and the re-imaging lens  6  are common to a pupil division stop aperture  5  and a re-imaging lens  6  in an optical system corresponding to other field aperture  3 . 
         [0105]    A condenser lens  4   2  is located near a field aperture  3   2 . A pupil division aperture stop  5   2  is located at a conjugate position of a virtual area  8   a  to a condenser lens  4   2 . A re-imaging lens  6   2  is located near that pupil division stop aperture  5   2 , and it is that re-imaging lens  6   2  that substantially projects an image on the field aperture  3   2  onto a photoelectric transformation surface  7   a1  through the condenser lens  4   2  and pupil division stop aperture  5   2 . 
         [0106]    A pupil division stop aperture  5   3  is located at a conjugate position of a virtual area  8   b  to a condenser lens  4   2 . A re-imaging lens  6   3  is located near that pupil division stop aperture  5   3 , and it is the re-imaging lens  6   3  that substantially projects an image on the field aperture  3   2  onto a photoelectric transformation surface  7   b2  through the condenser lens  4   2  and pupil division stop aperture  5   3 . 
         [0107]    A condenser lens  4   3  is located near a field aperture  3   3 . The pupil division stop aperture  5   3  is located at a conjugate position of the virtual area  8   a  to the condenser lens  4   3 . The re-imaging lens  6   3  is located near that pupil division stop aperture  5   3 , and it is the re-imaging lens  6   3  that substantially projects an image on the field aperture  3   3  onto a photoelectric transformation surface  7   a3  through the condenser lens  4   3  and pupil division stop aperture  5   3 . 
         [0108]    Further, a pupil division stop aperture  5   4  is located at a conjugate position of the virtual area  8   b  to the condenser lens  4   3 . A re-imaging lens  6   4  is located near that pupil division stop aperture  5   4 , and it is the pupil division stop aperture  5   4  that substantially projects on an image on the field aperture  3   3  onto a photoelectric transformation surface  7   4  through the condenser lens  4   3  and pupil division stop aperture  5   4 . 
         [0109]    Each photoelectric transformation plane  7  is formed on a CCD or CMOS that forms part of it, or a light receptor member, for instance, a member having an array of light receptor elements lined up in a row, thereby holding back fluctuations of sensitivity or other properties of the photoelectric transformation planes  7 . Of course, the light receptor elements may be lined up either in a single row or in multiple rows. The light receptor element array may be located at a position other than the position used for focus detection. 
         [0110]    A signal of the intensity distribution, obtained from each photoelectric transformation plane  7 , is guided to a signal processor S. At the signal processor S, a phase difference across the intensity distribution of each focus detection area  3   1 ,  3   2 ,  3   3  due to a light beam upon transmission through each virtual area  8   a ,  8   b  is detected, and on the basis of the detected phase difference, the signal processor S gives the taking lens  1  an instruction about the amount of movement for focusing. 
         [0111]    For instance, when only the focus detection area  3   2  is spotted for focus detection, a phase difference across an intensity distribution received at photoelectric transformation planes  7   a2  and  7   b2  is detected and depending on the magnitude of that phase difference, how much the taking lens  1  is to be moved is determined. In the example here, if the phase difference across the intensity distribution received at the photoelectric transformation planes  7   a2  and  7   b2  is zero, there is then an in-focus state, as shown in  FIG. 1(   a ). 
         [0112]    Where a subject image at the focus detection area  3   2  is shifted on the taking lens  1  side, the phase at the photoelectric transformation plane  7   a2  has a phase difference shown on the lower side of the drawing with respect to that at the photoelectric transformation plane  7   b2 . When there is out of focus in the opposite direction to the taking lens  1  side, the phase at the photoelectric transformation plane  7   a2  has a phase difference shown on the upper side of the drawing with respect to that at the photoelectric transformation plane  7   b2 . The same thing happens for other focus detection area  3 , too. 
         [0113]    It is also possible to implement focus detection using a plurality of focus detection areas  3 . For instance, when focus detection is implemented at three focus detection areas  3   1 ,  3   2  and  3   3 , the intensity distributions across the focus detection areas  3   1 ,  3   2  and  3   3  are detected on the basis of those at the photoelectric transformation planes  7   a1 ,  7   a2  and  7   a3 , and the intensity distributions across the focus detection areas  3   1 ,  3   2  and  3   3  are detected on the basis of those at the photoelectric transformation planes  7   b1 ,  7   b2  and  7   b3 . And depending on the magnitude of the phase differences of the respective detected intensity distributions, how much the taking lens  1  is to be moved is determined. Focus detection may also be implemented using any two focus detection areas  3  selected out of three. 
         [0114]    As shown in  FIGS. 1(   a ) and  1 ( b ), the pupil division stop aperture  5   2  and re-imaging lens  6   2  are commonly corresponding to the field apertures  3   1  and  3   2 . However, the corresponding virtual area  8  differs at the respective field apertures  3 : the field apertures  3   1  and  3   2  correspond to the virtual areas  8   b  and  8   a , respectively. 
         [0115]    Similarly, the pupil division stop aperture  5   3  and re-imaging lens  6   3  correspond commonly to the field apertures  3   2  and  3   3 . As in  FIGS. 17 and 18  referred to as comparative examples, there are three field apertures  3  as in  FIG. 18 , but there are four re-imaging lenses (six in Comparative Examples 1 and 2). 
         [0116]    Thus, no care should be taken of the overlaps of light beams from the respective field apertures  3  to the re-imaging lenses  6 ; it is possible to reduce the discreteness of the locations of the field apertures  3  to some considerable extents. It is also possible to diminish the angle of light on the re-imaging lenses  6 ; this works favorably for aberrations, and facilitates slimming down the optical system in consideration of a widening field of view as well. Further, it is possible to make the photoelectric transformation planes  7  flush with one another so that parts costs and assembling costs are easily cut short. Reduced discreteness also ensures that the outputs of  7   a1 ,  7   a2  and  7   a3  and the outputs of  7   b1 ,  7   b2  and  7   b3  are easily handleable as a successive output, helping make the range-finding areas wider and the measurable quantity of defocus much more. 
         [0117]      FIGS. 2(   a ) and  2 ( b ) are illustrative of the comparison of the embodiment of the invention with a conventional type:  FIG. 2(   a ) is one of the embodiments of the invention shown in  FIGS. 1(   a ) and  1 ( b ), and  FIG. 2(   b ) is a general prior example similar to that shown in  FIG. 15 . 
         [0118]    It is found that the powers of condenser lenses  4  and re-imaging lenses  6  can reasonably be increased, although there are field apertures  3  provided which are of much the same size as heretofore but have extremely reduced discreteness so that slimming-down is achievable. It is unlikely that the field apertures  203  become discrete as shown in  FIG. 16 , and it is not necessary to make sure some space for the reflective members  309 , either, as shown in  FIG. 17 . With  FIG. 17 , there are constraints under which optical paths leaving the field apertures cannot overlap: discreteness tends to grow substantially strong. In the embodiment here, however, it is found that this is overcome, too. 
         [0119]      FIG. 3  is illustrative of an embodiment wherein more field apertures than used in each of the aforesaid embodiments are provided in a specific direction so that the field of view is extended. In the embodiment shown in  FIGS. 1(   a ) and  1 ( b ), there are three field apertures in one direction; in the embodiment here, however, there are four or more lined up in a row. 
         [0120]    This embodiment is now explained on the basis of the optical axis of a taking lens  11 . The taking lens here is supposed to have a linear optical axis although, in actual applications, that optical path is often bent by a mirror or the like. The same will apply to other embodiments. 
         [0121]    Referring to  FIG. 3 , the taking lens  11 , virtual areas  18   a ,  18   b  and a predetermined imaging plane  12  are, or are supposed to be, located. Near the predetermined imaging plane  12  there are field apertures  13   1 ,  13   2 ,  13   3 , . . . ,  13   n−1  and  13   n  lined up, which are openings in n field stops for focal detection areas. About the optical axis, the field apertures  13  are lined up in the same direction as the virtual areas  18   a  and  18   b  are lined up. The field apertures  13 , each of the same size, are located equidistantly in an adjoining relation. This enables the focus detection system to be set up in order, working favorably for cost reductions. 
         [0122]    In the optical axis direction of, and near, the field apertures  13   1 ,  13   2 ,  13   3 , . . . ,  13   n−1  and  13   n , condenser lenses  14   1 ,  14   2 ,  14   3 , . . . ,  14   n−1  and  14   n  having varying optical axes are located corresponding to them. 
         [0123]    On a plane vertical to the optical axis and spaced away from the predetermined imaging plane  12  at a given distance in the opposite direction to the taking lens  11 , there are pupil division stop apertures  15   1 ,  15   2 ,  15   3 , . . . ,  15   n−1 ,  15   n  and  15   n+1  located, defining openings in (n+1) pupil division stops. The pupil division stop apertures  15 , each of the same size, are lined up at equal space. 
         [0124]    It is desired that a field aperture  13   i  lies between pupil division stop apertures  15   i  and  15   i+1 , as viewed through in the optical axis direction, where i is 1 to n. This facilitates diminishing the angle of light rays incident on the respective pupil division stop apertures  15 , working favorably for size reductions and aberration reductions. 
         [0125]    Except the condenser lenses  14   1  and  14   n , the condenser lens  14   i  is set up such that the virtual area  18   a  is in conjugate relation to the pupil division stop aperture  15   i  and the virtual area  18   b  is in conjugate relation to the pupil division stop aperture  15   i+1 . The condenser lens  14   1  is set up such that the virtual area  18   a  is in conjugate relation to the pupil division stop aperture  15   1 , and the condenser lens  14   n  is set up such that the virtual area  18   b  is in conjugate relation to the pupil division stop aperture  15   n+1 . 
         [0126]    To put this to practice, it is desired that the respective condenser lenses have equal power, and the center positions of the respective apertures corresponding to the optical axes of the respective condenser lenses differ little by little. 
         [0127]    A re-imaging lens group  16  comprising a plurality of re-imaging lenses  16   i  is located near positions where the respective pupil division stop apertures  15   i  (i is 1 to n+1) are offset in the optical axis direction. 
         [0128]    The re-imaging lens group  16  is going to define a two-dimensional imaging plane on a plane almost conjugate to the surface of each field aperture  13 , and on that imaging plane there is an array of light receptor elements located, which is the photoelectric transformation plane  17  of a light receptor member  17 A. 
         [0129]    A re-imaging lens  16   i  (where i is 1 to n) projects an image on the field aperture  13   i  onto a photoelectric transformation plane  17   ai  on the secondary imaging plane of the light receptor member through the condenser lens  14   i  and pupil division stop aperture  15   i , and a re-imaging lens  16   i  (where i is 2 to n+1) projects an image on a field aperture  13   i−1  onto a photoelectric transformation surface  17   bi  on the secondary imaging plane through a condenser lens  14   i−1  and a pupil division stop aperture  15   i . 
         [0130]    On the secondary imaging plane there are the photoelectric transformation planes  17  lined up in order of  17   a1 ,  17   a2 ,  17   b1 ,  17   a3 ,  17   b2 , . . . ,  17   ai ,  17   b(i−1) ,  17   a(i+1) ,  17   bi ,  17   a(i+2) ,  17   b(i+1) , . . . ,  17   an ,  17   b(n−1)  and  17   bn , and other than between 17 a1  and  17   a2  and  17   b(n−1)  and  17   bn , the photoelectric transformation planes  17  of the same length are lined up at equal space. The photoelectric transformation planes  17  here refer to an effective area from which phase difference information is amassed; they may be set up as an integral unit in practical fabrication processes. 
         [0131]    With the invention, slimming-down and a wide range-finding field of view are achievable. To narrow the space between a condenser lens group  14  and a pupil division stop aperture group  15 , it is required to make the powers of the condenser lenses  14  stronger; however, the individual apertures can be diminished and so can the diameters of the condenser lenses  14 . As a result, the powers of the condenser lenses  14  can easily be boosted up. Likewise, it is also possible to boost up the powers of the re-imaging lenses  16 . 
         [0132]    The field apertures  13  are intimately lined up so that the discreteness of the whole field of view is diminished; if the individual field apertures  13  are diminished, they can then be assembled so that they can be handled as one single field aperture  13 . Overall, they can be handled as a large field aperture  13  for focus detection. In this case, it is also possible to increase the quantity of defocus capable of detecting focus. 
         [0133]    In the embodiment here, a field frame is interposed between the respective field apertures  13  to prevent crosstalk, etc. 
         [0134]    In addition to, or instead of, this, a light block wall  19  may be interposed between the re-imaging lens  16  and the photoelectric transformation plane  17 , as shown in  FIG. 14 . To be particularly effective, the light block wall  19  is located such that there is a separation between the photoelectric transformation planes  17   b(i−2)  and  17   ai . In short, by providing the light block wall  19  between images formed by the re-imaging lenses  16  in re-imaging lens unit, a light beam from a certain re-imaging lens  16  can be blocked off while making sure the quantity of light from adjacent re-imaging lenses  16 . 
         [0135]    It is noted that when the photoelectric transformation planes  17  are located at equal space, the second photoelectric transformation planes as counted from both ends are not available. 
         [0136]      FIG. 5  is illustrative of the second embodiment wherein the embodiment of  FIG. 1  is modified such that the range-finding area is extended crosswise in two directions so that phase difference information for implementing focus detection is also used in two directions. The layout for the center re-imaging lens  6  has the merit of easily extending the range-finding area crosswise. 
         [0137]    In the embodiment shown in  FIG. 5 , a taking lens  21 , and two sets of virtual areas (a pair of  28   a  and  28   b , and a pair of  28   c  and  28   d ) are supposed to be provided. One set is supposed to comprise  28   a  and  28   b  and another  28   c  and  28   d.    
         [0138]    One set of virtual areas and another set of virtual areas are arranged in different directions (with  28   a  and  28   b  in the horizontal and  28   c  and  28   d  in the vertical). Field apertures  23   21 ,  23   22 ,  23   23 ,  23   12  and  23   32  that define openings in five field stops for focus detection areas are supposed to be located near a predetermined imaging plane  22  (not shown) equivalent to an imaging plane. Each field aperture  23  corresponds to each focus detection area, and two focus detection areas, vertical and horizontal, intersect at the center field aperture  23   22 . 
         [0139]    The field apertures  23   21 ,  23   22  and  23   23  are lined up straight along (here in the horizontal) in this order with the field aperture  23   22  as center, and the field apertures  23   12 ,  23   22  and  23   32  are lined up straight along (in the direction vertical to the direction that the field apertures  23   21 ,  23   22  and  23   23  are lined up) with the field aperture  23   22  as center. 
         [0140]    Here, the direction that the virtual areas  28   a  and  28   b  are lined up and the direction that the field apertures  23   21 ,  23   22  and  23   23  are lined up are the same with the optical axis of the taking lens  21  as center, and the direction that the virtual areas  28   c  and  28   d  are lined up and the direction that the field apertures  23   12 ,  23   22  and  23   32  are lined up are the same with the optical axis of the taking lens  21  as center. 
         [0141]    Near the field aperture  23   21  there is a condenser lens  24   21  located. Likewise, near the field apertures  23   22 ,  23   23 ,  23   12  and  23   32 , there are condenser lenses  24   22 ,  24   23 ,  24   12  and  24   32  located, respectively, which have different optical axes. 
         [0142]    At a conjugate position of the virtual area  28   a  to the condenser lens  24   22 , there is a pupil division stop aperture  25   h22  located that is an opening in the pupil division stop. Likewise, at conjugate positions of the virtual areas  28   b ,  28   c  and  28   d  to the condenser lens  24   22 , there are pupil division apertures  25   h23 ,  25   v22  and  25   v32  located, corresponding to the respective virtual areas. Further, at conjugate position of the virtual areas  28   a  and  28   b  to the condenser lens  24   21 , there are pupil division stop apertures  25   h21  (corresponding to the virtual area  28   a ) and  25   h22  (corresponding to the virtual area  28   b ) located. 
         [0143]    The virtual areas are to other condenser lenses  24   23 ,  24   12  and  24   32  what they are to the aforesaid condenser lens  24   21 , provided that they are rotated and moved about the optical axis of the taking lens  21 . 
         [0144]    That is to say, at conjugate positions of the virtual areas  28   a  and  28   b  to the condenser lens  24   23  there are pupil division stop apertures  25   h23  and  25   h24  located, and at conjugate positions of the virtual areas  28   c  and  28   d  to the condenser lens  24   12  there are pupil division stop apertures  25   v12  and  25   v22  located. Further at conjugate positions of the virtual areas  28   c  and  28   d  to the condenser lens  24   32 , there are pupil division stop apertures  25   v32  and  25   v42  located. 
         [0145]    Near the pupil division stop apertures  25   h21 ,  25   h22 ,  25   h23 ,  25   h24 ,  25   v22 ,  25   v32  and  25   v42  there are re-imaging lenses  26   h21 ,  26   h22 ,  26   h23 ,  26   h24 ,  2   6v12 ,  26   v22 ,  26   v32  and  26   v42  located, respectively. Although a re-imaging lens group  26  is not shown for simplification in  FIG. 5 , it should be understood that each pupil division stop aperture  25  is located integral with each re-imaging lens  26 . 
         [0146]    The function of the re-imaging lens in the horizontal direction is now explained. 
         [0147]    The re-imaging lens  26   h21  is operable to project an image on the field aperture  23   21  onto a photoelectric transformation plane  27   ha21  through the condenser lens  24   21  and pupil division stop aperture  25   h21 . 
         [0148]    The re-imaging lens  26   h22  is operable to project an image on the field aperture  23   21  onto a photoelectric transformation plane  27   hb21  through the condenser lens  24   21  and pupil division stop aperture  25   h22 , and project an image on the field aperture  23   22  onto a photoelectric transformation plane  27   ha22  through the condenser lens  24   22  and pupil division stop aperture  25   ha22 . 
         [0149]    The re-imaging lens  26   h22  is operable to project an image on the field aperture  23   22  onto a photoelectric transformation plane  27   hb22  through the condenser lens  24   22  and pupil division stop aperture  25   h22 , and project an image on the field aperture  23   22  onto a photoelectric transformation plane  27   ha23  through the condenser lens  24   23  and pupil division stop aperture  25   h23 . 
         [0150]    The re-imaging lens  26   h24  is operable to project an image on the field aperture  23   23  onto a photoelectric transformation plane  27   hb23  through the condenser lens  24   23  and pupil division stop aperture  25   h24 . 
         [0151]    The function of the re-imaging lens in the vertical direction is now explained. 
         [0152]    The re-imaging lens  26   v12  is operable to project an image on the field aperture  23   12  onto a photoelectric transformation plane  27   vc12  through the condenser lens  24   12  and pupil division stop aperture  25   v12 . 
         [0153]    The re-imaging lens  26   v22  is operable to project an image on the field aperture  23   12  onto a photoelectric transformation plane  27   vd12  through the condenser lens  24   12  and pupil division stop aperture  25   v22 , and project an image on the field aperture  23   22  onto a photoelectric transformation plane  27   vc22  through the condenser lens  24   22  and pupil division stop aperture  25   v22 . 
         [0154]    The re-imaging lens  26   v32  is operable to project an image on the field aperture  23   32  onto a photoelectric transformation plane  27   vd22  through the condenser lens  24   22  and pupil division stop aperture  25   v32 , and project an image on the field aperture  23   32  onto a photoelectric transformation plane  27   vc32  through the condenser lens  24   32  and pupil division stop aperture  25   v32 . 
         [0155]    The re-imaging lens  26   v42  is operable to project an image on the field aperture  23   32  onto a photoelectric transformation plane  27   vd32  through the condenser lens  24   32  and pupil division stop aperture  25   h42 . 
         [0156]    The respective photoelectric transformation planes on the light receptor member  27 A are located substantially at conjugate planes to the respective field aperture planes  23 . Light receptor elements on the photoelectric transformation plane  27  are lined up in a row, horizontal and vertical, so as to produce a phase difference for focus detection. Instead of each light receptor element array, there may be a single area sensor used, which comprises a light receptor element array. Parallel holds for other embodiments, too. 
         [0157]    Thus, the embodiment here may be applied to a focus detection optical system having the so-called crosswise range-finding field of view. 
         [0158]    In an embodiment shown in  FIG. 6 , the crosswise focus detection field such as the one according to the embodiment of  FIG. 5  is so extended that there can be the field of view broadened crossways. In the embodiment here, with the optical axis of a taking lens  32  as center, there are five field apertures  33  provided vertically, each defining an opening in a field stop in each focus detection area, and five provided horizontally, while the center apertures  33   33  intersect. As shown, there are a total of nine field apertures  33 , with the center field apertures  33   33  capable of detecting phase differences vertically and horizontally, enhancing focus detection precision. 
         [0159]    Corresponding to the respective focus detection areas, there are condenser lenses  34  located, which have different optical axes. Each condenser lens  34  is operable to guide a light beam toward a pair of adjoining re-imaging lenses  36  located in correspondence to each field aperture  33 . 
         [0160]    A re-imaging lens group  36  comprises vertically lined-up six re-imaging lenses  36  and horizontally lined-up six re-imaging lenses  36 , which are located symmetrically about the optical axis of the taking lens  31 . The re-imaging lenses except the outermost four are operable to guide a light beam incident from two adjoining field apertures  33  toward the subsequent light receptor element array. 
         [0161]    Virtual areas  38   a ,  38   b ,  38   c  and  38   d  of the taking lens  31 , field apertures  33 , condenser lenses  34 , pupil division stop apertures  35 , re-imaging lenses  36  and light receptor arrays are located in both vertical and horizontal relations as explained in conjunction with  FIG. 3 . 
         [0162]    The advantages of this embodiment: wider focus detection areas, smaller focus detection optical systems, and more increased defocus quantity capable of focus detection would be undisputed from the explanations of the already explained embodiments, and so any further details would be omitted. 
         [0163]      FIG. 7  is illustrative of the third embodiment of the invention wherein such focus detection systems as shown in  FIG. 3  are arranged in rows and columns. In  FIG. 7 , field apertures  43  that are openings in the field stops in a focus detection area, re-imaging lenses  46  and an array of light receptor elements  47  are schematically depicted. 
         [0164]    Briefly, there are such virtual areas  48   a ,  48   b ,  48   c  and  48   d  as in  FIG. 6  provided, and there are field apertures  43   ij  arrayed on m matrices in the direction that the virtual areas  48   a  and  48   b  are lined up and n matrices in the direction that the virtual areas  48   c  and  48   d  are lined up (1&lt;i&lt;m, 1&lt;j&lt;n). Near and corresponding to the respective field apertures  43 , there are condenser lens  44   ij  (1&lt;i&lt;m, 1&lt;j&lt;n) located. In  FIG. 7 , note that the condenser lenses are shown as being integral with the field apertures  43 . 
         [0165]    It is here noted that m and n will increase at an increment of 1 in the lined-up direction. Each field aperture  43  is just about the detection of a phase difference in the vertical and horizontal directions, and for this, use may be made of an crosswise aperture wherein long focus detection areas in the vertical and horizontal directions are put one upon another, or a square, rectangular or other aperture capable of covering a quantity detection area. 
         [0166]    A pupil division stop aperture  45  that is an opening in a pupil division step is located on a conjugate plane of the virtual area to each condenser lens  44 . The pupil division stop aperture  45  comprises, in order to obtain phase difference information in the horizontal direction, a pupil division stop aperture  45   hij  (1&lt;i&lt;m+1, 1&lt;j&lt;n) wherein pupil division apertures comprising m+1 openings in the horizontal direction are located parallel in n rows in the vertical direction and, in order to obtain phase difference information in the vertical direction, a pupil division stop aperture  45   vij  (1&lt;i&lt;m, 1&lt;j&lt;n+1) wherein pupil division apertures comprising n+1 openings in the vertical direction are located parallel in m rows in the horizontal direction. 
         [0167]    This arrangement is such that a condenser lens  44   ij  lets the virtual areas  48   a ,  48   b ,  48   c  and  48   d  have conjugate relations to the pupil division stop apertures  45   hij ,  45   h(i+1)j ,  45   vij  and  45   vi(j+1) . 
         [0168]    And corresponding re-imaging lenses  46   hij  (1&lt;i&lt;m+1, 1&lt;j&lt;n) and  46   hij  (1&lt;i&lt;m, 1&lt;j&lt;n+1) (shown as being integral with the pupil division stop apertures  45 ) are located proximately to the respective pupil division stop apertures  45 . 
         [0169]    Suppose here that the secondary imaging plane is defined by an almost conjugate plane to each field aperture  43   ij . Disposed on that plane are photoelectric transformation planes  47   ahij  (1&lt;i&lt;m, 1&lt;j&lt;n),  47   bhij  (i&lt;i&lt;m, 1&lt;j&lt;n),  47   cvij  (1&lt;i&lt;m, 1&lt;j&lt;n) and  47   dvij  (1&lt;i&lt;m, 1&lt;j&lt;n), and an image on the field aperture  43   ij  is again formed on each photoelectric transformation plane  47  by way of the re-imaging lenses  46   hij  and  46   vij . 
         [0170]    The photoelectric transformation planes  47  ahij (1&lt;i&lt;m, 1&lt;j&lt;n) and  47   bhij  (i&lt;i&lt;m, 1&lt;j&lt;n) for the detection of a phase difference in the horizontal direction are alternately arranged in the same row as shown in  FIG. 3 , except both ends, and the photoelectric transformation planes  47   cvij  (1&lt;i&lt;m, 1&lt;j&lt;n) and  47   dvij  (i&lt;i&lt;m, 1&lt;j&lt;n) for the detection of a phase difference in the vertical direction are alternately arranged in the same row as shown in  FIG. 3 , except both ends, 
         [0171]    A light beam passing through the virtual area  48   a  is guided onto the photoelectric transformation plane  47   ahij  located on the re-imaging plane through the field aperture  43   ij , condenser lens  44   ij , pupil division stop aperture  45   hij  and re-imaging lens  46   hij  so that an image near the field aperture  43   ij  is again formed. 
         [0172]    A light beam passing through the virtual area  48   b  is guided onto the photoelectric transformation plane  47   bhij  located on the re-imaging plane through the field aperture  43   ij , condenser lens  44   ij , pupil division stop aperture  45   hi(j+1)  and re-imaging lens  46   hi(j+1)  so that an image near the field aperture  43   ij  is again formed. 
         [0173]    Likewise, a light beam passing through the virtual area  48   c  is guided onto the photoelectric transformation plane  47   cvij  located on the re-imaging plane through the field aperture  43   ij , condenser lens  44   ij , pupil division stop aperture  45   vij  and re-imaging lens  46   vij  so that an image near the field aperture  43   ij  is again formed, and a light beam passing through the virtual area  48   d  is guided onto the photoelectric transformation plane  47   dvij  located on the re-imaging plane through the field aperture  43   ij , condenser lens  44   ij , pupil division stop aperture  45   v(i+1)j  and re-imaging lens  46   v(i+1)j  so that an image near the field aperture  43   ij  is again formed. 
         [0174]    According to the embodiment here making use of the invention of this application, it is thus possible to set up a large range-finding area with a slimmed-down focus detection system adapted to amass phase difference information in two directions. It is also possible to make fragmentation of the field aperture while discreteness is kept small, thereby setting up a focus detection system of high precision. Note here that to get around crosstalk or other inconvenience, a field frame may be formed on the field aperture or a light block wall  49  (not shown) may be interposed between the re-imaging lens and the photoelectric transformation plane. 
         [0175]    Preferably, the magnification of the re-imaging system of the invention is set at ½ or below in the event that focus detection is implemented using unidirectional phase difference information as shown in  FIG. 3  or at ⅓ or below in the event that focus detection is implemented using bidirectional phase difference information as shown in  FIGS. 5 and 6 , because the photoelectric transformation plane can efficiently be set up. 
         [0176]    A numerical example is now explained with reference to  FIG. 8 . Typically, an account is given of such a two-dimensional extent as shown in  FIG. 7 . For the purpose of explanation, reference is made to a part of the section in one h direction as shown in  FIG. 8 . In this numerical example, a virtual area is set at −100 mm from the predetermined imaging plane, and the central spacing (pitch) of a field aperture  53  defined by openings in adjoining field stops of a focus detection area is 0.02 mm in the (horizontal) h direction (fh) and 0.02 mm in the (vertical) v direction (fv), with an optical medium filling in between a condenser lens  54  and a re-imaging lens  56 . 
         [0177]    In other words, a condenser lens array  54   a  (plane) is provided on the taking lens side of a plate  50 , and a re-imaging lens array  56   a  is provided on the side of a photoelectric transformation plane  57 . 
         [0178]    The plate  50  here is formed of resin, and suppose that the refractive index of the medium is 1.5, the radius of curvature rc of the surface of the condenser lens  54  is 0.13 mm, the radius of curvature r of the re-imaging lens  56  (surface) is 0.034 mm, the thickness dp of the plate is 0.4 mm, a pupil division stop aperture  55  is integral with the re-imaging lens  56  (surface) and configured as a circle having a diameter φs of 0.013 mm, the spacing da between the re-imaging lens surface  56  and the photo-electric transformation plane  57  is 0.1 mm, and the magnification of the focal detection system is about ⅓. 
         [0179]    The spacing (pitch) of the re-imaging lenses lined up in the h and v directions, too, is about 0.02 mm as is the case with the condenser lens  54 . Suppose here that the condenser lens  54  has a focal length of 0.27 mm and the re-imaging lens  56  has a focal length of 0.09 mm. If h is a distance from the center of the field aperture  54  up to the optical axis  51  of the taking lens, then a distance hc from the condenser lens  54  up to the optical axis  51  is 0.9973×h. It is here understood that there is a midway point set between the adjoining re-imaging lenses  56  at a position of extending from the center of the field aperture  53  toward the optical axis  51 . The light reception range of each photoelectric transformation portion  57  is then given by sh (horizontal)×sv (vertical)=0.0067 mm×0.0067 mm. 
         [0180]    Further, a light block wall  50  may as well be located between the midway position between the adjoining re-imaging lenses  56  and the photoelectric transformation plane  57  used. Here, if the focus detection optical system has a magnification of ⅓, between the center of the midway position between adjoining re-imaging lenses  56  and the photoelectric transformation plane  57  there is then a space left through which none of normal light beams (used for focus detection) pass. In the example here, the light block wall  59  is located within an area of 0.0067 mm×0.0067 mm about the midway position between adjoining re-imaging lenses  56 . 
         [0181]    A field frame  53   a  is located at the field aperture  53 . With this light block wall  59  or the filed frame  53   a , crosstalk at the photoelectric transformation plane  57 , to which one re-imaging lens  56  corresponds, can efficiently be kept in check. 
         [0182]      FIG. 9  is illustrative of a layout of the photo-electric transformation member  47  as viewed from the optical axis direction:  FIG. 9  illustrates a part of the layout of the integral photoelectric transformation member  47  on an enlarged scale. The photoelectric transformation member  47  comprises light receptor element arrays arranged vertically and horizontally in a matrix fashion. 
         [0183]    In  FIG. 9 , the inside of a hatched rectangle is a portion that is not used for focus detection, and rectangles indicated at a, b, c and d are the ranges of photoelectric transformation portions corresponding to the virtual areas  48   a ,  48   b ,  48   c  and  48   d  in  FIG. 7 . 
         [0184]    A rectangular range surrounded by a thick line is a range of the field aperture  43  projected in the optical axis direction. It is found that the rectangular range surrounded by the thick line is triple as large as the range of the photoelectric transformation means  47 , both vertically and horizontally. 
         [0185]    What is encircled refers to an effective range of a pupil division stop aperture  45  projected in the optical axis direction and a re-imaging lens  46  located near it. A solid-line circle corresponds to a pupil division stop aperture  45  and a re-imaging lens  47  for detecting phase information in the horizontal direction, corresponding to the virtual areas  48   a  and  48   b  in  FIG. 7 , and a dotted-line circle corresponds to a pupil division stop aperture  45  for detecting phase information in the vertical direction, corresponding to the virtual areas  48   c  and  48   d  in  FIG. 7 . 
         [0186]      FIG. 10  is illustrative of another embodiment of the invention of this application:  FIG. 10(   a ) is a perspective view of another embodiment, and  FIG. 10(   b ) is a front view of another embodiment. 
         [0187]    For a better understanding of construction, a re-imaging lens group  66  and a light receptor element array  67  are exaggerated in terms of spacing and size. However, viable distances and sizes should be determined in such a way as to satisfy imaging capabilities as shown in  FIGS. 1(   a ) and  1 ( b ). 
         [0188]    Here, there are a taking lens  61  and two sets of virtual areas (a pair of  68   a  and  68   b  and a pair of  68   c  and  68   d ) supposed. One set is going to be the virtual areas  68   a  and  68   b , and another the virtual areas  68   c  and  68   d . In the respective sets, the virtual areas are lined up in different directions. Field apertures  63   α  and  63   β  that are openings in two field stops in a focus detection area are supposed to lie near a predetermined imaging plane  62  (not shown) equivalent to the imaging plane, and provided corresponding to the respective field apertures  63   α  and  63   β  are a condenser lens  64 , a pupil division stop aperture  65  that is an opening in a pupil division stop, a re-imaging lens group  66  and a photoelectric transformation plane  67  comprising a light receptor element array provided on a light receptor member  67 A which have different optical axes. 
         [0189]    And a condenser lens  64   α  is located near the field aperture  63   α , and a pupil division stop aperture  65   αβ  is located at a conjugate position of the virtual area  68   a  to the condenser lens  64   α . Near the pupil division stop aperture  65   αβ  there is a re-imaging lens  66   αβ  (shown as being integral with  65   αβ ) located, and the re-imaging lens  66   αβ  is operable to substantially project an image on the field aperture  63   α  onto the photoelectric transformation plane  67   a  through the condenser lens  64   α  and pupil division stop aperture  65   αβ . 
         [0190]    For the purpose of simplification,  FIG. 10  shows each re-imaging lens  66  as being integral with the pupil division stop aperture  65  and, at the same time, shows re-imaging lenses  66   αβ ,  66   αb  and  66   βd , with openings being defined by pupil division stop apertures  65   αβ ,  65   αb  and  65   βd . As shown in  FIG. 1(   a ) and so on, of course, the re-imaging lens  66  may be spaced slightly away from the opening  65  or, alternatively, the pupil division stop aperture  65  may be formed by coating on one surface of the re-imaging lens  66 . 
         [0191]    The pupil division stop aperture  65   αb  is located at a conjugate position of the virtual area  68   b  to the condenser lens  64   α , and near that there is the re-imaging lens  66   αb  (shown as being integral with the pupil division stop aperture  65   αb ) located, so that the re-imaging lens  66   αb  is operable to substantially project an image on the field aperture  63   α  onto the photoelectric transformation  67   b  through the condenser lens  64   α  and pupil division stop aperture  65   αb . 
         [0192]    And near the field aperture  63   β  there is the condenser lens  64   β  located, and the pupil division stop aperture  65   αβ  is located at a conjugate position of the virtual area  68   c  to the condenser lens  64   β . Near that there is the re-imaging lens  66   αβ  (shown as being integral with the pupil division stop aperture  65   αβ ) located, so that the re-imaging lens  66   αβ  is operable to refract an image on the field aperture  63   β  through the condenser lens  64   2β  and substantially project it onto the photoelectric transformation plane  67   c  through the pupil division stop aperture  65   αβ . 
         [0193]    The pupil division stop aperture  65   βd  is located at a conjugate position of the virtual area  68   d  to the condenser lens  64   β . Near that there is the re-imaging lens  66   βd  (shown as being integral with  65   βd ) located, so that the re-imaging lens  66   βd  is operable to refract an image on the field aperture  63   β  through the condenser lens  64   β  and substantially project it onto the photoelectric transformation plane  67   d  through the pupil division stop aperture  65   βd . 
         [0194]    The respective re-imaging lenses  66  are adjacent to one another. 
         [0195]    As shown in  FIG. 10 , the example here may also be applied to a focus detection apparatus capable of handling phase difference information in different directions. The field apertures  63   α  and  63   β  may be adjacent to each other. Further, if necessary, these focus detection areas may be crossed over one another to obtain the same range-finding point in practical applications. 
         [0196]    The example here being constructed like such, the pupil division stop apertures can be closer to the re-imaging lens as compared with a phase difference type of focus detection system of conventional construction, thereby using a lot more light and improving on performance. It is also easy to bring the re-imaging lens group and the photoelectric transformation plane having a light receptor element array closer to the predetermined imaging plane, working favorably for reducing the size of the focal detection system. 
         [0197]      FIG. 11(   a ) is illustrative of a layout for the pupil division stop aperture  65  and re-imaging lens  66  in the example of  FIG. 10 , and  FIG. 11(   b ) is illustrative of layout for a pupil division stop aperture  165  and a re-imaging lens  167  in the focus detection apparatus handling phase difference information in two directions. With the layout of  FIG. 11(   a ), the number of openings and re-imaging lenses can be decreased, and the re-imaging lenses can get closer to one another so that opening size can be increased, going in favor of light quantity. 
         [0198]      FIGS. 12 ,  13  and  14  are illustrative of applications of the focus detection system to a practical camera body. 
         [0199]      FIG. 12  is illustrative of an example of the imaging apparatus wherein a taking lens  71  may be integral with or interchangeable. This imaging apparatus comprises a quick return mirror  73  and a sub-mirror  77  that, upon focus detection or framing, enter an optical path and retracts out of an optical path at the time of taking images. A finder optical system  76  is provided on a path of light reflected off the quick return mirror  73 , and an inventive focus detection system  74  is located on a path taken by light reflected off the sub-mirror  77  after transmitting through the quick return mirror  73 . On the optical path out of which the quick return mirror  73  and sub-mirror  77  are retracted, there is the imaging plane of an imaging device  75  such as CCC or CMOS located. 
         [0200]    In  FIG. 13 , a half-mirror  83  is located between a taking lens  81  and an imaging plane  85 , and an inventive focus detection system  84  is located on a path taken by reflected light. Preferably in this case, the half-mirror  83  is of a thin pellicle construction or antireflection treated on a non-half-mirror surface. 
         [0201]    With this method, it is possible to extend the focus detection area across the whole imaging screen range. Here, the finder function for framing and so on may be shown on a liquid crystal screen or the like, using image information obtained from an imaging device. 
         [0202]      FIG. 14  is illustrative of one modification to  FIG. 12  wherein a quick return mirror  93  is used instead of the half-mirror. Upon focus detection or framing, the quick return mirror  93  enters an optical path and upon taking, it retracts out of the optical path. Preferably in this case, the quick return mirror should be of substantial total reflection construction so that the quantity of light entering the focus detection system can be increased. For framing or the like, image information about much the same area as an image range may be obtained from an output from the inventive focus detection apparatus for display on a liquid crystal screen or the like.