Abstract:
Goal: Providing a mirror unit that can decrease the wait time before distance measurement. 
     Means: A mirror unit comprising a first mirror; a second mirror; a first mirror holder that holds the first mirror, is rotatable on a rotational axis arranged above an optical path of the incident light, is lowered to a first mirror-down position, and is raised to a first mirror-up position; and a second mirror holder that includes an auxiliary component and a mirror holding component holding the second mirror, is lowered to a second mirror-down position, and is raised to a second mirror-up position, wherein the auxiliary component is provided below the first mirror holder in the direction of the lowering, is rotatable on a rotational axis that is the same as or parallel to the rotational axis of the first mirror holder, and can be lowered independently of the first mirror holder, and the mirror holding component is engaged with the auxiliary component to be relatively rotatable around a rotational axis that is parallel to the rotational axis of the auxiliary component.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present application claims priority from a Japanese Patent Application No. 2009-014052 filed on Jan. 26, 2009, the contents of which are incorporated herein by reference. 
         [0003]    The present invention relates to a mirror unit and an image capturing apparatus. 
         [0004]    2. Related Art 
         [0005]    A known mirror unit for a single lens reflex camera includes a main mirror, which is a half mirror, and a sub-mirror that reflects the light passed by the main mirror downward to a distance measuring sensor, and these mirrors are lowered into the optical path or raised above the optical path, as shown in Japanese Patent Application Publications No. 62-78536 and No. 63-95430. In such a mirror unit, a sub-frame holding the sub-mirror hangs down to be rotatable relative to a main frame holding the main mirror. 
         [0006]    To achieve accurate distance measurement, it is desirable that measurement be started after vibration of the lowered sub-mirror has stopped. However, in this mirror unit, the vibration of the sub-mirror does not stop until the vibration of the lowered main frame has stopped. In other words, initiation of distance measurement is delayed because of the vibration of the main frame. 
       SUMMARY 
       [0007]    To solve this problem, a first aspect of the present invention provides a mirror unit comprising a first mirror ( 102 ) that reflects and passes incident light from a subject side; a second mirror ( 104 ) that reflects the incident light passed by the first mirror; a first mirror holder ( 110 ) that holds the first mirror, is rotatable on a rotational axis arranged above an optical path of the incident light, is lowered to a first mirror-down position in which the first mirror is inserted into the optical path of the incident light, and is raised to a first mirror-up position in which the first mirror is removed from the optical path of the incident light; and a second mirror holder that includes an auxiliary component ( 114 ) and a mirror holding component ( 112 ) holding the second mirror, is lowered to a second mirror-down position in which the second mirror is inserted into the optical path of the incident light, and is raised to a second mirror-up position in which the second mirror is removed from the optical path of the incident light, wherein the auxiliary component is provided below the first mirror holder in the direction of the lowering, is rotatable on a rotational axis that is the same as or parallel to the rotational axis of the first mirror holder, and can be lowered independently of the first mirror holder, and the mirror holding component is engaged with the auxiliary component to be relatively rotatable around a rotational axis that is parallel to the rotational axis of the auxiliary component. 
         [0008]    The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. The above and other features and advantages of the present invention will become more apparent from the following description of the embodiments taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic cross-sectional side view of a single lens reflex digital camera  101  provided with a mirror unit  100  according to an embodiment of the present invention. 
           [0010]      FIG. 2  is a schematic view as seen from the right side of a photographer of the mirror unit  100  in a mirror-down state 
           [0011]      FIG. 3  is a schematic view as seen from the left side of a photographer of the mirror unit  100  in a mirror-down state. 
           [0012]      FIG. 4A  is a schematic view as seen from the right side of a photographer of the auxiliary frame  114  and the sub-frame  112  in a mirror-down state. 
           [0013]      FIG. 4B  is a schematic view as seen from the left side of a photographer of the auxiliary frame  114  and the sub-frame  112  in a mirror-down state. 
           [0014]      FIG. 5  is a plan view seen from below of the mirror unit  100  in a mirror-up state. 
           [0015]      FIG. 6A  is a side view as seen from the right side of a photographer of the mirror unit  100  in a mirror-down state. 
           [0016]      FIG. 6B  is a side view as seen from the left side of a photographer of the mirror unit  100  in a mirror-down state. 
           [0017]      FIG. 7A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-up operation is performed. 
           [0018]      FIG. 7B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-up operation is performed. 
           [0019]      FIG. 8A  is a side view as seen from the right side of a photographer of the mirror unit  100  in the mirror-up state. 
           [0020]      FIG. 8B  is a side view as seen from the left side of a photographer of the mirror unit  100  in the mirror-up state. 
           [0021]      FIG. 9A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-down operation is performed. 
           [0022]      FIG. 9B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-down operation is performed. 
           [0023]      FIG. 10A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-down operation is performed. 
           [0024]      FIG. 10B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-down operation is performed. 
           [0025]      FIG. 11A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-down operation is performed. 
           [0026]      FIG. 11B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-down operation is performed. 
           [0027]      FIG. 12A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-down operation is performed. 
           [0028]      FIG. 12B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-down operation is performed. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0029]    Hereinafter, some embodiments of the present invention will be described. The embodiments do not limit the invention according to the claims, and all the combinations of the features described in the embodiments are not necessarily essential to means provided by aspects of the invention. 
         [0030]      FIG. 1  is a schematic cross-sectional side view of a single lens reflex digital camera  101  provided with a mirror unit  100  according to an embodiment of the present invention. As shown in  FIG. 1 , the digital camera  101  is provided with an optical component  420 , a lens barrel  430 , an image capturing unit  500  such as a CCD, and a control section  550 . The lens barrel  430  houses the optical component  420 . The image capturing unit  500  captures an image of a subject that is focused by the optical component  420 . The control section  550  controls the image capturing unit  500 . 
         [0031]    The digital camera  101  includes a lens unit  410 , which contains the optical component  420  and the lens barrel  430 , and a body  460 . The lens unit  410  is detachably mounted on the body  460  via a mount  450 . 
         [0032]    The optical component  420  contains, in order from an incident end that is the left side of  FIG. 1 , a front lens  422 , a compensator lens  424 , a focusing lens  426 , and a main lens  428 . An iris unit  440  is arranged between the focusing lens  426  and the main lens  428 . 
         [0033]    The body  460  houses an optical component that includes the mirror unit  100 , a pentaprism  470 , and an eyepiece system  490 . The mirror unit  100  includes a main mirror  102 , which is a half mirror for reflecting and passing incident light passed through the lens unit  410 . The main mirror  102  moves between (i) a down position in which the main mirror  102  is arranged diagonally in the optical path of the incident light and (ii) an up position, shown by a dotted line in  FIG. 1 , in which the main mirror  102  is raised out of the optical path of the incident light. 
         [0034]    When in the down position, the main mirror  102  guides a majority of the incident light to the pentaprism  470 . The pentaprism  470  projects the reflection of the incident light toward the eyepiece system  490 , and therefore the image of the focusing screen can be seen as a real image from the eyepiece system  490 . The remaining incident light is guided to the light measuring unit  480  by the pentaprism  470 . The light measuring unit  480  measures the intensity of the incident light, and a distribution or the like of this intensity. 
         [0035]    A half mirror  492  is arranged between the pentaprism  470  and the eyepiece system  490  to superimpose a display image formed by a finder liquid crystal  494  onto the image of the focusing screen. The display image is displayed overlapping the image projected from the pentaprism  470 . 
         [0036]    The mirror unit  100  includes a sub-mirror  104  on a back side of the main mirror  102 , which is the side of the main mirror  102  not facing the incident light. The sub-mirror  104  moves between (i) a down position in which the sub-mirror  104  is arranged diagonally in the optical path of the incident light and (ii) an up position, shown by the dotted line in  FIG. 1 , in which the sub-mirror  104  is raised out of the optical path of the incident light. 
         [0037]    When in the down position, the sub-mirror  104  guides the incident light passed by the main mirror  102  to a distance measuring unit  530  arranged below the sub-mirror  104 . In other words, when the main mirror  102  and the sub-mirror  104  are in the down position, the distance measuring unit  530  measures the distance to the subject. When the main mirror  102  is moved to the up position, the sub-mirror  104  also moves to the up position. 
         [0038]    A focal plane shutter  540 , a low-pass filter  510 , and an image capturing unit  500  are arranged in the stated order behind the main mirror  102  in the direction of the incident light. When the focal plane shutter  540  is opened, the main mirror  102  and the sub-mirror  104  positioned immediately in front of the focal plane shutter  540  are moved to the up position, and so the incident light can enter directly into the image capturing unit  500 . In this way, the image formed by the incident light is converted into an electric signal. As a result, the image capturing unit  500  can capture the image focused by the lens unit  410 . 
         [0039]      FIG. 2  is a schematic view as seen from the right side of a photographer of the mirror unit  100  in a mirror-down state. In  FIG. 2 , a direction to the right of the camera is represented by the arrow X, a direction upward from the camera is represented by the arrow Y, and the direction in which the incident light proceeds is represented by the arrow Z. 
         [0040]    The mirror unit  100  is provided with a main frame  110  that holds the main mirror  102 , a sub-frame  112  that holds the sub-mirror  104 , and an auxiliary frame  114  that holds the sub-frame  112 . The main frame  110  is provided with a rectangular base part  116  on which the main mirror  102  is mounted and side parts  118  and  120  that are provided on the left and right sides of the base part  116  and extend at substantially right angles therefrom to curve toward a subject side. The base part  116  has a rectangular aperture  122  that passes the incident light passed through the main mirror  102 . 
         [0041]    Circular holes are formed on the tips of the side parts  118  and  120  and shafts  124  and  126  are inserted into these holes to be relatively rotatable. The shafts  124  and  126  are arranged along an axis that runs crosswise in the camera and that is positioned above the optical path of the incident light, and are fixed on a unit frame  111 . Therefore, the main frame  110  is supported in a manner to be rotatable on an axis positioned above the optical path of the incident light and extending crosswise in the camera. 
         [0042]    The shaft  126  is inserted into a torsion coil spring  140  serving as a biasing member. The ends of the torsion coil spring  140  are locked respectively by an upper portion of the side part  120  and a locking part  103  formed on the unit frame  111 . The ends of the torsion coil spring  140  can be elastically deformed in a direction to draw near each other, and the restorative force biases the side part  120  downward around the shaft  126 . Accordingly, the main frame  110  is biased by the torsion coil spring  140  downward around the shafts  124  and  126 . 
         [0043]    The auxiliary frame  114  includes a rectangular base part  128  that is arranged lower than the base part  116  and has a length extending crosswise in the camera, and side parts  130  and  132  that are provided on the left and right sides of the base part  128  and extend at substantially right angles in a curving manner. The side parts  130  and  132  have lengths that extend along the width of the base part  128 . One longitudinal end of the side part  130  has a circular hole into which the shaft  124  is inserted to be relatively rotatable, and one longitudinal end of the side part  132  has a circular hole into which the shaft  126  is inserted to be relatively rotatable. As a result, the auxiliary frame  114  can be supported in a manner to be rotatable relative to the unit frame  111  on an axis positioned above the optical path of the incident light and extending crosswise in the camera. Furthermore, the auxiliary frame  114  is supported to be rotatable with respect to the unit frame  111  on the same axis as the main frame  110 . 
         [0044]    The shaft  124  is inserted into a torsion coil spring  142  serving as a biasing member. The ends of the torsion coil spring  142  are locked respectively by an upper portion of the side part  130  and a locking part  105  formed on the unit frame  111 . The ends of the torsion coil spring  142  can be elastically deformed in a direction to draw near each other, and the restorative force biases the side part  130  downward around the shaft  124 . Accordingly, the auxiliary frame  114  is biased by the torsion coil spring  142  downward around the shafts  124  and  126 . 
         [0045]    The sub-frame  112  includes a rectangular base part  134  that has a length extending crosswise in the camera and on which is mounted the sub-mirror  104 , and side parts  136  and  138  that are provided on the left and right sides of the base part  134  and extend at substantially right angles in to curve toward a subject side. The side parts  136  and  138  have lengths that extend along the width of the base part  134 . One longitudinal end of the side part  136  is engaged with the side part  130  of the auxiliary frame  114  via a coupling axle  137  in a manner to be relatively rotatable, and one longitudinal end of the side part  138  is engaged with the side part  132  of the auxiliary frame  114  via the coupling axle  137  in a manner to be relatively rotatable. 
         [0046]    One longitudinal end of the side part  136  has a boss  144  protruding towards the center of the camera in the crosswise direction. A longitudinal central portion of the side part  130  of the auxiliary frame  114  has a boss  146  that protrudes outward in the crosswise direction. The base end of the boss  146  is inserted into a toggle spring  148  serving as a biasing member. One end of the toggle spring  148  is locked by the side part  130 , and the other end of the toggle spring  148  is locked by the boss  144 . The ends of the toggle spring  148  can be elastically deformed in a direction to draw near each other, and the restorative force biases the side part  136  in the rotational direction of the coupling axle  137 . The operation of the toggle spring  148  is described further below. 
         [0047]    The side part  118  of the main frame  110  has a cam  150  mounted via an axle  152 . The axle  152  is arranged in line with the boss  146  in the direction of rotation. The cam  150  is an elliptically shaped component, and one longitudinal end of the cam  150  is engaged with the axle  152 . The other longitudinal end of the cam  150  is arranged to oppose the end surface of the boss  146 . 
         [0048]    A mirror-up lever  154  is arranged below the cam  150  and the boss  146 . The mirror-up lever  154  pivots on a pivotal axis arranged toward the subject side and the bottom thereof to push up the cam  150  and the boss  146 . A detailed description of this operation is described further below. 
         [0049]    A positioning pin  156  is provided beneath the shaft  126  and below the auxiliary frame  114  in a direction in which the auxiliary frame  114  is lowered. The positioning pin  156  contacts the base part  128  when the auxiliary frame  114  is lowered. The down position of the auxiliary frame  114  is determined by the positioning pin  156  exerting pressure on the torsion coil spring  142 . 
         [0050]    A positioning pin  158  is arranged directly below the shaft  126 . The other longitudinal end of the side part  138  of the sub-frame  112 , i.e. the end on the outside of the rotational radial direction, has a U-shaped groove  160 , and the positioning pin  158  engages with this U-shaped groove  160 . The down position of the sub-frame  112  is determined by toggle spring  148  applying pressure to the positioning pin  158 . 
         [0051]    Here, the length of the auxiliary frame  114  from the shafts  124  and  126  to the outer end in the rotational radial direction is less than the length of the main frame  110  from the shafts  124  and  126  to the outer end in the rotational radial direction. In particular, in the mirror unit  100  of the present embodiment, the length of the auxiliary frame  114  from the shafts  124  and  126  to the outer end in the rotational radial direction is less than half of the length of the main frame  110  from the shafts  124  and  126  to the outer end in the rotational radial direction. As a result, the moment of inertia of the auxiliary frame  114  is less than the moment of inertia of the main frame  110 . 
         [0052]    The spring constants of the torsion coil springs  140  and  142  are set according to the difference in moment of inertia between the auxiliary frame  114  and the main frame  110 , such that the auxiliary frame  114  is lowered faster than the main frame  110 . For example, in the present embodiment, the moment of inertia of the main frame  110  is greater than that of the auxiliary frame  114 , and so the spring constants of the torsion coil springs  140  and  142  are set to be the same, or are set such that the spring constant of the torsion coil spring  142  biasing the auxiliary frame  114  is greater than the spring constant of the torsion coil spring  140  biasing the main frame  110 . Furthermore, the spring constant of the torsion coil spring  140  may be set greater than the spring constant of the torsion coil spring  142  as long as the lowering speed of the main frame  110  does not exceed the lowering speed of the auxiliary frame  114 . 
         [0053]      FIG. 3  is a schematic view as seen from the left side of a photographer of the mirror unit  100  in a mirror-down state. As shown in  FIG. 3 , a positioning pin  162  is arranged beneath the shaft  124  and below the main frame  110  in the direction in which the main frame  110  is lowered. The positioning pin  162  contacts the base part  116  of the lowered main frame  110 . The down position of the main frame  110  is determined by the torsion coil spring  140  exerting pressure on the positioning pin  162 . 
         [0054]    A cam  164  is provided on an upper end of the side part  138  of the sub-frame  112 . A cam groove  166  is formed in the cam  164 . The unit frame  111  has a cam pin  168  that protrudes to be inserted into the cam groove  166 . The operation of the cam  164  and the cam pin  168  is explained in detail further below. 
         [0055]      FIG. 4A  is a schematic view as seen from the right side of a photographer of the auxiliary frame  114  and the sub-frame  112  in a mirror-down state. As shown in  FIG. 4A , the auxiliary frame  114  is suspended from the shafts  124  and  126  in a diagonal orientation relative to the subject side. The sub-frame  112  is suspended from the coupling axle  137  in a diagonal orientation relative to the imaging side. 
         [0056]    Here, the auxiliary frame  114  is not engaged with the main frame  110 , and the base part  128  of the auxiliary frame  114  is arranged below the base part  116  of the main frame  110 . Therefore, when the auxiliary frame  114  is lowered to the down position, there is no interference with the main frame  110 . Accordingly, the auxiliary frame  114  can be lowered to the down position independently from the main frame  110 . 
         [0057]      FIG. 4B  is a schematic view as seen from the left side of a photographer of the auxiliary frame  114  and the sub-frame  112  in a mirror-down state. As shown in  FIG. 4B , the cam groove  166  of the cam  164  is formed on a free curve. The wall of the cam groove  166  contacts the cam pin  168  when the sub-frame  112  is raised from the down position to the up position, but the direction of the weight between the cam groove  166  and the cam pin  168  changes according to the angular position of the sub-frame  112 . 
         [0058]      FIG. 5  is a plan view seen from below of the mirror unit  100  in a mirror-up state. As shown in  FIG. 5 , the aperture  122  formed in the base part  116  of the main frame  110  is blocked by the main mirror  102 . However, a passing portion  1021  with an area smaller than that of the aperture  122  is formed in a region where the main mirror  102  overlaps the aperture  122 . The periphery of the passing portion  1021  of the main mirror  102  is a light blocking region covered with an opaque material. 
         [0059]    Here, the base part  134  of the sub-frame  112  has a greater area than the passing portion  1021 , and the base part  134  overlaps with the entire area of the passing portion  1021  when viewed in an up and down direction. 
         [0060]    As a result, with the main mirror  102  and the sub-mirror  104  in the up position, light is blocked from reaching the passing portion  1021  of the main mirror  102  by the sub-mirror  104  and the base part  134  of the sub-frame  112 . Accordingly, the inverse incident light from the finder is prevented from leaking into the mirror box. 
         [0061]    The following describes the operation of the mirror unit  100 . In the following description, the rotation of the main frame  110 , the sub-frame  112 , and the auxiliary frame  114  from the down position to the up position is referred to as the “up direction” and the rotation of the main frame  110 , the sub-frame  112 , and the auxiliary frame  114  from the up position to the down position is referred to as the “down direction.” 
         [0062]      FIG. 6A  is a side view as seen from the right side of a photographer of the mirror unit  100  in a mirror-down state, and  FIG. 6B  is a side view as seen from the left side of a photographer of the mirror unit  100  in a mirror-down state. As shown in  FIGS. 6A and 6B , by pressing a release switch, a drive motor of the mirror unit  100  is driven to pivot the mirror-up lever  154 . The mirror-up lever  154  pivots around a pivot point  155  that is arranged downward and toward the subject side from the mirror-up lever  154 . The mirror-up lever  154  pivots upward and toward the subject side around the pivot point  155 . 
         [0063]    With the main frame  110  and the auxiliary frame  114  in the down position, the mirror-up lever  154  does not contact the boss  146  or the cam  150 . Furthermore, the wall of the cam groove  166  does not contact the cam pin  168 . 
         [0064]      FIG. 7A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-up operation is performed, and  FIG. 7B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-up operation is performed. As shown in  FIGS. 7A and 7B , when the mirror-up operation is begun for the mirror unit  100 , the mirror-up lever  154  contacts the boss  146 . The auxiliary frame  114  responds to the bias of the torsion coil spring  142  to rotate on the shafts  124  and  126  in the up direction. Next, the mirror-up lever  154  contacts the cam  150 . The main frame  110  responds to the bias of the torsion coil spring  140  to rotate on the shafts  124  and  126  in the up direction. 
         [0065]    At this time, the sub-frame  112  follows the auxiliary frame  114  to rotate on the shafts  124  and  126  in the up direction. In this state, the bias of the toggle spring  148  is exerted against the sub-frame  112  in the down direction around the coupling axle  137 . Furthermore, the wall of the cam groove  166  contacts the cam pin  168 , and the cam pin  168  exerts a counter force on the cam groove  166 . This counter force is exerted on the cam  164  in the up direction around the coupling axle  137 . Due to this counter force, the sub-frame  112  responds to the bias of the toggle spring  148  to rotate in the up direction on the coupling axle  137 . 
         [0066]    When the auxiliary frame  114  is raised further, the cam groove  166  and the cam pin  168  cause the sub-frame  112  to rotate in the up direction on the coupling axle  137 . The bias direction of the toggle spring  148  changes to the up direction on the coupling axle  137 . Therefore, the sub-frame  112  responds to the bias of the toggle spring  148  to rotate in the up direction on the coupling axle  137 . 
         [0067]      FIG. 8A  is a side view as seen from the right side of a photographer of the mirror unit  100  in the mirror-up state, and  FIG. 8B  is a side view as seen from the left side of a photographer of the mirror unit  100  in the mirror-up state. As shown in  FIGS. 8A and 8B , in this state, the bias of the torsion coil spring  142  causes the boss  146  of the auxiliary frame  114  to exert pressure on the top surface of the mirror-up lever  154 . Furthermore, the bias of the torsion coil spring  140  causes the cam  150  of the main frame  110  to exert pressure on the top surface of the mirror-up lever  154 . As a result, the main frame  110  and the auxiliary frame  114  are stopped in a state of vertically overlapping each other. 
         [0068]    The bias of the toggle spring  148  causes the base part  134  of the sub-frame  112  to exert pressure on the base part  116  of the main frame  110 . As a result, the main frame  110  and the sub-frame  112  are stopped in a state of vertically overlapping each other. 
         [0069]      FIG. 9A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-down operation is performed, and  FIG. 9B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-down operation is performed. As shown in  FIGS. 9A and 9B , when the mirror-down operation is begun for the mirror unit  100 , the mirror-up lever  154  is lowered. Next, the auxiliary frame  114  and the main frame  110  are lowered by the bias of the torsion coil springs  142  and  140  exerting pressure on the mirror-up lever  154 . Furthermore, the cam groove  166  and the cam pin  168  cause the auxiliary frame  114  to rotate in the down direction on the coupling axle  137 . The bias direction of the toggle spring  148  then changes to the down direction on the coupling axle  137 . 
         [0070]      FIG. 10A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-down operation is performed, and  FIG. 10B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-down operation is performed. As shown in  FIGS. 10A and 10B , in this state, the auxiliary frame  114  rotates in the down direction, i.e. the auxiliary frame  114  is lowered, due to its own weight, the weight of the sub-frame  112 , and the bias of the torsion coil spring  142 . The sub-frame  112  rotates in the down direction due to its own weight and the bias of the toggle spring  148 . The main frame  110  rotates in the down direction, i.e. the main frame  110  is lowered, by its own weight and the bias of the torsion coil spring  140 . 
         [0071]    Here, the length of the auxiliary frame  114  from the shafts  124  and  126  to the outer end in the rotational radial direction is less than the length of the main frame  110  from the shafts  124  and  126  to the outer end in the rotational radial direction. As a result, the moment of inertia of the auxiliary frame  114  is less than the moment of inertia of the main frame  110 . The spring constants of the torsion coil springs  140  and  142  are set such that the rotational speed of the auxiliary frame  114  in the down direction is greater than the rotational speed of the main frame  110  in the down direction. Accordingly, when the mirror-down operation is performed for the mirror unit  100 , the auxiliary frame  114  begins rotating in the down direction earlier than the main frame  110 . 
         [0072]      FIG. 11A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-down operation is performed, and  FIG. 11B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-down operation is performed. As shown in  FIGS. 11A and 11B , the positioning pin  158  is engaged with the U-shaped groove  160  of the sub-frame  112  prior to the auxiliary frame  114  contacting the positioning pin  156 . 
         [0073]      FIG. 12A  is a side view as seen from the right side of a photographer of the mirror unit  100  when a mirror-down operation is performed, and  FIG. 12B  is a side view as seen from the left side of a photographer of the mirror unit  100  when a mirror-down operation is performed. As shown in  FIGS. 12A and 12B , the auxiliary frame  114  rotates in the down direction due to its own weight, the weight of the sub-frame  112 , and the bias of the torsion coil spring  142 . As a result, the auxiliary frame  114  comes into contact with the positioning pin  156 , and the positioning pin  158  enters deeply into the U-shaped groove  160 . In this state, the bias of the torsion coil spring  142  causes the auxiliary frame  114  to exert pressure on the positioning pin  156 , and the bias of the toggle spring  148  causes the sub-frame  112  to exert pressure on the positioning pin  158 . As a result, the auxiliary frame  114  and the sub-frame  112  are stopped in the down position. 
         [0074]    On the other hand, the main frame  110  rotates in the down direction, due to its own weight and the bias of the torsion coil spring  140 , later than the auxiliary frame  114 . The main frame  110  then contacts the positioning pin  162 . In this state, the bias of the torsion coil spring  140  causes the main frame  110  to exert pressure on the positioning pin  162 . As a result, the main frame  110  stops in the down position. 
         [0075]    After the main frame  110 , the sub-frame  112 , and the auxiliary frame  114  reach the down position, vibrate vibration is caused by elastic vibration of the torsion coil springs  140  and  142  and the toggle spring  148 . In other words, the main frame  110 , the sub-frame  112 , and the auxiliary frame  114  bounce upon reaching the down position. 
         [0076]    Here, the sub-mirror  104  held by the sub-frame  112  guides the incident light to the distance measuring sensor disposed therebelow, but when the sub-mirror  104  bounces, the length of the optical path and direction of the optical axis of the incident light directed toward the distance measuring sensor changes, thereby preventing accurate distance measurement. Therefore, it is necessary to wait for the sub-mirror  104  to stop bouncing before beginning the distance measurement. 
         [0077]    If the time from when the mirror-down operation begins to when the sub-mirror  104  stops bouncing can be shortened so that the distance measurement can begin earlier, the number of images that can be captured per unit time during continuous image capturing can be increased. 
         [0078]    In the mirror unit  100  of the present embodiment, the auxiliary frame  114  and the sub-frame  112  are lowered to the down position independently from the main frame  110 , and therefore the bouncing of the main frame  110  does not affect the sub-mirror  104 . Accordingly, the distance measurement can be begun without waiting for the bouncing of the main frame  110  to stop. 
         [0079]    In particular, since the rotational radius of the auxiliary frame  114  in the mirror unit  100  of the present embodiment is less than the rotational radius of the main frame  110 , the moment of inertia of the auxiliary frame  114  is less than the moment of inertia of the main frame  110 . Furthermore, the spring forces of the torsion coil springs  140  and  142  are set such that the rotational speed of the main frame  110  in the down direction does not exceed the rotational speed of the auxiliary frame  114  in the down direction. Yet further, the auxiliary frame  114  is arranged further downward in the direction of the lowering than the main frame  110 , and so the auxiliary frame  114  can rotate in the down direction independently from the main frame  110 . 
         [0080]    Since the auxiliary frame  114  and the sub-frame  112  move to the down position prior to the main frame  110 , as described above, the time from when the mirror-down operation begins to when the sub-mirror  104  stops bouncing is shortened. Accordingly, the distance measurement can be begun earlier and the number of images that can be captured per unit time during continuous image capturing can be increased. 
         [0081]    With the mirror unit  100  of the present embodiment, the mirror-up lever  154  contacts the boss  146  that is provided on the auxiliary frame  114  and the cam  150  provided on the main frame  110 . As a result, the main mirror  102  and the sub-mirror  104 , which move independently from each other during the mirror-up and mirror-down operations, can be driven by a shared mirror driving mechanism. Accordingly, an increase in the cost of the mirror driving mechanism is prevented. 
         [0082]    While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention. For example, the embodiments described the main frame  110  and the auxiliary frame  114  as being rotatable on the same axis, but the main frame  110  and the auxiliary frame  114  may instead be arranged to be rotatable on separate rotational axes that are parallel to each other. 
         [0083]    The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order.