Patent Publication Number: US-9423629-B2

Title: Imaging apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging apparatus equipped with an anti-shake (image shake correction/image stabilizing/shake reduction) system. 
     2. Description of the Related Art 
     In recent years, mobile electronic devices which are designed mainly for taking still/moving photographic images, such as digital cameras (still-video cameras) and digital camcorders (motion-video cameras), and other mobile electronic devices which are designed to be capable of taking such photographic images as a subsidiary function, such as mobile phones equipped with a camera and tablet computers, etc., equipped with a camera, have become widespread, and there has been a demand to miniaturize of the imaging units incorporated in these types of mobile electronic devices. In order to miniaturize an imaging unit, it is known to construct an optical system of an imaging unit out of a bending optical system which reflects (bends) light rays using a reflecting surface of a reflector such as a prism or a mirror. Using a bending optical system in an imaging unit makes it possible to achieve a reduction in thickness of the imaging unit, especially in the direction of travel of the incident light emanating from an object which is to be photographed. 
     In addition, there is a demand for imaging units to be equipped with a so-called anti-shake (image shake correction/image stabilizing/shake reduction) system that is designed to reduce image shake on an image plane that is caused by vibrations such as hand shake. The following four different types of imaging units are known in the art as imaging units using a bending optical system which are equipped with an anti-shake system: a first type (disclosed in Japanese Unexamined Patent Publication Nos. 2009-86319 and 2008-268700) in which an image sensor is moved in directions orthogonal to an image plane to reduce image shake, a second type (disclosed in Japanese Unexamined Patent Publication No. 2010-128384 and Japanese Patent No. 4,789,655) in which a lens disposed behind a reflector (on the image plane side) that has a reflecting surface is moved in directions orthogonal to an optical axis to reduce image shake, a third type (disclosed in Japanese Unexamined Patent Publication Nos. 2007-228005, 2010-204341, 2006-330439, and Japanese Patent No. 4,717,529) in which the angle of a reflector (a reflecting surface thereof) and the angle of a lens adjacent to the reflector are changed to reduce image shake, and a fourth type (disclosed in Japanese Unexamined Patent Publication Nos. 2006-166202 and 2006-259247) in which the entire imaging unit is tilted/inclined to reduce image shake. 
     An anti-shake system using voice coil motors (VCMs), which generate force (driving force) by application of a current (voltage) across the terminals of the coil positioned inside the magnetic field of a permanent magnet, for driving an optical element (anti-shake optical element) to reduce image shake is known in the art (disclosed in Japanese Unexamined Patent Publication Nos. 2009-86319, 2010-128384, 2007-228005, and Japanese Patent No. 4,789,655). Information on the position of the anti-shake optical element can be obtained with sensors (e.g., Hall sensors) that measure the change in the magnetic field. 
     The first type of anti-shake system tends to become complicated in structure and tends to increase in cost because a circuit board connected to the image sensor is moved in order to follow movements of the image sensor, which requires electrical components that are provided around the image sensor to also be movable components in addition to the image sensor. In addition, the periphery of the imaging surface of the image sensor is required to be dust tight; however, in small imaging units intended for being incorporated into a mobile phone or a tablet computer, etc., it is difficult to secure sufficient space for allowing the image sensor to perform an anti-shake (image shake correction/image-stabilizing/shake reduction) operation while maintaining the dust-tight structure of the image sensor. 
     The second type of anti-shake system has a structure such that the moving direction of the lens group, disposed behind the reflector, during an anti-shake operation corresponds to the direction of the thickness of the imaging unit (i.e., the forward/rearward direction of the imaging unit, wherein the direction toward an object to be photographed refers to the forward (front) direction of the imaging unit), and hence, there is a problem with providing enough space to house such an anti-shake structure in a slimmed-down imaging unit. In other words, the slimming-down of the imaging unit is limited if this type of anti-shake system is used. There is a similar problem also in the type of anti-shake system in which an image sensor is moved, instead of a lens group, in the direction of the thickness of the imaging unit. 
     The third type of anti-shake system requires a large space for allowing the reflector and the lens group to tilt/incline, and accordingly, the imaging unit is easily enlarged in size. The fourth type of anti-shake system requires a larger space for allowing the entire imaging unit to be tilted/inclined to reduce image shake. 
     Accordingly, there has been a demand for an anti-shake system that utilizes a different manner of driving an anti-shake optical element from those of the above described types of imaging units and that is advantageous for miniaturization and slimming-down of the imaging apparatus. In addition, in the case where voice coil motors (VCMs) are used as drive sources of an anti-shake system, arranging elements thereof such as permanent magnets, coils and sensors in a space-efficient manner in addition to an anti-shake driving manner is also important for achieving miniaturization of the imaging apparatus. 
     Voice coil motors (voice coil linear motors) that provide linear motion can transmit power with no need for a mechanism to convert rotational motion into linear motion, and therefore, if voice coil motors are used in an anti-shake system as drive sources which move an anti-shake optical element in a plane orthogonal to an optical axis, the anti-shake system can be easily simplified in structure. On the other hand, permanent magnets and coils which constitute voice coil motors are typically flat in shape, each having a wide surface along a plane (orthogonal to an optical axis of the anti-shake optical element) in which the anti-shake optical element moves, so that the installation space for the permanent magnets and the coils in this plane tends to be large. In the voice coil motors for use in an anti-shake system in particular, a combination of a permanent magnet and a coil and another combination of a permanent magnet and a coil, these combinations being orthogonal in linear moving direction to each other, are used and arranged around the anti-shake optical element, and accordingly, it is necessary to pay attention to the arrangement of the voice coil motors and associated sensors when it is attempted to miniaturize and reduce the thickness of an imaging unit equipped with an anti-shake system using voice coil motors as drive sources for driving an anti-shake optical element. 
     Additionally, in the case where voice coil motors in which permanent magnets are mounted on a movable member, i.e., so-called moving-magnet type voice coil motors, are used for an anti-shake system, there is a possibility of magnetic materials around the voice coil motors exerting an influence on the magnetic fields of the permanent magnets and thereby deteriorating the driving accuracy of the anti-shake system, so that countermeasures against this problem are required. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in view of the above mentioned drawbacks and provides an imaging apparatus in which a bending optical system is used and which is equipped with an anti-shake system that drives an anti-shake optical element using voice coil motors, wherein miniaturization and reduction in thickness of the imaging apparatus are achieved by the adoption of the anti-shake system which is superior in space efficiency and driving accuracy. 
     According to an aspect of the present invention, an imaging apparatus is provided, including a front lens group which constitutes part of an imaging optical system of the imaging apparatus and is provided at a fixed position with respect to an optical axis direction, wherein the front lens group includes at least one front lens element and a reflector, in that order from an object side, and wherein light rays exiting from the front lens element along a first optical axis are reflected by the reflector to travel along a second optical axis that is nonparallel to the first optical axis; at least one rear lens group which constitutes another part of the imaging optical system and is provided closer to an image plane than the front lens group; a base member which supports at least the reflector; a movable frame which supports the front lens element and is supported by the base member to be movable relative to the base member along a plane orthogonal to the first optical axis; and a driver which drives the movable frame, in response to vibrations applied to the imaging optical system, to reduce image shake on the image plane. The driver includes a first voice coil motor which includes a first coil and a first permanent magnet that are mounted to one and the other of the base member and the movable frame, respectively, so that the first coil and the first permanent magnet face each other in a direction parallel to the first optical axis, wherein the first voice coil motor generates a driving force in a first direction orthogonal to a magnetic pole boundary line of the first permanent magnet upon the first coil being energized; and a second voice coil motor which includes a second coil and a second permanent magnet that are mounted to the one and the other of the base member and the movable frame, respectively, so that the second coil and the second permanent magnet face each other in the direction parallel to the first optical axis, wherein the second voice coil motor generates a driving force in a second direction orthogonal to a magnetic pole boundary line of the second permanent magnet upon the second coil being energized. The first permanent magnet and the second permanent magnet are positioned so that directions of the magnetic pole boundary lines thereof are orthogonal to each other in a plane orthogonal to the first optical axis. The first voice coil motor and the second voice coil motor are positioned on opposite sides of a first reference plane in which the first optical axis and the second optical axis lie, respectively. Centers of the first permanent magnet and the second permanent magnet and centers of the first coil and the second coil are positioned on one side of a second reference plane, which includes the first optical axis and is orthogonal to the first reference plane, wherein the second optical axis extends on the other side of the second reference plane in a direction away from the second reference plane. The magnetic pole boundary line of the first permanent magnet and the magnetic pole boundary line of the second permanent magnet are inclined with respect to each other so as to approach the first reference plane in a direction away from the second reference plane. The imaging apparatus further includes a first magnetic sensor which detects a position of the movable frame in the first direction, in which the first voice coil motor generates the driving force; and a second magnetic sensor which detects a position of the movable frame in the second direction, in which the second voice coil motor generates the driving force. As viewed along the first optical axis, the first magnetic sensor is positioned on an opposite side of the first coil in the first direction from the front lens element side, and the second magnetic sensor is positioned on an opposite side of the second coil in the second direction from the front lens element side. 
     With this structure, the first magnetic sensor and the second magnetic sensor, in addition to the voice coil motors, can be arranged around the front lens element in a space-efficient manner. 
     It is desirable for first permanent magnet to be greater in width than the first coil in the first direction. At least a part of the first magnetic sensor and at least a part of the first permanent magnet overlap each other as viewed along the first optical axis. The second permanent magnet is greater in width than the second coil in the second direction. At least a part of the second magnetic sensor and at least a part of the second permanent magnet overlap each other as viewed along the first optical axis. 
     It is desirable for at least apart of the first magnetic sensor and at least a part of the first coil to overlap each other as viewed along the first direction, and for at least a part of the second magnetic sensor and at least a part of the second coil to overlap each other as viewed along the second direction. This configuration produces an effect of reducing the thickness of the imaging apparatus in a direction along the first optical axis. 
     It is desirable for the first coil to have an elongated shape having a pair of long sides extending in a direction orthogonal to the first direction, and for the second coil to have an elongated shape having a pair of long sides extending in a direction orthogonal to the second direction. The first magnetic sensor is positioned along one of the long sides of the first coil, and the second magnetic sensor is positioned along one of the long sides of the second coil. 
     It is desirable for the first permanent magnet and the second permanent magnet to be symmetrically arranged on both sides of the first reference plane. The first coil and the second coil are symmetrically arranged on both sides of the first reference plane. The first magnetic sensor and the second magnetic sensor are symmetrically arranged on both sides of the first reference plane. 
     The present invention is practical in either case where each permanent magnet and each coil are mounted to one or the other of the movable frame and the base member; however, the present invention is suitably applicable to a so-called moving magnet type anti-shake system (anti-shake system using moving-magnet type voice coil motors), in which the first permanent magnet and the second permanent magnet are mounted to the movable frame. The first coil, the second coil, the first magnetic sensor and the second magnetic sensor are fixedly supported by the base member. 
     In this case, if the imaging apparatus includes a cover member which is fixed to the base member to cover said movable frame, wherein the first coil, the second coil, the first magnetic sensor and the second magnetic sensor are provided on the cover member, each component can be easily mounted. 
     Sensors for use as the first magnetic sensor and the second magnetic sensor can be an optional type. For instance, each of the first magnetic sensor and the second magnetic sensor can be a Hall sensor. 
     The reflector of the front lens group can be of any type. For instance, the reflector of the front lens group can be a prism. 
     According to the present invention, the front lens element of the front lens group, which is positioned in front of the reflector of the front lens group, is moved in directions orthogonal to an optical axis (the first optical axis) to counteract image shake, which makes it possible to achieve miniaturization of the imaging apparatus in an efficient manner, even though an anti-shake system is incorporated, especially with respect to a reduction in thickness of the imaging apparatus in the forward/rearward direction along the first optical axis that passes through the front lens element. In addition, the arrangement in which two permanent magnets and the two coils are positioned in a space-efficient manner in an area around the front lens element which does not interfere with other optical elements contributes to miniaturization of the imaging apparatus that includes an anti-shake system. Additionally, since the driver that includes the permanent magnets is arranged in a section not easily influenced by other magnetic materials, a superior effect of ensuring the driving accuracy of the anti-shake system is also obtained. Additionally, the two magnetic sensors that are for detecting the position of the moving frame that supports the front lens element are arranged around the voice coil motors in a space-efficient manner, so that overall miniaturization of the imaging apparatus that includes the magnetic sensors is achieved. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2013-19481 (filed on Feb. 4, 2013) which is expressly incorporated herein by reference in its entirety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described below in detail with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of an embodiment of an imaging unit according to the present invention; 
         FIG. 2  is a perspective view of the imaging unit with the housing removed, illustrating the internal structure of the imaging unit; 
         FIG. 3  is a transverse sectional view of the imaging unit; 
         FIG. 4  is an exploded perspective view of a first lens-group unit of the imaging unit that constitutes a part of the imaging unit; 
         FIG. 5  is a front elevational view of the imaging unit with a covering member removed; 
         FIG. 6  is a sectional view taken along the line VI-VI shown in  FIG. 5 , illustrating the first lens-group unit; 
         FIG. 7  is a sectional view taken along the line VII-VII shown in  FIG. 5 , illustrating the first lens-group unit; 
         FIG. 8  is a sectional view taken along the line VIII-VIII shown in  FIG. 5 , illustrating a portion of an electromagnetic actuator provided in the first lens-group unit, and the vicinity thereof, with the covering member mounted; 
         FIG. 9  is a sectional view taken along the line IX-IX shown in  FIG. 5 , illustrating another portion of the electromagnetic actuator, and the vicinity thereof, with the covering member mounted; 
         FIG. 10  is a front elevational view of a first lens frame that holds a first lens element of the imaging optical system of the imaging unit; and 
         FIG. 11  is a front elevational view of a base member, and guide shafts and a first prism which are supported on the base member, viewed from the object side. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of an imaging unit (imaging apparatus)  10  according to the present invention will be discussed below with reference to  FIGS. 1 through 11 . In the following descriptions, forward and rearward directions, leftward and rightward directions, and upward and downward directions are determined with reference to the directions of the double-headed arrows shown in the drawings. The object side corresponds to the front side. As shown by the outward appearance of the imaging unit  10  in  FIG. 1 , the imaging unit  10  has a laterally elongated shape which is slim in the forward/rearward direction and long in the leftward/rightward direction. 
     As shown in  FIGS. 2 and 3 , an imaging optical system of the imaging unit  10  is provided with a first lens group (front lens group) G 1 , a second lens group (rear lens group) G 2 , a third lens group (rear lens group) G 3  and a fourth lens group (rear lens group) G 4 . The first lens group G 1  is provided with a first prism (reflector) L 11  and the imaging unit  10  is provided with a second prism L 12  on the right-hand side (image plane side) of the fourth lens group G 4 . The imaging optical system of the imaging unit  10  is configured as a bending optical system which reflects (bends) light rays at substantially right angles at each of the first prism L 11  and the second prism L 12 . As shown in  FIGS. 3 and 7 , the first lens group G 1  is configured of a first lens element (front lens element) L 1 , the first prism L 11  and a second lens element L 2 . The first lens element L 1  is positioned in front of (on the object side of) an incident surface L 11 - a  of the first prism L 11 , while the second lens element L 2  is positioned on the right-hand side (image plane side) of an exit surface L 11 - b  of the first prism L 11 . Each of the second lens group G 2 , the third lens group G 3  and the fourth lens group G 4  is a lens group including no reflector element such as a prism. 
     As shown in  FIG. 3 , light rays emanated from the photographic object and incident on the first lens element L 1  along a first optical axis O 1  extending in the rearward direction from the front of the imaging unit  10  enter the first prism L 11  through the incident surface L 11 - a  and are reflected by a reflecting surface L 11 - c  of the first prism L 11  in a direction along a second optical axis O 2  (extending in the rightward direction) to exit from the exit surface L 11 - b  of the first prism L 11 . Subsequently, the light rays exiting from the exit surface L 11 - b  pass through the second lens element L 2  of the first lens group G 1  and the second through fourth lens groups G 2 , G 3  and G 4 , which lie on the second optical axis O 2 , and are incident on the second prism L 12  through an incident surface L 12 - a  thereof. Subsequently, the light rays which are passed through the incident surface L 12   a  are reflected by a reflecting surface L 12 - c  of the second prism L 12  in a direction along a third optical axis O 3  (extending in the forward direction) and are incident on the imaging surface of an image sensor IS to form an object image thereon. The first optical axis O 1  and the third optical axis O 3  are substantially parallel to each other and lie, together with the second optical axis O 2 , on a common plane. This (imaginary) common plane defines a first reference plane P 1  (see  FIGS. 5 and 6 ) in which the first optical axis O 1 , the second optical axis O 2  and the third optical axis O 3  lie, and an imaginary plane which is orthogonal to the first reference plane P 1  and includes the first optical axis O 1  is represented by a second reference plane P 2  (see  FIG. 5 ). The imaging unit  10  has a shape elongated in a direction along the second optical axis O 2 , and the first lens element L 1  is positioned in the vicinity of an end (the left end) of the imaging unit  10  in the lengthwise direction thereof. 
     As shown in  FIGS. 1 through 3 , the imaging unit  10  is provided with a body module  11  which holds the second lens group G 2 , the third lens group G 3 , the fourth lens group G 4 , the second prism L 12  and the imaging sensor IS, and a first lens-group unit  12  which holds the first lens group G 1 . The body module  11  is provided with a box-shaped housing  13  which is elongated in the leftward/rightward direction and is small in thickness (slim) in the forward/rearward direction. The first lens-group unit  12  is fixed to one end (the left end) of the housing  13  in the lengthwise direction thereof, and the fourth lens group G 4 , the second prism L 12  and the imaging sensor IS are fixedly held at the other end (the right end) of the housing  13  in the lengthwise direction thereof. 
     As shown in  FIG. 2 , the second lens group G 2  and the third lens group G 3  are held by a second lens group frame  20  and a third lens group frame  21 , respectively, which are supported to be movable along the second optical axis O 2  by a pair of rods  22  and  23  provided in the housing  13 . The imaging unit  10  is provided with a first motor M 1  and a second motor M 2  that are supported by the housing  13 . When the first motor M 1  is driven to rotate a screw shaft Mia thereof which projects from the body of the first motor M 1 , this rotation is transmitted to the second lens group frame  20  to move the second lens group frame  20  along the pair of rods  22  and  23 . When the second motor M 2  is driven to rotate a screw shaft M 2   a  thereof which projects from the body of the second motor M 2 , this rotation is transmitted to the third lens group frame  21  to move the third lens group frame  21  along the pair of rods  22  and  23 . The imaging optical system of the imaging unit  10  is a zoom lens system (variable-focal length lens system), and a zooming operation (power-varying operation) is performed by moving the second lens group G 2  and the third lens group G 3  along the second optical axis O 2 . In addition, a focusing operation is performed by moving the third lens group G 3  along the second optical axis O 2 . 
     The imaging unit  10  is provided with an anti-shake (image shake correction/image-stabilizing/shake reduction) system that reduces image shake on an image plane which is caused by vibrations such as hand shake. This anti-shake system drives the first lens element L 1  of the first lens group G 1  in a plane orthogonal to the first optical axis O 1 . The first optical axis O 1  in the following descriptions and the drawings of the present embodiment of the imaging apparatus denotes the position of the first optical axis O 1  in a state where the first lens element L 1  is positioned at the center of the driving range thereof by the anti-shake system (i.e., at an initial optical-design position of the first lens element L 1  when no image shake correction operation is performed). 
     As shown in  FIG. 4 , the first lens-group unit  12  is provided with a first lens frame (movable frame)  30  which holds the first lens element L 1 , a base member  31  which holds the first prism L 11  and the second lens element L 2 , and a cover member  32  which covers the first lens frame  30  and the base member  31  from front. The base member  31  is substantially rectangular in shape as viewed from front as shown in  FIGS. 5 and 11  and is provided with a base plate  35 , a rear flange  36  and an exit-side flange  37 . As shown in  FIGS. 4, 6 and 7 , the base plate  35  lies in a plane substantially orthogonal to the first optical axis O 1 , the rear flange  36  projects rearward from the base plate  35 , and the exit-side flange  37  is positioned at the right end of the base plate  35 . The support position of the first lens-group unit  12  on the body module  11  is determined by making the rear flange  36  and the exit-side flange  37  abut against the housing  13  and by engaging ends of the pair of rods  22  and  23  in holes formed in the exit-side flange  37  (see  FIGS. 1 and 3 ). The first lens-group unit  12  is fixed to the body module  11  by screwing set screws which are inserted into holes  36   a  (see  FIGS. 1, 2 and 4 ) formed through the rear flange  36  of the base member  31 , into screw holes (not shown) formed in the housing  13 . The aforementioned set screws are not shown in the drawings. 
     As shown in  FIGS. 3, 4, 6, 7 and 11 , the base member  31  is provided with a prism mounting recess  38 . The front side of the prism mounting recess  38  is open and exposed on the top of the base plate  35 , while the right side of the prism mounting recess  38  is open and exposed toward the exit-side flange  37 . The first prism L 11  is fit-engaged into the prism mounting recess  38  and fixed thereto. The first prism L 11  is provided with the incident surface L 11 - a , the exit surface L 11 - b , the reflecting surface L 11 - c  and a pair of side surfaces L 11 - d . The incident surface L 11 - a  is positioned on the first optical axis O 1  and faces forward, the exit surface L 11 - b  is positioned on the second optical axis O 2  and faces rightward, the reflecting surface L 11 - c  is positioned at an angle of substantially 45 degrees with respect to the incident surface L 11 - a  and the exit surface L 11 - b , and the pair of side surfaces L 11 - d  are substantially orthogonal to both the incident surface L 11 - a  and the exit surface L 11 - b . The exit surface L 11 - b  is substantially parallel to the second reference plane P 2 , and the pair of side surfaces L 11 - d  are substantially parallel to the first reference plane P 1 . The base member  31  is further provided with a lens holding portion  39  which extends through the exit-side flange  37  in the rightward direction from the prism mounting recess  38 , and the second lens element L 2  is fit-engaged into the lens holding portion  39  to be held thereby. 
     As shown in  FIGS. 4, 10 and 11 , the incident surface L 11 - a  of the first prism L 11  is in the shape of a non-square rectangle which is defined by two pairs of sides (two long sides and two short sides). The first prism L 11  is positioned in the prism mounting recess  38  so that the long sides (a pair of opposite sides) of the incident surface L 11 - a  extend upward and downward and that the short sides (another pair of opposite sides) of the incident surface L 11 - a  extend leftward and rightward. In the following descriptions, the long side of the incident surface L 11 - a  which adjoins the exit surface L 11 - b  (and which constitutes the boundary between the incident surface L 11 - a  and the exit surface L 11 - b ) is referred to as the exit long-side of the incident surface L 11 - a , and the long side of the incident surface L 11 - a  that is on the opposite side of the exit long-side and far from the exit surface L 11 - b  (and which constitutes the boundary between the incident surface L 11 - a  and the reflecting surface L 11 - c ) is referred to as the end long-side of the incident surface L 11 - a . The pair of short sides of the incident surface L 11 - a , which connect the exit long-side and the end long-side of the incident surface L 11 - a , constitute the boundaries between the incident surface L 11 - a  and the pair of side surfaces L 11 - d.    
     The base member  31  is provided on the front of the base plate  35  with three guide support portions  40 A,  40 B and  40 C. As shown in  FIGS. 5 and 11 , the guide support portions  40 A and  40 B are arranged at positions along the pair of side surfaces L 11 - d  (the pair of short sides of the incident surface L 11 - a ) of the first prism L 11  and are symmetrical with respect to the first reference plane P 1 , and the guide support portion  40 C is positioned between the end long-side of the incident surface L 11 - a  and the left end of the base member  31 . In other words, the guide support portions  40 A,  40 B and  40 C are formed in a U-shaped area along the three sides of the incident surface L 11 - a  except for the exit long-side thereof. As shown in  FIG. 4 , each of the guide support portions  40 A,  40 B and  40 C is U-shaped in cross section and has an elongated open groove T 1  that is open toward the peripheral edge of the base member  31 . The elongated open grooves T 1  of the guide support portions  40 A and  40 B are elongated grooves which are elongated in a direction substantially parallel to the short sides of the incident surface L 11 - a  of the first prism L 11 , and the elongated open groove T 1  of the guide support portion  40 C is an elongated groove which is elongated in a direction substantially parallel to the long sides of the incident surface L 11 - a  of the first prism L 11 . 
     A guide shaft  41 A, a guide shaft  41 B and a guide shaft  41 C are inserted into and supported by the elongated open grooves T 1  of the guide support portions  40 A,  40 B and  40 C, respectively. The guide shafts  41 A,  41 B and  41 C are cylindrical columnar members which have a uniform cross section in the lengthwise direction and are made of metal, synthetic resin or the like. The elongated open groove T 1  of the guide support portion  40 A is open on the upper side thereof, and the guide shaft  41 A is inserted into the elongated open groove T 1  of the guide support portion  40 A in a direction to approach the first optical axis O 1  from this upper-side opening that faces upward. The elongated open groove T 1  of the guide support portion  40 B is open on the lower side thereof, and the guide shaft  41 B is inserted into the elongated open groove T 1  of the guide support portion  40 B in a direction to approach the first optical axis O 1  from this lower-side opening that faces downward. The elongated open groove T 1  of the guide support portion  40 C is open on the left side thereof, and the guide shaft  41 C is inserted into the elongated open groove T 1  of the guide support portion  400  in a direction to approach the first optical axis O 1  from this left-side opening that faces leftward. Each guide shaft  41 A,  41 B and  41 C can be inserted into the associated elongated open groove T 1  along a plane orthogonal to the first optical axis O 1 , and the axes of the guide shafts  41 A,  41 B and  41 C lie in a plane orthogonal to the first optical axis O 1  with each guide shaft  41 A,  41 B and  41 C inserted into the associated elongated open groove T 1 . More specifically, as shown in  FIGS. 5 and 11 , the axes of the guide shafts  41 A and the  41 B are substantially parallel to the short sides (the pair of side surfaces L 11 - d ) of the incident surface L 11 - a  of the first prism L 11  and the first reference plane P 1 , and the axis of the guide shaft  41 A and the axis of the guide shaft  41 B are substantially equi-distant from the first reference plane P 1 . In addition, the axis of the guide shaft  41 C is substantially parallel to the long sides of the incident surface L 11 - a  of the first prism L 11  and the second reference plane P 2 . Furthermore, as shown in  FIG. 5 , the centers of the guide shafts  41 A and  41 B with respect to the axial direction thereof lie in the second reference plane P 2 , and the center of the guide shaft  41 C with respect to the axial direction thereof lies in the first reference plane P 1 . Cutouts (recesses)  42 A,  42 B and  42 C are formed in central portions of the guide support portions  40 A,  40 B and  40 C, each of which has a shape so as not to hold the associated guide shaft  41 A,  41 B or  41 C. The cutouts  42 A and  42 B are positioned on the second reference plane P 2 , and the cutout  42 C is positioned on the first reference plane P 1 . 
     The base member  31  is provided on the front of the base plate  35  with a movement limit projection  43  and a swing pivot  44 , each of which projects forward. As shown in  FIGS. 4 and 11 , the movement limit projection  43  is a cylindrical columnar projection which is formed between the prism mounting recess  38  (the end long-side of the incident surface L 11 - a  of the prism L 11 ) and the cutout  42 C. The swing pivot  44  is a cylindrical columnar projection which is formed near the boundary between the guide support portion  40 B and the exit-side flange  37  (in the vicinity of the corner between the lower short side of the incident surface L 11 - a  and the exit long-side of the incident surface L 11 - a ) in the vicinity of the prism mounting recess  38 . 
     In the anti-shake system of the imaging unit  10 , the first lens frame  30  is supported by the base member  31  to be movable in a plane orthogonal to the first optical axis O 1  via the three guide shafts  41 A,  41 B and  41 C. As shown in  FIGS. 4 and 10 , the first lens frame  30  is provided with a cylindrical lens holding portion  50 , into which the first lens element L 1  is fitted to be fixed thereto, and a flange  55  which projects in a direction (leftward direction) opposite to the direction of extension of the second optical axis O 2 . The first lens frame  30  is further provided around the lens holding portion  50  and the flange  55  with three slidable support portions  51 A,  51 B and  51 C. As viewed from front as shown in  FIGS. 5 and 10 , the first lens element L 1  has a D-cut shape that is formed (defined) by cutting off a portion of the outer edge of the first lens element L 1  on the right side thereof (the side from which the second optical axis O 2  extends rightward from the reflecting surface L 11 - c  of the first prism L 11 ), and the lens holding portion  50  has a D-cut cylindrical shape which corresponds in outside shape to the first lens element L 1 . The three slidable support portions  51 A,  51 B and  51 C are formed on the first lens frame  30  along three sides thereof except for the side on which the D-cut portion is formed. 
     More specifically, the slidable support portions  51 A and  51 B are formed on the periphery of the lens holding portion  50  to be symmetrical with respect to the first reference plane  81 , and the slidable support portion  510  is formed at the left end of the flange  55 . In the state shown in  FIGS. 5 through 7 , in which the first lens frame  30  is supported by the base member  31 , the slidable support portion  51 A is positioned above the cutout  42 A, the slidable support portion  51 B is positioned above the cutout  42 B and the slidable support portion  51 C is positioned above the cutout  42 C. The cutouts  42 A,  42 B and  42 C serve as clearance recesses which prevent the guide support portions  40 A,  40 B and  40 C from interfering with the slidable support portions  51 A,  51 B and  51 C, respectively, when the first lens frame  30  moves relative to the base member  31  to perform an anti-shake operation. 
     As shown in  FIGS. 4, 6 and 7 , each of the three slidable support portions  51 A,  51 B and  51 C is U-shaped in cross section and has an elongated open groove T 2  that is open toward the peripheral edge of the first lens frame  30 . The elongated open grooves T 2  of the slidable support portions  51 A and  51 B are elongated grooves which are elongated in a direction substantially parallel to the short sides of the incident surface L 11 - a  of the first prism L 11 , and the elongated open groove T 2  of the slidable support portion  51 C is an elongated groove which is elongated in a direction substantially parallel to the long sides of the incident surface L 11 - a  of the first prism L 11 . The guide shaft  41 A is inserted into the elongated open groove T 2  of the slidable support portion  51 A from the upper-side opening of this elongated open groove that faces upward, the guide shaft  41 B is inserted into the elongated open groove T 2  of the slidable support portion  51 B from the lower-side opening of this elongated open groove that faces downward, and the guide shaft  41 C is inserted into the elongated open groove T 2  of the slidable support portion  51 C from the left-side opening of this elongated open groove that faces leftward. In an assembly process, it is advisable that the base member  31  and the first lens frame  30  be combined together and thereafter each guide shaft  41 A,  41 B and  41 C be inserted into the associated elongated open groove T 1  and the associated elongated open groove T 2 . When the first lens frame  30  is mounted on the base member  31  with the slidable support portions  51 A,  51 B and  51 C respectively aligned with the cutouts  42 A,  42 B and  42 C, the elongated open grooves T 2  of the slidable support portions  51 A,  51 B and  51 C are positioned relative to the elongated open grooves T 1  of the guide support portions  40 A,  40 B and  40 C such that the elongated open grooves T 1  are communicatively connected to, and coaxial with, the elongated open grooves T 2 , respectively (each elongated open groove T 2  is positioned at the midpoint of the associated elongated open groove T 1  in the elongated direction thereof). In this state, the guide shaft  41 A is inserted into the elongated open groove T 1  of the guide support portion  40 A and the elongated open groove T 2  of the slidable support portion  51 A in a direction to approach the first optical axis O 1  from the upper-side openings of these elongated open grooves T 1  and T 2  that face upward. Likewise, the guide shaft  41 B is inserted into the elongated open groove T 1  of the guide support portion  40 B and the elongated open groove T 2  of the slidable support portion  51 B in a direction to approach the first optical axis O 1  from the lower-side openings of these elongated open grooves T 1  and T 2  that face downward, and the guide shaft  41 C is inserted into the elongated open groove T 1  of the guide support portion  40 C and the elongated open groove T 2  of the slidable support portion  51 C in a direction to approach the first optical axis O 1  from the left-side openings of these elongated open grooves T 1  and T 2  that face leftward. Each guide shaft  41 A,  41 B and  41 C inserted into the associated elongated open groove T 1  is fixed, at both ends thereof, inside the associated elongated open groove T 1  by an adhesive, press-fitting or the like, and held so as not to come off the associated elongated open groove T 1  by an outer surrounding wall  57  of the cover member  32 . 
     As shown in  FIGS. 4, 6 and 7 , each slidable support portion  51 A,  51 B and  51 C is provided in the elongated open groove T 2  thereof with a pair of projections  52  which face each other in a direction parallel to the first optical axis O 1 , and the pair of projections  52  of each slidable support portion  51 A,  51 B and  51 C holds the associated guide shaft  41 A,  41 B or  41 C therebetween from both sides thereof in a direction parallel to the first optical axis O 1 . Each pair of projections  52  project in opposite directions toward each other so as to partially narrow the width of the associated elongated open groove T 2  in a direction parallel to the first optical axis O 1  to hold the associated guide shaft  41 A,  41 B or  41 C with substantially no clearance (specifically, with the presence of a minimum clearance allowing the associated slidable support portion  51 A,  51 B or  51 C to slide on the associated guide shaft  41 A,  41 B or  41 C). This structure prevents the first lens frame  30  from moving relative to the base member  31  in a direction along the first optical axis O 1 . As shown in  FIG. 4 , each projection  52  is trapezoidal in cross sectional shape, and the contact portion of each projection  52  which is in contact with the associated guide shaft  41 A,  41 B or  41 C is formed as a flat surface (the upper base of a trapezoid) lying in a plane substantially orthogonal to the first optical axis O 1 . Accordingly, each projection  52  is slidable on the associated guide shaft  41 A,  41 B or  41 C in a direction along a plane orthogonal to the first optical axis O 1 . 
     As shown in  FIG. 5 , a clearance D 1  is provided on each of the opposite sides of each slidable support portion  51 A,  51 B and  51 C in the sliding direction thereof with respect to the associated (adjacent) guide support portion  40 A,  40 B or  40 C to allow each slidable support portion  51 A,  51 B and  51 C to move in the axial direction of the associated guide shaft  41 A,  41 B or  41 C. In addition, as shown in  FIGS. 6 and 7 , a clearance D 2  is provided in the elongated open groove T 2  of each slidable support portion  51 A,  51 B and  51 C between the bottom of this elongated open groove T 2  and the associated guide shaft  41 A,  41 B or  41 C inserted therein to allow each slidable support portion  51 A,  51 B and  51 C to move in the direction of depth of the elongated open groove T 2  that is orthogonal to the axis of the associated guide shaft  41 A,  41 B or  41 C. Namely, the slidable support portions  51 A,  51 B and  51 C are supported to be movable along a plane orthogonal to the first optical axis O 1  via the guide shafts  41 A,  41 B and  41 C, respectively, that are fixedly supported on the base member  31 . 
     The flange  55  of the first lens frame  30  is provided with a movement limit hole  53  which is formed through the flange  55  in the forward/rearward direction and into which the movement limit projection  43  of the base member  31  is inserted. As shown in  FIGS. 5 and 10 , the inner wall of the movement limit hole  53  is generally rectangular in shape in a plane substantially orthogonal to the first optical axis O 1 . The first lens frame  30  can move relative to the base member  31  within a range until the movement limit projection  43  comes into contact with the inner wall of the movement limit hole  53 . The aforementioned clearances D 1  and D 2  that are set in each slidable support portion  51 A,  51 B and  51 C are set to be greater than the moving range of the first lens frame  30  that is allowed by the movement limit hole  53  and the movement limit projection  43 , and the moving range of the first lens frame  30  relative to the base member  31  is determined by the movement limit projection  43  and the movement limit hole  53 . 
     The first lens frame  30  is further provided with a pivot support groove  54  in which the swing pivot  44  of the base member  31  is engaged. The pivot support groove  54  is an elongated groove which is elongated in a radial direction which centers on the first optical axis O 1  and exposed radially outwards, toward the outer periphery of the first lens frame  30 . As shown in  FIG. 5 , the pivot support groove  54  is engaged with the swing pivot  44  with a clearance allowing the pivot support groove  54  to move relative to the swing pivot  44  in the lengthwise (depthwise) direction of the pivot support groove  54 , and the pivot support groove  54  is prevented from moving relative to the swing pivot  44  in a direction orthogonal to the lengthwise direction of the pivot support groove  54 . Although the first lens frame  30  is supported by the base member  31  to be movable in a plane orthogonal to the first optical axis O 1  due to the sliding engagement of the three guide shafts  41 A,  41 B and  41 C with the slidable support portions  51 A,  51 B and  51 C as mentioned above, the moving direction of the first lens frame  30  in the aforementioned orthogonal plane is defined by the engagement of the swing pivot  44  with the pivot support groove  54 . Specifically, the first lens frame  30  is supported by the base member  31  to be allowed to move in the rotational (swinging) direction about the swing pivot  44  and the lengthwise direction of the pivot support groove  54 . 
     The movement limit projection  43  and the swing pivot  44  are inserted into the movement limit hole  53  and the pivot support groove  54 , respectively, at the stage at which first lens frame  30  is mounted on the base member  31  before the installation of the guide shafts  41 A,  41 B and  41 C. 
     As shown in  FIG. 4 , the cover member  32  is provided with a plate-shaped front wall  56  that is orthogonal to the first optical axis O 1  and the outer surrounding wall  57  that projects rearward from the front wall  56 . The cover member  32  is fixed to the base member  31  so that the front wall  56  covers the first lens frame  30  from front. In this fixed state, the outer surrounding wall  57  is a U-shaped wall which surrounds the three guide support portions  40 A,  40 B and  40 C of the base member  31  from the outer side, and the side openings of the elongated open grooves T 1  of the guide support portions  40 A,  40 B and  40 C and the side openings of the elongated open grooves T 2  of the slidable support portions  51 A,  51 B and  51 C are all closed by the outer surrounding wall  57  (see  FIG. 3 ). The front wall  56  is provided with a photographic aperture  58  through which the first lens element L 1  is exposed forward. 
     The first lens frame  30  is driven by an electromagnetic actuator. This electromagnetic actuator includes two voice coil motors (VCMs) provided with two permanent magnets  60  and  61  and two coils  62  and  63 , respectively. 
     The two permanent magnets  60  and  61  are supported by the first lens frame  30  and the two coils  62  and  63  are supported by the cover member  32 . The permanent magnets  60  and  61  are fitted into and held by magnet holding holes formed in the flange  55  of the first lens frame  30  (see  FIGS. 8 and 9 ). Each of the permanent magnets  60  and  61  is in the shape of a non-square rectangular thin plate. The permanent magnets  60  and  61  are arranged symmetrically with respect to the first reference plane P 1 . More specifically, opposite sides of a magnetic pole boundary line Q 1  (see  FIGS. 5 and 10 ) of the permanent magnet  60 , which extends in the lengthwise direction thereof and passes through an approximate center of the permanent magnet  60  with respect to the width thereof, are magnetized into north and south poles, respectively, while opposite sides of a magnetic pole boundary line Q 2  (see  FIGS. 5 and 10 ) of the permanent magnet  61 , which extends in the lengthwise direction thereof and passes through an approximate center of the permanent magnet  61  with respect to the width thereof, are magnetized into north and south poles, respectively. In other words, the magnetic pole boundary line Q 1  defines a boundary between north and south poles of the permanent magnet  60 , while the magnetic pole boundary line Q 2  defines a boundary between north and south poles of the permanent magnet  61 . The magnetic pole boundary line Q 1  of the permanent magnet  60  and the magnetic pole boundary line Q 2  of the permanent magnet  61  are inclined to each other so that the distance therebetween (i.e., the distance from the first reference plane P 1 ) gradually increases in a direction from left to right. The inclination angles of the magnetic pole boundary lines Q 1  and Q 2  of the permanent magnets  60  and  61  with respect to the first reference plane P 1  are set to approximately ±45 degrees, respectively. Namely, the lengthwise directions (the magnetic pole boundary lines Q 1  and Q 2 ) of the permanent magnets  60  and  61  are substantially orthogonal to each other. 
     As shown in  FIG. 4 , a circuit board  59  is fixed to a portion of the front wall  56  of the cover member  32  which does not overlap the photographic aperture  58 . As shown in  FIGS. 8 and 9 , the coils  62  and  63  that constitute elements of the electromagnetic actuator are fixed to the rear of the front wall  56  and electrically connected to the circuit board  59 . As shown in  FIGS. 5 and 10 , each of the coils  62  and  63  is an air-core coil which includes a pair of linear portions (long sides) that are substantially parallel to each other and a pair of curved (U-shaped) portions which connect the pair of linear portions at the respective ends thereof. The coils  62  and  63  are substantially identical in shape and size to each other and are symmetrically arranged with respect to the first reference plane P 1 . Specifically, in a state where the first lens element L 1  is positioned at the center of the driving range thereof by the anti-shake system (i.e., at an initial optical-design position of the first lens element L 1  when no image shake correction operation is performed), the long axis (major axis) of the coil  62 , which is parallel to the linear portions of the coil  62  and passes through the air core of the coil  62 , and the long axis (major axis) of the coil  63 , which is parallel to the linear portions of the coil  63  and passes through the air core of the coil  63 , correspond to the magnetic pole boundary line Q 1  of the permanent magnet  60  and the magnetic pole boundary line Q 2  of the permanent magnet  61 , respectively, as viewed from front as shown in  FIGS. 5 and 10 . In other words, the coils  62  and  63  are arranged to be inclined to each other so that the distance between the long axis of the coil  62  and the long axis of the coil  63  gradually increases in a direction from left to right, similar to the permanent magnets  60  and  61 . The inclination angles of the long axes of the coils  62  and  63  with respect to the first reference plane P 1  are set to approximately ±45 degrees, respectively. Namely, the lengthwise directions (the long axes) of the coils  62  and  63  are substantially orthogonal to each other. 
     The energization of the coils  62  and  63  is controlled via the circuit board  59 . A driving force is generated in a direction substantially orthogonal to the magnetic pole boundary line Q 1  of the permanent magnet  60  (i.e., orthogonal to the direction of the long axis of the coil  62 ) in a plane orthogonal to the first optical axis O 1  upon the coil  62  being energized. The direction of action of this driving force is shown by a double-headed arrow F 1  in  FIGS. 5, 8 and 10 . On the other hand, a driving force is generated in a direction substantially orthogonal to the magnetic pole boundary line Q 2  of the permanent magnet  61  (i.e., orthogonal to the direction of the long axis of the coil  63 ) in a plane orthogonal to the first optical axis O 1  upon the coil  63  being energized. The direction of action of this driving force is shown by a double-headed arrow F 2  in  FIGS. 5, 9 and 10 . The direction of action F 1  of the driving force generated by energizing the coil  62  is substantially parallel to the lengthwise direction of the pivot support groove  54 , and the first lens frame  30  can move linearly along the lengthwise direction of the pivot support groove  54  relative to the base member  31  upon the coil  62  being energized. On the other hand, the direction of action F 2  of the driving force generated by energizing the coil  63  is substantially orthogonal to the lengthwise direction of the pivot support groove  54 , and the pivot groove  54  is prevented from moving relative to the swing pivot  44  in this orthogonal direction, and accordingly, the first lens frame  30  rotates (swings) about the swing pivot  44  relative to the base member  31  of the first lens frame  30  upon the coil  63  being energized. The first lens frame  30  can be moved to any arbitrary position in a plane orthogonal to the first optical axis O 1  with respect to the base member  31  by a combination of controlling the passage of current through the coils  62  and  63 . As described above, the moving range of the first lens frame  30  with respect to the base member  31  is limited by engagement of the movement limit projection  43  with the inner wall of the movement limit hole  53 . 
     Reference character U 1  shown in  FIGS. 5 and 10  designates the centers of the permanent magnet  60  and the coil (the centers of the outer shapes thereof) in a plane orthogonal to the first optical axis O 1 . Likewise, reference character U 2  shown in  FIGS. 5 and 10  designates the centers of the permanent magnet  61  and the coil  63  (the centers of the outer shapes thereof) in a plane orthogonal to the first optical axis O 1 . The center U 1  of the permanent magnet  60  corresponds to both the center of the permanent magnet  60  in the lengthwise (long-side) direction thereof along the magnetic pole boundary line Q 1  and the center of the permanent magnet  60  in the short-side direction thereof that is orthogonal to the magnetic pole boundary line Q 1 . The center U 2  of the permanent magnet  61  corresponds to both the center of the permanent magnet  61  in the lengthwise (long-side) direction thereof along the magnetic pole boundary line Q 2  and the center of the permanent magnet  61  in the short-side direction thereof that is orthogonal to the magnetic pole boundary line Q 2 . The center U 1  of the coil  62  corresponds to both the center of the coil  62  in the lengthwise (long-side) direction thereof along the long axis of the coil  62  and the center of the coil  62  in the short-side direction thereof that is orthogonal to the long axis of the coil  62 . The center U 2  of the coil  63  corresponds to both the center of the coil  63  in the lengthwise (long-side) direction thereof along the long axis of the coil  63  and the center of the coil  63  in the short-side direction thereof that is orthogonal to the long axis of the coil  63 .  FIGS. 5 and 10  each show a state where the first lens frame  30  is positioned at the center of the moving range thereof, which is mechanically defined (limited) by the movement limit projection  43  and the movement limit hole  53 . When the first lens frame  30  is positioned at the center of the moving range thereof, the center U 1  of the permanent magnet  60  and the center U 1  of the coil  62  are coincident with each other (i.e., the center U 1  of the permanent magnet  60  and the center U 1  of the coil  62  are aligned in the forward/rearward direction), and the center U 2  of the permanent magnet  61  and the center U 2  of the coil  63  are coincident with each other (i.e., the center U 2  of the permanent magnet  61  and the center U 2  of the coil  63  are aligned in the forward/rearward direction). A movement of the first lens frame  30  which is caused by the passage of current through the coils  62  and  63  causes the positions of the centers U 1  and U 2  of the permanent magnets  60  and  61  with respect to the centers U 1  and U 2  of the coils  62  and  63  to change, respectively. 
     In addition, two magnetic sensors  65  and  66  are mounted to and supported by the rear of the front wall  56  of the cover member  32  as shown in  FIGS. 8 and 9 . Each of the two magnetic sensors  65  and  66  is composed of a Hall sensor connected to the circuit board  59 . As viewed from front as shown in  FIGS. 5 and 10 , the magnetic sensor  65  is disposed on the opposite side of the coil  62  in the direction of action F 1  from the first lens element L 1  side (on the side farther from the first optical axis O 1 ) to be adjacent to the linear portion of the coil  62 , and the magnetic sensor  65  and the coil  62  overlap each other as viewed in the direction of action F 1  (see  FIG. 8 ). Additionally, as viewed from front as shown in  FIGS. 5 and 10 , the magnetic sensor  66  is disposed on the opposite side of the coil  63  in the direction of action F 2  from the first lens element L 1  side (on the side farther from the first optical axis O 1 ) to be adjacent to the linear portion of the coil  63 , and the magnetic sensor  66  and the coil  63  overlap each other as viewed in the direction of action F 2  (see  FIG. 9 ). The reference character K 1  shown in  FIG. 8  designates the overlapping range between the magnetic sensor  65  and the coil  62  and the reference character K 1  shown in  FIG. 9  designates the overlapping range between the magnetic sensor  66  and the coil  63 . 
     When the cover member  32  is mounted to the base member  31 , the magnetic sensors  65  and  66  are positioned in the vicinity of the permanent magnets  60  and  61 , respectively. As shown in  FIGS. 8 and 9 , the magnetic sensors  65  and  66  are positioned in front of the permanent magnets  60  and  61 , respectively, in the forward/rearward direction of the imaging unit  10  along the first optical axis O 1 . As shown in  FIG. 8 , in the direction of action F 1 , the width of the permanent magnet  60  in the short-side direction thereof is greater than the width of the coil  62  in the short-side direction thereof so that both ends of the permanent magnet  60  project from both ends of the coil  62  in the direction of action F 1 , and one of the projecting ends of the permanent magnet  60  which is farther from the first optical axis O 1  (farther from the first lens element L 1 ) (i.e., the right end of the permanent magnet  60  with respect to  FIG. 8 ) and the magnetic sensor  65  overlap each other as viewed from the front. As shown in  FIG. 9 , in the direction of action F 2 , the width of the permanent magnet  61  in the short-side direction thereof is greater than the width of the coil  63  in the short-side direction thereof so that both ends of the permanent magnet  61  project from both ends of the coil  63  in the direction of action F 2 , and one of the both projecting ends of the permanent magnet  61  which is farther from the first optical axis O 1  (farther from the first lens element L 1 ) (i.e., the left end of the permanent magnet  61  with respect to  FIG. 9 ) and the magnetic sensor  66  overlap each other as viewed from front. The reference character K 2  shown in  FIG. 8  designates the overlapping range between the magnetic sensor  65  and the permanent magnet  60  and the reference character K 2  shown in  FIG. 9  designates the overlapping range between the magnetic sensor  66  and the permanent magnet  61 . 
     As shown in  FIGS. 5 and 10 , each of the two magnetic sensors  65  and  66  has a narrow rectangular shape in a front orthographic projection, and the reference characters U 3  and U 4  shown in  FIGS. 5 and 10  designate the centers of the magnetic sensors  65  and  66  in a plane orthogonal to the first optical axis O 1 , respectively. The lengthwise direction of the magnetic sensor  65  is substantially parallel to the magnetic pole boundary line Q 1  and the lengthwise direction of the magnetic sensor  66  is substantially parallel to the magnetic pole boundary line Q 2 . As shown in  FIG. 5 , a straight line that passes through the center U 3  of the magnetic sensor  65  and extends in the direction of action F 1  of the driving force caused by the permanent magnet  60  and the coil  62  and a straight line that passes through the center U 4  of the magnetic sensor  66  and extends in the direction of action F 2  of the driving force caused by the permanent magnet  61  and the coil  63  intersect each other on the first reference plane P 1 . Due to this arrangement, variation in position of the permanent magnet  60  in accordance with movement of the first lens frame  30  that is caused by the electromagnetic actuator causes the output of the magnetic sensor  65  to vary, and variation in position of the permanent magnet  61  in accordance with movement of the first lens frame  30  that is caused by the electromagnetic actuator causes the output of the magnetic sensor  66  to vary. Hence, the position of the first lens frame  30  can be detected from the output variations of the two magnetic sensors  65  and  66 . Upon start-up of the imaging unit  10 , the calibration of each magnetic sensor  65  and  66  is performed by driving the first lens frame  30  to a moving end thereof defined by the movement limit projection  43  and the movement limit hole  53 . 
     If the imaging unit  10 , which is completely assembled by mounting the first lens-group unit  12  that has the above described structure to the body module  11 , is directed toward an object located in front of the imaging unit  10 , light reflected by the object (light emanating from a photographic object) enters the first prism L 11  through the incident surface L 11 - a  after passing through the first lens element L 1  and is reflected at an angle of 90 degrees by the reflecting surface L 11 - c  of the first prism L 11  and travels toward the exit surface L 11 - b . Subsequently, the reflected light that emerges from the exit surface L 11 - b  of the first prism L 11  enters the second prism L 12  from the incident surface L 12 - a  after passing through the second lens element L 2 , the second lens group G 2 , the third lens group G 3  and the fourth lens group G 4 , and is reflected at an angle of 90 degrees by the reflecting surface L 12 - c  of the second prism L 12  and travels toward the exit surface L 12 - b . Subsequently, the reflected light emerges from the exit surface L 12 - b  and is captured (received) by the imaging surface of the image sensor IS. A zooming operation of the imaging optical system of the imaging unit  10  is performed by moving the second lens group G 2  and the third lens group G 3  along the pair of rods  22  and  23  using the first motor M 1  and the second motor M 2 . A focusing operation of the imaging optical system of the imaging unit  10  is performed by moving the third lens group G 3  along the pair of rods  22  and  23  using the second motor M 2 . By performing these zooming and focusing operations, focused object images can be captured at selected angle of view. 
     Additionally, in the imaging unit  10 , an anti-shake (image shake correction/image-stabilizing/shake reduction) operation is performed using the first lens element L 1  of the first lens group G 1  that is positioned in front of the first prism L 11 . As described above, the anti-shake system supports the first lens frame  30  in a manner to allow the first lens frame  30  to move relative to the base member  31 , which is fixed with respect to the housing  13 , in a plane orthogonal to the first optical axis O 1  and drives the first lens frame  30  using the electromagnetic actuator. 
     The moving direction of the first lens element L 1  during an anti-shake operation is orthogonal to the first optical axis O 1 . Namely, the first lens frame  30  that holds the first lens element L 1  does not move in the forward/rearward direction that corresponds to the direction of the thickness of the imaging unit  10 . In addition, the support mechanism (which includes the guide support portions  40 A,  40 B and  40 C, the guide shafts  41 A,  41 B and  41 C, the cutouts  42 A,  42 B and  42 C, the movement limit projection  43 , the swing pivot  44 , the slidable support portions  51 A,  51 B and  51 C, the movement limit hole  53 , the pivot support groove  54 , etc.) and the driver (which includes the permanent magnets  60  and  61 , the coils  62  and  63 , etc.) that are for moving the first lens frame  30  relative to the base member  31  are arranged at positions about the first optical axis O 1  which surround the first lens element L 1 , so that the installation space for the support mechanism and the driver is small with respect to the forward/rearward direction of the imaging unit  10 . Accordingly, the selection of the first lens element L 1  as an anti-shake optical element makes it possible to slim down the imaging unit  10  in the forward/rearward direction even though the imaging unit  10  is provided with an anti-shake system. For instance, unlike the present embodiment, if an anti-shake system were to move the second lens group G 2  or the third lens group G 3  in directions orthogonal to the second optical axis O 2  to cancel out image shake, securing room for the second lens group frame  20  or the third lens group frame  21  and installing the driver for the second lens group frame  20  or the third lens group frame  21  would require a greater installation space for the anti-shake system in the housing  13  in the forward/rearward direction than in the case of the above described illustrated embodiment, thus increasing the thickness of the imaging unit  10 . 
     The first lens element L 1  that is supported by the first lens frame  30  does not need to be connected to any circuit board, unlike an electrical component such as the imaging sensor IS, so that the structure of the imaging unit  10  does not become complicated due to routing of a flexible wiring board, or a flexible wiring board does not exert resistance on the first lens element L 1  during an anti-shake operation. For instance, unlike the present embodiment, if the anti-shake system were to move the image sensor IS in directions orthogonal to the third optical axis O 3  to cancel out image shake, the flexible wiring board to which a circuit board and the imaging sensor IS are connected is required to have a sufficient length so as not to provide resistance to movement of the image sensor IS; however, there is not much space around the image sensor IS, so that the flexible wiring board would interfere with other members if the flexible wiring board is made long. If the image sensor IS and the circuit board are spaced from each other in the forward/rearward direction in order to prevent this problem from occurring, this spacing conflicts with the slimming down of the imaging unit  10 . 
     The selection of the first lens element L 1  as an anti-shake optical element avoids the above described problems and makes it possible to achieve a simple anti-shake system which contributes to the slimming down of the imaging unit  10 . Since only the first lens element L 1  is driven during the anti-shake control, rather than the entire first lens group G 1 , there is the advantage of the moving parts of the anti-shake system being able to be provided in a compact manner and the driving load thereon can be small. In typical anti-shake systems, if only a part (e.g., one lens element) of a lens group is driven in directions orthogonal to the optical axis thereof, there is a possibility of aberrations in the imaging optical system increasing (thereby deteriorating the optical performance of the imaging optical system) and thus causing the imaging optical system to become impractical to use. In this connection, since the first prism L 11  that functions only to reflect the incident light rays is disposed between the first lens element L 1  and the second lens element L 2  (that are optical elements having refractive powers) in the first lens group G 1  in the present embodiment, the distance between the first lens element L 1  and the second lens element L 2  is great, so that an increase in aberration is reduced (deterioration of the optical performance of the imaging optical system is minimalized) even if the first lens element L 1  is solely moved to perform anti-shake control. Accordingly, a satisfactory optical performance can be secured for an anti-shake operation even if the first lens element L 1  and the second lens element L 2 , which are spaced far from each other in the optical axis direction with the first prism L 11  positioned therebetween, are treated as different lens groups, even though the aberration is controlled over the entire first lens group G 1 , which includes the first lens element L 1 , the first prism L 11  and the second lens element L 2 , in an imaging optical system; hence, only the first lens element L 1  is set as an optical element used for anti-shake operation in the present embodiment. 
     Unlike telescopic lens barrels in which the length in an optical axis direction (the distance between the image plane and the lens element closest to the object side) varies when a zooming operation or a barrel retracting operation is performed, the length of the optical path from the incident surface of the first lens element L 1  to the image plane (the imaging surface of the image sensor IS) in the imaging unit  10  is constant at all times. Therefore, it is possible to embed the imaging unit  10  into a mobile electronic device and cover the front of the first lens element L 1  with a protection glass or the like, and no practical problem arises even if the first lens element L 1  of the optical system of the imaging unit  10 , which is located closest to the object side, is driven to cancel out image shake. 
     As mentioned above, even though only a part of a lens group is driven in the above described structure, in which the first lens element L 1 , which is an element of the first lens group LG1 and positioned in front of the first prism L 11 , is solely driven to reduce image shake, such a configuration does not easily influence the aberrations of the imaging optical system. However, since the first lens element L 1  is required to have a higher operating accuracy than that of an anti-shake system in which an entire lens group is driven to reduce image shake, it is required to precisely support and drive the first lens frame  30 , which holds the first lens element L 1 , to stabilize the anti-shake performance and the optical performance. Additionally, in regard to the driving of the first lens element L 1  to reduce image shake, which is the greatest in diameter among all the lens elements of the imaging optical system, it is required to make the anti-shake system as compact as possible so as to contribute to miniaturization of the imaging unit. Features of this anti-shake system will be described hereinafter. 
     When four quadrants V 1 , V 2 , V 3  and V 4  which can be divided into four by the first reference plane P 1  and the second reference plane P 2  are set in a front view as shown in  FIG. 5  on the premise of the following descriptions, the first quadrant V 1  and the fourth quadrant V 4  are positioned on a side of the second reference plane P 2  (the right side of the second reference plane P 2  with respect to  FIG. 5 ) toward the light-ray travelling direction along the second optical axis O 2  upon the light rays being reflected by the first prism L 11 , while the second quadrant V 2  and the third quadrant V 3  are positioned on the opposite side (the left side of the second reference plane P 2  with respect to  FIG. 5 ) of the second reference plane P 2  to the side of the first quadrant V 1  and the fourth quadrant V 4 . 
     Out of the left-side section and the right-side section of the second reference plane P 2 , optical elements of the imaging optical system such as the second lens element L 2 , the second lens group G 2 , the third lens group G 3 , the fourth lens group G 4  and the second prism L 12  are arranged along the second optical axis O 2  in the right-side section (which includes the first quadrant V 1  and the fourth quadrant V 4 ) of the second reference plane P 2 . The pair of rods  22  and  23 , the first motor M 1  and the second motor M 2 , which constitute elements of the advancing/retracting drive mechanism for moving the second lens group G 2  and the third lens group G 3  along the second optical axis O 2 , are also arranged in the right-side section of the second reference plane P 2 . 
     On the other hand, the permanent magnets  60  and  61  and the coils  62  and  63 , which constitute the electromagnetic actuator for driving the first lens element L 1  to reduce image shake, and the magnetic sensors  65  and  66 , which are for detecting the position of the first lens element L 1  during driving thereof, are arranged in the second and third quadrants V 2  and V 3 , which are positioned in the left-side section of the second reference plane P 2  on the opposite side of the second reference plane P 2  from the side toward the light-ray travelling direction along the second optical axis O 2 . More specifically, the permanent magnet  60 , the coil  62  and the magnetic sensor  65  are positioned in the second quadrant V 2 ; the permanent magnet  61 , the coil  63  and the magnetic sensor  66  are positioned in the third quadrant V 3 ; and each of the elements in the second quadrant V 2  and each of the elements in the third quadrant V 3  are arranged to be substantially symmetrical with respect to the first reference plane P 1 . The permanent magnets  60  and  61  are arranged so that the inclination angles of the magnetic pole boundary lines Q 1  and Q 2  thereof with respect to the first reference plane P 1  are approximately ±45 degrees, respectively, as described above, and the directions of inclination of the magnetic pole boundary lines Q 1  and Q 2  are set to approach the first reference plane P 1  (so as to reduce the distance between the magnetic pole boundary lines Q 1  and Q 2 ) in the leftward direction away from the second reference plane P 2 . Likewise, the coils  62  and  63  are arranged so that the inclination angles of the long axis of the coil  62  and the long axis of the coil  63  with respect to the first reference plane P 1  become approximately ±45 degrees, respectively, and the directions of inclination of the long axes of the coils  62  and  63  are set to approach the first reference plane P 1  (so as to reduce the distance between the long axes of the coils  62  and  63 ) in the leftward direction away from the second reference plane P 2 . In other words, the point of intersection between two straight lines respectively extending along the magnetic pole boundary lines Q 1  and Q 2  and the point of intersection between two straight lines respectively extending along the long axes of the coils  62  and  63  are positioned in the left-side section of the second reference plane P 2 , which is on the opposite side of the second reference plane P 2  to that in which the second optical axis O 2  extends. 
     The following effects are obtained by the above described arrangement of the permanent magnets  60  and  61  and the coils  62  and  63 , which constitute elements of the anti-shake system for driving the first lens element L 1 . The arrangement of the electromagnetic actuator is not easily subjected to space restrictions because the second quadrant V 2  and the third quadrant V 3  are sections on the opposite side of the second reference plane P 2  from the side toward the light-ray travelling direction along the second optical axis O 2  and because none of the optical elements of the imaging optical system which are positioned optically rearward from the first prism L 11  (rightward with respect to  FIG. 3 ) are arranged in either the second quadrant V 2  or the third quadrant V 3 . For instance, with the above described (illustrated) arrangement, it is possible to drive the first lens element L 1  even if a combination of the permanent magnet  60  and the coil  62  and a combination of the permanent magnet  61  and the coil  63  are arranged in the first quadrant V 1  and the fourth quadrant V 4 , respectively, to be symmetrical with respect to the second reference plane P 2 . However, the second lens element L 2  is positioned in the first quadrant V 1  and the fourth quadrant V 4  at a position adjacent to the exit surface L 11 - b  of the first prism L 11 , so that in this case there is a problem of it being difficult to secure space for installing the entire electromagnetic actuator without interfering with the second lens element L 2 . Whereas, there is no such a restriction in the arrangement of the illustrated embodiment in which a combination of the permanent magnet  60  and the coil  62  provided in the second quadrant V 2  and a combination of the permanent magnet  61  and the coil  63  provided in the third quadrant V 3 . 
     In general, to drive an object using voice coil motors, each of which includes a permanent magnet and a coil, two sets of permanent magnets and coils which have mutually different driving-force directions are used. The present embodiment of the imaging apparatus is provided with a combination of the permanent magnet  60  and the coil  62 , the lengthwise (long-side) directions (the magnetic boundary line Q 1  and the long axis of the coil  62 ) of which are parallel to each other, and a combination of the permanent magnet  61  and the coil  63 , the lengthwise (long-side) directions (the magnetic boundary line Q 2  and the long axis of the coil  63 ) of which are parallel to each other, and the direction of action F 1  of the driving force generated by the former combination (the permanent magnet  60  and the coil  62 ) and the direction of action F 2  of the driving force generated by the latter combination (the permanent magnet  61  and the coil  63 ) are orthogonal to each other. This arrangement makes it possible to move the first lens element L 1  freely in a plane orthogonal to the first optical axis O 1 . In addition, the directions of inclination of the former combination (the permanent magnet  60  and the coil  62 ) and the latter combination (the permanent magnet  61  and the coil  63 ) are set so that the distance between the magnetic pole boundary lines Q 1  and Q 2  and the distance between the long axes of the coils  62  and  63  increase in the rightward direction (in which the second optical axis O 2  extends) and so that the distance between the magnetic pole boundary lines Q 1  and Q 2  and the distance between the long axes of the coils  62  and  63  decrease in the opposite direction, i.e., the leftward direction. This arrangement makes it possible to accommodate the permanent magnet  60  and the coil  62  and the permanent magnet  61  and the coil  63  in the second quadrant V 2  and the third quadrant V 3  in the area that is peripheral to the lens holding portion  50  of the first lens frame  30 , which is cylindrical in shape. 
     Just for the purpose of driving the first lens element L 1 , it is possible to make the directions of inclination of the permanent magnet  60  and the coil  62  and the permanent magnet  61  and the coil  63  with respect to the first reference plane P 1  different from those in the above described embodiment. For instance, it is possible to drive the first lens element L 1  in a plane orthogonal to the first optical axis O 1  even if the magnetic pole boundary line Q 1  of the permanent magnet  60  and the long axis of the coil  62  are parallel to one of the reference planes P 1  and P 2  and the magnetic pole boundary line Q 2  of the permanent magnet  61  and the long axis of the coil  63  are parallel to the other reference plane P 1  or P 2 . However, this arrangement causes at least one of a combination of the permanent magnet  60  and the coil  62  and a combination of the permanent magnet  61  and the coil  63  to enter the first quadrant V 1  or the fourth quadrant V 4  by a large amount, which deteriorates the aforementioned effect of using the second quadrant V 2  and the third quadrant V 3  that are subjected to less space restrictions. In addition, there is also a demerit of increasing in size of the anti-shake system in a direction along the first reference plane P 1  because either a combination of the permanent magnet  60  and the coil  62  or a combination of the permanent magnet  61  and the coil  63  is positioned on the left-hand side of the movement limit projection  43  and the movement limit hole  53  with respect to  FIG. 5 . 
     In contrast, the anti-shake system can be installed in the second quadrant V 2  and the third quadrant V 3  in a space-efficient manner by setting the directions of inclination of the permanent magnets  60  and  61  and the coils  62  and  63  in a front view as shown in  FIG. 5  as described in the above illustrated embodiment. Although the inclination angles of the magnetic pole boundary lines Q 1  and Q 2  of the permanent magnets  60  and  61  with respect to the first reference plane P 1  are set to approximately ±45 degrees, respectively, and the inclination angles of the long axes of the coils  62  and  63  with respect to the first reference plane  91  are set to approximately ±45 degrees, respectively, in the above illustrated embodiment, the aforementioned effects of the space-saving design can also be obtained even if the inclination angles of the magnetic pole boundary lines Q 1  and Q 2  with respect to the first reference plane P 1  and the inclination angles of the long axes of the coils  62  and  63  with respect to the first reference plane P 1  are slightly changed. Specifically, if the inclination angles of the magnetic pole boundary lines Q 1  and Q 2  of the permanent magnets  60  and  61  with respect to the first reference plane P 1  and the inclination angles of the long axes of the coils  62  and  63  with respect to the first reference plane P 1  are each set to within ±35 to ±55 degrees, with the angle of the magnetic pole boundary lines Q 1  and Q 2  and the angle of the long axes of the coils  62  and  63  each maintained at 90 degrees, a space-saving arrangement of the anti-shake system is achieved. 
     Additionally, the second lens group G 2  and the third lens group G 3  that are movable along the second optical axis O 2  are provided on an optical path extending from the first prism L 11 ; the first motor M 1  and the second motor M 2  that respectively constitute drive systems of the second lens group G 2  and the third lens group G 3  contain metal parts; and the pair of rods  22  and  23  are also metal parts. If these metal parts are made of a magnetic material and are positioned near the electromagnetic actuator, there is a possibility of such metal parts exerting an adverse influence on the anti-shake driving operation of the electromagnetic actuator. In the moving-magnet electromagnetic actuator of the present embodiment of the anti-shake system in particular, in which the permanent magnets  60  and  61  are supported on the moveable first lens frame  30 , in order to make the electromagnetic actuator perform drive control with high precision, it is required to remove the adverse influence caused by external magnetic materials on the magnetic fields of the permanent magnets  60  and  61 . The permanent magnets  60  and  61  and the coils  62  and  63  that are arranged in the second quadrant V 2  and the third quadrant V 3  are farther from each motor M 1  and M 2  and each rod  22  and  23  than in the case where the permanent magnets  60  and  61  and the coils  62  and  63  were to be arranged in the first quadrant V 1  and the fourth quadrant V 4 ; therefore, any adverse influence of these parts of the electromagnetic actuator would not easily reach the driving of the electromagnetic actuator even when these parts contain magnetic metal. 
     As described above, the anti-shake system that is superior in space utilization and driving performance is obtained by the installation of the permanent magnets  60  and  61  and the coils  62  and  63  in the sections (the second quadrant V 2  and the third quadrant V 3 ) on the opposite side of the second reference plane P 2  from the side toward the light-ray travelling direction along the second optical axis O 2  and the arrangement of the permanent magnets  60  and  61  and the coils  62  and  63  in which the distance between the magnetic pole boundary lines Q 1  and Q 2  and the distance between the long axes of the coils  62  and  63  decrease in the direction opposite to the direction of extension of the second optical axis O 2  in the arrangement of the anti-shake system for driving the first lens element L 1 . 
     Although the permanent magnets  60  and  61  and the coils  62  and  63  are entirely arranged in the second quadrant V 2  and the third quadrant V 3  in the above illustrated embodiment of the imaging apparatus, the permanent magnets  60  and  61  and the coils  62  and  63  can be alternatively arranged so as to partly project into the first and fourth quadrants V 1  and V 4  beyond the second reference plane P 2 . In this case, as a condition for obtaining the aforementioned effects for space utilization and drive performance of the anti-shake system, it is desirable that at least the centers U 1  and U 2  of the permanent magnets  60  and  61  and the centers U 1  and U 2  of the coils  62  and  63  be positioned on the left-hand side of the second reference plane P 2 , i.e., within the second quadrant V 2  and the third quadrant V 3 , respectively. 
     Additionally, the slimming-down of the anti-shake system in the forward/rearward direction (depthwise direction) of the imaging unit  10  has been achieved. The permanent magnets  60  and  61 , which constitute elements of drive sources of the anti-shake system, are fixed onto the flange  55  of the first lens frame  30 . The flange  55  projects sideways from the lens holding portion  50 , which is cylindrical in shape and holds the first lens element L 1 , and is positioned rearward (downward with respect to  FIG. 7 ) from the position at which the first lens element L 1  is supported by the lens holding portion  50  in a direction along the first optical axis O 1  (i.e., in the forward/rearward direction; see  FIG. 7 ). Consequently, in the forward/rearward direction, the positions of the permanent magnets  60  and  61 , which are respectively supported by the flange  55  thereon, are set behind the first lens element L 1  in the vicinity of the incident surface L 11 - a  of the first prism L 11 . 
     The coils  62  and  63 , which constitute, together with the permanent magnets  60  and  61 , drive sources of the anti-shake system, and the magnetic sensors  65  and  66 , which detect the position of the first lens element L 1 , are held at positions overlapping the first lens element L 1  in the forward/rearward direction in a state where the cover member  32  is mounted to the base member  31  (see  FIG. 2 ). 
     As described above, in a space lateral to the first lens element L 1  and the first prism L 11 , the permanent magnets  60  and  61  and the coils  62  and  63  are arranged so that the permanent magnet  60  and the coil  62  superpose each other in the forward/rearward direction and so that the permanent magnet  61  and the coil  63  superpose each other in the forward/rearward direction, and accordingly, the electromagnetic actuator can be installed in a space-efficient manner in the forward/rearward direction, which contributes to slimming of the imaging unit  10 . 
     As shown in  FIGS. 5, 8, 9 and 10 , the arrangement of the magnetic sensors  65  and  66  on the outside of the coils  62  and  63  is also superior in space efficiency. The permanent magnets  60  and  61  and the coils  62  and  63  are arranged so that the directions of inclination of a combination of the permanent magnet  60  and the coil  62  and a combination of the permanent magnet  61  and the coil  63  approach the first reference plane P 1  in a direction away from the second reference plane P 2 , so that a substantially triangular space is obtained between each of the two left corners of the first lens group unit  12  (the upper left corner and the lower left corner of the base plate  35  of the base member  31  with respect to  FIGS. 5 and 11 ), which is substantially rectangular in shape in a front orthographic projection, and the installation area of the electromagnetic actuator. In other words, two substantially triangular spaces are obtained on the front of the base plate  35  in the vicinity of the upper left corner and the lower left corner of the base plate  35  as viewed from front, respectively. The magnetic sensors  65  and  66  are installed while utilizing these two triangular spaces. Specifically, since the permanent magnet  60  is greater in width than the coil  62  in the direction of action F 1  of the driving force generated by energizing the coil  62 , and the permanent magnet  61  is greater in width than the coil  63  in the direction of action F 2  of the driving force generated by energizing the coil  63  and since the magnetic sensor  65  is positioned adjacent to the outer linear portion of the coil  62  while the magnetic sensor  66  is positioned adjacent to the outer linear portion of the coil  66 , both the amount of projection of the magnetic sensor  65  from the permanent magnet  60  in a direction away from the first optical axis O 1  and the amount of projection of the magnetic sensor  66  from the permanent magnet  61  in a direction away from the first optical axis O 1  have been minimized. Accordingly, an increase in size of the front orthographic projection shape of the imaging unit  10  which may be caused by installation of the magnetic sensors  65  and  66  can be prevented from occurring. 
     Although the centers U 3  and U 4  of the magnetic sensors  65  and  66  are spaced from the centers U 1  and U 2  of the permanent magnets  60  and  61 , respectively, as shown in  FIGS. 5 and 10 , since each magnetic sensor  65  and  66  is positioned close to the associated permanent magnet  60  or  61  to a degree to be partly included in the front orthographic projection area of the associated permanent magnet  60  or  61  as shown in FIGS.  8  and  9  as the overlapping range K 2 , the magnetic sensors  65  and  66  can achieve a sufficient detection accuracy. 
     The position of the first lens frame  30  can be detected even if the magnetic sensors  65  and  66  are positioned behind the magnetic sensors  60  and  61  in a direction along the first optical axis O 1 , unlike the above described embodiment. However, in this case, it is required to provide space for the installation of the sensors behind the permanent magnets  60  and  61 . In contrast, according to the arrangement of the magnetic sensors  65  and  66  in the above illustrated embodiment of the imaging apparatus, no space for the installation of the sensors is required behind the permanent magnets  60  and  61 , and an effect of reducing the thickness of the imaging unit  10  in the forward/rearward direction can also be obtained. Specifically, further slimming of the imaging unit  10  is achieved because the magnetic sensor  65  and the coil  62  overlap each other and the magnetic sensor  66  and the coil  63  overlap each other in a direction along the first optical axis O 1  as shown in  FIGS. 8 and 9  as the overlapping range K 1 . 
     Features of the arrangement of the electromagnetic actuator in the first lens group unit  12  has been described above, and the support structure for the first lens frame  30  on the base member  31  will be discussed hereinafter. 
     First, the first lens frame  30  is supported to be movable along a plane orthogonal to the first optical axis O 1  and is prevented from coming off the base member  31  in a direction along the first optical axis O 1  by the above described structure in which each guide shaft  41 A,  41 B or and  41 C is held by the associated slidable support portion  51 A,  51 B or  51 C (between the pair of projections  52  thereof), which has two walls opposed in a direction along the first optical axis O 1  and is U-shaped in cross section. Therefore, a biaser such as a spring(s) is not required to be provided for holding the first lens frame  30  on the base member  31 , so that simplification of the support structure for the first lens frame  30  on the base member  31  has been achieved. Additionally, the support structure for the first lens frame  30  on the base member  31  is superior in assembling workability because the state of supporting the first lens frame  30  is completed by inserting each guide shaft  41 A,  41 B and  41 C into the elongated open groove T 1  of the associated guide support portion  40 A,  40 B or  40 C and the elongated open groove T 2  of the associated slidable support member  51 A,  51 B or  51 C from one side (the upper side, the lower side or the left side) of the first lens group unit  12 . 
     As described above, the optical elements of the imaging optical system which are positioned optically rearward from the first prism L 11  are arranged on the second optical axis O 2  that is deflected by the first prism L 11  (i.e., arranged in the first quadrant V 1  and the fourth quadrant V 4 ), so that space is limited on the second optical axis O 2  side. For instance, if a supporter for the first lens frame  30  is installed at a position along the exit surface L 11 - b  (the exit long-side of the incident surface L 11 - a ) of the first prism L 11 , there is a possibility of this supporter interfering with the second lens element L 2  or the lens holding portion  39 . Whereas, in the first lens group unit  12  of the above described embodiment of the imaging apparatus, the guide shafts  41 A,  41 B and  41 C and the guide support portions  40 A,  40 B and  40 C, which support the first lens frame  30  in a manner to allow the first lens frame  30  to move relative to the base member  31 , are arranged in a U-shaped area along the three sides of the incident surface L 11 - a  except the exit long-side thereof as viewed along the first optical axis O 1  from the front, as shown in  FIGS. 10 and 11 , thus not interfering with either the second lens element L 2  or the lens holding portion  39 . 
     Out of the guide shafts  41 A,  41 B and  41 C and the guide support portions  40 A,  40 B and  40 C, the two guide shafts  41 A and  41 B and the guide support portions  40 A and  40 B are arranged on either side of the first reference plane P 1  along the pair of side surfaces L 11 - d  (along the short sides of the incident surface L 11 - a ) of the first prism L 11  as viewed from the front, as shown in  FIGS. 5 and 10 . As shown in  FIGS. 10 and 11 , the axes of the guide shafts  41 A and  41 B are substantially parallel to the reference plate P 1  (the second optical axis O 2 ), and the length of each guide shaft  41 A and  41 B in the axial direction thereof substantially fits within the installation range of the first prism L 11 . Accordingly, the installation of the guide shafts  41 A and  41 B does not cause an increase in size of the first lens group unit  12  in the leftward/rightward direction (lengthwise direction) of the imaging unit  10 . Additionally, the amount of projection of the guide shafts  41 A and  41 B and the slidable support portions  51 A and  51 B from both sides of the lens holding portion  50  of the first lens frame  30  in the upward/downward direction (widthwise direction) of the imaging unit  10  (in directions away from the first reference plane P 1 ) have been reduced to the same degree as the permanent magnets  60  and  61  and the coils  62  and  63  that constitute an electromagnetic actuator as shown in  FIGS. 5 and 10 , which achieves a reduction in size of the imaging unit  10  in the upward/downward (widthwise) direction thereof. Due chiefly to the arrangement of the guide shafts  41 A and  41 B, in which the axes thereof extend substantially parallel to the first reference plane P 1  in a plane orthogonal to the first optical axis O 1 , the amount of projection of the guide shafts  41 A and  41 B in the upward/downward direction (widthwise direction) of the imaging unit  10  is to a minimum compared to the case where the guide shafts  41 A and  41 B are arranged so that the axes thereof extend in a direction intersecting the first reference plane P 1  in a plane orthogonal to the first optical axis O 1 . 
     In addition, support portions for the first lens frame  30  by the guide shafts  41 A and  41 B are arranged to be spaced from each other in the direction of the long sides of the incident surface L 11 - a  of the first prism L 11  to be positioned at substantially equal distances from the first reference plane P 1 ; this arrangement makes it possible to support the first lens frame  30  in a balanced manner in the upward/downward direction. Additionally, the support structure for the first lens frame  30  is also superior in supporting balance in the leftward/rightward direction (in the direction of the short sides of the incident surface L 11 - a  of the first prism L 11 ) because the pair of projections  52  of the slidable support portion  51 A that holds the guide shaft  41 A therebetween and the pair of projections  52  of the slidable support portion  51 B that holds the guide shaft  41 B therebetween lie in the second reference plane P 2 . 
     To support the first lens frame  30  on the base member  31  with stability, it is desirable to support the first lens frame  30  at three or more points of support. Accordingly, in addition to the two support points by the guide shafts  41 A and  41 B and the slidable support portions  51 A and  51 B that are arranged on both sides of the first reference plane P 1 , the first lens group unit  12  is provided with a combination of the guide shaft  41 C and the slidable support portion  51 C as a third support portion. As shown in  FIGS. 5 and 10 , the guide shaft  41 C and the slidable support portion  51 C are arranged in the vicinity of the left end of the first lens group unit  12  to substantially fit into the space between the permanent magnets  60  and  61  and the coils  62  and  63  in the upward/downward direction. Accordingly, the installation of the guide shaft  41 C does not cause an increase in size of the first lens group unit  12  in the upward/downward direction (widthwise direction) of the imaging unit  10 . In addition, due to the arrangement of the guide shaft  41 C, in which the axis thereof extends substantially parallel to the second reference plane P 2  in a plane orthogonal to the first optical axis O 1 , the amount of projection of the guide shaft  41 C in the leftward direction of the imaging unit  10  is to a minimum compared with the case where the guide shaft  41 C is arranged so that the axis thereof extends in a direction intersecting the second reference plane P 2  in a plane orthogonal to the first optical axis O 1 . 
     As can be seen from above, the above illustrated support structure for the first lens frame  30 , in which the first lens frame  30  is supported to be movable by the guide shafts  41 A,  41 B and  41 C that are arranged in an area (U-shaped area) along the three sides of the incident surface L 11 - a  of the first prism L 11  except the exit long-side thereof, contributes to miniaturization of the imaging unit  10 . 
     As a different embodiment of the imaging apparatus from the above illustrated embodiment of the imaging apparatus, instead of providing the first lens group unit  12  with the guide shaft  41 C, it is possible to provide a third support portion by making at least one of the guide shaft  41 A and the guide shaft  41 B extend in the axial direction thereof and making part of the first lens frame  30  engaged with this extended portion of the guide shaft  41 A and the guide shaft  41 B. However, if the guide shafts  41 A and  41 B are extended in the second quadrant V 2  and the third quadrant V 3 , respectively, the extended portions of the guide shafts  41 A and  41 B would interfere with the electromagnetic actuator because the permanent magnets  60  and  61 , the coils  62  and  63  and the magnetic sensors  65  and  66 , which constitute an electromagnetic actuator, are arranged in the second and third quadrants V 2  and V 3  around the first lens element L 1 , as can be seen from  FIGS. 5 and 10 . Additionally, the exit-side flange  37  that is in contact with the body module  11  is provided in the first and fourth quadrants V 1  and V 4 , so that there is almost no space for allowing the guide shafts  41 A and  41 B to extend on this side. Accordingly, the structure in which the guide shaft  41 C (third support portion), the axis of which extends in a direction parallel to the second reference plane P 2 , is installed in a space (the second quadrant V 2  and the third quadrant V 3 ) on the opposite side of the second reference plane P 2  from the side on which the second optical axis O 2  extends is superior in space efficiency. In other words, due to the arrangement in which the permanent magnets  60  and  61 , the coils  62  and  63  and the magnetic sensors  65  and  66 , which constitute an electromagnetic actuator, and the first lens element L 1 , etc., are arranged in a U-shaped area defined by the outer profiles of the guide shafts  41 A,  41 B and  41 C when the first lens group unit  12  is viewed from front as shown in  FIG. 5 , an optimum space efficiency has been achieved. 
     The movement limit projection  43  and the movement limit hole  53 , and the swing pivot  44  and the pivot support groove  54  also constitute elements of the support structure for the first lens frame  30 . As shown in  FIGS. 5, 10 and 11 , the movement limit projection  43  and the movement limit hole  53  are arranged with space efficiency at a position surrounded by the lens holding portion  50 , the electromagnetic actuator (the permanent magnets  60  and  61  and the coils  62  and  63 ) and the slidable support portion  51 C. 
     The swing pivot  44  and the pivot support groove  54  are provided in the fourth quadrant V 4 , thus not interfering with the electromagnetic actuator and other elements that are provided in the second quadrant V 2  and the third quadrant V 3 . In addition, the swing pivot  44  and the pivot support groove  54  are arranged outside the area near the boundary between the circular portion and the D-cut portion of the first lens element L 1  and nestled in the fourth quadrant V 4  with space efficiency without interfering with the second lens element L 2 . 
     Although the present invention has been described based on the above illustrated embodiment, the present invention is not limited solely thereto; various modifications to the above illustrated embodiment are possible. For instance, although the magnetic sensors  65  and  66  of the above illustrated embodiment of the imaging apparatus are Hall sensors that use Hall elements, it is possible that MR sensors that use MR (Magneto Resistive) elements be used as the magnetic sensors  65  and  66 . 
     The above illustrated embodiment of the imaging unit  10  has a structure such that the first lens frame  30  is supported by the base member  31  to be movable relative to the base member  31  via the guide shafts  41 A,  41 B and  41 C. Although this support structure contributes to miniaturization of the first lens group unit  12  as mentioned above, the arrangement configuration of the electromagnetic actuator and the magnetic sensors according to the present invention is also effective in the modified structure in which the guide shafts  41 A,  41 B and  41 C are replaced by spherical guide members (e.g., balls) which are installed between the first lens frame  30  and the base member  31  or replaced by any other guide device. 
     In addition, the present invention can also be applied to a type of imaging apparatus which has an L-shaped optical path without including a prism corresponding to the second prism L 12  in an imaging optical system. Alternatively, an imaging apparatus which contains a bending optical system including one or more reflectors in addition to the first prism L 11  and the second prism L 12  can also be an imaging apparatus to which the present invention is applicable. Although the imaging optical system provided in the above illustrated embodiment of the imaging apparatus uses prisms as reflectors for bending an optical path, each prism can be replaced by a different type of reflector such as a mirror. The bending angle (reflecting angle) of an optical axis by each reflector can be any angle other than 90 degrees. 
     Although the operating direction of the first lens frame  30  relative to the base member  31  is defined using the swing pivot  44  and the pivot support groove  54  in the above illustrated embodiment of the imaging apparatus, it is possible to define the operating direction by providing a different actuator instead of the swing pivot  44  and the pivot support groove  54 . 
     Additionally, the above illustrated embodiment of the electromagnetic actuator is a moving-magnet electromagnetic actuator in which the permanent magnets  60  and  61  are supported by the movable first lens frame  30  and the coils  62  and  63  are supported by the immovable cover member  32 . This type of electromagnetic actuator is superior in wiring routing for coils and magnetic sensors; however, the present invention is also applicable to a moving-coil electromagnetic actuator in which the coils  62  and  63  are supported by the first lens frame  30  that is movable and the permanent magnets  60  and  61  are supported by the base member  31  or the cover member  32  that is immovable. In such a case, it is advisable that the magnetic sensors  65  and  66  be also provided on the first lens frame  30 . 
     Although each of the permanent magnets  60  and  61  has a rectangular shape in a front view that is elongated in a direction along the associated magnetic boundary line Q 1  or Q 2  and each of the coils  62  and  63  has an elongated shape in a front view that is elongated in a direction along the associated magnetic boundary line Q 1  or Q 2  in the above illustrated embodiment of the anti-shake system, the present invention can also be applied to an anti-shake system having permanent magnets and coils which are different in shape from the permanent magnets  60  and  61  and the coils  62  and  63 . Specifically, the permanent magnets of the anti-shake system can be replaced by, e.g., permanent magnets that each has a square shape in a front view. 
     Although the center U 1  of the permanent magnet  60  and the center U 1  of the coil  62  substantially coincide with each other in a plane orthogonal to the first optical axis O 1  and the center U 2  of the permanent magnet  61  and the center U 2  of the coil  63  substantially coincide with each other in a plane orthogonal to the first optical axis O 1  when the first lens frame  30  is positioned at the center of the moving range thereof in the above illustrated embodiment of the anti-shake system, the present invention can also be applied to an imaging apparatus equipped with an anti-shake system in which the center of each permanent magnet and the center of the associated coil do not coincide (are not aligned) with each other in an initial state of the anti-shake system. 
     Although the second lens group G 2 , the third lens group G 3  and the fourth lens group G 4  are provided on the second optical axis O 2  in the above illustrated embodiment of the imaging apparatus, the present invention can also be applied to an imaging optical system in which less than or more than three lens groups are provided on an optical axis of the imaging optical system which corresponds to the second optical axis O 2 . 
     Additionally, in the first lens group G 1 , it is possible to change the number of lens elements arranged in front of the incident surface L 11 - a  of the first prism L 11  on the first optical axis O 1  and the number of lens elements arranged on the right-hand side of the exit surface L 11 - b  of the first prism L 11  on the second optical axis O 2 . For instance, the first lens element L 1  in the above illustrated embodiment can be replaced by two or more front lens elements which are arranged in front of the first prism L 11 . In such a case, the distances between the front lens elements arranged in front of the first prism L 11  are small, and accordingly, to prevent the aberrations from deteriorating, it is advisable to perform anti-shake control by moving all the plurality of front lens elements, arranged in front of the first prism L 11 , in directions orthogonal to the first optical axis O 1 . Additionally, although the second lens element L 2  is arranged on the right-hand side of the first prism L 11  in the above illustrated embodiment, the number of lens elements in the first lens group G 1  which are arranged on the optical path extending from the exit surface L 11 - b  of the first prism L 11  toward the second lens group G 2  can be more than one. Additionally, it is possible to modify the first lens group G 1  so as not to include any lens element on the optical path extending from the exit surface L 11 - b  of the first prism L 11  toward the second lens group G 2 . 
     The length of the optical path from the incident surface of the first lens element L 1  to the image plane in the imaging unit  10  is constant at all times in the above described embodiment. In this type of imaging optical system, the first lens element L 1  that is the closest to the object side is generally a negative lens. However, the lens element (front lens element) for use in anti-shake control in the imaging apparatus according to the present invention can be a positive lens. Regardless of whether the power of the front lens element is negative or positive, any lens element can be adopted as the front lens element as long as it has a refractive power. 
     Additionally, although the imaging optical system of the above illustrated embodiment of the imaging unit  10  is a zoom lens (variable power optical system) which performs a zooming operation (power varying operation) by moving the second lens group G 2  and the third lens group G 3  along the second optical axis O 2 , the present invention is also applicable to an imaging apparatus which incorporates an imaging optical system having no power varying capability. For instance, it is possible to modify the imaging unit  10  such that the second lens group G 2  and the third lens group G 3  do not move for a zooming operation and that the second lens group G 2  or the third lens group G 3  moves solely for a focusing operation. 
     Although the incident surface L 11 - a  of the first prism L 11  in the above illustrated embodiment of the imaging apparatus is in the shape of a laterally elongated rectangle, the present invention can also be applied to a type of imaging apparatus having a first prism (which corresponds to the first prism L 11 ), the incident surface thereof having a different shape such as a square or a trapezoid. 
     Obvious changes may be made in the specific embodiment of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.