Patent Publication Number: US-2011050982-A1

Title: Lens barrel and imaging device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2009-197819 filed on Aug. 28, 2009, and Japanese Patent Application No. 2009-197820 filed on Aug. 28, 2009. The entire disclosures of Japanese Patent Applications No. 2009-197819 and No. 2009-197820 are hereby incorporated herein by reference. 
     BACKGROUND  
     1. Technical Field 
     The technology disclosed herein relates to a lens barrel and an imaging device having a focusing lens and a driver for driving the focusing lens. 
     2. Background Information 
     The utility of a camera suffers when the lens takes a long time to focus. In view of this, the camera disclosed in Japanese Laid-Open Patent Application 2006-189506 to Ishige et al. was developed to increase the lens focusing time. According to the Ishige et al. patent application, the orientation of the camera is detected by a sensor, and if the camera is in a horizontal state, the lens is capable of being instantly moved along the optical axis for a faster focusing time. But if the camera is not in a horizontal state, movement of the lens along the optical axis is slower, and thus produces a much slower focusing time. 
     But regardless of whether the camera is in a horizontal state, the lens is still subject to a load that interferes with the lens as it moves along the optical axis. So even if the camera is positioned horizontally, the focusing time is still much slower than desired. 
     In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved lens barrel and imaging device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
     SUMMARY  
     The lens barrel disclosed herein comprises a focusing lens, a driver and a controller. The focusing lens changes a state of focus by moving in the direction of an optical axis. The focusing lens is subject to a load which is dependent upon the position of the focusing lens along the optical axis. The driver is coupled to the focusing lens and produces a driving force to move the focusing lens along the optical axis at a predetermined speed. The controller is coupled to the driver to adjust the driving speed of the driver relative to the position of the focusing lens. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
       Referring now to the attached drawings, which form a part of this original disclosure: 
         FIG. 1  is a simplified diagram of the configuration of a digital camera; 
         FIG. 2  is a block diagram of the configuration of a camera body; 
         FIG. 3  is an oblique view of a digital camera; 
         FIG. 4A  is a top view of a camera body, and 
         FIG. 4B  is a rear view of a camera body; 
         FIG. 5  is an oblique view of an interchangeable lens unit; 
         FIG. 6  is a cross section of an interchangeable lens unit; 
         FIG. 7  is a diagram of the configuration of an optical system; 
         FIG. 8  is an exploded oblique view of an aperture unit and its surrounding parts; 
         FIG. 9  is an exploded oblique view of a cam barrel and its surrounding parts; 
         FIG. 10  is another exploded oblique view of a cam barrel and its surrounding parts; 
         FIG. 11  is an exploded oblique view of a biasing member and its surrounding parts; 
         FIG. 12  is a diagram illustrating contrast autofocus operation; 
         FIG. 13  is a graph of the load torque produced by a biasing member and the maximum speed of a focus motor; 
         FIG. 14  is a flowchart of the processing pertaining to a variable set speed method; 
         FIG. 15  is an example of a speed switching table; 
         FIG. 16  is a graph of the relation of the set speed of the focus motor, the maximum speed of the focus motor, the load torque, the pressure angle of the cam groove, and the shape of the cam groove with respect to the position of a focus movable unit in a second embodiment; 
         FIG. 17  is a graph of the relation of the set speed of the focus motor, the maximum speed of the focus motor, the load torque, the pressure angle of the cam groove, and the shape of the cam groove with respect to the position of a focus movable unit in a third embodiment; and 
         FIG. 18  is a graph of the relation of the set speed of the focus motor, the maximum speed of the focus motor, the load torque, the pressure angle of the cam groove, and the shape of the cam groove with respect to the position of a focus movable unit in a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
     Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     First Embodiment  
     Overview of Digital Camera 
     A digital camera  1  will be described through reference to  FIGS. 1 to 11 .  FIG. 1  is a simplified diagram of the configuration of the digital camera  1 . As shown in  FIG. 1 , the digital camera  1  (one example of an imaging device) is a digital camera with an interchangeable lens, and mainly includes a camera body  3  and an interchangeable lens unit  2  (one example of a lens barrel) that is removably mounted to the camera body  3 . The interchangeable lens unit  2  is mounted via a lens mount  95  to a body mount  4  provided to the front face of the camera body  3 . 
       FIG. 2  is a block diagram of the configuration of the camera body  3 .  FIG. 3  is an oblique view of the digital camera  1 .  FIG. 4A  is a top view of the camera body  3 , and  FIG. 4B  is a rear view of the camera body  3 .  FIG. 5  is an oblique view of the interchangeable lens unit  2 .  FIG. 6  is a cross section of the interchangeable lens unit  2 .  FIG. 7  is a diagram of the configuration of an optical system L.  FIG. 8  is an exploded oblique view of an aperture unit  62  and its surrounding parts.  FIG. 9  is an exploded oblique view of a cam barrel  51  and its surrounding parts.  FIG. 10  is another exploded oblique view of the cam barrel  51  and its surrounding parts.  FIG. 11  is an exploded oblique view of a spring  98  (one example of a biasing member) and its surrounding parts. 
     In this embodiment, a three-dimensionally perpendicular coordinate system is set with respect to the digital camera  1 . The optical axis AZ direction of the optical system L (discussed below) coincides with the Z axis direction. The X axis direction coincides with the horizontal direction when the digital camera  1  is in its landscape orientation position. The Y axis direction coincides with the vertical direction when the digital camera  1  is in its landscape orientation position. In the following description, “front” means the subject side of the digital camera  1  (the Z axis direction positive side), and “rear” means the opposite side from the subject side of the digital camera  1  (the user side or the Z axis direction negative side). 
     Interchangeable Lens Unit 
     The configuration of the interchangeable lens unit  2  will be described through reference to  FIGS. 1 to 11 . As shown in  FIG. 1 , the interchangeable lens unit  2  has the optical system L, a lens support mechanism  71  that supports the optical system L, a focus adjusting unit  72 , an aperture adjusting unit  73 , and a lens microprocessor  40 . Each of these will be described in detail below. 
     (1) Optical System 
     The optical system L is a lens system for forming an optical image of a subject. More specifically, as shown in  FIG. 7 , the optical system L has seven lenses. The first lens L 1  is a meniscus lens having its convex side facing the subject side. The second lens L 2  is a meniscus lens having its convex side facing the subject. The opposing side of the second lens L 2 , which faces the imaging sensor  11 , is aspherical. The third lens L 3  is a biconcave lens. The fourth lens L 4  is a biconvex lens and is bonded to the third lens L 3  via an adhesive layer. The fifth lens L 5  is a biconvex lens. The sixth lens L 6  is a biconcave lens and is bonded to the fifth lens L 5  via an adhesive layer. The seventh lens L 7  is a biconvex lens, and the faces of the seventh lens L 7  on the subject side and the imaging sensor  11  side are both aspherical. 
     The aperture unit  62  is located between the second lens L 2  and the third lens L 3 . 
     The optical system L is not a so-called zoom lens, but rather a fixed focal lens. That is, the optical system L has a fixed focal distance. 
     During focusing from an infinity focal state to a close-up focal state, the optical system L and the aperture unit  62  maintain a constant distance between each other and move integrally towards the subject side. Conversely, during focusing from a close-up focal state to an infinity focal state, the optical system L and the aperture unit  62  maintain a constant distance between each other and move integrally towards the user side. That is, in this embodiment, the optical system L as a whole is a focusing lens. A focusing lens is a lens that moves in the optical axis direction in order to change and/or adjust the focal state of an optical image of a subject. The unit is composed of the optical system L and the aperture unit  62  that moves integrally during focusing is a movable focusing unit  94  (one example of a focusing lens). 
     (2) Lens Support Mechanism 
     The lens support mechanism  71  is for supporting the movable focusing unit  94  so that it is movable in the Z direction, and as shown in  FIG. 6 , it has the lens mount  95 , a fixed frame  50 , a cam barrel  51 , a thrust ring  52 , a first lens group support frame  53 , a second lens group support frame  54 , a focus ring unit  88 , and the biasing member  98 . Each of these will be described in detail below. 
     The lens mount  95  is mounted to the body mount  4  of the camera body  3  and has a lens-side contact  91 . A light blocking frame  96  that blocks out unwanted light is attached to the lens mount  95  ( FIGS. 6 and 11 ). 
     The fixed frame  50  is a member that rotatably supports the cam barrel  51  and is fixed to the lens mount  95 . The fixed frame  50  has a substantially cylindrical shape whose center axis is the optical axis AZ. Formed on the interior of the fixed frame  50  are three linear through-grooves  50   c  disposed at an equal pitch (in the circumferential direction) around the optical axis AZ. The linear through-grooves  50   c  each have a shape that extends in the Z axis direction. Also formed on the interior of the fixed frame  50  is a linear auxiliary through-groove  50   d  at a phase position that is in between two of the linear through-grooves  50   c,  that is, at a position that is in between two of the linear through-grooves  50   c  around the optical axis AZ (in the circumferential direction) ( FIG. 9 ). Two of these linear auxiliary through-grooves  50   d  are formed in the fixed frame  50 . The linear auxiliary through-grooves  50   d  each extend in the Z axis direction. A groove  50   f  into which the thrust ring  52  is inserted and fixed is formed in the fixed frame  50  ( FIG. 10 ). The width of the groove  50   f  is slightly greater than the thickness of the thrust ring  52 . 
     The cam barrel  51  has a substantially cylindrical shape whose center axis coincides with the optical axis AZ. Formed on the interior of the cam barrel  51  are three cam grooves  51   d  disposed at an equal pitch (in the circumferential direction) around the optical axis AZ. The cam grooves  51   d  each extend in both the circumferential direction and the Z axis direction. Also formed on the interior of the cam barrel  51  is an auxiliary cam groove  51   e  at a phase position that is in between two of the cam grooves  51   d,  that is, at a position that is in between two of the cam grooves  51   d  around the optical axis AZ (in the circumferential direction). Two of these auxiliary cam grooves  51   e  are formed in the fixed frame  50 . In the cam barrel  51 , a gear  51   a  that receives the rotational drive force of a focus motor  64  is formed, and a stopper  51   g  that defines the end of rotation of the cam barrel  51  is formed ( FIG. 10 ). The front side of the cam barrel  51  is in contact with a flange  50   e  of the fixed frame  50 , and the rear side is in contact with the thrust ring  52 . The cam barrel  51  is supported so that it is rotatable with respect to the fixed frame  50  and does not move in the optical axis direction. 
     As shown in  FIG. 10 , the thrust ring  52  has a shape in which part of the circular ring is cut out, that is, an arc shape, and its inside diameter is slightly smaller than the outside diameter of the fixed frame  50 . The thrust ring  52  is engaged with and fixed to the groove  50   f  that is formed in the fixed frame  50 . Near the part of the thrust ring  52  where the circular ring that is cut out, an end portion  52   a  of the thrust ring  52  is bent along the Z-axis direction. This end portion (or protrusion)  52   a  hits the stopper  51   g  of the cam barrel  51 , and thereby defines the end of the region in which the cam barrel  51  can rotate. 
     The first lens group support frame  53  supports the first lens L 1  and the second lens L 2 . Female threads  53   c  for attaching a conversion lens and an optical filter, such as a polarizing filter or a protective filter, are formed at the front of the first lens group support frame  53 . Screw holes (not shown) for fastening the first lens group support frame  53  and the second lens group support frame  54  together with screws are formed in the first lens group support frame  53 . 
     The second lens group support frame  54  supports the third lens L 3 , the fourth lens L 4 , the fifth lens L 5 , the sixth lens L 6 , and the seventh lens L 7 . As shown in  FIG. 8 , the second lens group support frame  54  has three convex components  54   b  disposed at an equal pitch around the optical axis AZ (in the circumferential direction), and three cam pins  54   c  formed so as to protrude outward in the radial direction from the three convex components  54   b,  respectively. The three cam pins  54   c  are respectively inserted into the three cam grooves  51   d  of the cam barrel  51 . The three convex components  54   b  are respectively inserted into the three linear through-grooves  50   c  of the fixed frame  50 . Since the cam grooves  51   d  extend in the circumferential direction and the Z axis direction, when the cam barrel  51  rotates with respect to the fixed frame  50 , the cam pins  54   c  are guided in the Z axis direction along the cam grooves  51   d.  Here, since the movement in the circumferential direction of the convex components  54   b  inserted in the linear through-grooves  50   c  is restricted, the cam pins  54   c  do not rotate with respect to the fixed frame  50 . As a result, the second lens group support frame  54  is able to move in the Z axis direction without rotating with respect to the fixed frame  50 . The amount of movement of the second lens group support frame  54  in the Z axis direction with respect to the fixed frame  50  per unit amount of rotation of the cam barrel  51  with respect to the fixed frame  50  is determined by the shape of the cam grooves  51 , that is, the inclination (or surface that forms a pressure angle) of the cam grooves  51   d.  In this embodiment, the inclination (or surface that forms the pressure angle) of the cam grooves  51   d  is constant over the entire range of movement of the cam pins  54   c  in the optical axis direction. That is, when the cam barrel  51  is seen from a plan view, the cam grooves  51   d  extend linearly. The first lens group support frame  53  is fixed to and moves integrally with the second lens group support frame  54 . 
     The second lens group support frame  54  has an auxiliary convex component  54   d  at a phase position that is in between two of the convex components  54   b,  that is, at a position that is in between two of the convex components  54   b  around the optical axis AZ (in the circumferential direction). The second lens group support frame  54  has two of these auxiliary convex components  54   d.  Further, the second lens group support frame  54  has two auxiliary cam pins  54   e  formed so as to protrude outward in the outer radial direction from the two auxiliary convex components  54   d,  respectively. The two auxiliary cam pins  54   e  are respectively inserted into the two auxiliary cam grooves  51   e  of the cam barrel  51 . The two auxiliary convex components  54   d  are respectively inserted into the two linear auxiliary through-grooves  50   d  of the fixed frame  50 . The spacing between the auxiliary cam pins  54   e  and the auxiliary cam grooves  51   e  is greater than the spacing between the cam pins  54   c  and the cam grooves  51   d.  Also, there is a space between the auxiliary convex components  54   d  and the linear auxiliary through-grooves  50   d.  If the interchangeable lens unit  2  should be subjected to impact because it is dropped, for example, the auxiliary cam pins  54   e  and the auxiliary cam grooves  51   e,  and/or the auxiliary convex components  54   d  and the linear auxiliary through-grooves  50   d,  come into contact with each other and cushion the impact exerted on the cam pins  54   c  or the convex components  54   b.    
     The focus ring unit  88  has a focus ring  89  and a focus ring angle detector  90  that detects the rotational angle of the focus ring  89 . The focus ring  89  has a cylindrical shape and is rotatably supported by the fixed frame  50  and a rear frame  97  around the optical axis AZ in a state in which movement in the Z axis direction is restricted. The rotational angle and rotational direction of the focus ring  89  can be detected by the focus ring angle detector  90 . In this embodiment, the focus ring angle detector  90  has two photosensors  90   a.  The focus ring  89  has a plurality of protrusions  89   a  that protrude inward in the radial direction and are equally spaced in the rotational direction. Each of these photosensors  90   a  has a light emitting component (not shown) and a light receiving component (not shown), and the plurality of protrusions  89   a  pass in between the light emitting components and the light receiving components, allowing the rotational angle and rotational direction of the focus ring  89  to be detected. It should be understood that the focus ring  89  can alternatively have another structure such as a movable lever, depending on the intended use of the disclosed embodiments. 
     In this embodiment, the biasing member  98  is a coil spring that biases the movable focus unit  94  in the optical axis direction. More specifically, one end of the biasing member  98  is contact with the light blocking frame  96  (which is fixed), and the other end is contact with the second lens group support frame  54 , with the biasing member  98  being disposed such that it is always shorter than its natural length. Consequently, the movable focus unit  94  supported by the second lens group support frame  54  and the first lens group support frame  53 , which moves integrally with the second lens group support frame  54 , is always in a state of being biased forward, so it is less likely that the optical system L will become tilted due to looseness between the cam pins  54   c  and the cam grooves  51   d,  or the like, and this is effective at improving optical performance. As shown in  FIG. 11 , in this embodiment the biasing member  98  is disposed such that its center coincides with the optical axis AZ, allowing the biasing member  98  to expand and contract in the Z axis direction. Even when the movable focus unit  94  comes closest to the imaging sensor  11 , the length of the biasing member  98  is greater than the minimum compressed length. Furthermore, even if the movable focus unit  94  moves closest to the subject side, the biasing member  98  will still provide a specific biasing force, such as a biasing force greater than the weight of the movable focus unit  94 . 
     (3) Focus Adjusting Unit 
     The focus adjusting unit  72  has the focus motor  64 , a gearbox  80 , a focus drive controller  41 , and the photosensor  67  (an example of a position sensor). The focus motor  64  and the gearbox  80  are fixed to the fixed frame  50 . The focus drive controller  41  controls the focus motor  64  according to commands from a lens microprocessor  40 . The focus motor  64  is a stepping motor, for example, and outputs rotational force to the gearbox  80 . The gearbox  80  changes the speed of rotation of the focus motor  64 , and outputs rotational force from a gearbox output component  80   a.  The gearbox output component  80   a  engages with the gear  51   a  of the cam barrel  51 , and drive of the focus motor  64  rotates the cam barrel  51 . That is, the cam barrel  51  is rotated by rotational force received from the gearbox output component  80   a  via the gear  51   a.  As discussed above, rotation of the cam barrel  51  is facilitated by the cam pins  54   c  being guided along the cam grooves  51   d  in the Z axis direction, and the movable focus unit  94  moves in the Z axis direction without rotating with respect to the lens mount  95 . Thus, the focus motor  64  functions as a driver that outputs drive force for driving the optical system L in the optical axis direction, and the cam grooves  51   d  of the cam barrel  51  and the cam pins  54   c  of the second lens group support frame  54  function as cam mechanisms that receive the drive force outputted from the focus motor  64  and guide the optical system L in the optical axis direction. 
     Also, a photosensor  67  that detects the home position of the movable focus unit  94  in the optical axis direction is fixed to the fixed frame  50 . This photosensor  67  has a light emitting component (not shown) and a light receiving component (not shown). When a focus home point detected component  54   f  of the second lens group support frame  54  passes between the light emitting component and the light receiving component, that is, when the focus home point detected component  54   f  is at the home position, the photosensor  67  can detect the presence of the focus home point detected component  54   f.  In other words, the photosensor  67  is able to detect the home position of the movable focus unit  94  with respect to the fixed frame  50 . 
     The lens microprocessor  40  is able to control the drive speed of the focus motor  64  so as to output a drive force that will drive (or move) the movable focus unit  94  to the desired position along the Z axis direction. For instance, the lens microprocessor  40  drives the movable focus unit  94  to the home position, and recognizes from a signal from the photosensor  67  that the movable focus unit  94  is in the home position. 
     The home position that can be detected by the photosensor  67  is the absolute position which never changes with respect to the fixed frame  50 . Accordingly, in resetting the position of the movable focus unit  94  to the home position with respect to the fixed frame  50 , the focus motor  64  drives (or moves) the movable focus unit  94  to the position at which the focus home point detected component  54   f  for detecting the home point is detected by the photosensor  67 . For example, when the power switch  25  of the digital camera  1  is turned off, the movable focus unit  94  is driven by the focus motor  64  to the position at which the focus home point detected component  54   f  of the second lens group support frame  54  is detected by the photosensor  67 , regardless of the current position of the movable focus unit  94 . Upon completion of the drive of the movable focus unit  94 , the power to the digital camera  1  is switched off. Conversely, when the power switch  25  of the digital camera  1  is turned on, the movable focus unit  94  is driven to a specific position by the focus motor  64 . The photosensor  67  is an example of a home detector. The home detector is not limited to being a photosensor, and can instead have a combination of a magnet and a magnetic sensor. 
     (4) Aperture Adjusting Unit 
     The aperture adjusting unit  73  has the aperture unit  62  (an example of an aperture device), an aperture drive motor that drives the aperture unit  62 , and an aperture drive controller  42  that controls the aperture drive motor. The aperture drive motor is a stepping motor, for example. The aperture drive motor is driven on the basis of a drive signal inputted from the aperture drive controller  42 . The drive force generated by the aperture drive motor drives aperture blades of the aperture unit  62  in the opening and closing directions, and changes the shape of the opening defined by the aperture blades. Therefore, the lens microprocessor  40  can vary the aperture value of the optical system L by driving the aperture blades via the aperture drive controller  42 . In this embodiment, a photosensor  62   b  can detect when the opening defined by the aperture blades has a specified opening diameter. The aperture unit  62  has a positioning hole (not shown) and an anti-rotation hole (not shown). The positioning hole (not shown) and the anti-rotation hole (not shown) engage respectively with a positioning boss (not shown) and an anti-rotation boss (not shown) formed on the second lens group support frame  54 , which determines the position of the aperture unit  62  in the X-Y plane. The aperture unit  62  is fixed by being fastened with screws to the second lens group support frame  54 . 
     (5) Lens Microprocessor 
     The lens microprocessor  40  has a CPU (not shown), a ROM (not shown), and a memory  40   a,  and various functions can be performed by reading programs stored in the ROM into the CPU. For instance, the lens microprocessor  40  can recognize that the movable focus unit  94  is in the home position by using a detection signal from the photosensor  67 . 
     The memory  40   a  is a nonvolatile memory, and can hold stored information even when the power supply has been halted. In this embodiment, information related to the interchangeable lens unit  2  (lens information) is held in the memory  40   a.    
     The lens microprocessor  40  has a counter  40   b  for counting the number of drive pulses of the focus motor  64 . The counter  40   b  counts “+1” when the movable focus unit  94  is driven by one drive pulse to the Z axis direction positive side, and counts “−1” when the movable focus unit  94  is driven by one drive pulse to the Z axis direction negative side. The lens microprocessor  40  can thus ascertain the relative position of the movable focus unit  94  with respect to the fixed frame  50  by counting the number of drive pulses of the focus motor  64  with the counter  40   b.  That is, the lens microprocessor  40  can ascertain the absolute position of the movable focus unit  94  with respect to the fixed frame  50  by combining recognition of the home position by the photosensor  67  with ascertaining the relative position found by counting the number of drive pulses. 
     Camera Body 
     The configuration of the camera body  3  will be described through reference to  FIGS. 1 to 4B . As shown in  FIGS. 1 to 4B , the camera body  3  has a housing  3   a,  the body mount  4 , an interface unit  39 , an image acquisition component  35 , an image display component  36 , a viewfinder component  38 , a body microprocessor  10 , and a battery  22  (an example of a main power supply). 
     (1) Housing 
     The housing  3   a  functions as the outer part of the camera body  3 . As shown in  FIGS. 4A and 4B , the body mount  4  is provided to the front face of the housing  3   a,  and the interface unit  39  is provided to the rear and top faces of the housing  3   a.  More specifically, a display component  20 , the power switch  25 , a mode selector dial  26 , a directional arrow key  27 , a menu setting button  28 , a set button  29 , an imaging mode selector button  34 , and a moving picture capture button  24  are provided to the rear face of the housing  3   a.  A shutter button  30  is provided to the top face of the housing  3   a.    
     (2) Body Mount 
     The body mount  4  is the portion where the lens mount  95  of the interchangeable lens unit  2  is mounted, and has a body-side contact (not shown) that can be electrically connected with the lens-side contact  91 . The camera body  3  is able to send and receive data to and from the interchangeable lens unit  2  via the body mount  4  and the lens mount  95 . For example, the body microprocessor  10  (discussed below) sends a control signal to the lens microprocessor  40 , such as an exposure synchronization signal, via the body mount  4  and the lens mount  95 . 
     (3) Interface Unit 
     As shown in  FIGS. 4A and 4B , the interface unit  39  has various operating members that the user can use to input operating information. For instance, the power switch  25  is a switch for turning the power on and off to the digital camera  1  or the camera body  3 . When the power is turned on with the power switch  25 , power is supplied to the various parts of the camera body  3  and the interchangeable lens unit  2 . 
     The mode selector dial  26  is used to switch the operating mode, such as still picture imaging mode, moving picture imaging mode, or play mode, and the user can turn the mode selector dial  26  to switch the operating mode. When the still picture imaging mode is selected with the mode selector dial  26 , the operating mode is switched to the still picture imaging mode, and when the moving picture imaging mode is selected with the mode selector dial  26 , the operating mode is switched to the moving picture imaging mode. In the moving picture imaging mode, basically moving picture imaging is possible. When the play mode is selected with the mode selector dial  26 , the operating mode is switched to the play mode, allowing the captured image to be displayed on the display component  20 . 
     The directional arrow key  27  is a button for the user to select the left, right, up, and down directions. The user can use the directional arrow key  27  to select the desired menu from various menu screens displayed on the display component  20 , for example. 
     The menu setting button  28  is for setting the various operations of the digital camera  1 . The set button  29  is for executing the operations corresponding to the various menus. 
     The moving picture imaging button  24  is for starting and stopping the capture of moving pictures. Even if the operating mode selected with the mode selector dial  26  is the still picture imaging mode or the play mode, when the moving picture imaging button  24  is pressed, the operating mode is forcibly changed to the moving picture imaging mode, and moving picture imaging begins, regardless of the setting on the mode selector dial  26 . When this moving picture imaging button  24  is pressed during the capture of a moving picture, the moving picture imaging ends and the operating mode changes to the one selected on the mode selector dial  26 , that is, to the one prior to the start of moving picture imaging. For example, if the still picture imaging mode has been selected with the mode selector dial  26  when the moving picture imaging button  24  is pressed, the operating mode automatically changes to the still picture imaging mode after the moving picture imaging button  24  is pressed again. 
     The shutter button  30  is pressed by the user to capture an image. When the shutter button  30  is pressed, a timing signal is outputted to the body microprocessor  10 . The shutter button  30  is a two-stage switch that can be pressed half way down or all the way down. Light measurement and ranging are commenced when the user presses the button half way down. When the user presses the shutter button  30  all the way down in a state in which the shutter button  30  has been pressed half way down, a timing signal is outputted, and image data is acquired by the image acquisition component  35 . 
     As shown in  FIG. 2 , a lens removal button  99  for removing the interchangeable lens unit  2  from the camera body  3  is provided to the front face of the camera body  3 . The lens removal button  99  has a contact (not shown) that is in its “on” state when the button is pressed by the user, for example, and is electrically connected to the body microprocessor  10 . When the lens removal button  99  is pressed, the built-in contact is switched on, and the body microprocessor  10  recognizes that the lens removal button  99  has been pressed. 
     (4) Image Acquisition Component 
     The image acquisition component  35  mainly includes the imaging sensor  11  (an example of an imaging element), a shutter unit  33  that adjusts the exposure state of the imaging sensor  11 , a shutter controller  31  that controls the drive of the shutter unit  33  on the basis of a control signal from the body microprocessor  10 , and an imaging sensor drive controller  12  that controls the operation of the imaging sensor  11  on the basis of a control signal from the body microprocessor  10 . 
     The imaging sensor  11  in this embodiment is a CCD (charge coupled device) sensor that converts the optical image formed by the optical system L into an electrical signal. The imaging sensor  11  is controlled so as to be driven by a timing signal produced by the imaging sensor drive controller  12 . The imaging sensor  11  can instead be a CMOS (complementary metal oxide semiconductor) sensor. 
     The shutter controller  31  drives a shutter drive actuator  32  and operates the shutter unit  33  according to a control signal outputted from the body microprocessor  10  that has received a timing signal. 
     The auto-focus method that is employed in this embodiment is a contrast detection method that makes use of image data produced by the imaging sensor  11 . Using a contrast detection method allows high-precision focal adjustment. 
     (5) Body Microprocessor 
     The body microprocessor  10  is a control device that is the command center of the camera body  3 , and controls the various components of the digital camera  1  according to operation information inputted to the interface unit  39 . More specifically, the body microprocessor  10  is equipped with a CPU, ROM, and RAM, and the programs held in the ROM are read by the CPU, allowing the body microprocessor  10  to perform a variety of functions. For instance, the body microprocessor  10  has the function of detecting that the interchangeable lens unit  2  has been mounted to the camera body  3 , and the function of acquiring information that is necessary for controlling the digital camera  1 , such as information about the focal distance from the interchangeable lens unit  2 . 
     The body microprocessor  10  is able to receive signals from the power switch  25 , the shutter button  30 , the mode selector dial  26 , the directional arrow key  27 , the menu setting button  28 , and the set button  29 . Different information related to the camera body  3  is held in a memory  10   a  inside the body microprocessor  10 . The memory  10   a  is a nonvolatile memory and can hold stored information even when the power supply has been halted. 
     Also, the body microprocessor  10  periodically produces a vertical synchronization signal, and produces an exposure synchronization signal on the basis of the vertical synchronization signal in parallel with the production of the vertical synchronization signal. The body microprocessor  10  can produce an exposure synchronization signal because the body microprocessor  10  ascertains beforehand the exposure start timing and the exposure stop timing based on the vertical synchronization signal. The body microprocessor  10  outputs a vertical synchronization signal to a timing generator (not shown), and outputs an exposure synchronization signal at a specific period to the lens microprocessor  40  via the body mount  4  and the lens mount  95 . The lens microprocessor  40  acquires position information about the movable focus unit  94  in synchronization with the exposure synchronization signal. 
     The imaging sensor drive controller  12  produces an electronic shutter drive signal and a read signal of the imaging sensor  11  at a specific period on the basis of the vertical synchronization signal. The imaging sensor drive controller  12  drives the imaging sensor  11  on the basis of the electronic shutter drive signal and the read signal. That is, the imaging sensor  11  outputs to a vertical transfer component (not shown) the pixel data produced by numerous opto-electrical conversion elements (not shown) present in the imaging sensor  11 , according to the read signal. 
     The body microprocessor  10  also controls the focus adjusting unit  72  via the lens microprocessor  40 . 
     The image signal outputted from the imaging sensor  11  is successively processed by an analog signal processor  13 , an A/D converter  14 , a digital signal processor  15 , a buffer memory  16 , and an image compressor  17 . The analog signal processor  13  subjects the image signal outputted from the imaging sensor  11  to gamma processing or other such analog signal processing. The A/D converter  14  converts the analog signal outputted from the analog signal processor  13  into a digital signal. The digital signal processor  15  subjects the image signal converted into a digital signal by the A/D converter  14  to digital signal processing such as noise elimination or contour enhancement. The buffer memory  16  is a RAM (Random Access Memory), and temporarily stores the image signal. The image signal stored in the buffer memory  16  is sent to and processed by first the image compressor  17  and then an image recorder  18 . The image signal stored in the buffer memory  16  is read at a command from an image recording controller  19  and sent to the image compressor  17 . The data of the image signal sent to the image compressor  17  is compressed according to a command from the image recording controller  19 . This compression adjusts the image signal to a smaller data size than that of the original data. An example of the method for compressing the image signal is the JPEG (Joint Photographic Experts Group) method in which compression is performed on the image signal for each frame. After this, the compressed image signal is recorded in the image recorder  18  by the image recording controller  19 . When a moving picture is recorded, JEPG can be used, in which compression is performed on an image signal corresponding to one frame, and an H.264/AVC method can also be used, in which compression is performed on image signals corresponding to some frames all at once. 
     The image recorder  18  produces a still picture file or moving picture file which includes specific information to be recorded and the image signal associated with the specific information to be recorded, on the basis of a command from the image recording controller  19 . The image recorder  18  also records the still picture file or moving picture file on the basis of a command from the image recording controller  19 . The image recorder  18  is a removable memory and/or an internal memory, for example. The specific information to be recorded with the image signal includes the date and time information when the image was captured, focal distance information, shutter speed information, aperture value information, and imaging mode information. Still picture files are in Exif® format or a format similar to Exif® format, for example. Moving picture files are in H.264/AVC format or a format similar to H.264/AVC format, for example. 
     (6) Image Display Component 
     The image display component  36  has the display component  20  and an image display controller  21 . The display component  20  is a liquid crystal monitor, for example. The display component  20  displays as a visible image the image signal recorded to the buffer memory  16  or the image recorder  18  on the basis of a command from the image display controller  21 . Possible display modes on the display component  20  include a display mode in which only the image signal is displayed as a visible image, and a display mode in which the image signal and information about the time of capture of the image signal are displayed as a visible image. 
     (7) Viewfinder 
     The viewfinder component  38  has a liquid crystal viewfinder  8  that displays the image acquired by the imaging sensor  11 , and a viewfinder eyepiece window  9  provided to the rear face of the housing  3   a.  The user looks into the viewfinder eyepiece window  9  to view the image displayed on the liquid crystal viewfinder  8 . 
     (8) Battery 
     The battery  22  supplies power to the various components of the camera body  3 , and also supplies power to the interchangeable lens unit  2  via the lens mount  95 . In this embodiment, the battery  22  is a rechargeable battery. The battery  22  can also be a dry cell, or an external power supply can be used, with which power is supplied from the outside through a power cord. 
     Operation of Digital Camera 
     The operation of the digital camera  1  will be described. 
     (1) Imaging Mode 
     This digital camera  1  has two imaging modes. More specifically, the digital camera  1  has a viewfinder imaging mode in which the user looks at the subject through the viewfinder eyepiece window  9 , and a monitor imaging mode in which the user looks at the subject on the display component  20 . 
     In viewfinder imaging mode, for example, the image display controller  21  drives the liquid crystal viewfinder  8 . As a result, an image of the subject acquired by the imaging sensor  11  (a so-called through-image) is displayed on the liquid crystal viewfinder  8 . 
     In monitor imaging mode, for example, the display component  20  is driven by the image display controller  21 , and a real-time image of the subject is displayed on the display component  20 . An imaging mode selector button  34  allows switching between these two imaging modes. 
     (2) Still Picture Imaging 
     When the user presses the shutter button  30  all the way down, a command is sent from the body microprocessor  10  to the lens microprocessor  40  so that the aperture value of the optical system L will be set to the aperture value calculated on the basis of the light measurement output of the imaging sensor  11 . The aperture drive controller  42  is controlled by the lens microprocessor  40 , and the aperture unit  62  is stopped down to the indicated aperture value. Simultaneously with the indication of the aperture value, a drive command is sent from the imaging sensor drive controller  12  to the imaging sensor  11 , and a drive command is sent from the shutter controller  31  to the shutter unit  33 . The imaging sensor  11  is exposed by the shutter unit  33  for a length of time corresponding to the shutter speed calculated on the basis of the light measurement output of the imaging sensor  11 . 
     The body microprocessor  10  executes imaging processing and, when the imaging is completed, sends a control signal to the image recording controller  19 . The image recorder  18  records an image signal to an internal memory and/or removable memory on the basis of the command of the image recording controller  19 . The image recorder  18  records imaging mode information (whether the auto-focus imaging mode or the manual focus imaging mode was used) and the image signal to the internal memory and/or removable memory on the basis of the command of the image recording controller  19 . 
     Upon completion of the exposure, the imaging sensor drive controller  12  reads image data from the imaging sensor  11 , and after specific image processing, image data is outputted via the body microprocessor  10  to the image display controller  21 . Consequently, the captured image is displayed on the display component  20 . 
     Also, upon completion of the exposure, the shutter unit  33  is reset to its initial position by the body microprocessor  10 . The body microprocessor  10  issues a command to the lens microprocessor  40  for the aperture drive controller  42  to reset the aperture  62  to its open position, and a reset command is sent from the lens microprocessor  40  to the various units. Upon completion of this resetting, the lens microprocessor  40  tells the body microprocessor  10  that resetting is complete. After the resetting completion information has been received from the lens microprocessor  40 , and after a series of post-exposure processing has been completed, the body microprocessor  10  confirms that the shutter button  30  has not been pressed, and the imaging sequence is concluded. 
     (3) Moving Picture Imaging 
     The digital camera  1  also has the function of capturing moving pictures. In the moving picture imaging mode, image data is produced by the imaging sensor  11  at a specific period, and the image data thus produced is utilized to continuously carry out auto-focusing by the contrast detection method. In the moving picture imaging mode, if the shutter button  30  is pressed, or if the moving picture imaging button  24  is pressed, a moving picture is recorded to the image recorder  18 , and when the shutter button  30  or the moving picture imaging button  24  is pressed again, recording of the moving picture by the image recorder  18  is stopped. 
     (4) Contrast AF Operation 
     Auto-focus operation of the digital camera  1  by contrast detection (contrast AF) will now be described through reference to  FIGS. 12 to 14 .  FIG. 12  is a diagram illustrating contrast AF operation. The vertical axis in  FIG. 12  is the contrast value, and the greater the contrast value, the better the focus. The horizontal axis in  FIG. 12  is the position of the movable focus unit  94  in the optical axis direction; moving to the right of the graph, there image of the subject is moving increasingly closer (i.e., the movable focus unit  94  is on the subject side), and moving to the left, the image of the subject is moving increasingly to infinity (i.e., the movable focus unit  94  is on the user side). 
     When the shutter button  30  is pushed half-way down by the user, a timing signal is sent to the body microprocessor  10 , and the digital camera  1  changes to contrast AF operation. 
     When the camera changes to contrast AF operation, the digital camera  1  performs a first focus drive operation, in which the peak contrast value is detected and the focal position is predicted. More specifically, in the first focus drive operation, the body microprocessor  10  issues commands to the lens microprocessor  40  for the speed of the focus motor  64  (contrast detection speed) and the detection end position F 12 , which is the target position to which the movable focus unit  94  is to be moved. The contrast detection speed, which is the speed of the focus motor  64  indicated by the body microprocessor  10  during the first focus drive operation, is a speed at which the body microprocessor  10  can accurately predict the focal position, and in this embodiment, it is faster than the “set speed” discussed below. As a result, in step S 4  discussed below, the actual drive speed of the focus motor  64  during the first focus drive operation becomes the “set speed” discussed below. The lens microprocessor  40  sends a command to the focus drive controller  41  on the basis of the command from the body microprocessor  10 , and the focus motor  64  is driven by the focus drive controller  41 . The focus motor  64  moves the movable focus unit  94  from the detection start position F 11  to the detection end position F 12  via the gearbox  80 , the cam barrel  51 , and the second lens group support frame  54 . While the movable focus unit  94  is being moved from the detection start position F 11  to the detection end position F 12 , the imaging sensor  11  outputs image data for each timing interval of the exposure synchronization signal. The body microprocessor  10  detects the contrast value for each image data. Furthermore, the body microprocessor  10  acquires position information about the position of the movable focus unit  94  from the lens microprocessor  40  for each timing interval of the exposure synchronization signal. The body microprocessor  10  associates the position information about the position of the movable focus unit  94  with the contrast value acquired for each timing interval of the exposure synchronization signal, and stores this in the memory  10   a.  The body microprocessor  10  predicts the position of the movable focus unit  94  at which the contrast value will be at its peak (the peak position F 14 ) on the basis of the distribution of the contrast values and the position information about the position of the movable focus unit  94  (that is, it predicts the focal position). When prediction of the peak position F 14  is finished, the digital camera  1  changes to a second focus drive operation. During the first focus drive operation, if the body microprocessor  10  determines that the contrast value has decreased through the movement of the movable focus unit  94 , then the body microprocessor  10  reverses the direction in which the movable focus unit  94  is moved and performs the first focus drive operation over again.  FIG. 12  is a diagram illustrating the operation when the focal position is more to the subject side than the initial position F 11  of the movable focus unit  94 . The contrast value indicates the degree of sharpness of the subject. The contrast value is an example of a value that expresses the degree of focus. 
     In the second focus drive operation, first the body microprocessor  10  issues commands to the lens microprocessor  40  for the speed of the focus motor  64  and the target position F 13  of the movable focus unit  94 , which is higher than the peak position F 14  of the contrast value when viewed from the current position F 12 . The actual drive speed of the focus motor  64  during the second focus drive operation becomes the “set speed” discussed below. The lens microprocessor  40  drives the focus motor  64  on the basis of the command from the body microprocessor  10  and the “set speed,” and when the movable focus unit  94  reaches the target position F 13 , the second focus drive operation ends and changes to a third focus drive operation. 
     In the third focus drive operation, the body microprocessor  10  issues commands to the lens microprocessor  40  for the speed of the focus motor  64  and the peak position F 14  of the contrast value (serving as a target position). The actual drive speed of the focus motor  64  during the third focus drive operation also becomes the “set speed” just as in the second focus drive operation. The lens microprocessor  40  drives the focus motor  64  on the basis of the command from the body microprocessor  10  and the “set speed,” and when the movable focus unit  94  reaches the target position F 14 , the third focus drive operation ends and so does contrast AF operation. The contrast value is neither calculated during the second focus drive operation nor during the third focus drive operation. 
     As discussed above, in the first, second, and third focus drive operations, the body microprocessor  10  sends a request to the lens microprocessor  40  for the speed of the focus motor  64  (command speed), and this command speed is determined as follows. First, if the body microprocessor  10  detects that the interchangeable lens unit  2  has been mounted to the camera body  3 , it acquires from the lens microprocessor  40  information about the characteristics of the interchangeable lens unit  2 , which will be necessary in the overall control of the digital camera  1 . This information about the characteristics of the interchangeable lens unit  2  includes the above-mentioned focal distance information as well as information indicating the “maximum speed” of the focus motor  64 , etc. The body microprocessor  10  decides the maximum speed at which the focus motor  64  will not go out of step under various restrictions, within a range that does not exceed said “maximum speed” during the drive of the movable focus unit  94 , including during the first, second, and third focus drive operations. The body microprocessor  10  then issues a command to the lens microprocessor  40  regarding this speed. Meanwhile, the lens microprocessor  40  compares the maximum speed decided by the body microprocessor  10  so that the focus motor  64  will not go out of step with the “set speed”, as in step S 4  discussed below, and employs the slower of the speeds as the actual drive speed for the focus motor  64 . In this embodiment, the maximum speed B in  FIG. 13  is sent as the “maximum speed” from the lens microprocessor  40  to the body microprocessor  10  when the interchangeable lens unit  2  is mounted to the camera body  3 . 
     Also, the reason the movable focus unit  94  is not driven to F 14 , which is the peak position of the contrast value, immediately after the end of the first focus drive operation is because the gearbox  80  experiences a backlash when the direction of movement of the movable focus unit  94  is changed and as a result, error corresponding to the backlash will occur. To reduce this error caused by backlash, the focal position detection direction (first focus drive operation) and the focal position movement direction (third focus drive operation) are made to be in the same direction, so that the error corresponding to backlash is smaller. Accordingly, when there is little variance in backlash due to orientation error, repetition error, or the like, the contrast AF operation can end by moving to a target position obtained by adding a backlash correction component to the focal position F 14  in the second focus drive operation. 
     In a contrast AF method, accurate positioning the movable focus unit  94  is required for the predicted focal position, so a stepping motor is used as the focus motor  64 . With a stepping motor, the rotational angle varies with the inputted drive pulses, so the position the movable focus unit  94  can be controlled with this motor without using an external sensor, and stepping motors are widely used for digital cameras that employ contrast AF. However, if the resistance to rotation (load torque) is too great, or the drive speed (output torque) of the focus motor  64  is too high, synchronization is lost between the number of drive pulses and the rotational angle (the drive is out of step). Accordingly, the drive speed (output torque) of the focus motor  64  must be set extra low to take into account the load torque, temperature characteristics, and so forth. 
     When the movable focus unit  94  is biased by the biasing member  98  as in this embodiment, the biasing force on the movable focus unit  94  is used as load torque on the focus motor  64  in the movement of the movable focus unit  94  in the optical axis direction. The biasing force on the movable focus unit  94  varies according to the position of the movable focus unit  94  in the optical axis direction. That is, the load torque of the biasing member  98  varies according to the position of the movable focus unit  94  in the optical axis direction. Accordingly, for example, a method can be used in which the speed of the stepping motor at which the drive of the focus motor  64  does not go out of step is set as the “set speed,” using the point at which the load torque of the movable focus unit  94  is at its maximum as a reference. With this method, when the movable focus unit  94  is moved at high speed to the target position, the movable focus unit  94  is driven at the constant “set speed” regardless of the position of the movable focus unit  94  in the optical axis direction. In this embodiment, though, the “set speed” of the focus motor  64  is made variable according to the position of the movable focus unit  94  in the optical axis direction as discussed below, so that the actual drive speed of the focus motor  64  is made variable according to the position of the movable focus unit  94  in the optical axis direction, and a higher focusing speed is attained. 
       FIG. 13  is a graph showing the relationship between the load torque produced by the biasing member  98  and the maximum speed of the focus motor  64  at which the drive does not go out of step even under such load torque, within the range of movement of the movable focus unit  94  in the optical axis direction. As shown in  FIG. 13 , when the movable focus unit  94  is positioned to focus on a subject on the infinity side, the biasing member  98  is greatly compressed and the magnitude of the load torque is large, and when the movable focus unit  94  is positioned to focus on a subject on the close-up side, compression of the biasing member  98  is reduced and the magnitude of the load torque is small. Specifically, the load torque is greater when the movable focus unit  94  is at position FH 21  than when the movable focus unit  94  is at position FH 22 . Accordingly, when the movable focus unit  94  is at the position FH 21 , the maximum speed A is lower than the maximum speed B when the movable focus unit  94  is at the position FH 22 . As a result, as shown in  FIG. 15 , when the movable focus unit  94  is in the position FH 21 , the “set speed” is set lower than when the movable focus unit  94  is in the position FH 22 . To put this another way, when the movable focus unit  94  is at the position FH 22 , a higher speed is achieved by setting the “set speed” at a higher “set speed” than when the movable focus unit  94  is in the position FH 21 . 
       FIG. 14  shows a flowchart of the process related to a variable set speed method, and  FIG. 15  shows an example of a speed switching table. In this embodiment, processing by the following variable set speed method is used in the above-mentioned first focus drive operation, second focus drive operation, and third focus drive operation. 
     First, the speed of the focus motor  64  and the target position are indicated by command from the body microprocessor  10  to the lens microprocessor  40  (step  1 ). The target position is, for example, the target position F 12  in the first focus drive operation, the target position F 13  in the second focus drive operation, or the target position F 14  in the third focus drive operation. 
     Next, the lens microprocessor  40  acquires the current position of the movable focus unit  94  (step  2 ). More specifically, the current position of the movable focus unit  94  is acquired by counting the number of drive pulses of the focus motor  64  after ascertaining the absolute position of the movable focus unit  94  as discussed above. 
     The lens microprocessor  40  then determines the “set speed” of the focus motor  64  corresponding to the current position on the basis of a speed switching table (step  3 ). A speed switching table shows the corresponding relationship between the “set speed” and the position of the movable focus unit  94  in the optical axis direction. The actual drive speed of the focus motor  64  is determined to be the “set speed” when the “set speed” is equal to or less than the command speed from the body microprocessor  10 , as in step S 4  discussed below, and is the “set speed” during the first, second, and third focus drive operations. Therefore, a speed switching table is information that expresses the corresponding relationship between the actual drive speed of the focus motor  64  and the position of the movable focus unit  94  in the optical axis direction. The “set speed” is defined for each position of the movable focus unit  94  as the speed at which the drive of the focus motor  64  does not go out of step. The “set speed” is also a value that varies according to the position of the movable focus unit  94 . More specifically, the “set speed” is lower when the load torque is greater and is higher when the load torque is less. The speed switching table is stored in the memory  40   a.    
     The lens microprocessor  40  compares the driving speed indicated by the body microprocessor  10  (an example of the command speed) with the “set speed” determined in step  3 . If the driving speed or command speed is the same as the “set speed” of the focus motor  64 , or is higher than the “set speed,” then the actual drive speed of the focus motor  64  is set to the “set speed” of the focus motor  64 , and the flow proceeds to step  6 . If the driving speed or command speed is slower than the “set speed” of the focus motor  64 , then the flow proceeds to step  5  (step  4 ). In step  5 , the lens microprocessor  40  sets the actual drive speed of the focus motor  64  to the command speed from the body microprocessor  10 , and the flow proceeds to step  6 . 
     The lens microprocessor  40  then drives the focus motor  64  at the set drive speed (step  6 ). More specifically, the number of drive pulses per unit of time transmitted to the focus motor  64  is made to correspond with the set drive speed. 
     The lens microprocessor  40  monitors the position of the movable focus unit  94 , determines whether or not the focus motor  64  has reached the speed switching position in the speed switching table of  FIG. 15  (the position at which the “set speed” changes), and if the speed switching position has been reached, the flow proceeds to step  3 , but if it has not been reached, the flow proceeds to step  8  (step  7 ). The lens microprocessor  40  acquires the current position of the movable focus unit  94  by counting the number of drive pulses after ascertaining the absolute position of the movable focus unit  94 . 
     In step  8 , the lens microprocessor  40  determines whether or not the movable focus unit  94  has reached the target position indicated by the body microprocessor  10 . If the movable focus unit  94  has not reached the target position, the flow proceeds to step  7 , but if it has reached the target position, the flow proceeds to step  9  and the lens microprocessor  40  halts the focus motor  64 . The lens microprocessor  40  acquires the current position of the movable focus unit  94  by counting the number of drive pulses after ascertaining the absolute position of the movable focus unit  94 . 
     With the variable set speed method discussed above, the focus motor  64  that drives the focusing lens can be prevented from going out of step, while the movement speed of the movable focus unit  94  (or the focusing lens) can be increased. 
     Second Embodiment  
     Only those points that differ from the first embodiment will be described, and description of points that are the same will be omitted. 
     The interchangeable lens unit  2  in the first embodiment had the biasing member  98 , but the interchangeable lens unit  2  in the second embodiment does not have the biasing member  98 . 
     Also, the cam grooves  51   d  of the interchangeable lens unit  2  in the first embodiment had a constant inclination (or surface that forms the pressure angle) over the entire range of movement of the movable focus unit  94  in the optical axis direction. That is, the load torque produced by the cam grooves  51   d  was constant over the entire range of movement of the movable focus unit  94  in the optical axis direction. On the other hand, the cam grooves  51   d  of the interchangeable lens unit  2  in the second embodiment are formed such that their inclination (surface and pressure angle) varies with the position in the optical axis direction. In other words, the cam grooves  51   d  are formed in such a way that the amount of movement of the movable focus unit  94  in Z axis direction per unit of rotational force outputted from the focus motor  64  varies with the position of the movable focus unit  94  in the optical axis direction. That is, the load torque produced by the cam grooves  51   d  varies with the position of the movable focus unit  94  in the optical axis direction. Furthermore, the greater the inclination (surface and pressure angle) of the cam grooves  51   d,  the greater the amount of movement of the movable focus unit  94  in the Z axis direction with respect to the amount of rotation of the cam barrel  51 . That is, the greater the inclination (surface and pressure angle) of the cam grooves  51   d,  the greater the amount of movement of the movable focus unit  94  with respect to the amount of rotation of the focus motor  64  (the same applies hereinafter). 
       FIG. 16  is a graph showing the relationship between the set speed of the focus motor  64 , the maximum speed of the focus motor  64 , the load torque, the surface and/or the pressure angle of the cam grooves  51   d,  and the shape of the cam grooves  51   d  with respect to the position of the movable focus unit  94 . The “shape of the cam grooves  51   d ” referred to here is approximately the shape of the cam grooves  51   d  when the cam barrel  51  is seen from a plan view. 
     The load torque is higher where the pressure angle of the cam grooves  51   d  is greater. Also, the load torque is lower where the pressure angle of the cam grooves  51   d  is smaller. The situation in which the load torque varies with the position of the movable focus unit  94  in the optical axis direction is the same as that in the first embodiment. And the same variable set speed method as in the first embodiment is used again in the second embodiment. Consequently, the focus motor  64  that drives the movable focus unit  94  (or the focusing lens) can be prevented from going out of step, while the movement speed of the focusing lens can be increased. 
     Third Embodiment  
     Only those points that differ from the first embodiment will be described, and description of points that are the same will be omitted. 
     The cam grooves  51   d  of the interchangeable lens unit  2  in the first embodiment had a constant inclination (or surface that forms the pressure angle) over the entire range of movement of the movable focus unit  94  in the optical axis direction. That is, the load torque produced by the cam grooves  51   d  was constant over the entire range of movement of the movable focus unit  94  in the optical axis direction. On the other hand, the cam grooves  51   d  of the interchangeable lens unit  2  in the third embodiment extend such that their inclination (surface and pressure angle) varies with the position in the optical axis direction. In other words, the cam grooves  51   d  are formed in such a way that the amount of movement of the movable focus unit  94  in the Z axis direction per unit of rotational force outputted from the focus motor  64  varies with the position of the movable focus unit  94  in the optical axis direction. That is, the load torque produced by the cam grooves  51   d  varies with the position of the movable focus unit  94  in the optical axis direction. 
     The interchangeable lens unit  2  of the third embodiment also has the biasing member  98 . Therefore, the load torque produced by the biasing member  98  varies with the position of the movable focus unit  94  in the optical axis direction. 
       FIG. 17  is a graph showing the relationship between the set speed of the focus motor  64 , the maximum speed of the focus motor  64 , the load torque, the surface and/or the pressure angle of the cam grooves  51   d,  and the shape of the cam grooves  51   d  with respect to the position of the focus movable unit  94 . The “shape of the cam grooves  51   d ” referred to here is substantially the shape of the cam grooves  51   d  when the cam barrel  51  is seen from a plan view. 
     The total load torque obtained by combining the load torque produced by the cam grooves  51   d  and the load torque produced by the biasing member  98  also varies with the position of the movable focus unit  94  in the optical axis direction. The situation in which the load torque varies with the position of the movable focus unit  94  in the optical axis direction is the same as that in the first embodiment. And the same variable set speed method as in the first embodiment is used again in the third embodiment. Consequently, the focus motor  64  that drives the movable focus unit  94  (or the focusing lens) can be prevented from going out of step, while the movement speed of the focusing lens can be increased. 
     Fourth Embodiment 
     Only those points that differ from the first embodiment will be described, and description of points that are the same will be omitted. 
     The interchangeable lens unit  2  of the fourth embodiment also has the biasing member  98 . Therefore, the load torque produced by the biasing member  98  varies with the position of the movable focus unit  94  in the optical axis direction. 
     The cam grooves  51   d  of the interchangeable lens unit  2  in the first embodiment had a constant inclination (or surface that forms the pressure angle) over the entire range of movement of the movable focus unit  94  in the optical axis direction. That is, the load torque produced by the cam grooves  51   d  was constant over the entire range of movement of the movable focus unit  94  in the optical axis direction. On the other hand, the cam grooves  51   d  of the interchangeable lens unit  2  in the fourth embodiment are formed such that their inclination (surface and pressure angle) varies with the position in the optical axis direction. In other words, the cam grooves  51   d  extend in such a way that the amount of movement of the movable focus unit  94  in the Z axis direction per unit of rotational force outputted from the focus motor  64  (an example of a unit output of the driver) varies with the position of the movable focus unit  94  in the optical axis direction. That is, the load torque produced by the cam grooves  51   d  varies with the position of the movable focus unit  94  in the optical axis direction. 
       FIG. 18  is a graph showing the relationship between the set speed of the focus motor  64 , the maximum speed of the focus motor  64 , the load torque, the pressure angle of the cam grooves  51   d,  and the shape of the cam grooves  51   d  with respect to the position of the focus movable unit  94 . The “shape of the cam grooves  51   d ” referred to here is substantially the shape of the cam grooves  51   d  when the cam barrel  51  is seen from a plan view. 
     In this embodiment and within the range of movement of the movable focus unit  94  in the optical axis direction, the inclination (i.e., the surface and/or the pressure angle) of the cam grooves  51   d  is lower at a position where the load torque produced by the biasing member  98  is relatively high; in other word, the surface and/or the pressure angle of the cam grooves  51   d  is lower at a position where the biasing force of the biasing member  98  is relatively large. The surface and/or the pressure angle of the cam grooves  51   d  is higher at a position where the load torque produced by the biasing member  98  is relatively low; in other word, the surface and/or the pressure angle of the cam grooves  51   d  is higher at a position where the biasing force of the biasing member  98  is relatively small. Therefore, the load torque obtained by combining the load torque produced by the cam grooves  51   d  and the load torque produced by the biasing member  98  fluctuate very little according to the position of the movable focus unit  94  in the optical axis direction. 
     In this embodiment, unlike in the first embodiment, the “set speed” of the focus motor  64  is set to be constant regardless of the position of the movable focus unit  94  in the optical axis direction. The “set speed” is set so that step-out will not occur regardless of the position of the movable focus unit  94  in the optical axis direction. 
     Even though the “set speed” of the focus motor  64  is set to be constant, the speed during movement of the movable focus unit  94  changes with the position of the movable focus unit  94  in the optical axis direction. That is, the speed during movement of the movable focus unit  94  changes according to the inclination (surface and/or pressure angle) of the cam grooves  51   d.  Of the positions of the movable focus unit  94  in the optical axis direction, the speed during movement of the movable focus unit  94  is lower at a position where the load torque produced by the biasing member  98  is high and is higher at a position where the load torque produced by the biasing member  98  is low. 
     Therefore, just as in the first embodiment, even when the load torque produced by the biasing member  98  changes according to the position of the movable focus unit  94 , the focus motor  64  that drives the movable focus unit  94  (or the focusing lens) can still be prevented from going out of step, and the movement speed of the focusing lens can be raised. 
     Processing pertaining to a variable set speed method can also be executed using the flowchart in  FIG. 14 , by storing the speed during movement of the movable focus unit  94  with respect to the position of the movable focus unit  94  in the optical axis direction (which is affected by the inclination (surface and/or pressure angle) of the cam grooves  51   d ) as a speed switching table. 
     Also, the cam grooves  51   d  can be formed so that the total load torque obtained by combining the load torque produced by the cam grooves  51   d  and the load torque produced by the biasing member  98  is constant over the entire range of the movable focus unit  94  in the optical axis direction. 
     Other Embodiments  
     Embodiments of the present invention are not limited to those given above and various changes and modifications are possible without departing from the gist of the present invention. Also, the embodiments given above are basically just preferred examples, and the scope of the present invention, objects that the present invention is applied to, and the use or purpose of the present invention are not limited to these embodiments. 
     (1) 
     In the above embodiments, the digital camera  1  was capable of capturing both moving and still pictures, but can instead be capable of capturing just still pictures, or just moving pictures. 
     (2) 
     In the above embodiments, the digital camera  1  can be, for example, a digital still camera, a digital video camera, a mobile telephone equipped with a camera, or a PDA equipped with a camera. 
     (3) 
     The above-mentioned digital camera  1  did not have a quick return mirror, but a quick return mirror can be installed as in a conventional single reflex lens camera. Also, the lens barrel and the camera body can be integrated in the digital camera  1 . 
     (4) 
     The configuration of the optical system L is not limited to that in the embodiments. For example, the fifth lens L 5  and the sixth lens L 6  may not be joined together. Also, the optical system L can be a zoom lens with which the focal distance can be changed. The focusing lens can be just one part of the optical system L, rather than the entire optical system L. 
     (5) 
     In the above embodiments, the biasing member  98  was a single coil spring, and its center was disposed so as to coincide with the optical axis AZ, but a plurality of biasing members can be disposed within the X-Y plane. Also, these do not necessarily have to be coil springs. Also, the biasing member  98  can bias the movable focus unit  94  to the rear. 
     (6) 
     In the above embodiments, contrast auto-focusing was used, but with a phase difference method of auto-focusing, a variable set speed method can be employed in driving the movable focus unit  94  to the predicted focal position. More specifically, in a phase difference method of auto-focusing, the actual drive speed of the focus motor  64  can be changed according to the position of the movable focus unit  94  in the optical axis direction in driving the movable focus unit  94  to the predicted focal position. 
     (7) 
     The variable set speed method may not be used in all of the first focus drive operation, second focus drive operation, and third focus drive operation discussed above, and just in one or two of them, the variable set speed method can be used. For instance, in the first focus drive operation, it may not be used. 
     (8) 
     In the above embodiments, the “set speed” of the speed switching table was made variable in three stages, but the switching points can be set as desired, and the switching of the “set speed” can be carried out continuously. 
     (9) 
     In the above embodiments, the movable focus unit  94  was driven by a force obtained by converting the output of the focus motor  64  with a cam mechanism, but this is the only option, and the output of the focus motor  64  can be converted into the rectilinear force of a nut via a screw and nut, and the movable focus unit  94  driven by this rectilinear force. Also, the output of the focus motor  64  can be converted into some other force, and the movable focus unit  94  driven by this force. 
     (10) 
     Steps  3 ,  4 ,  5 ,  7 , and  8  in the flowchart of processing pertaining to the variable set speed method can be executed by the body microprocessor  10  rather than by the lens microprocessor  40 . For example, information in the speed switching table is sent from the lens microprocessor  40  to the body microprocessor  10  at the point when the interchangeable lens unit  2  is mounted to the camera body  3 , etc. Then, in step  3 , the body microprocessor  10  determines the “set speed” by referring to the speed switching table, ant then the determined “set speed” is sent from the body microprocessor  10  to the lens microprocessor  40 . 
     (11) 
     In the above embodiments, during the first, second, and third focus drive operations, the speed of the focus motor  64  indicated by the body microprocessor  10  to the lens microprocessor  40  (command speed) was greater than the “set speed” that was placed in the speed switching table, and as a result, the “set speed” was employed as the actual drive speed of the focus motor  64 . However, the command speed can be slower than the “set speed” in at least one of the first, second, and third focus drive operations, and as a result, the command speed can be employed rather than the “set speed” as the actual drive speed of the focus motor  64 . The command speed is decided by the body microprocessor  10  as the maximum speed at which the focus motor  64  will not go out of step, according to information indicating the characteristics of the interchangeable lens unit  2 . Therefore, the command speed can be slower than the “set speed” depending on the characteristics of the interchangeable lens unit  2  mounted to the camera body  3 . 
     Features of Embodiments  
     Features of the above embodiments are listed below. The inventions encompassed by the above embodiments are not limited to what is given below. The parts in parentheses which the various components are followed by are specific examples of those components given to facilitate an understanding of the features. Those components are not limited to those specific examples. Also, to obtain the effects listed for the various features, a component other than that of the discussed features can be modified or eliminated. 
     (F1) 
     The lens barrel (interchangeable lens unit  2 ) pertaining to the first feature includes: 
     a focusing lens (optical system L) that changes its state of focus by moving in the optical axis direction; 
     a driver (focus motor  64 ) that outputs a drive force for driving the focusing lens in the optical axis direction; and 
     a controller (lens microprocessor  40 ) that controls the driving speed (the “set speed” • the number of drive pulses per unit of time) of the driver, 
     wherein the focusing lens is subject to a load as it moves in the optical axis direction, the load is dependent upon the position of the focusing lens in the optical axis direction, and 
     the controller controls the driver so that when the focusing lens is at a position where the load is small the driving speed (the “set speed” • the number of drive pulses per unit of time) is larger than the driving speed (the “set speed” • the number of drive pulses per unit of time) when the focusing lens is at a position where the load is large. 
     With this lens barrel, the motor that drives the focusing lens can be prevented from going out of step while the movement speed of the focusing lens can be raised. 
     (F2) 
     The lens barrel pertaining to the second feature is the lens barrel pertaining to the first feature, further including a biasing member that biases the focusing lens (optical system L) in the optical axis direction. 
     With this lens barrel, degradation of the optical performance of the focusing lens can be suppressed, while the same effect as with the lens barrel pertaining to the first feature can be obtained. 
     (F3) 
     The lens barrel pertaining to the third feature is the lens barrel pertaining to the first or second feature, 
     further including a cam mechanism (cam grooves  51   d,  cam pins  54   c ) that is subject to the drive force and guides the focusing lens (optical system L) in the optical axis direction, 
     wherein the cam mechanism has a cam groove ( 51   d ) and a cam follower (cam pins  54   c ) that is inserted into the cam groove, and 
     the cam groove ( 51   d ) extending in such a way that the amount of movement of the focusing lens (optical system L) in the optical axis direction resulting from a unit output driver force of the driver (focus motor  64 ) varies with the position of the focusing lens (optical system L) in the optical axis direction. 
     With this lens barrel, design latitude can be ensured for the cam mechanism or the optical system, while the same effect as with the lens barrel pertaining to the first feature can be obtained. 
     (F4) 
     The lens barrel pertaining to the fourth feature is the lens barrel pertaining to the third feature, 
     wherein the cam groove ( 51   d ) extends such that the surface and/or pressure angle varies with the position in the optical axis direction. 
     (F5) 
     The lens barrel pertaining to the fifth feature is the lens barrel pertaining to any of the first to fourth features, 
     further including a memory component (memory  40   a ) that stores the relationship between the drive speed (“set speed” • number of drive pulses) and the position of the focusing lens (optical system L) in the optical axis direction. 
     With this lens barrel, control with the controller is easier. 
     (F6) 
     The lens barrel pertaining to the sixth feature is the lens barrel pertaining to any of the first to fifth features, 
     wherein the driver (focus motor  64 ) is a stepping motor. 
     With this lens barrel, controlling the position of the focusing lens is easier. 
     (F7) 
     The lens barrel (interchangeable lens unit  2 ) pertaining to the seventh feature includes: 
     a focusing lens (optical system L) that changes its state of focus by moving in the optical axis direction; 
     a driver (focus motor  64 ) that outputs a drive force for driving the focusing lens in the optical axis direction; 
     a biasing member that biases the focusing lens (optical system L) in the optical axis direction; and 
     a cam mechanism (cam grooves  51   d,  cam pins  54   c ) that is subject to the drive force and guides the focusing lens (optical system L) in the optical axis direction, 
     wherein the cam mechanism has a cam groove ( 51   d ) and a cam follower (cam pins  54   c ) that is inserted into the cam groove, and 
     the cam groove ( 51   d ) extends in such a way that the amount of drive of the focusing lens (optical system L) in the optical axis direction resulting from a unit output driving force of the driver (focus motor  64 ) becomes relatively small when the focusing lens is at a position where the biasing force of the biasing member is relatively large, and becomes relatively large when the focusing lens is at a position where the biasing force of the biasing member is relatively small, within the range of movement of the focusing lens (optical system L) in the optical axis direction. 
     With this lens barrel, degradation of the optical performance of the focusing lens can be suppressed, while the motor that drives the focusing lens can be prevented from going out of step, and the movement speed of the focusing lens can be raised 
     (F8) 
     The lens barrel (interchangeable lens unit  2 ) pertaining to the eighth feature is the lens barrel pertaining to the seventh feature, 
     wherein the cam groove ( 51   d ) extends in such a way that the surface and/or the pressure angle becomes relatively small at a position where the biasing force of the biasing member is relatively large, and becomes relatively large at a position where the biasing force of the biasing member is relatively small. 
     (F9) 
     The lens barrel (interchangeable lens unit  2 ) pertaining to the ninth feature is the lens barrel pertaining to the seventh or eighth feature, 
     wherein the driver is a stepping motor. 
     With this lens barrel, controlling the position of the focusing lens is easier. 
     (F10) 
     The imaging device (digital camera  1 ) pertaining to the tenth feature includes: 
     a focusing lens (optical system L) that changes its state of focus by moving in the optical axis direction; 
     a driver (focus motor  64 ) that outputs a drive force for driving the focusing lens in the optical axis direction; and 
     a controller (lens microprocessor  40 ) that controls the driving speed (“set speed” • number of drive pulses per unit of time) of the driver, 
     wherein the focusing lens is subject to a load and moves in the optical axis direction, the load is dependent upon the position of the focusing lens in the optical axis direction, and 
     the controller controls the driver so that when the focusing lens is at a position where the load is small the driving speed (“set speed” • number of drive pulses per unit of time) is larger than the driving speed (“set speed” • number of drive pulses per unit of time) when the focusing lens is at a position where the load is large. 
     With this lens barrel, the motor that drives the focusing lens can be prevented from going out of step while the movement speed of the focusing lens can be raised. 
     GENERAL INTERPRETATION OF TERMS  
     In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms “including,” “having,” and their derivatives. Also, the terms “part,” “section,” “portion,” “member,” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. Also as used herein to describe the above embodiments, the following directional terms “forward”, “rearward”, “above”, “downward”, “vertical”, “horizontal”, “below” and “transverse” as well as any other similar directional terms refer to those directions of an imaging device and/or lens barrel equipped with a focusing lens and a driver for driving the focusing lens. Accordingly, these terms, as utilized to describe the above embodiments should be interpreted relative to an imaging device and/or lens barrel equipped with a focusing lens and a driver for driving the focusing lens. 
     Moreover, the term “configured” as used herein to describe a component, section, or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. 
     The term “detect” as used herein to describe an operation or function carried out by a component, a section, a device or the like includes a component, a section, a device or the like that does not require physical detection, but rather includes determining, measuring, modeling, predicting or computing or the like to carry out the operation or function. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope of the invention is not limited to the disclosed embodiments.