Abstract:
A lens barrel including: an imaging optical system; a tube configured to contain the imaging optical system; a holder configured to hold a lens included in the imaging optical system in the tube in such a way that the lens is movable along a direction of an optical axis of the imaging optical system; a sensor unit configured to output a cyclic detection signal whose peak value changes depending on a movement amount of the holder; a memory unit configured to store a relationship between peak values of the detection signal and a position of the holder in the direction of the optical axis in advance; and an arithmetic processor configured to calculate a position of the holder in the direction of the optical axis from a peak value of a detection signal detected by the sensor unit in movement of the holder based on the relationship stored in the memory unit.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2008-008689, filed in the Japan Patent Office on Jan. 18, 2008, the entire contents of which being incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to lens barrels and imaging devices, and particularly to a lens barrel including a position detecting mechanism that detects the position of an imaging optical system including a focus lens, a zoom lens, and so on, and an imaging device including this lens barrel. 
         [0004]    2. Description of the Related Art 
         [0005]    In a video camera device, a movable lens for zooming and a movable lens for focusing are disposed inside a tube in order to achieve a zoom function and an autofocus function, and a drive unit for moving these lenses along the optical axis direction is provided. For accurate control of the driving of the movable lens, the position of the movable lens needs to be accurately detected. 
         [0006]    In a related art, the position detection for the movable lens is carried out by a position detecting sensor. As shown in  FIG. 15 , the position detecting sensor is composed of a position detecting element  6  attached to a tube  4  as a fixed part and a position detection magnet  1  that is so attached to a lens holder  2  for an optical lens  3  as to face the position detecting element  6  and extend along the movement direction of the lens holder  2 . 
         [0007]    N and S poles of the position detection magnet  1  are so magnetized as to be alternately arranged along the extension direction of the position detection magnet  1  as shown in  FIG. 16 . 
         [0008]    As the position detecting element  6 , an MR sensor (magnetoresistive element) is used. The resistance of the magnetoresistive element changes in response to change in a magnetic field. Therefore, upon the movement of the position detection magnet  1  in linkage with the movement of the movable lens, the magnetic field that acts on the position detecting element  6  opposed to the position detection magnet  1  changes and the magnetoresistance value changes. 
         [0009]    In linkage with the change in the magnetoresistance value, as shown in  FIG. 17 , the position detecting element  6  outputs an A-phase detection signal A sin θ that changes in the manner of a sine wave with a predetermined cycle and a B-phase detection signal A cos θ that changes in the manner of a cosine wave that is different in the phase by λ/4 from the A-phase detection signal. The position of the movable lens is detected based on these two detection signals. 
         [0010]    The lens holder  2  is so held as to be movable in the tube  4  along the direction of an optical axis L by a guide shaft  5  provided in the tube  4  in parallel to the optical axis L. This lens holder  2  is driven along the direction of the optical axis L by a linear actuator. The linear actuator is composed of a drive coil  7 , a drive magnet  8 , a grounded yoke  9 , and an opposed yoke  10 . 
         [0011]    Examples of documents relating to the related art include Japanese Patent Laid-open No. 2006-10568 (Patent document 1, hereinafter), Japanese Patent Laid-open No. 2004-221527 (Patent document 2, hereinafter), and Japanese Patent No. 3177931. 
       SUMMARY OF THE INVENTION 
       [0012]    However, it is impossible to know the absolute position of the movable lens from only the above-described detection signals A sin θ and A cos θ. 
         [0013]    Therefore, in the above-described position detection, the position serving as the reference (reference position) needs to be detected in order to convert a measured position into an absolute value. Patent document 1 discloses a configuration for detecting the reference position. Specifically, in this configuration, a lens holder that moves together with an optical lens is provided with a light-blocking part, and a sensor for resetting such as a photointerrupter is disposed on a fixed part such as a tube. 
         [0014]    In the position detecting unit with this configuration, the sensor output is changed from High to Low or from Low to High in response to blocking of the optical path of the photointerrupter by the light-blocking part in the movement of the lens holder. The position corresponding to the timing of the change in the sensor output is detected, and the detected position is defined as the reference position. The position of the lens holder is detected based on this reference position information and the peak value of the output from the position detecting element. 
         [0015]    However, for the position detecting unit with this configuration, the reset sensor for the reference position detection is needed outside in addition to the position detecting sensor (the position detecting element  6  and the position detection magnet  1 ). This increases the size of the entire position detecting system, and thus leads to problems such as cost increase. 
         [0016]    As a system to address such a problem, the position detecting system disclosed in Patent document 2 is known. In this position detecting system, a lens holder is brought into contact with a mechanical mechanism such as a mechanical stopper, and the position corresponding to the contact is defined as the reference position. For this system, a reset sensor or the like for the reference position detection does not need to be provided outside. 
         [0017]    However, in both the schemes of Patent documents 1 and 2, the detection of the reference position is essential and it is impossible to know the absolute position of the lens holder from only the output signal from the position detecting element  6 . Under such a condition, if force such as external shock is applied to the lens barrel in the activation thereof, the stop position of the lens holder will be moved and the accurate position of the lens holder will be lost, so that the lens holder will be out of control. Furthermore, in order for the lens holder to revert to normal operation again, reset operation for detecting the reference position is necessary. In addition, the operation of detecting the reference position needs to be carried out also in the activation of a camera, which leads to a problem that the activation operation of the camera is slow. 
         [0018]    There is a need for the present invention to provide a lens barrel that allows achievement of the absolute position of an imaging optical system from only the output of a sensor unit without acquisition of reference position information, and an imaging device including the lens barrel. 
         [0019]    According to an embodiment of the present invention, there is provided a lens barrel including an imaging optical system, a tube configured to contain the imaging optical system, a holder configured to hold at least one lens included in the imaging optical system in the tube in such a way that the lens is movable along the direction of the optical axis of the imaging optical system, and a sensor unit configured to output at least one cyclic detection signal whose peak value changes depending on the movement amount of the holder. The lens barrel further includes a memory unit configured to store the relationship between peak values of the detection signal and the position of the holder in the direction of the optical axis in advance, and an arithmetic processor configured to calculate the position of the holder in the direction of the optical axis from a peak value of a detection signal detected by the sensor unit in movement of the holder based on the relationship stored in the memory unit. 
         [0020]    According to another embodiment of the present invention, there is provided an imaging device including a lens barrel. The lens barrel includes an imaging optical system, a tube configured to contain the imaging optical system, a holder configured to hold at least one lens included in the imaging optical system in the tube in such a way that the lens is movable along the direction of the optical axis of the imaging optical system, and a sensor unit configured to output at least one cyclic detection signal whose peak value changes depending on the movement amount of the holder. The lens barrel further includes a memory unit configured to store the relationship between peak values of the detection signal and the position of the holder in the direction of the optical axis in advance, and an arithmetic processor configured to calculate the position of the holder in the direction of the optical axis from a peak value of a detection signal detected by the sensor unit in movement of the holder based on the relationship stored in the memory unit. 
         [0021]    In the lens barrel and the imaging device according to the embodiments of the present invention, the sensor unit outputs at least one detection signal that cyclically changes depending on the movement amount of the holder, and the peak value of the detection signal changes in such a manner as to decrease or increase depending on the amount of the movement of the holder along the direction of the optical axis. Furthermore, the peak value of the detection signal detected first by the sensor unit at the time of activation of the drive unit is compared with the respective peak values recorded in the memory unit by the arithmetic processor. Subsequently, the position of the holder in the direction of the optical axis, i.e. the position of the imaging optical system, is calculated by using the point that matches the detected peak value as the reference. 
         [0022]    Thus, the lens barrel and the imaging device according to the embodiments of the present invention make it possible to know the absolute position of the imaging optical system from only the output of the sensor unit without operation of detecting the reference position in the related art, and detect the position of the imaging optical system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a perspective view of an imaging device according to a first embodiment of the present invention, 
           [0024]      FIG. 2  is a block diagram showing the configuration of the imaging device according to the first embodiment of the present invention; 
           [0025]      FIG. 3  is a schematic sectional view of a lens barrel according to the first embodiment of the present invention; 
           [0026]      FIG. 4  is a schematic sectional view of major part of the lens barrel according to the first embodiment of the present invention; 
           [0027]      FIG. 5  is a waveform diagram for explaining detection signals output from a position detecting element according to the first embodiment of the present invention; 
           [0028]      FIG. 6  is a flowchart showing position detecting operation according to the first embodiment of the present invention; 
           [0029]      FIG. 7  is a schematic sectional view of a lens barrel according to a second embodiment of the present invention; 
           [0030]      FIG. 8  is an explanatory diagram showing the relationship between the magnetization pattern of a position detection magnet and a position detecting element according to the second embodiment of the present invention; 
           [0031]      FIG. 9  is an explanatory diagram showing the configuration of a sensor unit in a lens barrel according to a third embodiment of the present invention; 
           [0032]      FIG. 10  is a schematic sectional view of a lens barrel according to a fourth embodiment of the present invention; 
           [0033]      FIG. 11  is a schematic sectional view of major part of the lens barrel according to the fourth embodiment of the present invention; 
           [0034]      FIG. 12  is a schematic sectional view of major part of a lens barrel according to a fifth embodiment of the present invention; 
           [0035]      FIG. 13  is a schematic sectional view of major part of a lens barrel according to a sixth embodiment of the present invention; 
           [0036]      FIG. 14  is a waveform diagram for explaining a detection signal output from a position detecting element according to the sixth embodiment of the present invention; 
           [0037]      FIG. 15  is a schematic sectional view of major part of a lens barrel in a related art; 
           [0038]      FIG. 16  is an explanatory diagram showing the relationship between the magnetization pattern of a position detection magnet and a position detecting element in the related art; and 
           [0039]      FIG. 17  is a waveform diagram for explaining detection signals output from the position detecting element in the related art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0040]    Embodiments of the present invention will be described below with reference to the drawings. 
         [0041]      FIG. 1  is a perspective view of an imaging device  30  according to a first embodiment of the present invention.  FIG. 2  is a block diagram showing the configuration of the imaging device  30  according to the first embodiment. 
         [0042]    As shown in  FIG. 1 , the imaging device  30  of the present embodiment is a digital still camera and has a case  12  serving as the external package. In the present specification, the subject side is defined as the “front” and the opposite side is defined as the “back”. 
         [0043]    On the right side of the front face of the case  12 , a lens barrel  20  is provided in which an imaging optical system  14 , a drive unit  16  for the imaging optical system  14 , and a sensor unit  18  used to detect the position of the imaging optical system  14  are incorporated. At the backside end of the lens barrel  20 , an imaging element  111  (see  FIG. 2 ) that captures a subject image guided by the imaging optical system  14  is provided. A detection signal output from the sensor unit  18  is captured in an arithmetic processor  38   a  in a controller  38  (see  FIG. 2 ) to be described later. 
         [0044]    On the upper side of the front face of the case  12 , a flash unit  22  that emits flash light, an objective lens  23  of an optical finder, and so on are provided. In the present specification, the “front” refers to the subject side and the “back” refers to the image-formation side. 
         [0045]    A shutter button  24  is provided on the top face of the case  12 . Provided on the back face of the case  12  are an eyepiece window  25  of the optical finder, plural operation switches  26  for various kinds of operation such as turning-on/off of the power supply and switching between the imaging mode and the reproduction mode, and a display  27  (see  FIG. 2 ) that displays captured video. 
         [0046]    As shown in  FIG. 2 , the imaging device  30  includes the imaging element  111 , a memory medium  32 , an image processor  34 , a display processor  36 , the controller  38 , a memory unit  39 , and so on. 
         [0047]    The imaging element  111  is formed of a CCD (charge coupled device), a CMOS (complementary metal oxide semiconductor) sensor, or the like that has an imaging plane  111 A (see  FIG. 3 ) and captures a subject image formed on the imaging plane  111 A by the imaging optical system  14  to produce an imaging signal. 
         [0048]    The image processor  34  produces image data based on the imaging signal output from the imaging element  111  and records the image data in the memory medium  32 . 
         [0049]    The memory medium  32  is formed of e.g. a memory card loaded/removed in/from a memory slot provided in the case  12  or a memory incorporated in the case  12 . 
         [0050]    The display processor  36  causes the display  27  to display the image corresponding to the image data supplied from the image processor  34 . 
         [0051]    The controller  38  is formed of a CPU (central processing unit) or the like that controls the image processor  34 , the display processor  36 , and the drive unit  16  in response to operation of the operation switches  26  and the shutter button  24 . The controller  38  includes the arithmetic processor  38   a  that calculates the position of the imaging optical system  14  based on the detection signal output from the sensor unit  18 . The memory unit  39  is formed of a ROM (read only memory) or the like and stores therein the respective peak values of the detection signal cyclically output from the sensor unit  18  depending on the movement amount of the imaging optical system  14  and data such as position data associated with the peak values. 
         [0052]    The configuration of the lens barrel  20  will be described below. 
         [0053]      FIG. 3  is an explanatory diagram showing the schematic configuration of the lens barrel  20 . 
         [0054]    As shown in  FIG. 3 , the lens barrel  20  includes the imaging optical system  14 , the drive unit  16  for the imaging optical system  14 , the sensor unit  18  used to detect the position of the imaging optical system  14 , a fixed tube  104 , and so on. 
         [0055]    The imaging optical system  14  is contained in the tube  104  and has at least one optical lens  103  such as a focus lens and a zoom lens. The optical lens  103  is held by a lens holder  102  (equivalent to the holder set forth in the claims). The lens holder  102  is so held as to be movable in the tube  104  along the direction of an optical axis L without rattling and rotation by guide shafts  105  and  106  provided in the tube  104  in parallel to the optical axis L. The lens holder  102  including the imaging optical system is so configured as to be driven along the direction of the optical axis L by the drive unit  16  based on a linear actuator system. 
         [0056]    The drive unit  16  based on the linear actuator system includes a drive coil  107  fixed to the lens holder  102 , a drive magnet  108  that moves the lens holder  102  along the direction of the optical axis L based on effects of magnetic attraction and repulsion with respect to the drive coil  107 , and a grounded yoke  109  and an opposed yoke  110  that form the magnetic path between the drive coil  107  and the drive magnet  108 . The grounded yoke  109  is disposed on the opposite side to the drive coil  107  across the magnet  108 , and the opposed yoke  110  passes through the drive coil  107  wound into a ring shape. 
         [0057]    The imaging element  111  that captures a subject image guided by the optical lens  103  is provided on the tube  104 . 
         [0058]    The sensor unit  18  outputs detection signals shown in  FIG. 5 . Specifically, it outputs an A-phase detection signal A sin θ in the manner of a sine wave that cyclically changes depending on the amount of the movement of the lens holder  102  including the optical lens  103  along the direction of the optical axis L, and a B-phase detection signal A cos θ in the manner of a cosine wave that is different in the phase by λ/4 from the A-phase detection signal. The peak values of the detection signals A sin θ and A cos θ change in such a manner as to decrease or increase depending on the amount of the movement of the lens holder  102  along the direction of the optical axis L. 
         [0059]    As shown in  FIGS. 3 and 4 , the sensor unit  18  that outputs such detection signals includes a magneto-sensitive position detecting element  100  formed of an MR element provided on the inner wall of the tube  104 , and a position detection magnet  101  that is so provided on the lens holder  102  as to face the position detecting element  100  and extend along the movement direction of the lens holder  102 . 
         [0060]    N and S poles of the position detection magnet  101  are so magnetized as to be alternately arranged along the extension direction of the position detection magnet  101  (the direction of the optical axis L) similarly to those shown in  FIG. 16 . Furthermore, the position detection magnet  101  is so disposed as to be inclined at a predetermined angle θ to the line parallel to the optical axis L (the movement direction of the lens holder  102 ) along the movement direction of the lens holder  102 . In contrast, the position detecting element  100  opposed to the position detection magnet  101  is disposed in parallel to the optical axis L. 
         [0061]    The operation of the present embodiment will be described below. 
         [0062]    Upon the flowing of current through the drive coil  107 , thrust force parallel to the direction of the optical axis L is applied to the drive coil  107  in accordance with Fleming&#39;s left-hand rule due to magnetic flux passing between the opposed yoke  110  and the drive magnet  108 . Thus, the lens holder  102  including the optical lens  103  moves together with the drive coil  107  in the direction of the optical axis L along the guide shafts  105  and  106 . 
         [0063]    The position detection magnet  101  is inclined at the angle θ to the optical axis L along the movement direction of the lens holder  102 . Therefore, in the movement of the lens holder  102  along the optical axis L, difference arises in the distance between the position detection magnet  101  and the position detecting element  100  depending on the position of the lens holder  102  in the optical axis direction. 
         [0064]    As the distance between the position detection magnet  101  and the position detecting element  100  becomes larger, the magnetic field from the position detection magnet  101  to the position detecting element  100  becomes weaken. This allows the position detecting element  100  to output the sine-wave detection signal A sin θ and the cosine-wave detection signal A cos θ shown in  FIG. 5 , which cyclically change depending on the movement amount of the lens holder  102 . 
         [0065]    Specifically, positive peak values HP 1 , HP 2 , . . . and negative peak values LP 1 , LP 2 , . . . of the detection signals A sin θ and A cos θ change in linkage with the movement of the lens holder  102  along the direction of the optical axis L. More specifically, the peak values decrease along with the movement of the optical lens  103  in L 1  direction (wide-direction) along the optical axis L, whereas the peak values increase along with the movement of the optical lens  103  in L 2  direction (tele-direction) along the optical axis L. Thus, the absolute position of the lens holder  102  can be known from the respective peak values of the detection signals A sin θ and A cos θ. 
         [0066]    Details on how to know the absolute position of the lens holder  102  will be described below with reference to  FIG. 6 . 
         [0067]    The following description is based on an assumption that the position of the lens holder  102  exists between the positions corresponding to the peak values LP 2  and HP 3  shown in  FIG. 5  at the time of activation of the imaging device  30  for the start of imaging. 
         [0068]    Initially, e.g. at the time of factory shipment of the imaging device  30  including the lens barrel  20 , the lens holder  102  is actually moved by the drive unit  16  in the wide-direction or the tele-direction, and the respective peak values HP 1 , HP 2 , . . . and LP 1 , LP 2 , . . . of the detection signals A sin θ and A cos θ output from the position detecting element  100  are loaded in the arithmetic processor  38   a  of the controller  38 . Furthermore, the peak values HP 1 , HP 2 , . . . and LP 1 , LP 2 , . . . and data on the positions of the lens holder  102  in the direction of the optical axis L, determined corresponding to these peak values, are associated with each other and a table of the associated data is created and stored in the memory unit  39  (step S 1 ). 
         [0069]    Subsequently, the drive unit  16  is activated by giving an activation instruction to the drive unit  16  via the controller  38  through operation of the operation switches  26  or the like. This moves the lens holder  102  in the L 1  direction (wide-direction) or the L 2  direction (tele-direction) along the optical axis L shown in  FIG. 4 . Thus, the value of the detection signal indicating the position of the lens holder  102  is shifted from a movement start point P 1  shown in  FIG. 5  in the arrowhead direction, and either the peak value LP 2  or HP 3  is detected by the position detecting element  100  (step S 2 ). 
         [0070]    Subsequently, the peak value detected by the position detecting element  100  is captured in the arithmetic processor  38   a  and is compared with the respective peak values HP 1 , HP 2 , . . . and LP 1 , LP 2 , . . . stored in the memory unit  39  in advance (step S 3 ). 
         [0071]    Subsequently, on the basis of the point that matches the detected peak value, i.e. on the basis of the position in the direction of the optical axis L corresponding to the peak value in the memory unit  39 , the position of the lens holder  102  in the optical axis direction resulting from the subsequent movement thereof along the direction of the optical axis L is calculated by the arithmetic processor  38   a.    
         [0072]    For example, if the detected peak value is HP 3 , the position of the lens holder  102  resulting from the subsequent movement thereof along the direction of the optical axis L can be detected on the basis of the data on the position in the direction of the optical axis L corresponding to the peak value HP 3  in the memory unit  39 . 
         [0073]    As described above, in the present embodiment, the sensor unit  18  is so configured that the peak values of the detection signals A sin θ and A cos θ output from the position detecting element  100  decrease or increase in linkage with the movement of the optical lens  103  along the direction of the optical axis L. Furthermore, on the basis of the point that matches the peak value detected first by the position detecting element  100  at the time of activation of the lens barrel, i.e. on the basis of the position in the direction of the optical axis L corresponding to the detected peak value, which matches a peak value recorded in the memory unit  39  in advance, the position of the lens holder  102  in the optical axis direction resulting from the movement thereof along the direction of the optical axis L subsequent to the activation is calculated by the arithmetic processor  38   a . These features provide the following advantages. 
         [0074]    a) In the position detection, the absolute position of the lens holder  102  can be detected without detecting the reference position for converting a measured position into an absolute value. In addition, the circuit that detects the reference position for converting a measured position into an absolute value is unnecessary. 
         [0075]    b) Because the reference position for converting a measured position into an absolute value does not need to be detected, a sensor for resetting such as a photointerrupter does not need to be disposed on the fixed part such as the tube and thus space saving and cost reduction can be achieved. 
         [0076]    c) Because the reference position for converting a measured position into an absolute value does not need to be detected, a light-blocking part does not need to be provided on the lens holder  102 . Thus, space saving and weight reduction can be achieved, and the power for driving the lens holder  102  can be decreased. 
         [0077]    d) Operation of temporarily bringing the lens holder in contact with a mechanical mechanism such as a mechanical stopper to thereby detect the reference position is unnecessary unlike the related art, and therefore fast activation operation is possible. 
         [0078]    e) Because the absolute position of the lens holder  102  can be detected without detecting the reference position, the present position will be not lost even when the lens barrel receives external shock or the like, and thus reset operation is unnecessary unlike the related art. 
         [0079]    f) Because the absolute position is detected through recording of the respective peak values of the output from the position detecting element, the movement direction can also be detected by reading the peak values on both the sides of the movement start point. 
         [0080]    g) Because the position of the optical lens is detected by using two detection signals A sin θ and A cos θ output from the position detecting element  100  with phase difference, the resolution of the position detection of the optical lens is high. 
       Second Embodiment 
       [0081]    A lens barrel  20  according to a second embodiment of the present invention will be described below with reference to  FIGS. 7 and 8 . 
         [0082]    In the description of the following embodiments, the same component as that in the first embodiment is given the same numeral and the description thereof is omitted, and the part different from the first embodiment will be mainly described. 
         [0083]    The second embodiment is different from the first embodiment in the configuration of a sensor unit  18 . 
         [0084]    Specifically, as is apparent from  FIGS. 7 and 8 , a position detection magnet  101  of the sensor unit  18  is so mounted on a lens holder  102  as to be parallel to the movement direction of the lens holder  102  (the direction of an optical axis L). 
         [0085]    N and S poles of the position detection magnet  101  are so magnetized as to be alternately arranged along the extension direction of the position detection magnet  101  similarly to those shown in  FIG. 16 . In addition, the intensity of the magnetic field by the N and S poles increases or decreases from one end toward the other end of the position detection magnet  101  along the extension direction thereof. 
         [0086]    In the lens barrel  20  according to the second embodiment, although the position detecting element  100  and the position detection magnet  101  are parallel to each other, the intensity of the magnetic field from the position detection magnet  101  to the position detecting element  100  changes depending on the position of the lens holder  102  in the optical axis direction, because the intensity of the magnetic field by the N and S poles alternately arranged along the extension direction of the position detection magnet  101  increases or decreases from one end toward the other end of the position detection magnet  101  along the extension direction thereof. 
         [0087]    As a result, the peak values of the A-phase and B-phase detection signals A sin θ and A cos θ output from the position detecting element  100  can be decreased or increased in linkage with the movement of the optical lens  103  along the direction of the optical axis L, similarly to the detection signals shown in  FIG. 5 . This feature provides the same advantages as those of the first embodiment. 
       Third Embodiment 
       [0088]    A sensor unit used for a lens barrel according to a third embodiment of the present invention will be described below with reference to  FIG. 9 . 
         [0089]    A sensor unit  200  of the present embodiment is based on an optical linear scale. As shown in  FIG. 9 , the sensor unit  200  includes a light-emitting element  202  formed of an LED (light-emitting diode) or the like for position detection, a slit plate  203 , a measurement plate  204 , a pair of position detecting elements  206  each formed of a photodiode or the like, and an optical filter  207 . 
         [0090]    The light-emitting element  202  is provided on a lens holder (not shown). The pair of position detecting elements  206  are so attached to a tube (not shown) as to be opposed to the light-emitting element  202 . 
         [0091]    The slit plate  203  is so provided close to the light-emitting element  202  as to face the light-emitting element  202  and extend along the direction of an optical axis L. In this slit plate  203 , plural slits  201  to allow A-phase and B-phase detection signals output from the pair of position detecting elements  206  to cyclically change as shown in  FIG. 5  are formed with constant intervals of λ. 
         [0092]    The measurement plate  204  is so provided close to the position detecting elements  206  as to face the position detecting elements  206  and extend along the direction of the optical axis L. At the position opposed to the position detecting elements  206  on this measurement plate  204 , a pair of slits  205  are formed with an interval of λ/4. The pair of slits  205  guide the light that has passed through the slits  201  of the slit plate  203  to the pair of position detecting elements  206  to thereby allow the position detecting elements  206  to output the A-phase and B-phase detection signals. 
         [0093]    The optical filter  207  is so provided between the light-emitting element  202  and the pair of position detecting elements  206  as to extend along the direction of the optical axis L. The optical filter  207  serves to cause the peak values of the detection signals, which cyclically change depending on the amount of the movement of the lens holder along the direction of the optical axis L, to change in such a manner as to decrease or increase in linkage with the movement of the lens holder along the direction of the optical axis L. For this purpose, the optical filter  207  is so configured that the light-transmission amount thereof increases or decreases from one end toward the other end of the optical filter  207  along the extension direction thereof. 
         [0094]    In the sensor unit  200  based on the optical linear scale system, the slit plate  203  and the measurement plate  204  are disposed between the light-emitting element  202  and the pair of position detecting elements  206 , and the light-transmission amount of the optical filter  207  interposed between the slit plate  203  and the measurement plate  204  varies depending on the position of the lens holder in the optical axis direction. Thus, similarly to the first embodiment, signals with waveforms similar to those of the detection signals A sin θ and A cos θ shown in  FIG. 5  can be achieved as the A-phase and B-phase detection signals output from the pair of position detecting elements  206 . 
         [0095]    Therefore, the third embodiment can also achieve the same advantages as those of the first embodiment. 
       Fourth Embodiment 
       [0096]    A lens barrel  20  according to a fourth embodiment of the present invention will be described below with reference to  FIGS. 10 and 11 . 
         [0097]    The fourth embodiment is different from the first embodiment in the configuration of a sensor unit  18 . 
         [0098]    Specifically, as is apparent from  FIGS. 10 and 11 , a position detecting element  100  of the sensor unit  18  is so provided on a tube  104  as to be inclined along the movement direction of a lens holder  102  (the direction of an optical axis L) similarly to a position detection magnet  101 . 
         [0099]    In the lens barrel  20  of this fourth embodiment, although the position detecting element  100  is inclined along the movement direction of the lens holder  102  similarly to the position detection magnet  101 , difference arises in the distance between the position detection magnet  101  and the position detecting element  100  depending on the position of the lens holder  102  in the optical axis direction. Thus, the intensity of the magnetic field that acts on the position detecting element  100  from the position detection magnet  101  changes depending on the position of the lens holder  102  in the optical axis direction. 
         [0100]    This allows the position detecting element  100  to output A-phase and B-phase detection signals A sin θ and A cos θ whose peak values decrease or increase in linkage with the movement of the optical lens  103  similarly to the detection signals shown in  FIG. 5 . This feature provides the same advantages as those of the first embodiment. 
       Fifth Embodiment 
       [0101]    A lens barrel  20  according to a fifth embodiment of the present invention will be described below with reference to  FIG. 12 . 
         [0102]    The fifth embodiment is different from the first embodiment in the configuration of a sensor unit  18 . 
         [0103]    Specifically, as is apparent from  FIG. 12 , a position detection magnet  101  of the sensor unit  18  is disposed in parallel to the direction of an optical axis L of an optical lens  103 . In addition, this position detection magnet  101  is supported by a guide component  112  different from guide shafts for a lens holder  102  so that the position detection magnet  101  can move in such a direction as to become closer to or farther from a position detecting element  100  along with the movement of the lens holder  102  in the direction of the optical axis L. 
         [0104]    This guide component  112  is inclined at an angle θ to the line parallel to the optical axis L along the movement direction of the lens holder  102 . 
         [0105]    N and S poles of the position detection magnet  101  are so magnetized as to be alternately arranged along the extension direction of the position detection magnet  101  similarly to those shown in  FIG. 16 . The position detecting element  100  is so provided on a tube  104  as to be parallel to the surfaces of the N and S poles of the position detection magnet  101 . 
         [0106]    In the lens barrel  20  of this fifth embodiment, because the position detection magnet  101  can move in such a direction as to become closer to or farther from the position detecting element  100  along with the movement of the lens holder  102  in the direction of the optical axis L, difference arises in the distance between the position detection magnet  101  and the position detecting element  100  depending on the position of the lens holder  102  in the optical axis direction. Thus, the intensity of the magnetic field that acts on the position detecting element  100  from the position detection magnet  101  changes depending on the position of the lens holder  102  in the optical axis direction. 
         [0107]    This allows the position detecting element  100  to output A-phase and B-phase detection signals A sin θ and A cos θ whose peak values decrease or increase in linkage with the movement of the optical lens  103  similarly to the detection signals shown in  FIG. 5 . This feature provides the same advantages as those of the first embodiment. 
       Sixth Embodiment 
       [0108]    A lens barrel  20  according to a sixth embodiment of the present invention will be described below with reference to  FIGS. 13 and 14 . 
         [0109]    The sixth embodiment is different from the first embodiment in the configuration of a sensor unit  18 . 
         [0110]    Specifically, as is apparent from  FIGS. 13 and 14 , a position detecting element  100  of the sensor unit  18  is so configured as to be capable of outputting a detection signal A sin θ of only one phase equivalent to the A phase. A position detection magnet  101  is so provided on a lens holder  102  as to be inclined at an angle θ to the line parallel to an optical axis L along the movement direction of the lens holder  102 . 
         [0111]    In the lens barrel  20  of this sixth embodiment, because the position detection magnet  101  is inclined along the movement direction of the lens holder  102 , difference arises in the distance between the position detection magnet  101  and the position detecting element  100  depending on the position of the lens holder  102  in the optical axis direction. Thus, the intensity of the magnetic field that acts on the position detecting element  100  from the position detection magnet  101  changes depending on the position of the lens holder  102  in the optical axis direction. 
         [0112]    This allows the position detecting element  100  to output the detection signal A sin θ in the manner of a sine wave that cyclically changes, like that shown in  FIG. 14 . Specifically, positive peak values HP 1 , HP 2 , . . . and negative peak values LP 1 , LP 2 , . . . of this detection signal A sin θ change in linkage with the movement of the lens holder  102  along the direction of the optical axis L. More specifically, the peak value decreases along with the movement of the optical lens  103  in L 1  direction (wide-direction) along the optical axis L. 
         [0113]    Therefore, the absolute position and the movement direction of the lens holder  102  can be known from the respective peak values of the detection signal A sin θ of only one phase. Thus, the same advantages as those of the first embodiment can be achieved by using the sensor unit that outputs the one-phase detection signal. 
         [0114]    In the above description of the respective embodiments, a digital still camera is employed as an example of the imaging device. However, it should be noted that the embodiments of the present invention can be applied to various imaging devices such as video cameras, camera-equipped cellular phones, PDAs, and portable electronic devices. 
         [0115]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.