Patent Publication Number: US-2021181457-A1

Title: Optical device and endoscope

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of PCT/JP2019/005364 filed on Feb. 14, 2019 and claims benefit of Japanese Application No. 2018-158411 filed in Japan on Aug. 27, 2018, the entire contents of which are incorporated herein by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical device capable of changing an optical function by moving a moving frame holding an optical member in an optical axis direction by magnetic force. 
     2. Description of the Related Art 
     In a conventionally known image pickup apparatus for which downsizing is requested, in particular, such as a portable terminal equipped with a camera or an endoscope, a two-focal-point switching optical device capable of switching the focal point of an image pickup optical system by using a magnetic actuator has been employed. 
     In such an optical device, a predetermined clearance is provided between an inner peripheral surface of a fixed frame and an outer peripheral surface of a moving frame so that the moving frame can slide inside the fixed frame. However, such a clearance potentially causes backlash of the moving frame and affects an optical property. 
     To avoid this issue, for example, Japanese Patent Application Laid-Open Publication No. 2017-63845 discloses a technology of an optical device including: an objective lens; a moving lens; a moving frame made of a magnetic body; a holding frame holding the moving frame so that the moving frame can freely move forward and backward; a pair of magnets provided on an outer periphery of the holding frame; a yoke provided between the pair of magnets; and a coil provided closer to the holding frame than the yoke. The yoke includes a frame part that covers an outer periphery of the coil, and a yoke convex portion partially formed in a circumferential direction of the frame part and protruding at an outer peripheral surface of the holding frame at front and back ends of the frame part to reduce a distance between the frame part and the outer peripheral surface. Accordingly, backlash when the moving frame is moved by using magnetic force is prevented with a simple configuration. 
     SUMMARY OF THE INVENTION 
     An optical device according to an aspect of the present invention includes: an optical system including a moving lens that is movable in a direction of an optical axis; a moving frame formed of a magnetic body material and holding the moving lens; a holding frame formed of a non-magnetic body material in a tubular shape and holding the moving frame on an inner peripheral surface so that the moving frame is movable along the optical axis; a first magnet and a second magnet disposed separately from each other in the direction of the optical axis on an outer periphery of the holding frame and each having an annular shape; and a coil wound between the first magnet and the second magnet on the outer periphery of the holding frame. The moving frame has a non-rotationally symmetric shape about the optical axis. 
     An endoscope according to another aspect of the present invention includes an optical device including: an optical system including a moving lens that is movable in a direction of an optical axis; a moving frame formed of a magnetic body material and holding the moving lens; a holding frame formed of a non-magnetic body material in a tubular shape and holding the moving frame on an inner peripheral surface so that the moving frame is movable along the optical axis; a first magnet and a second magnet disposed separately from each other in the direction of the optical axis on an outer periphery of the holding frame and each having an annular shape; and a coil wound between the first magnet and the second magnet on the outer periphery of the holding frame. The moving frame has a non-rotationally symmetric shape about the optical axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an endoscope; 
         FIG. 2  is a cross-sectional view illustrating a configuration of an image pickup apparatus in which a moving lens unit is moved to a distal end side; 
         FIG. 3  is a cross-sectional view illustrating the configuration of the image pickup apparatus in which the moving lens unit is moved to a proximal end side; 
         FIG. 4  is a perspective view of the moving lens unit; 
         FIG. 5  is an explanatory diagram schematically illustrating magnetic force received by a moving lens frame inside a fixed lens frame; 
         FIG. 6  is a perspective view of the moving lens unit according to a first modification; 
         FIG. 7  is a perspective view of the moving lens unit according to a second modification; 
         FIG. 8  is a main part cross-sectional view of an optical device according to a third modification at a section in a direction orthogonal to an optical axis; 
         FIG. 9  is a main part cross-sectional view of the optical device according to a fourth modification at a section in the direction orthogonal to the optical axis; 
         FIG. 10  is a main part cross-sectional view of the optical device according to a fifth modification at a section in the direction orthogonal to the optical axis; 
         FIG. 11  is a main part cross-sectional view of the optical device according to a sixth modification at a section in the direction orthogonal to the optical axis; 
         FIG. 12  is a main part cross-sectional view of the optical device according to a seventh modification at a section in the direction orthogonal to the optical axis; 
         FIG. 13  is a perspective view of the moving lens unit according to an eighth modification; 
         FIG. 14  is a perspective view of the moving lens unit according to a ninth modification; and 
         FIG. 15  is a perspective view of the moving lens unit according to a tenth modification. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Forms of the present invention will be described below with reference to the accompanying drawings. The drawings relate to an embodiment of the present invention,  FIG. 1  is a schematic configuration diagram of an endoscope,  FIG. 2  is a cross-sectional view illustrating a configuration of an image pickup apparatus in which a moving lens unit is moved to a distal end side,  FIG. 3  is a cross-sectional view illustrating the configuration of the image pickup apparatus in which the moving lens unit is moved to a proximal end side,  FIG. 4  is a perspective view of the moving lens unit, and  FIG. 5  is an explanatory diagram schematically illustrating magnetic force received by a moving lens frame inside a fixed lens frame. 
     An endoscope  101  of the present embodiment can be introduced into a subject such as a human body and is configured to optically pick up an image of a predetermined observation site in the subject. 
     Note that the subject into which the endoscope  101  is introduced is not limited to a human body but may be another living body or may be an artificial object such as a machine or a building. 
     The endoscope  101  mainly includes an insertion portion  102  that is introduced into the subject, an operation portion  103  positioned at a proximal end of the insertion portion  102 , and a universal cord  104  extending from a side part of the operation portion  103 . 
     The insertion portion  102  has a configuration in which a distal end portion  110  disposed at a distal end, a bending portion  109  that is bendable and disposed on the proximal end side of the distal end portion  110 , and a flexible tube portion  108  that is flexible, disposed on the proximal end side of the bending portion  109 , and connected with a distal end side of the operation portion  103  are continuously provided. 
     Although described later in detail, an image pickup apparatus  1  is provided at the distal end portion  110 . An angle operation knob  106  for operating bending of the bending portion  109  is provided at the operation portion  103 . 
     An endoscope connector  105  that is connected with an external device  120  is provided at a proximal end portion of the universal cord  104 . The external device  120  that is connected with the endoscope connector  105  is connected with an image display unit  121  such as a monitor through a cable. 
     The endoscope  101  also includes a composite cable  115  inserted into the universal cord  104 , the operation portion  103 , and the insertion portion  102 , and an optical fiber bundle (not illustrated) that transmits illumination light from a light source unit provided at the external device  120 . 
     The composite cable  115  electrically connects the endoscope connector  105  and the image pickup apparatus  1 . When the endoscope connector  105  is connected with the external device  120 , the image pickup apparatus  1  is electrically connected with the external device  120  through the composite cable  115 . 
     Electrical power supply from the external device  120  to the image pickup apparatus  1  and communication between the external device  120  and the image pickup apparatus  1  are performed through the composite cable  115 . 
     An image processing unit is provided at the external device  120 . The image processing unit generates a video signal based on an image pickup device output signal that is output from the image pickup apparatus  1 , and outputs the generated video signal to the image display unit  121 . Accordingly, in the present embodiment, an optical image (endoscope image) picked up by the image pickup apparatus  1  is displayed as a video on the image display unit  121 . 
     Note that the endoscope  101  does not necessarily need to be connected with the external device  120  or the image display unit  121 , but for example, may include part or whole of the image processing unit or the monitor. 
     The optical fiber bundle (not illustrated) has a configuration through which light emitted from the light source unit of the external device  120  is transmitted to an illumination window as an illumination light emission portion of the distal end portion  110 . The light source unit may be disposed in the operation portion  103  or the distal end portion  110  of the endoscope  101 . 
     Subsequently, the configuration of the image pickup apparatus  1  according to the present embodiment will be described below in detail. 
     As illustrated in  FIGS. 2 and 3 , the image pickup apparatus  1  includes, for example, a two-focal-point switching optical device  2 , and an image pickup device  3  continuously provided on a back side of the optical device  2 . 
     Note that the image pickup device  3  is an image sensor such as a CCD or a CMOS, fixed to an image pickup device holding frame (not illustrated), and continuously provided on the back side of the optical device  2  through the image pickup device holding frame. 
     The optical device  2  includes a fixed lens unit  10 , a moving lens unit  20 , and an actuator  30  as a drive mechanism. 
     The fixed lens unit  10  includes a fixed lens  11  that is an objective lens as an optical system through which light of an object image (optical image) is focused toward the image pickup device  3 , a substantially tubular fixed lens frame  12  that is a lens holding frame as a holding frame formed of a non-magnetic member, and two restriction members  13   a  and  13   b  each having an annular shape. Note that one of the restriction members  13   a  and  13   b  may be part of the lens holding frame  12  (in other words, may be integrally formed with the lens holding frame  12 ). 
     The fixed lens frame  12  is formed in a tube elongated along an image pickup optical axis O and holds, at a distal end part of the image pickup optical axis O, the fixed lens  11  provided with an optical aperture  14  as an optical member included in the optical system. Note that the fixed lens  11  may be configured as a group of a plurality of lenses. 
     The restriction member  13   a  configured to restrict a position of the distal end side of the moving lens unit  20  to sandwich the optical aperture  14  on the back side of the fixed lens  11  is fixed to an inner peripheral part of the fixed lens frame  12 . The restriction member  13   b  configured to restrict a position of a back end side of the moving lens unit  20  is fixed on a front side of the image pickup device  3 . 
     The moving lens unit  20  includes a moving lens frame  21  as a moving frame formed of a magnetic member and having a substantially tubular shape as a basic shape, and a moving lens  22  as an optical member that is included in the optical system and through which light of the object image is focused to a light receiving unit of the image pickup device  3 . 
     As illustrated in  FIGS. 2 to 4 , in the moving lens frame  21 , a lens holding portion  23 , a first sliding portion  24  as a sliding portion continuously provided on the distal end side of the lens holding portion  23 , and a second sliding portion  25  as a sliding portion continuously provided on the proximal end side of the lens holding portion  23  are integrally formed of a magnetic body. 
     The lens holding portion  23  has a substantially annular shape having an inner periphery formed as a lens holding hole  23   a , and the moving lens  22  is held in the lens holding hole  23   a  of the lens holding portion  23 . Note that the moving lens  22  may be configured as a group of a plurality of lenses. 
     The first and second sliding portions  24  and  25  are provided continuously and coaxially on a central axis O 1  of the lens holding hole  23   a  of the lens holding portion  23 . 
     The first and second sliding portions  24  and  25  are each configured as a substantially annular member having an outer diameter larger than an outer diameter of the lens holding portion  23  and slightly smaller than an inner diameter of the fixed lens frame  12 . Accordingly, outer peripheral surfaces of the first and second sliding portions  24  and  25  are set as sliding surfaces that are freely slidable relative to an inner peripheral surface of the fixed lens frame  12 . 
     Note that in the present embodiment, the outer diameters of the sliding portions  24  and  25  are, for example, 1.3 mm to 1.5 mm approximately, which are smaller than the inner diameter of the fixed lens frame  12  by, for example, 0.02 mm approximately. 
     The moving lens unit  20  is enclosed in the fixed lens frame  12  of the fixed lens unit  10  and is provided to be freely movable in a front-back direction along the image pickup optical axis O as the sliding portions  24  and  25  slide. 
     The moving lens frame  21  of the present embodiment includes a cutout portion  26  formed by cutting out part of the lens holding portion  23  and the first and second sliding portions  24  and  25  into an integrated planar shape by machining or the like in a direction orthogonal to the central axis O 1 , and accordingly, the moving lens frame  21  has a non-rotationally symmetric shape about the central axis O 1 . 
     In this case, for example, as illustrated in  FIG. 5 , the cutout portion  26  is formed on circumferences of the first and second sliding portions  24  and  25  in a range having a central angle of 120° or smaller. 
     With this configuration, the sliding surface of each of the sliding portions  24  and  25  can contact the inner peripheral surface of the fixed lens frame  12  at at least three points equally spaced from each other by 120° about the central axis O 1 , which enables stable forward-and-backward movement of the moving lens unit  20  in the fixed lens frame  12 . 
     The actuator  30  includes a yoke  31  provided on an outer periphery of the fixed lens frame  12 , a first magnet  33  having an annular shape and integrally fixed to a front end part of the yoke  31 , a second magnet  34  having an annular shape and integrally fixed to a back end part of the yoke  31 , and a solenoid coil (hereinafter simply referred to as coil)  35  wound and fixed between an inner periphery of the yoke  31  and the outer periphery of the fixed lens frame  12 . 
     The yoke  31  of the present embodiment is formed in divisions of a first yoke member  36  and a second yoke member  37  each having a substantially cylindrical shape. An inward flange  36   a  is formed on an entire circumference at a distal end of the first yoke member  36 , and the first magnet  33  is fixed to the inward flange  36   a . In addition, an inward flange  37   a  is formed on an entire circumference at a back end of the second yoke member  37 , and the second magnet  34  is fixed to the inward flange  37   a . The first yoke member  36  and the second yoke member  37  are externally inserted from the distal end side and the back end side, respectively, of the fixed lens frame  12  to entirely enclose and cover the coil  35  and are connected and fixed so that facing end parts of the first yoke member  36  and the second yoke member  37  are integrated with each other by a bonding agent, brazing, or the like. 
     The first yoke member  36  and the second yoke member  37  are magnetic members such as soft iron for amplifying magnetic force generated at the first magnet  33 , the second magnet  34 , and the coil  35 . 
     Note that the first yoke member  36  and the second yoke member  37  are formed as separate bodies for assembly to cover the coil  35  wound and fixed on the outer periphery of the fixed lens frame  12 , but may be formed as one integrated yoke as long as assembly to cover the coil  35  is possible. 
     In the actuator  30 , the first magnet  33  is a permanent magnet having a south pole magnetized on the front side and a north pole magnetized on the back side, and the second magnet  34  is a permanent magnet having a north pole magnetized on the front side and a south pole magnetized on the back side. In other words, the first magnet  33  and the second magnet  34  are disposed so that the same magnetic poles (north poles, in this example) face each other. 
     The actuator  30  thus configured can generate drive power to the moving lens unit  20  by switching a direction of energization to the coil  35  to switch a direction of the magnetic field generated at the coil  35  relative to the magnetic field of the first magnet  33  and the magnetic field of the second magnet  34 . 
     Specifically, when the coil  35  is energized in a first direction in which the coil  35  is excited to have a south pole on the distal end side and a north pole on the proximal end side, the magnetic field of the first magnet  33  is strengthened on the distal end side of the actuator  30  by the magnetic field generated at the coil  35  in the same direction. Simultaneously, the magnetic field of the second magnet  34  is canceled and weakened on the back end side of the actuator  30  by the magnetic field generated at the coil  35  in the opposite direction. 
     As a result, drive power (magnetic force) that attracts the moving lens frame  21  toward the distal end side is generated at the actuator  30  as a whole, and moves the moving lens frame  21  (moving lens unit  20 ) to a front-side movement position defined by the restriction member  13   a.    
     Note that after the energization to the coil  35  is canceled, the moving lens frame  21  after being moved to the distal end side is held by magnetic force of the first magnet  33 . 
     When the coil  35  is energized in a second direction in which the coil  35  is excited to have a north pole on the distal end side and a south pole on the proximal end side, the magnetic field of the first magnet  33  is canceled and weakened on the distal end side of the actuator  30  by the magnetic field generated at the coil  35  in the opposite direction. Simultaneously, the magnetic field of the second magnet  34  is strengthened on the back end side of the actuator  30  by the magnetic field generated at the coil  35  in the same direction. 
     As a result, drive power (magnetic force) that attracts the moving lens frame  21  to the proximal end side is generated at the actuator  30  as a whole, and moves the moving lens frame  21  (moving lens unit  20 ) to a back-side movement position defined by the restriction member  13   b.    
     Note that after the energization to the coil  35  is canceled, the moving lens frame  21  after being moved to the back end side is held by magnetic force of the second magnet  34 . 
     Note that, in the image pickup apparatus  1 , a state in which the moving lens unit  20  is moved forward and stopped on the front side is a wide end as a first stop position, and a state in which the moving lens unit  20  is moved backward and stopped on the back side is a tele end as a second stop position. 
     In this manner, the image pickup apparatus  1  has a configuration in which the moving lens unit  20  is moved forward and backward by drive of the actuator  30  to perform switching between two wide and tele optical properties. 
     The two wide and tele optical properties of the image pickup apparatus  1 , which depend on the front and back stop positions of the moving lens unit  20 , may be inverted by lens designing of the fixed lens  11  and the moving lens  22  and the like. 
     In movement and holding of the moving lens frame  21  (moving lens unit  20 ) as described above to and at the front-side movement position or in movement and holding of the moving lens frame  21  (moving lens unit  20 ) to and at the back-side movement position, magnetic force received by the moving lens frame  21  from the first and second magnets  33  and  34  and the coil  35  in each direction orthogonal to the central axis is ununiform since the moving lens frame  21  has a non-rotationally symmetric shape about a central axis of the lens holding portion  23  (lens holding hole  23   a ). 
     Specifically, a volume of a site at which the cutout portion  26  is provided is smaller than a volume of another site, and thus magnetic force (attraction force) received by the moving lens frame  21  at the site at which the cutout portion  26  is provided is relatively smaller than magnetic force (attraction force) received by the moving lens frame  21  at a site in another direction (refer to  FIG. 5 ). 
     Accordingly, in the moving lens frame  21  (moving lens unit  20 ), a site opposite to the site at which the cutout portion  26  is provided is attracted and pressed toward the inner peripheral surface of the fixed lens frame  12 . When the moving lens frame  21  is attracted in this manner to perform what is called backlash reduction, backlash of the moving lens unit  20  is appropriately prevented. 
     In this case, the cutout portion  26  of the moving lens frame  21  is desirably set with taken into account gravitational force received by the moving lens unit  20 . Specifically, a cutout amount of the cutout portion  26  is desirably set so that a difference (relative attraction force) between attraction force that the moving lens frame  21  is attracted in a direction opposite to the cutout portion  26  by magnetic force and attraction force that the moving lens frame  21  is attracted in a direction toward the cutout portion  26  by magnetic force is larger than the gravitational force. 
     In addition, for example, as illustrated in  FIGS. 2 and 3 , the moving lens frame  21  is desirably designed so that the central axis O 1  when backlash reduction of the moving lens frame  21  is performed is aligned with a central axis O of the optical device  2 . 
     According to such an embodiment, the moving lens frame  21  formed of a magnetic body and including the lens holding hole  23   a  holding the moving lens  22 , the fixed lens frame  12  formed of a non-magnetic body in a tubular shape and holding the moving lens frame  21  on an inner peripheral surface so that the moving lens frame  21  is movable along an optical axis O, the first magnet  33  and the second magnet  34  disposed at a predetermined distance from each other in a direction of the optical axis O on the outer periphery of the fixed lens frame  12  and each having an annular shape, and the coil  35  wound between the first magnet  33  and the second magnet  34  on the outer periphery of the fixed lens frame  12  are provided, and the moving lens frame  21  has a non-rotationally symmetric shape about the central axis O 1  so that magnetic force received from the first and second magnets  33  and  34  and the coil  35  at a site in at least one direction orthogonal to the central axis O 1  of the lens holding hole  23   a  is relatively smaller than magnetic force received from the first and second magnets  33  and  34  and the coil  35  at another site, thereby moving the moving lens frame  21  with sufficient drive power and appropriately preventing backlash of the moving lens frame  21 . 
     Specifically, since the moving lens frame  21  has a non-rotationally symmetric shape about the central axis O 1  and is configured so that magnetic force received from the first and second magnets  33  and  34  and the coil  35  at a site in at least one direction orthogonal to the central axis O 1  is relatively smaller than magnetic force received from the first and second magnets  33  and  34  and the coil  35  at another site, backlash reduction of the moving lens frame  21  can be performed without increasing or decreasing the magnetic forces of the first and second magnets  33  and  34  only at biased parts during energization of the coil  35 . In addition, sufficient drive power can be generated for the moving lens frame  21  by increasing or decreasing the magnetic forces of the first and second magnets  33  and  34  over the entire circumference during energization of the coil  35 . 
     In this case, since the cutout portion  26  is formed by cutting out part of the lens holding portion  23  and the first and second sliding portions  24  and  25  into a planar shape, the moving lens frame  21  can be easily formed into a non-rotationally symmetric shape. 
     For example, as illustrated in  FIG. 6 , a cutout portion  40  may be formed only at the first and second sliding portions  24  and  25  among the lens holding portion  23  and the first and second sliding portions  24  and  25  in the moving lens frame  21 . 
     With this configuration, the cutout portion can be more deeply formed while holding strength of the moving lens  22  is maintained, which improves the degree of freedom of attraction force amount designing for backlash reduction of the moving lens frame  21 . 
     For example, as illustrated in  FIG. 7 , a cutout portion  41  may be formed only at the lens holding portion  23  among the lens holding portion  23  and the first and second sliding portions  24  and  25  in the moving lens frame  21 . 
     With this configuration, it is possible to prevent backlash of the moving lens frame  21  while maintaining slidability of the first and second sliding portions  24  and  25  relative to an inner periphery of the fixed lens frame  12 . 
     For example, as illustrated in  FIG. 8 , a hole portion  42  may be drilled through the moving lens frame  21  in place of the cutout portion, thereby forming a non-rotationally symmetric shape of the moving lens frame  21  about the central axis O 1 . 
     With this configuration, it is possible to prevent backlash of the moving lens frame  21  without affecting an outer shape of the moving lens frame  21 . 
     For example, as illustrated in  FIG. 9 , the central axis O 1  of the lens holding hole  23   a  may be decentered relative to a central axis O 2  of the first and second sliding portions  24  and  25  and the like, thereby forming a non-rotationally symmetric shape of the moving lens frame  21  about the central axis O 1 . 
     In this case, a decentering amount e of the central axis O 1  relative to the central axis O 2  is set to be equal to a half value of a maximum clearance c (which is e=c/2) when the moving lens frame  21  is pressed against the inner periphery of the fixed lens frame  12 , thereby achieving backlash reduction of the moving lens frame  21  so that the central axis O 1  is constantly positioned at a center of the fixed lens frame  12  when the moving lens frame  21  rotates inside the fixed lens frame  12 . Thus, the optical axis O and the central axis O 1  can be constantly aligned with each other by setting the optical axis O of the optical device  2  at the center of the fixed lens frame  12 . 
     A plurality of cutout portions may be provided at the moving lens frame  21 . For example,  FIG. 10  illustrates a configuration of the moving lens frame  21  in which a first cutout portion  45  is provided at a site in one direction orthogonal to the central axis O 1 , and second and third cutout portions  46  and  47  smaller than the first cutout portion  45  are provided at rotational positions at 120° intervals with respect to the first cutout portion  45  about the central axis O 1 . 
     With this configuration, the magnetic forces received from the first and second magnets  33  and  34  and the coil  35  are smallest at the site at which the first cutout portion  45  is provided on a periphery of the moving lens frame  21 , and is next smallest at sites at which the second and third cutout portions  46  and  47  are provided. Thus, the moving lens frame  21  is subjected to backlash reduction in a direction opposite to the first cutout portion  45  through the central axis O 1  and is attracted toward sides opposite to the second and third cutout portions  46  and  47  through the central axis O 1 . Accordingly, the moving lens frame  21  is appropriately prevented from wobbling pivoted at a site of contact with the fixed lens frame  12  (in other words, a site opposite to the first cutout portion  45  through the central axis O 1  of the moving lens frame  21 ). 
     For example, as illustrated in  FIG. 11 , a rotation restriction portion  50  may be provided on the inner periphery of the fixed lens frame  12  at a position facing the cutout portion  26  of the moving lens frame  21 . 
     With this configuration, a rotational position of the moving lens frame  21  relative to the fixed lens frame  12  can be kept constant. 
     For example, as illustrated in  FIG. 12 , as for the shape of a cutout portion, a groove-shaped cutout portion  51  extending in the direction of the optical axis O (central axis O 1 ) may be provided in place of the cutout portion  26  having a planar shape. 
     In this case, a key-shaped rotation restriction portion  52  may be provided on the inner periphery of the fixed lens frame  12  at a position facing the cutout portion  51  of the moving lens frame  21 . 
     With this configuration, the rotational position of the moving lens frame  21  relative to the fixed lens frame  12  can be kept constant. 
     For example, as illustrated in  FIGS. 13 to 15 , the lens holding portion  23  may be formed to have an outer diameter equal to the outer diameters of the first and second sliding portions  24  and  25 . 
     With this configuration, an outer peripheral surface of the lens holding portion  23  can act as a sliding surface relative to the inner periphery of the fixed lens frame  12 . Thus, when a cutout portion  55  obtained by cutting out an optional amount is formed at the first and second sliding portions  24  and  25 , as well, a sliding surface of the moving lens frame  21  can contact the inner peripheral surface of the fixed lens frame  12  at at least three points equally spaced from each other by 120° about the central axis O 1 . 
     Note that the present invention is not limited to the above-described embodiment but may be provided with modifications and changes in various kinds of manners, and these modifications and changes are also included in the technical scope of the present invention. For example, components of the above-described embodiment and modifications may naturally be combined as appropriate.