Patent Publication Number: US-8526122-B2

Title: Lens barrel

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2010-134769 filed on Jun. 14, 2010, the entire disclosure of which is incorporated by reference herein. In addition, this application is related to Japanese Patent Application No. 2011-119594 filed on May 27, 2011, the entire disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     A technique disclosed herein relates to a lens barrel. 
     Conventionally, a lens barrel has been known, in which a position of a lens group is adjustable. As one of the lens barrels of this type, there is a lens barrel disclosed in Japanese Patent Publication No. 2010-008802. The lens barrel disclosed in Japanese Patent Publication No. 2010-008802 includes a first frame holding lenses, a second frame in which the first frame is housed, and a third frame in which the second frame is housed. Drive force driving such frames is transmitted from the third frame to the second frame through a second cam mechanism, and then is transmitted from the second frame to the first frame through a first cam mechanism. Each of the first and second cam mechanisms includes a cam follower and a cam groove to be engaged with the cam follower. Each of the cam grooves of the first and second cam mechanisms has a predetermined locus. The first frame moves in an optical axis direction according to shapes of the cam grooves of the first and second cam mechanisms. 
     SUMMARY 
     There is a lens barrel in which, when a focal distance is changed from a wide-angle end to a tele end, not only a first frame simply moves toward an object, but also the first frame moves toward the object after the first frame temporarily moves toward an imaging surface. That is, in the lens barrel, the first frame moves along a locus in which a distance from the imaging surface to the first frame has a local minimum. In the lens barrel having such a configuration, when the focal distance is set between the wide-angle end and the tele end, if an impact is applied to the first frame due to, e.g., dropping of the lens barrel etc., the first frame is pushed to a position where the distance from the imaging surface to the first frame is the local minimum. In such a position, impact force is transmitted to a second frame through a first cam mechanism, and is further transmitted to a third frame through a second cam mechanism. In such a state, force in a rotational direction may act on the first or second frame due to component force of the impact force transmitted from a cam follower to a cam groove. As a result, there is a possibility that the first to third frames and members therearound are damaged. 
     The technique disclosed herein has been made in view of the foregoing, and it is an objective of the technique to provide a lens barrel having high strength against external force. 
     A lens barrel disclosed herein includes a zoom optical system including a plurality of lens groups each having one or more lenses, in which a focal distance is changeable between a wide-angle end and a tele end by moving the lens groups along an optical axis; a first frame moving together with a first lens group of the plurality of lens groups and which is closest to an object in the zoom optical system; and first and second cam mechanisms transmitting drive force to the first frame. Each of the first and second cam mechanisms includes a cam follower and a cam groove engaged with the cam follower, and the cam groove includes a section in which the cam follower moves when the focal distance is changed between the wide-angle end and the tele end. The section of at least one of the cam grooves of the first and second cam mechanisms includes a first section which has a predetermined gradient to a circumferential direction about the optical axis, a second section which adjoins the first section and has an absolute value of a gradient smaller than that of the first section, and a third section which adjoins the second section and has an absolute value of a gradient larger than that of the second section. A distance from an imaging surface to the first frame in the optical axis direction has a local minimum when the cam follower for the at least one of the cam grooves is positioned within the second section. 
     According to the lens barrel, the lens barrel having high strength against external force can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a lens barrel of a first embodiment. 
         FIG. 2  illustrates a locus of a first group frame in an optical axis direction relative to a rotation amount, and development diagrams of a fixed cam groove, a cam slot, and a first group cam groove. 
         FIG. 3  illustrates a locus of a first group frame in an optical axis direction relative to a rotation amount, and development diagrams of a fixed cam groove, a cam slot, and a first group cam groove in a second embodiment. 
         FIG. 4  illustrates a locus of a first group frame in an optical axis direction relative to a rotation amount, and development diagrams of a fixed cam groove, a cam slot, and a first group cam groove in a third embodiment. 
         FIG. 5  illustrates a locus of a first group frame in an optical axis direction relative to a rotation amount, and development diagrams of a fixed cam groove, a cam slot, and a first group cam groove in a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will be described below in detail with reference to the drawings.  FIG. 1  is an exploded perspective view of a lens barrel of the embodiment. 
     &lt;&lt;First Embodiment&gt;&gt; 
     A lens barrel  100  of a first embodiment is mounted in a digital still camera. The lens barrel  100  includes a master flange  190 , various frames which will be described below, a zoom optical system formed by lenses held by the frames, and an imaging device  101  configured to convert light entering the imaging device  101  through the zoom optical system into an electrical signal and output the electrical signal. The lens barrel  100  is one example, and can be mounted not only in the digital still camera but also in a camcorder, a camera-equipped cell-phone, etc. The zoom optical system includes a first lens group  301 , a second lens group  302 , and a focus lens group  303 . The first lens group  301  is positioned closest to an object. Each of the first lens group  301 , the second lens group  302 , and the focus lens group  303  includes one or more lenses. The zoom optical system forms an optical image of an object on an imaging surface  101   a  of the imaging device  101 . The first lens group  301  forms a first lens group, and the second lens group  302  forms a second lens group. 
     As the various frames, the lens barrel  100  includes a first group frame  110  holding the first lens group  301 , a second group frame  120  holding the second lens group  302 , a cam slot frame  130 , a translational frame  140 , a cam frame  150 , a drive frame  160 , a fixed frame  170 , and a third group frame  180  holding the focus lens group  303 . The first group frame  110 , the second group frame  120 , the cam slot frame  130 , the translational frame  140 , the cam frame  150 , the drive frame  160 , the fixed frame  170 , and the third group frame  180  are concentrically arranged about an optical axis X. The first group frame  110  forms a first frame, and the cam frame  150  forms a second frame. In addition, the drive frame  160  forms a third frame. 
     The imaging device  101  is fixed to the master flange  190  so that the imaging surface  101   a  of the imaging device  101  faces the object. In addition, the fixed frame  170  is fixed to the master flange  190 . A fixed cam groove  171  extending in a predetermined pattern and having a bottom, and a translational groove extending in an optical axis direction (or along the optical axis X) and having a bottom are formed in an inner circumferential surface of the fixed frame  170 . A zoom motor unit  173  is attached to the fixed frame  170 . In the present specification, unless otherwise described, a “groove” includes a groove having a bottom and a groove not having a bottom. In addition, unless otherwise described, a “slot” means a groove not having a bottom. 
     A drive cam follower  161  and a gear portion are provided on an outer circumferential surface of the drive frame  160 . The drive frame  160  is housed in the fixed frame  170 . In such a state, the drive cam follower  161  of the drive frame  160  is engaged with the fixed cam groove  171  of the fixed frame  170 , and the gear portion of the drive frame  160  is engaged with the zoom motor unit  173 . The drive frame  160  is rotatably driven about the optical axis by the zoom motor unit  173 . The drive frame  160  relatively moves in the optical axis direction while relatively rotating about the optical axis with respect to the fixed frame  170  according to the fixed cam groove  171 . A circumferential groove extending in a circumferential direction and having a bottom, and a translational groove  162  extending in the optical axis direction and having a bottom are formed in an inner circumferential surface of the drive frame  160 . When a term “rotate” is simply used below, it means a rotation about the optical axis. 
     A first engagement protrusion  133  to be engaged with the translational groove of the fixed frame  170 , and a second engagement protrusion to be engaged with the circumferential groove of the drive frame  160  are provided on an outer circumferential surface of the cam slot frame  130 . The cam slot frame  130  is housed in the drive frame  160 . In such a state, the second engagement protrusion of the cam slot frame  130  is engaged with the circumferential groove of the drive frame  160 . This allows the cam slot frame  130  to relatively rotate with respect to the drive frame  160 , and not to relatively move in the optical axis direction with respect to the drive frame  160 . That is, the cam slot frame  130  moves together with the drive frame  160  in the optical axis direction. The cam slot frame  130  housed in the drive frame  160  is further housed in the fixed frame  170 . In such a state, the first engagement protrusion  133  is engaged with the translational groove of the fixed frame  170 . Since the translational groove of the fixed frame  170  extends in the optical axis direction, the cam slot frame  130  is supported so as to move in the optical axis direction in a state in which the cam slot frame  130  cannot relatively rotate with respect to the fixed frame  170 . A cam slot  131  extending in a predetermined pattern is formed in the cam slot frame  130 . In addition, a translational groove  132  extending in the optical axis direction and having a bottom is formed in an inner circumferential surface of the cam slot frame  130 . 
     Cam followers  151  are provided on an outer circumferential surface of the cam frame  150 . The cam frame  150  is housed in the cam slot frame  130 . In such a state, the cam followers  151  of the cam frame  150  penetrate the cam slot  131  of the cam slot frame  130 , and are engaged with the translational groove  162  of the drive frame  160 . In this manner, the cam frame  150  is held so as to rotate together with the drive frame  160  and translationally and relatively move in the optical axis direction with respect to the drive frame  160 . In addition, the cam followers  151  are also engaged with the cam slot  131  of the cam slot frame  130 . Thus, when the drive frame  160  is rotatably driven, the cam frame  150  relatively moves in the optical axis direction while relatively rotating with respect to the cam slot frame  130  according to a shape of the cam slot  131 . First group cam grooves  210  each having a bottom and second group cam grooves  220  each having a bottom are provided in an inner circumferential surface of the cam frame  150 . In addition, engagement protrusions are provided on the inner circumferential surface of the cam frame  150 . 
     The translational frame  140  is housed in the cam frame  150 . A circumferential groove extending in the circumferential direction and having a bottom is formed in an outer circumferential surface of the translational frame  140 . The engagement protrusions of the cam frame  150  are engaged with the circumferential groove. The engagement of the engagement protrusions with the circumferential groove allows the translational frame  140  to relatively rotate with respect to the cam frame  150 , and not to relatively move in the optical axis direction with respect to the cam frame  150 . That is, when the cam frame  150  moves in the optical axis direction and rotates, the translational frame  140  moves together with the cam frame  150  in the optical axis direction. The translational frame  140  includes engagement protrusions  141  outwardly protruding from a rear end portion of the translational frame  140 . The rear end portion of the translational frame  140  protrudes beyond the cam frame  150 , and the engagement protrusion  141  is engaged with the translational groove  132  of the cam slot frame  130 . The engagement of the engagement protrusion  141  with the translational groove  132  allows the translational frame  140  not to relatively rotate with respect to the cam slot frame  130 . In addition, translational slots  142 ,  143  are formed in the translational frame  140 . 
     The first group frame  110  includes first group cam followers  111 . The first group frame  110  is housed in the translational frame  140 . The first group frame  110  is engaged with the translational slots  142  of the translational frame  140 . The first group frame  110  is translationally and relatively movable in the optical axis direction with respect to the translational frame  140 , and does not relatively rotate with respect to the translational frame  140 . The first group cam followers  111  are engaged with the first group cam grooves  210  of the cam frame  150 . When the cam frame  150  rotates, the first group frame  110  relatively moves in the optical axis direction and relatively rotates with respect to the cam frame  150  through a first cam mechanism formed by the first group cam follower  111  and the first group cam groove  210 . 
     The second group frame  120  includes second group cam followers  121 . The second group frame  120  is housed in the translational frame  140 . The second group frame  120  is engaged with the translational slot  143  of the translational frame  140 . The second group frame  120  is translationally and relatively movable in the optical axis direction with respect to the translational frame  140 , and does not relatively rotate with respect to the translational frame  140 . The second group cam followers  121  are engaged with the second group cam grooves  220  of the cam frame  150 . When the cam frame  150  rotates, the second group frame  120  relatively moves in the optical axis direction with respect to the cam frame  150  and relatively rotates with respect to the cam frame  150  through a second cam mechanism formed by the second group cam follower  121  and the second group cam groove  220 . 
     The third group frame  180  is slidably engaged with a guide pole provided in the master flange  190 . The third group frame  180  is driven separately from other frames in the optical axis direction by an actuator provided in the master flange  190 . 
     The cam groove  171  of the fixed frame  170 , the cam slot  131  of the cam slot frame  130 , and the first group cam groove  210  and the second group cam groove  220  of the cam frame  150  are set so as to have a proper shape corresponding to a rotational angle. Thus, the first lens group  301  and the second lens group  302  can be arranged in proper positions corresponding to a rotation amount of the drive frame  160 . In addition, the length of the lens barrel  100  in the optical axis direction when the lens barrel  100  is not in a shootable state (i.e., in a refracted state) can be shortened. 
     In the lens barrel  100  configured as described above, when the zoom motor unit  173  rotatably drives the drive frame  160 , the first group frame  110  and the second group frame  120  move in the optical axis direction. This adjusts a focal distance in the zoom optical system. In addition to the foregoing, the third group frame  180  is moved to a proper position based on a distance between the object and the lens barrel  100 . In such a manner, light enters the imaging device  101  to form an image on the imaging device  101 , and such an image is captured. 
     Subsequently, movement of the first group frame  110  will be described in detail.  FIG. 2  illustrates a locus of the first group frame  110  in a case where a rotation amount of the drive frame  160  is plotted along a horizontal axis, and a position of the first group frame moving from a retracted position to a tele end through a wide-angle end in the optical axis direction relative to the rotation amount is plotted along a vertical axis. In addition,  FIG. 2  illustrates development diagrams of the fixed cam groove  171 , the cam slot  131 , and the first group cam groove  210  so that the retracted position, the wide-angle end, and the tele end of each of the development diagrams correspond to those of the locus. 
     As described above, drive force of the zoom motor unit  173  is first transmitted to the drive frame  160 . According to the drive cam follower  161  and the fixed cam groove  171 , the drive frame  160  moves in the optical axis direction while relatively rotating with respect to the fixed frame  170 . Meanwhile, the drive force is transmitted from the drive frame  160  to the cam frame  150  through the cam followers  151  and the cam slot  131 . As a result, the cam frame  150  relatively moves in the optical axis direction with respect to the drive frame  160  while rotating together with the drive frame  160 . Further meanwhile, the drive force is transmitted from the cam frame  150  to the first group frame  110  through the first group cam grooves  210  and the first group cam followers  111 . As a result, the first group frame  110  moves in the optical axis direction while relatively rotating with respect to the drive frame  160 . Switching of a rotational direction of the drive frame  160  allows switching of the first group frame  110  between movement toward the object and movement toward the imaging surface. 
     A displacement of the first group frame  110  in the optical axis direction is determined by a sum of a displacement of the drive cam follower  161  moving along the fixed cam groove  171  in the optical axis direction, a displacement of the cam follower  151  moving along the cam slot  131  in the optical axis direction, and a displacement of the first group cam follower  111  moving along the first group cam groove  210  in the optical axis direction. Loca of the drive cam follower  161 , the cam follower  151 , and the first group cam follower  111  are defined in accordance with shapes of the fixed cam groove  171 , the cam slot  131 , and the first group cam groove  210 , respectively. 
     Each of the fixed cam groove  171 , the cam slot  131 , and the first group cam groove  210  has a zoom region and a refracted region. The zoom region is a region where each of the cam followers is movable when an object image is zoomed in/out for shooting. The zoom region corresponds to a section in which each of the cam followers moves when the focal distance is changed between the wide-angle end and the tele end. On the other hand, the refracted region is a region where each of the cam followers moves when the lens barrel  100  is changed from a shooting state to the retracted state. One of end portions of the zoom region closer to the retracted region is the wide-angle end, and the other end portion of the zoom region apart from the retracted region is the tele end. In addition, an end portion of the refracted region apart from the zoom region is the retracted position. In the retracted position, the frames are retracted, and the lens barrel  100  is in the refracted state. In any of the fixed cam groove  171 , the cam slot  131 , and the first group cam groove  210 , the retracted position, the wide-angle end, and the tele end are arranged in this order from one side to the other side in the circumferential direction. That is, when the drive cam follower  161  is at the refracted position of the fixed cam groove  171 , the cam follower  151  is also at the retracted position of the cam slot  131 , and the first group cam follower  111  is also at the retracted position of the first group cam groove  210 . When the drive cam follower  161  is at the wide-angle end of the fixed cam groove  171 , the cam follower  151  is also at the wide-angle end of the cam slot  131 , and the first group cam follower  111  is also at the wide-angle end of the first group cam groove  210 . When the drive cam follower  161  is at the tele end of the fixed cam groove  171 , the cam follower  151  is also at the tele end of the cam slot  131 , and the first group cam follower  111  is also at the tele end of the first group cam groove  210 . Each of the fixed cam groove  171 , the cam slot  131 , and the first group cam groove  210  has a shape parallel to the circumferential direction at the retracted position, the wide-angle end, and the tele end. Note that, regarding the shape of the cam groove and the locus of the cam follower, when a change in position from the imaging surface to the object in the optical axis direction indicates a positive displacement and a change in position from the wide-angle end to the tele end in the circumferential direction indicates a positive displacement, a ratio of a change amount of a position in the optical axis direction to a change amount of a position in the circumferential direction (the change amount of the position in the optical axis direction/the change amount of the position in the circumferential direction) is referred to as a “gradient” in the present specification. In other words, the “gradient” of the cam groove and the “gradient” of the locus of the cam follower mean an inclination to the circumferential direction about the optical axis. The gradient in a case where the cam follower is positioned closer to the object in the optical axis direction as the cam follower moves toward the tele end in the circumferential direction indicates a positive gradient, and the gradient in a case where the cam follower is positioned closer to the imaging surface in the optical axis direction as the cam follower moves toward the tele end in the circumferential direction indicates a negative gradient. 
     The fixed cam groove  171  obliquely and linearly extends from the retracted position to the wide-angle end and the object in the retracted region. In addition, the fixed cam groove  171  substantially extends parallel to the circumferential direction from the wide-angle end to the tele end in the zoom region. That is, the drive cam follower  161  does not move in the optical axis direction while moving in the zoom region of the fixed cam groove  171 . 
     The cam slot  131  obliquely and linearly extends from the retracted position to the wide-angle end and the object in the retracted region. In addition, the cam slot  131  obliquely extends from the wide-angle end to the tele end and the object in the zoom region, and substantially extends parallel to the circumferential direction (i.e., substantially extends parallel to a plane perpendicular to the optical axis) toward the tele end in the middle of the zoom region. Then, the cam slot  131  obliquely extends toward the tele end and the object again, and reaches the tele end. Specifically, the cam slot  131  in the zoom region includes the following from the wide-angle end to the tele end: a first section  131   a  having the positive gradient, a second section  131   b  in which an absolute value of the gradient is smaller than that of the first section  131   a , and a third section  131   c  in which a sign of the gradient is positive as in the first section  131   a , and the absolute value of the gradient is larger than that of the second section  131   b . More specifically, the second section  131   b  includes a portion which is substantially parallel to the circumferential direction. The first section  131   a  includes an inflection point at which a change in gradient is turned from an increase to a decrease. In addition, the second section  131   b  includes an inflection point at which the change in gradient is turned from the decrease to the increase. Further, the third section  131   c  includes an inflection point at which the change in gradient is turned from the increase to the decrease. That is, when the cam follower  151  moves along the cam slot  131  from the wide-angle end to the tele end, the cam follower  151  moves toward the object in the optical axis direction, the displacement of the cam follower  151  in the optical axis direction becomes temporarily zero or extremely small, and the cam follower  151  moves toward the object in the optical axis direction again. 
     The first group cam groove  210  obliquely and linearly extends from the retracted position to the wide-angle end and the object in the retracted region. In addition, the first group cam groove  210  obliquely extends from the wide-angle end to the tele end and the imaging surface in the zoom region, and then substantially extends parallel to the circumferential direction toward the tele end. Subsequently, the first group cam groove  210  obliquely extends toward the tele end and the object, and reaches the tele end. Specifically, the first group cam groove  210  in the zoom region includes the following in the order from the wide-angle end: a first section  210   a  having the negative gradient, a second section  210   b  in which the absolute value of the gradient is smaller than that of the first section  210   a , and a third section  210   c  having the positive gradient, in which the absolute value of the gradient is larger than that of the second section  210   b . More specifically, the second section  210   b  includes a portion which is substantially parallel to the circumferential direction. The first section  210   a  includes an inflection point at which the change in gradient is turned from the decrease to the increase. In addition, the third section  210   c  includes an inflection point at which the change in gradient is turned from the increase to the decrease. That is, when the first group cam follower  111  moves along the first group cam groove  210  from the wide-angle end to the tele end, the first group cam follower  111  moves toward the imaging surface in the optical axis direction, the displacement of the first group cam follower  111  in the optical axis direction becomes temporarily zero, and the first group cam follower  111  moves toward the object in the optical axis direction. 
     As described above, in the zoom region, drive force in the rotational direction from the zoom motor unit  173  is transmitted from the drive frame  160  to the cam frame  150  as drive force in the rotational direction and the optical axis direction through the second cam mechanism formed by the cam slot  131  and the cam follower  151 . Further, the drive force transmitted to the cam frame  150  is transmitted to the first group frame  110  as drive force in the optical axis direction through the first cam mechanism formed by the first group cam groove  210  and the first group cam follower  111 . As a result, the first group frame  110  is driven in the optical axis direction. 
     The locus of the first group frame  110  according to the fixed cam groove  171 , the cam slot  131 , and the first group cam groove  210  in the optical axis direction relative to the rotation of the drive frame  160  is illustrated in  FIG. 2 . That is, according to the locus, the first group frame  110  is linearly displaced from the refracted position to the wide-angle end toward the object in the optical axis direction in the retracted region. In addition, in the zoom region, the first group frame  110  is displaced from the wide-angle end to the tele end toward the imaging surface in the optical axis direction, and then the displacement of the first group frame  110  in the optical axis direction becomes zero (i.e., the displacement in the optical axis direction becomes a local minimum). Subsequently, the first group frame  110  is displaced toward the object in the optical axis direction. 
     Actual movement of the first group frame  110  will be described. When the first group frame  110  moves from the refracted position to the wide-angle end, the first group frame  110  linearly moves toward the object in the optical axis direction relative to the rotation amount of the drive frame  160 . When the first group frame  110  moves from the wide-angle end to the tele end, the first group frame  110  temporarily moves toward the imaging surface in the optical axis direction, the displacement in the optical axis direction becomes zero, and the first group frame  110  moves toward the object in the optical axis direction. Note that the first group frame  110  is less likely to move in the optical axis direction at and near the refracted position, the wide-angle end, and the tele end. 
     As in the foregoing, the position of the first group frame  110  displaced in the optical axis direction relative to the rotation amount of the drive frame  160  has the local minimum in the zoom region. In other words, a distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction, which is changed relative to the rotation amount of the drive frame  160  has the local minimum in the zoom region. When the distance is the local minimum, the cam follower  151  is positioned in the second section  131   b  of the cam slot  131 , and the first group cam follower  111  is positioned in the second section  210   b  of the first group cam groove  210 . That is, the cam follower  151  is positioned in a portion of the cam slot  131 , which is substantially parallel to the circumferential direction, and the first group cam follower  111  is positioned in a portion of the first group cam groove  210 , which is substantially parallel to the circumferential direction. Note that the fixed cam groove  171  in which the drive cam follower  161  is positioned is parallel to the circumferential direction in the zoom region. Thus, in a state in which the value for the position of the first group frame  110  in the optical axis direction is the local minimum, even if external force in the optical axis direction acts on the first group frame  110 , little component force in the rotational direction about the optical axis acts on the first group frame  110 , the cam slot frame  130 , the cam frame  150 , and the drive frame  160 . When an impact is applied to the lens barrel  100  due to, e.g., dropping of the lens barrel  100  in the shootable state, and bumping of the lens barrel  100  against a wall etc. (in particular, when the impact acts on the first group frame  110 ), external force in a direction in which the length of the lens barrel  100  is shortened acts on the lens barrel  100 . Then, the first group frame  110  is pushed toward the imaging surface in the optical axis direction until the first group frame  110  reaches the position where the distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction is the local minimum. Since the first group frame  110  cannot further move toward the imaging surface beyond such a position, impact force and its reaction force act on the first group cam follower  111  and the first group cam groove  210  in this state. Similarly, the impact force and the reaction force act on the cam follower  151 , the cam slot  131 , and the translational groove  162 . However, the first group cam follower  111  is positioned in the portion of the first group cam groove  210 , which is substantially parallel to the circumferential direction, and the cam follower  151  is positioned in the portion of the cam slot  131 , which is substantially parallel to the circumferential direction. Thus, most of the impact force acts on the first group frame  110 , the cam frame  150 , and the cam slot frame  130  in the optical axis direction, and does not act in the rotational direction. Suppose that great impact force in the rotational direction acts on the first group frame  110 . Such great impact force in the rotational direction also acts on the translational frame  140  engaged with the first group frame  110 . Thus, there is a possibility that the engagement protrusion  141  engaged with the translational groove  132  of the cam slot frame  130  is damaged. If great force in the rotational direction acts on the cam slot  131 , there is a possibility that the engagement protrusion  141  of the translational frame  140 , which is engaged with the translational groove  132  is damaged, and the first engagement protrusion  133  of the cam slot frame  130 , which is engaged with the translational groove of the fixed frame  170  is damaged. Further, there is a possibility that other members such as the first group cam follower  111 , the cam follower  151 , etc. are damaged. On the other hand, according to the foregoing configuration, little force in the rotational direction acts on the first group frame  110 , the cam frame  150 , and the cam slot frame  130 , thereby reducing the possibility of causing the damage of the lens barrel  100 . 
     Thus, the lens barrel  100  of the present embodiment the zoom optical system including the first to third lens groups  301 - 303  each having one or more lenses, in which the focal distance is changeable by moving the lens groups in the optical axis direction; the first group frame  110  moving together with the first lens group  301  closest to the object in the zoom optical system; and the first and second cam mechanisms transmitting drive force to the first group frame  110 . Each of the first and second cam mechanisms includes the cam follower and the cam groove engaged with the cam follower and including the section in which each of the cam followers moves when the focal distance is changed between the wide-angle end and the tele end. The section of at least one of the cam grooves (cam slot  131 ) of the first and second cam mechanisms includes the first section  131   a  which has the predetermined gradient to the circumferential direction about the optical axis, the second section  131   b  which adjoins the first section  131   a  and has the absolute value of the gradient smaller than that of the first section  131   a , and the third section  131   c  which adjoins the second section  131   b  and has the absolute value of the gradient larger than that of the second section  131   b , and in which the sign of the gradient is the same as that of the first section  131   a . The distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction has the local minimum in the second section  131   b . According to such a configuration, when external force acts on the first group frame  110 , and the first group frame  110  is pushed to the position where the distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction is the local minimum, acting of great force in the rotational direction on the first group frame  110  and other members associated therewith can be reduced. Thus, strength of the lens barrel  100  against external force can be improved. 
     In all of the cam grooves involved in the movement of the first group frame  110  in the optical axis direction, the portion where the corresponding cam follower is positioned when the distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction is the local minimum extends along the circumferential direction. Thus, force in the rotational direction, which acts on the first group frame  110  and all of the frames including the cam grooves can be reduced. 
     Each of the first group cam groove  210  which is the cam groove of the first cam mechanism, and the cam slot  131  which is the cam groove of the second cam mechanism includes the second section (flat section) in which the absolute value of the gradient is smaller than those of adjoining sections sandwiching the second section. In at least one of the first group cam groove  210  and the cam slot  131 , the sign of the gradient is the same between the first and third sections which sandwich the second section. 
     &lt;&lt;Second Embodiment&gt;&gt; 
     Next, a lens barrel of a second embodiment will be described.  FIG. 3  corresponds to  FIG. 2 .  FIG. 3  illustrates a locus of a first group frame  110 , and a development diagram of each of a fixed cam groove  171 , a cam slot  131 , and a first group cam groove  210 . The lens barrel of the present embodiment is different from that of the first embodiment in shapes of a fixed cam groove  171  and a cam slot  131 . Thus, the shapes of the fixed cam groove  171  and the cam slot  131  will be mainly described. 
     The fixed cam groove  171  obliquely and linearly extends from a retracted position to a wide-angle end and an object in a retracted region. However, a change amount in an optical axis direction is small in the retracted region. In a zoom region, the fixed cam groove  171  obliquely extends from the wide-angle end to a tele end and the object, and substantially extends parallel to a circumferential direction toward the tele end in the middle of the zoom region. Then, the fixed cam groove  171  obliquely extends toward the tele end and the object again, and reaches the tele end. Specifically, the fixed cam groove  171  in the zoom region includes the followings from the wide-angle end to the tele end: a first section  171   a  having a positive gradient, a second section  171   b  in which an absolute value of the gradient is smaller than that of the first section  171   a , and a third section  171   c  in which a sign of the gradient is positive as in the first section  171   a , and the absolute value of the gradient is larger than that of the second section  171   b . More specifically, the second section  171   b  includes a portion which is substantially parallel to the circumferential direction. The first section  171   a  includes an inflection point at which a change in gradient is turned from an increase to a decrease. In addition, the second section  171   b  includes an inflection point at which the change in gradient is turned from the decrease to the increase. Further, the third section  171   c  includes an inflection point at which the change in gradient is turned from the increase to the decrease. That is, when a drive cam follower  161  moves along the fixed cam groove  171  from the wide-angle end to the tele end, the drive cam follower  161  moves toward the object in the optical axis direction, a displacement of the drive cam follower  161  in the optical axis direction becomes temporarily zero or extremely small, and the drive cam follower  161  moves toward the object in the optical axis direction again. 
     The cam slot  131  obliquely and linearly extends from the retracted position to the wide-angle end and the object in the retracted region. In the zoom region, the cam slot  131  substantially extends parallel to the circumferential direction from the wide-angle end to the tele end. That is, a cam follower  151  does not move in the optical axis direction while moving in the zoom region of the cam slot  131 . 
     A shape of a first group cam groove  210  of the present embodiment is similar to that of the first embodiment. 
     As described above, in the zoom region, drive force in a rotational direction from a zoom motor unit  173  is transmitted to a drive frame  160  as drive force in the rotational direction and the optical axis direction through a second cam mechanism formed by the fixed cam groove  171  and the drive cam follower  161 . Then, the drive force transmitted to the drive frame  160  is transmitted to the cam frame  150  without change through the cam slot  131  and the cam follower  151 . Further, the drive force transmitted to the cam frame  150  is transmitted to a first group frame  110  as drive force in the optical axis direction through a first cam mechanism formed by the first group cam groove  210  and a first group cam follower  111 . As a result, the first group frame  110  is driven in the optical axis direction. In the second embodiment, the first group frame  110  forms a first frame, and the drive frame  160  forms a second frame. In addition, a fixed frame  170  forms a third frame. 
     &lt;&lt;Third Embodiment&gt;&gt; 
     Next, a lens barrel of a third embodiment will be described.  FIG. 4  corresponds to  FIG. 2 .  FIG. 4  illustrates a locus of a first group frame  110 , and a development diagram of each of a fixed cam groove  171 , a cam slot  131 , and a first group cam groove  210 . The lens barrel of the present embodiment is different from that of the first embodiment in shapes of a fixed cam groove  171  and a cam slot  131 . Thus, the shapes of the fixed cam groove  171  and the cam slot  131  will be mainly described. 
     The fixed cam groove  171  obliquely and linearly extends from a retracted position to a wide-angle end and an object in a retracted region. However, a change amount in an optical axis direction is small in the retracted region. In a zoom region, the fixed cam groove  171  obliquely extends from the wide-angle end to a tele end and the object, and substantially extends parallel to a circumferential direction toward the tele end in the middle of the zoom region. Then, the fixed cam groove  171  obliquely extends toward the tele end and the object again, and reaches the tele end. Specifically, the fixed cam groove  171  in the zoom region includes the following from the wide-angle end to the tele end: a first section  171   a  having a positive gradient, a second section  171   b  in which an absolute value of the gradient is smaller than that of the first section  171   a , and a third section  171   c  in which a sign of the gradient is positive as in the first section  171   a , and the absolute value of the gradient is larger than that of the second section  171   b . More specifically, the second section  171   b  includes a portion which is substantially parallel to the circumferential direction. The first section  171   a  includes an inflection point at which a change in gradient is turned from an increase to a decrease. In addition, the second section  171   b  includes an inflection point at which the change in gradient is turned from the decrease to the increase. Further, the third section  171   c  includes an inflection point at which the change in gradient is turned from the increase to the decrease. That is, when a drive cam follower  161  moves along the fixed cam groove  171  from the wide-angle end to the tele end, the drive cam follower  161  moves toward the object in the optical axis direction. Then, a displacement of the drive cam follower  161  in the optical axis direction becomes temporarily zero or extremely small, and the drive cam follower  161  moves toward the object in the optical axis direction again. 
     The cam slot  131  obliquely and linearly extends from the retracted position to the wide-angle end and the object in the retracted region. In the zoom region, the cam slot  131  obliquely extends from the wide-angle end to the tele end and the object. 
     Although a degree of the gradient is different between the present embodiment and the first embodiment, an entire shape of a first group cam groove  210  of the present embodiment is similar to that of the first embodiment. 
     As described above, in the zoom region, drive force in a rotational direction from a zoom motor unit  173  is transmitted to a drive frame  160  as drive force in the rotational direction and the optical axis direction through a second cam mechanism formed by the fixed cam groove  171  and the drive cam follower  161 . Then, the drive force transmitted to the drive frame  160  is transmitted to the cam frame  150  as drive force in the rotational direction and the optical axis direction through the cam slot  131  and the cam follower  151 . Further, the drive force transmitted to the cam frame  150  is transmitted to a first group frame  110  as drive force in the optical axis direction through a first cam mechanism formed by the first group cam groove  210  and a first group cam follower  111 . As a result, the first group frame  110  is driven in the optical axis direction. In the third embodiment, the first group frame  110  forms a first frame, and the drive frame  160  forms a second frame. In addition, a fixed frame  170  forms a third frame. 
     &lt;&lt;Fourth Embodiment&gt;&gt; 
     Next, a lens barrel of a fourth embodiment will be described.  FIG. 5  corresponds to  FIG. 2 .  FIG. 5  illustrates a locus of a first group frame  110 , and a development diagram of each of a fixed cam groove  171 , a cam slot  131 , and a first group cam groove  210 . The lens barrel of the present embodiment is different from that of the first embodiment in shapes of a cam slot  131  and a first group cam groove  210 . Thus, the shapes of the cam slot  131  and the first group cam groove  210  will be mainly described. 
     A shape of a fixed cam groove  171  of the present embodiment is similar to that of the first embodiment. 
     The cam slot  131  obliquely and linearly extends from a retracted position to a wide-angle end and an object in a retracted region. In addition, the cam slot  131  obliquely extends from the wide-angle end to a tele end and the object in the zoom region, and extends along a circumferential direction toward the tele end in the middle of the zoom region. Then, the cam slot  131  obliquely extends toward the tele end and the object again, and reaches the tele end. The present embodiment is similar to the first embodiment in that the cam slot  131  in the zoom region includes the following from the wide-angle end to the tele end: a first section  131   a  having a positive gradient, a second section  131   b  in which an absolute value of the gradient is smaller than that of the first section  131   a , and a third section  131   c  in which a sign of the gradient is positive as in the first section  131   a , and the absolute value of the gradient is larger than that of the second section  131   b . However, in the first embodiment, the cam slot  131  in the second section  131   b  is substantially parallel to the circumferential direction. On the other hand, in the fourth embodiment, although the cam slot  131  in the second section  131   b  extends along the circumferential direction, the cam slot  131  is slightly inclined to the circumferential direction. In particular, at a point in the second section  131   b , at which a distance from an imaging surface  101   a  to a first group frame  110  in the optical axis direction is a local minimum, an angle of inclination to the circumferential direction is θ. 
     The first group cam groove  210  obliquely and linearly extends from the retracted position to the wide-angle end and the object in the retracted region. In addition, the first group cam groove  210  obliquely extends from the wide-angle end to the tele end and the imaging surface in the zoom region, and then extends along the circumferential direction toward the tele end. Subsequently, the first group cam groove  210  obliquely extends toward the tele end and the object, and reaches the tele end. The present embodiment is similar to the first embodiment in that the first group cam groove  210  in the zoom region includes the following from the wide-angle end to the tele end: a first section  210   a  having a negative gradient, a second section  210   b  in which the absolute value of the gradient is smaller than that of the first section  210   a , and a third section  210   c  in which the sign of the gradient is positive, and the absolute value of the gradient is larger than that of the second section  210   b . However, in the first embodiment, the first group cam groove  210  in the second section  210   b  is substantially parallel to the circumferential direction. On the other hand, in the fourth embodiment, although the first group cam groove  210  in the second section  210   b  extends along the circumferential direction, the first group cam groove  210  is slightly inclined to the circumferential direction. In particular, at a point in the second section  210   b , at which the distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction is the local minimum, the angle of inclination to the circumferential direction is −θ. That is, an inclination direction at such a point is opposite to an inclination direction of the second section  131   b  of the cam slot  131 , and an absolute value of such an inclination is the same between the second section  210   b  of the first group cam groove  210  and the second section  131   b  of the cam slot  131 . This results in the local minimum for the distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction at the foregoing point. 
     As in the foregoing case, the second sections  131   b ,  210   b  in which the angle of inclination is gradual (i.e., the absolute value of the gradient is smaller than those of adjoining sections) are provided in the cam slot  131  and the first group cam groove  210 , respectively. Thus, when external force in the optical axis direction acts on the first group frame  110 , force in a rotational direction, which acts on the first group frame  110  and other members associated therewith can be reduced as compared to a case where the second sections  131   b ,  210   b  are not provided. This improves strength of the lens barrel  100  against external force. 
     A coefficient of dynamic friction μ between a cam follower and a cam groove, and the angle of inclination θ are set so as to satisfy a relationship of tan θ≦μ. Thus, when external force in the optical axis direction acts on the first group frame  110 , force in the rotational direction, which acts on the first group frame  110  and other frames associated therewith can be further reduced. That is, by satisfying the relationship represented by the foregoing expression, friction force between the cam follower and the cam groove becomes larger than force rotating the frame, and therefore force in the rotational direction, which acts on the frame can be further reduced. When the coefficient of dynamic friction μ is, e.g., about 0.2, the angle of inclination θ may be equal to or less than about 11.3 degrees. When the coefficient of dynamic friction μ is about 0.1, the angle of inclination θ may be equal to or less than about 5.7 degrees. Fluorine lubricant etc. having a small coefficient of friction may be used for the lens barrel  100 . If such lubricant is used, the coefficient of dynamic friction μ is about 0.05-0.1. In such a case, if μ=0.1, it is preferred that the angle of inclination θ is equal to or less than about 5.7 degrees. 
     &lt;&lt;Other Embodiment&gt;&gt; 
     The foregoing embodiments may have the following configurations. 
     That is, a configuration of the lens barrel  100  is not limited to the foregoing configurations. Any configurations may be employed in the lens group and the frame. 
     The first group frame  110  moves according to the shapes of the fixed cam groove  171 , the cam slot  131 , and the first group cam groove  210 , but the present disclosure is not limited to such a configuration. A configuration may be employed, in which the first group frame  110  moves according to a cam groove provided in a frame other than the fixed cam frame  170 , the cam slot frame  130 , and the first group cam frame  110 . 
     In the first and fourth embodiments, the second sections  210   b ,  131   b  in which the absolute value of the gradient is smaller than those of the adjoining sections are provided in both of the first group cam groove  210  and the cam slot  131 , but the present disclosure is not limited to such a configuration. For example, a section in which an absolute value of the gradient is smaller that those of adjoining sections may be provided in either one of the first group cam groove  210  and the cam slot  131 , and a cam follower may be positioned in such a section when the distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction is the local minimum. Even in such a configuration, an advantage can be realized, in which force in the rotational direction, which acts on the first group frame  110  and the frames associated therewith can be reduced. In addition, in the second and third embodiments, the second sections  210   b ,  171   b  in which the absolute value of the gradient is smaller than those of the adjoining sections are provided in both of the first group cam groove  210  and the fixed cam groove  171 , but the present disclosure is not limited to such a configuration. As described above, the section in which the absolute value of the gradient is smaller that those of the adjoining sections may be provided in either one of the first group cam groove  210  and the fixed cam groove  171 , and the cam follower may be positioned in such a section when the distance from the imaging surface  101   a  to the first group frame  110  in the optical axis direction is the local minimum. 
     In the foregoing embodiments, each of the first group cam groove  210 , the cam slot  131 , and the fixed cam groove  171  includes the portion which substantially extends parallel to the circumferential direction, but such a portion is not necessarily parallel to the circumferential direction. That is, the portion may be extend along the circumferential direction. 
     In the foregoing embodiments, each of the cam grooves of the first and second cam mechanisms includes the second section in which the absolute value of the gradient is smaller than those of the adjoining sections sandwiching the second section. In one of the second sections, the sign of the gradient is the same between the first and third sections which sandwich the second section. In the other second section, the sign of the gradient is opposite between the first and third sections which sandwich the second section. However, the present disclosure is not limited to such a configuration. In both of the second sections of the cam grooves of the first and second cam mechanisms, the sign of the gradient may be the same between the first and third sections which sandwich the second section. 
     As described above, the present disclosure is useful for the lens barrel. 
     The description of the embodiments of the present disclosure is given above for the understanding of the present disclosure. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.