Abstract:
A lens barrier driving device that prevents an increase in the number of parts constituting a lens barrier driving mechanism and avoids light leakage from a penetrating portion. A lens barrier is disposed on an object side of a lens and is openable and closable. A transmission member transmits power to a barrier driving member. The barrier driving member opens and closes the lens barrier. A cam member is able to engage with the transmission member. A first member holds the cam member. A second member is provided with a penetrating portion. The penetrating portion is disposed between the lens barrier and the first member. A rib portion of the transmission member penetrates the penetrating portion. A light shielding portion of the transmission member shields a incident light through the penetrating portion.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a lens barrier driving device that drives a lens barrier in a lens barrel mounted in an image pickup apparatus such as a silver film camera or an electronic camera, as well as the lens barrel and the image pickup apparatus. 
   2. Description of the Related Art 
   Many conventional cameras have lens barrels in which lens barriers are mounted. The lens barrier shuts off a surface of a photographic lens from the outside while pictures are not taken. This thus protects the photographic lens from attachment of sebum resulting from the user&#39;s touch or attachment of dust or the like to prevent optical performance from being degraded. 
   Available lens barrier driving methods include a method in which a photographer manually opens and closes or mounts and removes the lens barrier and a method in which the lens barrier is automatically opened when the camera is ready for shooting and closed when the camera is in a standby or power-off state. An example of a mechanism is shown below which automatically opens and closes the lens barrier. 
     FIG. 12A  is an exploded perspective view showing the components of a lens barrel according to a conventional example.  FIG. 12B  is a perspective view showing the assembled lens barrel.  FIGS. 13A and 13C  are diagrams schematically showing how a driving mechanism operates to sink the lens barrel.  FIGS. 13B and 13D  are diagrams schematically showing how the driving mechanism operates to withdraw the lens barrel. 
   In  FIGS. 12A to 13D , a first group barrel  100  holds a first lens group and internally has a mechanism that drives a lens barrier  111 . A barrier driving lever  112  drivingly opens and closes the lens barrier  111 . A barrier opening spring  113  always biases the lens barrier  111  in a direction in which it is opened. A barrier closing spring  115  biases the lens barrier  111  in a direction in which it is closed. A lens barrier driving plate  114  drives the lens barrier  111  in the closing direction. The lens barrier driving plate  114  has a hole portion  114   a  and a notch portion  114   b . A tapered cam member  130   a  is disposed on a barrel base plate  130  to open and close the lens barrier  111 . Reference numerals  116  and  117  denote a barrier cover and an external cap respectively. 
   Using an image pickup apparatus mounted with the lens barrel described above, when a photographer suspends or temporarily pauses shooting, the lens barrel including the first group barrel  100  is housed (sunken) in the image pickup apparatus main body. During transition to the housed state, an abutting member  112   a  of the barrier driving lever  112  abuts against a slope near a tip of the cam member  130   a  (see  FIG. 13A ). The abutting member  112   a  slidably moves on the slope of the cam member  130   a  to rotatively drive the barrier driving lever  112 . Thus, the spring force charged in the barrier closing spring  115  applies a rotative driving force to the lens barrier driving plate  114 . The rotative driving force is utilized to close the lens barrier  111 . 
   Once the lens barrel is completely housed in the image pickup apparatus main body, the photographic lens is covered with the lens barrier  111 . If the state in which the lens barrel is housed in the image pickup apparatus main body is transited to an image taking enabled state, the first group barrel  100  is gradually separated from the barrel base plate  130 . At this time, the spring force charged in the barrier opening spring  113  is gradually released to pivot the barrier driving lever  112  counterclockwise in  FIG. 13D . Pivoting of the barrier driving lever  112  causes rotation of the lens barrier driving plate  114  and counterclockwise rotation of the lens barrier  111  to perform an opening operation. 
   On the other hand, a lens barrier driving method has been proposed which drives the lens barrier utilizing cam drive on the basis of the relationship between the relative positions of a first and second lens groups (see, for example, Japanese Laid-Open Patent Publication No. H07-159855). 
   Another lens barrier driving method has been proposed which transmits the rotating force of the rotating barrel to a transmission member and utilizes the transmitted force to drive the lens barrier (see, for example, Japanese Laid-Open Patent Publication No. H10-186453). 
   Another lens barrier driving method has been proposed which drives the lens barrier utilizing cam drive on the basis of the relationship between the relative positions of a first and third lens groups (see, for example, Japanese Laid-Open Patent Publication No. H07-005547). 
   With the conventional technique shown in  FIGS. 12A to 13D , the barrier driving lever  112  and the cam member  130   a  are arranged as follows. To reduce the size of the lens barrel, a penetrating area is formed in a part of a barrel such as a second or third group barrel (not shown) that holds another lens group so that the barrier driving lever  112  and cam member  130   a  can penetrate the penetrating area. Thus, with image taking enabled, appropriately arranging the lens groups results in a cavity in the penetrating area in the second or third group barrel, which area is penetrated by the barrier driving lever  112  and cam member  130   a.    
   This leads to a light leakage phenomenon in which external light travels through the cavity to an image forming surface, or ghosts caused by, for example, light reflected from an inner wall surface of the barrel. As a result, the optical performance may be degraded. To avoid this, the penetrating portion through which the barrier driving lever  112  and cam member  130   a  penetrate is provided in the direction of 12 o&#39;clock with respect to the optical axis as observed from an object. This thus suppresses the adverse effect of light leakage. However, this structure imposes a restriction in constructing the barrel, thus reducing the degree of freedom of barrel design. 
   With the conventional technique described in Japanese Laid-Open Patent Publication No. H07-159855, if a lens configuration is adopted in which the first and second lens groups are brought closer to each other in an image taking area near a telescope end than when the lens barrel is sunken, the following mechanism is provided. No transmission member is provided between a barrier opening and closing ring and a cam member. Instead, a mechanism for allowing the cam member to escape is provided so as to prevent the cam member from engaging with the barrier opening and closing ring. This disadvantageously increases the number of parts required. 
   With the conventional technique described in Japanese Laid-Open Patent Publication No. H10-186453, the barrel holding the transmission member requires a gap corresponding to the stroke of rotation of the barrel. This may disadvantageously lead to light leakage. 
   The conventional technique described in Japanese Laid-Open Patent Publication No. H07-005547 reduces a variation in the spacing between the first and third lens groups in the image taking area. This technique further opens and closes the barrier using the transmission member for transmitting power to the lens group. This requires a space to be formed in the second lens group, located between the first and third lens groups, so as to allow the penetration of the transmission member. This may disadvantageously lead to light leakage in that space. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a lens barrier driving device that suppresses an increase in the number of parts constituting a lens barrier driving mechanism and avoids light leakage from a penetrating portion, ghosts, and the like which may degrade the optical performance, as well as a lens barrel and a image pickup apparatus. 
   To attain the above object, in a first aspect of the present invention, there is provided a lens barrier driving device driving a lens barrier which is disposed on an object side of a lens and is openable and closable, the lens barrier driving device comprising a transmission member that transmits power to a barrier driving member which opens and closes the lens barrier, a cam member that is able to engage with the transmission member, a first member that holds the cam member, and a second member that is provided with a penetrating portion, wherein the penetrating portion is disposed between the lens barrier and the first member and the transmission member comprises a rib portion that penetrates the penetrating portion and a light shielding portion that shields a incident light through the penetrating portion. 
   Preferably, the transmission member is supported by the second member. 
   Preferably, the cam member and the transmission member are not engaged with each other during photographing. 
   Preferably, the cam member and the transmission member are engaged with each other during non-photographing, and a cam drive between the cam member and the transmission member causes the lens barrier to open or close. 
   Preferably, during non-photographing, the barrier driving member and the transmission member are engaged with each other before the cam member and the transmission member are engaged. 
   Preferably, during photographing, the cam member advances into the penetrating portion of the second member to shield the penetrating portion together with the light shielding portion of the transmission member. 
   Further preferably, the lens barrier driving device further comprises biasing means that biases the transmission member so that the light shielding portion shields a space in the penetrating portion into which the cam member advances. 
   With the arrangement of the first aspect of the present invention, the transmission member shields the penetrating portion of the second member supporting the transmission member. This makes it possible to suppress an increase in the number of parts constituting the lens barrier driving mechanism of the second member and avoid light leakage from the penetrating portion, ghosts, and the like which may degrade the optical performance. 
   To attain the above object, in a second aspect of the present invention, there is provided a lens barrel comprising the lens barrier driving device described above. 
   Preferably, the lens barrel is of a sink type that can be housed in a housing of an image pickup apparatus. 
   To attain the above object, in a third aspect of the present invention, there is provided an image pickup apparatus comprising the lens barrel described above. 
   The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view showing the components of a lens barrel according to an embodiment of the present invention. 
       FIG. 2  is an exploded perspective view showing the configuration of a second group barrel, a transmission member, a barrier driving lever, and the like which are shown in  FIG. 1 . 
       FIG. 3  is a diagram showing the positional relationship between a moving cam ring and a fixed cylinder which are shown in  FIG. 1 . 
       FIG. 4  is a diagram showing the positional relationship among the moving cam ring, an advancing cylinder, a first group barrel and a second group barrel which are shown in  FIG. 1 . 
       FIG. 5  is a sectional view showing the configuration of a barrier driving lever shown in  FIG. 1 . 
       FIG. 6  is a sectional view showing the configuration of the barrier driving lever shown in  FIG. 1 . 
       FIG. 7A  is a diagram showing that lens barriers are closed,  FIG. 7B  is a diagram showing how the two barrier driving levers are arranged, and  FIG. 7C  is a diagram showing how the two lens barriers are arranged. 
       FIG. 8  is a diagram showing that the lens barriers are open. 
       FIG. 9  is a diagram showing how gears of a zoom gear unit are arranged. 
       FIG. 10A  is a diagram showing how a photo interrupter for a worm gear shown in  FIG. 9  is placed,  FIG. 10B  is a diagram showing the output waveform of the photo interrupter obtained when the worm gear shown in  FIG. 9  rotates clockwise, and  FIG. 10C  is a diagram showing the output waveform of the photo interrupter obtained when the worm gear shown in  FIG. 9  rotates counterclockwise. 
       FIG. 11  is a diagram showing movement trajectories of the moving cam ring, first group barrel, and second group barrel which are shown in  FIG. 1 . 
       FIG. 12A  is an exploded perspective view showing the components of a lens barrel according to a conventional example, and  FIG. 12B  is a perspective view showing the assembled lens barrel. 
       FIGS. 13A and 13C  are diagrams schematically showing how a driving mechanism operates to sink the lens barrel, and  FIGS. 13B and 13D  are diagrams schematically showing how the driving mechanism operates to withdraw the lens barrel. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An embodiment of the present invention will be described below with reference to the drawings. 
     FIG. 1  is an exploded perspective view showing the components of a lens barrel according to an embodiment of the present invention.  FIG. 2  is an exploded perspective view showing the configuration of a second group barrel, a transmission member, a barrier driving lever, and the like which are shown in  FIG. 1 .  FIG. 3  is a diagram showing the positional relationship between a moving cam ring and a fixed cylinder which are shown in  FIG. 1 .  FIG. 4  is a diagram showing the positional relationship among the moving cam ring, an advancing cylinder, a first group barrel and a second group barrel which are shown in  FIG. 1 . 
   In  FIGS. 1 to 4 , the lens barrel (barrel unit) comprises a first group barrel  10 , a lens barrier  12 , a lens barrier  13 , a barrier driving lever  14 , a barrier driving lever  15 , a second group barrel  20 , a transmission member  21 , a barrel base plate  30 , a fixed cylinder  31 , a moving cam ring  32 , and an advancing cylinder  34 . The lens barrel is mounted in the image pickup apparatus. When the image pickup apparatus is powered off, the lens barrel is sunken in an axial direction and housed in the image pickup apparatus main body. 
   First, description will be given of the configuration of the lens barrel except for the lens barrier and lens barrier driving mechanism. 
   The first group barrel  10  holds a lens holder  3  holding a first lens group  1 . The second group barrel  20  holds a second lens group  2 . A barrier cover  4  is disposed in front of the first group barrel  10  to press the lens barriers  12  and  13 . An external cap  5  is mounted in front of the barrier cover  4 . The barrel base plate  30  comprises a tapered cam portion (rib portion)  30   a  as shown in  FIG. 2 . The tapered cam portion  30   a  has a tapered surface that abuts against a rib portion  21   b  of the transmission member  21  which will be described later. The barrel base plate  30  and fixed cylinder  31  are fixed by screws during assembly of the lens barrel. 
   Gear teeth  32   a  (see  FIG. 9 ) are formed on an outer peripheral portion of the moving cam ring  32 . The gear teeth  32   a  of the moving cam ring  32  engages with a gear  58  constituting a part of a zoom gear unit  50  comprising a DC motor  51  and speed reduction gear systems  53  to  58  described later. When the DC motor  51  is energized, the rotating force of the DC motor  51  is reduced by the zoom gear unit  50 . The reduced force is then transmitted to the gear teeth  32   a  of the moving cam ring  32  to rotatively drive the moving cam ring  32 . 
   Three holes (not shown) are formed on an outer peripheral surface of the moving cam ring  32  at equal intervals along a circumferential direction. Three moving cam followers  33  are pressed in the respective holes. The moving cam followers  33  are movable along a cam groove  31   a  (see  FIG. 3 ) formed in an inner peripheral surface of the fixed cylinder  31 . The moving cam ring  32  is thus rotatively driven by the DC motor  51  such that the moving cam ring  32  rotatively moves along a cam trajectory shown by arrows in  FIG. 3 . The gear  58  has teeth that are long enough along the direction of the optical axis so as not to disengage from the moving cam ring  32 , which rotatively moves in the optical axis direction. 
   Cam grooves  32   b  and  32   c  are also formed in an inner peripheral surface of the moving cam ring  32 . The cam grooves  32   b  and  32   c  are provided to move the first group barrel  10  and second group barrel  20  in the optical axis direction. 
   The advancing cylinder  34  has an outer peripheral surface fitted on the inner peripheral surface of the moving cam ring  32 . A protruding portion  34   a  (see  FIG. 4 ) is provided on one end of the advancing cylinder  34  in the axial direction. The protruding portion  34   a  is fitted into a groove formed at a tip of the moving cam ring  32 . The groove in the moving cam ring  32  is formed all over the circumference of the moving cam ring  32 . The advancing cylinder  34  thus rotates freely and moves integrally with the moving cam ring  32 . 
   A flange portion  34   b  and a protruding portion  34   c  are provided at the other end of the advancing cylinder  34  in the axial direction; the protruding portion  34   c  projects from the flange portion  34   b . The protruding portion  34   c  is fitted into an advancing key  31   b  formed in an inner peripheral surface of the fixed cylinder  31  along the optical axis direction. This regulates the movement of the advancing cylinder  34  in a direction in which it rotates when the moving cam ring  32  rotates. 
   Slot portions  34   d  are formed in an inner peripheral surface of the advancing cylinder  34  and parallel to the optical axis direction. Three elliptical protruding portions  10   a  are fitted into the slot portions  34   d ; the protruding portions  10   a  are formed on an outer peripheral surface of the first group barrel  10  at equal intervals. This enables the first group barrel  10  to non-rotatively move in the optical axis direction within the area in which the slot portions  34   d  in the advancing cylinder  34  are formed. 
   Slot portions  34   e  are formed in a cylindrical surface of the advancing cylinder  34  and parallel to the optical axis direction. Two cam follower portions  20   a  and a movable follower  23  are fitted into the slot portions  34   e ; the cam follower portions  20   a  and movable follower  23  are provided on an outer peripheral surface of the second group barrel  20 . This enables the second group barrel  20  to non-rotatively move in the optical axis direction within the area in which the slot portions  34   e  in the advancing cylinder  34  are formed. 
   The three elliptical protruding portions  10   a  are provided on the outer peripheral surface of the first group barrel  10  at equal intervals. A cam follower  35  is pressed in the center of each of the elliptical protruding portions  10   a . The cam followers  35  move along cam grooves  32   b  formed in the inner peripheral surface of the moving cam ring  32 . Thus, rotatively driving the moving cam ring  32  moves the first group barrel  10  along the cam grooves  32   b  in the moving cam ring  32  in the optical axis direction because of the rotational regulation on the first group barrel  10  (see  FIG. 4 ). 
   Cam follower portions  20   a  and a cam follower hole  20   b  (see  FIG. 4 ) are formed on the outer peripheral surface of the second group barrel  20 . A movable follower  23  and a follower spring  24  are also incorporated into the outer peripheral surface of the second group barrel  20 ; the movable follower  23  and follower spring  24  are fitted into the cam follower hole  20   b . The cam follower portions  20   a  and movable follower  23  move along cam grooves  32   c  formed in the inner peripheral surface of the moving cam ring  32 . Thus, rotatively driving the moving cam ring  32  moves the second group barrel  20  along the cam grooves  32   c  in the moving cam ring  32  in the optical axis direction because of the rotational regulation on the second group barrel  20  (see  FIG. 4 ). 
   Now, a detailed description will be given of the configuration of the lens barrier and lens barrier driving mechanism in the lens barrel. 
     FIGS. 5 and 6  are sectional views showing the configuration of the barrier driving levers  14  and  15 , shown in  FIG. 1 .  FIG. 7A  is a diagram showing that the lens barriers  12  and  13 , shown in  FIG. 1 , are closed.  FIG. 7B  is a diagram showing how the barrier driving levers  14  and  15  are arranged.  FIG. 7C  is a diagram showing how the lens barriers  12  and  13  are arranged.  FIG. 8  is a diagram showing that the lens barriers  12  and  13  are open. 
   In  FIGS. 1 and 5  to  8 , the above first group barrel  10  not only serves as a holding member holding the first lens group  1  but also constitutes a mechanism for drivingly opening and closing the lens barriers  12  and  13 , which protect the first lens group  1 . The barrier shaft plate  18  is fixed to the barrier base plate  11  by screws or the like. A through-hole  18   c  (see  FIG. 5 ) is formed in the barrier shaft plate  18  so that the barrier driving lever  14  can rotate axially through the through-hole  18   c . A shaft portion  11   a  and a fitting hole  11   b  (see  FIGS. 7A and 7C ) are provided on a front surface of the barrier base plate  11 . 
   The lens barrier  12  is provided with a fitting hole  12   a , a gear portion  12   b , and a slot  12   c . The lens barrier  13  is provided with a shaft portion  13   a  and a gear portion  13   b . The shaft portion  11   a  of the barrier base plate  11  is fitted into the fitting hole  12   a  in the lens barrier  12 . The fitting hole  11   b  in the barrier base plate  11  is fitted around the shaft portion  13   a  of the lens barrier  13 . The lens barriers  12  and  13  are engaged with each other via the gear portions  12   b  and  13   b  and rotate around the fitting hole  12   a  and shaft portion  13   a , respectively. 
   The barrier driving lever  14  is provided with a boss  14   a , a recess portion  14   b , and a shaft hole  14   c . The barrier driving lever  15  is provided with a pawl portion  15   a , a projecting portion  15   b , and a key portion  15   c . The barrier driving lever  14  is fitted into the through-hole  18   c  in the barrier shaft plate  18 . The boss  14   a  of the barrier driving lever  14  is fitted into the slot  12   c  in the lens barrier  12 . The barrier driving lever  15  is coaxially placed in the shaft hole  14   c  in the barrier driving lever  14 . To incorporate the barrier driving lever  15  into the shaft hole  14   c  in the barrier driving lever  14 , the pawl portion  15   a  of the barrier driving lever  15  is elastically deformed. 
   The tip  15   a  of the barrier driving lever  15  need not necessarily be shaped like a pawl. As shown in  FIG. 6 , a tightening member such as an E ring or a C ring may be used to incorporate the barrier driving lever  15  into the shaft hole  14   c  in the barrier driving lever  14 . 
   A barrier closing spring  16  is connected to the barrier driving lever  14  to exert a biasing force acting clockwise in  FIG. 7A . A barrier opening spring  17  is connected to the barrier driving lever  15  to exert a counterclockwise biasing force. One end of recess portion  14   b  of the barrier driving lever  14  abuts against the projecting portion  15   b  of the barrier driving lever  15 . 
   In this case, the recess portion  14   b  of the barrier driving lever  14  is formed to be wider than the projecting portion  15   b  of the barrier driving lever  15 . Thus, if the closed lens barriers  12  and  13  are forcibly opened, the other end of recess portion  14   b  of the barrier driving lever  14  does not abut against the projecting portion  15   b  of the barrier driving lever  15 . When the lens barriers  12  and  13  are in a normal state, the barrier closing spring  16  biases the recess portion  14   b  to the projecting portion  15   b.    
   The balance between a force exerted by the barrier closing spring  16  and a force exerted by the barrier opening spring  17  is set so that the barrier opening spring  17  exerts a stronger force than the barrier closing spring  16 . With no load imposed on the barrier driving lever  15 , the barrier driving lever  15  exerts a stronger rotating force than the barrier driving lever  14 . The barrel is thus designed so that the rotating force of the barrier driving lever  15  is transmitted to the barrier driving lever  14  to open the lens barriers  12  and  13 . 
     FIGS. 7A to 7C  shows the relationship between both lens barriers  12  and  13  and both barrier driving levers  14  and  15  observed while the lens barriers  12  and  13  are closed. Clockwise pivoting of the barrier driving lever  14  transits the state thereof shown in  FIG. 7A  to the one shown in  FIG. 8  to open the lens barriers  12  and  13 . Counterclockwise pivoting of the barrier driving lever  14  transits the state thereof shown in  FIG. 8  to the one shown in  FIG. 7A  to close the lens barriers  12  and  13 . A detailed description will be given below of the mechanism of opening and closing of the lens barrier. 
   A key portion  15   c  (see  FIG. 5 ) is provided at a shaft end of the barrier driving lever  15 . When the first group barrel  10  and the second group barrel  20  are closer to and within a predetermined distance from each other in the optical axis direction, the key portion  15   c  of the barrier driving lever  15  is fitted into the key groove portion  21   a  (see  FIG. 2 ), formed in the shaft of the transmission member  21  which is supported on the second group barrel  20  and will be described later. However, the key portion  15   c  is arbitrarily movable in the optical axis direction. 
   As shown in  FIG. 2 , the transmission member  21  comprises the key groove portion  21   a , the rib portion  21   b , and a light shielding portion  21   c . The transmission member  21  is supported on the second group barrel  20  and located coaxially with the barrier driving levers  14  and  15 , incorporated into the first group barrel  10 . The key groove portion  21   a  of the transmission member  21  engages with the key portion  15   c  of the barrier driving lever  15 . Once the first group barrel  10  and the second group barrel  20  approach each other in the optical axis direction to fit the barrier driving lever  15  into the transmission member  21 , the rotating force of the transmission member  21  is transmitted to the barrier driving lever  15 . 
   The transmission member  21  can be pivoted through a predetermined angle that is necessary and sufficient to open and close the lens barriers  12  and  13 . Specifically, a penetrating portion is formed in the second group barrel  20  by cutting away a part of the second group barrel  20 ; the penetrating potion corresponds to an area in which the transmission member  21  is pivoted through the predetermined angle. The penetrating portion penetrates the second group barrel  20  along the optical axis direction. The rib portion  21   b  of the transmission member  21  is disposed to penetrate the penetrating portion. The transmission member  21  is biased clockwise by a transmission member biasing spring  22 . 
   The light shielding portion  21   c  of the transmission member  21  serves to cover an area in which the rib portion  21   b  penetrates the second group barrel  20 , that is, the penetrating portion, before the rib portion  21   b  of the transmission member  21  abuts against the tapered cam portion  30   a  of the barrel base plate  30 , that is, in an image taking enabled state of the image pickup apparatus in which the lens barrel has been mounted. This is expected to produce a light shielding effect. 
   The rib portion  21   b  of the transmission member  21  is not engaged with the tapered cam portion  30   a  of the barrel base plate  30  during photographing with the image pickup apparatus in which the lens barrel has been mounted and is engaged with the tapered cam portion  30   a  in an image taking disabled state. In the image taking disabled state, the barrier driving lever  15  is fitted into the transmission member  21  before the rib portion  21   b  of the transmission member  21  engages with the tapered cam portion  30   a  of the barrel base plate  30 . 
     FIG. 9  is a diagram showing how gears of a zoom gear unit are arranged.  FIG. 10A  is a diagram showing how a photo interrupter for a worm gear shown in  FIG. 9  is placed.  FIG. 10B  is a diagram showing the output waveform of the photo interrupter obtained when the worm gear shown in  FIG. 9  rotates clockwise.  FIG. 10C  is a diagram showing the output waveform of the photo interrupter obtained when the worm gear shown in  FIG. 9  rotates counterclockwise.  FIG. 11  is a diagram showing movement trajectories of the moving cam ring, first group barrel, and second group barrel which are shown in  FIG. 1 . 
   In  FIGS. 9 to 11 , a zoom gear unit  50  comprises a DC motor  51 , a worm gear  53 , and gears  54  to  58  and is incorporated into the lens barrel. The DC motor  51  is fixed to a gear base by screws or the like. The worm gear  53  is fitted around the shaft of the DC motor  51 . The worm gear  53  engages with the gear  54 , and the gears  54  to  58  sequentially engage with one another. The gear  58  engages with the gear teeth  32   a  of the moving cam ring  32 . Driving the DC motor  51  rotates the worm gear  53  and thus the gears  54  to  58 . As a result, the moving cam ring  32  rotates. 
   The worm gear  53  is provided with three propellers  53   a  as shown in  FIG. 10A . Photo interrupters  61  and  62  are disposed near the three propellers  53   a  of the worm gear  53  and opposite each other across a predetermined slit. The zoom gear unit  50  extracts the rotative driving force of the DC motor  51  via the worm gear  53  and gears  54  to  58  to increase rotational torque. The rotational torque is then transmitted to the moving cam ring  32  via the gear  58  to generate power required to drive the lens barrel. 
   Rotation of the worm gear  53  around a shaft passes the three propellers  53   a  through the slits between the photo interrupters  61  and  62 . The passage of the propellers  53   a  cause photo diodes in the photo interrupters  61  and  62  to convert an optical signal indicating whether or not light is blocked, into an electric pulse signal. A controller (not shown) of the image pickup apparatus in which the lens barrel is mounted integrates the electric pulse signals to determine the rotation angle of the moving cam ring  32 . The controller thus calculates the focal distances of the image-taking optical system for telescope- and wide-angle-side image taking. 
   The combination of the components shown in  FIGS. 1 to 10A  constitutes the lens barrel (barrel unit) of the present embodiment. 
   Now, a detailed description will be given of the operation of the lens barrel of the present embodiment configured as described above and of the image pickup apparatus in which the lens barrel is mounted. 
   It is assumed that the image pickup apparatus is “powered off” and that the lens barrel is “sunken”. When the image pickup apparatus is then powered on, a current flow through the DC motor  51  to rotate the worm gear  53  clockwise (CW), which rotates the gears  54  to  58 . As a result, the moving cam ring  32  engaged with the gear  58  starts to rotate counterclockwise (CCW). Then, as described above, the moving cam ring  32  rotatively moves in the optical axis direction following the path of the cam groove  31   a , formed in the inner periphery of the fixed cylinder  31 . The first group barrel  10  and the second group barrel  20  correspondingly move in the optical axis direction following the path of the cam grooves  32   b  and  32   c , formed in the inner periphery of the moving cam ring  32 . 
     FIG. 11  shows the positional relationship among the moving cam ring  32 , the first group barrel  10  and the second group barrel  20 . Within a shaded area A between a sink area and a wide area which are shown in  FIG. 11 , the relationship between the relative positions of the first and second group barrels  10  and  20  remains almost unchanged. The barrier driving lever  15  remains fitted in the transmission member  21 , while the second group barrel  20  and the barrel base plate  30  gradually separate from each other. 
   The balance between a force exerted by the transmission member biasing spring  22 , a forces exerted by the barrier closing springs  16  and a force exerted by opening spring  17  results in a spring force acting to open the lens barriers  12  and  13 ; the transmission member biasing spring  22  biases the transmission member  21 . The gradual separation between the second group barrel  20  and the barrel base plate  30  opens the lens barriers  12  and  13 . 
   Once the lens barriers  12  and  13  are completely opened, the relationship between the relative positions of the first and second group barrels  10  and  20  changes. At this time, the key portion  15   c  of the barrier driving lever  15  connected to the first group barrel  10  separates from the key groove portion  21   a  in the transmission member  21  connected to the second group barrel  20 . However, under these conditions, the barrier opening spring  17  exerts a stronger force than the barrier closing spring  16 . This keeps the lens barriers  12  and  13  open. 
   On this occasion, the separated second group barrel  20  and barrel base plate  30  may approach each other again to engage the transmission member  21  with the tapered cam portion  30   a  of the barrel base plate  30 . The reason is as follows. The penetrating portion of the second group barrel  20  is only partly covered with the light shielding portion  21   c  of the transmission member  21 . However, the tapered cam portion  30   a  of the barrel base plate  30  advances into the free area (non-covered area) of the penetrating portion. Thus, both the light shielding portion  21   c  of the transmission member  21  and the tapered cam portion  30   a  of the barrel base plate  30  shield the penetrating portion of the second group barrel  20 . 
   In the present embodiment, in the image taking enabled area (wide area), the transmission member  21  and the tapered cam portion  30   a  of the barrel base plate  30  remain separate from each other. When the image taking enabled area is reached, the spring force of the transmission member biasing spring  22  biases the light shielding portion  21   c  of the transmission member  21  to a position where the light shielding portion  21   c  covers the space in the second group barrel  20  into which the tapered cam portion  30   a  of the barrel base plate  30  advances. This blocks light involved in light leakage or ghosts and which affects adversely the optical performance. 
   Now, it is assumed that the operator has operated a zoom button (not shown) on the image pickup apparatus to move the image-taking optical system from wide area to telescope area, thus performing the corresponding scale changing operation. A current flows through the DC motor  51  to rotate the worm gear  53  clockwise (CW), which rotates the gears  54  to  58 . As s result, the moving cam ring  32  engaged with the gear  58  starts rotating counterclockwise (CCW). On this occasion, the first group barrel  10  and second group barrel  20  follow the movement trajectories shown in  FIG. 11  move to the telescope area, thus performing the corresponding scale changing operation. 
   In a shaded area B between a certain area and an end of the telescope area, the key portion  15   c  of the barrier driving lever  15  is fitted into the key groove portion  21   a  of the transmission member  21 . However, in the area B, the relationship between the relative positions of the barrier driving lever  15  and transmission member  2  at the moment of separation of the barrier driving lever  15  and transmission member  21  during the transition from the sink area to the wide area is maintained intactly. This allows the key portion  15   c  of the barrier driving lever  15  to be fitted into the key groove portion  21   a  of the transmission member  21  without any stress. Further, on this occasion, the member corresponding to the tapered cam member  30   a  of the barrel base plate  30  which drives the lens barriers  12  and  13  does not advance into the penetrating portion of the second group barrel  20 . This prevents the lens barriers  12  and  13  from being closed. 
   Now, it is assumed that the operator has operated the zoom button (not shown) on the image pickup apparatus to move the image-taking optical system from telescope area to wide area, thus performing the corresponding scale changing operation. A current flows through the DC motor  51  to rotate the worm gear  53  counterclockwise (CCW), which rotates the gears  54  to  58 . As s result, the moving cam ring  32  engaged with the gear  58  starts rotating clockwise (CW). On this occasion, the first group barrel  10  and second group barrel  20  follow the movement trajectories shown in  FIG. 11  to move from telescope area to wide area, thus performing the corresponding scale changing operation. 
   Between a certain area and the wide area, an area is present in which the key portion  15   c  of the barrier driving lever  15  and the key groove portion  21   a  of the transmission member  21  which have been fitted together are separated from each other. However, in this area, the lens barriers  12  and  13  need not be driven. Consequently, image taking is not affected. 
   Now, it is assumed that the operator powers off the image pickup apparatus. A current flows through the DC motor  51  to rotate the worm gear  53  counterclockwise (CCW), which rotates the gears  54  to  58 . As s result, the moving cam ring  32  engaged with the gear  58  starts rotating clockwise (CW). On this occasion, the first group barrel  10  and second group barrel  20  follow the movement trajectories shown in  FIG. 11  to move from wide area to sink area. 
   In a shaded area A between a certain area and an end of the sink area, the key portion  15   c  of the barrier driving lever  15  is fitted into the key groove portion  21   a  of the transmission member  21 . Once this area is reached, the rib portion  21   b  of the transmission member  21  starts to abut against the tapered cam portion  30   a  of the barrel base plate  30 . The transmission member  21  starts to pivot along the taper of the tapered cam portion  30   a . This pivots the barrier driving lever  15  counterclockwise in  FIG. 7A  so that a protruding portion  15   b  of the barrier driving lever  15  leaves the end surface of recess portion  14   b  of the barrier driving lever  14 . 
   However, since the barrier closing spring  16  acts on the barrier driving lever  14 , the closing force of the barrier closing spring  16  drives the barrier driving lever  14  counterclockwise in  FIG. 7 . This driving force pivots the lens barrier  12  in the closing direction. This also pivots the lens barrier  13  engaged with the lens barrier  12  via the gear portions  12   b  and  13   b , in the closing direction. Consequently, immediately before the first group barrel  10  and second group barrel  20  are driven to the end of the sink area, the lens barriers  12  and  13  are completely closed. 
   As described above, according to the present embodiment, while the rib portion  21   b  of the transmission member  21  does not abut against the tapered cam portion  30   a  of the barrel base plate  30  (image taking enabled state), the light shielding portion  21   c  of the transmission member  21  and the tapered cam portion  30   a  of the barrel base plate  30  shields the penetrating portion of the second group barrel  20 . This makes it possible to provide a lens barrel that serves to suppress an increase in the number of parts constituting the lens barrier driving mechanism and avoid light leakage from the penetrating portion which is penetrated by the lens barrier driving mechanism, ghosts, and the like; the light leakage and ghosts have often occurred conventionally and may degrade the optical performance. 
   The type of the image pickup apparatus in which the lens barrel shown in the above embodiment is mounted is not particularly limited. The embodiment is applicable to either a silver film camera or an electronic camera. 
   This application claims the benefit of Japanese Patent Application No. 2005-193824 filed Jul. 1, 2005, which is hereby incorporated by reference herein in its entirety.