Abstract:
An apparatus, such as an image pickup apparatus, includes an optical unit a moving direction of which changes from one of a drawing-out direction and a drawing-in direction to the other of the drawing-out direction and the drawing-in direction in process of varying a focal length from one of a shortest focal length end and a longest focal length end to the other of the shortest focal length end and the longest focal length end, the optical unit having possibility of receiving an external force in the drawing-in direction, a driving device which drives the optical unit, and a control device which controls the driving device. When the optical unit is determined to move in the drawing-in direction to reach a stop position on the basis of a position of the optical unit and a direction of varying the focal length, the control device controls and causes the driving device to move the optical unit in the drawing-out direction after moving the optical unit in the drawing-in direction beyond the stop position and, then, to stop the optical unit at the stop position.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical apparatus, such as a camera, which performs the drawing-out and drawing-in of an optical unit. 
     2. Description of Related Art 
     Heretofore, in zoom lens cameras capable of performing a zooming action, a cam ring is used as a means for driving a lens to perform the zooming action. A cam groove provided in the cam ring is formed, for example, in the shape as shown in FIG.  10 ( a ). In FIG.  10 ( a ), the cam groove  101  in the cam ring  100  is provided for driving a variator lens. A cam pin provided on a lens barrel holding the variator lens is fitted in the cam groove  101  and the lens barrel is arranged to move between a stowage position where the lens barrel is stowed within a camera body and a telephoto end position, according to the rotation of the cam ring  100 . Incidentally, when the lens barrel is in the stowage position, a barrier (not shown) provided on the front end portion of the lens barrel is closed to protect the lens. 
     In the conventional zoom mechanism as described above, there is provided such a cam groove as to cause the variator lens to be drawn out monotonously (without being drawn in on its way) during the zooming action from the wide-angle side to the telephoto side and to be drawn in monotonously (without being drawn out on its way) during the zooming action from the telephoto side to the wide-angle side, and the lens barrel holding the variator lens is driven by a DC motor, serving as a zoom motor, and a train of gears so as to be moved in the optical axis direction. 
     In the meantime, there exists backlash in a train of gears. Specifically, when the lens barrel holding the variator lens is driven from the wide-angle side to the telephoto side, the backlash exists on the telephoto side. On the other hand, when the lens barrel holding the variator lens is driven from the telephoto side to the wide-angle side, the backlash exists on the wide-angle side. 
     In order to cause the lens barrel holding the variator lens to stop always at the same zoom position regardless of whether the zooming operation is performed by an operator from the telephoto side to the wide-angle side or from the wide-angle side to the telephoto side, it becomes necessary to perform a biasing operation for eliminating backlash. 
     A problem might arise in a case where the lens barrel is driven from the telephoto side, i.e., the position where the lens barrel protrudes from the camera body at the full length, to the wide-angle side, i.e., the position where the lens barrel is drawn within the camera body. In such a case, if the lens barrel happens to come into contact with an obstacle, or if the operator erroneously pushes in the lens barrel, exerting an external force on the lens barrel, the lens barrel would be pushed in as much as the backlash, so that the photo-taking magnification is caused to change. 
     In view of the above points of view, there has been proposed a camera in which, when zooming is effected from the telephoto side to the wide-angle side, a biasing operation, i.e., the operation of drawing in the lens barrel further by an extra amount from the position desired by the operator and, after that, drawing out the lens barrel, is performed to eliminate the backlash of a train of gears, so that the lens barrel can be stopped at the stop position desired by the operator, thereby solving the above conveniences. 
     On the other hand, there is another zoom mechanism in which, as shown in FIG.  10 ( b ), an extreme value is set between the wide-angle end and the telephoto end in a cam groove  201  provided in a cam ring  200 . In that zoom mechanism, when advancing from the wide-angle end to the telephoto end, the lens is moved toward the image side before reaching the extreme value, and is moved toward the object side after passing the extreme value. 
     In the above zoom mechanism, assuming that the zooming action is stopped between the wide-angle end and the extreme value during the process of zooming toward the telephoto end (herein, the rotating direction of a gear at this time is referred to as the normal rotating direction), the backlash occurring in a train of gears for driving the cam ring  200  is considered to exist on the side in the normal rotating direction. Then, since, in such a stop position, the direction in which the lens barrel advances toward the extreme value is the abovementioned normal rotating direction, if any external force is exerted on the lens barrel, the lens barrel would be drawn in, so that the photo-taking magnification is caused to change. 
     In order to remove such inconveniences, it is necessary to perform the biasing operation. However, since, in the conventional method, the biasing operation is performed only when the lens barrel is moved from the telephoto side to the wide-angle side, the biasing operation is not performed in a case where the lens barrel is moved from the wide-angle side to the telephoto side. Therefore, there is the possibility that, if the lens barrel happens to come into contact with an obstacle, or if the operator erroneously pushes in the lens barrel, exerting an external force on the lens barrel, the lens barrel would be pushed in slightly, so that the photo-taking magnification is caused to change. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, there is provided an apparatus, such as an image pickup apparatus, comprising an optical unit a moving direction of which changes from one of a drawing-out direction and a drawing-in direction to the other of the drawing-out direction and the drawing-in direction in process of varying a focal length from one of a shortest focal length end and a longest focal length end to the other of the shortest focal length end and the longest focal length end, the optical unit having possibility of receiving an external force in the drawing-in direction, a driving device which drives the optical unit, and a control device which controls the driving device, the control device, when the optical unit is determined to move in the drawing-in direction to reach a stop position on the basis of a position of the optical unit and a direction of varying the focal length, controlling and causing the driving device to move the optical unit in the drawing-out direction after moving the optical unit in the drawing-in direction beyond the stop position and, then, to stop the optical unit at the stop position, so that the optical unit can be prevented from being moved by an external force due to backlash, regardless of a stop position of the optical unit. 
     The above and other aspects and features of the invention will become apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is an exploded perspective view showing a zoom lens barrel of a camera according to an embodiment of the invention. 
     FIG. 2 is a development view of a fixed tube shown in FIG.  1 . 
     FIGS.  3 ( a ) and  3 ( b ) are diagrams for explaining an opening-and-closing mechanism for a barrier shown in FIG. 1, FIG.  3 ( a ) showing the closed state of the barrier and FIG.  3 ( b ) showing the opened state of the barrier. 
     FIG. 4 is a perspective view showing a driving ring shown in FIG. 1 and a driving mechanism therefor. 
     FIG.  5 ( a ) is a diagram showing a cam locus of the fixed tube shown in FIG. 1, 
     FIG.  5 ( b ) is a diagram showing a cam locus of a moving cam ring for a first lens-group tube, and 
     FIG.  5 ( c ) is a diagram showing a movement locus of the first-lens-group tube which is the sum of the cam locus shown in FIG.  5 ( a ) and the cam locus shown in FIG.  5 ( b ). 
     FIG. 6 is a development view showing the inner side of the fixed tube shown in FIG.  1 . 
     FIG. 7 is a flow chart showing the opening operation of the barrier shown in FIG.  1 . 
     FIG. 8 is a block diagram showing the electrical arrangement for controlling the lens barrel shown in FIG.  1 . 
     FIG. 9 is a flow chart showing the zooming action to be performed by a control part shown in FIG.  8 . 
     FIG.  10 ( a ) is a development view showing a cam groove of a moving cam ring of a conventional zoom lens barrel, and FIG.  10 ( b ) is a development view showing a cam groove of a moving cam ring to which the invention is applied. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a preferred embodiment of the invention will be described in detail with reference to the drawings. 
     Among FIGS. 1 to  9 , which show the embodiment of the invention, FIG. 1 is an exploded perspective view showing a lens barrel part of a camera according to the embodiment of the invention. 
     Referring to FIG. 1, a base  1  serving as a base part of a lens barrel unit constitutes a structural body of the lens barrel unit in conjunction with a fixed tube  2  which is secured to the front end of the base  1  with screws. A first-lens-group tube  3  holds a plurality of lenses (not shown). Three follower pins  7  which have tapered fore end parts are press-fitted into the outer circumferential side surface of the first-lens-group tube  3 . A cap (not shown) is secured by bonding to the front surface of the first-lens-group tube  3 . 
     A second-lens-group tube  9 , which holds a plurality of lenses (not shown), is secured by bonding to a diaphragm base plate of a diaphragm unit  13  integrally therewith. 
     A third-lens-group tube  15 , which holds a lens, is arranged to be guided by guide bars  17  and  18  which extend in the optical axis direction, to have its axial position restricted by a nut (not shown) having a female thread pinched by the arm part of the third-lens-group tube  15  and arranged to engage a screw  21 , and to be in a state of being biased by a tension spring  20  in the drawing-in direction of the lens barrel. Further, a projection which is provided on the third-lens-group tube  15  is fitted into a slit part provided on the nut in such a way as to restrict the rotating motion of the third-lens-group tube  15 . The screw  21  is formed integrally with a magnet  22  to have a male thread part arranged to engage the female thread part of the nut. A bearing metal piece is press-fitted into the base  1  and has one end of the screw  21  fitted therein in a rotatable manner. A stepping motor  24  is arranged to drive and move the third-lens-group tube  15 . The stepping motor  24  constitutes a magnetic circuit including the magnet  22 , and is secured to the base  1 . A slit plate  30  which is secured to the third-lens-group tube  15  integrally therewith is disposed in a position to be insertable and retractable into and from a slit part of a photo-interrupter (not shown), so that the position of the third-lens-group tube  15  can be detected. 
     An image sensor  32  is secured by bonding to a holding plate  33 , which is secured to the base  1  with screws. A flexible printed circuit board  34  is arranged to supply a photoelectrically-converted image signal to a signal processing circuit which will be described later herein. A dust-proof rubber piece  35  and an optical low-pass filter (LPF)  36  are both secured by bonding to the base  1 . 
     A driving ring  37  is rotatably fitted on the outer (circumferential) side of the fixed tube  2 . As shown in FIG. 4, the driving ring  37  has a gear part  37   a  formed at a part on the outer side thereof. A zoom motor  38  is arranged on the outer side of the driving ring  37  to have a pinion gear  39  firmly press-fitted to its output shaft. The driving force of the zoom motor  38  is transmitted from the pinion gear  39  to the gear part  37   a  of the driving ring  37  through gears  42 ,  41 ,  40 ,  43 ,  44  and  45  one after another. These gears  40  to  45  are placed in gear boxes  46  and  47  and are secured to the base  1  in that state. The zoom motor  38  is also secured to the gear box  46 . 
     A moving cam ring  48  is fitted in on the inner side of the fixed tube  2 . A rectilinear guide tube  49  is fitted in on the inner side of the moving cam ring  48 . On the outer side of the moving cam ring  48 , are mounted three driving pins  50  and three follower pins  51  having tapered parts, at equal intervals. As shown in FIGS. 2 and 6, the driving pins  50  pierce through hole parts  2   a  of the fixed tube  2  to fittingly engage groove parts  37   b  which are provided on the inner side of the driving ring  37 . 
     The follower pins  51  has their fore-end tapered parts in sliding contact with tapered cam grooves  2   b  which are formed on the inner side of the fixed tube  2 , as shown FIG.  6 . FIGS. 2 and 6 are inner circumferential-surface development views for explaining the above-described arrangement. 
     Two kinds of tapered cam grooves (not shown) are formed on the inner circumferential side of the moving cam ring  48 . Followers  7  which are provided on the first-lens-group tube  3  and followers which are provided on the diaphragm unit  13  are in sliding contact with the two kinds of tapered cam grooves. At the same time, the side surfaces of the respective followers are fittingly engaging the rectilinear grooves  49   a  and  49   b  of the rectilinear guide tube  49 , so that the positions of the respective followers in the direction of rotation are restricted. 
     Further, front-side projections  49   c  provided on the outer circumferential side of the rectilinear guide tube  49  are in contact with a groove part  48   c  provided on the inner circumferential side of the moving cam ring  48 , and a flange part  49   d  of the rectilinear guide tube  49  is in contact with an end part of the moving cam ring  48 . By this arrangement, the rectilinear guide tube  49  is restrained from relatively moving in the optical axis direction with respect to the moving cam ring  48 . At the same time, rear-side projections  49   e  provided on the outer circumferential side of the rectilinear guide tube  49  are fitted in linear groove parts  2   c  provided on the inner circumferential side of the fixed tube  2 , in such a way that the rectilinear guide tube  49  is rectilinearly movable while being restrained from moving in the rotating direction. 
     A cap  52  is arranged to hold a dust-proof sheet (not shown) between the fixed tube  2  and the cap  52 . The cap  52  has rail parts  52   a  and  52   b  formed on a front surface thereof to guide a barrier  54  which will be described later. 
     A linear sensor  56 , which is composed of a variable resistor, is secured to the base  1  with screws or the like. A lever  57  is arranged to pinch and carry a sliding piece of the variable resistor and to be guided by a guide bar  58 . A spring  59  urges the lever  57  to move toward one side. The rotational angle of the driving ring  37  is detected by A/D-converting the terminal voltage of the linear sensor  56 , so that the position of the first-lens-group tube  3  also can be detected accordingly. 
     Referring to FIG.  1  and FIGS.  3 ( a ) and  3 ( b ), the barrier  54  is supported to be rotatable around a shaft  63  mounted on a barrier base  62 . The barrier  54  is urged by a closing spring  64  hooked on its hook part  54   a  to move clockwise as viewed from the front of the camera. 
     A barrier driving lever  65  is supported to be rotatable around a shaft  66  mounted on the barrier base  62 . The barrier driving lever  65  is urged to move clockwise by an opening spring  67  which is hooked on a hook part  65   a  of the barrier driving lever  65 . Here, the urging forces of the two springs  64  and  67  are set as “the closing springs  64 &lt;&lt;the opening spring  67 ”. 
     A shaft  68  is mounted at one end of the barrier driving lever  65  in a position corresponding to one side face of the barrier  54 . A leaf switch  69  which is formed by integral molding is secured to the barrier base  62  with screws. The barrier base  62  is secured to the base  1  with screws. 
     FIG.  3 ( a ) shows the state where the barrier  54  is closed. In this state, a stepped part  37   f  of the driving ring  37  pushes a bent part  65   b  of the barrier driving lever  65  to cause the barrier driving lever  65  to be swung counterclockwise against the urging force of the opening spring  67  and to be locked at that position. The barrier  54  is, in the meantime, caused to swing in the direction of closing by the urging force of the closing spring  64  and to be in a closed state with its bent part  54   b  abutting on a stopper part  2   d  of the fixed tube  2 . 
     The lens barrel of the camera arranged according to the present embodiment as described above operates as follows. 
     When the zoom motor  38  is driven, the driving ring  37  is caused to rotate through the gears  39  to  45 . Then, the rotation of the driving ring  37  causes the moving cam ring  48  to rotate through the driving pins  50 . However, since the follower pins  51  of the moving cam ring  48  are engaging the cam grooves  2   b  of the fixed tube  2 , the moving cam ring  48  is allowed to also move in the optical axis direction along the cam grooves  2   b  of the fixed tube  2 . The movement of the moving cam ring  48  in the optical axis direction causes the rectilinear guide tube  49  to move also in the optical axis direction integrally with the moving cam ring  48 . Then, since the projections  49   e  on the outer circumferential side of the rectilinear guide tube  49  is under restriction of the groove parts  2   c  of the fixed tube  2 , the rectilinear guide tube  49  is allowed to move only in the optical axis direction without rotating. 
     When the moving cam ring  48  rotates, the first-lens-group tube  3  and the second-lens-group tube  9  which is secured to the diaphragm unit  13  relatively move in the optical axis direction respectively along the grooves  49   a  and  49   b  of the rectilinear guide tube  49  according to the lifts of the respective cams of the moving cam ring  48 . 
     FIGS.  5 ( a ) to  5 ( c ) are diagrams showing only the loci of the cam parts of the lens barrel. FIG.  5 ( a ) shows the cam locus of the fixed tube  2 . FIG.  5 ( b ) shows the cam locus of the moving cam tube  48  for the first-lens-group tube  3 . FIG.  5 ( c ) shows the locus of movement of the first-lens-group tube  3 , which is the sum of the cam loci shown in FIGS.  5 ( a ) and  5 ( b ). Referring to the locus of movement shown in FIG.  5 ( c ), the first-lens-group tube  3  is driven in the drawing-in direction and in the drawing-out direction with the boundary set to the position of an extreme value between the wide-angle end and the telephoto end. 
     In each of FIGS.  5 ( a ) to  5 ( c ), a point W represents a wide-angle end position, a point T represents a telephoto end position, and a point S represents a stowage position. Each cam is provided with a flat area extending from the stowage position S to a position B. 
     According to the loci shown in FIGS.  5 ( a ) to  5 ( c ), a change-over between the stowage position and a photo-taking position (S-W) and a zooming action in the photo-taking range (W-T) are carried out by driving the zoom motor  38 . 
     When the driving ring  37  rotates, the lever  57  moves along a cam groove (not shown) in the optical axis direction to displace the sliding piece of the linear sensor  56  and thus to vary the output of the linear sensor  56 . Thus, by detecting the output of the liner sensor  56 , every zoom position is successively detectable and the position of the extreme value is also detectable. 
     As mentioned in the foregoing, the stepped part  37   f  of the driving ring  37  locks the barrier driving lever  65  when the lens barrel is in the stowage position. However, when the driving ring  37  rotates, the barrier driving lever  65  is unlocked to allow the barrier driving lever  65  to be swung clockwise by the urging force of the opening spring  67 . The barrier driving lever  65  thus comes to push the side surface of the barrier  54  through the shaft  68 . 
     Since the urging force of the closing spring  64  is weaker than the urging force of the opening spring  67  as mentioned above, the barrier  54  is caused to be swung counterclockwise by the barrier driving lever  65  into an opened position as shown in FIG.  3 ( b ). At this time, if no external force is exerted on the barrier driving lever  65 , the L-shaped bent part  54   b  of the barrier  54  pushes a contact  69   a  of the leaf switch  69  to turn on the leaf switch  69 . Therefore, it is possible to electrically detect the state in which the barrier  54  is opened. 
     However, when the lens barrel is in the stowage position, i.e., when the camera is not in the photo-taking state, the stepped part  37   f  of the driving ring  37  acts on the bent part  65   b  of the barrier lever  65  to move the bent part  65   b  downward. At this time, the barrier  54  is closed by the action of the closing spring  64 . 
     The opening and closing actions on the barrier  54  are thus arranged to be carried out within the flat areas of the cam loci, i.e., between the positions S and B, shown in FIGS.  5 ( a ) to  5 ( c ). 
     FIG. 8 is a block diagram showing the electrical arrangement for driving the lens barrel according to the embodiment of the invention. 
     In FIG. 8, the lens barrel  71  is the same as the lens barrel that has been described above, and the components of the lens barrel  71  are indicated by the same reference numerals as those used in the foregoing description. 
     Referring to FIG. 8, an image signal obtained through photoelectric conversion by the image sensor  32  is supplied to a signal processing circuit  72  for a color-conversion process, a gamma correction process, etc. After these processes, the image signal is recorded in a memory  73  which is, for example, a card medium or the like. A control part  74 , which controls the whole camera, is arranged to control the stepping motor  24 , the zoom motor  38  and the diaphragm unit  13  while watching the outputs of the linear sensor  56 , the photo-interrupter  29 , the leaf switch  69 , etc., which are disposed within the lens barrel  71 , and also to control the signal processing circuit  72  and the memory  73 . A nonvolatile memory  75 , which is, for example, an EEPROM or the like, is arranged to permit electrical erasure and recording. A mode dial switch  76  is arranged to permit selection and setting of various operation modes, such as turning-off of the power supply, a photo-taking mode, a reproduction mode, a PC (personal computer) connection mode, etc. A zoom switch  77  is arranged to output signals indicative of the direction of varying magnification, i.e., a direction toward the telephoto end or a direction toward the wide-angle end. According to the output signals of the zoom switch  77 , the zoom motor  38  is driven to make a normal rotation or a reverse rotation. 
     The operation of the camera as described above is shown in the flow chart of FIG.  7 . 
     Referring to the flow chart of FIG. 7, when a power supply is turned on in step S 001 , the zoom motor  38  makes a normal rotation in step S 002 . The rotation of the zoom motor  38  is transmitted to the gear  37   a  of the driving gear  37  through the gears  39  to  45 , so that the driving gear  37  is made to rotate counterclockwise as viewed in FIG.  4 . 
     In step S 003 , a check is made to find if the barrier  54  is in the opened position. The opened position of the barrier  54  is decided according to the rotational angle of the driving ring  37 , and the rotational angle of the driving ring  37  is detectable by the linear sensor  56 . When the driving ring  37  has rotated up to the opened position of the barrier  54 , the barrier driving lever  65  rotates clockwise, as viewed in FIG. 1, by the action of the opening spring  67 , so that the shaft  68  of the barrier driving lever  65  pushes down the barrier  54  to bring the barrier  54  into the opened state as shown in FIG.  3 ( b ). 
     At this point of time, the zoom motor  38  is stopped in step S 004 . In step S 005 , a timer in which a predetermined time is set is started. 
     In step S 006 , a check is made to find if the leaf switch  69  is in an on-state, i.e., if the barrier  54  has been completely opened. 
     When the barrier  54  comes into the completely-opened state, the bent part  54   b  of the barrier  54  pushes the contact  69   a  of the leaf switch  69 , thereby turning on the leaf switch  69 . If the leaf switch  69  is not in the on-state, the flow proceeds to step S 007 . In step S 007 , a check is made to find if the time set in the timer in step S 005  has elapsed. If not, the flow returns to step S 006 , repeating the above operation until the leaf switch  69  is turned on within the predetermined time set in the timer, i.e., waiting until the barrier  54  is completely opened. 
     If it is not detected in step S 007  that the barrier  54  has been completely opened within the predetermined time set in the timer, assuming that an error has occurred in the opening operation of the barrier  54 , an error code (for example, code “15” indicative of the error of the opening operation of the barrier  54 ) is set to perform an error processing operation. 
     Incidentally, from the start of rotation of the driving ring  37  in response to the turning-on of the power supply until the barrier  54  is opened, the follower  7  of the first-lens-group tube  3  is moving on a section A-A′ of the cam groove  201  shown in FIG.  10 ( b ). Therefore, during that period, the first-lens-group tube  3  is made not to be drawn out. 
     Accordingly, since the rotation of the zoom motor  38  is stopped in the section A-A′, even if an error occurs in the barrier opening operation, the first-lens-group tube  3  is prevented from being drawn out to collide with the barrier  54 . 
     On the other hand, if it is detected that the leaf switch  69  has been turned on within the predetermined time set in the timer and the barrier  54  has been completely opened, the flow proceeds to step S 010 . 
     In step S 010 , with the on-state of the leaf switch  69  detected, the driving ring  37  is again rotated by the zoom motor  38 . Accordingly, the moving cam ring  48  is rotated and is also moved in the optical axis direction along the cam groove  2   b  provided on the inner side of the fixed tube  2 . 
     Further, according to the rotation of the moving cam ring  48 , the first-lens-group tube  3  is moved along the cam groove  48   a  provided on the inner side of the moving cam groove  48 . When the first-lens-group tube  3  is drawn out up to the wide-angle end position W shown in FIG.  5 ( c ), a photo-taking operation becomes possible. The moved position of the first-lens-group tube  3  is recognized on the basis of the detection information of the linear sensor  56 . When the first-lens-group tube  3  reaches the wide-angle end position W, the driving of the zoom motor  38  is stopped. 
     According to the present embodiment, the first-lens-group tube  3  is once driven from the stowage position to the barrier opening position in response to the turning-on of the power supply. Then, after it is confirmed that the barrier  54  has been completely opened, the first-lens-group tube  3  is driven up to the wide-angle end position. Therefore, the first-lens-group tube  3  is prevented from colliding with the barrier  54 . 
     After that, when the operator performs a zooming operation, the zoom motor  38  rotates and the driving cam  37  rotates, so that the variator lens is moved to vary the photo-taking magnification. Accordingly, the operator can select a desired zoom position. 
     In addition, A/D-converted values of the terminal voltages of the linear sensor  56  corresponding to the respective positions, the respective zoom positions and the extreme value of the first-lens-group tube  3  are beforehand stored in the nonvolatile memory  75 . The position of the first-lens-group tube  3  can be electrically detected by reading such A/D-converted values of the linear sensor  56  from the nonvolatile memory  75 . 
     Next, the zooming action of the camera according to the embodiment of the invention will be described with reference to the flow chart of FIG.  9 . 
     The characteristic feature of the zooming action resides in that, in a case where the first-lens-group tube  3  stops before reaching the extreme value P in process of moving from the wide-angle side to the telephoto side, and in a case where the first-lens-group tube  3  stops before reaching the extreme value P in process of moving from the telephoto side to the wide-angle side, because the backlash of the zoom gear train ( 39 - 45 ) exists on the side in the direction of drawing in the first-lens-group tube  3 , a biasing operation is made to be performed. The biasing operation is performed by driving the driving ring  37  toward the telephoto side from a target stop position extra as much as a predetermined amount to draw in the first-lens-group tube  3  and, after that, reversing the driving direction of the driving ring  37  and then stopping the first-lens-group tube  3  at the target stop position. As a result, the backlash of the gear train ( 39 - 45 ) is eliminated, so that, even if an external force is exerted on the first-lens-group tube  3  in the drawing-in direction, the first-lens-group tube  3  is prevented from moving. 
     Further, in a case where, during the zooming action from the telephoto side to the wide-angle side, the zooming action is stopped on the telephoto side of the extreme value provided between the wide-angle end and the telephoto end, because the backlash of the gear train ( 39 - 45 ) exists on the side in the direction of drawing in the first-lens-group tube  3 , a biasing operation is also performed. In this case, the biasing operation is performed by driving the driving ring  37  toward the wide-angle side from a target stop position extra as much as a predetermined amount to draw in the first-lens-group tube  3  and, after that, reversing the driving direction of the driving ring  37  and then stopping the first-lens-group tube  3  at the target stop position. 
     On the other hand, in a case where, during the zooming action from the wide-angle side to the telephoto side, the zooming action is stopped on the telephoto side of the extreme value provided between the wide-angle end and the telephoto end, because the backlash of the gear train ( 39 - 45 ) exists on the side in the direction of drawing out the first-lens-group tube  3 , a biasing operation is unnecessary. 
     Further, in a case where, during the zooming action from the telephoto side to the wide-angle side, the zooming action is stopped on the wide-angle side of the extreme value provided between the wide-angle end and the telephoto end, because the backlash of the gear train ( 39 - 45 ) exists on the side in the direction of drawing out the first-lens-group tube  3 , a biasing operation is also unnecessary. 
     In the flow chart shown in FIG. 9, with the zooming action started, a check is made in step S 011  to find if a telephoto switch T (not shown) of the zoom switch  77  for indicating the zooming action from the wide-angle side to the telephoto side is turned on or a wide-angle switch W (not shown) of the zoom switch  77  for indicating the zooming action from the telephoto side to the wide-angle side is turned on. If the telephoto switch T is turned on, the flow proceeds to step S 012 . If the wide-angle switch W is turned on, the flow proceeds to step S 017 . 
     In step S 012 , the zoom motor  38  is driven to make a normal rotation so as to move the zoom lens toward the telephoto side. When the zoom motor  38  makes the normal rotation, the first-lens-group tube  3  moves in the drawing-in direction toward the extreme value P, as shown in FIG.  5 ( c ). Then, in step S 013 , a check is made to find if the zoom switch  77  is turned off. If not, the zoom motor  38  remains driven in the same direction. If so, the flow proceeds to step S 014 . In step S 014 , the driving of the zoom motor  38  is stopped to stop the first-lens-group tube  3  and the second-lens-group tube  9  at the respective positions. 
     The position of the first-lens-group tube  3  is detected, as described in the foregoing, by the linear sensor  56  according to the rotational angle of the driving ring  37 . Accordingly, in step S 015 , a check is made to find if the stop position in the step S 014  is between the wide-angle end and the extreme value P. 
     If the stop position of the first-lens-group tube  3  is found in step S 015  to be between the extreme value P and the telephoto end during the driving from the wide-angle end to the telephoto end, the biasing operation for eliminating backlash is unnecessary, as described above. However, if the stop position of the first-lens-group tube  3  is found in step S 015  to be between the wide-angle end and the extreme value P during the driving from the wide-angle end to the telephoto end, the biasing operation for eliminating backlash (the operation for eliminating hysteresis) is necessary, as described above, so that the flow proceeds to step S 016 . 
     In step S 016 , the biasing operation for eliminating hysteresis in the zoom driving from the wide-angle end to the telephoto end. Since the backlash of the gear train ( 39 - 45 ) exists on the side in the direction of drawing in the first-lens-group tube  3 , the biasing operation is performed by driving the driving ring  37  toward the telephoto side (in the drawing-in direction) from a target stop position extra as much as a predetermined amount to draw in the first-lens-group tube  3  and, after that, causing the zoom motor  38  to rotate in the reverse direction and then stopping the first-lens-group tube  3  at the target stop position. Thus, as the stop position of the first-lens-group tube  3  obtained when the zoom switch  77  is turned off is stored, the first-lens-group tube  3  is made to be returned to the stop position as stored. 
     As a result, the backlash of the gear train ( 39 - 45 ) is eliminated, so that, even if an external force is exerted on the first-lens-group tube  3  in the drawing-in direction, the first-lens-group tube  3  is prevented from moving. 
     On the other hand, in step S 017 , the zoom motor  38  is driven to make a reverse rotation so as to move the zoom lens toward the wide-angle side. When the zoom motor  38  makes the reverse rotation, the first-lens-group tube  3  moves in the drawing-in direction toward the extreme value P, as shown in FIG.  5 ( c ). Then, in step S 018 , a check is made to find if the zoom switch  77  is turned off. If not, the zoom motor  38  remains driven in the same direction. If so, the flow proceeds to step S 014 . In step S 019 , the driving of the zoom motor  38  is stopped to stop the first-lens-group tube  3  and the second-lens-group tube  9  at the respective positions. 
     The position of the first-lens-group tube  3  is detected, as described in the foregoing, by the linear sensor  56  according to the rotational angle of the driving ring  37 . Accordingly, in step S 020 , a check is made to find if the stop position in the step S 019  is between the telephoto end and the extreme value P. 
     If the stop position of the first-lens-group tube  3  is found in step S 020  to be between the extreme value P and the wide-angle end during the driving from the telephoto end to the wide-angle end, the biasing operation for eliminating backlash is unnecessary, as described above. However, if the stop position of the first-lens-group tube  3  is found in step S 020  to be between the extreme value P and the telephoto end during the driving from the telephoto end to the wide-angle end, the biasing operation for eliminating backlash (the operation for eliminating hysteresis) is necessary, as described above, so that the flow proceeds to step S 016 . 
     In step S 016 , the biasing operation for eliminating hysteresis in the zoom driving from the telephoto end to the wide-angle end. Since the backlash of the gear train ( 39 - 45 ) exists on the side in the direction of drawing in the first-lens-group tube  3 , the biasing operation is performed by driving the driving ring  37  toward the wide-angle side (in the drawing-in direction) from a target stop position extra as much as a predetermined amount to draw in the first-lens-group tube  3  and, after that, causing the zoom motor  38  to rotate in the normal direction and then stopping the first-lens-group tube  3  at the target stop position. Thus, as the stop position of the first-lens-group tube  3  obtained when the zoom switch  77  is turned off is stored, the first-lens-group tube  3  is made to be returned to the stop position as stored. 
     As a result, the backlash of the gear train ( 39 - 45 ) is eliminated, so that, even if an external force is exerted on the first-lens-group tube  3  in the drawing-in direction, the first-lens-group tube  3  is prevented from moving. 
     The individual components shown in schematic or block form in the drawings are all well-known in the camera arts and their specific construction and operation are not critical to the operation or best mode for carrying out the invention. 
     While the present invention has been described with respect to what is presently considered to be the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     For example, although, in the above-described embodiment, the extreme value in the zoom area (zoom locus) of the first-lens-group tube  3  is on the top of a locus which is protrusive toward the image side, the invention is applicable also to an arrangement in which the extreme value is on the top of a locus which is protrusive toward the object side, or to an arrangement in which the zoom locus is composed of a plurality of protrusive loci. 
     Further, in the above-described embodiment, a rear focus zoom lens is employed as an optical arrangement. However, the invention is applicable also to another zoom arrangement or to a focal-length changeover optical arrangement other than the zoom arrangement. 
     Further, in the above-described embodiment, an optical system composed of three lens groups is employed. However, the invention is applicable also to an optical system composed of a plurality of, other than three, lens groups, such as two or four lens groups. 
     Further, in the above-described embodiment, an optical arrangement is composed of a magnification-varying lens group and a focusing lens group. However, the invention is applicable also to an optical arrangement composed of other lens groups or to another optical unit arrangement including a filter or the like. 
     Further, the software arrangement and the hardware arrangement in the above-described embodiment may be adaptively replaced with each other. 
     Further, in the invention, the technical elements of the above-described embodiment may be combined with each other according to necessity. 
     Further, the invention also applies to cases where each claim or the whole or a part of the arrangement of the embodiment constitutes one apparatus or is used in combination with another apparatus or as a component element of an apparatus. 
     Further, the invention is also applicable to various types of cameras, such as an electronic still camera, a video camera and a camera using a silver-halide film, various image pickup apparatuses other than cameras, various optical apparatuses, such as a lens barrel, other types of apparatuses, and, moreover, to apparatuses adapted for the cameras, the image pickup apparatuses, optical apparatuses and the other types of apparatuses, and elements constituting the above-mentioned apparatuses.