Abstract:
A lens barrel has its collapsed length that can be further decreased by removing unnecessary space and bringing lens groups close to each other and a photographic apparatus incorporates the lens barrel. When the lens barrel is collapsed, at least a fourth lens group is retracted from a photographic optical axis by an advance/retract mechanism. In that case, a first lens group and third lens group are aligned along the optical axis and stuffed into the lens barrel, the fourth lens group is placed on the top, bottom, right, or left side of these lenses, and a second lens group is placed on the top, bottom, right, or left side of the first and third lens groups. This reduces the length of the lens barrel. During extension, the second and fourth lens groups are advanced onto the photographic optical axis by the advance/retract mechanism to extend the lens barrel greatly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lens barrel which holds a zoom lens consisting of multiple lens groups, to a photographic apparatus which takes photographs by capturing light from a subject entering through the zoom lens held by the lens barrel, and to an optical apparatus which includes a lens consisting of multiple lens. 
     2. Description of the Related Art 
     Recently, digital cameras have been spreading rapidly, and increasingly higher image quality is demanded together with smaller size and thinner profiles. A thin card-size digital camera equipped with a lens barrel which holds a zoom lens consisting of multiple lens groups has been proposed and introduced commercially as one of digital cameras which meet the above needs. This camera is capable of high image quality zooming using optical zoom whereas earlier thin digital cameras use an electronic zoom function for magnification. 
     One of the trends in user needs is to have higher-powered optical zoom capabilities while achieving smaller size and thinner profiles. 
     Patent Document (Japanese Patent Laid-Open No. 2003-295031) proposes a technique for collapsing a lens barrel equipped with a zoom lens composed of four lens groups in a thin camera body using an ingenious method for forming cam grooves. 
     An internal configuration of the lens barrel disclosed in Patent Document 1 is described below. 
       FIGS. 1 ,  2 , and  3  are sectional views taken along the optical axis of the lens barrel mounted in a digital camera. Of these,  FIGS. 1 and 2  show the lens as it is extended. According to Patent Document 1,  FIG. 1  is a diagram showing a telephoto end while  FIG. 2  is a diagram showing a wide-angle end.  FIG. 3  is a diagram showing the lens barrel as it is collapsed.  FIG. 4  is a developed view illustrating cam grooves used to extend and collapse the lens barrel from/into the camera body. 
     A configuration of a lens barrel  100  is described with reference to  FIGS. 1 to 4 . 
     The lens barrel  100  holds a four-group zoom lens composed of a first lens group  210 , second lens group  220 , third lens group  230 , and fourth lens group  240 . Of the four lens groups, the second lens group  220  is moved along the optical axis for adjustment of focal distance while the fourth lens group  240  serving as a focus lens is moved along the optical axis for focus adjustment. 
     The first lens group  210  is held in an inner tube  110 . The inner tube  110  is equipped with cam pins  111 , which are engaged with cam grooves  121  (see  FIG. 4 ) formed in the inner surface of an outer tube  120 . Three cam pins  111  are installed on the outer wall of the inner tube  110  at unequal intervals and three cam grooves  121  are formed on the inner wall of the outer tube  120  at unequal intervals (see  FIG. 4 ) to engage with the cam pins  111 . Consequently, when rotation of a zoom motor  270  is transmitted to a gear  124  installed on the inner wall of the outer tube  120  via a coupling gear  271  (see  FIG. 2 ), rotating the outer tube  120 , the first lens group  210  extends together with the inner tube  110  following the shape of the cam grooves  121 . 
     The second lens group  220  is held by a lens group holding frame  221 , on whose circumference three cam pins  222  are installed at unequal intervals. The cam pins  222  are engaged with respective cam grooves  122  formed in the inner wall of the outer tube  120  (see  FIG. 4 ). A guide rod  1132  is passed through a through-hole  221   a  made in the lens group holding frame  221  of the second lens group  220 . Along with rotation of the outer tube  120 , the second lens group  220  moves along the optical axis, being guided by the guide rod  1132 . The guide rod  1132  is supported by a tip support  1132   a  while a guide rod  1133  which guides the third lens group  230  is supported by another tip support  1133   a . The tip supports  1132   a  and  1133   a  support the respective guide rods  1132  and  1133  and are also used as members which support the inner tube  110 . The tip supports  1132   a  and  1133   a  are equipped with an intermediate frame  1101  and a retainer ring  1102 . The intermediate frame  1101  is inserted slidably along the inner wall of the inner tube  110  and the retainer ring  1102  is installed at the rear end of the inner tube  110  to prevent the intermediate frame  1101  from moving backward. The intermediate frame  1101  and retainer ring  1102  are equipped with respective spring pegs  1101   a  and  1102   b . A spring  1103  is bridged between the spring pegs to restrict the movement of the intermediate frame  1101  by urging the intermediate frame  1101  forward along the sliding surface so that the intermediate frame  1101  will not move backward when it is extended together with the inner tube  110 . 
     Furthermore, a cam groove  123  (see  FIG. 4 ) is formed between the cam grooves  121  and  122  in the outer tube  120  to engage with a cam pin (not shown) installed on a lens group holding frame  131  of the third lens group  230 . Consequently, as the outer tube  120  rotates by receiving the driving force of the zoom motor  270  via the coupling gear  271  and gear  124  (see  FIG. 2 ), the third lens group  230  moves along the optical axis following the shape of the cam groove  123 . Incidentally, a shutter unit  131  is linked to the lens group holding frame  130  which holds the third lens group  230 . 
     A through-hole  131   a  is made in the lens group holding frame  130  as in the case of the lens group holding frame  221 . The guide rod  1132  is passed through the through-hole  131   a . Furthermore, the guide rod  1132  is also passed through a through-hole  141   a  made in a lens group holding frame  140  which holds the fourth lens group  240  described later. In this way, this example employs a configuration in which the second lens group  220 , third lens group  230 , and fourth lens group  240  are guided by the common guide rod  1132  to avoid misalignment of optical axes. 
     Extension operation of the lens barrel  100  with this configuration is described in detail with reference to  FIG. 4 . 
     When the outer tube  120  is rotated by a zoom motor  270 , the inner tube  110  extends from a collapsed state to an extended position (B-side end of the area indicated by symbol A) following the shape of the cam grooves  121  (area indicated by symbol A) and held at the extended position (area indicated by symbol B). Until the outer tube  120  is held at the extended position, the second lens group  220  moves along the area indicated by symbol C following the shape of the cam grooves  122  and reaches the end of the area indicated by symbol C when the inner tube  110  is extended to the extended position. As a zoom switch (not shown) is operated at this time, the second lens group  220  enters the area indicated by symbol D, and moves to the end of the area indicated by symbol D if the zoom switch continues to be operated. On the other hand, as the inner tube  110  rotates, the third lens group  230  leaves a collapsed position, moves along the cam groove  123  through an extension area (area indicated by symbol E) and reaches an extended position (intersection of areas indicated by symbols E and F). It remains held at the extended position (area indicated by symbol F) even if the zoom switch is operated. 
     In this way, by arranging cam grooves ingeniously in the inner wall of the outer tube  120 , it is possible to move the first lens group, second lens group  220 , and third lens group  230  among the four lens groups along the optical axis following the shape of the cam grooves by the rotation of the single tube  120  and move the second lens group along the optical axis by the operation of the zoom switch, and thereby carry out zooming. 
     In the lens barrel  100  shown in  FIGS. 1 to 3 , the fourth lens group  240  at the tail of the four lens groups composing the zoom lens is used as a focus lens. The zoom lens held in the lens barrel  100  has a high zoom ratio, and consequently the fourth lens group  240  acting as the focus lens must have a relatively long travel distance. Thus, in this example, a column screw  1131  (see  FIG. 1 ) as long as the long travel distance is installed along the optical axis, a nut  141   b  is fastened to the lens group holding frame  140  which holds the fourth lens group  240 , and the column screw  1131  is screwed into the nut  141   b  for accurate focus adjustment. 
     Focus is adjusted as the column screw  1131  is rotated by rotational driving force of a focus motor (not shown) transmitted via a gear train (not shown) and a lens group holding frame  141  moves along the optical axis by the distance equivalent to the rotation of the column screw  1131 , being guided by the column screw  1131  and guide rods  1132  and  1133 . When an image taking lens starts to catch a subject, focus is adjusted by generating image data by means of an image pickup device  280 , detecting a focus position based on the image data, and moving the fourth lens group  240  acting as the focus lens to the focus position through rotation of the column screw. 
     After focus adjustment, when a shutter button (not shown) is pressed, the shutter unit  131  provided in the lens group holding frame  130  which holds the third lens group  230  is operated in synchronization with full depression of the shutter button to take a photograph. Consequently, light from the subject passes through the first lens group  210 , second lens group  220 , third lens group  230 , and fourth lens group  240  (focus lens) and forms an image on a light-receiving surface of the image pickup device  280 , which generates an image signal which represents the subject image formed on the light-receiving surface. 
     In this way, by arranging multiple cam grooves ingeniously in the inner wall of the outer tube, bringing the outer tube into cam engagement with the inner tube, and using the fourth lens group at the tail of the zoom lens as a focus lens, it is possible to implement a lens barrel which can be housed in a relatively slim body with its length relatively reduced when collapsed and can achieve a high zoom ratio when extended. 
     However, to move lens groups located forward along the optical axis, such as the first lens group and second lens group, a long distance during extension for a high zoom ratio and stuff the lens groups including the above lens groups as close together as possible into the lens barrel during collapse, it is necessary to provide enough collapsed length to keep the lens groups clear of each other as well as to provide enough extended length to extend the lens groups located forward along the optical axis, such as the first lens group and second lens group, as far ahead as possible. This inevitably makes the lens barrel a little longer, creating an unnecessary space in the collapsed lens barrel. This unnecessary space is concentrated around the guide rods and column screw stretching out along the optical axis to move the fourth lens group accurately along the optical axis. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances and provides a lens barrel whose collapsed length can be further decreased as well as a photographic apparatus which incorporates the lens barrel into its body. 
     The present invention provides a first lens barrel which is extendable and collapsible and holds a zoom lens consisting of multiple lens groups, the lens barrel having: 
     a first lens group holding frame which holds a first lens group that is the foremost of the multiple lens groups composing the zoom lens and a second lens group holding frame which holds a second lens group that is the second foremost of the multiple lens groups composing the zoom lens, wherein: 
     the second lens group holding frame is spring-loaded forward along an optical axis, and 
     the first lens group holding frame has a pushing section which pushes the second lens group holding frame along the optical axis during collapse and separates from the second lens group holding frame during extension. 
     In the first lens barrel according to the present invention, the first lens group holding frame pushes the second lens group holding frame during collapse. Consequently, the spring which urges the second lens group holding frame forward along the optical axis is compressed in such a way as to gather repulsive force, bringing the first lens group and second lens group close to each other. This eliminates unnecessary space between the first lens group and second lens group, further reducing the length of the lens barrel. Also, while the first lens group holding frame is separated from the second lens group holding frame, the second lens group is extended along the optical axis by the force of the spring more greatly than the extension by rotation of a rotating tube. This has the advantage of an increased zoom ratio. 
     This makes it possible to implement a lens barrel which can not only reduce its collapsed length, but also increase the zoom ratio by increasing the travel distance of the second lens group. 
     Preferably, the lens barrel has a fixed tube, a straight-ahead key member advanceably/retractably engaged with the fixed tube, a middle tube which rotatably engages with the straight-ahead key member and advances/retracts along with rotation due to cam engagement with the fixed tube, and a front tube which advanceably/retractably engages with the straight-ahead key member and advances/retracts along with rotation due to cam engagement with the middle tube, 
     wherein the first lens group holding frame is fixed to the front tube. 
     This makes it possible to push the second lens group holding frame more securely by the front tube to which the first lens group holding frame is fixed. 
     Also, preferably the lens barrel has a second lens group supporting frame which advances/retracts along with rotation of the middle tube due to cam engagement with the middle tube, wherein the second lens group holding frame is supported by the second lens group supporting frame, being spring-loaded forward along an optical axis. 
     Thus, during extension or zooming, the zoom ratio is adjusted as the second lens group holding frame advances/retracts along the optical axis along with the rotation of the middle tube, being spring-loaded by the second lens group supporting frame. During collapse, the first lens group holding frame approaches the second lens group supporting frame and is housed in it while the second lens group holding frame gathers the spring force directed forward along the optical axis. 
     Furthermore, preferably the lens barrel has a second lens group retraction mechanism which retracts the second lens group from the optical axis of the zoom lens by turning the second lens group holding frame during collapse and advances the second lens group onto the optical axis during extension, 
     wherein the pushing section pushes the retracted second lens group holding frame during collapse. 
     On the other hand, the present invention provides a first photographic apparatus which takes photographs by capturing light from a subject entering through a zoom lens consisting of multiple lens groups and held by an extendable/collapsible lens barrel, wherein: 
     the lens barrel has a first lens group holding frame which holds a first lens group that is the foremost of the multiple lens groups composing the zoom lens and a second lens group holding frame which holds a second lens group that is the second foremost of the multiple lens groups composing the zoom lens, 
     the second lens group holding frame is spring-loaded forward along an optical axis, and 
     the first lens group holding frame has a pushing section which pushes the second lens group holding frame along the optical axis during collapse and separates from the second lens group holding frame during extension. 
     Preferably, the photographic apparatus has an image pickup device which generates image signals by capturing light from a subject entering through the zoom lens held by the lens barrel. 
     Also, the present invention provides a second lens barrel which holds a zoom lens consisting of a first group, a second group, a third group, and a fourth group in this order when viewed from ahead along an optical axis, having: 
     a lens advance/retract mechanism which retracts at least the fourth lens group of the zoom lens from a photographic optical axis during collapse and advances the retracted lens group onto the photographic optical axis during extension. 
     In the second lens barrel according to this invention, the advance/retract mechanism which makes the fourth lens group advanceable/retractable can retract the fourth lens group from the photographic optical axis and put the fourth lens group on a different plane from the first lens group, second lens group, and third lens group arranged on the photographic optical axis. Naturally, to change the conventional mechanism (see  FIGS. 1 to 4 ) and retract the fourth lens group, the guide rods and column screw shown in  FIG. 1  should be included in the advance/retract mechanism. It is easy to include the guide rods and column screw in the advance/retract mechanism. Specifically, it is conceivable to mount the guide rods and column screw on the straight-ahead key ring shown in  FIG. 1 . This eliminates the need to stretch out at great length the guide rods and column screw, allowing the freed space to be used for the fourth lens group retracted from the photographic optical axis. This makes it possible to decrease the length of the collapsed lens barrel. Besides, the lens advance/retract mechanism makes it possible to increase the length of the extended lens barrel by advancing the fourth lens group accurately onto the optical axis during extension. 
     This makes it possible to implement a lens barrel whose collapsed length is decreased and whose extended length is increased. 
     Preferably, the fourth lens group acts as a focus lens. 
     Conventionally, the approach of retracting the focus lens from the photographic optical axis has been avoided because it can lead to misalignment of the photographic optical axis. However, the present invention reduces the length of the lens barrel using the advance/retract mechanism which not only retracts the fourth lens group from the photographic optical axis to the retracted position, but also advances the fourth lens group accurately onto the photographic optical axis. 
     Preferably, the lens advance/retract mechanism retracts the fourth lens group from the photographic optical axis during collapse and retracts the second lens group to a place which coincides with the retracted fourth lens group when viewed along the optical axis. 
     In this way, the fourth lens group and second lens group can constitute a photographic optical system or the first lens group and third lens group together with the second lens group and fourth lens group can constitute a twin-lens photographic optical system. 
     Also, preferably the lens barrel has an image pickup device which generates image signals by capturing light from a subject entering through the zoom lens held by the lens barrel. 
     When the lens barrel is equipped with an image pickup device in this way, it is possible to adjust focus using a travel mechanism of the lens barrel based on the image signals generated by the image pickup device. Also, the combined use of electronic and optical zooms makes it possible to further increase the magnification of the photographic apparatus. 
     Also, the present invention provides a second photographic apparatus which takes photographs by capturing light from a subject entering through a zoom lens held by a lens barrel and consisting of a first lens group, a second lens group, a third lens group, and a fourth lens group in this order when viewed from ahead along an optical axis, having: 
     a lens advance/retract mechanism which retracts at least the fourth lens group of the zoom lens from a photographic optical axis during collapse and advances the retracted lens group onto the photographic optical axis during extension. 
     By incorporating the lens barrel according to the present invention into a photographic apparatus in this way, it is possible to reduce the thickness of the photographic apparatus. 
     Preferably, the fourth lens group acts as a focus lens, the lens advance/retract mechanism retracts the fourth lens group from the photographic optical axis during collapse and retracts the second lens group to a place which coincides with the retracted fourth lens group when viewed along the optical axis, and the photographic apparatus has an image pickup device which generates image signals by capturing light from a subject entering through the zoom lens held by the lens barrel. 
     Also, a lens barrel according to the present invention may include an image taking lens consisting of a plurality of lens groups and which is capable of changing a barrel length thereof between a housed state of relatively short length and a photographing state of relatively long length, the lens barrel having: 
     a first lens group holding frame which holds a first lens group that is the foremost of the plurality of lens groups and a second lens group holding frame which holds a second lens group that is the second foremost of the plurality of lens groups, wherein: 
     the second lens group holding frame may be urged by urging means forward along an optical axis, and 
     the first lens group holding frame may have a pushing section which pushes the second lens group holding frame along the optical axis during transition to the housed state and separates from the second lens group holding frame during the photographing state. 
     Further, this lens barrel may have: a fixed tube, a straight-ahead key member advanceably/retractably engaged with the fixed tube, a middle tube which rotatably engages with the straight-ahead key member and advances/retracts along with rotation due to cam engagement with the fixed tube, and a front tube which advanceably/retractably engages with the straight-ahead key member and advances/retracts along with rotation due to cam engagement with the middle tube, wherein the first lens group holding frame may be fixed to the front tube. In this case, the lens barrel may have a second lens group supporting frame which advances/retracts along with rotation of the middle tube due to cam engagement with the middle tube, wherein the second lens group holding frame may be supported by the second lens group supporting frame, being urged forward along an optical axis by the urging means. 
     Furthermore, this lens barrel may have: a second lens group retraction mechanism which retracts the second lens group from the optical axis of the image taking lens by turning the second lens group holding frame during transition to the housed state and advances the second lens group onto the optical axis during transition to the photographing state, 
     wherein the pushing section may push the retracted second lens group holding frame during collapse. 
     A lens barrel according to the present invention may hold an image taking lens consisting of a first lens group, a second lens group, a third lens group, and a fourth lens group in this order when viewed from ahead along an optical axis and which is capable of changing a barrel length thereof between a housed state of relatively short length and a photographing state of relatively long length, having: 
     a lens advance/retract mechanism which retracts at least the fourth lens group from a photographic optical axis during transition to the housed state and advances the retracted lens group onto the photographic optical axis during transition to the photographing state. 
     In this lens barrel, the fourth lens group may act as a focus lens, the lens advance/retract mechanism may retract the fourth lens group from the photographic optical axis during transition to the housed state and retract the second lens group to a place which coincides with the retracted fourth lens group when viewed along the optical axis. Also, this lens barrel may have an image pickup device which generates image signals by capturing light from a subject entering through the image taking lens held by the lens barrel. 
     Further, a lens barrel according to the present invention may hold an image taking lens consisting of a first lens group, a second lens group, a third lens group, and a fourth lens group in this order when viewed from ahead along an optical axis and which is capable of changing a barrel length thereof between a housed state of relatively short length and a photographing state of relatively long length, having: 
     a lens advance/retract mechanism which retracts at least the third lens group lens from a photographic optical axis during transition to the housed state and advances the retracted lens group onto the photographic optical axis during transition to the photographing state. 
     In this lens barrel, the third lens group may act as a lens for zooming. Also, this lens barrel may have an image pickup device which generates image signals by capturing light from a subject entering through the image taking lens held by the lens barrel. 
     A photographic apparatus according to the present invention may take photographs by capturing light from a subject entering through an image taking lens held by a lens barrel and consisting of a first lens group, a second lens group, a third lens group, and a fourth lens group in this order when viewed from ahead along an optical axis, the lens barrel being capable of changing a barrel length thereof between a housed state of relatively short length and a photographing state of relatively long length, the photographic apparatus having: 
     a lens advance/retract mechanism which retracts at least the third lens group from a photographic optical axis during transition to the housed state and advances the retracted lens group onto the photographic optical axis during transition to the photographing state. 
     In this photographic apparatus, the fourth lens group may act as a focus lens and the third lens group may act as a lens for zooming. Also, this photographic apparatus may have an image pickup device which generates image signals by capturing light from a subject entering through the image taking lens held by the lens barrel. 
     Furthermore, a lens barrel according to the present invention may hold an image taking lens consisting of a first lens group, a second lens group, a third lens group, and a fourth lens group in this order when viewed from ahead along an optical axis and which is capable of changing a barrel length thereof between a housed state of relatively short length and a photographing state of relatively long length, having: 
     a lens advance/retract mechanism which retracts at least one of the lens groups from a photographic optical axis during transition to the housed state and advances the retracted lens group onto the photographic optical axis during transition to the photographing state. 
     In this case, the lens barrel may have an image pickup device which generates image signals by capturing light from a subject entering through the image taking lens held by the lens barrel. 
     Still furthermore, a photographic apparatus according to present invention may take photographs by capturing light from a subject entering through an image taking lens held by a lens barrel and consisting of a first lens group, a second lens group, a third lens group, and a fourth lens group in this order when viewed from ahead along an optical axis, the lens barrel being capable of changing a barrel length thereof between a housed state of relatively short length and a photographing state of relatively long length, the photographic apparatus having: 
     a lens advance/retract mechanism which retracts at least one of the lens groups from a photographic optical axis during transition to the housed state and advances the retracted lens group onto the photographic optical axis during transition to the photographing state. In this case, the photographic apparatus may have an image pickup device which generates image signals by capturing light from a subject entering through the image taking lens held by the lens barrel. 
     Also, an optical apparatus according to the present invention may hold a lens unit that includes a lens consisting of a first lens group, a second lens group, a third lens group, and a fourth lens group in this order when viewed from ahead along an optical axis, the lens unit being capable of changing a barrel length thereof between a first state of relatively short length and a second state of relatively long length, the optical apparatus having: 
     a lens advance/retract mechanism which retracts at least one of the lens groups from a photographic optical axis during transition to the first state and advances the retracted lens group onto the photographic optical axis during transition to the second state. 
     As described above, the present invention implements a lens barrel whose collapsed length can be further decreased, photographic apparatus which incorporates the lens barrel into its body, and an optical apparatus which has a lens whose length in a state of short length is further decreased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view taken along the optical axis of a lens barrel mounted in a conventional digital camera, where the lens barrel is at a wide-angle end; 
         FIG. 2  is a sectional view taken along the optical axis of the lens barrel mounted in the conventional digital camera, where the lens barrel is at a telephoto end; 
         FIG. 3  is a sectional view taken along the optical axis of the lens barrel mounted in the conventional digital camera, where the lens barrel is collapsed; 
         FIG. 4  is a developed view illustrating cam grooves formed in the inner wall of an outer tube  120  and used to extend an inner tube  110  as well as to adjust focus by adjusting distance between a second lens group  220  and third lens group  230 ; 
         FIG. 5  is an external perspective view of a digital camera  300  which is an embodiment of a photographic apparatus according to the present invention and which incorporates an embodiment of a lens barrel according to the present invention into its body; 
         FIG. 6  is a diagram showing the digital camera in  FIG. 5  with the lens barrel extended; 
         FIG. 7  is a sectional view along the optical axis of the lens barrel  310  incorporated into the digital camera in  FIGS. 5 and 6  with the lens barrel at the wide-angle end; 
         FIG. 8  is a sectional view along the optical axis of the lens barrel  310  incorporated into the digital camera in  FIGS. 5 and 6  with the lens barrel at the telephoto end; 
         FIG. 9  is a sectional view along the optical axis of the lens barrel  310  incorporated into the digital camera in  FIGS. 5 and 6  with the lens barrel collapsed; 
         FIG. 10  is a sectional view of the lens barrel sliced along the line and viewed in the direction of the arrows in  FIG. 7 ; 
         FIG. 11  is a sectional view of the lens barrel sliced along the line and viewed in the direction of the arrows in  FIG. 8 ; 
         FIG. 12  is a sectional view of the lens barrel sliced along the line and viewed in the direction of the arrows in  FIG. 9 ; 
         FIG. 13  is a developed view illustrating cam grooves formed in the inner wall of a middle tube; 
         FIG. 14  is a block diagram showing a circuit configuration of the digital camera according to this embodiment; 
         FIG. 15  is a sectional view along the optical axis of an embodiment of a lens barrel incorporated into the digital camera in  FIGS. 5 and 6  with the lens barrel at the wide-angle end; 
         FIG. 16  is a sectional view along the optical axis of the lens barrel  310  incorporated into the digital camera in  FIGS. 5 and 6  with the lens barrel at the telephoto end; 
         FIG. 17  is a sectional view along the optical axis of the lens barrel  310  incorporated into the digital camera in  FIGS. 5 and 6  with the lens barrel collapsed; 
         FIG. 18  is a sectional view of the lens barrel taken along the line and viewed in the direction of the arrows in  FIG. 15 ; 
         FIG. 19  is a sectional view of the lens barrel taken along the line and viewed in the direction of the arrows in  FIG. 16 ; 
         FIG. 20  is a sectional view of the lens barrel taken along the line and viewed in the direction of the arrows in  FIG. 17 ; 
         FIG. 21  is a diagram showing a lens arrangement of an image taking lens consisting of four groups during extension; 
         FIG. 22(   a ) is a diagram showing a lens arrangement of lens groups during collapse; 
         FIG. 22(   b ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   c ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   d ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   e ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   f ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   g ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   h ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   i ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   j ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   k ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   l ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   m ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   n ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   o ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   p ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   q ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   r ) is a diagram showing another lens arrangement of lens groups during collapse; 
         FIG. 22(   s ) is a diagram showing another lens arrangement of lens groups during collapse; and 
         FIG. 23  is a diagram showing two photographic optical systems constituted of retracted lens groups and the remaining lens groups, respectively. 
         FIG. 24  is a diagram showing the configuration of an optical pickup apparatus as an example of the optical apparatus. 
         FIG. 25  is a diagram showing a liquid crystal projector  80  as another example of the optical apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention are described below. 
       FIGS. 5 and 6  are external perspective views of a digital camera  300  which is an embodiment of a photographic apparatus according to the present invention and which incorporates an embodiment of a lens barrel according to the present invention into its body. 
       FIG. 5  is an external view in the case where a lens barrel  310  incorporated into the body of the digital camera  300  is in a collapsed state while  FIG. 6  is an external view in the case where the lens barrel  310  is in an extended state. 
     The lens barrel  310  of the digital camera  300  shown in  FIGS. 5 and 6  incorporates an image taking lens consisting of four lens groups. The second of the four lens groups is moved along the optical axis for adjustment of focal distance while the fourth lens group serving as a focus lens is moved along the optical axis for focus adjustment. 
     On the upper front part of the digital camera  300  shown  FIGS. 5 and 6 , there are a fill flash window  302  and finder objective window  303 . On the top face, there is a shutter button  304 . 
     A zoom control switch is mounted on the back (not shown) of the digital camera  300 . While one end of the zoom control switch is depressed, the lens barrel  310  extends to the telephoto side. While the other end of the zoom control switch is depressed, the lens barrel  310  moves to the wide-angle side. 
       FIGS. 7 ,  8 , and  9  are sectional views taken along the optical axis of the lens barrel  310  incorporated into the digital camera in  FIGS. 5 and 6 .  FIGS. 7 ,  8 , and  9  show a wide-angle end, telephoto end, and collapsed state, respectively.  FIG. 10  is a sectional view of the lens barrel in  FIG. 7  sliced along line A–A′ and viewed in the direction of the arrows in  FIG. 7 ,  FIG. 11  is a sectional view of the lens barrel in  FIG. 8  sliced along line B–B′ and viewed in the direction of the arrows in  FIG. 8 ,  FIG. 12  is a sectional view of the lens barrel in  FIG. 9  sliced along line C–C′ and viewed in the direction of the arrows in  FIG. 9 .  FIG. 7  is a sectional view of the lens barrel sliced along line D–D′ and viewed in the direction of the arrows in  FIG. 10 ,  FIG. 8  is a sectional view of the lens barrel sliced along line E–E′ and viewed in the direction of the arrows in  FIG. 11 ,  FIG. 9  is a sectional view of the lens barrel sliced along line F–F′ and viewed in the direction of the arrows in  FIG. 12 .  FIG. 13  is a developed view illustrating cam grooves formed in the inner walls of the outermost and middle tubes of the three tubes which compose the lens barrel shown in  FIGS. 7 to 12 . Hereinafter, the outermost tube of the three tubes is referred to as a fixed tube  313 , the middle tube of the three tubes is referred to as a middle tube  312 , and the innermost tube is referred to as a front tube  311 . 
     The lens barrel  310  incorporates a four-group zoom lens composed of a first lens group  410 , second lens group  420 , third lens group  430 , and fourth lens group  440 . The fourth lens group  440  at the tail of the four-group zoom lens is used as a focus lens. 
     As shown in  FIGS. 7 ,  8 , and  9 , the first lens group  410  is held in the front tube  311 . The front tube  311  is equipped with cam pins  3111  (see  FIG. 8 ), which are engaged with cam grooves  3123  (see  FIGS. 8 and 13 ) formed in the inner wall of the middle tube  312 . The middle tube  312  is also equipped with cam pins  3124 , which are engaged with cam grooves  3131  (see  FIG. 13 ) formed in the inner wall of the fixed tube  313 . A straight-ahead key ring  320  has a straight-ahead groove  3204  into which a protrusion  3112  of the front tube  311  is fitted. The fit (see  FIG. 7 ) between the protrusion  3112  and straight-ahead groove  3204  serves to stop the rotation of the front tube  311 . Thus, the front tube  311  and straight-ahead key ring  320  are only capable of relative travel along the optical axis, and do not rotate around the optical axis. 
     The middle tube  312  is configured to rotate by means of a zoom motor  3300  (see  FIGS. 11 and 12 ). When the middle tube  312  rotates being driven by the zoom motor  3300 , it moves along the optical axis following the shape (see  FIG. 13 ) of the cam grooves  3131  in the inner surface of the fixed tube  313  and the front tube  311  moves along the optical axis following the shape of the cam grooves  3123  in cam engagement with the rotating and moving middle tube  312 .  FIGS. 11 and 12  show coupling between the zoom motor  3300  and middle tube  312 , where the zoom motor  3300  torques a gear  3125  on the inner wall of the middle tube  312  via a coupling gear  3302  (see  FIG. 9 ), causing the middle tube  312  to rotate. 
     A straight-ahead key  320 K installed on the straight-ahead key ring  320  is advanceably/retractably engaged with a straight-ahead groove  3132  in the fixed tube  313 . Furthermore, the middle tube  312  is rotatably engaged with the straight-ahead key ring  320  as cam pins  3203  installed on the straight-ahead key ring  320  are engaged with circumferentially extending cam grooves  3122  (see  FIGS. 8 and 13 ) formed in the middle tube  312 . Thus, as the middle tube  312  advances/retracts along with rotation due to cam engagement with the fixed tube  313 , the straight-ahead key ring  320  also advances/retracts along with the middle tube  312 . 
     In this way, the middle tube  312  is capable of relative travel around the optical axis and capable of traveling along the optical axis integrally with the straight-ahead key ring  320 . 
     Of the lens group holding frames which hold the lens groups, relationship among a second lens group holding frame  421  which holds the second lens group  420 , a fourth lens group holding frame  441  which holds the fourth lens group  440 , and the middle tube  312 , relationship between the straight-ahead key ring  320  and third lens group  430 , and relationship between the middle tube  312  and fixed tube  313  are described with reference to  FIGS. 7 to 13 . In this embodiment, to further reduce the length of the lens barrel, the second and fourth lens groups are retracted to the retracted position. Such retraction mechanism is described as required in the process of describing relationships among lens groups. 
     The second lens group holding frame  421  which holds the second lens group  420  is supported movably by a lens group supporting frame  422  extending from the straight-ahead key ring  320  and whose outer wall has cam pins  3211 . The cam pins  3211  penetrate a key way  320   a  of the straight-ahead key ring  320  and engages with cam grooves  3121  (see  FIGS. 7 and 13 ) formed in the inner wall of the middle tube  312 . Also, as shown in  FIGS. 7 to 12 , the straight-ahead key ring  320  supports the second lens group holding frame  421  via the second lens group supporting frame  422 . It also supports the third lens group  430  directly. Furthermore, it supports the fourth lens group  440  via a travel mechanism constituted of a column screw  3201  (see  FIG. 8 ). 
     As described above, the second lens group supporting frame  422  engaged with the straight-ahead key ring  320  has the cam pins  3211 . As described above, the cam pins  3211  penetrate the key way  320   a  of the straight-ahead key ring  320  and engages with the cam grooves  3121  in the inner wall of the middle tube  312 . As the cam pins  3211  move following the shape of the cam grooves  3121 , the second lens group  420  moves from the telephoto end to the wide-angle end or from the wide-angle end to the telephoto end, being guided by the key way  320   a.    
     The middle tube  312  has the cam pins  3124  implanted in its outer wall. The cam pins  3124  are engaged with the cam grooves  3131  formed in the fixed tube  313 . The cam grooves  3131  run in such a way as to rotate through a predetermined angle (indicated by symbol k in  FIG. 13 ) during extension from the collapsed state shown in  FIG. 9  to the telephoto end shown in  FIG. 8 . Thus, the middle tube  312  extends along the optical axis following the cam grooves  3131  while rotating by the predetermined angle from the collapsed state ( FIG. 9 ) to the telephoto end ( FIG. 8 ) by receiving the driving force of the zoom motor  3300  (see  FIGS. 11 and 12 ). Along with extension of the middle tube  312 , the front tube  311  is extended without rotation, following the shape (indicated by symbol m in  FIG. 13 ) of the cam grooves  3123 . With the middle tube  312  extended, if the zoom switch is turned to the wide-angle position, the middle tube  312  rotates at the extended position (indicated by symbol  1  in  FIG. 13 ), causing the second lens group  420  to move to the wide-angle end following the shape (indicated by symbol p in  FIG. 13 ) of the cam grooves  3121 . In this way, the lens barrel is zoomed according to operation of the zoom switch. 
     The second lens group holding frame  421  which holds the second lens group  420  is supported by the second lens group supporting frame  422 , which in turn is supported by the straight-ahead key ring  320 . The second lens group supporting frame  422  is equipped with a pivot shaft  422   a  in order for the second lens group holding frame  421  to rotate when retracting from the optical axis. On the other hand, the second lens group holding frame  421  has a through-hole  422   c  to accept the pivot shaft  422   a . The pivot shaft  422   a  is passed through the through-hole  422   c  in the second lens group holding frame  421  and a spring  422   b  is wound around a base end of the pivot shaft  422   a  to urge the second lens group holding frame  421  forward along the optical axis. The spring  422   b  acts as a torsion spring which urges the second lens group holding frame  421  to turn around the pivot shaft  422   a . When the lens barrel is collapsed, since the second lens group holding frame  421  is pushed by a pusher  311   a  on the front tube  311  due to the presence of the spring  422   b , the second lens group  420  moves toward the base end and is housed compactly in the lens barrel (see  FIG. 9 ). When the lens barrel is extended, since the pusher  311   a  (see  FIG. 7 ) on the front tube which holds the first lens group  410  separates from the second lens group supporting frame  422 , the second lens group  420  is urged forward along the optical axis by the spring  422   b . At this time, the second lens group holding frame  421  is urged also in the direction of rotation by the spring  422   b , the stub  4212  on the second lens group holding frame  421  abuts the stop bar  4221 , and the second lens group is held at the abutting position so that the center of the second lens group  420  will align accurately with the optical axis. 
     The second lens group holding frame  421  is located on one side of the pivot shaft  422   a  while on the other side, there is an extension member  4211 . The extension member  4211  engages with a switching protrusion  3133  when it collapses together with the straight-ahead key ring  320 . The switching protrusion  3133  is sloped in the retreating direction of the straight-ahead key ring  320 . 
     When the extension member  4211  moves along the slope of the switching protrusion  3133  during collapse, the second lens group holding frame  421  rotates around the pivot shaft  422   a  to a place where it abuts the straight-ahead key ring  320  (see  FIG. 12 ). 
     According to this example, an advance/retract mechanism of the second lens group  420  is composed of a series of members including the second lens group holding frame  421 , its extension member  4211 , the stub  4212  of the holding frame  421  opposite to the extension member, a torsion spring  421   b , the stop bar  4221  provided on the second lens group supporting frame  422  supported by the straight-ahead key ring  320 , and the switching protrusion  3133  provided on the main body. 
     The third lens group  430  is supported directly by the straight-ahead key ring  320 , which also supports the fourth lens group  440  via a travel mechanism consisting of a focus motor  3200  and a column screw  3201  meshing with a gear head of the focus motor  3200 . The fourth lens group  440  is supported by the lens group holding frame  441 . A through-hole  441   a  is made in the lens group holding frame  441  and a guide rod  3202  attached to the straight-ahead key ring  320  is passed through the through-hole  441   a . The column screw  3201  is screwed into a nut  3201   a , which is engaged with an extension member  4411  provided on the lens group holding frame  441 . On the other hand, a spring  441   b  is wound around the guide rod  3202  which guides the lens group holding frame  441  accurately along the optical axis. The spring  441   b  urges the lens group holding frame.  441  toward the nut  3201   a.    
     In this example, the fourth lens group holding frame  441  is equipped with an advance/retract mechanism as is the case with the second lens group holding frame  421 . When the extension member  4411  of the fourth lens group holding frame  441  engages with another switching protrusion  3134  during collapse, the fourth lens group  440  retracts from the optical axis (see  FIG. 12 ) as is the case with the second lens group  420 . This makes it possible to line up the first lens group  410  and third lens group  430  as closely as possible and then place the second lens group  420  and fourth lens group  440  closely above or below the first lens group  410  and third lens group  430 , further reducing the length of the lens barrel. 
     As described above, the focus motor  3200  which moves the fourth lens group  440  along the optical axis is also supported by the straight-ahead key ring  320 . The rotational driving force of the focus motor  3200  supported by the straight-ahead key ring  320  is transmitted via a gear train to rotate the column screw  3201  shown in  FIG. 8 . The nut  3201   a  fitted rotatably over the column screw  3201  moves by the distance equivalent to the rotation of the column screw  3201 , moving the lens group holding frame  441  engaged with the nut  3201   a , and consequently adjusting the focus. 
     When taking a photograph, a control apparatus (not shown) gives a drive command for focus adjustment to the focus motor  3200  supported by the straight-ahead key ring, based on image data generated by an image pickup device  500 . 
     After the focus adjustment, when a shutter button  304  is pressed, a shutter unit  330  supported by the straight-ahead key ring is operated and an electronic shutter is operated to take a photograph. Consequently, light from the subject passes through the first lens group  410 , second lens group  420 , third lens group  430 , and fourth lens group (focus lens)  440  and forms an image on a light-receiving surface of the image pickup device  500 , which generates an image signal which represents the subject image formed on the light-receiving surface. Incidentally, wiring cables for use to transmit commands to the focus motor and shutter unit are not shown in  FIGS. 7 to 12 . 
     In this way, the straight-ahead key ring  320  which moves along the optical axis supports the second lens group  420  and third lens group  430 . Also, it supports the fourth lens group  440  via the column screw  3201 . Furthermore, it supports the focus motor  3200  which rotates the column screw  3201 . In this configuration, the fourth lens group is moved together with the straight-ahead key ring to near the focus position and the position of the fourth lens group is fine-adjusted for focus adjustment by turning the column screw. 
     This eliminates the need to provide a column screw and guide rods stretching out along the optical axis from the light-receiving surface of the image pickup device and move the fourth lens group at great length along the guide rods by turning the column screw. This creates a free space between the image pickup device  500  and fourth lens group  440  without obstruction. When the lens barrel is collapsed, the second lens group  420  and fourth lens group  440  retracted from the optical axis can be stuffed into this free space ( FIG. 9 ) on a different plane from the other lens groups, reducing the length of the lens barrel greatly. Also, since the front tube  311  is equipped with the pusher  311   a  which pushes the second lens group holding frame  421  to the second lens group supporting frame  422 , compressing the spring  422   b  during collapse, it is possible to bring the first lens group  410  and second lens group  420  unlimitedly close to each other as well as to extend the second lens group greatly along the optical axis during extension by urging it forward along the optical axis under the force of the spring. 
     Consequently, even though the digital camera  300  shown in  FIGS. 5 and 6  has a slim body, the lens barrel  310  is housed in the camera body when it is collapsed and the digital camera  300  allows photography with a high zoom ratio when the lens barrel  310  holding the four-group zoom lens is extended from the camera body. 
     Finally, an internal configuration of the digital camera in  FIGS. 5 and 6  is described. 
       FIG. 14  is a block diagram showing a circuit configuration of the digital camera according to this embodiment. 
     The digital camera  300  in this figure is equipped with a four-group zoom lens  400  composed of the first lens group  410 , second lens group  420 , third lens group  430 , and fourth lens group  440  shown in  FIGS. 7 to 9  as well as with the shutter unit  330  and image pickup device  500 . A subject image formed on the image pickup device  500  via the zoom lens  400  and shutter unit  330  is converted into an analog image signal by the image pickup device  500 . The shutter unit  330  is composed of an aperture which controls amounts of light falling on the image pickup device and a shutter which prevents smearing caused by light when the analog signal is read from the image pickup device  500 . 
     The digital camera  300  is equipped with a fill flash section  600 , which emits fill light ahead of the digital camera through the fill flash window  302  shown in  FIGS. 5  and  6  in low-light conditions. The fill flash section  600  can be made to emit light other than in low-light conditions if necessary. 
     Also, the digital camera  300  is equipped with an analog signal processing section  501 , A/D converter section  502 , digital signal processing section  503 , temporary memory  504 , compression/decompression section  505 , built-in memory (or memory card)  506 , image monitor  507 , and a drive circuit  508 . The image pickup device  500  is driven based on timings generated by a timing generation circuit (not shown) in the drive circuit  508  and outputs an analog image signal. Also, the drive circuit  508  includes drive circuits which drive the image taking lens  400 , shutter unit, fill flash section  600 , etc. The analog image signal outputted from the image pickup device  500  is subjected to analog signal processing by the analog signal processing section  501 , to A/D conversion by the A/D converter section  502 , and to digital signal processing by the digital signal processing section  503 . Image data which represents the image after the digital signal processing is stored temporarily in the temporary memory  504 . The image data stored in the temporary memory  504  is compressed by the compression/decompression section  505  and recorded in the built-in memory (or memory card)  506 . Depending on image mode, the image data may be stored in the built-in memory (or memory card)  506  directly by omitting the process of compression. The image data stored in the temporary memory  504  is read out to display the image of the subject on the image monitor  507 . 
     Furthermore, the digital camera  300  is equipped with a CPU  509  which controls the entire digital camera  300 , control switch group  510  including a zoom control switch, and shutter button  304 . A photograph is taken, i.e., image data such as those described above is generated when the user sets desired photographic conditions including a desired angle of view by operating the control switch group  510  and presses the shutter button  304 . 
     Although in the above embodiment, the fourth lens group  440  and second lens group  420  are retracted, it is also possible to retract only the fourth lens group  440 . 
       FIGS. 15 to 20  are diagrams showing a configuration of another embodiment of a lens barrel equipped with an advance/retract mechanism which retracts the fourth lens group  440 . In the following description, it is assumed that a lens barrel  310 ′ shown in  FIGS. 15 to 20  is incorporated in the camera shown in  FIGS. 5 and 6 . The configuration of the lens barrel  310 ′ shown in  FIGS. 15 to 20  is the same as the one shown in  FIGS. 7 to 12  except that the switching protrusion  3133  which is a part of the advance/retract mechanism of the second lens group shown in  FIG. 10  has been removed and that there are some changes to the shape of the second lens group holding frame  421   a.    
       FIGS. 15 ,  16 , and  17  are sectional views taken along the optical axis of the lens barrel  310 ′ incorporated into the digital camera shown in  FIGS. 5 and 6 .  FIGS. 15 ,  16 , and  17  show a wide-angle end, telephoto end, and collapsed state, respectively.  FIG. 18  is a sectional view of the lens barrel in  FIG. 15  sliced along line A–A′ and viewed in the direction of the arrows in  FIG. 15 ,  FIG. 19  is a sectional view of the lens barrel in  FIG. 16  sliced along line B–B′ and viewed in the direction of the arrows in  FIG. 16 , and  FIG. 20  is a sectional view of the lens barrel in  FIG. 17  sliced along line C–C′ and viewed in the direction of the arrows in  FIG. 17 .  FIG. 15  is a sectional view taken along the arrows of line D–D′ in  FIG. 18 ,  FIG. 16  is a sectional view taken along the arrows of line E–E′ in  FIG. 19 , and  FIG. 17  is a sectional view taken along the arrows of line F–F′ in  FIG. 20 . 
     Since the fourth lens group  440  acts as a focus lens group, it is particularly important to align its optical axis. Although description is omitted in the embodiment shown in  FIGS. 7 to 12 , the fourth lens group holding frame  441  which holds the fourth lens group  440  has a mechanism which advances the fourth lens group  440  accurately to the photographic optical axis as is the case with the second lens group holding frame  421 . The mechanism has the same configuration as the second lens group holding frame  421 . It is configured such that an end  4412  of the fourth lens group holding frame  441  stops against a stop rod  3205  provided on the straight-ahead key ring  320  so that the center of the focus lens coincides accurately with the optical axis (see  FIG. 16 ). As is the case with the embodiment shown in  FIGS. 7 to 12 , the advance/retract mechanism according to the present embodiment has a torsion spring  441   b , switching protrusion  3134  (see  FIG. 15 ), extension member  4411  of the fourth lens group holding frame  441 , an end  4412  (see  FIG. 16 ) of the fourth lens group holding frame  441 , and stop rod  3205  (see  FIG. 16 ) supported by the straight-ahead key ring  320 . When the fourth lens group  440  collapses together with the straight-ahead key ring  320 , as the extension member  4411  engages with the switching protrusion  3134  and moves along the slope of the switching protrusion  3134 , the fourth lens group holding frame  441  rotates around the guide rod  3202  to a place (see  FIG. 20 ) where it abuts the straight-ahead key ring  320 . This makes it possible to line up the first lens group  410 , second lens group  420 , and third lens group  430  as closely as possible and then place the fourth lens group  440  closely above or below the first lens group  410 , second lens group  420 , and third lens group  430 , further reducing the length of the lens barrel. 
     As described above, by installing the advance/retract mechanism which retracts at least the fourth lens group  440  from the photographic optical axis, it is possible to reduce the length of the lens barrel and by incorporating the shortened lens barrel into a photographic apparatus, it is possible to reduce the thickness of the photographic apparatus. 
     Although in this embodiment, the advance/retract mechanism retracts at least the fourth lens group  440  which acts as a focus lens from the photographic optical axis, there can be any number of configurations in which a lens group is retracted from the photographic optical axis. Last, a brief description will be given of possible lens arrangements which result when lens groups are retracted. 
       FIG. 21  is a diagram showing a lens arrangement of an image taking lens consisting of four groups during extension while  FIGS. 22(   a ) to ( s ) are diagrams showing lens arrangements of an image taking lens consisting of four groups during collapse.  FIGS. 22(   a ) to  22 ( s ) show possible lens arrangements which can be used when retracting at least one of four lens groups from the photographic optical axis. 
       FIGS. 22(   a ) to  22 ( j ) show lens arrangements which result when any of the first lens group, second lens group, third lens group, and fourth lens group is retracted from the photographic optical axis toward the circumference of the tubes in the lens barrel using the same advance/retract mechanism as in  FIGS. 7 to 9 .  FIGS. 22(   k ) to  22 ( s ) show lens arrangements which result when at least one of the first lens group, second lens group, third lens group, and fourth lens group is retracted along the length of the tubes by the advance/retract mechanism.  FIGS. 22(   f ) to  22 ( s ) show examples in which the lens barrel is further reduced by retracting lens groups into a recess by the side of a CCD. 
     As described above, the fourth lens group holding frame  441  and the advance/retract mechanism consisting of the extension member  4411  and the like provided on the fourth lens group holding frame are supported by the straight-ahead key ring  320 , and when the lens barrel collapses, the extension member, retreating backward along the optical axis together with the straight-ahead key ring  320 , engages with the switching protrusion  3134 , thereby retracting the lens group. Thus, the advance/retract mechanism according to the present invention, in a broad sense, includes a cam mechanism. 
     To show that the at least one lens group turned by the advance/retract mechanism is caused to retreat further along the optical axis by the cam mechanism, the retreat caused by the cam mechanism is indicated by hollow arrows in distinction from the turning which is performed by the advance/retract mechanism and indicated by normal arrows. 
     The lens diameters of lens groups often differ from one another. Besides, they vary with the photographic optical system. Thus, when retracting lens groups of different diameters, it is sometimes necessary to retract them in a specific sequence. Numerals which indicate such sequences appear in some drawings herein. 
     In this way, it is possible to retract at least one of the lens groups from the photographic optical axis by turning it toward the circumference of the tubes in the lens barrel as well as to retract lens groups from the photographic optical axis in such a way as to be in parallel to the optical axis by turning them in the longitudinal direction of the lens barrel. Besides, since the lens group retracted from the optical axis by the advance/retract mechanism is caused to retreat further by being carried by the advance/retract mechanism, which retreats being supported by the straight-ahead key ring, it is possible to implement a far shorter lens barrel than conventional ones. 
     When two lens groups are retracted according to the advance/retract mechanism, the retracted lens groups and remaining lens groups may constitute respective photographic optical systems. 
       FIG. 23  is a diagram showing two photographic optical systems in that case. 
     As shown in  FIG. 23 , the retracted second and fourth groups constitute a photographic optical system and the remaining first and third groups constitute another photographic optical system. If a second image pickup device  500   a  is provided behind the photographic optical system of the retracted lens groups, a twin-lens photographic optical system results. Various extended configurations are conceivable if both retracted lens groups and remaining lens groups are used in this way instead of simply retracting lens groups. 
     In addition, since the above embodiments are described using a digital camera, the words “collapse” and “extend” are used to describe its lens barrel. However, the present invention is not limited to photographic apparatus such as digital camera and can be widely applied to optical apparatus having a lens consisting of multiple lens groups. 
     Since the present invention can be applied to optical apparatus having a lens unit consisting of multiple lens groups, such optical apparatus is described below as an example. 
       FIG. 24  is a diagram showing the configuration of an optical pickup apparatus as an example of the optical apparatus. 
     An optical pickup apparatus  70  shown in  FIG. 16  is comprised of a semiconductor laser  71  and an optical system for guiding light emitted from the semiconductor laser  71  onto an optical disk  73 . The optical system includes a lens unit  72  that is comprised of a lens consisting of the first lens group, the second lens group, the third lens group and the fourth lens group arranged in this order when viewed from ahead along the optical axis. The lens unit  72  has a tube extendable between the first state in which the tube length is relatively short and the second state in which the tube length is relatively long. The optical pickup apparatus  70  includes a lens advancing/retracting mechanism that retracts at least one of the multiple lens groups from the optical axis when the lens unit shifts from the second state to the first stated and advances the retracted lens group onto the optical axis when the lens unit shifts from the first state to the second stated. 
     A brief description of the operation performed by the optical pickup apparatus  70  shown in  FIG. 24  is given below. 
     The optical pickup apparatus  70  focuses the light emitted from the semiconductor laser  71  on the optical disk  73  by using the optical system including the lens unit so as to record an image signal on the optical disk  73 . In the optical pickup apparatus  70  shown in  FIG. 16 , part of the light emitted from the semiconductor laser  71  is reflected by a glass plate  74  arranged obliquely with respect to the light axis and detected by a light detector  76  so that a detection signal is generated. The detection signal is then sent back to a driving device (not shown) of the semiconductor laser so that the output level of the semiconductor laser is adjusted and the intensity of the light to be emitted is controlled. Also, in the optical pickup apparatus  70 , light returned from the optical disk  73  is reflected by glass plates  74  and  75  and detected by light detectors  77  and  78  respectively so that a focus error signal and a tracking error signal are generated. 
     If the above lens unit is incorporated into such an optical apparatus, it is housed in the optical apparatus when unused to make the optical apparatus compact in size. 
       FIG. 25  shows a liquid crystal projector  80  as another example of the optical apparatus. 
     As shown in  FIG. 25 , the liquid crystal projector  80  includes: a lamp  81 ; an oval reflector  82  that reflects white light emitted from the lamp  81  backward in the optical axis so as to make the white light going forward in the optical axis; a circular reflector  83  that has an irradiation hole and reflects light which is not reflected by the oval reflector  82  toward the oval reflector  82 ; a condensing lens  84  that changes the path of polarized light generated by the oval reflector  82  so as to condense it to a liquid crystal panel  85 ; the liquid crystal panel  85  that modulates light condensed by the condensing lens  84 ; and a projecting lens  86  that forms light of R, G and B modulated by the liquid crystal panel  85  on a screen  87 . A lens unit of which structure is similar to that of the above-described lens barrel can be applied to the projecting lens  86 . Then, as an optical apparatus, there is realized a liquid crystal projector having the projecting lens  86  housed therein when unused.