Patent Publication Number: US-2023152675-A1

Title: Camera accessory

Description:
INCORPORATION BY REFERENCE 
     This application is a continuation application of U.S. application Ser. No. 17/022,284, filed Sep. 16, 2020, which is a continuation application of U.S. application Ser. No. 16/677,229, filed Nov. 7, 2019, which in turn is a continuation application of U.S. application Ser. No. 15/918,135, filed Mar. 12, 2018, which in turn is a continuation application of U.S. application Ser. No. 15/252,731, filed Aug. 31, 2016, which in turn is a continuation application of U.S. application Ser. No. 14/850,331, filed Sep. 10, 2015, which in turn is a continuation application of U.S. application Ser. No. 13/225,919, filed Sep. 6, 2011, the entire contents of the priority applications being incorporated herein by reference. The entire disclosures of the following priority applications are herein incorporated by reference: 
     Japanese Patent Application No. 2010-198864 filed Sep. 6, 2010 
     Japanese Patent Application No. 2011-135245 filed Jun. 17, 2011 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a camera accessory that can be detachably engaged with a camera body equipped with a camera body mount assuming a bayonet structure. 
     2. Description of Related Art 
     A camera accessory, such as an exchangeable lens, and a camera body assuming structures that allow them to be detachably engaged with each other via a mount system adopting a bayonet structure are known in the related art. Such a camera accessory and a camera body are engaged with each other by first inserting a camera accessory mount through a camera body mount at the correct interlock phase and then rotating the camera accessory along a predetermined direction (mounting direction) around the incident light optical axis. The camera accessory can be dismounted from the camera body by first rotating the camera accessory mounted at the camera body along a direction (dismounting direction) opposite from the mounting direction and then pulling the camera accessory forward relative to the camera body. 
     Japanese Laid Open Patent Publication No. H3-229229 discloses a system that includes a camera accessory and a camera body structured as described above, whereby a projecting portion formed at the camera accessory comes in contact with a side end of a tab at the camera body mount when the camera accessory is being mounted at or dismounted from the camera body, so as to restrict the range of rotation of the camera accessory along the mounting direction or the dismounting direction. 
     SUMMARY OF THE INVENTION 
     In the mount system disclosed in the patent literature cited above, the portion (restricting portion) that restricts the rotation range by coming into contact with a side end of a tab at the camera body mount assumes a position close to a retainer lock groove via which the camera accessory is held fast. For this reason, backlash between the camera accessory and the camera body centered on a camera body-side pin inserted through the lock groove, which is attributable to the clearance formed as the camera accessory is mounted at the camera body, cannot be completely prevented via the restricting portion. 
     A camera accessory that can be detachably mounted at a camera body that includes a camera body mount, and includes an accessory mount assuming a bayonet structure with a first tab, a second tab and a third tab set over intervals in a circumferential direction running along a circumference of a circle with a predetermined diameter and projecting from the circumference along a radial direction, according to a first aspect of the present invention, the accessory mount can be inserted through the camera body mount without any of the first through third tabs being hindered by any of three camera body-side tabs at the camera body mount, as long as the accessory mount is inserted into the camera body mount at a correct interlock phase; the first tab, the second tab and the third tab extend over varying lengths along the circumferential direction; the first tab extends over a greatest length along the circumferential direction among the first through third tabs; the third tab extends over a smallest length along the circumferential direction among the first through third tabs; the accessory mount includes a restricting member disposed near the third tab, a fitting portion used to align an optical axis of the accessory mount with an optical axis of the camera body, and a lock pin hole through which a lock pin, retractably disposed at the camera body, is inserted so as to lock the camera accessory in a fully mounted state; and the restricting member is disposed at a position assumed on a side substantially opposite from the lock pin hole across the fitting portion. 
     According to a second aspect of the present invention, in the camera accessory according to the first aspect, it is preferable that when the first through third tabs are rotated along the circumference in a first rotating direction in order to mount the accessory mount, having been inserted at the correct interlock phase, at the camera body mount, the restricting member comes into contact with a side end of one camera body-side tab among the camera body-side tabs, thereby restricting a range of rotation in the first rotating direction; and when the first through third tabs are rotated along the circumference in a second rotating direction opposite from the first rotating direction in order to dismount the accessory mount from the camera body mount, the restricting member comes into contact with a side end of a camera body-side tab different from the one camera body-side tab, thereby restricting a rotation range in the second rotating direction. 
     According to a third aspect of the present invention, in the camera accessory according to the first aspect, it is preferable that insertion of the accessory mount through the camera body mount is disallowed if the accessory mount is positioned at a phase other than the correct interlock phase, with at least two tabs among the first tab, the second tab and the third tab coming into contact with at least two of the camera body-side tabs. 
     According to a fourth aspect of the present invention, in the camera accessory according to the first aspect, it is preferable that the first tab at the accessory mount having been inserted at the correct interlock phase and engaged with the camera body mount overlaps a camera body-side tab assuming an uppermost position when the camera body is set with a longitudinal orientation achieved by rotating the upright camera body assuming a lateral orientation by approximately 90°. 
     According to a fifth aspect of the present invention, in the camera accessory according to the first aspect, the first tab, the second tab and the third tab may be fixed to a main body of the camera accessory. 
     According to a sixth aspect of the present invention, in the camera accessory according to the first aspect, the first tab, the second tab and the third tab may be rotatably disposed so as to be allowed to rotate along the circumferential direction relative to a main body of the camera accessory. 
     According to a seventh aspect of the present invention, in the camera accessory according to the first aspect, it is preferable that the first tab, the second tab and the third tab are made up with a tab present over an angular range of 56.5° to 115°, a tab present over an angular range of 172.5° to 214.5° and a tab present over an angular range of 272° to 343.5° at the accessory mount engaged with the camera body mount, along a clockwise direction relative to a nine o&#39;clock direction viewed from a rear side of the accessory mount. 
     According to a eighth aspect of the present invention, in the camera accessory according to the first aspect, it is preferable that angles defining circular arcs representing lengths of the first tab, the second tab and the third tab extending along the circumferential direction are respectively 71.5°, 58.5° and 42°. 
     According to a ninth aspect of the present invention, in the camera accessory according to the seventh aspect, it is preferable that when the first tab, the second tab and the third tab are rotated along the circumferential direction by 40.5° in order to engage the accessory mount, having been inserted at the correct interlock phase, with the camera body mount, the accessory mount becomes engaged with the camera body mount. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective schematically illustrating a camera body with exchangeable lenses and a photographic lens that can be detachably mounted at the camera body, in conjunction with which the present invention may be adopted. 
         FIG.  2    is a perspective schematically illustrating the structure of the body mount, viewed along a diagonal direction from a front right viewpoint relative to the camera body. 
         FIG.  3    is a perspective schematically illustrating the structure of the accessory mount, viewed along a diagonal direction from a rear left viewpoint relative to the photographic lens. 
         FIG.  4    indicates the position at which a restricting member is disposed. 
         FIG.  5    indicates the positions of the body-side tabs and the body-side gaps at the body mount viewed from the front side of the camera body. 
         FIG.  6    indicates the positions of the accessory-side tabs and the accessory-side gaps at the accessory mount viewed from the rear side of the photographic lens. 
         FIG.  7    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  8    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  9    schematically illustrates the body-side first tab expanded along the circumference in a view taken from the outer side along the radial direction. 
         FIGS.  10 A through  10 F  illustrate how the state of engagement between the accessory-side second tab and the body-side second tab changes as the photographic lens is mounted and dismounted. 
         FIG.  11    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  12    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  13    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  14    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  15    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  16    illustrates the relationship between the body mount and the accessory mount that will manifest if an excessive external force is applied to the photographic lens along the direction matching that of gravitational force. 
         FIG.  17    indicates the contact points PD and PU and the distance LS assumed when the camera body is in a lateral orientation. 
         FIG.  18    indicates the contact points PD and PU and the distance LS assumed when the camera body is in a longitudinal orientation. 
         FIG.  19    indicates the contact points PD and PU and the distance LS assumed when the camera body is in another longitudinal orientation. 
         FIGS.  20 A and  20 B  present an example of a variation. 
         FIG.  21    illustrates the variation example. 
         FIGS.  22 A through  22 D  illustrate the variation example. 
         FIG.  23    is a perspective schematically illustrating a camera body with exchangeable lenses and a photographic lens that can be detachably mounted at the camera body, achieved in a second embodiment. 
         FIGS.  24 A and  24 B  present a perspective schematically illustrating the structure of the body mount  100 , viewed along a diagonal direction from a front right viewpoint relative to the camera body. 
         FIG.  25    is a perspective schematically illustrating the structure of the accessory mount, viewed along a diagonal direction from a rear left viewpoint relative to the photographic lens. 
         FIGS.  26 A and  26 B  indicate the position at which a restricting member is disposed. 
         FIG.  27    indicates the positions of the body-side tabs and the body-side gaps at the body mount viewed from the front side of the camera body. 
         FIG.  28    indicates the positions of the accessory-side tabs and the accessory-side gaps at the accessory mount viewed from the rear side of the photographic lens. 
         FIG.  29    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  30    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  31    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  32    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  33    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  34    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  35    shows a specific state of interference (state of overlap) between the body-side tabs and the accessory-side tabs. 
         FIG.  36    illustrates how the accessory mount may swing relative to the camera body mount when there is a significant clearance between the inner circumferential surface of the camera body mount and the fitting portion of the accessory mount. 
         FIG.  37    illustrates a range of the phase that the restricting member may assume in a fully mounted state. 
         FIG.  38    is a graph indicating a range of the phase that the restricting member may assume in a fully mounted state. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     In reference to  FIGS.  1  through  19   , the camera accessory, the accessory mount, the camera body and the body mount achieved in a first embodiment of the present invention are described.  FIG.  1    is a schematic perspective of a camera body  1  with an exchangeable photographic lens  2  that can be detachably mounted at the camera body  1 , in conjunction with which the present invention is adopted. A shutter release button  4 , an image sensor  5  and a control circuit  6  that controls various units of the camera body  1  are disposed at the camera body  1 . Reference numeral  3  indicates a photographic optical path through which a subject image departing the photographic lens  2  is guided to the image sensor  5 . 
     On the front side of the camera body  1 , a body mount  100  assuming a bayonet structure is disposed via which a camera accessory, e.g., the photographic lens  2  or a converter lens used to adjust the focal length, is mounted at the body mount  100 . On the rear side of the photographic lens  2 , a lens mount, i.e., an accessory mount  200 , via which the photographic lens  2  can be detachably mounted at the body mount  100 , is disposed. It is to be noted that  FIG.  1    does not include detailed illustrations of the structures adopted in the body mount  100  and the accessory mount  200 . In addition, in the description of the embodiment, the front side (toward the subject) of the camera body  1  on which the photographic lens  2  is mounted is referred to as “forward” and the rear side of the camera body  1  where the image sensor  5  is mounted, is referred to as “rearward”. A top-bottom direction, a left-right direction and a front-rear direction are defined as indicated in  FIG.  1    for the camera body  1 .  FIG.  1    shows that the top-bottom direction relative to the camera body  1  matches the vertical direction. The attitude of the camera  1  shown in  FIG.  1    is referred to as an upright attitude or a lateral orientation. The attitude of the camera body  1  assumed by rotating the camera body with the lateral orientation by 90° clockwise or counterclockwise viewed from the front side is referred to as a longitudinal orientation. 
     —Body Mount  100 — 
       FIG.  2    is a schematic perspective illustrating the structure of the body mount  100  viewed along a diagonal direction from a front right viewpoint relative to the camera body  1 . The body mount  100  adopts a bayonet structure that includes three tabs (body-side tabs) set apart from one another along the circumferential edge of a circular opening (photographic optical path) at the camera body  1 , each projecting inward from the outer side of the opening circumference. Among the three body-side tabs, the body-side tab located at the uppermost position is referred to as a body-side first tab  110 , the body-side tab located next to the body-side first tab  110  along the counterclockwise direction in the figure is referred to as a body-side second tab  120  and the body-side tab located next to the body-side second tab  120  along the counterclockwise direction is referred to as a body-side third tab  130 . 
     The space created between two consecutive body-side tabs, through which a tab (accessory-side tab) at the accessory mount  200  passes when the photographic lens  2  is being mounted or dismounted as explained later, is referred to as a body-side gap. The body-side gap present between the body-side third tab  130  and the body-side first tab  110  is referred to as a body-side first gap  151 , the body-side gap located next to the body-side first gap  151  along the counterclockwise direction in the figure is referred to as a body-side second gap  152  and the body-side gap located next to the body-side second gap along the counterclockwise direction is referred to as a body-side third gap  153 . 
     The surface of the body-side first tab  110  facing forward is referred to as a front surface  113 , the counterclockwise side end of the body-side first tab  110  facing the body-side second gap  152  is referred to as a first side end  111 , and the clockwise side end of the body-side first tab  110  facing the body-side first gap  151  is referred to as a second side end  112 . Likewise, the surface of the body-side second tab  120  facing forward is referred to as a front surface  123 , the side end of the body-side second tab  120  facing the body-side third gap  153  is referred to as a first side end  121  and the side end of the body-side second tab  120  facing the body-side second gap  152  is referred to as a second side end  122 . The surface of the body-side third tab  130  facing forward is referred to as a front surface  133 , the side end of the body-side third tab  130  facing the body-side first gap  151  is referred to as a first side end  131 , and the side end of the body-side third tab  130  facing the body-side third gap  153  is referred to as a second side end  132 . It is to be noted that the first side ends  111 ,  121  and  131  are tab ends formed facing toward a mounting direction, to be described in detail later, whereas the second side ends  112 ,  122  and  132  are tab ends formed facing toward a dismounting direction (opposite the mounting direction). 
     It is to be noted that, although not shown in  FIG.  2   , the surface of the body-side first tab  110  facing rearward is referred to as rear surface  114 . Likewise, the surfaces of the body-side second tab  120  and the body-side third tab  130  facing rearward are respectively referred as a rear surface  124  and a rear surface  134 . 
     Reference numeral  101  indicates a body mount reference surface. The body  5  mount reference surface  101  is a ring-shaped flat surface formed so as to face toward the front side. As the photographic lens  2  is mounted at the camera body  1 , the body mount reference surface  101  comes into contact with an accessory mount reference surface  201  of the accessory mount  200 , which is to be described in detail later, thereby regulating the position assumed by the photographic lens  2  along the front-rear direction. Reference  0  numeral  102  indicates an inner circumferential surface of the cylindrical body mount  100 . 
     The inner circumferential surface  102 , which interlocks with a fitting portion of the accessory mount  200 , to be detailed below, is used as a reference surface, in reference to which the optical axis of the photographic lens  2  is aligned with the optical axis of the camera body  1  (the central axis of the photographic optical path  3 ). Reference numeral  161  indicates a pin hole through which a pin (not shown) projects out or retracts in. The pin projecting out through the pin hole  161  disallows rotation of the mounted photographic lens  2  relative to the camera body  1 . 
     —Accessory Mount  200 — 
       FIG.  3    is a perspective schematically illustrating the structure of the accessory mount  200  viewed along a diagonal direction from a rear left viewpoint relative to the photographic lens  2 . It is to be noted that  FIG.  3    shows the accessory mount  200  with the photographic lens  2  fully mounted at the camera body  1  (mounting complete state), i.e., in a photographing-enabled state. This means that the top-bottom/left-right orientation of the accessory mount  200  shown in  FIG.  3    matches the top-bottom/left-right orientation of the camera body  1 . Unless specifically noted, the following description is given by assuming that the top-bottom/left-right orientation of the accessory mount  200  is the orientation in the mounting complete state. 
     The accessory mount  200  adopts a bayonet structure that includes three tabs (accessory-side tabs) set apart from one another along the circumferential direction, each projecting from the inner side of the circumference toward the outer side of the circumference. Among the three accessory-side tabs, the accessory-side tab located at the uppermost position is referred to as an accessory-side first tab  210 , the accessory-side tab located next to the accessory-side first tab  210  along the clockwise direction in the figure is referred to as an accessory-side second tab  220  and the accessory-side tab located next to the accessory-side second tab  220  along the clockwise direction is referred to as an accessory-side third tab  230 . 
     The space created between two consecutive accessory-side tabs, through which a body-side tab passes when the photographic lens  2  is being mounted or dismounted as explained later, is referred to as an accessory-side gap. The accessory-side gap present between the accessory-side first tab  210  and the accessory-side second tab  220  is referred to as an accessory-side first gap  251 , the accessory-side gap located next to the accessory-side first gap  251  along the clockwise direction in the figure is referred to as an accessory-side second gap  252  and the accessory-side gap located next to the accessory-side second gap along the clockwise direction is referred to as an accessory-side third gap  253 . 
     The surface of the accessory-side first tab  210  facing rearward is referred to as a rear surface  213 , the side end of the accessory-side first tab  210  facing the accessory-side first gap  251  is referred to as a first side end  211  and the side end of the accessory-side first tab  210  facing the accessory-side third gap  253  is referred to as a second side end  212 . Likewise, the surface of the accessory-side second tab  220  facing rearward is referred to as a rear surface  223 , the side end of the accessory-side second tab  220  facing the accessory-side second gap  252  is referred to as a first side end  221 , and the side end of the accessory-side second tab  220  facing the accessory-side first gap  251  is referred to as a second side end  222 . The surface of the accessory-side third tab  230  facing rearward is referred to as a rear surface  233 , the side end of the accessory-side third tab  230  facing the accessory-side third gap  253  is referred to as a first side end  231  and the side end of the accessory-side third tab  230  facing the accessory-side second gap  252  is referred to as a second side end  232 . The first side ends  211 ,  221  and  231  are tab ends formed toward the mounting direction to be described in detail later, whereas the second side ends  212 ,  222  and  232  are tab ends facing toward a direction (dismounting direction) opposite from the mounting direction. 
     It is to be noted that, although not shown in  FIG.  3   , the surface of the accessory-side first tab  210  facing forward is referred to as a front surface  214 . Likewise, the surfaces of the accessory-side second tab  220  and the accessory-side third tab  230  facing forward are respectively referred to as a front surface  224  and a front surface  234 . 
     Reference numeral  201  indicates an accessory mount reference surface. The accessory mount reference surface  201  is a ring-shaped flat surface formed so as to face rearward, and is in contact with the body mount reference surface  101  when the photographic lens  2  is fully mounted. Reference numeral  202  indicates a fitting portion. The fitting portion  202  is a cylindrical portion that fits at the inner circumferential surface  102  of the body mount  100  and is used for reference when aligning the optical axis of the photographic lens  2  with the optical axis of the camera body  1  (the central axis of the photographic optical path  3 ). Reference numeral  261  indicates a pin hole. As a pin (not shown), which projects out or retracts into the pin hole  161  at the body mount  100 , is inserted through the pin hole  261 , rotation of the fully mounted photographic lens  2  relative to the camera body  1  (body mount  100 ) becomes disallowed and the photographic lens  2  thus becomes locked. 
     Although not shown in  FIG.  3   , a restricting member  262 , which limits the rotation range of the photographic lens  2  (accessory mount  200 ) relative to the camera body  1  (body mount  100 ) while the photographic lens  2  is mounted or dismounted, is disposed frontward relative to the accessory-side second tab  220  near its second side end  222 , as shown in  FIG.  4   . The restriction of the rotation range achieved via the restricting member  262  will be described in detail later. The restricting member  262  may be, for instance, a headed pin screwed in from the outer side of the accessory mount  200  along the radial direction. 
     —Positions of Body-Side Tabs and Body-Side Gaps— 
       FIG.  5    indicates the positions of the body-side tabs and the body-side gaps at the body mount  100  viewed from the front side of the camera body  1 . A reference angular position is taken along the three o&#39;clock direction at the body mount  100  viewed from the front side of the camera body  1  and various angles representing the positions at which the body-side tabs are disposed, are assumed along the counterclockwise direction relative to the three o&#39;clock direction. Namely, the body-side first tab  110  extends over an angular range of 76° to 130°, the body-side second tab  120  extends over an angular range of 177.5° to 226° and the body-side third tab  130  extends over an angular range of 304.5° to) 15° (375°. 
     The sizes of the individual body-side tabs  110  to  130  can each be indicated by the corresponding angular range along the circumferential edge of the circular opening at the body mount  100 . Namely, the body-side first tab  110  has a 54° angular range, the body-side second tab  120  has a 48.5° angular range and the body-side third tab  130  has a 70.5° angular range. Likewise, the sizes of the individual body-side gaps  151  to  153  can each be indicated by the corresponding angular range along the circumferential edge of the circular opening at the camera body  1 . Namely, the body-side first gap  151  has a 61° angular range, the body-side second gap  152  has a 47.5° angular range and the body-side third gap  153  has a 78.5° angular range. 
     Reference numeral  7  in  FIG.  5    indicates an index mark at the camera body  1 . The index mark  7  is used as a reference mark when mounting the photographic lens  2  at the camera body  1 . The index mark  7  assumes a 90° angular position counterclockwise relative to the reference three o&#39;clock direction taken at the body mount  100  viewed from the front side of the camera body  1 . 
     —Positions of Accessory-Side Tabs and Accessory-Side Gaps— 
       FIG.  6    indicates the positions of the accessory-side tabs and the accessory-side gaps at the accessory mount  200  viewed from the rear side of the photographic lens  2 . A reference angular position is taken along the nine o&#39;clock direction at the accessory mount  200  viewed from the rear side of the photographic lens  2  fully mounted at the camera body. Various angles representing the positions at which the accessory-side first tabs are disposed, are assumed along the clockwise direction relative to the nine o&#39;clock direction. Namely, the accessory-side first tab  210  extends over an angular range of 56.5° to 115°, the accessory-side second tab  220  extends over an angular range of 172.5° to 214.5°, and the accessory-side third tab  230  extends over an angular range of 267.5° to 343.5°. 
     The ranges over which the accessory-side tabs  210  to  230  extend along the circumference of the accessory mount  200  can each be indicated by the corresponding angular range along the circumference of the accessory mount  200 . Namely, the accessory-side first tab  210  has a 58.5° angular range, the accessory-side second tab  220  has a 42° angular range and the accessory-side third tab  230  has a 76° angular range. Likewise, the sizes of the individual accessory-side gaps  251  to  253  can each be indicated by the corresponding angular range along the circumference of the accessory mount  200 . Namely, the accessory-side first gap  251  has a 57.5° angular range, the accessory-side second gap  252  has a 53° angular range and the accessory-side third gap  253  has a 73° angular range. 
     Reference numeral  8  in  FIG.  6    indicates an index mark at the photographic lens  2 . The index mark  8  is used as a reference mark when mounting the photographic lens  2  at the camera body  1 . The index mark  8  assumes a 130.5° angular position clockwise relative to the nine o&#39;clock direction taken at the accessory mount  200  viewed from the rear side of the photographic lens  2  in the mounting complete state. This angle is equal to a value representing the sum of 90° and a mounting angle of 40.5° to be described in detail later. 
     As explained earlier, the restricting member  262  is disposed frontward relative to the accessory-side second tab  220  near its second side end  222 . This means that the restricting member  262  is disposed near the accessory-side second tab  220  extending over the shortest range along the circumference of the accessory mount  200  (with the smallest angular range along the circumference of the accessory mount  200 ). 
     —Mounting the Photographic Lens  2  at the Camera Body  1 — 
     The photographic lens  2  structured as described above is mounted at the camera body  1  through the following procedure. It is to be noted that the rotational position assumed by the photographic lens  2  relative to the camera body  1  when the axis extending along the front-rear direction through the center of the photographic optical path  3  of the camera body  1  and the optical axis of the photographic lens  2  are aligned with each other is to be hereafter referred to simply as an interlock phase. While sustaining the alignment of the axis extending along the front-rear direction through the center of the photographic optical path  3  at the camera body  1  and the optical axis of the photographic lens  2 , the photographic lens  2  is mounted at the camera body  1  by aligning the rotational position of the index mark at the photographic lens  2  with the rotational position of the index mark at the camera body  1 . When the two index marks are aligned, the photographic lens  2  and the camera body  1  achieve a positional relation relative to each other in which the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  do not interfere with each other, as shown in  FIG.  7   , and the accessory mount  200  can be thus inserted through the body mount  100  until the accessory mount reference surface  201  contacts the body mount reference surface  101 . The interlock phase assumed in this state is to be referred to as a correct interlock phase. It is to be noted that in  FIG.  7   , showing a specific state of interference (state of overlap) between the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230 , the accessory-side tabs are shown as fan-shaped members in a schematic representation. 
     As the accessory mount  200  is inserted at the body mount  100  at the correct interlock phase, the accessory-side first tab  210  passes from front to back through the body-side first gap  151 , the accessory-side second tab  220  passes from front to back through the body-side second gap  152  and the accessory-side third tab  230  passes from front to back through the body-side third gap  153 . Likewise, the body-side first tab  110  passes from back to front through the accessory-side first gap  251  (no reference numeral appended in  FIG.  7   ), the body-side second tab  120  passes from back to front through the accessory-side second gap  252  (no reference numeral appended in  FIG.  7   ) and the body-side third tab  130  passes from back to front through the accessory-side third gap  253  (no reference numeral appended in  FIG.  7   ), as the accessory mount  200  is inserted through the body mount  100  at the correct interlock phase. 
     As the photographic lens  2  is turned along the counterclockwise direction viewed from the front side of the camera body  1  after the accessory mount  200  is inserted through the body mount  100  at the correct interlock phase, the accessory-side first tab  210  takes up a position to the rear of the body-side first tab  110 , the accessory-side second tab  220  takes up a position to the rear of the body-side second tab  120  and the accessory-side third tab  230  takes up a position to the rear of the body-side third tab  130 . When the photographic lens  2  is turned by 40.5° relative to the correct interlock phase along the counterclockwise direction viewed from the front side of the camera body  1  as shown in  FIG.  8   , the restricting member  262  comes in contact with the second side end  122  of the body-side second tab  120 , thereby preventing further rotation along the counterclockwise direction. In other words, once the photographic lens  2  is turned by 40.5° along the counterclockwise direction from the correct interlock phase, the accessory mount  200  becomes engaged with the body mount  100  (engagement of the accessory-side tabs and the body-side tabs is completed) and thus, the photographic lens  2  becomes fully mounted. The rotational angle of 40.5°, by which the photographic lens  2  is turned from the correct interlock phase to the phase at which mounting of the photographic lens  2  is completed, is to be referred to as a mounting angle. In addition, the counterclockwise turning direction mentioned above may be otherwise referred to as a mounting direction. 
     When the photographic lens  2  is fully mounted, the position of the pin hole  261  at the accessory mount  200  is aligned with the position of the pin hole  161  at the body mount  100 . The pin (not shown) projecting out through the pin hole  161  at the body mount  100  is thus inserted through the pin hole  261 . 
     —Dismounting the Photographic Lens  2  from the Camera Body  1 — 
     The photographic lens  2  is dismounted from the camera body  1  through the following procedure. A button (not shown) disposed at the camera body  1  is depressed so as to retract the pin mentioned above from the pin hole  261 . As the photographic lens  2  is turned back to the correct interlock phase along the clockwise direction viewed from the front side of the camera body  1  in this state, the accessory-side tabs and the body-side tabs become dismounted. When the photographic lens  2  is turned to the correct interlock phase, the restricting member  262  comes in contact with the first side end  111  of the body-side first tab  110 , thereby preventing further rotation along the clockwise direction. 
     In other words, the state shown in  FIG.  7    is resumed. Since the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  do not interfere with each other at the correct interlock position, as explained earlier, the photographic lens  2  can be pulled forward and separated from the camera body  1 . It is to be noted that the clockwise turning direction mentioned above may be otherwise referred to as a dismounting direction. 
     —State of Engagement between Accessory-Side Tabs and Body-Side Tabs— 
       FIG.  9    is a schematic view of the body-side first tab  110  expanded along the circumferential direction, taken from the outer circumferential side of the photographic lens  2 . The body-side first tab  110  includes a tapered surface  117  formed on the side where the rear surface  114  is located, near the second side end  112 . The tapered surface  117  is formed so as to allow the accessory-side first tab  210  to smoothly move into a position to the rear of the body-side first tab  110  while the photographic lens  2  is being mounted. A contact portion  115 , constituted with a surface ranging perpendicular to the direction extending from front to rear relative to the body mount  100 , is formed continuous to the tapered surface  117  at the body-side first tab  110  near the second side end  112 . At the rear surface  114 , a spring  116  is disposed at a position adjacent to the contact portion  115 . 
     The spring  116  is an elastic member that comes in contact with the front surface  214  (not shown in  FIG.  9   ) of the accessory-side first tab  210  having moved into the position to the rear of the body-side first tab  110  and presses the front surface  214  rearward. With the spring  116 , a state in which the accessory mount reference surface  201  and the body mount reference surface  101  remain in contact with each other is sustained. If an external force that would cause the photographic lens  2  to tilt relative to the camera body  1  against the force applied by the spring  116  is applied to the photographic lens  2  having become fully mounted, the contact portion  115  comes in contact with the front surface  214  of the accessory-side first tab  210  to restrict any forward displacement of the accessory-side first tab  210 . It is to be noted while the force applied by the spring  116  keeps the accessory mount reference surface  201  and the body mount reference surface  101  in contact (tight contact) with each other, the contact portion  115  is set slightly apart from the front surface  214  of the accessory-side first tab  210 . 
     The body-side second tab  120  and the body-side third tab  130  adopt structures similar to that described above. Namely, the body-side second tab  120  includes a tapered surface  127  formed on the side where the rear surface  124  is located, near the second side end  122 . A contact portion  125 , constituted with a surface ranging perpendicular to the direction extending from front to rear relative to the body mount  100 , is formed continuous to the tapered surface  127  at the body-side second tab  120  near the second side end  122 . At the rear surface  124 , a spring  126  is disposed at a position adjacent to the contact portion  125 . The body-side third tab  130  includes a tapered surface  137  formed on the side where the rear surface  134  is located, near the second side end  132 . A contact portion  135 , constituted with a surface ranging perpendicular to the direction extending from front to rear relative to the body mount  100 , is formed continuous to the tapered surface  137  at the body-side third tab  130  near the second side end  132 . At the rear surface  134 , a spring  136  is disposed at a position adjacent to the contact portion  135 . 
     A mark assigned with reference numeral  116   a  in  FIG.  8    indicates the position at which the spring  116  presses the front surface  214  of the accessory-side first tab  210 , i.e., the specific phase of the spring force pressing position assumed along the circumferential direction. Likewise, a mark assigned with reference numeral  126   a  indicates the specific phase of the position at which the spring  126  presses the front surface  224  of the accessory-side second tab  220  assumed along the circumferential direction, and a mark assigned with reference numeral  136   a  indicates the specific phase of the position at which the spring  136  presses the front surface  234  of the accessory-side third tab  230  assumed along the circumferential direction. 
     As indicated in  FIGS.  7  and  8   , the contact portion  115  is located substantially at the top center of the body mount  100 . The contact portion  125  is disposed at a position set apart from the second side end  122  of the body-side second tab  120  by a distance equal to the distance setting the contact portion  115  apart from the second side end  112  at the body-side first tab  110 . The contact portion  135  is disposed at a position set apart from the second side end  132  of the body-side third tab  130  by a distance equal to the distance setting the contact portion  115  apart from the second side end  112  at the body-side first tab  110 . 
       FIGS.  10 A through  10 F  show changes in the state of engagement between the accessory-side second tab  220  and the body-side second tab  120  occurring in time series as the photographic lens  2  is mounted at the camera body  1  and then is dismounted from the camera body  1  in schematic views of the body-side second tab  120  expanded along the circumferential direction, taken from the outer side along the radial direction.  FIG.  10 A  shows the accessory-side second tab  220  of the accessory mount  200  having been inserted at the correct interlock phase, passing through the body-side second gap  152 .  FIG.  10 B  shows the state of the accessory-side second tab  220  and the body-side second tab  120  assumed after the accessory-side second tab  220  passes through the body-side second gap  152  and the accessory mount reference surface  201  comes into contact with the body mount reference surface  101 . 
     Subsequently, the photographic lens  2  is turned along the mounting direction, causing the accessory-side second tab  220  to move into a position to the rear of the body-side second tab  120 , as shown in  FIG.  10 C . The accessory-side second tab  220  further moves to the rear of the spring  126 , resulting in an application of a rearward force by the spring  126  to the accessory-side second tab  220 , as illustrated in  FIG.  10 D . Once the photographic lens  2  is turned from the correct interlock phase by an extent matching the mounting angle along the mounting direction, the restricting member  262  comes into contact with the second side end  122  of the body-side second tab  120 , as shown in  FIG.  10 E , thereby preventing further rotation of the photographic lens  2  along the mounting direction. 
     In the state shown in  FIG.  10 E , the photographic lens  2  is turned along the dismounting direction in order to dismount the photographic lens  2  from the camera body  1 . Once the photographic lens  2  is turned along the dismounting direction by an extent matching the mounting angle, the restricting member  262  comes in contact with the first side end  111  of the body-side first tab  110 , as illustrated in  FIG.  10 F , thereby preventing further rotation of the photographic lens  2  along the dismounting direction. 
     —Insertion of the Photographic Lens  2  at a Phase other than the Correct Interlock phase— 
     If the user attempts to insert the photographic lens  2  at a phase other than the correct interlock phase, at least two tabs among the body-side tabs  110  to  130  and at least two tabs among the accessory-side tabs  210  to  230  come into contact with each other at, at least, two locations, and thus, insertion of the accessory mount  200  through the body mount  100  is disallowed (deterred) in the embodiment. 
     As can be surmised by viewing  FIG.  7   , if the user attempts to mount the photographic lens  2  slightly offset from the correct interlock phase along the counterclockwise direction in the figure, the rear surface  213  of the accessory-side first tab  210  will come into contact with the front surface  113  of the body-side first tab  110 , the rear surface  223  of the accessory-side second tab  220  will come into contact with the front surface  123  of the body-side second tab  120 , and the rear surface  233  of the accessory-side third tab  230  will come into contact with the front surface  133  of the body-side third tab  130 . As a result, the three body-side tabs  110  to  130  and the three accessory-side tabs  210  to  230  will be in contact with each other and insertion of the accessory mount  200  into the body mount  100  will be disallowed. This situation may arise when, for instance, the accessory-side tabs  210  to  230  in  FIG.  8    are positioned frontward relative to the body-side tabs  110  to  130 . 
     If the user attempts to mount the photographic lens  2  turned by a greater extent along the counterclockwise direction relative to the state described above, the accessory-side second tab  220  will move frontward relative to the body-side third gap  153 , as shown in  FIG.  11    and thus, the accessory-side second tab  220  will no longer be in contact with any of the body-side tabs  110  to  130 . However, the rear surface  213  of the accessory-side first tab  210  will be contact with the front surface  113  of the body-side first tab  110  and the rear surface  233  of the accessory-side third tab  230  will be contact with the front surface  133  of the body-side third tab  130 . In this situation, the contact achieved by the two body-side tabs among the body-side tabs  110  to  130  and the two accessory-side tabs among the accessory-side tabs  210  to  230  disallows insertion of the accessory mount  200  through the body mount  100 . The contact thus achieved by the body-side tabs  110  to  130  and the accessory-side tabs at a total of two contact locations, is indicated as the hatched areas in  FIG.  11   . 
     A slight counterclockwise rotation of the photographic lens  2  will alter the positional relationship of the two mounts relative to each other in  FIG.  11    to a positional relationship whereby the rear surface  213  of the accessory-side first tab  210  contacts the front surface  123  of the body-side second tab  120 . Namely, a slight counterclockwise turn of the photographic lens  2  will allow the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  to achieve contact at three contact locations. If, on the other hand, the photographic lens  2  in the state shown in  FIG.  11    is turned along the clockwise direction, the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  will contact each other over a larger area and the rear surface  233  of the accessory-side third tab  230  and the front surface  133  of the body-side third tab  130  will contact each other over a larger area. 
     In other words,  FIG.  11    shows a state in which the contact of the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  and the contact of the rear surface  233  of the accessory-side third tab  230  and the front surface  133  of the body-side third tab  130  are achieved at the two contact locations over the smallest contact area. The rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  contact each other at the interlock phase shown in  FIG.  11    over an angular range, assumed by viewing the body mount  100  from the front side of the camera body  1 , of 11°. 
     It is to be noted that there are other situations in which insertion of the accessory mount  200  through the body mount  100  is disallowed via a total of two contact locations where two tabs among the body-side tabs  110  to  130  and two tabs among the accessory-side tabs  210  to  230  contact each other, as described in detail later. Among the plurality of states in which insertion of the accessory mount  200  through the body mount  100  is disallowed via a total of two contact locations where two tabs among the body-side tabs  110  to  130  and two tabs among the accessory-side tabs  210  to  230  achieve contact, the smallest contact area is formed at either of the two contact locations is the in two specific conditions, i.e., the state shown in  FIG.  11    and the state (not shown) achieved by rotating the photographic lens  2  in the state shown in  FIG.  11    by 11° along the counterclockwise direction in the figure, as described below. 
     If the photographic lens  2  with the two mounts assuming the specific positional relationship relative to each other as shown in  FIG.  11    is turned by 11° along the counterclockwise direction in the figure, the contact of the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  will cease but the rear surface  213  of the accessory-side first tab  210  will come into contact with the front surface  123  of the body-side second tab  120 . It is to be noted that contact will also be achieved as the rear surface  233  of the accessory-side third tab  230  comes into contact with the front surface  133  of the body-side third tab  130 . The rear surface  213  of the accessory-side first tab  210  and the front surface  123  of the body-side second tab  120  will contact each other over an angular range, assumed by viewing the body mount  100  from the front side of the camera body  1 , of 11°, as in the state shown in  FIG.  11   . 
     Thus, in this case too, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations. Among various states in which contact is achieved by the rear surface  213  of the accessory-side first tab  210  and the front surface  123  of the body-side second tab  120  and contact is achieved by the rear surface  233  of the accessory-side third tab  230  and the front surface  133  of the body-side third tab  130  at a total of two contact locations, the accessory-side tabs and the body-side tabs contact each other over the smallest area in this state. 
     Namely, while there are various insertion disallowed states, in which insertion of the accessory mount  200  through the body mount  100  is disallowed via a total of two contact locations where two tabs among the body-side tabs  110  to  130  and two tabs among the accessory-side tabs  210  to  230  contact each other, the rear surface  213  the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  contact each other over the smallest contact area in the state shown in  FIG.  11   . Likewise, the smallest contact area is also formed by the rear surface  213  of the accessory-side first tab  210  and the front surface  123  of the body-side second tab  120  in the insertion disallowed state with the photographic lens  2  turned counterclockwise by 11° relative to the state shown in  FIG.  11   , among the insertion disallowed states in which insertion of the accessory mount  200  through the body mount  100  is disallowed via a total of two contact locations where two tabs among the body-side tabs  110  to  130  and 2 tabs among the accessory-side tabs  210  to  230  contact each other. The interlock phase shown in  FIG.  11    and the interlock phase assumed with the photographic lens  2  turned by 11° along the counterclockwise direction relative to the state shown in  FIG.  11    are each referred to as a specific interlock phase. 
     As described above, a counterclockwise rotation of the photographic lens  2  in the state shown in  FIG.  11    will cause the rear surface  213  of the accessory-side first tab  210  to come into contact with both the front surface  113  of the body-side first tab  110  and the front surface  123  of the body-side second tab  120  and cause the rear surface  233  of the accessory-side third tab  230  to come into contact with the front surface  133  of the body-side third tab  130 . In other words, the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  will achieve contact at a total of three contact locations. 
     As the photographic lens  2  is turned further counterclockwise in the figure until the photographic lens  2  is rotated by 11° along the counterclockwise direction relative to the state shown in  FIG.  11   , as described above, the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  will cease to be in contact with each other and thus, the number of contact locations where the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  contact each other will be reduced to two. 
     As the photographic lens  2  is turned further along the counterclockwise direction in the figure, the state shown in  FIG.  12    will be achieved. 
     As can be surmised by viewing  FIG.  12   , a slight counterclockwise turn of the photographic lens  2  will alter the positional relationship shown in  FIG.  12   , causing the rear surface  223  of the accessory-side second tab  220  to come into contact with the front surface  133  of the body-side third tab  130 . In other words, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of three contact locations. A further counterclockwise turn of the photographic lens  2  will cause the rear surface  233  of the accessory-side third tab  230  to come into contact with the front surface  113  of the body-side first tab  110  as well. In this situation, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  over a total of four contact locations. 
     As the photographic lens  2  is turned further counterclockwise, the rear surface  233  of the accessory-side third tab  230  and the front surface  133  of the body-side third tab  130  will cease contact with each other and then, the rear surface  213  of the accessory-side first tab  210  and the front surface  123  of the body-side second tab  120 , too, will cease contact with each other. In other words, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations under these circumstances, as shown in  FIG.  13   . 
     A further counterclockwise turn of the photographic lens  2  in the state shown in  FIG.  13    will cause a decrease in both the contact area over which the rear surface  223  of the accessory-side second tab  220  and the front surface  133  of the body-side third tab  130  are in contact with each other and the contact area over which the rear surface  233  of the accessory-side third tab  230  and the front surface  113  of the body-side first tab  110  are contact with each other. In other words,  FIG.  13    shows a state in which the contact of the rear surface  223  of the accessory-side second tab  220  and the front surface  133  of the body-side third tab  130  and the contact of the rear surface  233  of the accessory-side third tab  230  and the front surface  113  of the body-side first tab  110  are achieved at the two contact locations over the largest contact area. 
       FIG.  14    shows a state achieved by further turning the photographic lens  2  along the counterclockwise direction relative to the state shown in  FIG.  13   . Among various states in which the rear surface  223  of the accessory-side second tab  220  and the front surface  133  of the body-side third tab  130  contact each other and the rear surface  233  of the accessory-side third tab  230  and the front surface  113  of the body-side first tab  110  contact each other at a total of two contact locations, the accessory-side tabs and the body-side tabs contact each other over the smallest area in this state. 
     A slight counterclockwise turn of the photographic lens  2  relative to the state shown in  FIG.  14    will allow the rear surface  213  of the accessory-side first tab  210  and the front surface  133  of the body-side third tab  130  to come into contact with each other. At this time, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of three contact locations. As the photographic lens  2  in this state is turned further along the counterclockwise direction in the figure, the rear surface  233  of the accessory-side third tab  230  and the front surface  123  of the body-side second tab  120  will come into contact with each other. In this state, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of four contact locations. As the photographic lens  2  in this state is turned further along the counterclockwise direction in the figure, the rear surface  223  of the accessory-side second tab  220  and the front surface  133  of the body-side third tab  130  will cease contact with each other, as shown in  FIG.  15   . At this time, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of three contact locations. 
     As the photographic lens  2  is slightly turned along the counterclockwise direction relative to the state shown in  FIG.  15   , the rear surface  223  of the accessory-side second tab  220  and the front surface  113  of the body-side first tab  110  will come into contact with each other but substantially simultaneously, the contact of the rear surface  233  of the accessory-side third tab  230  and the front surface  113  of the body-side first tab  110  will cease. As the photographic lens  2  is turned further along the counterclockwise direction in the figure, the correct interlock phase shown in  FIG.  7    will be reestablished and any contact (interference) between the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  will cease. 
     —Accessory-Side Third Tab  230 — 
     When contact is achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations with the accessory-side third tab  230  in contact with a body-side tab at one of the contact locations, the accessory-side third tab  230  invariably contacts the body-side tab  110 ,  120  or  130  on the side where its second side end  232  is located (i.e., at the rear surface  233  near the second side end  232 ), rather than on the side where its first side end  231  is located (i.e., at the rear surface  233  near the first side end  231 ), as shown in  FIGS.  11  through  14   . In other words, when contact is achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations and the accessory-side third tab  230  achieves contact at one of the contact locations, the function of preventing erroneous insertion is achieved in the area near the second side end  232  of the accessory-side third tab  230 . The accessory-side third tab  230  is formed so that the area near its second side end  232  is positioned in the body-side third gap  153  without overlapping the body-side third tab  130  when the photographic lens  2  is mounted at the camera body  1  as shown in  FIG.  8   . Namely, when 0 the photographic lens  2  is mounted at the camera body  1 , the area near the second side end  232  of the accessory-side third tab  230  does not directly contribute to locking of the photographic lens  2  to the camera body  1 . 
     In addition, near the first side end  231  of the accessory-side third tab  230 , the front surface  234  is pressed by the spring  136  at the body-side third tab  130 , and whenever 5 an excessive external force is applied, the front surface  234  will come into contact with the contact portion  135 , as explained earlier. This means that when the photographic lens  2  is mounted at the camera body  1 , the force applied to lock the photographic lens  2  to the camera body  1  will be imparted primarily to the area near the first side end  231  at the accessory-side third tab  230 . 
     In summary, the accessory-side third tab  230  is formed so that different functions are achieved in the area near the first side end  231  and the area near the second side end  232  along the direction in which the accessory-side third tab  230  extends. 
     —Body-Side First Tab  110 — 
     When contact is achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations with the body-side first tab  110  in contact with an accessory-side tab at one of the contact locations, the body-side first tab  110  invariably contacts the accessory-side tab  210 ,  220  or  230  on the side where its first side end  111  is located (i.e., at the front surface  113  near the first side end  111 ), rather than on the side where its second side end  112  is located (i.e., at the front surface  113  near the second side end  112 ), as shown in  FIGS.  11 ,  13  and  14   . In other words, when contact is achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations and the body-side first tab  110  achieves contact at one of the contact locations, the function of preventing erroneous insertion is achieved in the area near the first side end  111  of the body-side first tab  110 . 
     The spring  116  is disposed adjacent to the contact portion  115  at the body-side first tab  110  over the area near its second side end  112 . Namely, the function of locking the photographic lens  2  having been mounted at the camera body  1  is achieved over the area near the second side end  112  of the body-side first tab  110 . 
     In summary, the body-side first tab  110  is formed so that different functions are achieved in the area near the first side end  111  and the area near the second side end  112  along the direction in which the body-side first tab  110  extends. 
     The camera body  1  and the photographic lens  2  structured as described above achieve the following advantages. 
     (1) Three accessory-side tabs  210  to  230  assuming a uniform size and disposed over equal intervals and three body-side tabs  110  to  130  assuming a uniform size and disposed over equal intervals will allow the photographic lens  2  to be mounted at the camera body  1  at a phase other than the correct interlock phase. Accordingly, the three accessory-side tabs  210  to  230  are formed in varying sizes and are disposed over varying intervals and the three body-side tabs  110  to  130  are also formed in varying sizes and disposed over varying intervals. In addition, even if the user attempts to insert the photographic lens  2  at a phase other than the correct interlock phase, at least two tabs among the body-side tabs  110  to  130  and at least two tabs among the accessory-side tabs  210  to  230  come into contact with each other at a total of at least two contact locations, so as to disallow insertion of the accessory mount  200  through the body mount  100 . If contact was achieved by a camera accessory mount tab and a camera body mount tab at a single contact location, the other two contact-free tabs on the accessory-side could assume positions between camera body-side tabs. In contrast, contact is achieved in the embodiment by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at, at least, two contact locations at a phase other than the correct interlock phase as described above, and consequently, erroneous mounting of the camera accessory can be reliably prevented. 
     (2) The restricting member  262 , which comes in contact with the second side end  122  of the body-side second tab  120  and the first side end  111  of the body-side first tab  110 , restricts the range over which the photographic lens  2  can be turned along the mounting direction and the dismounting direction. Since the rotational range of the photographic lens  2  along the mounting direction and the dismounting direction can be restricted via a single member assuming a simple shape, the manufacturing costs of the body mount  100  and the accessory mount  200  can be minimized. 
     (3) The restricting member  262  is disposed near a side end of the accessory-side second tab  220  assuming a smallest length along the circumference of the accessory mount  200 . At the correct interlock phase, the accessory-side second tab  220  passes through the body-side second gap  152 , which extends over an angular range of 47.5° along the circumferential edge of the circular opening of the camera body  1 . In addition, the mounting angle assumed in the embodiment is 40.5°. This means that a range, over which the restricting member  262  is allowed to move between the second side end  122  of the body-side second tab  120  and the first side end  111  of the body-side first tab  110  within the angular range (47.5° of the body-side second gap  152 , is equivalent to the mounting angle. Accordingly, by disposing the restricting member  262  near a side end of the accessory-side second tab  220  assuming the smallest length along the circumference of the accessory mount  200 , the mounting angle for the mount system constituted with the body mount  100  and the accessory mount  200  is defined. From a different perspective, the mounting angle setting can be rationalized as described below and the sizes of the body-side second gap  152  and the accessory-side second tab  220  may be regarded to have been set in correspondence to the mounting angle rationalized as described below. In any case, these structural features achieve a rational overall structure with no superfluous structural elements, to contribute toward minimization of the overall weight. 
     It is to be noted that at a smaller mounting angle, the accessory can be engaged with greater ease, the extent to which electrical contact points (not shown) at the camera body  1  and at the photographic lens  2  slide against each other can be reduced and the extent of wear of the electrical contact points occurring whenever the photographic lens  2  is exchanged can be reduced. However, if the mounting angle is very small, tabs engaged with each other may become inadvertently dismounted from each other, and in such a case, the photographic lens  2  may fall from the camera body. At the same time, the body mount  100  must assume a certain circumferential dimension in order to accommodate the springs  116 ,  126  and  136 , the contact portions  115 ,  125  and  135  and the tapered surfaces  117 ,  127  and  137 . Moreover, a sufficient mounting angle is required to ensure that the accessory-side tabs  210  to  230  are allowed to reliably mount the springs  116 ,  126  and  136  and the contact portions  115 ,  125  and  135  over the tapered surfaces  117 ,  127  and  137 . The mounting angle of 40.5° adopted in the embodiment has been determined by taking into consideration all these factors. 
     (4) While the weight distribution of the photographic lens  2  is such that the photographic lens  2  tends to pull down on its front side (subject side), the forces applied from the springs  116 ,  126  and  136  sustain the accessory mount reference surface  201  in contact with the body mount reference surface  101  under typical operating conditions. However, if an external force exceeding the force of the springs, working along a direction matching that of gravity is applied to the photographic lens  2  in a photographing state with the camera body  1  in the upright attitude fixed to, for instance, a tripod, the spring  116  of the uppermost body-side first tab  110  will flex further to allow the front surface  214  of the accessory-side first tab  210  to come in contact with the contact portion  115  of the body-side first tab  110 . 
       FIG.  16    illustrates this condition. It is to be noted that the intervals and the like formed between the individual parts are exaggerated in  FIG.  16    so as to provide a clear illustration. In the state shown in  FIG.  16   , the accessory mount reference surface  201  and the body mount reference surface  101  are in contact with each other at a point PD on the lower side, the front surface  214  of the accessory-side first tab  210  and the contact portion  115  of the body-side first tab  110  are in contact with each other at a point PU on the upper side as described above and the weight of the photographic lens  2  and the external force applied to the photographic lens  2  are supported at these two points. Accordingly, the force applied to the two points PD and PU can be reduced by maximizing the distance LS extending along the vertical direction (along the direction of gravitational force) between the two points PD and PU. 
     This situation will not arise in a normal photographing condition unless an unintended external force is applied. However, the photographic lens  2 , which normally assumes a cylindrical shape, tends to come into contact with various objects in real-world usage and may be employed in a range of circumstances. For this reason, the condition shown in  FIG.  16    may be expected to arise fairly often and, therefore it is important to devise measures to effectively address the issues arising from such circumstances. 
       FIGS.  17  through  19    each indicate the contact points PD and PU and the distance LS corresponding to a specific attitude assumed by the camera body  1 . It is to be noted that the camera body  1  assumes the upright attitude (lateral orientation) in  FIG.  17   , with the distance LS and the contact points PD and PU corresponding to this attitude respectively notated as LS 1 , PD 1  and PUL The camera body  1  assumes a longitudinal orientation in  FIG.  18    achieved by turning the left side of the camera body  1  upward, with the distance LS and the contact points PD and PU corresponding to this attitude respectively notated as LS 2 , PD 2  and PU 2 . The camera body  1  assumes a longitudinal orientation in  FIG.  19    achieved by turning the right side of the camera body  1  upward, with the distance LS and the contact points PD and PU corresponding to the particular attitude respectively notated as LS 3 , PD 3  and PU 3 . The distance LS 1  assumed in the lateral orientation in  FIG.  17    and the distance LS 2  assumed in the longitudinal orientation in  FIG.  18    are both substantially equal to the diameter of the body mount  100 . However, the distance LS 3  in the longitudinal orientation shown in  FIG.  19    is markedly smaller than the distances LS 1  and LS 2 . 
     For this reason, at a given level of external force applied to the photographic lens  2 , the level of force working at PU 3  will be greater than the levels of force working at PU 1  and PU 2 . 
     Accordingly, the body-side third tab  130  in the embodiment is formed so as to assume a greater length along the circumferential direction compared to the body-side first tab  110  and the body-side second tab  120 . Greater strength is thus assured for the body-side third tab  130  over the body-side first tab  110  and the body-side second tab  120 . In addition, the accessory-side third tab  230  at the accessory mount  200 , which is to come into contact with the contact portion  135  (i.e., PU 3 ) when the photographic lens  2  is fully mounted, is formed to achieve a greater length along the circumferential direction than the accessory-side first tab  210  and the accessory-side second tab  220 . Consequently, since ample strength is assured at the body-side third tab  130  and the accessory-side third tab  230 , the body-side third tab  130  and the accessory-side third tab  230 , which are to be subjected to the highest levels of force, will remain intact even when the camera body  1  assumes the longitudinal orientation shown in  FIG.  19   . 
     As described above, the advantage of reliably disallowing erroneous insertion at a phase other than the correct interlock phase and the advantage of assuring sufficient strength for the accessory mount  200  and the body mount  100  when the photographic lens  2  having been inserted at the correct interlock phase is fully mounted can both be achieved through the embodiment described above. 
     (5) As explained earlier, different functions are achieved in the area near the first side end  231  and in the area near the second side end  232  along the direction in which the accessory-side third tab  230  extends. As a result, the durability of the accessory-side third tab  230  is improved. The accessory-side third tab  230 , where the application of force at PU 3  occurs when the camera body  1  assumes the longitudinal orientation with the right side thereof turned upward, is subjected to the highest level of force. With the durability of this accessory-side third tab  230  coming under the heaviest onus improved as described above, the durability and the reliability of the accessory mount  200  itself are improved. 
     (6) As explained earlier, different functions are achieved in the area near the first side end  111  and in the area near the second side end  112  along the direction in which the body-side first tab  110  extends. The durability of the body-side first tab  110  is thus improved. The body-side first tab  110 , where the spring  116 , which, under the weight of the photographic lens  2 , is subjected to a more significant and continuous onus compared to the other springs  126  and  136 , and the application of force occurs at Pul as described earlier when the camera body  1  assumes the typical lateral orientation, is subjected to an onus most frequently. Since the durability of the body-side first tab  110  coming under the most frequent onus is improved, the durability and the reliability of the body mount  100  itself are improved. 
     (7) The contact portion  115  is set substantially at the top center of the body mount  100  in the structure achieved in the embodiment and thus, a large value can be set for LS 1  mentioned earlier. As a result, the level of force applied at the contact point PU 1  can be reduced, which improves the durability of the body-side first tab  110  and effectively prevents the photographic lens  2  from drooping down at its front side. At the same time, the body-side first tab  110  is set at the top center of the body mount  100  and, as a result, sufficient strength is assured for the contact portion  115 . As a result, the durability of the body-side first tab  110  is improved. This ultimately leads to improvements both in the durability and in the reliability of the body mount  100 . 
     (8) As indicated in  FIG.  16   , the lower areas of the rear surfaces of the body-side tabs  110  to  130  and the lower areas of the front surfaces of the accessory-side tabs  210  to  230  come into contact with each other least readily. This means that there is not a significant need for having the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  come into contact with each other over these areas. Accordingly, the largest body-side third gap  153  in the embodiment is positioned on the lower side of the body mount  100 , as illustrated in  FIG.  5   . This, in turn, ensures that the body-side tabs  110  to  130  can be positioned in a highly rational configuration. 
     (9) The following is the rationale for disposing the accessory-side tabs  210  to  230  at the specific positions indicated in  FIG.  6   . Namely, it is desirable to assure a high degree of strength with a greater length assumed along the circumferential direction at the accessory-side first tab  210 , which is positioned at the top center when the photographic lens  2  is mounted at the camera body  1  assuming the lateral orientation. However, since the largest body-side third gap  153  is set on the lower side of the body mount  100 , as described earlier, the accessory-side third tab  230 , which passes through the body-side third gap  153  when the accessory mount is inserted at the correct interlock phase, is formed to range over the greatest length along the circumferential direction. Accordingly, the accessory-side first tab  210  is formed to range over a second largest length along the circumferential direction. The accessory-side second tab  220 , which is not subjected to as many requirements, is formed so as to extend over a smallest range along the circumferential direction. The accessory-side tabs  210  to  230  formed as described above can be disposed with a high level of rationality. 
     Second Embodiment 
     In reference to  FIGS.  23  through  36   , the camera accessory, the accessory mount, the camera body and the camera body mount achieved in a second embodiment of the present invention are described.  FIG.  23    is a schematic perspective of a camera body  1  with the exchangeable lenses and an exchangeable photographic lens  2  that may be detachably mounted at the camera body  1 , achieved in the second embodiment. A shutter release button  4 , an image sensor  5  and a control circuit  6  that controls various units of the camera body  1  are disposed at the camera body  1 . Reference numeral  3  indicates a photographic optical path through which a subject image departing the photographic lens  2  is guided to the image sensor  5 . Reference numeral  9  indicates a lens retainer lock pin used to lock the photographic lens  2  having been mounted fully at the camera body  1 , so as to disallow rotation thereof. 
     —Body-Mount  100 — 
       FIG.  24 A  is a schematic perspective illustrating the structure of the body mount  100 , viewed along a diagonal direction from a front right viewpoint relative to the camera body  1  and  FIG.  24 B  is a sectional view of the body mount  100  over an area around the lens retainer lock pin  9 . The body mount  100  adopts a bayonet structure that includes three tabs (body-side tabs) set apart from one another along the circumferential edge of a circular opening (photographic optical path) at the camera body  1 , each projecting inward from the outer side of the opening circumference. Among the three body-side tabs, the body-side tab located at the uppermost position is referred to as a body-side first tab  110 , the body-side tab located next to the body-side first tab  110  along the counterclockwise direction in the figure is referred to as a body-side second tab  120  and the body-side tab located next to the body-side second tab  120  along the counterclockwise direction is referred to as a body-side third tab  130 . 
     Reference numeral  101  indicates a body mount reference surface. The body mount reference surface  101  is a ring-shaped flat surface formed so as to face toward the front side. As the photographic lens  2  is mounted at the camera body  1 , the body mount reference surface  101  comes into contact with an accessory mount reference surface  201  of the accessory mount  200 , which is to be described in detail later, thereby regulating the position assumed by the photographic lens  2  along the front-rear direction. Reference numeral  102  indicates an inner circumferential surface of the cylindrical body mount  100 . The inner circumferential surface  102 , which interlocks with a fitting portion  202  of the accessory mount  200  to be detailed below, is used as a reference surface, in reference to which the optical axis of the photographic lens  2  is aligned with the optical axis of the camera body  1  (the central axis of the photographic optical path  3 ). Reference numeral  161  indicates a pin hole through which the lens retainer lock pin  9  projects out or retracts in. It is to be noted that a force applied by a spring (not shown) pushes the lens retainer lock pin  9  to project out at the body mount reference surface  101 , as shown in  FIG.  24    B. As the user presses a lens release button (not shown), the lens retainer lock pin  9  retracts backward relative to the body mount reference surface  101  against the force imparted by the spring (not shown). 
     —Accessory Mount  200 — 
       FIG.  25    is a perspective schematically illustrating the structure of the accessory mount  200 , viewed along a diagonal direction from a rear left viewpoint relative to the photographic lens  2 . It is to be noted that  FIG.  25    shows the accessory mount  200  with the photographic lens  2  fully mounted at the camera body  1  (mounting complete state), i.e., in a photographing-enabled state. This means that the top-bottom/left-right orientation of the accessory mount  200  shown in  FIG.  25    matches the top-bottom/left-right orientation of the camera body  1 . Unless specifically noted, the following description is given by assuming that the top-bottom/left-right orientation of the accessory mount  200  is the orientation in the mounting complete state. 
     The accessory mount  200  adopts a bayonet structure that includes three tabs (accessory-side tabs) set apart from one another along the circumferential direction, each projecting from the inner side of the circumference toward the outer side of the circumference. Among the three accessory-side tabs, the accessory-side tab located at the uppermost position is referred to as an accessory-side first tab  210 , the accessory-side tab located next to the accessory-side first tab  210  along the clockwise direction in the figure is referred to as an accessory-side second tab  220  and the accessory-side tab located next to the accessory-side second tab  220  along the clockwise direction is referred to as an accessory-side third tab  230 . 
     The space created between two consecutive accessory-side tabs through which a body-side tabs passes when the photographic lens  2  is being mounted or dismounted as explained later, is referred to as an accessory-side gap. The accessory-side gap present between the accessory-side first tab  210  and the accessory-side second tab  220  is referred to as an accessory-side first gap  251 , the accessory-side gap located next to the accessory-side first gap  251  along the clockwise direction in the figure is referred to as an accessory-side second gap  252  and the accessory-side gap located next to the accessory-side second gap along the clockwise direction is referred to as an accessory-side third gap  253 . 
     The surface of the accessory-side first tab  210  facing rearward is referred to as a rear surface  213 , the side end of the accessory-side first tab  210  facing the accessory-side first gap  251  is referred to as a first side end  211  and the side end of the accessory-side first tab  210  facing the accessory-side third gap  253  is referred to as a second side end  212 . 
     Likewise, the surface of the accessory-side second tab  220  facing rearward is referred to as a rear surface  223 , the side end of the accessory-side second tab  220  facing the accessory-side second gap  252  is referred to as a first side end  221 , and the side end of the accessory-side second tab  220  facing the accessory-side first gap  251  is referred to as a second side end  222 . The surface of the accessory-side third tab  230  facing rearward is referred to as a rear surface  233 , the side end of the accessory-side third tab  230  facing the accessory-side third gap  253  is referred to as a first side end  231  and the side end of the accessory-side third tab  230  facing the accessory-side second gap  252  is referred to as a second side end  232 . The first side ends  211 ,  221  and  231  are tab ends facing toward the mounting direction, to be described in detail later, whereas the second side ends  212 ,  222  and  232  are tab ends facing toward a direction (dismounting direction) opposite the mounting direction. 
     It is to be noted that, although not shown in  FIG.  25   , the surface of the accessory-side first tab  210  facing forward is referred to as a front surface  214 . Likewise, the surface of the body-side second tab  220  facing forward and the surface of the accessory-side third tab  230  facing forward are respectively referred to a front surface  224  and a front surface  234 . 
     Reference numeral  201  indicates an accessory mount reference surface. The accessory mount reference surface  201  is a ring-shaped flat surface formed so as to face rearward, and is in contact with the body mount reference surface  101  when the photographic lens  2  is fully mounted at the camera body  1 . Reference numeral  202  indicates a fitting portion. The fitting portion  202  is a cylindrical portion that fits at the inner circumferential surface  102  of the body mount  100  and is used for reference when aligning the optical axis of the photographic lens  2  with the optical axis of the camera body  1  (the central axis of the photographic optical path  3 ). Reference numeral  261  indicates a pin hole. As the lens retainer lock pin  9 , which projects out or retracts into the pin hole  161  at the body mount  100 , is inserted through the pin hole  261 , rotation of the fully mounted photographic lens  2  relative to the camera body  1  (body mount  100 ) becomes disallowed and the photographic lens  2  thus becomes locked. 
     A restricting member  262 , which limits the rotation range of the photographic lens  2  (accessory mount  200 ) relative to the camera body  1  (body mount  100 ) while the photographic lens  2  is mounted or dismounted, is disposed frontward relative to the accessory-side second tab  220  near its second side end  222 , as shown in  FIG.  25    and in  FIGS.  26 A and  26 B . The restriction of the rotation range achieved via the restricting member  262  will be described in detail later. The restricting member  262  may be, for instance, a headed pin screwed in from the outer side of the accessory mount  200  along the radial direction. It is to be noted that  FIG.  26 A  shows the accessory-side second tab  220  near the second side end  222 , viewed from the rear-side, whereas  FIG.  26 B  shows the accessory-side second tab  220  near the second side and  222 , viewed from the left side. 
     —Positions of Body-Side Tabs and Body-Side Gaps— 
       FIG.  27    indicates the positions of the body-side tabs and body-side gaps at the body mount  100  viewed from the front side of the camera body  1 . A reference angular position is taken along the three o&#39;clock direction at the body mount  100  viewed from the front side of the camera body  1  and various angles representing the positions at which the body-side tabs are disposed, are assumed along the counterclockwise direction relative to the three o&#39;clock direction. Namely, the body-side first tab  110  extends over an angular range of 76° to 130°, the body-side second tab  120  extends over an angular range of 177.5° to 226° and the body-side third tab  130  extends over an angular range of 304.5° to) 15° (375°). 
     The sizes of the individual body-side tabs  110  to  130  can each be indicated by the corresponding angular range along the circumferential edge of the circular opening at the body mount  100 . Namely, the body-side first tab  110  has a 54° angular range, the body-side second tab  120  has a 48.5° angular range and the body-side third tab  130  has a 70.5° angular range. Likewise, the sizes of the individual body-side gaps  151  to  153  can each be indicated by the corresponding angular range along the circumferential edge of the circular opening at the camera body  1 . Namely, the body-side first gap  151  has a 61° angular range, the body-side second gap  152  has a 47.5° angular range and the body-side third gap  153  has a 78.5° angular range. The body-side second gap  152  (47.5° is thus the smallest gap at the camera body mount  100 . 
     Reference numeral  7  in  FIG.  27    indicates an index mark at the camera body  1 . The index mark  7  is used as a reference mark when mounting the photographic lens  2  at the camera body  1 . The index mark  7  assumes a 90° angular position counterclockwise relative to the reference three o&#39;clock direction taken at the body mount  100  viewed from the front side of the camera body. 
     —Positions of Accessory-Side Tabs and Accessory-Side Gaps— 
       FIG.  28    indicates the positions of the accessory-side tabs and the accessory-side gaps at the accessory mount  200  viewed from the rear side of the photographic lens  2 . A reference angular position is taken along the nine o&#39;clock direction at the accessory mount  200  viewed from the rear side of the photographic lens  2  fully mounted at the camera body. Various angles, representing the positions at which the accessory-side tabs are disposed, are assumed along the clockwise direction relative to the nine o&#39;clock direction. Namely, the accessory-side first tab  210  extends over an angular range of 56.5° to 115°, the accessory-side second tab  220  extends over an angular range of 172.5° to 214.5° and the accessory-side third tab  230  extends over an angular range of 272° to 343.5°. 
     The ranges over which the accessory-side tabs  210  to  230  extend along the circumference of the accessory mount  200  can each be indicated by the corresponding angular range along the circumference of the accessory mount  200 . Namely, the accessory-side first tab  210  has a 58.5° angular range, the accessory-side second tab  220  has a 42° angular range and the accessory-side third tab  230  has a 71.5° angular range. In other words, the angles indicating the ranges of the first through third accessory-side tabs, each representing the range of a circular arc extending along the circumferential direction, are; 58.7° for the accessory-side first tab  210 , 42° for the accessory-side second tab  220  and 71.5° for the accessory-side third tab  230 . This means that the smallest tab among the accessory-side tabs is the accessory-side second tab  220  (42°). 
     Likewise, the sizes of the individual accessory-side gaps  251  to  253  can each be indicated by the corresponding angular range along the circumference of the accessory mount  200 . Namely, the accessory-side first gap  251  has a 57.5° angular range, the accessory-side second gap  252  has a 57.5° angular range and the accessory-side third gap  253  has a 73° angular range. 
     Reference numeral  8  in  FIG.  28    indicates an index mark  8  at the photographic lens  2 . The index mark  8  is used as a reference mark when mounting the photographic lens  2  at the camera body  1 . The index mark  8  assumes a 130.5° angular position clockwise relative to the reference nine o&#39;clock direction taken at the accessory mount  200  viewed from the rear side of the photographic lens  2  in the mounting complete state. This angle is equal to a value representing the sum of 90° and a mounting angle of 40.5° to be described in detail later. 
     As explained earlier, the restricting member  262  is disposed frontward relative to the accessory-side second tab  220  near its second side end  222 . This means that the restricting member  262  is disposed near the accessory-side second tab  220  extending over the shortest range along the circumference of the accessory mount  200  (with the smallest angular range along the circumference of the accessory mount  200 ). 
     Mounting the Photographic Lens  2  at the Camera Body  1 — 
     The photographic lens  2  structured as described above is mounted at the camera body  1  through the following procedure. It is to be noted that the rotational position assumed by the photographic lens  2  relative to the camera body  1  when the axis extending along the front-rear direction through the center of the photographic optical path  3  of the camera body  1  and the optical axis of the photographic lens  2  are aligned with each other is to be hereafter referred to simply as an interlock phase. While sustaining the alignment of the axis extending along the front-rear direction through the center of the photographic optical path  3  at the camera body  1  and the optical axis of the photographic lens  2 , the photographic lens  2  is mounted at the camera body  1  by aligning the rotational position of the index mark at the photographic lens  2  with the rotational position of the index mark at the camera body  1 . When the two index marks are aligned, the photographic lens  2  and the camera body  1  achieve a positional relation relative to each other in which the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  do not interfere with each other, as shown in  FIG.  29   , and the accessory mount  200  can be thus inserted through the body mount  100  until the accessory mount reference surface  201  contacts the body mount reference surface  101 . The interlock phase assumed in this state is to be referred to as a correct interlock phase. It is to be noted that in  FIG.  29   , showing a specific state of interference (state of overlap) between the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230 , the accessory-side tabs are shown as fan-shaped members in a schematic representation. 
     As the accessory mount  200  is inserted at the body mount  100  at the correct interlock phase, the accessory-side first tab  210  passes from front to back through the body-side first gap  151 , the accessory-side second tab  220  passes from front to back through the body-side second gap  152  and the accessory-side third tab  230  passes from front to back through the body-side third gap  153 . Likewise, the body-side first tab  110  passes from back to front through the accessory-side first gap  251  (no reference numeral appended in  FIG.  29   ), the body-side second tab  120  passes from back to front through the accessory-side second gap  252  (no reference numeral appended in  FIG.  29   ) and the  0  body-side third tab  130  passes from back to front through the accessory-side third gap  253  (no reference numeral appended in  FIG.  29   ) as the accessory mount  200  is inserted through the body mount  100  at the correct interlock phase. 
     As the photographic lens  2  is turned along the counterclockwise direction viewed from the front side of the camera body  1  after the accessory mount  200  is inserted through  5  the body mount  100  at the correct interlock phase, the accessory-side first tab  210  takes up a position to the rear of the body-side first tab  110 , the accessory-side second tab  220  takes up a position to the rear of the body-side second tab  120  and the accessory-side third tab  230  takes up a position to the rear of the body-side third tab  130 . When the photographic lens  2  is turned by 40.5° relative to the correct interlock phase along the counterclockwise direction viewed from the front side of the camera body  1 , as shown in  FIG.  30   , the lens retainer lock pin  9  projecting out through the pin hole  161  at the body mount  100  is inserted through the pin hole  261  at the accessory mount  200 , thereby restricting counterclockwise rotation. It is to be noted that when the lens release button (not shown) is depressed so as to cause the lens retainer lock pin  9  to retract backward relative to the body mount reference surface  101  and the photographic lens  2  is rotated counterclockwise, the restricting member  262  comes in contact with the second side end  122  of the body-side second tab  120 , thereby restricting rotation along the counterclockwise direction. 
     In other words, once the photographic lens  2  is turned by 40.5° along the counterclockwise direction from the correct interlock phase, the accessory mount  200  becomes engaged with the body mount  100  (engagement of the accessory-side tabs and the body-side tabs is completed) and thus, the photographic lens  2  becomes fully mounted. The rotational angle of 40.5°, by which the photographic lens  2  is turned from the correct interlock phase to the phase at which mounting of the photographic lens  2  is completed, is to be referred to as a mounting angle. In addition, the counterclockwise turning direction mentioned above may be otherwise referred to as a mounting direction. 
     When the photographic lens  2  is fully mounted, the position of the pin hole  261  at the accessory mount  200  is aligned with the position of the pin hole  161  at the body mount  100 . The lens retainer lock pin  9  projecting out through the pin hole  161  at the body mount  100  is thus inserted through the pin hole  261 . 
     —Dismounting the Photographic Lens  2  from the Camera Body  1 — 
     The photographic lens  2  is dismounted from the camera body  1  through the following procedure. The lens on/off button (not shown) disposed at the camera body  1  is depressed so as to retract the lens retainer lock pin  9  from the pin hole  261 . As the photographic lens  2  is turned back to the correct interlock phase along the clockwise direction viewed from the front side of the camera body  1  in this state, an engagement of the accessory-side tabs and the camera body-side tabs is ceased. Namely, a gap equivalent to a predetermined clearance is formed between the restricting member  262  and the first side end  111  of the body-side first tab  120 . In other words, the state shown in  FIG.  29    is resumed. Since the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  do not interfere with each other at the correct interlock position, as explained earlier, the photographic lens  2  can be pulled forward and separated from the camera body  1 . It is to be noted that as the photographic lens  2 , having been rotated to the correct interlock phase, is further rotated clockwise, the restricting member  262  comes into contact with the first side end  111  of the body-side first tab  120  so as to restrict any further rotation along the clockwise direction. Also note that the clockwise turning direction mentioned above may be otherwise referred to as a dismounting direction. 
     As described above, if the user attempts to mount the photographic lens  2  onto the camera body  1  while holding down the lens release button (not shown), the restricting member  262  comes in contact with the second side end  122  of the body-side second tab  120 , thereby restricting rotation of the photographic lens  2  along the mounting direction. In addition, when the photographic lens  2  currently mounted at the camera body  1  is dismounted from the camera body  1 , the restricting member  262  contacts the first side end  111  of the body-side first tab  110  so as to restrict rotation along the dismounting direction. 
     In the description, the end surface of the restricting member  262  that comes in contact with the second side end  122  of the body-side second tab  120  as the user attempts to mount the photographic lens  2  at the camera body  1  while holding down the lens release button (not shown) will be referred to as a mounting side end surface. In addition, the end surface of the restricting member  262  that comes in contact with the first side end  111  of the body-side first tab  110  as the user dismounts the photographic lens  2  from the camera body  1  will be referred to as a dismounting side end surface. Namely, the mounting side end surface of the restricting member  262  is set apart, along the clockwise direction in  FIG.  29   , from the second side end  122  of the body-side second tab  120  by an extent matching the sum of the mounting angle and the angle representing a predetermined interval at the accessory mount  200  inserted at the body mount  100  at the correct interlock phase, as shown in  FIG.  29   . In addition, the dismounting side end surface of the restricting member  262  is set apart, along the counterclockwise direction in  FIG.  29   , from the first side end  111  of the body-side first tab  110  by an extent matching the angle representing the predetermined interval at the accessory mount  200  inserted through the body mount  100  at the correct interlock phase, as shown in  FIG.  29   . 
     —Insertion of the Photographic Lens  2  at a Phase Other than the Correct Interlock Phase— 
     If the user attempts to insert the photographic lens  2  at a phase other than the correct interlock phase, at least two tabs among the body-side tabs  110  to  130  at the body mount  100  and at least two tabs among the accessory-side tabs  210  to  230  at the accessory mount  200  come into contact with each other at, at least, two contact locations, and thus, insertion of the accessory mount  200  through the body mount  100  is disallowed (deterred) in the embodiment. 
     As can be surmised by viewing  FIG.  29   , if the user attempts to mount the photographic lens  2  slightly offset from the correct interlock phase along the counterclockwise direction in the figure, the rear surface  213  of the accessory-side first tab  210  will come into contact with the front surface  113  of the body-side first tab  110 , the rear surface  223  of the accessory-side second tab  220  will come into contact with the front surface  123  of the body-side second tab  120 , and the rear surface  233  of the accessory-side third tab  230  will come into contact with the front surface  133  of the body-side third tab  130 . As a result, the three body-side tabs  110  to  130  and the three accessory-side tabs  210  to  230  will be in contact with each other and insertion of the accessory mount  200  into the body mount  100  will thus be disallowed. This situation may arise when, for instance, the accessory-side tabs  210  to  230  in  FIG.  30    are positioned frontward relative to the body-side tabs  110  to  130 . 
     If the user attempts to mount the photographic lens  2  offset by a greater extent along the counterclockwise direction relative to the state described above, the accessory-side second tab  220  will move frontward relative to the body-side third gap  153 , as shown in  FIG.  31    and thus, the accessory-side second tab  220  will no longer be in contact with any of the body-side tabs  110  to  130 . However, the rear surface  213  of the accessory-side first tab  210  will be contact with the front surface  113  of the body-side first tab  110  and the rear surface  233  of the accessory-side third tab  230  will be contact with the front surface  133  of the body-side third tab  130 . In this situation, the contact achieved by the two body-side tabs among the body-side tabs  110  to  130  and the two accessory-side tabs among the accessory-side tabs  210  to  230  disallows insertion of the accessory mount  200  through the body mount  100 . The contact thus achieved by the body-side tabs  110  to  130  and the accessory-side tabs at a total of two contact locations is indicated as the hatched areas in  FIG.  31   . 
     A slight counterclockwise rotation of the photographic lens  2  will alter the positional relationship of the two mounts relative to each other in  FIG.  31    to a positional relationship whereby the rear surface  213  of the accessory-side first tab  210  contacts the front surface  123  of the body-side second tab  120 . Namely, a slight counterclockwise turn of the photographic lens  2  will allow the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  to achieve contact at three contact locations. If, on the other hand, the photographic lens  2  in the state shown in  FIG.  31    is turned along the clockwise direction, the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  will contact each other over a larger area and the rear surface  233  of the accessory-side third tab  230  and the front surface  133  of the body-side third tab  130  will contact each other over a larger area. 
     In other words,  FIG.  31    shows a state in which the contact of the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  and the contact of the rear surface  233  of the accessory-side third tab  230  and the front surface  133  of the body-side third tab  130  are achieved at the two contact locations over the smallest contact area. The rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  contact each other at the interlock phase shown in  FIG.  31    over an angular range, assumed by viewing the body mount  100  from the front side of the camera body  1 , of 11°. 
     It is to be noted that there are other situations in which insertion of the accessory mount  200  through the body mount  100  is disallowed via a total of two contact locations where two tabs among the body-side tabs  110  to  130  and two tabs among the accessory-side tabs  210  to  230  contact each other. Among the plurality of states in which insertion of the accessory mount  200  through the body mount  100  is disallowed via a total of two contact locations where two tabs among the body-side tabs  110  to  130  and two tabs among the accessory-side tabs  210  to  230  achieve contact, the smallest contact area is formed at either of the two contact locations in two specific conditions, i.e., the state shown in  FIG.  31    and the state (not shown) achieved by rotating the photographic lens  2  in the state shown in  FIG.  31    by 11° along the counterclockwise direction in the figure, as described below. 
     If the photographic lens  2  with the two mounts assuming the specific positional relationship relative to each other as shown in  FIG.  31    is turned by 11° along the counterclockwise direction in the figure, the contact of the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  will cease but the rear surface  213  of the accessory-side first tab  210  will come into contact with the front surface  123  of the body-side second tab  120 . It is to be noted that contact will also be achieved as the rear surface  233  of the accessory-side third tab  230  comes into contact with the front surface  133  of the body-side third tab  130 . The rear surface  213  of the accessory-side first tab  210  and the front surface  123  of the body-side second tab  120  will contact each other over an angular range, assumed by viewing the body mount  100  from the front side of the camera body  1 , of 11°, as in the state shown in  FIG.  31   . 
     Thus, in this case too, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations. Among various states in which the contact is achieved by the rear surface  213  of the accessory-side first tab  210  and the front surface  123  of the body-side second tab  120  and contact is achieved by the rear surface  233  of the accessory-side third tab  230  and the front surface  133  of the body-side third tab  130  at a total of two contact locations, the accessory-side tabs and the body-side tabs contact each other over the smallest area in this state. 
     Namely, while there are various insertion disallowed states, in which insertion of the accessory mount  200  through the body mount  100  is disallowed via a total of two contact locations where two tabs among the body-side tabs  110  to  130  and two tabs among the accessory-side tabs  210  to  230  contact each other, the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  contact each other over the smallest contact area in the state shown in  FIG.  31   . Likewise, the smallest contact area is also formed by the rear surface  213  of the accessory-side first tab  210  and the front surface  123  of the body-side second tab  120  in the insertion disallowed state with the photographic lens  2  turned counterclockwise by 11° relative to the state shown in  FIG.  31   , among the insertion disallowed states in which insertion of the accessory mount  200  through the body mount  100  is disallowed via a total of two contact locations among the body-side tabs  110  to  130  and two tabs among the accessory-side tabs  210  to  230  contact each other. The interlock phase shown in  FIG.  31    and the interlock phase assumed with the photographic lens  2  turned by 11° along the counterclockwise direction relative to the state shown in  FIG.  31    are each referred to as a specific interlock phase. 
     As described above, a counterclockwise rotation of the photographic lens  2  in the state shown in  FIG.  31    will cause the figure, the rear surface  213  of the accessory-side first tab  210  to come into contact with both the front surface  113  of the body-side first tab  110  and the front surface  123  of the body-side second tab  120  and cause the rear surface  233  of the accessory-side third tab  230  to come into contact with the front surface  133  of the body-side third tab  130 . In other words, the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  will achieve contact at a total of three contact locations. 
     As the photographic lens  2  is turned further counterclockwise in the figure until the photographic lens  2  is rotated by 11° along the counterclockwise direction relative to the state shown in  FIG.  31   , as described above, the rear surface  213  of the accessory-side first tab  210  and the front surface  113  of the body-side first tab  110  will cease to be in contact with each other and thus, the number of contact locations where the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  contact each other will be reduced to two. 
     As the photographic lens  2  is turned further along the counterclockwise direction in the figure, the state shown in  FIG.  32    will be achieved. 
     As can be surmised by viewing  FIG.  32   , a slight counterclockwise turn of the photographic lens  2  will alter the positional relationship shown in  FIG.  32   , causing the rear surface  223  of the accessory-side second tab  220  to come into contact with the front surface  133  of the body-side third tab  130 . In other words, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of three contact locations. A further counterclockwise turn of the photographic lens  2  will cause the rear surface  233  of the accessory-side third tab  230  to come into contact with the front surface  113  of the body-side first tab  110  as well. In this situation, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  over a total of four contact locations. 
     As the photographic lens  2  is turned further counterclockwise, the rear surface  233  of the accessory-side third tab  230  and the front surface  133  of the body-side third tab  130  will cease contact with each other and then, the rear surface  213  of the accessory-side first tab  210  and the front surface  123  of the body-side second tab  120 , too, will cease contact with each other. In other words, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations under these circumstances, as shown in  FIG.  33   . 
     A further counterclockwise turn of the photographic lens  2  in the state shown in  FIG.  33    will cause a decrease in both the contact area over which the rear surface  223  of the accessory-side second tab  220  and the front surface  133  of the body-side third tab  130  are in contact with each other and the contact area over which the rear surface  233  of the accessory-side third tab  230  and the front surface  113  of the body-side first tab  110  are contact with each other. In other words,  FIG.  33    shows a state in which the contact of the rear surface  223  of the accessory-side second tab  220  and the front surface  133  of the body-side third tab  130  and the contact of the rear surface  233  of the accessory-side third tab  230  and the front surface  113  of the body-side first tab  110  are achieved at two contact locations over the largest contact area. 
       FIG.  34    shows a state achieved by further turning the photographic lens  2  along the counterclockwise direction relative to the state shown in  FIG.  33   . Among various states in which the rear surface  223  of the accessory-side second tab  220  and the front surface  133  of the body-side third tab  130  contact each other and the rear surface  233  of the accessory-side third tab  230  and the front surface  113  of the body-side first tab  110  contact each other at a total of two contact locations, the accessory-side tabs and the body-side tabs contact each other over the smallest area in this state. 
     A slight counterclockwise turn of the photographic lens  2  relative to the state shown in  FIG.  34    will allow the rear surface  213  of the accessory-side first tab  210  and the front surface  133  of the body-side third tab  130  to come into contact with each other. At this time, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of three contact locations. As the photographic lens  2  in this state is turned further along the counterclockwise direction in the figure, the rear surface  233  of the accessory-side third tab  230  and the front surface  123  of the body-side second tab  120  will come into contact with each other. In this state, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of four contact locations. 
     As the photographic lens  2  in this state is turned further along the counterclockwise direction in the figure, the rear surface  223  of the accessory-side second tab  220  and the front surface  133  of the body-side third tab  130  will cease contact with each other. At this time, contact will be achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of three contact locations. As shown in  FIG.  35   , when the photographic lens  2  is further turned along the counterclockwise direction, the rear surface  233  of the accessory-side third tab  130  ceases contact with the front surface  113  of the body-side first tab  110 . At this point, the body-side tabs  110  through  130  and the accessory-side tabs  210  through  230  contact each other at a total of two contact locations. 
     As the photographic lens  2  is slightly turned along the counterclockwise direction relative to the state shown in  FIG.  35   , the rear surface  223  of the accessory-side second tab  220  and the front surface  113  of the body-side first tab  110  will come into contact with each other. At this time, the body-side tabs  110  through  130  and the accessory-side tabs  210  through  230  contact each other at a total of three contact locations. As the photographic lens  2  is turned further along the counterclockwise direction in the figure, the correct interlock phase shown in  FIG.  29    will be reestablished and any contact (interference) between the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  will cease. 
     —Rationale for Disposing the Restricting Member  262  at the Accessory-Side Second Tab  220 — 
     In the embodiment, the restricting member  262  is disposed at the accessory-side second tab  220  so as to achieve the following advantages. 
     (1) Among the three tabs ( 210 ,  220  and  230 ) present on the accessory-side, the accessory-side second tab  220  ranges over the shortest distance along the circumference of the accessory mount  200 . In other words, the accessory-side second tab  220  assumes the shortest circular arc length among the three tabs. This means that the exchangeable lens  2  can be mounted at the camera body  1  at the correct interlock phase simply by rotating the exchangeable lens  2  by an extent matching the circular arc length of the accessory-side second tab  220 . Likewise, the exchangeable lens  2  can be dismounted from the camera body  1  simply by rotating the exchangeable lens  2  (along the direction opposite from the direction in which it is rotated during the mounting operation) by the extent matching the circular arc length of the accessory-side second tab  220 . Namely, by disposing the restricting member at the accessory-side tab  220 , the extent to which the user needs to rotate the accessory to mount or dismount it can be minimized. Consequently, the rotating operation onus placed on the user while mounting or dismounting the accessory (exchangeable lens  2 ) can be reduced and the length of time required to mount or dismount the accessory (exchangeable lens  2 ) can be reduced as well. 
     (2) In the embodiment, a single restricting member  262  is used both to restrict the range of rotation when mounting the accessory and restrict the range of rotation when dismounting the accessory, instead of using two separate restricting members in correspondence to the mounting operation and the dismounting operation. As a result, the number of required parts can be minimized and the mount structure can be simplified. 
     (3) The restricting member  262  located at the second tab  220  physically contacts a tab end of a body-side tab, which is present at a position where damage does not occur readily while the camera is in use. In other words, the restricting member  262  restricts the range of rotation of the accessory (exchangeable lens  2 ) being mounted or dismounted by coming into physical contact with an end portion (an end portion of the body-side tab) located at a less vulnerable position. 
     This point is explained in further detail below. The accessory-side second tab  220  at which the restricting member  262  is disposed moves through the body-side second gap  152  with the narrowest angular range along the circumferential edge of the circular opening at the camera body  1  while the photographic lens  2  is being mounted (at the correct interlock phase). The angular range of the body-side second group  152  along the circumferential edge of the circular opening at the camera body  1  is 47.5°. This means that the accessory-side first tab  210  (with its angular range of 58.5° and the accessory-side third tab  230  (with its angular range of 71.5°, assuming greater angular ranges than the 47.5° angular range cannot pass through the body-side second gap  152 . Thus, at any phase other than the correct interlock phase, there is no risk of the accessory-side first tab  210  or the accessory-side third tab  230  moving into the body-side second gap  152  and consequently, the first side end  111  of the body-side first tab  110  and the second side end  122  of the body-side second tab  120 , which define the angular range of the body-side second gap  152 , are less likely to be damaged. 
     The following problem would arise if the first side end  111  of the body-side first tab  110  and the second side end  122  of the body-side second tab  120  were susceptible to damage. 
     As explained earlier, the first side end  111  of the body-side first tab  110  comes in contact with the restricting member  262  while the photographic lens  2  is being dismounted from the camera body  1  so that the photographic lens  2  does not rotate along the dismounting direction any more than necessary. Thus, if the first side end  111  of the body-side first tab  110  becomes damaged and, for instance, chipped, the photographic lens  2  will be allowed to rotate along the dismounting direction further than necessary. Under such circumstances, the second side end  212  of the accessory-side first tab  210  may move further rearward relative to the body-side third tab  130  or the second side end  232  of the accessory-side third tab  230  may move further rearward relative to the body-side second tab  120 , causing the photographic lens  2  to jam at the camera body  1 . 
     As explained earlier, the second side end  122  of the body-side second tab  120  comes in contact with the restricting member  262  while the photographic lens  2  is being mounted at the camera body  1  by the user holding down the lens release button so as to ensure that the photographic lens  2  is not allowed to rotate along the mounting direction any further than necessary. As the photographic lens  2  mounted at the camera body  1  while holding down the lens release button (not shown) is turned along the mounting direction until the restricting member  262  comes in contact with the second side end  122  of the body-side second tab  120 , at the position of the lens retainer lock pin  9  and the position of the pinhole  261  at the accessory mount  200  may become slightly offset relative to each other. 
     However, since the lens retainer lock pin  9  has a rounded end, the accessory mount  200 , having rotated by a greater extent than necessary along the mounting direction, will be repositioned to assume the correct phase, as the lens retainer lock pin  9 , driven with the force imparted from the spring (not shown) passes through the pin hole  261  once the user releases the lens release button, as long as the offset of the pin hole  261  is slight. Otherwise, as the user releases the lens release button and the photographic lens  2  is caused to move back slightly, the lens retainer lock pin  9  will slip through the pin hole  261 , thereby setting the accessory mount  200  at the correct phase. 
     If the second side end  122  at the body-side second tab  120  becomes damaged, however, the position of the lens retainer lock pin  9  and the position of the pin hole  261  at the accessory mount  200  may become offset relative to each other by a significant extent as the photographic lens  2  is mounted at the camera body  1  while the lens release button (not shown) is held down. Under such circumstances, even if the user releases the lens release button, the lens retainer lock pin  9  will be unable to pass through the pin hole  261  and thus the accessory mount  200 , having turned along the mounting direction by an excessive extent, will not resume the correct phase. As a result, problems such as disabled photographing operation due to an offset of electrical contact points (not shown) via which signals and the like are exchanged by the photographic lens  2  and the camera body  1 , will occur. 
     In the embodiment, it is ensured that the first side end  111  of the body-side first tab  110  and the second side end  122  of the body-side second tab  120  are less susceptible to damage as described above, so as to prevent such problems and provide a more reliable camera system. 
     —Positional Relation between the Restricting Member  262  and the Pin Hole  261 — 
     As shown in  FIG.  8   , in the mounting complete state, the position of the restricting member  262  is substantially opposite from the position of the pin hole  261 , that is, the position of the pin hole  161 , across the fitting portion  202  which is not show in FIG,  8 , i.e., the optical axis of the photographic lens  2 . In other words, the restricting member  262  and the pin hole  261  (or the pin hole  161 ) are arranged so that they are substantially axisymmetrically disposed with respect to the optical axis when viewed along the optical axis of the photographic lens  2 . 
     If, for instance, a gap between the inner circumferential surface  102  of the cylindrical body mount  100  and the fitting portion  202  of the accessory mount  200  is large, the accessory mount  200 , that is, the photographic lens  2 , is capable of swinging with respect to the body mount  100 , that is, the camera body  1 , about the lens retainer lock pin  9  in a direction as indicated by an arrow A. It is to be noted that the swinging of the accessory mount  200  is exaggerated in  FIG.  16    for the sake of illustration. 
     The swinging about the lens retainer lock pin  9  causes the position furthest from the lens retainer lock pin  9  to be displaced by a largest distance. Therefore, in the present embodiment, the largest positional change by the swinging around the lens retainer lock pin  9  occurs near the restricting member  262  which is substantially opposite from the pin hole  261  across the fitting portion  202  (or the optical axis of the photographic lens  2 ). The direction of the positional change by the swinging around the lens retainer lock pin  9  is a tangential directions of a circle centered at the lens retainer lock pin  9  as indicated by an arrow B in  FIG.  36   . 
     In other words, the direction of the positional change caused by the swinging around the lens retainer lock pin  9  near the restricting member  262  substantially corresponds to a direction in which the restricting member  262  comes in contact with and separates from the second side end  122  of the body-side second tab  120 . As described above, in the mounting completion state of the photographic lens  2 , the mounting side end surface of the restricting member  262  is merely set apart from the second side end  122  of the body-side second tab  120  by a predetermined clearance. Thus, in the present embodiment, the positional change in the photographic lens  2  caused by the swinging around the lens retainer lock pin  9  can be reduced effectively by the restricting member  262  and the second side end  122  of the body-side second tab  120  even when the gap between the cylindrical inner circumferential surface  102  of the body mount  100  and the fitting portion  202  of the accessory mount  200  becomes large. 
     It is to be noted that as long as the swinging movement centered on the lens retainer lock pin  9  can be effectively deterred, the restricting member  262  may assume a phase other than that described above in the fully mounted state. This point will be explained in reference to the model presented in  FIG.  37   . ΔR in the model in  FIG.  37    represents a hypothetical difference between the radius of the cylindrical camera body mount  100  measured to its inner circumferential surface  102  and the radius of the accessory mount  200  measured at its fitting portion  202 . In addition, θ in the model presented in  FIG.  37    denotes the angle formed around the center of the circle defined by the inner circumferential surface  102  (fitting portion  202 ), i.e., the angle centered on the optical axis, which represents the position of the restricting member  262  in the fully mounted state, measured along the clockwise direction relative to the nine o&#39;clock direction in the figure. In this model, it is hypothesized that ΔR is equal to one thousandth of R (=D/2), representing a virtual radius of the cylindrical camera body mount  100  measured at its inner circumferential surface  101  and also of the accessory mount  200  measured at its fitting portion  202 . It is also hypothesized that the distance from the center of the circle defined by the inner circumferential surface  102  to the center of the lens retainer lock pin  9  is R+L and that L is 0.14 times R (i.e., L/R=0.14). 
     In conjunction with the hypothetical dimensions assumed as described above, the distance by which the restricting member  262  moves on the circumference of the circle with the radius R along a tangential direction as the fitting portion  202 , the center of which has been aligned with the center of the circle defined by the inner circumferential surface  102 , swings upward around the lens retainer lock pin  9  by an extent allowed in correspondence to the difference AR between the radii explained above, is referred to as a “shift quantity”. The value obtained by dividing the “shift quantity” by the radius R for purposes of non-dimensional representation is referred to as a “shift quantity ratio”. The relationship between the “shift quantity ratio” and  0  can be graphed as the curve shown in  FIG.  38   . Since the radius R is a constant, the greater a value of the “shift quantity ratio”, the greater the “shift quantity” relative to the extent of swinging movement of the fitting portion  202  centered on the lens retainer lock pin  9 . Thus, the greater the value of the shift quantity ratio, the more effectively can the swinging movement of the fitting portion  202  around the center of the lens retainer lock pin  9 , be deterred by the restricting member  262  coming into contact with the second side end  122  of the body side second tab  120 . As the graph in  FIG.  38    indicates, the “shift quantity ratio” assumes a smaller value when assumes a greater value and that once the value of θ exceeds 30°, the “shift quantity ratio” decreases markedly. Accordingly, when the hypothetical dimensions are assumed as described above, the phase of the restricting member  262  in the fully mounted state should be set by ensuring that  0  does not assume a value exceeding 30°. 
     —Accessory-Side Third Tab  230 — 
     As explained earlier, located near the first side end  231  of the accessory-side third tab  230 , the front surface  234  is pressed by the spring  136  at the body-side third tab  130 , and whenever an excessive external force is applied, the front surface  234  will come into contact with the contact portion  135 . This means that when the photographic lens  2  is mounted at the camera body  1 , the force applied to lock the photographic lens  2  to the camera body  1  will be applied primarily to the area near the first side end  231  at the accessory-side third tab  230 . 
     —Body-Side First Tab  110 — 
     When contact is achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations with the body-side first tab  110  in contact with an accessory-side tab at one of the contact locations, the body-side first tab  110  invariably contacts the accessory-side tab  210 ,  220  or  230  on the side where its first side end  111  is located (i.e., at the front surface  113  near the first side end  111 ), rather than on the side where its second side end  112  is located (i.e., at the front surface  113  near the second side end  112 ), as shown in  FIGS.  31 ,  33  and  34   . In other words, when contact is achieved by the body-side tabs  110  to  130  and the accessory-side tabs  210  to  230  at a total of two contact locations and the body-side first tab  110  achieves contact at one of the contact locations, the function of preventing erroneous insertion is achieved in the area near the first side end  111  of the body-side first tab  110 . 
     The spring  116  is disposed adjacent to the contact portion  115  at the body-side first tab  110  over the area near its second side end  112 . Namely, the function of locking the photographic lens  2  having been mounted at the camera body  1  is achieved over the area near the second side end  112  of the body-side first tab  110 . 
     In summary, the body-side first tab  110  is formed so that different functions are achieved in the area near the first side end  111  and the area near the second side end  112  along the direction in which the body-side first tab  110  extends. 
     The camera body  1  and the photographic lens  2  achieved in the second embodiment as described above achieve the following advantages in addition to the advantages of the first embodiment. 
     (1) The restricting member  262  and the pin hole  261  (the pin hole  161 ) are arranged so that they are disposed substantially axisymmetrically with respect to the optical axis when viewed along the optical axis of the photographic lens  2 . In this manner, the positional change in the photographic lens  2  caused by the swinging around the lens retainer lock pin  9  can be reduced effectively by the restricting member  262  and the second side end  112  of the body-side first tab  110  even when the gap between the cylindrical inner circumferential surface  102  of the body mount  100  and the fitting portion  202  of the accessory mount  200  becomes large. 
     (2) In the present embodiment, the restricting member  262  is disposed at the accessory-side second tab  220  which ranges over the shortest distance along the circumference of the accessory mount  200  or assumes the shortest circular arc length. Consequently, the rotating operation onus placed on the user while mounting or dismounting the accessory can be reduced and the length of time required to mount or dismount the accessory can be reduced as well. 
     (3) In the embodiment, a single restricting member  262  is used both to restrict the range of rotation when mounting the accessory and restrict the range of rotation when dismounting the accessory. As a result, the number of required parts can be minimized and the mount structure can be simplified. 
     (4) In the embodiment, the restricting member  262  located at the second tab  220  physically contacts a tab end of a body-side tab, which is present at a position where damage does not occur readily. In other words, the restricting member  262  restricts the range of rotation of the accessory being mounted or dismounted by coming into physical contact with an end portion (an end portion of the body tab) located at a less vulnerable position. As a result, a rotation regulating structure assuring a high level of durability is achieved, which, in turn, makes it possible to improve the durability of the bayonet-type mount structure. 
     In other words, the body-side second gap  152  is formed so as to assume a narrower angular range along the circumferential edge of the circular opening at the camera body  1  compared to the other body-side gaps  151  and  153 , so as to ensure that the accessory-side first tab  210  or the accessory-side third tab  230  does not move into the body-side second gap  152  at any phase other than the correct interlock phase and thus, improve the durability of the bayonet mount structure as described above. 
     —Variations— 
     (1) While the accessory-side tabs  210  to  230  are fixed onto the main body of the photographic lens  2  and the photographic lens  2  is mounted or dismounted by turning it along the mounting direction or the dismounting direction relative to the camera body  1  in the embodiments described above, the present invention is not limited to this example. For instance, the present invention may be adopted in conjunction with a structure that includes accessory-side tabs  210  to  230  rotatably disposed relative to the main body of the photographic lens  2  and allows the photographic lens  2  to be mounted at the camera body  1  simply by rotating the accessory-side tabs  210  to  230  alone along the mounting direction once the photographic lens  2  is positioned at the correct interlock phase. 
       FIGS.  20 A and  20 B  present external views of a photographic lens  20  with accessory-side tabs  210  to  230  formed thereat so that they are able to rotate relative to the main body of the photographic lens  2 . The photographic lens  20  includes a lens barrel  21 , a bayonet tube  22  disposed rotatably relative to the lens barrel  21 , at which accessory-side tabs  210  to  230  are disposed, and an operation ring  23  via which the bayonet tube  22  is rotated relative to the lens barrel  21 . It is to be noted that in  FIGS.  20 A and  20 B  and other figures in reference to which the following description is provided, the same reference numerals are assigned to members similar to those already described so as to preclude the necessity for a repeated explanation thereof. 
       FIG.  21    presents an external view of the lens barrel  21 , whereas  FIGS.  22 A to  22 D  schematically illustrate the structure of the photographic lens  20 . As shown in FIG.  21 , a fitting groove  21   a  at which the operation ring  23  fits is formed at the rear of the lens barrel  21 . An elongated hole  21   b , through which a pin  24  connecting the operation ring  23  with the bayonet tube  22  is allowed to move, is formed at the bottom of the fitting groove  21   a . As indicated in the sectional views presented in  FIGS.  22 A and  22 D , the operation ring  23  fits into the fitting groove  21   a  at the lens barrel  21  and the operation ring  23  and the bayonet tube  22  are linked with each other via the pin  24 . Reference numeral  25  indicates a lens. As the operation ring  23  is rotated relative to the lens barrel  21  in the photographic lens  20  structured as described above, the bayonet tube  22 , i.e., the accessory-side tabs  210  to  230 , turns together with the operation ring  23  relative to the lens barrel  21 . The photographic lens  20  assuming this structure can be mounted at the camera body  1  without having to turn the lens barrel  21 , by simply operating the operation ring  23  to turn the bayonet tube  22  alone along the mounting direction once the photographic lens  20  is positioned at the correct interlock phase. It is to be noted that  FIG.  22 D  is a sectional view taken along C-C in  FIG.  22 A . 
     (2) The photographic lenses  2  and  20  described above each represent an example of a camera accessory. Namely, the present invention is not limited to these examples and may be adopted in conjunction with any of various types of camera accessories such as a converter lens used to adjust the focal length or an adapter used to adjust the distance from the photographic lens mount surface to the image capturing plane, as long as it includes an accessory mount  200 , which can be engaged with a camera body mount  100 . 
     (3) While the restricting member  262  described above is headed pin disposed at the accessory mount  200 , an area near the second side end  222 , for instance, may be formed in a specific shape so as to function as a restricting member  262 , instead of providing a separate member. Namely, an area near the second side end  222  may be formed to assume a shape that will allow the area to function as a restricting member  262  through injection molding at an accessory mount  200  constituted with a resin injection mold. In addition, the restricting member  262  may be constituted with two pins; a first pin that includes a mounting side end surface, such as that explained earlier, and a second pin, different from the first pin, which includes a dismounting side end surface such as that described earlier. As a further alternative, instead of disposing separate members such as the first pin and the second pin, an area near the second side end  222  may be formed so as to assume shapes resembling a first pin and a second pin. 
     (4) The camera body mount  100  described above may be disposed at the photographic lens  2  and the accessory mount  200  described above may be disposed at the camera body  1 . 
     (5) An explanation has been given above in reference to the embodiments on an example in which the present invention is adopted in a camera system comprising the camera body  1  and the exchangeable lens  2 . However, the present invention is not limited to this example and may be adopted in any system other than a camera system, as long as it includes a mount at which the exchangeable lens  2  can be detachably mounted. Such a system may be, for instance, an electronic device such as a projector. A system similar to those described in reference to the embodiments can be achieved by equipping the projector with an exchangeable projection lens that can be detachably mounted. 
     (6) Various embodiments and variations described above may be adopted in combination. 
     It is to be noted that the present invention is not limited to specific structural features of the embodiments described above in any way whatsoever and that a camera accessory, which can be detachably mounted at a camera body equipped with a camera body mount and includes an accessory mount assuming a bayonet structure with first through third tabs set over intervals in a circumferential direction running along a circumference of a circle with a specific diameter and projecting radially from the circumference, characterized in that the accessory mount is allowed to be inserted through the camera body mount without any of the first through third tabs being hindered by any of three camera body-side tabs at the camera body mount, as long as the accessory mount is inserted at the camera body mount at a correct interlock phase, that the first through third tabs extend along the circumferential direction over varying lengths, with the first tab extending over a greatest length along the circumferential direction among the first through third tabs and the third tab extending over a smallest length along the circumferential direction among the first through third tabs, that the accessory mount includes a restricting member disposed near the third tab, a fitting portion used to align an optical axis of the accessory mount with an optical axis of the camera body and a lock pin hole through which a lock pin, retractably disposed at the camera body, is inserted so as to lock the camera accessory in a fully mounted state, and that the restricting member is disposed at a position assumed on a side substantially opposite from the lock pin hole across the fitting portion, otherwise adopting any of various structures, are all within the scope of the present invention. 
     According to the present invention described above, backlash between a camera accessory and a camera body can be effectively prevented. 
     The above described embodiments are examples and various modifications can be made without departing from the scope of the invention.