Patent Publication Number: US-10317642-B2

Title: Camera filter frame and camera filter unit

Description:
This application is a divisional of patent application Ser. No. 14/917,355 filed on Mar. 8, 2016, which is National Stage Application of International Application No. PCT/JP2014/074316, filed on Sep. 12, 2014, the disclosure of which is incorporated herein by reference in its entirety. The International Application No. PCT/JP2014/074316 is entitled to and claims benefit of Japanese Patent Application No. 2013-190529, filed on Sep. 13, 2013, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     FIELD 
     The present invention relates to a camera filter frame in which a front ring for holding an optical element is rotatably held by a rear ring. The present invention also relates to a camera filter unit in which the camera filter frame holds an optical element such as a polarization filter. 
     BACKGROUND 
     A camera filter frame for holding a polarization filter is disclosed in Patent Literature 1. The camera filter frame of Patent Literature 1 includes a front ring for holding the polarization filter and a rear ring for holding the front ring. The front ring includes an annular recess that is recessed to the inner peripheral side on the outer peripheral surface of the front ring. The rear ring includes an annular recess that is recessed to the outer peripheral side on the inner peripheral surface of the rear ring. The front ring and the rear ring are combined in such a manner that the respective annular recesses face each other in the radial direction, and a washer is inserted into an annular space formed by the annular recess of the front ring and the annular recess of the rear ring. The washer has its outer peripheral portion positioned in the annular recess of the rear ring and its inner peripheral portion positioned in the annular recess of the front ring. This structure enables the front ring and the rear ring to rotate relatively to each other about the axis with the washer as a guide. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Publication No. 2005-301172 
     SUMMARY 
     Technical Problem 
     In order to insert a washer into an annular space formed by the annular recess of the front ring and the annular recess of the rear ring, a C-shaped washer is first placed in the annular recess provided to the outer peripheral surface of the front ring. Next, open ends of the washer are brought closer to each other to reduce the outside diameter of the washer so as to combine the front ring and the rear ring. The annular recess of the front ring and the annular recess of the rear ring are then caused to face each other. As a result, an annular space is formed between the front ring and the rear ring, and then the washer&#39;s own shape restoring force separates the open ends from each other to restore the outside diameter of the washer. Consequently, the washer has the outer peripheral portion positioned in the annular recess of the rear ring and the inner peripheral portion positioned in the annular recess of the front ring. 
     A washer that expands and contracts in the radial direction is easy to be distorted. It is also difficult to maintain the washer placed in the annular space in a position orthogonal to the axis of each ring. Therefore, in the structure in which the front ring and the rear ring are caused to rotate relatively to each other with the washer as a guide, the washer comes into contact with each ring at random, which might cause the front ring not to rotate smoothly relative to the rear ring. 
     In view of the foregoing, an object of the present invention is to provide a camera filter frame in which a rear ring rotatably holds a front ring for holding an optical element with a simple structure using no washer. Another object of the present invention is to provide a camera filter unit in which the camera filter frame holds an optical element. 
     Solution to Problem 
     To achieve the above object, a camera filter frame according to the present invention includes a front ring that holds an optical element on the inner peripheral side thereof and includes an annular recess on the outer peripheral surface thereof, and a rear ring that includes, on the inner peripheral surface thereof, an annular protrusion inserted into the annular recess and that holds the front ring rotatably. 
     According to the present invention, the front ring can rotate about the axis using the annular protrusion of the rear ring that is inserted into the annular recess as a guide. Consequently, no washer needs to be used to allow the rear ring to hold the front ring rotatably. In the case in which the front ring is rotated using the annular protrusion that is disposed in the rear ring as a guide, the guide (annular protrusion) does not become distorted, nor does the position of the guide (annular protrusion) change. The annular protrusion serving as a guide can be formed more accurately than a washer or the like that expands and contracts in the radial direction. Furthermore, because no member such as a washer is interposed between the front ring and the rear ring, the front ring and the rear ring can be combined accurately. The front ring held by the rear ring can therefore be rotated smoothly. 
     In the present invention, the front ring includes an annular wall portion extending in the radial direction, an annular plate portion extending rearward from the inner peripheral end of the annular wall portion, and an annular projection portion projecting for a shorter distance than the annular wall portion from the rear end of the annular plate portion toward the outer peripheral side. The annular recess is formed by the annular wall portion, the annular plate portion, and the annular projection portion. A position of the annular protrusion is between the annular wall portion and the annular projection portion in the fore-and-aft direction and the annular plate portion is bent to the outer peripheral side to displace the annular projection portion to the outer peripheral side and the state in which the annular protrusion is inserted into the annular recess is created. In other words, in the present invention, the front ring includes an annular wall portion extending in the radial direction, an annular plate portion extending rearward from the inner peripheral end of the annular wall portion, and an annular projection portion projecting for a shorter distance than the annular wall portion from the rear end of the annular plate portion toward the outer peripheral side. The annular recess is formed by the annular wall portion, the annular plate portion, and the annular projection portion. The annular wall portion includes a bent portion that is bent to the outer peripheral side. This structure facilitates creating the state in which the annular protrusion of the rear ring is inserted into the annular recess of the front ring. 
     In the present invention, it is preferable that the annular plate portion include an annular groove on at least one of the inner peripheral surface and the outer peripheral surface thereof, and that a portion in which the annular groove is formed in the annular plate portion be bent to displace the annular projection portion to the outer peripheral side. In other words, it is preferable that the annular plate portion include an annular groove on at least one of the inner peripheral surface and the outer peripheral surface thereof, and that a portion in which the annular groove is formed in the annular plate portion be a bent portion that is bent to the outer peripheral side. Because the portion in which the annular groove is formed is thinner than other portions, the rear annular plate portion can be bent accurately with relatively weak force by bending this thin portion. Consequently, annular projection portion can be displaced accurately. This can improve the accuracy of dimension between the annular protrusion of the rear ring and the annular recess of the front ring, preventing the front ring from rattling on the rear ring. The front ring can therefore be rotated smoothly. 
     In the present invention, the camera filter frame can include a front stopper that restricts the forward movement of the optical element, and a rear stopper that restricts the rearward movement of the optical element. The front ring can include a holding portion for holding the optical element in a position closer to the front side than the position of the annular wall portion is, the front stopper can be attached to the inner peripheral surface of the front ring, and the rear stopper can be the annular wall portion. This structure can restrict the back-and-forth movement of the optical element that is held to the front ring. 
     In this case, the front ring preferably holds the optical element between the front stopper and the rear stopper in a rotatable manner about the axis. In this manner, stress (pressure) to be placed on the optical element can be reduced compared with the case in which the optical element is held to the front ring in an unrotatable manner while being gripped by the front stopper and the rear stopper from the front and rear. Because distortion can be prevented from occurring in the optical element with lower stress placed on the optical element, finer images and videos can be obtained when, for example, images and videos the resolution of which is many times higher than full high-definition ones are shot. 
     It is preferable that the rear ring include an annular portion positioned on the outer peripheral side of the holding portion closer to the front side than the annular protrusion is, and that the holding portion include an adhesive injection hole passing therethrough in a direction intersecting the axis of the holding portion. In this manner, after the optical element is held to the holding portion, an adhesive can be injected through the adhesive injection hole to fix the optical element to the front ring. Because the annular portion of the rear ring is positioned on the outer peripheral side of the holding portion, the adhesive injection hole is not exposed to the outer peripheral side even in the case in which the adhesive injection hole is provided in the holding portion. Consequently, the adhesive injection hole does not impair the appearance of the camera filter frame. 
     In the present invention, the camera filter frame can include an inner ring that is inserted into the front ring, and a rear stopper that restricts the rearward movement of the optical element. The inner ring can include an annular frame for holding the optical element from the outer peripheral side, and a front stopper that protrudes from the front edge of the annular frame to the inner peripheral side to restrict the forward movement of the optical element. The front ring can hold the inner ring in a position closer to the front side than the position of the annular wall portion is, and the rear stopper can be the annular wall portion. This structure allows the front ring to hold the optical element via the inner ring. This structure can also restrict the back-and-forth movement of the optical element that is held to the front ring. 
     In this case, the front ring preferably holds the optical element between the front stopper and the rear stopper in a rotatable manner about the axis. In this manner, stress (pressure) to be placed on the optical element can be reduced compared with the case in which the optical element is held to the front ring in an unrotatable manner while being gripped by the front stopper and the rear stopper from the front and rear. Because distortion can be prevented from occurring in the optical element with lower stress placed on the optical element, finer images and videos can be obtained when, for example, images and videos the resolution of which is many times higher than full high-definition ones are shot. 
     The annular frame can include an adhesive injection hole passing therethrough in a direction intersecting the axis of the annular frame. In this manner, after the optical element is held to the annular frame, an adhesive can be injected through the adhesive injection hole to fix the optical element to the annular frame. Because the annular frame is held on the inner side of the front ring, the adhesive injection hole is not exposed to the outer peripheral side even in the case in which the adhesive injection hole is provided in the annular frame. Consequently, the adhesive injection hole does not impair the appearance of the camera filter frame. 
     In the present invention, it is preferable that the rear ring include an external thread in the rear end of the outer peripheral surface thereof, and that the annular protrusion be disposed in a position overlapping the external thread when seen from the radial direction. In this manner, the dimension of the camera filter frame can be reduced in the fore-and-aft direction (axial direction) compared with the case in which the annular protrusion is formed in a position different from that of the external thread in the axial direction. 
     In the present invention, the rear ring can hold a second optical element on the inner peripheral side thereof. Specifically, the camera filter frame can hold a first optical element to the front ring and a second optical element to the rear ring. 
     In the present invention, the camera filter frame can include a second front stopper that restricts the forward movement of the second optical element, and a second rear stopper that restricts the rearward movement of the second optical element. The rear ring can include a second annular projection portion that projects to the inner peripheral side closer to the rear than the front ring does, and a rear holding portion for holding the second optical element at the rear of the second annular projection portion. The second front stopper can be the second annular projection portion, and the second rear stopper can be attached to the inner peripheral surface of the rear ring. This structure can restrict the back-and-forth movement of the second optical element that is held to the rear ring. 
     In this case, the rear ring preferably holds the second optical element between the second front stopper and the second rear stopper in a rotatable manner about the axis. In this manner, stress (pressure) to be placed on the second optical element can be reduced compared with the case in which the second optical element is held to the rear ring in an unrotatable manner while being gripped by the second front stopper and the second rear stopper from the front and rear. Because distortion can be prevented from occurring in the second optical element with lower stress placed on the second optical element, finer images and videos can be obtained when, for example, images and videos the resolution of which is many times higher than full high-definition ones are shot. 
     In the present invention, the camera filter frame can include a rear inner ring that is inserted into the rear ring, and a second front stopper that restricts the forward movement of the second optical element. The rear ring can include a second annular projection portion that projects to the inner peripheral side at the rear of the front ring. The rear inner ring can include a rear annular frame that is positioned at the rear of the second annular projection portion and that holds the second optical element from the outer peripheral side, and a second rear stopper that projects from the rear annular frame to the inner peripheral side to restrict the rearward movement of the second optical element. The second front stopper can be the second annular projection portion. This structure can restrict the back-and-forth movement of the optical element that is held to the rear ring. 
     In this case, the rear ring preferably holds the second optical element between the second front stopper and the second rear stopper in a rotatable manner about the axis. In this manner, stress (pressure) to be placed on the second optical element can be reduced compared with the case in which the second optical element is held to the rear ring in an unrotatable manner while being gripped by the second front stopper and the second rear stopper from the front and rear. Because distortion can be prevented from occurring in the second optical element with lower stress placed on the second optical element, finer images and videos can be obtained when, for example, images and videos the resolution of which is many times higher than full high-definition ones are shot. 
     A camera filter unit according to the present invention includes the camera filter frame as described above and a polarization filter that is held by the front ring as the optical element. 
     According to the present invention, the front ring for holding the polarization filter can be rotated smoothly with respect to the rear ring. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a camera filter unit according to a first embodiment; 
         FIG. 2A  is a longitudinal sectional view of the camera filter unit according to the first embodiment; 
         FIG. 2B  is a partially enlarged sectional view of the camera filter unit according to the first embodiment; 
         FIGS. 3A, 3B and 3C  are diagrams for describing an assembly method of the camera filter unit; 
         FIG. 4A  is a longitudinal sectional view of the camera filter unit according to a second embodiment; 
         FIG. 4B  is a partially enlarged sectional view of the camera filter unit according to a second embodiment; 
         FIG. 5A  is a longitudinal sectional view of the camera filter unit according to a third embodiment; 
         FIG. 5B  is a partially enlarged sectional view of the camera filter unit according to a third embodiment; 
         FIG. 6A  is a longitudinal sectional view of the camera filter unit according to a fourth embodiment; 
         FIG. 6B  is a partially enlarged sectional view of the camera filter unit according to a fourth embodiment; 
         FIG. 7A  is a longitudinal sectional view of the camera filter unit according to a fifth embodiment; and 
         FIG. 7B  is a partially enlarged sectional view of the camera filter unit according to a fifth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes a camera filter unit to which the present invention is applied with reference to the drawings. 
     (First Embodiment) 
       FIG. 1  is a perspective view of a camera filter unit according to a first embodiment.  FIG. 2A  is a longitudinal sectional view of the camera filter unit in  FIG. 1 .  FIG. 2B  is a partially enlarged sectional view thereof. As illustrated in  FIG. 1  and  FIG. 2A , a camera filter unit  1  according to the present embodiment includes a disc-like polarization filter (optical element)  2 , an inner ring  3  for holding the polarization filter  2  coaxially, a front ring  4  for holding the inner ring  3  coaxially from the outer peripheral side, and a rear ring  5  for holding the front ring  4  coaxially. The front ring  4  holds the polarization filter  2  via the inner ring  3 . The rear ring  5  holds the front ring  4  rotatably about an axis L. The inner ring  3 , the front ring  4 , and the rear ring  5  constitute a camera filter frame  10 . In the following description, the direction of the inner ring  3 , the front ring  4 , and the rear ring  5  along the axis L is assumed to be a fore-and-aft direction X of the camera filter unit  1 . In the fore-and-aft direction X, the side on which the front ring  4  is positioned is assumed to be the front side (front X 1 ), and the side on which the rear ring  5  is positioned is assumed to be the rear side (rear X 2 ). 
     The front ring  4  holds the inner ring  3  in a position where the inner ring  3  is not exposed from the front end of the front ring  4 . On the inner peripheral surface of the front ring  4 , an internal thread  11  is provided in a region of a certain width from the front edge toward the rear X 2 . The exposed portion of the internal thread  11  that is exposed to the front side more than the inner ring  3  is a front mounting portion for mounting a cap or a hood into the front of the camera filter unit  1 . On the outer peripheral surface of the rear ring  5 , an external thread  12  is provided in a region of a certain width from the rear edge toward the front X 1 . The external thread  12  is a rear mounting portion for mounting the camera filter unit  1  into a lens-barrel of an imaging lens or a camera. The inner ring  3 , the front ring  4 , and the rear ring  5  are all formed of a metal base. In the present embodiment, the inner ring  3 , the front ring  4 , and the rear ring  5  are all made of aluminum. 
     (Front Ring) 
     As illustrated in  FIG. 2B , the front ring  4  includes a front annular plate portion  15  extending in the fore-and-aft direction X along the axis L, a front annular wall portion  16  extending from the rear end of the front annular plate portion  15  toward the inner peripheral side in a radial direction R orthogonal to the axis L, an intermediate annular plate portion  17  extending from the inner peripheral end of the front annular wall portion  16  to the rear X 2  along the axis L, a rear annular wall portion (annular wall portion)  18  extending from the rear end of the intermediate annular plate portion  17  toward the inner peripheral side in the radial direction R, a rear annular plate portion  19  extending from the inner peripheral end of the rear annular wall portion  18  to the rear X 2 , and an annular projection portion  20  projecting for a shorter distance than the rear annular wall portion  18  from the rear end of the rear annular plate portion  19  toward the outer peripheral side. On the outer peripheral surface of the front ring  4 , an annular recess  21  is formed by the rear annular wall portion  18 , the rear annular plate portion  19 , and the annular projection portion  20 . 
     The front annular plate portion  15  includes a thick portion  23  and a thin portion  24  in this order from the front X 1  toward the rear X 2 . The thick portion  23  projects to the outer peripheral side more than the thin portion  24  does. An annular step  25  is formed by the thick portion  23  and the thin portion  24  on the outer peripheral surface of the front annular plate portion  15 . The internal thread  11  serving as a front mounting portion is formed on the inner peripheral surface of the front annular plate portion  15 . 
     The front annular wall portion  16  has a constant thickness dimension in the fore-and-aft direction X. The intermediate annular plate portion  17  has a constant thickness in the radial direction R. The rear annular wall portion  18  has a constant thickness dimension in the fore-and-aft direction X. 
     The rear annular plate portion  19  inclines to the outer peripheral side toward the rear X 2 . More specifically, the front end of the outer peripheral surface of the rear annular plate portion  19  (portion adjacent to the rear annular wall portion  18 ) is provided with an annular groove  27 , and the portion in which the annular groove  27  is formed is bent to the outer peripheral side so that the rear annular plate portion  19  inclines to the outer peripheral side. Specifically, in the rear annular plate portion  19 , the portion in which the annular groove  27  is formed is a bent portion that is bent to the outer peripheral side. The rear portion of the outer peripheral surface of the rear annular plate portion  19  (portion adjacent to the annular projection portion  20 ) is provided with an annular notch  28 . The notch  28  is shallower than the annular groove  27 . 
     The annular projection portion  20  projects in a direction orthogonal to the rear annular plate portion  19 . Consequently, the annular projection portion  20  extends to the front X 1  toward the outer peripheral side. In the present embodiment, the annular projection portion  20  has a trapezoidal sectional shape tapering toward the tip. The inner peripheral portion of an annular protrusion  45  provided to the rear ring  5  is inserted into the annular recess  21  formed by the rear annular wall portion  18 , the rear annular plate portion  19 , and the annular projection portion  20 . 
     (Inner Ring) 
     The inner ring  3  includes an annular frame  31  for holding the polarization filter  2  coaxially from the outer peripheral side, and an annular front stopper  32  that projects from the front edge of the annular frame  31  to the inner peripheral side to restrict the movement of the polarization filter  2  to the front X 1 . The front stopper  32  is abuttable from the front X 1  on the outer peripheral edge of the polarization filter  2  that is held by the annular frame  31 . 
     The inner peripheral surface of the annular frame  31  is an annular surface that extends with a constant diameter dimension along the axis L, and is a filter holding surface  33  for holding the polarization filter  2  coaxially. The annular frame  31  includes a thick frame portion  34  and a thin frame portion  35  in this order from the front X 1  toward the rear X 2 . The outer peripheral surface of the thick frame portion  34  is positioned closer to the outer peripheral side than the outer peripheral surface of the thin frame portion  35  is, and between these portions, an annular rearward surface  34   a  is formed that extends along the radial direction R. On the outer peripheral surface of the thick frame portion  34 , an external thread  36  is formed that can be threadedly engaged with the internal thread  11  of the front ring  4 . In the thin frame portion  35 , an adhesive injection hole  37  is formed that passes through the thin frame portion in the radial direction R to be open on the filter holding surface  33 . 
     If an adhesive is injected from the outer peripheral side of the annular frame  31  through the adhesive injection hole  37  with the polarization filter  2  held to the filter holding surface  33 , the adhesive enables the polarization filter  2  to be fixed to the inner ring  3 . The number of adhesive injection holes  37  may be one, but a plurality of adhesive injection holes  37  are open on the thin frame portion  35  at equal angular intervals in the present embodiment. 
     The front stopper  32  is an annular protrusion that protrudes from the front edge of the annular frame  31  toward the inner peripheral side. The front stopper  32  includes an annular rear end surface  32   a  that is a flat surface orthogonal to the axis L. The front stopper  32  has a width dimension from the annular rear end surface  32   a  to the rear end of the annular frame  31  (width dimension W of the filter holding surface  33 ) larger than a thickness dimension D of the polarization filter  2  held by the filter holding surface  33 . The difference between the width dimension W of the filter holding surface  33  in the fore-and-aft direction X and the thickness dimension D of the polarization filter  2  is 0.03 mm or less. 
     The inner ring  3  is inserted from the front side into the front ring  4  with the polarization filter  2  held to the inner peripheral side of the front annular frame  31 . The external thread  36  of the inner ring  3  is then threadedly engaged with the internal thread  11  of the front ring  4  to be screwed until the rear end of the annular frame  31  abuts on the rear annular wall portion  18  of the front ring  4 . When the rear end of the annular frame  31  abuts on the rear annular wall portion  18 , the annular rearward surface  34   a  of the inner ring  3  faces the front annular wall portion  16  of the front ring  4  in the fore-and-aft direction X with a minute spacing allowed therebetween. The outer peripheral surface of the thin frame portion  35  of the annular frame  31  also faces the intermediate annular plate portion  17  in the radial direction R with a minute spacing therebetween. With the rear end of the annular frame  31  abutting on the rear annular wall portion  18 , the distance (width dimension W of the filter holding surface  33 ) between the front stopper  32  and the rear annular wall portion  18  is larger than the thickness dimension D of the polarization filter  2 . Consequently, the front ring  4  holds the polarization filter  2  rotatably about the axis L between the front stopper  32  and the rear annular wall portion  18 . Specifically, the front stopper  32  and the rear annular wall portion  18  do not grip the polarization filter  2  unrotatably. The polarization filter  2  is fixed to the inner ring  3  unrotatably by the adhesive. The rear annular wall portion  18  functions as a rear stopper that prevents the polarization filter  2  from moving to the rear X 2 . 
     When the rear end of the annular frame  31  abuts on the rear annular wall portion  18  by screwing the inner ring  3  into the front ring  4 , the annular rearward surface  34   a  of the inner ring  3  may abut on the front annular wall portion  16  of the front ring  4 . 
     (Rear Ring) 
     The rear ring  5  includes a front annular plate portion  41  extending in the fore-and-aft direction X along the axis L, an annular wall portion  42  extending from the rear end of the front annular plate portion  41  toward the inner peripheral side in the radial direction R, and a rear annular plate portion  43  extending from the inner peripheral end of the annular wall portion  42  to the rear X 2  along the axis L. The front annular plate portion  41  has a constant thickness dimension in the radial direction R. A grease holding recess  44  is formed on the front end surface of the annular wall portion  42 . The grease holding recess  44  holds grease to slide the front ring  4  and the rear ring  5  smoothly. The external thread  12  serving as a rear mounting portion is formed on the outer peripheral surface of the rear annular plate portion  43 . The annular protrusion  45  that protrudes toward the inner peripheral side in the radial direction R is disposed on the inner peripheral surface of the rear annular plate portion  43 . 
     The annular protrusion  45  includes a rectangular sectional shape. The annular protrusion  45  has a width dimension in the fore-and-aft direction X larger than the protrusion dimension to the inner peripheral side. The annular protrusion  45  is formed in a position overlapping the external thread  12  when seen from the radial direction R. 
     With the rear ring  5  holding the front ring  4 , the front annular plate portion  41  is slidably fitted into the annular step  25  of the front ring  4 . This structure allows the outer peripheral surface  41   a  of the front annular plate portion  41  to be steplessly continuous to the outer peripheral surface  15   a  of the thick portion  23  in the front annular plate portion  15  of the front ring  4 . The annular wall portion  42  abuts on the front annular wall portion  16  of the front ring  4  from the rear X 2 , while the rear annular plate portion  43  abuts on the intermediate annular plate portion  17  of the front ring  4  from the outer peripheral side. The annular protrusion  45  abuts on the rear annular wall portion  18  of the front ring  4  from the rear X 2 . The inner peripheral portion of the annular protrusion  45  is inserted into the annular recess  21  on the outer peripheral surface of the inner ring  3 . This structure allows the rear ring  5  to hold the front ring  4  rotatably about the axis L. 
     (Assembly of Camera Filter Unit) 
       FIGS. 3A, 3B and 3C  are diagrams for describing an assembling operation of the camera filter unit  1 . In order to assemble the camera filter unit  1 , the front ring  4  and the rear ring  5  are first combined with each other in the fore-and-aft direction X so that the position of the annular protrusion  45  of the rear ring  5  is between the rear annular wall portion  18  of the front ring  4  and the annular projection portion  20  in the fore-and-aft direction X. 
     In the state before the front ring  4  and the rear ring  5  are combined, the rear annular plate portion  19  of the front ring  4  extends along the axis L, as illustrated in  FIG. 3A . Thus, the annular projection portion  20  projects from the rear end of the rear annular plate portion  19  in the radial direction R, and an outer peripheral end surface  20   a  of the annular projection portion  20  extends parallel to the axis L. The annular groove  27  includes a rectangular section. The notch  28  is notched downward toward the rear X 2  and has a triangular sectional shape. When the front ring  4  and the rear ring  5  are disposed coaxially, the outer peripheral end surface  20   a  of the annular projection portion  20  of the front ring  4  is positioned flush with an inner peripheral end surface  45   a  of the annular protrusion  45  of the rear ring  5  or positioned slightly closer to the inner peripheral side than the inner peripheral end surface  45   a  is. Consequently, the front ring  4  and the rear ring  5  are brought closer to each other while being maintained coaxially, whereby the annular protrusion  45  of the rear ring  5  can be positioned between the rear annular wall portion  18  of the front ring  4  and the annular projection portion  20 . 
     As illustrated in  FIG. 3B , the annular protrusion  45  of the rear ring  5  is then caused to abut on the rear annular wall portion  18  of the front ring  4  from the rear X 2 . Subsequently, the rear annular plate portion  19  of the front ring  4  is bent (plastically deformed) to the outer peripheral side to displace the annular projection portion  20  to the outer peripheral side. This structure creates the state in which the annular protrusion  45  of the rear ring  5  is inserted into the annular recess  21  of the front ring  4 . 
     When the rear annular plate portion  19  of the front ring  4  is bent to the outer peripheral side, a force exerted from the inner peripheral side to the outer peripheral side in the radial direction R is applied to the rear annular plate portion  19 . As a result, the rear annular plate portion  19  is bent at the portion on the front end side the thickness of which is reduced by the formation of the annular groove  27 , as illustrated in  FIG. 3C . Additionally, the rear annular plate portion  19  is bent to the outer peripheral side, whereby the annular projection portion  20  projects to the outer peripheral side more than the inner peripheral end surface  45   a  of the annular protrusion  45  does. Consequently, the state in which the annular protrusion  45  is inserted into the annular recess  21  is created. The notch  28  serves as a relief part to avoid causing the rear annular plate portion  19  to abut on the annular protrusion  45  when the rear annular plate portion  19  is bent to the outer peripheral side. 
     In the present embodiment, the portion the thickness of which is reduced by the formation of the annular groove  27  is provided to the front end of the rear annular plate portion  19  so as to bend the thin portion to the outer peripheral side. Consequently, the rear annular plate portion  19  can be bent accurately with relatively weak force. This enables the annular projection portion  20  to be displaced accurately to the outer peripheral side, improving the accuracy of dimension between the annular protrusion  45  of the rear ring  5  and the annular recess  21  of the front ring  4 . The annular projection portion  20  of the front ring  4  can therefore be opposed to the annular protrusion  45  of the rear ring  5  at the rear X 2  with a minute spacing G that is set in advance allowed therebetween. In the present embodiment, the spacing G can be between 0.03 mm and 0.1 mm. Consequently, the front ring  4  can be prevented from rattling on the rear ring  5  when the front ring  4  is rotated. The front ring  4  can also be rotated smoothly with respect to the rear ring  5 . 
     Thereafter, the polarization filter  2  is inserted into the inner ring  3  from the rear X 2 , causing the polarization filter  2  to abut on the front stopper  32 . Additionally, an adhesive is injected through the adhesive injection hole  37  to fix the polarization filter  2  to the inner ring  3 . The inner ring  3  is then screwed into the front ring  4  from the front X 1 , causing the rear end of the annular frame  31  of the inner ring  3  to abut on the rear annular wall portion  18  of the front ring  4 . This completes the camera filter unit  1 . In the state in which the camera filter unit  1  is completed, the polarization filter  2  is held by the front ring  4  between the front stopper  32  and the rear annular wall portion  18  in a rotatable manner about the axis L while being fixed to the inner ring  3  unrotatably by the adhesive. 
     An adhesive may be applied to the annular rearward surface  34   a  of the inner ring  3  or the front annular wall portion  16  of the front ring  4  in advance to fix the inner ring  3  to the front ring  4  by the adhesive. 
     (Working Effect) 
     According to the present embodiment, the front ring  4  can rotate about the axis L using the annular protrusion  45  of the rear ring  5  that is inserted into the annular recess  21  as a guide. Consequently, no washer needs to be used to allow the rear ring  5  to hold the front ring  4  rotatably. In the case in which the front ring  4  is rotated using the annular protrusion  45  that is disposed in the rear ring  5  as a guide, the guide (annular protrusion  45 ) does not become distorted, nor does the position of the guide (annular protrusion  45 ) change. The annular protrusion  45  serving as a guide can be formed more accurately than a washer that expands and contracts in the radial direction, for example. Furthermore, because the present embodiment enables the annular projection portion  20  to be displaced accurately to the outer peripheral side, the accuracy of dimension between the annular protrusion  45  of the rear ring  5  and the annular recess  21  of the front ring  4  is high. In addition to this, because no member such as a washer is interposed between the front ring  4  and the rear ring  5 , the front ring  4  and the rear ring  5  can be combined accurately. The front ring  4  held by the rear ring  5  can therefore be rotated smoothly. 
     The annular protrusion  45  of the rear ring  5  is disposed in a position overlapping the external thread  36  when seen from the radial direction R. Consequently, the dimension of the camera filter frame  10  can be reduced in the fore-and-aft direction X compared with the case in which the annular protrusion  45  is formed in a position different from that of the external thread  36  in the direction of the axis L. 
     Furthermore, the polarization filter  2  is held by the front ring  4  between the front stopper  32  and the rear annular wall portion  18  in a rotatable manner about the axis L, and is fixed to the inner ring  3  unrotatably by the adhesive. Consequently, stress (pressure) to be placed on the polarization filter  2  can be reduced compared with the case in which the polarization filter  2  is held to the front ring  4  in an unrotatable manner while being gripped by the front stopper  32  and the rear annular wall portion  18  (rear stopper) from the front and rear. Because distortion can be prevented from occurring in the polarization filter  2  with lower stress placed on the polarization filter  2 , finer images and videos can be obtained when, for example, images and videos the resolution of which is many times higher than full high-definition ones are shot. 
     The adhesive injection hole  37  is formed in the inner ring  3  that is held on the inner peripheral side of the front ring  4 , which prevents the adhesive injection hole  37  from being exposed to the outer peripheral side not to impair the appearance of the camera filter frame  10 . 
     The width dimension W of the filter holding surface  33  can be the same as the thickness dimension D of the polarization filter  2 , and a structure can also be employed in which the polarization filter  2  is gripped unrotatably between the front stopper  32  and the rear annular wall portion  18  when the rear end of the annular frame  31  is caused to abut on the rear annular wall portion  18 . 
     (Second Embodiment) 
       FIG. 4A  is a longitudinal sectional view of a camera filter unit according to a second embodiment.  FIG. 4B  is a partially enlarged sectional view thereof. As illustrated in  FIG. 4A , a camera filter unit  1 A according to the present embodiment includes a disc-like polarization filter (optical element)  2 , a front ring  4  for holding the polarization filter  2  coaxially, and a rear ring  5  for holding the front ring  4  rotatably about the axis L. The camera filter unit  1 A also includes a front stopper  6  that is attached to the inner peripheral surface of the front ring  4  and restricts the movement of the polarization filter  2  to the front X 1 . The front ring  4 , the rear ring  5 , and the front stopper  6  constitute a camera filter frame  10 A. Note that the camera filter unit  1 A according to the present embodiment includes components corresponding to those of the camera filter unit  1 , and thus the same reference numerals are given to the corresponding components for description thereof. 
     As illustrated in  FIG. 4A , on the inner peripheral surface of the front ring  4 , an internal thread  11  is provided in a region of a certain width from the front edge toward the rear X 2 . The front portion of the internal thread  11  that is exposed to the front side more than the front stopper  6  is a front mounting portion for mounting a cap or a hood into the front of the camera filter unit  1 A. On the outer peripheral surface of the rear ring  5 , an external thread  36  is provided in a region of a certain width from the rear edge toward the front X 1 . The external thread  36  is a rear mounting portion for mounting the camera filter unit  1 A into a lens-barrel of an imaging lens or a camera. The front stopper  6 , the front ring  4 , and the rear ring  5  are all formed of a metal base. In the present embodiment, the front stopper  6 , the front ring  4 , and the rear ring  5  are made of aluminum. 
     (Front Ring) 
     As illustrated in  FIG. 4B , the front ring  4  includes a front annular plate portion  15  extending in the fore-and-aft direction X along the axis L, an intermediate annular plate portion (holding portion)  17  that is continuous to the rear end of the front annular plate portion  15  and extends on the inner peripheral side of the front annular wall portion  15  to the rear X 2  along the axis L, a rear annular wall portion (annular wall portion)  18  extending from the rear end of the intermediate annular plate portion  17  toward the inner peripheral side in the radial direction R orthogonal to the axis L, a rear annular plate portion  19  extending from the inner peripheral end of the rear annular wall portion  18  to the rear X 2 , and an annular projection portion  20  projecting for a shorter distance than the rear annular wall portion  18  from the rear end of the rear annular plate portion  19  toward the outer peripheral side. On the outer peripheral surface of the front ring  4 , an annular step  25  is formed by the front annular wall portion  15  and the intermediate annular plate portion  17 . On the outer peripheral surface of the front ring  4 , an annular recess  21  is also formed by the rear annular wall portion  18 , the rear annular plate portion  19 , and the annular projection portion  20 . 
     The internal thread  11  serving as a front mounting portion is formed on the inner peripheral surface of the front annular plate portion  15 . The intermediate annular plate portion  17  has a constant thickness in the radial direction R. The inner peripheral surface of the intermediate annular plate portion  17  (surface positioned between the front end surface of the intermediate annular plate portion  17  and the front end surface of the rear annular wall portion  18 ) is an annular surface that extends with a constant diameter dimension along the axis L, and is a filter holding surface  17   a  for holding the polarization filter  2  coaxially. The filter holding surface  17   a  has a width dimension W larger than a thickness dimension D of the polarization filter  2  held by the filter holding surface  17   a . The difference between the width dimension W of the filter holding surface  17   a  and the thickness dimension D of the polarization filter  2  is 0.03 mm or less. 
     The rear annular wall portion  18  has a constant thickness dimension in the fore-and-aft direction X. The rear annular wall portion  18  functions as a rear stopper that restricts the movement of the polarization filter  2  that is held by the filter holding surface  17   a  to the rear X 2 . 
     The rear annular plate portion  19  inclines to the outer peripheral side toward the rear X 2 . More specifically, the front end of the outer peripheral surface of the rear annular plate portion  19  (portion adjacent to the rear annular wall portion  18 ) is provided with an annular groove  27 , and the portion in which the annular groove  27  is formed is bent to the outer peripheral side so that the rear annular plate portion  19  inclines to the outer peripheral side. The rear portion of the outer peripheral surface of the rear annular plate portion  19  (portion adjacent to the annular projection portion  20 ) is provided with an annular notch  28 . The notch  28  is shallower than the annular groove  27 . 
     The annular projection portion  20  projects in a direction orthogonal to the rear annular plate portion  19 . Consequently, the annular projection portion  20  extends to the front X 1  toward the outer peripheral side. In the present embodiment, the annular projection portion  20  has a trapezoidal sectional shape tapering toward the tip. The inner peripheral portion of an annular protrusion  45  of the rear ring  5  is inserted into the annular recess  21  formed by the rear annular wall portion  18 , the rear annular plate portion  19 , and the annular projection portion  20 . 
     (Front Stopper) 
     The front stopper  6  has a ring shape. The front stopper  6  has a height dimension in the radial direction R larger than a height dimension of the intermediate annular plate portion  17  of the front ring in the radial direction R (thickness of the intermediate annular plate portion  17 ). The front stopper  6  also includes an annular rear end surface  6   a  that is a flat surface orthogonal to the axis L. On the entire outer peripheral surface of the front stopper  6 , an external thread  51  is formed that can be threadedly engaged with the internal thread  11  of the front ring  4 . The external thread  51  of the front stopper  6  is threadedly engaged with the internal thread  11  of the front ring  4  to be screwed until the annular rear end surface  6   a  of the front stopper  6  abuts on the front end surface of the intermediate annular plate portion  17 . In the state in which the front stopper  6  abuts on the intermediate annular plate portion  17 , the inner peripheral end of the front stopper  6  projects to the inner peripheral side more than the filter holding surface  17   a  does. Consequently, the front stopper  6  can restrict the movement of the polarization filter  2  that is held by the filter holding surface  17   a  to the front X 1 . 
     (Rear Ring) 
     The front ring  4  includes a front annular plate portion (annular portion)  41  extending in the fore-and-aft direction X along the axis L, an annular wall portion  42  extending from the rear end of the front annular plate portion  41  toward the inner peripheral side in the radial direction R, and a rear annular plate portion  43  extending from the inner peripheral end of the annular wall portion  42  to the rear X 2  along the axis L. The front annular plate portion  41  has a constant thickness dimension in the radial direction R. A grease holding recess  44  is formed on the front end surface of the annular wall portion  42 . The grease holding recess  44  holds grease to slide the front ring  4  and the rear ring  5  smoothly. The external thread  12  serving as a rear mounting portion is formed on the outer peripheral surface of the rear annular plate portion  43 . 
     The annular protrusion  45  that protrudes toward the inner peripheral side in the radial direction R is disposed on the front end of the inner peripheral surface of the rear annular plate portion  43 . The annular protrusion  45  includes a rectangular sectional shape. The front end surface of the annular protrusion  45  is steplessly continuous to the front end surface of the annular wall portion  42 . The annular protrusion  45  has a width dimension in the fore-and-aft direction X larger than the protrusion dimension to the inner peripheral side. The annular protrusion  45  is formed in a position partially overlapping the external thread  12  when seen from the radial direction R. 
     With the rear ring  5  holding the front ring  4 , the front annular plate portion  41  is fitted into the annular step  25  of the front ring  4  in a frictionally movable manner. This structure allows the outer peripheral side  41   a  of the front annular plate portion  41  to be steplessly continuous to the outer peripheral side  15   a  of the front annular plate portion  15  of the front ring  4 . The rear annular plate portion  43  abuts on the rear end of the intermediate annular plate portion  17  of the front ring  4  from the rear X 2 . The annular protrusion  45  abuts on the rear end of the rear annular wall portion  18  of the front ring  4  from the rear X 2 . The inner peripheral portion of the annular protrusion  45  is inserted into the annular recess  21  cut on the outer peripheral surface of the front stopper  6 . This structure allows the rear ring  5  to hold the front ring  4  rotatably about the axis L. 
     (Assembly of Camera Filter Unit) 
     In order to assemble the camera filter unit  1 A, the front ring  4  and the rear ring  5  are first combined with each other coaxially in the fore-and-aft direction X. The annular protrusion  45  of the rear ring  5  is then inserted into the annular recess  21  on the outer peripheral surface of the front ring  4 . The assembling operation of inserting annular protrusion  45  of the rear ring  5  into the annular recess  21  on the outer peripheral surface of the front ring  4  is the same as that for the camera filter frame  10  according to the first embodiment. The description thereof will thus be omitted. 
     Thereafter, the polarization filter  2  is inserted into the front ring  4  from the front X 1 , causing the filter holding surface  17   a  to hold the polarization filter  2 . The front stopper  6  is then screwed into the front ring  4  from the front X 1 , causing the annular rear end surface  6   a  of the front stopper  6  to abut on the intermediate annular plate portion  17  of the front ring  4 . This completes the camera filter unit  1 A. In the state in which the camera filter unit  1 A is completed, the polarization filter  2  is held by the front ring  4  between the front stopper  6  and the rear annular wall portion  18  in a rotatable manner about the axis L. 
     An adhesive may be applied to the annular rear end surface  6   a  of the front stopper  6  or the front end surface of the intermediate annular plate portion  17  of the front ring  4  in advance to fix the front stopper  6  to the front ring  4  by the adhesive. 
     (Working Effect) 
     According to the present embodiment, the front ring  4  can rotate about the axis L using the annular protrusion  45  of the rear ring  5  that is inserted into the annular recess  21  as a guide. Consequently, no washer needs to be used to allow the rear ring  5  to hold the front ring  4  rotatably. In the case in which the front ring  4  is rotated using the annular protrusion  45  that is disposed in the rear ring  5  as a guide, the guide (annular protrusion  45 ) does not become distorted, nor does the position of the guide (annular protrusion  45 ) change. The annular protrusion  45  serving as a guide can be formed more accurately than a washer that expands and contracts in the radial direction, for example. Furthermore, because the present embodiment enables the annular projection portion  20  to be displaced accurately to the outer peripheral side, the accuracy of dimension between the annular protrusion  45  of the rear ring  5  and the annular recess  21  of the front ring  4  is high. In addition to this, because no member such as a washer is interposed between the front ring  4  and the rear ring  5 , the front ring  4  and the rear ring  5  can be combined accurately. The front ring  4  held by the rear ring  5  can therefore be rotated smoothly. 
     Because the annular protrusion  45  of the rear ring  5  is disposed in a position overlapping the external thread  51  when seen from the radial direction R, the dimension of the camera filter frame  10 A can be reduced in the fore-and-aft direction X compared with the case in which the annular protrusion  45  is formed in a position different from that of the mounting portion in the direction of the axis L. 
     Furthermore, the front ring  4  holds the polarization filter  2  rotatably about the axis L between the front stopper  6  and the rear annular wall portion  18 . Consequently, stress (pressure) to be placed on the polarization filter  2  can be reduced compared with the case in which the polarization filter  2  is gripped by the front stopper  6  and the rear annular wall portion  18  (rear stopper) from the front and rear to be unrotatable, which can prevent distortion from occurring in the polarization filter  2 . 
     As shown by the dotted lines in  FIG. 4B , in the intermediate annular plate portion  17  of the front ring  4 , an adhesive injection hole  37  can be provided that passes through the intermediate annular plate portion in the direction intersecting the axis L to be open on the filter holding surface  17   a . In the case in which the adhesive injection hole  37  is provided, the polarization filter  2  can be fixed to the front ring  4  by injecting an adhesive through the adhesive injection hole  37 . The intermediate annular plate portion  17  is positioned on the inner peripheral side of the front annular plate portion  41  in the rear ring  5 . Consequently, even in the case in which the adhesive injection hole  37  is formed in the intermediate annular plate portion  17 , the adhesive injection hole  37  is not exposed to the outer peripheral side not to impair the appearance of the camera filter frame  10 . 
     The width dimension W of the filter holding surface  17   a  can be the same as the thickness dimension D of the polarization filter  2 , and a structure can also be employed in which the polarization filter  2  is gripped unrotatably between the front stopper  6  and the rear annular wall portion  18 . 
     (Third Embodiment) 
       FIG. 5A  is a longitudinal sectional view of a camera filter unit according to a third embodiment.  FIG. 5B  is a partially enlarged sectional view thereof. A camera filter unit  1 B according to the present embodiment includes a disc-like polarization filter (optical element)  2 , an inner ring  3  for holding the polarization filter  2  coaxially, a front ring  4  for holding the inner ring  3  coaxially from the outer peripheral side, and a rear ring  5  for holding the front ring  4  coaxially. The camera filter unit  1 B according to the present embodiment also includes a disc-like second polarization filter (second optical element)  60 , and a rear inner ring  61  that is inserted into the rear ring  5  from the rear to hold the second polarization filter  60  coaxially. The rear inner ring  61  is made of aluminum. 
     The front ring  4  holds the polarization filter  2  via the inner ring  3 . The rear ring  5  holds the second polarization filter  60  via the rear inner ring  61 . The rear ring  5  also holds the front ring  4  rotatably about the axis L. The inner ring  3 , the front ring  4 , the rear ring  5 , and the rear inner ring  61  constitute a camera filter frame  10 B. 
     In the camera filter unit  1 B according to the present embodiment, a holding structure for the polarization filter  2  in which the front ring  4  holds the polarization filter  2  is the same as that in the camera filter unit  1  according to the first embodiment. A holding structure for the front ring  4  in which the rear ring  5  holds the front ring  4  rotatably about the axis L is also the same as those in the camera filter units  1  and  1 A according to the first and the second embodiments, respectively. The following thus describes a holding structure in which the rear ring  5  holds the second polarization filter  60 . The same reference numerals are given to components in common with those of the camera filter unit  1  according to the first embodiment and description thereof will be omitted. In the camera filter unit  1 B according to the present embodiment, the holding structure for the polarization filter  2  in which the front ring  4  holds the polarization filter  2  is the same as the holding structure in the camera filter unit  1 A according to the second embodiment. 
     As illustrated in  FIGS. 5A and 5B , the rear ring  5  includes a second annular projection portion (a second annular projection portion and a second front stopper)  64  that projects to the inner peripheral side closer to the rear X 2  than the front ring  4  does. The rear ring  5  also includes a thick portion  65  and an internal thread  66  to the rear side of the second annular projection portion  64 . More specifically, a rear annular plate portion  43  of the rear ring  5  extends to the rear X 2  longer than those in the camera filter units  1  and  1 A according to the first and the second embodiments do, and includes the second annular projection portion  64 , the thick portion  65 , and the internal thread  66  on the inner peripheral surface thereof. 
     The second annular projection portion  64  projects toward the inner peripheral side in the radial direction R in a position that is separated from an annular protrusion  45  to the rear X 2 . The position of an annular projection portion  20  of the front ring  4  is between the annular protrusion  45  and the second annular projection portion  64  in the fore-and-aft direction X. The thick portion  65  extends from a midway position of the second annular projection portion  64  in the radial direction R to the rear X 2  with a constant thickness. The second annular projection portion  64  includes a first annular rearward surface  64   a  on the outer peripheral side of the thick portion  65 . The internal thread  66  is provided continuously to the rear X 2  of the thick portion  65 . The thick portion  65  includes a second annular rearward surface  65   a  between the thick portion  65  and the internal thread  66 . An external thread  12  serving as a rear mounting portion is provided on the outer peripheral surface of the rear end of the rear ring  5  (outer peripheral surface of the rear end of the rear annular plate portion  43 ). 
     The rear inner ring  61  includes a rear annular frame  70  for holding the second polarization filter  60  coaxially from the outer peripheral side, and a second rear stopper (second rear stopper)  71  that projects from the rear edge of the rear annular frame  70  to the inner peripheral side to restrict the movement of the second polarization filter  60  to the rear X 2 . The second rear stopper  71  is abuttable from the rear X 2  on the outer peripheral edge of the second polarization filter  60  that is held by the rear annular frame  70 . The inner peripheral surface of the rear annular frame  70  is an annular surface that extends with a constant diameter dimension along the axis L, and is a rear filter holding surface  72  for holding the second polarization filter  60  coaxially. 
     The rear inner ring  61  includes a thin ring portion  73  and a thick ring portion  74  in this order from the front X 1  toward the rear X 2 . The outer peripheral surface of the thick ring portion  74  is positioned closer to the outer peripheral side than the outer peripheral surface of the thin ring portion  73  is, and between these portions, an annular forward surface  74   a  is formed that extends along the radial direction R. On the outer peripheral surface of the thick ring portion  74 , an external thread  75  is formed that can be threadedly engaged with the internal thread  66  of the rear ring  5 . In the thin ring portion  73 , a rear adhesive injection hole  76  is formed that passes through the thin ring portion in the radial direction R to be open on the rear filter holding surface  72 . If an adhesive is injected from the outer peripheral side of the rear annular frame  70  through the rear adhesive injection hole  76  with the second polarization filter  60  held to the rear filter holding surface  72 , the adhesive enables the second polarization filter  60  to be fixed to the rear inner ring  61 . The number of rear adhesive injection holes  76  may be one, but a plurality of rear adhesive injection holes  76  are open on the thin ring portion  73  at equal angular intervals in the present embodiment. The rear adhesive injection hole  76  may also be omitted. 
     The second rear stopper  71  is an annular protrusion that protrudes from the rear edge of the rear annular frame  70  toward the inner peripheral side. The second rear stopper  71  includes an annular front end surface  71   a  that is a flat surface orthogonal to the axis L. The second rear stopper  71  has a width dimension from the annular front end surface  71   a  to a front end  70   a  of the rear annular frame  70  (width dimension of the rear filter holding surface  72 ) W 2  larger than a thickness dimension D 2  of the second polarization filter  60  held by the filter holding surface  33 . The difference between the width dimension W 2  of the rear filter holding surface  72  in the fore-and-aft direction X and the thickness dimension D 2  of the second polarization filter  60  is 0.03 mm or less. 
     The rear inner ring  61  is inserted from the rear side into the rear ring  5  with the second polarization filter  60  held to the inner peripheral side of the rear annular frame  70 . The external thread  75  of the rear inner ring  61  is then threadedly engaged with the internal thread  66  of the rear ring  5  to be screwed until the front end  70   a  of the rear annular frame  70  abuts on the second annular projection portion  64  (first annular rearward surface  64   a ) of the rear ring  5 . When the front end  70   a  of the rear annular frame  70  abuts on the second annular projection portion  64 , the annular forward surface  74   a  of the rear inner ring  61  faces the second annular rearward surface  65   a  of the rear ring  5  in the fore-and-aft direction X with a minute spacing allowed therebetween. The outer peripheral surface of the thin ring portion  73  of the rear annular frame  70  also faces the thick portion  65  of the rear ring  5  in the radial direction R with a minute spacing allowed therebetween. 
     With the front end  70   a  of the rear annular frame  70  abutting on the second annular projection portion  64 , the distance (width dimension of the rear filter holding surface  72 ) W 2  between the second rear stopper  71  and the second annular projection portion  64  is larger than the thickness dimension D 2  of the second polarization filter  60 . Consequently, the rear ring  5  holds the second polarization filter  60  rotatably about the axis L between the second rear stopper  71  and the second annular projection portion  64 . Specifically, the second rear stopper  71  and the second annular projection portion  64  do not grip the second polarization filter  60  unrotatably. The second polarization filter  60  is fixed to the rear inner ring  61  unrotatably by the adhesive. The second annular projection portion  64  functions as a second front stopper that prevents the second polarization filter  60  from moving to the front X 1 . 
     When the front end  70   a  of the rear annular frame  70  abuts on the second annular projection portion  64  of the rear ring  5  by screwing the rear inner ring  61  into the rear ring  5 , the annular forward surface  74   a  of the rear inner ring  61  may abut on the second annular rearward surface  65   a  of the rear ring  5 . 
     (Working Effect) 
     Also in the present embodiment, the working effect similar to the camera filter units  1  and  1 A according to the first and the second embodiments can be obtained. In the present embodiment, the second polarization filter  60  is also held to the rear ring  5 . In the present embodiment, two optical filters (the polarization filter  2  and the second polarization filter  60 ) can thus be held in the camera filter frame  10 B. Furthermore, the rear ring  5  holds the second polarization filter  60  rotatably about the axis L between the second rear stopper  71  and the second annular projection portion  64 . Consequently, stress (pressure) to be placed on the second polarization filter  60  can be reduced compared with the case in which the second polarization filter  60  is gripped by the second rear stopper  71  and the second annular projection portion  64  from the front and rear to be unrotatable. Therefore, distortion can be prevented from occurring in the second polarization filter  60 . 
     The rear adhesive injection hole  76  is formed in the rear inner ring  61  that is held on the inner peripheral side of the rear ring  5 , which prevents the rear adhesive injection hole  76  from being exposed to the outer peripheral side not to impair the appearance of the camera filter frame  10 B. 
     The width dimension W 2  of the rear filter holding surface  72  can be the same as the thickness dimension D 2  of the second polarization filter  60 , and a structure can also be employed in which the second polarization filter  60  is gripped unrotatably between the second rear stopper  71  and the second annular projection portion  64 . 
     The internal thread  66  of the rear ring  5  and the external thread  75  of the rear inner ring  61  may be omitted and the rear inner ring  61  may be fixed to the inner peripheral side of the rear ring  5  by the adhesive. 
     Furthermore, the optical elements held by the front ring  4  and the rear ring  5  are not limited to polarization filters. For example, a polarization filter can be held to the front ring  4  and a color filter can be held to the rear ring  5 . Alternatively, for example, a cross filter can be held to the front ring  4  and a color filter can be held to the rear ring  5 . 
     (Fourth Embodiment) 
       FIG. 6A  is a longitudinal sectional view of a camera filter unit according to a fourth embodiment.  FIG. 6B  is a partially enlarged sectional view thereof. A camera filter unit  1 C according to the present embodiment includes a disc-like polarization filter (optical element)  2 , an inner ring  3  for holding the polarization filter  2  coaxially, a front ring  4  for holding the inner ring  3  coaxially from the outer peripheral side, and a rear ring  5  for holding the front ring  4  coaxially. The camera filter unit  1 C according to the present embodiment also includes a disc-like second polarization filter (second optical element)  60 , and a rear inner ring  81  that is inserted into the rear ring  5  from the rear X 2  to hold the second polarization filter  60  coaxially. The rear inner ring  81  is made of aluminum. 
     The front ring  4  holds the polarization filter  2  via the inner ring  3 . The rear ring  5  holds the second polarization filter  60  via the rear inner ring  81 . The rear ring  5  also holds the front ring  4  rotatably about the axis L. The inner ring  3 , the front ring  4 , the rear ring  5 , and the rear inner ring  81  constitute a camera filter frame  10 C. 
     In the camera filter unit  1 C according to the present embodiment, a holding structure for the polarization filter  2  in which the front ring  4  holds the polarization filter  2  is the same as that in the camera filter unit  1  according to the first embodiment. A holding structure for the front ring  4  in which the rear ring  5  holds the front ring  4  rotatably about the axis L is also the same as those in the camera filter units  1  and  1 A according to the first and the second embodiments, respectively. The following thus describes a holding structure in which the rear ring  5  holds the second polarization filter  60 . The same reference numerals are given to components in common with those of the camera filter unit  1  according to the first embodiment and description thereof will be omitted. In the camera filter unit  1 C according to the present embodiment, the holding structure for the polarization filter  2  in which the front ring  4  holds the polarization filter  2  is the same as the holding structure in the camera filter unit  1 A according to the second embodiment. 
     As illustrated in  FIGS. 6A and 6B , the rear ring  5  includes a second annular projection portion (a second annular projection portion and a second front stopper)  84  that projects to the inner peripheral side closer to the rear X 2  than the front ring  4  does. The rear ring  5  also includes an internal thread  85  to the rear side of the second annular projection portion  84 . More specifically, a rear annular plate portion  43  of the rear ring  5  extends to the rear X 2  longer than those in the camera filter units  1  and  1 A according to the first and the second embodiments do, and includes the second annular projection portion  84  and the internal thread  85  on the inner peripheral surface thereof. 
     The second annular projection portion  84  projects toward the inner peripheral side in the radial direction R in a position that is separated from an annular protrusion  45  to the rear X 2 . The position of an annular projection portion  20  of the front ring  4  is between the annular protrusion  45  and the second annular projection portion  84  in the fore-and-aft direction X. The internal thread  85  is provided continuously to the rear X 2  of the second annular projection portion  84 . The second annular projection portion  84  includes an annular rearward surface  84   a  between the second annular projection portion  84  and the internal thread  85 . An external thread  12  serving as a rear mounting portion is provided on the outer peripheral surface of the rear end of the rear ring  5  (outer peripheral surface of the rear end of the rear annular plate portion  43 ). 
     The rear inner ring  81  includes a rear annular frame  86  for holding the second polarization filter  60  coaxially from the outer peripheral side, and a second rear stopper (second rear stopper)  87  that projects from the rear edge of the rear annular frame  86  to the inner peripheral side to restrict the movement of the second polarization filter  60  to the rear X 2 . The second rear stopper  87  is abuttable from the rear X 2  on the outer peripheral edge of the second polarization filter  60  that is held by the rear annular frame  86 . The inner peripheral surface of the rear annular frame  86  is an annular surface that extends with a constant diameter dimension along the axis L, and is a rear filter holding surface  88  for holding the second polarization filter  60  coaxially. On the outer peripheral surface of the rear inner ring  81 , an external thread  89  is formed that can be threadedly engaged with the internal thread  85  of the rear ring  5 . 
     In the rear annular frame  86 , a rear adhesive injection hole  90  is formed that passes through the rear annular frame in the radial direction R to be open on the rear filter holding surface  88 . If an adhesive is injected from the outer peripheral side of the rear annular frame  86  through the rear adhesive injection hole  90  with the second polarization filter  60  held to the rear filter holding surface  88 , the adhesive enables the second polarization filter  60  to be fixed to the rear inner ring  81 . The number of rear adhesive injection holes  90  may be one, but a plurality of rear adhesive injection holes  90  are open on the thin ring portion  73  at equal angular intervals in the present embodiment. The rear adhesive injection hole  90  may also be omitted. 
     The second rear stopper  87  is an annular protrusion that protrudes from the rear edge of the rear annular frame  86  toward the inner peripheral side. The second rear stopper  87  includes an annular front end surface  87   a  that is a flat surface orthogonal to the axis L. The second rear stopper  87  has a width dimension from the annular front end surface  87   a  to a front end  86   a  of the rear annular frame  86  (width dimension of the rear filter holding surface  88 ) W 2  larger than a thickness dimension D 2  of the second polarization filter  60  held by the filter holding surface  33 . The difference between the width dimension W 2  of the rear filter holding surface  88  in the fore-and-aft direction X and the thickness dimension D 2  of the second polarization filter  60  is 0.03 mm or less. 
     The rear inner ring  81  is inserted from the rear side into the rear ring  5  with the second polarization filter  60  held to the inner peripheral side of the rear annular frame  86 . The external thread  89  of the rear inner ring  81  is then threadedly engaged with the internal thread  85  of the rear ring  5  to be screwed until the front end  86   a  of the rear annular frame  86  abuts on the second annular projection portion  84  (annular rearward surface  84   a ) of the rear ring  5 . 
     With the front end  86   a  of the rear annular frame  86  abutting on the second annular projection portion  84 , the distance (width dimension of the rear filter holding surface  95 ) W 2  between the second rear stopper  87  and the second annular projection portion  84  is larger than the thickness dimension D 2  of the second polarization filter  60 . Consequently, the rear ring  5  holds the second polarization filter  60  rotatably about the axis L between the second rear stopper  87  and the second annular projection portion  84 . Specifically, the second rear stopper  87  and the second annular projection portion  84  do not grip the second polarization filter  60  unrotatably. The second polarization filter  60  is fixed to the rear inner ring  81  unrotatably by the adhesive. The second annular projection portion  84  functions as a second front stopper that prevents the second polarization filter  60  from moving to the front X 1 . 
     (Working Effect) 
     Also in the present embodiment, the working effect similar to the camera filter units  1  and  1 A according to the first and the second embodiments can be obtained. In the present embodiment, the second polarization filter  60  is also held to the rear ring  5 . Specifically, two optical filters can be held in the camera filter frame  10 C in the present embodiment. Furthermore, the rear ring  5  holds the second polarization filter  60  rotatably about the axis L between the second rear stopper  87  and the second annular projection portion  84 . Consequently, stress (pressure) to be placed on the second polarization filter  60  can be reduced compared with the case in which the second polarization filter  60  is gripped by the second rear stopper  87  and the second annular projection portion  84  from the front and rear to be unrotatable. Therefore, distortion can be prevented from occurring in the second polarization filter  60 . 
     The rear adhesive injection hole  90  is formed in the rear inner ring  81  that is held on the inner peripheral side of the rear ring  5 , which prevents the rear adhesive injection hole  90  from being exposed to the outer peripheral side not to impair the appearance of the camera filter frame  10 C. 
     The width dimension W 2  of the rear filter holding surface  88  can be the same as the thickness dimension D 2  of the second polarization filter  60 , and a structure can also be employed in which the second polarization filter  60  is gripped unrotatably between the second rear stopper  87  and the second annular projection portion  84 . The rear inner ring  81  may be fixed to the inner peripheral side of the rear ring  5  by the adhesive. Furthermore, the optical elements held by the front ring  4  and the rear ring  5  are not limited to polarization filters. 
     (Fifth Embodiment) 
       FIG. 7A  is a longitudinal sectional view of a camera filter unit according to a fifth embodiment.  FIG. 7B  is a partially enlarged sectional view thereof. A camera filter unit  1 D according to the present embodiment includes a disc-like polarization filter (optical element)  2 , an inner ring  3  for holding the polarization filter  2  coaxially, a front ring  4  for holding the inner ring  3  coaxially from the outer peripheral side, and a rear ring  5  for holding the front ring  4  coaxially. The camera filter unit  1 D according to the present embodiment also includes a disc-like second polarization filter (second optical element)  60 , and a second rear stopper  91  that is inserted into the rear ring  5  from the rear X 2  to restrict the movement of the second polarization filter  60  to the rear X 2 . The second rear stopper  91  is made of aluminum. 
     The front ring  4  holds the polarization filter  2  via the inner ring  3 . The rear ring  5  holds the second polarization filter  60 . The rear ring  5  also holds the front ring  4  rotatably about the axis L. The inner ring  3 , the front ring  4 , the rear ring  5 , and the second rear stopper  91  constitute a camera filter frame  10 D. 
     In the camera filter unit  1 D according to the present embodiment, a holding structure for the polarization filter  2  in which the front ring  4  holds the polarization filter  2  is the same as that in the camera filter unit  1  according to the first embodiment. A holding structure for the front ring  4  in which the rear ring  5  holds the front ring  4  rotatably about the axis L is also the same as those in the camera filter units  1  and  1 A according to the first and the second embodiments, respectively. The following thus describes a holding structure in which the rear ring  5  holds the second polarization filter  60 . The same reference numerals are given to components in common with those of the camera filter unit  1  according to the first embodiment and description thereof will be omitted. In the camera filter unit  1 D according to the present embodiment, the holding structure for the polarization filter  2  in which the front ring  4  holds the polarization filter  2  is the same as the holding structure in the camera filter unit  1 A according to the second embodiment. 
     As illustrated in  FIGS. 7A and 7B , the rear ring  5  includes a second annular projection portion (a second annular projection portion and a second front stopper)  92  that projects to the inner peripheral side closer to the rear X 2  than the front ring  4  does. The rear ring  5  also includes a thick portion  93  and an internal thread  94  to the rear side of the second annular projection portion  92 . More specifically, a rear annular plate portion  43  of the rear ring  5  extends to the rear X 2  longer than those in the camera filter units  1  and  1 A according to the first and the second embodiments do, and includes the second annular projection portion  92 , the thick portion  93 , and the internal thread  94  on the inner peripheral surface thereof. 
     The second annular projection portion  92  projects toward the inner peripheral side in the radial direction R in a position that is separated from an annular protrusion  45  to the rear X 2 . The position of an annular projection portion  20  of the front ring  4  is between the annular protrusion  45  and the second annular projection portion  92  in the fore-and-aft direction X. The thick portion  93  extends from a midway position of the second annular projection portion  92  in the radial direction R to the rear X 2  with a constant thickness. The second annular projection portion  92  includes a first annular rearward surface  92   a  on the outer peripheral side of the thick portion  93 . 
     The thick portion  93  is a rear holding portion for holding the second polarization filter  60  coaxially from the outer peripheral side. Specifically, the inner peripheral surface of the thick portion  93  is an annular surface that extends with a constant diameter dimension along the axis L, and is a rear filter holding surface  95  for holding the second polarization filter  60  coaxially. The rear filter holding surface  95  has a width dimension W 2  larger than a thickness dimension D 2  of the second polarization filter  60  held by the rear filter holding surface  95 . The difference between the width dimension W 2  of the rear filter holding surface  95  and the thickness dimension D 2  of the second polarization filter  60  is 0.03 mm or less. 
     The internal thread  94  is provided continuously to the rear X 2  of the thick portion  93 . The thick portion  93  includes a second annular rearward surface  93   a  between the thick portion  93  and the internal thread  94 . An external thread  12  serving as a rear mounting portion is provided on the outer peripheral surface of the rear end of the rear ring  5  (outer peripheral surface of the rear end of the rear annular plate portion  43 ). 
     The second rear stopper  91  has a ring shape. The second rear stopper  91  has a height dimension in the radial direction R larger than a height dimension of the thick portion  93  of the rear ring  5  in the radial direction R (thickness of the thick portion  93 ). The second rear stopper  91  also includes an annular front end surface  91   a  that is a flat surface orthogonal to the axis L. On the entire outer peripheral surface of the second rear stopper  91 , an external thread  96  is formed that can be threadedly engaged with the internal thread  94  of the rear ring  5 . The external thread  96  of the second rear stopper  91  is threadedly engaged with the internal thread  94  of the rear ring  5  to be screwed until the annular front end surface  91   a  of the second rear stopper  91  abuts on the second annular rearward surface  93   a  of the thick portion  93 . In the state in which the second rear stopper  91  abuts on the thick portion  93 , the inner peripheral end of the second rear stopper  91  projects to the inner peripheral side more than the rear filter holding surface  95  does. Consequently, the second rear stopper  91  can restrict the movement of the second polarization filter  60  that is held by the rear filter holding surface  95  to the rear X 2 . 
     In order for the rear ring  5  to hold the second polarization filter  60 , the second polarization filter  60  is first inserted into the rear ring  5  from the rear X 2  to cause the rear filter holding surface  95  to hold the second polarization filter  60 . The second rear stopper  91  is then screwed into the rear ring  5  from the rear X 2  to cause the annular front end surface  91   a  of the second rear stopper  91  to abut on the second annular rearward surface  93   a  of the thick portion  93 . This completes the camera filter unit  1 D. In the state in which the camera filter unit  1 D is completed, the second polarization filter  60  is held by the rear ring  5  between the second rear stopper  91  and the second annular projection portion  92  in a rotatable manner about the axis L. The second annular projection portion  92  functions as a second front stopper that prevents the second polarization filter  60  from moving to the front X 1 . 
     (Working Effect) 
     Also in the present embodiment, the working effect similar to the camera filter units  1  and  1 A according to the first and the second embodiments can be obtained. In the present embodiment, the second polarization filter  60  is also held to the rear ring  5 . Specifically, two optical filters can be held in the camera filter frame  10 C in the present embodiment. Furthermore, the rear ring  5  holds the second polarization filter  60  rotatably about the axis L between the second rear stopper  91  and the second annular projection portion  92 . Consequently, stress (pressure) to be placed on the second polarization filter  60  can be reduced compared with the case in which the second polarization filter  60  is gripped by the second rear stopper  91  and the second annular projection portion  92  from the front and rear to be unrotatable. Therefore, distortion can be prevented from occurring in the second polarization filter  60 . 
     The width dimension W 2  of the rear filter holding surface  95  can be the same as the thickness dimension D 2  of the second polarization filter  60 , and a structure can also be employed in which the second polarization filter  60  is gripped unrotatably between the second rear stopper  91  and the second annular projection portion  92 . The second rear stopper  91  may be fixed to the inner peripheral side of the rear ring  5  by the adhesive. Furthermore, the optical elements held by the front ring  4  and the rear ring  5  are not limited to polarization filters. 
     (Other Embodiments) 
     Although the annular groove  27  is cut on the outer peripheral surface of the rear annular plate portion  19  of the front ring  4  in the above embodiment, an annular groove may be cut on the inner peripheral surface of the rear annular plate portion  19 , and the rear annular plate portion  19  may be bent from the position in which the annular groove is formed to the outer peripheral side. Alternatively, annular grooves may be cut in respective positions on the inner peripheral surface and the outer peripheral surface of the rear annular plate portion  19  overlapping with each other when seen from the radial direction R, and the rear annular plate portion  19  may be bent from the positions in which the annular grooves are formed to the outer peripheral side. Furthermore, the annular groove  27  and the notch  28  may be omitted. 
     A cross filter can also be held to the front ring  4  as an optical element. Furthermore, a color filter may be held to the front ring  4  and a polarization filter or a cross filter may be held to the rear ring  5 . Alternatively, a color filter may be held to each of the front ring  4  and the rear ring  5 .