Abstract:
A lens device in which a first movable lens holding frame and a second movable lens holding frame are moved along an optical axis by rotating a cam cylinder rotatably provided on a lens barrel, an escape channel for escapement of a portion of the second movable lens holding frame being formed in the first movable lens holding frame, the lens device comprising:
       a light shielding member moved along the optical axis by the rotation of the cam cylinder, the light shielding member being moved to a position at which the light shielding member closes the escape channel of the first movable lens holding frame.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a lens device and, more particularly, to a lens device in which a first movable lens holding frame and a second movable lens holding frame are moved forward or rearward along an optical axis by rotating a cam cylinder to perform zooming. 
   2. Description of the Related Art 
   In zoom lens devices for electric news gathering (ENG) cameras, a type of zoom lens device is dominant in which zooming is performed by two movable lens group: a first movable lens group functioning as a variable-power lens group, and a second movable lens functioning as a focus correcting lens group. A cam cylinder is rotatably provided in the lens barrel of such a zoom lens device. In the cam cylinder are formed cam channels engaged with cam rollers projecting from a frame for holding the first movable lens group (hereinafter referred to as “first movable lens holding frame”) and a frame for holding the second movable lens group (hereinafter referred to as “second movable lens holding frame”). These cam rollers are passed through the cam channels to be engaged with straight-travel grooves formed in a fixed cylinder placed outside the cam cylinder. When the cam cylinder is rotated, the first and second movable lens holding frames are moved forward or rearward in predetermined paths along the optical axis by the straight-travel guide function of the cam channel and the straight-travel grooves, thereby adjusting the focal length. 
   On the other hand, a zoom lens device is known which is designed to reduce its overall size by reducing the lens optical path length while maintaining the desired image-taking power. In this zoom lens device, when the distance between the first and second movable lens holding frames is minimized, escape channels for escapement of the cam rollers on the second movable lens holding frame are formed in the first movable lens holding frame to set a longer stroke of movement between the first and second movable lens holding frames, thereby ensuring the desired image-taking power. 
   In the case where such escape channels are formed in the first movable lens holding frame, there is no problem when the cam rollers on the second movable lens holding frame are positioned in the escape channels. However, when the cam rollers are retreated from the escape channels, detrimental rays (stray light) enter the camera through the escape channels to generate ghost. 
   To solve this problem, a light shielding member for closing the escape channels of the first movable lens holding frame is provided in a lens device disclosed in Japanese Patent Application Laid-Open No. 2003-248157. The light shielding member prevents detrimental rays from entering the camera through the escape channels. When the distance between the first and second movable lens holding frames is minimized, the light shielding member is retreated from the escape channels by being forced by the cam rollers on the second movable lens holding frame. When the cam rollers are retreated from the escape channels, the light shielding member is returned by an urging force of a spring to the position at which it closes the escape channels. 
   In the lens device disclosed in Japanese Patent Application Laid-Open No. 2003-248157, however, impact sound and vibration are generated when the cam rollers on the second movable lens holding frame collides against the light shielding member. A cameraperson operating the lens device has a feeling of strangeness from the impact sound and vibration. 
   SUMMARY OF THE INVENTION 
   In view of the above-described circumstances, an object of the present invention is to provide a lens device in which an escape channel is formed in a movable lens holding frame, and which is capable of preventing detrimental rays from entering through the escape channel, without generating impact sound and vibration. 
   To achieve the above-described object, according to a first aspect of the present invention, there is provided a lens device in which a first movable lens holding frame and a second movable lens holding frame are moved along an optical axis by rotating a cam cylinder rotatably provided on a lens barrel, an escape channel for escapement of a portion of the second movable lens holding frame being formed in the first movable lens holding frame, the lens device having a light shielding member moved along the optical axis by the rotation of the cam cylinder, the light shielding member being moved to a position at which the light shielding member closes the escape channel of the first movable lens holding frame. 
   According to the first aspect of the present invention, the cam cylinder that moves the first and second movable lens holding frames moves the light shielding member to close the escape channel of the first movable lens holding frame, thereby preventing detrimental rays from entering through the escape channels, without generating impact sound and vibration. That is, when the first movable lens holding frame is moved to such a position that the portion of the second movable lens holding frame is retreated from the escape channel of the first movable lens holding frame, and that detrimental rays can enter through the escape channel, the light shielding member also moved by the cam cylinder is placed at the position at which it closes the escape channel. Also, since according to the first aspect of the present invention the light shielding member is moved by the common cam cylinder, the mechanism for moving the light shielding member can be constructed without largely increasing the number of component parts. 
   To achieve the above-described object, according to a second aspect of the present invention, there is provided a lens device having a lens barrel, a cam cylinder rotatably provided in the lens barrel, a first movable lens holding frame engaged with a first cam channel of the cam cylinder by means of a first cam follower, the first movable lens holding frame having an escape channel formed in its outer peripheral portion, a second movable lens holding frame engaged with a second cam channel of the cam cylinder by means of a second cam follower, the second cam follower being positioned in the escape channel of the first movable lens holding frame when the second movable lens holding frame is brought close to the first movable lens holding frame, and a light shielding member engaged with a third cam channel of the cam cylinder by means of a third cam follower, the moved light shielding member being moved to a position at which the light shielding member closes the escape channel of the first movable lens holding frame. 
   The lens device in the second aspect of the present invention is a more concrete form of the lens device in the first aspect of the invention. According to the second aspect of the invention, an escape channel in which the second cam follower of the second movable lens holding frame is positioned is formed in an outer peripheral portion of the first movable lens holding frame, and is closed by the light shielding member. That is, when the first movable lens holding frame is moved to such a position that a second cam follower of the second movable lens holding frame is retreated from the escape channel of the first movable lens holding frame, and that detrimental rays can enter through the escape channel, the light shielding member also moved by the cam cylinder is placed at the position at which it closes the escape channel. Also, since according to the second aspect of the present invention the light shielding member is moved by the common cam cylinder, the mechanism for moving the light shielding member can be constructed without largely increasing the number of component parts, as is that in the lens device according to the first aspect of the invention. 
   According to a third aspect of the present invention, when in the lens device in the first or second aspect of the present invention the first and second movable lens holding frames are positioned so that the distance between the first and second movable lens holding frames is maximized, the light shielding member is moved by the rotation of the cam cylinder to the position at which it closes the escape channel of the first movable lens holding frame. Also, when the first and second movable lens holding frames are positioned so that the distance between the first and second movable lens holding frames is minimized, the light shielding member is moved to a position at which it blocks unnecessary external light to prevent flare due to reflection on an internal surface of the cam cylinder while maintaining an effective optical path diameter. 
   In the third aspect of the present invention, the range through which the light shielding member is moved by the third cam channel of the cam cylinder is specified. The light shielding member is moved to the position at which it closes the escape channel of the first movable lens holding frame, when the first and second movable lens holding frames are positioned so that the distance between the first and second movable lens holding frames is maximized. That is, the light shielding member closes the escape channel when it is at one end of the range of movement. Also, the light shielding member is moved to a position at which it blocks unnecessary external light to prevent flare due to reflection on an internal surface of the cam cylinder while maintaining an effective optical path diameter, when the first and second movable lens holding frames are positioned so that the distance between the first and second movable lens holding frames is minimized. That is, the light shielding member blocks unnecessary external light while maintaining the effective optical path diameter when it is at the other end of the range of movement. The other end position of the light shielding member is determined by the third cam channel to block unnecessary external light and prevent flare due to reflection on the internal surface of the cam cylinder while maintaining the effective optical path diameter for setting the F value for example, as well as to prevent entrance of detrimental rays through the escape channel. 
   In the lens device in accordance with the present invention, as described above, the cam cylinder that moves the first and second movable lens holding frames moves the light shielding member to close the escape channel of the first movable lens holding frame, thereby preventing detrimental rays from entering through the escape channels, without generating impact sound and vibration. Also, since according to the present invention the light shielding member is moved by the common cam cylinder, the mechanism for moving the light shielding member can be constructed without largely increasing the number of component parts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partially fragmentary sectional view of a zoom lens device for an ENG camera to which the present invention is applied; 
       FIG. 2  is an enlarged sectional view of an essential portion of the lens device, showing the positions of lens holding frames when the lens device is at a wide-angle end position; 
       FIG. 3  is an enlarged sectional view of an essential portion of the lens device, showing the positions of lens holding frames when the lens device is at a telephoto end position; 
       FIG. 4  is a perspective view showing the lens holding frames and a light shielding ring; 
       FIG. 5  is an enlarged development of a cam cylinder; 
       FIG. 6  is an enlarged sectional view of an essential portion of the lens device, showing the positions of the lens holding frames and the light shielding ring when the lens device is at the telephoto end position; and 
       FIG. 7  is an enlarged sectional view of an essential portion of the lens device, showing the positions of the lens holding frames and the light shielding ring when the lens device is at the wide-angle end position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the lens device in accordance with the present invention will be described below in detail with reference to the accompanying drawings. 
     FIG. 1  is a partially fragmentary sectional view of a zoom lens device  10  for an ENG camera to which the present invention is applied. A focusing ring  14 , a zoom ring  15  and an iris ring  18  are rotatably provided on a fixed lens barrel  12  of the zoom lens device  10 . In the fixed lens barrel  12  are provided a focusing lens group (not shown), a variable-power lens group (first movable lens)  20 , a focus correcting lens group (second movable lens)  22 , an iris  24  and a relay lens (not shown) in this order from the left hand side of  FIG. 1 . 
   The focusing lens group is supported on a focusing barrel  26  formed integrally with the focusing ring  14 . The focusing barrel  26  is supported by a straight-travel mechanism such as a helicoid screw mechanism or a cam mechanism on a lens barrel  28  fixed on the fixed lens barrel  12 . The focusing barrel  26  is movable forward and rearward by the straight-travel mechanism. When the focusing barrel  26  is rotated through the focusing ring  14 , the focusing lens group is moved forward or rearward, thereby performing focusing. The forward direction is a direction from the camera toward an object to be imaged by the camera, and the rearward direction is a direction from the object to the camera. 
   The variable-power lens group  20  is supported on a lens holding frame  30 , as shown in  FIG. 3 . The position of the variable-power lens group  20  shown in  FIGS. 1 and 2  is a wide-angle end position determined by one ends  36 A of cam channels (first cam channels)  36  (see  FIG. 5 ) of a cam cylinder  34  described below. The position of the variable-power lens group  20  shown in  FIG. 3  is a telephoto end position determined by the other ends  36 B of the cam channels  36  of the cam cylinder  34 . 
   As shown in  FIG. 4 , three cam rollers (first cam followers, only two of which are shown in  FIG. 4 )  32  are provided at equal intervals on an outer peripheral portion of the lens holding frame  30  so as to project from the same. The cam rollers  32  are engaged with the cam channels  36  of the cam cylinder  34  shown in  FIGS. 2 and 5 . Top end portions of the cam rollers  32  project out of the cam channels  36  and are engaged with straight-travel grooves  38  formed in an inner peripheral surface of the fixed lens barrel  12 . The straight-travel grooves  38  are formed parallel to an image taking optical axis P of the zoom lens device  10 . 
   The focus correcting lens group  22  is supported on a lens holding frame  40 , as shown in  FIGS. 2 and 3 . The position of the focus correcting lens group  22  shown in  FIGS. 1 and 2  is a wide-angle end position determined by one ends  46 A of cam channels (second cam channels)  46  (see  FIG. 5 ) of the cam cylinder  34 . The position of the focus correcting lens group  22  shown in  FIG. 3  is a telephoto end position determined by the other ends  46 B of the cam channels  46  of the cam cylinder  34 . 
   Cam roller mount portions  42  in generally rectangular form as shown in  FIG. 4  are formed at equal intervals in three places on an outer peripheral portion of the lens holding frame  40 . Cam rollers (second cam followers)  44  are respectively formed on the cam roller mount portions  42  so as to project from the same. The cam rollers  44  are engaged with cam channels  46  of the cam cylinder  34  shown in  FIGS. 3 and 5 . Top end portions of the cam rollers  44  project out of the cam channels  46  and are engaged with straight-travel grooves  47  in the fixed lens barrel  12 . The straight-travel grooves  47  are also formed parallel to the image taking optical axis P of the zoom lens device  10 . The straight-travel grooves  47  are formed at positions offset from the straight-travel grooves  38  in the circumferential direction of the cam cylinder  34 . 
   The cam cylinder  34  is placed inside the fixed lens barrel  12  so as to be rotatable about the image taking optical axis P. A pin  48  projects from an outer peripheral surface of the cam cylinder  34  as shown in  FIG. 1 . The pin  48  is passed through a slit  50  formed in the fixed lens barrel  12  as shown in  FIG. 3  and is fixed on the zoom ring  16 , shown in  FIG. 1 . The slit  50  shown in  FIG. 3  is formed along a plane perpendicular to the image taking optical axis P. When the zoom ring  16  is rotated in a range corresponding to the length of the slit  50 , the cam cylinder  34  rotates about the image taking optical axis P by following the zoom ring  16 . The lens holding frame  30  is thereby moved forward or rearward along the loci of the cam channels  36  and between the ends  36 A and the other ends  36 B of the cam channels  36 . Also, the lens holding frame  40  is thereby moved forward or rearward along the loci of the cam channels  46  and between the ends  46 A and the other ends  46 B of the cam channels  46 . Zooming is thus performed between the wide-angle end shown in  FIG. 2  and the telephoto end shown in  FIG. 3 . 
   Escape channels  52  are formed at equal intervals in three places (only two of which are indicated in  FIG. 4 ) in an outer peripheral portion of the lens holding frame  30 , as shown in  FIG. 4 . The escape channels  52  enable the cam roller mount portions  42  formed on the lens holding frame  40  to escape as shown in  FIG. 6 , when the lens holding frame  30  and the lens holding frame  40  are brought close to each other, that is, when the telephoto end and a position close to the telephoto end are reached. The escape channels  52  are formed in a size and shape corresponding to the cam roller mount portions  42 . 
   Since the escape channels  52  are formed in the lens holding frame  30 , there is a problem that detrimental rays, e.g., those reflected by the inner peripheral surface of the lens barrel in the rays entering the lens from the front side can enter the imaging area by passing through the escape channels  52  rearward to generate ghost, if no improvement is made in the above-described arrangement. 
   In this embodiment, therefore, a light shielding ring (light shielding member)  60  is provided to close the escape channels  52  at the end position of the lens holding frame  30 .wide-angle 
   As shown in  FIGS. 4 and 7 , the light shielding ring  60  is constituted by a ring portion  61  for escapement of a frame body  31  of the lens holding frame  30 , shielding portions  64  formed at equal intervals in three places on an outer peripheral flange  62  of the ring portion  61  to close the escape channels  52  of the lens holding frame  30  at the wide-angle end position, and cam rollers  66  respectively projecting from the shielding portions  64 . 
   The cam rollers  66  of the light shielding ring  60  are engaged with cam channels (third cam channels)  68  of the cam cylinder  34  shown in  FIGS. 3 and 5 . Top end portions of the cam rollers  66  project out of the cam channels  68  and are engaged with straight-travel grooves  70  in the fixed lens barrel  12 . The straight-travel grooves  70  are also formed parallel to the image taking optical axis P of the zoom lens device  10 . The straight-travel grooves  70  are formed at positions offset from the straight-travel grooves  38  and  47  in the circumferential direction of the cam cylinder  34 . When the cam cylinder  34  is rotated, the light shielding ring  60  is moved forward or rearward along the loci of the cam channels  68  and between one ends  68 A and the other ends  68 B of the cam channels  68  in a linked relationship with the lens holding frames  30  and  40 . Also, when the cam rollers  66  reach the positions at the ends  68 A of the wide-angle end position, the shielding portions  64  enter the escape channels  52  of the lens holding frame  30 , as shown in  FIG. 7 , thus preventing detrimental rays Q from entering rearward through the escape channels  52  from penetrating rearward. 
   In the zoom lens device  10  of this embodiment, as described above, the cam cylinder  34  that moves the lens holding frames  30  and  40  moves the light shielding ring  60  to close the escape channels  52  of the lens holding frame  30  by the light shielding portions  64 , thus preventing detrimental rays Q from entering rearward through the escape channels  52 , without generating impact sound and vibration such as those generated in the conventional device. 
   In the zoom lens device  10 , the light shielding ring  60  is moved by the common cam cylinder  34 . Thus, The moving mechanism of the light shielding ring  60  can be constructed without largely increasing the number of component parts. 
   Further, the telephoto end position of the light shielding ring  60  to which the movement of the light shielding ring  60  is limited by the other ends  68 B of the cam channels  68  of the cam cylinder  34  is determined so that, as shown in  FIG. 6 , unnecessary external light is blocked to prevent flare due to reflection on the internal surface of the cam cylinder  34 , while the desired effective optical path diameter of subject light is ensured. If the ends  68 A and  68 B of the cam channels  68  are specified, entrance of detrimental rays P through the escape channels  52  at the wide-angle end position can be prevented and unnecessary external light can be blocked to prevent flare due to reflection on the internal surface of the cam cylinder  34 , while the effective optical path diameter for setting the F value for example is ensured.