Abstract:
The present invention provides a lens barrel having an optical system capable of change of a focal length, comprising: a diaphragm device which controls a light amount of light incident to the optical system and passing to an image forming surface; and an open F value varying device which is provided separately from the diaphragm device, controls a light amount of the light incident to the optical system and passing to the image forming surface and varies an open F value by limiting the light amount so that the open F value becomes a value according to a focal length of the optical system when the light amount which is controllable by the diaphragm device is at the maximum.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a lens barrel and particularly to a lens barrel having a diaphragm mechanism for changing an open F value according to a focal length in a zoom lens (image pickup lens with variable focal length).  
         [0003]     2. Description of the Related Art  
         [0004]     A zoom lens with a variable focal length in which an open F value is adjusted according to a focal length by limiting a maximum opening amount of a diaphragm is proposed in Japanese Patent No. 3610113. According to this, with a trend of size reduction and higher magnification of a zoom lens, a defocus amount (spherical aberration) tends to get bigger when the diaphragm is opened, and thus, it is proposed that the maximum opening amount of the diaphragm is limited to the extent that the defocus amount does not become poor. That is, an F value when the diaphragm is not opened to the maximum is set as an open F value, and it is so constituted that the diaphragm opening amount is changeable only in a range that a favorable optical performance can be obtained. Moreover, since the defocus amount differs depending on a focal length, it is also suggested that a maximum opening amount to be limited is changed according to a focal length.  
         [0005]     However, according to Japanese Patent No. 3610113, since the open F value is limited by limiting on control a maximum opening amount of a diaphragm for exposure control to be controlled by a motor such as an IG meter, there is a problem that diaphragm control gets complicated and delayed. There is also another problem that the diaphragm for exposure control can not be manually operated.  
         [0006]     The present invention has been made in view of the above circumstances and has an object to provide a lens barrel which can change the open F value according to the focal length with a simple mechanism and with a high accuracy without deteriorating an optical performance.  
       SUMMARY OF THE INVENTION  
       [0007]     In order to achieve the above object, a lens barrel described in a first aspect of the present invention is a lens barrel having an optical system capable of change of a focal length, comprising a diaphragm device which controls a light amount of light incident to the optical system and passing to an image forming surface, and an open F value varying device which is provided separately from the diaphragm device, controls the light amount of the light incident to the optical system and passing to the image forming surface and varies an open F value by limiting the light amount so that the open F value becomes a value according to a focal length of the optical system when the light amount which is controllable by the diaphragm device is at the maximum.  
         [0008]     According to the present invention, since the device which varies the open F value is provided separately from a usual diaphragm device for exposure control, individual construction and control are made possible, and construction in structure or control is simplified compared with a case where the open F value is limited by the usual diaphragm device.  
         [0009]     The lens barrel according to a second aspect of the present invention is characterized in that, in the invention described in the first aspect, the open F value varying device varies the open F value in mechanical conjunction with a member operating with a change of a focal length of the optical system.  
         [0010]     According to the present invention, when the focal length is changed, the open F value varying device varies the open F value to an appropriate value according to the focal length in mechanical conjunction with that. Thus, special control of the open F value varying device is not required and the open F value can be varied with a high accuracy.  
         [0011]     The lens barrel according to a third aspect of the present invention is characterized in that, in the invention described in the first aspect or the second aspect, the open F value varying device comprises an opening for passing light and a device which changes an opening amount of the opening.  
         [0012]     According to the present invention, the open F value varying device may be constituted similarly to a usual diaphragm mechanism which varies the opening amount of the opening by a diaphragm blade, for example.  
         [0013]     The lens barrel according to a fourth aspect of the present invention is characterized in that, in the invention according to the first, second or third aspect, the diaphragm device comprises an opening for passing light and a device which changes an opening amount of the opening in mechanical conjunction with an operation member operated manually.  
         [0014]     According to the present invention, since the open F value varying device is provided separately from the diaphragm device, the diaphragm device can be a manually operated diaphragm as in the present invention, and an F value can be set with a higher accuracy as compared with an electrically operated one. Also, since there is no need to provide a mechanism for limiting the open F value at the diaphragm device by providing the open F value varying device separately from the diaphragm device as in the present invention, there is no such a problem that the structure is complicated, but an appropriate open F value can be set without considering limitation of the opening amount at diaphragm control so that the optical performance is not deteriorated.  
         [0015]     According to the lens barrel of the present invention, change of the open F value according to the focal length can be carried out with a simple mechanism and with a high accuracy so that the optical performance is not deteriorated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a side sectional view showing a lens barrel of a zoom lens used in a monitor camera or the like, for example, to which the present invention is applied;  
         [0017]      FIG. 2  is a view of a cam plate of a rear diaphragm and major components arranged on its rear side shown from a front direction of an optical axis;  
         [0018]      FIG. 3  is a view showing diaphragm blades arranged on a front face side of the cam plate;  
         [0019]      FIG. 4  is a view of a cam plate of a front diaphragm and major components arranged on its front side shown from a rear direction of the optical axis;  
         [0020]      FIG. 5  is a view showing diaphragm blades arranged on a rear face side of the cam plate; and  
         [0021]      FIG. 6  is a view showing a relation between a focal length determined by a rotating position of a zoom ring and an open F value determined by an opening amount of the front diaphragm according to the rotating position of the zoom ring.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     A preferred embodiment of the lens barrel according to the present invention will be described below referring to the attached drawings.  
         [0023]      FIG. 1  is a side sectional view showing a lens barrel of a zoom lens used in an ENG camera, a video camera, a monitor camera or the like, for example, to which the present invention is applied.  
         [0024]     A lens barrel  10  in this figure is provided with a lens mount  12 , whose detail is omitted, formed at its rear end and is detachably attached to a camera head, not shown, by this lens mount  12 .  
         [0025]     A fixed cylinder of the lens barrel  10  comprises a front fixed cylinder  14 , a middle fixed cylinder  16  and a rear fixed cylinder  18  connected in series, and each of members constituting the lens barrel  10  is supported by these fixed cylinders  14 ,  16 ,  18 .  
         [0026]     On an optical axis  0  of the lens barrel  10 , there are arranged as an optical system for forming an image of a subject in order from the object side, a fixed lens (group)  20 , a zoom lens (group)  22 , a front diaphragm  24 , a rear diaphragm  26 , a fixed lens (group)  28 , a vibration-proof lens (group)  30 , a fixed lens (group)  32 , a focus lens (group)  34  and a fixed lens (group)  36 .  
         [0027]     The zoom lens  22  is to vary a zoom magnification (focal length) of the optical system and movably supported in the optical axial direction. A flange portion  42  is formed at a holding frame  40  of the zoom lens  22 , and the zoom lens  22  is connected to a moving cylinder  44  through the flange portion  42 . An intermediate cylinder  46  is fixed to an inner circumference portion of the front fixed cylinder  14 , a rotating cylinder  48  is rotatably held by an inner circumference portion of the intermediate cylinder  46 , and the moving cylinder  44  is held by an inner circumference portion of the rotating cylinder  48 .  
         [0028]     Moreover, a straight groove  46 A in the optical axial direction is formed on the inner circumference surface of the intermediate cylinder  46 , and a cam groove (cam-shaped hole)  48 A is formed in the rotating cylinder  48 . A cam pin  50  fixed to the moving cylinder  44  is inserted through the cam groove  48 A of the rotating cylinder  48  and is engaged with the straight groove  46 A of the intermediate cylinder  46 . By this, the moving cylinder  44  is moved straight in the optical axial direction in a state where rotation is restricted, and the cam pin  50  is held at a position engaged with the cam groove  48 A. When the rotating cylinder  48  is rotated, an intersection position between the cam groove  48 A of the rotating cylinder  48  and the straight groove  46 A of the intermediate cylinder  46  is changed to a position according to a cam shape, and the moving cylinder  44  is moved forward or backward in the optical axial direction by movement of the cam pin  50  to the intersection point.  
         [0029]     On the other hand, a zoom ring  52  is rotatably arranged on an outer circumference portion of the front fixed cylinder  14 , and a rod-state connection rod  54  is mounted inward in the radial direction on an inner circumferential surface of the zoom ring  52 . This connection rod  54  is inserted through a long hole in the circumferential direction formed at the front fixed cylinder  14  and the intermediate cylinder  46  and is connected to the rotating cylinder  48 . By this, when the zoom ring  52  is rotationally operated, the rotating cylinder  48  is rotationally moved in conjunction with that. When the rotating cylinder  48  is rotationally moved, the moving cylinder  44  is moved forward or backward as mentioned above, and the zoom lens  22  is moved in the optical axial direction in conjunction with the moving cylinder  44 . Therefore, zoom magnification is changed by rotational operation of the zoom ring  52 .  
         [0030]     The vibration proof lens  30  is supported by a support mechanism, not shown, movably in the vertical and horizontal directions in a surface crossing the optical axis. Moreover, driving in the vertical and horizontal directions of the vibration proof lens  30  is performed by a motor of a driving mechanism, not shown. And by controlling the motor, a position in the vertical and horizontal directions of the vibration proof lens  30  is controlled. When the vibration proof lens  30  is displaced in the vertical or horizontal direction, an image forming position of an image is also displaced in the vertical or horizontal direction according to the displacement. Thus, when the lens barrel  10  is vibrated, by displacing the vibration proof lens  30  so that image fluctuation by the vibration is cancelled, the image fluctuation is prevented. Detailed explanation on vibration insulation will be omitted.  
         [0031]     The focus lens  34  is to change a focal position of the optical system and is movably supported in the optical axial direction. Though detail will be omitted, guide rods  56 ,  56  are provided in the optical axial direction between the middle fixed cylinder  16  and the rear fixed cylinder  18 , and a holding frame  58  of the focus lens  34  is connected to the guide rods  56 ,  56 . By moving the holding frame  58  by a driving mechanism (an electric or a manual driving mechanism), not shown, along the guide rods  56 ,  56 , the focus lens  34  is moved forward or backward in the optical axial direction.  
         [0032]     The front diaphragm  24  and the rear diaphragm  26 , whose detail will be described later, are constituted capable of opening/closing operation by diaphragm blades  68 ,  72  arranged at a common base plate  60 . The rear diaphragm  26  is a usual diaphragm for exposure control and is opened/closed by rotational operation of an iris ring  62  rotatably arranged between the front fixed cylinder  14  and the middle fixed cylinder  16 .  
         [0033]     On the other hand, the front diaphragm  24  is an open F value varying device which varies an open F value according to the focal length of the optical system and is opened/closed in conjunction with rotational movement of the rotating cylinder  48 . Therefore, when the zoom ring  52  is rotationally operated, the zoom lens  22  is moved in the optical axial direction, and the focal length is changed. And at the same time, the front diaphragm  24  is opened/closed so as to have an opening amount corresponding to the focal length. That is, exposure control is performed by the rear diaphragm  26  in principle, but the open F value in which an exposure amount is not increased any more even if the opening amount of the rear diaphragm  26  is increased more than a certain amount (minimum F value) is limited by the opening amount of the front diaphragm  24 . Moreover, the open F value is varied to an appropriate value according to the focal length.  
         [0034]     The front diaphragm  24  and the rear diaphragm  26  will be described below in detail. In  FIG. 1 , the common base plate  60  of the front diaphragm  24  and the rear diaphragm  26  is fixed to the front fixed cylinder  14  by a screw  64  together with the intermediate cylinder  46 . A circular opening  60 A is provided at the base plate  60 , and the center of the opening  60 A is arranged at a point substantially matching the optical axis O. On the front face side of the base plate  60 , a cam plate (clip washer)  66  having an opening  66 A with a position and a size substantially matching the opening  60 A of the base plate  60  is rotatably mounted with holding diaphragm blades  68 ,  68  . . . , which will be described later, between them, and a cam plate (clip washer)  70  having an opening  70 A with a position and a size substantially matching the opening  60 A of the base plate  60  is rotatably mounted also on the back face side of the base plate  60  holding diaphragm blades  72 ,  72  . . . , which will be described later, between them.  
         [0035]     Next, a structure of a diaphragm mechanism in which the front diaphragm  24  and the rear diaphragm  26  are constituted integrally will be described in detail.  FIG. 2  is a view of the cam plate  70  of the rear diaphragm  26  and major components arranged on its rear side shown from the front direction of the optical axis. As shown in this figure, the cam plate  70  of the rear diaphragm  26  is formed in a disk state and rotatably held on the back face side of the base plate  60 . The opening  70 A is formed at the center of the cam plate  70 , and cam grooves (long holes forming a cam)  80 ,  80  . . . to be engaged with a cam pin projected at the diaphragm blades  72 ,  72  . . . , which will be shown later, are formed around the opening  70 A.  
         [0036]     Moreover, a connection portion  82  is projected at the cam plate  70 , and one end of a rod-state connection rod  84  is mounted to a portion  82 A (See  FIG. 1 ) bent in the optical axis direction of the connection portion  82 . On the other hand, the other end side of the connection rod  84  is mounted to the iris ring  62 . By this, when the iris ring  62  is rotatably operated, the cam plate  70  is rotationally moved. In this figure, a position of the connection rod  84  when the rear diaphragm  26  is fully opened and a position of the connection rod  84  when fully closed are shown, and the iris ring  62  is operated in a range of about 90 degrees.  
         [0037]      FIG. 3  is a view showing the diaphragm blades  72 ,  72  . . . arranged on the front face side of the cam plate  70 . As shown in this figure, the rear diaphragm  26  comprises six diaphragm blades  72 ,  72 . . . . On the base plate  60  (See  FIG. 1 ), pins  86 ,  86 . . . for rotatably supporting each of the diaphragm blades  72  are projected, and by fitting each of the pins  86  in a hole  72 A formed in each of the diaphragm blades  72 , each of the diaphragm blades  72  is rotatably supported around each of the pins  86 .  
         [0038]     On the other hand, on the back face side of each of the diaphragm blades  72 , a cam pin  72 B is projected, and each of the cam pins  72 B is fitted into each of the corresponding cam grooves  80  of the cam plate  70 . By this, when the cam plate  70  is rotated, each of the diaphragm blades  72  is rotationally moved around the pin  86  to a position where the cam pin  72 B is engaged with the cam groove  80  of the cam plate  70 . By rotational movement of each of the diaphragm blades  72  according to the cam groove  80 , the opening amount of the rear diaphragm  26  is varied.  
         [0039]      FIG. 4  is a view of major components arranged on the front side of the cam plate  66  of the front diaphragm  24  shown from the rear direction of the optical axis. As shown in this figyre, the cam plate  66  of the front diaphragm  24  is formed in a disk state and rotatably held on the front face side of the base plate  60 . The opening  66 A is formed at the center of the cam plate  66 , and cam grooves (long holes forming a cam)  90 ,  90  . . . to be engaged with cam pins projected on the diaphragm blades  68 ,  68  . . . , which will be shown later, are formed around the opening  66 A. Moreover, a connection portion  92  is projected on an outer circumference portion of the cam plate  66 , and a tip end of the connection portion  92  is connected to the rotating cylinder  48 . The rotating cylinder  48  is, as shown also in  FIG. 1 , connected by the connection rod  54  to the zoom ring  52  and is rotationally moved with the zoom ring  52 . By this, rotational operation of the zoom ring  52  rotationally moves the cam plate  66 .  
         [0040]      FIG. 5  is a view showing the diaphragm blades  68 ,  68  . . . arranged on the back face side of the cam plate  66 . As shown in this figure, the front diaphragm  24  comprises six diaphragm blades  68 ,  68  . . . . Pins  94 ,  94  . . . for rotatably supporting each of the diaphragm blades  68  are projected at the base plate  60 , and by fitting each of the pins  94  in a hole  68 A formed at each of the diaphragm blades  68 , each of the diaphragm blades  68  is rotatably supported around each of the pins  94 .  
         [0041]     On the other hand, a cam pin  68 B is projected on the front face side of each of the diaphragm blades  68 , and each of the cam pins  68 B is fitted in each of the corresponding cam grooves  90  of the cam plate  66 . By this, when the cam plate  66  is rotated, each of the diaphragm blades  68  is rotationally moved around the pin  94  to a position where the cam pin  68 B is engaged with the cam groove  90  of the cam plate  66 . Rotational movement of each of the diaphragm blades  68  according to the cam groove  90  varies the opening amount of the front diaphragm  24 .  
         [0042]     Here,  FIG. 6  shows a relation between a focal length determined by a rotational position of the zoom ring  52  and an open F value determined by the opening amount of the front diaphragm  24  according to the rotational position of the zoom ring  52 . As shown in this figure, in the case where the focal length is smaller than a predetermined focal length fa (wide side), the front diaphragm  24  is in the substantially open (maximum opening amount) state, and the open F value is held at the state of F1.9, for example. That is, in a section from the focal length at the wide end to the focal length fa, the shape of the cam groove  90  of the cam plate  66  is formed so that each of the diaphragm blades  68  is held substantially open even though the cam plate  66  of the front diaphragm  24  is rotated. On the other hand, if the focal length is larger than fa (tele side), when the focal length gets larger, the opening amount is gradually decreased, and when the focal length reaches the maximum (tele end), the open F value becomes F2.8. That is, in a section where the focal length is larger than fa, the shape of the cam groove  90  of the cam plate  66  is formed so that the opening amount of each of the diaphragm blades is decreased when the cam plate  66  is rotated to the tele side.  
         [0043]     In this way, by varying the opening amount of the front diaphragm  24  in conjunction with rotational operation of the zoom ring  52 , the open F value is regulated to an appropriate value according to the focal length, and in whatever state the opening amount of the rear diaphragm  26  is, the value is limited within a range of an F value larger than the open F value regulated by the front diaphragm  24 . That is, even if the opening amount of the rear diaphragm  26  for exposure control is controlled by an operator by operating the iris ring  62  without considering deterioration of the optical performance, the optical characteristics of the optical system is maintained in a favorable state. Moreover, in the case where the vibration proof lens  30  is included as in the lens barrel of this preferred embodiment, since deterioration of the optical performance can be remarkable due to displacement of the vibration proof lens  30 , prevention of deterioration in the optical performance by varying the open F value as above is particularly effective.  
         [0044]     In the above preferred embodiment, both the front diaphragm  24  for varying the open F value and the rear diaphragm  26  for exposure control are operated in conjunction with manual operation, but the invention is not limited to this. The front diaphragm  24  for varying the open F value may be an electric mechanism driven by a motor, for example, so that the opening amount of the front diaphragm  24  is changed by the motor according to the focal length detected by a sensor. Moreover, the rear diaphragm  26  may also be an electric mechanism driven by a motor. When the front diaphragm  24  is mechanically varied according to the focal length, a member connected to the cam plate  66  of the front diaphragm  24  may be a member operating with the change of the focal length.  
         [0045]     Moreover, the construction of front diaphragm  24  and the rear diaphragm  26  is not limited to diaphragm blades as in the above preferred embodiment but may be any structure only if it is a device which varies a passing light amount. Furthermore, as a device which varies the open F value, not as the front diaphragm  24  for varying the opening amount of the opening, but it may be a device which varies the open F value by moving a position of an opening with a fixed size in the fore-and-aft direction along the optical axis according to the focal length.