Abstract:
Disclosed is a method of assembling plural single lenses of a compound lens such that optical axes of the single lenses are aligned with each other. The method includes: pressing the single lenses disposed respectively on the opposite sides of a cylindrical spacer against the spacer by a pair of chucking units having center axes aligned with each other and capable of moving in directions parallel to the optical axes of the single lenses; and fixing the single lenses to the spacer by a fixing means with the curved surfaces of the single lenses on the opposite sides of the spacer in contact with contact surfaces of the chucking units and the spacer contained in planes perpendicular to the optical axis, respectively, and with the single lenses and the spacer automatically centered.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present invention contains subject matter related to Japanese Patent Application 2004-378000 filed in the Japanese Patent Office on Dec. 27, 2004, the entire contents of which being incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a compound lens assembling method, a compound lens assembled by the same compound lens assembling method, and a camera using the compound lens. More particularly, the present invention relates to a compound lens assembling method of assembling a plurality of lenses with their optical axis aligned with each other, a compound lens assembled by the same compound lens assembling method, and a camera provided with a lens system including compound lenses assembled by the compound lens assembling method.  
         [0004]     2. Description of the Related Art  
         [0005]     Improvements have been made in video cameras and digital still cameras for many themes including miniaturization, weight reduction, increasing zoom ratio, rapid focusing and rapid zooming. The lens system including moving lens groups for an optical instrument, such as a video camera or a digital still camera, is provided with a linear actuator or a linear motor for driving a coil combined with a movable zooming member having a zooming function or a movable focusing member having a focusing function by a magnetic circuit including a magnet and a yoke for linear movement.  
         [0006]      FIG. 13  shows a representative optical system including movable lenses for an optical instrument, such as a video camera or a digital still camera. Referring to  FIG. 13 , an objective lens  2  is fixedly mounted on a front lens barrel  1 , a zoom lens  4  is attached to a movable zooming member  3  capable of moving along the optical axis of the optical system, a fixed middle lens  6  is fixed to a middle lens barrel  5 , and a focusing lens  8  is attached to a movable focusing member  7  capable of moving along the optical axis. An image pickup device  10  including a CCD is attached to a back lens barrel  9 . An iris  11 , namely, an aperture stop, is placed in the fixed middle lens  6 . The movable zooming member  3  and the movable focusing member  7  are guided for axial movement along the optical axis by upper guide bars  12  and  13  and a lower guide bar  14 .  
         [0007]     The movable zooming member  3  holding the zoom lens  4  is moved for zooming to determine an image frame. The movable focusing member  7  holding the focusing lens  8  is moved for focusing according to the movement of the movable zooming member. The iris  11  adjusts optical aperture for proper exposure. An image of an object is formed on the surface of the image pickup device  10 . The image pickup device  10  provides electric signals representing the intensities of light rays fallen thereon.  
         [0008]     The performance of this optical system can be improved and aberrations can be corrected by using a compound lens formed by assembling a plurality of single lenses as the objective lens  2 , the zoom lens  4 , the fixed middle lens  6  or the focusing lens  8 . In assembling a plurality of single lenses to form a compound lens, the tilt and shift of the single lenses or the compound lens need to be accurately corrected and the single lenses need to be correctly fixed to the lens barrel.  
         [0009]     The alignment of a lens  96  is adjusted by an adjusting mechanism including a plate spring  93  as shown in  FIG. 14 . A reinforcing plate  91  is attached to the front side of a lens barrel  90 , a ring  92  is seated on the front surface of the reinforcing plate  91 . The plate spring  93  is fastened to the lens barrel  90  with screws  94  passed through the reinforcing plate  91  and screwed in threaded holes formed in the lens barrel  91 . The ring  92  is held between the plate spring  93  and the reinforcing plate  91 . The lens  96  is fixedly mounted on a lens holding frame  95 . The position of the lens holding frame  95  is adjusted by means of adjusting screws  97 .  
         [0010]     A structure holding the lens holding frame  95  by the plate spring  93  can correct only the tilt of the lens  96 . Since the adjusting screws  97  are arranged around the lens  96  and the plate spring  93  surrounds the lens  96 , the outside diameter of the lens barrel is large as compared with the diameter of the lens  96 . Thus this structure obstructs the reduction of the radial dimension of the lens barrel.  
         [0011]     A chart is displayed, and the tilt of the lens holding frame  95  is adjusted by means of the adjusting screw  97  and the plate spring  93  to adjust the optical axis of the lens  96  of this lens system. Since the adjusting screws  97  are on the side of the lens  96  and have axes parallel to the optical axis, an adjusting tool for turning the adjusting screws  97  is necessary. As obvious from  FIG. 14 , many parts are needed for adjustment and the adjusting mechanism is complex.  
         [0012]      FIG. 15  shows another adjusting mechanism. A lens  103  is mounted on a lens holding frame  104 . A tension coil spring  105  pulls the lens holding frame  104  in a radial direction. The lens holding frame  104  is moved for positioning against the resilience of the tension coil spring  105  to a predetermined position by adjusting screws  106 . A thrust spring  107  is used for adjusting the position of the lens holding frame  104  with respect to a direction parallel to the optical axis. A pressure member  108  presses the thrust spring  107 . Pressure applied by the pressure member  108  to the thrust spring  107  is adjusted to adjust the axial position of the lens  103  on the optical axis.  
         [0013]     The adjusting mechanism shown in  FIG. 15  preloads the lens holding frame  104  radially by the tension coil spring  105 , presses the lens holding frame  104  axially by the thrust spring  107 , and corrects only the position of the lens holding frame  105  with respect to a shifting direction in a plane perpendicular to the optical axis by means of the adjusting screws  106 . Since the adjusting screws  106  and the thrust plate  107  are disposed outside the lens  103 , the lens barrel is enlarged radially and axially relative to the lens  103  and it is difficult to form the lens barrel in small dimensions.  
         [0014]     An image of a chart is projected through the lens system on a screen when the shift of the lens holding frame  104  is adjusted by this adjusting mechanism. Although work for operating the adjusting mechanism shown in  FIG. 15  is easier than that for operating the adjusting mechanism shown in  FIG. 14 , an adjusting tool is needed to turn and move the adjusting screws  106  in directions perpendicular to the optical axis to adjust the shift of the lens holding frame  104 . Thus the adjusting mechanism causes difficulties in assembling equipment. The adjusting mechanism needs a plurality of adjusting members and many component parts, which causes difficulties in assembling work.  
         [0015]      FIG. 16  shows an adjusting mechanism for carrying out an adjusting method disclosed in JP-A No. 11-72768. This adjusting mechanism uses a bell chuck for optical axis alignment. A lower chucking unit  116  is fixedly mounted on a bell chuck base  115 . An upper chucking unit  117  is vertically movable along an axis. A lens holding frame  119  is supported on a support member  118 . Peripheral parts of the lens holding frame  119  is guided by two guide bars  120  and  121  for vertical movement. A lens held on the lens holding frame  119  is held between the lower bell chucking unit  116  and the upper bell chucking unit  117 . After aligning the optical axis of the lens  125  with the center axis of the lens holding frame  119 , a gap between the circumference of the lens  125  and the lens holding frame  119  filled up with an adhesive  122 . Then, the adhesive  122  is irradiated with ultraviolet rays emitted by irradiating devices  123  to set the adhesive  122 .  
         [0016]     The adjusting mechanism shown in  FIG. 16  holds the lens  125  fixedly by the lower bell chucking unit  116  and the upper bell chucking unit  117 , holds the lens holding frame  119  by the guide bars  120  and  121 , and bonds the lens  125  to the lens holding frame  119  with the adhesive  122 . It is possible that the lens  125  cannot be accurately positioned relative to the lens holding frame  119  due to the insufficient accuracy of the parallelism of the chucking units  116  and  117  with the guide bars  120  and  121  supporting the lens holding frame  119 , the insufficient positional accuracy of the chucking units  116  and  117  and the guide bars  120  and  121 , and plays between the lens holding frame  119  and the guide bars  120  and  121 .  
         [0017]     The adjusting mechanism shown in  FIG. 16  fixes the single lens  125  to the lens holding frame  119 . However, the adjusting mechanism shown in  FIG. 16  is unable to fix a plurality of lenses accurately to the lens holding frame with a spacer held between the adjacent lenses. A compound lens according to an embodiment of the present invention includes a plurality of lenses that can be fixed relative to each other with a spacer placed between the adjacent lenses and remarkably increases the degree of freedom of optical design for determining the distance between the surfaces of the adjacent lenses. The adjusting mechanism shown in  FIG. 16  fastens the lens  125  to the lens holding frame  119  only with the adhesive  122  filling up the gap between the lens  125  and the lens holding frame  119 . It is possible that lens holding strength and lens fixing accuracy deteriorate after the lens  125  has been bonded to the lens holding frame  119  with the adhesive  122 ,  
       SUMMARY OF THE INVENTION  
       [0018]     An embodiment of the present invention is directed to a compound lens assembling method of assembling a compound lens including a plurality of single lenses, capable of positioning the single lenses with their optical axes highly accurately aligned with each other without requiring any optical axis aligning process.  
         [0019]     Another embodiment of the present invention is directed to a compound lens assembling method of assembling a compound lens, capable of achieving an optical centering process for centering a plurality lenses without using any lens adjusting mechanism.  
         [0020]     Another embodiment of the present invention is directed to a compound lens assembling method that enables the dimensional reduction of a lens barrel.  
         [0021]     Another embodiment of the present invention is directed to a compound lens assembling method not requiring adjusting parts for optical axis alignment and capable of reduced assembling man-hours.  
         [0022]     Another embodiment of the present invention is directed to a compound lens assembling method capable of reducing man-hours for optical axis alignment.  
         [0023]     Another embodiment of the present invention is directed to a compound lens assembling method capable of highly accurately assembling a compound lens and including a compound lens assembling process needing simple tools and jigs to facilitate assembling work.  
         [0024]     Another embodiment of the present invention is directed to a compound lens assembled by a compound lens assembling method according to the embodiment of the present invention.  
         [0025]     Another embodiment of the present invention is directed to a lens system including compound lens assembled by a compound lens assembling method according to the embodiment of the present invention.  
         [0026]     A compound lens assembling method of assembling a plurality of single lenses of a compound lens such that their optical axes are aligned with each other in the embodiment of the present invention includes the steps of: pressing the single lenses disposed respectively on the opposite sides of a cylindrical spacer against the spacer by a pair of chucking units having center axes aligned with each other and capable of moving in directions parallel to the optical axes of the single lenses; and fixing the single lenses to the spacer by a fixing means with the curved surfaces of the single lenses on the opposite sides of the spacer in contact with contact surfaces of the chucking units and the spacer contained in flat planes perpendicular to the optical axis, respectively, and with the single lenses and the spacer automatically centered.  
         [0027]     At least one of the single lenses may be automatically centered relative to one of the chucking units with a curved surface thereof in contact with the contact surface of the corresponding chucking unit. At least one of the single lenses may be automatically centered relative to the spacer with a curved surface thereof in contact with a contact surface of the spacer. Contact parts of the single lenses in contact with the spacer may be contained in flat planes perpendicular to the optical axis, respectively. Contact surfaces of the single lenses in contact with the pair of chucking units may be contained in flat planes perpendicular to the optical axis, respectively. The fixing means may be adhesive bonding, staking, an annular holding spring or a fixing member of a heat shrinkable polymer. A compound lens in the embodiment of the present invention is assembled by the foregoing compound lens assembling method in the embodiment of the present invention.  
         [0028]     The present invention concerning a compound lens relates to a compound lens assembled by any one of the above constitutions. The present invention concerning a lens system relates to a lens system using the compound lens.  
         [0029]     In a lens system in a preferred embodiment of the present invention for an image pickup device, such as a video camera, the curved surfaces of a plurality of single lenses to be centered with each other are chucked with a cylindrical spacer placed between the adjacent single lenses from vertical directions by a chucking device, as a fixing means capable of fixing the single lenses in a highly accurate compound lens to a lens barrel without requiring optical axis aligning work, having centered first and second chucking surfaces capable of being moved parallel to the optical axes of the single lenses, the curved surfaces in contact with the spacers are fixed to the spacers by adhesive bonding, staking or clamping with annular holding springs, and in a compound lens holding method, a compound lens includes spacers and a chucking devices, each of the spacers or the chucking devices has flat contact surfaces perpendicular to the optical axis in contact with the single lenses, and the single lenses are fixed to the lens barrel by adhesive bonding, staking or clamping with annular holding springs.  
         [0030]     According to an embodiment of the present invention, in the lens system included in an image pickup system of a video camera or the like, the single lenses are accurately combined in a compound lens without requiring optical axis aligning work, the fixing means for fixing the single lenses to the lens barrel does not need any lens adjusting mechanism incorporated into the barrel, which is very advantageous for the dimensional reduction of the lens barrel. Adjusting parts and man-hours for assembling and optical axis alignment can be reduced. The compound lens can be highly accurately assembled, tools and jigs to be used by a lens assembling process can be simplified and the compound lens can be easily assembled.  
         [0031]     The compound lens assembling method of assembling the plurality of single lenses with the optical axes thereof aligned with each other according to the embodiment of the present invention presses the single lenses disposed respectively on the opposite sides of the cylindrical spacer against the spacer by the pair of chucking units having center axes aligned with each other and capable of moving in directions parallel to the optical axes of the single lenses, and fixes the single lenses by the fixing means with the curved surfaces of the single lenses on the opposite sides of the spacer in contact with flat contact surfaces perpendicular to the optical axis, respectively, of the chucking units or the spacer, and with the single lenses and the spacer automatically centered.  
         [0032]     Thus, when the compound lens is assembled by the compound lens assembling method, the spacer and the single lenses on the opposite sides of the spacer are automatically centered when the compound lenses are pressed against to the opposite end surfaces of the cylindrical spacer by the chucking units, and the thus centered compound lenses and the spacers are fixed by the fixing means to complete the compound lens. Therefore, any special adjusting parts and assembling work for the optical axis adjustment of the lenses and spacers are unnecessary. Thus the compound lens assembling method of the present invention is very simple and capable of assembling the plurality of single lenses with their optical axes aligned with each other in high accuracy. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     The above and other objects, features and advantages of the present invention will become apparent from the following description taken in connection with the accompanying drawings, in which:  
         [0034]      FIG. 1  is a perspective view of a lens system;  
         [0035]      FIG. 2  is a perspective view of the lens system shown in  FIG. 1  placed with its right side facing up;  
         [0036]      FIG. 3  is a side elevation of the lens system shown in  FIG. 1  with its case removed;  
         [0037]      FIG. 4  is a perspective view of an essential part of the lens system shown in  FIG. 1 ;  
         [0038]      FIG. 5  is a front elevation of the essential part shown in  FIG. 4  taken from the side of an objective lens;  
         [0039]      FIG. 6  is a rear view of the essential part shown in  FIG. 4  taken from the side of an image pickup device;  
         [0040]      FIG. 7  is a block diagram of an image pickup system including the lens system shown in  FIG. 1 ;  
         [0041]      FIG. 8  is an exploded perspective view of a fixed middle lens unit;  
         [0042]      FIG. 9  is sectional view of the fixed middle lens unit shown in  FIG. 8 ;  
         [0043]      FIG. 10  is a longitudinal sectional view of a fixed middle lens unit in a first modification;  
         [0044]      FIG. 11  is a longitudinal sectional view of a fixed middle lens unit in a second modification;  
         [0045]      FIG. 12  is a longitudinal sectional view of a fixed middle lens unit in a third modification;  
         [0046]      FIG. 13  is a longitudinal sectional view of an essential part of a lens system of related art;  
         [0047]      FIG. 14  is an exploded perspective view of an optical axis aligning mechanism included in the lens system shown in  FIG. 13 ;  
         [0048]      FIG. 15  is an exploded perspective view of another optical axis aligning mechanism; and  
         [0049]      FIG. 16  is a longitudinal sectional view of an essential part of an optical axis aligning mechanism including bell chuck units. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0050]     Preferred embodiment of the present invention will be described. FIGS.  1  to  7  show a lens system related with the present invention. Referring to  FIGS. 1 and 2 , the lens system has a case  20  made of a synthetic resin having a shape substantially resembling a rectangular solid. The open front end of the case  20  is covered with a front plate  21 . A front barrel  22  is formed integrally with the front plate  21  as best shown in FIGS.  1  to  3 . An objective lens  23  is fixedly held on the front barrel  22 . The open back end of the case  20  is covered with a back plate  24 .  
         [0051]     A pair of upper guide bars  26  and  27  is extended in an upper part of the interior of the case  20 . A lower guide bar  28  is extended in a lower part of the interior of the case  20 . The upper guide bars  26  and  27  and the lower guide bar  28  have front and back ends supported on the front plate  21  and the back plate  24 , respectively.  
         [0052]     As shown in FIGS.  3  to  5 , a movable zooming member  31 , namely, a lens frame, is guided for movement along the optical axis of the compound lens by the upper guide bar  26  and the lower guide bar  28 . The movable zooming member  31  holds a lens  32 . As shown in  FIGS. 3, 4  and  6 , a movable focusing member  33  is guided for movement along the optical axis the upper guide bar  27  and the lower guide bar  28 . The movable focusing member  33  holds a focusing lens  34  fixedly.  
         [0053]     As shown in  FIG. 3 , a partition wall  36  is disposed fixedly at a middle position with respect to a longitudinal direction between the movable zooming member  31  and the movable focusing member  33  in the case  20 . A middle barrel  37  is formed integrally with the partition wall  36  so as to protrude forward. Fixed middle lenses  38  are held fixedly on the middle barrel  37 . As shown in  FIG. 1 , an iris  39 , namely, an aperture stop, is inserted in a space between the fixed middle lenses  38 .  
         [0054]     A zooming linear motor  41  for moving the movable zooming member  31  will be described. Referring to  FIGS. 4 and 5 , the zooming linear motor  41  has a coil  42  having a shape resembling a rectangular tube. The coil  42  is attached to the movable zooming member  31  and is mounted for sliding on a guide core  43  substantially parallel to the upper guide bar  26 . A substantially U-shaped yoke  44  is mounted on the guide core  43  with the opposite ends thereof attached to the guide core  43 . A magnet  45  is attached to the lower surface of the yoke  44 . The magnetic flux of the magnet  45  penetrates the guide core  43 . Magnetic lines of force representing a magnetic field created by the magnet  45  extend through the yoke  44  and the guide core  43  to form a magnetic circuit.  
         [0055]     A position indicating magnet  46  is attached to the movable zooming member  31  fixedly holding the coil  42 . A magnetoresistive element  48  attached to a support plate  47  as shown in  FIG. 2  detects the position indicating magnet  46 . Power is fed to the coil  42  through a flexible printed cable  49  attached to the back surface, facing an image pickup device  61 , of the movable zooming member  31 . As shown in  FIGS. 1 and 2 , the yoke  44  is fitted in a slot  50  formed in the upper wall of the case  20 .  
         [0056]     A focusing linear motor  51  for moving the movable focusing member  33  holding the focusing lens  34  will be described with reference to  FIGS. 3, 4 ,  5  and  6 . The focusing linear motor  51  has a coil  52  having a shape resembling a rectangular tube. The coil  52  is guided for sliding movement by a guide core  53  parallel to the upper guide bar  27 . A substantially U-shaped yoke  54  is mounted on the guide core  53 . As shown in  FIGS. 1 and 2 , the yoke  54  is held in a slot  60  formed in the upper wall of the case  20 . A bar magnet  55  is attached to the lower surface of the yoke  54 .  
         [0057]     A position indicating magnet  56  is attached to a side surface, to which the coil  52  is attached, of the movable focusing member  33 . A magnetoresistive element  58  attached to the inner surface of a support plate  57  as shown in  FIG. 1  detects the position indicating magnet  56 . Power is fed to the coil  52  through a flexible printed cable  59  attached to a side surface, on the side of the objective lens  23 , of the movable focusing member  33 .  
         [0058]     An image forming unit included in an optical system provided with the compound lens is provided with an image pickup device  61 . The image pickup device  61  converts an optical image into electric image signals. The image pickup device  61  is, for example, a CCD. Image signals provided by the image pickup device  61  is converted into corresponding digital image signals by an A/D converter  62  shown in  FIG. 7 . A signal processor  63  processes the digital image signals to provide proper image signals. The image signals provided by the signal processor  63  are sent to an image pickup unit. If the image pickup unit is a video camera provided with a video signal recording device, the image signals received from the signal processor  63  are stored in the video signal recording device.  
         [0059]     The signal processor  63  is provided with a detecting circuit capable of detecting specific components of the image signals, such as luminance components in a high region. A detection signal provided by the detecting circuit is given to a controller  64  for controlling the image pickup operation of the image pickup device. The controller  64  determines a focusing condition, namely, a condition of focus, on the basis of the level of the detection signal and executes an automatic focus control operation to bring an object into focus.  
         [0060]     In the automatic focus control operation, the controller  64  executes a servo control process to make a servo circuit  66  generate a drive signal for driving the focusing linear motor  51  to move the focusing lens  34 . The drive signal is given to the focusing linear motor  51 . Then, the focusing linear motor  51  drives the movable focusing member  33  to adjust the position of the focusing lens  34  for focusing.  
         [0061]     When a zooming key is operated, a zooming signal is given to the controller  64 . The controller  64  operates on the basis of the zooming signal to give desired position information about a desired position of the zooming lens  32  to the servo circuit  66 . Then, the servo circuit  66  generates a drive signal for driving the zooming linear motor  41 , and the zooming linear motor  41  moves the movable zooming member  31  to place the zooming lens  32  at the desired position for framing.  
         [0062]     The moving speed of the movable zooming member  31  holding the zooming lens  32  can be varied by varying zooming key operating mode. When the zooming linear motor  41  is driven to move the movable zooming member  31  at the highest moving speed, the zooming lens  32  can be moved from a position corresponding to the shortest focal length for the widest angle of view to a position corresponding to the longest focal length for the narrowest angle of view in, for example, a very short time less than 1 s.  
         [0063]     Even if the position of the object relative to the camera is not changed, the focusing lens  34  is not in exact focus unless the position of the focusing lens  34  is changed according to the focal length when the image frame and hence the focal length is changed. Necessary information about the relation between focal length and the position of the focusing lens  34  in exact focus is stored beforehand in a storage device  65  connected to the controller  64 . When the focal length is changed, the controller  64  reads the information from the storage device  65 , gives a control signal specifying a focusing control operation to the servo circuit  66  to adjust the position of the focusing lens  34 .  
         [0064]     The focusing linear motor  51  is capable of driving the movable focusing member  33  holding the focusing lens  34  for movement at a very high moving speed. When the zooming lens  32  is moved at a high moving speed, the position of the focusing lens  34  is corrected according to the movement of the zooming lens  32 . Thus the position of the focusing lens  34  can be corrected to keep the focusing lens  34  in exact focus by a rapid position correcting operation.  
         [0065]     A compound lens assembling method of assembling the compound lenses, such as the objective lens  23 , the zooming lens  32 , the focusing lens  34  and the fixed middle lens  38 , will be described. Each of those lenses  23 ,  32 ,  34  and  38  are compound lens formed by assembling a plurality of single lenses and having improved optical characteristics. The lenses of the compound lens need to be arranged at predetermined axial intervals with their optical axis correctly aligned with each other. A compound lens assembling method will be described as applied to assembling the fixed middle lens  38  by way of example.  
         [0066]     Referring to  FIG. 8 , the fixed middle lens  38  includes a back lens  71 , namely, an upper lens as viewed in  FIG. 8 , a front lens  72 , namely, a lower lens as viewed in  FIG. 8 , and a spacer  73  spacing the back lens  71  and the front lens  72  from each other. The lenses  71  and  72  are fixed with their surfaces facing each other in contact with the opposite end surfaces of the spacer  73 , respectively.  
         [0067]     A bell chucking mechanism having an upper chucking unit  75  and a lower chucking unit  76  as shown in  FIG. 9  is used for assembling the lenses  71  and  72  and the spacer  73 . The respective center axes of the upper chucking unit  75  and the lower chucking unit  76  are aligned with each other. At least either of the upper chucking unit  75  and the lower chucking unit  76  is axially movable relative to the other with its center axis aligned with that of the other. The chucking units  75  and  76  can be moved toward each other for a pressing operation.  
         [0068]     The back lens  71  is seated on the spacer  73  with its curved surface in contact with a contact part  81  of the spacer  73 . The front lens  72  is in contact with the other contact part  82  of the spacer  73 . The chucking units  75  and  76  apply axial pressures to the lenses  71  and  72 . Consequently, the respective optical axes of the lenses  71  and  72  and the spacer  73  are aligned automatically with each other.  
         [0069]     When the chucking units  75  and  76  apply the axial pressures to the lenses  71  and  72  having curved surfaces symmetrical with respect to an axis, the lenses  71  and  72  are in the most dynamically stable state and at the axially shortest distance from each other as shown in  FIG. 9  when the respective optical axes of the lenses  71  and  72  and the spacer  73  are aligned with each other. Thus the lenses  71  and  72  and the spacer  73  can be automatically aligned by applying pressures to the lenses  71  and  72  by the chucking units  75  and  76 .  
         [0070]     The contact part  78  of the lower chucking unit  76  comes into contact with the front curved surface of the front lens  72  such that the center axis of the lower chucking unit  76  is aligned with the optical axis of the front lens  72 . The back curved surface of the front lens  72  is in stable contact with the contact part  82  of the spacer  73  such that the optical axis of the front lens  72  is aligned with the center axis of the spacer  73 . The upper chucking unit  75  presses the back lens  71  seated on the spacer  73  against the spacer  73 . Consequently, the curved front surface of the back lens  71  comes into close contact with the contact part  81  of the spacer  73  with the respective center axes of the spacer  73  and the upper chucking unit  75  and the optical axis of the back lens  71  aligned with each other.  
         [0071]     Thus the lenses  71  and  72  and the spacer  73  are combined stably with the respective optical axes of the lenses  71  and  72  and the center axis of the spacer  73  aligned with the respective center axes of the chucking units  75  and  76 . The lenses  71  and  72  and the spacer  73  thus combined are fixedly joined together with an adhesive  84 .  
         [0072]     The chucking mechanism having the centered chucking units  75  and  76  capable of being moved parallel to the optical axes of the lenses  71  and  72  clamps the lenses  71  and  72  with the spacer  73  held between the lenses  71  and  72 . The lenses  71  and  72  are bonded to the spacer  73  with an adhesive or are fastened to the spacer  73  by staking. When the lenses  71  and  72  are thus fastened to the spacer  73 , the tilt of the optical axes of the lenses  71  and  72  is corrected. The surfaces of each of the lenses  71  and  72  may be any one of combinations of curved surfaces.  
         [0073]     The lenses  71  and  72  of the fixed middle lens  38  shown in  FIG. 9  are bonded to the spacer  73  with the adhesive  84 . The adhesive  84  may be, for example, an ultraviolet curable adhesive that hardens when irradiated by ultraviolet rays. The combination of the lenses  71  and  72  and the spacer  73  may be fixedly clamped together by an annular clamping spring  85  as shown in  FIG. 10 .  
         [0074]     In assembling a fixed middle lens  38  shown in  FIG. 11 , a front lens  72 , a lower lens as viewed in  FIG. 11 , is put on a lower chucking unit  76  with the curved front surface thereof in contact with the contact part of the chucking unit  76 , a spacer  73  is placed on the flat back surface of the front lens  72  with its contact part  82  in contact with the back surface of the front lens  72 , a back lens  71 , namely, an upper lens as viewed in  FIG. 11 , is put on the spacer with the curved front surface thereof in contact with a contact part  81  of the spacer  73 , and an upper chucking unit  75  is pressed against the back surface of the back lens  71  to compress the assembly of the front lens  72 , the spacer  73  and the back lens  71  between the chucking units  75  and  76 . Thus the lenses  71  and  72  are held in contact with the spacer  73  in two contact planes containing the contact parts  81  and  82  of the spacer  73  and perpendicular to the optical axis.  
         [0075]     When the assembly of the front lens  72 , the spacer  73  and the back lens  71  are compressed between the chucking units  75  and  76 , the positions of the contact planes are adjusted automatically so as to be perpendicular to the center axes of the chucking units  75  and  76  and the optical axes of the lenses  71  and  72  on the principle of a bell chuck. After the lenses  71  and  72  and the spacer  73  have been thus aligned with each other, the lenses  71  and  72  are fixedly bonded to the spacer  73  with an adhesive  84 . The surfaces of the lenses  71  and  72  and the contact surfaces of the chucking units  75  and  76  to be brought into contact with the lenses  71  and  72  may be any suitable curved surfaces.  
         [0076]     A structure shown in  FIG. 12  will be described. A front lens  72  is put on a chucking unit  76  with its curved front surface in contact with the contact part  78  of the chucking unit  76 , a spacer  73  is put on the front lens  72  with its front contact part  82  in contact with the back surface of the front lens  72 , a back lens  71  is put on the spacer  73  with its front surface in contact with the back contact part  81  of the spacer  73 , and the lower end of a chucking unit  75  is brought into contact with the back surface of the back lens  71 .  
         [0077]     The chucking mechanism including the chucking units  75  and  76  and the lenses  71  and  72  are in contact at least in two contact planes perpendicular to the optical axis. The contact planes contain the contact part  77  of the upper chucking unit  75  in contact with the back lens  71 , and the contact part  78  of the lower chucking unit  76  in contact with the front lens  72 , respectively. The lenses  71  and  72  are bonded to the spacer  73  in a state where the two contact planes are perpendicular to the optical axis. The surfaces of each of the lenses  71  and  72  excluding the flat contact parts may be anyone of combinations of curved surfaces. The lenses  71  and  72  and the spacer  73  may be fixedly joined together by a holding spring  85  similar to that shown in  FIG. 10 .  
         [0078]     The automatic centering method using the chucking mechanism including the chucking units  75  and  76  can fixedly join together the lenses  71  and  72  and the spacer  73  in accurate alignment without performing optical centering using adjusting screws. The assembly of the lenses  71  and  72  and the spacer  73  is mounted on a lens barrel. Thus the lens barrel does not need to be provided with any lens adjusting mechanism and hence can be formed in a small size. Elimination of adjusting parts reduces component parts, assembling man-hours and adjusting man-hours. Accurate assembly of the compound lenses simplifies tools and jigs needed by a lens assembling process and facilitates assembling work.  
         [0079]     Although the invention has been described in its preferred embodiments, the present invention is not limited thereto in its practical application and many changes and variations are possible therein without departing from the scope and spirit thereof. Although the invention has been described as applied to the fixed middle lens of the lens system for a video camera, the present invention is applicable to compound lenses of other lens systems. The combination of the positions of the curved surfaces of the plurality of lenses and the chucking units and the spacer may be optionally changed within the technical scope of the present invention.  
         [0080]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.