Abstract:
A camera system is disclosed which allows different types of lens apparatuses to be used in combination with a single image-taking apparatus without requiring changes of focus control signals on the side of the image-taking apparatus depending on the type of a lens apparatus mounted on the image-taking apparatus. The camera system includes an image-taking apparatus which outputs a focus control signal and a lens apparatus which includes a focus lens and is mounted on the image-taking apparatus. The lens apparatus includes a controller which produces a drive signal according to a type of focus of the lens apparatus based on the focus control signal and controls drive of the focus lens based on the drive signal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a camera system which includes an image-taking apparatus, such as a television camera and a video camera, and a lens apparatus mounted on the image-taking apparatus. 
   2. Description of the Related Art 
   Camera systems for use in television broadcast or video recording include an auto-focus (hereinafter abbreviated as AF) type in which a video camera is integral with a lens and focusing is performed automatically, and a manual focus (hereinafter abbreviated as MF) type in which a lens is removably mounted on a camera and focusing control is performed in response to manual operation of a manipulation member. 
   Recently, the AF has also been used in camera systems which employ interchangeable lenses. 
   Conventionally, only AF-capable lenses can be used in AF camera systems and only MF-capable lenses can be used in MF camera systems. In recent years, however, there is an increasing need for camera systems in which both of MF-capable lenses and AF-capable interchangeable lenses can be used. 
   In the MF-capable lenses, however, a so-called front focus type is typically employed in which a focus lens is placed closer to an object side than a variable magnification lens. On the other hand, in the AF-capable lenses, a so-called rear focus type is usually employed in which a focus lens is placed closer to an image side than a variable magnification lens. This is because of the operability, controllability and portability of the lenses. 
   Japanese Patent Application Laid-Open No. H6 (1994)-62305 has disclosed an AF method in such a camera system, for example. In the AF method, a signal indicating the sharpness evaluation value of an object is extracted from a video signal taken by a camera to move a focus lens in a direction in which the strength of the signal is increased. The AF method is called a climbing method (a contrast detection method). 
   With a smaller size and a lighter weight of the entire camera system, zoom lenses of the rear focus type allowing reductions in size and weight are used in the AF-capable lenses. In the zoom lenses of the rear focus type, the position of an image plane changes with varied magnification even when the same distance to an object is maintained, so that the position of a focus lens needs to be adjusted in association with the varied magnification. Such a method of adjusting a focus lens has been disclosed, for example, in Japanese Patent Application Laid-Open No. H1 (1989)-280709, Japanese Patent Application Laid-Open No. H8 (1996)-220414 and the like. 
   Description is now made for the structure of a conventional zoom lens of the rear focus type. In  FIG. 4 , the zoom lens is comprised of four lens units including, in order from an object side, a fixed front lens unit (a first lens unit)  131 , a variable magnification lens unit  132  which is movable on an optical axis (a second lens unit, hereinafter referred to as a zoom lens unit), a fixed lens unit (a third lens unit)  136 , and a focus lens unit (a fourth lens unit)  118  which is movable on the optical axis for correcting image plane variations in varying magnification and for focusing. 
   Reference numeral  111  shows a zoom ring. When the zoom ring  111  is rotated, the zoom lens unit  132  is moved in the optical axis direction by a cam (not shown) formed on a fixed barrel  137  to perform variable magnification. 
   Reference numeral  115  shows a zoom motor which drives the zoom ring  111  through interlocking gears  133  and  134 . Reference numerals  119  and  120  show guide bars which guide a moving frame  122  for holding the focus lens unit  118  in the optical axis direction. 
   Reference numeral  121  shows a focus motor which is realized by a step motor, for example. The moving frame  122  has a screw portion formed thereon which engages with a screw shaft  123  which is driven by the focus motor  121 . Thus, when the screw shaft  123  is rotated by the focus motor  121 , the moving frame  122  is moved in the optical axis direction together with the focus lens unit  118 . Reference numeral  135  shows a stop unit which adjusts an amount of light. 
     FIG. 6  shows trajectory data which represents the positional relationship between the zoom lens unit  132  and the focus lens unit  118  on the optical axis for various object distances. The trajectory data shows the moving trajectories of the focus lens unit  118  for maintaining an in-focus state of an object at distances from INF (infinity) to MOD (minimum object distance). 
   For the rear focus lens, the focus lens unit  118  exists closer to an image side than the zoom lens unit  132 , so that simply driving the zoom lens unit  132  in the structure shown in  FIG. 4  causes the position of the image plane to be changed with varied magnification. To maintain the in-focus state, it is necessary to determine the position where the focus lens unit  118  should be placed from the position of the zoom lens unit  132  and the object distance in the trajectory data of  FIG. 6  to drive the focus lens unit  118  in association with varied magnification. 
   Next, description is made for AF processing in the aforementioned camera system which employs the rear focus lens with reference to a block diagram of  FIG. 5 . In  FIG. 5 , reference numeral  102  shows an image-pickup element such as a CCD sensor and a CMOS sensor, and  103  a camera processing circuit which produces a video signal based on an output signal from the image-pickup element  102 . The video signal is output to an AF circuit  104 . The AF circuit  104  extracts a high-frequency component from the video signal and outputs it as an AF evaluation value signal to a CPU  140 . 
   In normal AF processing (AF processing without varying magnification), the CPU  140  causes the focus lens unit  118  to wobble through a lens control section  139  on the zoom lens side to determine a drive direction of the focus lens unit  118  in which the strength of the AF evaluation value signal is increased. The CPU  140  outputs a focus control signal for driving the focus lens unit  118  by a predetermined drive amount in the determined direction and drives the focus lens unit  118  until the AF evaluation value signal is at the maximum. 
   In the AF processing with varied magnification, the CPU  140  calculates the drive amount of the focus lens unit  118  by using the trajectory data in  FIG. 6  stored in a trajectory data memory  145  and information about the current positions of the zoom lens unit  132  and the focus lens unit  118  detected as described later and outputs a focus control signal according to the calculation result. 
   On the zoom lens side, reference numeral  106  shows a zoom position detector which detects the position of the zoom lens unit  132  on the optical axis. Reference numeral  107  shows a zoom motor driver which drives the zoom motor  115 . Reference numeral  125  shows a focus motor driver which drives the focus motor  121  realized by the step motor. The number of drive pulses input to the focus motor  121  is counted by the CPU  140 . The count is used to detect the position of the focus lens unit  118 . 
   Reference numeral  138  shows a stop position detector which detects the position of blades (that is, a stop value) provided for the stop unit  135 . 
   The lens control section  139  produces a drive signal for the zoom lens unit  132  based on a zoom control signal produced in a zoom operation circuit  141  in response to manipulation of a zoom switch  142  such as a seesaw switch. The lens control section  139  also produces a drive signal for the focus lens unit  118  based on a focus control signal output from the CPU  140 . 
   The trajectory data shown in  FIG. 6  depends on optical characteristics of a zoom lens. For example, the moving amount of a zoom lens unit varies according to the magnification of a lens. For this reason, if interchangeable lenses having various optical characteristics are used for one camera, the trajectory data of each interchangeable lens is transmitted to the camera for storage in the trajectory data memory  145  when each interchangeable lens is mounted on the camera. 
   The focus control signal produced in the camera system which has the AF-capable lens of the rear focus lens type includes a signal which represents the drive amount of the focus lens unit  118  calculated by using the current positions of the zoom lens unit  132  and the focus lens unit  118  and the trajectory data in order to correct the image plane variations in varying magnification. 
   In contrast, in the camera system which has the MF-capable lens of the front focus type, the focus lens is placed closer to the object side than the variable magnification lens and thus the position of the image plane is not changed with varied magnification. This eliminates the need to drive the focus lens unit in varying magnification. 
   Now, description is made for the structure of a conventional front focus type zoom lens and a manual focus control system. In  FIG. 7 , on the side of a camera  201 , reference numeral  110  shows a zoom/focus operation circuit. Connected to the zoom/focus operation circuit  110  are a zoom operation member  112  which outputs an operation signal for servo-driving a zoom lens unit (not shown) and a focus operation member  113  which outputs an operation signal for servo-driving a focus lens unit (not shown). The zoom/focus operation circuit  110  produces and outputs control signals according to operation amounts of the respective operation members. Reference numeral  144  is a camera control section which is responsible for control of the camera. 
   On the other hand, on the zoom lens side, reference numeral  114  shows a focus motor which drives the focus lens unit through an interlocking gear (which has no reference numeral). Reference numeral  116  shows a focus motor driver which drives the focus motor  114  in response to a focus drive signal from a lens control section  143 . 
   Reference numeral  117  shows a focus position detector which detects the current position of the focus lens unit  118  and transmits it to the lens control section  143 . Reference numeral  124  shows a focus ring associated with the focus lens unit. The focus ring  124  is rotated by the focus motor  114  to move the focus lens unit, thereby performing manual focusing. 
   Reference numeral  143  shows the lens control section which controls the zoom motor  115  and the focus motor  114 . 
   When the focus operation member  113  is operated, an operation signal output from the focus operation member  113  is input to the lens control section  143  as a focus control signal through the zoom/focus operation circuit  110 . Similarly, when the zoom operation member  112  is operated, an operation signal output from the zoom operation member  112  is input to the lens control section  143  as a zoom control signal through the zoom/focus operation circuit  110 . 
   These control signals are converted by the lens control section  143  into a focus drive signal and a zoom drive signal for achieving drive of the motors and output to the drives  116  and  107 , respectively. In response to the drive signals, the drivers  116  and  107  drive the focus motor  114  and the zoom motor  115  to rotate the focus ring  124  and a zoom ring  111 , respectively. In this manner, the focus lens unit and the zoom lens unit are driven. 
   A controller  109  which can be used to perform zoom and focus operation is connected to the camera for use in such a system, besides the operation members  112  and  113 . The controller  109  outputs a control signal according to the amount of operation thereof, and the control signal is output to the lens control section  143  through the camera control section  144 . 
   As described above, when the rear focus lens is used to perform the AF control, the focus control signal output from the camera for correcting the image plane variations associated with varied magnification is a signal which represents the position to which the focus lens unit should be moved (the drive amount), determined from the trajectory data in  FIG. 6 , the current position of the zoom lens unit, the current position of the focus lens, and the object distance. 
   On the other hand, when the front focus lens is used to perform the MF control, the focus control signal output from the camera is a signal produced according to the operation amount of the focus operation member  113 . 
   In other words, the AF camera system with the rear focus lens and the MF camera system with the front focus lens produce the focus control signals on the camera sides in different manners, so that only dedicated interchangeable lenses can be used in each of the systems. 
   Next, description is made for a conventional camera system in which both of an MF-capable lens and an AF-capable lens can be used for one camera with reference to a block diagram of  FIG. 8 . In  FIG. 8 , components identical to those in  FIGS. 5 and 7  are designated with the same reference numerals as those in  FIGS. 5 and 7  and description thereof is omitted. 
   In  FIG. 8 , reference numeral  200  shows a lens control section which is responsible for control of a zoom lens. The zoom lens is an MF-capable interchangeable lens or an AF-capable interchangeable lens. 
   When the zoom lens is mounted on a camera  301 , a camera control section  305  provided in the camera  301  outputs a request for transmission of an identification signal for determining whether the zoom lens is an MF-capable lens or an AF-capable lens. In response to the request, the lens control section  200  transmits the identification signal back to the camera control section  305 . The camera control section  305  switches between methods of producing a focus control signal depending on the determination result. In this manner, the focus control signal appropriate for the mounted lens is produced and output to the lens. 
   It should be noted that some zoom lenses have no identification signals to be transmitted in response to the transmission request from the camera control section  305 , in which case an identification signal cannot be transmitted back. In this case, the camera control section  305  determines that no reply is made in a certain time period and considers the mounted lens as an MF-capable lens before it starts control. 
   The control of the MF-capable lens is performed as described above with reference to  FIG. 7 . The control of the AF-capable lens is performed as described above with reference to  FIG. 5 . When the AF-capable lens is mounted, the camera  101  takes the trajectory data shown in  FIG. 6  from the lens side and stores it in the trajectory data memory  145 . 
   As described above, in the conventional camera system, various types of lenses are used with different arrangements such as the front focus lens and the rear focus lens and with different focus control methods such as the MF control method and the AF control method. In a camera which can accept only a particular type of lens, if another type of lens is mounted thereon, normal focus control cannot be performed. 
   Specifically, since the focus control signal to be provided for the MF-capable front focus lens by the camera is not consistent with the focus control signal to be provided for the AF-capable rear focus lens, both types of lenses cannot be used in each of the MF-capable camera and the AF-capable camera. 
   As described in  FIG. 8 , it is possible that the type of the mounted lens is determined on the camera side to switch between the methods of producing the focus control signal. However, to produce the focus control signal appropriate for the AF-capable lens (the rear focus lens) on the camera side, the camera must take the large amount of trajectory data from the lens at power-up, and the camera needs to have a memory of large capacity for storing the trajectory data and a calculation function for producing the focus control signal suitable for the AF-capable lens. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a camera system which allows different types of lens apparatuses to be used in combination with a single image-taking apparatus (camera) without requiring changes of focus control signals on the side of the image-taking apparatus depending on the type of a lens apparatus mounted on the image-taking apparatus, a lens apparatus, and an interchangeable lens system. 
   According to an aspect, the present invention provides a camera system comprising an image-taking apparatus which outputs a focus control signal, and a lens apparatus which includes a focus lens and is mounted on the image-taking apparatus. The lens apparatus includes a controller which produces a drive signal according to a type of focus of the lens apparatus based on the focus control signal and controls drive of the focus lens based on the drive signal. 
   According to another aspect, the present invention provides a camera system comprising an image-taking apparatus which outputs a focus control signal, a first lens apparatus which includes a first focus lens and is mounted on the image-taking apparatus, and a second lens apparatus which includes a second focus lens and is mounted on the image-taking apparatus. The second lens apparatus employs a type of focus different from a type of focus of the first lens apparatus. The first lens apparatus includes a first controller which produces a first drive signal according to the type of the first lens apparatus based on the focus control signal and controls drive of the first focus lens based on the first drive signal. The second lens apparatus includes a second controller which produces a second drive signal according to the type of the second lens apparatus based on the focus control signal and controls drive of the second focus lens based on the second drive signal. 
   According to another aspect, the present invention provides a lens apparatus comprising a focus lens, and a controller which produces a drive signal according to a type of focus of the lens apparatus based on a focus control signal output from an image-taking apparatus and controls drive of the focus lens based on the drive signal. 
   According to yet another aspect, the present invention provides an interchangeable lens system comprising a first lens apparatus which includes a first focus lens and is mounted on the image-taking apparatus, a second lens apparatus which includes a second focus lens and is mounted on the image-taking apparatus. The second lens apparatus employs a type of focus different from a type of focus of the first lens apparatus. The first lens apparatus includes a first controller which produces a first drive signal according to the type of the first lens apparatus based on a focus control signal output from the image-taking apparatus and controls drive of the first focus lens based on the first drive signal. The second lens apparatus includes a second controller which produces a second drive signal according to the type of the second lens apparatus based on the focus control signal and controls drive of the second focus lens based on the second drive signal. 
   These and other characteristics of the camera system, the lens apparatus, and the interchangeable lens system of the present invention will be apparent from the following description of specific embodiments with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the structure of a camera system which includes a rear focus lens and a camera in an embodiment of the present invention; 
       FIG. 2  is a block diagram showing the structure of a camera system which includes a front focus lens and the camera in the embodiment of the present invention; 
       FIG. 3  is a flow chart showing a processing program performed in the rear focus lens and the front focus lens in the embodiment of the present invention; 
       FIG. 4  is a section view showing a conventional rear focus lens; 
       FIG. 5  is a block diagram showing the structure of a conventional camera system which includes the rear focus lens and a camera; 
       FIG. 6  is a graph showing trajectory data of a focus lens unit in connection with object distance and the position of a zoom lens unit; 
       FIG. 7  is a block diagram showing the structure of a conventional camera system which includes a front focus lens and a camera; and 
       FIG. 8  is a block diagram showing the structure of a conventional camera system which allows a manual focus support lens and an auto-focus support lens to be used. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention is hereinafter described with reference to the drawings. 
   The embodiment is described in connection with a camera system which is comprised of an image-taking apparatus such as a television camera and a video camera and a lens apparatus (an interchangeable lens) such as a television lens and a video lens. In the camera system, one camera can be used without changing focus control signals for any types of lenses including a rear focus type and a front focus type, and a manual focus (MF)-capable lens and an auto-focus (AF)-capable lens. 
     FIG. 1  shows the structure of the camera system in which a lens apparatus of the rear focus type (hereinafter referred to as a rear focus lens) is mounted on a camera.  FIG. 2  shows the structure of a camera system in which a lens of the front focus type (hereinafter referred to as a front focus lens) is mounted on the camera. In  FIGS. 1 and 2 , components identical to those described in the section “DESCRIPTION OF RELATED ART” are designated with the same reference numerals as those in  FIGS. 4 to 8 . 
   In  FIGS. 1 and 2 , reference numeral  101  shows the camera such as a video camera and a television camera (the image-taking apparatus). The camera can be used with both of an AF-capable lens and an MF-capable lens, similarly to that described in  FIG. 8 . However, the camera does not have the trajectory data memory  145  shown in  FIG. 8 . Reference numeral  401  shows a zoom lens which is realized by a rear focus lens. 
   On the side of the camera  101 , reference numeral  102  shows an image-pickup element such as a CCD sensor and a CMOS sensor, and  103  shows a camera processing circuit which produces a video signal based on an output signal from the image-pickup element  102 . The video signal is input to an AF circuit  104 . The AF circuit  104  extracts a high-frequency component from the video signal and outputs it as an AF evaluation value signal to a camera control section  105  serving as a controller. 
   Reference numeral  105  shows the camera control section which produces and outputs a focus control signal for performing AF control based on the AF evaluation value signal from the AF circuit  104 . The camera control section  105  also outputs toward the lens side a zoom control signal and a focus control signal based on operation signals for zoom and focus input from a zoom/focus controller  109  connected to the camera  101 . 
   The lens  401  shown in  FIG. 1  is the rear focus lens which has a focus lens unit  118  placed closer to an image side than a zoom lens unit  132 . The lens  401  has the same optical system structure as that of the optical system of the rear focus lens shown in  FIG. 4 . 
   On the side of the lens  401 , reference numeral  106  shows a zoom position detector which is coupled to a gear  115   a  engaging with a zoom ring  111  to detect the position of the zoom lens unit  132 . When the zoom ring  111  is rotated, the zoom lens unit  132  is driven by a cam formed on a fixed barrel (see  137  in  FIG. 4 ) in an optical axis direction to provide variable magnification. 
   Reference numeral  107  shows a zoom motor driver which drives a zoom motor  115  in response to a zoom drive signal from a lens control section  408 . 
   The lens control section  408  is responsible for control of the entire lens  401 . The lens control section  408  uses an AF control signal from the camera control section  105  and a zoom control signal and a focus control signal from a zoom/focus operation circuit  110  or the zoom/controller  109  to produce drive signals (a zoom drive signal and a focus drive signal) for the zoom lens unit  132  and the focus lens  118 . 
   The lens control section  408  relies on the focus control signal to produce the focus drive signal for driving the focus lens  118  depending on the type of focus based on lens arrangements such as the front focus type and the rear focus type or based on focus control methods such as manual focus and auto-focus. 
   The lens control section  408  has a memory, not shown, in which zoom tracking data matching the optical characteristics of the lens  401  is stored therein, corresponding to the trajectory data shown in  FIG. 6 . 
   A zoom operation member  112  and a focus operation member  113  are connected to the zoom/focus operation circuit  110 . The zoom operation member  112  produces and outputs the zoom control signal for servo-driving the zoom lens unit  132  according to an operation amount by a user. The focus operation member  113  produces and outputs the focus control signal for servo-driving the focus lens unit  118  according to an operation amount by a user. 
   Reference numeral  121  shows a focus motor which is realized by a step motor for driving the focus lens unit  118 . By the focus motor  121  is rotated a screw shaft  123 , to drive a moving frame  122  engaging therewith to the optical axis direction. The moving frame  122  holds the focus lens unit  118 . Reference numerals  119  and  120  show guide bars which guide the moving frame  122  in the optical axis direction. 
   Reference numeral  125  shows a focus motor driver which drives the focus motor  121  in response to the focus drive signal from the lens control section  408 . 
   Reference numeral  106  shows the zoom position detector which detects the position of the zoom lens unit on the optical axis. The number of drive pulses input to the focus motor  121  is counted by the lens control section  408 . The count is used to detect the position of the focus lens unit  118 . 
   On the other hand, a lens  501  shown in  FIG. 2  shows a front focus lens in which a focus lens unit  118  is placed closer to an object side than a zoom lens unit  132 . In  FIG. 2 , components identical to those in  FIG. 1  are designated with the same reference numerals as those in  FIG. 1 . A camera  101  is the same as the camera shown in  FIG. 1 . 
   In the lens  501 , reference numeral  114  shows a focus motor which drives the focus lens unit through interlocking gears  114   a  and  114   b . Reference numeral  116  shows focus motor driver which drives the focus motor  114  in response to a focus drive signal from a lens control section  508 . 
   Reference numeral  117  shows a focus position detector which detects the current position of the focus lens unit  118  and transmits it to the lens control section  508 . Reference numeral  124  shows a focus ring which engages with the focus lens unit  118 . The focus ring  124  is rotated by the focus motor  114  to move the focus lens unit  118  to perform manual focusing. 
   The lens control section  508  is responsible for control of the entire lens  501 , and produces and outputs a zoom drive signal and a focus drive signal for controlling a zoom motor  115  and the focus motor  114 , respectively. 
   When a focus operation member  113  is operated, an operation signal output from the focus operation member  113  is input to the lens control section  508  as a focus control signal through a zoom/focus operation circuit  110 . Similarly, when a zoom operation member  112  is operated, an operation signal output from the zoom operation member  112  is input to the lens control section  508  as a zoom control signal through the zoom/focus operation circuit  110 . The lens control section  508  produces a focus drive signal and a zoom drive signal based on the focus control signal and the zoom control signal and drives the focus motor  114  and the zoom motor  115  through the drivers  116  and  107 , respectively, to rotate the focus ring  124  and a zoom ring  111 . In this manner, the focus lens unit  118  and the zoom lens unit  132  are driven. 
   Next, description is made for a processing program used in common to the lens control section  401  of the rear focus lens  401  and the lens control section  508  of the front focus lens  501  in the camera system of the embodiment in which both of the rear focus lens  401  and the front focus lens  501  can be used for the one camera  101 , with reference to a flow chart of  FIG. 3 . 
   In the embodiment, the focus control signal output from the camera control section  105  of the camera  101  is a position signal which represents a drive position (a drive amount) of the focus lens unit  118  or a speed signal which represents a drive direction and a drive speed of the focus lens unit  118 . 
   The flow of the processing on the side of the AF-capable rear focus lens  401  is first described in the combination of the rear focus lens  401  and the camera  101  shown in  FIG. 1 . 
   At step  101 , the lens control section  408  takes a focus control signal from the camera control section  105 . Then, at step  102 , it determines whether the lens itself is a rear focus lens or a front focus lens. 
   Since the rear focus lens is used in this case, the flow proceeds to step  103  to determine the current focus mode is an AF mode or an MF mode. In the case of the AF mode, the flow proceeds to step  105  where the lens control section  408  monitors an output from the zoom position detector  106  to determine whether or not the zoom lens unit  132  is being driven (whether or not zooming is being driven). 
   When it is determined that zooming is not being driven at step  105 , the flow proceeds to step  106  to perform AF processing. The AF processing is later described. When the current focus mode is not the AF mode (that is, it is the MF mode) at step  103 , the flow proceeds to step  111 . At step  111 , it is determined whether or not zooming is being driven similarly to step  105 . 
   In the AF processing at step  106 , the following control is performed in the camera  101  and the rear focus lens  401 . When a luminous flux from an object passes through the rear focus lens  401  and arrives on a light-receiving surface of the image-pickup element  102 , the luminous flux is photoelectrically converted by the image-pickup element  102  and the resultant electric signal is output therefrom. The camera processing circuit  103  performs various types of processing on the output signal input thereto from the image-pickup element  102  to produce a video signal. 
   The AF circuit  104  extracts a high-frequency component from a portion of the video signal corresponding to a focus detection area set to the center of the image screen or the like to produce an AF evaluation value signal according to the sharpness (contrast) of the object image. For example, when the focus detection area is set to the center of the image screen, the camera system is always focused on an object present at the center of the image screen. 
   In normal AF processing without varying magnification (without zoom drive), the camera control section  105  outputs a command signal to the lens control section  408  for causing the focus lens unit  118  to wobble in order to determine the drive direction of the focus lens unit  118  in which the strength of the AF evaluation value signal output from the AF circuit  104  is increased. The camera control section  105  then outputs a focus control signal to the lens control section  408  such that the focus motor  121  is driven in the determined direction by a predetermined number of drive steps. The processing is repeated until the AF evaluation value signal is at the maximum, that is, until an in-focus state is achieved for the object. 
   When it is determined that zooming is being driven at step  105  or when it is determined that the current focus mode is not the AF mode (it is the MF mode) at step  103  and that zooming is being driven at step  111 , the flow proceeds to step  107 . 
   At steps  107  to  110 , the following AF processing (zoom tracking processing) is performed in order to correct image plane variations associated with zoom drive to maintain an in-focus state. In this case, the zoom tracking processing has higher priority, and focus control by the camera  101  is not performed. 
   First, at step  107 , the lens control section  408  takes the current position of the zoom lens unit  132  by the zoom position detector  106 . Next, at step  108 , it calculates the position of the focus lens unit  118  from the count of drive steps for the focus motor  121 . Specifically, the focus lens unit  118  is reset to a predetermined original position (a reference position) at power-up, and the drive steps from the reference position is counted to obtain the relative position of the focus lens unit  118  with respect to the reference position based on the count. 
   When the focus motor  121  is realized by a DC motor, the position of the focus lens unit  118  can be taken by using a focus position detector similar to the zoom position detector  106 . The detected positions of the zoom lens unit  132  and the focus lens unit  118  are stored in a memory, not shown, in the lens control section  408 . 
   Next, at step  109 , to perform focus control of moving the focus lens unit  118  on zoom tracking data in association with the zoom drive, the lens control section  408  first calculates a point (an in-focus position) on the zoom tracking data which matches the current position of the zoom lens unit  132  and the current position of the focus lens unit  118 . Next, the lens control section  408  calculates a drive amount of the focus lens unit  118  for moving the focus lens unit  118  corresponding to the movement amount of the zoom lens unit  132  on the zoom tracking data. 
   The lens control section  408  outputs a focus drive signal necessary for the drive of the calculated drive amount at step  110 , and drives the focus motor  121  through the focus motor driver  125  at step  120 . 
   When it is determined that zooming is not being driven at step  111 , the flow proceeds to step  112  where it is checked whether the focus control signal from the camera control section  105  is a position signal or a speed signal. In the case of the position signal, the position of the focus lens unit  118  is calculated from the count of drive steps for the focus motor  121  (step  113 ). Then, the lens control section  408  produces a focus drive signal based on the focus control signal and the calculated position of the focus lens unit  118  (step  114 ). 
   Since position servo is used in this case, the focus motor  121  is stopped when it is determined that the drive position indicated by the focus control signal is equal to the position of the focus lens unit  118  calculated from the count of the drive steps for the focus motor  121 . 
   On the other hand, when it is determined that the focus control signal is the speed signal (speed servo) at step  112 , the lens control section  408  considers the focus control signal as the speed signal and produces a focus drive signal (step  115 ). Then, it drives the focus motor  121  through the focus motor driver  125  (step  120 ). 
   Next, description is made for the flow of processing on the side of the front focus lens  501  in the combination of the MF-capable rear focus lens  501  and the camera  101  shown in  FIG. 2 . 
   When the lens control section  501  takes a focus control signal at step  101 , it determines whether the lens itself is a rear focus lens or a front focus lens. Since the front focus lens is used in this case, the flow proceeds to step  104  to check whether the focus control signal is a position signal or a speed signal. In the case of the position signal, the focus position detector  117  takes the position of the focus lens unit  118  (step  116 ). 
   The lens control section  508  produces a focus drive signal from the focus control signal and the taken position of the focus lens unit  118  (step  118 ). Since position servo is used in this case, the focus motor  114  is stopped when it is determined that the position indicated by the focus control signal is equal to the position of the focus lens unit  118  detected by the focus position detector  117 . 
   When it is determined that the focus control signal is the speed signal at step  104 , the lens control section  501  considers the focus control signal as the speed signal and produces a focus drive signal (step  117 ). 
   Then, it drives the focus motor  114  through the focus motor drive  116  (step  120 ). 
   In this manner, according to the embodiment, it is possible for the lens to determine the type of focus in the lens, that is, the type of the lens arrangement such as the front focus type and the rear focus type, and the type of the focus control method such as the manual focus support and the auto-focus support to produce the focus drive signal according to the determined type based on the focus control signal from the camera side. This eliminates the need to change the focus control signal output from the camera side depending on the type of the lens mounted on the camera. Consequently, a plurality of types of lenses can be used for one camera. 
   In addition, the focus control calculations are made by using the trajectory data (the zoom tracking data) in the AF-capable lens, so that it is not necessary to transmit the large amount of trajectory data from the lens to the camera. Moreover, it is not necessary for the camera to have a memory for storing the trajectory data or have the function of focus control calculations with the trajectory data. 
   While a preferred embodiment has been described, it is to be understood that modification and variation of the present invention may be made without departing from the scope of the following claims. 
   This application claims priority from Japanese Patent Application No. 2003-343923 filed on Oct. 2, 2003, which is hereby incorporated by reference herein.