Patent Publication Number: US-8542314-B2

Title: Camera system including camera body and interchangeable lens for performing focus control

Description:
TECHNICAL FIELD 
     The present invention relates to a lens-interchangeable camera system, and more particularly to a camera system capable of performing an autofocus operation in a contrast detection method. 
     BACKGROUND ART 
     Patent document 1 discloses autofocus control that employs both a phase-difference detection method and a contrast method. An imaging apparatus disclosed in the Patent document 1 corrects focus control information obtained by the phase-difference detection method, based on focus information obtained by the contrast method. Specifically, correction information for compensating for insufficient detection accuracy of the phase-difference detection which has excellent high-speed performance is obtained based on information indicating a focus status obtained by using contrast detection which enables focus determination at high accuracy. This arrangement can provide focus control by the phase-difference detection method with higher speed and higher accuracy compared to the hybrid method, and the like. 
     Patent Document 1: JP-A-2003-295047 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     As described above, the Patent document 1 discloses an improvement in the focus accuracy of a lens-interchangeable camera system. However, the Patent document 1 discloses an invention for improving the focus accuracy of the phase-difference detection method by correcting focus control information obtained by the phase-difference detection method based on focus information obtained by the contrast method. It does not disclose an improvement in the accuracy of focus information itself that is obtained by the contrast method. 
     An object of the present invention is to provide a lens-interchangeable camera system having improved accuracy of an autofocus operation in a contrast method. 
     Solving Means 
     A camera system according to the present invention includes an interchangeable lens and a camera body. The interchangeable lens includes a focus lens operable to move back and forth in an optical axis direction to change a focus status of a subject image; a driver operable to drive the focus lens; a position detector operable to detect a position of the focus lens; a storage unit operable to store information on the detected position; a position information transmission unit operable to transmit the position information stored in the storage unit to the camera body; and a lens controller operable to control an operation of the interchangeable lens under control of the camera body. The camera body includes an imaging unit operable to image a subject to generate image data; a timing signal transmission unit operable to generate a predetermined timing signal and transmit the timing signal to the interchangeable lens; a drive signal transmission unit operable to generate a drive signal for driving the focus lens and transmit the drive signal to the interchangeable lens; a position information request transmission unit operable to generate a position information request signal for requesting a transmission of position information stored in the storage unit of the interchangeable lens and transmit the position information request signal to the interchangeable lens; and a body controller operable to control an operation of the camera body. The body controller controls the drive signal transmission unit to drive the focus lens in a predetermined direction, and performs control to transmit the timing signal to the interchangeable lens. The lens controller performs control to detect a position of the focus lens in synchronization with the timing signal received from the camera body and store information on the detected position in the storage unit. Further, the lens controller, when receiving the position information request signal from the camera body, performs control to transmit the position information stored in the storage unit to the camera body. The body controller performs control so that a timing of transmission of the position information request signal does not overlap a timing of transmission of the timing signal. 
     According to camera system having the above-described configuration, a position of the focus lens can be accurately obtained with the focus lens being moved, and thus fast and accurate contrast autofocus operation can be achieved. Further the lens controller does not receive the timing signal and the position information request signal at the same time. As a result, the lens controller can perform a lens position detection process done when receiving the timing signal and so on, exclusively from a process for transmitting lens position information done when receiving the position information request signal. 
     EFFECT OF THE INVENTION 
     According to the present invention, in a lens-interchangeable camera system, the accuracy of a contrast autofocus operation can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a camera system according to embodiments of the invention. 
         FIG. 2  is an explanatory diagram for describing an imaging preparation operation of the camera system. 
         FIG. 3  is an explanatory diagram for describing a contrast AF operation. 
         FIGS. 4A to 4M  are timing charts for a contrast AF operation performed by a camera system according to a first embodiment of the invention. 
         FIG. 5  is a flowchart of a contrast AF operation performed by the camera system according to the first embodiment of the invention. 
         FIG. 6  is a flowchart of a contrast AD operation performed by a camera system according to a second embodiment of the invention. 
         FIGS. 7A to 7L  are timing charts for a contrast AF operation performed by the camera system according to the second embodiment of the invention. 
         FIG. 8  is a diagram for describing an example of a transmission prohibition period of an exposure synchronizing (sync) signal. 
         FIG. 9  is a flowchart of a contrast AF operation performed by a camera system according to a third embodiment of the invention. 
         FIGS. 10A to 10L  are timing charts for a contrast AF operation performed by the camera system according to the third embodiment of the invention. 
         FIG. 11  is a flowchart of a contrast AF operation performed by a camera system according to a fourth embodiment of the invention. 
         FIGS. 12A to 12L  are timing charts for a contrast AF operation performed by the camera system according to the fourth embodiment of the invention. 
     
    
    
     DESCRIPTION OF REFERENCE SIGNS 
       1  camera system 
       100  camera body 
       110  CCD image sensor 
       112  timing generator 
       130  release button 
       140  camera controller 
       150  body mount 
       200  interchangeable lens 
       230  focus lens 
       231  first encoder 
       232  second encoder 
       233  focus motor 
       240  lens controller 
       250  lens mount 
     Best Modes for Carrying Out the Invention 
     First Embodiment 
     1-1. Configuration 
     1-1-1. Overview 
       FIG. 1  is a block diagram showing a configuration of a camera system according to embodiments of the invention. A camera system  1  includes a camera body  100  and an interchangeable lens  200  mountable to the camera body  100 . The camera system  1  can perform an autofocus operation in a contrast method, based on image data generated by a CCD image sensor  110 . 
     1-1-2. Configuration of the Camera Body 
     The camera body  100  includes the CCD image sensor  110 , a liquid crystal display (LCD) monitor  120 , a camera controller  140 , a body mount  150 , a power supply  160 , and a card slot  170 . 
     The camera controller  140  controls the entire operation of the camera system  1  by controlling components such as the CCD image sensor  110  in response to an instruction from operation members such as a release button  130 . The camera controller  140  transmits a vertical synchronizing signal to a timing generator  112 . In parallel with this, the camera controller  140  generates an exposure synchronizing (sync) signal. The camera controller  140  periodically transmits the generated exposure synchronizing signal to a lens controller  240  through the body mount  150  and a lens mount  250 . The camera controller  140  uses a DRAM  141  as a work memory when performing a control operation or image processing operation. 
     The CCD image sensor  110  images a subject image that enters through the interchangeable lens  200  to generate image data. Specifically, the CCD image sensor  110  is exposed at a predetermined timing to capture a subject image and generate image data. The generated image data is digitized by an AD converter  111 . The digitized image data is subjected to predetermined image processing by the camera controller  140 . The predetermined image processing includes, for example, a gamma correction process, a white balance correction process, a flaw correction process, a YC conversion process, an electronic zoom process, and a JPEG compression process. 
     The CCD image sensor  110  operates at timing that is controlled by the timing generator  112 . The operations of the CCD image sensor  110  include a still-image capturing operation, a through-the-lens image (hereinafter, referred to as a “through image”) capturing operation, and the like. The through image is mainly a moving image and is displayed on the LCD monitor  120  so that a user can determine composition for capturing a still image. 
     The LCD monitor  120  displays an image represented by display image data that is subjected to image processing by the camera controller  140 . The LCD monitor  120  can selectively display both a moving image and a still image. 
     The card slot  170  allows the memory card  171  to be placed therein, and controls the memory card  171  under control of the camera controller  140 . The memory card  171  can store image data generated by image processing by the camera controller  140 . The memory card  171  can store, for example, JPEG image files. Image data or an image file stored in the memory card  171  can be read. The image data or image file read from the memory card  171  is subjected to image processing by the camera controller  140 . For example, the camera controller  140  decompresses image data or an image file obtained from the memory card  171  to generate display image data. 
     The power supply  160  supplies power to be consumed by the camera system  1 . The power supply  160  may be, for example, a dry battery or rechargeable battery. Alternatively, the power supply  160  may supply power to the camera system  1  from an external source through a power cable. 
     The body mount  150  can be mechanically and electrically connected to the lens mount  250  of the interchangeable lens  20 . The body mount  150  can transmit and receive data with the interchangeable lens  200  through the lens mount  250 . The body mount  150  transmits an exposure synchronizing signal received from the camera controller  140  to the lens controller  240  through the lens mount  250 . Also, the body mount  150  transmits other control signals received from the camera controller  140 , to the lens controller  240  through the lens mount  250 . Also, the body mount  150  transmits a signal received from the lens controller  240  through the lens mount  250  to the camera controller  140 . Also, the body mount  150  supplies power received from the power supply  160  to the entire interchangeable lens  200  through the lens mount  250 . 
     1-1-3. Configuration of the Interchangeable Lens 
     The interchangeable lens  200  includes an optical system, the lens controller  240 , and the lens mount  250 . The optical system includes a zoom lens  210 , an OIS lens  220 , and a focus lens  230 . 
     The zoom lens  210  is a lens for changing the magnification of a subject image to be formed by the optical system. The zoom lens  210  includes one or a plurality of lenses. A drive mechanism  211  includes a zoom ring or the like that can be operated by a user, and transmits an operation of the user to the zoom lens  210  to cause the zoom lens  210  to move along an optical axis direction of the optical system. A detector  212  detects a moving amount of the drive mechanism  211 . The lens controller  240  can grasp a zoom magnification of the optical system by receiving a result of the detection by the detector  212 . 
     The OIS lens  220  is a lens for correcting shake of a subject image to be formed by the optical system of die interchangeable lens  200 . The OIS lens  220  moves in a direction to cancel the shake of the camera system  1 , thereby reducing shake of a subject image on the CCD image sensor  110 . The CIS lens  220  includes one or a plurality of lenses. An actuator  221  drives the OIS lens  220  in a plane vertical to an optical axis of the optical system, under the control of an OIS IC  223 . The actuator  221  can be implemented by, for example, a magnet and a planar coil. A position detection sensor  222  is a sensor for detecting a position of the OIS lens  220  in the plane vertical to the optical axis of the optical system. The position detection sensor  222  can be implemented by, for example, a magnet and a Hall element. The OIS IC  223  controls the actuator  221  based on a detection result obtained by the position detection sensor  222  and a detection result obtained by a camera shake detector such as, for example, a gyro sensor. The OIS IC  223  obtains a detection result by the camera shake detector from the lens controller  240 . Also, the OIS IC  223  transmits a signal indicating a status of an optical image shake correction process to the lens controller  240 . 
     The focus lens  230  is a lens for changing a focus status of a subject image to be formed on the CCD image sensor  110  through the optical system. The focus lens  230  includes one or a plurality of lenses. 
     A focus motor  233  drives the focus lens  230  to move back and forth along the optical axis of the optical system under control of the lens controller  240 . By this, the focus status of a subject image to be formed on the CCD image sensor  110  through the optical system can be changed. In the present embodiment, as the focus motor  233 , a EC motor can be used. Note, however, that the invention is not limited thereto and the focus motor  233  can also be implemented by a stepping motor, a servo motor, an ultrasonic motor, and the like. 
     A first encoder  231  and a second encoder  232  are encoders that generate a signal indicating a driving status of the focus lens  230 . The first encoder  231  and the second encoder  232  can be implemented by, for example, a rotor mounted on a rotating shaft of the focus motor  233  and a photocoupler. The rotor is a disk having holes formed at predetermined intervals. The photocoupler emits a detection light from one side of the rotor and receives the light from the other side. Hence, along with rotation of the rotor, the photocoupler is alternately switched on and off. The lens controller  240  has a counter  243  provided therein. The counter  243  counts the number of times the photocoupler is switched on and off. The phases of the first encoder  231  and the second encoder  232  are shifted from each other. Therefore, a moving direction of the focus lens  230  when the first encoder  231  is switched from off to on can be determined. Specifically, when the first encoder  231  is switched from off to on, the second encoder  232  can have two statuses: on and off. When the first encoder  231  is switched from off to on with the second encoder  232  being on, the counter  243  determines this to be a forward rotation and thus counts up, as “+1”. On the other hand, when the first encoder  231  is switched from off to on with the second encoder  232  being off, the counter  243  determines this to be a reverse rotation and thus counts down, as “−1”. By thus counting, the lens controller  240  can grasp the amount of movement of the focus lens  230 . 
     The lens controller  240  controls the focus motor  233  in a first control mode or a second control mode. The first control mode is a control mode in which the lens controller  240  cannot detect a reversal of the driving direction of the focus lens  230  based on only a detection result of a position of the focus lens  230  obtained by a position detector. The second control mode is a control mode in which the lens controller  240  can detect a reversal of the driving direction of the focus lens  240  based on only a detection result of the position detector. 
     Specifically, in the first control mode, the lens controller  240  controls the focus motor  233  using only the first encoder  231 . Whether the focus motor  233  rotates forward or backward cannot be determined only by knowing that the first encoder  231  is switched from off to on. Hence, in the first control mode, the lens controller  240  is unable to detect a reversal of the driving direction of the focus lens  230 . 
     On the other hand, in the second control mode, the lens controller  240  controls the focus motor  233  using the first encoder  231  and the second encoder  232 . When using the first encoder  231  and the second encoder  232 , as described above, whether the focus motor  233  rotates forward or backward when the first encoder  231  is switched from off to on can be determined by the status of the second encoder  232 . Hence, in the second control mode, the lens controller  240  can detect a reversal of the drive direction of the focus lens  230 . 
     The lens controller  240  controls the entire interchangeable lens  200  by controlling the OIS IC  223 , the focus motor  233 , and the like, based on control signals from the camera controller  140 . Also, the lens controller  240  receives signals from the detector  212 , the OIS IC  223 , the first encoder  231 , the second encoder  232 , and the like, and transmits the signals to the camera controller  140 . Transmission and reception of data between the lens controller  240  and the camera controller  140  is performed through the lens mount  250  and the body mount  150 . The lens controller  240  uses a DRAM  241  as a work memory when performing control. A flash memory  242  stores programs and parameters to be used for the control by the lens control  240 . 
     1-1-4. Correspondence of Configurations Between the Embodiments and the Invention 
     The focus motor  233  is an example of a driver of the present invention. A configuration including the first encoder  231  and the counter  243  or a configuration including the first encoder  231 , the second encoder  232 , and the counter  243  is an example of a position detector of the present invention. A configuration including the DRAM  241  or the flash memory  242  is an example of a storage unit of the present invention. A configuration including the lens controller  240  and the lens mount  250  is an example of a position information transmission unit of the present invention. A configuration including the lens controller  240  and the lens mount  250  is an example of a first position detection cancellation unit of the present invention. The lens controller  240  is an example of a lens controller of the present invention. The CCD image sensor  110  is an example of an imaging unit of the present invention. A configuration including the camera controller  140  and the body mount  150  is an example of a timing signal transmission unit of the present invention. A configuration including the camera controller  140  and the body mount  150  is an example of a drive signal transmission unit of the present invention. The camera controller  140  is an example of a body controller of the present invention. A configuration including the camera controller  140  and the body mount  150  is an example of a second position detection cancellation unit of the present invention. 
     1-2. Operation 
     1-2-1. Imaging Preparation Operation 
     First, the operation of the camera system  1  for imaging preparation will be described.  FIG. 2  is a diagram showing signal transmission and reception in an imaging preparation operation of the camera system  1 . 
     When a user turns on power to the camera body  100  with the interchangeable lens  200  mounted on the camera body  100 , the power supply  160  supplies power to the interchangeable lens  200  through the body mount  150  and the lens mount  250  (S 11 ). Then, the camera controller  140  requests the lens controller  240  for authentication information of the interchangeable lens  200  (S 12 ). The authentication information of the interchangeable lens  200  includes information on whether the interchangeable lens  200  is mounted and information on whether accessories are mounted. The lens controller  240  responds to the lens authentication request from the camera controller  140  (S 13 ). 
     Then, the camera controller  140  requests the lens controller  240  to perform an initialization operation (S 14 ). In response to this, the lens controller  240  performs an initialization operation, such as reset of an iris and reset of the OIS lens  220 . The lens controller  240  then sends a response indicating that the lens initialization operation has been completed to the camera controller  140  (S 15 ). 
     Then, the camera controller  140  requests the lens controller  240  for lens data (S 16 ). The lens data is stored in the flash memory  242 . The lens controller  240  reads the lens data from the flash memory  242  and sends the lens data back to the camera controller  140  (S 17 ). The lens data includes characteristic values specific to the interchangeable lens  200 , such as, for example, a lens name, F-number, and a focal length. 
     When the camera controller  140  grasps the lens data of the interchangeable lens  200  mounted on the camera body  100 , the camera system  1  goes into a status capable of imaging. In this status, the camera controller  140  periodically requests the lens controller  240  for lens status data indicating a status of the interchangeable lens  200  (S 18 ). The lens status data includes, for example, information of zoom magnification of the zoom lens  210 , information of position of the focus lens  230 , and information of aperture value. In response to the request, the lens controller  240  sends back the requested lens status data to the camera controller  140  ( 519 ). 
     In this status, the camera system  1  can operate in a control mode in which an image represented by image data generated by the CCD image sensor  110  is displayed on the LCD monitor  120  as a through image. This control mode is called “live view mode”. In the live view mode, a through image, which is a moving image, is displayed on the liquid crystal monitor  120  so that the user can determine composition for imaging a still image while viewing the LCD monitor  120 . For the method of an autofocus operation in the live view mode, a contrast method is generally used. This is because in the live view mode, image data is steadily generated by the CCD image sensor  110 , and thus it is easy to perform a contrast autofocus operation using the generated image data. 
     When performing an autofocus operation in a contrast method, the camera controller  140  requests the lens controller  240  for data for contrast AF (S 20 ). The data for contrast AF is data required for performing an autofocus operation in a contrast method, and includes, for example, a focus drive speed, the amount of focus shift, an image magnification, and information indicating whether contrast AF can be performed. 
     1-2-2. Contrast Autofocus Operation 
     Next, an autofocus operation of the camera system  1  after the completion of imaging preparation will be described. Here, an autofocus operation in a contrast method (hereinafter, referred to as a “contrast AF operation”) will be described. 
       FIG. 3  is a diagram explaining the movement of a focus lens when a contrast AF operation of the camera system  1  according to the present embodiment is performed. As shown in the diagram, when the lens is moved from a position away from a focus position, the contrast value of an image becomes higher as the lens comes closer to the focus position, and reaches a maximum at the focus position. In the contrast AF operation according to the present embodiment, while the focus lens is continuously moved without being stopped, an image is captured at a timing that synchronizes with an edge change of an exposure synchronizing signal and position information of the focus lens at the timing is obtained. Then, a contrast value is obtained for each captured image. A position of the focus lens at which the contrast value reaches a maximum is obtained and then the position is determined to be a focus position. As shown in  FIG. 3 , a path of contrast values obtained when the focus lens is moved from a position away from a focus position toward the focus position represents a mountain shape. In the following description, an operation of causing the focus lens to move from a position away from a focus position toward the focus position is called “mountain-climbing (M/C)”. 
       FIGS. 4A to 4M  are timing charts when performing an autofocus operation according to the present embodiment. In  FIGS. 4A to 4M , it is assumed that the camera controller  140  operates in the live view mode. As shown in  FIG. 4A , the camera controller  140  periodically generates a vertical synchronizing signal (CCDVD). The camera controller  140  generates, as shown in  FIG. 4C , an exposure synchronizing signal from a predetermined time based on an electronic shutter driving signal and the vertical synchronizing signal. The camera controller  140  grasps in advance exposure start timing and exposure end timing with respect to the vertical synchronizing signal, and thus, can generate an exposure synchronizing signal. 
     The camera controller  140  outputs the vertical synchronizing signal to the timing generator  112  and transmits the exposure synchronizing signal to the lens controller  240 . Specifically, the camera controller  140  generates a timing signal at timing correlated with timing of the exposure and transmits the generated timing signal to the interchangeable lens  200  through the body mount  150 . Here, the timing signal corresponds to a rising edge and a falling edge of the exposure synchronizing signal. The lens controller  240  obtains position information of the focus lens  230  for a contrast autofocus operation, in synchronization with the received exposure synchronizing signal. Specifically, the lens controller  240  obtains a position of the focus lens  230  at timing of a rising edge and a falling edge of the received exposure synchronizing signal to store the obtained position in the DRAM  241  (see  FIG. 3 ). 
     The timing generator  112  periodically generates an electronic shutter drive driving signal (see  FIG. 4B ) based on the vertical synchronizing signal, and drives CCD image sensor  110  based on the vertical synchronizing signal and the electronic shutter drive signal. 
     The CCD image sensor  110  performs an electronic shutter operation according to the electronic shutter drive signal (see  FIG. 4B ). The electronic shutter operation causes the CCD image sensor  110  to sweep out unnecessary charges externally. 
     The CCD image sensor  110  sweeps out charges by the electronic shutter drive signal and reads pixel data to the vertical transferring unit (not shown) by a read signal. That is, during a period from the last one of electronic shutter drive signals to a vertical synchronizing signal, an exposure operation is performed for image data for a through image. The exposure synchronizing signal is a signal indicating an exposure period, going up at a timing the edge of the last electronic shutter goes up and going down at a timing the edge of the vertical synchronizing signal goes up. 
     1-2-2-1. Processing Flow 
     The flow of a contrast AF operation of the camera system  1  according to the first embodiment will be described with reference to a flowchart in  FIG. 5 . 
     The camera controller  140  monitors whether the release button  130  has been half-pressed (S 101 ). For example, in  FIGS. 4A to 4M , when the release button  130  is half-pressed at time t 1 , the camera controller  140  transmits an AF start command to the lens controller  240 , as shown in  FIG. 4D . The AF start command is a command for instructing the start of an autofocus operation in a contrast method. 
     In response to this command, the lens controller  240  changes the control mode of the focus motor  233  from the first control mode to the second control mode (S 102 ). In  FIG. 4E , a period of “Low” is a period of the first control mode, while a period of “High” is a period of the second control mode. Then, the lens controller  240  stores in the DRAM  241  a value (the number of pulses) of the counter  243  when the control mode is changed to the second control mode. Subsequently, the lens controller  240  controls the focus motor  233  with reference to the value. 
     The reason why the control mode is changed from the first control mode to the second control is that the second control mode is relatively superior to the first control mode in the accuracy of detection of a reversal of the driving direction of the focus lens  230  that is not intended by the lens controller  240 . 
     Thereafter, the camera controller  140  transmits, at time t 2 , a mountain-climbing (M/C) start point move command to the lens controller  240 . In the example of  FIGS. 4A to 4M , at time t 2 , the mountain-climbing start point move command is transmitted. This command is a command indicating a position to which the focus lens  230  is moved at the time of the start of a contrast AF operation and a moving direction of the focus lens  230  during detection of an AF evaluation value. In response to this, the lens controller  240  controls the focus motor  233 . The focus motor  233  causes the focus lens  230  to move to a position indicated by the mountain-climbing start point move command, under the control of the lens controller  240  (S 103 ). 
     Then, the camera controller  140  transmits a mountain-climbing (M/C) start command to the lens controller  240  (S 104 ). In the example of  FIGS. 4A to 4M , at time t 3 , the mountain-climbing start command is transmitted. The lens controller  240  drives the focus motor  233  in the second control mode, as shown in  FIG. 4E , in response to an instruction from the camera controller  140 . 
     After the transmission of the mountain-climbing start command, as shown in  FIG. 4C , the camera controller  140  transmits an exposure synchronizing signal to the lens controller  240  (S 105 ). Specifically, the exposure synchronizing signal is transmitted from the camera controller  140  to the lens controller  240  in synchronization with an exposure period from the first exposure timing after the transmission of the mountain-climb start command. By thus transmitting the exposure synchronizing signal after the transmission of the mountain-climbing start command, it can be avoided to transmit simultaneously the mountain-climbing start command and the exposure synchronizing signal. By this, the risk of losing information due to a collision between the mountain-climbing start command and the exposure synchronizing signal can be reduced. 
     The lens controller  240  sequentially stores, in the DRAM  241 , the number of pulses of the counter  243  obtained when the exposure synchronizing signal is switched from off to on or from on to off (S 105 ). The number of pulses of the counter  243  is information indicating a position of the focus lens  230 . The lens controller  240  stores position information of the focus lens  230  in the DRAM  241  in synchronization with switching of an edge of the exposure synchronizing signal (see  FIG. 3 ). 
     The CCD image sensor  110  is exposed during an exposure period and transmits generated image data to the camera controller  140  through the AD converter  111 . The camera controller  140  calculates an evaluation value for an autofocus operation (hereinafter, referred to as an “AF evaluation value”), based on the received image data (S 105 ). For an AF evaluation value calculation method, it is known that luminance signals are obtained from image data generated by the CCD image sensor  110  and high frequency components of the luminance signals in the image are added up, so that an AF evaluation value is obtained. The calculated AF evaluation value is stored in the DRAM  141  in which the calculated AF evaluation value is associated with the exposure synchronizing signal. 
     After the exposure synchronizing signal is transmitted from the camera controller  140  to the lens controller  240  and the position information of the focus lens  230  is stored in the DRAM  241  by the lens controller  240  and the camera controller  140  calculates the AF evaluation value of the image data obtained from the CCD image sensor  110 , the camera controller  140  and the lens controller  240  each determine whether one of them has received an end signal from the other of them (S 106 ). The end signal is a signal for terminating the communication for lens position detection between the camera controller  140  and the lens controller  240 . 
     Below, timing at which each of the camera controller  140  and the lens controller  240  transmits an end signal, i.e., condition of canceling a lens position detection, will be described. 
     The camera controller  140  can transmit an end signal to the lens controller  240 , when, for example, a calculated All evaluation value goes over a mountain of mountain-climbing and a focus position is found. Alternatively, the camera controller  140  may transmit an end signal to the lens controller  240 , when a user performs a playback operation of image data stored in the memory card  171  or a shooting-mode end operation, such as turning off power of the camera system  1 . 
     The lens controller  240  transmits an end signal to the camera controller  140 , when, for example, the focus lens  230  reaches a far limit or near limit. With this arrangement, when the focus lens  230  can no longer be driven, the camera controller  140  stops calculation of an AF evaluation value. As a result, the possibility that the camera controller  140  makes an erroneous focus determination based on a nonsense AF evaluation value can be reduced. 
     Alternatively, the lens controller  240  may transmit an end signal to the camera controller  140 , when the DRAM  241  runs short of its free space and thus no more number of pulses of the counter  243  can be stored. In this configuration, driving of the focus lens  230  stops when the DRAM  241  runs short of its free space. A case can be avoided, in which lens position information for the AF evaluation value cannot be obtained despite the fact that the camera body  100  obtains an AF evaluation value of image data. When the DRAM  241  has shortage of free space, the position information stored in the DRAM  241  may be transmitted immediately, resulting in free space in the DRAM  241 . This allows the lens position information to be stored continuously without stopping the lens drive. 
     For example, when the zoom lens  210  is moving or when relatively large motion such as camera shake occurs in the camera system  1 , the camera controller  140  does not need to calculate an AF evaluation value. The reason is as follows. In this case, the angle of view or a shooting range changes. Thus, even if AF evaluation value is calculated, the calculated AF evaluation value is not so useful when performing an autofocus operation after the angle of view or the imaging range is stabilized. Accordingly, when the lens controller  240  detects that the angle of view or the shooting range of an imaged image is relatively largely changed, the lens controller  240  may transmit an end signal to the camera controller  140 . By this, unnecessary calculation of AF evaluation value by the camera controller  140  is lessened. 
     As such, in the camera system  1 , end signal can be transmitted from both the camera body  100  and the interchangeable lens  200 . With this configuration, the camera system  1  can perform communication of position information of the focus lens  230  at optimal timing. This configuration can relatively reduce the number of communications compared to a configuration in which an AF evaluation value is transmitted to the camera controller  140  each time an AF evaluation value is calculated. As a result, the camera system  1  can implement of a relatively fast contrast AF operation. 
     Note that although, in the present embodiment, when communication for lens position detection between the camera body  100  and the interchangeable lens  200  ends, an end signal is transmitted from the camera body  100  or the interchangeable lens  200 . However the configuration does not necessarily need to be like this. For example, when the camera body  100  stops the transmission of an exposure synchronizing signal, the interchangeable lens  200  may transmit position information to the camera body  100 , so that the communication for lens position detection may end. Alternatively, by the camera body  100  stopping generation of an exposure synchronizing signal, the interchangeable lens  200  may transmit position information to the camera body  100 , so that the communication for lens position detection may end. Alternatively, the communication for lens position detection may end by controlling the interchangeable lens  200  to transmit position information to the camera body  100  without transmitting the end signal. 
     When neither the camera controller  140  nor the lens controller  240  receives an end signal, the camera controller  140  and the lens  2 C controller  240  repeat a process of calculating an AF evaluation value and obtaining lens position information (S 105  to S 106 ). 
     When at least one of the camera controller  140  and the lens controller  240  receives an end signal from the other of them, the lens controller  240  transmits to the camera controller  140  the number of pulses of the counter  243 , i.e., position information of the focus lens  230  stored in the DRAM  241  (S 107  and time t 7  in  FIG. 4K ). 
     When the position information of the focus lens  230  is obtained, the camera controller  140  associates the AF evaluation value stored in the DRAM  141  with the obtained position information (S 108 ). The associated information is stored in the DRAM  141 . The AF evaluation value and the position information of the focus lens  230  are associated with the exposure synchronization signal. Hence, the camera controller  140  can store the AF evaluation value and the lens position information which is associated with the AF evaluation value. For example, an AF evaluation value which is calculated using image data exposed during period “a” in  FIG. 4C  is stored in DRAM  141  at time  6 . Then, the calculated AF evaluation value is associated with an average value of a position of the focus lens  230  at time t 4  and a position of the focus lens  230  at time t 5 . In such a manner, the AF evaluation value calculated using image data exposed during period “a” in  FIG. 4C  is stored in the DRAM  141  at time t 7 . 
     When the association of the AF evaluation value with the position information of the focus lens  230  is made, the camera controller  140  determines whether a focus position of the focus lens  230  has been extracted (S 109 ). Specifically, a position of the focus lens  230  at which the AF evaluation value is maximum is extracted as a focus position. 
     When a focus position of the focus lens  230  has not been a extracted, the camera controller  140  and the lens controller  240  repeat the processing of steps S 104  to S 109 . 
     When a focus position of the focus lens  230  has been extracted, the camera controller  140  transmits a focus position move command to the lens controller  240  ( FIG. 4L , time t 8 ). The focus position move command is a command indicating a moving direction and a position to which the focus lens  230  is moved. The lens controller  240  drives the focus motor  233  according to the focus position move command (S 110 ). When the move to a focus point is completed, the camera controller  140  transmits an AF completion command to the lens controller  240  ( FIG. 4M , time t 9 ). In response to this, the lens controller  240  changes the control mode of the focus motor  233  from the second control mode back to the first control mode, as shown in  FIG. 4E . In this manner, the contrast autofocus operation is completed. 
     As described above, in the first embodiment, when the camera controller  140  and/or the lens controller  240  receives an end signal, position information of the focus lens  230  is transmitted from the interchangeable lens  200  to the camera body  100 . By this, in the camera system  1 , the camera body  100  can obtain position information of the focus lens  230  at optimal timing, and can associate the obtained position information with a calculated evaluation value of image data obtained at corresponding timing. As a result, the camera system  1  can improve accuracy of a contrast autofocus operation. 
     Second Embodiment 
     The present embodiment describes another example of the contrast AF operation. The configuration and basic operation of the camera system  1  are the same as those in the first embodiment. 
       FIG. 6  is a flowchart of a contrast AF operation according to the present embodiment. Steps S 201  to S 205  and steps S 211  to S 214  are respectively the same as steps S 101  to S 105  and steps S 107  to S 110  in the flowchart of  FIG. 5  according to the first embodiment. 
     An exposure synchronizing signal is transmitted from a camera controller  140  to a lens controller  240 , and position information of a focus lens  230  is stored in a DRAM  241  by the lens controller  240 . An AF evaluation value of image data obtained from a CCD image sensor  110  is calculated by the camera controller  140  (S 201  to S 205 ). Then, the camera controller  140  determines whether the CCD image sensor  110  has completed exposure for a predetermined number of frames (S 206 ). In the present embodiment, the predetermined number is two but any other number, e.g., three or four, may be used. 
     Until exposure for the predetermined number of frames is completed, the camera controller  140  and the lens controller  240  repeat transmission of an exposure synchronizing signal, calculation of an AF evaluation value, and acquisition of lens position information (S 205  to S 206 ). 
     When it is determined that exposure for the predetermined number of frames has been completed, the camera controller  140  transmits, to the lens controller  240 , a position information request command (see  FIG. 71 ) that requests for lens position information stored in the DRAM  241  (S 207 ). 
     As such, in the present embodiment, the camera controller  140  requests for lens position information when a predetermined number of pieces of lens position information are stored in the DRAM  241 . Specifically, the camera controller  140  transmits an exposure synchronizing signal to the lens controller  240  at a first period (see  FIG. 7C ) and transmits a position information request command to the lens controller  240  at a second period which is longer than the first period (see  FIG. 7I ). 
     With the above-described configuration, the frequency of communicating lens position information can be reduced compared to a configuration in which lens position information is requested each time lens position information is stored in the DRAM  241 . Thus, the number of frames captured by the CCD image sensor  110  per unit of time increases, and this configuration can support a case in which it is difficult to communicate lens position information each time lens position information is stored in the DRAM  241 . Also, the reduction in communication frequency can reduce burden of the camera controller  140  so that ability of the camera controller can also be assigned to the other function. 
     Also, by the above-described configuration, the camera controller  140  can obtain lens position information more frequently compared to a configuration in which the camera controller  140  requests for lens position information only after a focus position is detected. As a result, even when an operation by which the angle of view changes, such as a zoom operation, is performed during a contrast AF operation, or even when a shooting range is changed due to relatively large motion given to the camera system  1 , a focus position can be relatively easily detected. 
     After the transmission of the position information request command, the camera controller  140  prohibits (stops) the transmission of the exposure synchronizing signal to the lens controller  240  (S 208 ). The reason why the transmission of the exposure synchronizing signal is thus prohibited will be described below. 
     The lens controller  240  obtains position information of the focus lens  230  in synchronization with an edge change of the exposure synchronizing signal transmitted from the camera controller  140  and stores the position information in the CRAM  241 . Then, when the lens controller  240  receives a position information request command from the camera controller  140 , the lens controller  240  transmits to the camera controller  140  lens position information for two frames having been stored in the DRAM  241  up to that point. In such control, when an edge of the exposure synchronizing signal is changed during the transmission of a position information request command, the lens controller  240  obtains lens position information during the transmission of the position information request command and stores the lens position information in the DRAM  241 . Then, the lens controller  240  transmits, to the camera controller  140 , not only four pieces of lens position information for two frames already stored in the DRAM  241  as a response to the position information request command before receiving the position information request command, but also the lens position information obtained during the transmission of the position information request command. Although the camera controller  140  essentially expects to obtain four pieces of lens position information for two frames that are exposed immediately before the transmission of the position information request command, the camera controller  140  actually obtains six pieces of lens position information. This causes a problem that an association of lens position information with an AF evaluation value cannot be made. To avoid such a problem, in the present embodiment, transmission of an exposure synchronizing signal to the lens controller  240  is prohibited for a predetermined period after the transmission of a position information request command so that the timing of an edge change of an exposure synchronizing signal does not overlap the transmission timing of a position information request command. 
     After the prohibition of the transmission of an exposure synchronizing signal, the camera controller  140  determines whether a predetermined prohibition period has elapsed (S 209 ). The prohibition period will be described below. 
       FIG. 8  is a diagram describing the prohibition period. After a period of transmitting a position information request command by the camera controller  140 , a period required for receiving a checksum and a period required for receiving data by the camera controller follow. During the period of transmitting position information request command, the camera controller  140  transmits a position information request command to the lens controller  240 . During the period of receiving a checksum, the lens controller  240  considers the transmitted command to be a numeric string, calculates the sum of numeric values of the numeric string (checksum), and transmits the sum to the camera controller  140 . The camera controller  140  considers the obtained command to be a numeric string, calculates the sum of numeric values of the numeric string, and compares the sum of numeric values calculated by itself with the numeric value received from the lens controller  240 . If they match, then the communication is determined to be successful. If they do not match, then the communication is determined to be failed. During the period of receiving data, the camera controller  140  obtains lens position information stored in the DRAM  241  from the lens controller  240 . 
     In the present embodiment, during the period of transmitting position information request command and the period of receiving a checksum out of these periods, the camera controller  140  prohibits (stops) the transmission of an exposure synchronizing signal to the lens controller  240 . That is, the prohibition period at step S 209  during which the transmission of an exposure synchronizing signal is prohibited is set to at least a period obtained by summing the period of transmitting position information request command and the period of receiving a checksum. 
     By such a configuration, the lens controller  240  can be prevented from simultaneously receiving an exposure synchronizing signal and a position information request command from the camera controller  140 . As a result, the lens controller  240  can exclusively perform a lens position detection process, and the like, upon reception of an exposure synchronizing signal and a lens position information transmission process upon reception of a position information request command. Also, a risk of losing either of an exposure synchronizing signal and a position information request command due to simultaneous reception thereof can be reduced. 
     Returning to  FIG. 6 , after the prohibition of the transmission of an exposure synchronizing signal (S 208 ), when the prohibition period has elapsed, the camera controller  140  cancels the prohibition of the transmission of an exposure synchronizing signal (S 210 ). By this, the transmission of an exposure synchronizing signal is enabled. It is to be noted that the prohibition (stopping) of the transmission of an exposure synchronizing signal means to prohibit switching an exposure synchronizing signal from high to low or from low to high and that the cancellation of the prohibition of the transmission of an exposure synchronizing signal means to allow switching an exposure synchronizing signal from high to low or from low to high. For example, for the exposure synchronizing signal shown in  FIG. 7C , the exposure synchronizing signal is essentially suppose to rise to “high” at time ta. However, time ta is a time immediately after the transmission of the third position information request command C 3 , and the predetermined period has not elapsed since the transmission of the position information request command C 3 . Therefore, at time ta, the exposure synchronizing signal is not switched, and the exposure synchronizing signal is switched to “high” at time tb after waiting for the prohibition period to elapse. 
     Then, the lens controller  240  transmits position information of the focus lens  230  to the camera controller  140  (S 211 ). In the present embodiment, the camera controller  140  transmits a position information request command each time the CCD image sensor  110  captures two frames of images. Thus, when the lens controller  240  receives a position information request command from the camera controller  140 , the lens controller  240  transmits four pieces of lens position information stored in the DRAM  241 . It is to be noted that, in Step S 209 , when the switching timing of the exposure synchronizing signal is shifted, the lens position to be associated with the AF evaluation value should be corrected in associating the lens position with the AF evaluation value in Step S 212 . Specifically, an actual lens position is shifted by amount corresponding to the shift amount, and then the shifted lens position is associated with the AF evaluation value. 
     Thereafter, similar to the first embodiment, an association of an AF evaluation value and the lens position information is made (step S 212 ). Then, steps S 205  to S 213  are repeated until a focus position of the focus lens  230  can be extracted, and finally the focus lens  230  is moved to the focus position (S 214 ). 
     As described above, in the present embodiment, by providing an exposure synchronizing signal transmission prohibition period after the transmission of a position information request command (S 207  to S 209 ), a collision between the exposure synchronizing signal and the position information request command is prevented. By this, the camera controller  140  can correctly associate position information received from the lens controller  240  after the transmission of a position information request command, with a relevant AF evaluation value. 
     Third Embodiment 
     In the second embodiment, in order to prevent a collision between an exposure synchronizing signal and a position information request command, a predetermined prohibition period is provided after the transmission of a position information request command and during the prohibition period the transmission of an exposure synchronizing signal is prohibited (stopped). The present embodiment describes an example in which in order to prevent a collision between an exposure synchronizing signal and a position information request command, the transmission timing of a position information request command is shifted so that the transmission of an exposure synchronizing signal does not take place during a prohibition period. The configuration and basic operation of the camera system  1  are the same as those in the first embodiment. 
       FIG. 9  is a flowchart of a contrast AF operation according to the present embodiment. Steps S 301  to S 305  and steps S 309  to S 312  are respectively the same as steps S 101  to S 105  and steps S 107  to S 110  in the flowchart of  FIG. 5  according to the first embodiment. 
     In the present embodiment, in order that a position information request command is not transmitted at timing of an edge change of an exposure synchronizing signal, the transmission timing of the position information request command is varied. Specifically, after exposure for a predetermined number of frames are completed (S 306 ), a timing for transmitting a position information request command is determined (S 307 ). Specifically, a camera controller  140  determines whether a prohibition period expected if a position information request command would be transmitted at that time overlaps a timing at which an edge of an exposure synchronizing signal changes (S 307 ). If determined that they overlap, then the camera controller  140  does not transmit a position information request command. The camera controller  140  transmits a position information request command when it is determined that there is no overlap (S 307  and S 308 ). As such, control is performed such that the transmission timing of a position information request command is shifted to prevent the transmission timing of the position information request command from overlapping the timing of an edge change of an exposure synchronizing signal. 
     For example, in  FIGS. 10A to 10L , it is assumed that a timing at which exposure for the predetermined number of frames is completed is time ta. As shown in  FIG. 105 , at time ta, the exposure synchronizing signal rises up. Hence, the camera controller  140  determines that the transmission timing of a position information request command overlaps the timing of an edge change of the exposure synchronizing signal. Thus, the camera controller  140  does not transmit a position information request command at time ta. By this, as shown in  FIG. 101 , the transmission of a position information request command is delayed to time tb, so that a collision between an edge of the exposure synchronizing signal and the position information request command is avoided. 
     Fourth Embodiment 
     The present embodiment shows still another configuration for preventing a collision between an exposure synchronizing signal and a position information request command. The configuration and basic operation of the camera system  1  are the same as those in the first embodiment. 
     In the present embodiment, in order that an exposure synchronizing signal and a position information request command do not collide with each other, the position information request command is transmitted at predetermined timing. Specifically, timing of a position information request command is set such that timing at which an edge of an exposure synchronizing signal changes does not overlap a position information request command transmission prohibition period. For example, an exposure synchronizing signal may be controlled so that an edge of the exposure synchronizing signal is not changed during a predetermined period of one frame period, and a position information request command may be controlled to be transmitted during the predetermined period. 
       FIG. 11  is a flowchart of a contrast AF operation according to the present embodiment. The flowchart according to the present embodiment is the same as the flowchart of  FIG. 9  shown in the third embodiment with step S 307  eliminated. Specifically, in the present embodiment, since an exposure synchronizing signal and a position information request command are transmitted at predetermined timing, step S 307  in the flowchart of  FIG. 9  in which the transmission timing of a position information request command is shifted can be eliminated. 
       FIG. 12  is a timing chart of the camera system  1  according to the present embodiment. In an example of  FIG. 12 , a specified period immediately after the vertical synchronizing signal (CCDVD) rises up is set to a period for which an edge of an exposure synchronizing signal is not changed, and the position information request command is transmitted during the specified period. As shown in the figure, the position information request command is transmitted at predetermined timing so as not to overlap an edge of an exposure synchronizing signal. 
     Other Embodiments 
     Although the first to fourth embodiments are described above as exemplary embodiments of the present invention, the invention is not limited thereto. Examples of some other possible embodiments of the invention will be summarized in this section. 
     Although the above-described embodiments exemplify, as the second control mode, a control mode using the first encoder  231  and the second encoder  232 , the second control mode is not limited thereto. For example, in the second control mode, one encoder and a linear position detection sensor may be used. The linear position detection sensor is, for example, a position sensor implemented by a linear resistor whose length is the same as a driving range of the focus lens  230  and a contact that moves in conjunction with the drive of the focus lens  230  while being in contact over the resistor. Namely, the second control mode should be such a control mode that is relatively superior to the first control mode the position detection accuracy of a reversal of the driving direction of the focus lens  230  that is not intended by the lens controller  240 . It is not necessary to provide the first control mode, and it is enough to provide only the second control mode. 
     Although the above-described embodiments exemplify a configuration having the zoom lens  210  and the CIS lens  220 , such lenses are not essential to the present invention. That is, the idea of the embodiments can also be applied to a camera system having mounted thereon a single-vision lens with no zoom function. The idea of the embodiments can also be applied to a camera system having mounted thereon an interchangeable lens with no camera shake correction function. 
     Although the above-described embodiments exemplify a camera body with no movable mirror, the invention is not limited thereto. For example, a camera body may be provided with a movable mirror or a prism for splitting a subject image. Alternatively, a movable mirror may be included in an adapter instead of in a camera body. 
     In the above-described embodiments, a position of the focus lens  230  is not directly detected, but indirectly detected by detecting an angle of rotation of the rotating shaft of the focus motor  233 . A position of the focus lens  230  may be indirectly detected by detecting a position of a mechanism member that operates in conjunction with the focus lens  230 . That is, detection can be performed in any manner as long as a position of the focus lens  230  can be identified eventually. 
     The above-described embodiments exemplify a camera system that does not incorporate a phase-difference detection sensor. However, a camera system may incorporate a phase-difference detection sensor so that a phase-difference autofocus operation and a contrast autofocus operation can be selectively performed. In this case, when a contrast autofocus operation is performed, the idea of the embodiments is applicable. 
     Although in the above-described embodiments a position of the focus lens  230  is obtained in synchronization with a timing of an edge of an exposure synchronizing signal, a position of the focus lens  230  may be obtained in synchronization with a timing shifted from a timing of an edge of an exposure synchronizing signal This is because the exact lens position can be determined with linear interpolation as long as the shift amount from the edge of the exposure synchronizing signal is recognized. Accordingly, as a timing signal of the invention, any signal can be employed as long as it correlates with the exposure synchronizing signal. 
     For example, a vertical synchronizing signal and electronic shutter drive signals for the CCD image sensor  110  may be transmitted to the lens controller  240  as timing signal. By this, the camera controller  140  does not need to transmit an exposure synchronizing signal, enabling to facilitate control. However, in this case, the specification of the electronic shutter drive signal (transmission intervals in one group of electronic shutter drive signals, the number of transmissions, and the like) need to be notified in advance from the camera controller  140  to the lens controller  240 . According to the notified specification, the lens controller  240  reads a pulse value of the counter  243  based on the electronic shutter drive signal and the vertical synchronizing signal. 
     Although the above-described embodiments exemplify the CCD image sensor  110  as an imaging element, the invention is not limited thereto. The imaging element may be configured by a CMOS image sensor or an NMOS image sensor. 
     Although specified embodiments of the present invention are described above, many other modifications, corrections and applications apparent to those skilled in the art. Therefore, the present invention is not limited by the disclosure provided herein. The present disclosure relates to subject matter contained in Japanese Patent Application Nos. 2008-086079, 2008-086080, and 2008-086081 filed on Mar. 28, 2009, which are expressly incorporated herein by reference in its entirety. 
     Industrial Applicability 
     The present invention can be applied to lens-interchangeable camera systems. Specifically, it can be applied to digital still cameras, digital movie cameras, and the like.