Patent Publication Number: US-2009237554-A1

Title: Autofocus system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the Japanese Patent Application Nos. 2008-72241, 2008-72243 and 2008-72244 filed on Mar. 19, 2008; the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to an autofocus system, and more particularly to an autofocus system capable of performing focus adjustment while tracking a desired subject (person) by automatically moving an autofocus (AF) target position (range) in a shooting range of a camera. 
     2. Description of the Related Art 
     There has been known an AF area auto-tracking system that is adapted to change autofocus (AF) area in accordance with movement of a subject in order to bring the subject into focus when a quickly moving subject in sporting scenes such as a soccer and a figure skating is shot by, for example, a TV camera and the like (for example, see JP 2006-267221 A (corresponding to US 2006/0140612 A). 
     In an imaging system (like a TV camera) that converts an image of a subject to an electric signal (image signal) by an imaging device (CCD or the like), a contrast mode is generally adopted as an autofocus (AF) mode. The contrast mode is to detect the contrast of a subject image from an image signal acquired by an imaging device, and control focus of a taking lens so that the highest contrast can be obtained, thereby automatically bring the subject into best focus. 
     In the AF in the contrast mode, the entire subject in the shooting range of a camera is not set to the target of the AF, but only a subject in a part of the shooting range is processed as the AF target in many cases. In the AF in the contrast mode, the image signals in the target range of the AF are extracted from the image signals acquired from the entire imaging area of the imaging device, and the target of the AF is limited only to the subject in a part of the range by controlling the focus so that the contrast of the extracted image signal in that range is maximized. In this specification, a subject range, being a target of the AF, in a shooting range of a camera or an image range of a target subject, being a target of the AF, in a shot image of a camera will be referred to as an “AF area”, and a frame indicating an outline of the AF area will be referred to as an “AF frame”. 
     In a lens system for broadcast, a cameraman performs a pan/tilt (P/T) operation and a zooming operation in order to capture a quickly moving subject like a figure skater. However, in such a situation, it is very difficult to move the AF area. In this case, the above-mentioned AF area auto-tracking system is useful. 
     In the above-mentioned AF area auto-tracking system, for example, it is possible to perform the tracking when the subject is moving at least in the shooting range. However, if the subject goes out of the shooting range and disappears from the screen (i.e. goes out of a frame), the tracking target is lost. Thus, if the subject comes into the shooting range again (i.e. appears in a frame), the subject may not be recognized by a simple pattern recognition, and the subject may not be tracked again. 
     Also, for example, in the case where a soccer game is shot, for example, the case where a large number of subjects move similarly in a shooting range, it is difficult to continuously track a specific subject that is previously recognized by means of a simple pattern recognition technique, and the tracking target would be changed. As a result, there is a possibility that an image of the subject to be tracked originally might be blurred. 
     Furthermore, in a face recognition technique provided to recent digital cameras, it is effective to simply extract a person in a shooting range. However, in the case where not only a specific person but also a plurality of persons exist in a frame, it is difficult to recognize the specific person just by setting an AF frame onto a face. Thus, there is a possibility that the target of the AF may be changed to another person. 
     SUMMARY OF THE INVENTION 
     The invention has been made in view of the above situations so as to solve the problems and provides an autofocus system that can perform an AF control by automatically tracking a desired AF target in response to the case where a person who is an AF target disappears from a shooting range or appears in the shooting range. 
     [1] According to an aspect of the invention, an autofocus system includes an imaging unit, an autofocus unit, a tracking unit, a pattern registration unit, a tracking target detection unit and a tracking target selection unit. The imaging unit takes a subject image formed by an optical system. The autofocus unit performs focus adjustment of the optical system so as to bring into focus a subject in an AF area in a shot image taken by the imaging unit. The tracking unit moves the AF area in accordance with movement of a tracking target subject in the image taken by the imaging unit. The pattern registration unit registers registration patterns of subjects among subjects in the image taken by the imaging unit. The tracking target detection unit detects the registration patterns, which are registered in the pattern registration unit, from among the subjects in the image taken by the imaging unit. The tracking target selection unit selects a registration pattern to be tracked from among the registration patterns registered in the pattern registration unit. If a subject having another registration pattern exists in the image taken by the imaging unit as well as the tracking target subject having the registration pattern selected by the tracking target selection unit in the course of tracking the tracking target subject having the selected registration pattern and if the tracking target detection unit detects that the subject having the selected registration pattern disappears from the taken image, the subject having said another registration pattern and existing in the taken image is tracked.
 
[2] In the autofocus system of [1], even if the tracking target detection unit detects that the tracking target subject, which disappeared from the taken image, appears in the taken image again after the tracking target detection unit detects that the tracking target subject disappeared from the taken image, it may be continued to track the subject having said another registration pattern.
 
     With this configuration, if another registration pattern exits in the image as well as the target currently being tracked, even if the tracking target disappears from the image, a subject having said another registration pattern is automatically tracked, and the tracking of the subject having said another registration pattern is continued. Therefore, additional operation by an operator is not required, and it is possible to easily perform the auto-tracking. 
     [3] In the autofocus system of [1], if the tracking target detection unit detects that the tracking target subject, which disappeared from the taken image, appears in the taken image again after the tracking target detection unit detects that the tracking target subject disappeared from the taken image, it may be stopped to track the subject having said another registration pattern, and it is started again to track the tracking target subject. 
     With this configuration, if one tracking target is selected, it possible to automatically track the target so long as the target exists in the image even if the target repeatedly disappears from and appears in the image. 
     [4] The autofocus system of [1] may further include a mode switch that switches between first and second tracking modes. In the first tracking mode, if a subject having another registration pattern exists in the image taken by the imaging unit as well as the tracking target subject having the registration pattern selected by the tracking target selection unit in the course of tracking the tracking target subject having the selected registration pattern and if the tracking target detection unit detects that the subject having the selected registration pattern disappears from the taken image, the subject having said another registration pattern and existing in the taken image is tracked, and thereafter, even if the tracking target detection unit detects that the tracking target subject, which disappeared from the taken image, appears in the taken image again, it is continued to track the subject having said another registration pattern. In the second mode, if a subject having another registration pattern exists in the image taken by the imaging unit as well as the tracking target subject having the registration pattern selected by the tracking target selection unit in the course of tracking the tracking target subject having the selected registration pattern and if the tracking target detection unit detects that the subject having the selected registration pattern disappears from the taken image, the subject having said another registration pattern and existing in the taken image is tracked, and thereafter, if the tracking target detection unit detects that the tracking target subject, which disappeared from the taken image, appears in the taken image again, it is stopped to track the subject having said another registration pattern, and it is started again to track the tracking target subject. 
     With this configuration, the tracking mode can be changed in accordance with a situation, and thus it is possible to perform the auto-tracking in accordance with scenes. 
     As described above, with the configurations of [1] to [4], if a tracking target disappears from or appears in a shooting range, it is possible to perform the AF control by automatically tracking a desired tracking target in response thereto. Thus, it is possible to perform the tracking control in accordance with scenes. 
     Furthermore, the inventor of this application studied how to select a next target to be tracked if a person who is an AF target disappears from a shooting range while a plurality of persons still exist in the shooting range. 
     The invention may provide an autofocus system that can perform an appropriate AF control by selecting a desired AF target from among a plurality of targets existing in a shooting range even if a person who is an AF target disappears from the shooting range. 
     [5] In the autofocus system of [1], the pattern registration unit may give priorities to the registration patterns of the subjects in registering the registration patterns of the subjects among the subjects in the taken image. If a plurality of subjects having other registration patterns exist in the image taken by the imaging unit as well as the tracking target subject having the registration pattern selected by the tracking target selection unit in the course of tracking the tracking target subject having the selected registration pattern and if the tracking target detection unit detects that the subject having the selected registration pattern disappears from the taken image, the tracking target selection unit may select a registration pattern having the highest priority among said other registration patterns as the registration pattern to be tracked. 
     With this configuration, by determining the priorities in advance, another registration pattern is automatically tracked as a tracking target even if a target that is previously tracked disappears from a shooting range. Therefore, even if a person who is an AF target disappears from the shooting range, it is possible to perform an appropriate AF control by selecting a desired AF target among a plurality of targets existing in the shooting range. 
     [6] In the autofocus system of [5], the priorities may be defined as an order of registering into the pattern registration unit. 
     With this configuration, if targets desired to be tracked are registered in order in advance without particular priorities given thereto and if a person who is an AF target disappears from a shooting range, it is possible to perform an appropriate AF control by selecting a desired AF target among a plurality of targets existing in the shooting range. 
     [7] In the autofocus system of [1], the pattern registration unit may automatically number the registration patterns of the subjects in registering the registration patterns of the subjects among the subjects in the taken image. If a plurality of subjects having other registration patterns exist in the image taken by the imaging unit as well as the tracking target subject having the registration pattern selected by the tracking target selection unit in the course of tracking the tracking target subject having the selected registration pattern and if the tracking target detection unit detects that the subject having the selected registration pattern disappears from the taken image, the tracking target selection unit may select one of said other registration patterns as a new registration pattern to be tracked by designating number corresponding to the one of said other registration patterns. 
     With this configuration, even if a person who is an AF target disappears from a shooting range, it is possible to continue the AF control by selecting a desired AF target among a plurality of targets existing in the shooting range. 
     [8] In the autofocus system of [7], the tracking target selection unit may include a plurality of switches that are numbered and select the registration pattern to be tracked. The pattern registration unit may register the registration patterns of the subjects existing in the taken image so as to correspond to the plurality of switches and automatically number the registration patterns of the subjects. The tracking target selection unit may designate number by use of any of the switches to select as the registration pattern to be tracked the registration pattern assigned to the switch corresponding to the designated number. 
     With this configuration, an operator can select a next target to be tracked by simply pressing a switch corresponding to a target desired to be tracked. 
     As described above, with the configurations of [5] to [8], even if a person who is an AF target disappears from a shooting range, it is possible to perform an appropriate AF control by selecting a desired AF target among a plurality of targets existing in the shooting range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the overall configuration of an imaging system to which an autofocus system according to an embodiment of the invention is applied; 
         FIG. 2  is an explanatory diagram showing an example of an AF frame. 
         FIGS. 3A and 3B  are a flowchart showing a basic operation of the autofocus system of one embodiment. 
         FIG. 4  is a block diagram showing another exemplary configuration of an imaging system to which the autofocus system according to an embodiment of the invention is applied. 
         FIG. 5  is an explanatory diagram showing an example of an image displayed on a screen of a viewfinder. 
         FIG. 6  is a flowchart showing a tracking control after a target being tracked disappears from a screen. 
         FIG. 7  is an explanatory diagram showing a tracking control by summarizing the tracking control shown in  FIG. 6 . 
         FIGS. 8A and 8B  is a flowchart showing a process flow for the case where an operator selects a tracking target from among a plurality of registration patterns in the screen by using a switch. 
         FIG. 9  is a flowchart showing a process flow for the case where the operator selects the tracking target from among the plurality of registration patterns in the screen by using the switch in the same manner. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Hereinafter, with referring to the accompanying drawings, an autofocus system according to embodiments of the invention will be described in detail. 
       FIG. 1  is a block diagram showing the overall configuration of the imaging system to which the autofocus system according to an embodiment of the invention is applied. The imaging system shown in the drawing is, for example, an imaging system that is used to shoot an image by a TV camera for broadcast.  FIG. 1  shows a camera head  10  whose lens can be replaced, a lens device  12  that has a taking lens (optical system) attached to a lens mount of the camera head  10 , a tracking device  16  that includes an operation unit  14 , and the like. 
     The camera head  10  is mounted with an imaging device (for example, a CCD), a signal processing circuit, and the like. An image formed by the taking lens of the lens device  12  is photoelectrically converted by the imaging device, and then signal processing is performed for the image by the signal processing circuit. A video signal of a certain format is generated by the signal processing circuit, and is output from a video signal output terminal or the like of the camera head  10  to an external equipment. Furthermore, a viewfinder (monitor)  18  is adapted to display a video taken by the camera head  10  and be used to check composition thereof, and is installed in the tracking device  16 . The video signal from the camera head  10  is sent to the viewfinder  18 , and a real-time image (video) currently being taken by the camera head  10  is displayed on the screen of the viewfinder  18 . Furthermore it is possible to display on the screen of the viewfinder  18  information such as an AF frame indicating an AF area range that is a autofocus (AF) target. 
     The lens device  12  is provided with the taking lens (optical system), which is not shown in the drawings and is attached to the lens mount of the camera head  10 . The taking lens forms an image of a subject on an imaging surface of the imaging device of the camera head  10 . The taking lens includes, as its elements, movable portions for adjusting a shooting condition. The movable portions such as a focus lens group, a zoom lens group, and an aperture diaphragm are electrically driven by a motor (servomechanism) not shown in the drawings. For example, the focus lens group and the zoom lens group move in an optical axis direction, the movement of the focus lens group adjusts a focus (subject distance), and the movement of the zoom lens group adjusts a focal length (zoom power). Like this exemplary embodiment, in a system that relates to control of autofocus, at least the focus lens group may be electrically driven, and the other movable portions may be only driven manually. When a certain movable portion is electrically driven by an operator&#39;s operation, the certain movable portion is controlled based on control signals output from an operation unit (an operation unit of a controller connected to the lens device  12  or the like), which is not shown in the drawings, in accordance with the operator&#39;s operation, but details thereon are omitted here. 
     As shown in the drawing, the lens device  12  is mounted with a lens CPU  20  for integrally controlling the entire lens device  12 , an AF processing unit  22  that executes autofocus (AF) processing, an AF imaging circuit  24 , and the like. The AF imaging circuit  24  is disposed on the lens device  12  in order to acquire a video signal for AF process. The AF imaging circuit  24  includes the imaging device (CCD or the like) and the processing circuit for outputting an output signal of the imaging device as the video signal in the certain format, and the like. The imaging device of the AF imaging circuit  24  may be referred to as an “AF imaging device”. It is assumed that the video signal output from the AF imaging circuit  24  is a luminance signal. Light is branched from subject light incident on the imaging device of the camera head  10  by the use of a half mirror or the like disposed on a light path of the taking lens, and the branched light forms an image on the imaging surface of the AF imaging device. A shooting range and a subject distance (a distance at which a subject can be focused to) for an imaging area of the AF imaging device are configured to match a shooting range and a subject distance for an imaging area of the imaging device of the camera head  10 . A subject image acquired by the AF imaging device matches a subject image acquired by the imaging device of the camera head  10 . It is not necessary for the both shooting ranges to be completely matching. For example, the shooting range of the AF imaging device may be large enough to contain include the shooting range of the imaging device of the camera head  10 . 
     The AF processing unit  22  acquires the video signal from the AF imaging circuit  24 , and calculates a focus evaluation value indicating a level of a contrast of the subject image based on the video signal. For example, high-frequency component signals of the video signal acquired from the AF imaging device are extracted by a high-pass filter, and then of the high-frequency component signals, signals in a range corresponding to an AF area that is an AF target are integrated for each screen (frame). An integrated value thus acquired for each screen indicates the level of the contrast of the subject image, and is given to the lens CPU  20  as the focus evaluation value. Furthermore, the range of the AF area is designated by the lens CPU  20  as will be described later. 
     As will be described in detail later, the lens CPU  20  acquires information (AF frame information) of the AF frame indicating a range (outline) of the AF area from an operation unit  14 , and informs of the AF processing circuit  22  a range in the AF frame that is designated by the AF range information as the AF area. Then, the focus evaluation value, which is obtained from the image (video signal) in the AF area, is acquired from the AF processing unit  22 . In this manner, whenever a video signal for each screen is acquired from the AF imaging circuit  24  (whenever a focus evaluation value is acquired from the AF processing unit  22 ), the lens CPU  20  acquires the focus evaluation value from the AF processing unit  22 , and controls the focus lens group so as to maximize the acquired focus evaluation value (to take a local maximum value), that is, maximize the contrast of the subject image in the AF area. For example, a climbing mode is generally known as a mode of controlling the focus lens group based on the focus evaluation value. In this mode, the focus lens group is moved in a direction for increasing the focus evaluation value, and then if a point at which the focus evaluation value starts to decrease is detected, the focus lens group is set to the detected point. Thus, it is possible to automatically bring the subject in the AF frame into focus. 
     The tracking device  16  is a device for moving an AF point in accordance with a movement of a target object specified by an operator while tracking on the screen the target object, and for automatically changing a position of the AF frame. The tracking device  16  includes the operation unit  14 , an image processing unit  26 , an image input unit  28 , a tracking processing unit  30 , a pattern data storage unit  32 , a frame information generation unit  34 , a video synthesizing unit  36 , and the like. 
     If the operator does not perform an operation for changing the AF frame, the tracking device  16  automatically changes the AF frame. In contrast, if the operator performs the operation for changing the AF frame, the operation is prioritized over the tracking device  16 , and thus the AF frame is changed in accordance with the operator&#39;s operation. 
     The operation unit  14  includes a track ball  14   a  for specifying a moving direction of the AF frame and a moving amount of the AF frame as an operation member that allows the operator to specify settings of the AF frame such as a position, a shape, and a size of the AF frame. Further, the operation unit  14  includes a press-button type switch  14   b  as an operation member for registering or selecting which subject in the screen is set as a tracking target to be tracked by performing the AF. As shown in  FIG. 1 , a plurality of the switches  14   b  may be provided. Tracking targets may be registered in association with the switches  14   b , respectively, and thereby the plural tracking targets to be tracked by performing the AF may be registered. Pattern data (registration pattern) of the registered tracking target is stored in a database of the pattern data storage unit  32  (which will described later). 
       FIG. 2  shows an example of the AF frame. As shown in  FIG. 2 , the AF frame indicates the outline of the AF area indicating an AF target range in a shot image or in a shooting range of the imaging device of the camera head  10 . 
     If the operator operates the operation member; the operation unit  14  sets and changes the settings such as a position of the AF frame in accordance with the operation. In this embodiment, it is assumed that the shape of the AF frame is limited to a rectangle. Changing of the settings of the AF frame such as the position of the AF frame and the shape of the AF frame by the operation member of the operation unit  14  is performed by changing the settings of the AF frame by a change amount corresponding to an operation amount of the operation member with reference to the settings of the AF frame before the operator&#39;s operation. For example, a position of the AF frame is moved vertically and horizontally in a direction corresponding to a rotation direction of the trackball  14   a  and by a movement amount corresponding to a rotation amount of the trackball  14   a . If an AF point is defined as a point for determining the position of the AF frame (center position of the rectangular AF frame in this embodiment) in the shooting range (shot image), the position of the AF frame is determined by setting the position of the AF point. 
     On the other hand, during a period in which the operation member is not operated and the operator does not give an instruction of changing the settings of the AF frame, the AF frame is set and/or changed in the tracking processing unit  30  (which will be described later). The operation unit  14  transmits to the lens CPU  20  and the tracking processing unit  30  the AF frame information indicating the settings of the AF frame, which are set and changed by the operator&#39;s operation, in accordance with a request from the lens CPU  20 . The AF frame information indicating the settings of the AF frame, which are set and changed in tracking processing unit  30 , is transmitted to the lens CPU  20  through the operation unit  14 . Thereby, an AF target range in which the tracking processing unit  30  performs the tracking process is set to the range of the AF frame. 
     The image input unit  28  obtains the video signal (luminance signal) acquired by the AF imaging circuit  24  through the AF processing unit  22 , and provides the image processing unit  26  with an image (image data) that is in a range specified by the image processing unit  26  and is in the shooting range. 
     The image processing unit  26  performs predetermined image processing for the image data acquired from the image input unit  28 , and sends the processed image data to the tracking processing unit  30 . 
     In the AF area, which is determined from the sent image data based on the set AF frame, the tracking processing unit  30  identifies and recognizes a person&#39;s face (object) based on a well-known face (object) authentication technique for security by using the pattern data (registration pattern) stored in the pattern data storage unit  32 . Then, the tracking process unit  30  tracks a specific person (object) who is the tracking target in the AF area and performs the AF for the specific person (object), and moves the AF point so as to follow a movement of the target object. 
     The pattern data storage unit  32  registers and stores the pattern data that is required to recognize the specific person (object) in the AF area as a tracking target to be tracked by performing the AF. At this time, if one pattern data (registration pattern) is registered so as to correspond to each of the plurality of switches  14   b , it is possible to register plural pieces of (plural persons&#39;) pattern data (registration pattern) of the tracking targets. 
     As for the registration of the pattern data, the operator may register the pattern data in advance while associating the pattern data with each switch  14   b . Alternatively, during shooting, the tracking processing unit  30  may automatically assign pattern data of persons in the screen to the switches  14   b  for registration. In the example shown in  FIG. 1 , since the number of switches  14   b  is five, it is possible to resister five persons at maximum. However, the number of the registerable persons is not particularly limited. 
     If the operator presses one of the switches  14   b  when the pattern data indicating the tracking target is registered in the pattern data storage unit  32 , the tracking target of the pattern data corresponding to the pressed switches  14   b  is selected. When the tracking target is selected, whether or not the selected tracking target exists in the screen is searched all the time during shooting. When the selected tracking target is detected, the tracking target subject is tracked and simultaneously the AF is started. The operation unit  14  is further provided with an auto-tracking switch (not shown in the drawing) for giving instructions of starting and ending the auto-tracking. 
     Furthermore, there is a case that the plurality of tracking targets are registered in the pattern data storage unit  32 . In this case, in order to make it possible to automatically select which registration pattern is to be tracked when a plurality of registration patterns exist in the screen or when a registration pattern disappears from or appears in the screen, priorities may be assigned to the plural registration patterns, respectively. An order in which the tracking targets are registered in association with the switches  14   b  (numbers of switches) may be used as priorities as they are. 
     The frame information generation unit  34  receives the AF frame information from the tracking processing unit  30 , and generates AF frame display data for superimposing and displaying the AF frame on an image displayed on the display screen of the viewfinder  18 . 
     The video synthesizing unit  36  synthesizes the image data (video signal) received from the camera head  10  and frame information data that is generated for display and is received from the frame information generation unit  34 , generates a synthesized video for display on the viewfinder  18 , and sends the synthesized video to the viewfinder  18 . 
     In the viewfinder  18 , the image is displayed with the AF frame being superimposed and display in a corresponding position on the screen. Furthermore, a frame may be displayed on a face of the person of the registration pattern, which is the tracking target in the AF frame. The switches  14   b  may also serve as a switch for turning ON/OFF display of the face frame. Furthermore, as shown in  FIG. 1 , for example, a title such as “FUJI#1” for specifying a tracking target may be display for the face frame, and an indication that the AF is running or the like may be displayed on a lower side of the screen. 
     Next, a basic operation of the autofocus system of this embodiment will be described. 
       FIGS. 3A and 3B  are a flowchart showing a basic operation of the autofocus system of the embodiment. 
     In the embodiment shown in  FIGS. 3A and 3B , one person subject (one object) is set as the tracking target, a registration pattern of the tracking target is detected from a shot image and is automatically tracked. 
     First, an initial setting is performed in step S 100  of  FIG. 3A . 
     Next, in step S 102 , it is determined as to whether or not the auto-tracking is performed. This determination is made based on ON/OFF state of the tracking switch for giving instructions of starting and ending that the AF area automatically tracks a target. The tracking switch is not shown in the drawing, but for example, is installed in the operation unit  14 . 
     If it is determined in step S 102  that the auto-tracking is OFF, it is determined in the following step S 104  as to whether or not the tracking target is to be registered and its pattern data (registration pattern) is to be stored. This determination is made based on ON/OFF state of the switches  14   b  mentioned above. 
     If the determination in step S 104  results in that the tracking target is not to be registered and its registration pattern is not to be stored, the operator manually sets the AF area in the following step S 106 . 
     When the AF area is set manually, it is determined in the following step S 108  as to whether the AF (autofocus). This determination is based on ON/OFF state of the AF switch, which is also not shown in the drawing and is adapted to give instructions of starting and ending the AF. 
     If the AF is ON, the normal AF process is performed in the following step S 110 , and the focus lens is automatically controlled to be focused on the subject in the set AF area. 
     On the other hand, if it is determined in step S 108  that the AF is OFF, the operator manually performs a focus control in step S 112 . 
     Furthermore, if the determination made in step S 104  results in that the tracking target is to be registered and its registration pattern is to be stored, the registration pattern of the tracking target is registered and stored by the following procedure. Specifically, at first, in step S 114 , the image processing unit  26  of the tracking device  16  acquires an image through the image input unit  28 . 
     Next, a pattern sampling is performed from the image, which is acquired by the image processing unit  26  in step S 116 . A face of the subject person is detected in the image, and pattern data is extracted, which is required to recognize a specific person as a tracking target. 
     In step S 118 , the above extracted pattern data is registered (stored) in the database of the pattern data storage unit  32  through the tracking processing unit  30 . 
     In step S 120 , the tracking processing unit  30  tracks the tracking target indicated by the registration pattern just registered and starts the AF. The operator checks the tracking target by using the viewfinder  18 . 
     In step S 122 , the operator determines as to whether or not the sampling is terminated, based on whether or not the current auto-tracking target is appropriate. 
     If it is determined in step S 122  that the sampling is terminated, the switches  14   b  for giving an instruction of registering (storing) the registration pattern is turned OFF in step S 124 . 
     As described above, when the registration (storage) of the registration pattern is terminated, the tracking processing unit  30  starts the auto-tracking process by turning ON an internal switch for compulsorily starting to track the just registered tracking target as the target of the AF area in the following step S 126   
     Subsequently, a title (name, number, or the like) for the tracking target is registered in step S 128 . When the title of the tracking target is registered, the title (name, number, and the like), which indicates the tracking target, like “FUJI#1” is displayed on the display screen of the viewfinder  18  as shown in  FIG. 1 . The registration of the title is performed, for example, by using an alphanumeric keyboard that is not shown in the drawing and is provided in the operation unit  14 . After the registration of the title, the flow advances to step S 130 , and the auto-tracking process is performed. 
     Meanwhile, if it is determined in step S 122  that the sampling is not yet to be terminated because the sampling is required to be performed again, for example, because the pattern, which is sampled and registered in the database, is different from a target that the operator intends to track, the flow advances to step S 108 , and returns to step S 102 . 
     Furthermore, if it is determined in step S 102  that the auto-tracking is ON, the flow advances to step S 130 , and the following auto-tracking process is performed. 
     First, the image processing unit  26  acquires an image currently being shot in the AF area from the lens device  12  through the image input unit  28  in step S 130 . The image processing unit  26  detects a face from the acquired image, and acquires data required for comparison to the registration pattern. 
     Although the AF area auto-tracking is now being performed, it is determined as to whether or not a subject in the AF area of the shot image has already been found as a registration pattern in step S 132 . 
     If the determination result shows that the subject has already been found as the registration pattern and a tracking target corresponding to the pattern is automatically tracked, the flow advances to step S 134 . Then, it is further determined as to whether or not a title of the tracking target is displayed. Whether or not the title is displayed is determined based on whether the switch  14   b  corresponding to the registration pattern is in a display ON state or a display OFF state or the like. Furthermore, in this embodiment, the switch  14   b  serves as a switch for giving instructions of the pattern registration and a switch for giving instructions of the title display. However, those switches are formed as individual switches. 
     If the title display is ON in step S 134 , it is determined in the following step S 136  as to whether or not the title for the tracking target is registered. 
     If the title is registered, the title of the registered tacking target is displayed as, for example, “FUJI#1” in step S 138 . If the title is not registered, for example, “#??” is displayed in step S 140 . Thereby, the operator easily checks as to whether or not the subject(s) being displayed on the screen of the viewfinder  18  are registered. 
     Then, the auto-tracking is continuously preformed for the tracking target in step S 142 . 
     Furthermore, if the registration pattern, which is the tracking target, has not yet been found in step S 132 , the tracking processing unit  30  searches the image of the AF area received from the image processing unit  26  for a pattern that is the same as any of the registration pattern(s), which are sampled and registered in the database of the pattern data storage unit  32 , in step S 144 . 
     If the pattern, which is the same as any of the registration pattern(s) are found in step S 146 , the flow advances to step S 134 , and the process for the case where the above-mentioned registration pattern has already found is performed. Furthermore, if the pattern being the same as the registration pattern is not found, it is determined in step S 148  as to whether or not checking the sampled pattern against all the registration pattern data has been completed. Further, if the checking against all the data has not been completed, the flow returns to step S 144 . In contrast, if the checking against all the data has been completed, a pattern search error is displayed in step S 150 . Subsequently, the auto-tracking for the target of the AF area is compulsorily turned OFF in step S 152 , and the flow advances to step S 108 . 
     Next, another exemplary configuration of an imaging system to which the autofocus system according to an embodiment of the invention is applied will be described. 
       FIG. 4  is a block diagram showing another exemplary configuration of the imaging system to which the autofocus system according to this embodiment of the invention is applied. 
     The imaging system shown in  FIG. 4  has substantially the same configuration as the imaging system shown in  FIG. 1 . The imaging system includes a camera head  110 , a lens device  112 , a tracking device  116  including an operation unit  114 , a viewfinder  118  and the like. 
     The camera head  110  is mounted with an imaging device (for example, a CCD), a required signal processing circuit, and the like. The lens device  112  is provided with a taking lens not shown in the drawings, and the taking lens is attached to a lens mount of the camera head  110 . An image of a subject is formed by the taking lens on an imaging surface of the imaging device of the camera head  110 . Furthermore, the lens device  112  is mounted with a lens CPU  120 , an AF processing unit  122 , an AF imaging circuit  124 , and the like. 
     The tracking device  116  includes the operation unit  114 , an image processing unit  126 , an image input unit  128 , a tracking processing unit  130 , a pattern data storage unit  132 , a frame information generation unit  134 , a video synthesizing unit  136 , and the like. 
     Furthermore, the example shown in  FIG. 4  is different only in only the configuration of the operation unit  114  from that shown in  FIG. 1 . The configuration of this example is the same as that of  FIG. 1  mentioned above except the operation unit  114 . Accordingly, detailed description of the configuration of each part except the operation unit  114  will be omitted. 
     The operation unit  114  in  FIG. 4  includes: an arrow key  114   a  as an operation member for manually specifying a moving direction of the AF frame and a moving amount of the AF frame, instead of the track ball  14   a ; switches  114   b  for registering and selecting a tracking target; a display panel  114   c  for checking registration of a title of the tracking target; an indicator  114   d  indicating states of zooming (Z) and focusing (F); a slide switch  114   e  for selecting a tracking mode (which will be described later). 
     The switches  114   b  are used to register or select a tracking target(s) similarly to the switches  14   b  of the operation unit  14  in  FIG. 1 . Tracking targets can be registered in association with the respective switches  114   b . Also, titles for identifying the tracking targets can be registered for the respective tracking targets. The registration of the title is performed by inputting alphanumeric characters by a keyboard not shown in the drawings. On the display panel  114   c  for checking, the input title is displayed. 
     In the example shown in  FIG. 4 , a tracking target having a title of “XXXX#1” is registered in No. 1 of the switches  114   b , and a tracking target having a title of “YYYY#2” is registered in No. 2 of the switches  114   b.    
     In the indicator  114   d , one group of Z (zooming) and F (focusing) is provided for each switch  114   b , and each group is adapted to display the zooming and focusing conditions. 
     The slide switch  114   e  (an example of a tracking mode switch unit) is adapted to switch between tracking modes. The tracking modes will be described later in detail. In  FIG. 4 , any of three modes, that is, a preset function mode (Z+F mode) of zooming and focusing, a face recognition auto-tracking mode (OBJ mode), and a combination mode (Z+F+OBJ mode) can be selected by changing a setting position of the slide switch  114   e . However, the mode setting is not limited thereto but more modes such as modes M 1  to M 4  may be set so as to enable switching between those modes as will be described later. 
       FIG. 5  shows an example of an image displayed on the screen of the viewfinder  118 .  FIG. 5  shows an image in the case where the auto-tracking is performed for a video of a soccer game, and face frames are displayed for the recognized faces in the screen, respectively. 
     Furthermore, in the case of the auto-tracking, a registered face pattern(s) are automatically recognized in a frame, and a title (name, number, and the like) is registered for the tracking target, and in the case of display ON, the title is displayed in the vicinity of the face frame. 
     In  FIG. 5 , registration patterns of two persons exist in the frame, and the titles for the patterns are registered, respectively. Thus, the titles (name, number, and the like) of “XXXX#1” and “YYYY#2” are displayed for the face frames, respectively. Furthermore, since the face images of the persons who seem to be cameramen located behind in the screen have no registered pattern data, titles for those face frames are displayed as “#??”, and the operator can see that those persons are not registered. 
     Further, in some cases, the title display may be disabled (OFF) since it may be difficult to see the screen if the title is displayed as described above. 
     Furthermore, when a plurality of registration patterns exist in the frame, it might be hard to know which one is being tracked if the face frames are being displayed for all of the plurality of registration patterns. Thus, a color of the face frame of the tracking target (AF target) may be changed and displayed. 
     As described above, in  FIG. 5 , there is a plurality (two persons) of registration patterns in a frame, and it is possible to select which one of them is set as a face pattern of a tracking target to be tracked by performing the AF. 
     There are various methods for selecting the AF area as the tracking target to be tracked by performing the AF. For example, the operator may select by pressing the switches  114   b  of the operation unit  114 . Alternatively, when priorities are given to the registration patterns in advance and when a plurality of registration patterns exist in the screen, the registration pattern having the highest priority may be automatically selected. Further alternatively, the selection may be automatically performed in a registration order (order of registration No.) of patterns that are registered in association with the switches  114   b . The registration order and the priorities are similar to each other, but do not always coincide with each other. The priorities can be set so as to be different from the registration order after the registration, and it is also possible to change the priorities. 
     The tracking target (AF area) to be tracked by performing the AF is determined, the auto-tracking is started in the screen, and simultaneously the AF is automatically started for the tracking target. When the auto-tracking is being performed, sometimes the tracking target disappears from a frame. However, various cases can be expected as the situation thereafter. 
     For example, a different registration pattern may not exist in the screen when the tracking target disappears from a frame. Also, in another case, a different registration target may still exist in the screen even when the tracking target disappears from a frame. 
     In addition, when a different registration pattern does not exist in the screen at that time, the registration pattern being tracked hitherto may appear in a frame again. Also, in another case, a different registration pattern that is not the tracking target may appear in a frame. In further another case, if a different registration target exists in the screen when the tracking target disappears from a frame, the tracking target being tracked hitherto may appear in a frame again. 
     In these cases, various tracking control methods (tracking modes) of how to select the tracking target and how to perform the auto-tracking can be considered. Hereinafter, the respective methods will be described while classifying these cases. 
     In a flowchart of  FIG. 6 , if a registration pattern A currently being tracked disappears from the screen when the registration pattern A is automatically being tracked as a tracking target, various situations thereafter are classified with mainly paying attention to a registration pattern that appears on the screen. 
     As shown in step S 201  of  FIG. 6 , the registration pattern (tracking pattern) A currently being tracked disappears from the screen. 
     In this case, there may be two situations, that is, one is that any registration pattern does not exist in the screen, and the other is that a registration pattern other than the pattern A exists. 
     First, let consider the situation where another registration pattern does not exist in the screen after the pattern A disappears from the screen as shown in step S 202 . In this case, the AF (autofocus) is turned OFF as shown in step S 203  since there is already no pattern to be tracked in the screen. Then, the AF is kept being in the OFF state as it is unless a registration pattern exists in the screen. 
     It is assumed that, subsequently, a registration pattern appears in the screen. In this case, there may be two situations, that is, one is that the pattern A, which was tracked previously, appears in the screen as shown in step S 204 , and the other is that a registration pattern B other than registration pattern A appears in the screen as shown in step S 205 . 
     In step S 204 , as for a response to the case where the pattern A appears in the screen again, there may be two situations: one is that it is started to track the pattern A again as shown in step S 206  since the pattern A being tracked hitherto appears again on the screen, and the other is that the AF may kept being in the OFF state without the pattern A, which disappeared from the screen once, being tracked as shown in step S 207 . 
     Furthermore, when the pattern A initially being tracked disappears from the screen, and when any registration pattern to be tracked does not exist in the screen as shown in step S 205 , another registration pattern B appears in the screen. In this case, there may be two situations: one is that the AF is kept being in the OFF sate as shown in step S 207  because the pattern A is hitherto tracked and because the other registration pattern B has no relation, and the other is that the registration pattern B is tracked as shown in step S 208  because the pattern A disappears and the registration pattern B enters instead. 
     Here, the pattern A is tracked again in step S 206  only when the pattern A has priority as the tracking target. Thus, even if another registration pattern appears in the screen, such another pattern is not set to a tracking target. Furthermore, the tracking of another registration pattern B in step S 208  represents that no matter which registration pattern appears after the pattern A disappears from the screen, the appearing pattern is set to a tracking target. 
     Furthermore, in step S 205 , after any registration pattern does not exist in the screen since the pattern A initially being tracked disappears from the screen, then only one person of the registration pattern B appears in the screen. However, a plurality of registration patterns other than the pattern A may simultaneously appear in the screen, and so the plurality of registration patterns may exist in the screen. In this case, any one pattern is selected as a tracking target from among the plurality of registration patterns in step S 208 . The selection method will be described later. 
     Next, let consider the situation where a registration pattern other than the pattern A exists in the screen after the pattern A disappears from the screen, as shown in step S 209 . 
     As for a response to this case, there may be two situations: one is that the AF is be turned OFF as shown in step S 210  because the tracking pattern A does not exist, and the other is that another registration pattern B existing in the screen is tracked as shown in step S 211 . 
     If no registration pattern exists in the screen except the other registration pattern B when it is started to track the other registration pattern B in step S 211 , the pattern B is tracked. However, if a plurality of the registration patterns exist in the screen, one tracking pattern should be determined in some way. The method for selecting one registration pattern from among the plurality of registration patterns as a tracking target (tracking pattern) will be described later. 
     If the pattern A appears in the screen again as shown in step S 212  when the AF has been turned OFF in step S 210 , the current situation is the same as the above-mentioned situation in step S 204 . 
     In this case, there may be two situations: one is that the AF is kept being in the OFF state as shown in step S 213  even if the pattern A appears in the screen again, and the other is that it is started to track the pattern A again as shown in step  214 . 
     Furthermore, if the pattern A appears in the screen as shown in step S 215  when the other registration pattern B has been set to be tracked in step S 211 , there may be two situations: one is that it is started to track the pattern A again by changing the tracking target from the pattern B currently being tracked to the pattern A originally being tracked as shown in step S 214 , and the other is that the tracking of the pattern B is continued as shown in step S 216 . 
     As a result, the auto-tracking control methods shown in  FIG. 6  can be summarized as follows: 
     In the flow of the steps S 201 -&gt;S 202 -&gt;S 203 -&gt;S 204 -&gt; 8206  and the flow of the steps S 201 -&gt;S 209 -&gt;S 210 -&gt;S 212 -&gt;S 214 , the pattern A is treated as a main tracking target, and the tracking of the pattern A is prioritized at any time in the following manner. That is, if the pattern A disappears from the screen, the AF is turned OFF irrespective of whether or not another registration pattern exists in the screen. Also, if the pattern A appears in the screen again, it is started again to track the pattern A. 
     Furthermore, in the flow of steps S 201 -&gt;S 202 -&gt;S 203 -&gt;S 205 -&gt;S 208 , if another registration pattern B appears in the screen after the pattern A disappears from the screen, the pattern B is tracked. 
     Further, in the flow of the steps S 201 -&gt;S 202 -&gt;S 203 -&gt;S 204  (S 205 )-&gt;S 207  and the flow of step S 201 -&gt;S 209 -&gt;S 210 -&gt;S 212 -&gt;S 213 , once the pattern A, which is tracked, disappears from the screen, the AF is kept being in the OFF state even if the pattern A or another registration pattern appears in the screen. 
     In the above flows of the tracking control, the tracking of the pattern A is prioritized. 
     In contrast, in the flow of the steps S 201 -&gt;S 209 -&gt;S 211 , if the pattern A, which is tracked, disappears from the screen and if another registration pattern exists in the screen, first, another registration pattern is tentatively tracked. In this case, there are the following two situations. One is that the tracking of the other registration pattern is just temporal as shown in the flow of the steps S 215 -&gt;S 214 , and the tracking target is changed to the pattern A when the pattern A appears in the screen again. The other is that the tracking target is completely changed to the other registration pattern as shown in the flow of the steps S 215 -&gt;S 216 , and the tracking of the other registration pattern is continued even if the pattern A appears in the screen again. 
     Accordingly, the tracking control shown in  FIG. 6  can be briefly expressed as that shown in  FIG. 7 . 
     Specifically, if a tracking pattern A disappears from a screen when the registration pattern A is tracked as a tracking target, either “AF is tuned OFF” or “another registration pattern B is tracked”. If a registration pattern appears in the screen when “AF is turned OFF”, the current state (“AF is turned OFF”) is maintained, or it is started to track the registration pattern appearing in the screen. If the registration pattern A appears in the screen when “another registration pattern B is tracked”, the current state (“another registration pattern B is tracked”) is maintained, or it is started again to track the pattern A appearing in the screen again. 
     As described above, the following modes are considered: a mode M 1  in which the AF is kept being in the OFF state even if any registration pattern appears in the screen after the pattern A disappears from the screen; a mode M 2  in which if any registration pattern appears in the screen after the pattern A disappears from the screen (especially, the pattern A, which was tracked previously, is prioritized over the other registration patterns), it is started to track the appearing registration pattern; a mode M 3  in which if the pattern A disappears from the screen, a tracking target is changed in any case to track another registration pattern B; and a mode M 4  in which once the pattern A appears in the screen again, it is restarted to track the pattern A even if another registration pattern B is being tracked. 
     These are the basic tracking control methods in this embodiment. Furthermore, there may be actually various complicate cases, but the tracking control is performed based on the above-mentioned basic cases. 
     In practice, the system may include all the above modes, and may include only some of those modes. In addition, when a plurality of modes are provided, the modes are switchable. Thus, it is preferable that the operator perform various tracking process by changing between the modes, for example, in accordance with scenes. 
     For example, the system may include two modes of: the mode M 1  in which the AF is kept being in the OFF state even if a registration pattern appears in the screen after the tracking pattern A disappears from the screen; and the mode M 2  in which if a registration pattern appears in the screen, the appearing registration pattern is tracked. The system may be configured to change between these modes. 
     Alternatively, the system may be configured to change between two modes of: the mode M 3  in which even if the pattern A appears in the screen again when another registration pattern B is being tracked, it is continued to track the other registration pattern B; and the mode M 4  in which if the pattern A appears in the screen again, the pattern A is tracked again. 
     Further alternatively, a combination of the modes M 1  and M 2  may be changed to a combination of the modes M 3  and M 4 . 
     Next, when a plurality of registration patterns exist in a screen as shown in the step S 209  of  FIG. 6 , a selection method (how to select a registration pattern as a tracking target from among the plurality of registration patterns) will be described. 
     In the selection method of selecting a registration pattern as a tracking target when a plurality of registration patterns exist in the screen, there may be three situations: first one is that the operator may perform the selection, second one is that the system may automatically perform the selection in order of registering the registration patterns, and third one is that similarly, the system may automatically perform the selection in priority order that is set in advance for the registration patterns. 
     First, in the pattern registration, for example during shooting, the tracking processing unit  30  assigns subjects in the screen to the switches  14   b  of the operation unit  14  in  FIG. 1  or the switches  114   b  in  FIG. 4 , respectively, and automatically numbers and registers the subjects. In this manner, it is possible to register patterns as many as the number of switches. For example, as shown in  FIG. 5 , numbers (# 1 ,  42 , and the like) of the switches corresponding to the registered patterns are displayed for the registered pattern on the viewfinder  118 . The operator watches this display, and can correct the registration manually in such a case where an undesired pattern is unnecessarily registered. 
     Furthermore, when the operator selects a registration pattern as a tracking target, if the operator presses, for example, the second switch (# 2 ) of the switches  14   b  (or  114   b ) while watching the viewfinder  18 , the registration pattern assigned to the second switch is selected as the tracking target. 
     Next, a process flow for the case where the system automatically assigns registration patterns to the respective switches  114   b  in advance and an operator selects a tracking target from among a plurality of registration patterns in the screen by using the switches  114   b  will be described with reference to flowcharts in  FIGS. 8A ,  8 B and  9 . 
     First, an initial setting is performed in step S 300  of  FIG. 8A . 
     Next, it is determined as to whether or not auto-tracking is performed in step S 302 . If it is determined in step S 302  that the auto-tracking is ON, the flow advances to step S 348  (connector ( 1 )) of  FIG. 9 . If it is determined in step S 302  that the auto-tracking is OFF, it is determined in the following step S 304  as to whether or not the current mode is a pattern registration mode. 
     As a result, if the pattern registration mode is OFF, an operator manually sets an AF area in the following step S 306 . 
     Next, an ON/OFF state of the area auto-tracking switch (tracking switch) is checked in step S 308 . If the switch is ON, the auto-tracking is turned ON in step S 310 . If the switch is OFF, the auto-tracking is turned OFF in step S 312  (connector ( 2 )) of  FIG. 8B . 
     Next, it is determined in step S 314  as to whether or not number of the switch  114   b  to which a registration pattern is assigned is reset. 
     If the reset is OFF, an ON/OFF state of the AF (autofocus) is checked in step S 316 . Then, if the determination made in step S 314  results in that the reset of the assigned No. is ON, the AF is turned OFF in step S 318 , and the switch No. of the switch  114   b  is reset in step S 320 . Accordingly, the assignment of the registration pattern to the switch of the No. is canceled, and it is possible to assign another registration pattern thereto. 
     Furthermore, if the determination made in step S 316  results in that the AF is OFF, the operator manually performs the focus control in step S 322 . If the AF is ON, the normal AF process is performed in step S 324 . 
     Furthermore, if it is determined in step S 304  that the pattern registration mode is ON, the pattern is registered in the database by the processes in step S 326  and subsequent steps. 
     First, in step S 326 , the image processing unit  126  of the tracking device  116  (see  FIG. 4 ) acquires an image through the image input unit  128 . The image processing unit  126  performs the predetermined image process for the acquired image, and the processed image is sent to the tracking processing unit  130 . 
     Next, the tracking processing unit  130  performs the face recognition process for a subject in the image in step S 328 . Then, the auto-tracking of the recognized subject is performed in step S 330 . 
     In step S 332 , it is determined as to whether or not the face recognition is terminated. If the face recognition is not yet terminated, the flow returns to step S 326 , and an image is acquired again. 
     Furthermore, if it is determined in step S 332  that the face recognition is terminated, the operator selects a face to be registered while watching the viewfinder  118  in the following step S 334 . 
     Next, the pattern sampling is performed based on the acquired image in step S 336 . The tracking processing unit  130  performs the auto-tracking by setting the subject having the currently selected face as a tracking target in step S 338 . 
     The operator determines in step S 340  as to whether or not the sampling is completed, in accordance with whether or not the current auto-tracking target is appropriate. 
     As a result, if it is determined that the sampling is still needed, the flow returns to step S 336 , and the sampling is continued. 
     In contrast, if it is determined that the sampling is completed, a title for the tracking target is registered in the following step S 342 . 
     Then, the pattern registration mode is turned OFF in step S 344 , the area auto-tracking is turned ON in step S 346 , and the flow advances to step S 314  (connector ( 2 )) of  FIG. 8A . 
     On the other hand, if it is determined in step S 302  that the auto tracking is ON, the flow advances to step S 348  (connector ( 1 )) of  FIG. 9 . 
       FIG. 9  shows a part of the process flow in which the system automatically assigns the registration patterns to the respective numbers of the switches  114   b.    
     First, the image processing unit  126  acquires an image in step S 348  of  FIG. 9 . In step S 350 , the tracking processing unit  130  performs the face recognition process for a subject in the image. 
     Then, in step S 352 , the tracking processing unit  130  searches the database for a pattern that is identical to the sampled pattern of the face, which is detected from the subject in the image. 
     As a result, if the identical pattern is found in step S 354 , a number of the switch  114   b  is assigned to the found pattern in step S 356 , and as shown in  FIG. 5 , the number such as # 1  and # 2  is displayed on the viewfinder  118  in step S 358 . Then, the auto-tracking is performed in step S 360 . 
     Furthermore, if the pattern is not found in step S 354 , the above-mentioned process is not performed, and the flow advances to the following step S 362 . 
     In step S 362 , it is determined whether or not all the faces have been changed against the data registered in the database. If the checking process has not yet completed, the flow returns to step S 348 , and the above-mentioned process is repeated. 
     Furthermore, if the checking process has been completed, it is determined in the following step S 364  as to whether or not one or more registered patterns are found. 
     If no registered pattern is found, the AF is turned OFF in step S 366 , and the flow advances to step S 314  (connector ( 2 )) of  FIG. 8B . 
     Furthermore, if one or more the registered patterns are found in step S 364 , the operator selects as an AF target a number of any of the switches  114   b  to which the registration patterns are assigned, in step S 368 . 
     In step S 370 , it is determined as to whether or not the AF target pattern before is changed to another target pattern by the operator&#39;s selection of the tracking target (AF target). If the AF target pattern is changed, change of the AF target is performed in step S 372 . Further, if the AF target is not changed, the AF is kept being in the ON state in step S 374 , and the flow advances to step S 314  (connector ( 2 )) of  FIG. 8B . 
     In this manner, when a registration pattern is registered in the database in advance, the system automatically assigns the registration pattern of some of subjects in the screen to the respective numbers of the switches  114   b . Then, when the operator selects a tracking target from among the plurality of registration patterns in the screen, the operator can select a registration patterns as a tracking target by using the numbers assigned to the respective registration patterns displayed on the viewfinder  118 . 
     Furthermore, in the pattern registration, the operator may manually register a title(s) as described above and additionally may give priorities in registration. 
     Furthermore, the registration order is formed in an arrangement order of the switches  14   b  (or  114   b ). The registration order does not necessarily coincide with the priorities. For example, when the above-mentioned system registers patterns by automatically numbering the patterns and assigning the patterns to the switches, the operator can give to the registration patterns priorities that are different from the order of the switches. 
     For example, in the tracking control in  FIG. 6  as described above, when the tracking pattern A disappears from the screen, the selection of the tracking target of the pattern A is canceled, the AF is turned OFF, and another registration pattern will not be selected as a tracking target in step S 210 . Accordingly, even if the pattern A appears in the screen again, the AF is kept being in the OFF state. Thereby, the mode M 1  shown in  FIG. 7  is realized. 
     Likewise, if the tracking pattern A disappears from the screen in step S 201  of  FIG. 6  and the AF is turned OFF once, a registration pattern is tracked in any case, and especially, a priority is given to the pattern A so that the pattern A is selected as a tracking target. Then, a registration pattern is searched for in the screen all the time. Therefore, if a registration pattern appears in the screen, that registration pattern is detected, and it is started to track the detected registration pattern. In this case, especially the tracking of the pattern A is prioritized. In this manner, the mode M 2  shown in  FIG. 7  is realized. 
     Furthermore, for example, if a registration pattern having the highest priority is set to be tracked at all time and if a pattern currently being tracked disappears from the screen, the priority of the disappearing pattern is changed to the lowest priority. Then, when a pattern B having the next highest priority is being tracked in step S 211  of  FIG. 6  after the pattern A being tracked hitherto disappears from the screen, even if the pattern A appears in the screen again in step S 215 , it is still continued to track the pattern B in step S 216  without the pattern A being tracked. This is because the priority of the pattern A has already been lowered. Thereby, the mode M 3  shown in  FIG. 7  is realized. 
     Further, for example, when the pattern A initially being tracked appears in the screen during the tracking of the pattern B in step S 215  of  FIG. 6 , settings are made so that the pattern A is prioritized over the other registration patterns and that if a plurality of registration patterns exist in the screen, a registration pattern having higher priority is tracked on a priority basis. Accordingly, the tracking of the pattern A instead of the pattern B is started in step S 214 . Thereby, the mode M 4  shown in  FIG. 7  is realized. 
     As described above, by setting priorities for the selection of the tracking target, it is possible to automatically change the tracking target. The mode change is performed by the slide switch  114   e  of the operation unit  114 . Furthermore, the modes may be set or changed in a way that the operator manually selects the tracking target through the operation unit  14  ( 114 ). 
     As described above, the autofocus system according to the embodiments of the invention has described in detail. However, the invention is not limited to the embodiments mentioned above, and it is apparent that various modifications and variations may be made thereto without departing from the technical scope of the invention.