Patent Publication Number: US-7725019-B2

Title: Apparatus and method for deciding in-focus position of imaging lens

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for deciding the in-focus position of an imaging lens. 
     2. Description of the Related Art 
     In an example of the prior art (see the specification of Japanese Patent Application Laid-Open No. 11-146405), a skin-tone area contained in the image of a subject is detected and the skin-tone area detected is adopted as a focusing area. 
     Even in a case where a skin-tone area is detected, however, it is difficult to detect the skin-tone area accurately when the very subject image in which the skin tone is to be detected is extremely out of focus. Consequently, there are occasions where focusing cannot be achieved accurately even when a skin-tone area is adopted as a focusing area. Similarly, even in a case where a target is detected solely from contrast (density) information without using color information, it is difficult to detect the target accurately in an extremely out-of-focus state. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to achieve focusing comparatively accurately. 
     According to a first aspect of the present invention, the foregoing object is attained by providing an apparatus for deciding an in-focus position of an imaging lens, comprising: a solid-state electronic image sensing device for sensing the image of a subject and outputting image data representing the image of the subject; a first focusing data output device for moving an imaging lens, which has been placed in front of the solid-state electronic image sensing device, along the direction of the optic axis in increments of a first prescribed distance and outputting focusing data representing degree of focus of the image of the subject at every position to which the lens is moved; a target-image detecting device for detecting the image of a target, which is contained in the image of the subject represented by image data that is output from the solid-state electronic image sensing device at least at one position among positions of the imaging lens at which the level of the focusing data that has been output from the focusing data output device is equal to or greater than a prescribed threshold value, in increments of a second prescribed distance (at each position of the imaging lens) that is less than the first prescribed distance; and an in-focus position deciding device for deciding on the position of the imaging lens as the in-focus position based upon data that corresponds to the target detected by the target detecting device. 
     The first aspect of the present invention also provides a method suited to the above-described apparatus for deciding an in-focus position of an imaging lens. More specifically, there is provided a method of deciding an in-focus position of an imaging lens comprising the steps of: sensing the image of a subject using a solid-state electronic image sensing device and obtaining image data representing the image of the subject; moving an imaging lens, which has been placed in front of the solid-state electronic image sensing device, along the direction of the optic axis in increments of a first prescribed distance and obtaining focusing data representing degree of focus of the image of the subject at every position to which the lens is moved; detecting the image of a target, which is contained in the image of the subject represented by image data that is obtained from the solid-state electronic image sensing device at least at one position among positions of the imaging lens at which the level of the focusing data obtained is equal to or greater than a prescribed threshold value, in increments of a second prescribed distance that is less than the first prescribed distance; and deciding on the position of the imaging lens as the in-focus position based upon data that corresponds to the target detected. 
     In accordance with the first aspect of the present invention, the imaging lens is moved in increments of a first prescribed distance along the direction of the optic axis and focusing data representing degree of focus of the image of the subject is obtained at every position to which the lens is moved. The image of a target, which is contained in the image of the subject represented by image data obtained from the solid-state electronic image sensing device at least at one position among positions of the imaging lens at which the level of the focusing data is equal to or greater than a prescribed threshold value, is detected in increments of a second prescribed distance, which is less than the first prescribed distance. The position of the imaging lens is decided on as the in-focus position based upon data that corresponds to the target detected by the target detecting device. 
     Since it is attempted to detect the target from the image of the subject obtained in a case where the focusing data is equal to or greater than a prescribed threshold value, it is easier to detect the target accurately. The position of the imaging lens is decided on based upon the image data representing the image of the detected target. The position of the imaging lens can therefore be decided so as to bring the image of the target into focus. 
     The apparatus may further comprise a focusing target area deciding device for deciding one or a plurality of focusing target areas based upon the target detected by the target detecting device; and a weighting coefficient deciding device for deciding a weighting coefficient in such a manner that a weighting coefficient of the target is enlarged in the one or plurality of focusing target areas decided by the focusing target area deciding device. In this case the in-focus position deciding device would decide on the position of the imaging lens as the in-focus position based upon image data, which is obtained from the focusing target area decided by the focusing target area deciding device, from among the image data that is output from the solid-state electronic image sensing device. 
     According to a second aspect of the present invention, the foregoing object is attained by providing an apparatus for deciding an in-focus position of an imaging lens, comprising: a solid-state electronic image sensing device for sensing the image of a subject and outputting image data representing the image of the subject; a first focusing data output device for moving an imaging lens, which has been placed in front of the solid-state electronic image sensing device, along the direction of the optic axis in increments of a first prescribed distance and outputting first focusing data representing degree of focus of the image of the subject at every position to which the lens is moved; a target-image determination device for determining whether the image of a target is contained in the image of the subject represented by image data that is output from the solid-state electronic image sensing device at least at one position among positions of the imaging lens at which the level of the first focusing data that has been output from the first focusing data output device is equal to or greater than a prescribed threshold value; a focusing range deciding device for deciding a focusing range, which contains the position of the imaging lens prevailing when the target-image determination device has determined that the target is contained in the image of the subject, among ranges in which the imaging lens is situated in such a manner that the level of the first focusing data that has been output from the first focusing data output device is equal to or greater than a threshold value continuously; a second focusing data output device for moving the imaging lens along the direction of the optic axis in increments of a second prescribed distance, which is less than the first prescribed distance, in the focusing range that has been decided by the focusing range deciding device, and outputting second focusing data representing degree of focus of the image of the subject at every position to which the lens is moved; and an in-focus position deciding device for deciding on the position of the imaging lens, at which the image of the subject is brought into focus based upon the second focusing data that has been output from the second focusing data output device, as the in-focus position. 
     The second aspect of the present invention also provides a method suited to the above-described apparatus for deciding an in-focus position of an imaging lens. More specifically, there is provided a method of deciding an in-focus position of an imaging lens comprising the steps of: sensing the image of a subject using a solid-state electronic image sensing device and obtaining image data representing the image of the subject; moving an imaging lens, which has been placed in front of the solid-state electronic image sensing device, along the direction of the optic axis in increments of a first prescribed distance and obtaining first focusing data representing degree of focus of the image of the subject at every position to which the lens is moved; determining whether the image of a target is contained in the image of the subject represented by image data that is obtained from the solid-state electronic image sensing device at least at one position among positions of the imaging lens at which the level of the first focusing data obtained is equal to or greater than a prescribed threshold value; deciding a focusing range, which contains the position of the imaging lens prevailing when it has been determined that the target is contained in the image of the subject, among ranges in which the imaging lens is situated in such a manner that the level of the first focusing data obtained will be equal to or greater than a threshold value continuously; moving the imaging lens along the direction of the optic axis in increments of a second prescribed distance, which is less than the first prescribed distance, in the focusing range decided, and obtaining second focusing data representing degree of focus of the image of the subject at every position to which the lens is moved; and deciding on the position of the imaging lens, at which the image of the subject is brought into focus based upon the second focusing data, as the in-focus position. 
     In accordance with the second aspect of the present invention, the imaging lens is moved in increments of a first prescribed distance along the direction of the optic axis and first focusing data representing degree of focus of the image of the subject is obtained at every position to which the lens is moved. It is determined whether the image of a target is contained in the image of the subject represented by image data obtained from the solid-state electronic image sensing device at least at one position among positions of the imaging lens at which the level of the first focusing data is equal to or greater than a prescribed threshold value. A focusing range is decided, namely a range that contains the position of the imaging lens prevailing when it has been determined that the target is contained in the image of the subject, among ranges in which the imaging lens is situated in such a manner that the level of the first focusing data obtained will be equal to or greater than a threshold value continuously. The imaging lens is moved along the direction of the optic axis in increments of a second prescribed distance, which is less than the first prescribed distance, in the focusing range decided, and second focusing data is obtained at every position to which the lens is moved. The position of the imaging lens at which the image of the subject is brought into focus based upon the second focusing data obtained is decided upon as the in-focus position. 
     In accordance with the second aspect of the present invention, a target area is detected in the image of a subject obtained in a case where first focusing data is equal to or greater than a prescribed threshold value. A focusing range within which the image of a subject that contains the target will be sensed is decided, the imaging lens is moved a prescribed distance at a time in the focusing range thus decided, second focusing data is obtained and the in-focus position of the imaging lens is decided. Thus the position of the imaging lens is decided in such a manner that the target will come into focus. 
     The target-image determination device may be so adapted as to detect and determine, for every position of the imaging lens, the target contained in the image of the subject represented by image data that is output from the solid-state electronic image sensing device at a position of the imaging lens at which the level of the first focusing data that has been output from the first focusing data output device is equal to or greater than the prescribed threshold value. In this case the second focusing data output device would move the imaging lens along the direction of the optic axis within the focusing range that has been decided by the focusing range deciding device and would output the second focusing data, which represents the degree of focus of the target detected by the target determination device among the images of the subject, at every position to which the lens is moved. 
     The second focusing data output device may be so adapted as to move the imaging lens along the direction of the optic axis in the focusing range that has been decided by the focusing range deciding device, and to output the second focusing data, which represents the degree of focus of the image within the target area decided based upon all targets that have been detected by the target determination device among images of the subject, at every position to which the lens is moved. 
     The apparatus may further comprise a sub-target area deciding device for deciding one or a plurality of sub-target areas based upon all of the targets; and a weighting coefficient deciding device for deciding a weighting coefficient in such a manner that a weighting coefficient of the target area is enlarged in the one or plurality of sub-target areas decided by the sub-target area deciding device. In such case the second focusing data output device would move the imaging lens (move the lens again) along the direction of the optic axis within the focusing range that has been decided by the focusing range deciding device and would output the second focusing data, which represents the degree of focus of the target detected by the target determination device among images of the subject, at every position to which the lens is moved while adjusting the level of the second focusing data based upon the weighting coefficient that has been decided by the weighting coefficient deciding device. 
     It may be so arranged that movement of the imaging lens in the first focusing data output device is performed from the NEAR side. In such case the target determination device would execute processing to determine whether the image of the subject in the subject determination device contains a target whenever the imaging lens is moved. The apparatus may further comprise an imaging-lens movement control device for controlling the first focusing data output device so as to halt movement of the imaging lens in response to detection of the target. It may be construed that an important target is close to the image sensing device. Since movement of the imaging lens is halted in response to detection of such an important target, the position of the imaging lens can be decided comparatively quickly. 
     Naturally, it may be so arranged that movement of the imaging lens in the first focusing data output device is performed from the INF (infinity) side, or it may be so arranged that movement of the imaging lens in the first focusing data output device is performed from either the NEAR side or INF side in accordance with a setting. The apparatus may further comprise a setting device for setting direction of movement of the imaging lens in the first focusing data output device. In such case movement of the imaging lens in the first focusing data output device would be performed from the NEAR side or INF side in accordance with the setting made by the setting device. 
     The apparatus may further comprise an image sensing mode determination device for determining whether a short-distance image sensing mode or a long-distance image sensing mode has been set; and a device for setting movement of the imaging lens in the first focusing data output device in such a manner that movement is performed from the NEAR side in response to a determination by the image sensing mode determination device that the short-distance image sensing mode has been set and from the INF side in response to a determination by the image sensing mode determination device that the long-distance image sensing mode has been set. 
     Furthermore, in a case where the imaging lens is a zoom lens, movement of the imaging lens in the first focusing data output device is performed from the NEAR side or INF side in accordance with the zoom magnification of the zoom lens. 
     The target-image determination device would determine whether the image of the subject represented by the image data that is output from the solid-state electronic image sensing device contains a target at a position of the imaging lens that corresponds to a first maximal value (maximal value of an envelope signal obtained from the focusing data) equal to or greater than the prescribed threshold value in the first focusing data that has been output from the first focusing data output device. 
     Distance travelled by the imaging lens in the first focusing data output device may be greater than distance travelled by the imaging lens in the second focusing data output device. 
     Detection of the target is performed based upon at least one among target likeliness in the target area, size of the target, brightness of the target and position of the target area, by way of example. 
     The apparatus may further comprise a display device for displaying the image of the subject, which is represented by the image data that has been output from the solid-state electronic image sensing device, on a display screen; and a display control device for controlling the display device so as to display the area of the detected target on the image of the subject. 
     The apparatus may further comprise a display device for displaying the image of the subject, which is represented by the image data that has been output from the solid-state electronic image sensing device, on a display screen; and a display control device for controlling the display device so as to display the target area on the image of the subject. The user can ascertain at a glance where the target area and the focus target area are located. 
     The target is a face or an eye, by way of example. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the electrical structure of a digital still camera; 
         FIG. 2  illustrates the relationship between a digital still camera and subjects; 
         FIGS. 3 and 4  are flowcharts illustrating recording processing; 
         FIG. 5  is a flowchart illustrating preliminary search processing for autofocus; 
         FIG. 6  is a flowchart illustrating face detection processing; 
         FIG. 7  is a flowchart illustrating processing for registering a face area; 
         FIG. 8A  illustrates the relationship between the position of an imaging lens and an overall focus evaluation value, and  FIG. 8B  illustrates the relationship between the position of an imaging lens and a face-area focus evaluation value; 
         FIG. 9A  illustrates the area of the image of a subject when an overall focus evaluation value is obtained, and  FIG. 9B  illustrates an area when a face-area focus evaluation value is obtained; 
         FIG. 10  illustrates the registration of faces of face information; 
         FIG. 11  is a flowchart of other processing for face detection; 
         FIG. 12  is a flowchart of other processing for face-area registration; 
         FIGS. 13 to 15  illustrate ways to decide weighting; 
         FIGS. 16A and 16B  illustrate relationships between the position of an imaging lens and an overall focus evaluation value, and  FIG. 16C  illustrates the relationship between the position of an imaging lens and a face-area focus evaluation value in another embodiment; and 
         FIG. 17A  illustrates the relationship between the position of an imaging lens and an overall focus evaluation value, and  FIG. 17B  illustrates the relationship between the position of an imaging lens and a face-area focus evaluation value in a further embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the drawings. 
       FIG. 1  is a block diagram illustrating the electrical configuration of a digital still camera according to an embodiment of the present invention. 
     The digital still camera according to this embodiment is so adapted that a face in the image of a subject will come into focus in suitable fashion. 
     The operation of the overall digital still camera is controlled by a control circuit  1 . The control circuit  1  includes a CPU  2 , a ROM  3  in which the operating program of the digital still camera and prescribed data, etc., have been stored, and a RAM  4  for storing data temporarily. 
     The digital still camera is provided with an operating device  5  having switches and buttons such as a shutter-release button of two-step stroke type. An operating signal that is output from the operating device  5  is input to the control circuit  1 . 
     A diaphragm  10  and imaging lens  8  are provided in front of a CCD  12 . The imaging lens  8  is supported so as to be freely movable along the direction of the optic axis of the lens. [The side of the lens facing the CCD  12  is referred to as the “NEAR” side, and the side facing away from the CCD  12  is referred to as the “INF” (infinity) side.] The position of the lens is controlled by a lens driving circuit  7  in such a manner that the image of the subject (the image of a face in a case where the image of the subject contains the image of a face, as will be described later) is focused and formed on the photoreceptor surface of the CCD  12 . The diaphragm  10  has its f-stop number controlled by a diaphragm driving circuit  9  in such a manner that an appropriate amount of exposure is obtained. When the image of a subject is sensed, a video signal representing the image of the subject that has been formed on the photoreceptor surface of the CCD  12  is output under the control of a control circuit  11  for controlling the image sensing device. The video signal that has been output from the CCD  12  is input to an analog signal processing circuit  13 . The lens driving circuit  7 , diaphragm driving circuit  9  and image sensing device control circuit  11  are controlled by the control circuit  1 . 
     The video signal is subjected to prescribed analog signal processing such as a white balance adjustment in the analog signal processing circuit  13 . The video signal that has been output from the analog signal processing circuit  13  is converted to digital image data in an analog/digital signal converting circuit  14  and the digital image data is input to a digital signal processing circuit  15  and to the control circuit  1 . 
     High-frequency components are extracted from the input digital image data in the control circuit  1 , whereby focusing data is obtained. The focusing data obtained is integrated over the entire image or in an area that is part of the image to thereby obtain a focus evaluation value. The imaging lens  8  is controlled by the lens driving circuit  7  based upon the obtained focus evaluation value in such a manner that the image of the subject is focused upon the photoreceptor surface of the CCD  12 . 
     Further, the digital image data is subjected to prescribed digital signal processing such as a gamma correction in the digital signal processing circuit  15 . The image data that has been output from the digital signal processing circuit  15  is applied to a display unit  17  via a memory  16 . The image of the subject is displayed on the display screen of the display unit  17 . Image data that has been output from the digital signal processing circuit  15  is input also to a face extraction processing circuit  6 . The latter extracts the image of a face contained in the image of the subject represented by the image data that has been output from the digital signal processing circuit  15 . Data representing the extracted image of the face and data such as position and size indicative of the area of the extracted face is applied to the control circuit  1 . As will be described later in greater detail, focusing control is carried out using the data representing the image of the face. There is no particular limitation on the method of extracting the face. Methods which may be used include a method that relies upon brightness, a method using color or a method using both brightness and color. 
     If the shutter-release button is pressed through the first step of its stroke, focusing control is performed in a manner described later. If the shutter-release button is pressed through the second step of its stroke, the image data that has been output from the digital signal processing circuit  15  is applied to and stored temporarily in a memory  16 , as described above. The image data is recorded by reading it from the memory  16  and applying it to a memory card  19  via an interface  18 . 
       FIG. 2  illustrates the relationship between subjects and a digital still camera. 
     As illustrated in  FIG. 2 , a digital still camera  20  is arranged facing subjects  21  and  22 . Subject  21  is a person and subject  22  a vehicle. 
     In this embodiment, the direction in which the imaging lens of the digital still camera  20  moves along the optic axis of the lens is defined as the direction along the Z axis. The plane in which the Z axis is the normal line is defined as the XY plane. 
       FIGS. 3 and 4  are flowcharts illustrating recording processing executed by the digital still camera. 
     Sensing of the image of the subject by the CCD  12  continues and image data representing the image of the subject is obtained in the manner described above. If the shutter-release button is pressed through the first step of its stroke (“YES” at step  31 ), automatic exposure (AE) processing in which luminance components are extracted from the obtained image data is executed (step  32 ). An appropriate amount of exposure is calculated based upon the luminance components extracted (step  33 ). The diaphragm  10  and the shutter speed (so-called “electronic shutter”) of the CCD  12  are controlled in such a manner that the calculated amount of exposure is achieved. 
     Next, a preliminary search for autofocus (AF) is carried out (step  34 ). 
       FIG. 5  is a flowchart illustrating preliminary search processing for AF. 
     The imaging lens  8  is freely movable within a prescribed range along the Z direction and is initially positioned at an initial position. The image of the subject is sensed in a state in which the imaging lens  8  is at the initial position, and image data representing the entirety I of the image of the subject is obtained, as indicated by the hatching in  FIG. 9A . An overall focus evaluation value is obtained by extracting and integrating high-frequency components from the image data obtained (step  51 ). 
     If the overall focus evaluation value obtained is equal to or greater than a prescribed threshold value (“YES” at step  52 ), then it is construed that the image of the subject obtained at the position of the imaging lens  8  that prevailed when the overall focus evaluation value was obtained is in comparatively good focus. This position of the imaging lens  8  is stored as a face-detection execution position (step  53 ). Processing for detecting a face in the image of a subject is executed, as will be described later, at the face-detection execution positions. When a face is detected, high-frequency components are extracted from the detected face to thereby obtain a face-area focus evaluation value. A face-detection execution position at which the face-area focus evaluation value is maximized is decided upon as the position of the imaging lens  8  where the face of the image of the subject is in best focus. If the overall focus evaluation value obtained is less than the threshold value (“NO” at step  52 ), then the processing of step  53  is skipped. 
     If the imaging lens  8  is not at the terminus of the range of movement thereof (“NO” at step  54 ), then the imaging lens  8  is moved a prescribed distance and is positioned at the next position (step  55 ). The processing of steps  51  to  53  is repeated until the imaging lens  8  arrives at the terminus in the range of movement (“YES” at step  54 ). 
       FIG. 8A  indicates focus evaluation values and face-detection execution positions obtained by the preliminary search processing for AF. 
     The horizontal axis in  FIG. 8A  is the direction (Z direction) of movement of the imaging lens  8 , and the vertical axis is the plot of overall focus evaluation values. 
     A graph G 1  of overall focus evaluation values is obtained by obtaining the overall focus evaluation values (first focusing data) while moving the imaging lens  8  in increments of the prescribed distance, as described above. In the graph G 1 , the positions of the imaging lens  8  corresponding to the portion of the graph equal to or greater than a threshold value are Z 1 , Z 2 , Z 3 , Z 4 , Z 5 , Z 6 , Z 7  and Z 8 . These positions Z 1 , Z 2 , Z 3 , Z 4 , Z 5 , Z 6 , Z 7  and Z 8  of the imaging lens are positions stored as face-detection execution positions. 
     With reference again to  FIG. 3 , when the preliminary search for AF ends, face detection processing (which may be processing for detecting an eye) is executed (step  35 ) at the face-detection execution position obtained in the preliminary search for AF. 
       FIG. 6  is a flowchart illustrating face detection processing. 
     The image of the subject is sensed upon positioning the imaging lens  8  at the initial face-detection execution position among the face-detection execution positions that have been stored. The image data representing the image of the subject is input to the face extraction processing circuit  6 , which proceeds to detect a face contained in the image of the subject (step  61 ). If a face is detected (“YES” at step  62 ), the result of face detection processing (position and size, etc., of the detected image area) is stored in correspondence with the face-detection execution position at which the imaging lens  8  is situated. If a face is not detected (“NO” at step  62 ), then the processing of step  63  is skipped. 
     If the imaging lens is not at the final face-detection execution position (“NO” at step  64 ), the imaging lens  8  is moved to the next face-detection execution position (step  65 ). The processing of steps  61  to  63  is repeated until the imaging lens  8  arrives at the final face-detection execution position (“YES” at step  64 ). 
     The left side of  FIG. 10  shows entire images obtained by face detection processing and faces contained in respective ones of these entire images. 
     As mentioned above, the imaging lens  8  is positioned at each of the face-detection execution positions Z 1 , Z 2 , Z 3 , Z 4 , Z 5 , Z 6 , Z 7  and Z 8  and the image of the subject is sensed at each position, whereby subject images I 1 , I 2 , I 3 , I 4 , I 5 , I 6 , I 7  and I 8  are obtained. Among the subject images I 1 , I 2 , I 3 , I 4 , I 5 , I 6 , I 7  and I 8  in  FIG. 10  obtained by imaging, the subject images I 2 , I 3  and I 4  contain faces f 2 , f 3  and f 4 , respectively. 
     In the face detection processing, the faces f 2 , f 3  and f 4  are stored in correspondence with the face-detection execution positions Z 2 , Z 3  and Z 4 . 
     With reference again to  FIG. 3 , processing (XY-plane processing) is executed to register the face area, detected when face detection processing is executed, in face information regardless of the position of the imaging lens (step  36 ). In addition, processing (Z-direction processing) for deciding a focusing range that includes the face-detection execution positions at which the face was detected is executed (step  37 ). 
       FIG. 7  is a flowchart illustrating processing for registering a face area. 
     First, the stored result of face detection processing is read (step  71 ). The read result of face detection processing is registered in the face information (step  72 ). This reading of the results of face detection processing and registration of the results of face detection processing in the face information continues up to the final result of face detection processing (step  73 ). 
     The right side of  FIG. 10  shows an example of face information. 
     As described above, faces f 2 , f 3  and f 4  are contained in subject images I 2 , I 3  and I 4 , respectively, obtained by positioning the imaging lens  8  at the face-detection execution positions Z 2 , Z 3  and Z 4 , respectively, and sensing the image of the subject at these positions. The positions and sizes of the images of these faces are registered as results of face detection processing in the same face information I face  of one frame. 
     Thus, faces contained in the images of the subjects obtained at different positions of the imaging lens  8  are registered in the face information I face . As a result, a face area A 1  produced by overlapping the images f 2 , f 3  and f 4  of the faces obtained when the imaging lens  8  is situated at each of the face-detection execution positions Z 2 , Z 3  and Z 4 , respectively, is registered as an area in which the faces are contained. 
     With reference again to  FIG. 3 , a focusing range that includes the face-detection execution positions at which faces are detected is decided (step  37 ) in concurrence with the processing (step  36 ) for registering face information. 
     Since faces have been detected when the imaging lens  8  is situated at the face-detection execution positions Z 2 , Z 3  and Z 4  among the positions Z 1  to Z 4  and Z 5  to Z 8  of the imaging lens  8  at which the overall focus evaluation value is equal to or greater than the threshold value, as illustrated in  FIG. 8A  and the left side of  FIG. 10 , the positions Z 5  to Z 8  of imaging lens  8  at which the overall focus evaluation value is equal to or greater than the threshold value are decided upon as the focusing range. 
     With reference to  FIG. 4 , face areas A 1 , A 2  that have been registered in the face information I face  undergo a main search in the focusing range that has been decided (step  38 ), and the position (in-focus position) of the imaging lens  8  at which the image of the face contained in the image of the subject is in focus is decided (step  39 ). 
     As shown in  FIG. 8B , the focusing range is a range over which the positions of the imaging lens  8  are defined by Z 1  to Z 4 . The imaging lens  8  is moved in prescribed increments in the focusing range Z 1  to Z 4  and the image of the subject is sensed at each position to which the lens is moved. Focusing data (second focusing data) is extracted from image data representing the interior of face area A 1  (indicated by the hatching in  FIG. 9B ) from the sensed image of the subject. An evaluation value of the extracted focusing data is obtained as the face-area focus evaluation value. By obtaining the face-area focus evaluation value while the imaging lens  8  is moved within the focusing range Z 1  to Z 4 , a graph G 2  of face-area focus evaluation values is obtained. This is the main search mentioned above. In the graph G 2  obtained by the main search, position Zf of the imaging lens  8  at which the face-area focus evaluation value is maximized is decided upon as an in-focus position Zf. 
     With reference again to  FIG. 4 , when the in-focus position is decided, the imaging lens  8  is moved so as to be positioned at the in-focus position decided (step  40 ). The position to which the imaging lens  8  has been moved is that at which the face in the image of the subject is in best focus. 
     If the shutter-release button is pressed through the second step of its stroke (“YES” at step  41 ), actual picture-taking is performed at step  43 . The image data obtained by actual picture-taking is recorded on the memory card  19 . 
     If the shutter-release button continues to be pressed through the first step of its stroke (“YES” at step  42 ) without being pressed through the second step of its stroke (“NO” at step  41 ), then the determination as to whether or not the shutter-release button has been pressed through the second step of its stroke is performed repeatedly. If the shutter-release button stops being pressed through the first step of its stroke (“NO” at step  42 ) without being depressed through the second step of its stroke (“NO” at step  41 ), then control returns to the processing of step  31 . 
     In the embodiment described above, the traveling distance of the imaging lens  8  in the preliminary search (step  34 ) for AF may be made larger than the traveling distance of the imaging lens  8  in the main search (step  38 ). This makes it possible to shorten the time needed to perform the preliminary search for AF. 
       FIG. 11  is a flowchart of other processing for face detection. Here processing steps identical with those shown in  FIG. 6  are designated by like step numbers. 
     In the face detection processing illustrated in  FIG. 11 , the imaging lens  8  is moved in prescribed increments from the NEAR side to the INF side along the direction of the optic axis and processing for face detection is executed (step  66 ). If a face is detected (“YES” at step  62 ), the result of face detection processing is stored in correspondence with the face-detection execution position prevailing at this time (step  63 ) and process for detecting a face image ends. 
     Processing for detecting a face image ends in response to the first detection of a face among the images of the subjects obtained while the imaging lens  8  is moved. This initially detected face is that nearest the digital still camera  20 . The imaging lens  8  can be positioned so as to bring the image of this face into focus. Further, since processing for detecting the face image ends in response to first detection of a face, the processing for detecting the face image can be terminated quickly. 
       FIG. 12  is a flowchart of other processing for registering a face area in face information. Here processing steps identical with those shown in  FIG. 7  are designated by like step numbers and need not be described again. 
     In this processing for registering a face area, weighting is applied to a face-image area that has been registered in the face information in the manner described above (step  74 ). 
     How weighting is applied will be described with reference to  FIGS. 13 to 15 . 
     As shown in  FIG. 13 , it is assumed that subject images I 11 , I 12  and I 13  have been obtained by sensing the image of a subject while changing the position of the imaging lens  8  in the manner described above. It is assumed that subject image I 11  contains faces a 11  and a 12 , that subject image I 12  contains faces a 21  and a 22 , and that subject image I 13  contains a face a 31 . 
     If the faces a 11 , a 12 , a 21 , a 22  and a 31  are registered in face information I face1 , the result is as shown on the right side of  FIG. 13 . 
     A face area (target area) A 11  that has been registered in the face information I face1  can be divided into three areas (sub-target image areas)  81 ,  82  and  83 . The area  81  is an area that is contained in the two faces all and a 31 . The area  82  is an area contained in all three of the areas of faces a 11 , a 21  and a 31 . The area  83  is an area contained in the two face images of faces a 11  and a 21 . 
     Further, a face area A 12  that has been registered in the face information I face1  can also be divided into three areas  84 ,  85  and  86 . The area  84  is an area contained in face a 22 , the area  85  is an area contained in faces a 12  and a 22 , and the area  86  is an area contained in face a 12 . 
     The weighting in this case is the product of the number of face-detection execution positions at which a face has been detected and a constant k 1 . Hence, weighting coefficients of the areas  81 ,  82 ,  83 ,  84 ,  85  and  86  are 2k 1 , 3k 1 , 2k 1 , k 1 , 2k 1  and k 1 , respectively. 
     By multiplying the face-area focus evaluation values obtained in the main search by these weighting coefficients, the face-area focus evaluation values are corrected and the above-mentioned graph G 2  regarding the face-area focus evaluation values is obtained. 
     When the faces a 11 , a 12 , a 21 , a 22  and a 31  are registered in face information I face2 , the result is as shown on the right side of  FIG. 14 . 
     Face area A 31  registered in face information I face2  is the result of adding areas  87  and  88  to the top and bottom, respectively, of the face area A 11  shown in  FIG. 13 . Similarly, face area A 41  registered in face information I face2  is the result of adding areas  89  and  90  to the top and bottom, respectively, of the face area A 12  shown in  FIG. 13 . 
     Weighting is calculated as follows: (number of face-detection execution positions at which a face has been detected)×constant k 1 +(distance from face area)×constant k 2 . The weighting coefficients of areas  81 ,  82 ,  83 ,  84 ,  85 ,  86 ,  87 ,  88 ,  89  and  90  are 2k 1 +k 2 , 3k 1 +k 2 , 2k 1 +k 2 , k 1 +k 2 , 2k 1 +k 2 , k 1 +k 2 , k 2 , k 2 , k 2  and k 2 , respectively. 
     When the faces a 11 , a 12 , a 21 , a 22  and a 31  are registered in face information I face2 , as shown in  FIG. 15 , the result is I face3  indicated on the right side of  FIG. 15 . 
     Face area A 51  registered in face information I face3  is such that the areas  87  and  88  contained in the face area A 31  of  FIG. 14  have each been divided into areas  91  and  92 . Similarly, the areas  89  and  90  contained in the face area A 41  of  FIG. 14  have each been divided into areas  93  and  94 . 
     Weighting is calculated as follows: (number of face-detection execution positions at which a face has been detected)×constant k 1 ×(distance from face area)×constant k 2 . The weighting coefficients of areas  81 ,  82 ,  83 ,  84 ,  85 ,  86 ,  87 ,  88 ,  89 ,  90 ,  91 ,  92 ,  93  and  94  are 2k 1 , 3k 1 , 2k 1 , k 1 , 2k 1 , k 1 , k 2 /2, k 2 , k 2 /2 and k 2 , respectively. 
       FIGS. 16A to 16C  illustrate another embodiment of the present invention. 
     In the embodiment described above, a range containing a face is decided as a focusing range from a range of overall focus evaluation values equal to or greater than a threshold value, and a main search is conducted within this focusing range. By contrast, in this embodiment in  FIGS. 16A to 16C , maximal values are found from the graph G 1  of overall focus evaluation values and face detection processing is executed at the positions of the imaging lens  8  that correspond to these maximal values. Positions of the imaging lens  8  in the vicinity of a maximal value at which a face has been detected are decided upon as the focusing range. The main search is conducted in the focusing range decided. 
     As shown in  FIG. 16A , a graph G 1  of overall focus evaluation values (the graph of an envelope signal) is obtained by conducting an AF preliminary search. Maximal values M 1  and M 2  of the graph G 1  are detected and positions Z 11  and Z 13  of the imaging lens  8  corresponding to these maximal values M 1  and M 2 , respectively, are detected. The imaging lens  8  is positioned at each of the positions Z 11  and Z 13  and the image of the subject is sensed, whereby subject images I 21  and I 23 , respectively, are obtained. A face detection search to determine whether a face is contained in the obtained subject images I 21  and I 23  is conducted. Here it will be assumed that a face a 41  is contained in the subject image I 21 . 
     If the face a 41  is contained in the subject image I 21  obtained when the imaging lens  8  is positioned at the position Z 11 , then, as shown in  FIG. 16B , a range Z 10  to Z 12  is decided upon as the focusing range. The range Z 10  to Z 12  is one defined by points of intersection between the graph G 1  and an overall focus integration value p 2  obtained when an overall focus evaluation value p 1  at the maximal value M 1  is lowered a prescribed level q. Moreover, this range contains the maximal value M 1 . 
     If the focusing range is decided, then, as shown in  FIG. 16C , a main search is conducted in this focusing range and the graph G 2  of face-area focus evaluation values is obtained in the manner described above. An in-focus position Z 1   f  of the imaging lens  8  is decided from the graph G 2  obtained. 
       FIGS. 17A and 17B  illustrate a further embodiment of the present invention. 
     In  FIGS. 16A to 16C  described above, the range decided upon as the focusing range is one defined by points of intersection between the graph G 1  and the overall focus integration value p 2  obtained when the overall focus evaluation value p 1  at the maximal value M 1  is lowered a prescribed level q. In  FIGS. 17A and 17B , however, the range decided upon as the focusing range is a range Z 15  to Z 16  defined by positions Z 15 , and Z 16  obtained when the imaging lens  8  is moved by ΔZ from the imaging-lens position Z 11 , which corresponds to the maximal value M 1 , toward the NEAR side and toward the INF side, as illustrated in  FIG. 17B . 
     If the focusing range is decided, then, as shown in  FIG. 17B , a main search is conducted in this focusing range and a graph G 3  is obtained in the same manner as illustrated in  FIG. 16C . An in-focus position Z 2   f  of the imaging lens  8  is decided from the graph G 3  obtained. 
     In the embodiment described above, the imaging lens is set at an initial position on the NEAR side to obtain the focus evaluation value, then the imaging lens is moved to the next position to obtain the focus evaluation value again. However, it may be so arranged that the imaging lens is set at an initial position on the INF (infinity) side to obtain the focus evaluation value and is then moved to the next position to obtain the focus evaluation value again. It may be so arranged that whether the initial position of the imaging lens is on the NEAR side or INF side is set in advance, or it may be so arranged that the user is capable of making this setting. If the arrangement is such that the user can make the setting, then the operating device  5  of the above-described digital still camera would be provided with a setting device such as a button or switch to set the NEAR side or INF side. The signal resulting from the setting is applied to the control circuit  1 , whereby the initial position of the imaging lens  8  is set to the NEAR side or INF side. 
     Furthermore, it may be so arranged that in a case where a short-distance image sensing mode such as a portrait shooting mode or macro shooting mode or a long-distance image sensing mode such as a scene shooting mode or night-scene shooting mode can be set as the shooting mode of the digital still camera, the initial position of the imaging lens  8  is set in accordance with the shooting mode. For example, in a case where the short-distance image sensing mode has been set, it can be construed that the main subject is at a location close to the digital still camera and therefore the imaging lens  8  is set at an initial position on the NEAR side. In a case where the long-distance image sensing mode has been set, it can be construed that the main subject is at a location far from the digital still camera and therefore the imaging lens  8  is set at an initial position on the INF side. Thus, the time it takes to detect the main subject in accordance with movement of the imaging lens is shortened in conformity with each image sensing mode. 
     Furthermore, in a case where the imaging lens  8  is a zoom lens, the initial position of the imaging lens (zoom lens)  8  may be set to the NEAR side or INF side in dependence upon the zoom magnification of the zoom lens. For example, in a case where zoom magnification of the zoom lens is set high (on the telephoto side), it can be construed that the main subject is far away and therefore the initial position is placed on the INF side. In a case where zoom magnification of the zoom lens is set low (on the close-up side), it can be construed that the main subject is near and therefore the initial position is placed on the NEAR side. Of course, this does not mean that the initial position must necessarily be placed on the INF side if the zoom magnification of the zoom lens is high and that it must necessarily be placed on the NEAR side if the zoom magnification of the zoom lens is low. The initial position may conversely be placed on the INF side if the magnification of the zoom lens is low and on the NEAR side if the zoom magnification is high. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.