Patent Publication Number: US-8121404-B2

Title: Exposure control apparatus and image pickup apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. JP 2006-020136, filed in the Japanese Patent Office on Jan. 30, 2006, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to exposure control apparatuses for controlling an exposure adjustment mechanism on the basis of an image signal captured by a solid-state image pickup device, and image pickup apparatuses having an exposure control function. More specifically, the present invention relates to an exposure control apparatus and an image pickup apparatus suitably used for capturing face images. 
     2. Description of the Related Art 
     An increasing number of image pickup apparatuses using solid-state image pickup devices, such as digital still cameras and digital video cameras, having a function called a “scene mode” that provides optimum control, such as automatic exposure control, according to various scenes during the capture process have been available. For example, a “portrait mode,” which is suitably used for photography of the human face, is a mode whose exposure control target is set higher than general modes to thereby allow a human face image to be captured with optimum brightness. Therefore, images captured with beautiful skin tones can be obtained. 
     A related technique for detecting a face from a captured image signal is also known. In one of various known face detection techniques, an average face image is used as a template to perform matching with an input image signal (see, for example, Japanese Unexamined Patent Application Publication No. 2003-271933 (paragraph numbers [0051] to [0053] and FIG. 6)). If a face appearing in a captured image is tilted at an angle, it is difficult to correctly detect such a face. Therefore, there have been proposed techniques, such as determining the angle of the face and rotating the face image according to the determined angle before performing the matching. In one known technique, a center line of the face is detected, the degree of similarity to the center line is determined on the basis of the face symmetry, and the angle of the center line with a high degree of similarity is output as the angle of the face (see, for example, Japanese Unexamined Patent Application Publication No. 9-171560 (paragraph numbers [0032] to [0044] and FIG. 4)). 
     SUMMARY OF THE INVENTION 
     In general, the portrait mode is manually set by a user, and it is time-consuming for a photographer to perform the setting operation. In particular, a user who usually captures images with fully automatic settings without caring about the settings of an image pickup apparatus may avoid using the portrait mode due to the time-consuming nature of the operation. 
     Another challenging issue is to improve the quality of face images in the portrait mode. The portrait mode further has a problem of impaired balance of brightness between a face and a background. For example, when a face image is captured in a backlight situation, the face may be dark, or, when the background is dark, the face may be excessively bright, resulting in an “overexposed highlight” phenomenon. 
     It is therefore desirable to provide an image processing apparatus that allows a user to obtain a high-quality face image without performing any special setting operation during capturing of an image of a person&#39;s face, and an image pickup apparatus having such a control function. 
     According to an embodiment of the present invention, an exposure control apparatus for controlling an exposure adjustment mechanism on the basis of an image signal captured by a solid-state image pickup device includes the following elements. Face detecting means detects a face of a person appearing in a captured image on the basis of the captured image signal. Histogram data determining means determines histogram data by setting a face detection region in the captured image according to detection information obtained by the face detecting means and detecting at least a brightness value of the face detection region from the captured image signal. Target brightness setting means sets a target brightness range for desired brightness of the face. When the face detecting mean detects a face, exposure control amount determining means determines an amount of control for the exposure adjustment mechanism on the basis of a comparison result obtained by comparing the target brightness range with a brightness value at a peak of histogram data corresponding to the face detection region or a brightness region in the vicinity of the peak. 
     In the exposure control apparatus, the face detecting means detects a face of a person appearing in a captured image on the basis of an image captured by the solid-state image pickup device. The histogram data determining means determines histogram data by setting a face detection region in the captured image according to detection information obtained by the face detecting means and detecting at least a brightness level of the face detection region from the captured image signal. The target brightness setting means sets a target brightness range for desired brightness of the face. When the face detecting mean detects a face, the exposure control amount determining means determines an amount of control for the exposure adjustment mechanism on the basis of a comparison result obtained by comparing the target brightness range with a brightness level at a peak of histogram data corresponding to the face detection region or a brightness region in the vicinity of the peak. 
     According to the embodiment of the present invention, therefore, in the exposure control apparatus, a target brightness range that is set for desired brightness of a face is compared with a brightness level at a peak of histogram data corresponding to a face region or a brightness region in the vicinity of the peak, and the amount of control for the exposure adjustment mechanism is determined on the basis of the comparison result. Thus, the brightness of the face can be adjusted with high accuracy. Further, when a face of a person appearing in a captured image is detected, exposure control for adjusting the brightness of the face is automatically performed using brightness histogram data detected from the face region in the image. Therefore, when a face image is captured, the quality of the face image can automatically be improved without a special operation by a user. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the structure of the main part of an image pickup apparatus according to an embodiment of the present invention; 
         FIG. 2  is a flowchart showing the overall flow of exposure control and flash emission control; 
         FIG. 3  is a flowchart showing a procedure for calculating an importance level of a face; 
         FIG. 4  is a graph showing the correspondence between the face proportion and the face size importance level factor; 
         FIG. 5  is a diagram showing a method for calculating the distance between the screen center region and the face; 
         FIG. 6  is a diagram showing a method for calculating the included angle; 
         FIGS. 7A and 7B  are graphs showing a method for calculating the face position importance level factor and a method for calculating the position-posture importance level factor, respectively; 
         FIGS. 8A and 8B  are diagrams showing methods for setting a detection frame on the basis of face detection results; 
         FIG. 9  is a flowchart showing the flow of a process for determining a peak of histogram data; 
         FIG. 10  is a graph showing a first example of the detection of brightness histogram data of the face and the background; 
         FIG. 11  is a graph showing a second example of the detection of brightness histogram data of the face and the background; 
         FIG. 12  is a graph showing a third example of the detection of brightness histogram data of the face and the background; 
         FIG. 13  is a flowchart showing the flow of a process for setting a target brightness range; 
         FIG. 14  is a graph showing the relationship between the amount of external light and the shift amount of the target brightness range; 
         FIG. 15  is a graph showing the relationship between the importance level of the face and the adjustment ratio for the width of the target brightness range; 
         FIG. 16  is a flowchart showing a first portion of the flow of a process for re-calculating the amount of exposure control and flash control information; 
         FIG. 17  is a flowchart showing a second portion of the flow of the process for re-calculating the amount of exposure control and flash control information; 
         FIG. 18  is a diagram showing histogram data obtained when the adjustment amount is applied in step S 511  in the state shown in  FIG. 12 ; 
         FIG. 19  is a correction curve to be used in step S 513  shown in  FIG. 17 ; 
         FIG. 20  is a diagram showing histogram data obtained when an adjustment amount is applied in the state shown in  FIG. 12  after gradation correction is performed; 
         FIG. 21  is a diagram showing histogram data obtained when the adjustment amount is applied in the state shown in  FIG. 10 ; 
         FIG. 22  is a diagram showing histogram data obtained when an adjustment amount lower than the determined adjustment amount is applied in the state shown in  FIG. 10 ; 
         FIG. 23  is a correction curve used in step S 520  shown in  FIG. 17 ; 
         FIG. 24  is a diagram showing histogram data obtained when a corrected adjustment amount is used in the state shown in  FIG. 10  and gradation correction is performed; and 
         FIG. 25  is a diagram showing an example of the setting of an eye detection region. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described in detail with reference to the drawings. 
       FIG. 1  is a block diagram showing the structure of the main part of an image pickup apparatus according to an embodiment of the present invention. 
     The image pickup apparatus shown in  FIG. 1  includes an optical block  11 , a driver  11   a , an image pickup device  12 , a timing generator (TG)  12   a , an analog front end (AFE) circuit  13 , a camera signal processing circuit  14 , a microcomputer  15 , a graphic interface circuit  16 , a display  17 , an input unit  18 , and a flash emitter  19 . 
     The optical block  11  includes a lens for focusing light from an object onto the image pickup device  12 , a driving mechanism for moving the lens for focusing or zooming, a shutter mechanism, and an iris mechanism. The driver  11   a  controls the driving of those mechanisms in the optical block  11  according to a control signal from the microcomputer  15 . 
     The image pickup device  12  is a solid-state image pickup device, such as a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) image pickup device, and is driven by a timing signal output from the TG  12   a  to convert incident light from the object into an electrical signal. The TG  12   a  outputs a timing signal under the control of the microcomputer  15 . In the embodiment, an analog image signal obtained from the image pickup device  12  is a primary color signal of RGB colors. For example, the analog image signal may be any other color signal such as a complementary color signal. 
     The AFE circuit  13  is formed of, for example, a single integrated circuit (IC) for performing correlated double sampling (CDS) on the image signal output from the image pickup device  12  to sample-and-hold it so that a high signal-to-noise (S/N) ratio can be maintained. The AFE circuit  13  further performs auto gain control (AGC) to control the gain, and performs analog-to-digital (A/D) conversion to output a digital image signal. 
     The camera signal processing circuit  14  is formed of, for example, a single IC for performing camera signal processing, such as various types of signal correction, auto focus (AF), and auto exposure (AE), on the image signal output from the AFE circuit  13  or performing a portion of the processing. In the embodiment, the camera signal processing circuit  14  includes an image preprocessor  21 , an image processor  22 , a histogram detector  23 , a color detector  24 , and a face detector  25 . 
     The image preprocessor  21  performs preprocessing, such as defective pixel correction, digital clamping, and digital gain control, on the image signal output from the AFE circuit  13 . The image processor  22  performs various image quality correction such as white-balance adjustment, and sharpness, color saturation, and contrast adjustment. The operation of the image preprocessor  21  and the image processor  22  is controlled by the microcomputer  15 . 
     The histogram detector  23  detects brightness information on the basis of the image signal output from the image preprocessor  21  to determine a histogram for each predetermined range, and supplies the histograms to the microcomputer  15 . The color detector  24  detects color information for each predetermined range on the basis of the signal output from the image preprocessor  21 , and supplies the color information to the microcomputer  15 . 
     The face detector  25  detects a human face on the basis of the image signal output from the image processor  22 , and supplies information including the number of faces detected, and the size, position, and tilt of each face to the microcomputer  15 . A method for face detection may be to perform matching between an input image and predetermined templates representing average face images. 
     The microcomputer  15  includes, for example, a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM), and executes a program stored in the ROM or the like to generally control the respective sections of the image pickup apparatus. The microcomputer  15  further communicates with the camera signal processing circuit  14  to execute a portion of the calculation for various types of camera signal processing described above. In the embodiment, the microcomputer  15  performs processing, such as calculating an amount of exposure control and an amount of gradation correction for exposure control, and determining whether or not flash light is necessary, according to the amount of external light and using the histogram detector  23 , the color detector  24 , and the face detector  25 . 
     The graphic interface circuit  16  converts the image data output from the camera signal processing circuit  14  into a signal to be displayed on the display  17 , and supplies the signal to the display  17 . The graphic interface circuit  16  further superimposes a menu screen, various setting screens and various warnings, and so forth on an image according to a request from the microcomputer  15 . The display  17  is formed of, for example, a liquid crystal display (LCD) for displaying an image based on the image signal from the graphic interface circuit  16 . 
     The input unit  18  includes, for example, a shutter release button, and keys and dials for performing various settings such as turning on or off flash light, and supplies control information corresponding to a user&#39;s operation of the button, keys and dials to the microcomputer  15 . 
     The flash emitter  19  emits flash light according to flash control information from the microcomputer  15 . 
     In the image pickup apparatus, the image pickup device  12  receives light and photoelectrically converts the light into a signal. The signal is sequentially supplied to the AFE circuit  13  for CDS processing or AGC processing, and is then converted into a digital signal. The camera signal processing circuit  14  performs image quality correction on the digital image signal supplied from the AFE circuit  13 , and converts the result into a brightness signal (Y) and color difference signals (R−Y and B−Y) to finally output them. 
     The image data output from the camera signal processing circuit  14  is supplied to the graphic interface circuit  16  for conversion into an image signal for display, and a camera-through image is therefore displayed on the display  17 . When a user operates the input unit  18  to issue an instruction to record an image to the microcomputer  15 , the image data output from the camera signal processing circuit  14  is supplied to an encoder (not shown) for predetermined compression encoding, and the encoded data is recorded onto a recording medium (not shown). When a still image is to be recorded, image data of one frame is supplied from the camera signal processing circuit  14  to the encoder. When a moving image is to be recorded, processed image data is consecutively supplied to the encoder. 
     The procedure for exposure control and flash emission control in the image pickup apparatus will be described. 
     Overview of Exposure Control Process 
       FIG. 2  is a flowchart showing the overall flow of exposure control and flash emission control. The overview of exposure control and flash emission control using face detection results will be described with reference to  FIG. 2 . 
     In step S 101 , the microcomputer  15  calculates an amount of exposure control and flash control information using a standard AE control method on the basis of a detected value of an image signal from the camera signal processing circuit  14 . 
     The standard AE control method is a process for calculating an amount of exposure control and flash control information using a general process without using face detection results such as those obtained in steps S 102  to S 110  described below. Basically, the amount of exposure control and flash control information are calculated on the basis of brightness information detected from a captured image signal for an entire screen. The amount of exposure control is a value for controlling exposure-related parameters such as the opening of an iris in the optical block  11 , the shutter speed of a mechanical shutter in the optical block  11  and an electronic shutter in the image pickup device  12  (or one of those shutters), and a gain amount of AGC in the AFE circuit  13 . The flash control information is information for controlling the emission operation of the flash emitter  19 . 
     In step S 102 , the face detector  25  of the camera signal processing circuit  14  detects a human face from an image output from the image processor  22 . 
     In step S 103 , the microcomputer  15  determines whether or not the face detector  25  has detected a face. If a face is detected, the process proceeds to step S 104 . If no face is detected, the amount of exposure control and flash control information calculated in step S 101  are determined as the final values, and the process ends. 
     In step S 104 , the microcomputer  15  obtains detection information of the face from the face detector  25 , and calculates an importance level of each detected face on the basis of the detection information. 
     In step S 105 , the microcomputer  15  determines whether or not a face having a higher importance level than a predetermined value is found. If such a face is found, the process proceeds to step S 106 . If such a face is not found, the amount of exposure control and flash control information calculated in step S 101  are determined as the final values, and the process ends. 
     In step S 106 , the microcomputer  15  sets a detection frame for the histogram detector  23  and the color detector  24  for brightness detection and color detection, respectively, for each face having a higher importance level found in step S 105 . 
     In step S 107 , the microcomputer  15  obtains brightness histogram data and color detection value for each face from the histogram detector  23  and the color detector  24 , respectively, using the detection frame set in step S 106 . If a plurality of faces having a higher importance level are found in step S 105 , brightness histogram data for the individual faces are normalized with respect to the entire face region according to the individual importance levels of the faces. 
     In step S 108 , the microcomputer  15  determines a peak of the histogram data of the face. The microcomputer  15  further searches for limit brightness values Yf_l and Yf_h defining a peak region, described below, on the basis of the frequency value and brightness value at the peak. 
     In step S 109 , the microcomputer  15  sets a target brightness range of the face according to the importance level and color of the face and external light information. 
     In step S 110 , the microcomputer  15  calculates a final amount of exposure control and flash control information on the basis of the histogram data of the face (in the case of the divisional detection method, the histogram data normalized in step S 107 ), the target brightness range set in step S 109 , and the amount of exposure control and flash control information calculated in step S 101 . 
     Accordingly, exposure control can be performed so that a face image, in particular, a face image that is important for a photographer, can be captured with appropriate brightness while achieving the desired brightness balance between the face and the background. Further, once a face is detected, such exposure control taking the brightness balance between the face and the background into consideration can automatically be performed in a normal auto-shooting mode without performing the setting operation by a photographer. 
     That is, once a face is detected from a captured image (step S 103 ), only when a face having a higher importance level than a predetermined value is found from among the detected faces (step S 105 ), exposure control taking the brightness balance between the face and the background into consideration is performed (steps S 106  to S 110 ); otherwise, general exposure control in the auto-shooting mode is performed. 
     The above-described process will be described in more detail. 
     Calculation of Control Value Using Standard AE Control Method 
     First, in step S 101 , an amount of exposure control and flash control information are calculated using a standard AE control method. As described above, the AE control method is a general control method in the auto-shooting mode. For example, the control method is performed according to Eq. (1) as follows:
 
 Ev =log 2 ( Fno   2 )+log 2 (1/ T )+log 2 (100/ ISO )  Eq. (1)
 
where Ev denotes the total amount of exposure control, Fno denotes the amount of control indicating the amount of aperture of the iris, T denotes the exposure time using at least one of the mechanical shutter and the electronic shutter, and ISO denotes the exposure sensitivity.
 
     According to Eq. (1), the higher the shutter speed, the larger the amount of exposure control Ev, resulting in a smaller amount of exposure. If the amount of light reflected from the subject does not change, the brightness of the captured image becomes low. If the exposure sensitivity decrease, the amount of exposure control Ev increases, resulting in a reduction in the amount of exposure. If the amount of aperture of the iris increases, the amount of exposure control Ev increases, resulting in a reduction in the amount of exposure. In the case of the same amount of external light (brightness of the subject), the larger the amount of exposure control Ev, the smaller the amount of exposure, resulting in a darker image captured. Conversely, the smaller the amount of exposure control Ev, the larger the amount of exposure, resulting in a brighter image captured. 
     Determination of Importance Level of Face 
     Then, the face detector  25  detects a face from a captured image signal (step S 102  shown in  FIG. 2 ), and the microcomputer  15  calculates an importance level of the detected face on the basis of detection results obtained by the face detector  25  (step S 104  shown in  FIG. 2 ) according to a procedure described below with reference to  FIG. 3 . 
       FIG. 3  is a flowchart showing a procedure for calculating an importance level of a face, which corresponds to the processing of step S 104 . 
     In step S 201 , the microcomputer  15  obtains face size, position, and tilt information as detection information of each face detected by the face detector  25 . 
     In step S 202 , the microcomputer  15  calculates a face proportion r_size representing the ratio of the size of the face to the size of the entire screen, and stores the result in the RAM (not shown) or the like. 
     In step S 203 , the microcomputer  15  calculates a face size importance level factor Wsize of the face on the basis of the calculation result obtained in step S 202 . The face size importance level factor Wsize represents an importance level to be estimated from the ratio of the size of the face to the size of the entire screen. The face size importance level factor Wsize is calculated according to the face proportion r_size using Eqs. (2) to (4) as follows: 
     for r_size&gt;Rfb 2 ,
 
 W size=100 [%]  Eq. (2)
 
for Rfb 1 ≦r_size≦Rfb 2 ,
 
 W size=( r _size− Rfb 1)/( Rfb 2− Rfb 1)  Eq. (3)
 
for r_size&lt;Rfb 1 ,
 
 W size=0  Eq. (4)
 
where Rfb 1  and Rfb 2  are threshold values satisfying Rfb 1 &lt;Rfb 2 .
 
       FIG. 4  is a graph showing the correspondence between the face proportion r_size and the face size importance level factor Wsize. 
     According to Eqs. (2) to (4) given above and  FIG. 4 , if the face proportion r_size relative to the screen exceeds the predetermined threshold value Rfb 2 , it is determined that the photographer intentionally selects the face as the target, and the face size importance level factor Wsize is set to a maximum value of 100%. Conversely, if the face proportion r_size is less than the predetermined threshold value Rfb 1 , it is determined that not the face but another face or the background is the target, and the face size importance level factor Wsize is set to 0. If the face proportion r_size is within a range (between the threshold values Rfb 1  and Rfb 2 ), it is difficult to clearly determine whether or not the face is the target, and the size importance level factor Wsize successively changes according to the proportion. Specifically, the larger the face proportion r_size, the higher the importance level because of a higher possibility of being the target. 
     Referring back to  FIG. 3 , in step S 204 , the microcomputer  15  determines whether or not the face size importance level factor Wsize is 0. If the face size importance level factor Wsize is not 0, the processing of step S 205  is performed. If the face size importance level factor Wsize is 0, the processing of step S 210  is performed. 
     In step S 205 , the microcomputer  15  calculates a distance L between a predetermined screen center region and the face. 
     In step S 206 , the microcomputer  15  calculates a face position importance level factor PosW indicating an importance level to be estimated from the position of the face within the screen. In this case, the microcomputer  15  calculates a maximum face position importance level factor value PosWh and a minimum face position importance level factor value PosW 1  according to the distance L determined in step S 205 . 
     In step S 207 , the microcomputer  15  calculates an included angle ang. The included angle ang is defined between a predicted centroid ray LineA directed from the face toward the feet and a ray LineB directed from the face toward the center of the screen on the basis of the face position and tilt information. 
     In step S 208 , the microcomputer  15  calculates a face position-posture importance level factor Wpos on the basis of the calculation results obtained in steps S 206  and S 207 . The face position-posture importance level factor Wpos represents an importance level to be estimated from the position and tilt of the face within the screen. As described below, the available range of the face position-posture importance level factor Wpos is restricted according to the distance L between the screen center region and the face, which is determined in step S 205 . 
     In step S 209 , the microcomputer  15  calculates a final importance level Wf of the face using Eq. (5) as follows:
 
 Wf=W size× Wpos   Eq. (5)
 
     By using Eq. (5) given above, the importance level Wf of the face can be determined on the basis of three parameters, namely, the size, position, and tilt of the face within the screen. Therefore, it can be suitably determined whether or not the detected face is intentionally selected as the target by the user. 
     If it is determined in step S 204  that the face size importance level factor Wsize is 0, then in step S 210 , the microcomputer  15  sets the final importance level Wf of the face to 0. 
     After the processing of step S 209  or S 210  is performed, in step S 211 , the microcomputer  15  determines whether or not the calculation of the importance level Wf has been completed for all the faces detected by the face detector  25 . If the calculation has not yet been completed, the process returns to step S 203 , and the importance level Wf of another face is calculated. If the calculation has been completed for all the faces, this sub-routine ends. 
     The processing of steps S 205  to S 208  will be described in more detail. 
       FIG. 5  is a diagram showing a method for calculating the distance L between the screen center region and the face, which corresponds to the processing of step S 205 . 
     As shown in  FIG. 5 , in the embodiment, a region  101  having a predetermined area is defined around the center of the screen, and a distance L between the boundary of the region  101  and a position of a face  102  (typically, the center of the face  102 ) within the screen is calculated. As described below, the smaller the distance L, the higher the importance level (the position importance level factor PosW) because the face  102  is closer to the center of the screen. In this example, it is determined that a face located in the region  101  around the center of the screen will equivalently have a high possibility of being the target. The distance L may be defined between the center of the screen and the face. Further, the size of the region  101  may be variably set by the user. 
       FIG. 6  is a diagram showing a method for calculating the included angle ang, which corresponds to the processing of step S 207 . In  FIG. 6 , for the convenience of easy understanding, face images are illustrated as being shifted from the center nodes of the images. Actually, however, the center nodes are positioned at the center of the faces (e.g., around the nose). 
     If the tilt of a given face is represented by an angle relative to the vertical center line of the face, a predicted centroid ray LineA directed from the face toward the feet is a ray that is drawn downward from the face along its center line with respect to the center of the face (the position corresponding to the position information of the face) as the origin. The angle between the predicted centroid ray LineA and a ray LineB directed from the center of the face toward a screen center C 0  is referred to as an included angle ang, where 0≦ang≦180. In  FIG. 6 , for example, a face  111  located at an upper left position with respect to the screen center C 0  is detected, and has a center node C 1 . If the face  111  is tilted at an angle of 0°, the predicted centroid ray LineA of the face  111  is drawn downward in the screen, and the ray LineB directed toward the screen center C 0  is drawn toward the lower right of the screen. The included angle ang is defined between the predicted centroid ray LineA and the ray LineB. 
     In the embodiment, as described below, the lower the included angle ang, the higher the importance level (the position-posture importance level factor Wpos). For example, if faces  111  and  112  are located at an upper left position and a lower left position with respect to the screen center C 0 , respectively, and are tilted at an angle of 0°, the included angle ang of the face  112  is greater than that of the face  111 , and it is therefore determined that the importance level of the face  112  is lower. This determination is based on a determination rule that the distance between the center node C 1  of the face  111  and the edge of the screen in the direction of the predicted centroid ray LineA is larger so that the whole body of the person corresponding to the face  111  may highly possibly appear in the screen, and therefore the possibility of that person being the target is higher. 
     According to a similar determination rule, when faces  113  and  114  are located at above and below the screen center C 0 , respectively, and are tilted at an angle of 0°, the included angles of the faces  113  and  114  are 0° and 180°, respectively. Therefore, the importance level of the face  113  is higher. Further, for faces  115  and  116  that are tilted at an angle greater than 0°, the predicted centroid rays LineA and the rays LineB toward the screen center C 0  are directed in the same direction, and the included angles ang of both faces are 0°. Therefore, the importance levels of the faces  115  and  116  are high. In the case of the faces  115  and  116 , for example, when the capturing operation is performed with the image pickup apparatus tilted about the lens optical axis, the same determination rule may apply to determine the importance levels. That is, regardless of the tilt of the face, the larger the space extending downward from the face (toward the feet) within the screen, the higher the possibility of that person being the target because of a higher possibility that the whole body of the person corresponding to the face will appear in the screen. 
     The above-described determination rule is merely an example, and a determination rule different from that described above (or a determination rule in which other conditions are added to the above-described determination rule) may be used on the basis the position and tilt of faces to calculate the importance levels. Alternatively, a plurality of determination rules may be provided so that the user can change them by a setting operation. 
     A method for calculating the face position importance level factor PosW and the position-posture importance level factor Wpos will be described in detail. 
     First, in step S 206  shown in  FIG. 3 , the face position importance level factor Posw is calculated using Eqs. (6) to (8) as follows:
 
 PosWh ={( PosW 2− PosW 1)/ L 2}× L+PosW 1  Eq. (6)
 
for 0≦L≦L 1 ,
 
 PosW 1={( PosW 3− PosW 1)/ L 1}× L+PosW 1  Eq. (7)
 
for L 1 &lt;L≦L 2 ,
 
 PosW 1 =PosW 3  Eq. (8)
 
where L 1  denotes a predetermined threshold value, L 2  denotes a maximum value of the distance L between the face and the screen center region, and PosW 1 , PosW 2 , and PosW 3  denote predetermined threshold values, where 0≦PosW 3 ≦PosW 2 ≦PosW 1 .
 
     In step S 208  shown in  FIG. 3 , the face position-posture importance level factor Wpos is calculated using Eqs. (9) and (10) as follows: 
     for 0≦ang≦Ang 1 ,
 
 Wpos=PosWh   Eq. (9)
 
for Ang 1 &lt;ang≦180,
 
 Wpos =−{( PosWh−PosW 1)/(180− Ang 1)}×( ang−Ang 1)+ PosWh   Eq. (10)
 
where Ang 1  denotes a predetermined threshold value satisfying 0≦Ang 1 ≦180.
 
       FIGS. 7A and 7B  are graphs showing a method for calculating the face position importance level factor PosW and a method for calculating the position-posture importance level factor Wpos, respectively. 
       FIG. 7A  shows the relationship between the distance L between the face and the screen center region and the face position importance level factors PosWh and PosW 1  according to Eqs. (6) to (8) given above. As can be seen from  FIG. 7A , basically, the larger the distance L between the face and the screen center region, the smaller the face position importance level factors PosWh and PosW 1 . 
       FIG. 7B  shows the relationship between the included angle ang and the face position-posture importance level factor wpos according to Eqs. (9) and (10) given above. As can be seen from  FIG. 7B , basically, the larger the included angle ang, the smaller the face position-posture importance level factor Wpos. As can also be seen from  FIGS. 7A and 7B , the face position importance level factors PosWh and PosW 1  define the upper and lower limits of the face position-posture importance level factor Wpos, respectively. 
     According to  FIGS. 7A and 7B  and Eqs. (2) to (5), if the size of a detected face is large or the position of the face is close to the screen center region, and if the space extending downward from the face in the screen is large, the importance level of the face is high. Therefore, the degree of possibility that a detected face is intentionally selected as the target by the user can be determined with high accuracy. 
     In  FIG. 7B , when the included angle ang is equal to or lower than Ang 1 , the face position-posture importance level factor Wpos is set constant at the upper limit. Thereby, even if the face is tilted to some extent with respect to the ray drawn from the face toward the feet, it can be determined that the importance level is high. 
     In  FIG. 7A , by setting the face position importance level factor PosWh to be larger than 0, the lower limit of the face position-posture importance level factor Wpos can be larger than 0 to prevent the final importance level Wf of the face from being equal to 0. Therefore, for example, when the user captures an image so that a face can intentionally be positioned at the edge of the screen, the brightness of the face can possibly be adjusted. 
     Brightness Histogram Data and Color Detection 
     The processing of steps S 106  and S 107  shown in  FIG. 2  will be described in detail. The brightness histogram data and the detection of color information differ depending on the method for setting a face detection frame on the screen. The following description will be made in the context of a normal frame method and a divisional detection method. In the normal frame method, a detection frame is flexibly set according to the position and size of a detected face. The divisional detection method uses a plurality of detection frames into which the screen is divided in advance. 
       FIGS. 8A and 8B  are diagrams showing methods for setting a detection frame on the basis of face detection results, which correspond to the processing of step S 106 .  FIG. 8A  shows the normal frame method, and  FIG. 8B  shows the divisional detection method. 
     As shown in  FIG. 8A , in the normal frame method, the microcomputer  15  sets a face detection frame  121  for the histogram detector  23  and the color detector  24  so as to contour the face according to the face position and size information obtained from the face detector  25 . The brightness information and color information of the detected face are detected from an image signal inside the detection frame  121  and a background, except for the face, is detected from an image signal outside the detection frame  121 . 
     In the divisional detection method, on the other hand, the brightness information and the color information are generally detected in units of a plurality of divisional frames configured in the screen. In the example shown in  FIG. 8B , the screen is divided into nine detection frames in the horizontal direction and eight detection frames in the vertical direction. In step S 106 , the microcomputer  15  selects a detection frame that is mostly occupied by the face on the basis of the face position and size information obtained from the face detector  25 , and sets the selected detection frame for the histogram detector  23  and the color detector  24  as a face detection frame. In the example shown in  FIG. 8B , brightness information and color information from four detection frames, namely, X 6 Y 5 , X 7 Y 5 , X 6 Y 6 , and X 7 Y 6 , are designated as face detection values, and brightness information and color information from the remaining detection frames are designated as background detection values. 
     In either the normal frame method or the divisional detection method, if a plurality of faces are detected, a region that is not included in all the face detection frames are designated as a background detection region. 
     After a face detection frame is set for the histogram detector  23  and the color detector  24  in the manner described above, the microcomputer  15  obtains the brightness histogram data and color information of each detected face from the histogram detector  23  and the color detector  24  on the basis of the detection frame (step S 107 ). In step S 107 , the brightness histogram data and color information of the background region, except for the face, or the brightness histogram data and color information of the entire screen are also obtained, if necessary. 
     In a case where a plurality of faces are detected, a coefficient (weighting) is set according to the importance level of each face determined in step S 104 . The coefficient is multiplied by the brightness histogram data for each face region, and the results are added, thereby normalizing the brightness histogram data with respect to all the face regions. The normalized histogram data is stored in the RAM (not shown) or the like, and is used as brightness histogram data for the faces after the processing of step S 107 . Among the color information detected from the face regions, the color information of the face having the highest importance level is stored in the RAM or the like, and is used as face color information after the processing of step S 107 . 
     Determination of Peak of Histogram Data 
     A process for determining a peak of the histogram data in step S 108  shown in  FIG. 2  will be described in detail. 
       FIG. 9  is a flowchart showing the flow of the peak determination process. 
     In step S 301 , the microcomputer  15  reads the brightness histogram data of the face determined in step S 107  shown in  FIG. 2  from, for example, the RAM or the like. The histogram data is obtained as a sequence of frequency information corresponding to N brightness values (or N steps of brightness range). 
     In step S 302 , the microcomputer  15  extracts the maximum frequency and the corresponding brightness information from the read brightness histogram data of the face. 
     In step S 303 , the microcomputer  15  calculates threshold values indicating the lower limit and upper limit of the frequency at the extracted maximum frequency of the face (at which the frequency peaks). 
     In step S 304 , the microcomputer  15  detects limit brightness values at the peak value of the frequency and based on the peak for adjusting the amount of exposure control in the subsequent processing. 
       FIGS. 10 to 12  are diagrams showing examples of the detection of brightness histogram data of the face and the background.  FIG. 10  shows a case where the background is relatively brighter than the face,  FIG. 11  shows a case where, conversely, the face is brighter than the background, and  FIG. 12  shows a case where the face is significantly brighter than the background. The above-described processing will be specifically described with reference to  FIGS. 10 to 12 . 
     In the examples shown in  FIGS. 10 to 12 , the brightness value at the peak of the histogram data of the face extracted in step S 302  is represented by Yf, and the peak value of the frequency at the brightness value Yf is represented by Ff. In step S 303 , an upper limit threshold value Ff_max and a lower limit threshold value Ff_lmt of the frequency at the peak for the face are calculated. The upper limit threshold value Ff_max is designated as the peak value of the frequency, and the lower limit threshold value Ff_lmt is determined using Eq. (11) as follows:
 
 Ff   —   lmt=Ff _max× Rf   —   lmt   Eq. (11)
 
where Rf_lmt denotes a coefficient, where 0≦Rf_lmt≦1. In  FIGS. 10 to 12 , the coefficient is ½, by way of example.
 
     In step S 304 , a limit brightness value for determining a control target range in the subsequent processing is searched for based on the lower limit threshold value Ff_lmt extracted in step S 303 . The limit brightness value determines a brightness region (peak region) including the peak. The histogram data of the face is scanned from the peak at the brightness value Yf toward the low-brightness side and the high-brightness side, and the brightness values obtained at the lower limit threshold value Ff_lmt (in this example, Ff/2) of the frequency are designated as lower and upper limit brightness values Yf_l and Yf_h, respectively. The limit brightness values Yf_l and Yf_h are stored in the RAM or the like. 
     Setting of Target Brightness Range of Face 
     A process for setting a target brightness range in step S 109  shown in  FIG. 2  will be described in detail. 
       FIG. 13  is a flowchart showing the flow of the target brightness range setting process. 
     In step S 401 , the microcomputer  15  provisionally sets a target brightness range (with a lower limit value Ya_l and an upper limit value Ya_h) for adjusting the brightness of the face. In step S 401 , initial values defining a certain target brightness range that is determined in advance so that a face image can be captured with optimum brightness. For example, for a maximum brightness value of 255 (brightness values for 8-bit data), Ya_l=140 and Ya_h=200 are set. In  FIGS. 10 to 12 , an example of the setting of a target brightness range is also illustrated for reference. 
     Then, the microcomputer  15  performs the processing of steps S 402  to S 406  to finely adjust the target brightness range of the face provisionally set in step S 401  according to various parameters such as the amount of external light, the brightness histogram data, and the importance level so that the desired brightness balance between the face and the background can be achieved. 
     In step S 402 , the microcomputer  15  shifts the target brightness range according to the color information detected from the face region (the color information for the face having the highest importance level, which is stored in the RAM or the like in step S 107  shown in  FIG. 2 ). 
     For example, if the color saturation of the face is high, the target brightness range is shifted toward the low-brightness side. If the color saturation of the face is low, the reflectivity of the face is determined. If the reflectivity is high, the target brightness range is shifted toward the high-brightness side. If the reflectivity is low, the target brightness range is shifted toward the low-brightness side. Therefore, the control target value can be changed according to the face skin color. 
     In step S 403 , the microcomputer  15  calculates an amount of external light Lv according to Eq. (12) as follows:
 
 Lv =log 2 ( Fno   2 )+log 2 (1 /T )+log 2 (100/ ISO )+log 2 ( Yave/Y   —   N )  Eq. (12)
 
where Y_N denotes a predetermined control target value of brightness relative to the entire screen, and Yave denotes the average of detected brightness values.
 
     In step S 404 , the microcomputer  15  further shifts the target brightness range of the face according to the determined amount of external light Lv. 
     A shift amount Yshift by which the target brightness range is to be shifted is determined using Eqs. (13) to (17) below, and the shift amount Yshift is added to the current lower limit value Ya_l and upper limit value Ya_h of the target brightness range: 
     for Lv&lt;LV 1 ,
 
 Y shift=SHIFT2  Eq. (13)
 
for LV 1 ≦Lv&lt;LV 2 ,
 
 Y shift={SHIFT2*( LV 2− Lv )}/( LV 2− LV 1)  Eq. (14)
 
for LV 2 ≦Lv&lt;LV 3 ,
 
 Y shift=0  Eq. (15)
 
for LV 3 ≦Lv&lt;LV 4 ,
 
 Y shift={SHIFT1*( Lv−LV 3)}/( LV 4− LV 3)  Eq. (16)
 
for LV 4 ≦Lv,
 
 Y shift=SHIFT1  Eq. (17)
 
where LV 1 , LV 2 , LV 3 , and LV 4  denote threshold values of the amount of external light satisfying 0≦LV 1 &lt;LV 2 &lt;LV 3 &lt;LV 4 , and SHIFT 1  and SHIFT 2  denote constants indicating the upper and lower limits of the shift amount Yshift, respectively, satisfying SHIFT 2 &lt;0&lt;SHIFT 1 .
 
       FIG. 14  is a graph showing the relationship between the amount of external light Lv and the shift amount Yshift of the target brightness range according to Eqs. (13) to (17) given above. 
     As shown in Eqs. (13) to (17) and  FIG. 14 , basically, the target brightness range is shifted toward the low-brightness side as the amount of external light decreases, and the target brightness range is shifted toward the high-brightness side as the amount of external light increases. Therefore, the brightness is controlled so that the photographic style can strongly be reflected to allow the face and the background to appear with a more natural balance. The possible range of the shift amount Yshift is controlled between the SHIFT 1  and the SHIFT 2  in order to prevent an unnatural image in which the brightness of the face is too adjusted to the photographic environment. 
     Referring back to  FIG. 13 , in step S 405 , the microcomputer  15  calculates an amount of overexposure and an amount of underexposure from the histogram data of the entire screen, and further shifts the target brightness range according to the obtained amount of overexposure and underexposure. 
     The amount of overexposure indicates the degree of occurrence of overexposed highlights. The amount of overexposure is calculated by, for example, determining how much the frequency of occurrence of overexposed highlights in a predetermined high-brightness region is higher than a threshold value. The amount of underexposure indicates the degree of occurrence of underexposed shadows. The amount of underexposure is calculated by, for example, determining how much the frequency of occurrence of underexposed shadows in a predetermined low-brightness region is higher than a threshold value. The target brightness range is largely shifted toward the low-brightness side when the amount of overexposed areas is high; whereas the target brightness range is largely shifted toward the high-brightness side as the amount of underexposed areas is high. Therefore, the occurrence of overexposed highlights or underexposed shadows can be prevented over the entire screen. 
     For example, the histogram data of the entire screen may be obtained from the histogram detector  23  in step S 107  shown in  FIG. 2 . Instead of the entire screen, the histogram data of the background, except for the face, may be used to calculate the amount of underexposure and the amount of overexposure. 
     In step S 406 , the microcomputer  15  adjusts the width of the target brightness range according to the importance level Wf of the face determined in step S 104  shown in  FIG. 2  (if a plurality of faces exist, for example, the maximum value of the importance levels Wf of those faces). In this case, an adjustment ratio (scale) for the width of the target brightness range is determined, and the width of the target brightness range is changed according to the adjustment ratio (scale) to calculate the lower limit value Ya_l and upper limit value Ya_h of the target brightness range. The adjustment ratio (scale) for the width of the target brightness range can be determined using Eqs. (18) to (20) as follows: 
     for Wf≦WL,
 
scale=SCALE_MAX  Eq. (18)
 
for WL&lt;Wf≦WH,
 
scale={(SCALE_MAX−1)×( WH−Wf )}/( WH−WL )+1  Eq. (19)
 
for WH&lt;Wf,
 
scale=1  Eq. (20)
 
where WL and WH denote threshold values for the importance level Wf of the face, satisfying 0≦WL&lt;WH, and SCALE_MAX denotes the upper limit threshold value of the adjustment ratio (scale), satisfying SCALE_MAX&gt;1.
 
       FIG. 15  is a graph showing the relationship between the importance level Wf of the face and the adjustment ratio (scale) for the width of the target brightness range according to Eqs. (18) to (20) given above. 
     As shown in Eqs. (18) to (20) given above and  FIG. 15 , basically, the lower the importance level Wf of the face, the larger the target brightness range is adjusted to be. With this control, if the importance level Wf of the face is high, exposure adjustment is preferentially performed on the face; whereas if the importance level Wf is low, the target brightness range is adjusted to be large so that the effect of exposure control adjustment based on the amount of detection of the face can be reduced to perform exposure adjustment on both the face and the background in a well-balanced manner. 
     For example, a small target brightness range may be set in step S 401 . In step S 406 , if the importance level Wf of the face is high, the target brightness range may be increased so that the adjustment ratio (scale) can be equal to or more than 1. 
     The process shown in  FIG. 13  allows the target brightness range to be adjusted so that the desired brightness balance between the face and the background, as well as appropriate brightness of the face, can be achieved. 
     Re-Calculation of Amount of Exposure Control and Flash Control Information 
     A process for re-calculating the amount of exposure control and the flash control information, which corresponds to the processing of step S 110  shown in  FIG. 2 , will be described in detail. In this process, a final amount of exposure control and flash control information are calculated on the basis of the amount of exposure control and flash control information determined using the standard AE control method in step S 101 , the information such as the peak of the histogram data determined in step S 108 , and the target brightness range determined in step S 109 . Therefore, exposure control can be performed so that the brightness of the face can be appropriately adjusted while the desired brightness balance between the face and the background can be achieved without the occurrence of underexposed shadows or overexposed highlights in the background or the like. 
       FIGS. 16 and 17  are flowcharts showing the flow of the process for re-calculating the amount of exposure control and the flash control information. 
     In step S 501 , the microcomputer  15  uses the limit brightness values Yf_l and Yf_h based on the peak, which are determined in step S 304  shown in  FIG. 9 , and the lower limit value Ya_l and the upper limit value Ya_h of the target brightness range, which are determined in the process shown in  FIG. 13 , to compare the width of the peak region of the histogram data of the face (Yf_h-Yf_l) with the width of the target brightness range (Ya_h-Ya_l). If the width of the peak region is larger than that of the target brightness range, the processing of step S 502  is performed. If the width of the peak region is equal to or smaller than the width of the target brightness range, the processing of step S 506  is performed. 
     After the comparison processing is performed, the microcomputer  15  adjusts the amount of exposure control so that the peak or the peak region is included in the target brightness range. In the embodiment, if the width of the peak region is larger than that of the target brightness range, the amount of exposure control is adjusted so that the peak is included in the target brightness range; whereas if the width of the peak region is smaller than that of the target brightness range, the amount of exposure control is adjusted so that the peak region itself is included in the target brightness range. 
     In step S 502 , the microcomputer  15  compares the brightness value Yf at the peak of the histogram data of the face with the upper limit value Ya_h of the target brightness range. If the brightness value Yf at the peak is higher, the processing of step S 503  is performed. If the brightness value Yf at the peak is equal to or lower than the upper limit value Ya_h, the processing of step S 504  is performed. 
     In step S 503 , since the brightness value Yf at the peak of the histogram data of the face is higher than the upper limit value Ya_h of the target brightness range, the microcomputer  15  calculates an adjustment amount ΔEv by which the amount of exposure control is to be adjusted using Eq. (21) below so that the peak can be shifted toward the low-brightness side, that is, the amount of incident light can decrease. The adjustment amount ΔEv can therefore be determined so that the peak can be included in the target brightness range.
 
Δ Ev =−log 2 ( Yf/Ya   —   h )  Eq. (21)
 
     In step S 504 , the microcomputer  15  compares the brightness value Yf at the peak of the histogram data of the face with the lower limit value Ya_l of the target brightness range. 
     If the brightness value Yf at the peak is equal to or higher than the lower limit value Ya_l, it can be considered that the peak is included in the target brightness range, resulting in an exposure control state in which the balance between the face and the background can be appropriately adjusted. Thus, the process ends, and the exposure mechanism and the flash emission operation are controlled using the amount of exposure control and flash control information determined in step S 101  shown in  FIG. 2 . If the brightness value Yf at the peak is lower than the lower limit value Ya_l, the processing of step S 505  is performed. 
     In step S 505 , since the brightness value Yf at the peak of the histogram data of the face is lower than the lower limit value Ya_l of the target brightness range, the microcomputer  15  calculates an adjustment amount ΔEv by which the amount of exposure control is to be adjusted using Eq. (22) below so that the peak can be shifted toward the high-brightness side, that is, the amount of incident light can increase. The adjustment amount ΔEv can therefore be determined so that the peak can be included in the target brightness range.
 
Δ Ev =−log 2 ( Yf/Ya   —   l )  Eq. (22)
 
     In step S 506 , the microcomputer  15  compares the limit brightness value Yf_l based on the peak of the histogram data of the face with the lower limit value Ya_l of the target brightness range. If the limit brightness value Yf_l is lower, the processing of step S 507  is performed. If the limit brightness value Yf_l is equal to or higher than the lower limit value Ya_l, the processing of step S 508  is performed. 
     In step S 507 , since at least the low-brightness-side peak region of the histogram data of the face is outside the target brightness range, it is determined that it is in an underexposure state such as the state shown in  FIG. 10 . Thus, the microcomputer  15  calculates an adjustment amount ΔEv by which the amount of exposure control is to be adjusted using Eq. (23) below so that the peak can be shifted toward the high-brightness side, that is, the amount of incident light can increase. The adjustment amount ΔEv can therefore be determined so that the peak can be included in the target brightness range.
 
Δ Ev =−log 2 ( Yf   —   l/Ya   —   l )  Eq. (23)
 
     In step S 508 , the microcomputer  15  compares the limit brightness value Yf_h based on the peak with the upper limit value Ya_h of the target brightness range. 
     If the limit brightness value Yf_h is equal to or lower than the upper limit value Ya_h, as in the state shown in  FIG. 11 , the whole peak region is inside the target brightness range. Then, the process ends, and the exposure mechanism and the flash emission operation are controlled using the amount of exposure control and flash control information determined in step S 101  shown in  FIG. 2 . If the limit brightness value Yf_h is higher than the upper limit value Ya_h, the processing of step S 509  is performed. 
     In step S 509 , since at least the high-brightness-side peak region of the histogram data if the face is outside the target brightness range, it is determined that it is in an overexposure state such as the state shown in  FIG. 12 . Thus, the microcomputer  15  calculates an adjustment amount ΔEv by which the amount of exposure control is to be adjusted using Eq. (24) below so that the peak can be shifted toward the low-brightness side, that is, the amount of incident light can decrease. The adjustment amount ΔEv can therefore be determined so that the peak region can be included in the target brightness range.
 
Δ Ev =−log 2 ( Yf   —   h/Ya   —   h )  Eq. (24)
 
     In step S 510 , the microcomputer  15  determines whether or not the adjustment amount ΔEv determined in step S 503 , S 505 , S 507 , or S 509  is larger than 0. 
     If the adjustment amount ΔEv is smaller than 0, the amount of exposure control Ev determined in step S 101  shown in  FIG. 2  is smaller than the ideal value, resulting in overexposure if control is performed using the current amount of exposure control Ev. If the adjustment amount ΔEv is applied to the amount of exposure control Ev (in this case, the adjustment amount ΔEv is subtracted from the amount of exposure control Ev), the amount of exposure will be low, resulting in a possibility of underexposed shadows. If the adjustment amount ΔEv is smaller than 0 (in this case, including the case of ΔEv=0), therefore, the processing of steps S 511  to S 513  is performed to correct brightness gradation so as not to cause underexposed shadows. 
     If the adjustment amount ΔEv is larger than 0, the amount of exposure control Ev determined in step S 101  shown in  FIG. 2  is higher than the ideal value, resulting in underexposure if control is performed using the current amount of exposure control Ev. If the adjustment amount ΔEv is applied to the amount of exposure control Ev, the amount of exposure will be high, resulting in a possibility of overexposed highlights. If the adjustment amount ΔEv is larger than 0, therefore, the processing of steps S 515  to S 521  is performed to perform processing so as not to cause overexposed highlights, such as correcting brightness gradation, correcting the adjustment amount ΔEv, and determining whether emission of flash light is allowed. 
     In step S 511 , the microcomputer  15  calculates a predicted underexposure ratio Ri on the basis of the brightness histogram data of the entire screen. The predicted underexposure ratio Ri represents the proportion of a screen region having a possibility of underexposed shadows due to an insufficient amount of exposure when the adjustment amount ΔEv is applied. Specifically, for example, as shown in the graph of  FIG. 12 , a low-brightness region VL 1  whose brightness level is lower than a predetermined threshold value is defined, and the number of pixels included in the low-brightness region VL 1  is determined from the histogram data of the entire screen. The ratio of the number of pixels in the low-brightness region VL 1  to the number of pixels of the entire screen is designated as the predicted underexposure ratio Ri. 
     In step S 512 , the microcomputer  15  compares the determined predicted underexposure ratio Ri with a threshold value Ri_thr. If the predicted underexposure ratio Ri is higher than the threshold value Ri_thr, the processing of step S 513  is performed. If the predicted underexposure ratio Ri is equal to or lower than the threshold value Ri_thr, the processing of step S 514  is performed. 
     In step S 513 , since it is determined that the subject corresponding to the low-brightness region VL 1  has a high possibility of underexposed shadows, the microcomputer  15  converts the brightness of the input image signal using a gamma curve or the like, and controls the camera signal processing circuit  14  so that the gradation in the low-brightness side can be increased. Therefore, the captured image signal to which the adjustment amount ΔEv is applied is subjected to gradation correction, and the image quality of the low-brightness region is improved. Then, the processing of step S 514  is performed. 
     The gradation correction function used in step S 513  is included in the image preprocessor  21  or image processor  22  of the camera signal processing circuit  14 . However, the histogram detector  23  detects brightness information from the image signal that has not been subjected to gradation correction. 
     In step S 514 , the microcomputer  15  updates the amount of exposure control Ev by applying the current adjustment amount ΔEv to the amount of exposure control Ev. In this case, the adjustment amount ΔEv is subtracted from the amount of exposure control Ev. The exposure mechanism is controlled using the updated amount of exposure control Ev. 
     The processing of steps S 512  to S 514  will further be described with respect to an example. 
       FIG. 18  is a diagram showing histogram data obtained when the adjustment amount ΔEv is applied in step S 511  in the state shown in  FIG. 12 . 
     In the state shown in  FIG. 12 , the amount of exposure of the face is excessively high, and a captured image whose brightness is low as a whole is obtained, as shown in  FIG. 18 , if the adjustment amount ΔEv is applied. The pixels included in the low-brightness region VL 1  shown in  FIG. 12  before the exposure correction are included in a low-brightness region VL 2  shown in  FIG. 18 . The upper limit of the low-brightness region VL 2  is lower than the upper limit of the low-brightness region VL 1  before the exposure correction. Thus, when the adjustment amount ΔEv is applied, the pixels included in the low-brightness region VL 2  have an insufficient amount of exposure, resulting in a high possibility of underexposed shadows. 
     In step S 512  shown in  FIG. 17 , therefore, if the proportion of the screen region included in the low-brightness region VL 1  before the exposure correction is high, it is determined that the possibility of underexposed shadows occurring in that region is high, and the processing of step S 513  is performed to increase the brightness gradation in the low-brightness side. 
       FIG. 19  shows an example of a correction curve to be used in step S 513  shown in  FIG. 17 . 
     As shown in  FIG. 19 , in the gradation correction in step S 513 , for example, a gamma curve that increases the low-brightness component in the input brightness signal can be used. Such a correction curve is determined in advance based on a test result obtained by changing the predicted underexposure ratio Ri, the peak of the histogram data of the face, or the like. 
       FIG. 20  shows histogram data obtained when the adjustment amount ΔEv is applied in the state shown in  FIG. 12  after the gradation correction is performed. 
     A captured image having the histogram shown in  FIG. 20  is obtained by, for example, performing gradation correction on the image corresponding the graph of  FIG. 12  and then performing exposure adjustment using the adjustment amount ΔEv. The obtained image is based on an output image signal from the camera signal processing circuit  14 . 
     The pixels included in the low-brightness region VL 1  shown in  FIG. 12  are included in a low-brightness region VL 3  shown in  FIG. 20 ; however, the upper limit of the low-brightness region VL 3  is not substantially different from that of the low-brightness region VL 1 . Therefore, the low-brightness component can have gradation even after exposure adjustment is performed, and the occurrence of underexposed shadows is reduced. 
     Referring back to  FIG. 17 , in step S 515 , the microcomputer  15  calculates a predicted overexposure ratio Ro on the basis of the brightness histogram data of the entire screen. The predicted overexposure ratio Ro represents the proportion of a screen region having a possibility of overexposed highlights due to an excessive amount of exposure when the adjustment amount ΔEv is applied. Specifically, for example, as shown in the graph of  FIG. 10 , a high-brightness region VH 1  whose brightness level is higher than a predetermined threshold value is defined, and the number of pixels included in the high-brightness region VH 1  is determined from the histogram data of the entire screen. The ratio of the number of pixels in the high-brightness region VH 1  to the number of pixels of the entire screen is designated as the predicted overexposure ratio Ro. 
     In step S 516 , the microcomputer  15  compares the determined predicted overexposure ratio Ro with a threshold value Ro_thr. 
     If the predicted overexposure ratio Ro is equal to or lower than the threshold value Ro_thr, it is determined that the possibility of overexposed highlights is low. Then, the process proceeds to step S 514 , and the current adjustment amount ΔEv is applied to perform exposure control. If the predicted overexposure ratio Ro is larger than the threshold value Ro_thr, the processing of step S 517  is performed. 
     In step S 517 , the microcomputer  15  determines whether or not the flash light reaches the subject. If it is determined that the flash light reaches the subject, the processing of step S 518  is performed. If it is determined that the flash light does not reach, the processing of step S 520  is performed. For example, the microcomputer  15  can predict the distance to the subject on the basis of the amount of control by the lens position for focusing, and can determine that the flash light reaches when the predicted distance is equal to or lower than a predetermined threshold value. 
     In step S 518 , the microcomputer  15  determines whether or not a flash emission prohibition mode is currently set by the user. If the flash emission prohibition mode is set, the processing of step S 519  is performed. If the flash emission prohibition mode is not set, the processing of step S 521  is performed. 
     In step S 519 , the microcomputer  15  performs control so as to give a warning to the user to perform an operation for emitting the flash light. For example, control information for displaying warning information on the display  17  is output to the graphic interface circuit  16 . 
     In step S 520 , the microcomputer  15  converts the brightness of the input image signal using a gamma (γ) curve or the like, and controls the camera signal processing circuit  14  so that the gradation in the vicinity of the face brightness component can be increased. The microcomputer  15  further updates the adjustment amount ΔEv to reduce its value. Therefore, the captured image signal to which the updated adjustment amount ΔEv is applied is subjected to gradation correction, and the brightness of the face is corrected. Then, the process proceeds to step S 514 , and exposure control is performed by applying the updated adjustment amount ΔEv. 
     In step S 521 , the microcomputer  15  outputs control information for requesting the flash emitter  19  to emit flash light. Then, exposure control is performed using the amount of exposure control Ev determined by the standard AE control method. At this time, if the shutter release button is pressed, exposure control is performed using the amount of exposure control Ev determined by the standard AE control method, and the flash light is emitted. 
     The processing of steps S 515  to S 521  described above will further be described with respect to an example. 
       FIG. 21  shows histogram data obtained when the adjustment amount ΔEv is applied in the state shown in  FIG. 10 . 
     In the state shown in  FIG. 10 , the amount of exposure of the face is excessively low, and a captured image whose brightness is high as a whole is obtained, as shown in  FIG. 21 , if the adjustment amount ΔEv is applied. The pixels included in the high-brightness region VH 1  shown in  FIG. 10  before the exposure correction are included in a high-brightness region VH 2  shown in  FIG. 21 . The lower limit of the high-brightness region VH 2  is higher than the lower limit of the high-brightness region VH 1  before the exposure correction. Thus, when the adjustment amount ΔEv is applied, the pixels included in the high-brightness region VH 2  have an excessive amount of exposure, resulting in a high possibility of overexposed highlights. 
     However, if the flash light reaches the subject, i.e., the face, an image of the face can be captured with higher brightness, and the occurrence of overexposed highlights in the background or the like can be prevented even if the adjustment amount ΔEv is not applied. Therefore, if the face is within the range of the flash light (step S 517 ) and if emission of the flash light is allowed (step S 518 ), an image with the desired balance of brightness between the face and the background and without the occurrence of overexposed highlights can be captured by performing exposure correction using the amount of exposure control Ev determined by the standard AE control method and emitting the flash light (step S 521 ). 
     If the flash light does not reach the face or if the flash emission prohibition mode is set, on the other hand, the processing of step S 520 . That is, the adjustment amount ΔEv is corrected, and brightness gradation correction is performed, as described below, to capture an image of the face with appropriate brightness while preventing the occurrence of overexposed highlights. 
       FIG. 22  shows histogram data obtained when a value lower than the adjustment amount ΔEv (hereinafter referred to as an “adjustment amount ΔEv′”) is applied in the state shown in  FIG. 10 . 
     If the adjustment amount ΔEv′ lower than the determined adjustment amount ΔEv is applied to the captured image shown in  FIG. 10 , the pixels included in the high-brightness region VH 1  shown in  FIG. 10  before the exposure correction are included in a high-brightness region VH 3  shown in  FIG. 22 . If the adjustment amount ΔEv′ is determined so that the lower limit of the high-brightness region VH 3  is not substantially different from the lower limit of the high-brightness region VH 1  before the exposure correction, the gradation of the high-brightness region can remain, and the occurrence of overexposed highlights can be prevented. 
     In this case, however, the amount of exposure of the face is smaller than the ideal value. If the brightness value at the peak of the histogram data of the face in the graph of  FIG. 22  is represented by Yf′ and the limit brightness value based on the peak is represented by Yf_l′, the control target value may be deviated by a value (Ya_l-Yf_l′). In step S 520 , therefore, the occurrence of overexposed highlights is prevented by the adjustment amount ΔEv′, and the deviation of the control target value of the face can be overcome by correcting for the gradation in the vicinity of the face brightness component. 
     The adjustment amount ΔEv′ is determined by Eq. (25) as follows:
 
Δ Ev′=ΔEv +log 2 ( Yf   —   l′/Ya   —   l )  Eq. (25)
 
       FIG. 23  shows an example of the correction curve used in step S 520  shown in  FIG. 17 . 
     As shown in  FIG. 23 , in the gradation correction in step S 520 , for example, a gamma curve that increases the brightness output in the vicinity of the face brightness component can be used. More specifically, as shown in  FIG. 23 , the gradient of the gamma curve is increased in a region between the lower limit value Ya_l of the target brightness region and the limit brightness value Yf_l′. The image signal is corrected in this manner, thereby increasing the brightness of the face. 
     In step S 520 , the adjustment amount ΔEv′ is calculated using Eq. (25) given above and the correction curve, and the current adjustment amount ΔEv is updated by the determined adjustment amount ΔEv′. Further, the camera signal processing circuit  14  is requested to perform gradation correction based on the correction curve. Specifically, for example, a plurality of correction curves (gamma curves) with different gradients are provided in advance, and the adjustment amount ΔEv is applied to the individual correction curves to determine the limit brightness values Yf_l′. The correction curve having the optimum limit brightness value Yf_l′ is selected and is used for the camera signal processing circuit  14 , and the optimum limit brightness value Yf_l′ is used in Eq. (25) to determine the adjustment amount ΔEv′ to update the adjustment amount ΔEv. 
       FIG. 24  shows histogram data obtained when the adjustment amount ΔEv′ is applied in the state shown in  FIG. 10  and the gradation correction is performed. 
     As described above, exposure correction is performed on the basis of the adjustment amount ΔEv′, and gradation correction is performed. Thereby, as shown in  FIG. 24 , the peak of the histogram data of the face can be included in the target brightness range without reducing a high-brightness region VH 4  corresponding to the high-brightness region VH 1  before the exposure correction. Therefore, the brightness of the face can appropriately be adjusted while preventing the occurrence of overexposed highlights in the background or the like, and the desired brightness balance between the face and the background can be obtained. 
     The processing of steps S 515  to S 521  has been described in the context of a case in which the adjustment amount ΔEv is determined by the processing of step S 507  in the flowchart of  FIG. 16 . If the adjustment amount ΔEv is determined by the processing of step S 505 , in step S 520 , the adjustment amount ΔEv′ is calculated using Eq. (26) as follows:
 
Δ Ev′=ΔEv +log 2 ( Yf′/Ya   —   l )  Eq. (26)
 
where Yf′ denotes the brightness value at the peak of the histogram data of the face when the adjustment amount ΔEv′ is applied. The brightness value Yf′ can be determined in a similar manner to the limit brightness value Yf_l′ in Eq. (25) given above.
 
     In the gradation correction in step S 520 , for example, instead of the limit brightness value Yf_l′ shown in  FIG. 23 , the brightness value Yf′ at the peak may be used. That is, the gradient of the gamma curve may be increased in the area between the lower limit value Ya_l of the target brightness region and the peak in order to increase the brightness output. 
     In general, the camera signal processing circuit  14  has a function for performing gradation correction on an image signal using a gamma curve or the like. The histogram detector  23  detects brightness on the basis of the image signal that has not been subjected to gradation correction, thus allowing more appropriate exposure control according to the processing described above. In a case where the histogram detector  23  detects brightness on the basis of the image signal that has been subjected to gradation correction, the lower limit value Ya_l and the upper limit value Ya_h of the target brightness range for the face are set in consideration of the characteristics for the gradation correction. 
     Advantages of Embodiment 
     According to the image pickup apparatus of the embodiment, therefore, when a face is detected within a captured image, exposure control for appropriately adjusting the brightness of the face can automatically be performed in the auto-shooting mode without setting an operation mode for appropriately adjusting the brightness of the face, such as a portrait mode. Thus, even a novice user who is not familiar with the setting operation can capture an image with improved quality of a face image. 
     Further, it can be determined whether or not exposure control for performing adjustment according to an importance level of a detected face is to be performed. Moreover, by changing the shift amount of the target brightness range, the object intended by the user can be estimated, and various exposure control can be automatically selected and performed, such as exposure control that places more importance on the brightness of the face, exposure control that places more importance on the brightness of the background, or exposure control that places more importance on the brightness balance between the face and the background. Therefore, an image whose brightness is adjusted can be captured under various conditions. 
     Further, exposure adjustment is performed based on a peak or a peak region of histogram data of a face, thus allowing high-accuracy adjustment of the brightness of the face. For example, in the related art, exposure control is performed using an average brightness value in a face region as a brightness value representing the face region. However, if an error occurs during the face detection process, the average brightness value of the face region may largely vary, resulting in a high possibility of incorrect exposure control. In contrast, a peak or a peak region of histogram data of a face may not largely vary even if an error occurs during the face detection process (in particular if a face having a low importance level is not detected by an error), and high-accuracy exposure control can be achieved. 
     Further, exposure control is performed based on not only a peak of histogram data of a face but also a peak region of the histogram data, as necessary, and a target brightness range is set. Therefore, a certain range of the control target of brightness can be obtained, and a natural image taking the brightness balance between the face and the background into consideration can be captured without extreme exposure control that places importance on only the brightness of the face. Moreover, the possibility of underexposed shadows or overexposed highlights can be predicted on the basis of a detected brightness value, and can be used for exposure control, thereby attaining an appropriate brightness balance between the face and the background. 
     Other Process for Shifting Target Brightness Range 
     In step S 402  shown in  FIG. 13 , the target brightness range is shifted on the basis of the color information detected from the face region. For example, the target brightness range may be shifted by, as described above, determining the reflectivity of the face according to the color saturation of the face. However, in particular, when the color saturation of the face is low, the reflectivity may not accurately be determined. By detecting the brightness of an eye region and comparing the average brightness of the face with the detected brightness, the reflectivity of the face can be more accurately determined using the comparison result. 
       FIG. 25  is a diagram showing an example of the setting of an eye detection region. In this example, a detection frame for color detection is set using the normal frame method, by way of example. 
     The microcomputer  15  sets a face-region detection frame  131  and eye detection frames  132  and  133  for the color detector  24  on the basis of face detection results obtained from the face detector  25 , such as according to a predetermined face contour pattern. Color detection information is obtained from the detection frames  131  to  133 , and the average brightness of the eye regions is determined. Then, the ratio of the average brightness to the average brightness of the face region is calculated. If the calculated ratio is small, it is determined that the reflectivity of the face is high, and the target brightness range is shifted toward the high-brightness side. If the average brightness of the face region is significantly smaller than the average brightness of the eye regions, it is determined that the reflectivity of the face is low, and the target brightness range is shifted toward the low-brightness side. This process allows more accurate adjustment of the brightness of the face regardless of the skin color of the subject. 
     Other Modifications 
     In the above-described embodiment, therefore, a target brightness range is set, thereby providing a certain range of the control target of brightness and achieving exposure control in consideration of the balance of brightness between the face and the background. By narrowing the target brightness range, exposure control that places more importance on the brightness of the face can be achieved. Thus, the target brightness range may be variably set by the user. For example, if the portrait mode is set, the target brightness can be limited to one point; whereas if a shooting mode suitable for landscape photography is set, the target brightness range can be increased. 
     Further, the importance level of the face may be set higher when the portrait mode is set. Accordingly, the target brightness range can be shifted so that more importance can be placed on the brightness of the face. 
     In the above-described underexposure and overexposure determination processing, the brightness in the entire screen or a detection value of the brightness histogram data is used. Alternatively, a detection value from the background, expect for the face, may be used, by way of example. Furthermore, the occurrence of underexposed shadows or overexposed highlights may be individually determined using detection values from both the face and the background. In a case where a face detection frame is set for the histogram detector  23 , the microcomputer  15  calculates a detection value of the background from among detection values of the face and the entire screen. If the determination processing is performed on the basis of a detection value of the background, it is advantageous that underexposed shadows and overexposed highlights in the background can be accurately determined particularly when the face region is large. 
     The process for determining an importance level of a face described above with reference to  FIG. 3  can also be used for the following applications, as well as exposure control in the above-described embodiment. For example, a focus lens is controlled to focus on a face having a high importance level. Further, in a case where a face having a high importance level exists, color gradation correction is performed using a special gamma curve for adjusting the color of the person, or parameters suitable for portrait photography are used for an aperture controller. Further, a face having a high importance level and the surrounding area are cut off or cropped, and a picture frame or the like is combined. Further, those operations can be performed not only by an image signal obtained from an image pickup device but also by an image signal recorded on a recording medium or an image signal received over a network. 
     Various image pickup apparatuses using solid-state image pickup devices, such as digital video cameras and digital still cameras, and apparatuses having an image capturing function, such as mobile phones and personal digital assistants (PDAs), may fall within the scope of the present invention. Various controllers, such as controllers for videophone applications connected to personal computers (PCs) and controllers used with miniature cameras for game software, may also fall within the scope of the present invention. 
     Image processing apparatuses configured to perform image quality correction in response to an input of an image signal may also fall within the scope of the present invention. In this case, after a target brightness range is set by the above-described processing, instead of controlling the exposure mechanism on the basis of the target brightness range and the information such as the peak or peak region of the histogram data, the brightness of the input image signal is adjusted to thereby correct the brightness of the face and the background. 
     The above-described processing functions can be implemented by a computer. In this case, a program describing the processing of the functions to be achieved by the apparatus (such as the above-described exposure control function, function for adjusting the brightness of the input image signal, and function for determining an importance level of a face) is provided. The program is executed by a computer to thereby implement the above-described processing functions on the computer. The program describing the processing can be recorded on a computer-readable recording medium. Examples of the computer-readable recording medium include a magnetic recording device, an optical disk, a magneto-optical disk, and a semiconductor memory. 
     The program may be distributed by, for example, selling portable recording media having the program recorded thereon, such as optical disks and semiconductor memories. The program may be stored in a storage device of a server computer so that the program can be transferred to another computer from the server computer over a network. 
     For example, a computer that is to execute the program stores the program recorded on a portable recording medium or the program transferred from the server computer in a storage device of the computer. The computer reads the program from the storage device, and performs processing in accordance with the program. The computer may read the program directly from the portable recording medium, and may perform processing in accordance with the program. Alternatively, each time the program is transferred from the server computer, the computer may perform processing in accordance with the received program. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.