Abstract:
An apparatus for imaging an object has an image sensor that comprises a plurality of pixels. The plurality of pixels is matrix-arrayed along vertical and horizontal directions. The apparatus also has an image sensor driver that drives the image sensor, and the image sensor driver is capable of reading image-pixel signals of neighboring pixels among the plurality of pixels while mixing the image-pixel signals. The apparatus also has a pixel addition setting processor that sets the number of pixel addition with respect to at least one of at least one row and at least one column. The pixel addition setting processor sets different numbers of pixel addition to different pixel areas. The image sensor driver reads the image-pixel signals in response to the set number of pixel addition.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to an image pickup apparatus that is utilizable to such as a camera, and particularly it relates to a reading process for image-pixel signals generated by an image sensor. 
         [0003]    2. Description of the Related Art 
         [0004]    As for a digital camera with an image sensor, it is required to produce a subject image by a wide dynamic range. For example, when an image sensor is used as a light-metering sensor or AE sensor, the brightness of an object should be precisely detected between a low luminance level and a high luminance level to determine an appropriate exposure value such as a shutter speed. 
         [0005]    Various methods for enlarging the dynamic range of an image sensor are known or proposed. For example, an exposure time is changed in each pixel, a plurality of images is acquired in time series, or an amount of incident light is changed in each pixel by dividing a light path. Also, it is known that pixels having low sensitivities and pixels having high sensitivities are mixture-arrayed. In this case, OB image-pixel signals that are read from OB (Optical Black) pixels having high sensitivity are utilized to calculate output values of OB image-pixel signals that are read from OB pixels having low sensitivity. Thus, values of image-pixel signals generated in a light-receiving area are corrected adequately. 
         [0006]    On the other hand, a method that reads out pixel signals while mixing or adding pixel signals is known. For example, a light-metering sensor incorporated in an SLR type digital camera outputs image-pixel signals while adding pixels. JP2012-253462A1 discloses a camera having an AE sensor with a color filter array, in which R, G, and B color elements are arrayed in a stripe along a vertical direction of the AE sensor. Then, five pixel signals that are neighboring one another along the vertical direction are mixed in each of the R, G, and B color filter elements. The five mixed pixel signals of each color element are output as one pixel signal. 
         [0007]    Such a pixel adding method improves the sensitivity in a low luminance range; however, a real effect that enlarges a dynamic range totally cannot be obtained since a pixel adding process is uniformly performed on the entire light-receiving area of the image sensor. 
         [0008]    On the other hand, a metering sensor in the camera is utilized to pursue a subject by extracting characteristics of the subject, such as a face, and high resolution is required to extract the characteristics of the subject. In the above pixel adding method disclosed in JP2012-253462A1, to prevent a decrease in resolution the pixel adding process is not performed when pursuing a subject using the metering sensor. In this case, the sensitivity in the low luminance level is not increased. Also, the number of image-pixel signals to mix, i.e., the number of added pixels, is constant under any photography condition when performing the pixel addition process. 
         [0009]    Furthermore, in the above pixel adding method disclosed in JP2012-253462A1, output levels of OB image-pixel signals corresponding to output levels of image-pixel signals generated by the pixel addition process are not considered sufficiently. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is directed to carry out a pixel addition process that allows a dynamic range of an image sensor to be enlarged. 
         [0011]    An apparatus for imaging an object, according to the present invention has an image sensor that comprises a plurality of pixels. The plurality of pixels is matrix-arrayed along vertical and horizontal directions. The apparatus also has an image sensor driver that drives the image sensor, and the image sensor driver is capable of reading image-pixel signals of neighboring pixels among the plurality of pixels while mixing the image-pixel signals. The apparatus also has a pixel addition setting processor that sets the number of pixel addition with respect to at least one of at least one row and at least one column. The pixel addition setting processor sets different numbers of pixel addition to different pixel areas. The image sensor driver reads the image-pixel signals in response to the set number of pixel addition. 
         [0012]    An apparatus for imaging an object, according to another aspect of the present invention, has a metering image sensor that comprises a plurality of pixels, the plurality of pixels being matrix-arrayed along vertical and horizontal directions; a photography image sensor; an image sensor driver that drives the metering image sensor, the image sensor driver being capable of reading image-pixel signals of neighboring pixels among the plurality of pixels while mixing the image-pixel signals; and a pixel addition setting processor that sets the number of pixel addition of the metering image sensor with respect to at least one of at least one row and at least one column, the pixel addition setting processor being capable of setting different numbers of pixel addition to different pixel areas; and an exposure controller that controls an exposure of the metering image sensor and the photography image sensor, the pixel addition setting processor being capable of switching the number of pixel addition, the image sensor driver reading the image-pixel signals in response to the set number of pixel addition, the exposure controller setting an exposure value of the photography image sensor on the basis of the brightness of an object image that is detected by the metering image sensor. 
         [0013]    An apparatus for imaging an object, according to another aspect of the present invention, has an image sensor that comprises a plurality of effective pixels and a plurality of OB (optical black) pixels, the plurality of effective pixels being matrix-arrayed along vertical and horizontal directions, the plurality of OB pixels being arrayed along at least one of the row and column of the effective pixels; an image sensor driver that drives the image sensor, the image sensor being capable of reading image-pixel signals of neighboring pixels among the plurality of pixels and the plurality of OB pixels while mixing the image-pixel signals; a pixel addition setting processor that sets the number of pixel addition of the effective pixels and the plurality of OB pixels with respect to at least one of at least one row and at least one column; and an image-pixel signal correcting processor that corrects effective image-pixel signals that are read from the effective pixels on the basis of corresponding OB image-pixel signals, the pixel addition setting processor setting different numbers of pixel addition to different pixel areas, the image sensor driver reading the image-pixel signals in response to the set number of pixel addition, the image-pixel signal correcting processor correcting the effective image-pixel signals using OB image-pixel signals corresponding to the set number of pixel addition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The present invention will be better understood from the description of the preferred embodiments of the invention set forth below, together with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a block diagram of a digital camera according to the first embodiment; 
           [0016]      FIG. 2  is a view showing part of a pixel-array and a part of an electric circuit for the AE sensor; 
           [0017]      FIG. 3  is a view showing a mixture of image-pixel signals; 
           [0018]      FIG. 4  is a timing chart of drive signals for reading image-pixel signals; 
           [0019]      FIG. 5  is a view showing sensitivity characteristics of the AE sensor; 
           [0020]      FIG. 6  is a view showing a dynamic range of the AE sensor that is enlarged by a pixel addition process based on different numbers of pixel addition 
           [0021]      FIG. 7  is a view showing part of a pixel-array and a part of an electric circuit for the AE sensor, according to the second embodiment; 
           [0022]      FIG. 8  is a view showing a timing chart of drive signals according to the second embodiment; 
           [0023]      FIG. 9  is a view illustrating a pixel addition process according to the third embodiment; 
           [0024]      FIGS. 10A and 10B  are views illustrate a pixel addition process along the row and column directions, respectively; 
           [0025]      FIG. 11  is a flowchart of a sequence of a recording process; 
           [0026]      FIG. 12  is a subroutine of Step S 103  in  FIG. 11 ; 
           [0027]      FIG. 13  is a view showing a pixel addition process when starting the metering process; 
           [0028]      FIG. 14  is a view showing a pixel addition process when the brightness of the object image rapidly changes; 
           [0029]      FIG. 15  is a view showing a pixel array of the AE sensor according to the fifth embodiment; 
           [0030]      FIG. 16  is a view showing a pixel addition process along the row direction by different numbers of pixel addition; and 
           [0031]      FIG. 17  is a view showing a pixel addition process along the column direction by different numbers of pixel addition. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Hereinafter, the preferred embodiments of the present invention are described with reference to the attached drawings. 
         [0033]      FIG. 1  is a block diagram of a digital camera according to the first embodiment. 
         [0034]    An SLR-type digital camera  10  is equipped with a body  15  and a photographing optical system  20  detachably attached to the body  15 . A user can select and set a mode from various modes such as a photography mode, a replay mode, etc., by operating a mode dial or mode button (not shown). When electric power is turned on, the photography mode is set. 
         [0035]    Light reflected off a target subject enters into the photographing optical system  20  and an iris  22 , and a portion of the light is directed to an optical finder  12  by a quick return mirror  24 . Also, a portion of the light passes through the quick return mirror  24  and is directed to an AF sensor  32  by a half mirror  26 . 
         [0036]    The optical finder  12  forms light that entered the optical finder  12  so that a user can confirm an object image via an eyepiece lens (not shown). Also, an AE sensor  34  is disposed near to the optical finder  12  and an object image is formed on a light-receiving area of the AE sensor  34 . 
         [0037]    A signal processor  40  that is constructed of a DSP outputs control signals to a shutter  27 , an image sensor driver  28 , an LCD  48 , a timing generator (not shown), etc.; and controls the motion of the camera  10 , including an AF process, a photographing/recording process, and a replay process, on the basis of an input operation that is detected by such as a button switch  45 . A program for controlling the digital camera  10  is stored in a ROM unit  44 . 
         [0038]    When a release button  19  is depressed halfway, light metering, an AF (Auto-Focus) process, and an auto-exposure operation are carried out. In the AF process, a focusing lens in the photographing optical system  20  is driven in accordance to a phase difference that is detected by the AF sensor  32 . An exposure controller  50  carries out the auto exposure operation, i.e., an exposure period (shutter speed) and an aperture value of the iris  22  are automatically set on the basis of the brightness of a target object that is detected by the AE sensor  34 . The AE sensor  34  is herein a solid state image sensor such as a CCD and is driven by an AE sensor driver  29 . 
         [0039]    When the release button  19  is depressed completely, a photographing process is carried out. Concretely, the quick return mirror  24  and the half mirror  26  move to the outside of a light path and the iris  22  and the shutter  19  are driven in accordance to the calculated exposure values. Thus, one frame&#39;s object image is formed on the photo-receiving area of the image sensor  30 . 
         [0040]    The image sensor  30  is, for example, a charge-transfer type image sensor such as a CCD image sensor, or may be an X-Y address type image sensor such as a CMOS image sensor. Also, a color filter array  35  is disposed on the light-receiving area of the image sensor  30 . One frame&#39;s worth of image-pixel signals that are generated by the image sensor  30  are read out by the image sensor driver  28  and are fed to the signal processor  40 . 
         [0041]    The signal processor  40  applies processing such as white-balance processing to one-frame&#39;s worth of image-pixel signals to generate color still-image data. The generated still-image data are temporarily stored in an inner memory  42  such as a RAM, and are recorded in an outer memory  46  such as a memory card either directly or compressed. 
         [0042]    The exposure controller  50  outputs control signals to the image sensor driver  28  and the AE sensor driver  29  to adjust an output timing of drive signals that are output to the image sensor  30  and the AE sensor  34 . The AE sensor driver  29  can output drive signals that allow neighboring pixels to be mixed or when reading the neighboring pixel signals. 
         [0043]    Also, the exposure controller  50  can adjust an exposure value (hereinafter, called a “metering exposure value”) by controlling an exposure such that an amount of exposure of the AE sensor  34  becomes an amount that is appropriate with respect to the brightness of the object. Concretely, an exposure time that is set based on an electronic shutter function is adjusted. Furthermore, the exposure controller  50  can adjust a gain value of the image-pixel signals read from the AE sensor  34 . 
         [0044]    On the other hand, when a face detection mode that automatically detects a face of an object is selected, the exposure controller  50  detects a face of an object by using the AE sensor  34  when light-metering is carried out, in accordance to a well-known detection method. A user may set the face detection mode in a state when a menu screen is displayed on the LCD  48 . 
         [0045]    In the present embodiment, a number of pixel signals that is mixed or added when reading the one frame&#39;s worth of image-pixel signals is adjusted so as to enlarge a dynamic range, and the number is different in response to an area defined in the light-receiving area of the AE sensor  34 . Hereinafter, a pixel-signal mixture process in the AE sensor  34  is explained with reference to  FIGS. 2-6 . Note that the term “a pixel mixture” and the term “pixel signal mixture” used in the explanation below have the same meaning, and the term “pixel addition” is used similarly to the term “pixel mixture” or “image-pixel signal mixture. 
         [0046]      FIG. 2  is a view showing part of a pixel-array and a part of an electric circuit for the AE sensor  34 . 
         [0047]    As described above, the AE sensor  34  is herein a CCD image sensor in which a plurality of pixels PD are matrix-arrayed along the vertical direction and the horizontal direction. In  FIG. 2 , twenty-four (6×4) pixels PD are shown, and the pixels PD are represented by reference numerals “A1-A6”, “B1-B6”, “C1-C6”, and “D1-D6”, respectively (hereinafter, the same reference numeral are used for image-pixel signals generated in a corresponding pixel). 
         [0048]    The color filter array  35  has color filter elements R, G1, G2, and B, which have different spectrums respectively, and which are arranged so as to be opposite to pixels A1-A6, B1-B6, C1-C6, and D1-D6. The color filter elements R are arrayed in the same column, and the other color filter elements G1, G2, or B are also arrayed in the same columns, respectively. Electric charges that are generated in each photodiode by photoelectric conversion are transferred to a horizontal transform circuit HT by a vertical transform circuit VT, and are output from the horizontal transform circuit HT toward the signal processor  40  via an amplifier AP. 
         [0049]      FIG. 3  is a view showing a mixture of image-pixel signals. Herein, a mixture method of image-pixel signals in a pixel area composed of twenty-four pixels is shown. The other image-pixel signals in other pixel areas are subjected to the same pixel mixture process. 
         [0050]    The mixture process for image-pixel signals is applied to the color filter elements having the same color along the vertical/column direction. At this time, the number of image-pixel signals to be mixed, i.e., the number of pixel addition, is not uniform but different in each pixel area. Concretely, a mixed pixel area in which two image-pixel signals are mixed and a non-mixed pixel area in which each image-pixel signal is output directly (not mixed) are defined respectively such that the mixed pixel area and the non-mixed pixel area are lined up alternately. In the mixed pixel areas R1 and R3 shown in  FIG. 3 , image-pixel signals (A1, A2), (B1, B2), (C1, C2), and (D1, D2) that are neighboring one another along the vertical direction are mixed and read out, respectively. On the other hand, image-pixel signals (A3, B3, C3, and D3) and pixels (A6, B6, C6, and D6) in a non-mixed pixel area R2 and R4 are transferred vertically without a mixture with neighboring pixels. Consequently, image-pixel signals are read out from the AE sensor  34  as “four row&#39;s worth of image-pixel signals”. For example, four image-pixel signals “A1′, A2′, A3′, and A4′” are output from the first column. The similar pixel addition process is performed for the second to fourth rows and the other rows. 
         [0051]    The read image-pixel signals are separated in accordance to the belonging pixel area, i.e., the number of pixels added to generate a frame&#39;s worth of image data for each of the different number of pixel additions. Consequently, two single frame&#39;s worth of pixel datum that have different sensitivity are generated from one frame&#39;s worth of image-pixel signals. Note that the pixel datum may be generated without a separation process of the image-pixel signals. 
         [0052]    Hereinafter, a reading of image-pixel signals without the pixel addition process is designated as a “reading of image-pixel signals by the number of pixel addition “1”. The term “one pixel addition” or “the number of pixel addition “1” means a reading of image-pixel signals without the pixel addition process. 
         [0053]      FIG. 4  is a timing chart of drive signals for reading image-pixel signals. 
         [0054]    An image-pixel signal generated in each pixel is transferred to the vertical transform circuit VT by a drive signal F0. Then, image-pixel signals in the sixth row are read out directly by a drive signal F1, and image-pixel signals in the fourth and fifth rows are mixed and read out simultaneously. Similarly, image-pixel signals in the third row are read out directly, whereas image-pixel signals in the first and second rows are mixed and read out simultaneously. Such a control of driving signals corresponds to different number settings of the pixel addition pixel in accordance to the pixel areas. The exposure controller  50  determines the appropriate pixel addition. 
         [0055]      FIG. 5  is a view showing sensitivity characteristics of the AE sensor  34 .  FIG. 6  is a view showing a dynamic range of the AE sensor  34  that is enlarged by a pixel addition process based on different numbers of pixel addition. 
         [0056]    In  FIG. 5 , a relationship between an input and an output of the AE sensor  34  is shown, and a relationship between an amount of incident light and output voltage regarding the one-pixel addition (no addition) and the two-pixel addition are represented by lines P1 and P2, respectively. Note that a graph shown in  FIG. 5  is designated with a logarithmic scale. 
         [0057]    As can be seen from a compression the line P1 with the line P2, an amount of light obtained by the two-pixel addition is twice as great as that obtained by the one-pixel addition. Therefore, when light having an amount of light “IN1” enters the AE sensor  34 , an output value “OUT1” and an output value “OUT2” that is twice the output value of “OUT1” are acquired by the AE sensor  34 . 
         [0058]    Also, when the ratio of the input value “IN1” to the input value “IN2” that produces the same output value “OUT2” is two, a dynamic range based on the two-pixel addition differs from a dynamic range based on the one-pixel addition by a magnitude of two. The difference between the input value “IN1” and the input value “IN2” corresponds to an enlarged portion of the dynamic range of the AE sensor  34  (see  FIG. 6 ). 
         [0059]    In  FIG. 6 , the dynamic range “DL1” based on the one-pixel addition (no addition) and the dynamic range “DL2” based on the two-pixel addition are shown. The sensitivity obtained when performing the pixel addition process by the different numbers of pixel addition (e.g., 1 and 2) is enlarged compared with the sensitivity without performing the pixel addition process. By obtaining both the sensitivity with the number of pixel addition “2” and the sensitivity with the number of pixel addition “1”, a dynamic range that covers both a low luminance portion, such as a dark portion, and a high luminance portion can be acquired. This enlarged dynamic range is caused by a sensitivity difference between neighboring pixels. Therefore, by alternately setting neighboring pixel areas that have different numbers of pixel addition in the AE sensor  34 , two pixel datum that have different sensitivities and have the same image area corresponding to one frame&#39;s worth are generated from a single object. Consequently, a total object captured by the photographing optical system  20  is imaged or picked up by an enlarged dynamic range. 
         [0060]    Since the two pixel data corresponding to one frame&#39;s worth are obtained with the same shutter timing, the brightness of an object can be detected adequately even though the object is a moving subject. This effect is not obtainable from a method that photographs an object while changing an exposure in time series. Furthermore, pixel areas that have different numbers of pixel addition are defined uniformly with respect to the total light-receiving area; a degree of enlargement of a dynamic range becomes uniform with respect to an object image formed on the entire light-receiving area. 
         [0061]    Also, since the pixel areas that have different numbers of pixel addition are defined with respect to only a column/vertical direction to which electric charges are transferred, image-pixel signals having two sensitivities are read out so that the brightness of an object image can be easily detected after the reading of image-pixel signals. Especially, since the vertical direction is a direction in which the same color filter elements R, G1, G2 or B are arrayed, an enlargement of a dynamic range reaches each color of image-pixel signals. 
         [0062]    In this way, the digital camera  10  according to the present embodiment is equipped with the AE sensor  34  with a color filter array  35  in which R, G, and B color filter elements are arrayed along the vertical direction separately. Then, in the pixel-signal reading process, the image sensor driver  28  performs a reading process that mixes neighboring pixel signals (the number of pixel addition “2”) and a reading process that does not mix pixel signals (the number of pixel addition “1”) alternately. 
         [0063]    Next, a digital camera according to the second embodiment is explained with reference to  FIGS. 7 and 8 . In the second embodiment, an AE sensor is constructed of a CMOS image sensor. Other constructions are substantially the same as those in the first embodiment. 
         [0064]      FIG. 7  is a view showing part of a pixel-array and a part of an electric circuit for the AE sensor, according to the second embodiment.  FIG. 8  is a view showing a timing chart of drive signals according to the second embodiment. 
         [0065]    An AE sensor  134  is a CMOS image sensor that is an X-Y address type image sensor. In  FIG. 7 , the part of pixel array  134 R composed of (4×4) pixels (A1-A4, B1-B4, C1-C4, and D1-D4) are shown. Similarly to the first embodiment, a color filter array is disposed on the AE sensor  134  and the same color filter elements are arrayed in the column direction. 
         [0066]    Each pixel has an image-pixel signal generating and outputting circuit S1. Also, a series of switch circuits S2 are provided along the column direction and the switch circuits S2 connect an output of image-pixel signals generated by a pixel with an output of image-pixel signals generated by neighboring pixels. When the switch circuits S2 are turned on, image-pixel signals are mixed between neighboring pixels along the column direction. 
         [0067]    When reading image-pixel signals based on the number of pixel addition “1” (no addition), image-pixel signals are read out along a line (row) directly so that image-pixel signals are read out line by line. On the other hand, when reading image-pixel signals based on the number of pixel addition “2”, two lines&#39; worth of image-pixel signals are mixed and read out. 
         [0068]    In the second embodiment, a reading of image-pixel signals based on the number of pixel addition “2” is performed once and then a reading of image-pixel signals based on the number of pixel addition “2” is performed two times continuously after that. Such a reading process is performed along the column direction repeatedly. This reading method is different from an alternate reading of image-pixel signals based on the number of pixel addition “2” and image-pixel signals based on the number of pixel addition “1” described in the first embodiment. 
         [0069]    Concretely speaking, as shown in  FIG. 8 , image-pixel signals in the 1st row are read out and image-pixel signals in the 2nd and 3rd rows are read out while mixing. Image-pixel signals in the 4 th  row are read out directly. Such a reading causes a sensitivity difference in the column direction so that a dynamic range is enlarged. 
         [0070]    Next, the third embodiment is explained with reference to  FIGS. 9 and 10 . The third embodiment is different from the first embodiment in that pixel areas that have different numbers of pixel addition are defined along the row direction or the row and column directions. An image sensor is applied to a CMOS image sensor similarly to the second embodiment. Note a CCD image sensor may be also applied. 
         [0071]      FIG. 9  is a view illustrating a pixel addition process according to the third embodiment. 
         [0072]    In  FIG. 9 , part of a 6×4 pixel array  134 ′R in the AE sensor is illustrated. A color filter array  135 ′ is disposed on the AE sensor  134 ′ and the color filter elements  135 ′ R, G1, G2, and B are arrayed in the horizontal direction, i.e., the row direction in each color. Then, a pixel addition process using different numbers of pixel addition is performed with respect to the row direction. 
         [0073]    Concretely speaking, the number of pixel addition “2” is applied to neighboring pixel areas R2 and R4 along the column direction, and the number of pixel addition “1” is applied to pixel areas R2 and R3 located between the pixel areas R1 and R4. Consequently, four image-pixel signals “A1′-A4′” along the row direction are read out. The AE sensor  134  has switch circuits that are the same as those shown in  FIG. 7  according to the second embodiment. The switch circuits allows neighboring image-pixel signals along the column to be mixed, and a pixel addition process based on the number of pixel addition “1” and the number of pixel addition “2” is performed. Note that, when the AE sensor  134  is a CCD image sensor, a pixel addition process along the column is performed by utilizing a horizontal transfer circuit. 
         [0074]      FIGS. 10A and 10B  illustrate a pixel addition process along the row and column directions, respectively. 
         [0075]    Herein, a pixel addition process based on different numbers of pixel addition is applied to both the row and column direction. As for the row direction, a reading of image-pixel signals based on the number of pixel addition “1” and a reading of image-pixel signals based on the number of pixel addition “2” are performed alternately similarly to the first embodiment. Furthermore, a similar alternate reading process is performed for the column direction. Consequently, image-pixel signals based on three different numbers of pixel addition “1”, “2”, and “4” are read out for the whole pixel area. 
         [0076]    On the other hand, different numbers of pixel addition may be set for the row direction and the column direction separately. As shown in  FIG. 10B , a reading of image pixel signals based on the number of pixel addition “1” and the number of pixel addition “2” is performed for the row direction, whereas a reading of image-pixel signals based on the number of pixel addition “1” and the number of pixel addition is performed for the column direction. Consequently, image-pixel signals based on four different numbers of pixel addition “1”, “2”, “4”, and “6” are read out for the whole pixel area. 
         [0077]    When pixel areas having different numbers of pixel addition are defined alternately such that the ratio of different numbers of pixel addition along the row direction is “1:k” and the ratio of different numbers of pixel addition along the column direction is “1:l” (l and k are integers greater than or equal to 2), image-pixel signals based on the ratio of numbers of pixel addition “1:k:k×l” are read out. Note that in a pixel addition process as shown in  FIGS. 10A and 10B , an image sensor without a color filter array may be applied. 
         [0078]    The exposure controller  50  controls the image sensor driver  28  to allow a given number of pixel addition along the row to be set. A setting of the number of pixel addition may be switched by a photography program stored in the ROM memory in advance. For example, the number of pixel addition set in accordance to an operation input by a user. 
         [0079]    Furthermore, instead of setting a pixel area in which image-pixel signals are mixed and a pixel area in which image-pixel signals are not mixed, pixel areas in which image-pixel signals are mixed, i.e., the number of pixel addition is equal to or more than 2, may be defined for all of the light-receiving area (e.g., the number of pixel addition “2” and the number of pixel addition “4”). This pixel addition process causes an enlargement of the dynamic range. Generally speaking, a pixel addition process based on the number of pixel addition “m” and the number of pixel addition “n” along the row and/or column direction may be performed, regarding the first to third embodiments. Note that “m” and “n” are integers that satisfy “m≧1” and “m &lt;n”. 
         [0080]    In the first to third embodiments, the pixel addition process is performed for the whole light-receiving area. However, the pixel addition process may be performed for a part of the light-receiving area. 
         [0081]    In the first embodiment, image-pixel signals are mixed when transferring the image-pixel signals to the horizontal transfer circuit in the AE sensor that is constructed from a CCD. However, the mixture of image-pixel signals may be carried out on the vertical transfer circuit, the horizontal transfer circuit, or an output circuit. 
         [0082]    In the case of the second and third embodiments that use an X-Y address type image sensor, pixel areas having different numbers of pixel addition may be defined along a diagonal direction instead of the row or column direction. In this case, a color filter array in which color filter elements are checkered is used. Note that a setting of the number of pixel addition along the diagonal direction is the same as a setting of the number of pixel addition along the row and column directions, and may be applied to an image sensor without a color filter array. 
         [0083]    Next, the fourth embodiment is explained with reference to  FIGS. 11-14 . In the fourth embodiment, a setting of a pixel addition process is switched in accordance to a photographic situation, an exposure of an object, etc. The other constructions are substantially the same as those in the first to third embodiments. 
         [0084]    A circuit diagram of the digital camera according to the fourth embodiment is substantially the same as that according to the first embodiment shown in  FIG. 1 , and light metering is carried out by the AE sensor  34 . An object image captured by the photographing optical system  20  is formed on the image sensor  30  by an operation of the release button  19 , and still image data is recorded. An exposure value for the image sensor  30  when recording the still image is determined on the basis of the brightness of the object image detected by the AE sensor  34 . 
         [0085]      FIG. 11  is a flowchart of a sequence of a recording process.  FIG. 12  is a subroutine of Step S 103  in  FIG. 11 . 
         [0086]    When release button  19  is depressed halfway, the AF process is carried out and the brightness of an object image is detected (S 101 -S 103 ). Then, exposure values such as a shutter speed, an aperture value of the iris (F number) are decided (S 104 ). Note that when electric power is turned on, the brightness of an object image is detected similarly to the process of Step S 103 . 
         [0087]    When performing the brightness detection of the object image, i.e., a metering process, a pixel addition process is carried out as required to enlarge a dynamic range of the AE sensor  34 . Then, as described below, an exposure adjustment process for the AE sensor  34  is carried out to correct or modify an exposure value (a metering exposure value). The brightness of an object image is then detected on the basis of the corrected exposure value. At this time, the pixel addition process is not carried out. 
         [0088]    When the release button  19  is depressed completely, the sequence of a recording process including an exposure control such as the opening/closing of the shutter and the generation of still image data is carried out (S 105 -S 107 ). At this time, the recording process is carried out on the basis of the set exposure value for the image sensor  30 . 
         [0089]    Hereinafter, a metering process is explained in detail with reference to  FIG. 12 . In the present embodiment, a “1:1” pixel addition process in which the number of pixel addition “1” is set for an entire pixel area (no pixel addition), a “1:3” pixel addition process in which the number of pixel addition “1” and the number of pixel addition “3” are set for a pixel area alternately, or a “1:19” pixel addition process in which the number of pixel addition “1” and the number of pixel addition “19” are set for a pixel area alternately is selectively carried out in accordance to a photographic situation. 
         [0090]    When the metering process is started, an initial flag IF and an exposure difference ΔEv that represents a previous exposure amount and a present exposure amount are subject to an initial setting; concretely, IF is set to 1 and ΔEv is set to zero (S 201 ). Then, it is determined whether or not the flag is 1 unless the metering process is not terminated. 
         [0091]    The initial flag IF is a flag that determines whether or not a present metering process in the sequence of a photographing/recording process is a first metering process. When it is determined that the initial flag IF=1, the initial flag IF is changed to 0, and the process goes to Step S 208  to enlarge a dynamic range of the AE sensor  34 . 
         [0092]    In Step S 208 , a predetermined initial value is set with respect to an exposure value of the AE sensor  34 . Then, In Step S 209  and S 210 , the above “1:19” pixel addition process is performed. Herein, an exposure period for the electronic shutter function is adjusted for an exposure. Image-pixel signals read from the entire light-receiving area are converted to detection data corresponding to an exposure operation and a representative value (such as an average value) is calculated from the detection data (S 211 , S 212 ). Then, an exposure difference ΔEv is calculated (S 223 ). Note that ΔEv obtained by the first metering process represents a difference between the representative value and a reference or target value. 
         [0093]    In the case of a photographic situation in which a luminance level of a target object does not change sharply, the process proceeds to Steps S 203 , S 206 , and S 218  in the 2nd metering process to carry out the “1:1” pixel addition process. Then, an exposure value is corrected on the basis of the calculated exposure difference ΔEv (S 218 ), and the “1:1” pixel addition process is carried out (S 219 , S 220 ). Consequently, a representative value that represents an accurate brightness of the object image is detected through the performance of Steps S 221  and S 222 . 
         [0094]      FIG. 13  is a view showing a pixel addition process when starting the metering process. 
         [0095]    When performing the first metering process using the AE sensor  34 , an actual brightness level of an object has not been determined before the brightness detection process. Therefore, the “1:19” pixel addition process is carried out as described above to obtain two pixel data with different sensitivities that correspond to one frame&#39;s worth, respectively. Thus, a dynamic range covering the difference between a low luminance level and high luminance level is acquired and the exposure difference ΔEv at the first metering process is calculated as described above. 
         [0096]    Then, the initially set metering exposure values of the AE sensor  34 , such as an exposure time, a gain value, etc., are corrected in response to the calculated the exposure difference ΔEv. Then, in the next metering process, the “1:1” pixel addition process is carried out. Thus, image-pixel signals are generated in the AE sensor  34  on the basis of exposure values accurate for the actual brightness of an object image and read out from the AE sensor  34  so that a luminance value is precisely detected. 
         [0097]    On the other hand, when a luminance level of the object image changes rapidly during metering, detection data occasionally becomes saturated, resulting in inaccurate detection of the brightness of the object image. When this occurs, it is determined that the luminance level of the object image suddenly changes when the luminance level exceeds a given range, and the “1:19” pixel addition process is carried out to enlarge the dynamic range of the AE sensor  34 . 
         [0098]    Concretely speaking, when the absolute value of an exposure difference ΔEv is greater than or equal to a predetermined threshold value TE (S 206 ), the process proceeds to Step S 208 -S 212  in a condition that the face detection mode is not set. By performing the “1:19” pixel addition process, an exposure difference ΔEv is obtained similarly to the first metering process described above. Then, when the exposure difference ΔEv becomes smaller than the threshold value TE by correcting the exposure value of the AE sensor  34 , the process advances to Step S 218 - 222  to perform the “1:1” pixel addition process. Thus, the brightness of an object image can be detected precisely. 
         [0099]      FIG. 14  is a view showing a pixel addition process when the brightness of the object image rapidly changes. 
         [0100]    When the luminance level of an subject changes abruptly in a situation where the “1:1” pixel addition process is performed, the “1:19” pixel addition process is carried out to enlarge the dynamic range. Then, the exposure value of the AE sensor  34  is corrected and the luminance level of the object image is measured using the “1:1” pixel addition process. 
         [0101]    On the other hand, when the face detection mode is set, the light-receiving area of the AE sensor  34  is divided into a plurality of areas. If the number of pixel addition is relatively large, occasionally a situation occurs in which a face of an object cannot be detected due to a reduction in the resolution of an object image. Therefore, when the brightness of the object image rapidly changes in a situation in which the face detection mode is set, the “1:3” pixel addition process in which the number of pixel addition is relatively small is carried out. 
         [0102]    Concretely, when it is determined at Step S 207  that the face detection mode is set, the “1:3” pixel addition process is carried out (S 213 -S 217 ). Such a small number of pixel addition allows the AE sensor  34  to enlarge the dynamic range and detect the face of an object. 
         [0103]    In this way, in the fourth embodiment, the “1:19” pixel addition process is carried out when the metering process is started and an object luminance is calculated. Then, the exposure difference ΔEv is calculated and the exposure time of the AE sensor  34  is adjusted based on the exposure difference ΔEv. The “1:1” pixel addition process (not pixel addition) is carried out after the adjustment of the exposure time. An exposure value to the image sensor  30  is operated on the basis of an object luminance that is obtained by the “1:1” pixel addition process. 
         [0104]    Also, when an object luminance changes abruptly, the “1:19” pixel addition process is carried out and the exposure values including the exposure time and the gain value is adjusted. Then, the “1:1” pixel addition process is carried out after the adjustment of the exposure values. Furthermore, when the face detection mode is set, the “1:3” pixel addition process in which the number of pixel addition is relatively small is carried out. 
         [0105]    Since the number of pixel addition is switched or changed in response to a photograph situation, a change of an object luminance, a metering mode, and so on, the brightness of an object can be measured simultaneously. Especially, since the number of pixel addition is switched with respect to the number of pixel addition that is equal to or more than 2, an enlarged width of the dynamic range can be set minutely. Also, when electric power is turned on, the “1:19” pixel addition process is carried out at the same time. Thus, a user can photograph quickly after turning electric power on. 
         [0106]    The “1:19” pixel addition process is herein carried out only once, however, this process may be carried out repeatedly (e.g., twice or three times) so as to perform a feedback control. Especially, an exposure may be adjusted such that ΔEv becomes equal to or less than a tolerance value. The pixel addition process may be carried out along the column direction or the column and row directions by different numbers of pixel addition, similarly to the third embodiment. 
         [0107]    As for the number of pixel addition, numerical value other than “3” and “19” may be set. Also, as for the switching process of the number of pixel addition, a pixel addition process that performs for the whole of the light-receiving area by the same number including “1” may be included in the switching between some pixel addition processes. For example, the number of pixel addition “3”, “5”, or “9” may be selectively set for the whole of the light-receiving area in accordance to a photograph situation, and the number of pixel addition can be selected from the above numbers of pixel addition “3”, “5”, “9” and the different numbers of pixel addition (e.g., 1:3, 1:9). The pixel addition process that is performed along the row or column uniformly by using the same number of pixel addition causes effect of an enlargement of the dynamic range with respect to a specific object. Therefore, by switching between the pixel addition process based on the number of pixel addition (≧2) that is uniform for the whole of the light-receiving area and the pixel addition process based on the number of pixel addition that is different in the pixel area, various enlargements of the dynamic range can be realized. 
         [0108]    Next, the fifth embodiment is explained with reference to  FIGS. 15 to 17 . In the fifth embodiment, in a dark current reduction process based on OB (optical black) pixels, a pixel value correction based on the number of pixel addition is carried out. Other constructions are substantially the same as those in the first embodiment. 
         [0109]      FIG. 15  is a view showing a pixel array of the AE sensor according to the fifth embodiment. 
         [0110]    An AE sensor  234  has an effective pixel area  234 A and a shaded OB pixel area  234 B. A luminance level of the object image is calculated on the basis of image-pixel signals (effective image-pixel signals) read from pixels in the effective pixel area  234 A. On the other hand, dark current components included in the effective image-pixel signals are removed on the basis of OB image-pixel signals read from pixels in the OB pixel area  234 B.  FIG. 15  illustrates a portion of OB pixels OB1-OB6 that are arrayed at the right side of the effective pixel area  234 A and a portion of OB pixels OB7-OB10 that are arrayed at the bottom side of the effective pixel area  234 A. 
         [0111]      FIG. 16  is a view showing a pixel addition process along the row direction by different numbers of pixel addition.  FIG. 17  is a view showing a pixel addition process along the column direction by different numbers of pixel addition. 
         [0112]    In the present embodiment, OB image-pixel signals are mixed in response to the number of pixel addition. Concretely, as for mixed image-pixel signals A1+A2, B1+B2, C1+C2, D1+D2, . . . , OB image-pixel signals OB1 and OB2 are mixed and read out. ON the other hand, as for image-pixel signals A3, B3, C3, D3, . . . , OB image-pixel signals are directly read out. 
         [0113]    Then, pixel values are corrected, i.e., the mixed OB image-pixel signals OB1+OB2 are subtracted from the mixed image-pixel signals A1+A2, B1+B2, C1+C2, D1+D2, . . . , respectively. On the other hand, the OB image-pixel signal OB3 is subtracted from the image-pixel signals A3, B3, C3, D3, . . . , respectively. Image-pixel signals in other pixel areas are also subtracted or reduced by the corresponding OB image-pixel signals to remove dark current components. 
         [0114]      FIG. 17  depicts a reading process of OB image-pixel signals when image-pixel signals a read along the column direction by different numbers of pixel addition. As for image-pixel signals A1+B1, A2+B2, . . . , that are mixed along the column direction, OB image-pixel signals OB7 and 068 are mixed, and pixel values are corrected with the mixed OB image-pixel signals. As for image-pixel signals C1, C2, . . . , pixel values are corrected with the OB image-pixel signal OB9. 
         [0115]    In this way, in the fifth embodiment, the reading of image-pixel signals based on the number of pixel addition “2” and the reading of image-pixel signals based on the number of pixel addition “1” are performed for the effective pixels alternately. Accordingly, the reading of image-pixel signals based on the number of pixel addition that corresponds to the corresponding effective pixels is performed for the OB pixels. Then, the OB image pixel signal components are subtracted from the effective pixel signal components to remove dark current components. 
         [0116]    In the present embodiment, the reduction of a dark current component may be carried out by multiplying a given OB image-pixel signal by the number of pixel addition, instead of an mixture of neighboring image-pixel signals. For example, when the number of pixel addition is “2”, a pixel value of an OB image-pixel signals is multiplied by 2 to remove a dark current. 
         [0117]    Also, pixel values of image-pixel signals may be corrected on the basis of a representative value such as an average value of OB image-pixel signals. For example, an average value OBA of all of OB image-pixel signals along the row is calculated. Then, when image-pixel signals are based on the number of pixel addition is “2”, the average value is multiplied by 2 to correct a pixel value of an image-pixel signal, whereas the average value is directly used when image-pixel signals are based on the number of pixel addition is “1”. 
         [0118]    In the first to fifth embodiments, the above pixel addition process is performed by using the AE sensor for the SLR type digital camera that is provided in the optical finder. However, an external AE sensor for a compact type camera that is not provided in an optical finder may be applied. Also, the above pixel addition process may be a camera that detects the brightness of an object by an image sensor for photographing. 
         [0119]    Finally, it will be understood by those skilled in the arts that the foregoing description is of preferred embodiments of the device, and that various changes and modifications may be made to the present invention without departing from the spirit and scope thereof. 
         [0120]    The present disclosure relates to subject matter contained in Japanese Patent Applications No. 2013-227496 (filed on Oct. 31, 2013), No. 2013-227463 (filed on Oct. 31, 2013), and No. 2013-227405 (filed on Oct. 31, 2013), which are expressly incorporated herein by reference, in their entireties.