Patent Publication Number: US-2011050965-A1

Title: Image capture device and control method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image capture device that can take an image having a wide dynamic range, and a control method of the image capture device. 
     2. Description Related to the Prior Art 
     An image capture device that is provided with a solid-state image sensor such as a CCD or CMOS image sensor, e.g. a digital camera is widely available. The solid-state image sensor is typically required to have a large number of pixels and a wide dynamic range. As for the number of pixels, fine light receiving elements contribute to development of the solid-state image sensor of over ten million pixels, and the requirement is almost satisfied. As for the dynamic range, on the other hand, only structural improvement of the light receiving elements is not enough to adequately widen the dynamic range, because a charge storage capacity is decreased with reduction in size of the light receiving elements. Thus, an additional technique is necessary for widening the dynamic range. 
     Regarding the additional technique, the applicant discloses in Japanese Patent Laid-Open Publication No. 2007-235656 a solid-state image sensor that has a plurality of pairs of a first light receiving element and a second light receiving element, exposure times of which are separately controllable. In this solid-state image sensor, while the first light receiving elements capture a long exposure image with high sensitivity by being exposed for long time, the second light receiving elements capture a short exposure image with low sensitivity by being exposed for short time. In other words, the long exposure image captures a darker part of a scene, while the short exposure image captures a brighter part of the scene. Superimposing the long and short exposure images on each other produces a composite image having the wide dynamic range. Also, since the second light receiving elements are exposed during the exposure of the first light receiving elements, the simultaneousness between the long and short exposure images is obtained. 
     According to this technique, varying the ratio between the exposure time of the first light receiving elements and the exposure time of the second light receiving elements allows obtainment of the desired dynamic range. When the wide dynamic range is unnecessary, on the other hand, the exposure times of the first and second light receiving elements are equated. Output signals from the first and second light receiving elements are handled as separate pixel data that provides an image of high resolution. 
     In the Japanese Patent Laid-Open Publication No. 2007-235656, it is also proposed to emit flash light within the long exposure time and without the short exposure time, for the purpose of acquiring the wide dynamic range in flash photography. By emitting the flash light at this timing, a light amount (integrated exposure energy) is increased only during the exposure of the first light receiving elements, though the light amount is not changed during the exposure of the second light receiving elements. 
     In this case, the amount of flash light is determined based on the exposure time of the first light receiving elements. Although the first light receiving elements can receive an appropriate amount of flash light, the second light receiving elements cannot. Thus, this technique cannot achieve the desired dynamic range in the flash photography. 
     Accordingly, the applicant proposed in Japanese Patent Application No. 2009-203486 to adjust the timing of flash light emission, so that the ratio between the flash light amount produced during the long exposure and the flash light amount produced during the short exposure coincides with the ratio between the long exposure time and the short exposure time.  FIG. 10  shows an example of a timing chart according to the Japanese Patent Application No. 2009-203486. According to  FIG. 10 , the first light receiving elements of the CCD image sensor start being exposed at T 0 , and the second light receiving elements start being exposed at T 2 . Then, both of the first and second light receiving elements end the exposure at T 4 . A rising edge T 1  and a falling edge T 3  of a flash trigger signal is so determined that the ratio between the flash light amount produced during the long exposure time TL (T 0  to T 4 ) of the first light receiving elements and the flash light amount produced during the short exposure time TS (T 2  to T 4 ) of the second light receiving elements coincides with the ratio between the long exposure time TL and the short exposure time TS. 
     In this case, the timing T 4  of ending the exposure of the first and second light receiving elements is regulated by using a mechanical shutter. Thus, electric charges that are needlessly accumulated in vertical charge coupled devices (VCCDs) are abandoned by idle transfer operation and the VCCDs are refreshed, before read of signal charges accumulated in the first and second light receiving elements to the VCCDs. Therefore, the low noise composite image can be obtained with preventing the occurrence of smear and blooming. 
     The flash light amount, however, is gradually reduced at the falling edge, while being sharply increased at the rising edge, in general. Although this control method as shown in  FIG. 10  is effective at producing the low noise image, the flash light amount is likely to vary in the short exposure time TS that contains the falling edge T 3  of the flash trigger signal. Variations in the flash light amount produced during the short exposure time TS bring about variations in the dynamic range of the composite image. 
     Accordingly, a control method as shown in  FIG. 11  is conceivable. In this method, both of the first and second light receiving elements start being exposed at T 0 . The first light receiving elements end the exposure at T 4 , and the second light receiving elements end the exposure at T 2 . The rising edge of the flash light emission is set at T 1  within the short exposure time TS (T 0  to T 2 ), and the falling edge is set at T 3  without the short exposure time TS, in order to prevent the variations in the dynamic range. In this method, however, the signal charges of the second light receiving elements are read to the VCCDs at T 2 . Thus, the idle transfer operation of the VCCDs cannot be carried out after completion of the long exposure time TL, that is, after T 4 . Also, since the flash light emission is continued even after the read of the signal charges from the second light receiving elements to the VCCDs, electric charges that are generated in the second light receiving elements and peripheral circuits thereof flood into the VCCDs, and are added to the signal charges. Therefore, the control method of  FIG. 11  tends to cause the smear and the blooming, while can prevent the adverse effect of the variations in the flash light amount. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image capture device and a control method of the image capture device that can produce an image having a wide dynamic range and low noise in flash photography. 
     To achieve the above object, an image capture device according to the present invention includes a flash lamp unit for emitting flash light, a CCD image sensor, an exposure control section for controlling exposure of the CCD image sensor, a flash control section, a noise correction section, and an image composition section. The CCD image sensor includes first light receiving elements for capturing a long exposure image, second light receiving elements for capturing a short exposure image, first VCCDs to which first signal charges are read out from the first light receiving elements to transfer the read first signal charges in a vertical direction, second VCCDs to which second signal charges are read out from the second light receiving elements at a time different from the readout of the first signal charges in order to transfer the read second signal charges in the vertical direction, a HCCD connected to an end of each of the first and second VCCDs for horizontally transferring the first and second signal charges transferred through the first and second VCCDs, and an output section for converting the first and second signal charges transferred through the HCCD into an analog signal and outputting the analog signal. The exposure control section makes the CCD image sensor produce a first image signal from the first signal charges read out from the first light receiving elements after the exposure for a long exposure time, and makes the CCD image sensor produce a second image signal from the second signal charges read out from the second light receiving elements after the exposure for a short exposure time. The exposure control section makes the CCD image sensor produce a first noise signal from first noise charges accumulated in the first VCCDs, and makes the CCD image sensor produce a second noise signal from second noise charges accumulated in the second VCCDs. The flash control section controls timing of flash light emission by the flash lamp unit, so as to equate a ratio between a flash light amount produced during the long exposure time and a flash light amount produced during the short exposure time with a ratio between the long exposure time and the short exposure time. The noise correction section removes noise from the second image signal based on the second noise signal. The image composition section merges the first image signal with the second image signal after correction by the noise correction section, to produce image data. 
     The exposure control section may start exposing the first and second light receiving elements at the same time. The second signal charges may be read out from the second light receiving elements to the second VCCDs after a lapse of the short exposure time, and held in the second VCCDs. The first signal charges may be read out from the first light receiving elements to the first VCCDs after a lapse of the long exposure time. The first signal charges are transferred by normal transfer operation together with the second signal charges that have been held in the second VCCDs. Just before the readout of the second signal charges from the second light receiving elements, the first and second VCCDs and the HCCD may be driven to transfer the first and second noise charges accumulated during the short exposure time in the first and second VCCDs by idle transfer operation. The noise correction section may calculate a second correction signal by multiplying the second noise signal produced from the second noise charges by a coefficient obtained based on the ratio between the long exposure time and the short exposure time. The noise correction section subtracts the second correction signal from the second image signal, and outputs the corrected second image signal. The second correction signal corresponds to the amount of noise charges added to the second signal charges, while the second signal charges are held in the second VCCDs. 
     The noise correction section may calculate a first correction signal by multiplying the first noise signal produced from the first noise charges by the coefficient obtained based on the ratio between the long exposure time and the short exposure time. The noise correction section subtracts the first correction signal from the first image signal, and outputs the corrected first image signal. The first correction signal corresponds to the amount of noise charges accumulated in the first VCCDs until an end of the long exposure time. The image composite section merges the corrected first image signal with the corrected second image signal to produce the image data. 
     It is preferable that a speed of the idle transfer operation be higher than that of the normal transfer operation. 
     Each of the first and second VCCDs may have a plurality of rows. In the idle transfer operation, out of all of the rows included in each of the first and second VCCDs, the first and second noise charges accumulated in a beginning predetermined number of rows may be transferred at a normal speed, while the first and second noise charges accumulated in the remaining rows are transferred at a high speed. Otherwise, the first and second noise charges accumulated in the beginning predetermined number of rows may be transferred at the normal speed, while the first and second noise charges accumulated in the remaining rows are left in the first and second VCCDs without being transferred. In either case, the first and second noise signals are produced from the first and second noise charges transferred at the normal speed. 
     It is preferable that the CCD image sensor have an electronic shutter function for simultaneously discharging the first and second signal charges accumulated in the first and second light receiving elements into a semiconductor substrate for reset. The electronic shutter function is activated before starting the exposure of the first and second light receiving elements. 
     The image capture device may further include an operation unit for setting a value of a dynamic range. The ratio between the long exposure time and the short exposure time is determined based on the set value of the dynamic range. 
     In the CCD image sensor, the first light receiving elements may be arranged in a matrix along the vertical and horizontal directions, and the second light receiving elements may be arranged in a matrix at a same pitch as that of the first light receiving elements along the vertical and horizontal directions. The first and second light receiving elements may be staggered in the vertical and horizontal directions. The first and second VCCDs extending in the vertical direction may be disposed alternately in the horizontal direction. The HCCD may extend in the horizontal direction. 
     A Bayer color filter including blue, green, and red may be disposed on the first light receiving elements, and another Bayer color filter including blue, green, and red may be disposed on the second light receiving elements. 
     A method for controlling an image capture device, having a flash lamp unit and a CCD image sensor, includes the steps of starting exposing the first and second light receiving elements at the same time; just before a lapse of a short exposure time, driving first and second VCCDs and a HCCD to transfer first noise charges accumulated in the first VCCDs and second noise charges accumulated in the second VCCDs by idle transfer operation, and producing a first noise signal from the first noise charges and producing a second noise signal from the second noise charges; after the lapse of the short exposure time, reading out second signal charges from the second light receiving elements to the second VCCDs, and holding the second signal charges in the second VCCDs; after a lapse of a long exposure time, reading out first signal charges from the first light receiving elements to the first VCCDs; after the readout of the first signal charges, driving the first and second VCCDs and the HCCD to transfer the first signal charges read out from the first light receiving elements and the second signal charges held in the second VCCDs by normal transfer operation, and producing a first image signal from the first signal charges and producing a second image signal from the second signal charges; controlling timing of flash light emission from a flash lamp unit so as to equate the ratio between a flash light amount produced during the long exposure time and a flash light amount produced during the short exposure time with the ratio between the long exposure time and the short exposure time; calculating a second correction signal by multiplying the second noise signal by a coefficient based on the ratio between the long exposure time and the short exposure time; subtracting the second correction signal from the second image signal, and outputting the corrected second image signal; and merging the first image signal with the corrected second image signal to produce image data. 
     According to the present invention, it is possible to obtain an image having a wide dynamic range and low noise in flash photography. In the present invention, the idle transfer operation for obtaining the noise signals is carried out while the first and second light receiving elements are exposed, and hence does not require increase in processing time for noise correction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For more complete understanding of the present invention, and the advantage thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a digital camera according to a first embodiment of the present invention; 
         FIG. 2  is a top plan view of a CCD image sensor; 
         FIG. 3  is a timing chart for explaining a control method of the digital camera; 
         FIG. 4  is a block diagram of a noise correction section and an image composition section; 
         FIGS. 5A to 5C  are explanatory views of noise correction processing; 
         FIG. 6  is a flowchart of the operation of the digital camera; 
         FIG. 7  is a timing chart for explaining a control method of a digital camera according to a second embodiment; 
         FIG. 8  is a timing chart for explaining a control method of a digital camera according to a third embodiment; 
         FIG. 9  is a block diagram of a noise correction section according to a fourth embodiment; 
         FIG. 10  is a timing chart of a prior art; and 
         FIG. 11  is a timing chart of another prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     As shown in  FIG. 1 , a digital camera  10  is provided with a taking lens  11 , a CCD image sensor  12 , an aperture stop  13  disposed between the taking lens  11  and the CCD image sensor  12 , and a mechanical shutter  14  disposed in front of the taking lens  11 . To the taking lens  11 , a lens driver  15  is connected. To the CCD image sensor  12 , an image sensor driver  16  is connected. An aperture stop driver  17  is connected to the aperture stop  13 , and a shutter driver  18  is connected to the mechanical shutter  14 . 
     A CPU  19  controls the whole electric control system of the digital camera  10  based on an operation signal from an operation unit  20 . The CPU  19  includes a flash control section  19   a , a focus control section  19   b , and an exposure control section  19   c . The flash control section  19   a  controls the timing and amount of flash light emission from a flash lamp unit  21 . The focus control section  19   b  commands the lens driver  15  to shift the taking lens  11  to obtain correct focus. The exposure control section  19   c  determines an f-stop number and a shutter speed (exposure value EV), and sends command signals to the aperture stop driver  17  and the image sensor driver  16 . The image sensor driver  16  drives the CCD image sensor  12  based on the command signal. Thus, the CCD image sensor  12  captures an object image through the taking lens  11 , and outputs an image signal. 
     The digital camera  10  is provided with an analog signal processing section  22  and an analog-to-digital converter (A/D)  23 , which are controlled by the CPU  19 . The analog signal processing section  22  applies various analog signal processes including correlated double sampling to the image signal outputted from the CCD image sensor  12 . The A/D  23  converts an output signal (RGB color signals) from the analog signal processing section  22  into a digital signal. 
     The digital camera  10  is also provided with a memory control section  25  connected to an image memory  24 , a digital signal processing section  26 , a compression/decompression processing section  27 , a recording medium control section  29 , and a display control section  31 . The digital signal processing section  26  carries out a color interpolation process, a gamma correction process, an RGB/YC conversion process, and the like, in addition to a noise correction process and an image composition process described later on. The compression/decompression processing section  27  compresses image data into a JPEG file, and decompresses the JPEG file. The recording medium control section  29  writes the JPEG file to a removable recording medium  28 , and reads the JPEG file from the recording medium  28 . The display control section  31  controls display of the image data and the like on a liquid crystal display (LCD)  30  provided on a rear face of a camera body. Every part described above is connected to one another through a control bus  32  and a data bus  33 , and is controlled by the CPU  19 . 
     The operation unit  20  includes a shutter release button for carrying out shutter release operation, a mode dial for switching among a plurality of operation modes, a menu button for displaying setting items on the LCD  30 , an enter button for choosing and entering the setting item, and the like. A user&#39;s command from the operation unit  20  is inputted to the CPU  19  as the operation signal. The CPU  19  carries out various control operations in response to the operation signal. 
     The shutter release button is a two-step button switch. With the use of auto-focusing (AF) and auto-exposure (AE) functions, the focus control section  19   b  and the exposure control section  19   c  carry out an AF process and an AE process, respectively, in response to a half push of the shutter release button. Then, upon a full push of the shutter release button, the CCD image sensor  12  captures the object image. 
     The digital camera  10  has a plurality of operation modes, among which the digital camera  10  is switchable with the mode dial. The operation modes include “a wide dynamic range mode” for capturing an image having a wide dynamic range, and “a high resolution mode” for capturing an image of high resolution without widening the dynamic range, and the like. 
     In the wide dynamic range mode, the dynamic range is chosen among, for example, 200%, 400%, and 800%. Also, whether or not to emit flash light from the flash lamp unit  21  is set with the operation unit  20 . 
     In flash photography, the flash control section  19   a  makes the flash lamp unit  21  emit the flash light with timing described later, in synchronization with the full push of the shutter release button. 
     As shown in  FIG. 2 , the CCD image sensor  12  is fabricated on a semiconductor substrate along vertical and horizontal directions. The CCD image sensor  12  is constituted of a plurality of light receiving elements (photodiodes)  40 , a plurality of vertical charge coupled devices (VCCDs)  41 , a horizontal charge coupled device (HCCD)  42 , and an output section  43 . Each light receiving element  40  photoelectrically converts object light into a signal charge. Each VCCD  41  transfers the signal charges generated by the light receiving elements  40  in a vertical direction. The HCCD  42  is connected to an end of every VCCD  41 , to transfer the signal charges that have been vertically transferred by the VCCDs  41  in the horizontal direction. The output section  43  converts the signal charges transferred by the HCCD  42  into an analog signal, and outputs the analog signal. 
     The light receiving elements  40  include first light receiving elements  40   a  indicated with R 1 , G 1 , and B 1 , and second light receiving elements  40   b  indicated with R 2 , G 2 , and B 2 . The first light receiving elements  40   a  are arranged in a tetragonal lattice structure. The second light receiving elements  40   b  are also arranged in the tetragonal lattice structure at the same intervals as that of the first light receiving elements  40   a . The first and second light receiving elements  40   a  and  40   b  are staggered in both of the vertical and horizontal directions, so as to have a so-called honeycomb structure on the whole. 
     The first light receiving elements  40   a  have a Bayer color filter, in which green (G 1 ) and blue (B 1 ) are alternately arranged in one direction, and the green (G 1 ) and red (R 1 ) are alternately arranged in a direction orthogonal thereto. Likewise, the second light receiving elements  40   b  also have a Bayer color filter, in which green (G 2 ) and blue (B 2 ) are alternately arranged in one direction, and green (G 2 ) and red (R 2 ) are alternately arranged in a direction orthogonal thereto. Thus, the first and second light receiving elements  40   a  and  40   b  have a double-Bayer color filter on the whole, into which the two Bayer color filters are combined. Every light receiving element  40   a ,  40   b  is composed of a photodiode, and basically has the same structure (same device size, opening size, junction depth, charge storage capacity, and the like), except for the color of the color filter. 
     A first VCCD  41   a  is provided for every single column of the first light receiving elements  40   a  in the vertical direction. A second VCCD  41   b  is provided for every single column of the second light receiving elements  40   b  in the vertical direction. As schematically shown by arrows in  FIG. 2 , charge reading gates are formed between each first light receiving element  40   a  and a charge transfer channel (not illustrated) of the first VCCD  41   a , and between each second light receiving element  40   b  and a charge transfer channel (not illustrated) of the second VCCD  41   b . Thus, first and second signal charges generated by and accumulated in the first and second light receiving elements  40   a  and  40   b , respectively, for the exposure times are read into the charge transfer channels of the first and second VCCDs  41   a  and  41   b  through the charge reading gates. 
     The charge transfer channels of the first and second VCCDs  41   a  and  41   b  are curvedly routed along the vertical direction so as to navigate around the first and second light receiving elements  40   a  and  40   b  in a surface layer of the semiconductor substrate. On a surface of the semiconductor substrate, vertical transfer electrodes V 1  to V 8  are curvedly formed along the horizontal direction across the charge transfer channels of the first and second VCCDs  41   a  and  41   b  so as to navigate around the first and second light receiving elements  40   a  and  40   b . The first and second VCCDs  41   a  and  41   b  are driven by vertical transfer pulses φV 1  to φV 8 , which are supplied by the image sensor driver  16  to the vertical transfer electrodes V 1  to V 8 , respectively. 
     The charge reading gates of the first light receiving elements  40   a  adjoin the vertical transfer electrodes V 3  and V 7 . The charge reading gates of the second light receiving elements  40   b  adjoin the vertical transfer electrodes V 1  and V 5 . Thus, to read the first signal charges from the first light receiving elements  40   a  to the charge transfer channels of the first VCCDs  41   a , a readout pulse is applied to the vertical transfer electrodes V 3  and V 7 . Likewise, to read the second signal charges from the second light receiving elements  40   b  to the charge transfer channels of the second VCCDs  41   b , a readout pulse is applied to the vertical transfer electrodes V 1  and V 5 . The signal charges, as described above, are separately read out from the first and second light receiving elements  40   a  and  40   b  with different timing by the application of the readout pulse to the different vertical transfer electrodes. 
     The HCCD  42  is constituted of a charge transfer channel and a plurality of horizontal transfer electrodes formed on the charge transfer channel, though neither is illustrated. The HCCD  42  is driven by two phase horizontal transfer pulses φH 1  and φH 2  outputted from the image sensor driver  16 . The output section  43  connected to an end of the HCCD  42  is constituted of an FD amplifier. The FD amplifier includes a floating diffusion section for converting the signal charge into a voltage and a source follower circuit. 
     Also, the CCD image sensor  12  has vertical overflow drains (VODs) through which electric charges needlessly accumulated in the first and second light receiving elements  40   a  and  40   b  are discharged into the semiconductor substrate. A charge reset function by the VODs is referred to as an electronic shutter. In response to electronic shutter pulses φSUB inputted from the image sensor driver  16  to the semiconductor substrate, a potential barrier formed in the bottom of each of the first and second light receiving elements  40   a  and  40   b  is reduced, so as to discharge the accumulated electric charge into the semiconductor substrate at a time. 
     Next, a control method of the CCD image sensor  12 , the mechanical shutter  14 , and the flash lamp unit  21  in the flash photography in a wide dynamic range mode will be described. As shown in  FIG. 3 , while the mechanical shutter  14  is kept open, application of the electronic shutter pulses φSUB is stopped at T 0  to start exposure of the first and second light receiving elements  40   a  and  40   b.    
     The mechanical shutter  14  is closed at T 4  after a lapse of predetermined time from start of the exposure. At T 5  after T 4 , the first signal charges are read out from the first light receiving elements  40   a  in response to the application of the readout pulse to the vertical transfer electrodes V 3  and V 7 . Thus, the exposure time (long exposure time) TL of the first light receiving elements  40   a  is defined as a period from T 0  to T 4 . 
     The second signal charges, on the other hand, are read out from the second light receiving elements  40   b  at T 2 , within the exposure time TL of the first light receiving elements  40   a , in response to the application of the readout pulse to the vertical transfer electrodes V 1  and V 5 . The exposure time (short exposure time) TS of the second light receiving elements  40   b  is defined as a period from T 0  to T 2 . 
     The second signal charges that have been readout at T 2  from the second light receiving elements  40   b  are held in the second VCCDs  41   b . After the first signal charges are read out at T 5  to the first VCCDs  41   a , vertical transfer pulses φV 1  to φV 8  and horizontal transfer pulses φH 1  and φH 2  are applied, so that the first and second VCCDs  41   a  and  41   b  and the HCCD  42  transfer the first and second signal charges to the output section  43  (normal transfer operation). The second signal charges readout from the second light receiving elements  40   b , however, contain noise charges that have occurred by intensive incident light due to the flash light, while the second signal charges are being held in the second VCCDs  41   b , that is, between T 2  and T 4 , as it will be described later in detail. The output section  43  converts the signal charges into an image signal of a single frame, and outputs the image signal. 
     Immediately before T 2 , in other words, immediately before the read of the second signal charges from the second light receiving elements  40   b , high-speed idle transfer operation is carried out in the first and second VCCDs  41   a  and  41   b  and the HCCD  42 , by the application of the vertical transfer pulses φV 1  to φV 8  and the horizontal transfer pulses φH 1  and φH 2  at higher frequency than that of the normal transfer operation. Thus, a noise signal produced by noise charges accumulated by smear or blooming in the first and second VCCDs  41   a  and  41   b  for the short exposure time TS is outputted from the output section  43 . 
     A flash trigger signal for actuating the flash lamp unit  21  is applied from T 1  to  13 , so that a period of flash light emission is within the long exposure time TL and partially overlaps the short exposure time TS. The timing of T 1  and T 3  is so determined by the flash control section  19   a  that the ratio between the flash light amount produced during the long exposure time TL and the flash light amount produced during the short exposure time TS coincides with the ratio between the long exposure time TL and the short exposure time TS. The flash control section  19   a  may determine the timing of T 1  and T 3  with referring to a table that shows the relation between the timing of T 1  and T 3  and the ratio between the long and short exposure times TL and TS. 
     The ratio between the long exposure time TL and the short exposure time TS is determined by the CPU  19  in accordance with a set value of the dynamic range. If the set value of the dynamic range is 400%, for example, the ratio between the long exposure time TL and the short exposure time TS is set at 4:1. 
     As shown in  FIG. 4 , the digital signal processing section  26  includes a noise correction section  50  and an image composition section  51 . The noise correction section  50  is constituted of an averaging circuit  52 , a coefficient setting circuit  53 , a multiplier  54 , and a subtractor  55 . 
       FIG. 5A  schematically shows first noise charges  104  accumulated in the first VCCDs  41   a  for the short exposure time TS, and second noise charges  106  accumulated in the second VCCDs  41   b  for the short exposure time TS. By the high-speed idle transfer operation just before T 2 , the noise signal is outputted. The noise signal includes a first noise signal produced from the first noise charges  104  and a second noise signal produced from the second noise charges  106 . The first and second noise signals are written to the image memory  24 . The second noise signal is also inputted to the averaging circuit  52  of the noise correction section  50 . The averaging circuit  52 , as shown in  FIG. 5B , averages the second noise signal, that is, noise charge amounts accumulated in the second VCCDs  41   b , on a second VCCD  41   b  basis, to calculate an average noise signal. In this embodiment, the averaging circuit  52  averages, for example, the amounts of two thousand noise charges, the number of which corresponds to the total number of the second light receiving elements  40   b  aligned in the vertical direction, on a second VCCD  41   b  basis. 
     As described above, the ratio between the flash light amount produced during the long exposure time TL and the flash light amount produced during the short exposure time TS is equal to the ratio between the long exposure time TL and the short exposure time TS. Thus, the ratio between the noise charge amount accumulated in the second VCCD  41   b  for the long exposure time TL and that for the short exposure time TS due to the flare and blooming equates to the ratio between the long exposure time TL and the short exposure time TS. 
     The CPU  19  sets an exposure time coefficient R on the coefficient setting circuit  53 . The exposure time coefficient R is defined as TL/TS−1, and is calculated by the ratio between the long exposure time TL and the short exposure time TS. Taking a case where the set value of the dynamic range is 400% as an example, since the ratio between the long exposure time TL and the short exposure time TS is 4:1, “3” is set on the coefficient setting circuit  53  as the exposure time coefficient R. The multiplier  54  multiplies the average noise signal by the exposure time coefficient R set on the coefficient setting circuit  53 , to produce a correction signal (second correction signal). In the case of the exposure time coefficient R of “3”, the correction signal is a triple of the average noise signal. This correction signal mathematically corresponds to the noise charge amounts accumulated in the second VCCD  41   b  in a period from T 2  to T 4 . 
       FIG. 5C  schematically shows electric charges held at T 5  by the first and second VCCDs  41   a  and  41   b . To the first VCCDs  41   a , first signal charges  100  generated by the first light receiving elements  40   a  during the long exposure time TL are read out in response to a readout pulse at T 5 . Each electric charge  102  held in the second VCCDs  41   b  is an addition of the noise charge that has accumulated in the second VCCDs  41   b  between T 2  and T 4  to the second signal charge read out at T 2  from the second light receiving element  40   b.    
     A first image signal is produced from the first signal charges  100  of the first VCCDs  41   a , and a second image signal is produced from the electric charges  102  of the second VCCDs  41   b . The first and second image signals are recorded on the image memory  24 . The subtractor  55  subtracts the corresponding correction signal from the second image signal on a second VCCD  41   b  basis. The correction signal, as described above, corresponds to the noise charges accumulated in each second VCCD  41   b  between T 2  and T 4 . Accordingly, the subtraction eliminates the effect of the noise charges from the second image signal, and brings about obtainment of the corrected second image signal that corresponds to only the second signal charges. 
     The image composition section  51  merges the first image signal (long exposure image signal) with the corrected second image signal (short exposure image signal), so as to merge the electric charges from the first and second light receiving elements  40   a  and  40   b  having the same color filter from pair to pair, as shown by broken lines in  FIG. 5C . The first image signal is high-sensitivity image data by the long exposure, and the second image signal is low-sensitivity image data by the short exposure. To carry out a merge process, as disclosed in Japanese Patent Laid-Open Publication No. 2007-235656, after a saturation voltage of the high-sensitivity image data is equalized with that of the low-sensitivity image data by signal slicing, data of the corresponding pixels of the same color is added up, and a composite signal becomes image data having a wide dynamic range. The digital signal processing section  26  applies to the image data the color interpolation process, the gamma correction process, the RGB/YC conversion process, and the like, as described above. 
     In the high resolution mode, the CPU  19  drives the second light receiving elements  40   b  at the same timing as the first light receiving elements  40   a , to equate the exposure times of both of the first and second light receiving elements  40   a  and  40   b . In this case, the high-speed idle transfer operation is not carried out before T 2 . The high-speed idle transfer operation is carried out between T 4  and T 5  instead, to refresh the VCCDs  41 . In the high resolution mode, the digital signal processing section  26  does not carry out the noise correction process and the image composition process as described above, but treats every first and second light receiving elements  40   a  and  40   b  as an equal pixel to produce image data of high resolution. 
     Next, the operation of the digital camera  10  will be described with referring to a flowchart of  FIG. 6 . The CPU  19  first judges whether or not the wide dynamic range mode is chosen with the mode dial (S 1 ). If the wide dynamic range mode is chosen (YES in S 1 ), steps S 3  to S 12  are carried out. If another mode is chosen (NO in S 1 ), processes of the chosen mode are carried out (S 2 ). 
     Upon detecting the half push of the shutter release button (YES in S 3 ), the CPU  19  notifies the focus control section  19   b  and the exposure control section  19   c  of the detection of the half push. In response to the notification, the exposure control section  19   c  carries out the AE process, and the focus control section  19   b  carries out the AF process (S 4 ). The CPU  19  sets the f-stop number and the shutter speed (EV) based on a result of the AE process (S 5 ). 
     The shutter speed determines the long exposure time TL of the first light receiving elements  40   a . The short exposure time TS of the second light receiving elements  40   b  is determined based on the set value of the dynamic range. For example, if the set value of the dynamic range is 200%, “TS=TL/2” holds. If the set value of the dynamic range is 400%, “TS=TL/4” holds. If the set value of the dynamic range is 800%, “TS=TL/8” holds. The set value of the dynamic range is manually inputted with the operation unit  20 , but may be automatically set in accordance with a photographed scene or the like. 
     Then, in response to detection of the full push of the shutter release button (YES in S 6 ), the CPU  19  judges whether or not the flash light from the flash lamp unit  21  is necessary (S 7 ). If the flash light is unnecessary (NO in S 7 ), processes of a non-flash light mode are carried out (S 2 ). 
     If the flash light is necessary (YES in S 7 ), on the other hand, the CPU  19  determines the timing T 1  to T 5  of actuation of individual parts illustrated in  FIG. 3  (S 8 ). The CPU  19  actuates the CCD image sensor  12 , the mechanical shutter  14 , and the flash lamp unit  21  to carry out flash photography operation as described above (S 9 ). The first and second noise signals and the first and second image signals outputted from the CCD image sensor  12  are processed by the analog signal processing section  22  and the A/D converter  23 , and are written to the image memory  24  by the memory control section  25 . 
     Then, the digital signal processing section  26  reads the first and second noise signals and the first and second image signals from the image memory  24 . The noise correction section  50  carries out the above noise correction process (S 10 ). In the noise correction process, the correction signal is produced based on the second noise signal obtained by the high-speed idle transfer operation. This correction signal corresponds to the amount of noise charges that are needlessly accumulated between T 2  and T 4  shown in  FIG. 3  due to the effect of the flash light. The correction signal is subtracted from the second image signal outputted from the second light receiving elements  40   b.    
     The first image signal and the corrected second image signal are merged by the image composition section  51  to obtain the image data having the wide dynamic range (S 11 ). The image data is subjected to the various signal processes and a compression process, and is then written to the recording medium  28  by the recording medium control section  29  (S 12 ). 
     Second Embodiment 
     In the first embodiment, the second noise signal is produced from a total row number (for example, two thousand) of noise charges that are transferred by the high-speed idle transfer operation just before T 2 , that is, just before reading out the signal charges from the second light receiving elements  40   b . In a second embodiment, when M refers to the total row number of the second light receiving elements  40   b , a first N number (for example, twenty) of noise charges are transferred by the idle transfer operation at a normal frequency, and then a remaining (M−N) number of noise charges are transferred by the high-speed idle transfer operation at a high frequency just before T 2 , as shown in  FIG. 7 . 
     The second noise signal is produced from the N number of noise charges transferred by the idle transfer operation at normal speed. The CPU  19  writes the second noise signal to the image memory  24 , and abandons a signal produced from the noise charges transferred by the high-speed idle transfer operation. The averaging circuit  52  of the noise correction section  50  averages the second noise signal on a second VCCD  41   b  basis, and outputs the average noise signal. The multiplier  54  multiplies the average noise signal by the exposure time coefficient R set on the coefficient setting circuit  53 , to obtain the correction signal. Since the noise charges accumulated in the same VCCD hardly vary in general in the vertical direction, even the correction signal produced from only the N number of noise charges has sufficient accuracy. The other components of the second embodiment are the same as those of the first embodiment, and description thereof will be omitted. 
     As described above, in the second embodiment, the second noise signal is produced from the N number of noise charges transferred at the normal speed. Thus, the noise charges are less prone to degradation in comparison with the case of transferring a large number of noise charges at the high speed, and hence the correction signal with high accuracy is obtained. This results in improvement in accuracy of the image data having the wide dynamic range. 
     Third Embodiment 
     In a third embodiment, when M refers to the total row number of the second light receiving elements  40   b , a first N number of noise charges are transferred at the normal speed just before T 2 , that is, just before reading out the signal charges from the second light receiving elements  40   b , as shown in  FIG. 8 , while a remaining (M−N) number of noise charges are not transferred. In this case, out of the noise charges accumulated in the second VCCD  41   b  by T 2 , only the N number of noise charges are transferred, while the remaining (M−N) number of noise charges remain in the second VCCD  41   b . Thus, the (M−N) number of noise charges remaining in the second VCCD  41   b  are added to the second signal charges read out from the second light receiving elements  40   b.    
     The number N is set smaller than the number M (for example, M=2000 and N=20). Thus, at T 2 , most of the noise charges that have been accumulated during the short exposure time TS remain in the second VCCD  41   b . Accordingly, in the third embodiment, the noise correction section  50  produces the correction signal that corresponds to the noise charges accumulated in a period between T 0  and T 4  in the second VCCD  41   b . To be more specific, an exposure time coefficient R′=TL/TS is set on the coefficient setting circuit  53 , instead of the exposure time coefficient R=TL/TS−1. The other components of the third embodiment are the same as those of the first embodiment, and description thereof will be omitted. 
     As described above, in the third embodiment, since only the N number of noise charges are transferred between T 0  and T 2 , the short exposure time TS of the second light receiving elements  40   b  can be more shortened. Therefore, the variable range of the ratio between the long exposure time TL and the short exposure time TS becomes wider, and a wider dynamic range can be obtained. 
     Fourth Embodiment 
     In the above first embodiment, out of the first and second image signals outputted from the CCD image sensor  12 , noise correction is applied to only the second image signal, being the short exposure image signal. In a fourth embodiment, the noise correction is applied not only to the second image signal but also to the first image signal, being the long exposure image signal. 
       FIG. 9  shows a noise correction section  60  according to the fourth embodiment. The noise correction section  60  is identical to the noise correction section  50  of the first embodiment, except that it has another averaging circuit  61 , multiplier  62 , and subtractor  63  for processing the first image signal. 
     The averaging circuit  61 , as with the averaging circuit  52 , averages the noise charges that have been accumulated in the first VCCDs  41   a , that is, the first noise signal on a first VCCD  41   a  basis, and calculates an average noise signal. The multiplier  62  multiplies the average noise signal by the exposure time coefficient R set on the coefficient setting circuit  53  to obtain a correction signal (first correction signal). The subtractor  63  subtracts the correction signal from the first image signal. As for the second image signal, the averaging circuit  52 , the multiplier  54 , and the subtractor  55  carry out the noise correction, as in the case of the first embodiment. 
     To the image composition section  51 , the first and second image signals corrected by the noise correction section  60  are inputted. The image composition section  51  produces image data having a wide dynamic range by the composition process as described above. The other components of the fourth embodiment are the same as those of the first embodiment, and description thereof will be omitted. 
     As described above, in the fourth embodiment, since the noise correction is applied not only to the second image signal being the short exposure image signal but also to the first image signal being the long exposure image signal. This results in improvement in accuracy of the image data having the wide dynamic range. The noise correction section  60  may be apply to the second and third embodiments, in order to remove noise from the first image signal, in addition to the second image signal. 
     In the first to fourth embodiments, before reading out the signal charges from the first light receiving elements  40   a , the mechanical shutter  14  is closed at T 4  to define an end of the long exposure time TL. However, the end of the long exposure time TL may be defined by the timing T 5  of input of the readout pulse for reading out the signal charges from the first light receiving elements  40   a , instead of closing the mechanical shutter  14 . 
     Although the present invention has been fully described by the way of the preferred embodiment thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.