Abstract:
An image sensor driving unit, comprising a first controller, a second controller and a third controller, is provided. The image sensor driving unit drives an image sensor to carry out the capture of an image. The capture is carried out by ordering pixels to generate signal charges and the charge-transfer channel to transfer the signal charges. The first controller orders the image sensor to carry out a rapid discharge operation before the charge-transfer channel transfers the signal charges. The second controller controls the first controller to order the image sensor to carry out the rapid discharge operation when light is made incident for capture after the first capture with the image sensor operating in continuous photographing mode. The third controller decreases the discharge number for capture after the first capture.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an image sensor driving unit that orders an image sensor to rapidly discharge an unnecessary electrical charge generated by a charge-transfer channel, such as a CCD, before a photographing operation. 
         [0003]    2. Description of the Related Art 
         [0004]    Various kinds of image sensors that generate an image signal corresponding to an optical image of a subject are known. Among the various kinds of image sensors, a CCD image sensor is commonly used owing to its adjustable size, relatively high S/N ratio, sensitivity, and various other desirable attributes. 
         [0005]    A CCD image sensor outputs pixel signals according to the amount of light received by the pixels by ordering a vertical CCD to receive signal charges generated by a plurality of pixels separately, by ordering the vertical CCD to transfer the signal charges to a horizontal CCD, and by ordering the horizontal CCD to transfer the signal charges to an output amplifier. 
         [0006]    The vertical CCD happens to store the electrical charge which the vertical CCD generates itself when light is leaked to the vertical CCD, and from an electrical charge left upon transferring an electrical charge exceeding the transferring capacity, from an electrical charge leaked from a pixel, and so on. Such an electrical charge becomes noise in a signal charge, and should be discharged to display an accurate image. 
         [0007]    Japanese Unexamined Patent Publication No. H04-356879 discloses the rapid discharge of electrical charges that remain in the vertical CCD before the vertical CCD receives and transfers signal charges from pixels. 
         [0008]    The influence of noise can be reduced by the rapid discharge. However, the time required to discharge electrical charges from the vertical CCD is added to the time to complete a photographing operation from depressing a release button. Accordingly, the entire time to complete the photographing operation is undesirably prolonged. Especially in the case of continuous photographing, it is desirable to increase the amount of photographing per a certain time by completing the photographing operation rapidly. 
       SUMMARY OF THE INVENTION 
       [0009]    Therefore, an object of the present invention is to provide an image sensor driving unit that shortens the time it takes to complete a photographing operation by discharging an electrical charge stored in a charge-transfer channel, such as a CCD. 
         [0010]    According to the present invention, an image sensor driving unit, comprising a first controller, a second controller, and a third controller, is provided. The image sensor driving unit drives an image sensor to carry out a capture of an image. The image sensor has a plurality of pixels and a charge-transfer channel. The pixels generate signal charges according to amounts of received light. The charge-transfer channel reads out the signal charges from the pixels and transfers the signal charges. The capture is carried out by ordering the pixels to generate the signal charges and the charge-transfer channel to transfer the signal charges. The first controller orders the image sensor to carry out a rapid discharge operation before the charge-transfer channel reads out and transfers the signal charges. An electrical charge remaining in the charge-transfer channel is discharged in the rapid discharge operation. The second controller controls the first controller to order the image sensor to carry out the rapid discharge operation when light is made incident on the pixels for the capture after the first capture with the image sensor operating in continuous photographing mode. The third controller varies a discharge number. The discharge number is the number of the rapid discharge operations to be carried out. The third controller decreases the discharge number for the capture after the first capture from the discharge number for the first capture. 
         [0011]    According to the present invention, an imaging apparatus, comprising an image sensor, a first controller, a second controller, and a third controller, is provided. The image sensor has a plurality of pixels and a charge-transfer channel. The pixels generate signal charges according to amounts of received light. The charge-transfer channel reads out the signal charges from the pixels and transfers the signal charges. The image sensor carries out a capture of an image. The capture is carried out by ordering the pixels to generate the signal charges and the charge-transfer channel to transfer the signal charges. The first controller orders the image sensor to carry out a rapid discharge operation before the charge-transfer channel reads out and transfers the signal charges. An electrical charge remaining in the charge-transfer channel is discharged in the rapid discharge operation. The second controller controls the first controller to order the image sensor to carry out the rapid discharge operation when light is made incident on the pixels for the capture after the first capture with the image sensor operating in continuous photographing mode. The third controller varies a discharge number. The discharge number is the number of the rapid discharge operations to be carried out. The third controller decreases the discharge number for the capture after the first capture from the discharge number for the first capture. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
           [0013]      FIG. 1  is a block diagram showing the internal structure of a single-lens reflex camera having the image sensor driving unit of the embodiments of the present invention; 
           [0014]      FIG. 2  is a schematic diagram showing the structure of the image sensor; 
           [0015]      FIG. 3  is a deployment diagram showing the first-fourth electrodes; 
           [0016]      FIG. 4  is a timing chart illustrating the release operation in the single photographing mode; 
           [0017]      FIG. 5  is a timing chart illustrating the release operation in the continuous photographing mode; 
           [0018]      FIG. 6  is a flowchart illustrating the process for the single release control carried out by the CPU; and 
           [0019]      FIG. 7  is a flowchart illustrating the process for the continuous release control carried out by the CPU. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The present invention is described below with reference to the embodiment shown in the drawings. 
         [0021]    In  FIG. 1 , the single-lens reflex camera  10  comprises a photographic optical system  11 , an image sensor  30 , a timing generator (TG)  12  (signal generator), a digital signal processor (DSP)  13 , a CPU  14 , and other components. 
         [0022]    The photographic optical system  11  comprises a plurality of lenses, including a focus lens (not depicted) and a zoom lens (not depicted). The image sensor  30  is arranged on the optical axis of the photographic optical system  11  so that the light-receiving surface of the image sensor  30  is perpendicular to the optical axis. The photographic optical system  11  enables an optical image of a subject to be in focus on the light-receiving surface. 
         [0023]    A diaphragm  16 , a mirror  17 , and a shutter  18  are positioned between the photographic optical system  11  and the image sensor  30 . By varying the size of the aperture of the diaphragm  16 , the amount of light incident on the image sensor  30  may be adjusted. In ready mode for photographing, the mirror  17  intersects the optical axis, and an optical image is reflected by the mirror  17  to a pentaprism  19  and transmitted to a viewfinder (not depicted). Upon a release operation, the mirror  17  is turned upward, and the optical image arrives at the shutter  18 . By opening and closing the shutter, light arriving at the image sensor  30  may be controlled. 
         [0024]    The diaphragm  16 , the mirror  17 , and the shutter  18  are driven by the optical system driver  20 . The optical system driver  20  drives each of the components on the basis of the control of the CPU  14 . 
         [0025]    The TG  12  drives the image sensor  30  to generate an image signal corresponding to the optical image reaching the light-receiving surface. The TG  12  is controlled by the DSP  13 . The generated image signal is transmitted to the DSP  13  via an AFE  21 . 
         [0026]    The AFE  21  carries out correlated double sampling processing, auto gain control processing, and analog-to-digital conversion processing on the received image signal. The DSP  13  carries out predetermined signal processing on the received image signal. The image signal, having undergone predetermined signal processing, is stored in a memory  22  or transmitted to an LCD  23 , where a captured image is displayed. 
         [0027]    The DSP  13  is connected to the CPU  14 . The DSP  13  drives the TG  12 , carries out predetermined signal processing on the image signal, stores the image signal in the memory  22 , and conducts other operations on the basis of orders from the CPU  14 . 
         [0028]    Furthermore, the CPU  14  controls the operations of each component of the single-lens reflex camera  10 . The CPU  14  is connected to an input block  24  that comprises a release button (not depicted), a power button (not depicted), a multifunctional cross key (not depicted), and so on. The CPU  14  controls the components according to various commands input by a user to the input block  24 . 
         [0029]    Next, the operation of the image sensor  30  upon the release operation, and the structure of the image sensor  30  are explained. 
         [0030]    The image sensor  30  is a CCD image sensor. As shown in  FIG. 2 , the image sensor  30  comprises pixels  31 , vertical CCDs  32 , a horizontal CCD  33 , an output amplifier  34 , and other components. 
         [0031]    A plurality of pixels  31  are arranged on the light-receiving surface in two dimensions. The vertical CCDs  32  are arranged in each column of the arranged pixels  31 . All the pixels  31  are connected to their respective adjacent vertical CCD  32 . The horizontal CCD  33  is arranged at the lower end of the vertical CCDs  32 . All the vertical CCDs  32  are connected to the horizontal CCD  33 . One end of the horizontal CCD  33  is connected to the output amplifier  34 . 
         [0032]    Each pixel  31  generates and accumulates a signal charge corresponding to the amount of light received. A substrate (not depicted) where the pixels  31  are arranged is connected to an electronic shutter terminal  35   sub.  When an electronic shutter pulse, hereinafter referred to as ΦSUB, is input to the electronic shutter terminal  35   sub,  an accumulated electrical charge is discharged from all the pixels  31 . In addition, a sensor gate (not depicted) is arranged between the pixels  31  and the vertical CCDs  32 . The sensor gate comprises a sensor gate terminal  35   sg,  hereinafter referred to as SG terminal. When an SG pulse, hereinafter referred to as ΦSG, is input to the SG terminal  35   sg,  the accumulated signal charge in each pixel is output to the vertical CCDs  32 . 
         [0033]    As shown in  FIG. 3 , first, second, third, and fourth electrodes  36   a,    36   b,    36   c,  and  36   d  are arranged repeatedly in order along the column direction of the vertical CCDs  32 . In addition, the first, second, third, and fourth electrodes  36   a,    36   b,    36   c,  and  36   d  are connected to first, second, third, and fourth vertical transfer terminals  35   v   1 ,  35   v   2 ,  35   v   3 , and  35   v   4  (see  FIG. 2 ), respectively. By inputting vertical transfer pulses, hereinafter referred to as ΦV, to the first through fourth electrodes  36   a - 36   d  while shifting their phases, the electrical charge stored in the vertical CCDs  32  is transferred to the horizontal CCD  33 . 
         [0034]    The speed at which the vertical CCDs  32  transfer an electrical charge varies in proportion to the frequency of ΦV. The frequency of ΦV is adjusted to the first frequency that is predetermined so that the signal charges can be transferred when the signal charges should be transferred, without the transfer error. The frequency of ΦV is adjusted to the second frequency, which is predetermined to be greater than the first frequency, when a rapid discharge is to be carried out. 
         [0035]    The fifth and sixth electrodes (not depicted) are arranged repeatedly in order along the row direction of the horizontal CCD  33 . In addition, the fifth and sixth electrodes are connected to first and second horizontal transfer terminals  35   h   1  and  35   h   2  (see  FIG. 2 ), respectively. By inputting horizontal transfer pulses, hereinafter referred to as ΦH, to the fifth and sixth electrodes while shifting their phase, the electronic charge received by the horizontal CCD  33  is transferred to the output amplifier  34 . 
         [0036]    ΦSUB, ΦSG, ΦV, and ΦH are generated by the TG  12  and input to their respective terminals. 
         [0037]    The output amplifier  34  comprises a capacitor (not depicted), that converts a received signal charge into a signal voltage, and outputs the converted signal voltage. 
         [0038]    The single-lens reflex camera  10  has single and continuous photographing modes. Release operations in the single and continuous photographing modes are explained using  FIGS. 4 and 5 . 
         [0039]    In the single photographing mode, a single photograph is taken by fully depressing the release button, and one frame of an image signal is generated. In the continuous photographing mode, a plurality of sequential photographs is taken automatically upon fully-depressing the release button, and a plurality of frames of image signals is generated. 
         [0040]    The CPU  14  commences the single release control, which is a sequential control, when the fully depressed release button is detected. 
         [0041]    At time t 1  after detection of full depression of the release button, the mirror  17  is turned upward. 
         [0042]    At time t 2  following time t 1 , the input of ΦV of the first frequency to the first to fourth vertical transfer terminals  35   v   1 - 35   v   4 , the input of ΦH to the first and second horizontal transfer terminals  35   h   1  and  35   h   2 , and the input of ΦSUB to the electrical shutter terminal  35   sub  commence. Then, electrical charges remaining in the vertical CCDs  32 , the horizontal CCD  33 , and the pixels  31  are discharged. 
         [0043]    At time t 3 , the input of ΦSUB is suspended and all the pixels  31  become capable of accumulating signal charges. In addition, at time t 3 , the shutter  18  is opened and the exposure of an optical image to the image sensor  30  commences. At time t 4 , after a set exposure time has elapsed since time t 3 , the shutter  18  is closed and the exposure is completed. 
         [0044]    As described later, the image sensor  30  is driven with an interlace scan. And the signal charges generated in one exposure are read out in two separate field periods, which are an even field period and an odd field period. During the even field period, the signal charges generated by the pixels  31  arranged in the even rows are read out from the image sensor  30 . During the odd field period which follows the even field period, the signal charges generated by the pixels  31  arranged in the odd rows are read out from the image sensor  30 . 
         [0045]    Before reading out the signal charges during the even field period, the rapid discharge from the vertical CCDs  32  is carried out. At time t 3 , the frequency of ΦV is changed to the second frequency, and then electrical charges stored in the vertical CCDs  32  are rapidly discharged. 
         [0046]    An electrical charge can be stored in any location of the vertical CCDs  32 . A single rapid discharge operation is carried out by transferring the electrical charges stored in each location to the horizontal CCD  33 , in order from the nearest to the farthest locations from the horizontal CCD  33 . 
         [0047]    Before reading out the signal charges during the odd field period, two rapid discharge operations are carried out. At time t 5  when the second rapid discharge is completed, the frequency of ΦV is changed to the first frequency. 
         [0048]    The time it takes to carry out two rapid discharge operations from the vertical CCDs  32  (i.e. time t 3 -time t 5 ) is constant because the second frequency is predetermined. On the other hand, the period for opening the shutter  18  varies. Accordingly, the period for two rapid discharge operations happens to be shorter than that for opening the shutter  18 . If the period required to carry out two rapid discharge operations is shorter than that for opening the shutter  18 , the rapid discharge continues until the shutter  18  is closed, even if two rapid discharge operations have been completed. 
         [0049]    If the exposure time is long, two rapid discharge operations can be carried out after the exposure. It is preferable to carry out the rapid discharge from the vertical CCDs  32  until just before starting the transfer of the signal charges. However, the continuous rapid discharge carried out during a long exposure causes power consumption to increase. In addition, in a long exposure a high speed is generally not required for a release operation. Accordingly, the rapid discharge operation during the exposure time can be suspended on the basis of the Tv value, and the rapid discharge operation can be carried out after exposure, as described above. 
         [0050]    After completion of the rapid discharge operation from the vertical CCDs  32 , ΦSG is input to the sensor gate terminal  35   sg  (see time t 6 ). In addition, ΦV, which is adjusted so that the vertical CCDs  32  read out the signal charges accumulated in the pixels arranged in the even rows, is input to the first through fourth vertical transfer terminals  35   v   1 - 35   v   4 . By inputting ΦSG and ΦV described above, the signal charges generated by and accumulated in the pixels  31  of the even rows during the period from time t 3  to time t 4  are read out by the vertical CCDs  32 . 
         [0051]    The frequency of ΦV is changed again to the first frequency after reading out the signal charges. By changing the frequency of ΦV to the first frequency, signal charges can be transferred to the horizontal CCD  33  without transfer error. 
         [0052]    When the transfer of the signal charges in all the pixels  31  in the even rows by the vertical CCDs  32  and the horizontal CCD  33  is completed, reading out from the even field finishes (see time t 7 ). After finishing the reading out from the even field, the frequency of ΦV is changed again to the second frequency, and electrical charges stored in the vertical CCDs  32  are rapidly discharged (see the period from time t 7  to time t 8 ). 
         [0053]    Unlike reading the signal charges from the even field, only a single rapid discharge operation is carried out before reading out the signal charges from the odd field. At time t 8 , when the single rapid discharge operation is completed, the frequency of ΦV is changed to the first frequency. 
         [0054]    After completion of the rapid discharge from the vertical CCDs  32 , ΦSG is input to the sensor gate terminal  35   sg  (see time t 9 ). In addition, ΦV, which is adjusted so that the vertical CCDs  32  read out the signal charges accumulated in the pixels arranged in the odd rows, is input to the first-fourth vertical transfer terminals  35   v   1  through  35   v   4 . By inputting ΦSG and ΦV described above, the signal charges generated by and accumulated in the pixels  31  of the odd rows during the period from time t 3  to time t 4  are read out by the vertical CCDs  32 . 
         [0055]    When the transfer of the signal charges in all the pixels  31  in the odd rows by the vertical CCDs  32  and the horizontal CCD  33  is completed, reading out from the odd field finishes (see time t 10 ). Then, by generating and reading out one frame of an image signal, one capture of an image is completed. After completing the capture of an image, electrical charges accumulated by the pixels  31  are discharged until the next single release control is started by inputting ΦSUB to the electrical shutter terminal  35   sub  again. 
         [0056]    Next, the release operation in the continuous photographing mode is explained. As in the single photographing mode, the CPU  14  commences the continuous release control, which is also a sequential control, when the fully depressed release button is detected. 
         [0057]    In the continuous release control, the same operations as in the single photographing mode are carried out at the same time for the first capture of an image (i.e. generating and reading out the first frame of an image signal) (see time t 1 -time t 10  in  FIG. 5 ). 
         [0058]    After the first capture of an image (time t 10 ), by inputting ΦSUB to the electrical shutter terminal  35   sub,  electrical charges accumulated by the pixels  31  are discharged. 
         [0059]    After a predetermined discharge period elapses from the beginning of the electrical charges, a second capture of an image can be started. At time t 11 , the input of ΦSUB is suspended, and all the pixels  31  become capable of accumulating signal charges. 
         [0060]    In addition, at time t 11 , the shutter  18  is opened and the second exposure of an optical image to the image sensor  30  commences. At time t 12 , after a set exposure time passes from time t 11 , the shutter  18  is closed and the exposure is completed. 
         [0061]    In addition, at time t 11 , the frequency of ΦV is changed to the second frequency, and electrical charges stored in the vertical CCDs  32  are rapidly discharged. Unlike the first exposure, a rapid discharge operation is carried out one time, as a rule, for the second capture of an image before reading out the signal charges during the even field period. If the exposure is not completed after one rapid discharge operation, the rapid discharge continues until the exposure is completed. 
         [0062]    At time t 12 , when the exposure and the rapid discharge are completed, the frequency of ΦV is changed to the first frequency. After completion of the rapid discharge, ΦSG is input to the sensor gate terminal  35   sg  (see time t 13 ). 
         [0063]    Thereafter, by carrying out the same operations that were conducted during the period from time t 6  to time t 10  for the first capture of an image, the second frame of an image signal is generated and read out. In addition, while the release button remains fully depressed the subsequent frames of image signals are generated and read out, similar to the generating and reading out the second frame of an image signal. 
         [0064]    Next, the single release control carried out by the CPU  14  is explained below using the flowchart of  FIG. 6 . The single release control commences when the CPU  14  detects the fully depressed release button. 
         [0065]    At step S 100 , the CPU  14  sets the number of the rapid discharge operation from the vertical CCDs  32  to two. After the number has been set, the process proceeds to step S 101 . 
         [0066]    At step S 101 , exposure of the light-receiving surface commences. The CPU  14  orders the optical system driver  20  to turn the mirror  17  upward and to open the shutter  18  for the duration of the set exposure time. After completion of the exposure, the process proceeds to step S 102 . 
         [0067]    At step S 102 , the CPU  14  orders the TG  12  to set the frequency of ΦV to the second frequency. The CPU  14  also orders the TG  12  to carry out the rapid discharge operations from the vertical CCDs  32  for the number of times that was set at step S 100 . After completion of the rapid discharge operation, the process proceeds to step S 103 . 
         [0068]    At step S 103 , the signal charges generated and accumulated by the pixels  31  in step S 101  are transferred to the output amplifier  34 . As described above, the CPU  14  controls the TG  12  so that the vertical CCDs  32  read out the signal charges, the vertical CCDs  32  transfer the signal charges to the horizontal CCD  33  by changing the frequency of ΦV to the first frequency, and the horizontal CCD  33  transfers the signal charges to the output amplifier  34 . After completion of the transfer of the signal charges to the output amplifier  34 , the process proceeds to step S 104 . 
         [0069]    At step S 104 , the CPU  14  determines whether or not the reading of the signal charges from the odd field has been completed. If the reading from the odd field has not been completed, the process proceeds to step S 105 . (0064) 
         [0070]    At step S 105 , the CPU sets the number of the rapid discharge operation from the vertical CCDs  32  to one. After the number has been set, the process returns to step S 102 . 
         [0071]    If the reading from the odd field is completed, the single release control terminates. 
         [0072]    Next, the continuous release control carried out by the CPU  14  is explained below using the flowchart of  FIG. 7 . The continuous release control commences when the CPU  14  detects the fully depressed release button. 
         [0073]    At step S 200 , the CPU  14  determines whether or not it is the first time the capture of an image is to be carried out. If it is the first time for the capture of an image, the process proceeds to step S 201 . If it is not the first time for the capture of an image, the process proceeds to step S 202 . 
         [0074]    At step S 201 , the CPU  14  sets the number of the rapid discharge operation from the vertical CCDs  32  to two. At step S 202 , the CPU  14  sets the number of the rapid discharge operation from the vertical CCDs  32  to the number of rapid discharge operations that can be carried out during the determined exposure time. After the number has been set at step S 201  or S 202 , the process proceeds to step S 203 . 
         [0075]    At step S 203 , the exposure of the light-receiving surface commences. The CPU  14  orders the optical system driver  20  to turn the mirror  17  upward and to open the shutter  18  for the duration of the set exposure time. After the completion of the exposure, the process proceeds to step S 204 . 
         [0076]    At step S 204 , the CPU  14  changes the frequency of ΦV to the second frequency, and then the rapid discharge operation is carried out for the number of times that was set at step S 201  or S 202 . In addition, at step S 204  the signal charges generated and accumulated by the pixels  31  in step S 203  are transferred to the output amplifier  34 . Namely, the CPU  14  controls the TG  12  so that the vertical CCDs  32  read out the signal charges, the vertical CCDs  32  transfer the signal charges to the horizontal CCD  33  by changing the frequency of ΦV to the first frequency, and the horizontal CCD  33  transfers the signal charges to the output amplifier  34 . After completion of the rapid discharge and transfer of the signal charges to the output amplifier  34 , the process proceeds to step S 205 . 
         [0077]    At step S 205 , the CPU  14  sets the number of the rapid discharge operation from the vertical CCDs  32  to one. After the number has been set, the process proceeds to step S 206 . 
         [0078]    At step S 206 , the CPU  14  orders the TG  12  to carry out the rapid discharge operation for the number of times that was set at step S 205 , and to transfer the signal charges to the output amplifier  34 . After completion of the rapid discharge operation and transfer of the signal charges to the output amplifier  34 , the process proceeds to step S 207 . 
         [0079]    At step S 207 , the CPU  14  determines whether or not the release button is still fully depressed. If the release button remains fully depressed, the process returns to step S 200  and steps S 200 -S 207  are repeated. If the release button does not remain fully depressed, the continuous release control terminates. 
         [0080]    In the above embodiment, the period of the release operation is shorter than it was for the prior camera, as explained below. 
         [0081]    In a general camera, the rapid discharge from the vertical CCDs does not commence until the exposure has been completed. After the rapid discharge operation, the signal charges are then read out from the pixels and transferred. On the other hand, in the above embodiment, the rapid discharge from the vertical CCDs is carried out at the same time as the exposure, and the period for the release operation is shortened accordingly. 
         [0082]    In addition, in the above embodiment, the amount of photographing per a determined period can be increased because the period of the time required for one capture of an image can be shortened in the continuous photographing mode. 
         [0083]    In the single photographing mode, the rapid discharge operation is carried out twice before transferring the signal charges from the even field. On the other hand, in the continuous photographing mode the number of the rapid discharge operation is decreased upon the capture of an image after the first capture of an image. Accordingly, the period for one capture of an image after the first capture of an image is shortened by the period for one rapid discharge operation per one capture of an image. 
         [0084]    If the captures of an image are repeated in the single photographing operation mode, the interval between the successive captures may be long enough to allow a large amount of electrical charges to accumulate in the vertical CCDs. In order to remove a sufficient amount noise, many rapid discharge operations are necessary. On the other hand, the interval between the successive captures in the continuous photographing mode is shorter than that in the single photographing operation mode. Accordingly, electrical charges accumulated in the vertical CCDs can be sufficiently discharged even if the number of rapid discharge operations is low. 
         [0085]    The exposure of an optical image to the image sensor and the rapid discharge are simultaneously carried out for the first capture of an image in the continuous photographing mode, in the above embodiment. However, the exposure and rapid discharge do not have to be simultaneously carried out for the first capture as long as the exposure and rapid discharge are simultaneously carried out for the captures after the first capture. 
         [0086]    Of course, in order to increase the amount of photographing it is preferable to carry out the exposure and the rapid discharge for the first capture simultaneously. However, the period for the capture of an image can be shortened through removing the electrical charge accumulated in the vertical CCDs by partially or entirely overlapping the periods in which the exposure and the rapid discharge operations are carried out, as long as the overlap occurs by the capture after the first capture at the latest. 
         [0087]    All the signal charges are transferred to the output amplifier  34  by twice interlace scanning, in the above embodiment. However, the number for transferring the signal charges is not limited to two, the transfer of the signal charges may be divided by three or more times. Or all the signal charges can be transferred to the output amplifier  34  at once, according to progressive scanning. If the signal charges are transferred according to progressive scanning, the same effect as the above embodiment can be achieved as long as the number of the rapid discharge operation for the first capture of an image is more than that for the captures after the first capture in the continuous photographing mode. 
         [0088]    The rapid discharge operation is carried out twice before transferring the signal charges in the first field period for the first capture, but the rapid discharge operation is carried out once per each subsequent transfer in the above embodiment. However, the same effect as the above embodiment can be achieved as long as the number of rapid discharge operations carried out before transferring the signal charges in the first field period for the first capture is greater than that per required time in subsequent captures. 
         [0089]    Four electrodes  36   a - 36   d  are arranged for the vertical CCDs  32  in the above embodiment. However, the number of electrodes for the vertical CCDs  32  is not limited to four. In addition, two electrodes are arranged for the horizontal CCD  33  in the above embodiment. However, the number of electrodes for the horizontal CCD  33  is not limited to two. 
         [0090]    The image sensor  30  is a CCD image sensor in the above embodiment. However, other kinds of charge-transfer image sensors can be used. 
         [0091]    Although the embodiments of the present invention have been described herein with reference to the accompanying drawings, obviously many modifications and changes may be made by those skilled in this art without departing from the scope of the invention. 
         [0092]    The present disclosure relates to subject matter contained in Japanese Patent Application No. 2008-239856 (filed on Sep. 18, 2008), which is expressly incorporated herein, by reference, in its entirety.