Abstract:
An electronic camera includes: an image-capturing device that stores a signal electrical charge achieved by performing photoelectric conversion on a subject image projected on a photosensitive surface and reads out the signal electrical charge to generate an image signal; and a photographing preparation device that executes photographing preparations for photoelectric conversion performed at the image-capturing device. And the photographic preparation device executes at least some of the photographing preparations for photographing a next frame during an electrical charge read period at the image-capturing device.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of the following priority application is herein incorporated by reference: 
     Japanese Patent Application No. 10-213207 filed Jul. 28, 1998. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic camera. To be more specific, it relates to an electronic camera which is capable of photographing a higher number of frames per unit time (hereafter referred to as the “frame speed”). 
     2. Description of the Related Art 
     There are single lens reflex type electronic cameras in the known art that separate the light flux from the photographing optical system to a viewfinder-side light flux and an image-capturing element-side light flux via a quick-return mirror or the like.  FIG. 11  shows the sequence of operations performed in this type of electronic camera. In  FIG. 11 , when the shutter release switch at the electronic camera is fully pressed, the internal sequence motor starts to rotate in a state in which power is supplied to the front curtain magnet and the rear curtain magnet of the shutter. 
     This rotation of the sequence motor causes the quick-return mirror to swing upward and the mechanical retention of the shutter to be released. At this point, the front curtain and the rear curtain at the shutter are held in a retained state respectively by the front curtain magnet and the rear curtain magnet mentioned earlier. 
     When the sequence motor has rotated by a specific amount and the operation described above is completed, the sequence switch shifts to an ON state and the sequence motor is stopped temporarily. In synchronization with the shift of the sequence switch to the ON state, the image-capturing element discharges unnecessary electrical charge on the photosensitive surface and starts electrical charge storage. However, since the shutter is closed at this point, no substantial signal electrical charge storage is achieved. 
     After rebounding to some extent at the top of its upswing, the quick-return mirror stops. After allowing a specific length of time (t 31  in the figure) to elapse to allow the rebounding to settle, power supply to the front curtain magnet is cut off and the front curtain at the shutter starts its run. When a length of time corresponding to a value set for the exposure time has elapsed after the front curtain starts its run, power supply to the rear curtain magnet is cut off and the rear curtain at the shutter starts its run. 
     With the two curtains caused to run one after the other in this manner, the photosensitive surface of the image-capturing element is exposed over the specific length of exposure time. When the rear curtain completes its run, the rear curtain closing switch shifts to an ON state. At the image-capturing element, a read of the signal electrical charge starts in synchronization with this shift of the rear curtain closing switch to the ON state. 
     After the read operation of the signal electrical charge is completed, the sequence motor starts to rotate again. This rotation of the sequence motor allows the mechanical operations (mirror down, shutter charge) to be executed in preparations for photographing of the next frame. When the mirror moves down, the path through which the subject light advances is changed, and the subject light is guided to the focal point detection block and the photometric unit of the electronic camera. 
     As the path of the subject light is changed in this manner, the focal point detection block and the photometric unit start measuring operations (focal point detection operation, photometry operation) in preparations for photographing of the next frame. 
     By repeating the sequence of operations while the shutter release switch is fully pressed down, continuous shooting is performed. The following details are known in the prior art as technologies that achieve higher frame speed in such continuous shooting. 
     (1) Japanese Laid-Open Patent Publication No. H 6-54252 discloses a technology that achieves an increase in the frame speed by exposing a succeeding frame at the image-capturing element while the image in the preceding frame is being compressed. 
     (2) In addition, Japanese Laid-Open Patent Publication No. H 7-135589 discloses a technology that achieves a higher frame speed by recording image data of an image that has been captured separately in a plurality of recording media to reduce the length of time required for image recording. 
     Now, higher resolution at the image-capturing element has become a vital requirement in an electronic camera in recent years to ensure that the captured image is of the highest possible quality. Higher resolution at the image-capturing element has caused the tendency of increases in the length of time required for reading the electrical charges from the image-capturing element (hereafter referred to as the “electrical charge read period”). 
     The increase in the electrical charge read period has caused the length of time required for photographing a single frame to increase correspondingly, which presents a problem in that the frame speed of the electronic camera becomes lower. It is to be noted that since the prior art technologies for achieving higher speed (Japanese Laid-Open Patent Publication No. H6-54252, Japanese Laid-Open Patent Publication No. H7-135589) do not specifically deal with the longer electrical charge read period, it has not been possible to solve the problem of the lower frame speed occurring for the reason described above. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an electronic camera with which the problem of lowered frame speed resulting from longer electrical charge read period can be alleviated. Another object of the present invention is to provide an electronic camera that is capable of maintaining accurate and correct operation timing even when a higher frame speed is achieved. 
     In order to attain the above objects, an electronic camera comprises: an image-capturing device that stores a signal electrical charge achieved by performing photoelectric conversion on a subject image projected on a photosensitive surface and reads out the signal electrical charge to generate an image signal; and a photographing preparation device that executes photographing preparations for photoelectric conversion performed at the image-capturing device. And, the photographic preparation device executes at least some of the photographing preparations for photographing a next frame during an electrical charge read period at the image-capturing device. 
     Preferably, this electronic camera further comprises: at least one of a shutter mechanism, a mirror mechanism, an aperture mechanism, a focal adjustment device and a photometric device. And the photographing preparations for the next frame include at least one of a shutter charge performed by the shutter mechanism, a mirror down performed by the mirror mechanism, a mirror up performed by the mirror mechanism, aperture control performed by the aperture mechanism, focal adjustment performed by the focal adjustment device and photometry performed by the photometric device, to enable photographing of the next frame. 
     Preferably, the electronic camera further comprises: a shutter mechanism, a mirror mechanism and an aperture mechanism. And the photographing preparation device completes a shutter charge performed by the shutter mechanism and a mirror down performed by the mirror mechanism to enable photographing of the next frame and starts a mirror up performed by the mirror mechanism and aperture control performed by the aperture mechanism to enable photographing of the next frame during an electrical charge read period at the image-capturing device. 
     Preferably, the electronic camera further comprises: a mirror mechanism, a focal adjustment device and a photometric device. And the photographing preparation device completes a mirror down performed by the mirror mechanism to enable photographing of the next frame and then implements focal adjustment by the focal adjustment device and photometry by the photometric device, during an electrical charge read period at the image-capturing device. 
     Preferably, the electronic camera further comprises: a continuous shooting command device that issues a command to perform continuous shooting. And, the photographic preparation device executes at least some of the photographing preparations for the next frame during the electrical charge read period at the image-capturing device while a command to perform continuous shooting issued by the continuous shooting command device is in effect. 
     Preferably, in the electronic camera, the photographing preparation device includes a drive motor that drives the photographing preparation and implements rotational drive of the drive motor during the electrical charge read period at the image-capturing device. 
     Preferably, in the electronic camera, the photographing preparation device includes a drive motor that sequentially drives a plurality of photographing preparations in correspondence to a rotating angle and implements rotational drive of the drive motor during the electrical charge read period at the image-capturing device. 
     Preferably, in the electronic camera, operation timing is set in advance at least at either the photographing preparation device or the image-capturing device to ensure that the electrical charge read period does not overlap a period over which a subject image of the next frame is projected onto the photosensitive surface. 
     Preferably, in the electronic camera, the photographing preparation device performs detection of a completion of a signal electrical charge read operation performed by the image-capturing device and following the detection, projects a subject image onto the photosensitive surface. 
     Preferably, the electronic camera further comprises: a time count device that measures at least a part of a length of time required for the photographing preparation. And at least either the photographing preparation device or the image-capturing device adjusts operation timing to ensure that a signal electrical charge read period does not overlap a period over which a subject image for the next frame is projected onto the photosensitive surface based upon results of count of the length of required time performed by the time count device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the positional relationships in the internal structure of the electronic camera  11 ; 
         FIG. 2  is a block diagram illustrating the electrical system in the electronic camera  11 ; 
         FIG. 3  is a timing chart of a first embodiment; 
         FIG. 4  is a flowchart of the operations performed by the microprocessor  20  in the first embodiment; 
         FIG. 5  is a flowchart of the operations performed by the microprocessor  20  in a second embodiment; 
         FIG. 6  is a block diagram illustrating the electrical system in the electronic camera  50  in a third embodiment; 
         FIG. 7  is a timing chart of the third embodiment; 
         FIG. 8  is a flowchart of the operations performed by the microprocessor  20  in the third embodiment; 
         FIG. 9  is a flowchart of the operations performed by the microprocessor  20  in a fourth embodiment; and 
         FIG. 10  is a timing chart of the focal point detection operation and the photometric operation started during the electrical charge read period. 
         FIG. 11  shows the sequence of operations during continuous shooting in the prior art electronic camera. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is an explanation of the preferred embodiments of the present invention, given in reference to the drawings. 
     First Embodiment 
       FIG. 1  illustrates the positioning of the main parts inside the camera in the first embodiment. In  FIG. 1 , a photographing optical system  12  is mounted at the body of an electronic camera  11 . An aperture drive mechanism  13  and a quick-return mirror  14  are positioned in this order at the photographing optical system  12  toward the image space. 
     A viewfinder optical system is provided along the direction in which the quick-return mirror  14  reflects light. A photometric unit  15  that takes in subject light is provided at a portion of the viewfinder optical system. In addition, a sub-mirror  16  is provided at the rear surface of the quick-return mirror  14  in its central transmission area. A focal point detection unit  17  is provided along the direction in which the sub-mirror  16  reflects light. Also, an image-capturing element  19  is provided to the rear of the quick-return mirror  14  via a shutter mechanism  18  which is provided in between. 
       FIG. 2  is a block diagram illustrating the electrical system in the electronic camera  11  described above. In  FIG. 2 , a microprocessor  20  that engages in the control of the entire system is provided inside the electronic camera  11 . The microprocessor  20  controls an AF motor  22  via a motor drive circuit  21 . The AF motor  22  drives the optical system back and forth for focus control provided inside the photographing optical system  12  and executes focal adjustment through the contrast method whereby the focal adjustment is assumed to be achieved when the contrast is at the highest. 
     In addition, the microprocessor  20  provides a drive pulse for electrical charge read to the image-capturing element  19  via a drive pulse generating circuit (not shown). An image signal output by the image-capturing element  19  is processed at an image-capturing circuit  23  which performs color signal processing and A/D conversion and then is input to the microprocessor  20  as image data. Furthermore, the microprocessor  20  implements control on a sequence motor  25  via a motor drive circuit  24 . Depending upon the rotating angle of the sequence motor  25 , the aperture drive mechanism  13 , a mirror drive mechanism  26  and a charge mechanism  27  are driven in a specific order. 
     The mirror drive mechanism  26  raises and lowers the quick-return mirror  14 . The charge mechanism  27  performs a sequence of operations, that is shutter charge, through which a front curtain  28  and a rear curtain  29  at the shutter mechanism  18  are reset to their pre-run positions and are mechanically retained. The front curtain  28  and the rear curtain  29  are respectively provided with a front curtain magnet  30  and a rear curtain magnet  31  that achieve curtain retention through magnetic force. The microprocessor  20  controls the electrical current supplied to the magnets  30  and  31  with a front curtain drive circuit  32  and a rear curtain drive circuit  33 . 
     Furthermore, a PC interface  34  is provided inside the electronic camera  11 . The microprocessor  20  exchanges data, instructions and the like with an external device  35  via the PC interface  34 . In addition, two timers A and B for counting time are provided inside the electronic camera  11 . By initializing the count values at the timers A and B, the microprocessor  20  can measure the lengths of time elapsed after the initialization. 
     Moreover, the microprocessor  20  obtains photometric data indicating the subject brightness via the photometric unit  15 . By engaging in exposure calculation based upon the photometric data and the photosensitivity of the image-capturing element, the microprocessor  20  determines the correct aperture value. Also, a connector  38  is provided at the electronic camera  11 . Image files and the like are transferred to and recorded in a memory card  39  by the microprocessor  20  via the connector  38 . 
     In addition, an image memory  40  that is provided to temporarily record image data, a compression/expansion circuit  41  that compresses and expands the image data, a memory  42  that stores a control program and data, and the like are connected to the microprocessor  20 . Also, a switch group that includes a power switch  45 , a shutter release switch  46 , a rear curtain closing switch  47  and a sequence switch  48 , is connected to the microprocessor  20 . 
     Operation of the First Embodiment 
       FIG. 3  is a timing chart of the first embodiment. In addition,  FIG. 4  is a flowchart of the operations performed by the microprocessor  20 . The following is an explanation of the operation achieved in the first embodiment given in reference to  FIGS. 3 and 4 . 
     First, a microprocessor  20  makes a decision as to whether or not the shutter release switch  46  is in a fully pressed state ( FIG. 4 , S 1 ). It is to be noted that the shutter release switch  46  is a switch that may be pressed halfway down or fully down. Before the routine in  FIG. 4  is started, automatic focal adjustment (AF) and photometry are performed while the shutter release switch  46  is pressed halfway down. 
     When the shutter release switch  46  is pressed fully down, the microprocessor  20  first starts power supply to the front curtain magnet  30  and the rear curtain magnet  31  and then starts rotation of the sequence motor  25  ( FIG. 4 , S 2 ). 
     This rotation of the sequence motor  25  causes the mirror drive mechanism  26  to raise the quick-return mirror  14 . In addition, the aperture drive mechanism  13  implements aperture control in correspondence to the correct aperture value. At the same time, the charge mechanism  27  releases the mechanical retention of the shutter mechanism  18 . 
     When these operations effected by the sequence motor  25  are completed, the sequence switch  48  shifts to an ON state. After verifying that the sequence switch  48  is now in an ON state ( FIG. 4 , S 3 ), the microprocessor  20  temporarily stops the sequence motor  25 . At this point, the microprocessor  20  discharges any unnecessary electrical charge from the image-capturing element  19  to start an electrical charge storage period. At the same time, the microprocessor  20  initializes the count value at the timer A and starts time count ( FIG. 4 , S 4 ). 
     In this state, the microprocessor  20  waits for a length of time t 11  to elapse by monitoring the count value at the timer A ( FIG. 4 , S 5 ). When the length of time t 11  elapses, the microprocessor  20  decides that the rebounding of the quick-return mirror  14  has subsided to a sufficient degree and cuts off the power supply to the front curtain magnet  30 . In response, the front curtain  28  starts its run due to the force applied to it and, as a result, the shutter mechanism  18  starts to open. At this point, the microprocessor  20  initializes the count value at the timer A to restart time count ( FIG. 4 , S 6 ). 
     The microprocessor  20  waits for the exposure time to elapse by monitoring the count value at the timer A ( FIG. 4 , S 7 ). When the exposure time elapses, the microprocessor  20  cuts off the power supply to the rear curtain magnet  31 , as well ( FIG. 4 , S 8 ). In response, the rear curtain  29  starts its run following the front curtain  28 . 
     When the rear curtain  29  completes its run and the shutter mechanism  18  becomes completely closed, the rear curtain closing switch  47  switches to an ON state. When the microprocessor  20  verifies that the rear curtain closing switch  47  has shifted to an ON state ( FIG. 4 , S 9 ), it supplies a drive pulse for electrical charge read to the image-capturing element  19  to start a read of the signal electrical charge. At this point, the microprocessor  20  initializes the count values at the timers A and B and starts time count ( FIG. 4 , S 10 ). 
     In this state, the microprocessor  20  waits for a length of time t 12  to elapse by monitoring the count value at the timer A ( FIG. 4 , S 11 ). When the length of time t 12  elapses, the microprocessor  20  restarts the rotation of the sequence motor  25  ( FIG. 4 , S 12 ). 
     The “electrical charge read start” and the “restart of the rotation of the sequence motor  25 ” are shifted from each other by the length of time t 12  in this manner. As a result, the source voltage is prevented from suddenly dropping and it becomes possible to reduce the noise interference between the two operations with great reliability. 
     As the sequence motor  25  restarts its rotation in this manner, the mirror drive mechanism  26  lowers the quick-return mirror  14 . In addition, the charge mechanism  27  resets the front curtain  28  and the rear curtain  29  to their pre-run positions and mechanically retains them (shutter charge operation). In addition, in order to improve the degree of accuracy of the count in the photometric operation and the focal point detection operation, the aperture drive mechanism  13  may be temporarily reset to the open aperture in response to the restart of rotation of the sequence motor  25 . 
     When these operations effected by the sequence motor  25  are completed, the sequence switch  48  shifts to an ON state. After verifying that the sequence switch  48  has shifted to an ON state ( FIG. 4 , S 13 ), the microprocessor  20  temporarily stops the sequence motor  25  ( FIG. 4 , S 14 ). 
     In this state, the microprocessor  20  waits for a length of time tf 1  to elapse on the timer B ( FIG. 4 , S 15 ). This length of time tf 1  represents the length of wait time set by anticipating a margin (tr 1  in  FIG. 3 ) in advance in order to end the electrical charge read operation currently in progress before the exposure of the next frame starts with great reliability. It is to be noted that as explained later, it is mechanically assured that even when the control returns to step S 2  to restart rotation of the sequence motor  25  and the like, the exposure of the next frame is not started for at least the length of time ts 1 . Consequently, a length of time that is shorter than the read period ty 1  by the length of time ts 1  can be set for the wait time tf 1 . 
     When the length of time tf 1  elapses, the microprocessor  20  makes a decision as to whether or not the shutter release switch  46  is in a fully pressed state ( FIG. 4 , S 16 ). At this point, if the fully pressed state has already been cleared, the microprocessor  20  ends the continuous shooting operation. If, on the other hand, the shutter release switch is still in the fully pressed state, the microprocessor  20  returns to the operation in step S 2  to continue with the sequence of continuous shooting operations described above. 
     In the first embodiment, the descent of the quick-return mirror  13  and the shutter charge are completed during the electrical charge read period, and then the quick-return mirror  14  is raised and the aperture control is started. 
     Advantages of the First Embodiment 
     As explained above, in the first embodiment, the preparations for photographing the next frame (mirror down, shutter charge, mirror up, aperture control) are started during the electrical charge read period and the preparations for photographing the next frame (mirror down, shutter charge, mirror up, aperture control) are performed concurrently with the electrical charge read. By achieving such parallel operations, it is possible to reduce the length of time required for photographing a single frame in an effective manner and a higher frame speed can be achieved with great reliability and with ease. 
     In addition, since the shutter is mechanically closed by allowing the wait time tf 1  to elapse during the operation, the subject image in the next frame is not projected onto the photosensitive surface during the electrical charge read period. Thus, the smear phenomenon resulting from the effect of the light at the photosensitive surface during the electrical charge read, i.e., during the electrical charge transfer, does not occur, so that the signal electrical charge can be read out with an even higher quality result. 
     In the case of the technologies for achieving higher speed in the prior art, it is difficult to achieve a higher frame speed if the electrical charge read period is lengthened as a result of assuring a higher resolution at the means for image-capturing. However, the first embodiment, in which the photographing preparations for the next frame are started in advance during the electrical charge read period and the preparations are made concurrently with the electrical charge, achieves a higher frame speed with great reliability. 
     Second Embodiment 
     Next, the second embodiment is explained. It is to be noted that since the structural features of the second embodiment are identical to those in the first embodiment except that the program operation at the microprocessor  20  is partially different, an explanation of the structural features is omitted here. 
       FIG. 5  is a flowchart of the operations performed by the microprocessor  20  in the second embodiment. The operation achieved in the second embodiment is characterized in that a front curtain run is started after verifying that the electrical charge read period has elapsed, as indicated in S 5   a  in  FIG. 5 . 
     In the second embodiment, in which operational verification is performed in this manner, no problems such as the subject image in the next frame being projected onto the photosensitive surface during the electrical charge read period occur, even if the operation timing changes due to fluctuations in the source or the like. Furthermore, since it is not necessary to set a great length of time for the wait time tf 1  in anticipation of fluctuations in the operation timing, the frame speed can be increased to the maximum possible. 
     Third Embodiment 
     Next, the third embodiment is explained.  FIG. 6  is a block diagram illustrating the electrical system of an electronic camera  50  in the third embodiment. The structural features of the third embodiment are as described below. 
     A shutter mechanism  51  of the electronic camera  50  is provided with a single curtain  52 . A curtain magnet  53  that retains the curtain  52  in an open state is controlled by the microprocessor  20  via a curtain drive circuit  54 . In addition, a curtain closing switch  56  whose switch position state changes in correspondence to the open/closed state of the curtain  52 . It is to be noted that since other structural features are identical to those in the first embodiment ( FIG. 2 ), the same reference numbers are assigned to the relevant components in  FIG. 6  to preclude the necessity for a repeated explanation thereof. 
       FIG. 7  is a timing chart of the third embodiment. In addition,  FIG. 8  is a flowchart of the operations performed by the microprocessor  20 . The following is an explanation of the operation achieved in the third embodiment, given in reference to  FIGS. 7 and 8 . 
     First, the microprocessor  20  makes a decision as to whether or not the shutter release switch  46  is in a fully pressed state ( FIG. 8 , S 101 ). When the shutter release switch  46  is pressed fully down, the microprocessor  20  first starts power supply to the curtain magnet  53  and then starts rotation of the sequence motor  25  ( FIG. 8 , S 102 ). It is to be noted that in this state, the curtain  52  sustains the closed state because of the mechanical retention effected by the charge mechanism  55 . 
     This rotation of the sequence motor  25  causes the mirror drive mechanism  26  to raise the quick-return mirror  14 . In addition, the aperture drive mechanism  13  implements aperture control in correspondence to the correct aperture value. At the same time, the charge mechanism  55  releases the mechanical retention of the shutter mechanism  51  to reset the curtain  52  in an open state (shutter charge operation). As a result, the curtain  52  is retained by the curtain magnet  53  to which power is being supplied and the shutter mechanism  51  is held in an open state. 
     When these operations effected by the sequence motor  25  are completed, the sequence switch  48  shifts to an ON state. After verifying that the sequence switch  48  is now in an ON state ( FIG. 8 , S 103 ), the microprocessor  20  temporarily stops the sequence motor  25 . At this point, the microprocessor  20  initializes the count value at the timer A and starts time count ( FIG. 8 , S 104 ). 
     The microprocessor  20  waits for a length of time t 21  to elapse by monitoring the count value at the timer A ( FIG. 8 , S 105 ). When the length of time t 21  elapses, the microprocessor  20  decides that the rebounding of the quick return mirror  14  has subsided to a sufficient degree and starts an electrical charge storage period by forcibly discharging any unnecessary electrical charge from the image capturing element  19 . At this point, the microprocessor  20  initializes the count value at the timer A to restart time count ( FIG. 8 , S 106 ). 
     The microprocessor  20  waits for the exposure time to elapse by monitoring the count value at the timer A ( FIG. 8 , S 107 ). During this process, when the exposure time elapses, the microprocessor  20  ends the electrical charge storage period by transferring the signal electrical charges stored at the photosensitive surface in a batch onto the transfer line. In addition, the microprocessor  20  cuts off the power supply to the curtain magnet  53  to cause the curtain  52  to run in the closing direction by the force applied to it ( FIG. 8 , S 108 ). 
     When the curtain  52  completes its run and the shutter mechanism  51  becomes completely closed, the curtain closing switch  56  switches to an ON state. When the microprocessor  20  verifies that the curtain closing switch  56  has shifted to an ON state ( FIG. 8 , S 109 ), it supplies a drive pulse for electrical charge read to the image capturing element  19  to start a read of the signal electrical charge. In addition, the microprocessor  20  restarts the rotation of the sequence motor  25  at this point and then initializes the count value at the timer A to start time count ( FIG. 8 , S 110 ). 
     As the sequence motor  25  restarts its rotation in this manner, the mirror drive mechanism  26  lowers the quick return mirror  14 . It is to be noted that the aperture drive mechanism  13  may reset the aperture to an open state during this period. When these operations effected by the sequence motor  25  are completed, the sequence switch  48  shifts to an ON state. After verifying that the sequence switch  48  has shifted to an ON state ( FIG. 8 , S 111 ), the microprocessor  20  temporarily stops the sequence motor  25  ( FIG. 8 , S 112 ). 
     In this state, the microprocessor  20  waits for a length of time tf 2  to elapse by monitoring the count value at the timer A ( FIG. 8 , S 113 ). This length of time tf 2  represents the length of wait time set in advance in order to end the electrical charge read operation currently in progress before the exposure of the next frame starts with great reliability. 
     When the length of time tf 2  elapses, the microprocessor  20  makes a decision as to whether or not the shutter release switch  46  is in a fully pressed state ( FIG. 8 , S 114 ). At this point, if the fully pressed state has already been cleared, the microprocessor  20  ends the continuous shooting operation. If, on the other hand, the shutter release switch is still in the fully pressed state, the microprocessor  20  returns to the operation in step S 2  to sustain the continuous shooting operation. 
     Advantages of the Third Embodiment 
     As explained above, in the third embodiment, too, the preparations for photographing the next frame (mirror down, shutter charge, mirror up, aperture control) are performed during concurrently the electrical charge read period. Thus, it is possible to reduce the length of time required for photographing a single frame correspondingly and a higher frame speed can be achieved with great reliability and with ease. 
     In addition, since the wait time tf 2  is allowed to elapse during the operation, the subject image in the next frame is not projected onto the photosensitive surface during the electrical charge read period. Thus, the smear phenomenon resulting from the effect of the light at the photosensitive surface during the electrical charge read, i.e., during the electrical charge transfer, does not occur, so that the signal electrical charge can be read out with even higher quality results. 
     Furthermore, in the third embodiment, the shutter mechanism  51  is constituted of a single curtain to partially incorporate the electronic shutter operation. Thus, it is possible to do without the length of time required for opening and closing the front curtain  28  in the first embodiment to achieve a corresponding reduction in the length of time required to photograph a single frame. Consequently, an even higher frame speed is achieved. 
     Fourth Embodiment 
     Next, the fourth embodiment is explained. It is to be noted that since the structure and the timing chart of the fourth embodiment are identical to those of the third embodiment ( FIGS. 6 and 7 ), their explanation is omitted here. 
       FIG. 9  is a flowchart of the operations performed by the microprocessor  20  in the fourth embodiment. The operation achieved in the fourth embodiment is characterized as follows. 
     (1) As indicated in  FIG. 9 , S 201 , the microprocessor  20  starts the timer A at the point in time at which the rotation of the sequence motor  25  starts. 
     (2) In this state, the microprocessor  20  monitors the switching state of the curtain closing switch ( FIG. 9 , S 202 ). 
     (3) The microprocessor  20  stores the count value at the timer A as a variable is when the curtain closing switch  56  shifts to an OFF state. This length of time is represents the length of time elapsing between the start of rotation of the sequence motor  25  and the completion of the shutter charge operation performed by the charge mechanism  55 . Based upon the length of time ts, tf 2 =ty 2 −(0.8·ts) is calculated to determine the wait time tf 2  ( FIG. 9 , S 203 ). 
     It is to be noted that ty 2  in the formula above represents the length of time required for the signal electrical charge read, which is uniquely determined by the resolution at the image capturing element  19  and the frequency of the drive pulse. In addition, “0.8” in the formula above is a coefficient set to assure the safety of the operation timing and is a value slightly smaller than 1. 
     (4) In conformance to the wait time tf 2  thus determined, the timing with which the rotation of the sequence motor  25  is started for the next frame is determined ( FIG. 9 , S 113 ). 
     Thus, in the fourth embodiment, the operation timing is automatically adjusted based upon the time count performed in the photographing preparations. Consequently, even when the operation timing gradually changes due to fluctuations in the battery voltage and the like, problems such as the subject image in the next frame becoming projected onto the photosensitive surface during the electrical charge read period do not occur. 
     In addition, since it is not necessary to set the wait time tf 2  with a margin in correspondence to the fluctuations of the operation timing, the frame speed can be increased to the maximum degree. 
     It is to be noted that while shutter charge, mirror down, mirror up and aperture control are started during the electrical charge read period as preparations for photographing in the embodiments explained above, the present invention is not limited to this example. 
     For instance, as illustrated in  FIG. 10 , photographing preparations such as the focal point detection operation and the photometric operation may be started concurrently during the electrical charge read period. 
     It is to be noted that in this example, the focal point detection operation and the photometric operation are executed while the mirror is down, since the camera structure in which the subject light is guided to the focal point detection unit and the photometric unit while the mirror is down, as in the electronic camera  11  in  FIG. 1 , is the prerequisite in the example. However, in an electronic camera that is free of such limitation, the focal point detection operation and photometric operation may be started regardless of the state of the mirror to achieve an even higher frame speed. 
     In addition, while an operation that is performed when the continuous shooting mode is set has been explained in reference to the embodiments above, the present invention is not limited to this example. For instance, the photographing preparations for the next frame may be started during an electrical charge read period in a single-shot mode. Through such an operation, it becomes possible to reduce the length of time required to elapse before the next frame can be photographed even in the single-shot mode. It is to be noted that in this case, the length of the time interval elapsing before the shutter release for the next frame may be measured so that the photographing preparations for the next frame can be performed again if this time interval exceeds a specific length of time. By implementing such an operation, it becomes possible to achieve an optimal balance between higher frame speed and correct photographing preparations for the next frame in the single-shot mode. 
     The explanation has been given in reference to the embodiments on single lens reflex type electronic camera such as the electronic camera  11  illustrated in  FIG. 1  having a quick-return mirror, a shutter mechanism and the like. However, the details of the electronic camera do not need to be limited to these particulars. The particulars of the present invention may be adopted in an electronic camera that is not provided with a quick-return mirror, a shutter mechanism and the like. In other words, the present invention may be adopted in the entire range of electronic cameras that require specific photographing preparations to enable photographing of the next frame.