BACKGROUND OF THE INVENTION 
     The present invention relates to a camera, and relates, more particularly, to a camera using a film cartridge equipped with a light-shielding door to be electrically opened and closed, for example, at a film inlet and outlet opening. 
     A film cartridge of a film feed-out type currently available in the market is provided with an open/close system light-shielding door at a position near an opening for inlet and outlet of a film. 
     Therefore, for using this type of film cartridge, a camera needs to be provided with a mechanism for opening/closing this light-shielding door at the time of mounting or dismounting this film cartridge into or from the camera, as disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 7-253616 or the like. 
     This light-shielding door is provided movably between a closing position covering a film inlet and outlet opening for shielding an external light directed toward the inside of a film cartridge main body and an opening position for making it possible to feed out or wind back a film at a position sheltered from the film inlet and outlet opening. 
     However, in order to prevent a film from being exposed to a light by a careless opening of the light-shielding door while the film cartridge is being carried, the cartridge of this type needs to be equipped with a light-shielding door locking member for securely locking the light-shielding door at the closed position by restricting the move of the light-shielding door. 
     A film cartridge actually commercialized in the market uses a locking mechanism utilizing an elastic member as a mechanism of this locking member. 
     In a general camera or the like using this type of film cartridge, when the film cartridge is mounted in a cartridge chamber, a light-shielding door driving member provided in the cartridge chamber becomes engaged with a light-shielding door driving groove to make the light-shielding door locking member elastically deformed to cancel a locking state, and thereafter turns the light-shielding door to open it. 
     Then, when the light-shielding door is in completely opened state, the force for elastically deforming the locking member is removed. 
     Further, when a film winding has been completed and a film take-out operation is carried out to open a cartridge chamber lid, the light-shielding door is returned to a closed state before the cartridge chamber lid is opened. 
     Cameras recently available in the market are fully automated progressively. For example, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-306465 and others, not only a film feeding operation and a focus adjustment operation, but also an opening/closing operation of a film cartridge chamber lid and an opening/closing operation of a light-shielding door have come to be electrically driven by plunger or by motor. 
     According to these cameras, however, there are considered many cases where the power levels of power source batteries are lowered in the course of electrically driving various kinds of camera mechanisms so that it becomes impossible to operate the cameras themselves. 
     Therefore, in order to prevent a camera from being placed in such a situation in advance, a power source voltage level of a battery operation is checked prior to the starting of the operation. Based on a result of a detection of the check, a decision is made as to whether it is possible or not to operate the camera thereafter. When a decision has been made that it is not possible to operate the camera thereafter, the subsequent operation of the camera is prohibited. 
     However, the above camera operation control involves other problems. 
     These problems will be explained with reference to FIGS. 7A,  7 B and  7 C. 
     FIG. 7A is a view for illustrating a film cartridge with its light-shielding door in a closed state. 
     FIG. 7B is a view for illustrating the film cartridge with its light-shielding door in an opened state. 
     FIG. 7C is a partially enlarged view of FIG. 7B to show a state that a light-shielding door locking member is kept elastically deformed. 
     There will now be considered a case where pictures have been taken into all the film frames of a film in a camera using the above-described film feed-out type film cartridge, and after a film wind-back has been completed, a cartridge chamber lid opening button not shown is operated in an attempt to take out the film cartridge from the camera. 
     As shown in FIGS. 7A and 7B, a light-shielding door  3  is provided near an opening at a side surface  2  of a film cartridge  1 . 
     In the normal operation, at first, an actuator not shown for electrically driving the light-shielding door  3  operates according to the operation of the cartridge chamber lid opening button, thereby to make a light-shielding door driving member  6  engaged with an engaging member  4  turn to a predetermined direction. 
     Then, the light-shielding door  3  is set in a closed state as shown in FIG.  7 A. 
     Subsequently, an actuator not shown for opening/closing the cartridge chamber lid is operated so that the cartridge chamber lid is set to an opened state. 
     Further, for setting the light-shielding door  3  to an opened state, as shown in FIG. 7B, the actuator not shown for electrically driving the light-shielding door  3  operates to make the light-shielding door  3  turn through the light-shielding door driving member  6  and the engaging member  4 , thereby to set the light-shielding door  3  to the opened state. 
     As explained above, in the normal operation, when the film is mounted, the light-shielding door  3  is driven to an opened state, and then a film automatic loading operation is executed. 
     In the camera of this type, a battery check operation is carried out when a new film cartridge has been mounted into the camera, in order to prevent the camera from being put in an inoperable situation during the film automatic loading operation. 
     As a result of this battery check operation, when a decision has been made that a battery voltage level is not sufficient for the automatic loading, the operation of a change from a closing to an opening of the light-shielding door and the automatic loading operation are prohibited. 
     In general, the open operation of the cartridge chamber lid requires a larger driving force than the close operation of the light-shielding door  3 . 
     Accordingly, a current to be supplied to the actuator for opening the cartridge chamber lid is much larger than a current to be supplied to the actuator for closing the cartridge chamber lid. 
     Therefore, when the capacity of the power source battery is lowered, there could arise a situation that it is not possible to open the cartridge chamber lid although it is possible to close the light-shielding door  3 . 
     In this situation, the light-shielding door locking member  5  is kept in an elastically deformed state, as shown in FIG.  7 C. 
     Therefore, when this state is left as it is, a creep phenomenon occurs so that this light-shielding door locking member  5  is plastically deformed, which may result in a risk of it becoming difficult to execute a subsequent locking sufficiently. 
     Furthermore, there arises a similar problem to the above when some failure has occurred in the opening mechanism of the cartridge chamber lid, for example, when there arises a situation that it is not possible to open the cartridge chamber lid despite an attempt to make it in the opened state. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a camera which can avoid a plastic deformation of a light-shielding door locking member followed by a subsequent insufficient locking operation, when the capacity of a power source battery has been reduced to a lower level or when some failure has occurred in the opening mechanism of a cartridge chamber lid. 
     In order to achieve the above object of the invention, according to one aspect of the invention, for example, there is provided a camera adapted to use a film cartridge having a light-shielding door for opening/closing a film inlet and outlet opening, wherein the camera comprises: a cartridge chamber for accommodating the film cartridge; a cartridge chamber lid disposed at an opening of the cartridge chamber; a first driving mechanism for driving the cartridge chamber lid; a second driving mechanism for driving the light-shielding door; and control means for controlling the second driving mechanism to maintain the light-shielding door in an opened state when it is not possible to take out the film cartridge from the cartridge chamber. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a block diagram for showing a structure of whole key parts of a camera according to a first embodiment of the present invention. 
     FIG. 2 is a flowchart for explaining the open operation of a cartridge chamber lid in the camera according to the first embodiment of the invention. 
     FIG. 3 is a block diagram for showing a structure of whole key parts of a camera according to a second embodiment of the present invention. 
     FIG. 4 is a flowchart for explaining a film loading operation to be executed after a photographer has mounted a new film cartridge into a cartridge chamber by opening a cartridge chamber lid and then has closed the cartridge chamber lid again, according to the second embodiment of the invention. 
     FIG. 5 is a flowchart for explaining the film loading operation according to the second embodiment of the invention. 
     FIG. 6 is a flowchart for explaining a film loading operation according to a third embodiment of the present invention. 
     FIGS. 7A,  7 B and  7 C are views for illustrating a structure of a conventional film cartridge respectively, wherein 
     FIG. 7A is a view for illustrating a film cartridge with its light-shielding door in a closed state, 
     FIG. 7B is a view for illustrating the film cartridge with its light-shielding door in an opened state, and 
     FIG. 7C is a partially enlarged view of FIG. 7B to show a state that a light-shielding door locking member is kept elastically deformed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention as illustrated in the accompanying drawings, in which like reference characters designate like or corresponding parts throughout the several drawings. 
     Embodiments of the present invention will be explained below with reference to the drawings. 
     First Embodiment 
     FIG. 1 is a block diagram for showing a structure of whole key parts of a camera according to a first embodiment of the present invention. 
     In FIG. 1, a control circuit  11  is control means constituted by a microcomputer and others, for controlling a sequence and the like relating to the operations of a camera. 
     To this control circuit  11 , there are connected various kinds of circuits and mechanisms including mechanisms relating to mounting/dismounting of a cartridge, and other mechanisms including, for example, a mechanism relating to photographing. 
     More specifically, the control circuit  11  is connected with a cartridge chamber lid opening/closing mechanism  13  as a first driving mechanism for opening/closing a cartridge chamber lid  12  by receiving a driving force from a motor (not shown) and others, a cartridge chamber lid open/close detecting switch (SW)  14  as detecting means for detecting a state that the cartridge chamber lid  12  has been driven by the cartridge chamber lid opening/closing mechanism  13  and for transmitting this state to the control circuit  11 , a light-shielding door opening/closing mechanism  16  as a second driving mechanism for electrically opening/closing a light-shielding door ( 3  in FIG. 7)  15  of a film cartridge ( 1  in FIG. 7)  10  as shown in FIGS. 7A,  7 B and  7 C, and a light-shielding door open/close detecting switch (SW)  17  for detecting an open/close state of the light-shielding door  15  and for transmitting this state to the control circuit  11 . 
     The light-shielding door opening/closing mechanism  16  includes a driving member for being engaged with an engaging section of the film cartridge  10 , and drives this member by motor not shown. 
     The control circuit  11  is further connected with a power source voltage detecting circuit  18  as decision means for converting a voltage of a power source battery into digital data by an analog/digital (A/D) converter and for transmitting this digital data to the control circuit  11 , and a cartridge chamber lid open/close operation switch (SW)  21  for transmitting a signal to the control circuit  11  in co-operation with a cartridge chamber lid open/close operation button  20  provided on the external surface of a camera main body (not shown). 
     The cartridge chamber lid opening/closing mechanism  13  and the light-shielding door opening/closing mechanism  16  may be structured to be driven by an electric actuator A like a plunger, other than the motor as their driving source. 
     Further, instead of providing exclusive actuators for respective mechanisms, a common actuator may be provided to be able to switch over a driving force from one mechanism to the other during the operation of a driving mechanism. 
     The control circuit  11  is further connected with an image-picked-up object evaluating mechanism  22  for detecting information of an image-picked-up object such as luminance of the object and distance from the object necessary for photographing, and for transmitting this information to the control circuit  11 , an information input mechanism  23  for inputting information such as ISO sensitivity and exposure correction, etc. displayed on the film cartridge  10 , and for transmitting this information to the control circuit  11 , and camera element driving mechanism  24  for driving camera elements  25  such as a focus adjusting mechanism including an image pick-up lens and a focus adjusting lens, a focusing mechanism, a shutter mechanism and a film feeding mechanism, that are not shown, according to control signals from the control circuit  11 . 
     The control circuit  11  is further connected with a state detecting mechanism  26  consisting of a plurality of switches for detecting a state of the camera elements  25  at a current time, such as, for example, a current position of the focus adjusting lens, a shutter open/close state, a stop level, a film feeding state, etc., and a display circuit  27  including an alarm display, for displaying various kinds of state of the camera. 
     As a detecting operation of the power source voltage detecting circuit  18 , it is general to measure a voltage reduction when a current is flown to a pseudo load. However, an open voltage may also be measured depending on the characteristics of the battery. 
     A result of the measurement by the power source voltage detecting circuit  18  is used as a material for making a decision of a battery remaining capacity. 
     In the above structure, the mechanism relating to the mounting/dismounting of the cartridge is constituted by the cartridge chamber lid opening/closing mechanism  13 , the cartridge chamber lid open/close detecting switch  14 , the light-shielding door opening/closing mechanism  16 , the light-shielding door open/close detecting switch  17  and the cartridge chamber lid open/close operation switch  21 . 
     On the other hand, the mechanism relating to the photographing is constituted by the image-picked-up object evaluating mechanism  22 , the information input mechanism  23  and the camera element driving mechanism  24 . 
     The open operation of the cartridge chamber lid  12  in the camera according to the first embodiment of the present invention will be explained next with reference to a flowchart shown in FIG.  2 . 
     Normally, this operation is carried out when the film cartridge  10  containing an unexposed film is to be mounted in the camera main body not shown, or when the film cartridge  10  containing a film completely wound back after the exposure of the film by picking up images is to be taken out from the camera main body not shown. The execution of this operation is started when an open operation signal is input from the cartridge chamber lid open/close operation switch  21  in a switch input waiting state. 
     All the operations including this operation are executed by the control of the control circuit  11  according to a program stored in advance in the read-only memory (ROM) within the control circuit  11 . 
     At first, in step S 1 , based on a result of a detection of the state detecting mechanism  26 , a decision is made as to whether or not the film in the film cartridge  10  has been in the state of being fed out from the cartridge into the camera main body, that is, whether or not the film wind-back operation is yet to be completed. 
     In this case, when a decision has been made that the film is not being fed out from the cartridge into the camera main body, the process proceeds to step S 2 , and a result of a measurement of a current power source voltage is obtained from the power source voltage detecting circuit  18 . 
     On the other hand, when a decision has been made in the step S 1  that the film is being fed out from the cartridge into the camera main body, the cartridge chamber lid  12  cannot be opened, and therefore, this sequence finishes without any action. 
     In step S 3 , a decision is made as to whether or not the film cartridge  10  has already been mounted in the cartridge chamber  30  of the camera main body, that is, whether or not the film cartridge  10  remains in the cartridge chamber  30  in the state that the film wind-back has been completed. 
     In this case, a state that the cartridge chamber lid open/close button  20  has been operated in the state that the film cartridge  10  has already been mounted in the cartridge chamber means a state that the film cartridge  10  containing the film after completion of the image pick-up of all frames is going to be taken out from the cartridge chamber  30 . 
     On the other hand, a state that the film cartridge  10  has not yet been mounted means a state that a new film cartridge  10  is going to be mounted in the cartridge chamber  30 . 
     Accordingly, when the film cartridge  10  has been mounted in the above step S 3 , the process goes to step S 4 , and a decision is made as to whether or not the battery voltage is at a first decision level L 1  or above. 
     The level L 1  is a level of the battery voltage at which the film cartridge  10  after finishing the photographing can be taken out from the cartridge chamber  30   m , that is, a level of the battery remaining capacity that can guarantee the close driving of the light-shielding door  15  and the open operation of the cartridge chamber  30 . 
     When the battery voltage is at the first voltage decision level L 1  or above in the above step S 4 , the process proceeds to step S 5 , and the light-shielding door  15  is set to an opened state. 
     This operation is executed when a control signal has been sent from the control circuit  11  to the light-shielding door opening/closing mechanism  16  and the light-shielding door opening/closing mechanism  16  drives the actuator for driving the light-shielding door according to this signal. 
     Next, in step S 6 , the cartridge chamber lid  12  is set to the opened state. 
     This operation is executed when a control signal has been sent from the control circuit  11  to the cartridge chamber lid opening/closing mechanism  13  and the cartridge chamber lid opening/closing mechanism  13  drives the actuator for driving the cartridge chamber lid according to this signal. 
     Next, in step S 7 , a decision is made as to whether or not the cartridge chamber lid  12  has been actually opened based on an output of the cartridge chamber lid open/close detecting switch  14 . 
     In this case, when a decision has been made that the cartridge chamber lid  12  is in the opened state, the sequence of “open operation of the cartridge” finishes. 
     On the other hand, when the cartridge chamber lid  12  is not in the opened state in the above step S 7 , it is considered that the opening mechanism of the cartridge chamber lid opening/closing mechanism  13  is in a fault state or the cartridge chamber lid  12  is being pressed by some force from the outside. 
     Accordingly, the step proceeds to step S 8 , and a control signal is sent from the control circuit  11  to the light-shielding door opening/closing mechanism  13  so that the light-shielding door  12  is set to the opened state again. 
     This is for preventing the light-shielding door locking member ( 5 ) from being plastically deformed in the film cartridge  10  as shown in FIG.  7 C. 
     Thereafter, no action is taken, and the sequence of this “open operation of the cartridge” finishes. 
     When a decision is made that there is no cartridge in the above step S 3 , a new film cartridge  10  is being mounted. Therefore, the process proceeds to step S 9 , and a decision is made as to whether or not the battery voltage is at the second voltage decision level L 2  or above. 
     This level L 2  is a level of a battery remaining capacity which guarantees the execution of a predetermined operation after the film cartridge  10  containing the new film has been mounted, for example, the execution of the film feeding operation and the image pick-up operation. 
     In the above step S 9 , when the battery voltage level is L 2  or above, the process proceeds to step S 5 , and the above-described operation is executed. 
     On the other hand, when the power source voltage level is less than L 2  in the step S 9 , the process proceeds to step S 10 , and a control signal from the control circuit  11  for alarming that the power source voltage level is lower than the L 2  level is displayed in the display circuit  27 . Then, the display circuit  27  finishes the operation. 
     Upon receiving the control signal, the display circuit  27  executes an alarm display operation to show that the photographing operation is not possible. 
     When a decision has been made in the step S 4  that the battery voltage is not L 1  or above, the process proceeds to step S 10 . 
     Then, a control signal for alarming that the battery voltage is lower than the L 1  level is output to the display circuit  27 . After the display circuit  27  has made a predetermined display, the operation finishes. 
     In this case, upon receiving the control signal, the display circuit  27  executes an alarm display operation to show that the open operation of the cartridge chamber lid is not possible. 
     Second Embodiment 
     Next, a camera according to a second embodiment of the present invention will be explained. 
     FIG. 3 is a block diagram for showing a structure of whole key parts of a camera according to the second embodiment of the invention. 
     In FIG. 3, a control circuit  111  is control means constituted by a microcomputer and others, for controlling a sequence and the like relating to the operations of a camera. 
     To this control circuit  111 , there are connected various kinds of circuits and mechanisms including mechanisms relating to mounting/dismounting of a cartridge, and other mechanisms including, for example, a mechanism relating to photographing. 
     The control circuit  111  is connected with a cartridge chamber lid open driving mechanism  113  for electrically opening a cartridge chamber lid  112  by receiving a driving force from a motor (not shown), a cartridge chamber lid open/close operation detecting mechanism  114  for detecting a state that the cartridge chamber lid  112  has been driven by the cartridge chamber lid open driving mechanism  113  and for transmitting this state to the control circuit  111 , and a cartridge chamber lid open/close operation detecting switch (SW)  116  for detecting a state that the cartridge chamber lid  112  has been opened by operating a cartridge chamber lid open/close operation button  115  or a state that the cartridge chamber lid has been closed manually, and for transmitting this state to the control circuit  111 . 
     The cartridge chamber lid open driving mechanism  113  is operated when the cartridge chamber lid open operation button  115  has been operated by a photographer and the cartridge chamber lid open operation detecting switch  116  has been turned on. 
     The control circuit  111  is connected with a film feeding mechanism  118  as film loading means for carrying out a loading of a film  117  (the operation of feeding out the film  117  from a film cartridge  110  and a winding to a first frame), a one-frame winding operation after an image pick-up, and a wind-back operation after picking up the whole frames, a film feed volume detecting mechanism  119  for detecting a feed volume of the film  117 , an light-shielding door opening/closing mechanism  122  as open/close means for electrically opening/closing a light-shielding door  121  of the film cartridge  110 , and a light-shielding door open/close detecting mechanism  123  for detecting an open/close state of the light-shielding door  121  and transmitting this state to the control circuit  111 . 
     The light-shielding door opening/closing mechanism  122  includes a driving member for being engaged with an engaging section of the film cartridge  110 , and drives this member by motor not shown. 
     The cartridge chamber lid open driving mechanism  113  and the light-shielding door opening/closing mechanism  122  may be structured by an electric actuator A like a plunger or the like, other than the motor. 
     Further, instead of providing exclusive actuators for respective mechanisms, a common actuator may be provided to be able to switch over a driving force from one mechanism to the other during the operation of a driving mechanism. 
     The control circuit  111  is further connected with a battery checking circuit  125  as battery checking means for converting a voltage of a power source battery not shown into digital data by an analog/digital (A/D) converter and for transmitting the digital data to the control circuit  111 , an image-picked-up object evaluating means  126  for detecting information on the image-picked-up object such as luminance of the object and a distance from the object necessary for photographing, and for transmitting this information to the control circuit  111 , a camera element driving mechanism  127  as camera operation means for driving a camera mechanism  128  including an image pick-up lens, a focus adjusting mechanism, a stopping mechanism, a shutter mechanism, etc., other than the film feeding mechanism  118 , according to control signals from the control circuit  111 , a state detecting mechanism  129  structured by a plurality of switches for detecting a state at a current time of the camera mechanism  128 , such as, for example, a current position of the focus adjusting lens, an open/close state of the shutter, a stopping volume, etc., an information input mechanism  131  for inputting information of ISO sensitivity, an exposure correction, etc. displayed on the film cartridge  110  and for transmitting this information to the control circuit  111 , and an alarm display circuit  132  for displaying various states of the camera. 
     As a detecting operation of the battery checking circuit  125 , it is general to measure a voltage reduction when a current is flown to a pseudo load. However, an open voltage may also be measured depending on the characteristics of the battery. A measured voltage is compared with a predetermined voltage, and a remaining battery capacity can be estimated from a result of the comparison. 
     For the operation of the battery checking circuit  125 , the battery remaining capacity can also be decided by a comparator circuit by dividing a power source voltage, in addition to the above-described detection using the A/D converter. 
     The alarm display circuit  132  is for alarm displaying a state of the camera based on a result of a battery check. This alarm display is carried out, for example, in voice using a PCV or by light flicker using a LED, a LCD or the like. 
     In the above-described structure, the mechanism relating to the mounting/dismounting of the cartridge is constituted by the cartridge chamber lid open driving mechanism  113 , the cartridge chamber lid open/close operation detecting mechanism  114 , the cartridge chamber lid open/close operation detecting switch  116 , the film feeding mechanism  118 , the film feed volume detecting mechanism  119 , the light-shielding door opening/closing mechanism  112 , and the light-shielding door open/close detecting mechanism  123 . 
     On the other hand, the mechanism relating to the photographing is constituted by the image-picked-up object evaluating means  126 , the camera element driving mechanism  127 , the state detecting mechanism  129 , and the information input mechanism  131 . 
     The open/close operation of the cartridge chamber lid  112  will be explained next with reference to a flowchart shown in FIG.  4 . 
     FIG. 4 shows a film loading operation sequence to be executed after a photographer has mounted the new film cartridge  110  into a cartridge chamber  130  by opening the cartridge chamber lid  112  and then has closed the cartridge chamber lid  112  again. 
     Normally, this operation is carried out in response to a signal, that is, a signal for showing that the cartridge chamber lid  112  has been closed, detected by the cartridge chamber lid open/close operation detecting mechanism  114 , in a waiting state of the switch input in the main routine. 
     All the operations including this operation are executed by the control of the control circuit  111  according to a program stored in the read-only memory (ROM) within the control circuit  111 . 
     At first, in step S 11 , a battery check operation is carried out. 
     This is an operation for measuring by the A/D converter a reduction in voltage when a current is flown to a pseudo load, such as, for example, a resistor, as described above. 
     Then, in step S 12 , a result of the measuring is compared with a decision level L 1  that is a predetermined level. 
     The decision level L 1  is a level for deciding whether or not the remaining capacity of the power source battery is sufficient enough to execute the “film loading operation”, that is, whether the remaining capacity of the power source battery can complete the film loading operation without stopping in the middle. 
     This value is set at the time of designing the camera, together with the value of the pseudo load. 
     When the measured voltage is L 1  or above in the step S 12 , the process proceeds to step S 13 , and the “light-shielding door open operation” is executed prior to the “film loading operation”. 
     The light-shielding door open operation is an operation for opening the light-shielding door  121  of the film cartridge  110  by the light-shielding door opening/closing mechanism  122 . 
     As the driving load of the light-shielding door opening/closing mechanism  122  is smaller than that of the film feeding mechanism  118 , when the voltage level checked in the step S 1  is L 1  or above, the light-shielding door opening/closing mechanism  122  can operate without problem. 
     Whether the light-shielding door  121  has been actually opened or not is detected by the light-shielding door open/close detecting mechanism  123 . When the opened state has been detected, this operation is finished. 
     Then, the “loading operation” is executed in step S 14 . 
     The loading operation is an operation for feeding out the film  117  from within the film cartridge  110  and bringing the first frame of the film  117  to an image pick-up open position by the film feeding mechanism  118 . 
     This operation is controlled according to a result of a detection by the film feed volume detecting mechanism  119 . 
     When the remaining capacity of the power source battery is sufficient, the loading operation is finished with the above-described flow. 
     However, when the battery remaining capacity becomes smaller, that is, when a decision has been made in the step S 12  that the voltage is not L 1  or above (that is, less than L 1 ), the process proceeds to step S 15 . In other words, when a decision has been made that it is not possible to execute the “film loading operation”, the process proceeds to step S 15 . 
     In the step S 15 , it is checked whether the voltage level is at a predetermined decision level L 2  or above, the L 2  being different from the decision level L 1 . 
     This decision level L 2  is for deciding whether it is possible or not to carry out the “open operation of the light-shielding door”, and the voltage value of L 2  is smaller than the voltage value of L 1  (L 1 &gt;L 2 ). 
     As a result of the comparison in the step S 15 , when the measured voltage is L 2  or above, it is possible to carry out the “open operation of the light-shielding door” although it is not possible to carry out the “film loading operation”. Accordingly, the process proceeds to step S 16 , and the “open operation of the light-shielding door” is executed. 
     Then, the process proceeds to the “alarm display” in step S 17 , and a state that the remaining capacity of the battery is small is displayed in the alarm display circuit  132 . Then, the process returns. 
     On the other hand, when the measured voltage is not L 2  or above, that is, when the measured voltage is less than L 2 , in the step S 5 , this indicates a state that the remaining capacity of the battery has been reduced to a level at which it is not possible to drive any one of the actuators. 
     Accordingly, in this case, the process proceeds to step S 17 , and the “alarm display” for indicating that the battery capacity has been exhausted is displayed in the alarm display circuit  132 , and then the process returns. 
     When the process has returned to the main routine after carrying out the alarm display, the subsequent camera operations are not possible. Therefore, it is desirable that the camera is directly put in the open locked state. 
     As explained above, according to the second embodiment of the invention, when a measured voltage level of the battery at the battery check time is less than the first decision level (L 1 ) at which it is possible to carry out the “film loading operation”, the camera is not directly put to the locked state. Instead, it is checked whether the measured voltage level is equal to or above the second decision level (L 2 ) at which it is possible to carry out the “open operation of the light-shielding door”. If the measured voltage level is the second decision level or above, only the “open operation of the light-shielding door” is carried out without carrying out the “film loading operation”. 
     Third Embodiment 
     Next, a camera according to a third embodiment of the present invention will be explained. 
     FIG. 5 is a flowchart for explaining a film loading operation in a camera according to the third embodiment of the invention. 
     The structure of the camera in the third embodiment is the same as the structure of the camera in the second embodiment shown in FIG.  3 . 
     In this example, at first, the “battery check  1 ” for deciding whether it is possible or not to carry out the “open operation of the light-shielding door” is executed in step S 21 . 
     Next, in step S 22 , a measured voltage is compared with the decision level L 2  that is the lower decision value. 
     When the measured voltage is less than L 2 , the process proceeds to step S 27 , and the “alarm display” for indicating that the battery has been exhausted is displayed in the alarm display circuit  132 . Then, the process returns. 
     On the other hand, when the measured voltage is L 2  or above, the “open operation of the light-shielding door” is executed in step S 23 . Then, in step S 24 , the “battery check  2 ” is executed for deciding whether it is possible or not to carry out the “film feeding operation”. 
     Then, in step S 25 , the measured voltage is compared with the decision level L 1 . 
     In this case, when the measured voltage is less than L 1 , the process proceeds to step S 27 , and the “alarm display” for indicating that the battery has been exhausted is displayed in the alarm display circuit  132 . Then, the process returns. 
     On the other hand, when the measured voltage is L 1  or above, the “loading operation” is executed in step S 26 , and the process returns. 
     As explained above, according to the third embodiment, at first, the “battery check  1 ” is executed for deciding whether it is possible or not to carry out the “open operation of the light-shielding door”. After the execution of the “open operation of the light-shielding door”, the “battery check  2 ” is executed for deciding whether it is possible or not to carry out the “film feeding operation”. In this respect, the third embodiment is different from the second embodiment. 
     According to the third embodiment, battery check is carried out twice. Thus, consideration is given to battery exhausting in the “open operation of the light-shielding door”. 
     Fourth Embodiment 
     Next, a camera according to a fourth embodiment of the present invention will be explained. 
     FIG. 6 is a flowchart for explaining a film loading operation in a camera according to the fourth embodiment of the invention. 
     The structure of the camera in the fourth embodiment is the same as the structure of the camera in the second embodiment shown in FIG.  3 . 
     At first, the “open operation of the light-shielding door” is executed in step S 31 . Then, the “battery check” for deciding whether it is possible or not to carry out the “film feeding operation is carried out in step S 32 . 
     Next, in step S 33 , a measured voltage is compared with the decision level L 1 . 
     When the measured voltage is less than L 1 , the process proceeds to step S 35 , and the “alarm display” for indicating that the battery has been exhausted is displayed in the alarm display circuit  132 . Then, the process returns. 
     On the other hand, when the measured voltage is L 1  or above, the process proceeds to step S 34 , and the “loading operation” is executed. Then, the process returns. 
     As explained above, according to the fourth embodiment, the battery check relating to the “open operation of the light-shielding door” is abolished. Regardless of the battery remaining capacity, the “open operation of the light-shielding door” is executed and then the battery check is carried out, for the following reason. 
     The battery check for deciding whether it is possible or not to carry out the “open operation of the light-shielding door” is abolished to save time and process, based on a fact that it is possible to carry out the “open operation of the light-shielding door” at considerably high probability levels regardless of a small battery remaining capacity as the load of the light-shielding door opening/closing mechanism is small, that is, the power consumption for the operation of the light-shielding door opening/closing mechanism is small. 
     As explained above, according to the present invention, it is possible to provide a camera which can avoid a plastic deformation of the light-shielding door locking member followed by a subsequent insufficient locking operation, when the capacity of the power source battery has been reduced to a lower level or when some fault has occurred in the opening mechanism of the cartridge chamber lid, as shown particularly in the first embodiment. 
     Further, according to the present invention, when a decision has been made as a result of a battery checking operation that it is possible to open the light-shielding door even though it is not possible to feed the film, the film feeding operation is prohibited and only the open operation of the light-shielding door is permitted, as shown particularly in the second to fourth embodiments. Accordingly, it is possible to reduce the frequency of the occurrence of the inconvenience that the light-shielding door locking member is plastically deformed because of the leaving of the light-shielding door in the opened state. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
A camera uses a film cartridge having a light-shielding door for opening/closing a film inlet and outlet opening. A cartridge chamber accommodates the film cartridge. A cartridge chamber lid is provided at the opening of the cartridge chamber. A first driving mechanism drives the cartridge chamber lid. A second driving mechanism drives the light-shielding door. A control section controls the second driving mechanism to maintain the light-shielding door in an opened state when it is not possible to take out the film cartridge from the cartridge chamber.