Abstract:
An image capture device includes a light source, an image capture unit, a brightness detector, an object distance obtaining unit which obtains an object distance to the object, first and second controllers, and an amplifier which amplifies an image signal output from the image capture unit by an amplifying factor according to a sensitivity set by one of the first and second controllers. The first controller disables the light source and sets a sensitivity which is in accordance with the brightness of the object and which is lower than a predetermined sensitivity. The second controller enables the light source and sets a sensitivity which is lower than the predetermined sensitivity. The second controller includes a sensitivity obtaining unit which obtains a sensitivity according to the object distance and a maximum amount of light emitted from the light source, and a setting unit which sets the obtained sensitivity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-008905, filed Jan. 16, 2004, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image capture device to be used for a digital camera comprising, for example, an ISO sensitivity automatic adjusting function, and a sensitivity setting method therefor. 
   2. Description of the Related Art 
   Conventionally, a digital camera comprising a stroboscope image capturing function is known. If a shutter key is half pressed, the digital camera calculates a distance to an object, and calculates and sets an amplifying factor (ISO sensitivity) of an AGC amplifier and a light measuring value of a stroboscope device based on the calculated distance. Then, if the shutter key is fully pressed, the stroboscope emits a light beam for the time according to the above-mentioned light measuring value as well as an image signal output from a CCD is amplified by the above-mentioned amplifying factor for recording the obtained image data. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention has been achieved in view of the above-mentioned conventional problem, and an object thereof is to provide an image capture device capable of image capturing an object with an adequate brightness while reducing influence on a battery life as much as possible, and a sensitivity setting method therefor. 
   According to an embodiment of the present invention, an image capture device comprises: 
   a light source which emits light; 
   an image capture unit which outputs an image signal; 
   a brightness detector which detecting a brightness of an object; 
   a first controller which disables the light source and sets a sensitivity which is in accordance with the brightness of the object detected by the brightness detector and which is lower than a predetermined sensitivity; 
   a second controller which enables the light source and sets a sensitivity which is lower than the predetermined sensitivity; and 
   an amplifier which amplifies the image signal output form the image capture unit by an amplifying factor according to the sensitivity set by one of the first and second controllers. 
   According to another embodiment of the present invention, a sensitivity setting method for an image capture device which causes a light source to operate according to a brightness of an object and amplifies an image signal by an amplifying factor according to the set sensitivity, the method comprises: 
   disabling the light source, setting a sensitivity which is in accordance with the brightness of the object detected by a detector and which is lower than a predetermined sensitivity; and 
   enabling the light source and setting a sensitivity which is lower than the predetermined sensitivity. 
   Additional objects and advantages of the present 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 present invention. 
   The objects and advantages of the present 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 embodiments of the present invention and, together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the present invention in which: 
       FIG. 1  is a block diagram schematically showing a configuration of a digital camera according to an embodiment of the present invention; 
       FIG. 2  is a program diagram; 
       FIG. 3  is a flow chart showing a process procedure at the time of half pressing a shutter key; 
       FIG. 4  is a flow chart showing a process procedure at the time of fully pressing the shutter key; 
       FIG. 5A  is a correspondence table of an ISO sensitivity, an object distance, or the like and an amount of stroboscope emission light; 
       FIG. 5B  is a flow chart showing a modified embodiment of the process procedure at the time of fully pressing the shutter key; 
       FIG. 6A  is a correspondence table of a brightness of the object (luminance value of an image signal), and the ISO sensitivity and an amount of stroboscope emission light at the time of the pre-emission of light; and 
       FIG. 6B  is a flow chart showing a modified embodiment of the process procedure at the time of half pressing the shutter key. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of an image capture device according to the present invention will now be described with reference to the accompanying drawings.  FIG. 1  is a block diagram schematically showing a configuration of a digital camera  1  according to an embodiment of the present invention. The digital camera  1  mainly comprises a CCD  2  as image capture means, and an MPU  3 . The MPU  3  has an image processing function of compressing and decompressing an image captured by the CCD  2  and producing and reproducing an image file of a format corresponding to a predetermined format, such as a universal standard DCF (design rule for camera file system) and a JPEG (joint photographic expert group). An optical image of an object is focused onto a light receiving surface of the CCD  2  through an image capture lens  4 , a focus lens  5  and a diaphragm  6 . The focus lens  5  is supported by a driving mechanism  7  comprising an AF motor and the like, and executes a focusing operation of reciprocally moving on the optical axis by supplying a driving signal output from an AF driver  8  to the driving mechanism  7  by a control signal from the MPU  3 . The diaphragm  6  is driven by a driving signal generated by a diaphragm driving unit  9  based on the control signal from the MPU  3  so as to adjust an amount of the light of the object image to be incident on the CCD  2 . 
   Moreover, the MPU  3  is connected with a TG (timing generator)  10  for generating a timing signal such that the CCD  2  is driven by a V driver  11  (vertical direction driver) based on the timing signal generated by the TG  10  so as to thereby output an analog image signal according to the luminance of the object image from the CCD  2  and send the same to a unit circuit  12 . The unit circuit  12  comprises a CDS (correlation double sampler) for storing an image signal output from the CCD  2 , au automatic gain controller (AGC) as an analog amplifier to which the image signal is supplied from the CDS, and an A/D converter (AD) for converting the analog image signal amplified and adjusted by the AGC into a digital signal, and the output signal of the CCD  2  is sampled according to the black level so as to be sent to the MPU  3  as the digital signal. The AGC amplifies the image signal to be supplied from the CDS by the amplifying factor according to an ISO sensitivity to be described later. 
   The MPU  3  is connected with an operation key unit  13  comprising various key groups including a shutter key, a TFT liquid crystal monitor  14 , a stroboscope  15 , a DRAM  16 , an MROM (mask ROM)  17 , and a flash memory  18 . The stroboscope  15  emits the light of which amount is controlled by the MPU  3 . 
   The DRAM  16  is an operation memory and the flash memory  18  is an image recording memory. The digital signal (image signal) output from the CCD  2  to the MPU  3  is stored temporarily in the DRAM  16  so as to be recorded in the flash memory  18  finally as the compressed image data after the MPU  3  has executed various image processes. The flash memory  18  may be of a card type detachable on the camera main body. 
   The image data recorded in the flash memory  18  are read out to the MPU  3  as needed. They are converted to a digital video signal or an analog video signal via a process such as a decompressing process, addition of a luminance signal and a color signal, or the like so as to be displayed by the TFT liquid crystal monitor  14 . Moreover, an image periodically captured in the recording mode is displayed on the TFT liquid crystal monitor  14  as a through image. 
   The MROM  17  is a program ROM for recording various kinds of operation programs needed for the control and the data process of each unit in the MPU  3 . That is, the MROM  17  stores various kinds of data such as a program diagram  171  showing a combination of an ISO sensitivity and a shutter speed corresponding to an appropriate exposure value (EV) (if the diaphragm value F is 2.8) at the time of image capturing shown in  FIG. 2  together with the above-mentioned programs. In the program linear diagram  171 , “▪” denotes a stroboscope automatic light emission point for automatically emitting a light beam from the stroboscope  15 . 
   The MPU  3  carries out the AE control by a charge accumulation time of the CCD  2  as the shutter speed, an opening degree of the diaphragm  6  and a gain control of the AGC of the unit circuit  12  according to the above-mentioned program diagram as well as it carries out the AF (auto focus) control for driving the focus lens  5  according to a predetermined program and the AWB (auto white balance) control. Further, if the shutter key is operated (at the time of image capturing), as needed, it controls the light emission of an image capture auxiliary light beam by the stroboscope  15  and an amount of emission light. The captured image data is recorded in the flash memory  18 . 
   A power source needed for each unit is supplied from a battery (not shown). 
   Next, an operation in a case of a half pressing operation of the shutter key at the time of image capturing by the digital camera  1  having the above-mentioned configuration will be explained with reference to the flow chart of  FIG. 3 . That is, in the case of the half pressing operation of the shutter key, the MPU  3  starts the operation based on the above programs so as to measure a brightness of the object based on the through image (step S 1 ). Next, an object distance D-af (m) as a distance to the object is obtained (step S 2 ). At step S 2 , the focus control of the focus lens  5  is executed by a known contrast AF method so as to obtain the object distance D-af using a focus setting table stored in the MROM  17  from the zoom ratio of the image capture lens  4  and the position of the focus lens  5  at the time, or distance information set by the mode of a manual focus, a macro mode, or the like is provided as the object distance D-af. 
   Subsequently, based on the program diagram  171 , the processes of step S 3  to step S 7  are executed. That is, based on the exposure value EV corresponding to the brightness of the object, the value being calculated in the above-mentioned step S 1 , it is determined whether or not the shutter speed set by the lowest ISO sensitivity (ISO 50) on the program diagram  171  is slower than 1/60 (step S 3 ). Here, the shutter speed 1/60 is the limit value of the slowest shutter speed capable of restraining the occurrence of blurring of images due to camera shake. Then, if the shutter speed set by the lowest ISO sensitivity is not slower than 1/60 (step S 3 ; NO), the shutter speed, the diaphragm and the sensitivity of the image capturing setting are set (step S 4 ). 
   However, if the shutter speed is slower than 1/60, it is determined whether or not the ISO sensitivity of the image capturing setting is the maximum value (ISO 800 in this embodiment) (step S 5 ). If it is not the maximum value, with the ISO sensitivity raised by one grade (step S 6 ), determination in the above-mentioned step S 3  is executed again. If the shutter speed becomes not slower than 1/60 (step S 3 ; NO), the shutter speed, the diaphragm and the sensitivity of the image capturing setting are set (step S 4 ). However, if the shutter speed is slower than 1/60 again, with the ISO sensitivity raised by one grade in the step S 6  process if the determination in step S 5  is NO, the shutter speed, the diaphragm and the sensitivity of the image capturing setting are set in step S 4 . 
   However, if the determination in step S 5  becomes YES, that is, if the shutter speed is slower than 1/60 even if the ISO sensitivity is the maximum value (ISO800), a mode of stroboscope light emission is set to be the automatic light emission mode as well as the diaphragm and the sensitivity are set with the shutter speed 1/60 (step S 7 ). At the time, as to the sensitivity, it is selected optionally from the ISO sensitivities 50, 100, 200, 400 and 800 and set. 
   Then, in step S 8  subsequent to step S 4  or step S 7 , it is determined whether or not the image capturing condition set by the above-mentioned image capturing setting includes a stroboscope light emission (step S 8 ). That is, since the stroboscope light emission is not included in the image capturing condition in the case of the procedure from step S 4  to step S 8 , the determination in step S 8  is NO. Therefore, in this case, the process according to the flow at the time of half pressing the shutter key is finished. If the shutter key is fully pressed, the image data captured with the shutter speed (the charge accumulation time of the CCD  2 ), the diaphragm and the ISO sensitivity set in the above-mentioned step S 4  without the stroboscope light emission is recorded in the flash memory  18 . Accordingly, since the image capturing operation is executed without the stroboscope light emission, shortening of the battery life by the frequent use of the stroboscope light emission can be prevented. 
   On the other hand, in the case of the procedure from step S 7  to step S 8 , since the stroboscope light emission is included in the image capturing condition, the determination in step S 8  is YES. In this case, a stroboscope reaching distance D-ae is calculated by the below-mentioned equation (1) (step S 9 ): 
   
     
       
         
           
             
               
                 
                   D 
                   ⁢ 
                   
                     - 
                   
                   ⁢ 
                   ae 
                 
                 = 
                 
                   
                     GN 
                     F 
                   
                   × 
                   
                     
                       
                         ISO 
                         ⁢ 
                         
                           - 
                         
                         ⁢ 
                         capt 
                       
                       100 
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   wherein GN is the maximum amount of light inherent to the digital camera (guide number), F is the diaphragm value and ISO-capt is the image capturing sensitivity (ISO sensitivity). 
   Next, it is determined whether or not D-ae&gt;D-af (step S 10 ). As a result of this determination, if D-ae&gt;D-af (step S 10 ; YES), since the stroboscope reaching distance D-ae (m) of the stroboscope in the ISO sensitivity (ISO-capt) is longer than the object distance D-af (m), it means that there is a possibility of clearly image capturing the object at the object distance D-af even if the ISO sensitivity (ISO-capt) is lowered. Therefore, in this case, with the setting sensitivity lowered by one grade, the ISO-capt substituted in the equation (1) is lowered by one grade (step S 11 ). Next, using the ISO-capt lowered by one grade, that is, with the ISO-capt lowered by one grade substituted in the equation (1), the reaching distance D-ae is calculated (step S 12 ). 
   Furthermore, it is determined whether or not the relationship between the reaching distance D-ae calculated by substituting the ISO-capt lowered by one grade in the equation (1) and the above-mentioned object distance D-af still satisfies the condition: D-ae&gt;D-af (step S 13 ). If it still satisfies the condition: D-ae&gt;D-af, it is determined whether or not the ISO-capt substituted in the equation (1) reaches the lowest sensitivity (in this embodiment, ISO 50) (step S 14 ). If it does not reach that, the processes from step S 11  are repeated. 
   Then, in the case of repeating the processes of steps S 11  to S 14 , at the time if the condition: D-ae&gt;D-af at the setting sensitivity is not satisfied (step S 13 ; NO), that is, at the time if the stroboscope reaching distance D-ae calculated by using the ISO-capt is not longer than the object distance D-af, in other words, at the time if the ISO-capt is not excessive, the process according to the flow at the time of half pressing the shutter key is finished. In addition, in the case of repeating the processes of steps S 11  to S 14 , also in the case where the ISO-capt substituted in the equation (1) reaches at the lowest sensitivity (step S 14 ; YES), since the ISO sensitivity cannot be further lowered, the process according to the flow at the time of half pressing the shutter key is finished. 
   On the other hand, if the condition: D-ae&gt;D-af is not satisfied as a result of the determination in step S 10  (step S 10 ; NO), since the stroboscope reaching distance D-ae at the ISO sensitivity (ISO-capt) is shorter than the object distance D-af, it is necessary to increase the stroboscope reaching distance D-ae by raising the ISO sensitivity (ISO-capt). Therefore, it is determined whether or not the ISO-capt substituted in the equation (1) so as to be used for the calculation of the stroboscope reaching distance D-ae reaches the maximum sensitivity (in this embodiment, ISO 800) (step S 15 ). If it does not reach that, with the ISO-capt to be substituted in the equation (1) raised by one grade (step S 16 ), the processes from step S 9  are repeated. If the condition: D-ae&gt;D-af is not satisfied, the determination in step S 10  becomes YES so as to proceed to the above-mentioned step S 11 . 
   However, if the ISO-capt substituted in the equation (1) so as to be used for the calculation of the stroboscope reaching distance D-ae reaches the maximum sensitivity without satisfying the condition: D-ae&gt;D-af (step S 10 ; NO and the step S 15 ; YES), it means that the condition: D-ae&gt;D-af cannot be satisfied even with the maximum amount of stroboscope light (GN) and the ISO-capt maximum sensitivity. Therefore, since the stroboscope light beam of the digital camera  1  cannot be reached to the object at the object distance D-af, the warning alert is provided on the TFT liquid crystal monitor  14  and the process according to the flow at the time of half pressing the shutter key is finished. 
   If the ISO-capt (ISO sensitivity) is determined with the stroboscope light emission set as the image capturing condition according to the shutter key half pressing operation as mentioned above, after setting the determined ISO-capt as the gain of the AGC of the unit circuit  12 , with the pre-emission of light of the stroboscope  15  according to the shutter key fully pressing operation, the appropriate amount of stroboscope emission light is decided based on the brightness of the digital signal (image signal) sent to the MPU  3  by the image capturing at the time of the pre-emission of light. As a result, an amount of stroboscope emission light is decided according to the decided ISO-capt. Thereafter, using the decided ISO-capt and amount of stroboscope emission light, the image capturing operation is executed with the above-mentioned set shutter speed and diaphragm. Moreover, in the case of the condition without the stroboscope light emission (in the case of proceeding from step S 4  to step S 8 ), the image capturing operation is executed with the shutter speed, diaphragm and sensitivity set in step S 4 . 
   The flow chart at the time of fully pressing the shutter key is shown in  FIG. 4 . 
   At step SA 1 , it is determined whether or not the image capturing condition includes the stroboscope light emission. If the image capturing condition does not include the stroboscope light emission, an image is captured without stroboscope light emission and with the above shutter speed, diaphragm value, and ISO sensitivity set during the shutter key half pressing operation (step SA 8 ). 
   If the image capturing condition includes the stroboscope light emission, ISO-capt is set as a gain of the AGC at step SA 2 . The stroboscope  15  pre-emits light at step SA 3 . An amount of light emission of the stroboscope  15  is determined based on the brightness of the image signal obtained at the time of pre-emission of light at step SA 4 . An image is captured with stroboscope light emission of which amount is determined at step SA 4  and with the above shutter speed and diaphragm value set during the shutter key half pressing operation and ISO-capt (step SA 5 ). 
   The captured image data is compressed (step SA 6 ), and the compressed image data is recorded in the flash memory  18  (step SA 7 ). 
   Of course, the appropriate ISO sensitivity is decided as mentioned above and the object distance is known. Accordingly, it is also possible to store in the MROM  17  the correspondence table of the ISO sensitivity, the object distance or the like and an amount of stroboscope emission light as shown in  FIG. 5A , and read out an amount of stroboscope emission light corresponding to the ISO sensitivity and the object distance from the correspondence table so as to control an amount of light emitted from the stroboscope  15  at the time of image capturing without executing the pre-emission of light (see the flow chart of  FIG. 5B ). 
   At step SB 1 , it is determined whether or not the image capturing condition includes the stroboscope light emission. If the image capturing condition does not include the stroboscope light emission, an image is captured without stroboscope light emission and with the above shutter speed, diaphragm value, and ISO sensitivity set during the shutter key half pressing operation (step SB 6 ). 
   If the image capturing condition includes the stroboscope light emission, an amount of stroboscope emission light corresponding to ISO-capt and the object distance D-af are read from the correspondence table (step SB 2 ). An image is captured with stroboscope light emission of which amount is read at step SB 2  and with the above shutter speed and diaphragm value set during the shutter key half pressing operation and ISO-capt (step SB 3 ). 
   The captured image data is compressed (step SB 4 ), and the compressed image data is recorded in the flash memory  18  (step SB 5 ). 
   Moreover, in this case, it is also possible to store the calculation formula instead of storing the correspondence table and calculate an amount of stroboscope emission light based on the ISO sensitivity and the object distance by using the calculation formula. 
   Although the ISO sensitivity is decided by the comparison of the stroboscope maximum reaching distance and the object distance in this embodiment, it is also possible to decide an amount of stroboscope emission light and the ISO sensitivity based on the brightness (an amount of light) of the object obtained by executing the pre-emission of light. For example, it is also possible to store in the MROM  17  the correspondence table of the object brightness (luminance value of the image signal) at the time of the pre-emission of light, the ISO sensitivity, and an amount of stroboscope emission light as shown in  FIG. 6A , and read out the ISO sensitivity and an amount of stroboscope emission light corresponding to the luminance level of the image signal obtained at the time of the pre-emission of light so as to control the amplifying factor of the image signal (ISO sensitivity) at the time of image capturing and an amount of light emitted from the stroboscope  15  (see the flow chart of  FIG. 6B ). 
   At step SC 1 , it is determined whether or not the image capturing condition includes the stroboscope light emission. If the image capturing condition does not include the stroboscope light emission, an image is captured without stroboscope light emission and with the above shutter speed, diaphragm value, and ISO sensitivity set during the shutter key half pressing operation (step SC 7 ). 
   If the image capturing condition includes the stroboscope light emission, the stroboscope  15  pre-emits light at step SC 2 . An amount of light emission of the stroboscope  15  and ISO sensitivity corresponding to the brightness of the image signal obtained at the time of pre-emission of light are read out from the correspondence table (step SC 3 ). An image is captured with stroboscope light emission of which amount is read at step SC 3  and with the above shutter speed and diaphragm value set during the shutter key half pressing operation and ISO-capt (step SC 4 ). 
   The captured image data is compressed (step SC 5 ), and the compressed image data is recorded in the flash memory  18  (step SC 6 ). 
   Further, it is also possible to store in the MROM  17  the ISO sensitivity, on or off of the stroboscope light emission, and an amount of stroboscope emission light (only in the case of the stroboscope light emission is on) corresponding to the object brightness (luminance value of the image signal), and read out the ISO sensitivity value, on or off of the stroboscope light emission, and an amount of stroboscope emission light corresponding to the measured object brightness so as to execute the image capturing process based on the readout information. 
   Moreover, although the object brightness is calculated based on the through image in step S 1  of  FIG. 3 , it is also possible to provide an optical sensor and obtain the object brightness by correcting as needed the brightness detected by the optical sensor. 
   In addition, although the object distance D-af is calculated by using the focus setting table by the contrast AF method in step S 2  of  FIG. 3 , it is also possible to provide a distance measuring sensor such as an external phase difference sensor and obtain the object distance D-af by the detection value of the distance measuring sensor. 
   While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. For example, the present invention can be practiced as a computer readable recording medium in which a program for allowing the computer to function as predetermined means, allowing the computer to realize a predetermined function, or allowing the computer to conduct predetermined means. Furthermore, although the case of applying the present invention in the digital camera  1  as a dedicated machine has been explained in this embodiment, it is not limited thereto, and the present invention can also be applied in a camera stored in a portable information terminal such as a portable phone.