Patent Publication Number: US-7589769-B2

Title: Camera capable of canceling noise in image data and signal processing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional Application of U.S. application Ser. No. 09/532,817, filed Mar. 21, 2000 now U.S. Pat. No. 6,747,696, which is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-082767, filed Mar. 26, 1999, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a camera and a signal processing method thereof. More particularly, the present invention relates to a camera for canceling a dark signal component resulted from an image pickup device such as a CCD (Charge Coupled Device), and relates to a signal processing method of the camera. 
     In recent years, mobile tools such as electronic still cameras and notebook personal computers have become widespread as the development of the semiconductor technology has surged forward. Especially, the electronic still camera picks up an image of a subject incident through an objective lens to generate electric signals by using an image sensor, and based on the electric signals displays the image on a liquid crystal display, or stores the image data in a nonvolatile semiconductor memory card. Further, the image data can be captured into a personal computer and subjected to an image processing. Therefore, the electronic still cameras have become widespread rapidly. 
     Meanwhile, it is known that the CCD which is mounted in the above-described electronic still camera has so-called dark output (dark voltage) characteristics in which output voltage is generated even when incident light is intercepted, and with this dark output, a very small current called dark current is generated. 
     This dark voltage in the CCD has temperature dependence in which dark voltage is approximately doubled if environmental temperature is varied (increased) about 8° C. The dark voltage has the exposure time (electric charge accumulation period) dependence indicative of tendency that the dark voltage is increased as the exposure time is longer under the same temperature condition. 
     The dark voltage of the CCD becomes noise component and as a result, this is the factor that affects the image data and deteriorates image quality. 
     As one of techniques for solving this problem, there is a known technique in which image data (dark output component) generated when a mechanical shutter is closed is subtracted from image data (subject image data including dark output component) generated when the mechanical shutter is opened, thereby canceling the noise component from the image data. 
     However, according to this technique, there is a problem that if the subtraction is carried out using image data which have been subjected to gamma-correction processing having nonlinear characteristics or saturated image data, or if the subtraction is carried out even when the level of the dark output component is low, excellent image data can not be obtained. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a camera capable of carrying out a correcting processing for canceling noise component from image data without deteriorating image quality so as to obtain excellent image data. 
     According to the present invention, there is provided a camera comprises image pick-up means (an image sensor) for outputting image data in accordance with incident light, first pick-up control means for obtaining first image data output from the image pick-up means under a condition where an incident light path of the image pick-up means is opened, second pick-up control means for obtaining second image data output from the image pick-up means under a condition where the incident light path of the image pick-up means is closed, means for correcting the first image data obtained by the first pick-up control means based on the second image data obtained by the second-pick-up control means, nonlinear processing means (a gamma correcting circuit) for subjecting the image data output from the image pick-up means to nonlinear processing, and means for prohibiting the nonlinear processing for the image data by the nonlinear processing means when the first and second image data are obtained by the first and second pick-up control means. 
     With this structure, the first and second pick-up control means can obtain the first and second image data which are not subjected to the nonlinear processing by the nonlinear processing means. With this feature, it is possible to provide a camera capable of carrying out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and capable of obtaining excellent image data. 
     According to the present invention, there is provided another camera comprises image pick-up means (an image sensor) for outputting image data in accordance with incident light, first pick-up control means for obtaining first image data output from the image pick-up means under a condition where an incident light path of the image pick-up means is opened, second pick-up control means for obtaining second image data output from the image pick-up means under a condition where the incident light path of the image pick-up means is closed, means for determining whether an isolation point is included in the second image data obtained by the second pick-up control means, and means for correcting the first image data obtained by the first pick-up control means based on a result of the determination made by the determining means. 
     With this feature, it is possible to provide a camera capable of carrying out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and capable of obtaining excellent image data. 
     According to the present invention, there is provided a further camera comprises image pick-up means (an image sensor) for outputting image data in accordance with incident light, first pick-up control means for obtaining first image data output from the image pick-up means under a condition where an incident light path of the image pick-up means is opened, second pick-up control means for obtaining second image data output from the image pick-up means under a condition where the incident light path of the image pick-up means is closed, means for determining whether a level of the first image data obtained by the first pick-up control means is saturated, and means for correcting the first image data obtained by the first pick-up control means based on a result of determination made by the determining means and based on the second image data obtained by the second pick-up control means. 
     With this feature, it is possible to provide a camera capable of carrying out the correction; processing for canceling the noise component from the image data without deteriorating the image quality, and capable of obtaining excellent image data. 
     According to the present invention, there is provided a still another camera comprising image pick-up means (an image sensor) for outputting image data in accordance with incident light, first pick-up control means for obtaining first image data output from the image pick-up means under a condition where an incident light path of the image pick-up means is opened, second pick-up control means for obtaining second image data output from the image pick-up means under a condition where the incident light path of the image pick-up means is closed, means for detecting a temperature around the image pick-up means, and means for correcting the first image data obtained by the first pick-up control means based on the temperature detected by the temperature detecting means and based on the second image data obtained by the second pick-up control means. 
     With this feature, it is possible to provide a camera capable of carrying out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and capable of obtaining excellent image data. 
     According to the present invention, there is provided a still further camera comprises image pick-up means (an image sensor) for accumulating electric charges corresponding to incident light, converting accumulated electric charges into image data, and outputting the image data, first pick-up control means for obtaining first image data output from the image pick-up means under a condition where an incident light path of the image pick-up means is opened, second pick-up control means for obtaining second image data output from the image pick-up means under a condition where the incident light path of the image pick-up means is closed, means for detecting an electric charge accumulation period of the image pick-up means, and means for correcting the first image data obtained by the first pick-up control means based on the electric charge accumulation period detected by the detecting means and based on the second image data obtained by the second pick-up control means. 
     With this feature, it is possible to provide a camera capable of carrying out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and capable of obtaining excellent image data. 
     According to the present invention, there is provided a signal processing method for a camera which carries out nonlinear processing for image data output from an image sensor, comprises a step of prohibiting the nonlinear processing for the image data output from the image sensor under a condition where an incident light path of the image sensor is opened, thereby obtaining first image data which are not subjected to the nonlinear processing, a step of prohibiting the nonlinear processing for the image data output from the image sensor under a condition where the incident light path of the image sensor is closed, thereby obtaining second image data which are not subjected to the nonlinear processing, and a step of correcting the first image data based on the second image data. 
     With this feature, it is possible to carry out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and to obtain excellent image data. 
     According to the present invention, there is provided another signal processing method for a camera, comprises a step of obtaining first image data output from an image sensor under a condition where an incident light path of the image sensor is opened, a step of obtaining second image data output from an image sensor under a condition where an incident light path of the image sensor is closed, a step of determining whether an isolation point is included in the second image data, and a step of correcting the first image data based on a result of the determination. 
     With this feature, it is possible to carry out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and to obtain excellent image data. 
     According to the present invention, there is provided a further signal processing method for a camera, comprises a step of obtaining first image data output from an image sensor under a condition where an incident light path of the image sensor is opened, a step of obtaining second image data output from an image sensor under a condition where an incident light path of the image sensor is closed, a step of determining whether a level of the first image data is saturated, and a step of correcting the first image data based on a result of the determination and the second image data. 
     With this feature, it is possible to carry out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and to obtain excellent image data. 
     According to the present invention, there is provided a still another signal processing method for a camera, comprises a step of obtaining first image data output from an image sensor under a condition where an incident light path of the image sensor is opened, a step of obtaining second image data output from an image sensor under a condition where an incident light path of the image sensor is closed, a step of detecting a temperature around the image sensor, and a step of correcting the first image data based on the second image data when the detected temperature satisfies a predetermined condition. 
     With this feature, it is possible to carry out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and to obtain excellent image data. 
     According to the present invention, there is provided a still further signal processing method for a camera, comprises a step of obtaining first image data output from an image sensor under a condition where an incident light path of the image sensor is opened, a step of obtaining second image data output from an image sensor under a condition where an incident light path of the image sensor is closed, a step of detecting an electric charge accumulation period of the image sensor, and a step of correcting the first image data based on the second image data when the detected electric charge accumulation period satisfies a predetermined condition. 
     With this feature, it is possible to carry out the correction processing for canceling the noise component from the image data without deteriorating the image quality, and to obtain excellent image data. 
     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 presently preferred embodiments of the present 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 present invention in which: 
         FIG. 1  is a block diagram showing a brief structure of a first embodiment of a camera according to the present invention; 
         FIG. 2  is a schematic block diagram of a CCD having n columns×m rows pixels applied to the camera of the first embodiment; 
         FIG. 3  is a flowchart showing a processing operation of a normal pick-up mode in the camera of the first embodiment; 
         FIG. 4  is a flowchart showing a processing operation of a special effect pick-up mode in the camera of the first embodiment; 
         FIGS. 5A and 5B  are graphs showing effect of a subtracting processing when a nonlinear γ-correction processing is carried out; 
         FIGS. 6A and 6B  are graphs showing effect of a subtracting processing when a linear γ-correction processing is carried out; 
         FIG. 7  is a block diagram showing a brief structure of a signal processing LSI including a γ-correction processing circuit; 
         FIG. 8  is a block diagram showing a structure of an essential portion of a second embodiment of the camera according to the invention; 
         FIG. 9  is a flowchart showing a main processing operation of the second embodiment of the camera according to the invention; 
         FIG. 10  is a block diagram showing a structure of an essential portion of a third embodiment of the camera according to the invention; 
         FIGS. 11A and 11B  are flowcharts showing a main processing operation of the third embodiment of the camera according to the invention; 
         FIG. 12  is a block diagram showing a structure of an essential portion of a fourth embodiment of the camera according to the invention; 
         FIG. 13  is a flowchart showing a main processing operation of the fourth embodiment of the camera according to the invention; 
         FIG. 14  is a flowchart showing a main processing operation of a fifth embodiment of the camera according to the invention; and 
         FIG. 15  is a flowchart showing a main processing operation of a sixth embodiment of the camera according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of a camera according to the present invention will now be described with reference to the accompanying drawings. 
     Embodiments of a camera according to the present invention will be explained with reference to the drawings below. 
     First Embodiment 
       FIG. 1  is a block diagram showing a brief structure of a first embodiment of a camera according to the present invention. A CCD  13  is provided behind an objective lens  11  with a mechanical shutter  12  interposed therebetween, and a driver  17  is connected to the CCD  13 . Output of the CCD  13  is supplied to a video transfer circuit  23  through a correlation double sampling circuit (CDS)  14 , an analog-digital converter  15 , a γ-correction circuit (nonlinear processing means)  21  and a color process circuit  22 . Output of a timing generator  18  is supplied to the color process circuit  22  also. A compression/decompression circuit  24  is connected to the video transfer circuit  23  and a buffer memory (storing means)  31 . The video transfer circuit  23  and the compression/decompression circuit  24  are connected to a bus line  70  of a CPU (Central Processing Unit)  60  together with a flash memory (memory means)  32  as an image memory. A key input device  50  is connected to the CPU  60 . Output of the video transfer circuit  32  is supplied to a liquid crystal display (display means)  40  through a digital video encoder  25 . The mechanical shutter  12  is controlled by an actuator  16  under control of the CPU  60 . The objective lens  11 , the mechanical shutter  12 , the CCD  13 , the driver  17 , the timing generator  18 , the CDS  14 , the analog-digital converter  15 , the γ-correction circuit  21  and the color process circuit  22  constitute an image pick-up means. The digital video encoder  25  and the liquid crystal display  40  constitute a display means. 
     Outlines of functions of the above-described individual parts are as follows: 
     Objective Lens  11 : 
     It is for forming an image of a subject on a light-receptive surface of the CCD  13 , and includes a focusing mechanism for an automatic focusing function. The objective lens  11  may include a zooming function and may be of retractable type. 
     Mechanical Shutter  12  and Actuator  16 : 
     They are for mechanically intercepting incident light to the light-receptive surface of the CCD  13 . The actuator  16  opens and closes the mechanical shutter  12  in accordance with a control signal from the CPU  60 . The mechanical shutter  12  opens an optical path to the light-receptive surface of the CCD  13 , and closes the optical path at the time of closing state. 
     CCD  13 : 
     It is a CCD of interlace type, and can switch the reading of field and the reading of frame by externally controlling. The exposure time at the time of reading of the field is controlled by opening/closing operation (application timing of SUB and XSG) of the electrical shutter, and the exposure time at the time of reading of the frame is controlled by opening/closing operation (application timing of SUB) and the mechanical shutter  12 . The reading of the field (which is also called accumulation of field), and the reading of the frame (which is also called accumulation of frame) will be explained later. 
     A concrete structure of the CCD which is excellently applied to the camera of the present embodiment will be explained with reference to the drawings. 
       FIG. 2  is a schematic block diagram of the CCD having n columns×m rows pixels. 
     As shown in  FIG. 2 , in the CCD, n×m matrix of photoelectric converters  131  are arranged. The photoelectric converters  131  accumulate electric charge corresponding to incident light amount. Total n-number of vertical transfer means  132  are arranged between adjacent columns to form a pick-up region  133 , and a horizontal transfer means  134  is disposed on one end of the vertical transfer means  132 . 
     Signal charges accumulated in the photoelectric converters  131  are sent to the adjacent vertical transfer means  132  in replay to a reading out signal XSG applied from the driver  17  ( FIG. 1 ) which is not shown in  FIG. 2 , and are sequentially transferred downward as viewed in the drawing in synchronization with vertical transfer clock φV inside the vertical transfer means  132 . 
     All output ends of the vertical transfer means  132  are connected to the horizontal transfer means  134 . One horizontal line (row) of signal charges are sequentially sent to the horizontal transfer means  134  in synchronization with the vertical transfer clock φV. The signal charges sent to the horizontal transfer means  134  are sequentially transferred leftward in the drawing in synchronization with horizontal transfer clock φH. The signal charges which have reached the output end of the horizontal transfer means  134  are converted into electric signals by charge detector  135 , and amplified by an amplifier  136  and then sent out as CCD outputs. The SUB is signal voltage (so-called charge discharge pulse) for pulling the accumulated charge of all the photoelectric converters  131  to a substrate. Time from application of this SUB to application of XSG is exposure time of the electric shutter (“electrical shutter”, hereinafter) of the CCD. 
     The above-described reading of field means a system for mixing and outputting pixel signal of one odd-numbered line and one even-numbered line as one signal, for example, an odd-numbered line O 1  and an even-numbered line E 1 , an odd-numbered line O 2  and an even-numbered line E 2 , . . . , of the photoelectric converter  131 . The reading of frame means a system for dividing frames into two, i.e., odd-numbered frames (O 1 , O 2 , O 3 , . . . ) and even-numbered frames (E 1 , E 2 , E 3 , . . . ), and outputting the CCD. 
     A recent electronic still camera in which LCD is mounted employs a through image mode which displays an image of a subject on an LCD screen as through image, thereby adjusting the composition. In such an operation mode, the through image is renewed every exposure time by the electrical shutter. 
     On the other hand, in the through image mode, when a desired composition is obtained, a capture operation is carried out in which a shutter key is pressed to store an image of a subject in a memory or the like as a capture image. In the capture operation, a mechanical shutter mechanism (the above-described mechanical shutter) for intercepting incident light to the CCD is indispensable for freezing the subject image formed on the CCD. A period from time when the exposure of the electrical shutter is started by pushing the shutter key (key input device  50 ) to time when the mechanical shutter is closed to intercept the incident light is exposure time of the mechanical shutter. 
     Driver  17  and TG  18 : 
     They generate driving signals necessary for reading of CCD  13  (e.g., φV, XSG, φH, SUB), and the CCD  13  outputs image signals in synchronization with these driving signals. 
     CDS  14 : 
     It carries out correlation double sampling processing for time-series signals read out from the CCD  13  at frequency corresponding to resolution of the CCD  13 . Automatic gain control is carried out after sampling in some cases. 
     A/D  15 : 
     It converts sampled analog signal into digital signal. 
     γ-Correction Circuit  21 : 
     It includes function to carry out a first correction processing for carrying out γ-correction based on γ-correction table having normal nonlinear characteristics, and to carry out a second correction processing for changing the γ-correction table into linear characteristics to carry out the γ-correction. 
     Here, γ-correction circuit  21  may integrally formed with other signal processing circuits such as a digital clamping circuit and a white balance correction processing circuit which will be explained later. 
     Color Process Circuit  22 : 
     It carries out known color signal processing such as interpolation processing of the three primary colors including R, G and B, exposure calculation (AE) and white balance processing (AWB) based on output signal from the γ-correction circuit  21 , and generation of brightness and color-difference multiplex signals (YUV signals) based on image information which were converted into digital signals. 
     The reason why the YUV signal is generated is that since the size of color data (RGB data) output from camera system is great, the YUV signal is used as data amount reducing signal for the purpose of effectively utilizing the limited memory and reducing the processing time. The color signal processing in the color process circuit  22  may be carried out by a signal processing circuit including the γ-correction circuit  21 , or may be carried out by providing another signal processing circuit behind the color process circuit  22 . 
     Video Transfer Circuit  23 : 
     It controls flow of data skipping between the color process circuit  22 , the buffer memory  31 , the compression/decompression circuit  24 , the flash memory  32 , the digital video encoder  25  and the LCD  40 . 
     More specifically, the video transfer circuit  23  writes or reads image data between the flash memory  32  and the buffer memory  31  which stores image data generated by the color process circuit  22  through the compression/decompression circuit. 24 , and displays the image data stored by the buffer memory  31  on the LCD  40 . 
     Compression/Decompression Circuit  24 : 
     It carries out compression and decompression in JPEG (Joint Photographic Experts Group) encoding manner. Compression parameter of JPEG may be fixed or given by the CPU  60  whenever the compression is carried out. 
     Digital Video Encoder  25 : 
     It converts image data read out from the buffer memory  31  through the video transfer circuit  23  into analog voltage, and sequentially outputs the voltage at timing corresponding to scanning system mode of the LCD  40 . 
     Buffer Memory  31 : 
     It comprises a DRAM which is one kind of rewritable semiconductor memory. However, the buffer memory  31  in the present invention is not limited to the DRAM, and may be any kind of rewritable semiconductor memory. 
     Flash Memory  32 : 
     It is a PROM (programmable read only memory) but which must be memory capable of electrically erasing and rewriting data in all bits (or the unit of block). This memory is also called a flash EEPROM (flash electrically erasable PROM). The flash memory  32  of the present embodiment may be fixed memory which is embedded in the camera body or may be of removable type such as card type or package type. 
     CPU  60 : 
     It concentrically controls operation of the entire camera by executing a predetermined program. The program is written in an instruction ROM in the CPU  60 . At the time of recording mode, a recording mode program is loaded into an internal RAM and executed, and the program outputs a control signal which opens or closes the mechanical shutter, a control signal which drives the CCD  13  and a control signal which changes the setting of characteristics of γ-correction table of the γ-correction process circuit  21 . 
     Bus  70 : 
     It is a transfer path for data (and addresses) commonly possessed by the above-described various portions. 
     Next, an image recording operation of the camera of the present embodiment will be explained with reference to the drawings. Here, a case in which the image recording operation of the camera of the present embodiment is applied to a camera (electronic still camera including LCD) having the above-described through image mode will be explained in detail. 
     &lt;Normal Pick-Up Mode&gt; 
     First, a normal pick-up mode will be explained with reference to the drawings. 
       FIG. 3  is a flowchart showing a processing operation of a normal pick-up mode in the camera of the first embodiment. Here, in  FIG. 3 , only points of the image recording operation will be shown using simplified terms. 
     Through Operation 
     After a user of the electronic still camera switches a mode switch included in the key input device  50  into a recording mode (REC), if he or she selects the normal pick-up mode from a menu displayed on the LCD  40 , the mechanical shutter  12  is opened, and image information of a subject formed on the light-receptive surface of the CCD  13  is displayed and output on the LCD  40  as the through image every fixed period (exposure time). 
     That is, by switching the mode into the recording mode, the through image mode is set (S 101 ), and the exposure time T 0  is set based on various information such as brightness, focus length obtained from pick-up environment of the subject (S 102 ). In the through image mode, opening and closing operation of the driver  17  and the TG  18  whenever the exposure time TO is elapsed (S 103 ), electric signal (CCD data) which is output from the CCD  13  every time when the electrical shutter is opened or closed is taken (S 104 ), and it is converted into digital signal by the A/D  15  and subjected to the nonlinear γ-correction processing by the γ-correction circuit  21  (S 105 ). 
     Then, color signal processing such as interpolation processing of the three primary colors including R, G and B, exposure calculation (AE) white balance processing (AWB), generation of brightness and color-difference multiplex signals are executed (S 106 , S 107 ), and a frame of image data are generated. 
     The image data generated by the color process circuit  22  are transferred to the buffer memory  31  by the video transfer circuit  23  and then, are subjected to video processing by the digital video encoder  25  (S 108 ), and converted into signal corresponding to display mode of the LCD  40  and displayed and output as the through image (S 109 ). 
     Capture Operation 
     In the through image operation, if a viewing direction of the camera body or the objective lens is changed, composition of the through image displayed on the LCD  40  is changed. When a through image having desired composition is obtained, if the shutter key provided on the key input device  50  is “halfway pushed” (S 110 ), exposure time T 1  and the focus are set (S 111 ) based on information obtained from the pick-up environment of the subject, and if the shutter key is “fully pushed”, the electrical shutter is opened (S 112 , S 113 ). After the exposure time T 1  set in step S 111  is elapsed (S 114 ), the mechanical shutter  12  is closed by the driver  17  and the TG  18  (S 115 ), CCD data output from the CCD  13  are captured (S 116 ), the data are converted into digital signals by the A/D  15  and are subjected to the nonlinear γ-correction processing by the γ-correction circuit  21  (S 117 ). 
     Next, the color signal processing such as interpolation processing of the three primary colors including R, G and B, exposure calculation (AE) white balance processing (AWB) are executed (S 118 ), a frame of image data is generated, and it is transferred to the buffer memory  31  by the video transfer circuit  23 . At that time, the image data captured into the buffer memory  31  is frozen as the subject image formed on the CCD  13  at the instant when the mechanical shutter  12  is closed, and the through image displayed on the LCD  40  is also frozen as the image at the same instant. 
     Further, after compression processing such as JPEG encoding is carried out by the video transfer circuit  23  through the compression/decompression circuit  24  (S 119 ), the image is recorded as a frame of captured image in the flash memory  32  (S 120 ). When pick-up is continued, i.e., when the normal pick-up mode is not canceled (S 121 ), the flow proceeds back to step S 101  where the mechanical shutter  12  is opened, and the through image mode is set (S 101 ). A series of processing operations in the above-described steps S 101  to S 121  are repeated. 
     According to the camera having such a series of processing operations, the exposure time (accumulation period) T 1  suitable for relatively bright normal pick-up condition, and after image data of a subject are captured, predetermined signal processing such as the nonlinear γ-correction processing is executed, and the pick-up and image recording operations are carried out excellently. 
     &lt;Special Effect Pick-Up Mode&gt; 
     Next, a special effect pick-up mode suitable for pick-up a dark object, such as nightscape will be explained with reference to the drawings. 
       FIG. 4  is a flowchart showing a processing operation of the special effect pick-up mode in the camera of the present embodiment. Here, in  FIG. 4 , only points of the image recording operation will be shown using simplified terms. 
     Through Operation 
     After a user of the electronic still camera switches a mode switch included in the key input device  50  into a recording mode (REC), if he or she selects the special effect pick-up mode from a menu displayed on the LCD  40 , the mechanical shutter  12  is opened, and a through image mode is set (S 201 ). Exposure time T 0  is set (S 202 ) based on various information such as brightness and focus length obtained from the pick-up environment of the subject. In the through image mode, whenever the exposure time T 0  is elapsed (S 203 ), opening and closing operation of the electrical shutter of the CCD  13  is controlled by the driver  17  and the TG  18  whenever the exposure time T 0  is elapsed (S 203 ), electric signal (CCD data) which is output from the CCD  13  every time when the electrical shutter is opened or closed is taken (S 204 ), and it is converted into digital signal by the A/D  15  and subjected to the nonlinear γ-correction processing by the γ-correction circuit  21  (S 205 ) as in the normal pick-up mode. 
     Then, color signal processing such as interpolation processing of the three primary colors including R, G and B, exposure calculation (AE) white balance processing (AWB), generation of brightness and color-difference multiplex signals are executed (S 206 , S 207 ), and a frame of image data is generated. 
     The image data generated by the color process circuit  22  is transferred to the buffer memory  31  by the video transfer circuit  23  and then, is subjected to video processing by the digital video encoder  25  (S 208 ), and converted into signal corresponding to display mode of the LCD  40  and displayed and output as the through image (S 209 ). 
     Capture Operation 
     Next, when a through image having a desired composition is displayed on the LCD  40 , if the shutter key provided on the key input device  50  is “halfway pushed” (S 210 ), the CPU  60  detects the start of the recording operation, outputs a control signal to the γ-correction circuit  21 , and changes the characteristic setting of the γ-correction table from the nonlinear characteristic to the linear characteristic (S 211 ). Exposure time T 1  and focus are set based on information obtained from the pick-up environment of the subject (S 212 ). 
     If the shutter key is further pushed, i.e., “fully pushed”, the electrical shutter is opened. After the exposure time T 1  set in step S 212  is elapsed (S 215 ), the mechanical shutter  12  is closed by the driver  17  and the TG  18  (S 216 ), CCD data (DATA 1 : first image data) of the subject image output from the CCD  13  is captured (S 217 ), the data is converted into digital data by the A/D  15  and is subjected to the linear γ-correction processing by the γ-correction circuit  21  based on the γ-correction table which was changed into linear characteristic in step S 211  (S 218 ). When the mechanical shutter  12  is closed in steps S 216  and S 217  and the CCD output data (DATA 1 ) is captured from the CCD  13 , the electrical shutter is closed. 
     Next, under a condition where the mechanical shutter  12  is closed (S 220 ), the electrical shutter is again opened (S 214 ), each of the processing of steps S 214  to  218  is executed using the exposure time T 1  set in step S 212 , thereby capturing CCD output data (DATA 2 : second image data) including dark output component generated by the CCD  13  in the current pick-up condition but not including the subject image, and the data is subjected to the linear γ-correction processing by the γ-correction circuit  21  based on the γ-correction table having the linear characteristic. The CCD output data (DATA 1 , DATA 2 ) captured by each of the processing of the above-described steps S 214  to S 218  and subjected to the γ-correction may be temporarily stored in the buffer memory  31 , or may be stored in an RAM (not shown) added to the CPU  60  which executes subtraction processing which will be described later. 
     As described above, when the CCD output data (DATA 1 , DATA 2 ) have been captured twice (S 219 ), it is determined whether pixel data (pixel which is to be subjected to the determining processing) in the CCD output data (DATA 2 ) which was captured at the second time is a white scratch of a predetermined level or higher i.e., a n isolation point (S 221 ). When it is determined that the pixel data is the white scratch, it is determined whether the level of the pixel data in the DATA 1  corresponding to that pixel is saturated (S 222 ). 
     As a result of such a series of determining processing (S 221 , S 222 ), if it is determined that the pixel data in the CCD output data (DATA 2 ) which was captured at the second time is not the white scratch of the predetermined level or higher, the pixel data corresponding to the DATA 1  is regarded as effective data which need not correction processing. Even when the pixel data in the CCD output data (DATA 2 ) which was captured at the second time is the white scratch of the predetermined level or higher, if the level of the pixel data in the DATA 1  corresponding to that pixel is saturated, the pixel data corresponding to the DATA 1  is regarded as data which need not correction processing. Details of the determining processing in steps S 221  and S 222  will be described later. 
     If the pixel data in the CCD output data (DATA 2 ) which was captured at the second time is the white scratch of the predetermined level or higher and the level of the pixel data in the DATA 1  corresponding to that pixel is not in the saturated state, it is determined that the corresponding pixel data in DATA 1  is data which should be subjected to the correction processing, correction processing (subtraction processing) in which CCD output data (DATA 2 ) captured at the second time is subtracted from CCD output data (DATA 1 ) captured at the first time, i.e., correction processing in which dark output component possessed by the CCD  13  commonly included in the CCD output data (DATA 1 ) and the CCD output data (DATA 2 ) is executed (S 223 ). 
     The pixel which is to be subjected to the determining processing of the white scratch and the correction processing is sequentially changed (S 225 ), and after a frame of all the pixels output from the CCD  13  is subjected to the processing (S 224 ), each of the pixel data is subjected to the nonlinear γ-correction processing (S 226 ), and the color signal processing such as interpolation processing of the three primary colors including R, G and B, exposure calculation (AE) white balance processing (AWB) are executed (S 227 ), and a frame of image data are transferred to the buffer memory  31  by the video transfer circuit  23 . The nonlinear γ-correction processing in step S 226  does not use the above-described γ-correction processing circuit  21 , but executes the nonlinear γ-correction processing (S 205 ) equal to that of the through operation using software by the CPU  60 . 
     Further, after compression processing such as JPEG encoding is carried out by the video transfer circuit  23  through the compression/decompression circuit A (S 228 ), the image is recorded as a frame of captured image in the flash memory  32  (S 229 ). When pick-up is continued, i.e., when the special effect pick-up mode is not canceled (S 230 ), the flow proceeds back to step S 201  where the mechanical shutter  12  is opened, and the through image mode (including change of settings to the nonlinear characteristic of the γ-correction table) is set (S 201 ). A series of processing operations in the above-described steps S 201  to S 230  are repeated. 
     Next, the relation between the characteristic setting of the above-described γ-correction processing and the subtraction processing (dark output component of the CCD is cancelled from the image data of the subject image) will be explained with reference to the drawings. 
       FIGS. 5A and 5B  are graphs showing effect of a subtracting processing when the nonlinear γ-correction processing is carried out, i.e., when step  211  shown in the flowchart of  FIG. 4  is not performed so that the γ-correction table of the γ-correction circuit  21  is not changed to the linear characteristic.  FIGS. 6A  and  6 B are graphs showing effect of the subtracting processing when the linear γ-correction processing is carried out for the DATA  1  and DATA  2 , i.e., when the γ-correction table of the γ-correction circuit  21  was changed to the linear characteristic in step S 211  shown in the flowchart of  FIG. 4 . 
     First, as shown in  FIG. 5A , when input/output signal characteristic in the γ-correction circuit  21  is nonlinear, the relation between gradients Pa and Pb on characteristic curve with respect to input levels “a” and “b” is Pa&lt;Pb. 
     On the other hand, as shown in  FIG. 5B , data level of the CCD output data DATA 1  captured at the first time under a condition where the mechanical shutter  12  is opened is equal to a sum total of original data component “C” of the subject image, random noise component “A” and dark voltage component (or white scratch component) “B”. Data level of the CCD output data DATA 2  captured at the second time under a condition where the mechanical shutter  12  is closed is equal to a sum total of random noise component “A′” and dark voltage component “B′”, since the data component “C” of the subject image is not included. 
     Therefore, the DATA 1  having greater data level is gently inclined, i.e., γ-correction processing is carried out with small γ-coefficient. Whereas, the DATA 2  having smaller data level is steeply inclined, i.e., γ-correction processing is carried out with great γ-coefficient. Therefore, the dark voltage components “B” and “B′” do not coincide with each other due to the nonlinear γ-correction processing, and there is a problem that the dark voltage component can not be cancelled properly by the subtraction processing which subtracts the DATA 2  from the DATA 1 . 
     Whereas, as shown in  FIG. 6A , when input/output signal characteristic in the γ-correction circuit  21  is linear, the relation between gradients Pa and Pb on characteristic curve with respect to input levels “a” and “b”, is always equal to each other. 
     Therefore, in both the DATA 1  having great data level and the DATA 2  having small data level, γ-correction processing is carried out with constant γ-coefficient and thus, the dark voltage components “B” and “B′” are always equal to each other, and the dark voltage component can be cancelled properly by the subtraction processing which subtracts the DATA 2  from the DATA 1 . 
     Next, the above-described γ-correction circuit  21  will be explained with reference to the drawings. 
       FIG. 7  is a block diagram showing a brief structure of the signal processing large-scale integrated (LSI) circuit  20  including the γ-correction processing circuit  21 . 
     As described above, characteristic setting of the γ-correction circuit  21  should be variable. Further, a recent camera such as an electronic still camera is supplied under a condition where the γ-correction circuit is incorporated in one chip LSI together other another signal processing circuit in many cases. 
     Therefore, as shown in  FIG. 7 , the signal processing LSI  20  comprises a digital/clamp processing unit  20 A which inputs a signal (CCD output data in the drawing) which is output from the CCD  13  and sampled and converted into digital data by the CDS  14  and the A/D  15  and which clamps at a predetermined black level, a shading correction processing unit  20 B for correcting shading when the shading is generated in optical system such as the objective lens  11 , a white balance correcting unit  20 C and a γ-correcting processing unit  20 D including a γ-correction table  20 E capable of changing the characteristic settings. The γ-correction table  20 E is capable of changing (or switching) the settings of γ-characteristic into nonlinear or linear. The γ-correcting processing unit  20 D and γ-correction table  20 E correspond to the γ-correction processing circuit  21 . 
     In the present embodiment, by the signal processing LSI  20 , the CCD output data output from the CCD  13  is subjected to the normal nonlinear γ-correction processing (first correction processing) at the time of the through operation and the capture operation in the normal pick-up mode, and is subjected to the linear γ-correction processing (second correction processing) at the time of the capture operation in the special effect pick-up mode. 
     As described above, at the time of the capture operation in the special effect pick-up mode, normal nonlinear γ-correction processing similar to that of the through operation is executed using software in a later processing (e.g., step S 226  in  FIG. 4 ). 
     In the present embodiment, reason why it is determined whether the level of the pixel in the DATA 1  corresponding to pixel determined as having white scratch in DATA 2  is in the saturated state as a condition which determines whether the subtraction processing should be executed is that irrespective of saturated state of the pixel data in DATA 1  (e.g., even if the level of the pixel data is substantially equal to a threshold value for determining the saturated state or largely exceeds the threshold value), in the above subtraction processing, since level of white scratch of the corresponding DATA 2  is always subtracted from the saturated level, level calculated form the pixel data which is in the saturated state is lower than original level of the pixel data, and there is a problem that image quality is deteriorated, e.g., tone is displayed lower as compared with peripheral pixels. 
     Especially, human eye has characteristics to sensitively sense a fine black point (lower gradation) in the white background (higher gradation) strongly, as compared with a fine white point (higher gradation) in the black background (lower gradation). Therefore, to the subtraction processing as described above, the level of the pixel is displayed black (lower gradation) as compared with surrounding pixels, and the deterioration of image quality is sensed strongly. 
     For this reason, image data in DATA 1  whose level is in saturated state are not subjected to the subtraction processing, and level of white scratch pixel of the corresponding DATA 2  of only image data whose level is in saturated state is subtracted, thereby appropriately correcting the image data level in the DATA 1  to the original level, so that the deterioration of image quality is suppressed. 
     According to the camera having the above-described series of processing operations, under a condition where the settings of the γ-correction table are changed to linear characteristic and γ-correction processing (nonlinear processing) by the γ-correction circuit is substantially prohibited (linear processing is carried out), if the CCD output data (DATA 2 ) obtained under a condition where the optical path of incident light to the CCD  13  is closed is subtracted from the CCD output data (DATA 1 ) obtained under a condition where the optical path of incident light (mechanical shutter  12 ) to the CCD  13  is opened, it is possible to excellently eliminate the noise component caused by dark voltage even if the image was picked-up with long exposure time and therefore, it is possible to excellently pick-up even under dark environment such as nightscape. 
     Even when pick-up condition or pick-up environment is changed, it is possible to capture CCD output data (DATA 1 , DATA 2 ) suitable for the condition or environment, and it is possible to appropriately eliminate the varying dark voltage component by the subtraction processing. Therefore, it is possible to appropriately pick-up a subject image and store the image with the simple structure. 
     Further, since it is determined whether there exists a white scratch and whether the data level is in saturated state for the CCD output data (DATA 1 , DATA 2 ), and the subtraction processing is carried out to lower the affect of the white scratch against the image data of the subject, isolation point (white scratch) caused by dark voltage is eliminate, and it is possible to appropriately pick-up a subject image and store the image. 
     In the above-described embodiment, it is determined whether there exists a white scratch (S 221 ) and whether the data level is in saturated state (S 222 ) per unit of pixel data. Alternatively, the DATA 2  may be directly subtracted from the DATA  1  without carrying out the processing (S 221  to S 225 ). 
     Other embodiments of the camera according to the present invention will be described. The same portions as those of the first embodiment will be indicated in the same reference numerals and their detailed description will be omitted. 
     Second Embodiment 
       FIG. 8  is a block diagram showing a structure of an essential portion of the second embodiment of the camera according to the invention, and  FIG. 9  is a flowchart showing a main processing operation of the second embodiment of the camera according to the invention. The same structures and processing operation as those in the above-described embodiment are designated with the same symbols, and explanation thereof will be omitted or simplified. 
     As shown in  FIG. 8 , the camera of the present embodiment includes, in addition to the structure of the above-described first embodiment ( FIG. 1 ), a bypass path Ln 2  bypassing a γ-correction circuit  21   a , and a switch SW 1  for selectively switching a first signal path Ln 1  passing through the γ-correction circuit  21   a  and the bypass path (second signal path) Ln 2 . 
     The switch SW 1  is controlled by a control signal from the CPU  60 . That is, the CPU  60  controls such that in the normal pick-up mode and the through image mode, the first signal path Ln 1  passing through the γ-correction circuit  21   a  is selected, and when the CCD output data (DATA 1 , DATA 2 ) used for the correction processing to eliminate the dark, voltage component of the CCD  13  are captured, the bypass path Ln 2  which does not include the γ-correction circuit  21   a  is selected. The γ-correction circuit  21   a  is arranged to execute the γ-correction processing based on the γ-correction table having nonlinear characteristic which is previously fixedly set. 
     Concrete processing operation in the special effect pick-up mode will be explained with reference to the flowchart in  FIG. 9 . The flowchart in  FIG. 4  is also referred to if necessary. 
     Through Operation 
     After a user of the electronic still camera switches a mode switch included in the key input device  50  into a recording mode (REC), if he or she selects the special effect pick-up mode from a menu displayed on the LCD  40 , the mechanical shutter  12  is opened, the through image mode is set. At that time, a control signal is output from the CPU  60 , and the switch SW 1  is switched such as to select the first signal path Ln 1  which passes through the γ-correction circuit  21   a . In the through image mode, as in the processing operation (S 201  to S 209 ) shown in  FIG. 4 , CCD output data based on a subject image is captured whenever a predetermined exposure time T 0  is elapsed, and the data is subjected to the nonlinear γ-correction processing by the γ-correction circuit  21   a  and the color signal processing by the color process circuit  22 , and displayed on the LCD  40  as the through image. 
     Capture Operation 
     Next, when a through image having a desired composition is displayed on the LCD  40 , if the shutter key provided on the key input device  50  is “halfway pushed” (S 210 ), the CPU  60  detects the start of the recording operation, outputs a control signal to the switch SW 1 , and switches such as to select the second signal path Ln 2  which does not pass through the γ-correction circuit  21   a  (S 301 ). Exposure time T 1  and focus are set based on information obtained from the pick-up environment of the subject (S 302 ). 
     If the shutter key is further pushed, i.e., “fully pushed” (S 303 ), the electrical shutter is opened (S 304 ). After the exposure time T 1  set in step S 302  is elapsed (S 305 ), the mechanical shutter  12  is closed by the driver  17  and the TG  18  (S 306 ), CCD data (DATA 1 ) of the subject image output from the CCD  13  is captured (S 307 ), the data is converted into digital signals by the A/D  15  (S 308 ). When the mechanical shutter  12  is closed in steps S 306  and S 307  and the CCD output data (DATA 1 ) is captured from the CCD  13 , the electrical shutter is closed. 
     Next, under a condition where the mechanical shutter  12  is closed (S 310 ), the electrical shutter is again opened (S 304 ), each of the processing of steps S 304  to S 308  is executed using the exposure time T 1  set in step S 302 , thereby capturing CCD output data (DATA 2 ) including dark output component generated by the CCD  13  in the current pick-up condition but not including the subject image, and the data is converted into digital data, and the data is subjected to the linear γ-correction processing by the γ-correction circuit  21  based on the γ-correction table having the linear characteristic. The CCD output data (DATA 1 , DATA 2 ) captured by each of the processing of the above-described steps S 214  to S 218  and subjected to the γ-correction may be temporarily stored in the buffer memory  31 , or may be stored in an RAM (not shown) added to the CPU  60  which executes subtraction processing which will be described later. 
     As described above, when the CCD output data (DATA 1 , DATA 2 ) have been captured twice (S 309 ), like the processing operation (S 221  to S 230 ) shown in  FIG. 4 , it is determined whether pixel data in the CCD output data (DATA 2 ) which was captured at the second time is a white scratch of a predetermined level or higher, and whether the level of the pixel data in the DATA 1  corresponding to that pixel is saturated, and it is determined whether the subtraction processing (correction processing for canceling dark output component) should be conducted. 
     The elimination of the dark output component based on the need of the subtraction processing is determined and executed and then, the linear γ-correction processing and the predetermined color signal processing are carried out by software, and compression processing such as encoding of JPEG is carried out, and the data is stored in the flash memory  32  as captured image. When the pick-up is to be continued, the through image mode is again set, thereby switching to select the first signal path Ln 1  which passes through the γ-correction circuit  21   a.    
     With this operation, in the circuit processing for canceling the dark voltage component, the second signal path Ln 2  bypassing the γ-correction circuit  21   a  is selected, the γ-correction processing (nonlinear processing) of the CCD output data (DATA 1 , DATA 2 ) is prohibited. Since the DATA 1  and the DATA 2  are not subjected to the nonlinear processing as in the case explained with reference to  FIG. 6 , the noise component caused by the dark voltage is excellently cancelled by subtracting the DATA 2  from DATA 1 . In this case, since it is possible to prohibit the correction processing for the CCD output data (DATA 1 , DATA 2 ) by switching the switch SW 1  without changing the settings of the γ-correction table of the γ-correction circuit  21   a , the noise component caused by the dark voltage can be excellently cancelled with simple structure. 
     In the normal mode, the same operation processing as that shown in the flowchart in  FIG. 3  is carried out, and at the instant when the through image mode is set (S 101 ), a control signal is output from the CPU  60 , and the switch SW 1  is switched to the first signal path Ln 1  which passes through the γ-correction circuit  21   a , so that the through operation and the capture operation are excellently carried out. 
     Third Embodiment 
     Next, a third embodiment of the camera of the present invention will be explained with reference to the drawings. 
       FIG. 10  is a block diagram showing a structure of an essential portion of the third embodiment of the camera according to the invention, and  FIGS. 11A and 11B  are flowcharts showing a main processing operation of the third embodiment of the camera according to the invention. The same structures and processing operation as those in the above-described embodiment are designated with the same symbols, and explanation thereof will be omitted or simplified. 
     As shown in  FIG. 10 , the camera of the present embodiment includes, in addition to the structure of the above-described first embodiment ( FIG. 1 ), a signal path Ln 3  for capturing image data from which dark voltage component is cancelled (image data in which DATA 2  is subtracted from DATA 1 ) into a γ-correction circuit  21   b  from the video transfer circuit  23 , and a switch SW 2  for selectively switching signals to be captured into the γ-correction circuit  21   b  (CCD output data (DATA 1 , DATA 2 ) or image data from which the dark voltage component captured through the signal path Ln 3  is cancelled). 
     The switch SW 2  is switched and the settings of characteristic of the γ-correction table of the γ-correction circuit  21   b  is changed by control signals from the CPU  60 . 
     That is, in the normal pick-up mode and through image mode, the switch SW 2  is controlled toward a first contact to receive the CCD data, and the γ-correction table of the γ-correction circuit  21   b  is set such as to have the nonlinear characteristic. In the processing for canceling the dark voltage component of the special effect pick-up mode, the switch SW 2  is controlled toward the first contact to receive the CCD data (DATA 1 , DATA 2 ), and the γ-correction table of the γ-correction circuit  21   b  is set such as to have the linear characteristic. Further, after the dark voltage component is cancelled, the switch SW 2  is controlled toward a second contact to receive the image data from the video transfer circuit  23  through the signal path Ln 3 , and the γ-correction table of the γ-correction circuit  21   b  is set such as to have the nonlinear characteristic. 
     Concrete processing operation in the special effect pick-up mode will be explained with reference to the flowchart in  FIGS. 11A and 11B . The flowchart in  FIG. 4  is also referred to if necessary. 
     Through Operation 
     After a user of the electronic still camera switches a mode switch included in the key input device  50  into a recording mode (REC), if he or she selects the special effect pick-up mode from a menu displayed on the LCD  40 , the mechanical shutter  12  is opened, the through image mode is set. At that time, a control signal is output from the CPU  60 , and the switch SW 2  is controlled toward the first contact to receive the CCD output data (S 402 ), and the γ-correction table of the γ-correction circuit  21   b  is set so as to have the nonlinear characteristic (S 403 ). In the through image mode, as in the processing operation (S 201  to S 209 ) shown in  FIG. 4 , CCD output data based on a subject image are captured whenever a predetermined exposure time T 0  is elapsed, and the data is subjected to the nonlinear γ-correction processing by the γ-correction circuit  21   a  and the color signal processing by the color process circuit  22 , and displayed on the LCD  40  as the through image. 
     Capture Operation 
     Next, as in the same manner as the capture operation (S 210  to S 225 ) shown in  FIG. 4 , when a through image having a desired composition is displayed on the LCD  40 , if the shutter key provided on the key input device  50  is “halfway pushed” (S 210 ), the CPU  60  detects the start of the recording operation, the γ-correction table of the γ-correction circuit  21   b  is changed such as to have linear characteristic. Exposure time T 1  and focus are set based on information obtained from the pick-up environment of the subject. 
     Thereafter, as in the same manner as the processing operation in the above-described first embodiment, the CCD output data (DATA 1 ) under a condition where the mechanical shutter  12  is opened and the CCD output data (DATA 2 ) under a condition where the mechanical shutter  12  is closed is captured and is subjected to the linear γ-correction processing by the γ-correction circuit  21   b , and it is determined whether the pixel data in the captured CCD output data (DATA 2 ) is white scratch having a predetermined level or higher, and whether the level of the pixel data in the DATA 1  corresponding to the pixel data is in saturated state, thereby determining whether the subtraction processing (correction processing for canceling the dark voltage component) should be carried out. The pixel data from which the dark voltage component is cancelled based on the determination whether the subtraction processing should be carried out is temporarily stored in, e.g., the buffer memory  31  through the video transfer circuit  23 . 
     When the above-described correction processing for a frame of all the pixels is completed (Yes in S 224  in  FIG. 4 ), as shown in  FIG. 11B , a control signal is output from the CPU  60 , the switch SW 2  is controlled toward the second contact to receive the image data stored in the buffer memory  31  through the signal path Ln 3  (S 411 ), and the setting of the γ-correction table of the γ-correction circuit  21   b  is changed such as to have the nonlinear characteristic (S 412 ). The image data stored in the buffer memory  31  is captured in the γ-correction circuit  21   b  through the video transfer circuit  23  and the signal path Ln 3 , and the nonlinear γ-correction correction processing is carried out by the γ-correction circuit  21   b  based on the γ-correction table which is set such as to have the nonlinear characteristic in step S 412  (S 413 ). Thereafter, as in the same manner as the capture operation (S 227  to S 230 ) shown in  FIG. 4 , the predetermined color signal processing and compression processing such as encoding of JPEG are carried out and then, the data is stored in the flash memory  32  as the captured image. When the pick-up is to be continued, the through image mode is again set, so that the switch SW 2  is controlled toward the first contact to receive the CCD output data as shown in  FIG. 11A , and the γ-correction table which is set such as to have the nonlinear characteristic. 
     With the above operation, after the correction processing for canceling the dark voltage component, the image data is again captured in the γ-correction circuit  21   b  in which the γ-correction table is set to have the nonlinear characteristic, and the nonlinear γ-correction can be carried out. Since it is unnecessary to carry out nonlinear γ-correction using software, it is possible to reduce the load of the CPU  60  for controlling the processing. 
     In the normal pick-up mode, the same operation processing as that shown in the flowchart in  FIG. 3  is carried out, and at the instant when the through image mode is set (S 101 ), a control signal is output from the CPU  60 , and the switch SW 2  is fixedly controlled toward the first contact to receive the CCD output data and the γ-correction table of the γ-correction circuit  21   b  is fixedly set to have the nonlinear characteristic, so that the through operation and the capture operation are excellently carried out. 
     Fourth Embodiment 
     Next, a fourth embodiment of the camera of the present invention will be explained with reference to the drawings. 
       FIG. 12  is a block diagram showing a structure of an essential portion of the fourth embodiment of the camera according to the invention, and  FIG. 13  is a flowchart showing a main processing operation of the fourth embodiment of the camera according to the invention. The same structures and processing operation as those in the above-described embodiment are designated with the same symbols, and explanation thereof will be omitted or simplified. 
     As shown in.  FIG. 12 , the camera of the present embodiment includes, in addition to the structure of the above-described first embodiment ( FIG. 1 ), a temperature sensor  80  for detecting temperature around the CCD  13  at the time of image pick-up of a subject or when electric signal (CCD output data) is captured. 
     The temperature sensor  80  detects an ambient temperature around the CCD  13  and outputs a temperature information to the CPU  60 , and the CPU  60  controls whether the correction processing (steps S 211  to S 225 ) for canceling the dark voltage component shown in the first embodiment should be carried out based on the detected temperature. 
     More specifically, the same through operation as that shown in steps S 201  to S 210  in  FIG. 4  is carried out, and if the shutter key is halfway pushed in step S 210 , the CPU  60  receives the ambient temperature information from the temperature sensor  80  (S 501 ), and the CPU  60  compares the detected temperature with a preset reference temperature (S 502 ). When a temperature raises to such an extent that the dark voltage component is increased, it determined that the dark voltage component included in the image data affects the CCD output data, the above-described processing (steps S 211  to S 225 ) for canceling the dark voltage component is carried out. 
     On the other hand, if the temperature does not greatly raise so that the affect of the dark voltage component is not great, the above-described correction processing is not carried out, and the capture operation (steps S 111  to S 116  in  FIG. 3 ) in the normal pick-up mode is carried out. 
     This embodiment aims to suppress the affect of dependence of temperature of dark voltage in which dark voltage is doubled if a temperature is increased about 8° C. Since it is known that the white scratch also has the similar dependence of temperature, it is possible to avoid the affect of white scratch by this countermeasure. Therefore, since it is possible to selectively perform the correction processing in accordance with the environmental variation (temperature variation), it is possible to reduce the load of the CPU  60  for controlling the processing. 
     Fifth Embodiment 
     Next, a fifth embodiment of the camera of the present invention will be explained with reference to the drawings. 
       FIG. 14  is flowchart showing a main processing operation of the fifth embodiment of the camera according to the invention. The same structures and processing operation as those in the above-described embodiment are designated with the same symbols, and explanation thereof will be omitted or simplified. 
     The camera of the present embodiment is characterized in that the CPU  60  is provided with a supervisory function of electric charge accumulating period (accumulating period detecting function) for driving the CCD  13 . 
     The CPU  60  controls whether the above-described correction processing (steps S 211  to S 225 ) should be executed based on the charge accumulating period of the CCD  13  at the time of pick-up, i.e., based on the exposure time. 
     More specifically, as shown in the flowchart in  FIG. 14 , the exposure time T 1  is set in a halfway pushing state of the shutter key in step S 210  (S 601 ), and the exposure time T 1  and a preset reference time are compared with each other (S 602 ). If it is determined that the exposure time T 1  is elongated to such an extent that the dark voltage is increased, it is determined that the affect of the dark voltage component included in the image data is great, setting of the γ-correction table of the γ-correction circuit  21  is changed to have the linear characteristic (S 603 ), the above-described processing (steps S 211  to S 225 ) for canceling the dark voltage component is carried out. 
     On the other hand, if the exposure time is the same as or shorter than the reference value, the correction processing is not carried out, and the capture operation (steps S 112  to S 121  in  FIG. 3 ) in the normal pick-up mode is carried out. 
     This embodiment aims to suppress the affect of dependence of temperature of dark voltage in which dark voltage is increased depending on the exposure time. Since it is known that the white scratch also has the similar dependence of the exposure time, it is possible to avoid the affect of white scratch by this countermeasure. Therefore, since it is possible to selectively perform the correction processing in accordance with the environmental variation (exposure time variation), it is possible to reduce the load of the CPU  60  for controlling the processing. 
     Sixth Embodiment 
     Next, a sixth embodiment of the camera of the present invention will be explained with reference to the drawings. 
       FIG. 15  is a flowchart showing a main processing operation of the sixth embodiment of the camera according to the invention. The same structures and processing operation as those in the above-described embodiment are designated with the same symbols, and explanation thereof will be omitted or simplified. 
     The camera according to the present embodiment is characterized in that in the structure of the above-described embodiment ( FIG. 1 ), the CPU  60  is provided with a function for detecting an isolation point (isolation point detecting function) existing in image data which was subjected to the correction processing (steps S 211  to S 225  in  FIG. 4 ) shown in the first embodiment. 
     The CPU  60  detects pixel (isolation point; black scratch when it is bright or white scratch when it is dark) whose data level included in image data which was subjected to the correction processing (steps S 211  to S 225 ), and based on the detection result, the pixel data of the isolation point is corrected. 
     More specifically, from the image data generated after subtraction processing of the DATA 1  and DATA 2  of steps S 221  to S 225  in  FIG. 4 , isolation point in which data level projects from surrounding pixels is detected (S 701 ). Then, the pixel data of the isolation point detected by the CPU  60  is determined as being erinaceous data, and this data is replaced by adjacent pixel data based on the address of the isolation point (S 702 ). Thereafter, as in the same manner as the capture operation (S 226  to S 230 ) shown in  FIG. 4 , the predetermined color signal processing and compression processing such as encoding of JPEG are carried out and then, the data is stored in the flash memory  32  as the captured image. 
     With the above operation, image data which is not to be subjected to the correction processing (subtraction processing) for canceling the dark voltage component and which has erinaceous data level, e.g., image data which has pixel data of DATA 1  equal to or higher than the predetermined level and which is determined that data level of the DATA 1  is in saturated state is also determined as being isolation point and is subjected to the correction processing (replacement processing), and it is possible to appropriately pick-up a subject and store the image. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the present invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples 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. For example, each of the embodiments were explained individually, some or all of them may be combined appropriately. In each of the embodiments, the explanation is made based on a case in which as technique of the correction processing for canceling dark voltage component included in the DATA 1  under a condition where the γ-correction processing is prohibited by the CPU  60 , the CCD output data (DATA 2 ) captured by closing the mechanical shutter  12  is subtracted from the CCD output data (DATA 1 ) of the subject image captured by opening the mechanical shutter  12  by computation processing (subtraction processing) in the CPU  60 . However, the present invention should not be limited to this, and other correction processing technique may be applied to the processing method for canceling the dark voltage component such as processing using software or processing using hardware such as a subtractor.