an embodiment of the present invention will be hereinafter described with reference to the drawings . the same reference numbers as in the conventional art refer to identical components . fig2 is a block diagram of a black level correcting device according to this embodiment . a black level correcting device 50 has a subtracter 12 , a pga 14 , a hold circuit 16 , an a / d converter 18 , a d / a converter 20 , a switch 60 , a pga 62 , a comparator 26 , a reference setting section 24 , and a subtracter 36 . the function of each part of the black level correcting device 50 will be hereinafter described . the subtracter 12 receives an analog pixel signal from an image sensor 70 , and subtracts a hold level of the hold circuit 16 from the pixel signal to input the result into the pga 14 . the pga 14 amplifies the input analog pixel signal , and inputs the amplified pixel signal into the a / d converter 18 . the a / d converter 18 converts the input pixel signal to a digital pixel value , and inputs the pixel value into the comparator 26 and the subtracter 36 . the reference setting section 24 inputs a reference value ( positive value ) into the comparator 26 and the subtracter 36 . the reference value is a target value which indicates how much value each pixel signal from ob pixels of the image sensor 70 is expected to be just after a / d conversion . the comparator 26 inputs a result of subtracting the reference value from the pixel value into the pga 62 . when pixel signals from the ob pixels of the image sensor 70 are input into the subtracter 12 ( when an ob timing signal in the drawing is at a high level ), the pga 62 changes gain in accordance with an input pixel value to shorten clamp time of the hold circuit 16 . the details of this operation will be described later . the switch 60 is turned on only over a period when the pixel signals from the ob pixels are input into the subtracter 12 , in order to input the pixel values from the pga 62 into the d / a converter 20 . the d / a converter 20 converts the input pixel value into an analog signal , and inputs the analog signal into the hold circuit 16 . the hold circuit 16 brings its hold level near to an input signal level , while the switch 60 is on . when the switch 60 is turned off , the hold circuit 16 holds and keeps outputting the hold level . the subtracter 12 subtracts the hold level from an analog pixel signal input from the image sensor 70 . this operation is carried out in both cases where pixel signals from the ob pixels are input and where pixel signals from valid pixels are input . the subtracter 36 ( also called a standard value subtracting section in the present invention ) subtracts the reference value from a pixel value input from the a / d converter 18 and outputs a result to a subsequent stage . in fig2 , a signal path routed through the comparator 26 , the pga 62 , the switch 60 , the d / a converter 20 , the hold circuit 16 , and the subtracter 12 in this order is also referred to as a feedback loop in the present invention . a signal which is output from the comparator 26 as a pixel value and input into the subtracter 12 from the hold circuit 16 through the pga 62 and the d / a converter 20 is referred to as a feedback signal in the present invention . before the explanation of clamp accelerating operation by the pga 62 , the structure of the image sensor 70 will be briefly described to explain order of reading out pixels , which is necessary for explaining the clamp accelerating operation . the image sensor 70 has pixels with “ m ” rows and “ n ” columns in the form of , for example , a bayer pattern , “ n ” number of vertical ccds which are formed for each column , a horizontal ccd , and a reading amplifier ( not illustrated ). signal charge ( pixel output ) generated by each pixel is first transferred to the vertical ccd . the “ n ” number of vertical ccds successively transfer the pixel output to the horizontal ccd on a row basis . the horizontal ccd successively transfers the transferred pixel output to the reading amplifier . then , the reading amplifier inputs the transferred pixel output to the subtracter 12 of the black level correcting device 50 as analog pixel signals . to simplify the explanation , ob pixels in an upper portion and the like are not considered in this embodiment , and it is assumed that of all the pixels in each row ( the number of pixels is “ n ”), a few pixels read out first are ob pixels ( lateral ob ) and the rest are valid pixels . therefore , signals are read out in order of the ob pixels in a first row , the valid pixels in the first row , the ob pixels in a second row and the valid pixels in the second row . . . . fig3 is a timing chart showing gain of the pga 62 . fig3 ( a ) shows a readout period of each row . fig3 ( b ) shows a pixel value output from the a / d converter 18 , and fig3 ( c ) shows variation in the gain of the pga 62 . the operation of the pga 62 will be hereinafter described with the use of timing of times t 1 to t 6 shown in the drawing . in this embodiment , idling operation is not carried out before reading out pixel signals from the ob pixels in the first row . at the time t 1 , the pixel signals from the ob pixels in the first row start being input into the subtracter 12 . in synchronization with this , the ob timing signal is changed into high level , and the switch 60 is turned on . this time is the start of a clamp , so that the hold level of the hold circuit 16 has not reached a convergence level . since the subtracter 12 forms a negative feedback loop , the convergence level of the hold level is a level of an analog signal obtained by converting , into an analog signal , “ a value subtracting the reference value from a value into which each pixel signal of the ob pixels is simply converted by a / d conversion ”. here , “ the value simply converted by the a / d conversion ” is a value obtained in such a case where the pixel signals of the ob pixels are input into the a / d converter 18 through the pga 14 without any subtraction by the subtracter 12 and converted by the a / d conversion . the absolute value of a pixel value output from the comparator 26 is at the maximum at the start of the clamp , and is reduced as the hold level approaches the convergence level . the absolute value of the pixel value becomes zero when the hold level reaches the convergence level . accordingly , as shown in fig3 ( b ), at the time t1 , the difference between a pixel value output from the a / d converter 18 and the reference value , that is , the absolute value of a pixel value output from the comparator 26 is equal to or more than a predetermined value “ a ” ( positive value ). in this embodiment , by way of example , the predetermined value “ a ” is a value in which the difference between the pixel value output from the a / d converter 18 and the reference value becomes 50 % of the reference value , when the reference value is regarded as a standard ( 100 %). while the absolute value of the pixel value output from the comparator 26 is equal to or more than the predetermined value “ a ”, the pga 62 , as shown in fig3 ( c ), brings the gain to “ l ” ( value at least larger than 1 ). a gain “ l ” is a value which is large to such an extent as to sufficiently shorten the clamp time , and is , for example , 16 . the clamp time is time from the start of inputting the pixel signals of the ob pixels in the first row into the subtracter 12 till the hold level reaches the convergence level . to be more specific , the hold circuit 16 is composed of , for example , a publicly known time constant circuit ( not illustrated ). namely , a charging voltage of a capacitor in the time constant circuit is brought near to an input voltage level during a sample period ( a period in which the switch 60 is on , in this embodiment ). the charging voltage of the capacitor is held and kept being output during a hold period ( a period in which the switch 60 is off , in this embodiment ). the capacitor is also called a maintaining section in the present invention . the charging voltage of the capacitor is also called a maintaining level in the present invention , and corresponds to the hold level of this embodiment . the larger the difference between the input level and the charging voltage of the capacitor in the sample period is , the more sharply the charging voltage of the capacitor increases or decreases . thus , by making the gain of the pga 62 high at the start of the clamp of the hold circuit 16 , the charging voltage of the capacitor of the hold circuit 16 rapidly varies because it is assumed that the pixel value output from the comparator 26 is large . as a result , the clamp time of the hold circuit 16 is shortened . next , at the time t 2 , the absolute value of the pixel value output from the comparator 26 becomes less than the predetermined value “ a ”. in synchronization with this , the pga 62 reduces the gain from “ l ” to “ m ”. the gain ““ m ” is a value , for example , 8 which is larger than 1 and smaller than the gain “ l ”. the pga 62 maintains the gain at “ m ” in a period of time when the absolute value of the pixel value output from the comparator 26 is equal to or more than a predetermined value “ b ” and less than the predetermined value “ a ”. this gain “ m ” and the foregoing gain “ l ” are also called clamp accelerating gain in the present invention . then , at the time t 3 , the absolute value of the pixel value output from the comparator 26 becomes less than the predetermined value “ b ” ( positive value less than the predetermined value “ a ”). in synchronization with this , the pga 62 lowers its gain to “ s ” ( for example , 4 ). by way of example , in this embodiment , the predetermined value “ b ” is a value in which the difference between the pixel value output from the a / d converter 18 and the reference value becomes 15 % of the reference value , when the reference value is regarded as a standard ( 100 %). after that , the hold level reaches the convergence level , and the pixel value output from the comparator 26 becomes zero . the moment when the hold level reaches the convergence level means “ the completion of the clamp operation ”. then , at the time t 4 , the readout of the pixel signals from the ob pixels in the first row is completed , the pixel signals from the valid pixels in the first row start being input into the subtracter 12 . in synchronization with this , the ob timing signal is changed into a low level , so that the switch 60 is turned off . while the switch 60 is off , the hold circuit 16 maintains and keeps outputting its hold level . the pga 62 keeps its gain at “ s ” while the switch 60 is off . since the switch 60 is off , the operation of the circuit does not change even if the gain of the pga 62 is set at any value . then , at the time t 5 , the readout of the pixel signals from the valid pixels in the first row is completed . then , at the time t 6 , pixel signals from the ob pixels in the second row start being input into the subtracter 12 . in synchronization with this , the ob timing signal changes into the high level , and the switch 60 is turned on . at this time , the hold level has already reached the convergence level . thus , the pga 62 keeps the gain at “ s ”. the pga 62 shortens the clamp time of the hold circuit 16 by changing the gain as described above . thus , the hold level of the hold circuit 16 is stabilized at the convergence level before completing the readout of the pixel signals from all of the ob pixels in the first row . therefore , a pixel signal of the black level ( an average level of the pixel signals from the ob pixels ) is output from the a / d converter 18 as a pixel value which is equal to the reference value . a sag does not occur because the hold level is rapidly stabilized at the convergence level . accordingly , the pixel signal from the image sensor 70 is changed into a pixel value with a black level of zero , by means of subtracting the reference value from the pixel value output from the a / d converter 18 by the subtracter 36 in the subsequent stage , as shown in fig2 . for comparison , fig3 ( d ) shows a pixel value output from an a / d converter 18 in a case where pixel signals of an image sensor 70 are read out similarly to above by use of a conventional black level correcting device 10 shown in fig1 . in the conventional black level correcting device 10 , as shown in fig3 ( d ), clamping is not completed at the time t 4 when the readout of the pixel signals from the ob pixels in the first row is completed , and even at a time when the readout of the pixel signals from the ob pixels from the second row is completed . therefore , there are problems that the valid pixels in at least first , second , and third rows are changed into pixel values having different black levels from one another and the like . in this embodiment , the pga 62 gradually reduces its gain after increasing the gain just after the start of the clamping . accordingly , it is possible to greatly shorten the clamp time ( convergence time ) of the hold circuit 16 . thus , even if the number of the ob pixels in each row is small , it is possible to stabilize the hold level of the hold circuit 16 at the convergence level before starting the readout of the pixel signals from the valid pixels . as a result of this , the sag hardly occurs . also , since it is unnecessary to carry out the idling operation before reading out the pixel signals after exposure , circuit stability time can be shortened . therefore , installing the black level correcting device 50 according to this embodiment in an electronic camera makes it possible to shorten a release time lag compared to the conventional art . furthermore , the gain of the pga 62 is reduced just before convergence . thus , it does not occur that noise in the pixel signals from the ob pixels is amplified just before the convergence and the hold level of the hold circuit 16 varies . accordingly , it is possible to precisely correct the black level . therefore , since banding noise is reduced , it is possible to improve image quality . in the case of long - term exposure , a direct - current level of the pixel signal of the ob pixel , being a noise component , is increased . also in such a case , however , it is possible to precisely correct the black level as described above . in other words , after a pixel value is output from the a / d converter 18 in such a manner that an average level of pixel signals of the ob pixels precisely becomes the reference value , the subtracter 36 in the subsequent stage subtracts the reference value from the pixel value . thus , the noise component included in the valid pixel is uniformly deleted , so that it is possible to precisely correct dark current irrespective of a level of the pixel signals from the ob pixels . since the sag occurs in a conventional example , it is necessary to subtract a reference value , which is added by a subtracter 12 in a previous stage , in a subsequent stage after sampling variation in the level again . accordingly , two synchronization signals with different timing , which indicate the input timing of pixel signals from ob pixels , are necessary for a negative feedback loop in a previous stage and a sampling and holding section 32 , respectively . on the other hand , since the sag does not occur in this embodiment , the pixel signal from the image sensor 70 is converted into the pixel value with a black level of zero just by subtracting the reference value from the pixel value output from the a / d converter 18 by the subtracter 36 in the subsequent stage . therefore , the single synchronization signal , which indicates the input timing of the pixel signals from the ob pixels , is necessary only for the negative feedback loop . therefore , it is possible to simplify the structure of the circuit . in this embodiment , the pga 62 is inserted between the comparator 26 and the switch 60 in series . the present invention , however , is not limited to this embodiment . the pga 62 may be inserted between , for example , the hold circuit 16 and the d / a converter 20 , or between the switch 60 and the d / a converter 20 . in this embodiment , by way of example , the predetermined value “ b ” is a value in which the difference between a pixel value output from the a / d converter 18 and a reference value is 15 % of the reference value , when the reference value is regarded as a standard ( 100 %). in other words , when a convergence level is regarded as a standard ( 100 %), large gain ( gain “ l ” and “ m ”) is used for accelerating clamping till the difference between the convergence level and a hold level is reduced to 15 %. the present invention , however , is not limited to this embodiment . the large gain may be used for accelerating the clamping till the difference between the convergence level and the hold level is reduced to , for example , 10 % or 20 %. in this embodiment , by way of example , a hold level reaches a convergence level before completing the readout of pixel signals from the ob pixels in the first row , namely , an output pixel value is changed in the first row . the present invention , however , is not limited to such an embodiment . there is a case where pixel output of several rows is read out before the hold level reaches the convergence level , namely , the output pixel values vary in a single image . also in this case , it is possible to shorten clamp time compared to the conventional art by varying the gain of the pga 62 as in the case of this embodiment . fig4 shows an example of a timing chart of the gain of the pga 62 according to this case . in the example of fig4 , pixel output of each row is read out in order of the ob pixels and the valid pixels . the switch 60 is turned on while the pixel output of the ob pixels is read out , and is turned off while the pixel output of the valid pixels is read out . while the pixel output of the ob pixels is read out , the pga 62 varies its gain in accordance with the absolute value of a pixel value output from the comparator 26 . while the switch 60 is on , and the absolute value of the pixel value output from the comparator 26 ( the difference between an output value from the a / d converter 18 corresponding to the pixel output from the ob pixels and a reference value ) is equal to or more than the predetermined value “ a ” ( up to a third row in this example ), the pga 62 sets the gain at “ l ” as shown in fig4 ( c ). in this period , a hold level of the hold circuit 16 rapidly approaches a convergence level . in a period of time after this when the switch 60 is on , if the absolute value of the pixel value output from the comparator 26 becomes less than the predetermined value “ a ”, the pga 62 lowers the gain from “ l ” to “ m ” in synchronization with this , and maintains it ( fourth and fifth rows in this example ). in a period of time after this when the switch 60 is on , if the absolute value of the pixel value output from the comparator 26 becomes less than the predetermined value “ b ”, the pga 62 lowers the gain from “ m ” to “ s ” in synchronization with this , and maintains it ( from a sixth row in this example ). after that , the hold level reaches the convergence level ( a period of reading out the pixel output from the ob pixels in an eighth row in this example ), and the pixel value output from the comparator 26 becomes zero . in an embodiment shown in fig4 , as described above , it is possible to obtain similar effect to the embodiment shown in fig3 . as shown in fig5 , the black level correcting device according to this embodiment may be installed in the electronic camera . in the drawing , an electronic camera 100 has a photographic lens 104 , the image sensor 70 , the black level correcting device 50 , a timing generator 108 , an mpu ( micro processing unit ) 112 , an operation section 116 , a system bus 120 , an image processing section 124 , an image - data memory 128 , a program memory 132 , a card interface 136 , and an exchangeable memory card 140 . the mpu 112 controls the system of the electronic camera 100 by use of the system bus 120 . the operation section 116 has a button group for setup including a power button , a release button , and the like ( not illustrated ). the black level correcting device 50 receives an analog pixel signal from the image sensor 70 together with an ob timing signal from the timing generator 108 , and outputs digital pixel values . the pixel values of all valid pixels after exposure are subjected to image processing in the image processing section 124 as image data , and then recorded on the memory card 140 . a major feature of the electronic camera 100 is that the black level correcting device 50 according to the present invention is installed therein , and explanation about the detailed imaging operation thereof will be omitted because it is publicly known . the invention is not limited to the above embodiments and various modifications may be made without departing from the spirit and scope of the invention . any improvement may be made in part or all of the components .