JP2011009834A - Imager and imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imager capable of easily obtaining an optimal image quality according to photographing without spoiling gradation more than required, and to provide an imaging method.SOLUTION: The imager includes an imaging element 5, an amplifier 6b, and an AD converter 7. The amplifier 6b amplifies an analog imaging signal outputted from the imaging element 5 by a predetermined analog gain. The AD converter 7 converts the imaging signal outputted from the amplifier 6b into a digital signal. The AD converter 7 clamps a black pixel signal to a setting level, and converts into a digital signal, wherein the black pixel signal is a signal not owing to light from an object among imaging signals outputted from the amplifier 6b. The AD converter 7 includes a black pixel signal output control function, an output lower-limit proportion operation function, and a system controller 11. The black pixel signal output control function controls the imaging element 5 to output a plurality of black pixel signals. The output lower-limit proportion operation function obtains a proportion occupied by signals which serves as output lower-limit of the AD converter 7 among the plurality of black pixel signals converted into digital signals. The system controller 11 has a level adjustment function to adjust a clamp level according to the proportion.

Description

  The present invention relates to an imaging apparatus and an imaging method.

  Imaging devices such as CCD type and CMOS type image sensors generally have a configuration in which a large number of photoelectric conversion element lines composed of a large number of photoelectric conversion elements arranged in the horizontal direction are arranged in the vertical direction orthogonal to the horizontal direction. The plurality of photoelectric conversion elements at the end of each photoelectric conversion element line are shielded from light. In an image pickup apparatus equipped with such an image pickup device, a black pixel signal, which is a signal obtained from a light-shielded photoelectric conversion element, among predetermined image pickup signals for one horizontal line output from the image pickup device is set to a predetermined target level (hereinafter referred to as a target level). , Referred to as a clamp level), and the subsequent signal processing is performed based on the clamp level.

  As a black pixel signal clamping method, every time an imaging signal is output from each photoelectric conversion element line, the black pixel signal included in the imaging signal for one line is compared with the clamp level. In general, a method of calculating an error from the clamp level of the black pixel signal and adding a correction amount that eliminates the error to the imaging signal before digital conversion is used.

  In addition, the imaging apparatus includes an amplifier that amplifies an analog signal output from the imaging element, and an AD converter that converts the amplified analog signal into a digital signal. The clamping process described above is performed by an AD converter. With the recent increase in sensitivity of imaging devices, gain values that can be set by an amplifier have increased. As the gain increases, signal level variation due to random noise increases. FIG. 13 is a diagram illustrating an example of a histogram of black pixel signals output from the AD converter. The horizontal axis in FIG. 13 indicates the signal value after AD conversion. The vertical axis in FIG. 13 indicates the number of each output signal value.

  A histogram h1 shown in FIG. 13A shows an example of a histogram when signal level variation due to random noise is large. When the variation in signal level is large, the width of the histogram is widened, and as shown in FIG. 13A, a black pixel signal having a negative output value is generated particularly in the low luminance portion. However, the AD converter has an upper limit value and a lower limit value that can be output. For example, if the AD conversion output is unsigned 16 bits, the upper limit output and the lower limit output are 65535 and 0, respectively. For this reason, the negative black pixel signal is clipped to 0 which is the lower limit value, and the histogram h1 is actually a histogram h2 shown in FIG. 13B. As a result, a phenomenon in which the low luminance part is digitally converted to be higher than the actual signal level (hereinafter, this phenomenon is referred to as a lower limit clip) occurs.

  The above-described clamping process performed by the AD converter is also called a process of fixing the input black pixel signal to the clamp level so that the peaks of the histograms h1 and h2 shown in FIG. be able to. The histogram h1 is a histogram obtained by AD conversion when the clamp processing is performed with the clamp level set to “a”. The average value of each black pixel signal indicated by the histogram h1 is obtained, and this average value is obtained by subsequent black level correction (correction for subtracting the black level from the signal generated by the photoelectric conversion element in response to light from the subject). The black level to be used. If the AD conversion output is as shown in the histogram h1, the difference between the average value of the black pixel signal after AD conversion and the clamp level a is small, so that the black level can be corrected with high accuracy. However, in actuality, as shown in FIG. 13B, the histogram is h2, so the average value of each black pixel signal indicated by the histogram h2 is larger than the clamp level a. As a result, the black level correction is not accurately performed, and the image quality is deteriorated.

  Patent Document 1 discloses a technique for preventing image quality deterioration due to the occurrence of a lower limit clip by raising the clamp level only under an exposure condition in which the lower limit clip is likely to occur. However, since this technique raises the clamp level according to exposure conditions such as gain, temperature, and exposure time, there are many variables and control becomes difficult. In addition, since the conditions for increasing the clamp level differ depending on the exposure conditions and individual differences of the imaging devices, the control is further complicated in order to give individual increases to these conditions. There is also a method in which the clamp level is set to be large in advance so that the lower limit clip does not occur under any exposure conditions. However, in this case, even under exposure conditions where the clamp level does not have to be increased, the histogram distribution approaches the upper limit value of the AD conversion output, so that the gradation of the high luminance part is lost.

  Patent Document 2 discloses a technique for preventing blackout by using a standard deviation of data obtained from light-shielded pixels. However, the data that can be handled by this technique is realistic after AD conversion, and the influence of the lower limit clip at the time of AD conversion cannot be excluded. Rather, since the standard deviation is obtained using the data in which the lower limit clipping has occurred, if the signal component clipped to zero increases, there is a risk that undercorrection or overcorrection will occur during black level correction.

JP 2007-110486 A JP 2006-67011 A

  The present invention has been made in view of the above circumstances, and provides an imaging apparatus and an imaging method capable of easily obtaining an optimum image quality according to shooting without impairing gradation more than necessary. Objective.

  An imaging apparatus according to the present invention includes an imaging device, an amplification unit that amplifies an analog imaging signal output from the imaging device with a predetermined analog gain, and an AD that converts the imaging signal output from the amplification unit into a digital signal. A conversion unit, wherein the AD conversion unit clamps a black pixel signal, which is a signal that does not depend on light from the subject, from the imaging signal output from the amplification unit to a set level and converts the signal to a digital signal A black pixel signal output control unit for performing control to output a plurality of black pixel signals from the image sensor, and an output of the AD conversion unit among the plurality of black pixel signals converted into the digital signal An output lower limit value ratio calculating unit for determining a ratio of a signal serving as a lower limit value, a level adjusting unit for adjusting the set level according to the ratio, and the analog gain according to the ratio And at least one of the analog gain adjustment unit for adjusting.

  The imaging method of the present invention includes an amplification step of amplifying an analog imaging signal output from the imaging device with a predetermined analog gain, and an AD conversion step of converting the imaging signal amplified in the amplification step into a digital signal. In the AD conversion step, a black pixel signal that is a signal that does not depend on light from the subject is clamped to a set level among the amplified image pickup signal and converted into a digital signal, and the black pixel signal is converted from the image pickup device. A black pixel signal output control step for performing control to output a plurality of signals, and a ratio of a signal serving as an output lower limit value in the AD conversion step among the plurality of black pixel signals converted into the digital signal is obtained. An output lower limit ratio calculation step, a level adjustment step for adjusting the set level according to the ratio, and the analog according to the ratio. And at least one of the analog gain adjustment step of adjusting the gain.

  According to the present invention, it is possible to provide an imaging apparatus and an imaging method capable of easily obtaining an optimum image quality according to shooting without impairing gradation more than necessary.

The figure which shows schematic structure of the digital camera which is an example of the imaging device for describing one Embodiment of this invention Plane schematic diagram showing a configuration example of the image sensor shown in FIG. The figure for demonstrating the change of the output histogram of AD conversion part by adjusting a clamp level The flowchart for demonstrating operation | movement of the digital camera shown in FIG. Timing chart when the digital camera of FIG. 1 performs imaging in the first mode Timing chart when the digital camera of FIG. 1 performs imaging in the second mode Timing chart when the digital camera of FIG. 1 performs imaging in the third mode The figure for demonstrating the change of the output histogram of AD conversion part by adjusting analog gain Flowchart for explaining the operation of the digital camera of the modified example Timing chart when the digital camera of the modified example performs imaging in the first mode Timing chart when the digital camera of the modification performs imaging in the second mode Timing chart when the digital camera of the modified example performs imaging in the third mode The figure which showed an example of the histogram of the black pixel signal output from AD converter

  Embodiments of the present invention will be described below with reference to the drawings. An imaging apparatus described below is an imaging unit incorporated in an imaging apparatus such as a digital camera or a digital video camera, an electronic endoscope, a mobile phone, or the like.

FIG. 1 is a diagram showing a schematic configuration of a digital camera which is an example of an imaging apparatus for explaining an embodiment of the present invention.
The imaging system of the illustrated digital camera includes a photographing lens 1, a CCD (Charge Coupled Device) type imaging device 5, a diaphragm 2 provided between the two, an infrared cut filter 3, an optical low-pass filter 4, and the like. And a mechanical shutter MS disposed in front of the photographing lens 1.

  A system control unit 11 that performs overall control of the electrical control system of the digital camera controls the flash light emitting unit 12 and the light receiving unit 13 and controls the lens driving unit 8 to adjust the position of the photographing lens 1 to the focus position and zoom. The exposure amount is adjusted by adjusting the aperture amount of the aperture 2 via the aperture drive unit 9.

  In addition, the system control unit 11 controls the shutter driving unit S to open the mechanical shutter MS so that light enters the image sensor 5 or closes the mechanical shutter MS so that the image sensor 5 is completely shielded from light. To do.

  Further, the system control unit 11 drives the imaging device 5 via the imaging device driving unit 10 and outputs a subject image captured through the photographing lens 1 as an imaging signal. An instruction signal from the user is input to the system control unit 11 through the operation unit 14.

  The electric control system of the digital camera further includes a correlated double sampling processing unit (CDS) 6a, an amplification unit (PGA) 6b, and an AD conversion unit 7, which are controlled by the system control unit 11.

  The CDS 6a performs correlated double sampling processing on the image signal output from the image sensor 5 to remove reset noise.

  The PGA 6b amplifies the imaging signal output from the CDS 6a with a predetermined analog gain. This analog gain is set according to the ISO sensitivity that can be set in the digital camera.

  The AD conversion unit 7 converts the analog imaging signal output from the PGA 6b into a digital signal and outputs the digital signal.

  Further, the electric control system of the digital camera performs predetermined digital signal processing on the main memory 16, the memory control unit 15 connected to the main memory 16, and the imaging signal output from the AD conversion unit 7. The digital signal processing unit 17 that generates image data, the compression / decompression processing unit 18 that compresses the image data generated by the digital signal processing unit 17 into a JPEG format or expands the compressed image data, and photometric data are integrated. An integration unit 19 for obtaining a gain for white balance correction performed by the digital signal processing unit 17, an external memory control unit 20 to which a detachable recording medium 21 is connected, and a liquid crystal display unit 23 mounted on the back of the camera or the like are connected. Display control unit 22. The memory control unit 15, digital signal processing unit 17, compression / decompression processing unit 18, integration unit 19, external memory control unit 20, and display control unit 22 are connected to each other by a control bus 24 and a data bus 25, and a system control unit 11 is controlled by a command from 11.

  FIG. 2 is a schematic plan view showing a configuration example of the image sensor shown in FIG.

  The imaging element 5 shown in FIG. 2 includes a photoelectric conversion element 51, a black level detection photoelectric conversion element 52, a vertical charge transfer unit 53, a horizontal charge transfer unit 54, and an output unit 55.

  The photoelectric conversion element 51 is for obtaining a signal corresponding to light from a subject, and an opening is provided in a light shielding film (not shown) provided above the light receiving surface so that light enters from the opening. It has become.

  The black level detecting photoelectric conversion element 52 is for detecting a black level (hereinafter referred to as a black level) as a reference of an imaging signal, and light is not incident by a light shielding film provided thereabove. Yes.

  The photoelectric conversion elements 51 and the black level detection photoelectric conversion elements 52 are arranged two-dimensionally (in the example of FIG. 2 in the form of a square lattice) in the row direction of the semiconductor substrate and in the column direction perpendicular thereto. When the photoelectric conversion element 51 and the black level detection photoelectric conversion element 52 are collectively referred to as a photoelectric conversion element, in the example of FIG. 2, a plurality of photoelectric conversion element rows including a plurality of photoelectric conversion elements arranged in the row direction are arranged in the column direction. It has a configuration in which a plurality of photoelectric conversion element columns formed of a plurality of photoelectric conversion elements arranged in the column direction are arranged in the row direction.

  Among the photoelectric conversion element rows, all the photoelectric conversion elements in the photoelectric conversion element rows up to the Mth line from the horizontal charge transfer unit 54 side are the black level detecting photoelectric conversion elements 52.

  Among the photoelectric conversion element rows, each photoelectric conversion element row from the (M + 1) th row to the Nth row counted from the horizontal charge transfer unit 54 side has a photoelectric conversion device 51 and a black level detection photoelectric conversion device 52. include. Each of the photoelectric conversion element rows includes a plurality of black level detection photoelectric conversion elements 52, and the plurality of black level detection photoelectric conversion elements 52 are arranged at end portions (horizontal charge transfer units) of the photoelectric conversion element rows. 54 at the upstream end in the charge transfer direction, the right end in the figure.

  The vertical charge transfer unit 53 is provided corresponding to each photoelectric conversion element array, and transfers the charge generated in each photoelectric conversion element of the corresponding photoelectric conversion element array to the horizontal charge transfer unit 54 in the column direction. . The vertical charge transfer unit 53 includes a vertical transfer channel (not shown) formed in the semiconductor substrate so as to extend in the column direction, and a plurality of vertical transfer electrodes 53a arranged in the column direction above the vertical transfer channel. Has been.

  Between the vertical transfer channel and the corresponding photoelectric conversion element, a charge reading unit (not shown) for reading charges from the photoelectric conversion element to the vertical transfer channel is formed.

  The vertical transfer electrode 53a includes a connection to the wiring V1, a connection to the wiring V2, a connection to the wiring V3, and a connection to the wiring V4. By applying four-phase pulses to these wirings V1 to V4, the charge transfer operation in the vertical transfer channel is controlled. The vertical transfer electrode 53a to which the wiring V1 is connected also covers the charge reading portion, and when a read pulse is applied thereto, the charge is read from the photoelectric conversion element to the vertical transfer channel.

  The horizontal charge transfer unit 54 transfers the charges transferred by the vertical charge transfer units 53 toward the output unit 55 in the row direction. The horizontal charge transfer unit 54 includes a horizontal transfer channel (not shown) formed in the semiconductor substrate so as to extend in the row direction, and a plurality of horizontal transfer electrodes 54a arranged in the row direction above the horizontal transfer channel. ing.

  The horizontal transfer electrode 54a includes one connected to the wiring H1 and one connected to the wiring H2. By applying a two-phase pulse to these wirings H1 and H2, the charge transfer operation in the horizontal transfer channel is controlled.

  The output unit 55 converts the charge transferred from the horizontal charge transfer unit 54 into a signal corresponding to the charge amount and outputs the signal.

  The system control unit 11 also functions as a black pixel signal output control unit that performs control to output n (n is a natural number of 2 or more) black pixel signals from the image sensor 5 during imaging. The system control unit 11 drives the imaging device 5 in one of three modes (first mode, second mode, and third mode) in response to an imaging instruction performed via the operation unit 14. The subject is imaged.

  In the first mode, after the exposure of the image sensor 5 performed in accordance with the imaging instruction is completed, charges are read from all the photoelectric conversion elements, transferred, and a black pixel signal (a signal that does not depend on light from the subject) ( In this control, an image pickup signal including a signal obtained from the photoelectric conversion element 52 for black level detection) and a signal corresponding to light from the subject (a signal obtained from the photoelectric conversion element 51) is output from the image pickup element 5. .

  In the second mode, after the exposure of the image sensor 5 performed in accordance with the imaging instruction is completed, before the charge is read from all the photoelectric conversion elements, the vertical charge transfer unit 53 and the horizontal charge transfer unit 53 are read without reading the charges from all the photoelectric conversion elements. The charge transfer unit 54 is driven to perform idle transfer. Then, by this empty transfer, only the black pixel signal, which is an imaging signal that does not depend on light from the subject, is output from the imaging device 5. Thereafter, the charge is read from all the photoelectric conversion elements, transferred, and the image pickup signal is output from the image pickup element 5.

  In the third mode, the mechanical shutter MS is closed and the image sensor 5 is shielded from light before the exposure of the image sensor 5 performed according to the imaging instruction. In this state, charges are read from all the photoelectric conversion elements, and the vertical charge transfer unit 53 and the horizontal charge transfer unit 54 are driven. As a result, only the black pixel signal, which is an imaging signal that does not depend on light from the subject, is output from the imaging device 5. Thereafter, the mechanical shutter MS is opened to expose the image sensor 5, and after the exposure is completed, electric charges are read from all the photoelectric conversion elements and transferred to output an image signal from the image sensor 5.

  The AD conversion unit 7 clamps the black pixel signal, which is a signal that does not depend on light from the subject, of the imaging signal output from the PGA 6b to a set level (hereinafter also referred to as a clamp level). Convert it to a digital signal.

  The system control unit 11 obtains a ratio (hereinafter, also referred to as a clip probability) of a signal serving as an output lower limit value of the AD conversion unit 7 among n black pixel signals converted into digital signals by the AD conversion unit 7. Also functions as a lower limit ratio calculation unit. This clip probability is obtained by creating a histogram of n black pixel signals and performing the calculation of {(number of black pixel signals whose signal value is “0”) / n} × 100. Can be sought.

  The system control unit 11 also functions as a level adjustment unit that adjusts the clamp level of the AD conversion unit 7 according to the clip probability.

  When the histogram of n black pixel signals output from the AD conversion unit 7 is as shown in h2 in FIG. 3A, the clip probability is greater than 0%. For this reason, the black level to be subtracted from the image pickup signal obtained from the photoelectric conversion element 51 becomes larger than the clamp level a, and it is difficult to perform black level correction with high accuracy. Therefore, the system control unit 11 adjusts the clamp level to a ′ that is larger than a. Thereby, as shown in FIG. 3B, the histogram h2 is shifted to the upper limit value side of the AD conversion output, and the clip probability can be reduced to 0%. When the clip probability is 0%, the clamp level and the black level almost coincide with each other, so that the black level can be corrected with high accuracy.

  As a specific method of adjusting the clamp level in accordance with the clip probability, a clip probability threshold (for example, 5%) that allows black level accuracy is set, and the clip probability exceeds the threshold when the clip probability is exceeded. There is a method of increasing the clamp level to a level where the probability is less than or equal to this threshold. Note that when the clamp level is increased, the gradation level may be impaired by increasing the clamp level too much. Therefore, an allowable range (for example, 2% to 5%) of the clip probability that can allow the black level accuracy is set, and when the clip probability is out of the allowable range, the clip probability is within the allowable range. The clamp level may be adjusted. For example, when the clip probability is higher than 5%, the clamp level is increased so that the clamp level is 2% to 5%, and when the clip probability is lower than 2%, the clamp level is 2% to 5%. Reduce the clamp level so that In this way, the gradation expression can be enriched while preventing the black level accuracy from being lowered, and the overall image quality can be improved.

  The clamp level value after adjustment may be obtained by calculation by the system control unit 11 or may be obtained by the system control unit 11 bringing data from a table. When calculating by calculation, clip probability data at each clamp level when the clamp level is changed by the minimum unit is obtained in advance, and from this data, the clip probability when the clamp level is changed by the minimum unit Find the amount of change. Then, from this amount of change, a clip probability is input, and a function that outputs an adjustment amount of the clamp level such that the clip probability is below a threshold value or within an allowable range is created and stored in the digital camera. good. Then, after the system control unit 11 inputs the clip probability to this function and obtains the adjustment amount of the clamp level, the adjustment amount is added to the already set clamp level to adjust the clamp level. good.

  If the table is obtained by a table, a table in which the clip probability and the clamp level are associated with each other is stored in the digital camera, and the system control unit 11 determines that the obtained clip probability exceeds the threshold value or is outside the allowable range. If there is, the clamp level at which the clip probability is equal to or less than the threshold value or within the allowable range may be read from the table and set in the AD conversion unit 7.

  Next, the operation at the time of shooting by the digital camera will be described. FIG. 4 is a flowchart for explaining the operation of the digital camera shown in FIG.

  When the shooting mode is set, the system control unit 11 determines whether or not a shooting instruction is given by pressing a release button included in the operation unit 14 (step S1). When a shooting instruction is given (step S1: YES), the system control unit 11 determines whether the analog gain set by the PGA 6b during shooting is smaller than a threshold value (step S2).

  When the analog gain is smaller than the threshold value (step S2: YES), the system control unit 11 drives the image sensor 5 in the first mode and images the subject.

  Hereinafter, an operation when performing imaging in the first mode will be described.

  FIG. 5 is a timing chart when the digital camera of FIG. 1 performs imaging in the first mode. In FIG. 5, “V1 to V4” indicate signals supplied to the wirings V1 to V4. “H1 and H2” indicate signals supplied to the wirings H1 and H2. “CLP” indicates a signal supplied from the system control unit 11 to the AD conversion unit 7. When this signal is at a high level, the AD conversion unit 7 performs a clamping process.

  First, the system control unit 11 opens the mechanical shutter MS and starts exposure of the image sensor 5 under the set exposure conditions (step S3). Next, the system control unit 11 closes the mechanical shutter MS to complete the exposure, supplies a read pulse to the wiring V <b> 1, and reads charges from all the photoelectric conversion elements of the image sensor 5 to the vertical charge transfer unit 53. Thereafter, a transfer pulse is supplied to the wirings V <b> 1 to V <b> 4 to transfer the charge of the first photoelectric conversion element row counted from the horizontal charge transfer unit 54 side to the horizontal charge transfer unit 54. Next, a transfer pulse is supplied to the wirings H1 and H2, the charge in the first photoelectric conversion element row is transferred to the output unit 55, and a signal corresponding to the charge is output from the imaging element 5 as a black pixel signal. . The system control unit 11 repeats such driving, and outputs signals from the photoelectric conversion element rows after the second photoelectric conversion element row.

  The black pixel signals obtained from the first photoelectric conversion element row are subjected to the output order, the reset noise is removed by the CDS 6 a, amplified by the PGA 6 b, and input to the AD conversion unit 7. The AD converter 7 performs a process of clamping each input black pixel signal to a clamp level. The clamp level is set to a predetermined value A in the initial state. The black pixel signal from the first photoelectric conversion element row output from the AD conversion unit 7 is temporarily stored in the main memory 16.

  Next, when the signal from the first photoelectric conversion element row is stored in the main memory 16, the system control unit 11 refers to the photoelectric conversion element row from which the stored signal is read. When the referenced photoelectric conversion element row is one of the 1st to Mth photoelectric conversion element rows, that is, when the photoelectric conversion element row is configured by only the black level detection photoelectric conversion element 52 (step) (S4: YES), the system control unit 11 uses the black pixel signal for one photoelectric conversion element row obtained from the referenced photoelectric conversion element row and stored in the main memory 16, and the histogram as shown in FIG. Is created (step S5).

  Next, the system control unit 11 calculates [{(number of black pixel signals with a signal value of 0) / (total number of black pixel signals for one photoelectric conversion element row)} × 100] from the created histogram. The clip probability is calculated (step S6).

  Next, the system control unit 11 adjusts the clamp level according to the calculated clip probability. Specifically, the adjustment amount of the clamp level set in the AD conversion unit 7 is calculated from the clip probability (step S7), and the calculated adjustment amount is added to the initial value A of the clamp level to obtain the AD conversion unit. The clamp level set to 7 is updated to A ′ (step S8). The clamp level update operation is performed until a signal is output from the next photoelectric conversion element (second photoelectric conversion element row).

  After the clamp level is adjusted, when a black pixel signal is output from the second photoelectric conversion element row, the black pixel signal is removed from the reset noise by the CDS 6a and amplified by the PGA 6b according to the output order. Input to the AD converter 7. The black pixel signal is clamped at the clamp level A ′ by the AD conversion unit 7, converted into a digital signal, and stored in the main memory 16.

  After step S8, the system control unit 11 sets the referenced photoelectric conversion element row as the next row (step S9), and returns the process to step S4. Steps S4 to S9 are repeatedly executed until the photoelectric conversion element rows to be referenced are up to the Mth row. When the (M + 1) -th row becomes a reference row, a clamp level (this clamp is applied) to an image-capturing signal (image-capturing signals output from the (M + 1) -th and subsequent photoelectric conversion element rows) used to generate image data. The level is A ″).

  After the clamp level is determined as A ″, the imaging signal used for generating the image data (imaging signal of each photoelectric conversion element row after the (M + 1) th row) is output from the imaging device 5 (step S10).

  According to the output order, the imaging signals output from the (M + 1) th and subsequent photoelectric conversion element rows are reset in the CDS 6a, amplified by the PGA 6b, and input to the AD converter 7. In the AD conversion unit 7, among the photoelectric conversion element rows, with respect to the image pickup signal output from the black level detection photoelectric conversion element 52, the image pickup signal is clamped at the clamp level A ″ and converted into a digital signal. Is done. The imaging signal after digital conversion is stored in the main memory 16.

  The digital signal processing unit 17 selects an imaging signal (obtained from the photoelectric conversion element 51) corresponding to light from the subject among the imaging signals obtained from the photoelectric conversion element rows from the (M + 1) th row to the (M + N) th row. The black level of the captured image signal is corrected. Specifically, the average of the values of the imaging signals obtained from the black level detecting photoelectric conversion elements 52 of each photoelectric conversion element row is obtained and set as the black level of the photoelectric conversion element row. Then, the black level is corrected by subtracting the black level from the value of the imaging signal obtained from each photoelectric conversion element 51 of the photoelectric conversion element row.

  After the black level correction, the digital signal processing unit 17 performs synchronization processing, RGB / YC conversion processing, and the like to generate image data. The generated image data is compressed and recorded on the recording medium 21. With the above operation, the imaging in the first mode is completed.

  Returning to step S2, if the analog gain is equal to or greater than the threshold (step S2: NO), the system control unit 11 calculates the luminance of the subject based on the monitor display image data and the like captured by the image sensor 5. . Then, it is determined whether or not the calculated luminance of the subject is smaller than a threshold value (step S11).

  When the brightness of the subject is smaller than the threshold (step S11: YES), the system control unit 11 drives the image sensor 5 in the second mode to capture the subject.

  Hereinafter, an operation when performing imaging in the second mode will be described.

  FIG. 6 is a timing chart when the digital camera of FIG. 1 performs imaging in the second mode. In FIG. 6, “V1 to V4” indicate signals supplied to the wirings V1 to V4. “H1 and H2” indicate signals supplied to the wirings H1 and H2. “CLP” indicates a signal supplied from the system control unit 11 to the AD conversion unit 7. When this signal is at a high level, the AD conversion unit 7 performs a clamping process. “Mechanical shutter” indicates the open / close state of the mechanical shutter MS.

  First, the system control unit 11 starts exposure of the image sensor 5 under the set exposure conditions with the mechanical shutter MS opened (step S12). Next, the system control unit 11 closes the mechanical shutter MS and ends the exposure.

  After the exposure is completed, the system control unit 11 performs the idle transfer by driving the vertical charge transfer unit 53 and the horizontal charge transfer unit 54 without supplying a read pulse to the wiring V1. By this empty transfer, a signal corresponding to the charge generated in the vertical transfer channel of the vertical charge transfer unit 53 is output from the image sensor 5 as a black pixel signal between the start of exposure and the end of exposure (step S13). The black pixel signal output here is treated as being obtained from the position of each photoelectric conversion element of the image sensor 5.

  The black pixel signal corresponding to each photoelectric conversion element in each photoelectric conversion element row from the first line to the (M + N) line is, in accordance with the output order, the reset noise is removed by the CDS 6a, amplified by the PGA 6b, and the AD conversion unit 7 is input. The AD converter 7 performs a process of clamping each input black pixel signal to a clamp level. The clamp level is set to a predetermined value A in the initial state. The black pixel signal output from the AD conversion unit 7 is temporarily stored in the main memory 16.

  When the black pixel signals corresponding to all the photoelectric conversion elements are stored in the main memory 16, the system control unit 11 creates a histogram as shown in FIG. 3 from the black pixel signals (step S14).

  Next, the system control unit 11 calculates [{(number of black pixel signals whose signal value is 0) / (total number of black pixel signals for all photoelectric conversion element rows)} × 100}] from the created histogram. Then, the clip probability is calculated (step S15).

  Next, the system control unit 11 adjusts the clamp level according to the calculated clip probability. Specifically, the adjustment amount of the clamp level set in the AD conversion unit 7 is calculated from the clip probability (step S16), and the calculated adjustment amount is added to the initial value A of the clamp level to obtain the AD conversion unit. The clamp level set to 7 is updated to A ′ (step S17).

  When the adjustment of the clamp level is completed, the system control unit 11 supplies a read pulse to the wiring V1, and reads out charges from all the photoelectric conversion elements of the image sensor 5 to the vertical charge transfer unit 53 (step S18). After that, the system control unit 11 drives the vertical charge transfer unit 53 and the horizontal charge transfer unit 54 to capture an image signal corresponding to the electric charges read from all the photoelectric conversion element rows (an image used for image data generation). Signal) is output from the image sensor 5 (step S10).

  In accordance with the output order, the image pickup signal output from the image pickup device 5 is removed from the reset noise by the CDS 6 a, amplified by the PGA 6 b, and input to the AD conversion unit 7. Of the image signals, the image signal output from the black level detection photoelectric conversion element 52 is clamped at the clamp level A ′ and converted into a digital signal. The imaging signal after digital conversion is stored in the main memory 16.

  The digital signal processing unit 17 selects an imaging signal (obtained from the photoelectric conversion element 51) corresponding to light from the subject among the imaging signals obtained from the photoelectric conversion element rows from the (M + 1) th row to the (M + N) th row. The black level of the captured image signal is corrected. Specifically, the average of the imaging signals obtained from the black level detecting photoelectric conversion elements 52 of each photoelectric conversion element row is obtained and set as the black level of the photoelectric conversion element row. Then, the black level is corrected by subtracting the black level from the imaging signal obtained from each photoelectric conversion element 51 of the photoelectric conversion element row.

  After the black level correction, the digital signal processing unit 17 performs synchronization processing, RGB / YC conversion processing, and the like to generate image data. The generated image data is compressed and recorded on the recording medium 21. With the above operation, the imaging in the second mode is completed.

  Returning to step S11, when the luminance of the subject is equal to or higher than the threshold (step S11: NO), the system control unit 11 drives the image sensor 5 in the third mode to image the subject.

  Hereinafter, an operation when performing imaging in the third mode will be described.

  FIG. 7 is a timing chart when the digital camera of FIG. 1 performs imaging in the third mode. In FIG. 7, “V1 to V4” indicate signals supplied to the wirings V1 to V4. “H1 and H2” indicate signals supplied to the wirings H1 and H2. “CLP” indicates a signal supplied from the system control unit 11 to the AD conversion unit 7. When this signal is at a high level, the AD conversion unit 7 performs a clamping process. “Mechanical shutter” indicates the open / close state of the mechanical shutter MS.

  First, the system control unit 11 closes the mechanical shutter MS and shields the image sensor 5 from light (step S19). Next, the vertical charge transfer unit 53 and the horizontal charge transfer unit 54 are driven to sweep out unnecessary charges existing in the vertical transfer channel at a high speed (step S20).

  Next, the system control unit 11 applies a high voltage to the semiconductor substrate, performs an electronic shutter operation to draw out the charge of each photoelectric conversion element to the semiconductor substrate, and then supplies a read pulse to the wiring V1 to obtain an image sensor. Charges are read from all the photoelectric conversion elements 5 to the vertical charge transfer unit 53 (step S21).

  Thereafter, the system control unit 11 drives the vertical charge transfer unit 53 and the horizontal charge transfer unit 54, and the image pickup signals corresponding to the charges read from all the photoelectric conversion element rows from the image pickup element 5 as black pixel signals. Output. The black pixel signals output from the image pickup device 5 are removed from the reset noise by the CDS 6 a according to the output order, amplified by the PGA 6 b, and input to the AD conversion unit 7. The clamp level is set to a predetermined value A in the initial state. The black pixel signal output from the AD conversion unit 7 is temporarily stored in the main memory 16.

  When the black pixel signals corresponding to all the photoelectric conversion elements are stored in the main memory 16, the system control unit 11 creates a histogram as shown in FIG. 3 from the black pixel signals (step S22).

  Next, the system control unit 11 calculates [{(number of black pixel signals whose signal value is 0) / (total number of black pixel signals for all photoelectric conversion element rows)} × 100}] from the created histogram. Then, the clip probability is calculated (step S23).

  Next, the system control unit 11 adjusts the clamp level according to the calculated clip probability. Specifically, the adjustment amount of the clamp level set in the AD converter 7 is calculated from the clip probability (step S24), and the calculated adjustment amount is added to the initial value A of the clamp level. The clamp level set to 7 is updated to A ′ (step S25).

  When the adjustment of the clamp level is completed, the system control unit 11 starts exposure of the image sensor 5 after opening the mechanical shutter MS, closes the mechanical shutter MS, and ends the exposure (step S26).

  When the exposure is completed, the system control unit 11 supplies a read pulse to the wiring V1, and reads out charges from all the photoelectric conversion elements of the image sensor 5 to the vertical charge transfer unit 53 (step S27). After that, the system control unit 11 drives the vertical charge transfer unit 53 and the horizontal charge transfer unit 54 to capture an image signal corresponding to the electric charges read from all the photoelectric conversion element rows (an image used for image data generation). Signal) is output from the image sensor 5 (step S10).

  In accordance with the output order, the image pickup signal output from the image pickup device 5 is removed from the reset noise by the CDS 6 a, amplified by the PGA 6 b, and input to the AD conversion unit 7. Of the image signals, the image signal output from the black level detection photoelectric conversion element 52 is clamped at the clamp level A ′ and converted into a digital signal. The imaging signal after digital conversion is stored in the main memory 16.

  In the digital signal processing unit 17, among the imaging signals obtained from the photoelectric conversion element rows from the (M + 1) th row to the (M + N) th row, the imaging signal (obtained from the photoelectric conversion device 51) according to the light from the subject. The black level of the captured image signal is corrected. Specifically, the average of the imaging signals obtained from the black level detecting photoelectric conversion elements 52 of each photoelectric conversion element row is obtained and set as the black level of the photoelectric conversion element row. Then, the black level is corrected by subtracting the black level from the imaging signal obtained from each photoelectric conversion element 51 of the photoelectric conversion element row.

  After the black level correction, the digital signal processing unit 17 performs synchronization processing, RGB / YC conversion processing, and the like to generate image data. The generated image data is compressed and recorded on the recording medium 21. With the above operation, the imaging in the third mode is completed.

  As described above, according to this digital camera, the clamp level corresponding to the clip probability, which is the ratio of the black pixel signal whose output value is zero, among the plurality of black pixel signals output from the AD converter 7. Can be adjusted.

  In general, the lower limit clip generated in the AD conversion unit 7 changes in degree depending on a combination of analog gain, temperature, and exposure time. Therefore, if the clamp level is adjusted according to these three conditions, the control becomes complicated. Moreover, it is difficult to obtain temperature information accurately. The correlation between these three conditions and the clip probability varies depending on the digital camera, and it is difficult to optimize the configuration for converting the three conditions into the clamp level adjustment amount for each digital camera.

  According to the digital camera of FIG. 1, the clamp level is adjusted using only the clip probability as an input value, so that the control is not complicated and the image quality can be improved with a simple configuration. In addition, since the clamp level is adjusted according to the clip probability obtained at the time of imaging or close to the time of imaging, the clamp level can be accurately adjusted even if there is an individual difference for each digital camera.

  Also, according to this digital camera, the clamp level is adjusted according to the three patterns of the first mode, the second mode, and the third mode according to the analog gain and the luminance of the subject. Optimal processing according to conditions can be performed.

  When the analog gain is lower than the threshold value, the variation of the black pixel signal amplified by the PGA 6b is not so large. For this reason, even if the number of black pixel signals output from the image sensor 5 for obtaining the clip probability is small, a highly reliable calculation result can be obtained. Therefore, in such a case, imaging is performed in the first mode in which the clip probability is obtained using only the black pixel signals obtained from the first to Mth photoelectric conversion element rows. By doing so, it is possible to shorten the time from the start to the end of imaging while improving the black level correction accuracy.

  When the analog gain is equal to or greater than the threshold value, the black pixel signal amplified by the PGA 6b varies greatly. For this reason, in order to obtain a highly reliable calculation result, it is preferable to increase the number of black pixel signals output from the image sensor 5 in order to obtain the clip probability. Therefore, in such a case, the second mode or the third mode for obtaining the clip probability using the black pixel signal for one screen obtained from the photoelectric conversion element rows from the first row to the (M + N) th row. Imaging is to be performed in the mode. By doing so, the black level correction accuracy can be improved even when the analog gain is high, that is, in the case of high-sensitivity shooting.

  In the CCD type image pickup device 5, smear may occur when the luminance of the subject is higher than the threshold value. In the second mode, since the black pixel signal is obtained by performing the empty transfer after the exposure is completed, the value of the black pixel signal cannot be obtained accurately if smear occurs. Therefore, when the luminance of the subject is higher than the threshold, imaging is performed in a third mode in which a black pixel signal for one screen is acquired with the imaging element 5 shielded before exposure. By doing so, the clamp level can be accurately adjusted without being affected by smear.

  In the above description, the image sensor 5 is a CCD type, but it may be a MOS type. When the MOS type is used, since smear does not occur, it is not necessary to separate the second mode and the third mode according to the luminance of the subject, and either one may be performed. In addition, the MOS type image pickup element is convenient because it has a high degree of freedom in the output reading order and can repeatedly read out only the black pixel signal.

  Further, in each of the second mode and the third mode, not a black pixel signal corresponding to the number of all the photoelectric conversion elements included in the image sensor 5 (one screen) but a part of the black pixel signal (for example, It is also possible to obtain a half photoelectric conversion element row signal) and calculate the clip probability from this black pixel signal. At this time, the black pixel signal that is not used for the calculation of the clip probability may be removed by high-speed sweeping driving or the like. By doing in this way, the time until the end of imaging can be shortened.

  In the second mode and the third mode, the black pixel signal acquisition and the clip probability calculation may be performed any number of times before the readout pulse is applied. By calculating the clip probability a plurality of times, the black level correction accuracy can be further improved.

  Next, a modified example of the digital camera described above will be described.

  The configuration of the digital camera of this modification is almost the same as that shown in FIG. The digital camera of this modification is different from the digital camera of FIG. 1 in that the system control unit 11 functions as a clamp level adjustment unit that adjusts the clamp level according to the clip probability, instead of the PGA 6b according to the clip probability. It is a point that functions as an analog gain adjustment unit for adjusting the analog gain.

  Further, in the digital camera of this modification, the system control unit 11 adjusts the analog gain, so that the total gain deviates from the set gain as it is. Therefore, in order to make the total gain constant, when the analog gain of the PGA 6b is adjusted, the digital signal processor 17 amplifies the digital signal output from the AD converter 7 with a predetermined digital gain. It also functions as a part. The digital signal processing unit 17 sets the digital gain such that the sum of the digital gain and the analog gain adjusted by the system control unit 11 is constant (total gain set at the time of shooting). Yes.

  When the histogram of n black pixel signals output from the AD conversion unit 7 is as shown in h2 in FIG. 8A, the clip probability is greater than 0%. For this reason, the black level to be subtracted from the image pickup signal obtained from the photoelectric conversion element 51 becomes larger than the clamp level a, and it is difficult to perform black level correction with high accuracy. In such a case, the system control unit 11 of the digital camera according to this modification performs adjustment to make the analog gain set by the PGA 6b smaller than that in FIG. As a result, the variation in the signal input to the AD conversion unit 7 is reduced, and as shown in FIG. 8B, the distribution width of the histogram h2 becomes narrow, and the clip probability can be set to 0%. When the clip probability is 0%, the clamp level and the black level almost coincide with each other, so that the black level can be corrected with high accuracy.

  As a specific method for adjusting the analog gain according to the clip probability, a clip probability threshold (for example, 5%) that allows black level accuracy is set, and if the clip probability exceeds this threshold, the clip is clipped. There is a method of reducing the analog gain to a level at which the probability is less than or equal to this threshold. Note that if the analog gain is too small, the digital gain needs to be increased correspondingly, so that the SN may deteriorate. Therefore, an allowable range (for example, 2% to 5%) of the clip probability that can allow the black level accuracy is set, and when the clip probability is out of the allowable range, the clip probability is within the allowable range. The analog gain may be adjusted. For example, when the clip probability is higher than 5%, the analog gain is decreased so that the clip probability is 2% to 5%. When the clip probability is lower than 2%, the clip probability is 2% to 5%. Increase the analog gain so that By doing so, noise can be reduced while preventing a decrease in black level accuracy, and the overall image quality can be improved.

  The adjusted analog gain value may be obtained by calculation by the system control unit 11 or may be obtained by the system control unit 11 bringing data from a table. In the case of obtaining by calculation, clip probability data at each analog gain when the analog gain is changed by a minimum unit is obtained in advance. From this data, the amount of change in clip probability when the analog gain is changed in the smallest unit is obtained. Then, from this amount of change, input the clip probability, create a function that outputs the analog gain adjustment amount so that the clip probability is below the threshold or within the allowable range, and store it in the digital camera. good. The system control unit 11 may input the clip probability to this function to obtain the analog gain adjustment amount, and then add the adjustment amount to the already set analog gain to adjust the analog gain.

  In the case of obtaining by a table, a table in which the clip probability and the analog gain are associated is stored in the digital camera. Then, when the determined clip probability exceeds the threshold value, or is outside the allowable range, the system control unit 11 reads an analog gain whose clip probability is equal to or less than the threshold value or within the allowable range from the table. You may make it set to PGA6b.

  Next, the operation at the time of shooting by the digital camera will be described. FIG. 9 is a flowchart for explaining the operation of the digital camera according to the modification. In FIG. 9, the same processes as those in FIG. 4 are denoted by the same reference numerals. The flowchart shown in FIG. 9 performs steps S7 ′, S8 ′, and S9 ′ instead of steps S7 and S8 shown in FIG. 4, and steps S16 ′ and S17 ′ instead of steps S16 and S17 shown in FIG. , S18 ′, steps S24 and S25 shown in FIG. 4 are set as steps S24 ′ and S25 ′, and step S26 ′ is added between steps S26 and S27 shown in FIG. Hereinafter, the changes will be described separately for the first mode, the second mode, and the third mode.

(First mode)
FIG. 10 is a timing chart when the digital camera according to the modification performs imaging in the first mode. In FIG. 10, “V1 to V4” indicate signals supplied to the wirings V1 to V4. “H1 and H2” indicate signals supplied to the wirings H1 and H2. “CLP” indicates a signal supplied from the system control unit 11 to the AD conversion unit 7. When this signal is at a high level, the AD conversion unit 7 performs a clamping process.

  After calculating the clip probability in step S6, the system control unit 11 adjusts the analog gain according to the calculated clip probability. Specifically, the analog gain adjustment amount set in the PGA 6b is calculated from the clip probability (step S7 ′), and the calculated adjustment amount is added to the initial value B of the analog gain to be set in the PGA 6b. The gain is updated to B ′ (step S8 ′). This analog gain update operation is performed until a signal is output from the next photoelectric conversion element (second photoelectric conversion element row).

  When the adjustment of the analog gain is completed, the digital signal processing unit 17 sets the digital gain according to the adjustment amount of the analog gain (step S9 '). For example, when the total set gain to be applied to the imaging signal is X dB and the adjusted analog gain is 0.5 X dB, the digital signal processing unit 17 sets the digital gain so that the total gain is X. Set to 0.5X dB.

  After the analog gain is adjusted, when a black pixel signal is output from the second photoelectric conversion element row, the reset noise is removed from the black pixel signal by the CDS 6a according to the output order, and the analog gain B is output from the PGA 6b. The signal is amplified by 'and input to the AD converter 7. The black pixel signal is clamped to a predetermined level by the AD conversion unit 7 and converted into a digital signal, which is stored in the main memory 16.

  After step S9 ', the system control unit 11 sets the referenced photoelectric conversion element row as the next row (step S9), and returns the process to step S4. Steps S4 to S9 are repeatedly executed until the photoelectric conversion element rows to be referenced are up to the Mth row. When the (M + 1) th row is set as a reference row, an analog gain (this analog gain) to be applied to an imaging signal used for generating image data (an imaging signal output from each photoelectric conversion element row after the (M + 1) th row) And B ″), the digital gain is determined.

  After the analog gain is determined to be B ′, an image pickup signal (an image pickup signal of each photoelectric conversion element row after the (M + 1) -th row) used for generating image data is output from the image pickup device 5 (step S10).

  In accordance with the output order, the imaging signals output from the (M + 1) th and subsequent rows of photoelectric conversion element rows are removed from the reset noise by the CDS 6 a, amplified by the analog gain B ″ in the PGA 6 b, and input to the AD conversion unit 7. Is done. In the AD conversion unit 7, among the photoelectric conversion element rows, the image pickup signal output from the black level detection photoelectric conversion element 52 is clamped at a predetermined clamp level and converted into a digital signal. The The imaging signal after digital conversion is stored in the main memory 16.

  The digital signal processing unit 17 selects an imaging signal (obtained from the photoelectric conversion element 51) corresponding to light from the subject among the imaging signals obtained from the photoelectric conversion element rows from the (M + 1) th row to the (M + N) th row. The black level of the captured image signal is corrected. Specifically, the average of the values of the imaging signals obtained from the black level detecting photoelectric conversion elements 52 of each photoelectric conversion element row is obtained and set as the black level of the photoelectric conversion element row. Then, the black level is corrected by subtracting the black level from the value of the imaging signal obtained from each photoelectric conversion element 51 of the photoelectric conversion element row.

  After the black level correction, the digital signal processing unit 17 amplifies the imaging signal obtained from each photoelectric conversion element 51 with the digital gain set in step S9 '. Thereafter, image data is generated by performing synchronization processing, RGB / YC conversion processing, and the like. The generated image data is compressed and recorded on the recording medium 21. With the above operation, the imaging in the first mode is completed.

(Second mode)
FIG. 11 is a timing chart when the digital camera according to the modification performs imaging in the second mode. In FIG. 11, “V1 to V4” indicate signals supplied to the wirings V1 to V4. “H1 and H2” indicate signals supplied to the wirings H1 and H2. “CLP” indicates a signal supplied from the system control unit 11 to the AD conversion unit 7. When this signal is at a high level, the AD conversion unit 7 performs a clamping process. “Mechanical shutter” indicates the open / close state of the mechanical shutter MS.

  After calculating the clip probability in step S15, the system control unit 11 adjusts the analog gain according to the calculated clip probability. Specifically, the analog gain adjustment amount set in the PGA 6b is calculated from the clip probability (step S16 ′), and the calculated adjustment amount is added to the initial value B of the clamp level to set the analog gain to be set in the PGA 6b. The gain is updated to B ′ (step S17 ′).

  When the adjustment of the analog gain is finished, the digital signal processing unit 17 sets the digital gain according to the adjustment amount of the analog gain (step S18 ').

  When the setting of the digital gain is completed, the system control unit 11 supplies a read pulse to the wiring V1, and reads charges from all the photoelectric conversion elements of the image sensor 5 to the vertical charge transfer unit 53 (step S18). After that, the system control unit 11 drives the vertical charge transfer unit 53 and the horizontal charge transfer unit 54 to capture an image signal corresponding to the electric charges read from all the photoelectric conversion element rows (an image used for image data generation). Signal) is output from the image sensor 5 (step S10).

  In accordance with the output order, the image pickup signal output from the image pickup device 5 is removed from the reset noise by the CDS 6 a, amplified by the analog gain B ′ in the PGA 6 b, and input to the AD conversion unit 7. Of these image pickup signals, the image pickup signal output from the black level detecting photoelectric conversion element 52 is clamped at a predetermined level and converted into a digital signal. The imaging signal after digital conversion is stored in the main memory 16.

  The digital signal processing unit 17 selects an imaging signal (obtained from the photoelectric conversion element 51) corresponding to light from the subject among the imaging signals obtained from the photoelectric conversion element rows from the (M + 1) th row to the (M + N) th row. The black level of the captured image signal is corrected. Specifically, the average of the imaging signals obtained from the black level detecting photoelectric conversion elements 52 of each photoelectric conversion element row is obtained and set as the black level of the photoelectric conversion element row. Then, the black level is corrected by subtracting the black level from the imaging signal obtained from each photoelectric conversion element 51 of the photoelectric conversion element row.

  After the black level correction, the digital signal processing unit 17 amplifies the imaging signal obtained from each photoelectric conversion element 51 with the digital gain set in step S18 '. Thereafter, image data is generated by performing synchronization processing, RGB / YC conversion processing, and the like. The generated image data is compressed and recorded on the recording medium 21. With the above operation, the imaging in the second mode is completed.

(Third mode)
FIG. 12 is a timing chart when the digital camera according to the modification performs imaging in the third mode. In FIG. 12, “V1 to V4” indicate signals supplied to the wirings V1 to V4. “H1 and H2” indicate signals supplied to the wirings H1 and H2. “CLP” indicates a signal supplied from the system control unit 11 to the AD conversion unit 7. When this signal is at a high level, the AD conversion unit 7 performs a clamping process. “Mechanical shutter” indicates the open / close state of the mechanical shutter MS.

  After calculating the clip probability in step S23, the system control unit 11 adjusts the analog gain according to the calculated clip probability. Specifically, the analog gain adjustment amount set in the PGA 6b is calculated from the clip probability (step S24 ′), and the calculated adjustment amount is added to the initial value B of the analog gain to be set in the PGA 6b. The gain is updated to B ′ (step S25 ′).

  When the adjustment of the analog gain is completed, the system control unit 11 opens the mechanical shutter MS to start exposure of the image sensor 5, closes the mechanical shutter MS, and ends the exposure (step S26).

  When the exposure ends, the digital signal processing unit 17 sets the digital gain according to the analog gain adjustment amount (step S26 ').

  When the setting of the digital gain is completed, the system control unit 11 supplies a read pulse to the wiring V1, and reads charges from all the photoelectric conversion elements of the image sensor 5 to the vertical charge transfer unit 53 (step S27). After that, the system control unit 11 drives the vertical charge transfer unit 53 and the horizontal charge transfer unit 54 to capture an image signal corresponding to the electric charges read from all the photoelectric conversion element rows (an image used for image data generation). Signal) is output from the image sensor 5 (step S10).

  In accordance with the output order, the image pickup signal output from the image pickup device 5 is removed from the reset noise by the CDS 6 a, amplified by the analog gain B ′ in the PGA 6 b, and input to the AD conversion unit 7. Of these image pickup signals, the image pickup signal output from the black level detecting photoelectric conversion element 52 is clamped at a predetermined level and converted into a digital signal. The imaging signal after digital conversion is stored in the main memory 16.

  In the digital signal processing unit 17, among the imaging signals obtained from the photoelectric conversion element rows from the (M + 1) th row to the (M + N) th row, the imaging signal (obtained from the photoelectric conversion device 51) according to the light from the subject. The black level of the captured image signal is corrected. Specifically, the average of the imaging signals obtained from the black level detecting photoelectric conversion elements 52 of each photoelectric conversion element row is obtained and set as the black level of the photoelectric conversion element row. Then, the black level is corrected by subtracting the black level from the imaging signal obtained from each photoelectric conversion element 51 of the photoelectric conversion element row.

  After the black level correction, the digital signal processing unit 17 amplifies the imaging signal obtained from each photoelectric conversion element 51 with the digital gain set in step S26 '. Thereafter, image data is generated by performing synchronization processing, RGB / YC conversion processing, and the like. The generated image data is compressed and recorded on the recording medium 21. With the above operation, the imaging in the third mode is completed.

  As described above, by adjusting the analog gain instead of the clamp level, the accuracy of the black level correction can be improved and the image quality can be improved. Compared with the method of adjusting the clamp level, the method of adjusting the analog gain according to the clip probability can enrich the gradation expression because the histogram distribution does not spread to the upper limit value side of the AD conversion unit 7. . On the other hand, the method of adjusting the clamp level according to the clip probability does not need to increase the digital gain as compared with the method of adjusting the analog gain, so that the SN can be improved.

  Note that the digital camera may perform both a method of adjusting the clamp level according to the clip probability and a method of adjusting the analog gain according to the clip probability. For example, the SN priority mode that operates according to the flow shown in FIG. 4 and the gradation priority mode that operates according to the flow shown in FIG. 9 may be provided in the digital camera, and imaging may be performed in the set mode. In this way, an image that suits the user's preference can be obtained.

  As described above, the following items are disclosed in this specification.

  The disclosed imaging apparatus includes an imaging device, an amplification unit that amplifies an analog imaging signal output from the imaging device with a predetermined analog gain, and an AD that converts the imaging signal output from the amplification unit into a digital signal. A conversion unit, wherein the AD conversion unit clamps a black pixel signal, which is a signal that does not depend on light from the subject, from the imaging signal output from the amplification unit to a set level and converts the signal to a digital signal A black pixel signal output control unit for performing control to output a plurality of black pixel signals from the image sensor, and an output of the AD conversion unit among the plurality of black pixel signals converted into the digital signal An output lower limit value ratio calculating unit for determining a ratio of a signal serving as a lower limit value, a level adjusting unit for adjusting the set level according to the ratio, and the analog gain according to the ratio And at least one of the analog gain adjustment unit for adjusting.

  With this configuration, at least one of the setting level and the analog gain is adjusted according to the ratio of the signal serving as the output lower limit value of the AD conversion unit in the black pixel signal, so that black level correction can be improved and color reproduction can be performed with simple control. Improvement, dynamic range optimization, and the like can be achieved, and image quality can be improved. For example, when the set level is increased, the output histogram of the AD conversion unit moves to the upper limit value side, so that the probability of occurrence of the lower limit clip can be reduced and black level correction can be performed with high accuracy. In addition, if the analog gain is reduced, the random noise variation is reduced, so that the output histogram width of the AD conversion unit is narrowed, the probability of occurrence of the lower limit clip can be reduced, and black level correction can be performed with high accuracy. it can. Also, for example, under exposure conditions with a small ratio, it is considered that the lower limit clip has little effect on the image quality, so by leaving the setting level and analog gain at the default values, the black level is maintained without impairing the gradation. Correction can be performed with high accuracy.

  In the disclosed imaging device, the analog gain adjustment unit reduces the analog gain to a level at which the ratio is equal to or less than the first threshold when the ratio is larger than the first threshold.

  With this configuration, black level correction can be performed with high accuracy.

  In the disclosed imaging device, when the analog gain adjustment unit has the ratio smaller than a second threshold smaller than the first threshold, the ratio is greater than or equal to the second threshold and less than or equal to the first threshold. The analog gain is increased to a level.

  With this configuration, it is possible to enrich the gradation expression without significantly reducing the black level accuracy.

  In the disclosed imaging apparatus, the level adjustment unit increases the setting level to a level at which the ratio is equal to or less than the third threshold when the ratio is greater than the third threshold.

  With this configuration, black level correction can be performed with high accuracy.

  In the disclosed imaging device, when the level adjustment unit has the ratio smaller than a fourth threshold smaller than the third threshold, the ratio is equal to or greater than the fourth threshold and equal to or less than the third threshold. Reduce the set level to the level.

  With this configuration, it is possible to enrich the gradation expression without significantly reducing the black level accuracy.

  The disclosed imaging apparatus includes a digital signal amplification unit that amplifies a digital signal output from the AD conversion unit with a predetermined digital gain when the analog gain adjustment is performed by the analog gain adjustment unit, The digital signal amplifying unit sets the digital gain so that a sum of the digital gain and the analog gain adjusted by the analog gain adjusting unit is constant.

  With this configuration, even if the analog gain is increased or decreased according to the above ratio, the increase or decrease can be adjusted with the digital gain, and the signal can be amplified with the total gain kept constant.

  In the disclosed imaging apparatus, the output lower limit value ratio calculation unit calculates the ratio by obtaining histograms of the plurality of black pixel signals output from the AD conversion unit.

  In the disclosed imaging device, the imaging element includes a photoelectric conversion element for obtaining a signal corresponding to light from a subject, and a black level detection photoelectric conversion element for detecting a black level, and the black pixel The signal output control unit outputs a signal corresponding to the electric charge generated in the black level detecting photoelectric conversion element as the black pixel signal.

  In the disclosed imaging device, the imaging element is a CCD type, and the black pixel signal output control unit reads charges from a photoelectric conversion element included in the imaging element to a charge transfer path in a state where the imaging element is shielded from light. The charge generated in the charge transfer path is transferred, and a signal corresponding to the charge is output as the black pixel signal.

  In the disclosed imaging apparatus, the imaging element is a CCD type, and the black pixel signal output control unit is configured to obtain a signal corresponding to light from a subject in a state where the imaging element is shielded from light, and a photoelectric conversion element An image pickup apparatus for transferring a charge read from a black level detection photoelectric conversion element for detecting a black level to a charge transfer path and outputting a signal corresponding to the charge as the black pixel signal.

  In the disclosed imaging apparatus, the imaging element is a CCD type, a photoelectric conversion element for obtaining a signal corresponding to light from a subject, and a black level detection photoelectric conversion element for detecting a black level The black pixel signal output control unit can perform the first control, the second control, and the third control in order to output the black pixel signal, and the first control includes: The black level detection photoelectric conversion element outputs a signal corresponding to the electric charge generated as the black pixel signal, and the second control includes a photoelectric element included in the imaging element in a state where the imaging element is shielded from light. The third control is a control for transferring the charge generated in the charge transfer path without reading the charge from the conversion element to the charge transfer path and outputting a signal corresponding to the charge as the black pixel signal. But shielded the image sensor And transferring the charge read from the photoelectric conversion element and the black level detection photoelectric conversion element to a charge transfer path and outputting a signal corresponding to the charge as the black pixel signal. The output control unit performs the first control when the analog gain is smaller than a threshold value, and performs the second control when the analog gain is equal to or greater than the threshold value and the subject brightness is smaller than a predetermined value. Then, the third control is performed when the analog gain is equal to or greater than the threshold value and the subject luminance is equal to or greater than the predetermined value.

  The disclosed imaging apparatus includes an adjustment amount calculation unit that calculates an adjustment amount of at least one of the setting gain and the analog gain from the ratio.

  The disclosed imaging method includes an amplification step of amplifying an analog imaging signal output from the imaging element with a predetermined analog gain, and an AD conversion step of converting the imaging signal amplified in the amplification step into a digital signal. In the AD conversion step, a black pixel signal that is a signal that does not depend on light from the subject is clamped to a set level among the amplified image pickup signal and converted into a digital signal, and the black pixel signal is converted from the image pickup device. A black pixel signal output control step for performing control to output a plurality of signals, and a ratio of a signal serving as an output lower limit value in the AD conversion step among the plurality of black pixel signals converted into the digital signal is obtained. An output lower limit ratio calculation step, a level adjustment step for adjusting the set level according to the ratio, and the analog according to the ratio. And at least one of the analog gain adjustment step of adjusting the Gugein.

  In the disclosed imaging method, in the analog gain adjustment step, when the ratio is greater than the first threshold, the analog gain is reduced to a level at which the ratio is equal to or less than the first threshold.

  In the disclosed imaging method, in the analog gain adjustment step, when the ratio is smaller than a second threshold smaller than the first threshold, the ratio is greater than or equal to the second threshold and less than or equal to the first threshold. The analog gain is increased to a level.

  In the disclosed imaging method, in the level adjustment step, when the ratio is greater than a third threshold, the set level is increased to a level at which the ratio is equal to or less than the third threshold.

  In the disclosed imaging method, in the level adjustment step, when the ratio is smaller than a fourth threshold that is smaller than the third threshold, the ratio is not less than the fourth threshold and not more than the third threshold. Reduce the set level to the level.

  The disclosed imaging method includes a digital signal amplification step of amplifying the digital signal converted in the AD conversion step with a predetermined digital gain when the analog gain is adjusted in the analog gain adjustment step, In the digital signal amplification step, the digital gain is set so that the sum of the digital gain and the analog gain adjusted in the analog gain adjustment step is constant.

  In the disclosed imaging method, in the output lower limit ratio calculation step, histograms of the plurality of black pixel signals converted in the AD conversion step are obtained, and the ratio is calculated.

  In the disclosed imaging method, the imaging element includes a photoelectric conversion element for obtaining a signal corresponding to light from a subject, and a black level detection photoelectric conversion element for detecting a black level, and the black pixel In the signal output control step, a signal corresponding to the charge generated in the black level detecting photoelectric conversion element is output as the black pixel signal.

  In the disclosed imaging method, the imaging device is a CCD type, and in the black pixel signal output control step, charges are read from a photoelectric conversion element included in the imaging device to a charge transfer path in a state where the imaging device is shielded from light. The charge generated in the charge transfer path is transferred, and a signal corresponding to the charge is output as the black pixel signal.

  In the disclosed imaging method, the imaging element is a CCD type, and in the black pixel signal output control step, a photoelectric conversion element for obtaining a signal corresponding to light from a subject in a state where the imaging element is shielded from light, and The charge read from the black level detection photoelectric conversion element for detecting the black level is transferred to the charge transfer path, and a signal corresponding to the charge is output as the black pixel signal.

  In the disclosed imaging method, the imaging element is a CCD type, and a photoelectric conversion element for obtaining a signal corresponding to light from a subject, and a black level detection photoelectric conversion element for detecting a black level In the black pixel signal output control step, in order to output the black pixel signal, the first control, the second control, and the third control can be performed, and the first control includes: The black level detection photoelectric conversion element outputs a signal corresponding to the electric charge generated as the black pixel signal, and the second control includes a photoelectric element included in the imaging element in a state where the imaging element is shielded from light. The third control is a control for transferring the charge generated in the charge transfer path without reading the charge from the conversion element to the charge transfer path and outputting a signal corresponding to the charge as the black pixel signal. But the image sensor The control is to transfer charges read from the photoelectric conversion elements and the black level detection photoelectric conversion elements to a charge transfer path in a lighted state, and to output a signal corresponding to the charges as the black pixel signal. In the pixel signal output control step, the first control is performed when the analog gain is smaller than a threshold value, and the second control is performed when the analog gain is equal to or larger than the threshold value and the subject luminance is smaller than a predetermined value. The third control is performed when the analog gain is equal to or greater than the threshold value and the subject brightness is equal to or greater than the predetermined value.

  The disclosed imaging method includes an adjustment amount calculation step of calculating an adjustment amount of at least one of the set gain and the analog gain from the ratio.

5 Imaging device 6b Amplifying unit 7 AD conversion unit 11 System control unit

Claims (24)

An image sensor;
An amplification unit that amplifies an analog imaging signal output from the imaging element with a predetermined analog gain;
An AD conversion unit that converts an imaging signal output from the amplification unit into a digital signal;
The AD conversion unit clamps a black pixel signal, which is a signal that does not depend on light from a subject, from the imaging signal output from the amplification unit to a set level and converts the signal to a digital signal.
A black pixel signal output control unit that performs control to output a plurality of the black pixel signals from the imaging device;
An output lower limit value ratio calculating unit for obtaining a ratio of a signal that becomes an output lower limit value of the AD conversion unit among the plurality of black pixel signals converted into the digital signal;
An imaging apparatus comprising at least one of a level adjustment unit that adjusts the set level according to the ratio and an analog gain adjustment unit that adjusts the analog gain according to the ratio. The imaging apparatus according to claim 1,
An imaging apparatus, wherein the analog gain adjustment unit reduces the analog gain to a level at which the ratio is equal to or less than the first threshold when the ratio is greater than a first threshold. The imaging apparatus according to claim 2,
When the analog gain adjustment unit has the ratio smaller than a second threshold smaller than the first threshold, the analog gain is adjusted to a level at which the ratio is greater than or equal to the second threshold and less than or equal to the first threshold. An imaging device to enlarge. The imaging apparatus according to any one of claims 1 to 3,
The imaging apparatus, wherein when the ratio is greater than a third threshold, the level adjustment unit increases the setting level to a level at which the ratio is equal to or less than the third threshold. The imaging apparatus according to claim 4,
When the level adjustment unit has the ratio smaller than the fourth threshold value smaller than the third threshold value, the setting level is decreased to a level at which the ratio is not less than the fourth threshold value and not more than the third threshold value. An imaging device. The imaging device according to any one of claims 1 to 5,
A digital signal amplifying unit that amplifies a digital signal output from the AD conversion unit with a predetermined digital gain when the analog gain is adjusted by the analog gain adjustment unit;
The imaging apparatus, wherein the digital signal amplifying unit sets the digital gain so that a sum of the digital gain and the analog gain adjusted by the analog gain adjusting unit is constant. The imaging apparatus according to any one of claims 1 to 6,
An imaging apparatus in which the output lower limit value ratio calculation unit calculates histograms by obtaining histograms of the plurality of black pixel signals output from the AD conversion unit. The imaging device according to any one of claims 1 to 7,
The imaging element includes a photoelectric conversion element for obtaining a signal corresponding to light from a subject, and a black level detection photoelectric conversion element for detecting a black level,
An imaging apparatus in which the black pixel signal output control unit outputs a signal corresponding to a charge generated by the black level detection photoelectric conversion element as the black pixel signal. The imaging apparatus according to any one of claims 1 to 8,
The imaging element is of a CCD type;
The black pixel signal output control unit transfers the charge generated in the charge transfer path without reading the charge from the photoelectric conversion element included in the image pickup element to the charge transfer path in a state where the image pickup element is shielded from light. An image pickup apparatus that outputs a signal corresponding to the electric charge as the black pixel signal. The imaging apparatus according to any one of claims 1 to 9,
The imaging element is of a CCD type;
The black pixel signal output control unit is charged from a photoelectric conversion element for obtaining a signal corresponding to light from a subject and a black level detection photoelectric conversion element for detecting a black level in a state where the imaging element is shielded from light. An image pickup apparatus that transfers charges read to a transfer path and outputs a signal corresponding to the charges as the black pixel signal. The imaging device according to any one of claims 1 to 7,
The imaging element is a CCD type, and includes a photoelectric conversion element for obtaining a signal according to light from a subject, and a black level detection photoelectric conversion element for detecting a black level,
The black pixel signal output control unit can perform the first control, the second control, and the third control in order to output the black pixel signal,
The first control is a control for outputting a signal corresponding to the charge generated in the black level detection photoelectric conversion element as the black pixel signal,
The second control transfers the charge generated in the charge transfer path without reading the charge from the photoelectric conversion element included in the image sensor to the charge transfer path in a state where the image sensor is shielded from light, and Control to output a signal corresponding to the charge as the black pixel signal,
The third control transfers charges read from the photoelectric conversion element and the black level detection photoelectric conversion element to a charge transfer path in a state where the image pickup element is shielded from light, and a signal corresponding to the charge is transferred to the black signal. It is a control to output as a pixel signal,
The black pixel signal output control unit performs the first control when the analog gain is smaller than a threshold, and the second control when the analog gain is equal to or greater than the threshold and the subject brightness is smaller than a predetermined value. An imaging apparatus that performs the third control when the analog gain is equal to or greater than the threshold value and the subject luminance is equal to or greater than the predetermined value. The imaging device according to any one of claims 1 to 11,
An imaging apparatus comprising an adjustment amount calculation unit that calculates an adjustment amount of at least one of the set gain and the analog gain from the ratio. An amplification step of amplifying an analog imaging signal output from the imaging device with a predetermined analog gain;
An AD conversion step of converting the imaging signal amplified in the amplification step into a digital signal,
In the AD conversion step, among the amplified imaging signals, a black pixel signal that is a signal that does not depend on light from a subject is clamped to a set level and converted into a digital signal,
A black pixel signal output control step for performing control to output a plurality of the black pixel signals from the imaging device;
An output lower limit value ratio calculating step for obtaining a ratio of a signal which becomes an output lower limit value in the AD conversion step among the plurality of black pixel signals converted into the digital signal;
An imaging method comprising at least one of a level adjustment step of adjusting the set level according to the ratio and an analog gain adjustment step of adjusting the analog gain according to the ratio. The imaging method according to claim 13,
In the analog gain adjustment step, when the ratio is greater than a first threshold, the analog gain is reduced to a level at which the ratio is equal to or less than the first threshold. The imaging method according to claim 14, wherein
In the analog gain adjustment step, when the ratio is smaller than a second threshold smaller than the first threshold, the analog gain is adjusted to a level at which the ratio is greater than or equal to the second threshold and less than or equal to the first threshold. Imaging method to enlarge. The imaging method according to any one of claims 13 to 15,
In the level adjustment step, when the ratio is larger than a third threshold, the imaging method increases the setting level to a level at which the ratio is equal to or less than the third threshold. The imaging method according to claim 16, wherein
In the level adjustment step, when the ratio is smaller than a fourth threshold value that is smaller than the third threshold value, the setting level is decreased to a level at which the ratio is not less than the fourth threshold value and not more than the third threshold value. Imaging method. The imaging method according to any one of claims 13 to 17,
A digital signal amplification step for amplifying the digital signal converted in the AD conversion step with a predetermined digital gain when the analog gain adjustment is performed in the analog gain adjustment step;
In the digital signal amplification step, the digital gain is set such that the sum of the digital gain and the analog gain adjusted in the analog gain adjustment step is constant. The imaging method according to any one of claims 13 to 18, comprising:
In the output lower limit ratio calculation step, an imaging method for calculating histograms by obtaining histograms of the plurality of black pixel signals converted in the AD conversion step. The imaging method according to any one of claims 13 to 19, comprising:
The imaging element includes a photoelectric conversion element for obtaining a signal corresponding to light from a subject, and a black level detection photoelectric conversion element for detecting a black level,
In the black pixel signal output control step, an imaging method for outputting a signal corresponding to the electric charge generated in the black level detection photoelectric conversion element as the black pixel signal. The imaging method according to any one of claims 13 to 20, comprising:
The imaging element is of a CCD type;
In the black pixel signal output control step, the charge generated in the charge transfer path is transferred without reading the charge from the photoelectric conversion element included in the image sensor to the charge transfer path in a state where the image sensor is shielded from light. An imaging method for outputting a signal corresponding to the electric charge as the black pixel signal. The imaging method according to any one of claims 13 to 21,
The imaging element is of a CCD type;
In the black pixel signal output control step, the charge from the photoelectric conversion element for obtaining a signal corresponding to light from the subject and the black level detection photoelectric conversion element for detecting a black level in a state where the image pickup element is shielded from light. An imaging method for transferring a read charge to a transfer path and outputting a signal corresponding to the charge as the black pixel signal. The imaging method according to any one of claims 13 to 19, comprising:
The imaging element is a CCD type, and includes a photoelectric conversion element for obtaining a signal according to light from a subject, and a black level detection photoelectric conversion element for detecting a black level,
In the black pixel signal output control step, in order to output the black pixel signal, the first control, the second control, and the third control can be performed,
The first control is a control for outputting a signal corresponding to the charge generated in the black level detection photoelectric conversion element as the black pixel signal,
The second control transfers the charge generated in the charge transfer path without reading the charge from the photoelectric conversion element included in the image sensor to the charge transfer path in a state where the image sensor is shielded from light, and Control to output a signal corresponding to the charge as the black pixel signal,
The third control transfers charges read from the photoelectric conversion element and the black level detection photoelectric conversion element to a charge transfer path in a state where the image pickup element is shielded from light, and a signal corresponding to the charge is transferred to the black signal. It is a control to output as a pixel signal,
In the black pixel signal output control step, the first control is performed when the analog gain is smaller than a threshold value, and the second control is performed when the analog gain is equal to or greater than the threshold value and the subject brightness is smaller than a predetermined value. An imaging method in which the third control is performed when the analog gain is equal to or greater than the threshold and the subject brightness is equal to or greater than the predetermined value. The imaging method according to any one of claims 13 to 23, wherein:
An imaging method comprising an adjustment amount calculating step of calculating an adjustment amount of at least one of the set gain and the analog gain from the ratio.

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