JP4871664B2 - IMAGING DEVICE AND IMAGING DEVICE CONTROL METHOD - Google Patents

Description

  The present invention relates to white balance control of digital cameras and digital video cameras.

  There is white balance (hereinafter referred to as WB) control performed to produce an appropriate color or a color intended by the photographer for an image captured by a digital camera.

  FIG. 8 shows an example in which the screen is divided into a plurality of arbitrary blocks, and FIG. 9 shows a white detection range.

  FIG. 10 shows an example of photographing a face up as a subject.

  The operation of a conventional WB coefficient calculation circuit used in a digital camera or the like will be described with reference to FIGS.

  As shown in FIG. 8, the screen is divided into a plurality of arbitrary blocks (m) in advance. Next, for each of the plurality of blocks, a color evaluation value (Cx [m], Cy [m]) and a luminance value Y [m] are calculated using the following formula, for example.

  As shown in FIG. 9, a white detection range is set by photographing a white object in advance under various light sources and calculating a color evaluation value. When the color evaluation value calculated above is included in the white detection range shown in FIG. 9, it is determined that the block including the color evaluation value is white, and the pixel values of the block determined as white are integrated (this) This determination process is called white search). From the integrated pixel values (sumR, sumG, sumB), a WB coefficient is calculated using the following equation.

  In the above WB coefficient calculation method, when a face is large as a subject as shown in FIG. 10, the color evaluation value (FIG. 9-1) of the human skin area photographed under sunlight and the photographed under tungsten light The color evaluation value (FIG. 9-2) of the white subject is almost the same value. For this reason, human skin may be misidentified as white under tungsten light, and the skin color may be corrected to white.

  In view of this, a proposal has been made to exclude the face area detected by the face detection circuit from the white detection search target (Patent Document 1). In addition, a proposal has been made to perform different WB controls for the background area and the main subject area (Patent Document 2).

  FIG. 11 shows the imaging operation timing in the imaging apparatus inferred from the processing configuration of the conventional WB coefficient calculation described in Patent Document 1 or 2 described above. Thus, the reference numerals in the drawings of the present invention are shown in parentheses as shown by (150). In FIG. 11, when the user fully presses the shutter button (hereinafter, SW2 is pressed), the main exposure is performed, and the image signal is immediately read from the sensor. Note that the main exposure is an exposure operation performed in response to an instruction for the imaging apparatus to perform shooting for the purpose of recording.

  Thereafter, the imaging device simply develops the image signal, and the face detection circuit performs face detection.

  The development is image processing for converting the output of the image sensor into data that can be viewed as an image. On the other hand, the above-described simple development means processing excluding WB processing among processing performed in the development processing.

  In the present invention, face detection is performed on an image signal after simple development without performing WB processing.

The imaging device uses the detected face information to calculate the WB coefficient by the method disclosed in Patent Document 1 or 2, and then performs the main development process to capture the captured image on an external recording medium such as a CF. Record.
JP 2003-189325 A JP 2000-308068 A

  As shown in the imaging operation timing of FIG. 11, signal readout and block integration (150) are performed using the image signal (149) after the main exposure.

  Thereafter, simple development (151) is performed for face detection input, and the face is detected by the face detection means (152).

  Next, the processing procedure is such that a WB coefficient is calculated (153) using the result of face detection, main development (154) is performed, and recording is performed on an external recording medium (155).

  In other words, in the conventional processing configuration, in addition to the processing time for performing face detection, almost the same time is required for calculating the WB coefficient using the face detection result, so that it takes a very long time to complete photographing. There are drawbacks.

  In order to solve the above-described problems and achieve the object, the present invention provides an imaging device including an imaging device that performs photoelectric conversion on a subject image formed by a photographing optical system, based on a signal obtained from the imaging device. Subject detection means for repeatedly detecting, white balance coefficient calculation means for calculating a white balance coefficient based on subject information output from the subject detection means, a scene from which the white balance coefficient is obtained, and a source for performing main exposure A scene change determination unit that compares the scenes, and switching whether to newly calculate a white balance coefficient by the white balance coefficient calculation unit after the main exposure according to the comparison result of the scene, To do.

  In order to solve the above-described problems and achieve the object, the present invention provides a method for controlling an imaging apparatus including an imaging element that photoelectrically converts a subject image formed by a photographing optical system, based on a signal obtained from the imaging element. A subject detection step for repeatedly detecting a subject region, a white balance factor calculation step for calculating a white balance factor based on subject information output from the subject detection step, a scene and a main exposure from which the white balance factor is obtained A scene change determination step for comparing the scenes that are the basis for performing the switching, and switching whether or not to newly calculate a white balance coefficient by the white balance coefficient calculation step after the main exposure according to the comparison result of the scenes It is characterized by.

  By determining whether or not there is a scene change and switching whether to newly calculate the WB coefficient according to the result, it is possible to speed up the shooting operation.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 shows the imaging operation timing in the imaging apparatus of the present invention, and FIG. 2 shows a simple block diagram of signal processing.

  In FIG. 2, reference numeral 201 denotes an image sensor such as a CCD sensor or a CMOS sensor that converts a light beam as a subject image that has passed through the imaging lens group into an electric signal. An analog / digital (hereinafter referred to as A / D) conversion circuit 202 converts an analog signal output obtained from the image sensor 201 into a digital signal. Reference numeral 203 denotes a buffer memory such as a DRAM that temporarily stores the digital signal output from the A / D conversion circuit 202. Reference numeral 204 denotes an optical black (hereinafter referred to as OB) subtraction that performs black subtraction from the image signal read from the buffer memory 203. Circuit.

  Here, FIG. 4 is a diagram illustrating an example of block division of the image sensor 201. In FIG. 4, the area is divided into 8 × 8, but the division method is not limited to this.

  Reference numeral 205 denotes an OB block integration circuit that performs pixel integration in the OB area and pixel integration in units of arbitrary small block areas set in advance as shown in FIG. 4 with respect to the image signal read from the buffer memory 203. Reference numeral 206 denotes a WB coefficient calculation circuit that calculates a WB coefficient obtained by converting the WB control value into a coefficient. As the WB coefficient calculated by the WB coefficient calculation circuit 206, a preset WB coefficient is used when the image pickup apparatus is turned on, and the WB coefficient is already calculated from an electronic viewfinder (hereinafter referred to as EVF) image signal before the current frame. If so, the WB coefficient is used.

  The EVF image signal is a signal generated by thinning out an image signal generated by performing various kinds of signal processing on a signal obtained from an image pickup element and displayed on a display unit such as a liquid crystal monitor of the image pickup apparatus.

  Here, an example of setting the WB coefficient will be described with reference to FIG.

  In FIG. 1, a preset WB coefficient (not shown) is used when developing image signals from EVF 101 to EVF 105. When developing image signals from EVF 106 to EVF 108, the result of WB calculation 1 (122) is used, and when developing image signals from EVF 109 to EVF 11, the result of WB calculation 2 (123) is used. In this way, the latest WB calculation result is used during image signal development. The result of the WB calculation is recorded in the buffer memory 203.

  Returning to FIG. 2, reference numeral 207 denotes a WB gain circuit that applies a WB gain to the RGB signals output from the OB subtraction circuit 204. A digital signal processing circuit 208 performs predetermined pixel interpolation processing and color conversion processing on the output data of the WB gain circuit 207. Reference numeral 209 denotes a feature detection circuit for performing face detection. This feature detection circuit 209 detects candidate groups of characteristic parts of subjects such as eyes, nose, mouth, and ears from texture groups detected by a bandpass filter (not shown) that detects each texture from the frequency characteristics of the image signal. Perform detection. Furthermore, the feature detection circuit 209 includes a feature detection register (not shown) that records the detected feature information.

  In the present embodiment, face detection is performed using the feature detection circuit 209.

  FIGS. 5A and 5B show face detection information such as detected face and eye coordinate values and reliability of detection results.

  The feature detection circuit 209 performs face detection using a luminance signal obtained from the EVF image signal periodically transferred from the digital signal processing circuit 208, and generates face detection information as shown in FIG. Recorded in a feature detection register (not shown).

  In the present embodiment, the face detection is performed by the feature detection circuit 209 according to the above-described processing flow, but a known face detection method may be programmed into the feature detection circuit 209 and incorporated.

  As a known face detection technique, a method using learning typified by a neural network, a part having a characteristic shape such as an eye, nose, or mouth is searched from an image area using template matching, and is regarded as a face if the similarity is high. Techniques are listed.

  In addition, many other methods have been proposed, such as methods that detect image features such as skin color and eye shape, and use statistical analysis. is there.

  Specific examples include a face detection method using wavelet transform and image feature amount described in JP-A-2002-251380.

  Reference numeral 210 denotes an external recording device such as a flash memory, a micro drive, or a hard disk for storing image data and the like. Reference numeral 211 denotes a display device such as a liquid crystal, which can realize a function as an EVF by sequentially displaying captured image data.

  The image signal multiplied by the WB coefficient is converted into a YUV signal by the digital signal processing circuit 208 and transferred to the feature detection circuit 209, the external recording device, and the display device 211.

  The digital signal processing circuit 208 detects a scene change, which will be described later, with respect to a luminance integral value calculated in arbitrary small block units and an exposure control value (Av, Tv value) that is a photographing condition at the time of photographing when the YUV signal is created. For use in (134), it is stored in the buffer memory 203.

  In addition, autofocus (hereinafter referred to as AF) distance measurement information, zoom position, and the like are similarly stored in the buffer memory 203. Reference numeral 212 denotes a CPU that controls the shutter, aperture, and focus lens based on the output of the feature detection circuit 209. A device control circuit 213 controls the shutter, aperture, and focus lens based on instructions from the CPU.

  Thus, the imaging operation before the shutter button is half-pressed (hereinafter SW1 is pressed) will be described with reference to FIG.

  For example, when the display frame rate of the display device 211 is 30 fps, the following imaging operation is performed at a rate of once every 30 seconds.

  EVF101 to EVF116 in FIG. 1 respectively indicate EVF display periods.

  FIG. 3 shows an outline of signal readout in the pixel addition mode of the image sensor.

  In FIG. 3, what is represented by R, B, G1, and G2 are photodiodes that convert incident light into charges corresponding to the amount of light. Then, the charges in the vertically aligned photodiode rows are read for each color of R, B, G1, and G2, and transferred in the vertical direction. The transferred charge is transferred horizontally for each horizontal line, the charge is converted into a digital signal by the A / D conversion circuit 202, and temporarily recorded in the buffer memory 203. In this way, the signal of the image sensor is read.

  By driving the image sensor 201 in a pixel addition mode (EVF readout mode) as shown in FIG. 3 and controlling an electronic shutter (not shown), an image signal is photographed under an optimal exposure condition for the subject.

  The signal processing performed during the EVF 101 display period will be described. During the display period of the EVF 101, the pixel signal read from the image sensor 201 is converted into, for example, a 10-bit digital signal by the A / D conversion circuit 202 and temporarily recorded in the buffer memory 203. The image signal read from the buffer memory 203 is subjected to pixel integration by the OB block integration circuit 205.

  The image signal read from the buffer memory 203 is blacked by the OB subtraction circuit 204, and the WB gain is applied to the RGB signals by the WB gain circuit 207. In parallel, the WB coefficient calculation circuit 206 calculates WB coefficients to be used in the subsequent frames. The image signal multiplied by the WB coefficient is converted into a YUV signal by the digital signal processing circuit 208 and transferred to the feature detection circuit 209 and the display device 211.

  In the present embodiment, since the feature detection circuit 209 uses only the luminance (Y) signal at the time of feature detection, only the output of the Y component of the image signal converted by the digital signal processing circuit 208 is sent to the feature detection circuit 209. Transferred.

  The feature detection circuit 209 detects the face of the transferred image signal. This is the end of the description of the signal processing performed during the EVF 101 display period.

  Next, signal processing performed during the display period of the EVF 102 will be described.

  In the display period of the EVF 102, the image signal read from the image sensor 201 is subjected to the same processing as the signal processing performed in the display period of the EVF 101, and converted into a YUV signal.

  At this timing, the face detection 1 (117) is performed using the image signal obtained in the display period of the EVF 101. However, since the face detection 1 does not end when the image signal development starts in the display period of the EVF 102, WB The coefficient is not yet calculated.

  Further, as the WB coefficient used during the development performed during the display period of the EVF 102, the WB coefficient used during the development performed during the display period of the EVF 101 is used as it is. This is the end of the description of the signal processing performed during the EVF 102 display period.

  The signal processing performed during the display period of the EVF 103 is the same as the signal processing performed during the display period of the EVF 102.

Next, signal processing performed during the display period of the EVF 104 will be described.
In the display period of the EVF 104, processing similar to the signal processing performed in the display period of the EVF 102 is performed until the image signal is read from the image sensor 201 and developed into a YUV signal.

  However, at the timing (146) at the end of reading the image signal, face detection 1 (117) is completed, and then face information is recorded in the face register.

  Therefore, at this timing, the WB calculation 1 (122) is started from the image signal obtained during the display period of the EVF 101 and the result of the face detection 1.

  Here, as another example of the WB coefficient calculation method using the face detection information, a plurality of fixed WB coefficients set in advance for each face area are used to correct image signals included in each face area. There is one that obtains a corrected image signal and calculates it based on the corrected image signal.

  As yet another method, the WB coefficient calculated by the above method and the WB coefficient obtained by a normal WB coefficient calculation method that does not use face detection are weighted using a weighted addition coefficient calculated from face detection information. There is one that calculates a WB coefficient by adding.

  In the present invention, the method for calculating the WB coefficient using the face detection information is not a matter of discussion and will not be described in detail. This is the end of the description of the signal processing performed during the EVF 104 display period.

The signal processing performed during the display period of the EVF 105 performs the same processing as the signal processing performed during the display period of the EVF 103.
Next, signal processing performed during the display period of the EVF 106 will be described.

  In the display period of the EVF 106, the WB coefficient 1 is calculated by the WB coefficient calculation 1 (122) at the timing (148) when the reading of the image signal ends.

  Therefore, the latest WB coefficient WB coefficient 1 is used to generate an image signal obtained during the display period of the EVF 106. However, at this time, if the WB coefficient used when generating the image signal obtained during the display period of the EVF 101 and the WB coefficient 1 calculated this time are rapidly changed, the color of the screen is greatly increased. The harmful effect of changing to Therefore, the WB coefficient used when generating the image signal obtained during the display period of the EVF 101 and the WB coefficient 1 calculated this time are weighted and added with an arbitrary weighted addition coefficient to calculate a new WB coefficient. WB coefficient during the display period.

  The signal processing performed during the display period of EVF 101 to EVF 106 is repeated until SW1 is pressed by the user.

  Next, an imaging operation from when SW1 is pressed until the shutter button is fully pressed (hereinafter, SW2 is pressed) will be described with reference to FIG.

  When SW1 is pressed by the user, the image sensor 201 is driven for AF / automatic exposure (hereinafter referred to as AE) with a higher-speed clock than usual, and the AF / AE operation (128) is performed. Since the details of the AF / AE operation are not the main point of the present invention, the description thereof will be omitted.

  By performing the AF / AE operation (128), a main exposure condition is calculated by a photometric circuit (not shown), and when the focusing is completed, a focusing sign (129) is issued.

  Next, the operation from when SW2 is pressed to when the captured image is recorded will be described with reference to FIG. When SW2 is pressed (127) by the user, the image sensor 201 is driven in an all-pixel readout mode, which will be described later, and actual shooting for still images is performed. Here, a driving method of the image sensor in the all-pixel reading mode will be described.

  FIG. 6 shows a conceptual diagram of readout of an image sensor that performs 3-field readout. For example, in the case of an image sensor that performs three-field readout, the electrical signals accumulated in the image sensor 201 are read out in the order of 6-1, 6-2, 6-3 in FIG. It is converted into a digital signal by 202 and temporarily recorded in the buffer memory 203. In this manner, the electrical signals accumulated for all the pixels are sequentially read out. In FIG. 1, reference numerals 131 to 133 correspond to the description of the three-field reading.

  Upon completion of the first field reading (131), the OB block integration circuit 205 causes the OB value and the preset blocks (Y (3, 3), Y (4, 3), Y (3, 3) as shown in FIG. 4), Y (4, 4)) luminance signal average value Yave_Fr is calculated.

  Here, the WB calculation is optimized using the face detection result, but the face detection and the WB calculation take time compared to the AE / AF processing. For this reason, a change between the scene for which the WB gain was calculated last time and the actual shooting scene is detected (134), and if there is no or little scene change, the WB coefficient used when creating the latest EVF image signal is read from the memory ( 135), used as it is.

  According to FIG. 1, the WB coefficient used at the time of actual photographing is the result of WB calculation 4 (125), and WB calculation 4 is calculated from face detection 4 (120), and the image signal used for face detection 4 is the EVF 111. This is a signal obtained during the display period. That is, a scene change may be detected based on the image signal obtained during the EVF 111 display period and the main exposure image signal.

FIG. 7 shows the relationship between the luminance difference value and the scene change amount.
As a method of detecting a scene change, an average luminance value (Yave_Fi) of a predesignated area recorded in the buffer memory 203 at the time of creating an image signal obtained during the EVF 111 display period and an average luminance of the same coordinate area in the main exposure image A luminance difference value ΔY of the value (Yave_Fr) is calculated.

  The scene change value (k) is calculated using a preset relationship diagram between the luminance difference value and the scene change amount as shown in FIG. Then, the presence / absence of a scene change is determined based on the scene change value (k).

  In other words, by determining whether the comparison result of the luminance of the image signal used for obtaining the WB coefficient used in the main photographing and the luminance of the image signal obtained in the main exposure exceeds the reference value, Judgment is made.

  Further, as a method for determining whether or not there is a scene change by using the luminance change of the subject as a scene change, the luminance difference value of the subject photometric value at the time of AE control (ΔBv = Bv_EVF111 shooting time−Bv_main shooting time) If the value is greater than the set value, there are cases where it is determined that there is a scene change.

  Further, as another method, there is a method in which it is determined that there is a scene change when the AF control value (focus lens position information) changes greatly. Specifically, the presence / absence of a scene change is determined by comparing the AF information during the display period of the EVF 111 with the AF information during actual photographing. There is also a method for detecting a scene change using a zoom position difference signal. More specifically, the presence / absence of a scene change is determined based on the zoom position change amount during the EVF 111 display period and the zoom position change amount during actual photographing. Further, the presence or absence of a scene change may be determined by arbitrarily combining these determination methods.

  Next, an example in which the WB coefficient calculation method is changed based on the determination result of the presence / absence of a scene change will be described.

  First, a method for calculating a WB coefficient when it is determined that there is no scene change will be described.

  If it is determined that there is no scene change as a result of determining whether there is a scene change, the WB calculation is not performed, and the WB coefficient 4 calculated by the WB calculation 4 (125) is obtained from the image signal obtained during the display period of the EVF 111. Reading from the buffer memory 203 (135), this is applied to the WB coefficient of the main exposure image signal.

  Thereafter, development processing (136) is performed, and recording (137) is performed in the external recording device 210. In this case, the time for generating the main exposure WB coefficient can be saved, so that the image generation time can be greatly shortened.

  Here, another example will be given when it is determined that there is no scene change. The WB coefficient 5 is calculated (138) using the latest face detection information (face detection 5 in FIG. 1) existing at the time of development and the first field signal of the main exposure image. Thereafter, development processing (139) is performed, and an image is recorded in the external recording device 210 (140). In the above example, although it takes time to calculate the WB coefficient, the spectral sensitivity of the image sensor may differ between the EVF readout mode and the still image readout mode, and the WB coefficient calculated for creating the EVF image signal cannot be used as it is. It is effective in the case.

Next, a method for calculating the WB coefficient when it is determined that there is a scene change will be described.
If it is determined that there is a scene change as a result of determining whether there is a scene change, face detection (142) is performed from the main exposure image signal, and a new WB coefficient is calculated using the detection result and the main exposure image signal. (143).

Thereafter, a weighted addition process is performed on the WB coefficient 4 during the EVF operation and the calculated new WB coefficient using the scene change rate (k) to calculate a WB coefficient 5 used for the main development.
WBCo5 = (1-k) × WBCo4 + k × WBCo5
Thereafter, a main development process (144) is performed using a WB coefficient of 5, and an image is recorded in the external recording device 210 (145). In the present embodiment, when an image signal used for face detection is obtained from, for example, an image sensor that performs three-field reading, as shown in FIG. A luminance signal is generated from one field signal (141). In an image sensor that performs multi-field readout, the RGB signals necessary for calculating the WB coefficient are prepared using only the first field signal. Therefore, the WB coefficient can be calculated without waiting for all the field signals to be read. Is possible.

  Therefore, in an imaging device that performs three-field readout, comparing the case where the WB coefficient is calculated from only the first field signal with the case where the WB coefficient is calculated from all field signals, the former is one third of the latter. It is possible to calculate the WB coefficient at a speed.

  The generated luminance signal is input to the feature detection circuit 209, and face detection is performed (142), thereby reducing the face detection time. This speeds up processing by starting development (141) and face detection (142) of the face detection image during the period during which the second and third fields are read (132, 133). Can do.

  Further, in the case of an image sensor that performs even-field readout, all the RGB colors are not prepared with only the first field signal, and colors cannot be generated. Therefore, the feature detection circuit 209 is used as an input using the first and second field signals. .

  Furthermore, when a feature detection circuit that does not use color information for face detection is used for speedup, a luminance signal is generated only from the first field image signal even in the case of an image sensor that performs even field readout. The input signal of the feature detection circuit.

  As described above, according to the present embodiment, the presence / absence of a scene change is determined, and whether or not the WB coefficient is newly calculated according to the determination result is switched, whereby the face detection simple image creation time and the face detection time are determined. It is possible to shorten.

It is a figure which shows the imaging operation timing in the imaging device of this invention. It is a signal processing simplified block diagram of the present invention. FIG. 10 is a schematic diagram of signal readout in the pixel addition mode of the image sensor. It is a figure showing an example of block division of an image sensor. It is a figure for demonstrating the face information in the 1st Embodiment of this invention. It is a conceptual diagram of the reading of the image pick-up element which performs 3 field reading. It is a figure which shows the relationship between a luminance difference value and a scene change amount. It is a figure which shows the example which divided | segmented the screen into arbitrary several blocks. It is a figure which shows a white detection range. It is a figure which shows the example which image | photographed the face-up as a to-be-photographed object. It is a figure which shows the imaging operation timing in the conventional imaging device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 201 Image pick-up element 202 A / D conversion circuit 203 Buffer memory 204 OB subtraction circuit 205 OB block integration circuit 206 WB coefficient calculation circuit 207 WB gain circuit 208 Digital signal processing circuit 209 Feature detection circuit 210 External recording device 211 Display device 212 CPU
213 Device control circuit

Claims (14)

In an imaging apparatus including an imaging element that photoelectrically converts a subject image formed by an imaging optical system,
Subject detection means for repeatedly detecting a subject region based on a signal obtained from the image sensor;
White balance coefficient calculating means for calculating a white balance coefficient based on subject information output from the subject detecting means;
A scene change determination means for comparing the scene from which the white balance coefficient is obtained and the scene from which the main exposure is performed;
An image pickup apparatus that switches whether or not a white balance coefficient is newly calculated by the white balance coefficient calculation means after the main exposure in accordance with a comparison result of the scene.   In accordance with the result of the scene comparison, an already obtained white balance coefficient is used as a white balance coefficient used for white balance processing performed after reading out a signal obtained from an image sensor, or an image obtained during main exposure. 2. The imaging apparatus according to claim 1, wherein subject information is detected from the signal, and a white balance coefficient is newly calculated based on the subject information.   2. The imaging apparatus including an imaging device capable of reading out a plurality of fields, wherein the subject detection unit performs subject detection from a first field signal or first and second field signals. Imaging device.   The imaging apparatus according to claim 1, wherein the scene change determination unit compares an average luminance value of an area designated in advance from a captured image with an average luminance value of the same coordinate area in the main exposure image.   The scene change determination means obtains a difference between a subject photometric value at the time of photographing an image signal from which the latest white balance coefficient is obtained and a subject photometric value at the time of photographing the main exposure image signal, or a latest white balance coefficient. The difference between the focus position information at the time of original image signal shooting and the focus position information at the time of main exposure image signal shooting, or the lens zoom position at the time of image signal shooting from which the latest white balance coefficient is obtained The imaging apparatus according to claim 1, wherein at least one of the difference between the zoom zoom position and the lens zoom position at the time of photographing the exposure image signal is compared.   If the scene comparison results indicate that there is a scene change, subject information is detected from the image signal obtained during the main exposure, and a new white balance coefficient is calculated and used based on the subject information. 2. The imaging apparatus according to claim 1, wherein when it can be considered that there is no white balance coefficient, a white balance coefficient that has already been calculated is used.   If the spectral sensitivity of the image sensor differs between when reading data to the electronic viewfinder and when reading it as still image data, the white balance coefficient is calculated from the first field image of the image signal obtained during the main exposure. The imaging device according to claim 1, wherein the imaging device is calculated. In a control method of an imaging apparatus including an imaging element that photoelectrically converts a subject image formed by a photographing optical system,
A subject detection step of repeatedly detecting a subject region based on a signal obtained from the image sensor;
A white balance coefficient calculation step of calculating a white balance coefficient based on the subject information output from the subject detection step;
A scene change determination process for comparing the scene from which the white balance coefficient is obtained and the scene from which the main exposure is performed;
A control method for an imaging apparatus, wherein whether or not a white balance coefficient is newly calculated by the white balance coefficient calculating step is switched after the main exposure according to the comparison result of the scene.   Depending on the comparison result of the scene, the white balance coefficient already calculated is used as the white balance coefficient used for the white balance processing performed after developing the signal obtained from the image sensor, or obtained during the main exposure. 9. The method of controlling an imaging apparatus according to claim 8, wherein subject information is detected from an image signal, and a white balance coefficient is newly calculated based on the subject information.   9. The method of controlling an imaging apparatus including an imaging device capable of reading out a plurality of fields, wherein the subject detection step performs subject detection from a first field signal or first and second field signals. The control method of the imaging device described in 1.   9. The control method for an imaging apparatus according to claim 8, wherein the scene change determination step compares an average luminance value of an area designated in advance from a captured image with an average luminance value of the same coordinate area in the main exposure image. .   The scene change determination step obtains a difference between a subject photometric value at the time of image signal photographing that is a basis for obtaining the latest white balance coefficient and a subject photometric value at the time of photographing the main exposure image signal, or obtains the latest white balance coefficient. The difference between the focus position information at the time of original image signal shooting and the focus position information at the time of main exposure image signal shooting, or the lens zoom position at the time of image signal shooting from which the latest white balance coefficient is obtained 9. The method of controlling an image pickup apparatus according to claim 8, wherein at least one of the difference between the image zoom position and the lens zoom position at the time of photographing the exposure image signal is compared.   If the scene comparison results indicate that there is a scene change, subject information is detected from the image signal obtained during the main exposure, and a new white balance coefficient is calculated and used based on the subject information. 9. The method of controlling an imaging apparatus according to claim 8, wherein a white balance coefficient that has already been calculated is used when it can be considered that there is no image. If the spectral sensitivity of the image sensor differs between when reading data to the electronic viewfinder and when reading it as still image data, the white balance coefficient is calculated from the first field image of the image signal obtained during the main exposure. The method of controlling an imaging apparatus according to claim 8, 9, 12, or 13, wherein the calculation is performed.

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