JP2018072489A - Imaging device and control method thereof - Google Patents

Abstract

An imaging apparatus capable of reducing an error in focus detection caused by saturation of an image signal of a pixel and a control method thereof are provided. An image pickup device having a pixel including a pair of subpixels that receive a light beam that has passed through different pupil partial regions of an imaging optical system, and signal acquisition for acquiring a pair of image signals based on signals output from the subpixels Means, saturation determination means for determining an image signal from a saturated pixel whose magnitude exceeds the first upper limit value, and shading correction of the image signal according to the image height based on the optical information of the imaging optical system A focus detection unit that detects a focus by performing a correlation operation on the pair of image signals, and the focus detection unit converts an image signal from a saturated pixel to an image signal at a corresponding image height of a non-saturated pixel. The shading correction is applied more weakly than the shading correction to be applied, or no shading correction is performed. [Selection] Figure 1

Description

  The present invention relates to an imaging apparatus having a focus detection function and a control method thereof.

  There has been proposed a technique for performing automatic focus detection by a phase difference detection method using an imaging device having a pair of sub-pixels that are pupil-divided. For example, Patent Document 1 describes a technique for performing focus detection by calculating a correlation between a pair of image signals generated based on a signal output from a sub-pixel to obtain an image shift amount.

JP 2013-171257 A

  In the phase difference detection method, when a pair of image signals is subjected to correlation calculation to obtain an image shift amount, shading correction for aligning the signal levels of the pair of image signals A and B is performed. However, in a saturated pixel where the amount of received light is large and the image signal is saturated, the ratio between the signal levels of the image signal A and the image signal B may be increased by shading correction. As a result, there is a problem that the degree of coincidence between the image signal A and the image signal B becomes low in the correlation calculation, and an error occurs in focus detection.

  An imaging apparatus according to the present invention includes an imaging device having a pixel including a pair of subpixels that receive a light beam that has passed through different pupil partial regions of an imaging optical system, and a pair of image signals based on signals output from the subpixels. A signal acquisition unit for acquiring, a saturation determination unit for determining an image signal from a saturated pixel whose magnitude exceeds the first upper limit value, and an image signal corresponding to the image height based on optical information of the imaging optical system And a focus detection unit that detects a focus by performing correlation calculation on the pair of image signals, and the focus detection unit detects a corresponding image height of the unsaturated pixel with respect to the image signal from the saturated pixel. The shading correction is applied more weakly than the shading correction applied to the image signal in FIG. 1, or no shading correction is performed.

  The image pickup apparatus control method according to the present invention is an image pickup apparatus control method including an image pickup element having a pair of sub-pixels that receive a light beam that has passed through different pupil partial regions of the image pickup optical system. A signal acquisition step of acquiring a pair of image signals based on signals output from the sub-pixels, a saturation determination step of determining an image signal from a saturated pixel whose magnitude exceeds the first upper limit value, and imaging A step of correcting shading of an image signal according to the image height based on optical information of the optical system, and for an image signal from a saturated pixel, a shading correction applied to the image signal at an image height corresponding to a non-saturated pixel A correction step in which the shading correction is weaker than that or no shading correction is performed, and a focus detection step in which a focus is detected by performing a correlation operation on the pair of image signals. And wherein the door.

  ADVANTAGE OF THE INVENTION According to this invention, the imaging device which can reduce the focus detection error which arises when the image signal of a pixel is saturated, and its control method can be obtained.

It is the schematic which shows the structure of the imaging device which concerns on 1st Embodiment. It is the schematic which shows the arrangement | sequence and structure of a pixel in the imaging device which concerns on 1st Embodiment. It is a figure which shows the relationship between the subpixel and pupil partial area | region in the imaging device which concerns on 1st Embodiment. It is a figure which shows an example of the imaging signal A + B, the image signal A, and the image signal B in the imaging device which concerns on 1st Embodiment. It is a figure for demonstrating the shading of the image signal in the imaging device which concerns on 1st Embodiment. It is a figure for demonstrating the clip process in the imaging device which concerns on 1st Embodiment. It is a figure for demonstrating the shading correction | amendment in the imaging device which concerns on 1st Embodiment. It is a figure for demonstrating the clip process in the imaging device which concerns on 2nd Embodiment. It is a figure for demonstrating the shading correction | amendment in the imaging device which concerns on 2nd Embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment, In the range which does not deviate from the summary, it can change suitably. In the drawings described below, components having the same function are denoted by the same reference numerals, and the description thereof may be omitted or simplified.

(First embodiment)
[Configuration of imaging device]
The imaging apparatus according to the first embodiment will be described with reference to FIGS. FIG. 1 is a schematic diagram illustrating a configuration of an imaging apparatus. FIG. 1 shows an example in which the imaging apparatus is applied to an interchangeable lens still camera, but the present embodiment is not necessarily limited to such a configuration. The imaging apparatus of the present embodiment may be a lens fixed video camera, for example.

  The first lens group 101 is disposed on the subject side of an imaging optical system such as a photographing lens, and is held so as to be able to advance and retreat in the optical axis direction. The aperture shutter 102 adjusts the amount of light from the subject irradiated on the image sensor 107 by changing the aperture diameter. The aperture shutter 102 also functions as a shutter for adjusting the exposure time during still image shooting. The second lens group 103 moves forward and backward in the optical axis direction integrally with the aperture shutter 102, and performs a zooming function (zoom function) in conjunction with the forward and backward movement of the first lens group 101. The third lens group 105 is disposed on the imaging element 107 side of the imaging optical system, and moves forward and backward in the optical axis direction to adjust the focus. The low-pass filter 106 removes the short wavelength component of the light irradiated to the image sensor 107 to reduce the false color and moire of the image.

  The image sensor 107 includes a CMOS sensor and a peripheral circuit, and is disposed on the imaging plane of the image pickup optical system. As the image sensor 107, for example, a two-dimensional single-plate color sensor in which a Bayer array color filter is formed on-chip on the photoelectric conversion units of pixels arranged in a matrix is used. The lens ROM 110 stores optical information necessary for focus adjustment and the like. The optical information includes, for example, an F value and an exit pupil distance. The CPU 121 on the camera body side acquires optical information stored in the lens ROM 110 by communicating with the interchangeable lens side.

  The zoom actuator 111 performs a zooming operation by driving the first lens group 101 or the second lens group 103 back and forth in the optical axis direction by rotating a cam cylinder (not shown), for example. The aperture shutter actuator 112 controls the aperture diameter of the aperture shutter 102 to adjust the amount of photographing light, and also controls the exposure time during still image photographing. The focus actuator 114 adjusts the focus by driving the third lens group 105 back and forth in the optical axis direction.

  The illumination unit 115 is an electronic flash for illuminating a subject at the time of photographing, and a flash illumination device using a xenon tube is suitable, but an illumination device including LEDs that emit light continuously may be used. The AF auxiliary light means 116 projects a mask image having a predetermined aperture pattern onto a subject via a light projection lens, and improves the focus detection capability for a dark subject or a low-contrast subject.

  The CPU 121 includes a calculation unit (not shown), ROM, RAM, A / D converter, D / A converter, communication interface circuit, and the like, and controls the camera body and the imaging optical system. The CPU 121 executes a program stored in the ROM, and executes a series of processes such as focus adjustment, shooting, image processing, and recording.

  The electronic flash control circuit 122 controls lighting of the illumination unit 115 in synchronization with the photographing operation. The auxiliary light source driving circuit 123 controls the lighting of the AF auxiliary light unit 116 in synchronization with the photographing operation. The image sensor driving circuit 124 is controlled by the CPU 121 and controls the image sensor 107. The image processing circuit 125 acquires an imaging signal based on a signal output from the pixel of the imaging element 107, and performs γ conversion, color interpolation, JPEG compression, and the like on an image obtained from the imaging signal.

  The focus drive circuit 126 is controlled by the CPU 121 and drives the focus actuator 114 to adjust the focus of the imaging optical system. The aperture shutter drive circuit 128 is controlled by the CPU 121 and drives the aperture shutter actuator 112 to adjust the aperture of the imaging optical system. The zoom drive circuit 129 is controlled by the CPU 121 and drives the zoom actuator 111 according to the zoom operation of the photographer to adjust the zoom of the imaging optical system.

  The display 131 is a display device such as an LCD, and displays information related to the shooting mode of the camera, a preview image before shooting, a confirmation image after shooting, a focus state display image at the time of focus detection, and the like. The operation switch 132 includes a power switch, a release (shooting trigger) switch, a zoom operation switch, a shooting mode selection switch, and the like. The storage medium 133 is a detachable storage medium such as a flash memory, and records captured images.

[Structure of image sensor]
FIG. 2 is a schematic diagram illustrating the arrangement and structure of the pixels 210 in the imaging apparatus. FIG. 2A shows 4 columns × 4 rows of pixels 210, but the actual image sensor 107 has more columns and rows of pixels 210. In FIG. 2A, a pixel group composed of 2 columns × 2 rows of pixels 210R, 210G, and 210B having different color filters is configured. For example, in a Bayer array pixel group, a pixel 210R having R (red) spectral sensitivity is located on the upper left, a pixel 210G having G (green) spectral sensitivity is located on the upper right and lower left, and a pixel having B (blue) spectral sensitivity. 210B is arranged at the lower right.

  FIG. 2B shows a plan view of the pixel 210 shown in FIG. FIG. 2C is a cross-sectional view taken along the line aa ′ of the pixel 210 shown in FIG. The pixel 210 of this embodiment shown in FIG. 2 is divided into a pair of sub-pixels 210a and 210b. The sub-pixels 210a and 210b receive light beams that have passed through different pupil partial regions of the optical system through the same microlens 202, and output a pair of image signals having different viewpoint information. The division direction and the number of divisions of the sub-pixels 210a and 210b are not limited to the configuration shown in FIG. 2, and can be changed as appropriate.

  Above the pixel 210 (on the + Z side), a microlens 202 for condensing light incident along the optical axis is formed. Further, a color filter 203 is formed between the microlens 202 and the photoelectric conversion units 201a and 201b. If necessary, the spectral transmittance of the color filter 203 may be changed for each sub-pixel, or the color filter 203 may be omitted.

  On the light receiving surfaces of the photoelectric conversion units 201a and 201b, electrons and holes are generated in pairs according to the amount of light received and separated by the depletion layer, and then negatively charged electrons are accumulated in the n-type layer. On the other hand, holes are discharged to the outside of the pixel 210 through a p-type layer connected to a constant voltage source (not shown). Electrons accumulated in the n-type layers of the photoelectric conversion units 201a and 201b are transferred to a floating diffusion unit (floating diffusion) via a transfer gate and read by a read circuit (not shown). The photoelectric conversion units 201a and 201b may be a pin structure photodiode in which an intrinsic layer is sandwiched between a p-type layer and an n-type layer, or may be a pn junction photodiode in which an intrinsic layer is omitted. Good.

  Note that not all the pixels 210 of the image sensor 107 need to be divided into sub-pixels 210a and 210b, and only a part of the pixels 210 in the image sensor 107 may be divided into sub-pixels 210a and 210b. Good. Further, instead of the pair of sub-pixels 210a and 210b sharing the same microlens 202, one pixel decentered per microlens is arranged, and pupil division is performed using a pair of pixels having different eccentricity. Focus detection may be performed.

[Concept of pupil division of image sensor]
FIG. 3 is a diagram illustrating a relationship between the sub-pixels 210a and 210b and the pupil partial areas 302a and 302b in the imaging apparatus. 3 shows a cross-sectional view of a pixel 210 similar to that in FIG. 3 shows the exit pupil plane of the imaging optical system.

  The pupil region 300 represents a pupil region that can be received by the pixel 210. The pupil partial areas 302a and 302b represent pupil areas that can be received by the sub-pixels 210a and 210b, respectively. The pupil partial region 302a has a center of gravity decentered toward the + X side on the pupil surface, and has a conjugate relationship with the light receiving surface of the photoelectric conversion unit 201a whose center of gravity is decentered in the −X direction by the microlens 202. Similarly, the pupil partial region 302b has a center of gravity decentered toward the −X side on the pupil plane, and has a conjugate relationship with the light receiving surface of the photoelectric conversion unit 201b whose center of gravity is decentered in the + X direction by the microlens 202.

  The pupil region 303 is a pupil region in consideration of vignetting due to a frame (a generic term for a lens frame and a diaphragm frame). The size, position, and shape of the pupil region 303 vary depending on the combination of the aperture value of the aperture shutter 102, the exit pupil distance determined by the distance from the exit pupil to the light receiving surface 304, and the image height of the image sensor 107. The sub-pixel 210a receives light that has passed through the pupil partial region 302a and outputs an image signal A corresponding to the amount of received light. The sub-pixel 210b receives light that has passed through the pupil partial region 302b and outputs an image signal B corresponding to the amount of received light.

  The image signal A and the image signal B are also used as a phase difference signal for detecting a focus, in addition to being used as an imaging signal A + B for generating a captured image. Specifically, the focus detection unit executed by the image processing circuit 125 or the CPU 121 acquires the distribution of the image signal A in the X direction from the plurality of sub-pixels 210a arranged at regular intervals in the X direction. Similarly, the distribution in the X direction of the image signal B is acquired from a plurality of subpixels 210b arranged at regular intervals in the X direction. Then, the defocus amount is calculated from the image shift amount of the distribution of the image signal A in the X direction and the distribution of the image signal B in the X direction.

  Thereafter, the focus detection unit controls the focus drive circuit 126 based on the defocus amount, and drives the third lens group 105 forward and backward in the optical axis direction to perform focus adjustment. Instead of calculating the image shift amount in the X direction of the image signals A and B, the same method is used to calculate the image shift amount in the Y direction of the image signals A and B to calculate the defocus amount. It may be. 3 shows an example in which the sub-pixels 210a and 210b of the pixel 210 are divided in the X direction. However, the pixel 210 may be divided in the Y direction, or may be divided in both the X direction and the Y direction. May be. Alternatively, these pixels 210 having different division directions may be mixed.

  FIG. 4 is a diagram illustrating an example of the imaging signal A + B, the image signal A, and the image signal B in the imaging apparatus. In FIG. 4, the image signal A is indicated by a dotted line, and the image signal B is indicated by a broken line. Further, an image signal A + B obtained by adding the image signal A and the image signal B is indicated by a solid line. The horizontal axis in FIG. 4 indicates the row number or column number of the pixels 210 arranged in a matrix, and the vertical axis in FIG. 4 indicates the relative magnitude of each signal in each pixel 210 in arbitrary units. .

  Of the three signals shown in FIG. 4, there are two independent signals. That is, if two of the three signals are acquired, the remaining one can be generated by computation. In practice, first, an imaging signal A + B for generating a captured image is preferentially acquired. Next, of the image signal A and the image signal B, for example, the image signal A is acquired. The image signal B is obtained by subtracting the image signal A from the imaging signal A + B. Thereafter, the focus detection is performed by calculating the defocus amount from the image shift amount between the image signal A and the image signal B.

[shading]
FIG. 5 is a diagram for explaining shading of image signals in the imaging apparatus. Depending on the state of an optical element such as a lens or a diaphragm, or a photographing optical system including a lens barrel that holds these, so-called vignetting may occur in which a light beam from a subject is blocked. The vignetting causes unevenness in the light receiving sensitivity of at least one of the sub-pixels 210a and 210b, and generates so-called shading in which the amount of light that can be received by the sub-pixels 210a and 210b changes according to the image height X. If the signal levels of the image signal A and the image signal B at the same image height X are different from each other due to the influence of shading, the image signal is calculated when performing a correlation operation for calculating the image shift amount between the image signal A and the image signal B. The degree of coincidence between A and the image signal B decreases, and the accuracy of focus detection decreases.

  FIG. 5A shows an incident angle light receiving characteristic 601a of the sub-pixel 210a and an incident angle light receiving characteristic 601b of the sub-pixel 210b. For example, the pixel 210 arranged at the image height −X 1 sees the pupil plane at the pupil coordinate + X 2 through the exit pupil frame 602. Here, at the pupil coordinate + X2, the incident angle light receiving characteristic 601b of the sub-pixel 210b is larger than the incident angle light receiving characteristic 601a of the sub-pixel 210a. That is, the signal level of the image signal B acquired from the sub-pixel 210b is relatively higher than the signal level of the image signal A acquired from the sub-pixel 210a.

  On the other hand, the pixel 210 arranged at the image height + X 1 sees the pupil plane at the pupil coordinate −X 2 through the exit pupil frame 602. Here, at the pupil coordinate −X2, the incident angle light receiving characteristic 601a of the subpixel 210a is larger than the incident angle light receiving characteristic 601b of the subpixel 210b. That is, the signal level of the image signal A acquired from the sub-pixel 210a is relatively higher than the signal level of the image signal B acquired from the sub-pixel 210b.

  FIG. 5B shows shading in the X direction of the image signal A received by the sub-pixel 210a arranged at the image height X. Similarly, shading in the X direction of the image signal B received by the subpixel 210b arranged at the image height X is shown. The shading of the image signals A and B is obtained by integrating the amount of light that can be received by the sub-pixels 210a and 210b having the image height X through the exit pupil frame 602 in the range of the corresponding pupil partial regions 302a and 302b. The shading of the image signals A and B obtained in this way can be handled approximately as a function that continuously changes according to the image height X, as shown in FIG.

[Shading correction]
The shading correction corrects shading caused by such unevenness in the light receiving sensitivity of the sub-pixels 210a and 210b. Similar to shading, shading correction can be handled as a function that continuously changes in accordance with the image height X.

  Here, it should be noted that the shading of the image signals A and B changes according to the position and size of the exit pupil frame 602 in addition to the image height X. For example, when the exit pupil distance determined by the distance from the exit pupil to the light receiving surface becomes small, -X2 on the pupil plane viewed from the same image height + X1 is further away from the center. Further, when the aperture value is decreased, the area of the pupil partial regions 302a and 302b viewed by the sub-pixels 210a and 210b through the exit pupil frame 602 is reduced, so that the integration range of the pupil partial region 302a is reduced, and the image signal A and the image signal are reduced. The ratio with B increases. Conversely, as the aperture value increases, the ratio between the image signal A and the image signal B decreases.

  Therefore, for example, when shading correction is performed with an interchangeable lens camera or the like, in addition to the image height X, a shading correction value that matches the position and size of the exit pupil frame 602 of the imaging optical system is stored in advance. It is necessary to keep. For this reason, the capacity of the ROM and RAM of the CPU 121 becomes enormous. Alternatively, instead of storing the shading correction value in advance, it is possible to calculate the shading correction value from the relationship between the incident angle light receiving characteristics 601a and 601b and the exit pupil frame 602, but this time the calculation amount of the shading correction is enormous. End up.

Therefore, in the present embodiment, a shading correction value corresponding to optical information such as the F value and exit pupil distance of a typical imaging optical system is calculated in advance, and the coefficient of the approximate function is stored. For example, the shading correction functions for the image signals A and B are respectively stored in the following equations (1) and (2) using correction coefficients S _A0 , S _A1 , S _A2 , S _B0 , S _B1 , and S _B2 . Thus, approximation is performed by a polynomial up to the second order of the image height X.
SA (X) = S _A0 + S _A1 · X + S _A2 · X ² (1)
SB (X) = S _B0 + S _B1 · X + S _B2 · X ² (2)

By multiplying the correction functions SA (X) and SB (X) by the image signals A and B, respectively, it is possible to obtain the image signals A and B having the same signal level after shading correction. As a result, the amount of image shift between the image signal A and the image signal B can be easily calculated, and the accuracy of focus detection is improved. In particular, in this embodiment, the correction coefficients S _A0 , S _A1 , S _A2 , S _B0 , S _B1 , and S _B2 are calculated in advance for each optical information and defined as a table, so that the capacity of the ROM or RAM is reduced. Alternatively, the calculation amount in the image processing circuit 125 and the CPU 121 can be reduced.

[Shading correction at saturation]
Next, clip processing and shading correction performed by the image processing circuit 125 or the CPU 121 when the imaging signal or the image signal is saturated will be described. For example, in the process of accumulating or amplifying signal charges when reading the signal output from the pixel 210, there is an upper limit value of the signal that can be handled. For this reason, when the amount of light received by the pixel 210 is large, the magnitudes of the image signals A and B and the imaging signal A + B are limited to this upper limit value.

  In particular, when the image signal A is obtained by subtracting the image signal A from the imaging signal A + B, the imaging signal A + B first reaches the upper limit value. At this time, the value of the image signal B obtained by subtracting the image signal A from the imaging signal A + B becomes smaller than the actual value when the magnitude of the image signal A exceeds the half value of the imaging signal A + B. As a result, the value of the image signal B in the pixel having the large light reception amount is inverted with respect to the distribution of the value of the image signal B in the pixel having the small light reception amount, and the image signal A and the image signal B match when performing the correlation calculation. The degree will be low. Here, the correlation calculation refers to a calculation for obtaining an image shift amount between the image signal A and the image signal B based on the degree of coincidence between the image signal A and the image signal B when performing focus detection.

  Therefore, when the magnitudes of the image signals A and B exceed the half value of the upper limit value of the imaging signal A + B (hereinafter referred to as “saturation”), the magnitude of the image signal is set to the half value of the upper limit value of the imaging signal A + B (hereinafter referred to as “saturation”). , “First upper limit value”). FIG. 6 is a diagram for explaining clip processing in the imaging apparatus. In FIG. 6, both the image signal A and the image signal B are saturated and clipped to the same first upper limit value (= 800) in a saturated pixel near the image height X = 15 where the amount of received light is large. In other than the saturated pixels, the signal level of the image signal A is approximately twice the signal level of the image signal B due to the influence of shading.

  FIG. 7 is a diagram for explaining shading correction in the imaging apparatus. FIG. 7A shows the result of conventional shading correction performed on the clipped image signals A and B shown in FIG. In FIG. 7A, in order to perform correlation calculation, gains are applied to the image signal A and the image signal B, respectively, so that the signal levels of the pair of image signals A and B are aligned. As a result, the signal levels of the image signals A and B are uniform except in the vicinity of the image height X = 15. However, in the saturated pixel near the image height X = 15, since the image signal A and the image signal B are clipped, the ratio of the signal level between the image signal A and the image signal B is increased by shading correction. Yes. If the correlation calculation is performed as it is, the degree of coincidence between the image signal A and the image signal B in the saturated pixel becomes low, and an error occurs in focus detection.

  Here, in the present embodiment, the image processing circuit 125 or the CPU 121 uses hardware configured to process a predetermined number of consecutive image signals when calculating the image shift amount. For this reason, when performing the correlation calculation, in order to perform the process of excluding only the image signal from the saturated pixel with respect to the image signal sequence in the X direction, a circuit is separately provided in the previous stage, or the normalization process is performed in the subsequent stage. It is difficult to do it. Therefore, this is dealt with at the stage of performing shading correction on saturated pixels, and the influence on the subsequent focus detection is reduced. Of course, even in the embodiment in which the image shift amount is processed by software, the countermeasure as in the present embodiment can be taken, and therefore, there is no particular limitation.

[Shading correction for saturated pixels]
Therefore, in the present embodiment, the CPU 121 performs a process different from the shading correction on the saturated pixels in which one of the image signals A and B is saturated, by slightly applying shading correction on the non-saturated pixels. . For example, in FIG. 7B, for the saturated pixels, gains are applied to the image signal A and the image signal B, respectively, so that the magnitudes of the pair of image signals A and B are set to the same correction value. This correction value may be a predetermined constant value, or may be equal to the first upper limit value (= 800) in the clip processing shown in FIG. Or you may determine according to the ratio of the magnitude | size of a pair of image signal A and B in the saturation pixel after shading correction | amendment. Alternatively, processing including shading correction may not be performed on saturated pixels. When shading correction is not performed, SA (X = 15 vicinity) and SB (X = 15 vicinity) are set to the same constant value in the above equations (1) and (2). In FIG. 7B, SA (near X = 15) and SB (near X = 15) are set to a constant value = 1.

  As a result, the signal levels of the image signals A and B after shading correction coincide with each other in the saturated pixel, or the difference between the signal levels is less likely to occur, so that the degree of coincidence between the image signal A and the image signal B is reduced in the correlation calculation. It is possible to reduce the focus detection error caused by the above. In particular, in this embodiment, the correlation calculation is performed in a form including the saturated pixels. Therefore, compared to the case where the correlation calculation is performed by excluding the saturated pixels, for example, the number of saturated pixels increases and can be used for the correlation calculation. A decrease in the number of pixels can be suppressed.

  As described above, the imaging apparatus according to the present embodiment includes the signal acquisition unit (image processing circuit) that acquires a pair of image signals based on the signals output from the subpixels, and the magnitude of the image signal has the first upper limit value. Saturation determination means (image processing circuit) is provided for determining an image signal from a saturated pixel that has exceeded. Further, the image processing apparatus includes a focus detection unit (image processing circuit, CPU) that detects the focus by correcting the shading of the image signal according to the image height based on the optical information of the imaging optical system, and calculating the correlation between the pair of image signals. . For the image signal from the saturated pixel, the shading correction is made weaker than the shading correction applied to the image signal at the image height corresponding to the non-saturated pixel, or the shading correction is not performed. With such a configuration, it is possible to obtain an imaging apparatus capable of reducing focus detection errors caused by saturation of pixel image signals and a control method thereof.

  In the present embodiment, it is not always necessary to perform clip processing before shading correction. In this embodiment, the image signal from the saturated pixel is subjected to clip processing by applying a weaker correction than the shading correction applied to the image signal corresponding to the image height corresponding to the non-saturated pixel or by performing no correction. The above-described excellent effects can be obtained regardless of the presence or absence.

(Second Embodiment)
Next, an imaging apparatus according to the second embodiment will be described with reference to FIGS. 8 and 9. In the first embodiment, the case where both the image signal A and the image signal B are saturated has been described. However, depending on the conditions of the image height, the exit pupil distance of the lens, and the F value, there may be a large difference between the amount of received light of the image signal A and the amount of received light of the image signal B. There are cases where B is not saturated.

  FIG. 8 is a diagram for explaining clip processing in the imaging apparatus. In FIG. 8, the image signal A is saturated and clipped to the first upper limit value (= 800) at the saturated pixel near the image height X = 15 where the amount of received light is large, but the image signal B is not saturated. In other than the saturated pixels, the signal level of the image signal A is about four times the signal level of the image signal B due to the influence of shading.

  FIG. 9 is a diagram for explaining shading correction in the imaging apparatus. FIG. 9A shows the result of performing conventional shading correction on the image signals A and B shown in FIG. In FIG. 9A, in order to perform correlation calculation, gains are applied to the image signal A and the image signal B, respectively, so that the signal levels of the pair of image signals A and B are aligned. As a result, the signal levels of the image signals A and B are uniform except in the vicinity of the image height X = 15. However, at the saturated pixel near the image height X = 15, since the image signal A is clipped, the ratio of the signal level between the image signal A and the image signal B is increased by shading correction. If the correlation calculation is performed as it is, the degree of coincidence between the image signal A and the image signal B in the saturated pixel becomes low, and an error occurs in focus detection.

  As in the first embodiment, when the ratio between the signal level of the image signal A and the signal level of the image signal B (for example, about twice in FIG. 6) is not so large, the image pickup signal A + B is saturated when the image signal A is saturated. Often to do. In this case, both the image signal A and the image signal B are clipped to the same first upper limit value (= 800). However, when the ratio between the signal level of the image signal A and the signal level of the image signal B (for example, about 4 times in FIG. 8) increases as in this embodiment, for example, only the image signal A may be saturated. . At this time, the value of the image signal B obtained by subtracting the saturated image signal A from the non-saturated imaging signal A + B is indefinite. That is, as in the first embodiment, the image signal A and the image signal B do not necessarily have the same value.

  Therefore, in this embodiment, when only one of the image signal A and the image signal B is saturated, the upper limit of both the image signal A and the image signal B after shading correction is limited to the second upper limit value. A second clip process is performed. In FIG. 9B, the second clip processing similar to that in FIG. 8 is performed on the image signals A and B after shading correction in saturated pixels. The second upper limit value that limits the magnitude of the image signal at this time may be a predetermined constant value, or determined according to the ratio of the magnitudes of the pair of image signals A and B in the saturated pixel after shading correction. May be. In FIG. 9B, the second upper limit value is made equal to the first upper limit value (= 800) in the clip processing shown in FIG.

  As a result, the signal levels of the image signals A and B are not greatly different in the saturated pixel, and therefore, it is possible to reduce the focus detection error caused by the low degree of coincidence between the image signal A and the image signal B in the correlation calculation. it can. In particular, in this embodiment, the correlation calculation is performed in a form including the saturated pixels. Therefore, compared to the case where the correlation calculation is performed by excluding the saturated pixels, for example, the number of saturated pixels increases and can be used for the correlation calculation. A decrease in the number of pixels can be suppressed.

  As described above, the imaging apparatus according to the present embodiment includes the saturation determination unit (image processing circuit) that determines a saturated pixel in which the magnitude of the image signal exceeds the first upper limit value, and sets the upper limit of the image signal to the first upper limit value. Clip means (image processing circuit) for restricting to the above. Further, based on the optical information of the image pickup optical system, the image signal limited to the first upper limit value is subjected to shading correction, and a focus detection unit (image) that detects a focus by performing a correlation operation on the pair of image signals after shading correction. Processing circuit, CPU). The correlation calculation is performed after the upper limit of the image signal after shading correction in the saturated pixel is limited to the second upper limit value. With such a configuration, it is possible to obtain an imaging apparatus capable of reducing focus detection errors caused by saturation of pixel image signals and a control method thereof.

[Modified Embodiment]
The present invention is not limited to the above embodiment, and various modifications can be made. For example, the configuration of the imaging apparatus described in the above embodiment is an example, and the imaging apparatus to which the present invention can be applied is not limited to the configuration of the above embodiment.

  The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

  The above-described embodiments are merely examples of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

107: Image sensor 121: CPU
125: Image processing circuit 210: Pixels 210a, 210b: Sub-pixel

Claims (15)

An imaging device having a pixel including a pair of sub-pixels that receive a light beam that has passed through different pupil partial regions of the imaging optical system;
Signal acquisition means for acquiring a pair of image signals based on signals output by the sub-pixels;
Saturation determination means for determining the image signal from a saturated pixel in which the magnitude of the image signal exceeds a first upper limit;
A focus detection unit that performs shading correction on the image signal according to image height based on optical information of the imaging optical system, and detects a focus by performing a correlation operation on the pair of image signals;
With
The focus detection unit applies shading correction to the image signal from the saturated pixel more weakly than shading correction applied to the image signal at the image height corresponding to the unsaturated pixel, or does not perform the shading correction. An imaging apparatus characterized by that. The focus detection unit performs shading correction by multiplying the image signal by a shading correction function, and for the pair of image signals from the saturated pixels, the value of the shading correction function is a constant value. The imaging apparatus according to claim 1, wherein: The imaging apparatus according to claim 1, wherein the focus detection unit performs the correlation calculation after aligning the magnitudes of the pair of image signals from the saturated pixels to the same correction value. The imaging apparatus according to claim 3, wherein the correction value is equal to the first upper limit value. The imaging apparatus according to claim 3, wherein the focus detection unit determines the correction value according to a ratio of magnitudes of the pair of image signals after shading correction in the saturated pixel. The imaging apparatus according to any one of claims 1 to 5, further comprising clip means for limiting an upper limit of the image signal to the first upper limit value. The focus detection unit performs shading correction on the image signal limited to the first upper limit value, and sets a second upper limit of the image signal after shading correction for the pair of image signals from the saturated pixels. The imaging apparatus according to claim 6, wherein the correlation calculation is performed after limiting to an upper limit value. The imaging apparatus according to claim 7, wherein the second upper limit value is equal to the first upper limit value. The imaging apparatus according to claim 7, wherein the focus detection unit determines the second upper limit value according to a ratio of magnitudes of a pair of the image signals after shading correction in the saturated pixels. The signal acquisition unit first acquires an imaging signal obtained by adding signals output from the pair of subpixels and an image signal A of the pair of image signals, and then acquires the image from the imaging signal. The image pickup apparatus according to any one of claims 1 to 9, wherein the signal A is subtracted to obtain an image signal B of the pair of image signals. The imaging apparatus according to any one of claims 1 to 10, wherein the optical information includes at least one of an F value and an exit pupil distance of the imaging optical system. The imaging apparatus according to any one of claims 1 to 11, wherein the focus detection unit performs shading correction on the image signal in accordance with an image height and an exit pupil distance in addition to the optical information. A method for controlling an imaging apparatus including an imaging element having a pixel including a pair of sub-pixels that receive a light beam that has passed through different pupil partial regions of an imaging optical system,
A signal acquisition step of acquiring a pair of image signals based on signals output by the sub-pixels;
A saturation determination step of determining the image signal from a saturated pixel in which the magnitude of the image signal exceeds a first upper limit;
Shading correction of the image signal according to image height based on optical information of the imaging optical system, wherein the image signal from the saturated pixel is the image signal at the corresponding image height of the unsaturated pixel. A correction step in which the shading correction is weaker than the shading correction applied to the image signal or the shading correction is not performed;
A focus detection step of detecting a focus by performing a correlation operation on the pair of image signals;
A control method characterized by comprising: In an imaging apparatus including an imaging device having a pixel including a pair of sub-pixels that receive a light beam that has passed through different pupil partial regions of an imaging optical system,
Signal acquisition means for acquiring a pair of image signals based on signals output by the sub-pixels;
Saturation determination means for determining the image signal from a saturated pixel in which the magnitude of the image signal exceeds a first upper limit;
A means for correcting shading of the image signal according to image height based on optical information of the imaging optical system, wherein the image signal from the saturated pixel is the image signal at a corresponding image height of a non-saturated pixel. Correction means for applying a shading correction weaker than the shading correction applied to the image signal or not performing the shading correction;
A focus detection means for detecting a focus by performing a correlation operation on the pair of image signals;
A program characterized by functioning as   A computer-readable storage medium storing the program according to claim 14.

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