JP2019028142A - Imaging apparatus and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image pickup apparatus capable of performing focus detection while maintaining focus detection performance at a certain level or higher, and enabling good focus detection even in small aperture continuous shooting. An image pickup apparatus has a first focus detection pixel and a second focus detection pixel that receive light rays that pass through a first pupil portion region of an imaging optical system and a second pupil portion region different from the first pupil portion region, respectively. Generates a first focus detection signal based on the light receiving signal of the first focus detection pixel and an image pickup element in which a plurality of the and are arranged, and generates a second focus detection signal based on the light receiving signal of the second focus detection pixel. The image shift amount is calculated based on the means, the first focus detection signal and the second focus detection signal, and the calculated image shift amount is determined according to the calculated image shift amount, the optical conditions of the photographing optical system at the time of photographing, and the optical conditions of the image pickup element. At the time of focus detection, based on the result of comparing the focus detecting means for detecting the defocus amount from the calculated conversion coefficient, the setting means for setting the threshold value for the conversion coefficient, and the conversion coefficient and the threshold value set by the setting means. It is provided with a determination means for determining the aperture value of. [Selection diagram] FIG. 11

Description

  The present invention relates to an imaging apparatus such as a digital camera or a video camera and a control method thereof, and more particularly to an imaging apparatus equipped with an imaging element having a focus detection function and a control method thereof.

  An imaging device such as a digital camera is generally equipped with an imaging device having an automatic focus detection (AF) function. As a focus detection method, a contrast detection method and a phase difference detection method capable of detecting a focus faster than the contrast detection method are known. However, the phase difference detection method requires a dedicated sensor having a pupil division function. For this reason, it has been used in relatively large and expensive imaging devices. However, in recent years, there has been proposed an image sensor that can realize focus detection by a phase difference detection method without using a dedicated sensor by adding a pupil division function to the pixels of the image sensor.

  When performing continuous shooting using the phase difference detection method using the output signal of such an image sensor, it is determined that there is vignetting due to the effects of the front and rear frames of the imaging optical system, especially at high image heights. In such a case, a technique for controlling the aperture value to be equal to or greater than a predetermined value has been proposed (Patent Document 1). In this proposal, when it is determined that “there is vignetting”, the focus detection performance is prevented from changing due to vignetting, and the focus detection accuracy is improved by controlling the aperture value to be a predetermined value or more. It can be made to.

JP 2013-239787 A

  Here, as a result of intensive studies by the present applicant, in addition to the influence of vignetting described in Patent Document 1, a baseline determined by conditions such as the exit pupil distance of the lens, the set pupil distance of the image sensor, the aperture value, and the image height It was found that the focus detection performance changes with the length. Therefore, in Patent Document 1, there is a limit to performing focus detection while maintaining the focus detection performance above a certain level, and it is necessary to determine the aperture value for focus detection in consideration of the above-described conditions.

  The present invention has been made on the basis of such knowledge, and enables focus detection while maintaining focus detection performance at a certain level or more, and enables good focus detection even in small-aperture continuous shooting. An object is to provide an imaging device and a control method thereof.

  In order to achieve the above object, an imaging apparatus according to the present invention receives a first light beam that passes through a first pupil partial region and a second pupil partial region different from the first pupil partial region of a photographing optical system. A first focus detection signal is generated based on an image sensor in which a plurality of pixels and second focus detection pixels are arranged, and a light reception signal of the first focus detection pixel, and based on the light reception signal of the second focus detection pixel A generating means for generating a second focus detection signal; an image shift amount is calculated based on the first focus detection signal and the second focus detection signal; the calculated image shift amount; and the photographing optical at the time of shooting. Focus detection means for detecting a defocus amount from the optical conditions of the system and the conversion coefficient calculated according to the optical conditions of the image sensor, setting means for setting a threshold for the conversion coefficient, the conversion coefficient and the setting To the means Based on the result of comparing the set the threshold value I, to a determining means for determining an aperture value at the time of focus detection, comprising: a.

  According to the present invention, since focus detection can be performed while maintaining the focus detection performance at a certain level or more, good focus detection can be performed in small aperture continuous shooting.

1 is a block diagram illustrating an outline of a system configuration example of a digital camera that is a first embodiment of an imaging apparatus of the present invention. It is the schematic which shows the arrangement | sequence of the imaging pixel containing the focus detection pixel of an image sensor. (A) is the top view which looked at one image pick-up pixel of an image sensor from the light-receiving surface side of an image sensor, (b) is the sectional view on the aa line of (a). It is the schematic which shows the aa sectional view of the pixel structure shown to Fig.3 (a), and the exit pupil plane of an imaging optical system. It is the schematic which shows the correspondence of an image pick-up element and pupil division. It is the schematic which shows the relationship between the defocus amount of a 1st focus detection signal and a 2nd focus detection signal, and the image shift amount between a 1st focus detection signal and a 2nd focus detection signal. It is a flowchart explaining the focus detection process of an imaging surface phase difference detection system. It is a figure which shows the relationship between the base line length, the 1st pupil partial area | region of a 1st focus detection pixel in the peripheral image height of an image pick-up element, the 2nd pupil partial area | region of a 2nd focus detection pixel, and the exit pupil of an imaging optical system. It is a graph which shows the relationship between the conversion coefficient K for every imaging | photography optical system, and image height. FIG. 6 is a graph showing the change in image height of the conversion coefficient for each aperture value of the lens when using a lens having the largest change in the conversion coefficient for each image height. It is a flowchart explaining a series of processes from the start to the end of continuous shooting. 6 is a flowchart illustrating a series of processes from the start to the end of continuous shooting in a digital camera that is a second embodiment of the imaging apparatus of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing an outline of a system configuration example of a digital camera which is a first embodiment of an imaging apparatus of the present invention.

  As shown in FIG. 1, the digital camera of the present embodiment (hereinafter referred to as a camera) has a first lens group 101 that constitutes a photographing optical system. The first lens group 101 is disposed on the most object side of the photographing optical system, and is held so as to be movable back and forth in the optical axis direction. The aperture / shutter 102 adjusts the aperture diameter to adjust the amount of light during shooting, and also functions as an exposure time adjustment shutter during still image shooting. The second lens group 103 moves forward and backward in the optical axis direction integrally with the diaphragm / shutter 102, and performs a zoom operation in conjunction with the forward / backward movement of the first lens group 101.

  The third lens group 105 is a focus lens that adjusts the focus by moving forward and backward in the optical axis direction. The optical low-pass filter 106 is an optical element for reducing false colors and moire in the captured image. The image sensor 107 includes a two-dimensional CMOS photosensor and a peripheral circuit, and has an image plane on which a subject light beam that has passed through the photographing optical system forms an image.

  The zoom actuator 111 rotates the cam cylinder (not shown) to drive the first lens group 101 or the second lens group 103 forward and backward in the optical axis direction, and performs a zoom operation. The aperture shutter actuator 112 controls the aperture diameter of the aperture / shutter 102 to adjust the amount of photographing light, and 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 strobe unit 115 is a light emitting device using a xenon tube, an LED, or the like, and illuminates a subject at the time of shooting. The AF auxiliary light source 116 projects an image of a mask having a predetermined aperture pattern onto a subject field via a projection lens, and improves focus detection capability for a dark subject or a low-contrast subject.

  The system control circuit 121 includes a CPU that controls the entire camera, a memory such as a ROM and a RAM, an A / D converter, a D / A converter, and a communication interface circuit. Then, the CPU of the system control circuit 121 drives various circuits of the camera based on a predetermined program stored in the ROM, and executes a series of operations such as AF, imaging, image processing, and recording.

  Further, the ROM or the like of the system control circuit 121 stores correction value calculation coefficients necessary for focus adjustment using an output signal of the image sensor 107 described later. This correction value calculation coefficient includes a focus state corresponding to the position of the third lens group 105, a zoom state corresponding to the positions of the first lens group 101 to the second lens group 103, the F value of the photographing optical system, and the imaging element 107. A plurality is prepared for each set pupil distance and pixel size.

  When performing focus adjustment, an optimum correction value calculation coefficient is selected according to the combination of the focus adjustment state (focus state, zoom state) and aperture value of the photographing optical system, the pupil distance set for the image sensor 107, and the pixel size. . Then, a correction value is calculated from the selected correction value calculation coefficient and the image height of the image sensor 107.

  In the present embodiment, the correction value calculation coefficient is stored in the ROM or the like of the system control circuit 121. For example, in an interchangeable lens imaging apparatus, the correction value calculation coefficient is stored in the ROM or the like of the interchangeable lens. You may do it. In this case, a correction value calculation coefficient may be transmitted to the camera according to the focus adjustment state of the photographing optical system.

  The strobe control circuit 122 controls lighting of the light emitting unit of the strobe unit 115 in synchronization with the photographing operation. The auxiliary light driving circuit 123 controls the lighting of the AF auxiliary light source 116 in synchronization with the focus detection operation. The image sensor drive circuit 124 controls the image capturing operation of the image sensor 107, A / D converts the acquired image signal, and transmits it to the system control circuit 121. The image processing circuit 125 performs processes such as γ conversion, color interpolation, and JPEG compression of the image acquired by the image sensor 107.

  The focus drive circuit 126 controls the focus actuator 114 based on the focus detection result, and adjusts the focus by driving the third lens group 105 back and forth in the optical axis direction. The aperture shutter drive circuit 128 controls the aperture of the aperture / shutter 102 by drivingly controlling the aperture shutter actuator 112. The zoom drive circuit 129 drives the zoom actuator 111 according to the zoom operation of the photographer.

  The display unit 131 is configured by an LCD or the like, and displays information related to a camera shooting mode, a preview image before shooting and a confirmation image after shooting, a frame of a focus detection area at the time of focus detection, an image in a focused state, and the like. . The operation switch group 132 includes a power switch, a release (shooting trigger) switch, a zoom operation switch, a shooting mode selection switch, and the like. The recording medium 133 is composed of, for example, a removable memory card or the like, and records the acquired image.

  Next, with reference to FIG. 2, the arrangement of the imaging pixels including the focus detection pixels of the imaging element 107 will be described. FIG. 2 is a schematic diagram illustrating an array of imaging pixels of the imaging element 107 configured by a two-dimensional CMOS sensor in a range of 4 columns × 4 rows (an array of focus detection pixels is a range of 8 columns × 4 rows).

  In FIG. 2, an imaging pixel group 200 of 2 columns × 2 rows has an imaging pixel 200R having a spectral sensitivity of R (red) on the upper left, and an imaging pixel 200G having a spectral sensitivity of G (green) on the upper right and lower left. Imaging pixels 200B having a spectral sensitivity of B (blue) are arranged at the lower right. Each imaging pixel has a first focus detection pixel 201 and a second focus detection pixel 202 arranged in 2 columns × 1 row.

  A large number of 4 columns × 4 rows of imaging pixels (8 columns × 4 rows of focus detection pixels) are arranged on the surface to enable acquisition of imaging signals (focus detection signals). In the image sensor 107 of the present embodiment, the period P of the imaging pixels is 4 μm, the number N of the imaging pixels is 5575 columns × 3725 rows = approximately 20.75 million pixels, the column direction period PAF of the focus detection pixels is 2 μm, and the number of focus detection pixels It is assumed that NAF is 11150 columns wide × 3725 rows tall = about 41.5 million pixels.

  3A is a plan view of one imaging pixel 200G of the image sensor 107 as viewed from the light receiving surface side (+ z side) of the image sensor 107, and FIG. 3B is a cross-sectional view taken along the line aa in FIG. It is sectional drawing which looked at from the -y side.

  As shown in FIG. 3, in the imaging pixel 200G, a microlens 305 for collecting incident light is formed on the light receiving side of each imaging pixel, and NH division (two divisions) is performed in the x direction, and in the y direction. A photoelectric conversion unit 301 and a photoelectric conversion unit 302 that are divided into NVs (one division) are formed. The photoelectric conversion unit 301 and the photoelectric conversion unit 302 correspond to the first focus detection pixel 201 and the second focus detection pixel 202, respectively.

  The photoelectric conversion unit 301 and the photoelectric conversion unit 302 may be a pin structure photodiode in which an intrinsic layer is sandwiched between a p-type layer and an n-type layer, or an intrinsic layer may be omitted and a pn junction if necessary. A photodiode may be used.

  In each imaging pixel, a color filter 306 is formed between the microlens 305, the photoelectric conversion unit 301, and the photoelectric conversion unit 302. If necessary, the spectral transmittance of the color filter may be changed for each sub-pixel, or the color filter may be omitted. The light incident on the imaging pixel 200 </ b> G is collected by the microlens 305, dispersed by the color filter 306, and then received by the photoelectric conversion unit 301 and the photoelectric conversion unit 302.

  In the photoelectric conversion unit 301 and the photoelectric conversion unit 302, a pair of electrons and holes are generated according to the amount of received light and separated by a depletion layer, and then negatively charged electrons are accumulated in an n-type layer (not shown). The light is discharged out of the image sensor 107 through a p-type layer connected to a constant voltage source (not shown). Electrons accumulated in the n-type layer (not shown) of the photoelectric conversion unit 301 and the photoelectric conversion unit 302 are transferred to the electrostatic capacitance unit (FD) through the transfer gate and converted into a voltage signal.

  Next, the correspondence between the pixel structure shown in FIG. 3 and pupil division will be described with reference to FIG. FIG. 4 is a cross-sectional view of the cross section taken along the line aa of the pixel structure shown in FIG. 3A from the + y side and a schematic view showing the exit pupil plane of the imaging optical system. In FIG. 4, the x axis and the y axis are inverted with respect to FIG. 3B in order to correspond to the coordinate axis of the exit pupil plane.

  In FIG. 4, the first pupil partial region 501 of the first focus detection pixel 201 has a substantially conjugate relationship by the light receiving surface of the photoelectric conversion unit 301 whose center of gravity is decentered in the −x direction and the microlens 305. A pupil region that can be received by the first focus detection pixel 201 is shown. The first pupil partial region 501 of the first focus detection pixel 201 has an eccentric center of gravity on the + X side on the pupil plane.

  On the other hand, the second pupil partial region 502 of the second focus detection pixel 202 has a substantially conjugate relationship by the light receiving surface of the photoelectric conversion unit 302 whose center of gravity is decentered in the + x direction and the microlens 305, and the second focus detection. A pupil region that can be received by the pixel 202 is shown. The second pupil partial region 502 of the second focus detection pixel 202 has an eccentric center of gravity on the −X side on the pupil plane.

  The pupil region 500 is a pupil region that can receive light in the entire pixel 200G when the photoelectric conversion unit 301 (first focus detection pixel 201) and the photoelectric conversion unit 302 (second focus detection pixel 202) are all combined. The imaging plane phase difference AF is affected by diffraction because pupil division is performed using the microlens 305 of the imaging element 107.

  In FIG. 4, the pupil distance to the exit pupil plane is several tens of mm, whereas the diameter of the microlens 305 is several μm. Therefore, the aperture value of the microlens 305 becomes several tens of thousands, and diffraction blur of several tens mm level is generated. Therefore, the image of the light receiving surface of the photoelectric conversion units 301 and 302 does not become a clear pupil region or pupil partial region, but has a pupil intensity distribution (incident angle distribution of light reception rate).

  FIG. 5 is a schematic diagram showing the correspondence between the image sensor 107 and pupil division. The light beams that have passed through different pupil partial regions of the first pupil partial region 501 and the second pupil partial region 502 are incident on the respective imaging pixels of the image sensor 107 at different angles, and are divided into 2 × 1 first. Light is received by the focus detection pixel 201 and the second focus detection pixel 202. In the present embodiment, the pupil region is divided into two pupils in the horizontal direction, but pupil division may be performed in the vertical direction as necessary.

  In the present embodiment, an imaging element 107 in which a plurality of imaging pixels each having a first focus detection pixel 201 and a second focus detection pixel 202 are arranged is used. The first focus detection pixel 201 receives a light beam passing through the first pupil partial region 501 of the photographing optical system, and the second focus detection pixel 202 is a second pupil portion of the photographing optical system different from the first pupil partial region 501. A light beam passing through the region 502 is received. In addition, the imaging pixel receives a light beam that passes through a pupil region that is a combination of the first pupil partial region 501 and the second pupil partial region 502 of the photographing optical system.

  The first focus detection signal 201 is collected by collecting the light reception signals of the first focus detection pixels 201 of the respective imaging pixels of the image sensor 107, and the second focus is collected by collecting the light reception signals of the second focus detection pixels 202 of the respective imaging pixels. A detection signal is generated to perform focus detection. Further, for each image pickup pixel of the image pickup element 107, an image pickup signal (captured image) having a resolution of N effective pixels is generated by adding the light reception signals of the first focus detection pixel 201 and the second focus detection pixel 202. .

  In the present embodiment, each imaging pixel of the imaging element 107 is configured by the first focus detection pixel 201 and the second focus detection pixel 202. However, the imaging pixel, the first focus detection pixel, The second focus detection pixel may have an individual pixel configuration. For example, the first focus detection pixel and the second focus detection pixel may be partially arranged in a part of the array of imaging pixels.

  Next, the relationship between the defocus amount and the image shift amount of the first focus detection signal and the second focus detection signal acquired by the image sensor 107 will be described with reference to FIG. FIG. 6 is a schematic diagram illustrating the relationship between the defocus amounts of the first focus detection signal and the second focus detection signal and the image shift amount between the first focus detection signal and the second focus detection signal.

  In FIG. 6, the exit pupil of the photographing optical system is divided into two parts, a first pupil partial region 501 and a second pupil partial region 502, as in FIGS. 4 and 5. The defocus amount d is the distance from the imaging position of the subject to the imaging surface 800 of the image sensor 107 having a magnitude | d |, and the front pin state where the imaging position of the subject is closer to the subject side than the imaging surface 800 is a negative sign. (D <0). Further, a rear pin state in which the imaging position of the subject is on the opposite side of the subject from the imaging surface 800 is defined as a positive sign (d> 0).

  An in-focus state where the imaging position of the subject is on the imaging surface 800 (in-focus position) is d = 0. FIG. 5 shows an example in which the subject 801 is in focus (d = 0), and the subject 802 is in the front pin state (d <0). The front pin state (d <0) and the rear pin state (d> 0) are combined to form a defocus state (| d |> 0).

  In the front pin state (d <0), the luminous flux that has passed through the first pupil partial area 501 (second pupil partial area 502) out of the luminous flux from the subject 802 is once condensed and then the gravity center position G1 of the luminous flux. The image spreads in the width Γ 1 (Γ 2) with (G 2) as the center, resulting in a blurred image on the imaging surface 800. The blurred image is received by the first focus detection pixel 201 (second focus detection pixel 202) constituting each image pickup pixel arranged in the image sensor 107, and thereby the first focus detection signal (second focus detection signal). ) Is generated.

  Therefore, the first focus detection signal (second focus detection signal) is recorded as a subject image in which the subject 802 is blurred by the width Γ1 (Γ2) at the gravity center position G1 (G2) on the imaging surface 800. The blur width Γ1 (Γ2) of the subject image increases substantially proportionally as the magnitude | d | of the defocus amount d increases.

  Similarly, the magnitude | p | of the object image displacement amount p (= difference G1-G2 in the center of gravity of the light beam) between the first focus detection signal and the second focus detection signal is also the size of the defocus amount d. As | d | increases, it increases approximately proportionally. Even in the rear pin state (d> 0), the image shift direction of the subject image between the first focus detection signal and the second focus detection signal is opposite to that in the front pin state, but the same.

  Therefore, in this embodiment, as the magnitude of the defocus amount of the imaging signal obtained by adding the first focus detection signal and the second focus detection signal or the first focus detection signal and the second focus detection signal increases, The magnitude of the image shift amount p between the one focus detection signal and the second focus detection signal increases.

  In the focus detection of the phase difference detection method of the present embodiment, the first focus detection signal and the second focus detection signal are relatively shifted to calculate the correlation amount indicating the degree of coincidence of the signals, and the shift amount that improves the correlation amount. From this, the image shift amount p is detected. As the magnitude of the defocus amount d of the imaging signal increases, the image shift amount p is converted into a conversion coefficient because of the relationship that the image shift amount p between the first focus detection signal and the second focus detection signal increases. To convert the detection defocus amount into focus detection.

  FIG. 7 is a flowchart for explaining focus detection processing of the imaging surface phase difference detection method. Each process in FIG. 7 is executed when a program stored in the ROM or the like of the system control circuit 121 is expanded in the RAM and the CPU or the like controls the image processing circuit 125.

  In FIG. 7, in step S701, the system control circuit 121 sets a focus detection area centered on the image height (X, Y) for focus adjustment from the effective pixel area of the image sensor 107, and the process proceeds to step S702. move on. In step S702, the system control circuit 121 generates a first focus detection signal from the light reception signal of the first focus detection pixel 201 in the focus detection area, and the second focus from the light reception signal of the second focus detection pixel 202 in the focus detection area. A detection signal is generated, and the process proceeds to step S703.

  In step S703, the system control circuit 121 performs pixel addition processing on each of the first focus detection signal and the second focus detection signal, and the process proceeds to step S704. Specifically, for the first focus detection signal and the second focus detection signal, a three-pixel addition process is performed in the column direction in order to suppress the amount of signal data, respectively, and further, the RGB signal is converted into a luminance signal. A Bayer (RGB) addition process is performed. These two addition processes are combined into a pixel addition process.

  In step S704, the system control circuit 121 performs shading correction processing (optical correction processing) on the first focus detection signal and the second focus detection signal, respectively, and the process proceeds to step S705.

  Here, with reference to FIG. 8, the change of the conversion coefficient necessary for conversion from the image shift amount p due to the pupil shift of the first focus detection signal and the second focus detection signal to the detected defocus amount and the shading correction processing will be described. To do. 8 shows baseline lengths BL0, BL1, and BL2, the first pupil partial region 501 of the first focus detection pixel 201 at the peripheral image height of the image sensor 107, the second pupil partial region 502 of the second focus detection pixel 202, and imaging. It is a figure which shows the relationship of the exit pupil 400 of an optical system.

  FIG. 8A shows a case where the exit pupil distance Dl of the photographing optical system and the set pupil distance Ds of the image sensor are the same. When the exit pupil distance Dl is the same as the set pupil distance Ds of the image sensor 107, the set pupil distance Ds of the image sensor 107 is the first focus detection signal and the second focus in any image pickup pixel of the image sensor 107. This is a parameter that makes the detected signal intensity substantially the same. This parameter is a value determined by the positional relationship between the photoelectric conversion units 301 and 302 and the microlens 305.

  When the exit pupil distance Dl of the imaging optical system and the set pupil distance Ds of the image sensor are the same, the exit pupil 400 of the imaging optical system is substantially equally divided by the first pupil partial region 501 and the second pupil partial region 502. Is done. The base line length, which is the distance between the center of gravity of the first pupil partial region 501 and the center of gravity of the second pupil partial region 502, inside the exit pupil 400 is indicated by BL0. At this time, the conversion coefficient K0 necessary for conversion from the image shift amount p to the detected defocus amount is obtained by K0 = Ds / BL0.

  On the other hand, FIG. 8B shows a case where the exit pupil distance Dl of the photographing optical system is shorter than the set pupil distance Ds of the image sensor 107. In this case, at the peripheral image height of the imaging device 107, a pupil shift occurs between the exit pupil 400 of the imaging optical system and the entrance pupil of the imaging device 107, and the exit pupil 400 of the imaging optical system is non-uniformly pupil-divided. Accordingly, the base line length is BL1 biased to one side, and accordingly, the conversion coefficient K1 = Ds / BL1.

  FIG. 8C shows a case where the exit pupil distance Dl of the photographing optical system is longer than the set pupil distance Ds of the image sensor 107. Also in this case, similarly to FIG. 8B, at the peripheral image height of the image sensor 107, a pupil shift occurs between the exit pupil 400 of the imaging optical system and the entrance pupil of the image sensor 107, and the exit pupil 400 of the imaging optical system is The pupil is divided unevenly. Therefore, the base line length is BL2 biased to the opposite side of FIG. 8B, and accordingly, the conversion coefficient K2 = Ds / BL2.

  Further, as the pupil division becomes nonuniform at the peripheral image height of the image sensor 107, the intensity of the first focus detection signal and the second focus detection signal also becomes nonuniform, and the first focus detection signal and the second focus detection are detected. Shading occurs in which the intensity of one of the signals increases and the intensity of the other decreases. Furthermore, it can be seen that when the aperture value of the photographing optical system changes, the size of the exit pupil 400 in FIG. 8 changes, so that the conversion coefficient K and shading also change according to the aperture value.

  Therefore, the conversion coefficient K and the shading from the image shift amount p to the detected defocus amount depend on the aperture value and exit pupil distance Dl of the photographing optical system, the pupil intensity distribution (optical characteristics) of the image sensor 107, and the image height. You can see that it changes.

  Therefore, the system control circuit 121 determines the first shading correction coefficient and the second focus detection of the first focus detection signal according to the image height of the focus detection area of the image sensor 107, the F value of the photographing optical system, and the exit pupil distance Dl. A second shading correction coefficient of the signal is generated respectively. Then, the first focus detection signal is multiplied by the first focus detection signal, the second shading correction coefficient is multiplied by the second focus detection signal, and shading correction processing (optical) of the first focus detection signal and the second focus detection signal is performed. Correction process).

  In focus detection using the imaging surface phase difference detection method, the detection defocus amount is detected based on the correlation amount (signal coincidence) between the first focus detection signal and the second focus detection signal. When shading due to pupil shift occurs, the correlation amount (the degree of signal coincidence) between the first focus detection signal and the second focus detection signal may decrease. Therefore, in the imaging surface phase difference type focus detection, in order to improve the correlation amount (signal coincidence) between the first focus detection signal and the second focus detection signal and to improve the focus detection performance, the shading correction processing ( It is desirable to perform optical correction processing.

  Here, the pupil shift has been described in an example in which the set pupil distance Ds of the image sensor 107 is the same and the exit pupil distance Dl of the photographing optical system changes, but conversely, the exit pupil of the photographing optical system. The same applies when the distance Dl is the same and the set pupil distance Ds of the image sensor 107 changes. In focus detection using the imaging surface phase difference detection method, a light beam received by a focus detection pixel (first focus detection pixel, second focus detection pixel) and an imaging pixel receive light as the set pupil distance Ds of the image sensor 107 changes. The luminous flux that changes also changes.

  In step S705 of FIG. 7, the system control circuit 121 performs a specific pass with respect to the first focus detection signal and the second focus detection signal in order to improve the focus detection accuracy by improving the correlation amount (signal coincidence). A band-pass filter process having a frequency band is performed, and the process proceeds to step S706. Examples of band-pass filters include differential filters such as (1, 0, −1) that perform edge extraction by cutting DC components, and addition types such as (1, 2, 1) that suppress high-frequency noise components. There is a filter.

  In step S706, the system control circuit 121 performs a shift process for relatively shifting the first focus detection signal and the second focus detection signal after the filter process in the pupil division direction, and calculates a correlation amount indicating the degree of coincidence of the signals. Then, the process proceeds to step S707.

  Here, the k-th first focus detection signal after filtering is A (k), the second focus detection signal is B (k), and the range of number k corresponding to the focus detection area is W. The correlation amount COR is calculated by the following equation (1), where s is the shift amount by the shift process and Γ is the shift range of the shift amount s.

  By shifting the shift amount s, the k-th first focus detection signal A (k) and the k-s-th second focus detection signal B (k-s) are matched and subtracted to generate a shift subtraction signal. The absolute value of the generated shift subtraction signal is calculated, the sum of the numbers k is calculated within the range W corresponding to the focus detection area, and the correlation amount COR (s) is calculated. If necessary, the correlation amount calculated for each row may be added over a plurality of rows for each shift amount.

  In step S <b> 707, the system control circuit 121 calculates a real-valued shift amount at which the correlation amount correlation amount COR (s) is the minimum value from the correlation amount COR (s) by sub-pixel calculation to obtain an image shift amount p. . The system control circuit 121 detects the image shift amount p by multiplying the image height of the focus detection area, the F value of the photographing optical system, and the conversion coefficient K corresponding to the exit pupil distance Dl of the photographing optical system. The defocus amount (Def) is detected, and the focus detection process is terminated. Thereafter, the system control circuit 121 drives the lens until the detected defocus amount (Def) is equal to or smaller than a predetermined value, and when it is equal to or smaller than the predetermined value, calculates an exposure amount and performs an imaging operation.

  Next, the relationship between the conversion coefficient K and the focus detection performance will be described with reference to FIG. FIG. 9 is a graph showing the relationship between the conversion coefficient K and the image height for each photographing optical system (photographing lens). As described above, the conversion coefficient K is determined by the relationship between the exit pupil distance D1 of the photographing optical system, the set pupil distance Ds of the image sensor 107, and the image height. Depending on the photographing optical system, the conversion coefficient K is large even at the same image height. May be different. As the value of the conversion coefficient K increases, the focus detection performance tends to decrease.

  As described above, the detected defocus amount (Def) is calculated by multiplying the image shift amount p by the conversion coefficient K. However, if an error occurs in the image shift amount p due to noise or the like, the conversion coefficient By applying K, the error is expanded K times. For this reason, the focus detection performance decreases as the enlargement ratio, that is, the conversion coefficient K increases. In this way, the conversion coefficient K can be used as an index for performance when performing focus detection.

  Next, focus detection when performing continuous shooting will be described with reference to FIGS. In order to perform continuous shooting in a so-called small aperture state, in order to perform focus detection with higher accuracy, it is desirable to perform focus detection with an open aperture value that takes as long a baseline length as possible. In this case, as described above, the conversion coefficient K can be used as an index of focus detection performance.

  Referring to FIG. 9, lenses A, B, and C that are imaging optical systems have different exit pupil distances D1, and the relationship between the image height, the exit pupil distance Dl, and the set pupil distance Ds of the image sensor 107. Therefore, the conversion coefficient K is different for each image height. Since the focus detection accuracy varies depending on the conversion coefficient K, in the present embodiment, in order to keep the focus detection performance above a certain level, the conversion coefficient K is controlled by providing a threshold value Kth.

  FIG. 10 is a graph showing the change in the image height of the conversion coefficient K for each aperture value of the lens A when, for example, the lens A in FIG. 9 having the largest change in the conversion coefficient K for each image height is used. Since the conversion coefficient K is derived from the base line length on the exit pupil plane of the photographing optical system, the conversion coefficient K is determined by the F value. At this time, the smaller the aperture value, the longer the base line length can be taken, and the value of the conversion coefficient K becomes smaller.

  Here, as described above, it is assumed that the threshold Kth of the conversion coefficient K for maintaining the focus detection performance at a certain level or more is 1. From FIG. 10, since the conversion coefficient K is equal to or less than the threshold value Kth even in the small aperture state of F11 in the vicinity of the image height of 0 mm, that is, in the vicinity of the central image height, focus detection can be performed with good focus detection performance. On the other hand, when the image height exceeds 5 mm, the conversion coefficient K exceeds the threshold value Kth, and the focus detection performance decreases. For this reason, it can be seen that it is necessary to perform focus detection at F2.8, which is full aperture.

  As described above, by setting the conversion coefficient K with the threshold value Kth and selecting whether focus detection is performed in a small aperture state based on the threshold value Kth or when the focus detection is performed after the aperture is fully opened, a focus of a certain level or more is obtained. Small aperture continuous shooting can be performed while maintaining detection performance.

  FIG. 11 is a flowchart for explaining a series of processes from the start to the end of continuous shooting. Each process in FIG. 11 is executed by a CPU or the like by developing a program stored in the ROM or the like of the system control circuit 121 in the RAM.

  In FIG. 11, in step S1101, the system control circuit 121 determines an exposure condition for focus detection, and proceeds to step S1102. In step S1102, the system control circuit 121 acquires the conversion coefficient K when performing focus detection, and proceeds to step S1103.

  The conversion coefficient K acquired here is a system control of a conversion coefficient table in which values corresponding to optical conditions such as image height, F value, exit pupil distance D1 of the imaging optical system, and set pupil distance Ds of the image sensor 107 are calculated in advance. The data is stored in the ROM of the circuit 121 or the like. Alternatively, in each case, the system control circuit 121 may calculate and use the conversion coefficient from optical conditions such as the image height, F value, exit pupil distance D1, and set pupil distance Ds of the image sensor 107.

  In addition, information on so-called frame vignetting by a front frame and a rear frame of a lens that is a photographing optical system may be used. For example, the conversion coefficient K is calculated from optical conditions such as the exit pupil distance D1, the set pupil distance Ds of the image sensor 107, and the image height, and the conversion coefficient at the peripheral image height is used there by using frame vignetting information for each lens. If correction is applied to, a more accurate conversion coefficient K can be calculated. Further, the switching of the aperture driving by the conversion coefficient K can be made more accurate.

  In step S1103, the system control circuit 121 determines whether or not the conversion coefficient K is equal to or less than the threshold value Kth (less than or equal to the threshold value). If the conversion coefficient K is equal to or less than the threshold value Kth, the process proceeds to step S1104, where the conversion coefficient K is equal to the threshold value Kth. If it exceeds, the process proceeds to step S1108. In step S1104, since it can be said that there is sufficient focus detection performance, the system control circuit 121 performs focus detection with the aperture value, and proceeds to step S1105.

  In this state, since the in-focus state is already good, the system control circuit 121 performs an imaging operation in step S1106, determines whether or not continuous shooting is ended in step S1107, and ends the processing if it is ended. If not finished, the process returns to step S1101.

  On the other hand, in step S1108, the system control circuit 121 sets the aperture value where the conversion coefficient K is equal to or less than the threshold value Kth or the full aperture value, and proceeds to step S1109. By setting the aperture value to the full aperture value, the conversion coefficient K can be lowered as much as possible, and good focus detection performance can be obtained.

  In step S1009, the system control circuit 121 determines the exposure condition for the aperture value set in step S1008, and the process proceeds to step S1010. The system control circuit 121 detects the focus in step S1010, drives the lens in step S1011, returns the aperture value set in step S1008 to the shooting aperture value in step S1012, Imaging is performed in S1106.

  As described above, in the present embodiment, since focus detection can be performed while maintaining the focus detection performance at a certain level or more, good focus detection can be performed even in small aperture continuous shooting, and during continuous shooting. It is possible to obtain an image in a good focus state.

(Second Embodiment)
Next, a digital camera which is a second embodiment of the imaging apparatus of the present invention will be described with reference to FIG. In addition, about the part which overlaps with respect to the said 1st Embodiment, a difference is mainly demonstrated, using a figure and a code | symbol.

  In the first embodiment, the aperture value at the time of focus detection is determined according to the comparison result between the conversion coefficient K and the threshold value Kth in step S1103 in FIG. 11, but the present invention is not limited to this. For example, when performing focus detection, the aperture value for the next focus detection may be determined in consideration of the defocus amount detected each time. In focus detection by the imaging surface phase difference detection method, the larger the aperture value, the clearer the resulting image becomes and the clearer the image, the better the focus detection performance when the defocus amount is large.

  FIG. 12 is a flowchart for explaining a series of processes from the start to the end of continuous shooting. Each process in FIG. 12 is executed by a CPU or the like after a program stored in the ROM or the like of the system control circuit 121 is expanded in the RAM. Note that the processing in steps S1201 to S1212 in FIG. 12 is the same as the processing in steps S1101 to S1102 in FIG. 11 of the first embodiment, and therefore step S1203a will be described.

  Even if the conversion coefficient K exceeds the threshold value Kth in step S1203, if the detected defocus amount Def detected immediately before or exceeds the threshold value Dth exceeds the threshold value Dth, focus detection is performed with a small aperture. It is more accurate to go.

  The smaller the conversion coefficient K is, the better the high detection performance is in the vicinity of the in-focus state, but this is not always the case when the detected defocus amount Def is large.

  Therefore, in step S1203a, if the detected defocus amount Def exceeds the threshold value Dth, the system control circuit 121 proceeds to step S1204 to perform focus detection with a small aperture, and if the detected defocus amount Def is less than the threshold value Dth, the system control circuit 121 proceeds to step S1208. move on. If the aperture value for focus detection is determined in this way, focus detection can be performed satisfactorily in any defocus state. Other configurations and operational effects are the same as those in the first embodiment.

  In addition, this invention is not limited to what was illustrated by said each embodiment, In the range which does not deviate from the summary of this invention, it can change suitably.

  By the way, in recent years, when performing continuous shooting, an increasing number of cameras can be used to select whether a photographer places importance on focusing accuracy or continuous shooting speed. For this reason, when the photographer places importance on the focusing accuracy, when the conversion coefficient K exceeds the threshold value Kth, the aperture value is changed to the threshold value Kth or less, and when the continuous shooting speed is to be emphasized, the aperture drive itself is omitted. Therefore, the aperture may be taken by performing focus detection while keeping the aperture value for shooting. As a result, when the photographer places importance on focusing accuracy or when focusing on continuous shooting speed, the appropriate aperture drive is performed, and continuous shooting with a balance between focusing accuracy and continuous shooting speed is performed. Is possible.

  In each of the above-described embodiments, the method for determining the aperture value for focus detection when performing continuous shooting has been described. However, the present invention is not limited to continuous shooting, and single image shooting may be performed.

  Further, the application range of the present invention can be applied to all of an imaging device for capturing a still image, an imaging device for capturing a moving image, or an imaging device for capturing both. For example, the present invention can be applied to an imaging apparatus that performs image blur correction such as a general single-lens reflex camera, a compact camera, a video camera, or a surveillance camera.

  The present invention can also be realized by executing the following processing. That is, 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 the computer of the system or apparatus read the program. It can also be realized by processing to be executed. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

101 First lens group 102 Shutter combined shutter 103 Second lens 105 Third lens 107 Image sensor 112 Aperture shutter actuator 121 System control circuit 124 Image sensor drive circuit 125 Image processing circuit 128 Aperture shutter drive circuit

Claims (9)

Imaging in which a plurality of first focus detection pixels and second focus detection pixels that receive a light beam passing through a first pupil partial region and a second pupil partial region different from the first pupil partial region of the photographing optical system are arranged. Elements,
Generating means for generating a first focus detection signal based on the light reception signal of the first focus detection pixel, and generating a second focus detection signal based on the light reception signal of the second focus detection pixel;
An image shift amount is calculated based on the first focus detection signal and the second focus detection signal, and the calculated image shift amount, the optical condition of the shooting optical system at the time of shooting, and the optical condition of the imaging element are calculated. A focus detection means for detecting a defocus amount from the conversion coefficient calculated in response,
Setting means for setting a threshold for the conversion coefficient;
An imaging apparatus comprising: a determination unit that determines an aperture value at the time of focus detection based on a result of comparing the conversion coefficient and the threshold set by the setting unit.   The said conversion coefficient is selected from the conversion coefficient table previously recorded on memory according to the optical condition of the said imaging optical system at the time of imaging | photography, and the optical condition of the said image pick-up element. Imaging device.   The imaging apparatus according to claim 1, wherein the optical condition of the photographing optical system includes an exit pupil distance of the photographing optical system.   The imaging apparatus according to claim 1, wherein the optical condition of the photographing optical system is frame vignetting information of the photographing optical system.   5. The image pickup apparatus according to claim 1, wherein the optical condition of the image pickup element is a set pupil distance of the image pickup element.   The imaging apparatus according to claim 1, further comprising a selection unit that allows a photographer to select an aperture value at the time of focus detection according to a shooting mode.   7. The determination unit according to claim 1, wherein, when the conversion coefficient exceeds the threshold value, the aperture value at the time of focus detection is set to an aperture value such that the conversion coefficient is equal to or less than the threshold value. The imaging device according to any one of the above.   The determining means sets the aperture value at the time of focus detection to an aperture value at which the conversion coefficient is equal to or less than the threshold value when the conversion coefficient exceeds the threshold value and the detected defocus amount is equal to or less than the threshold value. The imaging apparatus according to claim 1, wherein the imaging apparatus is set. An imaging device in which a plurality of first focus detection pixels and second focus detection pixels that receive a light beam that passes through a first pupil partial region and a second pupil partial region different from the first pupil partial region of an imaging optical system are arranged An imaging apparatus control method comprising:
Generating a first focus detection signal based on the light reception signal of the first focus detection pixel, and generating a second focus detection signal based on the light reception signal of the second focus detection pixel;
An image shift amount is calculated based on the first focus detection signal and the second focus detection signal, and the calculated image shift amount, the optical condition of the shooting optical system at the time of shooting, and the optical condition of the imaging element are calculated. A focus detection step for detecting a defocus amount from a conversion coefficient calculated according to the
A setting step for setting a threshold for the conversion coefficient;
And a determination step of determining an aperture value at the time of focus detection based on a result of comparing the conversion coefficient with the threshold value set in the setting step.

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