JP2013037143A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve focus detection accuracy for a low brightness object at any distance measurement position without increasing a distance measurement time, regardless of an aperture value and the kind of a lens of a photographic lens.SOLUTION: An imaging apparatus comprises: an imaging element in which imaging pixels receiving a flux of light passing through an exit pupil of a photographic lens and focus detection pixels receiving fluxes of the light passing through partial areas of the exit pupil of the photographic lens are arranged; a focus detection section that detects a defocus amount of the photographic lens by a phase difference method using output signals of the focus detection pixels; an acquisition section that acquires aperture information of the photographic lens; a threshold setting section that sets a prescribed threshold on the basis of the acquired aperture information and positions of the focus detection pixels; a reading section that reads pixel signals from the imaging element with a predetermined period; an addition section that adds the pixel signals of the focus detection pixels read from the imaging element with the predetermined period; a comparison section that compares the added addition signal with the prescribed threshold; and a control section that ends the adding process when the comparison determines that the level of the addition signal exceeds the prescribed threshold. The imaging apparatus detects the defocus amount of the photographic lens using the addition signal for which the adding process has ended.

Description

  The present invention relates to an imaging apparatus.

  As an image pickup apparatus that performs image pickup and focus detection using an image pickup element having an image pickup pixel and a focus detection pixel, those shown in Patent Document 1 and Patent Document 2 are known. Japanese Patent Application Laid-Open No. 2004-133867 acquires image signal outputs of imaging pixels and focus detection pixels from an image sensor, and attempts to achieve both improvement of the refresh rate of the display device and focus detection accuracy of a low-luminance subject.

  Japanese Patent Application Laid-Open No. 2004-228561 describes defocus information on the image shift amount of a focus detection pixel based on the distribution of luminous flux of the focus detection pixel, the exit pupil of the aperture of the photographing lens, and the aperture information of the exit pupil corresponding to the lens end. It is to convert to.

JP 2008-85738 A JP 2007-121896 A

  In Patent Document 1, pixel signal outputs of imaging pixels and focus detection pixels are acquired from an imaging element operated at a predetermined frame rate. Using the acquired image signals of the imaging pixels, display data for the display device is sequentially created and the display is updated to ensure the frame rate of the display device. On the other hand, for the image signal of the focus detection pixel, the maximum value P is compared with the threshold level Pth, and if the maximum value P does not exceed Pth, it is added to the focus detection signal of the previous frame. Then, the focus detection pixel signal is added retroactively until the maximum value P of the addition output satisfies P> Pth, thereby improving the focus detection accuracy of the low-luminance subject.

  Further, as described in Patent Document 2, the light beam incident on the focus detection pixel described in Patent Document 1 is generated by the exit pupil of the stop of the photographing lens and the exit pupil corresponding to the lens end portion. Different vignetting occurs depending on the change of the aperture value, the distance of the focus detection pixel from the photographic lens optical axis (hereinafter referred to as the image height position), and the type of the interchangeable lens. At the same time, when the centroid distance between the two images changes due to the occurrence of vignetting and vignetting occurs, the focus detection accuracy is deteriorated even if the image signal levels of the focus detection pixels are the same.

  Therefore, in Patent Document 1, frame addition is performed until the maximum value of the focus detection signal becomes equal to or higher than a predetermined threshold level, and focus detection accuracy is improved. It does not set a threshold level. For this reason, if the threshold value is determined at the center distance measuring point at the center of the optical axis with high accuracy, the accuracy of the peripheral distance measuring points at the position where the image height is high cannot be sufficiently ensured. Further, when the threshold value is determined at the peripheral ranging point with the lowest accuracy, the frame is unnecessarily added at the central ranging point, and the ranging time is unnecessarily long. Further, there is no description about the case where the aperture value of the photographing lens changes or the correspondence to various interchangeable lenses.

  The present invention has been made in view of the above-described problems, and its object is to achieve low luminance without increasing the distance measurement time at all distance measurement positions regardless of the aperture value of the photographing lens and the type of lens. It is to improve the focus detection accuracy of the subject.

  In the imaging apparatus according to the present invention, an imaging pixel that receives a light beam that has passed through an exit pupil of a photographing lens and a focus detection pixel that receives a light beam that has passed through a partial region of the exit pupil of the photographing lens are arranged. An imaging device, a focus detection unit that detects a defocus amount of the photographing lens by a phase difference method using an output signal of the focus detection pixel, an acquisition unit that acquires aperture information of the photographing lens, and the acquisition From the aperture information acquired by the means and the position of the focus detection pixel, threshold setting means for setting a predetermined threshold, reading means for reading out a pixel signal at a predetermined cycle from the image sensor, and from the image sensor Adder for adding pixel signals of the focus detection pixels read out at the predetermined period, and a comparator for comparing the addition signal added by the adder with the predetermined threshold And a control means for ending addition by the adding means when the level of the addition signal exceeds the predetermined threshold by comparison of the comparing means, and the focus detecting means is determined by the adding means. The defocus amount of the photographing lens is detected using the addition signal after the addition.

  According to the present invention, it is possible to improve the focus detection accuracy of a low-luminance object without lengthening the distance measurement time at all the distance measurement points regardless of the aperture value of the photographing lens and the type of lens. .

The block diagram of the camera concerning one Embodiment of this invention. The block diagram of an image sensor. The top view and sectional drawing of the pixel for an imaging of an image pick-up element. The top view and sectional drawing of the pixel for focus detection of an image sensor. The top view and sectional drawing of the pixel for focus detection of an image sensor. FIG. 3 is an enlarged view of an arrangement region of focus detection pixels of the image sensor. The figure explaining the vignetting of the pixel for focus detection, and the gravity center space | interval of a focus detection light beam. The figure which shows the relationship between the gravity center space | interval of a focus detection light beam, and the effective diameter of a taking lens. The figure which shows the addition method of the pixel for a focus detection of an image pick-up element. The figure explaining the vignetting of the pixel for focus detection, and the gravity center space | interval of a focus detection light beam.

  FIG. 1 is a block diagram showing an electrical configuration of a “single-lens reflex digital camera system” which is an embodiment of an imaging apparatus of the present invention. As shown in FIG. 1, in the digital camera 100 of this embodiment, a photographic lens unit 200 is detachably attached via a mount mechanism (not shown). The mount portion has an electrical contact group 210. The contact group 210 has a function of transmitting control signals, status signals, data signals, and the like between the camera body 100 and the photographing lens unit 200 and supplying currents of various voltages. Further, it has a function of transmitting a signal to the system controller 120 when the photographing lens unit 200 is connected. As a result, communication can be performed between the digital camera 100 and the photographic lens unit 200 to drive the photographic lens 201 and the diaphragm 202 in the photographic lens unit. The contact group 210 may be configured to transmit not only electrical communication but also optical communication, voice communication, and the like. In the present embodiment, the photographing lens 201 is shown as a single lens for convenience, but it is well known that it is actually composed of a larger number of lenses. The contact group 210 and the system controller 120 constitute lens detection means.

  An imaging light beam from a subject image (not shown) is guided to a quick return mirror 102 that can be driven in a direction indicated by an arrow in FIG. The central portion of the quick return mirror 102 is a half mirror, and a part of the light beam is transmitted when the quick return mirror 102 is lowered. Then, the transmitted light beam is reflected downward by the sub mirror 103 installed on the quick return mirror 102.

  Reference numeral 104 denotes a well-known phase difference AF sensor unit including a field lens, a reflection mirror, a secondary imaging lens, a diaphragm, a line sensor composed of a plurality of CCDs, and the like disposed in the vicinity of the imaging surface. is there. The focus detection circuit 105 controls the AF sensor unit 104 based on a control signal from the system controller 120 to perform focus detection by a known phase difference detection method. The AF sensor unit 104 and the focus detection circuit 105 constitute a focus detection unit.

  On the other hand, the photographing light beam reflected by the quick return mirror 102 reaches the eyes of the photographer via the pentaprism 101 and the eyepiece 106. A photometric sensor (not shown) disposed in the vicinity of the eyepiece 106 is a sensor for measuring the luminance of the subject, and its output is supplied to the system controller 120 via the photometric circuit 107. The photometric sensor, the photometric circuit 107 and the system controller 120 constitute a photometric means.

  When the quick return mirror 102 is raised, the light beam from the photographing lens 201 reaches the image sensor 112 typified by a CMOS or the like as an image sensor through a focal plane shutter 108 and a filter 109 that are mechanical shutters. .

  The filter 109 has two functions, one is a function of cutting infrared rays and guiding only visible light to the image sensor 112, and the other is a function as an optical low-pass filter. The focal plane shutter 108 has a front curtain and a rear curtain, and controls transmission and blocking of a light beam from the photographing lens 201. The sub mirror 103 is folded when the quick return mirror 102 is up.

  The digital camera 100 according to the present embodiment is a control unit for the entire digital camera, and includes a system controller 120 including a CPU that controls the control, and appropriately controls the operation of each unit described below. The system controller 120 corresponds to correction means.

  The system controller 120 controls a lens control circuit 204 that controls a lens driving mechanism 203 for moving the photographing lens 201 in the optical axis direction and focusing, and an aperture driving mechanism 205 for driving the diaphragm 202. A diaphragm control circuit 206 is connected via a lens control microcomputer 207. The system controller 120 is connected to a shutter charge / mirror drive mechanism 110 that controls the up / down drive of the quick return mirror 102 and the shutter charge of the focal plane shutter 108. The system controller 120 is connected to a shutter control circuit 111 for controlling the traveling of the front curtain and rear curtain of the focal plane shutter 108. Further, an EEPROM 122 and the like are connected to the system controller 120. The EEPROM 122 is a storage means for storing parameters that need to be adjusted to control the digital camera 100, camera ID information that enables individual identification of the digital camera, AF correction data adjusted by a reference lens, automatic exposure correction values, and the like. It is.

  The lens control microcomputer 207 also includes a lens storage device that stores lens-specific information, for example, information such as focal length, wide aperture, lens ID assigned to each lens, and information received from the system controller 120. In addition, the system controller 120 controls the lens driving mechanism 203 via the lens control microcomputer 207 to form a subject image on the image sensor 112. Further, based on the set Av value, the diaphragm drive mechanism 205 that drives the diaphragm 202 is controlled, and furthermore, based on the set Tv value, a control signal is output to the shutter control circuit 111 to perform exposure control. Do.

  The drive source of the front curtain and rear curtain of the focal plane shutter 108 is constituted by a spring, and a spring charge is required for the next operation after the shutter travels. The shutter charge / mirror drive mechanism 110 controls the spring charge. Further, the click return mirror 102 is raised and lowered by the shutter charge / mirror drive mechanism 110.

  An image data controller 115 is connected to the system controller 120. The image data controller 115 is constituted by a DSP (digital signal processor). Then, control of the image sensor 112 and correction and processing of image data input from the image sensor 112 are executed based on a command from the system controller 120. Auto white balance is also included in the image data correction and processing items. Auto white balance is a function that corrects a portion of maximum brightness in a captured image to a predetermined color (white). In auto white balance, the correction amount can be changed by a command from the system controller 120.

  Further, the system controller 120 and the image data controller 115 constitute second photometric means. The second photometric means divides the image signal into regions by the image data controller 115, supplies the values integrated for each Bayer pixel in each region to the system controller 120, and evaluates the integrated signal by the system controller 120. Perform photometry.

  The image data controller 115 is connected with a timing pulse generation circuit 114, an A / D converter 113, a DRAM 121, a D / A converter 116, an image compression circuit 119, and a contrast detection circuit 140. The timing pulse generation circuit 114 outputs a pulse signal necessary for driving the image sensor 112. The A / D converter 113 receives the timing pulse generated by the timing pulse generation circuit 114 together with the image sensor 112, and converts an analog signal corresponding to the subject image output from the image sensor 112 into a digital signal. The DRAM 121 temporarily stores the obtained image data (digital data). The DRAM 121 is used for temporarily storing image data before processing or data conversion into a predetermined format.

  Further, a recording medium 401 serving as a recording unit is connected to the image compression circuit 119. The image compression circuit 119 is a circuit for performing compression and conversion (for example, JPEG) of image data stored in the DRAM 121. The converted image data is stored in the recording medium 401. As this recording medium, a hard disk, a flash memory, a floppy (registered trademark) disk or the like is used. The image data controller 115, the image compression circuit 119, and the recording medium 401 constitute a recording unit.

  In addition, an image display circuit 118 is connected to the D / A converter 116 via an encoder circuit 117. The image display circuit 118 is a circuit for displaying image data picked up by the image sensor 112, and is generally composed of a color liquid crystal display element.

  The image data controller 115 converts the image data on the DRAM 121 into an analog signal by the D / A converter 116 and outputs the analog signal to the encoder circuit 117. The encoder circuit 117 converts the output of the D / A converter 116 into a video signal (for example, NTSC signal) necessary for driving the image display circuit 118. The D / A converter 215, the image display circuit 118, and the encoder circuit 117 constitute image display means.

  Further, the system controller 120 and the image data controller 115 constitute second focus detection means. The image data controller 115 passes a filter having a predetermined frequency characteristic to the corrected image data, evaluates the contrast in a predetermined direction of an image signal obtained by performing a predetermined gamma process, and the result is sent to the system controller 120. Supplied. The system controller 120 communicates with the lens control circuit 204, adjusts the focal position, and adjusts the focal position so that the contrast evaluation value is higher than a predetermined level. The image data controller 115, the system controller 120, the lens control circuit 204, the lens driving mechanism 203, and the photographic lens 201 constitute a second automatic focus adjusting unit.

  Further, the system controller 120 includes an operation display circuit 123, a shooting mode selection button 130, a main electronic dial 131, a determination SW 132, a distance measurement point selection button 133 (corresponding to distance measurement point selection means), and an AF mode. A selection button 134 is connected. Further, a metering mode selection button 135, a release SW1 (136) for starting photographing preparation operations such as metering and distance measurement, a release SW2 (137) for starting an imaging operation, and a viewfinder mode selection SW138 are provided. It is connected. The operation display circuit 123 causes the external liquid crystal display device 124 and the internal liquid crystal display device 125 to display information on the operation mode of the digital camera, exposure information (shutter time, aperture value, etc.) and the like. The shooting mode selection button 130 is a button for setting a mode so that the user can perform a desired operation on the digital camera. The ranging point selection button 133 is a button for selecting a focus detection position to be used from a plurality of focus detection positions of the AF sensor unit 104. The release SW1 (136) is a switch for starting a photographing preparation operation such as photometry and distance measurement, and the release SW2 (137) is a switch for starting an imaging operation.

  The external liquid crystal display device 124 and the internal liquid crystal display device 125 correspond to operation display means, the external liquid crystal display device 124 corresponds to external display means, and the internal liquid crystal display device 125 corresponds to internal display means. Further, the operation display circuit 123 and the system controller 120 constitute display control means.

  The viewfinder mode selection SW 138 corresponds to viewfinder mode selection means for switching between the optical viewfinder mode and the live view display mode. In the optical finder mode, it is possible to check the light beam passing through the eyepiece lens 106. In the live view display mode, the elephant signal received by the image sensor 112 is sequentially displayed by the image display circuit 118.

  Furthermore, the digital camera 100 is detachably attached to the strobe device 300 via a mount mechanism (not shown). The mount portion has an electrical contact group 310. The contact group 310 includes an X terminal (light emitting terminal) that transmits a control signal, a status signal, a data signal, and the like between the camera body 100 and the flash device 300 and controls the light emission timing. Further, it has a function of transmitting a signal to the system controller 120 when the strobe device 300 is connected. Accordingly, communication between the digital camera 100 and the strobe device 300 can be performed to control the light emission of the strobe. The contact group 310 may be configured to transmit not only electrical communication but also optical communication, voice communication, and the like.

  The strobe device 300 generates a voltage of about 300 V to supply power to the light emission control circuit 303 and the Xe tube 301 configured by an xenon (Xe) tube 301, a reflective shade 302, an IGBT for controlling light emission of the Xe tube 301, and the like. A charging circuit 304 is provided. Furthermore, a power supply 305 such as a battery for supplying power to the charging circuit 304, a flash emission and charging, and the like, and a strobe control microcomputer 306 for controlling communication with the system controller 120 on the camera side are also provided.

  FIG. 2 is a block diagram of the image sensor. Note that the block diagram of FIG. 2 illustrates a minimum configuration that can explain the reading operation, and a pixel reset signal and the like are omitted. 2, 201 is a photoelectric conversion unit (hereinafter abbreviated as PDmn. M is an X-direction address, m = 0, 1,..., M−1, n are Y-direction addresses, and n = 0. , 1,..., N−1). The photoelectric conversion unit 201 includes a photodiode, a pixel amplifier, a reset switch, and the like. In the imaging device of the present embodiment, m × n photoelectric conversion units are two-dimensionally arranged. Since the code is complicated, it is added only in the vicinity of the upper left photoelectric conversion unit PD00.

  Reference numeral 202 denotes a switch for selecting the output of the photoelectric conversion unit PDmn, which is selected for each row by the vertical scanning circuit 208. Reference numeral 203 denotes a line memory for temporarily storing the output of the photoelectric conversion unit PDmn, which stores the output signal of the photoelectric conversion unit for one row selected by the vertical scanning circuit. Usually, a capacitor is used. A switch 204 is connected to the horizontal output line to reset the horizontal output line to a predetermined potential VHRST, and is controlled by a signal HRST.

  Reference numeral 205 denotes a switch for sequentially outputting the output of the photoelectric conversion unit PDmn stored in the line memory 203 to the horizontal output line. The switch from H0 to Hm-1 is sequentially scanned by the horizontal scanning circuit 206 described later. Thus, the photoelectric conversion output signal for one row is read out.

  Reference numeral 206 denotes a horizontal scanning circuit, which sequentially scans the output of the photoelectric conversion unit stored in the line memory and outputs it to a horizontal output line. The signal PHST is a data input of the horizontal scanning circuit, PH1 and PH2 are shift clock inputs, and data is set when PH1 = H, and data is latched at PH2. Then, by inputting a shift clock to PH1 and PH2, PHST can be sequentially shifted and the switches from H0 to Hm-1 can be sequentially turned on. SKIP is a control terminal input for setting during thinning readout. By setting the SKIP terminal to the H level, the horizontal scanning circuit can be skipped at predetermined intervals.

  Reference numeral 208 denotes a vertical scanning circuit, which can select the selection switch 202 of the photoelectric conversion unit PDmn by sequentially scanning and outputting V0 to Vn-1. The control signal is controlled by the data input PVST, the shift clocks PV1 and PV2, and the thinning reading setting SKIP as in the horizontal scanning circuit. Since the operation is the same as that of the horizontal scanning circuit, detailed description thereof is omitted. In the figure, the control signal is not shown.

  3, 4, and 5 are diagrams illustrating the structures of the imaging pixels and the focus detection pixels. In the present embodiment, among the 2 × 2 four pixels, pixels having G (green) spectral sensitivity are arranged in two diagonal pixels, and R (red) and B (blue) spectral are arranged in the other two pixels. A Bayer arrangement in which one pixel having sensitivity is arranged is employed. In addition, focus detection pixels having a structure described later are distributed and arranged in a predetermined rule between the Bayer arrays.

  FIG. 3 shows the arrangement and structure of the imaging pixels. FIG. 3A is a plan view of 2 × 2 imaging pixels. As is well known, in the Bayer array, G pixels are arranged diagonally, and R and B pixels are arranged in the other two pixels. The 2 rows × 2 columns structure is repeatedly arranged.

  FIG. 3B shows a cross section AA of FIG. ML is an on-chip microlens disposed on the forefront of each pixel, CFR is an R (red) color filter, and CFG is a G (green) color filter. PD is a schematic diagram showing the photoelectric conversion unit of the C-MOS sensor, and CL is a wiring layer for forming signal lines for transmitting various signals in the C-MOS sensor. TL schematically shows the photographing optical system.

  Here, the on-chip microlens ML and the photoelectric conversion unit PD of the imaging pixel are configured to capture the light flux that has passed through the photographing optical system TL as effectively as possible. In other words, the exit pupil EP of the photographing optical system TL and the photoelectric conversion unit PD are conjugated with each other by the microlens ML, and the effective area of the photoelectric conversion unit is designed to be large. 3B, the incident light flux of the R pixel has been described, but the G pixel and the B (blue) pixel have the same structure. Accordingly, the exit pupil EP corresponding to each of the RGB pixels for imaging has a large diameter, and the S / N of the image signal is improved by efficiently capturing the light flux from the subject.

  FIG. 4 shows the arrangement and structure of focus detection pixels for performing pupil division in the horizontal direction (lateral direction) of the photographing optical system. Here, the definition of the horizontal direction or the horizontal direction indicates a direction along a straight line orthogonal to the optical axis and extending in the horizontal direction when the camera is held so that the optical axis of the photographing optical system is horizontal. FIG. 4A is a plan view of pixels of 2 rows × 2 columns including focus detection pixels. When obtaining an image signal for recording or viewing, the main component of luminance information is acquired with G pixels. Since human image recognition characteristics are sensitive to luminance information, image quality degradation is easily recognized when G pixels are lost. On the other hand, the R or B pixel is a pixel that acquires color information (color difference information). However, since human visual characteristics are insensitive to color information, the pixel that acquires color information may have some loss. Image quality degradation is difficult to recognize. Therefore, in the present embodiment, among the pixels of 2 rows × 2 columns, the G pixel is left as an imaging pixel, and the R and B pixels are replaced with focus detection pixels. This is indicated by SHA and SHB in FIG.

  FIG. 4B shows a cross section AA in FIG. The microlens ML and the photoelectric conversion unit PD have the same structure as the imaging pixel shown in FIG. In the present embodiment, since the signal of the focus detection pixel is not used for image creation, a transparent film CFW (White) is arranged instead of the color separation color filter. Moreover, since pupil division is performed by the image sensor, the opening of the wiring layer CL is biased in one direction with respect to the center line of the microlens ML. Specifically, since the opening OPHA of the pixel SHA is biased to the right side, the light beam that has passed through the left exit pupil EPHA of the imaging optical system TL is received. Similarly, since the opening OPHB of the pixel SHB is biased to the left side, the light beam that has passed through the right exit pupil EPHB of the imaging optical system TL is received. Therefore, the pixels SHA are regularly arranged in the horizontal direction, and the subject image acquired by these pixel groups is defined as an A image. Also, the pixels SHB are regularly arranged in the horizontal direction, and when the subject image acquired by these pixel groups is a B image, the relative positions of the A image and the B image are detected, and the conversion coefficient is calculated for the image shift amount. By multiplying by, the focus shift amount (defocus amount) of the subject image can be calculated.

  In the pixels SHA and SHB, focus detection is possible for an object having a luminance distribution in the horizontal direction of the shooting screen, for example, a vertical line, but focus detection is not possible for a horizontal line having a luminance distribution in the vertical direction. Therefore, in the present embodiment, pixels that perform pupil division are also provided in the vertical direction (longitudinal direction) of the photographing optical system so that the focus can be detected for the latter.

  FIG. 5 shows the arrangement and structure of focus detection pixels for performing pupil division in the vertical direction of the photographing optical system (in other words, the vertical direction or the vertical direction). Here, the definition of the vertical direction or up / down or vertical / horizontal direction refers to a direction along a straight line perpendicular to the optical axis and extending in the vertical direction when the camera is set so that the optical axis of the photographing optical system is horizontal. FIG. 5A is a plan view of pixels of 2 rows × 2 columns including focus detection pixels. As in FIG. 4A, the G pixel is left as an imaging pixel, and the R and B pixels are in focus. Detection pixels are used. This is indicated by SVC and SVD in FIG.

  A cross section AA of FIG. 5A is shown in FIG. 5B. The pixel of FIG. 4B has a structure in which pupil separation is performed in the horizontal direction, whereas the pixel of FIG. The pupil separation direction is just the vertical direction, and the pixel structure does not change. That is, since the opening OPVC of the pixel SVC is biased downward, the light beam that has passed through the upper exit pupil EPVC of the imaging optical system TL is received. Similarly, since the opening OPVD of the pixel SVD is biased upward, the light beam that has passed through the lower exit pupil EPVD of the imaging optical system TL is received. Therefore, the pixels SVC are regularly arranged in the vertical direction, and the subject image acquired by these pixel groups is defined as a C image. In addition, the pixels SVD are also regularly arranged in the vertical direction, and if the subject image acquired by these pixel groups is a D image, it has a luminance distribution in the vertical direction by detecting the relative position of the C image and the D image. A focus shift amount (defocus amount) of the subject image can be detected.

  FIG. 6 is a diagram illustrating an arrangement of imaging pixels and focus detection pixels. In FIG. 6, G is a pixel coated with a green filter, R is a pixel coated with a red filter, and B is a pixel coated with a blue filter. SHA in the figure is a focus detection pixel formed by horizontally deviating the aperture of the pixel portion, and a reference pixel group for detecting the amount of horizontal image deviation from the SHB pixel group described later. It is. Further, SHB is a pixel formed by deviating the opening of the pixel in the direction opposite to that of the SHA pixel, and is a reference pixel group for detecting a horizontal image shift amount with the SHA pixel group. The white portions of the SHA and SHB pixels indicate the opening positions of the biased pixels.

  Here, how to obtain the conversion coefficient for calculating the defocus amount from the image shift amount will be described. The conversion coefficient can be calculated based on the aperture information of the imaging optical system and the sensitivity distribution of the focus detection pixels. The image sensor 112 receives a light beam limited by several constituent members such as the lens holding frame of the photographing lens TL and the diaphragm 202. FIG. 7 is a diagram showing a state in which the light beam used for focus detection is limited by the vignetting of the photographing optical system. FIG. 7 shows a state in which the light beam is limited by the stop 702 of the imaging optical system at the position of the exit pupil plane 701 with respect to the pixel near the center of the image sensor. In FIG. 7A, 703 and 704 are image sensors (703 is a planned imaging plane position), 705 is an optical axis, 706 is an optical axis position on the image sensor, and 707 and 708 are limited by a diaphragm. The luminous fluxes 709 and 710 in the case indicate luminous fluxes when the luminous flux is not limited. The focus detection light beams 711 and 712 with respect to the light beams 707 and 708 are indicated by 711 and 712, and the barycentric positions of the focus detection light beams are indicated by 715 and 716, respectively. Similarly, the focus detection light beams 713 and 714 with respect to the light beams 709 and 710 are indicated by 713 and 714, and the gravity center positions of the focus detection light beams are indicated by 717 and 718, respectively.

  FIG. 7B is a diagram showing a change in the position of the center of gravity due to vignetting on the exit pupil plane 1 of the focus detection pixel at the center of the image sensor. In FIG. 7B, 723 and 724 indicate the pupil regions of the limited light beams 707 and 708 and the non-limited light beams 709 and 710 with respect to the central pixel of the image sensor, and 725 and 726 indicate focus detection. The incident angle characteristics of the pixels SHA and SHB are shown. Light beams that have passed through the shapes indicated by 721 and 722 are incident on the focus detection pixels SHA and SHB with sensitivity distributions indicated by 723 and 724. Therefore, by calculating the distribution centroids of the focus detection light fluxes that have passed through the insides of the shapes indicated by 723 and 724, the centroid distance between when the light flux used for focus detection is limited and when it is not limited is determined. be able to. By obtaining and storing the sensitivity distribution information of the focus detection pixels and the aperture information of the imaging optical system from measurement and calculation, a conversion coefficient for calculating the defocus amount can be obtained from the image shift amount. .

  In FIG. 7A, the defocus amount 719 is DEF, the distance 720 from the image sensor to the exit pupil plane 701 is L, and the center-of-gravity interval when the light beam used for focus detection is limited and when it is not limited, respectively. G1 (distance between 715 and 716), G2 (distance between 717 and 718), image displacement amounts are PRED1 (721) and PRED2 (722), and conversion coefficients for converting image displacement amounts into defocus amounts are K1 and K2. In this case, the defocus amount is obtained by the following equation.

DEF = K1 × PRED1 = K2 × PRED2
Conversion coefficients K1 and K2 for converting the image shift amount into the defocus amount are obtained by the following equations, respectively.

K1 = L / G1
K2 = L / G2
Therefore, by changing the aperture, the G value changes and the defocus detection accuracy changes.

  FIG. 8 is a diagram showing a change in G of the effective diameter D of the stop formed on the exit pupil and the conversion coefficient K that changes with the stop (L does not change even if the stop changes). In FIG. 8, the vertical axis represents the relative values of the effective diameters D and G. The dotted line in the figure represents the effective diameter D, and the solid line represents the relative change in G. The reference aperture size is the aperture value of 724 in FIG. 7 and corresponds to, for example, the aperture of F5.6. Further, the stop 723 in FIG. 7 corresponds to, for example, F11. The horizontal axis is the F value (FNo). The figure shows that the effective diameter D is inversely proportional to FNo. Further, even if the aperture is brighter than 724, G is restricted by the sensitivity distribution of the focus detection pixels and does not increase to 1 or more. Also, at an aperture value darker than 724, the sensitivity distribution of the substantially elliptical shape of the focus detection pixels is vignetted by the circular aperture, so that the change in G shows a greater decrease than the decrease in effective diameter.

Usually, in an imaging apparatus such as a camera, if the allowable circle of confusion is δ, the defocus detection accuracy DEF required to satisfy this is
DEF = Fδ
Thus, the required detection defocus accuracy may be increased by reducing F (= D / L). That is, if the result of dividing the value of G by the effective diameter D is 1 or more, the defocus detection accuracy does not deteriorate. The one-dot chain line in FIG. 8 is obtained by dividing the value of G by the effective diameter D. As can be seen from the figure, the defocus detection accuracy decreases as the aperture is made brighter than 724 and the aperture is made darker than 724. Therefore, in order not to deteriorate the defocus detection accuracy, it is necessary to improve the SN of the focus detection pixel to compensate for the accuracy deterioration due to the change in G.

  FIG. 9 is a diagram for explaining a frame addition operation for improving the SN of the focus detection pixel. In the present embodiment, it is assumed that the focus detection operation can be performed while the lens moves to the in-focus position, and FIG. 9 shows a state in which the lens approaches the in-focus position as time elapses. Yes. In FIG. 9, the time elapsed for reading out the image signal is shown at the top. The predetermined period for reading the image signal is Ts, the current frame read cycle is n, and the current frame time is nTs. The display is refreshed with the latest image data read in each frame. The image data of the focus detection pixels read out in each frame is stored in a stack in the internal memory. The image signal of the focus detection pixel read out at the nth read-out of the image signal is stored as a focus detection pixel signal a (n). As described above, a pair of pixel data is obtained from each pixel group of SHA and SHB as image data obtained in the focus detection area. In performing addition processing of image data, the number of added frames is controlled by an image having a smaller difference (peak-bottom value: hereinafter referred to as PB value) between the maximum value and the minimum value of each image data. Therefore, here, the image data with the smaller PB value is described as a (n). When the addition number of image data (hereinafter referred to as the frame addition number) is determined from the image data with the smaller PB value, the same number of image data is added even with the image data with the larger PB value.

  As a method of the addition processing, first, the PB value of the image data of the focus detection pixel in the current frame is compared with the threshold value PBth. And In FIG. 9, it is indicated at time nTs, and since the PB value of the focus detection pixel signal a (n) exceeds the threshold value PBth, a (n) is used as the addition pixel signal A (n). .

  In addition, when the focus detection pixel signal of the latest frame does not exceed PBth, the image data of the focus detection pixels acquired in the past frame is sequentially added, and the point when the PB value of the addition pixel signal exceeds PBth Then, the addition process is terminated, and the addition result is set as an addition pixel signal. In FIG. 9, the time (n-2) Ts is indicated. At time (n−1) Ts, the PB value of the focus detection pixel signal b (n−1) does not exceed the threshold value PBth, and the focus detection pixel signal c (n−2) acquired in the previous frame is used. When added to b (n−1), the PB value of the added pixel signal exceeds the threshold value PBth. Therefore, b (n−1) + c (n−2) is used as the addition pixel signal B (n−1).

  In addition, an upper limit is set for the number of frame additions, and when more than the upper limit is required, addition data up to the upper limit is used as the addition pixel signal. In FIG. 9, the upper limit of the number of frame additions is 3, which is indicated by time (n−2) Ts. The above is the method of addition processing.

  As described above, the SN of the focus detection pixel can be improved by adding the focus detection pixels over a plurality of frames. Furthermore, in order to prevent the detection accuracy from deteriorating due to the change of the stop, it is possible to prevent the detection accuracy from deteriorating by setting (threshold setting) the frame addition threshold PBth according to the stop of the taking lens. . For example, the threshold value may be set by multiplying the threshold level at F5.6 by the square of F / G. The reason for multiplying the square of F / G is that the SN ratio is improved by the route of the signal amount S. F and G are the F value of the photographing lens being accumulated by frame addition and the center of gravity interval of the focus detection pixels at that time.

  By setting the threshold in this way, it is possible to maintain the defocus detection accuracy regardless of the aperture value of the photographing lens. Further, it is possible to set an optimal number of frames to be added according to the F value of the photographing lens, and it is possible to improve the focus detection accuracy of a low-luminance subject without unnecessarily increasing the distance measurement time.

  FIG. 10 shows a state in which the luminous flux is limited by the lens holding frame and the diaphragm with respect to the pixel at the position having the image height from the center of the image sensor. Moreover, the same code | symbol is provided to the component same as FIG. In FIG. 10A, 703 and 704 are image sensors, 705 is an optical axis, and 1001 is an optical axis position on the image sensor. Reference numerals 1002 and 1003 denote light beams when the light beam is limited by the lens holding frame 730 that is closest to the image sensor and the lens holding frame 731 that is closest to the subject. The focus detection light fluxes 1004 and 1005 with respect to the light fluxes 1002 and 1003 are indicated by 1004 and 1005, and the gravity center positions of the focus detection light fluxes are indicated by 1006 and 1007.

  FIG. 10B is a diagram showing the position of the center of gravity due to vignetting on the exit pupil plane 1 of the focus detection pixel at a position having an image height from the center of the image sensor. In FIG. 10B, reference numeral 1008 denotes a pupil region of limited light beams 1002 and 1003 with respect to a pixel having an image height from the center of the image sensor, and reference numerals 1009 and 1010 denote incidences of focus detection pixels SHA and SHB. Angular characteristics are shown. Light beams that have passed through the inside of the shape indicated by 1008 are incident on the focus detection pixels SHA and SHB with sensitivity distributions indicated by 1009 and 1010. Therefore, by determining the distribution centroids of the focus detection light fluxes that have passed through the inside of the shape indicated by 1008, the centroid distance when the light flux used for focus detection is limited by the lens holding frame can be obtained. By obtaining and storing the sensitivity distribution information of the focus detection pixels and the aperture information of the imaging optical system from measurement and calculation, a conversion coefficient for calculating the defocus amount can be obtained from the image shift amount. .

  Also, reference numerals 723 and 724 shown in FIG. 10B indicate the pupil regions of the light flux limited by the stop 702 described in FIG. When the stop is viewed from a pixel at a high image height, the circular stop has a shape like an ellipse. As is apparent from the figure, the pixel at the position having the image height is limited by the lens holding frame when the aperture is brighter than the predetermined aperture, and by the aperture when the aperture is darker than the predetermined aperture. Which factor is dominant is determined by the position and size of the lens holding frame, the size position of the entrance pupil of the taking lens, and in an interchangeable lens system such as a single lens reflex, for each lens, Must be set. Needless to say, in a photographing lens such as a zoom lens, it is necessary to set for each zoom position.

  In FIG. 10A, when the luminous flux is limited by the lens holding frame, the defocus amount 719 is DEF, the distance 720 from the imaging surface of the image sensor 703 to the exit pupil plane 701 is L, and the luminous flux used for focus detection is the lens. G3 (distance between 1006 and 1007), the image shift amount PRED3 (1011), and the conversion coefficient for converting the image shift amount into the defocus amount are set to K3. The defocus amount is obtained by the following formula.

DEF = K3 x PRED3
A conversion coefficient K3 for converting the image shift amount into the defocus amount is obtained by the following equation.

K3 = L / G3
Further, when the stop is stopped more than a predetermined range, the light flux is limited by the stop, and the center-of-gravity interval becomes shorter than the center of the optical axis depending on the image height position. When the image shift amount is PRED4 and the conversion coefficient for converting the image shift amount into the defocus amount is K4, the defocus amount is obtained by the following equation.

DEF = K4 x PRED4
A conversion coefficient K4 for converting the image shift amount into the defocus amount is obtained by the following equation.

K4 = L / G4
When the image height position becomes high, it is necessary to set a frame addition threshold in consideration of the lens holding frame in addition to the aperture value to prevent the focus detection accuracy from deteriorating. In addition, even if the aperture is the same, the center-of-gravity interval differs at the image height position, so it is necessary to set a threshold value in consideration of this. Since the setting can be made in the same way as the method described with reference to FIG. 7, the description is omitted.

  Furthermore, in recent image sensors, the ISO sensitivity is further improved, and a setting exceeding ISO 12800 is possible. When shooting with ISO sensitivity set to high sensitivity, the SN of the pixel itself deteriorates, and the focus detection accuracy decreases. Therefore, it is desirable to set the threshold value in consideration of the ISO sensitivity in addition to the aperture information described above. In other words, if the ISO is increased by one stage, it is needless to say that the deterioration of the focus detection accuracy can be prevented by setting the threshold value to twice and the frame addition number to be doubled.

Claims (7)

An imaging device in which an imaging pixel that receives a light beam that has passed through an exit pupil of a photographing lens and a focus detection pixel that receives a light beam that has passed through a partial region of the exit pupil of the photographing lens;
Focus detection means for detecting a defocus amount of the photographing lens by a phase difference method using an output signal of the focus detection pixel;
Acquisition means for acquiring aperture information of the photographing lens;
Threshold setting means for setting a predetermined threshold based on the aperture information acquired by the acquisition means and the position of the focus detection pixel;
Reading means for reading out pixel signals from the image sensor at a predetermined period;
Adding means for adding pixel signals of the focus detection pixels read out from the image sensor at the predetermined period;
Comparing means for comparing the addition signal added by the adding means with the predetermined threshold;
Control means for ending addition by the addition means when the level of the addition signal exceeds the predetermined threshold by comparison of the comparison means,
The imaging apparatus according to claim 1, wherein the focus detection unit detects a defocus amount of the photographing lens by using an addition signal that has been added by the addition unit.   The imaging apparatus according to claim 1, wherein the aperture information of the photographing lens includes information on an F value of the photographing lens.   The imaging apparatus according to claim 1, wherein the aperture information of the photographing lens includes information on an exit pupil of the photographing lens.   The imaging apparatus according to claim 3, wherein the information on the exit pupil is information on a distance from an imaging surface of the imaging element to the exit pupil.   5. The image pickup apparatus according to claim 1, wherein the aperture information of the photographing lens includes information of a lens holding frame of the photographing lens.   The imaging apparatus according to claim 5, wherein the information on the lens holding frame is information on a position and a size of the lens holding frame.   The imaging apparatus according to claim 1, wherein the threshold setting unit further sets the predetermined threshold using information on ISO sensitivity.