Patent Publication Number: US-8976289-B2

Title: Imaging device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a PCT Bypass continuation application and claims the priority benefit under 35 U.S.C. §120 of PCT Application No. PCT/JP2011/059257 filed on Apr. 14, 2011 which application designates the U.S., and also claims the priority benefit under 35 U.S.C. §119 of Japanese Patent Application No. 2010-103903 filed on Apr. 28, 2010, which applications are all hereby incorporated in their entireties by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging device and especially relates to a technique of performing automatic focus adjustment using a phase difference detection scheme. 
     2. Description of the Related Art 
     Japanese Patent Application Laid-Open No. 2002-258142 discloses an imaging device that performs focus detection in a phase difference detection scheme, performs pixel combination of four adjacent right, left, up and down pixels in the case of low brightness to output them and performs focusing using this output signal. 
     Japanese Patent Application Laid-Open No. 2006-324760 discloses an imaging device that performs pixel combination by performing adjustment of larger pixel numbers in the case of lower brightness and performs contrast AF based on a light receiving signal after the pixel combination. 
     SUMMARY OF INVENTION 
     However, in the invention disclosed in Japanese Patent Application Laid-Open No. 2002-258142, pixels in the right and left directions are subjected to pixel combination in the case of lower brightness, and therefore there is a problem that the accuracy of AF processing degrades. Also, no processing applies to the signal of each pixel in the case of higher brightness, and therefore there is a problem that it is not possible to reduce noise. 
     Also, in the invention disclosed in Japanese Patent Application Laid-Open No. 2006-324760, because of a contrast AF scheme, there is a problem that it takes a long time to perform AF processing in the phase detection scheme. 
     The present invention is made in view of the above, and it is an object to provide an imaging device that can shorten a time required for AF processing and perform an accurate focus adjustment at lower cost and with space-saving. 
     The imaging device of a first aspect of the present invention includes: an imaging optical system; an imaging element in which a first pixel and a second pixel are arranged in a two-dimensional manner, the first pixel receiving a light on a partial area biased to a predetermined direction from a light axis of a light flux passing an exit pupil of the imaging optical system, and the second pixel arranged so as to be adjacent to the first pixel and receiving a light on a partial area biased to an opposite direction to the predetermined direction from the light axis of the light flux passing the exit pupil of the imaging optical system; an imaging element drive unit configured to read signal charges output from the first pixel and the second pixel as a voltage signal, the imaging element drive unit configured to read and combine signal charges of a first number of adjacent pixels with respect to the first pixel and the second pixel; an arithmetic mean calculation unit configured to calculate an arithmetic mean of a second number of adjacent voltage signals with respect to voltage signals of the first pixel and the second pixel combined and read by the imaging element drive unit; and an automatic focus adjustment unit configured to detect a phase difference between a voltage signal of the first pixel and a voltage signal of the second pixel subjected to arithmetic mean in the arithmetic mean calculation unit, and to automatically perform a focus adjustment of the imaging optical system based on the phase difference. 
     According to the imaging device of the first aspect, when reading, as voltage signals, the signal charges output from the first pixel receiving the light on the partial area biased to the predetermined direction from the light axis of the light flux passing the exit pupil of the imaging optical system and the second pixel arranged so as to be adjacent to the first pixel and receiving the light on the partial area biased to the opposite direction to the predetermined direction from the light axis of the light flux passing the exit pupil of the imaging optical system, the signal charges of adjacent first-number pixels are combined and read with respect to the first pixel and the second pixel. The arithmetic mean of adjacent second-number voltage signals are calculated with respect to the combined and read voltage signals of the first pixel and the second pixel. Subsequently, a phase difference is detected between the voltage signal of the first pixel and the voltage signal of the second pixel subjected to arithmetic mean, and a focus adjustment of the imaging optical system is automatically performed based on this phase difference. By this means, by using both the pixel combination and the arithmetic mean, it is possible to reduce noise regardless of the subject brightness and improve the SN ratio. Therefore, it is possible to perform an accurate automatic focus adjustment. 
     Further, since the automatic focus adjustment (AF) is performed using the imaging element in which phase difference pixels formed with the first pixel and the second pixel are arranged, it is possible to shorten the time required for AF processing. Since this imaging element is similar to an imaging element used for conventional phase difference AF and new hardware is not required to implement the present invention, it is possible to perform an accurate focus adjustment at lower cost and with space-saving. 
     The imaging device of a second aspect of the present invention is configured such that, in the first aspect, the arithmetic mean calculation unit determines the first number and the second number such that a product of the first number and the second number is constant. 
     According to the imaging device of the second aspect, the first number and the second number are determined such that the product of the first number and the second number is constant, and therefore it is possible to maintain the size of an area in which a phase difference and a defocus amount are calculated, regardless of the subject brightness. Therefore, a stable automatic focus adjustment is possible. 
     The imaging device of a third aspect of the present invention is configured such that, in the first or second aspect, the imaging element drive unit combines the first number of signal charges adjacent in a vertical direction, and the arithmetic mean calculation unit calculates an arithmetic mean of the second number of voltage signals adjacent in a vertical direction. 
     According to the imaging device of the third aspect, the first-number pixels adjacent in the vertical direction are subjected to pixel combination. Also, the arithmetic mean is calculated for the second-numbers adjacent in the vertical direction, with respect to the read voltage signals subjected to pixel combination. Also, a phase difference is detected between the voltage signal of the first pixel and the voltage signal of the second pixel subjected to arithmetic mean, and a focus adjustment of the imaging optical system is automatically performed based on this phase difference. That is, the pixel combination and the arithmetic mean are performed only in the vertical direction, and none of the pixel combination and the arithmetic mean is performed in the horizontal direction. By this means, it is possible to reduce noise without degrading the accuracy of AF processing. Therefore, it is possible to perform an automatic focus adjustment more stably. 
     The imaging device of a fourth aspect of the present invention includes, in one of the first to third aspects, a brightness acquisition unit configured to acquire a brightness of a subject, wherein the imaging element drive unit sets the first number to 1 when the acquired brightness of the subject is equal to or greater than a predetermined brightness, and increases the first number as the measured brightness of the subject becomes lower. 
     According to the imaging device of the fourth aspect, the first number is set to 1 when the brightness of the subject is equal to or greater than the predetermined brightness, and the pixel combination is not performed. Also, the first number is increased as the brightness of the subject becomes lower, and the combined pixel number (pixel combination number) is increased. By this means, even if the brightness is low, it is possible to adequately reduce noise and improve the SN ratio. Therefore, even in a darker brightness area, an automatic focus adjustment is possible. 
     The imaging device of a fifth aspect of the present invention includes, in the fourth aspect, an area division unit configured to divide the imaging element into a plurality of areas when the acquired brightness of the subject is higher than a predetermined threshold, wherein: the arithmetic mean calculation unit calculates an arithmetic mean of the voltage signals for each of the divided areas; and the automatic focus adjustment unit detects a phase difference between the first-pixel voltage signal and the second-pixel voltage signal for each of the areas based on the calculated arithmetic mean of the voltage signals for each of the areas, and automatically performs a focus adjustment of the imaging optical system based on the most reliable phase difference among the phase differences detected in the areas. 
     According to the imaging device of the fifth aspect, when the brightness of the subject is higher than the predetermined threshold, the imaging element is divided into a plurality of areas, and the arithmetic mean of voltage signals is calculated for each of the divided areas. Based on the calculated arithmetic mean of electric signals for each of the areas, the phase difference between the first-pixel voltage signal and the second-pixel voltage signal is detected for each of the areas, and an automatic focus adjustment is performed based on the most reliable phase difference among the phase differences detected in these areas. By this means, it is possible to perform an automatic focus adjustment more accurately. 
     The imaging device of a sixth aspect of the present invention is configured such that, in the fourth or fifth aspect, the arithmetic mean calculation unit sets the second number to 2 when the measured brightness of the subject is equal to or greater than a predetermined brightness, and increases the second number as the measured brightness of the subject becomes lower. 
     According to the imaging device of the sixth aspect, the second number is set to 2 when the brightness of the subject is equal to or greater than the predetermined brightness, that is, an arithmetic mean of two voltage signals is calculated. Also, the second number is increased as the measured brightness of the subject becomes lower, and the number of voltage signals used to calculate the arithmetic mean is increased. By this means, it is possible to perform an accurate automatic focus adjustment in a darker brightness area. 
     According to the present invention, it is possible to shorten a time required for AF processing and perform an accurate focus adjustment at lower cost and with space-saving. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an internal configuration of a digital camera  10  according to the first embodiment of the present invention. 
         FIG. 2A  is a plan view illustrating a configuration example of an imaging element. 
         FIG. 2B  is a plan view enlarging and illustrating the imaging element in  FIG. 2A . 
         FIG. 3  is a flowchart illustrating automatic focus adjustment processing according to the first embodiment of the present invention. 
         FIG. 4A  is a pattern view illustrating pixel combination and arithmetic mean processing in the automatic focus adjustment processing according to the first embodiment of the present invention. 
         FIG. 4B  is a pattern view illustrating pixel combination and arithmetic mean processing in the automatic focus adjustment processing according to the first embodiment of the present invention. 
         FIG. 4C  is a pattern view illustrating pixel arithmetic mean processing in the automatic focus adjustment processing according to the first embodiment of the present invention. 
         FIG. 5  is a graph illustrating relationships between an Ev value, shutter speed and pixel combination number. 
         FIG. 6  is a flowchart illustrating automatic focus adjustment processing according to the second embodiment of the present invention. 
         FIG. 7A  is a pattern view illustrating pixel combination and arithmetic mean processing in the automatic focus adjustment processing according to the second embodiment of the present invention. 
         FIG. 7B  is a pattern view illustrating pixel combination and arithmetic mean processing in the automatic focus adjustment processing according to the second embodiment of the present invention. 
         FIG. 7C  is a pattern view illustrating pixel arithmetic mean processing in the automatic focus adjustment processing according to the second embodiment of the present invention. 
         FIG. 8A  is a flowchart illustrating automatic focus adjustment processing according to the third embodiment of the present invention. 
         FIG. 8B  is a (subsequent) flowchart illustrating the automatic focus adjustment processing according to the third embodiment of the present invention. 
         FIG. 9  is a pattern view illustrating pixel combination and arithmetic mean processing in the automatic focus adjustment processing according to the third embodiment of the present invention. 
         FIG. 10A  is a plan view illustrating a configuration example of a phase difference CCD. 
         FIG. 10B  is a plan view illustrating a primary pixel of the phase difference CCD. 
         FIG. 10C  is a plan view illustrating a secondary pixel of the phase difference CCD. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following, embodiments of a monocular stereoscopic imaging device according to the present invention will be explained with reference to the accompanying drawings. 
       FIG. 1  is a block diagram illustrating an embodiment of a digital camera to which an imaging device according to the first embodiment of the present invention is applied. 
     This digital camera  10  records an imaged image in a memory card  54  and the entire operation of the camera is integrally controlled by a CPU (Central Processing Unit)  40 . 
     The digital camera  10  is provided with an operation unit  38  including a shutter button, and a mode dial to set an imaging mode, playback mode, and so on. A signal based on an operation using this operation unit  38  is input in the CPU  40 . 
     The image light indicating a subject is formed on a light receiving surface of an imaging element  16  such as a CCD (Charge-Coupled Device) image sensor and a CMOS (Complementary Metal-Oxide Semiconductor) image sensor, via an imaging lens  12  and a diaphragm  14 . The imaging lens  12  is driven by a lens drive unit  36  controlled by the CPU  40  to perform focus control or the like. The diaphragm  14  is formed with, for example, five diaphragm blades, driven by a diaphragm drive unit  34  controlled by the CPU  40  and diaphragm-controlled in five levels at 1 AV intervals in aperture values of F2.8 to F11, for example. 
     Also, the CPU  40  controls the diaphragm  14  via the diaphragm drive unit  34 , controls charge accumulation time (i.e. shutter speed) in the imaging element  16  via an imaging element control unit  32  and controls a pixel combination described later. 
       FIG. 2A  is a view illustrating a configuration example of the imaging element  16 , and  FIG. 2B  is a view enlarging and illustrating a part (central area  16 C) of  FIG. 2A . The imaging element  16  has odd-numbered line pixels and even-numbered line pixels arranged in a matrix manner, and the odd-numbered line pixels are arranged and shifted by a half pitch in the line direction with respect to the even-numbered line pixels. 
     In the imaging element  16 , among pixels having color filters of R (Red), G (Green) and B (Blue), a pixel alignment line of GBGB . . . and a pixel alignment line of RGRG . . . are provided every two lines. In the enlarged view in  FIG. 2 , the pixel alignment line of GBGB . . . is provided in the first, second, fifth, sixth, ninth, tenth, thirteenth, fourteenth, seventeenth, eighteenth, twenty-first, twenty-second, . . . columns, and the pixel alignment line of RGRG . . . is provided in the third, fourth, seventh, eighth, eleventh, twelfth, fifteenth, sixteenth, nineteenth, twentieth . . . columns. Here, the color filter alignment is not limited to this, and it is possible to adopt honeycomb alignment in which the pixel alignment line of GBGB . . . and the pixel alignment line of RGRG . . . are alternately arranged, Bayer alignment or other alignments. 
     As illustrated in  FIG. 2B , in the central area (focus area)  16 C of the imaging element  16 , phase different pixels using an output signal as an AF pixel signal for phase difference computation are arranged in addition to normal pixels not having a pupil division function. The phase difference pixels are arranged only in G pixels and formed with pixels subjected to light in a partial area biased to a predetermined direction from the light axis of the light flux passing an exit pupil of the imaging lens  12  and pixels subjected to light in a partial area biased to an opposite direction to the predetermined direction from the light axis of the light flux passing the exit pupil of the imaging lens  12 . In the present embodiment, the phase difference pixels include left shift pixels B 11 , B 12 , . . . , whose right half is covered and which are subjected to light only on the left side of the light axis of the light flux passing an exit pupil, and right shift pixels A 11 , A 12 , . . . , whose left half is covered and which are subjected to light only on the right side of the light axis of the light flux passing the exit pupil. 
     The right shift pixels A 11 , A 12 , . . . and the left shift pixels B 11 , B 12 , . . . , are arranged in an adjacent manner, and the right shift pixels A 11 , A 12 , . . . and the left shift pixels B 11 , B 12 , . . . , are arranged every four pixels in the horizontal direction and the vertical direction. In the present embodiment, it is assumed that the upper-left-most right shift pixel and the upper-left-most left shift pixel are A 11  and B 11 , respectively, the right shift pixel adjacent to the right shift pixel A 11  in the down direction is A 12  and the left shift pixel adjacent to the left shift pixel B 11  in the down direction is B 12 . Also, it is assumed that the right shift pixel adjacent to the right shift pixel A 11  in the right direction is A 21  and the left shift pixel adjacent to the left shift pixel B 11  in the right direction is B 21 . 
     In the present embodiment, although the central area  16 C of the imaging element  16  is used as a focus area and a phase difference pixel is arranged in this focus area, the phase difference pixel may be arranged in a focus area and the focus area is not limited to the central area of the imaging element  16 . The whole surface of the imaging element  16  may be used as the focus area or nine areas of three areas in the vertical direction×three areas in the horizontal direction in total may be used as the focus area. 
     In the present embodiment, the imaging element  16  in which phase difference pixels are arranged is used, where the phase difference pixels include left shift pixels which are subjected to light only on the left side of the light axis of the light flux passing an exit pupil and right shift pixels whose left half is covered and which are subjected to light only on the right side of the light axis of the light flux passing the exit pupil, but the phase difference pixel form is not limited to this. For example, phase difference pixels may be arranged, which are formed with pixels subjected to light only on the upper side of the light axis of the light flux passing the exit pupil and pixels subjected to light only on the lower side of the light axis of the light flux passing the exit pupil. Also, phase difference pixels may be arranged, which are formed with a partial area on the left side of the light axis of the light flux passing the exit pupil and a partial area on the right side of the light axis of the light flux passing the exit pupil (here, it is desirable that these areas are line-symmetric). 
     An explanation of  FIG. 1  is given again. A signal charge accumulated in the imaging element  16  is read as a voltage signal according to the signal charge, based on a reading signal added from the imaging element control unit  32 . The voltage signal read from the imaging element  16  is added to an analog signal processing unit  18 , subjected to the sampling hold of R, G and B signals every pixel here, and amplified and added to an A/D converter  20 . The A/D converter  20  converts the R, G and B signals input in order, into digital R, G and B signals and outputs them to an image input controller  22 . 
     A digital signal processing unit  24  performs predetermined signal processing such as offset processing, white balance correction, gain control processing including sensitivity correction, gamma correction processing and YC processing on the digital image signals input via the image input controller  22 . 
     The image data processed by the digital signal processing unit  24  is input in a VRAM (Video Random Access Memory)  50 . The VRAM  50  includes an A area and B area to store image data representing an image of one frame. In the VRAM  50 , image data representing an image of one frame is alternately overwritten between the A area and the B area. In the A area and B area of the VRAM  50 , written image data is read from an area different from an area in which image data is being rewritten. The image data read from the VRAM  50  is encoded in a video encoder  28  and output to a liquid crystal monitor  30  provided on the rear surface of the camera. By this means, the subject is displayed on a display screen of the liquid crystal monitor  30 . 
     Also, when a shutter button of the operation unit  38  is pressed at the first stage (half-pressed), an AE (Automatic Exposure) operation and an AF operation start. That is, the image data output from the A/D converter  20  is imported in an AF (Automatic Focus) detection unit  42  and AE detection unit  44 . 
     The AF detection unit  42  acquires voltage signals output from the right shift pixel and the left shift pixel, performs a correlation computation and detects a phase difference. Subsequently, the AF detection unit  42  calculates a defocus amount from the phase difference and outputs this defocus amount to the CPU  40 . The image signal acquired in the AF detection unit  42  is subjected to pixel combination and arithmetic mean if necessary. The pixel combination and arithmetic mean will be described later in detail. 
     The AE detection unit  44  integrates G signals on the whole screen or integrates G signals weighted in a different way between the screen central part and the peripheral part, and outputs the integration value to the CPU  40 . 
     The CPU  40  calculates a shift amount of the focus lens of the imaging lens  12  based on the defocus amount input from the AF detection unit  42 , and shifts the imaging lens  12  by the calculated shift amount via the lens drive unit  36 . Also, the CPU  40  calculates the brightness (imaging Ev value) of the subject by the integration value input from the AE detection unit  44 , and, based on this imaging Ev value, determines an aperture value of the diaphragm  14  and an electric shutter (shutter speed) of the imaging element  16  according to a predetermined program line figure. The CPU  40  controls the diaphragm  14  via the diaphragm drive unit  34  based on the determined aperture value and controls an electric accumulation time in the imaging element  16  via the imaging element control unit  32  based on the determined shutter speed. 
     When the AE operation and the AF operation end and the shutter button is pressed at the second stage (full-pressed), in response to the press, image data of one frame output from the A/D converter  20  is input from the image input controller  22  to a memory (SDRAM: Synchronous Dynamic Random Access Memory)  48  and temporarily stored. 
     The image data is read from the memory  48  and subjected to predetermined signal processing including generation processing (YC processing) of brightness data and color-difference data, in the digital signal processing unit  24 . The image data (YC data) subjected to YC processing is read from the digital signal processing unit  24  and stored in the memory  48  again. Subsequently, the YC data is output to a compression/decompression processing unit  26  and subjected to predetermined compression processing such as JPEG (Joint Photographic Experts Group). The compressed YC data is output to and stored in the memory  48  again, read by a media controller  52  and stored in a memory card  54 . 
     Next, an automatic focus adjustment method in the digital camera  10  employing the configuration will be explained. 
     First Embodiment 
     The first embodiment combines pixel combination and arithmetic mean according to the brightness of a subject.  FIG. 3  is a flowchart illustrating AF processing according to the first embodiment. 
     The CPU  40  acquires the brightness of the subject (subject brightness) based on an integration value acquired from the AE detection unit  44  and decides which of low brightness, medium brightness and high brightness this subject brightness corresponds to (step S 10 ). For example, the CPU  40  decides the low brightness when the subject brightness is less than a first threshold, decides the medium brightness when the subject brightness is equal to or greater than the first threshold and less than a second threshold (which is a higher value than the first threshold), and decides the high brightness when the subject brightness is equal to or greater than the second threshold. For example, when the maximum output of each pixel is 16,000, it is assumed that the first threshold is 4,000 and the second threshold is 8,000. Here, the subject brightness is not limited to one acquired based on the integration value acquired from the AE detection unit  44  but may be acquired using a sensor or the like. 
     When the subject brightness is decided as the low brightness (“low brightness” in step S 10 ), as illustrated in  FIG. 4A , the imaging element control unit  32  performs four-pixel combination of the right shift pixels and the left shift pixels in the vertical direction (step S 11 ). That is, the imaging element control unit  32  combines performs pixel combination of right shift pixels A 11 , A 12 , A 13  and A 14  and performs pixel combination of left shift pixels B 11 , B 12 , B 13  and B 14 . Similarly, right shift pixels A 21 , A 22 , A 23  and A 24  are subjected to pixel combination, and left shift pixels B 21 , B 22 , B 23  and B 24  are subjected to pixel combination. This applies to all right shift pixels and left shift pixels. 
     The pixel combination denotes processing to combine image signals of right shift pixels and left shift pixels adjacent to each other in the vertical direction of the imaging element  16  and acquire an image signal of a higher signal level than an image signal of one pixel. 
     The signal charge output from each pixel includes noise. The noise is roughly classified into dark noise (which is noise by charges caused in a state where no light is given to the imaging element), fixed pattern noise (which is noise depending on dark current characteristics of silicon) and shot noise (which is noise due to a light amount). Since the signal charge output by pixel combination becomes large, an influence of the shot noise is reduced and the SN ratio is improved. 
     When the subject brightness is the low brightness, since it is not possible to acquire a sufficient exposure amount, the signal charge of each pixel is small and the SN ratio degrades. Therefore, the signal charge output by pixel combination is increased to improve the SN ratio. For example, when the maximum output is 16,000 and the first threshold is 4,000, it is possible to increase the upper-limit output within a range not exceeding the maximum output by four-pixel combination. 
     The signal charges subjected to pixel combination in step S 11  are read as a voltage signal and input in the AF detection unit  42 . As illustrated in  FIG. 4A , the AF detection unit  42  calculates the arithmetic mean of two vertically adjacent output values with respect to the read voltage signals of right shift pixels and left shift pixels subjected to four-pixel combination in step S 11  (step S 12 ). That is, the AF detection unit  42  calculates the arithmetic mean of an output voltage signal acquired by pixel combination of right shift pixels A 11 , A 12 , A 13  and A 14  and an output voltage signal acquired by pixel combination of right shift pixels A 15 , A 16 , A 17  and A 18 , the arithmetic mean of an output voltage signal acquired by pixel combination of right shift pixels A 21 , A 22 , A 23  and A 24  and an output voltage signal acquired by pixel combination of right shift pixels A 25 , A 26 , A 27  and A 28 , and so on. Similarly, the arithmetic mean of voltage signals is calculated for the left shift pixels. 
     The arithmetic mean has an effect of causing noise to be 1/√N in a case where the arithmetic addition number is N. By performing arithmetic mean of voltage signals in which the noise is reduced by pixel combination, it is possible to further reduce noise. 
     When the subject brightness is decided as the medium brightness (“medium brightness” in step S 10 ), as illustrated in  FIG. 4B , the imaging element control unit  32  performs two-pixel combination of the right shift pixels and the left shift pixels in the vertical direction (step S 13 ). In the case of the medium brightness, the signal charge of each pixel is larger than that in the case of the low brightness. Therefore, compared to the case of the low brightness, the pixel arithmetic addition number decreases. For example, when the maximum output is 16,000 and the second threshold is 8,000, it is possible to increase the upper-limit output within a range not exceeding the maximum output by two-pixel combination. 
     As illustrated in  FIG. 4B , the AF detection unit  42  acquires signal charges read by two-pixel combination and calculates the arithmetic mean of three vertically-adjacent voltage signals of the right shift pixels or the left shift pixels subjected to two-pixel combination and read in step S 13  (step S 14 ). 
     When the subject brightness is decided as the high brightness (“high brightness” in step S 10 ), the signal amount of each pixel has a sufficient value and therefore pixel combination is not performed. Therefore, as illustrated in  FIG. 4C , voltage signals of the right shift pixels and the left shift pixels are read, and the AF detection unit  42  calculates the arithmetic mean of four vertically-adjacent read voltage signals of the right shift pixels or the left shift pixels (step S 15 ). Thus, in a brightness area in which it is possible to acquire a sufficient exposure amount, it is possible to reduce noise by arithmetic mean. 
     The AF detection unit  42  performs a correlation computation of the signal charges subjected to arithmetic mean in steps S 12 , S 14  and S 15 , and calculates a defocus amount from the phase difference (step S 16 ). The CPU  40  acquires the defocus amount from the AF detection unit  42 , calculates a shift amount of the focus lens of the imaging lens  12  based on this defocus amount and shifts the imaging lens  12  by the calculated shift amount via the lens drive unit  36  (step S 17 ). By this means, automatic focus adjustment is performed. 
     In the present embodiment, by using both the pixel combination and the arithmetic mean, it is possible to reduce noise and improve the SN ratio, regardless of the subject brightness. Also, by separately using the pixel combination and the arithmetic mean according to the subject brightness, even when imaging any subject, it is possible to adequately reduce noise and acquire a phase difference computation result at high accuracy. Therefore, it is possible to perform an accurate automatic focus adjustment. 
     Also, in the present embodiment, the number of pixels subjected to pixel combination is increased as the brightness degrades. By this means, even in the case of the low brightness, it is possible to adequately reduce noise and improve the SN ratio. Therefore, even in a darker brightness area, an automatic focus adjustment is possible. 
     Also, in the present embodiment, since the pixel combination and the arithmetic mean calculation are performed only in the vertical direction and are not performed in the horizontal direction, it is possible to reduce noise without degrading the accuracy of AF processing. 
     Also, in the present embodiment, for a phase difference AF, it is possible to shorten the time required for AF processing. Also, since the same imaging element as an imaging element used for the phase difference AF in the related art is used and new hardware is not required, it is possible to perform an accurate focus adjustment at lower cost and with space-saving. 
     Also, in the present embodiment, although pixel combination is performed on four pixels in the case of the low brightness and two pixels in the case of the medium brightness, it is needless to say that the pixel combination number is not limited to these. Also, even in the case of the high brightness, it may be possible to perform pixel combination based on accumulated signal charges. 
     Also, in the present embodiment, the pixel combination number is determined by relationships between the maximum output of each pixel and thresholds to separate the low brightness, the medium brightness and the high brightness, but the way of determining the pixel combination number is not limited to this.  FIG. 5  illustrates an example of a method of changing the pixel combination number on many stages according to the Ev value or the shutter speed. 
     According to  FIG. 5 , in a case where the shutter speed is equal to or less than 1/15 second, the pixel combination number is set to 32 regardless of the Ev value. The pixel combination number changes by the Ev value in a case where the shutter speed is between 1/15 second to 1/60 second, and the pixel combination number is 32 in the case of Ev8, the pixel combination number is 16 in the case of Ev9 and Ev10, the pixel combination number is 8 in the case of Ev11 and Ev12, the pixel combination number is 4 in the case of Ev13, the pixel combination number is 2 in the case of Ev14 and the pixel combination number is 1 in the case of Ev15. In a case where the shutter speed is 1/125 second, the pixel combination number is set to 1 regardless of the Ev value. Thus, by changing the pixel combination number on many stages, it is possible to determine the pixel combination number more adequately. 
     Also, in the present embodiment, although the arithmetic addition number used for arithmetic mean is increased as the subject brightness becomes higher, in order to perform an accurate automatic focus adjustment in a darker brightness area, the arithmetic addition number used for arithmetic mean may be increased as the subject brightness becomes lower. 
     Second Embodiment 
     The second embodiment maintains the product of the pixel number for pixel combination and the number of voltage signals used for arithmetic mean.  FIG. 6  is a flowchart illustrating a flow of AF processing according to the second embodiment. Also, the same reference numerals are assigned to the same components as in the first embodiment and detailed explanation will be omitted. 
     The CPU  40  designates a vertical width H of the focus area, that is, the value of the product of the pixel number for pixel combination and the number of voltage signals used for arithmetic mean (step S 20 ). In this processing, it may be designated based on an input from the operation unit  38  or a predetermined value may be always designated. In the present embodiment, “8” is designated as H. 
     The CPU  40  acquires the brightness of the subject (subject brightness) based on an integration value acquired from the AE detection unit  44  and decides which of low brightness, medium brightness and high brightness this subject brightness corresponds to (step S 10 ). 
     When the subject brightness is decided as the low brightness (“low brightness” in step S 10 ), as illustrated in  FIG. 7A , the imaging element control unit  32  performs four-pixel combination of the right shift pixels and the left shift pixels in the vertical direction (step S 11 ). That is, the pixel combination number GN in step S 11  is 4. 
     The signal charges subjected to pixel combination in step S 11  are read as a voltage signal and input in the AF detection unit  42 . The AF detection unit  42  determines a voltage signal number AN used to calculate an arithmetic mean based on H designated in step S 20  and the pixel number for pixel combination performed in step S 11 , and calculates the arithmetic mean using the determined voltage signal number. In the present embodiment, since H is 8 and the pixel number for pixel combination in step S 11  is 4, AN is calculated as H/GN=8/4=2. Therefore, as illustrated in  FIG. 7A , the AF detection unit  42  calculates the arithmetic mean of two vertically-adjacent output values with respect to the read voltage signals of right shift pixels and left shift pixels subjected to four-pixel combination in step S 11  (step S 21 ). 
     When the subject brightness is decided as the medium brightness (“medium brightness” in step S 10 ), as illustrated in  FIG. 7B , the imaging element control unit  32  performs two-pixel combination of the right shift pixels and the left shift pixels in the vertical direction (step S 13 ). That is, the pixel combination number GN in step S 13  is 2. 
     The signal charges subjected to pixel combination in step S 13  are read as a voltage signal and input in the AF detection unit  42 . The AF detection unit  42  determines a voltage signal number AN used to calculate an arithmetic mean based on H designated in step S 20  and the pixel number for pixel combination performed in step S 13 , and calculates the arithmetic mean using the determined number. In the present embodiment, since H is 8 and the pixel number for pixel combination in step S 13  is 2, AN is calculated as H/GN=8/2=4. Therefore, as illustrated in  FIG. 7B , the AF detection unit  42  calculates the arithmetic mean of four vertically-adjacent output values with respect to the read voltage signals of right shift pixels and left shift pixels subjected to two-pixel combination in step S 11  (step S 22 ). 
     When the subject brightness is decided as the high brightness (“high brightness” in step S 10 ), the signal amount of each pixel has a sufficient value and therefore pixel combination is not performed. Therefore, as illustrated in  FIG. 7C , voltage signals of the right shift pixels and the left shift pixels are read. That is, GN is 1 (step S 23 ). 
     The AF detection unit  42  determines a voltage signal number AN used to calculate an arithmetic mean based on H designated in step S 20  and the pixel number for pixel combination in step S 23 , and calculates the arithmetic mean using the determined number. In the present embodiment, since H is 8 and the pixel number for pixel combination in step S 23  is 1, AN is calculated as H/GN=8/1=8. Therefore, as illustrated in  FIG. 7C , the AF detection unit  42  calculates the arithmetic mean of eight vertically-adjacent output values with respect to the read voltage signals of right shift pixels and left shift pixels without pixel combination. 
     The AF detection unit  42  performs correlation computation on the signal charges subjected to arithmetic mean in steps S 21 , S 22  and S 24 , and calculates a defocus amount from the phase difference (step S 16 ). The CPU  40  acquires the defocus amount from the AF detection unit  42  and calculates a shift amount of the focus lens of the imaging lens  12  based on this defocus amount. Subsequently, the CPU  40  shifts the imaging lens  12  by the calculated shift amount via the lens drive unit  36  (step S 17 ). By this means, automatic focus adjustment is performed. 
     According to the present embodiment, it is possible to maintain the product of the pixel number for pixel combination and the number of voltage signals used for arithmetic mean, that is, the pixel number based on a voltage signal calculated by the arithmetic mean, while changing the pixel combination number according to the subject brightness. Therefore, it is possible to maintain a phase difference and the size of an area in which a defocus amount is calculated, regardless of the subject brightness. Therefore, a stable automatic focus adjustment is possible. 
     Third Embodiment 
     The third embodiment performs accurate automatic focus adjustment by dividing a focus area into a plurality of areas in the case of high brightness.  FIG. 8A  and  FIG. 8B  are flowcharts illustrating a flow of AF processing in the third embodiment. Also, the same reference numerals are assigned to the same components as in the first embodiment and the second embodiment, and detailed explanation will be omitted. 
     The CPU  40  designates a vertical width H (8 in the present embodiment) of the focus area, that is, the value of the product of the pixel number for pixel combination and the number of voltage signals used for arithmetic mean (step S 20 ). 
     The CPU  40  acquires the brightness of the subject (subject brightness) based on an integration value acquired from the AE detection unit  44  and decides which of low brightness, medium brightness and high brightness this subject brightness corresponds to (step S 10 ). 
     When the subject brightness is decided as the low brightness (“low brightness” in step S 10 ), a pixel combination number GN is set to 4 and the imaging element control unit  32  performs four-pixel combination of the right shift pixels and the left shift pixels in the vertical direction (step S 11 ). 
     The signal charges subjected to pixel combination in step S 11  are read as a voltage signal and input in the AF detection unit  42 . The AF detection unit  42  determines a voltage signal number AN (in the present step, AN=H/GN=8/4=2) used to calculate an arithmetic mean based on H designated in step S 20  and the pixel number for pixel combination performed in step S 11 , and calculates the arithmetic mean of two vertically adjacent output values with respect to the read voltage signals of right shift pixels and left shift pixels subjected to four-pixel combination in step S 11  (step S 21 ). 
     When the subject brightness is decided as the medium brightness (“medium brightness” in step S 10 ), the pixel combination number GN is set to 2 and the imaging element control unit  32  performs two-pixel combination of the right shift pixels and the left shift pixels in the vertical direction (step S 13 ). 
     The signal charges subjected to pixel combination in step S 13  are read as a voltage signal and input in the AF detection unit  42 . The AF detection unit  42  determines a voltage signal number AN (in the present step, AN=H/GN=8/2=4) used to calculate an arithmetic mean based on H designated in step S 20  and the pixel number for pixel combination performed in step S 13 , and calculates the arithmetic mean of four vertically adjacent output values with respect to the read voltage signals of right shift pixels and left shift pixels subjected to two-pixel combination in step S 11  (step S 22 ). 
     The AF detection unit  42  performs correlation computation on the signal charges subjected to arithmetic mean in steps S 21  and S 22 , and calculates a defocus amount from the phase difference (step S 16 ). 
     When the subject brightness is decided as the high brightness (“high brightness” in step S 10 ), the signal amount of each pixel has a sufficient value and therefore pixel combination is not performed (GN=1) (step S 23 ). 
     The AF detection unit  42  divides the focus area into a plurality of areas. Subsequently, the AF detection unit  42  calculates the arithmetic mean of voltage signals every divided area (step S 30 ,  32 , . . . ), performs correlation computation on the voltage signals subjected to arithmetic mean and calculates a defocus amount from the phase difference (step S 31 , S 33 , . . . ). 
     In the present embodiment, as illustrated in  FIG. 9 , the pixel number of phase difference pixels of a focus area is 8 pixels and is divided into four areas in the vertical direction. Therefore, the AF detection unit  42  calculates the arithmetic mean of 2-pixel vertically-adjacent voltage signals. That is, the arithmetic mean of voltage signals of right shift pixels A 11  and A 12  (first group) is performed (step S 30 ) and the arithmetic mean of voltage signals of right shift pixels A 13  and A 14  (second group) is performed (step S 32 ). This processing is performed on all groups. Similarly, the arithmetic mean is calculated for the left shift pixels. 
     A method of dividing a focus area into a plurality of areas in the AF detection unit  42  is arbitrary. For example, it may be possible to set the size of a divided area to a predetermined size (which can be arbitrarily changed) and determine the division number to provide this size. Also, it may be possible to arbitrarily determine the pixel number for arithmetic mean and divide a focus area based on this pixel number. 
     Subsequently, the AF detection unit  42  performs correlation computation on the first-group voltage signals calculated in step S 30 , calculates a defocus amount from the phase difference (step S 31 ), performs correlation computation on the second-group voltage signals calculated in step S 32  and calculates a defocus amount from the phase difference (step S 33 ). This processing is performed on all groups. By this means, the defocus amount for each group is calculated. 
     The AF detection unit  42  decides the reliability of the defocus amount calculated for each group in steps S 31 , S 33 , . . . , and determines the most reliable defocus amount (step S 35 ). For example, the reliability may be decided based on how it is close to the average value of the defocus amounts calculated for the groups, and the defocus amount closest to the average value may be decided as the most reliable defocus amount. Also, the reliability may be decided based on how many times the number of defocus amounts calculated for each group is included in all defocus amounts, and the defocus amount that is included most times may be decided as the most reliable defocus amount. Thus, a plurality of methods are possible to determine the most reliable defocus amount, it is arbitrary which method is adopted. 
     The CPU  40  acquires the defocus amount calculated and determined in steps S 16  and S 35 , from the AF detection unit  42 . The CPU  40  calculates a shift amount of the focus lens of the imaging lens  12  based on this defocus amount and shifts the imaging lens  12  by the calculated shift amount via the lens drive unit  36  (step S 17 ). By this means, automatic focus adjustment is performed. 
     According to the present embodiment, in a case where the subject brightness is the high brightness, by dividing a focus area into a plurality of areas, performing a specific phase difference computation and using the most reliable computation result, it is possible to perform automatic focus adjustment more accurately. 
     According to the present invention, although an imaging element in which a phase difference pixel is arranged in a focus area is used, a used imaging element is not limited to this. For example, a phase difference CCD in which all pixels are formed with phase difference pixels may be used.  FIG. 10A  to  FIG. 10C  are views illustrating a configuration example of a phase difference CCD. 
     As illustrated in  FIG. 10A , the phase CCD includes odd-numbered line pixels (primary pixels) and even-numbered line pixels (secondary pixels) arranged in a matrix manner, and image signals of two surfaces subjected to photoelectric conversion by these primary and secondary pixels can be independently read. 
     As illustrated in  FIG. 10B , among pixels having color filters of R (Red), G (Green) and B (Blue), the pixel alignment line of GRGR . . . and the pixel alignment line of BGBG . . . are alternately arranged in the odd-numbered lines (1, 3, 5, . . . ) in the phase difference CCD  16 , while, as illustrated in  FIG. 10C , similar to the odd-numbered lines, the pixel alignment line of GRGR . . . and the pixel alignment line of BGBG . . . are alternately arranged in the even-numbered line (2, 4, 6, . . . ) pixels, where the pixels of the even-numbered lines are shifted each other by half pitch in the line direction. 
     On the front surface side (micro lens L side) of the primary pixels of the phase difference CCD  16 , a light shielding member  16 A to shield light on the right half of a light receiving surface of the primary pixels (photodiode PD) is arranged, and, on the front surface side of the secondary pixels, a light shielding member  16 B to shield light on the left half of a light receiving surface of the secondary pixels (photodiode PD) is arranged. Therefore, in the primary pixels, light is received only on the left side of the light axis of the light flux passing an exit pupil, and, in the secondary pixels, light is received only on the right side of the light axis of the light flux passing the exit pupil. 
     By using such a phase difference CCD, generating left-view image data based on image signals output from primary pixels and generating right-view image data based on image signals output from secondary pixels, it is possible to image a stereoscopic view image. Also, by using the central area of the phase difference CCD as a focus area and using the primary pixels as the left shift pixels and the secondary pixels as the right shift pixels among G pixels in the focus area, it is possible to perform automatic focus adjustment by the same method as in the present embodiment. 
     In the present invention, although an explanation has been given with an example of using a CCD as an imaging element, it is not limited to the CCD. The present invention is also applicable to other image sensors such as a CMOS. 
     REFERENCE SIGNS LIST 
       10 : digital camera;  12  . . . imaging lens;  14  . . . diaphragm;  16  . . . imaging element;  24  . . . digital signal processing unit;  32  . . . imaging element control unit;  34  . . . diaphragm drive unit;  36  . . . lens drive unit;  38  . . . operation unit;  40  . . . CPU;  42  . . . AF detection unit;  44  . . . AE detection unit;  48  . . . memory