JP6429724B2 - Imaging apparatus and control method thereof - Google Patents

Description

  The present invention relates to a technique for performing focus adjustment using an image signal acquired by an image sensor.

  As a focus detection and focus adjustment method of the imaging apparatus, there are a contrast detection AF (AutoFocus) and a phase difference detection AF. In contrast detection AF (hereinafter referred to as “contrast AF”), a focus lens position where the AF evaluation value indicating contrast information is maximized by extracting a high-frequency component from the output signal of the image sensor is set as the in-focus position. In contrast AF, since it is necessary to pass through the focus position when searching for the focus position, it takes time to adjust the focus. Furthermore, when the driving direction of the focus lens for performing focus adjustment cannot be determined, there is a problem that if the direction detection takes time, the focusing time is extended.

  One of the phase difference detection AFs is an imaging surface phase difference detection AF (hereinafter referred to as imaging surface phase difference AF). In imaging plane phase difference AF, phase difference detection is performed based on the imaging signal. However, there is a problem in that the structure is complicated because the imaging pixel must have a special shape.

  On the other hand, there is a technique referred to as a depth from focus (hereinafter abbreviated as DFD) method. The DFD method is a method of detecting a current defocus amount by obtaining a blur evaluation value by performing arithmetic processing on two images having different blur states. In the DFD AF method (hereinafter referred to as DFD AF), the imaging pixel does not need to have a special shape, and thus the structure is simpler than the imaging plane phase difference AF. Specifically, in the DFD AF, data representing the correlation between the blur evaluation value and the defocus amount is stored in the storage unit as a reference table (hereinafter referred to as a correlation table). The defocus amount can be obtained from the blur evaluation value by referring to the correlation table. In the case of the DFD AF, the influence of noise is greatly affected as blurring becomes significant. For this reason, there is a problem in that focus detection fails if an accurate blur evaluation value cannot be calculated. For this problem, it is possible to perform good focus detection by excluding a noisy region and limiting a defocus region where focus detection is possible (Patent Document 1).

JP 2006-3803 A

In the conventional technique disclosed in Patent Document 1, the defocus area where focus detection is possible is narrow because the correlation table limits the blur evaluation value and the defocus amount to a one-to-one correspondence range. That is, due to the narrow focus detection range, when AF operation is started from a range where focus detection is not possible, the time until focus detection ends (hereinafter referred to as focus detection time) becomes long. There was a problem that.
An object of the present invention is to reduce the focus detection time by expanding the range of the correlation table that represents the relationship between the evaluation value of the blur state and the defocus amount in focus detection using data of a plurality of images with different blur states. And

  An apparatus according to the present invention includes an image pickup optical system including a focus lens, an image pickup element that photoelectrically converts a subject image formed by the image pickup optical system, and outputs data of a plurality of images having different blur states from the image pickup element. A calculation means for obtaining an evaluation value of a blur state obtained from the signal, a storage means for storing correlation data indicating a correspondence relationship between the evaluation value and the defocus amount, and an evaluation value calculated by the calculation means, Determining means for determining a defocus amount by collating with the correlation data stored in the storage means; and the focus lens according to the driving amount of the focus lens corresponding to the defocus amount determined by the determining means Driving means for driving the. When a plurality of defocus amount candidates are detected by the collation, the determining unit drives the focus lens by the driving unit to determine a change in the evaluation value and determines one defocus amount from the candidate. To decide.

  According to the present invention, in focus detection using data of a plurality of images having different blur states, the range of the correlation table representing the relationship between the evaluation value of the blur state and the defocus amount is expanded, and the focus detection time is shortened. Can do.

It is a block diagram which shows the structural example of the imaging device concerning embodiment of this invention. 6 is a flowchart illustrating an AF operation of the imaging apparatus according to the embodiment of the invention. It is a flowchart which shows the process following FIG. It is a figure which shows the correlation of the blur evaluation value and defocus amount in embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. The present invention can be applied to an imaging apparatus mounted on a digital still camera, a digital video camera, a mobile phone, or the like. In the present embodiment, an example of an apparatus that performs a shooting operation in a state where the lens apparatus is mounted on the camera body is described, but the present invention is applicable to an apparatus in which the lens is integrated with the camera body.

  With reference to FIG. 1, a schematic configuration of the entire imaging apparatus will be described. FIG. 1 is a block diagram illustrating a configuration example of a digital camera as an imaging apparatus to which the focus adjustment apparatus according to the present embodiment is applied. The digital camera exemplified in this embodiment is an interchangeable lens type single-lens reflex camera, and includes a lens unit 100 and a camera main body 120. The lens unit 100 is detachably connected to the camera body 120 via a mount M indicated by a dotted line in the center of FIG.

  The lens unit 100 includes a first lens group 101, a diaphragm shutter 102, a second lens group 103, a focus lens group (hereinafter simply referred to as “focus lens”) 104, and a control unit described later. The imaging optical system of the lens unit 100 includes a focus lens 104 and forms an image of a subject. In the following, the subject side is defined as the front side, and the positional relationship between the respective parts will be described.

  The first lens group 101 is disposed at the front end of the lens unit 100 and is held so as to be able to advance and retract along the optical axis direction OA. The aperture / shutter 102 is located behind the first lens group 101 and adjusts the light amount at the time of shooting by adjusting the aperture diameter, and also functions as an exposure time adjustment shutter at the time of still image shooting. The iris / shutter 102 and the second lens group 103 behind it can be moved forward and backward in the optical axis direction OA as one body, and a zoom function is realized by interlocking with the forward / backward movement of the first lens group 101.

  The focus lens 104 is positioned behind the second lens group 103, and performs focus adjustment by moving along the optical axis direction OA. In the present embodiment, among the two ends of the range in which the focus lens 104 is movable, the focus lens position on the infinity side is referred to as the infinity end, and the focus lens position on the closest side is referred to as the closest end.

  The control unit of the lens unit 100 includes a lens MPU (microprocessing unit) 117 and a lens memory 118. The lens MPU 117 controls the zoom driving unit 114, the aperture shutter driving unit 115, and the focus driving unit 116. The zoom actuator 111 performs a zoom operation by moving the first lens group 101 and the second lens group 103 in the optical axis direction OA. The zoom actuator 111 is driven by the zoom drive unit 114 according to the control command of the lens MPU 117 in accordance with the zoom operation of the photographer. The aperture shutter actuator 112 controls the aperture diameter of the aperture / shutter 102 to adjust the amount of photographing light, and also controls the exposure time during still image photographing. The aperture shutter actuator 112 is driven by the aperture shutter drive unit 115 in accordance with a control command from the lens MPU 117. The focus actuator 113 performs focus adjustment by moving the focus lens 104 in the optical axis direction OA. The focus actuator 113 has a function of a position detection unit that detects the current position of the focus lens 104. The focus driving unit 116 drives the focus actuator 113 according to the control command of the lens MPU 117 based on the focus detection result.

  The lens MPU 117 performs all calculations and controls related to the imaging optical system. The lens MPU 117 acquires the current lens position information and notifies the lens position information in response to a request from the camera MPU 125 described later. The lens memory 118 is a storage unit that stores various optical information necessary for automatic focus adjustment. Specifically, the lens memory 118 stores, for example, data on the correspondence between the position information of the focus lens 104 and the defocus amount. When the lens MPU 117 is requested to change the defocus amount by a predetermined amount from the camera MPU 125 described later, the lens MPU 117 refers to the correspondence data stored in the lens memory 118. The lens MPU 117 sends a control command to the focus driving unit 116 to control the focus actuator 113 so as to drive the focus lens 104 by a distance corresponding to a predetermined defocus amount.

  The camera body 120 includes an optical low-pass filter 121, an image sensor 122, and a controller. The optical low-pass filter 121 is an optical element that reduces false colors and moire in the captured image. The image sensor 122 photoelectrically converts a subject image formed by the imaging optical system. The image sensor 122 is composed of, for example, a C-MOS (complementary metal oxide semiconductor) sensor and its peripheral circuit section. The C-MOS sensor has a pixel array in which photoelectric conversion elements are arranged on light receiving pixels of m pixels in the horizontal direction and n pixels in the vertical direction. The image sensor 122 has a configuration capable of outputting an independent signal for each pixel in the pixel array. More specifically, the pixel array of the image sensor 122 has a plurality of shooting pixels that each receive a light beam passing through the entire exit pupil of the imaging optical system that forms an image of the subject and generate an image of the subject. . The pixel array further includes a plurality of focus detection pixels that respectively receive light beams passing through different exit pupil regions of the imaging optical system. The plurality of focus detection pixels as a whole can receive a light beam passing through the entire exit pupil of the imaging optical system, and corresponds to one photographing pixel. For example, in a 2 × 2 pixel arrangement, a pair of diagonally arranged G (green) pixels is used as a shooting pixel, and the other two R (red) pixels and B (blue) pixels are used for focus detection. Replaced by a pixel.

  The image sensor driving unit 123 performs drive control of the image sensor 122 and transmits the image signal acquired from the image sensor 122 to the camera MPU 125 after A (analog) / D (digital) conversion. The image processing unit 124 performs γ conversion, color interpolation, JPEG (Joint Photographic Experts Group) compression, and the like of image data acquired from the image sensor 122.

  The camera MPU 125 performs all calculations and control related to the camera body 120. The camera MPU 125 controls the image sensor driving unit 123, the image processing unit 124, the display unit 126, the operation unit 127, the memory 128, and the focus detection unit 129. The camera MPU 125 is connected to the lens MPU 117 via the signal line of the mount M. As a result, the camera MPU 125 acquires the lens position, issues a lens drive request with a predetermined drive amount, or acquires optical information specific to the lens unit 100 to the lens MPU 117. The camera MPU 125 includes a ROM (Read Only Memory) 125a that stores a program for controlling camera operations, and a RAM (Random Access Memory) 125b that stores data, variables, and the like. The camera MPU 125 incorporates an EEPROM (Electrically Erasable Programmable Read-Only Memory) 125c for storing various parameters. Further, the camera MPU 125 loads and executes a program stored in the ROM 125a to execute a focus adjustment process based on the focus detection state.

  The display unit 126 includes an LCD (liquid crystal display) or the like, and displays information related to the shooting mode of the camera, a preview image before shooting and a confirmation image after shooting, a display image in a focused state when focus is detected, and the like. The operation unit 127 includes a power switch, a release (shooting trigger) switch, a zoom operation switch, a shooting mode selection switch, and the like, and outputs a user operation instruction signal to the camera MPU 125. The release switch is a two-stage switch having a first switch (denoted as SW1) that is turned on by a user's first stroke operation and a second switch (denoted as SW2) that is turned on by a second stroke operation. Consists of. An instruction signal for starting AE (automatic exposure) processing and AF (automatic focus adjustment) processing performed prior to the photographing operation is an ON signal of the first switch SW1. The instruction signal for starting the actual exposure operation is an ON signal of the second switch SW2. The memory 128 is, for example, a flash memory that can be attached to and detached from the camera body 120, and records data of captured images.

  The focus detection unit 129 performs DFD focus detection. That is, focus detection is performed based on the blur evaluation value calculated from the image information obtained by the image processing unit 124. The image information obtained by the image processing unit 124 is, for example, information on two images that differ by a predetermined defocus amount. A blur evaluation value is calculated by performing arithmetic processing on the image information. The arithmetic processing is processing for extracting a specific frequency band component from, for example, luminance information of an image. The blur evaluation value according to the present embodiment is a value indicating the blurring state of the captured image, and is a value having a correlation with the variance of the point image intensity distribution function of the imaging optical system. The point image intensity distribution function is a function of the degree of spread after the point image passes through the lens. The dispersion of the point image intensity distribution function of the imaging optical system is also correlated with the defocus amount. From the above, there is a correlation between the blur evaluation value and the defocus amount. This correlation is referred to as a blur evaluation value / defocus amount correlation. The blur evaluation value / defocus amount correlation will be described in detail later.

  The focus detection unit 129 needs to acquire two images that differ by a predetermined defocus amount used for the DFD AF. For this purpose, the camera MPU 125 controls the shooting operation by changing shooting parameters such as the position of the focus lens, the aperture amount, and the focal length that affect the blurring state of the captured image. Regarding the change of the imaging parameter, any one or more parameters may be changed. In the present embodiment, a case will be described in which two images with different defocus amounts are obtained by changing the position of the focus lens 104 on the optical axis.

  The in-focus position determination process will be described with reference to FIGS. 2 and 3 are flowcharts illustrating the AF processing of the imaging apparatus to which the focus adjustment apparatus according to this embodiment is applied. The following focusing operation (AF operation) processing is performed according to a control program executed by the camera MPU 125. In FIG. 2 and FIG. 3, “S” is an abbreviation for step.

  When the AF operation starts in S200 of FIG. 2, an AF evaluation range for performing focus adjustment on the subject is set in the area setting process performed by the camera MPU 125 (S201). In the process of S201, for example, one AF evaluation range is set in the image. The AF evaluation range is a range in which an image signal for performing focus adjustment by an AF operation is evaluated. The purpose of the AF operation is to focus on the subject intended by the photographer within the AF evaluation range as the focus state detection area.

  Next, exposure to the image sensor 122 is performed, and captured image data is accumulated in the RAM 125b (S202). As a result, it is possible to obtain output signals used for focus detection and information on nearby subjects including the AF evaluation range. When the focus detection unit 129 acquires image data for focus detection, the number of output signals of the imaging pixels may be thinned out according to the required focus adjustment accuracy. Further, the exposure to the image sensor 122 may be performed while the focus lens 104 is continuously moving, or may be performed while the focus lens 104 is stopped.

  In step S203, the camera MPU 125 determines the number of exposures. If the number of exposures is 2 or more, the process proceeds to S205 (Yes in S203), and if the number of exposures is 1 or less (No in S203), the process proceeds to S204. In step S205, the focus detection unit 129 performs a process of calculating a blur evaluation value (denoted as C) used for focus detection. Then, the process proceeds to S206.

  In order to calculate the blur evaluation value C, image data obtained by two or more exposures with different blur states must be accumulated in the RAM 125b. That is, since the blur evaluation value cannot be calculated when the number of exposures is 1 or less, the process of S204 is executed in order to repeat the exposure once more. The lens MPU 117 performs drive control of the focus lens 104 with a predetermined drive amount (S204), and returns to the exposure step (S202). Specifically, the lens MPU 117 moves the focus lens 104 by a distance corresponding to a predetermined drive amount, for example, in a preset direction (usually the closest direction). The reason for setting the lens driving direction to the close direction is that the subject to be focused is often present on the close side. However, the direction is not limited to this. The predetermined drive amount in S204 corresponds to the difference between the blur states of two images having different blur states, which are necessary for performing the DFD AF. In the present embodiment, the drive amount related to the drive control of the focus lens 104 is described as being always constant, but may be changed each time S204 is executed. In that case, it is necessary to store a correlation table corresponding to each predetermined driving amount in the memory. Hereinafter, the predetermined drive amount related to the drive control of the focus lens 104 is denoted as Lk.

  The blur evaluation value C obtained in S205 is calculated from, for example, image data for two exposures with different blur states stored in the RAM 125b. In the calculation process of the blur evaluation value C, the same filter process is performed on the data of two images having different blur states, and a predetermined frequency band component is extracted. For example, a square sum operation is performed after a convolution operation in the real space or a multiplication in the frequency space, and the power of the image signal in the frequency space (hereinafter referred to as P) is calculated. By comparing the P values of the two images, a blur evaluation value C for evaluating the difference in the degree of blur between the two images can be calculated. The two images with different blur states indicate images captured at positions before and after the focus lens 104 is driven by a predetermined drive amount Lk. A change in the true defocus amount corresponding to the predetermined drive amount Lk is denoted as Dk. That is, the change amount of the true defocus amount of two images having different blur states is Dk.

  In this embodiment, it is assumed that the blur evaluation value C is zero when the subject is in focus. For the process of calculating the blur evaluation value C, a known method is used. For example, the blur evaluation value C is obtained by multiplying a value obtained by dividing the difference between the P values of the first image signal and the second image signal by the sum of the P values of the first and second image signals by a coefficient. It can be calculated. In this case, the blur evaluation value C is either a positive value, a negative value, or zero.

  In step S206, the camera MPU 125 determines the reliability of the blur evaluation value calculated in step S205. In S206, it is determined whether or not the blur evaluation value is reliable. The blur evaluation value C is an index for evaluating the difference in the degree of blur between the two images. By the way, in the state where the blur of the image where the contrast of the image is greatly reduced as in the vicinity of the infinite end or the close end, the difference in the blur condition between the two images is almost eliminated. For this reason, the S / N ratio (signal to noise ratio) decreases. Therefore, in step S206, the camera MPU 125 determines whether or not the blur is large, and determines that the blur evaluation value C calculated from the two images obtained when the blur is large is low in reliability. A known method is used for the reliability determination process of the blur evaluation value C. If it is determined that the blur evaluation value C is reliable (Yes in S206), the process proceeds to S208. If it is determined that the blur evaluation value C is not reliable (No in S206), the process proceeds to S207.

  In S207, the focus lens 104 for driving the blur evaluation value C again is driven. The lens MPU 117 drives and controls the focus lens 104 to drive the predetermined driving amount Lk or more. Further, the camera MPU 125 resets the number of exposures, returns to S202, and starts the first exposure (S202). In S207, the purpose of driving the focus lens 104 with the predetermined driving amount Lk or more is to reach a reliable lens position with respect to the blur evaluation value C at a higher speed. Note that the focus lens 104 may be driven with a predetermined drive amount Lk as necessary in the same manner as in S204. The purpose of resetting the number of exposures is to recalculate the blur evaluation value C at the position after driving the focus lens 104 in S207. In step S207, the lens MPU 117 controls the drive of the focus lens 104 in, for example, a preset direction (usually the closest direction). The reason for setting the lens driving direction to the closest direction is as described above.

  In S208, the camera MPU 125 collates the blur evaluation value C determined to be reliable in S206 with the data (correlation data) in the correlation table stored in the RAM 125b, and determines the defocus amount. Although details will be described later, as a result of the table reference in S208, a plurality of defocus amounts may be obtained. Therefore, in order to distinguish the defocus amount obtained in this step from the “true defocus amount” and avoid confusion of terms, the defocus amount obtained by referring to the correlation table is referred to as “detection defocus” below. Referred to as “amount”. A correct defocus amount corresponding to the position of the focus lens 104 is referred to as a true defocus amount. That is, the lens memory 118 stores data representing the correspondence between the position information of the focus lens 104 and the true defocus amount.

  With reference to FIG. 4, the process in S208 will be described. FIG. 4 is a diagram illustrating the correlation between the blur evaluation value C and the true defocus amount. The horizontal axis represents the true defocus amount, and the right side is the infinite side. The vertical axis represents the blur evaluation value C. The graph line represents the correlation table data (correlation data). Ds shown in FIG. 4 is a true defocus amount on the closest side, and Dm is a true defocus amount on the infinite side. Each value of Ds and Dm is assumed to be a value equal to or less than the true defocus amount corresponding to the close end and the infinite end. Also, Do shown in FIG. 4 is a true defocus amount corresponding to the in-focus position, and in this embodiment, Do = 0. The blur evaluation value Co corresponds to the true defocus amount Do at the in-focus position, and Co = 0 in this embodiment.

  The correlation table as a reference table is used when the true defocus amount is obtained from the blur evaluation value C. For example, the blur evaluation value determined to be reliable in S206 of FIG. In this case, referring to the correlation table shown in FIG. 4, Do = 0 is derived. That is, it can be seen that the current focus state is an in-focus state. In this way, the value of the blur evaluation value C, which is the vertical axis of FIG. 4, is compared with the graph line shown in FIG. 4, and the true defocus amount value, which is the horizontal axis of FIG. The read value on the horizontal axis is a value detected by the camera using the blur evaluation value C, and is a detected defocus amount.

Next, the signs of the blur evaluation value C and the true defocus amount shown in FIG. 4 will be described.
Regarding the sign of the true defocus amount, the sign of the true defocus amount on the near side from Do is defined as minus, and the sign of the true defocus amount on the infinite side from Do is defined as plus. As for the sign of the blur evaluation value C, a value smaller than Co = 0 is defined as minus. Cb corresponding to the true defocus amount Db indicates the minimum value of the blur evaluation value and is a minus sign. The sign of the blur evaluation value C greater than Co = 0 is defined as plus. Ca corresponding to the true defocus amount Da indicates the maximum value of the blur evaluation value and is a plus sign.

  Thus, FIG. 4 illustrates a correlation table in which the sign of the blur evaluation value C and the sign of the true defocus amount match. In the present embodiment, the description will be made on the assumption that the sign of the blur evaluation value C matches the sign of the true defocus amount. However, the sign of the blur evaluation value C changes depending on the calculation formula used when calculating the blur evaluation value C in S205 of FIG. Depending on the calculation formula, the sign of the blur evaluation value C may not always match the sign of the true defocus amount.

  In general, in the relationship between the blur evaluation value C and the true defocus amount, the absolute value of the blur evaluation value C increases as the absolute value of the true defocus amount increases from the focus position Do = 0. Become. For example, in FIG. 4, a range in which the true defocus amount is larger than Db and smaller than Da (see the third Def range) shows this relationship. In general, when the absolute value of the defocus amount is a predetermined value or more away from the focus position Do = 0, the absolute value of the blur evaluation value C decreases as the absolute value of the true defocus amount increases. . As an example, a range in which the true defocus amount is Db or less or Da or more (see the second Def range) in FIG. 4 shows this relationship. The reason for this relationship is that there is a difference in the power P of the two images that are used when calculating the blur evaluation value C and the defocus amount is different by Dk. A change amount of the power P of two images having different defocus amounts by Dk is denoted by ΔP. In the vicinity of focus, ΔP is small, but ΔP gradually increases as the distance from the focus position increases, and ΔP decreases again as the blur increases with a distance more than a predetermined distance from the focus position. In general, the power P hardly changes even when the defocus amount is changed by the same Dk in the state near the in-focus state and the state where the blur is large. When the blur evaluation value C is calculated using the power P having such properties, the relationship between the blur evaluation value C and the true defocus amount is as shown in FIG.

Hereinafter, for convenience of explanation, the true defocus amount according to the present embodiment is classified into a plurality of defocus areas. The classification method will be described with reference to FIG.
(1) First Defocus (Def) Range The first defocus range is a true defocus amount that is determined to have a large blur in the blur evaluation value reliability determination described in S206 of FIG. It is defined as the range that can be taken. As shown in FIG. 4, the first defocus range is a region near the infinite end or the closest end (range less than Ds and range beyond Dm). When the blur evaluation value is not reliable or is in the first defocus range, the correlation table is not referred to. Therefore, in FIG. 4, the blur evaluation value C corresponding to the first defocus range is not plotted, and only the blur evaluation value C corresponding to the reliable defocus range is shown.
(2) Second Defocus (Def) Range The second defocus range is defined as a range greater than or equal to Ds and less than or equal to Db, or a range greater than or equal to Da and less than or equal to Dm. That is, the second defocus range is a range outside the position corresponding to the two inflection points on the graph line in FIG. 4 (the side away from the focus position).
(3) Third Defocus (Def) Range The third defocus range is a range that is larger than Db and smaller than Da, and is defined as a range including the in-focus position Do.

  The point to be noted is that, as shown in FIG. 4, the signs of the blur evaluation value C and the true defocus amount coincide with each other in the range obtained by combining the second defocus range and the third defocus range. Is a point. That is, in the range including the second defocus range and the third defocus range, it can be said that the sign of the blur evaluation value is reliable. In other words, the reliability of the sign of the blur evaluation value means that it can be determined at the time when the blur evaluation value is calculated in which direction the in-focus side is located on the near side or the infinite side. There is nothing else.

  Next, a specific example of processing for determining the true defocus amount from the blur evaluation value C by referring to the correlation table will be described with reference to FIG. Assume that the blur evaluation value C calculated in S205 of FIG. 2 is Cx. Referring to the correlation table illustrated in FIG. 4, two detection defocus amounts Dx1 and Dx2 are obtained. Dx1 belongs to the third defocus range, and Dx2 belongs to the second defocus range. While the detected defocus amount is two in FIG. 4, the true defocus amount is one of the detected defocus amounts Dx1 and Dx2.

  In this way, as a result of referring to the correlation table for one blur evaluation value C, two detection defocus amounts are obtained. In other words, the correlation table is characterized in that two detected defocus amounts correspond to one blur evaluation value C (that is, one-to-two correspondence). The superiority of this feature over the prior art will be described in detail later. In this embodiment, for convenience of explanation, a one-to-two correlation table is illustrated. However, even if a correlation table that obtains a plurality of defocus amounts of three or more with respect to one blur evaluation value C is used, a focal point is used. Detection is possible. In short, a feature is to use a correlation table that can obtain a plurality of detected defocus amounts corresponding to one blur evaluation value C, and this is hereinafter referred to as one-to-many correspondence.

  When a one-to-two correlation table is used, two detection defocus amounts are obtained by reference. Of the two, the detected defocus amount (Dx1 in FIG. 4) that is included in the third defocus range, that is, closer to the in-focus position is referred to as a first detected defocus amount. Further, the detected defocus amount (Dx2 in FIG. 4) that is included in the second defocus range, that is, the farther from the in-focus position is referred to as a second detected defocus amount. In general, when a one-to-many correlation table is used, the defocus amount at the position closest to the in-focus position is set as the first detection defocus amount, and the nth detection defocus is increased as the distance from the in-focus position increases. The quantities may be defined sequentially (n ≧ 2).

  Here, there will be a case where the first detection defocus amount and the second detection defocus amount coincide with each other. The case where the first detected defocus amount and the second defocus amount coincide with each other is, for example, the correspondence between Ca and Da in FIG. 4 or the correspondence between Co and Do. That is, this is a one-to-one correspondence in which one detected defocus amount corresponds to one blur evaluation value C. Also in this case, focus detection can be performed by executing the processing shown in the flowcharts of FIGS. Therefore, even in the case of one-to-one correspondence, the same processing as that in the case of one-to-two correspondence is performed.

Next, the superiority of using a one-to-many correlation table over the prior art will be described.
In the case of the prior art, the correlation table stored in the RAM 125b has only data within the third defocus range shown in FIG. In this case, when the blur evaluation value C is calculated in the second defocus range, it cannot be determined whether the in-focus position is in the near side or the infinite side, so that it takes time to detect the focus. On the other hand, in this embodiment, the correlation table stored in the RAM 125b is a correlation table including not only the third defocus range but also the second defocus range. In this case, since the blur evaluation value C and the defocus amount correspond to one-to-many rather than one-to-one, a plurality of detected defocus amounts can be obtained as a result of table reference. That is, a plurality of candidates are obtained for the true defocus amount, but candidate selection is solved by the processing described later. Therefore, in the present embodiment, since the defocus amount can be detected in the second defocus range, in which the defocus amount cannot be detected by the conventional technique, the speed of the DFD AF can be increased.

  In step S209 of FIG. 2, the lens MPU 117 focuses in the direction of the in-focus position by an amount obtained by subtracting the predetermined drive amount Lk from the lens position corresponding to the first detected defocus amount Dx1 in accordance with the control command of the camera MPU 125. The lens 104 is moved. The true defocus amount change corresponding to the predetermined drive amount Lk is Dk. That is, in the processing in S209, the true defocus amount change is an amount “Dx1−Dk” obtained by subtracting Dk from Dx1. The purpose of the processing in this step will be described later, but the change in the true defocus amount will be described with reference to FIG. In the case of the first detected defocus amount Dx1 in FIG. 4, the focus lens 104 is driven toward the in-focus position (Do) by an amount corresponding to “Dx1−Dk”. As a result, the true defocus amount moves to Dx3. Further, in the case of the second detected defocus amount Dx2 in FIG. 4, as a result of the same lens driving, the true defocus amount moves to Dx4. Hereinafter, Dx3 is referred to as a third detected defocus amount, and Dx4 is referred to as a fourth detected defocus amount. After driving the focus lens 104, the process proceeds to S210 in FIG.

  In step S <b> 210, the camera MPU 125 controls the image sensor driving unit 123 to expose the image sensor 122. In order to obtain the blur evaluation value again using the two images before and after the driving of the focus lens 104 in S209, the first exposure is performed on the image sensor 122. Thereafter, the focus lens 104 is driven by the predetermined driving amount Lk to change the blurring state (S211), and the second exposure to the image sensor 122 is performed (S212). The data of the two images captured in S210 and S212 are accumulated in the RAM 125b. As a result, not only the focus detection output signal but also information on a nearby subject including the AF evaluation range can be obtained. In that case, the number of output signals of the image pickup pixels may be thinned and used for focus detection in accordance with the required focus adjustment accuracy. When the above process is completed, the process proceeds to S213.

  In step S213, the focus detection unit 129 calculates the blur evaluation value C. A blur evaluation value C is calculated from data of two images obtained by performing exposure before and after driving the focus lens 104 in S209. As described in S208, the true defocus amount is either the first detected defocus amount Dx1 or the second detected defocus amount Dx2. When the true defocus amount is the first detected defocus amount Dx1 at the time of the processing in S208, the state corresponds to the third detected defocus amount Dx3 after the focus lens is driven in S209. That is, since the focus lens 104 is in a position corresponding to the vicinity of the in-focus position indicated by Do, the value of the blur evaluation value C is close to zero. On the other hand, when the true defocus amount is the second detected defocus amount Dx2 at the time of the processing in S208, the state corresponds to the fourth detected defocus amount Dx4 after the focus lens is driven in S209. That is, since the focus lens 104 is located away from the position corresponding to the focus position indicated by Do, the blur evaluation value C does not become zero.

  In next step S214, the camera MPU 125 compares the absolute value of the blur evaluation value C with a predetermined threshold value. The threshold value set for the absolute value of the blur evaluation value C is determined in advance based on a range in which the viewer of the captured image feels in focus most. If the absolute value | C | of the blur evaluation value is equal to or smaller than the threshold value in S214 (Yes in S214), the process proceeds to S216. If | C | is larger than the threshold value (No in S214), the process proceeds to S215.

  When the absolute value of the blur evaluation value C is equal to or smaller than the threshold value, the blur evaluation value C is a value close to zero and is determined to be in focus. In-focus display is performed in S216, and the display unit 126 displays that it is in focus and presents it to the user. For example, when it is the first detected defocus amount Dx1 at the time of processing in S208 and the third detected defocus amount Dx3 at the time of processing in S209, the blur evaluation value at Dx3 (denoted as C3) is close to zero. . Therefore, the absolute value | C3 | is determined to be equal to or smaller than the threshold value, and the true defocus amount is within the threshold range including Do shown in FIG. After the process of S216, the process proceeds to S217.

  On the other hand, if the absolute value of the blur evaluation value C is larger than the threshold value, it is determined that the in-focus state is not achieved, and the process proceeds to S215 to drive the focus lens 104. For example, if the detected defocus amount Dx2 is the second detected defocus amount Dx2 at the time of processing in S208 and the fourth detected defocus amount Dx4 is the processed time of S209, the absolute value of the blur evaluation value (denoted as C4) at Dx4 | C4 | is larger than the threshold value. In S215, the lens MPU 117 moves the focus lens 104 toward the in-focus position (in-focus direction) by an amount obtained by subtracting the predetermined drive amount Lk from the lens position corresponding to the fourth detected defocus amount Dx4. I do. The process of S215 is executed only in the case of the fourth detected defocus amount Dx4 (No in S214). In this case, the remaining defocus amount up to the in-focus position Do is Dx4. That is, if the focus lens 104 is driven in the in-focus direction by the focus drive amount corresponding to Dx4, the focus lens 104 can be moved to the in-focus position. However, in the present embodiment, the focus lens 104 is driven to a position corresponding to Dx3 that is slightly out of focus with respect to the focus position Do in consideration of the drive error of the focus lens 104 and the detection error of the detected defocus amount. Is done. Then, the process returns to S210 and the exposure process is executed again. After the blur evaluation value C is calculated again in S213, the focus determination is performed again in S214. When the focus display in S216 is finished, the process proceeds to S217, and the AF operation is finished.

  In this embodiment, in the focus determination by the DFD method, with respect to the defocus amounts that exist in the case of one-to-many correspondence with the blur evaluation value C, the true defocus amount is changed by performing minute driving and the blur evaluation value changes. The amount of focus can be determined. By this determination, one is determined from a plurality of defocus amount candidates corresponding to one blur evaluation value. In the example of one-to-two correspondence shown in FIG. 4, there are two detected defocus amounts Dx1 and Dx2 corresponding to the blur evaluation value Cx. In this case, in Dx1, the blur evaluation value decreases due to the minute driving corresponding to Dk, whereas in Dx2, the blur evaluation value increases due to the minute driving corresponding to Dk. According to the present embodiment, one defocus amount corresponding to one blur evaluation value can be determined in a defocus range wider than the range of the conventional correlation table. Therefore, since the DFD AF operation can be performed at high speed, the focus detection time can be shortened.

  In the present embodiment, the correlation table used in S208 of FIG. 2 changes corresponding to the predetermined drive amount Lk of the focus lens 104 in S204. In the present embodiment, the example in which the value of the predetermined drive amount Lk is always constant has been described, but the drive amount Lk may be changed as a variable value. In that case, it is necessary to store data of a plurality of correlation tables corresponding to the number of changes in the drive amount Lk in the storage unit. Further, in this embodiment, when the absolute value of the blur evaluation value C is larger than the threshold value in S214 of FIG. 3, the process returns to S210 after performing the process of S215, and the blur evaluation value is calculated again. Not limited to this, even if the focus lens 104 is moved in the in-focus direction by the focus drive amount corresponding to the fourth detected defocus amount Dx4 in S215, the process may proceed to the in-focus display in S216. Good.

104 Focus lens 116 Focus drive unit 125 Camera MPU
129 Focus detection unit

Claims (7)

An imaging optical system including a focus lens;
An image sensor that photoelectrically converts a subject image formed by the imaging optical system;
Calculation means for obtaining data of a plurality of images having different blur states from an output signal of the image sensor and calculating an evaluation value of the blur state;
Storage means for storing correlation data indicating a correspondence relationship between the evaluation value and the defocus amount;
Determining means for determining a defocus amount by collating the evaluation value calculated by the calculating means with the correlation data stored in the storage means;
Drive means for driving the focus lens according to the drive amount of the focus lens corresponding to the defocus amount determined by the determination means;
When a plurality of defocus amount candidates are detected by the collation, the determining unit drives the focus lens by the driving unit to determine a change in the evaluation value and determines one defocus amount from the candidate. An imaging apparatus characterized by determining   The imaging apparatus according to claim 1, wherein the determining unit determines one defocus amount by selecting a defocus amount having a small absolute value from the candidates.   The determination means drives the focus lens to determine a change in the evaluation value, and among the candidates, the absolute value of the evaluation value decreases as the focus lens is driven in a focusing direction. The imaging apparatus according to claim 1, wherein a focus amount is determined. The calculating means calculates the evaluation value by extracting a frequency band component from the luminance information of the first and second images,
The determination means is a drive corresponding to a defocus amount that is a result of subtracting a difference between the defocus amount related to the first image and the defocus amount related to the second image from the candidate defocus amount. The image pickup apparatus according to claim 1, wherein the focus lens is driven by the driving unit by an amount. The storage means stores the correlation data corresponding to a plurality of defocus amounts for one evaluation value,
The correlation data includes data in a first range in which the absolute value of the evaluation value increases as the distance from the focus position where the defocus amount is zero, and the absolute value of the evaluation value as the distance from the focus position increases. 5. The image pickup apparatus according to claim 1, wherein the image pickup apparatus has data in a second range in which becomes smaller. An imaging apparatus that performs focus adjustment by moving the focus lens in a state where a lens apparatus including a focus lens and a driving unit that drives the focus lens is mounted on a main body,
An image sensor that photoelectrically converts a subject image formed by an imaging optical system including the focus lens;
Calculation means for obtaining data of a plurality of images having different blur states from an output signal of the image sensor and calculating an evaluation value of the blur state;
Storage means for storing correlation data indicating a correspondence relationship between the evaluation value and the defocus amount;
Determining means for determining a defocus amount by collating the evaluation value calculated by the calculating means with the correlation data stored in the storage means;
When a plurality of defocus amount candidates are detected by the collation, the determining unit drives the focus lens by the driving unit to determine a change in the evaluation value and determines one defocus amount from the candidate. And an output of a control command corresponding to the determined defocus amount to the drive means. A control method executed by an imaging apparatus including an imaging element that photoelectrically converts a subject image formed by an imaging optical system including a focus lens,
A calculation step of obtaining data of a plurality of images having different blur states from an output signal of the image sensor and calculating an evaluation value of the blur state;
A determination step of determining the defocus amount by comparing the evaluation value calculated in the calculation step with correlation data indicating a correspondence relationship between the evaluation value and the defocus amount;
A drive step of driving the focus lens according to the drive amount of the focus lens corresponding to the defocus amount determined in the determination step;
In the determining step, when a plurality of defocus amount candidates are detected by the collation, the focus lens is driven to determine a change in the evaluation value, and one defocus amount is determined from the candidates. And a method of controlling the imaging apparatus.

Images