JP2014202799A - Image capturing device and control method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image capturing device capable of preventing false focusing when performing focus detection of an image plane phase difference detection type using filtered signals; and a control method therefor.SOLUTION: A defocus amount for an image capturing optical system is obtained using image signals, which is generated by an image sensor and used for automatic focus detection of a phase difference detection type, after applying a filtering process thereon for removing predetermined high-frequency and low-frequency components. A contrast evaluation value is also obtained from the signals acquired from the image sensor. A focusing lens is kept driven as long as the contrast evaluation value keeps increasing even if a trend of reliability of the defocus amount obtained while driving the focusing lens in a certain direction switches from upward to downward.

Description

  The present invention relates to an imaging apparatus and a control method thereof, and more particularly to a focus detection adjustment technique using imaging plane phase difference AF.

  In recent years, an imaging apparatus represented by a single-lens reflex camera has been frequently photographed while viewing a live view (LV) screen (hereinafter referred to as LV photographing). It is demanded.

There are a phase difference detection method and a contrast detection method as main automatic focus detection (autofocus: AF) methods of the imaging apparatus.
In the phase difference detection method, the defocus amount of the photographic optical system is calculated from the phase difference between a pair of image signals obtained by imaging light beams from a subject that has passed through different exit pupil regions in the photographic optical system on a pair of line sensors. Ask. If the focus lens is moved by an amount corresponding to the defocus amount, the photographing optical system is brought into focus on the subject (see Patent Document 1). However, in a general configuration in which the optical path to the image sensor is interrupted when a light beam is imaged on the phase difference detection line sensor, focus adjustment by the phase difference detection method cannot be performed while performing LV imaging.

  On the other hand, in the contrast detection method, an in-focus state is obtained by searching for a focus lens position where a contrast evaluation value generated from an image pickup signal obtained using an image pickup element is maximized (see Patent Document 2). The contrast detection method is suitable for AF during LV shooting because it performs focus adjustment based on the imaging signal, and is the most mainstream AF method during LV shooting in recent years. However, the contrast detection method cannot easily determine the position and direction in which the focus lens is moved to focus on the subject. For this reason, the contrast detection method may require time for focusing, may cause the focus lens to move in the wrong direction, or may pass through the focus position. In particular, at the time of moving image shooting, an image shot while the focus lens is moving is also recorded. Therefore, the moving time and moving operation of the focus lens up to the in-focus state affect the image quality of the recorded moving image.

  Therefore, in AF control during moving image shooting, a high-quality focus adjustment operation that has a small influence on a recorded image is required rather than a speed (responsiveness) until a focused state is reached. For this reason, an imaging surface phase difference detection method has been proposed as a method capable of high-quality focus adjustment operation even during LV shooting. The imaging surface phase difference detection method is a method for obtaining a defocus amount based on a phase difference between a pair of signals obtained from an imaging element.

  As a method for realizing the imaging surface phase difference detection method, there is a method of detecting the defocus amount at the same time as imaging by dividing the pupil of the imaging pixel of the imaging element by a microlens and receiving subject light by a plurality of focus detection pixels. Proposed. In Patent Document 3, each photodiode is configured to receive light from a different pupil plane of an imaging lens by dividing photodiodes collected by one microlens in one pixel. Has been. This makes it possible to adjust the focus by the imaging plane phase difference detection method from the outputs of the two photodiodes. By using the imaging surface phase difference detection method, it is possible to obtain the defocus amount and know the moving direction and the moving amount of the focus lens even during LV shooting, so that the focus lens can be moved at high speed and with high quality. .

Japanese Patent Laid-Open No. 09-054242 JP 2001-004914 A JP 2001-083407 A

  Conventionally, before detecting the defocus amount, filtering is performed on a signal (A image signal) obtained from one of two photodiodes provided in one pixel and a signal (B image signal) obtained from the other, Specifically, band pass filter processing is generally performed. The purpose of the filter processing is, for example, noise removal, correction of a level difference between the A image signal and the B image signal, signal amplification, and the like.

  FIG. 6A is a diagram schematically showing how the noise component is reduced and the SN ratio of the image signal is improved by the high-frequency component cut function of the bandpass filter. By providing a band-pass filter that blocks high-frequency components that are considered to be noise components, the influence of noise in phase difference detection can be suppressed, and detection accuracy can be improved.

  FIG. 6B is a diagram showing the edge enhancement effect of a signal by the DC wave component (low frequency component) cut function of the bandpass filter. The DC wave component cut improves the focus detection accuracy for subjects that change in luminance only in one direction, improving the deterioration of focus detection accuracy due to residual corrections such as shading correction, and increasing / decreasing the luminance monotonously. There is an effect.

  Normally, the closer to the in-focus state, the higher the degree of coincidence between the A image signal and the B image signal, so the phase difference detection accuracy between them and the accuracy of the defocus amount obtained from the phase difference also increase. However, if there is a subject having a luminance change similar to the signal changed by applying the bandpass filter, the focus position may be erroneously detected.

  FIG. 7 shows an example of signal waveforms before and after application of the bandpass filter. As described above, since the bandpass filter has an edge enhancement effect, small edges existing in the signal before applying the filter shown in FIG. 7A are emphasized in the signal after applying the filter in FIG. 7B. It is in the shape. In this case, if there is an object with vertical stripes as shown in FIG. 7C, it has a luminance change similar to a mountain-shaped part deformed by the filter, and therefore it may be erroneously determined that the object is in focus. . In this case, the focus lens stops before reaching the true in-focus position, or passes through the in-focus position.

  This phenomenon will be further described with reference to FIG. When a vertical stripe reference chart is photographed, the waveforms of the A image signal and B image signal after application of the bandpass filter according to the example of the A image signal and B image signal before application of the bandpass filter and the degree of focus are obtained. An example is shown. By applying a bandpass filter, the phase difference of the waveform can be detected more clearly when the degree of focus is high, but when the degree of focus is low, it is different from the original focus position. This causes a false focus state in which the phase difference looks small.

  The present invention has been made in view of such a problem of the prior art, and an imaging apparatus capable of suppressing false focusing when performing focus detection by an imaging surface phase difference detection method using a signal after applying a filter, and the imaging apparatus The purpose is to provide a control method.

  An object of the present invention is to provide an image sensor capable of generating an image signal used for phase difference detection type automatic focus detection, and filter means for applying a filter process for removing predetermined high-frequency components and low-frequency components to the image signal. First acquisition means for acquiring the defocus amount of the photographing optical system based on the image signal to which the filter processing is applied, and second for acquiring the contrast evaluation value based on the signal output from the image sensor An acquisition unit, and a control unit that controls driving of a focus lens included in the photographing optical system, and the control unit has reliability of a defocus amount obtained when the focus lens is driven in a fixed direction. If the contrast evaluation value continues to rise even when the rise goes to the fall, the image pickup apparatus is characterized in that the drive of the focus lens is continued.

  With such a configuration, according to the present invention, it is possible to provide an imaging apparatus capable of suppressing false focusing when performing focus detection by an imaging plane phase difference detection method using a signal after applying a filter, and a control method thereof. Can do.

1 is a block diagram illustrating an example of a functional configuration of an interchangeable lens camera as an example of an imaging apparatus according to an embodiment. The figure which shows the pixel structural example of a non-imaging surface phase difference detection system and an imaging surface phase difference detection system The figure which shows the example of the reliability information table in embodiment of this invention The flowchart which shows the focus detection operation | movement in embodiment of this invention. The figure which shows typically the focus detection operation | movement in embodiment of this invention. Illustration of bandpass filter high frequency component cut and DC wave component cut The figure explaining the change of the signal waveform by the band pass filter The figure explaining the phenomenon that the image signal which applied the band pass filter falsely focuses

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiment described below is merely an example, and the present invention is not limited to the configuration described in the embodiment.

FIG. 1 is a block diagram illustrating a functional configuration example of an interchangeable lens camera as an example of an imaging apparatus according to an embodiment of the present invention.
The imaging apparatus according to the present embodiment includes a replaceable lens 10 and a main body 20. The lens control unit 106 that controls the overall operation of the lens and the camera control unit 212 that controls the overall operation of the camera system including the lens 10 can communicate with each other through terminals provided on the lens mount.

  First, the configuration of the lens 10 will be described. The fixed lens 101, the diaphragm 102, and the focus lens 103 constitute a photographing optical system. The diaphragm 102 is driven by the diaphragm driving unit 104 and controls the amount of light incident on the image sensor 201 described later. The focus lens 103 is driven by the focus lens driving unit 105, and the focus distance of the photographing optical system changes according to the position of the focus lens 103. The aperture driving unit 104 and the focus lens driving unit 105 are controlled by the lens control unit 106 to determine the aperture amount of the aperture 102 and the position of the focus lens 103.

  The lens operation unit 107 is an input device group for the user to make settings related to the operation of the lens 10 such as AF / MF mode switching, shooting distance range setting, camera shake correction mode setting, and the like. When the lens operation unit 107 is operated, the lens control unit 106 performs control according to the operation.

  The lens control unit 106 controls the aperture driving unit 104 and the focus lens driving unit 105 according to a control command and control information received from a camera control unit 212 described later, and transmits lens control information to the camera control unit 212. .

Next, the configuration of the main body 20 will be described. The main body 20 is configured to acquire an imaging signal from a light beam that has passed through the imaging optical system of the lens 10.
The image sensor 201 is composed of a CCD or a CMOS sensor. The light beam incident from the photographing optical system of the lens 10 forms an image on the light receiving surface of the image sensor 201 and is converted into a signal charge corresponding to the amount of incident light by a photodiode provided in a pixel arranged in the image sensor 201. . The signal charge accumulated in each photodiode is sequentially read out from the image sensor 201 as a voltage signal corresponding to the signal charge from a drive pulse output from the timing generator 215 in accordance with a command from the camera control unit 212.

  The image sensor 201 according to the present embodiment includes two photodiodes in one pixel, and generates an image signal used for automatic focus adjustment (hereinafter referred to as an imaging surface phase difference AF) by an imaging surface phase difference detection method. Is possible. 2A schematically illustrates an example of a pixel configuration that does not correspond to the imaging surface phase difference AF, and FIG. 2B schematically illustrates an example of a pixel configuration that corresponds to the imaging surface phase difference AF. Here, in any case, it is assumed that a Bayer array primary color filter is provided. In the pixel configuration of FIG. 2B corresponding to the imaging surface phase difference AF, one pixel in FIG. 2A is divided into two in the horizontal direction on the paper surface, and AB photodiodes (light receiving regions) are provided. . Note that the division method illustrated in FIG. 2B is an example, and other methods may be used, or different division methods may be applied depending on the pixels.

  The light beam incident on each pixel is separated by a microlens and received by two photodiodes provided in the pixel, whereby two signals for imaging and AF can be acquired by one pixel. That is, signals (A, B) obtained by two photodiodes A, B (A pixel, B pixel) in a pixel are two image signals for AF, and an addition signal (A + B) is an imaging signal. is there. Note that the pair of image signals used in the imaging plane phase difference AF is also a plurality of A so that the pair of image signals used in the normal phase difference detection AF is generated by a pair of line sensors having a plurality of pixels. It is obtained from the output of a pixel and a plurality of B pixels. Based on the AF signal, an AF signal processing unit 204 (to be described later) performs a correlation operation on the two image signals to calculate a defocus amount and various types of reliability information.

  The CDS / AGC / AD converter 202 performs correlated double sampling for removing reset noise, gain adjustment, and signal digitization on the imaging signal and AF signal read from the imaging element 201. The CDS / AGC / AD converter 202 outputs the imaging signal to the image input controller 203 and the imaging surface phase difference AF signal to the AF signal processing unit 204.

  The image input controller 203 stores the imaging signal output from the CDS / AGC / AD converter 202 in the SDRAM 209 via the bus 21. The image signal stored in the SDRAM 209 is read by the display control unit 205 via the bus 21 and displayed on the display unit 206. In the operation mode in which the image pickup signal is recorded, the image signal stored in the SDRAM 209 is recorded on the recording medium 208 by the recording medium control unit 207.

  The ROM 210 stores a control program executed by the camera control unit 212 and various data necessary for the control, and the flash ROM 211 stores various setting information regarding the operation of the main body 20 such as user setting information. .

  The AF signal processing unit 204 (filter unit) removes predetermined high-frequency components and low-frequency components from the two image signals for AF output from the CDS / AGC / AD converter 202 (band-pass filter). Processing). Further, the AF signal processing unit 204 (first acquisition unit and second acquisition unit) performs a correlation operation on the image signal to which the filter processing is applied, and performs defocus amount and reliability information (two-image coincidence degree). 2 image steepness, contrast evaluation value, saturation information, scratch information, etc.). The AF signal processing unit 204 outputs the calculated defocus amount and reliability information to the camera control unit 212. The contrast evaluation value can be obtained in the same manner as an evaluation value conventionally used in contrast detection type automatic focus detection.

  The camera control unit 212 stores the reliability information (reliability) received from the AF signal processing unit 204 in the SDRAM 209 in the form of a table. FIG. 3 shows an example of the reliability information table. The reliability information table of the present embodiment includes at least one reliability information (for example, two-image coincidence degree) representing the reliability of the contrast evaluation value and the defocus amount among the reliability information output from the AF signal processing unit 204. Are stored in time series (in the order output by the AF signal processing unit 204). The camera control unit 212 changes the setting of the AF signal processing unit 204 as necessary based on the defocus amount and reliability information obtained by the AF signal processing unit 204. For example, when the defocus amount is greater than or equal to a predetermined amount, the region for performing the correlation calculation is set to be wide, or the type of the band pass filter is changed according to the contrast evaluation value.

  In the present embodiment, a total of three signals of the imaging signal and the two AF image signals are acquired from the imaging element 201, but the present invention is not limited to this method. Considering the load of the image sensor 201, for example, a total of two signals of an imaging signal and one AF image signal may be taken out, and the difference between the imaging signal and the AF signal may be used as another AF image signal.

  The camera control unit 212 performs control by exchanging information with each functional block in the main body 20. The camera control unit 212 performs not only processing in the main body 20 but also power ON / OFF, setting change, recording start, AF control start, confirmation of recorded video, etc. according to input from the camera operation unit 214. Execute various camera functions operated by the user. Further, the camera control unit 212 sends a control command / control information for the lens 10 to the lens control unit 106, and acquires information about the lens 10 from the lens control unit 106.

  The camera control unit 212 is, for example, one or more programmable processors, and realizes the operation of the entire camera system including the lens 10 by executing a control program stored in the ROM 210, for example.

  The lens drive speed setting unit 213 represents one part of the function of the camera control unit 212, and determines the drive speed of the focus lens 103 via the lens control unit 106 and the focus lens drive unit 105 in the lens 10. Details will be described later with reference to a flowchart illustrating control of the main body 20.

Next, the focus detection control operation executed by the main body 20 will be described using the flowchart of FIG.
In S <b> 1, the camera control unit 212 uses the image sensor 201, the CDS / AGC / AD converter 202, and the timing generator 215 to read out the charge accumulation control (sensor accumulation operation) of the image sensor 201 and the image sensor 201. Perform preprocessing on the signal.
In the processing of S 1, among the pixel signals accumulated and read out in the image sensor 201, for example, a focus detection signal obtained from a pixel in a preset focus detection area is input to the CDS / AGC / AD converter 202. Is done. The CDS / AGC / AD converter 202 outputs a pair of signals (A image signal, B image signal) for phase difference AF to the AF signal processing unit 204.

In S2, the camera control unit 212 determines whether sensor accumulation and preprocessing have been completed. If not completed, the process proceeds to S1. If completed, the process proceeds to S3.
In S3, the AF signal processing unit 204 calculates a defocus amount from the A image signal and the B image signal output from the CDS / AGC / AD converter 202.
In S4, the AF signal processing unit 204 calculates reliability information (two-image coincidence, two-image steepness, contrast evaluation value, saturation information, flaw information, etc.) from the A image signal and the B image signal.

  In S <b> 5, the camera control unit 212 obtains at least one of reliability information (here, the two-image coincidence level) that is output from the AF signal processing unit 204 and represents the reliability of the defocus amount, and the contrast evaluation value. Stored in the reliability information table (FIG. 3). The reliability in FIG. 3 may be a value of reliability information, or may be a value obtained by further converting reliability information into reliability. Here, in order to facilitate explanation and understanding, reliability information = reliability. For example, when the two-image coincidence is used as the reliability information, the two-image coincidence becomes higher as the defocus amount reliability becomes higher. Therefore, the value of the degree of coincidence between the two images can be used as an index of the reliability of the defocus amount.

  In S6, the camera control unit 212 determines whether or not the reliability is higher than a threshold value determined in advance based on simulation, actual measurement, or the like. If it is higher, the process proceeds to S7, and if not higher, the process proceeds to S8.

  In S7, the camera control unit 212 communicates with the lens control unit 106, drives the focus lens 103 using the defocus amount calculated in S3 through the focus lens driving unit 105, and returns the process to S1.

  In S <b> 8, the camera control unit 212 determines whether driving of the focus lens 103 is started. If the focus lens 103 is not driven, the camera control unit 212 starts search driving (drive for searching the subject by driving the focus lens by a predetermined amount in a certain direction regardless of the defocus amount) in S14 (one step is executed). ).

When the search driving of the focus lens 103 is started, the camera control unit 212 refers to the reliability information table in S9, and determines the (latest) reliability calculated in S4 and the reliability calculated last time. Compare.
In S10, the camera control unit 212 determines whether or not the reliability is higher than the previous time. If the reliability is higher, the process proceeds to S14. If not, the process proceeds to S11.

In S11, the camera control unit 212 and the reliability information table are referred to, and the (latest) contrast evaluation value calculated in S4 is compared with the contrast evaluation value calculated last time.
In S12, the camera control unit 212 advances the process to S14 when the contrast evaluation value is higher than the previous time, and advances to S13 when the contrast evaluation value is not higher.

In step S13, the camera control unit 212 determines that the focus is achieved, stops the lens search drive, and ends the process.
In S14, the camera control unit 212 communicates with the lens control unit 106, executes search driving of the focus lens 103 (one step) through the focus lens driving unit 105, and returns the process to S1.

  FIG. 5 schematically shows the focus detection operation executed in the flowchart of FIG. Here, the case where the two-image coincidence is used as the reliability of the defocus amount is shown, but another index may be used. In FIG. 5, the up and down arrows indicating the two-image coincidence degree and the contrast evaluation value indicate a scale in which the upper side of the paper indicates a good value and the lower side indicates a bad value. The position of does not mean the best value or the worst value. Further, FIG. 5 shows only a state where the reliability of the defocus amount is equal to or less than the threshold from the viewpoint of explaining the effect of preventing false focusing.

  First, at the beginning (time t0), in a state where the blur is large, both the two-image coincidence degree and the contrast evaluation value are low (bad) values, and the search drive is not started, so the flow of S6, S8, and S14 Search drive is started. If the search direction of the lens is correct, each time the search drive is executed step by step in S14, the in-focus state is approached, so the two-image coincidence degree and the contrast evaluation value are better than the previous value (higher values). ).

  At time t1, the two-image coincidence reaches a maximum and then decreases. As described above, this maximum is an incorrect maximum as in the case where there is a subject having a luminance change similar to that of an image signal to which the bandpass filter is applied. In some cases, the position was erroneously determined as the in-focus position.

  However, as described with reference to FIG. 4, in this embodiment, even if the reliability is changed from increasing to decreasing and becomes no higher than the previous value (S10, NO), if the contrast evaluation value continues to increase (S12). , YES), the search drive is continuously executed. Therefore, the search drive is continued even after the time t1, and the two-image coincidence starts increasing again as the correct focus position is approached thereafter (time t2).

  As described above, in this embodiment, if the contrast evaluation value continues to increase even when the reliability of the defocus amount changes from an increase to a decrease, the reliability peak is an apparent in-focus position and is correct. The focus lens is driven by determining that the focus position is not reached. Therefore, even when the defocus amount is acquired based on an image signal to which a filter process that cuts high frequencies and low frequencies is applied, it is possible to suppress poor driving of the focus lens due to a change in the signal waveform due to the filter process. Can do. The drive with poor quality here is a drive that exerts an undesirable influence on a moving image obtained during the driving of the focus lens, such as temporarily stopping the driving of the focus lens or the driving direction being not constant.

  Although the present invention has been described based on the exemplary embodiments, the present invention is not limited to the specific embodiments described herein, and various modifications and changes can be made within the scope described in the claims. Is possible.

For example, the band-pass filter may be a combination of a high-pass filter and a low-pass filter, and the reliability of the defocus amount may be an index other than the degree of coincidence of two images such as the degree of steepness of two images.
In addition, the imaging element 201 may, for example, provide a pair of focus detection pixels (pixel pairs) with a limited aperture between imaging pixels that can receive the entire pupil region without limiting the aperture, at a predetermined pitch. A plurality of arrangements may be used. In this case, a signal obtained from the imaging pixel is an imaging signal, and a signal obtained from the pair of focus detection pixels is two image signals for AF.

  In the above-described embodiment, the contrast evaluation value is acquired from the image signal to which the filter process is applied. However, the contrast evaluation value may not be acquired from the image signal to which the filter process is applied. For example, it is possible to use a contrast evaluation value acquired based on an arbitrary signal output from the image sensor 201 such as an image signal before filter processing or an image signal before or after filter processing.

  The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

Claims (6)

An image sensor capable of generating an image signal used for automatic focus detection of a phase difference detection method;
Filter means for applying filter processing for removing predetermined high-frequency components and low-frequency components to the image signal;
First acquisition means for acquiring a defocus amount of the photographing optical system based on the image signal to which the filter processing is applied;
Second acquisition means for acquiring a contrast evaluation value based on a signal output from the image sensor;
Control means for controlling driving of a focus lens included in the photographing optical system,
If the contrast evaluation value continues to increase even if the reliability of the defocus amount obtained when the focus lens is driven in a certain direction changes from increasing to decreasing, the control means An imaging apparatus characterized by continuing to drive a lens.   The imaging apparatus according to claim 1, wherein the control unit drives the focus lens in a certain direction when reliability of the defocus amount is equal to or less than a predetermined threshold value.   3. The control unit according to claim 1, wherein when the reliability of the defocus amount is higher than a predetermined threshold, the control unit drives the focus lens based on the defocus amount. The imaging device described in 1. The first acquisition means further acquires the reliability of the defocus amount;
4. The imaging apparatus according to claim 1, wherein the control unit uses a reliability of the defocus amount acquired by the first acquisition unit. 5.   The imaging apparatus according to any one of claims 1 to 4, further comprising the imaging optical system. A method for controlling an imaging apparatus including an imaging element capable of generating an image signal used for automatic focus detection of a phase difference detection method,
A filtering step of applying a filtering process to remove predetermined high frequency components and low frequency components to the image signal;
A first acquisition step of acquiring a defocus amount of the photographing optical system based on the image signal to which the filtering process is applied;
A second acquisition step of acquiring a contrast evaluation value based on a signal output from the image sensor;
Even if the reliability of the defocus amount obtained when the focus lens included in the photographing optical system is driven in a certain direction is changed from an increase to a decrease, if the contrast evaluation value continues to increase, And a control step of continuing to drive the focus lens.

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