Image capturing device, method of controlling the same, program, and recording medium

Abstract

To provide an image capturing device that enables accurate focusing.SOLUTION: An image capturing device is provided, comprising an image capturing unit for capturing an image of an object formed by an image capturing optical system, a signal processing unit for executing noise reduction processing on the image according to the images of a plurality of frames captured by the image capturing unit, an acquisition unit for acquiring an evaluation value indicative of contrast of an image subjected to noise reduction processing by the signal processing unit, and a control unit configured to control the focus position of the image capturing optical system according to the evaluation value, where the control unit determines at least either of a driving speed and focus position of the image capturing optical system according to intensity of the noise reduction processing.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an imaging device, a control method for an imaging device, a program, and a storage medium. [Background technology]

[0002] 2. Description of the Related Art Conventionally, 3DNR (Digital Noise Reduction) processing is known as a type of digital signal processing for reducing image noise, which detects and removes noise by comparing multiple consecutive frames.

[0003] Patent Document 1 discloses a technique for displaying focus information using an evaluation value stabilized by time filtering processing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2015-34869 Summary of the Invention [Problem to be solved by the invention]

[0005] When performing autofocus (hereinafter referred to as AF) control to control the focus position of a lens using an evaluation value stabilized by a time filtering process as in Patent Document 1, the evaluation value is smoothed over multiple frames. As a result, a focus position that is shifted from the original focus position becomes a peak position (a position where the evaluation value is maximum), and there are cases where the focus position cannot be detected with high accuracy.

[0006] Therefore, an object of the present invention is to provide an imaging device capable of focusing with high accuracy when AF control is performed based on an image that has been subjected to 3DNR processing. [Means for solving the problem]

[0007] In order to solve the above problem, an imaging device according to one aspect of the present invention has an imaging unit that captures an image of a subject using an imaging optical system, a signal processing unit that performs noise reduction processing on the image based on multiple frames of images captured by the imaging unit, an acquisition unit that acquires an evaluation value indicating the contrast of the image on which noise reduction processing has been performed by the signal processing unit, and a control unit that controls a focus position of the imaging optical system based on the evaluation value, and is characterized in that the control unit determines at least one of a drive speed of the imaging optical system and a focus position of the imaging optical system based on the strength of the noise reduction processing. Effect of the Invention

[0008] According to the present invention, it is possible to provide an imaging apparatus capable of focusing with high accuracy. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing a configuration of an imaging device according to a first embodiment; [Diagram 2] Flowchart showing AF operation of a conventional imaging device [Diagram 3] A diagram showing the relationship between AF evaluation value and 3DNR [Figure 4] A diagram showing the relationship between AF evaluation value and 3DNR [Diagram 5] 1 is a flowchart showing an AF operation of an imaging apparatus according to a first embodiment. [Figure 6] FIG. 1 is a diagram showing a hardware configuration of an imaging apparatus according to a first embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiment for carrying out the present invention will be described in detail with reference to the attached drawings. The embodiment described below is one example of a means for realizing the present invention, and should be appropriately modified or changed depending on the configuration of the device to which the present invention is applied and various conditions, and the present invention is not limited to the following embodiment.

[0011] <Embodiment 1> (Device configuration) 1 shows the configuration of an image capturing apparatus according to this embodiment. The image capturing apparatus 100 includes a zoom lens 101, a focus lens 102, an aperture unit 103, a bandpass filter 104, a color filter 105, an image sensor 106, an AGC (Auto Gain Control) 107, an AD conversion unit 108, a signal processing unit 109, a communication unit 110, an acquisition unit 112, a control unit 113, and a drive unit 114.

[0012] The zoom lens 101, the focus lens 102, and the aperture unit 103 constitute the imaging optical system of the imaging device 100. The zoom lens 101 changes the focal length by moving in the optical axis direction. The focus lens 102 changes the focus position by moving in the optical axis direction. The aperture unit 103 adjusts the amount of light incident on the imaging element 106.

[0013] Light passing through the imaging optical system forms an image of a subject on an imaging surface of an imaging element 106 via a bandpass filter 104 and a color filter 105. The bandpass filter 104 may be removable from the optical path of the imaging optical system.

[0014] The image sensor 106 is an image capturing section that captures an image of a subject using an imaging optical system. Specifically, the image sensor 106 photoelectrically converts light incident on the image sensor 106 and outputs the image of the subject as an analog electrical signal (image signal).

[0015] The AGC 107 amplifies the electrical signal output from the image sensor 106 based on a set amplification factor and outputs the amplified signal.

[0016] The AD conversion unit 108 converts the analog image signal amplified by the AGC 107 into a digital image signal and outputs the digital image signal to the signal processing unit 109 .

[0017] The signal processing unit 109 performs various image processing on the digital image signal to generate an image. The image processing can perform 3DNR processing to reduce noise contained in the digital imaging signal. In addition, noise reduction processing by spatial filtering and white balance adjustment can also be performed. The 3DNR processing is a process of detecting noise contained in the digital image signal based on a comparison of signals of multiple consecutive frames output by the AD conversion unit 108, and smoothing the noise between multiple frames. This makes it possible to reduce noise on a pixel-by-pixel basis. The user can arbitrarily set whether or not to perform 3DNR processing and the strength of the 3DNR processing. The number of frames used for smoothing in the 3DNR processing depends on the set strength. The higher the strength, the more frames are used, and therefore a stronger noise reduction effect can be obtained. On the other hand, the higher the strength (i.e., the more frames are used), the more blurring may occur in the image when capturing a moving object, and the responsiveness of the evaluation value indicating the contrast may decrease.

[0018] The communication unit 110 is a network interface for outputting the image generated by the signal processing unit 109 to the monitor device 110 connected by wired or wireless communication, etc. Note that the image does not necessarily need to be output to the monitor device 110, and may be stored in a main storage device (not shown) such as a memory or an auxiliary storage device (not shown) such as a hard disk.

[0019] The acquisition unit 112 acquires an evaluation value indicating the contrast of the image generated by the signal processing unit 109. Specifically, the evaluation value can be acquired by generating a specific frequency component (e.g., a high frequency component) from among the spatial frequency components of the image as the evaluation value. Here, it is assumed that the evaluation value of the image on which the signal processing unit 109 has performed 3DNR processing is acquired.

[0020] The control unit 113 performs AF control based on the evaluation value acquired by the acquisition unit 112. In addition, in the case of manual focus control, the control unit 113 performs focus control based on user operation information input via the communication unit 110. More specifically, the focus control is performed by driving the focus position of the focus lens 101 of the imaging optical system in the optical axis direction. Although the switching between AF control and manual is also performed based on user operation information via the communication unit 110, the switching between AF and manual may be automatically performed when a predetermined condition is satisfied (for example, temperature focus correction, etc.). Although the focus control will be described in more detail later, at least one of the driving speed and the focus position of the imaging optical system is determined by the control unit 113 based on the strength of the 3DNR processing.

[0021] The driving unit 114 controls the focus lens 102 to the focus position instructed by the control unit 113 .

[0022] (Conventional AF control) 2 is a flowchart showing an example of conventional AF control. The AF control by the control unit 113 is executed by a CPU (Central Processing Unit) of the imaging device reading a program stored in a non-volatile storage medium such as a ROM (Read Only Memory). The hardware configuration of the imaging device 100 will be described later.

[0023] AF control is performed by combining a method of moving the imaging optical system widely at high speed to identify the approximate focus position (hereinafter referred to as the hill-climbing method) and a method of moving the imaging optical system minutely to identify the focus position with high precision (hereinafter referred to as the wobbling method).

[0024] After the AF operation starts, the wobbling method is performed. In the wobbling method, the focus lens 102 is slightly driven from the current focus position, and the evaluation values ​​before and after the drive are compared to identify the direction in which the evaluation value increases. The identified direction is set as the direction of the in-focus position as viewed from the focus position (focus direction), and the focus lens 102 is gradually driven in the focus direction.

[0025] In step S201, a wobbling amount according to depth is set. Generally, the wobbling amount is kept within one depth to make image changes caused by focus control less noticeable, but this is not always the case when the current focus position is not near the in-focus position.

[0026] In step S202, the lens is driven to the front focus position, and in step S203, the change in the evaluation value before and after the front focus drive (whether it increased or decreased) is obtained. The front focus position is the position obtained by subtracting the amount of wobbling from the wobbling center position (the current focus position in the first step).

[0027] Next, in step S204, the lens is driven to the rear focus position, and in step S205, the change in the evaluation value due to the rear focus drive (whether it has increased or decreased) is obtained. The rear focus position is a position obtained by adding the amount of wobbling to the wobbling center position.

[0028] In step S206, the focus direction is specified based on the results of the change in evaluation value before and after front focus drive and the change in evaluation value before and after rear focus drive. When the evaluation value increases before and after front focus drive and decreases before and after rear focus drive, the focus direction is specified as the front focus direction. When the change in evaluation value is opposite between front focus and rear focus, the rear focus direction is specified as the focus direction. At the same time, the wobbling center position is shifted in the focus direction.

[0029] In step S207, it is determined whether the focus direction is the same continuously. If it is the same continuously, the process proceeds to the hill-climbing method (step S208), and if not, it is determined whether the number of reversals of the focus direction is equal to or greater than a predetermined threshold (step S209). If the number of reversals is less than the predetermined threshold, the process returns to step S201 to continue the wobbling method, and if it is equal to or greater than the predetermined threshold, it is considered that the focus position has been reached, and the AF process is terminated.

[0030] In the hill climbing method (step S208), the focus lens 102 is driven at high speed in the focusing direction to detect the focus position (peak position) at which the evaluation value is maximized. While the focus lens 102 is driven, the evaluation value and focus position are stored, and it is determined whether the difference between the stored evaluation value and the current evaluation value is equal to or greater than a predetermined threshold (step S210). If it is equal to or greater than the predetermined threshold, the hill climbing method is terminated, and the stored focus position is detected as the peak position (step S211). When the peak position is detected, the focus lens 102 is driven to the peak position (S212), and the process returns to the wobbling method (step S201).

[0031] If the evaluation value is less than the predetermined threshold in step S210, it is determined in step S213 whether the evaluation value has decreased a predetermined number of times in succession. If the evaluation value has decreased in succession, it is considered that the lens is being driven in the opposite direction to the focusing direction, so the direction of the hill climbing is reversed (step S214) and the process returns to step S209.

[0032] (Relationship between evaluation value and 3DNR processing) 3 shows the transition of the evaluation value for each 3DNR processing strength (off / weak / strong) when the focus position of the imaging optical system (focus lens 102) is changed from 0 to 280 at a drive speed of 10 per frame. Note that the in-focus position is set to 140 here.

[0033] Graph 301 shows the evaluation value when the strength of 3DNR processing is off, i.e., when 3DNR processing is not performed. When 3DNR processing is not performed, there is no influence from the previous frame, so the responsiveness is high and the in-focus position 140 is detected as the peak position.

[0034] Graph 302 shows the evaluation value when the strength of 3DNR processing is weak, and graph 303 shows the evaluation value when the strength of 3DNR processing is strong. 3DNR processing is an image noise reduction process based on images of multiple frames, and smooths an image at the current focus position and at least one past focus position. Therefore, the evaluation value obtained from an image on which 3DNR processing has been performed is affected by past frames (previous frames), so the responsiveness of the evaluation value deteriorates, and a focus position that is shifted from the original focus position is detected as the peak position. The amount of deviation becomes larger as the strength of 3DNR processing becomes stronger (i.e., the more frames are used in 3DNR processing).

[0035] Due to this deterioration in responsiveness, the timing at which the hill-climbing method ends is delayed in the above-mentioned hill-climbing method, making it easier for overshooting to occur. Also, even if the camera is driven to the peak position detected by the hill-climbing method, the image will remain blurred because it is not aligned with the original focus position. Furthermore, in the subsequent wobbling method, the focus direction is continuously in the same direction, so there is a possibility that a hunting phenomenon will occur in which the camera will switch back to the hill-climbing method.

[0036] Fig. 4 is a diagram showing the transition of the evaluation value when the drive speed of the imaging optical system (focus lens 102) is halved (at a speed of driving 5 times per frame) in comparison with the example of Fig. 3. When 3DNR processing is performed (graphs 402, 403), the amount of deviation between the in-focus position and the peak position is reduced more than in Fig. 3. In other words, the drive speed of the imaging optical system is determined so that the stronger the strength of 3DNR (i.e., the more frames there are in the 3DNR processing), the slower the drive speed of the imaging optical system becomes. This makes it possible to reduce the amount of deviation from the peak position to the in-focus position.

[0037] However, slowing down the focus drive speed has side effects such as an increased focus time and vulnerability to disturbances and noise. In addition, as shown in FIG. 4, even if the drive speed of the imaging optical system is slowed down, the deviation of the peak position from the original focus position cannot be completely suppressed. Therefore, the detected peak position is corrected. In this embodiment, the stronger the strength of the 3DNR processing (i.e., the more frames there are in the 3DNR processing), the larger the correction amount of the peak position is. For example, when the strength of the 3DNR processing is weak (graph 302), correction is performed for one frame, and when the strength of the 3DNR processing is strong (graph 303), correction is performed for three frames. This allows the original focus position to be detected as the peak position, so that accurate focusing can be achieved by driving the focus position of the focus lens 102 to the peak position obtained by correction.

[0038] (Operation description) 5 is a flowchart showing an example of the AF operation of the imaging device according to this embodiment. The operation of this flowchart is executed by the CPU of the imaging device 100 loading a program from the ROM. Note that steps similar to those in the description of the conventional AF operation are given the same reference numerals and will not be described.

[0039] Before executing the hill climbing method in step S209, the driving speed of the imaging optical system is set according to the set strength of the 3DNR processing (step S501). As described above, the strength of the 3DNR processing can be arbitrarily set by the user. The strength of the 3DNR processing is acquired based on a user setting value (e.g., off / weak / strong), an internal setting value set within the imaging device, the amount of noise detected or estimated from the image, the number of frames used in the 3DNR processing, and the like. These may be acquired by the acquisition unit 112 together with the evaluation value.

[0040] In step S501, as described above, the driving speed of the imaging optical system is set to be slower as the strength of the 3DNR increases. For example, the deceleration coefficient may be set using table data according to a setting value set by the user, or may be calculated from the user's setting value, the number of frames used in the 3DNR process, etc.

[0041] When the peak position is detected in step S211, the peak position is corrected according to the strength of the 3DNR (S502). In other words, at least one of the driving speed of the imaging optical system and the peak position (focus position) is determined based on the strength of the noise reduction process. The peak position is determined by correcting the current focus position (i.e., the detected peak position). As described above, the stronger the strength of the 3DNR, the larger the correction amount is. Alternatively, the focus position before (past) the current focus position among the multiple focus positions controlled by the control unit 113 is corrected to be the peak position. For example, if the number of frames used in the 3DNR process is three frames, the focus position controlled three frames before the current frame is set as the peak position. In this way, the focus position of the imaging optical system is determined based on the strength of the 3DNR process so that the focus position driven (controlled) in the past before the current focus position is set as the peak position. Note that the multiple focus positions in the multiple frames past the current frame are temporarily stored in a storage unit such as a RAM (Random Access Memory) in the imaging device. As described above, the imaging device according to this embodiment is capable of focusing with high accuracy.

[0042] (Hardware configuration) 6 is a diagram showing an example of the hardware configuration of an imaging device according to this embodiment. The imaging device 100 has an imaging optical system 601, a drive unit 114, an imaging element 106, a CPU 602, a RAM 603, and a ROM 604. The drive unit 114 and the imaging element 106 are the same as those in FIG. 1. The imaging optical system 601 includes at least the zoom lens 101, the focus lens 102, and the aperture unit 103 in FIG. 1.

[0043] The CPU 602 is a central processing unit for executing AF control of the imaging device according to this embodiment. Of the blocks shown in Fig. 1, functions that are realized by software (e.g., the acquisition unit 112 and the control unit 113) are realized by the CPU 602 loading a program stored in the ROM 604 and executing it using the RAM 603 as a workspace. The RAM 603 stores, for example, a focus position for each of a plurality of frames.

[0044] <Other embodiments> The present invention can be realized by a process of reading and executing a program that realizes one or more functions of the above-mentioned embodiment 1. The program is supplied to a system or device via a network or a storage medium, and is read and executed by one or more processors in the computer of the system or device. The present invention can also be realized by a circuit (e.g., ASIC) that realizes one or more functions. [Explanation of symbols]

[0045] 100 Imaging device 101 Zoom Lens 102 Focus Lens 103 Aperture unit 104 Bandpass Filter 105 Color Filter 106 Image sensor 107 AGC 108 AD conversion section 109 Signal Processing Section 110 Communications Department 111 Monitor device 112 Acquisition Department 113 Control Unit 114 Drive unit

Claims

1. an imaging unit that captures an image of a subject using an imaging optical system; a signal processing unit that performs noise reduction processing on an image based on a plurality of frames of images captured by the imaging unit; an acquisition unit that acquires an evaluation value indicating the contrast of the image on which the noise reduction processing has been performed by the signal processing unit; a control unit that controls a focus position of the imaging optical system based on the evaluation value, the control unit determines at least one of a drive speed of the imaging optical system and a focus position of the imaging optical system based on the intensity of the noise reduction processing.

2. The imaging device according to claim 1 , wherein the acquisition unit acquires the intensity of the noise reduction processing.

3. 2. The imaging device according to claim 1, wherein the number of images of the plurality of frames used in the noise reduction process is determined based on the strength of the noise reduction process.

4. 4. The imaging apparatus according to claim 3, wherein the control unit determines the drive speed of the imaging optical system or the focus position based on the number of images in the plurality of frames.

5. The imaging device according to claim 1, characterized in that, when determining the drive speed of the imaging optical system, the control unit determines the drive speed of the imaging optical system so that the drive speed of the imaging optical system becomes slower as the strength of the noise reduction processing increases.

6. The imaging device according to claim 1, characterized in that, when determining the focus position of the imaging optical system, the control unit determines the focus position of the imaging optical system by correcting the current focus position based on the strength of the noise reduction processing.

7. The imaging device described in Claim 6, characterized in that the control unit determines the focus position of the imaging optical system so that the amount of correction increases as the intensity of the noise reduction processing increases.

8. a memory unit that stores a plurality of focus positions controlled by the control unit; 7. The imaging device according to claim 6, wherein the control unit corrects the current focus position so that the current focus position becomes one of the plurality of focus positions based on the strength of the noise reduction processing.

9. an imaging step of capturing an image of a subject; a signal processing step of performing noise reduction processing on the image based on the multiple frame images captured in the step; an acquisition step of acquiring an evaluation value indicating the contrast of the image on which the noise reduction processing has been performed; a control step of controlling a focus position of the imaging optical system based on the evaluation value, The control method for an imaging apparatus, wherein the control step determines a drive speed of the imaging optical system or a focus position of the imaging optical system based on the strength of the noise reduction processing.

10. 10. The method for controlling an imaging apparatus according to claim 9, wherein the acquiring step acquires the strength of the noise reduction processing.

11. 10. The method for controlling an image capturing apparatus according to claim 9, wherein the number of images of the plurality of frames used in the noise reduction process is determined based on the strength of the noise reduction process.

12. 12. The method of controlling an image pickup apparatus according to claim 11, wherein the control step determines a drive speed of the image pickup optical system or the focus position based on the number of images in the plurality of frames.

13. 10. The control method for an imaging device according to claim 9, wherein, in the control step, when determining a drive speed of the imaging optical system, the drive speed of the imaging optical system is determined so that the drive speed of the imaging optical system becomes slower as the strength of the noise reduction processing increases.

14. 10. The control method for an imaging device according to claim 9, wherein the control step, when determining a focus position of the imaging optical system, determines the focus position of the imaging optical system by correcting a current focus position based on the strength of the noise reduction processing.

15. The imaging method described in Claim 14, characterized in that the control process determines the focus position of the imaging optical system so that the amount of correction increases as the intensity of the noise reduction processing increases.

16. The control method for an imaging device according to claim 15, wherein the control unit corrects the focus position to one of a plurality of focus positions controlled before the current focus position based on the intensity of the noise reduction processing.

17. A program for executing the control method for an imaging device according to any one of claims 9 to 16.

18. A computer-readable storage medium storing the program according to claim 17.

Images