JP2018146773A - Focus detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a focus detection device capable of switching exposure control depending on a state of an object to realize more accurate focus detection.SOLUTION: A focus detection device comprises image capturing means having a plurality of two-dimensionally arranged pixels, and exposure control means for controlling exposure for each row of the image capturing means, and is configured to detect focus based on pixel signals acquired by the image capturing means. The exposure control means has a first control mode for providing control that keeps exposure the same for all the rows, and a second control mode for providing control that uses different exposure for each of adjacent rows.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a focus detection device, and more particularly to a focus detection sensor.

  Conventionally, as a focus detection device, a phase difference detection type focus detection device is known. In the phase difference detection method, light from a subject is separated into two image signals by an eyeglass lens, and a focus state is detected from the phase difference between them. Patent Document 1 discloses a focus detection device using a two-dimensional image sensor as a focus detection sensor. In Reference 1, in order to expand the signal dynamic range (D range) of a two-dimensional image sensor, accumulation control is repeated a plurality of times while changing the accumulation time. Further, the correlation calculation is performed on the image signals obtained at each accumulation time, and the focus adjustment control is performed from the calculation result with the highest reliability.

JP 2010-122356 A

  However, in the prior art disclosed in the above-mentioned patent document, since the accumulation control is performed a plurality of times, the responsiveness is lowered. In addition, in the method of expanding the D range by changing the accumulation time for each row, the accuracy of focus detection is reduced in a scene where the subject brightness is known in advance or in a scene where the subject imaged by the focus detection sensor is small.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a focus detection device that has a plurality of exposure controls and does not deteriorate responsiveness, and that achieves both an expansion of the D range capable of focus detection and high-precision focus detection.

  In order to achieve the above object, the present invention comprises an imaging means having a plurality of pixels arranged two-dimensionally and an exposure control means for controlling an exposure amount for each row of the imaging means. In the focus detection apparatus that performs focus detection based on the received pixel signal, the exposure amount control means controls the exposure so that all the rows have the same exposure amount, and different exposure for each adjacent row. It has the 2nd control means which controls so that it may become quantity, It is characterized by the above-mentioned.

  According to the present invention, focus adjustment accuracy can be improved.

It is a figure which shows the structure of a digital still camera. It is a figure which shows a focus detection sensor. It is a figure which shows the brightness | luminance range which can detect a focus. It is a flowchart showing a focus detection process. It is a figure which shows the image signal in a focus detection operation | movement. It is a flowchart showing 2nd exposure control. It is a figure which shows the exposure control by the magnitude | size of a to-be-photographed object.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a diagram showing a configuration of a digital still camera according to an embodiment of the present invention.

[Example 1]
In FIG. 1, the imaging apparatus according to the present embodiment is a single-lens reflex camera (hereinafter simply referred to as “camera”) 100 with an interchangeable lens. The camera 100 includes a camera body 30 and a photographing lens unit 20 configured to be detachable from the camera body 30. The photographic lens unit 20 and the camera body 30 are configured to be detachable via a mount indicated by a dotted line in the center of the figure. The photographing lens unit 20 includes a photographing lens 21, a diaphragm 22, a lens side MPU (microprocessing unit) 1, a lens driving unit 2, a diaphragm driving unit 3, a photographing lens position detecting unit 4, and an optical information table. 5.

  The lens side MPU 1 performs all calculations and controls related to the operation of the photographic lens unit 20. The lens driving unit 2 is a driving unit that drives the photographing lens 21 in accordance with control by the lens side MPU 1. The aperture drive unit 3 is a drive unit that drives the aperture 22 according to control by the lens side MPU 1. The photographing lens position detection unit 4 is a detection unit that detects the focus position of the photographing lens. The optical information table 5 is optical information necessary for automatic focus adjustment, and is stored in a memory (not shown).

  The camera body 30 includes a camera-side MPU 6, a focus detection unit 7, a shutter drive unit 8, a dial unit 10, and a photometry unit 11. The camera body 30 includes a main mirror 12, a sub mirror 13, a focus plate 14, a penta mirror 15, a finder 16, an image sensor (image sensor) 101, a switch SW1_18, and a switch SW2_19.

  The camera side MPU 6 performs all calculations and controls related to the operation of the camera body 30. The camera-side MPU 6 is connected to the lens-side MPU 1 via a mount signal line, and acquires lens position information from the lens-side MPU 1 or acquires unique optical information for each lens driving and interchangeable lens.

  The camera-side MPU 6 includes a ROM (not shown) that stores a program for controlling the operation of the camera body 30, a RAM (not shown) that stores variables, and an EEPROM (electrical erasure, A writable memory (not shown) is incorporated. A focus detection process, which will be described later, is executed by a program stored in the ROM. The display unit 17 also displays information related to the shooting mode of the camera, a preview image before shooting, a confirmation image after shooting, and the like.

  The focus detection unit 7 includes a focus detection sensor, performs focus detection processing, and performs focus detection by a phase difference detection method. A part of the light beam from the subject passes through the main mirror 12, is bent downward by the rear sub-mirror 13, and enters the focus detection unit 7. In the focus detection unit 7, an image formed on the primary image plane is divided in the horizontal direction by an optical system (glasses lens) (not shown) in the unit and is formed on the focus detection sensor 210. The focus state is detected by calculating the phase difference of the divided image signals.

  The focus detection sensor is a two-dimensional CMOS image sensor, and has a configuration capable of global electronic shuttering. In response to a charge accumulation start instruction from the camera side MPU 6, a circuit reset operation and a photodiode reset operation are performed to start a charge accumulation operation. Further, when the accumulation time set in advance for each row from the camera-side MPU 6 is reached, the charge accumulated in the photodiode portion is transferred to a memory portion (not shown) of a peripheral circuit of the photodiode. By shifting the transfer timing, the accumulation time for each row can be controlled. When the transfer of the charges of all the pixels to the memory unit is completed, the camera-side MPU 6 is notified of the completion of charge accumulation. It is also possible to control the accumulation time by shifting the timing of the circuit reset operation and the photodiode reset with a rolling shutter.

  When the camera side MPU 6 receives the end of the charge accumulation, it reads out a signal from the focus detection sensor. The shutter drive unit 8 is a drive unit for driving a shutter (not shown). The dial unit 10 is an operation unit for changing various settings of the camera 100. For example, switching between continuous shooting speed (continuous shooting speed), shutter speed, aperture value, shooting mode, etc., and determination of tracking operation in focus detection And so on.

  The photometric unit 11 includes a photometric sensor, and performs photometric processing via a photometric sensor (not shown) based on the light flux from the pentamirror 15 in response to a half-press operation on a release button (not shown). These are all connected to the camera-side MPU 6. The photometric sensor comprises a photoelectric conversion element such as a photodiode and its signal processing circuit, and outputs a signal related to the luminance level of the subject. The output signal is input to the camera-side MPU 6.

  The main mirror 12 has a function of folding most of the light beam incident through the photographing lens unit 20 upward and forming a subject image on the focus plate 14. The subject image on the focus plate 14 is converted and reflected into an erect image by the pentamirror 15 and guided to the viewfinder 16. Thereby, it functions as an optical viewfinder. Part of the light transmitted through the pentamirror 15 is guided to the photometry unit 11.

  When the camera 100 is in a shooting state, the main mirror 12 and the sub mirror 13 are retracted, and a light beam from a subject incident through the shooting lens unit 20 is imaged on the image sensor 101. The switch SW1_18 is a switch that is turned on by a first stroke operation (half press) of a release button (not shown). The switch SW2_19 is a switch that is turned on by a second stroke operation (full press) of a release button (not shown).

  Next, the focus detection sensor will be described with reference to FIG. The focus detection sensor 210 in FIG. 2 is a two-dimensional image sensor, and is configured to set a charge accumulation time and a read gain for each row in accordance with an instruction from the camera side MPU 6. L1, L2,.・ ・ L16. Here, the horizontal direction is defined as a row, and the vertical direction is defined as a column.

  Reference numerals 211a and 211b denote regions for receiving a subject image obtained by dividing an image on the primary imaging plane in the horizontal direction by an eyeglass lens. The image signals generated from 211a and 211b are out of phase in the horizontal direction according to the focus state. Therefore, the focus state can be detected by detecting the phase difference in the horizontal direction of the image signal by a defocus calculation described later. Here, an image signal generated by 211a is an A image, and an image signal generated by 211b is a B image.

  The focus detection sensor expands the brightness range in which focus detection is possible by changing the first exposure control for setting the same charge accumulation time and readout gain for all rows and changing the readout gain or charge accumulation time for each row. The exposure amount is controlled by any one of the two exposure control methods. In the first exposure control, one of the charge accumulation times t1, t2, and t3 is set for all rows.

  On the other hand, in the second exposure control, the charge accumulation times t1, t2, and t3 are periodically set in order from L1. Here, the accumulation time is assumed to be t1 <t2 <t3. The luminance range determined by the storage time set in this way will be described with reference to FIG.

  In FIG. 3, the horizontal axis indicates a row and the accumulation time of each row, the vertical axis indicates luminance, and the luminance range for each row is indicated by a white arrow. 3A shows the luminance range when the first exposure control is performed, and FIG. 3B shows the luminance range when the second exposure control is performed. Since the first exposure control is set to the accumulation time t2 for all rows, the luminance range in which focus detection is possible is equivalent to the luminance range of each row. If the subject luminance is within a luminance range in which focus detection is possible, focus detection can be performed on many rows, and focus detection results calculated from a plurality of rows can be averaged or selected to perform focus detection with high accuracy.

  On the other hand, since the second exposure control is set to an accumulation time different from the accumulation times t1, t2, and t3, the luminance range in which focus detection is possible differs for each row. Accordingly, since any of the rows can be detected for the focus with respect to the subject brightness, the focus can be detected in the entire brightness range. The above is the difference in the brightness range in which focus detection is possible by exposure control.

  Next, focus detection processing in the embodiment of the present invention will be described with reference to FIG. The focus detection process starts when the release button is pressed and the switch SW1_18 is turned on. In step S301, the camera-side MPU 6 sets the memory contents and the execution program initialization state, and executes a focus detection preparation operation such as turning on the power of the focus detection sensor. Also, the tracking unit ON / OFF set by the user in the dial unit 10 of FIG. 1 is read. In the tracking operation, a subject is detected from an image in the screen, and an area for focus detection is determined according to the detected position of the subject. The subject is detected based on information such as the color and shape of the subject.

  In step S302, the camera side MPU 6 determines whether or not it is the first focus detection operation after the focus detection process is started. If it is determined that the focus detection process is the first time, since the subject brightness is not known, the process proceeds to step S307 where the brightness range in which focus detection is possible is wide. On the other hand, in the second and subsequent focus detection processes, since the subject brightness of the previous frame is known, the process proceeds to step S303. When the first exposure control is performed in the second and subsequent focus detection operations, it is possible to set the brightness range in which focus detection is possible by using the accumulation time in which appropriate exposure is performed in the focus detection of the previous frame.

  In step S303, the camera side MPU 6 determines whether or not to perform a tracking operation. When it is determined that the tracking operation is set to ON by the user, the process proceeds to step S304. On the other hand, if it is determined that the tracking operation is set to OFF, the process proceeds to step S306. When the tracking operation is performed and the focus detection area is changed according to the subject position, the subject brightness may change with respect to the previous frame, so when the accumulation time is determined based on the subject brightness of the previous frame, There is a possibility that it cannot be controlled with the correct exposure. Therefore, when the tracking operation is ON, the second exposure control is performed.

  In step S304, the camera side MPU 6 performs a tracking operation. In the tracking operation, an accumulation instruction is issued from the camera-side MPU 6 to the photometric sensor, a subject is detected from an image obtained by the photometric sensor, and an area for focus detection is determined. In step S305, the camera-side MPU 6 determines whether the main subject to be tracked is a moving subject. If the main subject to be tracked moves within the screen larger than the set predetermined amount with respect to the previous frame, it is determined as a moving subject and the process proceeds to step S307.

  On the other hand, if the movement within the screen is equal to or less than the predetermined amount, it is determined that the subject is not a moving subject, and the process proceeds to step S306. When the subject moves greatly, the subject brightness may change with respect to the previous frame. Therefore, if the accumulation time is determined based on the subject brightness of the previous frame, it may not be possible to control with the correct exposure amount. Therefore, when the subject moves greatly, the second exposure control is performed.

  In step S306, the camera side MPU 6 sends an exposure control instruction to the focus detection sensor 210 in order to perform the first exposure control. Upon receiving the exposure control instruction, the focus detection sensor 210 performs charge accumulation in the same accumulation time in all rows according to the instruction. The accumulation time set here is set based on the accumulation time determined as appropriate exposure in step S311 described later.

  In step S307, the camera-side MPU 6 sends an instruction for performing the second exposure control to the focus detection sensor 210. A detailed description of the second exposure control will be given later. In step S308, when the camera side MPU 6 receives the charge accumulation completion flag in step S306, the camera side MPU 6 reads a signal from the focus detection sensor 210. In step S309, the camera side MPU 6 performs a defocus calculation.

  The defocus calculation is a known defocus calculation for detecting the focus state (defocus amount) of the photographing lens 21 based on the image signal read out in step S309. Here, the defocus amount (mm) is obtained by multiplying the phase difference (number of bits) of the focus detection sensor 210 by an optical coefficient such as the sensor pitch (mm) and the base length of the autofocus system.

  In step S310, the camera side MPU 6 determines whether or not the defocus amount has been calculated. If the image signal is detected in step S309 and the defocus amount can be calculated, the process proceeds to step S310. On the other hand, if the image signal cannot be detected in step S309 and the defocus amount cannot be calculated, the process proceeds to step S312 to search for a position where the image signal can be detected. Whether or not the defocus amount can be calculated will be described with reference to FIG.

  The horizontal axis in FIG. 5 indicates the pixel position, and the vertical axis indicates the pixel signal. An image signal indicated by a solid line is an A image, and an image signal indicated by a broken line is a B image. The 0th to 9th pixels are image signals imaged on 211a and 211b in FIG. 2, and the pixels before and after that are out of the focus detection range, so the image signals cannot be detected.

  FIG. 5A shows a state where the image A and the image B are largely defocused, the image signal indicating the subject cannot be detected, and the focus cannot be detected, so the defocus amount cannot be calculated. It is determined. On the other hand, in FIG. 5B, since the image signal is in the image signal detection range, it is determined that the defocus amount can be calculated because the image signal indicating the subject can be detected and the focus can be detected. The above is the determination method regarding whether or not the defocus amount is acceptable.

  In step S311, the camera side MPU 6 determines the final defocus amount from the defocus amount calculated based on the image signal of each row. In the second exposure control in which the control is performed with different accumulation times for each row, the defocus amount of the row in which the detected image signal is appropriate exposure is set as the final defocus amount. An image signal that is appropriate exposure means that the image signal is within a predetermined range.

  In addition, when the image signals of a plurality of rows are within a predetermined range in the first exposure control and the second exposure control, the defocus amount is averaged or the defocus amount calculated from the image signal having a large signal amount is final. It may be a defocus amount. Here, the accumulation time of the row having the final defocus amount is set as the accumulation time of the first exposure control in the next focus detection operation. If a row controlled with a different accumulation time in the second exposure control enters a predetermined range, the short accumulation time is set as the accumulation time of the first exposure control in the next focus detection operation.

  In step S312, the camera side MPU 6 performs search driving. The search operation is an operation of searching for a position where the image signal can be detected by driving the lens and moving the image signal. In step S313, the camera-side MPU 6 determines whether or not the focus state of the photographing lens 21 is in focus based on the defocus amount calculated in step S311. Whether or not it is in focus is determined to be in focus if the defocus amount is within a desired range, for example, within 1/4 Fδ (F: lens aperture value, δ: constant (20 μm)). For example, if the lens aperture value F = 2.0, it is determined that the lens is in focus when the defocus amount is 10 μm or less, and the focus detection operation is terminated. On the other hand, when the defocus amount is larger than 10 μm and it is determined that the in-focus state is out of focus, the process proceeds to step S314 in order to adjust the focus state of the photographing lens 21 to the in-focus position.

  In step S314, the camera side MPU 6 transmits the defocus amount to the lens side MPU1. The lens side MPU 1 instructs the photographing lens to drive the lens based on the defocus amount. Then, the camera MPU 6 repeats the operations in steps S302 to S314 described above until it determines that the in-focus state is obtained in step S313. The above is a series of flows in the focus detection operation.

  Next, the second exposure control will be described in detail with reference to FIG. FIG. 6 is a flowchart showing the second exposure control in step S307 of FIG. 4 in the embodiment of the present invention. In step S401, the camera side MPU 6 detects the size of the subject. The camera side MPU 6 sends an accumulation instruction to the photometric sensor and detects the main subject from the captured image. In step S402, the camera side MPU 6 determines whether or not the size of the subject detected in step S401 is larger than the predetermined value set.

  If it is determined that the subject is equal to or greater than the predetermined value, the process proceeds to step S403. On the other hand, when it determines with it being smaller than predetermined value, it transfers to step S405. The standard for determining the size of the subject will be described for a case where exposure control is performed in three different accumulation times to expand the luminance range. When the subject is smaller than the area corresponding to the three rows of focus detection sensors, focus detection cannot be performed on the subject in three different luminance ranges. Therefore, it is necessary to change the type of accumulation time according to the size of the subject.

  Here, the determination based on the size of the subject will be described in detail with reference to FIG. FIG. 7A shows a case where the subject is larger than the predetermined amount set with respect to the focus detection sensor of FIG. 2, and FIG. 6B shows a case where the subject is less than the predetermined amount. In FIG. 7A, the subject is on L6, L7, L8, L9, L10, and L11. On the other hand, in FIG. 7B, the subject covers L7 and L8.

  Therefore, in FIG. 7A, exposure control can be performed with all types of accumulation times t1, t2, and t3, whereas in FIG. 6B, exposure control is performed only at t1 and t2. In this way, it is necessary to change the accumulation control according to the size of the subject. Returning to the flowchart of FIG.

  In step S <b> 403, the camera side MPU 6 sends a charge accumulation instruction to the focus detection sensor 210. The accumulation time indicated here is set to three types, t1, t2, and t3. Upon receiving the charge accumulation instruction, the focus detection sensor 210 performs charge accumulation so that adjacent rows have different accumulation times according to the instruction. In step S404, the camera-side MPU 6 performs charge accumulation set in step S403, and reads a signal from the focus detection sensor 210 when it receives a charge accumulation completion flag.

  In step S <b> 405, the camera side MPU 6 sends a charge accumulation instruction to the focus detection sensor 210. Two types of accumulation time, t1 and t2, are set here. Upon receiving the charge accumulation instruction, the focus detection sensor 210 performs charge accumulation so that adjacent rows have different accumulation times according to the instruction. In step S406, the camera-side MPU 6 performs charge accumulation set in step S405, and reads a signal from the focus detection sensor 210 when a charge accumulation completion flag is received.

  In step S407, the camera-side MPU 6 determines whether or not the image signal read in step S406 is within a brightness range in which focus detection is possible. When the accumulation times t1 and t2 are appropriate accumulation times and the image signal is in the luminance range, the second exposure control is terminated. On the other hand, if the image signal is not within the luminance range, the process proceeds to step S408. In step S405, since the subject is smaller than a predetermined size and is accumulated only in two types of accumulation times t1 and t2, the entire luminance range cannot be covered. Therefore, when the image signals accumulated at the accumulation times t1 and t2 are not within the luminance range where focus detection is possible, it is necessary to change the accumulation time and perform charge accumulation again.

  In step S <b> 408, the camera side MPU 6 sends a charge accumulation instruction to the focus detection sensor 210. The accumulation time instructed here is set to t3 which is not accumulated in step S405. Upon receiving the charge accumulation instruction, the focus detection sensor 210 performs charge accumulation at the accumulation time t3 in accordance with the instruction. In step S409, the camera-side MPU 6 performs charge accumulation set in step S403, and reads a signal from the focus detection sensor 210 when a charge accumulation completion flag is received.

  The above is the flowchart regarding the second exposure control. As explained above, set the type of accumulation control with an appropriate period for the size of the subject, change the accumulation time if necessary and change the accumulation time if focus detection is possible and it does not fall within the luminance range, and accumulate again By performing time, the entire luminance range can be covered.

  In the above-described embodiment, the method for performing the focus detection process using the focus detection sensor has been described. However, the present invention is not limited to the focus detection process using the focus detection sensor, and is applicable to the focus detection using the image sensor (image sensor) 101. it can.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

100 Camera 30 Camera body 20 Shooting lens unit 21 Shooting lens

Claims (7)

An image pickup means (FIG. 2) having a plurality of pixels arranged two-dimensionally and an exposure control means (FIG. 2) for controlling an exposure amount for each row of the image pickup means, and a pixel signal obtained by the image pickup means In the focus detection device (7) that performs focus detection based on the first exposure mode, the exposure amount control means controls the first exposure mode (S306) in which all the rows have the same exposure amount, and every adjacent row Second control means (S307) for controlling the exposure amount to be different.
A focus detection device characterized by comprising: The exposure control unit performs control using the second control unit when the number of focus detection operations is the first focus detection operation, and the first control unit when the second focus detection operation is performed. The focus detection apparatus according to claim 1, wherein control is performed using the focus detection apparatus (S 302). The exposure control means performs control using the second control means when performing the subject tracking operation for detecting the position of the main subject in the screen and detecting the focus of the main subject, and does not perform the subject tracking operation. The focus detection apparatus according to claim 1, wherein control is performed using the first control means (S <b> 303). The exposure control unit performs control using the second control unit when performing a subject search operation for searching for a subject position with respect to the depth direction in the screen, and does not perform the subject search operation. The focus detection apparatus according to claim 1, wherein control is performed using the first control means (S310). The second control means extends a control period to a different exposure amount if the subject in the screen is large, and shortens a control period to a different exposure amount if the subject in the screen is small. The focus detection apparatus according to 1 The subject tracking operation includes a moving subject tracking operation that moves in the screen and a stationary subject tracking operation that is stationary in the screen,
The moving subject tracking operation is controlled using the second control unit, and the stationary subject tracking operation is controlled using the first control unit (S305). Focus detection device.   7. The focus detection apparatus according to claim 1, wherein the exposure control unit sets a different accumulation time or gain for each row (S <b> 307).