Imaging device, control method thereof, program, and storage medium

Abstract

To provide an imaging apparatus that can stably detect flicker even with presence of disturbance such as a change in an exposure condition.SOLUTION: An imaging apparatus comprises: an imaging unit that picks up an image of a subject; and an exposure control unit that adjusts at least storage time, a diameter of a diaphragm, and photographic sensitivity to perform exposure control in picking up an image of a subject by using the imaging unit. When picking up an image of the subject to obtain an image for detection being an image for detecting flicker by using the imaging unit, the exposure control unit performs the exposure control by increasing the photographic sensitivity and opening an opening of the diaphragm preferentially over extending the storage time of the imaging unit.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a technique for detecting flicker in an imaging device. [Background technology]

[0002] In recent years, digital cameras have become higher ISO (higher sensitivity) and can now shoot with a high shutter speed even under artificial light sources that cause flicker. While a high shutter speed has the advantage of being able to take blur-free photos when shooting indoor sports, etc., when shooting under a flickering light source, the flicker can cause uneven exposure and color in the image, either from frame to frame or even within a single frame.

[0003] To address this problem, methods are known that reduce the effect of flicker on images by performing exposure at the peak position of the flicker where the change in brightness is the smallest, or by setting the exposure time to an integer multiple of the flicker period.

[0004] In order to use a method for reducing the influence of such flicker, it is necessary to detect the flicker and its frequency. Regarding the detection of the flicker and its frequency, for example, a technique disclosed in Patent Document 1 is known. Patent Document 1 shows an example in which photometry is performed multiple times at a constant cycle, and the presence or absence of flicker and its frequency are determined from a first evaluation value obtained from photometric values ​​acquired at a first interval close to the same phase as the flicker and a second evaluation value obtained from photometric values ​​acquired at a second interval close to the opposite phase of the flicker among the multiple photometric values ​​obtained.

[0005] Moreover, Patent Document 2 discloses an imaging device that makes it possible to carry out a flicker detection operation during so-called live view display, without stopping the live view display. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2017-11352 A [Patent Document 2] JP 2020-80512 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, the conventional technology disclosed in the above-mentioned Patent Document 1 is premised on the fact that the exposure conditions such as the aperture do not change when exposing an image for flicker detection, and only the brightness of the subject due to the flicker light source changes. If this precondition is not met, the performance of flicker detection will be degraded.

[0008] On the other hand, in the technology disclosed in Patent Document 2, it is necessary to constantly control the exposure and focus to a state that is preferable for the user during live view display. Therefore, if an attempt is made to perform a flicker detection operation during live view display, there is a problem that the exposure conditions of the image for flicker detection change due to the influence of changes in exposure and focus during live view display.

[0009] The present invention has been made in consideration of the above-mentioned problems, and has as its object to provide an imaging device that can stably detect flicker even when disturbances such as changes in exposure conditions occur. [Means for solving the problem]

[0010] The imaging device according to the present invention comprises an imaging means for imaging a subject, and at least exposure and an exposure control means for controlling exposure when an object is imaged using the imaging means by adjusting exposure time, aperture diameter, and imaging sensitivity, the exposure control means controlling exposure based on a program diagram showing a correspondence relationship between the amount of light incident on the imaging means and the exposure time, imaging sensitivity, and aperture value of the imaging means, and the exposure control means controls exposure based on the amount of light incident on the imaging means and the exposure time, imaging sensitivity, and aperture value of the imaging means when an object is imaged using the imaging means to obtain a detection image that is an image for detecting flicker. exposure Rather than lengthening the time, it is better to increase the sensitivity and open the aperture. Based on the program diagram It is characterized by performing exposure control. Effect of the Invention

[0011] According to the present invention, it is possible to stably detect flicker even when disturbances such as changes in exposure conditions occur. [Brief description of the drawings]

[0012] [Figure 1] 1 is a diagram showing a schematic configuration of a digital mirrorless camera as an embodiment of an imaging apparatus of the present invention. [Diagram 2] 4A and 4B are diagrams for explaining driving of an image sensor during live view. [Diagram 3] 11 is a flowchart showing a flicker detection operation during live view. [Figure 4] FIG. 11 is a graph showing the relationship between exposure time and the obtained output in a flicker environment. [Diagram 5] 6 is a program diagram for determining an exposure control value of an image for flicker detection. [Figure 6] 6 is a diagram showing the relationship between the timing of acquiring an image for flicker detection and a photometric output. [Figure 7] 1 is a diagram showing a method for determining the presence of flicker. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0014] In one embodiment of the present invention, a so-called digital mirrorless camera is taken as an example of an imaging device to be described. However, the present invention is not limited to this, and may be various electronic devices equipped with a camera function. For example, the imaging device according to the present invention may be a mobile communication terminal with a camera function such as a mobile phone or a smartphone, a mobile computer with a camera function, or a mobile game machine with a camera function.

[0015] FIG. 1 is a diagram showing a schematic configuration of a digital mirrorless camera (hereinafter, simply referred to as a camera) which is an embodiment of an imaging device of the present invention.

[0016] 1, the camera is broadly composed of a camera body 100 and a photographing lens 200, which are mechanically and electrically connected via a lens mount mechanism 110. In this embodiment, the photographing lens 200 that forms a subject image is configured to be detachable from the camera body 100, but may also be integrated with the camera body 100.

[0017] First, the camera body 100 will be described. The imaging element 101 is an image sensor including an infrared cut filter, a low-pass filter, etc., and equipped with a photoelectric conversion element such as a CMOS type. The optical image of the subject is photoelectrically converted by the imaging element 101 and output as an image signal. The shutter 104 closes to block light from the imaging element 101 when not shooting, and opens during live view (LV) or shooting to guide incident light that has passed through the shooting lens 200 to the imaging element 101.

[0018] The system control unit 102 is an arithmetic processing unit that controls each unit of the camera body 100. The memory 103 includes a ROM that stores programs and variables executed by the system control unit 102, and a RAM that has a working area in which the system control unit 102 expands programs and a storage area for temporary image data, etc.

[0019] The system control unit 102 is also connected to a lens control unit 201 through a lens mount mechanism 110. The lens control unit 201 controls the position of the focusing lens 202 through a lens driving unit 203. It also controls an aperture 204 through an aperture control unit 205. More specifically, the system control unit 102 performs calculations related to AE (automatic exposure) / AF (automatic focusing) from the image acquired by the image sensor 101, and communicates the position of the focusing lens and the aperture value according to the calculation results to the lens control unit 201 for control.

[0020] The shutter 104 is also connected to the system control unit 102 via a shutter control unit 105 , and exposes the image sensor 101 for an exposure time according to the calculation result in the system control unit 102 .

[0021] Next, a description will be given of an operation for displaying an image acquired by the image sensor 101. Here, a description will be given of an operation in a so-called live view mode in which the image sensor 101 continuously captures images and displays the captured images in real time on the display unit, allowing the user to observe a subject.

[0022] There are two display destinations for the image captured by the image sensor 101: rear monitor 106 and viewfinder display unit 107, and the image is displayed by switching between these. The switching is performed based on the detection result of eyepiece detection unit 109. Specifically, if it is detected that the user has their eye in close contact, the image is displayed on the viewfinder display unit 107. The user looks into the viewfinder and checks the subject displayed on the viewfinder display unit 107 through eyepiece lens 108. On the other hand, if eyepiece detection unit 109 detects that the user has not placed their eye in close contact, the image is displayed on the rear monitor 106.

[0023] Next, the driving of the image sensor 101 in live view mode will be described with reference to Fig. 2. In this embodiment, the image sensor 101 has a drive mode for acquiring a live view display image in which pixel rows are read out at a predetermined interval, and a drive mode for flicker detection in which pixel rows that are not read out in the drive mode for live view display are read out at a predetermined interval. Also, it is possible to control the drive mode for flicker detection to be executed multiple times within one cycle of the drive mode for live view display.

[0024] FIG. 2 shows this driving mode, with the horizontal direction indicating time and the vertical direction indicating the vertical position of the image sensor 101, and the diagonal lines in the figure indicating the readout timing of the image sensor 101 in live view mode.

[0025] As shown in Fig. 2, first, one image for live view display is read out, and then multiple images for flicker detection are read out (period for capturing images for flicker detection). The acquisition of an image for live view and multiple images for flicker detection is considered as one set, and this set of operations is repeated. Note that the acquisition of the image for live view and the images for flicker detection is performed by the electronic shutter operation of the image sensor 101.

[0026] A plurality of images for flicker detection are captured at 600 fps (1.66 ms intervals). In this embodiment, if one set is repeated at a cycle of 33.3 [ms], this cycle becomes the frame rate of the live view display, which is 1 (s) / 33.3 (ms) = 30 [fps]. Also, 18 images for flicker detection can be acquired in one set. Note that reading out the images for flicker detection in the latter half of one set is not essential and can be omitted. Although the details will be described later, the state of flicker in the shooting environment is generally determined by the light source present in that environment, so it is unlikely to change in a short time. Therefore, the frequency of the flicker detection operation does not need to be very high. In this embodiment, it is performed once every six frames of the live view.

[0027] Next, the flicker detection operation during live view in the camera of this embodiment will be described with reference to Fig. 3. The operation of the flowchart in Fig. 3 is realized by the system control unit 102 expanding a program stored in the ROM of the memory 103 into the RAM and executing it.

[0028] When an instruction to start live view operation is received in step S101, the system control unit 102 first performs photometry before starting to display the live view in step S102 to capture the first image of the live view to be displayed initially, so as to prevent poor exposure such as whiteout or blackout.

[0029] Specifically, first, one image is captured with a predetermined fixed exposure control value (a combination of AV (aperture value), TV (shutter speed, accumulation time), and ISO sensitivity (photography sensitivity)). For example, if this image output exceeds a predetermined appropriate exposure level, the exposure control value is fed back so that the image output has an appropriate exposure level, for example by shortening the TV for the next image. Multiple captures are performed as necessary until an exposure control value that provides appropriate exposure is determined, but live view is not displayed. Once the exposure control value that provides appropriate exposure is determined, the process proceeds to step S103. The operation of determining the exposure control value that provides appropriate exposure from the image output and the exposure control value when the image was acquired will hereinafter be referred to as "photometry."

[0030] In step S103, the system control unit 102 uses the control value measured in step S102 to capture the first image of the live view and displays it on the rear monitor 106 or the viewfinder display unit 107. After step S102, the user can observe the subject and frame the image by checking the rear monitor 106.

[0031] In step S104, the system control unit 102 determines the time that has elapsed since the last photometry timing, and if a predetermined time has elapsed, the process proceeds to step S105, otherwise the process proceeds to step S109.

[0032] In the environment in which a user takes a photograph, brightness is constantly changing due to factors such as changes in the subject and panning by the user, so photometry needs to be performed at a certain frequency. Also, since a user may move from indoors where flicker is present to outdoors where there is sunlight, it is desirable to similarly perform flicker detection at a certain frequency.

[0033] For the above reasons, in this embodiment, the predetermined time in step S104 is set to 200 ms. As described above, since the frame rate of the live view display is 30 fps here, photometry and flicker detection operations are performed once every six frames.

[0034] Once exposure metering is performed, the next six frames of live view display are shot with the same exposure control value, and exposure metering is performed again using the sixth frame image to determine the exposure control value for the next six frames. In this case, the drive of the image sensor for the sixth frame is an operation to shoot a set of an image for live view display and an image for flicker detection, as shown in Figure 2. For other frames, flicker detection is not performed, so the acquisition of the image for flicker detection in the latter half of the set operation is omitted.

[0035] In this embodiment, the frequency of both photometry and flicker detection is set to once every six frames, but the frequency may be changed to once every four frames, or the frequencies of photometry and flicker detection may be set separately. If the system control unit 102 determines in step S104 that 200 ms or more have elapsed since the last photometry, the process proceeds to step S105 where photometry and flicker detection operations are performed. If the system control unit 102 determines that 200 ms has not elapsed, the process proceeds to step S109.

[0036] Next, the flicker detection operation and photometry operation in steps S105 to S108 will be described.

[0037] In step S105, the system control unit 102 determines an exposure control value for an image for flicker detection. Details will be described in step S107, but generally, in a flicker detection operation, the greater the amplitude ratio of light and dark of the flicker to be detected, the more stable the flicker detection can be. The amplitude ratio of the flicker is determined by the light source and the subject in the shooting environment, so it cannot be controlled on the camera side, but by devising an exposure operation for flicker detection, the apparent amplitude ratio in detection can be increased.

[0038] Specifically, shortening the exposure time of an image acquired for flicker detection increases the apparent amplitude ratio, which will be explained with reference to Figure 4. Figure 4 is a diagram showing the change in light intensity due to standard 100 Hz flicker, with time on the horizontal axis and relative light intensity on the vertical axis. In this diagram, T1 to T8 indicate the timings for exposing an image for flicker detection under the light source described above.

[0039] In detecting flicker, multiple images are captured at a cycle that is sufficiently fast relative to the frequency of the flicker, and the flicker is detected from the average output of each of the multiple images obtained. Here, consider a case where an image for flicker detection is exposed near the peak and bottom timings of the light amount of the flicker. In FIG. 4, the full-screen average output of an image captured in a period T2 to T3 (time span T_short) near the peak timing of the flicker is set to A(T2, T3). Similarly, the full-screen average output of an image captured in a period T6 to T7 near the bottom timing of the flicker with the same time span T_short is set to A(T6, T7). In this case, the apparent amplitude ratio AMP_short in detection is calculated by the following formula.

[0040] AMP_short=A(T2,T3) / A(T6,T7) On the other hand, the amplitude ratio AMP_long in the case where exposure is performed for a time width T_long that is longer than the above at the peak timing and bottom timing of the flicker is calculated by the following formula.

[0041] AMP_long = A(T1,T4) / A(T5,T8) Here, comparing A(T2, T3) and A(T1, T4), A(T2, T3) has a shorter exposure time and is therefore only exposed near the peak of the flicker, whereas A(T1, T4) includes the period before and after the peak in its exposure time. Therefore, when the exposure time is converted to a unit time, A(T2, T3) has a larger value than A(T1, T4).

[0042] Similarly, comparing A(T6,T7) and A(T5,T8) near the bottom, A(T6,T7) is exposed only near the bottom, whereas A(T5,T8) includes the period before and after it reaches the bottom in its exposure time. Therefore, when converted to exposure time per unit time, A(T6,T7) is smaller than A(T5,T8).

[0043] From the above, it can be seen that AMP_short>AMP_long, and that when acquiring an image for flicker detection, shortening the exposure time increases the apparent amplitude ratio. Based on this, step S105 is a step for determining an exposure control value for an image for flicker detection. The exposure control value is determined by using a so-called program diagram based on the brightness determined from the photometry results. An example of the program diagram is shown in FIG. 5.

[0044] 5 is a graph showing a program diagram with brightness on the horizontal axis and exposure time, ISO sensitivity, and lens aperture value on the vertical axis. The program diagram shows the relationship between the amount of light reaching the image sensor 101 and the exposure time, shooting sensitivity, and aperture value of the image sensor 101. Once brightness is determined by photometry, an exposure control value is uniquely determined according to this graph.

[0045] In Fig. 5, a program diagram for capturing an image for flicker detection is shown by a solid line indicating a diagram for obtaining an effect specific to this embodiment (hereinafter referred to as a "flicker diagram") and a conventional diagram (hereinafter referred to as a "normal diagram") by a dashed line, and the two are compared. Note that the shortest exposure time of the image sensor 101 in this embodiment is 1 / 16384 sec (≒61 μs), and since the image for flicker detection is captured at 600 fps, the longest exposure time is 1666 μs. The minimum ISO sensitivity is 100 and the maximum ISO sensitivity is 25600. As for the aperture, a lens with an open F value of F1.8 is used.

[0046] First, in the bright area of ​​BV11 or more in Fig. 5, both the flicker diagram and the normal diagram fix the exposure time to the shortest 61μs and the ISO sensitivity to the lowest 100, and the aperture value is adjusted to obtain an appropriate exposure. When it becomes darker than BV11, the aperture opens to the maximum F1.8, so in the normal diagram, the ISO sensitivity is fixed at 100 and the exposure time is extended to maintain an appropriate exposure. In the normal diagram, when it becomes BV5 or less, the exposure time also becomes the longest 1666μs, so the ISO sensitivity is increased from here, and when it becomes darker to BV-2, the ISO sensitivity also becomes the maximum 25600. Therefore, in the area below BV-2, the exposure time is already the longest, the ISO sensitivity is maximum, and the aperture is already open, so the exposure cannot be adjusted and the resulting image is underexposed. In the normal diagram, in order to avoid image quality degradation due to the increase in ISO sensitivity as much as possible, the aperture opens as it becomes darker, and the ISO sensitivity is only increased after the exposure time reaches its longest.

[0047] In normal still image shooting and live view image shooting, in order to avoid image quality degradation due to an increase in ISO sensitivity as much as possible, the normal diagram as described above is used, and the ISO sensitivity is controlled to be increased only after the exposure time has reached its maximum.

[0048] Next, considering the flicker diagram, as already explained, it is desirable to make the exposure time as short as possible in flicker detection, so as it gets darker, after the aperture diameter is fully opened at BV11, the ISO sensitivity is first increased while keeping the exposure time at the shortest 61μs. Only when the ISO sensitivity reaches its maximum of 25600 (the upper limit on the high sensitivity side) at BV2 are the exposure time extended. At BV-2 and below, the exposure time is at its longest, the ISO sensitivity is at its maximum, and the aperture is fully open, so the exposure control values ​​are the same as those on the normal diagram, and the resulting image is underexposed.

[0049] In shooting (exposure) for flicker detection, in order to make it easier to detect flicker, a short exposure time is given priority, and instead, the ISO sensitivity is controlled to be increased.

[0050] 5, in the range of BV-2 to BV11, the flicker diagram has a shorter exposure time than the normal diagram, making it easier to detect flicker. Thus, the gist of this embodiment is that when comparing exposure control in shooting an image for flicker detection with exposure control in shooting a still image or an image for live view display, a shorter shutter speed (exposure time) is set for a subject of the same luminance in shooting for flicker detection.

[0051] In step S105 of FIG. 3, the exposure control value of the flicker detection image is determined based on the above-mentioned flicker diagram.

[0052] After the exposure control value of the image for flicker detection is determined in step S105, the system control unit 102 actually acquires the image for flicker detection in step S106.

[0053] FIG. 6(a) is a diagram showing a change in signal value over time when accumulation for flicker detection is performed and a 100 Hz flicker is present. As already shown in FIG. 2, in this embodiment, after reading out an image for live view display, the image sensor 101 is driven by reading out an image for flicker detection 18 times at 1.66 ms intervals as one set. For flicker detection, the first 12 images out of the 18 images captured at 1.66 ms intervals are used. Accumulation and readout are performed for each of the 12 images, and as shown in the figure, the nth accumulation is described as "accumulation n", the signal readout of accumulation n is described as "readout n", and the photometric value (signal value) obtained from the result of readout n is described as "AE(n)". Here, the first 12 accumulations are focused on, so photometric values ​​AE(1) to AE(12) are obtained. Furthermore, because accumulation is performed over a finite period of time, the acquisition times of the photometric values ​​are represented by the median values ​​during the accumulation period (the center of gravity of the parallelogram in the drawing), and are set to t(1) to t(12), respectively. In step S106, the system control unit 102 causes the image sensor 101 to acquire images for flicker detection at intervals of 1.66 ms in this manner.

[0054] After acquiring the images for flicker detection in step S106, the system control unit 102 performs flicker detection calculations using AE(1) to AE(12) in step S107.

[0055] In the flicker detection calculation, the system control unit 102 first calculates an evaluation value used in determining the frequency of the flicker from AE(1) to AE(12). In this embodiment, the evaluation value used in determining the flicker frequency is defined by the following equation.

[0056]

number

[0057] SAD stands for Sum of Absolute Difference, and is an index of similarity used in fields such as pattern matching. m is a value indicating how many photometric values ​​from the nth photometric result AE(n) out of 12 photometric measurements are to be calculated for similarity, so SAD(m) is a formula for calculating the similarity with the photometric value after (1.66×m) ms. As can be seen from the formula, the higher the similarity, the smaller the SAD(m) value.

[0058] For example, in an environment where a 100 Hz flicker exists, the flicker period is about 10 ms, and the relationship with the photometric period of 1.66 ms is 10÷1.66≒6, so as shown in FIG. 6(a), the same photometric value is obtained in six periods regardless of the accumulation timing. In other words, the relationship is AE(n)≒AE(n+6). Due to this property, when SAD(6) is calculated in an environment where a 100 Hz flicker exists, SAD(6)≒0. Furthermore, in order to detect the presence of a 100 Hz flicker, SAD(3) is also calculated. SAD(3) is a value calculated to be similar to the photometric value after 1.66×3=5 ms has elapsed. In an environment where a 100 Hz flicker exists, the photometric value at a timing shifted by 5 ms has an inverse phase relationship, so SAD(3) is a very large value compared to SAD(6). In other words, when SAD(3) is large and SAD(6) is small, it is believed that 100 Hz flicker is present. Also, because SAD(3) is the difference between photometric values ​​at timings with an anti-phase relationship, the value of SAD(3) will be large in an environment with large amplitude flicker.

[0059] Using the same concept, in an environment where 120 Hz flicker exists, SAD(5) and SAD(3) can be calculated. In an environment where 120 Hz flicker exists, the light source lighting period is 8.333 ms, so AE(n) ≒ AE(n+5), and SAD(5) ≒ 0, as shown in Figure 6(b). In addition, in a 120 Hz flicker, the completely opposite phase relationship occurs after 4.16 ms has elapsed, so it is ideal to determine the similarity with the waveform after 4.16 ms has elapsed. However, since 4.16 ms is not an integer multiple of the frame period of 1.66 ms, the value of SAD(3), which indicates the similarity with the waveform after 5 ms has elapsed, is used instead as a value relatively close to this. Even in an environment where 120 Hz flicker exists, SAD(3) indicates a similarity close to the opposite phase, so SAD(3) has a very large value compared to SAD(5), and the larger the amplitude of the flicker, the larger the value of SAD(3) becomes.

[0060] From the above, SAD(6), SAD(5), and SAD(3) are calculated, and the final flicker frequency determination is performed using these. As already explained, in an environment where a 100 Hz flicker exists, SAD(3) will be a much larger value than SAD(6). Therefore, if we consider a plane as shown in FIG. 7(a) with SAD(3) on the horizontal axis and SAD(6) on the vertical axis, in an environment where a 100 Hz flicker exists, a plot will be obtained in the relatively lower right area of ​​this plane. In other words, if an area determined to be 100 Hz and an area determined not to be 100 Hz are set by dividing the area as shown in FIG. 7(a), it is possible to accurately determine whether or not the flicker is 100 Hz based on the position of the plot.

[0061] Similarly, by dividing the plane shown in FIG. 7(b) with SAD(3) on the horizontal axis and SAD(5) on the vertical axis, it is possible to determine whether or not there is a 120 Hz flicker.

[0062] It should be noted that the area dividing lines shown in FIGS. 7(a) and 7(b) are merely examples, and the inclinations and bending points of the lines are not limited to these.

[0063] The presence or absence of 100 Hz flicker can be determined from the plot positions on the plane shown in FIG. 7(a), and the presence or absence of 120 Hz flicker can be determined from the plot positions on the plane shown in FIG. 7(b). Finally, a process is performed to combine these determination results.

[0064] The integration process is performed based on the table shown in Figure 7(c). This table will be explained below. If 100 Hz flicker is present, the determination result in Figure 7(a) will be "100 Hz," and the determination result in Figure 7(b) will be "not 120 Hz," so the box in the lower left of Figure 7(c) will be "100 Hz." Using the same concept, the box in the upper right of Figure 7(c) will be "120 Hz."

[0065] If there is no flicker and the subject is constant DC light, the photometric value does not change over time. AE(1)≒AE(2)≒AE(3)≒ … ≒AE(12) This results in SAD(6) ≒ SAD(5) ≒ SAD(3) ≒ 0 In other words, in a DC environment, plots are obtained near the origin of both the planes in Fig. 7(a) and Fig. 7(b), and the judgment result in Fig. 7(a) is "not 100Hz," and the judgment result in Fig. 7(b) is "not 120Hz." Therefore, the box in the bottom right of the table in Fig. 7(c) is a DC judgment with no change in luminance.

[0066] Moreover, the box at the top left of the table in FIG. 7(c) is a case where it is both "100 Hz" and "120 Hz." Normally, it is unlikely that such a judgment result would be obtained, but if the subject being acquired from AE(1) to AE(12) is not the same due to subject movement or panning, etc., such a result may occur. Therefore, in such a case, a DC judgment is made, which means a flicker detection error. In this way, in step S107, system control unit 102 judges the presence or absence of flicker and its frequency.

[0067] The above is the details of flicker determination. Here, using Fig. 7(a), it will be explained that the larger the amplitude of the flicker, the more stably the flicker can be detected. As already explained, the larger the amplitude of the flicker, the larger the value of SAD(3). As the flicker determination threshold, there is SAD(3)=A shown in Fig. 7(a), and this threshold determines the amplitude of the detectable flicker. That is, a flicker with a small amplitude results in SAD(3)<A and cannot be determined as a flicker. Therefore, if the value of A is decreased, smaller flickers can be detected. However, as already explained, in a DC environment (without flicker), plots can be obtained near the origin. Even in a DC environment, especially in a dark state, AE(1)≒AE(2)≒AE(3)≒ … ≒AE(12) may not hold due to noise, and it may be distributed with an expansion relative to the origin. Therefore, if the value of A is decreased too much, in a DC environment, it will be misjudged as having a flicker. That is, there is a limit to decreasing the value of A. However, if the exposure time is made as short as possible as in the flicker diagram of Fig. 5, the amplitude of the flicker is apparently detected as larger, and accordingly, the value of SAD(3) also becomes larger. With the same value of A, even smaller amplitude flickers can be detected.

[0068] Also, assume that when flicker is detected multiple times in an environment where 100 Hz flicker exists, plots are obtained in the area shown by B in FIG. 7(a). In an ideal environment where 100 Hz flicker exists, SAD(6)≒0, so the area shown by B is located at the bottom end of the graph. However, when actually capturing an image for flicker detection in step S106, there are disturbances such as the focusing lens 202 of the shooting lens moving to keep the focus of the live view display appropriate, the user panning, and changes in the subject. If there is a disturbance, the shape of the graph changes between the first and second cycles, rather than the same shape of the change in light amount being observed for two cycles as in FIG. 6(a). Therefore, SAD(6)≒0 does not hold, SAD(6)>0, and the plot shifts to the area shown by C in FIG. 7(a), for example. Since the area C crosses the judgment threshold, there are cases where it is erroneously judged that there is no flicker. However, even in such a case, if the exposure time is shortened as much as possible by using the flicker diagram in Figure 5, the value of SAD(3) will become larger. This has the effect of observing areas B and C as areas B' and C', respectively, making it less likely that there will be a false determination that there is no flicker. The above is an example of an environment where a 100 Hz flicker is present, but the same applies to an environment where a 120 Hz flicker is present.

[0069] After completing the flicker detection in step S107, in step S108, the system control unit 102 performs photometry using the image for live view acquired together with the image for flicker detection acquired in step S106. Then, the process returns to step S103.

[0070] On the other hand, if 200 ms has not elapsed since the last photometry in step S104, the system control unit 102 advances the process to step S109. If 200 ms has not elapsed since the last photometry, the process basically returns to step S103 in order to perform live view shooting of the next frame. By repeating steps S103 to S109, a process of continuing live view display while performing photometry and flicker detection once every six frames is realized. However, if a shooting instruction operation is input by the user in step S109, the system control unit 102 interrupts the live view operation and switches the process to the shooting operation sequence in step S110. The shooting operation in step S110 is not directly related to the gist of the present invention, so a description thereof will be omitted.

[0071] As described above, in this embodiment, when capturing an image for flicker detection, a shutter speed (exposure time) is set to be shorter than when capturing a still image or a live view image for a subject of the same luminance. This increases the apparent amplitude of the flicker, making it possible to stably detect the flicker.

[0072] (Other embodiments) The present invention can also be realized by a process in which a program for implementing one or more of the functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., ASIC) that implements one or more of the functions.

[0073] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0074] 101: image sensor, 102: system control unit, 103: memory, 201: lens control unit, 203: lens driving unit, 204: aperture, 205: aperture driving unit

Claims

1. An imaging means for imaging a subject; and an exposure control means for controlling exposure when capturing an image of a subject using the imaging means by adjusting at least an exposure time, an aperture diameter, and a photographing sensitivity, the exposure control means performs exposure control based on a program diagram that indicates a correspondence relationship between the amount of light incident on the image capture means and an exposure time, a photographing sensitivity, and an aperture value of the image capture means; an exposure control means for controlling exposure based on a program chart that prioritizes increasing the imaging sensitivity and opening the aperture of the diaphragm over lengthening the exposure time of the imaging means when imaging a subject using the imaging means to obtain a detection image for detecting flicker;

2. 2. The imaging apparatus according to claim 1, wherein the exposure control means performs the exposure control so that the detection image has a proper exposure.

3. The imaging device according to claim 1 or 2, characterized in that, in exposure control when photographing a subject of the same luminance, the exposure control means performs exposure control based on a program chart that prioritizes increasing the shooting sensitivity and opening the aperture of the diaphragm over lengthening the exposure time of the imaging means when using the imaging means to obtain the image for detection, compared to a program chart used when using the imaging means to capture a still image or obtain an image for live view display.

4. 4. The image pickup apparatus according to claim 1, wherein the exposure control means is capable of controlling the exposure time by an electronic shutter operation using the image pickup means.

5. The imaging device according to any one of claims 1 to 4, characterized in that, when acquiring the detection image using the imaging means, the exposure control means fixes the exposure time to the shortest exposure time of the imaging means, and when the brightness of the subject becomes dark, opens the aperture of the diaphragm and increases the shooting sensitivity.

6. 6. The imaging device according to claim 5, wherein the exposure control means extends an exposure time of the imaging means when the luminance of the subject becomes dark while the aperture of the diaphragm is fully opened and the photographing sensitivity is at the upper limit on the high sensitivity side.

7. The imaging device according to any one of claims 1 to 6, further comprising a control means for causing the imaging means to repeatedly execute a set of operations, which includes multiple images taken to obtain one image for displaying a live view and multiple images for detection, and for causing the imaging means to repeatedly execute this set of operations.

8. A method for controlling an imaging device including an imaging unit that captures an image of a subject, comprising: an exposure control step of controlling exposure when capturing an image of a subject using the imaging means by adjusting at least an exposure time, an aperture diameter, and a photographing sensitivity; In the exposure control step, exposure control is performed based on a program diagram that indicates a correspondence relationship between an amount of light incident on the image capture means and an exposure time, a photographing sensitivity, and an aperture value of the image capture means; a control method for an imaging device, characterized in that, in the exposure control step, when an object is imaged using the imaging means to obtain a detection image that is an image for detecting flicker, exposure control is performed based on a program diagram that prioritizes increasing the imaging sensitivity and opening the aperture of the diaphragm over lengthening the exposure time of the imaging means.

9. A program for causing a computer to execute the control method according to claim 8.

10. A computer-readable storage medium storing a program for causing a computer to execute the control method according to claim 8.

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