Exposure control device, imaging device, control method and program

Abstract

To acquire a still picture in which an influence of a flicker is preferably reduced.SOLUTION: An exposure control device includes: reception means for receiving a designation of a shutter speed; first determination means for determining a first accumulation time for accumulating an electric charge by an imaging element in imaging at the time of a live view on the basis of the designated shutter speed; second determination means for determining a second accumulation time for accumulating the electric charge by the imaging element for the image at the time of acquiring a still picture on the basis of the designated shutter speed; and control means for controlling the imaging element so as to accumulate the electric charge at the first accumulation time determined by the first determination means or the second accumulation time determined by the second determination means. In the imaging element, the settable accumulation time is different between the live view and the accumulation of the still picture. The first determination means makes a determination method of the first accumulation time different in accordance with whether the designated shutter speed is longer than a predetermined shutter speed.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to an exposure control device, an imaging device, a control method, and a program, and more particularly to an imaging technique that reduces the effects of flicker caused by lighting. [Background technology]

[0002] When capturing images in an environment where fluorescent lights are installed, the subject will flicker in the captured image according to the driving frequency of the fluorescent lights. More specifically, because fluorescent lights are driven by a commercial power frequency of 50 / 60 Hz, the light intensity change period of the flicker caused by the fluorescent lights is 100 / 120 Hz.

[0003] Patent Document 1 discloses an imaging device that can switch to a shutter speed that reduces the effects of flicker when flicker is detected during imaging with a manually set shutter speed, in order to reduce the effects of flicker on captured images. When the shutter speed is switched to a speed that can reduce the effects of flicker, the imaging device of Patent Document 1 controls the operation of the image sensor so that readout is performed with an accumulation time that corresponds to the shutter speed, thereby enabling imaging with reduced effects of flicker. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-042352 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, LED light sources, in addition to fluorescent lights, have become common artificial light sources in photography environments. Because flicker caused by LED light sources occurs at frequencies ranging from several kHz to several hundred kHz, which are higher than those caused by fluorescent lights, reducing the effects of this flicker requires setting the shutter speed in smaller increments than under fluorescent lights, i.e., controlling the storage time of the image sensor.

[0006] On the other hand, the step width for controlling the accumulation time of the image sensor differs between capturing images for so-called live view and capturing still images. Therefore, if the same accumulation time cannot be set for both live view and still image capture, the reduction in the effect of flicker on the captured image under an LED light source may differ between live view and still image capture. This is because the number of horizontal lines read from the image sensor is limited during live view to reduce power consumption. Therefore, even if a user sets the shutter speed based on a captured image displayed during live view, if the accumulation time conditions of the image sensor are different during still image capture, the still image recorded by the capture may contain unintended flicker effects.

[0007] The present invention has been made in consideration of the above-mentioned problems, and aims to provide an exposure control device, an imaging device, a control method, and a program that can acquire a still image in which the effects of flicker are suitably reduced. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, an exposure control device of the present invention has an accepting means for accepting a shutter time specification, a first determining means for determining a first accumulation time for an image sensor to accumulate charge for imaging during live view based on the specified shutter time, a second determining means for determining a second accumulation time for an image sensor to accumulate charge for imaging during still image shooting based on the specified shutter time, and a control means for controlling the image sensor to accumulate charge for the first accumulation time determined by the first determining means or the second accumulation time determined by the second determining means, and when the specified shutter time is longer than a predetermined shutter time, the first determining means determines an accumulation time that does not exceed the specified shutter time among the accumulation times that can be set during live view. Longest Accumulation time of , the first accumulation time asIf the specified shutter speed is not longer than the predetermined shutter speed, the accumulation time that exceeds the specified shutter speed among the accumulation times that can be set during live view is selected. The accumulation time closest to the specified shutter speed is selected. , the first accumulation time as decision The second determination means determines, as the second accumulation time, an accumulation time that is closest to the specified shutter time among accumulation times that can be set when acquiring a still image. It is characterized by: [Effects of the Invention]

[0009] With this configuration, according to the present invention, it is possible to obtain a still image in which the influence of flicker is suitably reduced. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram showing the functional configuration of a digital mirrorless camera according to an embodiment and a modification of the present invention. [Figure 2] FIG. 10 is a diagram illustrating shutter speeds that can be manually set in a high-frequency flickerless mode according to an embodiment and a modification of the present invention; [Figure 3A] FIG. 10 is a diagram showing the relationship between a specified shutter speed and an accumulation time that can be set in the image sensor 101 according to the embodiment and a modification of the present invention. [Figure 3B] FIG. 10 is another diagram showing the relationship between the specified shutter speed and the accumulation time that can be set in the image sensor 101 according to the embodiment and the modified example of the present invention. [Figure 4A] FIG. 10 is a diagram illustrating the difference in accumulation time between still image capture and live view in accordance with an embodiment and a modified example of the present invention. [Figure 4B] FIG. 10 is another diagram for explaining the difference in accumulation time between still image capture and live view in accordance with the embodiment and modified examples of the present invention. [Figure 5] FIG. 10 is a diagram illustrating a data flow of processing related to exposure control performed by the camera body 100 according to the embodiment and modified examples of the present invention. [Figure 6] 1 is a diagram illustrating a digital gain related to exposure in accordance with a method for determining the accumulation time of the image sensor 101 according to an embodiment and a modification of the present invention; [Figure 7]1 is a flowchart illustrating an example of an imaging process executed by the camera body 100 according to an embodiment of the present invention. [Figure 8] A flowchart illustrating exposure calculation processing executed by the camera body 100 during still image shooting according to an embodiment of the present invention. [Figure 9] A flowchart illustrating an example of exposure calculation processing during live view executed by the camera body 100 according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Embodiment] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0012] In the embodiment described below, the present invention is applied to a digital mirrorless camera, which is an example of an imaging device, and which has an image sensor that can set different accumulation times for live view and still image capture and has a function for controlling the exposure operation of the image sensor. However, the present invention can be applied to any device that can control the operation of the image sensor, and is not limited to specific types of devices with an imaging function, such as mobile phones, smartphones, communication terminals, and game consoles.

[0013] <Functional configuration of digital mirrorless cameras> Fig. 1 is a block diagram showing the functional configuration of a digital mirrorless camera according to this embodiment. In Fig. 1, the digital mirrorless camera is composed of a camera body 100 and an imaging lens 200, which are mechanically and electrically connected via a lens mount mechanism 111. In this embodiment, the imaging lens 200 is described as being configured to be detachable from the camera body 100, but the present invention is not limited to this, and it goes without saying that these may also be configured as an integrated unit.

[0014] <Configuration of camera body 100> The system control unit 103 controls the operation of each block included in the camera body 100. Specifically, the system control unit 103 controls the operation of each block by reading out the operation program for each block stored in ROM, expanding it in a working area of RAM, and executing it. The system control unit 103 is also connected to the lens control unit 201 of the imaging lens 200 via the lens mount mechanism 111, and is configured to be able to send and receive information.

[0015] In this embodiment, the memory 104 is described as having both a ROM and a RAM, as well as a recording area for recording still images obtained by shooting. More specifically, the memory 104 includes a ROM for storing operation programs and parameters required for executing the operation programs. The memory 104 also includes a RAM, which is a working area for program expansion and calculation, and a storage area for temporarily storing intermediate data output in the processing of each block.

[0016] The image sensor 101 is a sensor equipped with a photoelectric conversion element, such as a CCD or CMOS sensor. The image sensor 101 captures an image by photoelectrically converting an optical image of a subject formed on an imaging surface via an imaging lens 200, and outputs an analog image signal. More specifically, the image sensor 101 outputs an analog image signal by accumulating and reading out electric charges over a set accumulation time. The analog image output by the image sensor 101 is converted into a digital image signal (hereinafter simply referred to as a captured image) by applying various image processes, including A / D conversion and development, to the signal processing unit 102. The image sensor 101 may also include an infrared cut filter, a low-pass filter, etc.

[0017] A shutter 105 is provided on the optical path of light incident through the imaging lens 200, and controls the exposure state of the image sensor 101. The shutter 105 is closed when not capturing images, blocking light from the image sensor 101, and is open when capturing images (during live view, still image shooting, or still image acquisition), directing incident light to the image sensor 101. The operation of the shutter 105 is controlled by a shutter control unit 106, which controls the opening and closing of the shutter 105 according to the exposure time determined for capturing images. More specifically, the shutter control unit 106 controls the opening and closing of the shutter 105 based on shutter time information related to the exposure conditions determined by the system control unit 103, thereby exposing the image sensor 101 for a predetermined time and receiving a light beam related to an optical image of a subject. While the example in FIG. 1 shows a configuration in which exposure control of the image sensor 101 is performed by a so-called mechanical shutter, exposure control may also be performed by a so-called electronic shutter, which is achieved by accumulation control of the image sensor 101.

[0018] The rear display unit 107 and the viewfinder display unit 108 are display devices such as TFT liquid crystal panels. The rear display unit 107 is provided on the exterior of the camera body 100, allowing the user to easily check its display. On the other hand, the viewfinder display unit 108 is an electronic viewfinder configured to be visible through an eyepiece 109, and the user must place their eye on the eyepiece 109 to check its display. When the camera body 100 is set to imaging mode and activated, these display devices provide a live view display by sequentially displaying captured images.

[0019] During live view, control is exercised so that display is shown on either the rear display unit 107 or the viewfinder display unit 108, and which is displayed is switched depending on the user's eyepiece position. In the camera body 100 of this embodiment, an eyepiece detection unit 110 is provided near the eyepiece lens 109, making it possible to detect whether an eyepiece is placed near the eyepiece. Therefore, while the eyepiece detection unit 110 detects that an eyepiece is placed near the eyepiece, the viewfinder display unit 108 is used for live view display, and while not detecting the presence of an eyepiece, the rear display unit 107 is used for live view display. Alternatively, the rear display unit 107 and the viewfinder display unit 108 may be used to display the shooting results (still images) when shooting a still image.

[0020] <Configuration of imaging lens 200> The lens control unit 201 is a control device such as a microchip equipped with a ROM and RAM (not shown), and controls the operation of each block of the imaging lens 200. The lens control unit 201 also communicates with the system control unit 103 to exchange information related to focus position and aperture (AE / AF) control.

[0021] The focusing lens 202 changes the focus state of the captured image by being moved in the optical axis direction based on information about the focal position. The movement of the focusing lens 202 is controlled by a lens drive unit 203. The aperture 204 adjusts the amount of light incident on the image sensor 101 of the camera body 100 by changing its open state according to the aperture value. The open state of the aperture 204 is controlled by an aperture control unit 205.

[0022] In this embodiment, the processing is described as being realized by circuits and processors corresponding to the blocks provided as hardware in the camera body 100 or the imaging lens 200. However, the implementation of the present invention is not limited to this, and the processing of at least some of the blocks may be realized by a program that performs the same processing as that of the block.

[0023] <<Exposure Control Overview>> Hereinafter, the exposure control performed by controlling the accumulation time of the image sensor 101 in the digital mirrorless camera of this embodiment will be described with reference to the drawings.

[0024] The digital mirrorless camera of this embodiment includes a manual mode as one of its imaging modes, allowing manual setting of exposure conditions. In this embodiment, it is assumed that at least the shutter speed can be set to a specified value in the manual mode. In addition to the imaging modes, the digital mirrorless camera also includes a mode (high-frequency flicker-free mode) for reducing the effects of flicker when capturing images in a shooting environment that may cause high-frequency flicker, such as with an LED light source. While the manual mode typically allows the shutter speed to be specified in 1 / 2 or 1 / 3-stop increments, the high-frequency flicker-free mode allows the shutter speed to be specified in finer increments. In other words, by allowing the shutter speed to be specified in finer increments than normal, the user can make more precise adjustments to determine a shutter speed that reduces the effects of flicker. In other words, because flicker from LED light sources and the like varies in frequency, the digital mirrorless camera of this embodiment allows the user to finely adjust the shutter speed to suit the shooting environment while checking the live view display. In other words, the user can set different shutter speeds, check the captured images obtained using those shutter speeds on the live view display, identify the optimal shutter speed that reduces the effects of flicker, and then take the final still image.

[0025] The shutter speed that can be specified by the user in the high-frequency flickerless mode may be as shown in FIG. 2. As shown in the figure, the number of steps that can be specified varies depending on the length of the reference shutter speed, and the longer the reference shutter speed, the finer the adjustment. In the example shown in the figure, the shutter speed can be adjusted in 1 / 4-stop increments between 1 / 8192.0 [s] and 1 / 4871.0 [s]. The shutter speed can be adjusted in 1 / 8-stop increments between 1 / 4096.0 [s] and 1 / 2233.4 [s]. The shutter speed can be adjusted in 1 / 16-stop increments between 1 / 2048.0 [s] and 1 / 1069.3 [s]. The shutter speed can be adjusted in 1 / 32-stop increments between 1 / 1024.0 [s] and 1 / 523.2 [s]. Furthermore, when the shutter speed is between 1 / 512.0 [s] and 1 / 258.8 [s], it can be adjusted in 1 / 64-stop increments. When the shutter speed is between 1 / 256.0 [s] and 1 / 128.7 [s], it can be adjusted in 1 / 128-stop increments. When the shutter speed is between 1 / 128.0 [s] and 1 / 50.0 [s], it can be adjusted in 1 / 256-stop increments. Hereinafter, in this embodiment, it is assumed that exposure according to such shutter speed is achieved by controlling the accumulation time of the image sensor 101 to a corresponding value.

[0026] The accumulation time that can be set for the image sensor 101 differs depending on the operating conditions. More specifically, the readout mode of the image sensor 101 differs between live view, in which captured images obtained by repeated image capture are displayed sequentially, and still image capture, which is performed to record a single still image, and therefore the settable accumulation time step width differs.

[0027] During live view, image capture is frequent and power consumption related to image capture operations can increase, so generally, during live view, the image sensor 101 is controlled to perform thinning readout or averaging readout. In other words, power consumption is reduced by limiting the number of horizontal lines read out by the image sensor 101 to a number less than when capturing still images.

[0028] Here, the control cycle for charge accumulation in the image sensor 101 can only be controlled based on the readout timing of horizontal lines, so the control cycle is longer during live view, when the number of horizontal lines to be read out is fewer than during still image capture. Therefore, the accumulation time that can be set in the image sensor 101 during live view has a larger step width than that that can be set during still image capture, which can reduce the accuracy of selecting an accumulation time that corresponds to a specified shutter speed.

[0029] Below, we will explain the relationship between the shutter speed and the accumulation time that can be set in the image sensor 101, assuming that the time required to read one horizontal line is 14.39 μs and that during live view, the number of horizontal lines read is reduced by thinning out one line or adding two lines. That is, the accumulation time step width that can be set in the image sensor 101 during still image capture is ΔT1=14.39 μs, and the accumulation time step width that can be set in the image sensor 101 during live view is ΔT2=28.78 μs.

[0030] For example, when a shutter speed between 1 / 8192.0 [s] and 1 / 4871.0 [s] is specified in the high-frequency flickerless mode, the relationship between the accumulation times that can be set in the image sensor 101 is as shown in FIG. 3A. Similarly, when a shutter speed between 1 / 2048.0 [s] and 1 / 1024.0 [s] is specified, the relationship between the accumulation times that can be set in the image sensor 101 is as shown in FIG. 3B. These figures show, from left to right, the specified shutter speed, the control period of the accumulation time that can be set when capturing a still image, and the control period of the accumulation time that can be set when capturing a live view. More specifically, the accumulation time that is actually set in the image sensor 101 for the specified shutter speed is selected from the corresponding accumulation times that can be set.

[0031] Simply put, by selecting the accumulation time closest to the specified shutter time, it is possible to capture an image with exposure conditions (shutter time) close to the user's intended. If there is a difference between the specified shutter time and the selected accumulation time, the resulting captured image will exhibit an exposure state different from that of the specified shutter time (an exposure difference occurs). This exposure difference is not a problem, for example, for still images to which the user can apply any image processing after capture. However, for captured images intended for temporary display as live view, image processing to reduce the exposure difference is required during live view, since the exposure representation should be changed evenly in response to changes in the shutter time. Therefore, during live view, the camera body 100 of this embodiment applies a digital gain to the captured image according to the difference between the specified shutter time and the accumulation time actually set in the image sensor 101, thereby adjusting the exposure state of the image displayed in live view. As a result, the exposure state of the image displayed in live view is adjusted to the exposure equivalent to the specified shutter time.

[0032] Incidentally, changing the exposure state by applying digital gain is performed by multiplying the signal value (pixel value) of the digital image signal (captured image) by the numerical value of the digital gain. That is, if the set accumulation time is shorter than the specified shutter time, a digital gain that increases the signal value (increases exposure) (hereinafter referred to as a positive digital gain) is applied. On the other hand, if the set accumulation time is longer than the specified shutter time, a digital gain that decreases the signal value (decreases exposure) (hereinafter referred to as a negative digital gain) is applied.

[0033] However, when a negative digital gain is applied, degradation of image quality (such as reduced resolution and pixel value compression) occurs in the resulting image. For this reason, it is generally considered preferable to select an accumulation time shorter than the shutter time so as not to apply a negative digital gain, and to generate an image for live view display by applying a positive digital gain. Therefore, the criteria for selecting the accumulation time of the image sensor 101 corresponding to a specified shutter time differ in that, when capturing a still image, the accumulation time closest to the shutter time is selected, and, when capturing a live view, the longest accumulation time among accumulation times shorter than the shutter time is selected.

[0034] On the other hand, as described above, the accumulation time that can be set in the image sensor 101 differs between still image shooting and live view, and therefore, if the criteria for selecting the accumulation time are different between still image shooting and live view, the following problems may arise.

[0035] 4A, the accumulation time 401 selected for still image shooting and the accumulation time 402 selected for live view will be the same. On the other hand, if the specified shutter time and the accumulation time selectable for still image shooting and the accumulation time 402 selected for live view have the relationship shown in FIG. 4B, the accumulation time 411 selected for still image shooting and the accumulation time 412 selected for live view will be different. In the readout mode for live view of this embodiment, as shown in FIG. 4B, there can be a difference in accumulation time of up to ΔT2 minutes between still image shooting and live view.

[0036] That is, in a situation where a user sets a shutter speed that reduces the effects of flicker while checking the live view display, such differences in the criteria for selecting the accumulation time make it difficult to set an appropriate shutter speed. More specifically, if different accumulation times are set in the image sensor 101 for still image capture and live view, even if the effects of flicker are suitably reduced in an image displayed as live view, the reduction may be different in a still image captured thereafter. This occurs regardless of whether digital gain is applied to change the exposure to a value equivalent to the specified shutter speed.

[0037] In order to reduce the difference in the manner in which the effects of flicker are reduced between images displayed in live view and still images, camera body 100 of this embodiment performs processing to change the method for determining the accumulation time during live view depending on the specified shutter speed. In other words, camera body 100 performs exposure control so that the difference in accumulation time set in image sensor 101 during live view and still image capture does not reach a level that causes a difference in the visibility of the effects of flicker in the captured image.

[0038] Experiments conducted by the applicant have confirmed that if the difference in accumulation time between live view and still image capture is less than 1 / 16 stop, the difference in the effect of flicker in captured images is less noticeable. Therefore, if the shutter speed is such that the maximum difference in accumulation time (ΔT2) resulting from the accumulation time selection criteria described above is less than 1 / 16 stop, the flicker effect reduction characteristics between live view and still image capture can be considered similar even when the selection criteria are adopted. On the other hand, if the shutter speed is such that ΔT2, i.e., the control cycle of charge accumulation during live view, is greater than 1 / 16 stop, a non-negligible difference in the effect of flicker will occur when the selection criteria are adopted. In other words, the selection criteria are effective when the shutter speed is longer than the specified shutter speed (a difference in the effect of flicker reduction can be tolerated), but are not effective when the shutter speed is shorter than the specified shutter speed. Here, in the case where ΔT2=28.78 [μs], the predetermined shutter speed is 1 / 693.4 [s], and the camera body 100 of this embodiment uses this shutter speed as the threshold TVth for switching the method of determining the accumulation time.

[0039] Therefore, when the specified shutter time is shorter than the predetermined shutter time, the camera body 100 of this embodiment changes the criteria for selecting the accumulation time during live view so that the difference in accumulation time between live view and still image capture is smaller. Specifically, for the corresponding shutter time, control is performed to select the accumulation time closest to the specified shutter time during live view, just as with still image capture. Note that in this case, because an accumulation time longer than the shutter time may be selected during live view, a fixed offset ΔK is added to the digital gain to prevent negative digital gain, and exposure control of other elements is performed to cancel out the increase in exposure amount.

[0040] <<Data flow related to exposure control>> The exposure control processing performed by the camera body 100 of this embodiment will be described below with reference to the data flow diagram of Fig. 5. Note that in this embodiment, the various processes shown in Fig. 5 will be described as being executed by the system control unit 103, but the present invention is not limited to this implementation and may be realized by other circuits, etc. Also, information on the manually specified shutter time TV and offset ΔK used in the various processes is assumed to be acquired from the memory 104.

[0041] First, when a shutter speed is specified, a photometric value (subject brightness) BV is derived in a photometric calculation 501, and a target exposure that serves as a reference for exposure control is determined. The photometric value BV may be derived, for example, based on the brightness of the entire surface of the captured image. Here, brightness may refer to a brightness signal calculated by multiplying the R, G, and B signals of a Bayer array RAW image by a certain coefficient, or may refer to the R, G, and B signals themselves. Also, a sensor array other than the Bayer array may be used.

[0042] As described above, in the camera body 100 of this embodiment, if the specified shutter time is shorter than the threshold, an offset ΔK is added to the digital gain during live view. Therefore, the target exposure determined by the photometry calculation 501 is the photometric value BV if the specified shutter time is longer than the threshold, and is the photometric value BV + offset ΔK if the specified shutter time is shorter than the threshold.

[0043] Once the target exposure is determined, exposure calculation 502 uses the designated shutter time TV and target exposure BV(+ΔK) to perform exposure control calculation based on the following APEX relational expression. BV(+ΔK)=AV+TV-SV That is, the aperture value AV and the ISO sensitivity SV are determined in the exposure calculation 502. As a result, various control values used for exposure control are determined. The aperture value AV is transmitted to the imaging lens 200.

[0044] Once the exposure control value is determined, a gain calculation 503 derives an analog gain to be set in the image sensor 101 based on the sensitivity SV. Due to operational constraints of the image sensor 101, the analog gain that can be set in the image sensor 101 is limited, and therefore the analog gain may be insufficient for the sensitivity SV derived in the exposure calculation 502. Therefore, a digital gain SVDig is derived in the gain calculation 503 to compensate for the exposure (related to sensitivity) that is insufficient in the analog gain. That is, in the gain calculation 503, a digital gain SvDig related to sensitivity is derived based on the difference between the sensitivity SV and the analog gain.

[0045] Furthermore, once the exposure control value is determined, an accumulation time AccumTime to be set in the image sensor 101 is determined in accumulation time calculation 504. As described above, the accumulation time determined in accumulation time calculation 504 is selected from among settable accumulation times (which differ between live view and still image shooting) based on the specified shutter time TV. Furthermore, in accumulation time calculation 504, a digital gain AccumGain related to exposure is derived to compensate for the exposure difference resulting from the difference between the determined accumulation time and the specified shutter time TV.

[0046] As described above, the method of deriving the exposure-related digital gain AccumGain differs depending on whether the specified shutter time is longer than the threshold value, so as to avoid a negative digital gain. Here, the differences in exposure-related digital gain AccumGain depending on the method of determining the accumulation time during live view will be described with reference to FIG. 6.

[0047] When the accumulation time is determined by selecting the longest accumulation time among the settable accumulation times that does not exceed the specified shutter time TV, the digital gain AccumGain associated with exposure is as shown by the dashed line 601. As shown in the figure, the dashed line 601 results in a positive digital gain regardless of the specified shutter time. On the other hand, when the accumulation time is determined by selecting the closest accumulation time among the settable accumulation times, including accumulation times that exceed the specified shutter time, the digital gain AccumGain associated with exposure is as shown by the solid line 602. As shown in the figure, the solid line 602 includes both positive and negative digital gains depending on the specified shutter time. Also, as shown in the figure, the shorter the specified shutter time, the greater the deviation between the specified shutter time and the set accumulation time tends to be, and the greater the amount of compensation by the digital gain AccumGain associated with exposure. Hereinafter, the former accumulation time determination method will be referred to as the first determination method, and the latter accumulation time determination method will be referred to as the second determination method.

[0048] As described above, in the camera body 100 of this embodiment, if the specified shutter speed is longer than the threshold, the accumulation time is determined using the first determination method; if the specified shutter speed is not longer than the threshold, the accumulation time is determined using the second determination method. Therefore, in the latter case, since the exposure-related digital gain AccumGain may be a negative digital gain, the offset ΔK is added to the value, as shown by the dashed-dotted line 603, to make the digital gain positive regardless of the shutter speed. Therefore, the fixed offset ΔK is set to a value that does not apply a negative digital gain, even when the shutter speed is near the fastest that can be specified. In other words, the offset ΔK is determined to be a value that, when the exposure-related digital gain AccumGain is applied to the image signal, the signal value after application exceeds the signal value before application.

[0049] The analog gain derived in the gain calculation 503 and the accumulation time AccumTime determined in the accumulation time calculation 504 are supplied to the image sensor 101 and used in image capture processing that performs charge accumulation and outputs analog image signals.

[0050] Furthermore, when an image is captured by the image sensor 101 and a digital image signal relating to the captured image is output by the signal processing unit 102, a digital gain is applied in the gain application 505. More specifically, in the gain application 505, a final digital gain obtained by multiplying the digital gain SVDig relating to sensitivity and the digital gain AccumGain relating to exposure is applied to the digital image signal.

[0051] Note that application of digital gain amplifies digital image signals using software, and is more susceptible to noise than analog gain, which amplifies electric charges in the image sensor 101. Therefore, various data flows related to digital gain may be performed only during live view.

[0052] <<Image capture processing>> Specific imaging processing in high frequency flicker-free mode of the digital mirrorless camera of this embodiment having such a configuration will be described using the flowchart in Fig. 7. The processing corresponding to this flowchart can be realized by the system control unit 103 reading out a corresponding processing program stored in, for example, ROM, expanding it into RAM, and executing it.

[0053] This imaging process will be described assuming that it is started, for example, when the camera body 100 is started in imaging mode and then set to high-frequency flickerless mode. Setting the high-frequency flickerless mode may be performed, for example, as follows: When started in imaging mode, the system control unit 103 detects the frequency of the flicker that is occurring by converting a group of flicker detection images captured at different frame rates into frequency components. The system control unit 103 may then suggest a shutter speed that will reduce the effects of the flicker via the rear display unit 107, and set the high-frequency flickerless mode when the user changes the setting to the suggested shutter speed.

[0054] In S701, the system control unit 103 performs photometric calculation to measure the brightness of the subject and derives a photometric value BV.

[0055] In S702, the system control unit 103 determines whether or not it is time to shoot a still image. If the system control unit 103 determines that it is time to shoot a still image, it shifts the process to S703 and executes exposure calculation processing for still image shooting. If the system control unit 103 determines that it is not time to shoot a still image, that is, that it is time to perform live view, it shifts the process to S704 and executes exposure calculation processing for live view.

[0056] <Exposure calculation processing (when shooting still images)> The exposure calculation process executed in step S703 for still image shooting will now be further described with reference to the flowchart in Fig. 8. As described above, digital gain is not applied when shooting still images, so the exposure calculation process only determines the exposure control values, analog gain related to sensitivity, and accumulation time of the image sensor 101, as shown below.

[0057] In S801, the system control unit 103 acquires a shutter speed TV specified by the user. The shutter speed TV may be specified, for example, by displaying a GUI that allows the user to select the shutter speed via the rear display unit 107, and by operating the GUI to specify the shutter speed. In other words, the system control unit 103 functions as a receiving unit according to the present invention and receives the specified shutter speed.

[0058] In S802, the system control unit 103 performs exposure calculation based on the photometric value BV derived in S701 of the imaging process and the designated shutter speed TV, and derives exposure control values (AV, SV) related to the aperture and sensitivity. Here, in the exposure calculation process for still image shooting, the photometric value BV is used as the target exposure.

[0059] In S803, the system control unit 103 performs gain calculation based on the exposure control value SV related to the sensitivity derived in S802, and derives an analog gain to be set in the image sensor 101.

[0060] In S804, the system control unit 103 determines an accumulation time to be set in the image sensor 101 as a second accumulation time according to the present invention based on the specified shutter time TV. More specifically, the system control unit 103 determines, among the accumulation times that can be set when capturing a still image, the accumulation time that has the smallest difference from the specified shutter time TV as the accumulation time to be set in the image sensor 101. In other words, when capturing a still image, the accumulation time to be set in the image sensor 101 is determined by the second determination method.

[0061] <Exposure calculation processing (live view)> Next, the exposure calculation process during live view, which is executed in S704, will be further described using the flowchart in Fig. 9. As described above, digital gain is applied during live view, so the exposure calculation process determines each exposure control value, an analog gain related to sensitivity, and the accumulation time of the image sensor 101, as well as a digital gain related to exposure and application of the digital gain, as shown below. Note that in the exposure calculation process during live view, steps that perform the same processing as when capturing still images are given the same reference numbers, and their description will be omitted.

[0062] When the specified shutter time TV is acquired in S801, the system control unit 103 determines in S901 whether the specified shutter time TV is longer than the threshold value TVth. If the system control unit 103 determines that the specified shutter time TV is longer than the threshold value TVth, it proceeds to S902, and if it determines that the specified shutter time TV is not long, it proceeds to S904.

[0063] In S902, the system control unit 103 determines the accumulation time to be set in the image sensor 101 as the first accumulation time according to the present invention based on the specified shutter time TV. More specifically, the system control unit 103 determines the longest accumulation time that does not exceed the specified shutter time TV, among the accumulation times that can be set during live view, as the accumulation time to be set in the image sensor 101. In other words, when during live view and the shutter time is longer than the threshold value TVth, the accumulation time to be set in the image sensor 101 is determined by the first determination method.

[0064] Then, in S903, the system control unit 103 determines the offset ΔK used to derive the digital gain AccumGain related to exposure to be 0, and stores this in the memory 104.

[0065] On the other hand, if the specified shutter time TV is not longer than the threshold value TVth, the system control unit 103 determines in S904 the accumulation time to be set in the image sensor 101 as the first accumulation time according to the present invention based on the specified shutter time. More specifically, the system control unit 103 determines, among the accumulation times that can be set during live view, the accumulation time that has the smallest difference from the specified shutter time TV as the accumulation time to be set in the image sensor 101. As described above, the accumulation times to be selected in this step include accumulation times that exceed the specified shutter time TV, and the accumulation time closest to the specified shutter time is selected. In other words, if the shutter time is during live view and is not longer than the threshold value TVth, the accumulation time to be set in the image sensor 101 is determined using the second determination method.

[0066] Then, in S905, the system control unit 103 determines the offset ΔK used to derive the digital gain AccumGain related to exposure to be a predetermined value greater than 0, and stores the determined value in the memory 104. Here, the predetermined value is a fixed value determined in advance so that the digital gain AccumGain related to exposure, which will be derived later, does not become a negative digital gain.

[0067] After determining the accumulation time and offset ΔK to be set in the image sensor 101, the system control unit 103 derives the target exposure for image capture during live view in S906. Specifically, the image sensor 101 derives the target exposure (BV+ΔK) by adding the offset ΔK to the photometric value BV derived in S701.

[0068] In S907, the system control unit 103 performs exposure calculation based on the target exposure derived in S906 and the designated shutter time TV, derives exposure control values (AV, SV) related to the aperture and sensitivity, and moves the process to S803.

[0069] After determining the analog gain related to sensitivity in S803, the system control unit 103 determines, in S908, a digital gain to be applied to a digital image signal (captured image) obtained by capturing an image. More specifically, the system control unit 103 derives a digital gain related to sensitivity, SVDig, to compensate for any exposure deficiency due to the analog gain derived in S803. The system control unit 103 also derives a digital gain related to exposure, AccumGain, to compensate for any exposure deficiency due to the accumulation time determined in S902 or S904. When deriving the digital gain related to exposure, AccumGain, an offset ΔK is added to the digital gain to compensate for the exposure equivalent to the specified shutter time. The system control unit 103 then multiplies these derived digital gains (SVDig and AccumGain) together to ultimately determine the digital gain to be applied to the digital image signal.

[0070] When exposure calculation processing is executed in this way during still image shooting or live view, the system control unit 103 sets the accumulation time and analog gain determined in the exposure calculation processing in the image sensor 101 in S705 of the imaging processing, and causes the image sensor 101 to capture an image based on these settings. At this time, the system control unit 103 transmits an exposure control value AV related to the aperture to the imaging lens 200, and causes the aperture control unit 205 to control the aperture 204. Then, when an image is captured by the image sensor 101, the signal processing unit 102 A / D converts the output analog image signal to generate a digital image signal.

[0071] In S706, the system control unit 103 performs processing to apply digital gain to the digital image signal generated in S705. Note that, as described above, digital gain is not derived in the exposure control processing when capturing still images, and therefore the processing in this step is performed only during live view. In other words, in order to suppress the S / N ratio of the still images to be recorded, processing to reduce exposure differences by applying digital gain is not performed during still image capture.

[0072] By doing this, the imaging device of this embodiment can reduce the difference in accumulation time of the image sensor 101 between live view and still image shooting in high-frequency flickerless mode, thereby achieving still image shooting with the effects of flicker suitably reduced.

[0073] In the imaging process of this embodiment, the high-frequency flicker-free mode is selected, and the method of determining the accumulation time is described as being different depending on the specified shutter speed. However, the present invention is not limited to this. That is, the present invention can be applied to any mode in which the control cycle of the image sensor's charge accumulation is different between live view and still image capture, thereby achieving the effect of providing a live view display with reduced exposure differences compared to the recorded still image. For example, when the image sensor's readout line count is thinned out more than one line during live view, the effect of flicker at a frequency lower than that of an LED light source can be reduced by applying the present invention. Therefore, the present invention can achieve a live view display with exposure conditions similar to those of the recorded still image, regardless of the flicker frequency.

[0074] [Variation 1] In the above-described embodiment, a fixed offset ΔK is added to derive the digital gain when a shutter speed longer than the specified shutter speed can be set during live view. However, the present invention is not limited to this. As shown by the solid line 602 in FIG. 6 , the degree of exposure difference based on the difference between the specified shutter speed and the correspondingly set accumulation time varies depending on the shutter speed and the specifications of the image sensor. Therefore, when target exposure control is not performed, the offset ΔK may be added to derive the digital gain related to exposure only when it is necessary to apply a negative digital gain (hereinafter referred to as the reference gain) to reduce the exposure difference. In other words, the digital gain related to exposure with the offset added and the target exposure with the offset added may be derived only at shutter speeds where the reference gain is equal to or less than 0.

[0075] Here, a suitable reduction in exposure difference can be achieved by not making the digital gain related to exposure a negative digital gain, and by setting the negative amount as an offset at the shutter speed at which the basic gain is a negative value. In this case, for the designated shutter speed TV (at which the basic gain is a negative value), using the determined accumulation time AccumTime, The offset ΔK can be derived as JPEG0007720715000001.jpg1250. By using the offset ΔK to offset the negative amount of the basic gain, it is possible to eliminate the digital gain associated with exposure. In other words, the offset does not have to be a fixed value, but may be a value that is dynamically derived according to the shutter speed.

[0076] [Variation 2] In the above-described embodiment, the method for determining the accumulation time and whether to add an offset to the digital gain associated with exposure are switched depending on whether the specified shutter time is longer than a threshold value TVth determined based on the visibility of differences in the effects of flicker. However, the present invention is not limited to this. The switching of the method for determining the accumulation time and the switching of whether to add an offset to the digital gain associated with exposure may be controlled independently. For example, considering that the S / N ratio of a captured image may decrease when a digital gain with a fixed offset is applied, the offset may be controlled to vary in stages depending on the shutter time, taking into account the expected degree of the negative contribution to the basic gain. In such an embodiment, because the degree of the negative contribution to the basic gain is small when the shutter time is long, the accumulation time of the image sensor may be determined using the second determination method regardless of the shutter time.

[0077] [Other embodiments] The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the 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 realizes one or more functions.

[0078] The invention is not limited to the above-described embodiments, and various changes and modifications can be made 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]

[0079] 100: camera body, 101: image sensor, 102: signal processing unit, 103: system control unit, 104: memory

Claims

1. Accepting means for accepting a shutter speed designation; a first determination means for determining a first accumulation time for the image sensor to accumulate electric charges for capturing an image during live view, based on the specified shutter speed; a second determination means for determining a second accumulation time for the image sensor to accumulate electric charges when capturing a still image based on the specified shutter time; a control means for controlling the image sensor so as to accumulate electric charges for the first accumulation time determined by the first determination means or the second accumulation time determined by the second determination means; and The first determining means When the specified shutter time is longer than a predetermined shutter time, the longest accumulation time that does not exceed the specified shutter time among accumulation times that can be set during live view is determined as the first accumulation time; When the specified shutter time is not longer than the predetermined shutter time, an accumulation time that is closest to the specified shutter time among accumulation times that can be set during live view and that include an accumulation time that exceeds the specified shutter time is determined as the first accumulation time; The second determination means determines, as the second accumulation time, an accumulation time that is closest to the specified shutter time among accumulation times that can be set when the still image is acquired. An exposure control device characterized by:

2. an adjustment unit that applies a digital gain to an image signal captured by the image sensor at least during live view; a third determination means for determining the digital gain to be applied by the adjustment means based on the specified shutter time; 2. The exposure control device according to claim 1, further comprising:

3. The third determination means When the designated shutter time is longer than the predetermined shutter time, the digital gain is determined to a value that adjusts the signal value of the image signal to an exposure equivalent to the designated shutter time; When the designated shutter time is not longer than the predetermined shutter time, the digital gain is determined to be a value obtained by adding a predetermined offset to a value for adjusting the signal value of the image signal to an exposure equivalent to the designated shutter time.

3. The exposure control device according to claim 2.

4. an acquisition means for acquiring a photometric value; an exposure control unit that performs exposure control for imaging based on the photometric value acquired by the acquisition unit; and 4. The exposure control device according to claim 3, wherein, when the digital gain is determined by adding the predetermined offset, the exposure control means performs exposure control by changing the photometric value based on the predetermined offset.

5. 5. The exposure control device according to claim 3, wherein the predetermined offset is determined so that the signal value after application of the digital gain exceeds the signal value of the image signal.

6. 6. The exposure control device according to claim 1, wherein a step width of the accumulation time that can be set during the live view is larger than a step width of the accumulation time that can be set during the acquisition of the still image.

7. 7. The exposure control device according to claim 6, wherein the predetermined shutter time is set based on a step width of an accumulation time that can be set during the live view mode.

8. The apparatus further includes a setting unit for setting whether or not to perform processing to reduce the effect of flicker caused by a light source disposed in the shooting environment, 8. The exposure control device according to claim 1, wherein, when the setting means is set to perform processing to reduce the effects of flicker, the first determining means changes the method of determining the first accumulation time depending on whether the specified shutter time is longer than the predetermined shutter time.

9. 9. The exposure control device according to claim 1, wherein the image sensor has different settable accumulation times for live view and for capturing still images.

10. An exposure control device according to any one of claims 1 to 9; imaging means for capturing an image using the imaging element; a recording means for recording a still image obtained by capturing the still image; An imaging device comprising:

11. a receiving step of receiving a designation of a shutter speed; a first determination step of determining a first accumulation time for an image sensor to accumulate electric charge for capturing an image during live view, based on the specified shutter speed; a second determination step of determining a second accumulation time for the image sensor to accumulate electric charges for capturing a still image based on the specified shutter time; a control step of controlling the image sensor so as to accumulate electric charges for the first accumulation time determined in the first determination step or the second accumulation time determined in the second determination step; and In the first determination step, When the specified shutter time is longer than a predetermined shutter time, the longest accumulation time that does not exceed the specified shutter time among accumulation times that can be set during live view is determined as the first accumulation time, when the specified shutter time is not longer than the predetermined shutter time, an accumulation time that is closest to the specified shutter time among accumulation times that can be set during the live view mode and that include an accumulation time that exceeds the specified shutter time is determined as the first accumulation time; In the second determination step, an accumulation time that is closest to the specified shutter time among accumulation times that can be set when the still image is acquired is determined as the second accumulation time.

2. A control method for an exposure control device comprising:

12. A program that causes a computer to function as each of the means of the exposure control device according to any one of claims 1 to 9.

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