JP2007027984A - Photographic apparatus and control method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photographic apparatus, capable of reducing exposure variations, when displaying a through-image by exposure correction, and to provide a control method thereof. <P>SOLUTION: When through-image display is started, the photographic apparatus calculates an AE evaluation value on the basis of an image signal from a CCD, determines the optimum exposure value so that a value multiplying the AE evaluation value with N (N>1) agrees with a target value when an exposure correction value set by an operation section is greater than a prescribed value S toward the positive correction side, brings the gain of the AWB to a multiple of N of the normal gain to multiply a level of the image signal by N fold for carrying out the through-image display. Thereafter, the exposure correction value is reflected on the proper exposure value, and the AE operation is executed repeatedly until a shutter button is depressed. Since the level of the image signal becomes 1/N fold, exposure variations due to the exposure correction is lesslikely to take place. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photographing apparatus having an automatic exposure function, an exposure correction function, and a through image display function, and a control method therefor.

  2. Description of the Related Art Digital cameras that capture a subject image with a solid-state imaging device such as a CCD image sensor, digitally convert the obtained image signal, and record it on a recording medium are widely used. Many digital cameras in recent years have an automatic exposure (AE) function that automatically determines an optimal exposure value (aperture value and shutter speed) for the brightness of the subject, and exposure with respect to an appropriate exposure value determined by AE. It has an exposure compensation function that compensates for over or under.

  In a digital camera having an exposure correction function, for example, when still image shooting is performed with the exposure correction value set to +2 Ev, first, as shown in FIG. 7A, the average value of the image signal (AE evaluation value) The appropriate exposure value (AEv) is determined so that the value matches the target value. Thereafter, as shown in FIG. 7B, the proper exposure value is corrected by the exposure correction value (AEv-2Ev), and photographing is performed.

The range of the amount of light that can be received by the solid-state imaging device is called a dynamic range. When the exposure correction value becomes larger than the dynamic range, the image signal is clamped at the upper limit or the lower limit of the dynamic range. In the above example, the range of −3.5 Ev to 2.5 Ev with respect to the target value of the AE evaluation value is the dynamic range of the solid-state imaging device, and the image is set at the upper limit (2.5 Ev) of the dynamic range by +2 Ev plus correction. The signal is clamped (the light intensity is saturated). In order to reduce this clamping, a method of changing the dynamic range of the solid-state image sensor according to the exposure correction value is known (see Patent Document 1).
JP 2004-222183 A

  Many digital cameras include an image display device such as a liquid crystal display (LCD). The image display device functions as a viewfinder at the time of shooting in addition to reproducing and displaying an image signal recorded on a recording medium. A live view is displayed. In the through image display, the image signal output from the solid-state imaging device is displayed on the image display device sequentially for each screen without being recorded on the recording medium.

  When exposure correction is performed in this through-image display state, after AE correction is performed as shown in FIG. 7B, an AE evaluation value is further calculated, and the calculated AE evaluation value becomes a new target value (originally A feedback AE operation for determining an appropriate exposure value so as to coincide with a level obtained by adding an exposure correction value to the target value of () is performed.

  At this time, since the image signal is clamped at the upper limit of the dynamic range of the solid-state imaging device, the AE evaluation value is below the new target value as shown in FIG. For this reason, as shown in FIG. 8B, the appropriate exposure value determined so that the AE evaluation value matches the new target value is shifted to the plus correction side from the appropriate exposure value before the feedback AE operation, A through image is overexposed compared to an image actually obtained by still image shooting.

  As described above, when exposure compensation is performed in the live view display state, the exposure of the live view image fluctuates due to restrictions on the dynamic range of the solid-state imaging device, and the exposure compensation value reflected in the live view image is actually set to the exposure compensation value. It will be different. In order to solve this problem, it is conceivable to change the dynamic range of the solid-state imaging device as described in Patent Document 1 described above, but in order to implement this, it is composed of high-sensitivity pixels and low-sensitivity pixels. It is necessary to use a special solid-state imaging device, which is problematic because of high cost.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a photographing apparatus capable of reducing exposure fluctuations during live view display due to exposure correction, and a control method therefor.

  In order to achieve the above object, an imaging apparatus of the present invention has an automatic exposure function for determining an optimal exposure value so that an evaluation value calculated based on an image signal from a solid-state image sensor matches a predetermined target value. In the photographing apparatus having an exposure correction function for correcting the optimum exposure value based on an exposure correction value set by an operation means, and a through image display function for sequentially displaying the image signal for each screen, When the exposure correction value is larger than a predetermined value on the plus correction side, an optimum exposure value is determined such that a value obtained by multiplying the evaluation value by N times (N> 1) matches the target value, and the image signal Control means for performing the through image display at a level N times is provided.

  In order to achieve the above object, the photographing apparatus of the present invention is an automatic exposure that determines an optimum exposure value so that an evaluation value calculated based on an image signal from a solid-state image sensor matches a predetermined target value. In a photographing apparatus provided with a function, an exposure correction function for correcting the optimum exposure value based on an exposure correction value set by an operation means, and a through image display function for sequentially displaying the image signal for each screen When the exposure correction value is smaller than a predetermined value on the minus correction side, an optimum exposure value is determined such that a value obtained by multiplying the evaluation value by 1 / N times (N> 1) matches the target value, Control means for performing the through image display by setting the level of the image signal to 1 / N times is provided.

  In addition, it is preferable that a gain adjustment unit that performs white balance correction is provided, and the control unit controls the gain adjustment unit to change the level of the image signal. In addition, gradation conversion means for performing gradation conversion of the image signal based on a predetermined parameter is provided, and the control means controls the gradation conversion means and changes the parameter to change the level of the image signal. Is preferably changed.

  In order to achieve the above object, the method for controlling an imaging apparatus according to the present invention determines an optimum exposure value so that an evaluation value calculated based on an image signal from a solid-state image sensor matches a predetermined target value. An automatic exposure function, an exposure correction function for correcting the optimum exposure value based on an exposure correction value set by an operation means, and a through image display function for sequentially displaying the image signal for each screen. In the control method of the photographing apparatus, when the exposure correction value is larger than a predetermined value on the plus correction side, an optimum exposure value is set such that a value obtained by multiplying the evaluation value by N times (N> 1) matches the target value. And the through image display is performed by increasing the level of the image signal by N times.

  In order to achieve the above object, the method for controlling an imaging apparatus according to the present invention determines an optimum exposure value so that an evaluation value calculated based on an image signal from a solid-state image sensor matches a predetermined target value. An automatic exposure function, an exposure correction function for correcting the optimum exposure value based on an exposure correction value set by an operation means, and a through image display function for sequentially displaying the image signal for each screen. In the control method of the photographing apparatus, when the exposure correction value is smaller than a predetermined value on the minus correction side, the value obtained by multiplying the evaluation value by 1 / N times (N> 1) matches the target value. The exposure value is determined, and the through image display is performed by increasing the level of the image signal by 1 / N times.

  According to the present invention, when the exposure correction value is larger than the predetermined value on the plus correction side, the optimum exposure value is determined so that the value obtained by multiplying the evaluation value by N times (N> 1) matches the target value. Since the through-image display is performed with the image signal level set to N times, the over-exposure fluctuation at the time of the through-image display caused by the automatic exposure function again after the exposure correction is reduced.

  Further, according to the present invention, when the exposure correction value is smaller than the predetermined value on the minus correction side, the optimum exposure value is set such that the value obtained by multiplying the evaluation value by 1 / N (N> 1) matches the target value. And the through image display is performed with the image signal level set to 1 / N times, so that the fluctuation in the exposure to the under side during the through image display caused by the automatic exposure function again after the exposure correction is reduced. .

  In FIG. 1 showing the configuration of the digital camera 10, a CPU (control means) 11 is stored in the ROM 13 in response to an operation signal input from an operation unit 12 including a shutter button, a mode switching button, an exposure correction value setting button, and the like. The operation of each part is controlled based on the control program. On the subject side of the CCD image sensor (CCD) 14, a photographing lens 15 and a diaphragm 16 are provided, and a lens motor 17 and an iris motor 18 are connected to each. The motors 17 and 18 are stepping motors and are driven by motor drivers 19 and 20, respectively. The CPU 11 controls the motor drivers 19 and 20 to adjust the lens position of the taking lens 15 and the aperture value of the aperture 16.

  The CCD 14 is controlled by a driving pulse transmitted from a timing generator (TG) 21 and outputs a captured image captured through the photographing lens 15 and the diaphragm 16 as an electrical image signal. The image signal includes an RGB color signal from a photodiode provided with primary color filters of R (red), G (green), and B (blue), and a noise signal from a light-shielded photodiode (optical black). And are included. The CCD 14 has an electronic shutter function, and the CPU 11 controls the shutter speed (charge accumulation time) of the CCD 14 via the TG 21.

  The image signal output from the CCD 14 is subjected to analog signal processing by an analog signal processing unit 25 including a correlated double sampling circuit (CDS) 22, an auto gain control amplifier (AGC) 23, and an A / D converter 24. The The CDS 22 removes amplifier noise and reset noise caused by the CCD 14. The AGC 23 is an amplifier that can change the gain, and the CPU 11 controls the AGC 23 to adjust the sensitivity. The A / D converter 24 digitizes the image signal that has been subjected to noise removal and gain adjustment by the CDS 22 and the AGC 23. The image signal (RAW data) output from the A / D converter 24 is input to the RAM 26.

  The digital signal processing circuit 27 performs digital signal processing, which will be described later, on the image signal for one screen stored in the RAM 26. The LCD driver 28 causes the liquid crystal display (LCD) 29 to sequentially display the image signals subjected to the digital signal processing as a through image.

  The compression / decompression processing circuit 30 performs compression processing in a predetermined compression format (for example, JPEG format) on the image signal subjected to digital signal processing when the shutter button in the operation unit 12 is pressed. The media controller 31 controls the memory card 32 and records the image signal compressed by the compression / decompression processing circuit 30 on the memory card 32 as image data. In the playback mode, the media controller 31 reads image data stored in the memory card 32. In this case, the compression / decompression processing circuit 30 performs decompression processing on the image data read by the media controller 31. The expanded image data is reproduced and displayed on the LCD 29 by the LCD driver 28.

  In FIG. 2 showing the configuration of the digital signal processing circuit 27, an offset correction circuit 40 reads an image signal for one screen from the RAM 26, and performs offset correction by subtracting a noise signal uniformly from RGB color signals included in the image signal. .

  The evaluation value calculation circuit 41 calculates evaluation values for AE, AF (automatic focus), and AWB (automatic white balance) based on the image signal output from the offset correction circuit 40, and inputs each evaluation value to the CPU 11. . In the case of AE, an average value (photometric data) of the image signal is calculated as an AE evaluation value, and the CPU 11 performs an operation to determine an appropriate exposure value so that the AE evaluation value matches a predetermined target value. The AE evaluation value is a positive value indicating brightness, and an integrated value of image signals may be used as the AE evaluation value. For calculating the AE evaluation value, center-weighted average metering or spot metering may be used.

  In the case of AF, the contrast value is calculated as the AF evaluation value, and the CPU 11 determines the lens position of the photographing lens 15 so that the AF evaluation value is maximized. In the case of AWB, the hue distribution is calculated as an AWB evaluation value, and the CPU 11 estimates the RGB ratio of the light source from the AWB evaluation value and adjusts the gain of the image signal (RGB ratio) so that the optimum color temperature is obtained. Adjustment).

  The gain correction circuit 42 performs the AWB gain adjustment on the image signal output from the offset correction circuit 40 based on the control signal transmitted from the CPU 11 (for example, R / G using the G image signal as a reference). Is α times and B / G is β times). The gamma conversion circuit (gradation conversion means) 43 performs gradation conversion on the image signal output from the gain correction circuit 42 according to a predetermined gamma conversion parameter stored in the ROM 13.

  The RGB interpolation calculation circuit 44 performs interpolation calculation on the image signal output from the gamma conversion circuit 43 to obtain RGB three-color signals at each pixel position. The YC conversion circuit 45 converts the image signal output from the RGB interpolation calculation circuit 44 into a luminance signal Y and color difference signals Cr and Cb. The image signal composed of the luminance signal Y and the color difference signals Cr and Cb generated by the YC conversion circuit 45 is stored in the RAM 26.

  In addition, the digital camera 10 has an exposure correction function, and the user operates an exposure correction value setting button included in the operation unit 12 to perform exposure in a range of −2 Ev to +2 Ev (0.5 Ev steps), for example. A correction value can be set. When the exposure correction value is set, the CPU 11 reflects the exposure correction value on the appropriate exposure value determined by the AE operation.

  Next, the AE operation during live view display will be described with reference to the flowchart of FIG. First, when the digital camera 10 is set to the photographing mode, image signals picked up by the CCD 14 and subjected to predetermined processing are sequentially displayed on the LCD 29 as a through image (step S1).

  At this time, in the digital signal processing circuit 27, the evaluation value calculation circuit 41 calculates an AE evaluation value (step S2). The CPU 11 determines whether or not the set exposure correction value is greater than or equal to a predetermined value S (for example, +1 Ev) (step S3). If the exposure correction value is smaller than the predetermined value S (No in step S3). The calculation is performed (step S4), and an appropriate exposure value is determined so that the AE evaluation value matches a predetermined target value (step S5).

  On the other hand, when the exposure correction value is greater than or equal to the predetermined value S (Yes in step S3), the CPU 11 performs an operation with the AE evaluation value multiplied by N (N> 1) (steps S6 and S7), and sets the AE evaluation value. An appropriate exposure value is determined so that the value N times matches the target value (step S8). As a result, the level of the image signal is 1 / N times the normal level. At this time, the CPU 11 sets the AWB gain amount in the gain correction circuit 42 in the digital signal processing circuit 27 to N times normal (for example, the R / G gain amount α is N times and the B / G gain amount β is The level (light quantity) of the through image displayed on the LCD 29 is increased N times (step S9).

  Next, the CPU 11 reflects the exposure correction value on the appropriate exposure value determined in step S5 or step S8 (step S10). Thereby, the level of the through image is corrected by the exposure correction value.

  Thereafter, the CPU 11 repeatedly executes the AE operations in steps S2 to S10 until the shutter button is pressed (this is called a feedback AE operation). During the feedback AE operation, the target value in steps S5 and S8 is set to a level obtained by adding the exposure correction value to the original target value, and the exposure correction value in step S10 is not reflected.

  If the shutter button is pressed during the feedback AE operation (Yes in step S11), the live view display ends (step S12), and the shooting process is executed (step S13). Is displayed (post-view display) on the LCD 29 (step S14). In the photographing process in step S13, the normal exposure correction process similar to that in steps S4, S5, and S10 is performed on the image signal regardless of the setting of the exposure correction value.

  The determination reference value S in step S3 and the magnification N in steps S6 and S9 are determined in consideration of the dynamic range of the CCD 14. When the dynamic range of the CCD 14 is in the range of −3.5 Ev to 2.5 Ev with respect to the target value of the AE evaluation value, for example, the determination reference value S is “+1 Ev” and the magnification N is “2”. . Note that + 1Ev corresponds to a double light amount, and -1Ev corresponds to a half light amount.

  Next, in this example, the feedback AE operation when the exposure correction value is set to +2 Ev will be specifically described. In the first AE operation before the feedback AE operation, as shown in FIG. 4A, the appropriate exposure value is determined so that the value obtained by doubling the AE evaluation value matches the target value. The through image display gained by a factor of 2 is then performed by the gain correction circuit 42. This through image matches the image displayed as no exposure correction (exposure correction value 0 Ev) at the original appropriate exposure determined so that the AE evaluation value matches the target value.

  Thereafter, when the exposure correction value + 2Ev is reflected in the appropriate exposure value, as shown in FIG. 4B, the level of the obtained image signal increases by + 2Ev. On the LCD 29, a through image display in which the image signal is gained by a factor of 2 is performed, so that the through image is an image subjected to +2 Ev exposure correction.

  Then, until the shutter button is pressed, the feedback AE operation is performed with the exposure correction value reflected, and the second transition AE operation is performed. At this time, as shown in FIG. 4B, the image signal does not have a level clamped due to the restriction due to the upper limit of the dynamic range of the CCD 14 (the amount of light is saturated). The value obtained by doubling the AE evaluation value is substantially equal to + 2Ev of the new target value (the level obtained by adding the exposure correction value to the original target value). Therefore, the exposure of the through image hardly fluctuates by the feedback AE operation.

  If the image signal has a level higher than 0.5 Ev in FIG. 4A before exposure correction, naturally, the level in which the light amount is saturated in the state of FIG. 4B after +2 Ev exposure correction. Therefore, the through image exposure fluctuates due to the feedback AE operation. However, in the above example, the level of the image signal is halved by determining the appropriate exposure value so that the value obtained by doubling the AE evaluation value matches the target value. The fluctuation is reduced than usual.

  In the above embodiment, when the appropriate exposure value is determined by setting the AE evaluation value to N times, the gain correction circuit 42 is controlled to set the level of the image signal for displaying the through image to N times. The present invention is not limited to this, and instead of this, by controlling the gamma conversion circuit 43, the level of the image signal for displaying the through image may be set to N times. Specifically, instead of increasing the AWB gain to the normal N times in step S9, the gamma conversion parameter is set to the normal N times as shown in FIG.

  FIG. 6 is an example of a gamma conversion parameter for converting the gradation of an image signal from 10 bits to 8 bits. The gamma conversion parameter 50 is used at the time of normal gamma conversion, and the gamma conversion parameter 51 is used for a normal gain. Used to double the gamma conversion. However, the gamma conversion parameter 51 is obtained by doubling the gamma conversion parameter 50 and clamping the portion where the gradation after the conversion exceeds the maximum value of 255 with 255.

  In the above embodiment, the level of the image signal to be displayed as a through image is changed by the gain correction circuit 42 used for AWB or the gamma conversion circuit 43 used for gamma conversion. However, the present invention is not limited to this. Instead, a circuit for changing the level may be provided separately from the gain correction circuit 42 and the gamma conversion circuit 43.

  Further, in the above embodiment, the case of the plus correction for correcting the appropriate exposure value to the over side has been described as an example, but the present invention is not limited to this, and the case of the minus correction for correcting the appropriate exposure value to the under side. It can also be applied to. If the appropriate exposure value is corrected to the under side in the through image display state, the image signal is clamped at the lower limit of the dynamic range of the CCD 14 contrary to the above case, so that the through image is underexposed by the feedback AE operation. Fluctuate to the side. In order to reduce this variation, when the set exposure correction value is equal to or smaller than a predetermined value, the AE evaluation value is set to 1 / N times (N> 1), the appropriate exposure value is determined, and the level of the image signal is set to N. The level of the image signal for displaying the through image may be set to 1 / N times.

  Further, in the through image display state of the above embodiment, in order to display the luminance value of the image signal on the LCD 29 as a histogram, the luminance signal Y output from the YC conversion circuit may be used as data. Thereby, accurate histogram display corresponding to the brightness of the through image and the image obtained by photographing can be performed.

It is a block diagram which shows the structure of a digital camera. It is a block diagram which shows the structure of a digital signal processing circuit. It is a flowchart which shows AE operation | movement at the time of a through image display. It is a graph which shows the example of the image signal before and behind exposure correction. It is a flowchart which shows the modification of AE operation | movement at the time of a through image display. It is a graph which shows the example of a gamma conversion parameter. It is a graph which shows the example of the image signal before and after exposure amendment at the time of still picture photography. It is a graph which shows the example of the image signal before and behind the conventional feedback AE operation | movement.

Explanation of symbols

10 Digital camera 11 CPU
DESCRIPTION OF SYMBOLS 12 Operation part 14 CCD image sensor 25 Analog signal processing part 27 Digital signal processing circuit 29 Liquid crystal display 40 Offset correction circuit 41 Evaluation value calculation circuit 42 Gain correction circuit 43 Gamma conversion circuit 44 RGB interpolation calculation circuit 45 YC conversion circuit 50, 51 Gamma Conversion parameter

Claims (6)

An automatic exposure function for determining an optimum exposure value so that an evaluation value calculated based on an image signal from a solid-state image sensor matches a predetermined target value, and the optimum value based on an exposure correction value set by an operation means In an imaging apparatus having an exposure correction function for correcting an exposure value and a through image display function for sequentially displaying the image signal for each screen,
When the exposure correction value is larger than a predetermined value on the plus correction side, an optimum exposure value is determined such that a value obtained by multiplying the evaluation value by N times (N> 1) matches the target value, and the image signal And a control means for displaying the through image at a N level. An automatic exposure function for determining an optimum exposure value so that an evaluation value calculated based on an image signal from a solid-state image sensor matches a predetermined target value, and the optimum value based on an exposure correction value set by an operation means In an imaging apparatus having an exposure correction function for correcting an exposure value and a through image display function for sequentially displaying the image signal for each screen,
When the exposure correction value is smaller than a predetermined value on the minus correction side, an optimum exposure value is determined such that a value obtained by multiplying the evaluation value by 1 / N times (N> 1) matches the target value, An imaging apparatus, comprising: a control means for performing a through image display by setting a level of an image signal to 1 / N times.   3. The photographing apparatus according to claim 1, further comprising a gain adjusting unit that performs white balance correction, wherein the control unit controls the gain adjusting unit to change the level of the image signal.   A gradation conversion unit that performs gradation conversion of the image signal based on a predetermined parameter is provided, and the control unit controls the gradation conversion unit and changes the level of the image signal by changing the parameter. The photographing apparatus according to claim 1 or 2, wherein An automatic exposure function for determining an optimum exposure value so that an evaluation value calculated based on an image signal from a solid-state image sensor matches a predetermined target value, and the optimum value based on an exposure correction value set by an operation means In a control method of an imaging apparatus having an exposure correction function for correcting an exposure value and a through image display function for sequentially displaying the image signal for each screen,
When the exposure correction value is larger than a predetermined value on the plus correction side, an optimum exposure value is determined such that a value obtained by multiplying the evaluation value by N times (N> 1) matches the target value, and the image signal A method for controlling a photographing apparatus, wherein the through image display is performed by increasing the level of N by N times. An automatic exposure function for determining an optimum exposure value so that an evaluation value calculated based on an image signal from a solid-state image sensor matches a predetermined target value, and the optimum value based on an exposure correction value set by an operation means In a control method of an imaging apparatus having an exposure correction function for correcting an exposure value and a through image display function for sequentially displaying the image signal for each screen,
When the exposure correction value is smaller than a predetermined value on the minus correction side, an optimum exposure value is determined such that a value obtained by multiplying the evaluation value by 1 / N times (N> 1) matches the target value, A method for controlling a photographing apparatus, wherein the through image display is performed by setting the level of an image signal to 1 / N times.

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