JP2005260738A - Imaging apparatus and its exposure control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform photographing using a proper exposure value without using complicated arithmetic. <P>SOLUTION: An object light that is made incident through an imaging lens 13 of a digital camera 10 is photoelectric-converted by a CCD 36 and then stored in an SDRAM 44 as image data. In the case of AE control, an AE detection circuit 52 performs divided photometry of image data 55 stored in the SDRAM 44 and defines a result as a photometric value (Lv) for each area. A CPU 33 weights the photometric values of areas, calculates a temporary exposure value, calculates a threshold resulting from adding +2.0Ev to the temporary exposure value and calculates a differential amount between the threshold and a maximum photometric value. The CPU 33 then calculates an exposure correction amount using a membership function (not shown in Fig.) stored in an EEPROM 49 based on the calculated differential amount and corrects the temporary exposure value based on a result of this calculation to calculate a proper exposure value, so that photographing can be performed with proper exposure without using complicated arithmetic. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image pickup apparatus suitable for photographing with an appropriate exposure value and an exposure control method thereof.

  As a photographing device, a digital camera that converts a subject image captured by an image sensor such as a CCD image sensor (CCD) into digital image data and stores the digital image data in a storage medium such as a built-in memory or a memory card is widely used. The digital camera automatically obtains an appropriate exposure value so that anyone can obtain a good shot image, and further sets the aperture value and shutter speed of the camera based on the obtained exposure value. In general, an exposure (AE) adjustment function is provided.

  As a method for obtaining an appropriate exposure value, for example, as described in Patent Document 1, a luminance histogram in which the luminance value for each pixel of the CCD is classified according to its size is created, and a plurality of luminance histograms are generated. Is divided into luminance areas (ranges). Next, after weighting the pixels on the high luminance side in each luminance region, the number of pixels in the luminance region is counted for each luminance region. Then, there is a method for obtaining an exposure value matched with a luminance range where the number of pixels is the largest as an appropriate exposure value. When this method is used, since the exposure is adjusted to the main subject that occupies the largest area (ratio) in the shooting screen, a good shot image can be obtained.

  In addition, when the area of the main subject in the shooting screen is small, such as when shooting outdoors, the above brightness histogram is created and the largest subject (outdoors such as the sky) in the shooting screen is created from this histogram. The first peak corresponding to the luminance range of the second landscape and the second peak corresponding to the luminance range of the main subject (person) occupying the second and subsequent sizes are detected. There is also a method of ignoring and obtaining an exposure value matched with the luminance range of the second peak as an appropriate exposure value (see Patent Document 2).

Japanese Patent No. 2823926 (pages 3-4) Japanese Patent No. 2823927 (pages 2 to 3)

  In the method described in Patent Document 1, after a luminance histogram is created, the luminance histogram is divided into a plurality of luminance regions, and further, each high luminance side pixel in each luminance region is weighted, and then each luminance region. Since it is necessary to count the number of pixels in the luminance range every time, the calculation for obtaining the exposure value becomes very complicated. As a result, the AE adjustment time may become long. Further, in the method described in Patent Document 2, the luminance range corresponding to the largest subject on the photographing screen is ignored, so that when the photographer intentionally shoots a desired subject at a large size, an appropriate value is obtained. Exposure value may not be obtained. In addition, since it is necessary to detect the first peak and the second peak from the luminance histogram, the calculation for obtaining the appropriate exposure value becomes very complicated and the AE adjustment time becomes long as in the case of Patent Document 1. There is a risk that.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide an imaging apparatus that can perform imaging with an appropriate exposure value without using a complicated calculation, and an exposure control method thereof.

  The present invention divides an imaging screen into a plurality of areas and divides the photometry for measuring the luminance for each area, and the exposure value calculation means for calculating the first exposure value by weighting the photometric value for each area. Difference amount detection means for obtaining a difference amount between a second exposure value that is larger than the first exposure value by a predetermined value and a maximum value among the photometric values for each area, and the difference amount detection means The correction value storage means that stores the correction value in association with each difference value obtained in step (i), and the correction value storage means corresponding to the difference amount obtained by the difference amount detection means. Exposure value correcting means for correcting the first exposure value with a correction value; and exposure control means for controlling at least one of an aperture and an exposure time so as to obtain an exposure value corrected by the exposure value correcting means. It is characterized by having.

  Further, when the maximum value among the photometric values is smaller than the second exposure value, the corrected exposure value is smaller than the first exposure value, and the maximum value is the first exposure value. When the value is larger than the value, it is preferable that the correction value is associated with the value of the difference amount so that the corrected exposure value is larger than the first exposure value. Further, the predetermined value is set so that the second exposure value becomes an upper limit value capable of reproducing the gradation of the captured image when imaging is performed with the first exposure value. It is preferable.

  Further, for each value of the calculated first exposure value, an upper limit value of the correction value that does not overcorrect the first exposure value is associated in advance and stored in the correction value storage means, When the correction value obtained when the first exposure value is calculated exceeds the upper limit value corresponding to the first exposure value, the exposure value correction means uses the upper limit value to determine the first exposure value. It is preferable to correct the exposure value of 1. Further, for each value of the calculated first exposure value, a lower limit value of the correction value that does not overcorrect the first exposure value is associated in advance and stored in the correction value storage means, When the correction value obtained when calculating the first exposure value is below the lower limit value corresponding to the first exposure value, the exposure value correcting means uses the lower limit value to determine the first exposure value. It is preferable to correct the exposure value of 1. Further, the exposure value includes a plurality of shooting modes including a normal shooting mode, a manual shooting mode in which a user manually sets an exposure value, and a specific shooting mode in which shooting is performed under a specific condition or shooting of a specific subject. It is preferable that the correction means does not correct the first exposure value when the shooting mode is switched to at least the manual shooting mode or the specific shooting mode.

  In the present invention, the imaging screen is divided into a plurality of areas, the luminance of each area is photometrically measured, the first exposure value is calculated by weighting the photometric value of each area, and the first exposure is calculated. A difference amount between the second exposure value that is larger than the value by a predetermined value and the maximum value among the photometric values for each area is obtained, and a correction value is associated in advance for each value of the difference amount. The first exposure value is corrected with the correction value corresponding to the obtained difference amount, and at least one of the aperture and the exposure time is controlled so that the corrected exposure value is obtained. To do.

  Further, when the maximum value among the photometric values is smaller than the second exposure value, the corrected exposure value is smaller than the first exposure value, and the maximum value is the first exposure value. When the value is larger than the value, it is preferable that the correction value is associated with the value of the difference amount so that the corrected exposure value is larger than the first exposure value. Further, the predetermined value is set so that the second exposure value becomes an upper limit value capable of reproducing the gradation of the captured image when imaging is performed with the first exposure value. It is preferable.

  Further, when the first exposure value is calculated by associating in advance an upper limit value of the correction value that does not overcorrect the first exposure value for each value of the calculated first exposure value. When the correction value obtained in (1) exceeds the upper limit value corresponding to the first exposure value, it is preferable to correct the first exposure value using the upper limit value. Further, when the first exposure value is calculated by associating in advance a lower limit value of the correction value that does not overcorrect the first exposure value for each value of the calculated first exposure value. When the correction value obtained in step (b) is below the lower limit value corresponding to the first exposure value, it is preferable to correct the first exposure value using this lower limit value. Further, the photographing mode includes a plurality of photographing modes including a normal photographing mode, a manual photographing mode in which a user manually sets an exposure value, and a specific photographing mode for photographing under a specific condition or photographing a specific subject. However, it is preferable that the first exposure value is not corrected when at least the manual photographing mode or the specific photographing mode is switched.

  According to the imaging apparatus and the exposure control method of the present invention, the imaging screen is divided into a plurality of areas, the luminance for each area is measured, the first exposure value is calculated by weighting the photometric value for each area. In addition, a difference amount between a second exposure value that is larger than the first exposure value by a predetermined value and the maximum value among the photometric values for each area is obtained, and a correction value for each value of the difference amount. In advance, the first exposure value is corrected with the correction value corresponding to the obtained difference amount, and at least one of the aperture and the exposure time is set so that the corrected exposure value is obtained. Since the control is performed, it is not necessary to perform a complicated calculation to calculate an appropriate exposure value, and the AE adjustment time can be shortened.

  In addition, when the first exposure value is obtained, weighting is performed by the division photometry method, so that a proper exposure can be obtained even when the photographer intentionally increases the proportion of the desired subject in the photographing screen. Can do. Further, since the correction value is obtained using the maximum value of the photometric value, it is possible to reduce the angle-of-view dependency in which the exposure changes depending on the angle of view. Further, a second exposure value that is larger than the first exposure value by a predetermined value is calculated, and the first exposure value is corrected according to the difference between the second exposure value and the maximum photometric value. As a result, underexposure can be suppressed even when the subject has a high brightness such as the sun.

  1 and 2 are a front perspective view and a rear perspective view of a digital camera 10 embodying the present invention. A lens barrier 12 that is slidable in the direction of the arrow is provided in front of the digital camera 10. By sliding the lens barrier 12 to the open position shown in FIG. 1, the imaging lens 13 and the strobe light emitting unit 14 are exposed so that the photographing can be performed.

  A CCD 36 (see FIG. 3) is disposed behind the imaging lens 13. The imaging signal from the CCD 36 is converted into digital image data by an A / D converter 40 (see FIG. 3). This image data is displayed as a so-called through image on a liquid crystal display (LCD) 15 provided on the back of the digital camera 10. In addition, the digital camera 10 is provided with a finder objective window 16 and a finder eyepiece window 17 constituting an optical finder, through which a subject image is framed.

  A release button 19 is provided on the upper surface of the digital camera 10. The release button 19 is a two-step push switch. When the release button 19 is lightly pressed (half-pressed) after framing by the LCD 15 or the optical viewfinder, automatic exposure (AE) adjustment (or control), automatic focus ( Various shooting preparation processes such as (AF) adjustment (or control) are performed.

  The image pickup signal that has been subjected to the shooting preparation process is data-locked until the release button 19 is released. In this state, when the release button 19 is pressed once more (fully pressed), the imaging signal for one screen subjected to the imaging preparation processing is converted into image data, and after various image processing and compression processing are performed. The data is stored in a storage medium such as a memory card 22 detachably loaded in the memory card slot 21.

  On the back of the digital camera 10, in addition to the LCD 15, a power button 24 for switching on / off the power, a zoom operation button 25 for changing the zoom lens of the imaging lens 13 to the wide side and the tele side, and various modes are selected. An operation key 27 such as a cross key for moving a cursor in a menu screen displayed on the LCD 15 and a mode changeover switch 26 to be operated is provided.

  The operation mode selected by the mode changeover switch 26 includes, for example, a shooting mode for shooting a still image, a playback mode for displaying the shot still image on the LCD 15, and a setup mode for performing various settings. These operation modes can be switched by sliding the mode changeover switch 26.

  As a shooting mode of the digital camera 10, an auto mode which is a normal shooting mode, a manual mode in which a photographer manually adjusts exposure and focus without performing AE and AF, a landscape, a sport, a night view, and the like There is a scene position mode in which a specific scene is specifically shot. In each of these shooting modes, a shooting mode selection menu (not shown) is displayed on the LCD 15, and the photographer selects a desired shooting mode from the shooting mode selection menu (not shown) using the operation keys 27. Can be switched. Here, instead of displaying a shooting mode selection menu (not shown), for example, a shooting mode switch may be provided.

  In FIG. 3 showing the electrical configuration of the digital camera 10, a lens motor 31 and an iris motor 32 are connected to the imaging lens 13 and the diaphragm 30. These motors 31 and 32 are driven and controlled via motor drivers 34 and 35 connected to the CPU 33, and perform shooting preparation processing by half-pressing the release button 19.

  The lens motor 31 performs zooming of the imaging lens 13 by moving the zoom lens of the imaging lens 13 to the wide side or the tele side in conjunction with the operation of the zoom operation button 25. Further, the focus lens is moved to adjust the focus of the imaging lens 13 so that the photographing conditions are optimized. The iris motor 32 operates the diaphragm 30 to adjust the exposure of the imaging lens 13.

  Behind the imaging lens 13 is a CCD 36 that captures the subject light that has passed through the imaging lens 13. A timing generator (TG) 37 controlled by the CPU 33 is connected to the CCD 36, and the shutter speed of the electronic shutter is determined by a timing signal (clock pulse) input from the TG 37.

  The imaging signal output from the CCD 36 is input to a correlated double sampling circuit (CDS) 38, and is output as R, G, B image data that accurately corresponds to the accumulated charge amount of each cell of the CCD 36. The image data output from the CDS 38 is amplified by an amplifier (AMP) 39 and converted into digital image data by an A / D converter (A / D) 40.

  The image input controller 41 is connected to the CPU 33 via the bus 42 and controls the CCD 36, CDS 38, AMP 39, and A / D converter 40 in accordance with a control command from the CPU 33. The image data output from the A / D converter 40 is displayed on the LCD 15 via the LCD driver 43 and stored in the SDRAM 44.

  The image signal processing circuit 45 performs various types of image processing such as gradation conversion, white balance correction, and γ correction processing on the image data. Image data that has been subjected to various processes in the image signal processing circuit 45 is converted into a luminance signal Y and color difference signals Cr and Cb by a YC conversion processing circuit 46. The compression / decompression processing circuit 47 performs image compression on the converted image data in a predetermined compression format (for example, JPEG format). The compressed image data is recorded on the memory card 22 via the media controller 48.

  In addition to the release button 19, the mode switch 26, and the operation key 27 described above, an EEPROM 49 is connected to the CPU 33. The EEPROM 49 stores various control programs and setting information. As will be described in detail later, the EEPROM 49 stores arithmetic expressions, function expressions, and the like for calculating an appropriate exposure value. The CPU 33 reads these pieces of information from the EEPROM 49 to the SDRAM 44, which is a working memory, and executes various processes.

  The bus 42 includes an AWB detection circuit 50 for detecting whether the white balance correction is appropriate for shooting, an AF detection circuit 51 for detecting whether the focus of the shooting lens 13 is appropriate for shooting, An AE detection circuit 52 for detecting whether the exposure value, that is, the shutter speed of the electronic shutter of the CCD 36, the photographing sensitivity, and the aperture value of the diaphragm 30 is appropriate for photographing is connected. The detection circuits 50 to 52 sequentially transmit detection results to the CPU 33 via the bus 42 when the release button 19 is half-pressed. The CPU 33 controls operations of the imaging lens 13, the diaphragm 30, and the CCD 36 based on the detection results transmitted from the detection circuits 50 to 52.

  The AWB detection circuit 50 performs integration processing for each color of RGB on the image data stored in the SDRAM 44 when the release button 19 is half-pressed. The CPU 33 analyzes the integrated image data, identifies an illumination light source (daylight, fluorescent light, tungsten light source, etc.) that illuminates the subject, and determines the white balance correction values (RGB) for each color corresponding to the identified light source. WB correction value) is calculated. The AF detection circuit 51 determines that the focus lens focus adjustment of the photographing lens 13 is optimal for photographing based on the image data output from the CCD 36 and digitized by the A / D converter 40 when the release button 19 is half-pressed. The detected AF value is detected. The CPU 33 drives the lens motor 31 based on this AF detection value to move the focus lens of the photographing lens 13 to an optimal position. The AE detection circuit 52 relates to the present invention. As will be described in detail later, when the release button 19 is half-pressed, the integrated value for each luminance component of the R, G, and B signals necessary for AE adjustment is obtained. To detect.

  In this embodiment, when obtaining an appropriate exposure value, weighting based on a well-known divided photometry method is performed first without performing complicated calculations as described in Patent Document 1 and Patent Document 2. The temporary exposure value is obtained. Next, as will be described in detail later, a threshold value (upper limit value) of an exposure value at which the captured image (captured image) does not overshoot is calculated from the temporary exposure value. The difference from the maximum value (hereinafter simply referred to as the maximum photometric value) is calculated. Then, an appropriate exposure value is obtained by correcting the temporary exposure value depending on whether the maximum photometric value is above or below the threshold value. Hereinafter, a method for obtaining the appropriate exposure value will be specifically described.

  The AE detection circuit 52 divides the shooting screen (subject image) 55 into 8 × 8 = 64 areas as shown in FIG. 4, for example, by half-pressing the release button 19. The AE detection circuit 52 integrates the luminance components of the R, G, and B signals for each area based on the image data stored in the SDRAM 44, and the integrated value and the aperture value of the aperture 30 at that time. From the shutter speed of the electronic shutter, the photometric value (Lv) of each area at the standard photographing sensitivity is calculated and stored in the SDRAM 44. Here, FIG. 4 shows an example of calculating the photometric value for each area. Naturally, the number of divided areas and the photometric value of each area are limited to those shown in FIG. is not. Further, each area may be divided so as to have an arbitrary shape such as a circle, an ellipse, a rectangle, or a half-moon shape. Further, as a method for calculating the photometric value, the luminance signal Y of the image data converted by the YC conversion processing circuit 46 may be integrated instead of integrating the luminance components of the R, G, B signals by color. .

  The CPU 33 calculates a proper exposure value by weighting the photometric value of each area stored in the SDRAM 44. As shown in FIG. 5, this weighting is performed according to a weighting pattern 56 that defines a weighting coefficient for each area. The weighting pattern information is stored in advance in the SDRAM 44 or the like. The weighting pattern 56 shows a pattern example when the above-described auto mode is selected, and information on this pattern is stored in the SDRAM 44 or the like in advance. Note that the weighting pattern stored in the SDRAM 44 is not limited to this, and weighting pattern information dedicated to night scene shooting and sports shooting is also stored in the SDRAM 44. Therefore, it is possible to perform weighting corresponding to various shootings such as night scenes, sports, and landscapes.

The CPU 33 stores the photometric value Lv _i (i = 0 to 63) of each area stored in the SDRAM 44 and the weighting coefficient W _i (i = 0 to 63) of the weighting pattern 56 in advance in the EEPROM 49 described below. Substituting into the temporary exposure value calculation formula,
Temporary exposure value (Ev) = Log ₂ [(Σ (W _i × 2 ^Lvi ) / ΣW _i ])
(I = 0-63)
A temporary exposure value at a standard photographing sensitivity is calculated. In this embodiment, the calculated temporary exposure value is described as 14.0 Ev. Further, the arithmetic expression for calculating the temporary exposure value is not limited to the above expression, and any arithmetic expression may be used as necessary.

  Here, in general, in a digital camera, when imaging is performed with an exposure value calculated by weighting, gradation reproduction is possible even if the actual exposure amount is about +2.0 Ev, but +2.0 Ev or more. Then, overexposure occurs and image data cannot be acquired. For this reason, in order to prevent overexposure of the photographed image, the shutter speed of the electronic shutter of the CCD 36, the photographing sensitivity, and the aperture 30 are often adjusted so as to be slightly underexposed. When imaging is performed, the captured image may become dark. Also, if there is a sun or moon (when shooting night scenes) in the subject, it will be pulled by the brightness, so even if weighting is performed, there is a risk of underexposure.

  Therefore, in this embodiment, a value higher by +2.0 Ev than the temporary exposure value is set as the threshold value (Ev). Then, it is determined whether the above-mentioned maximum photometric value exceeds or falls below the threshold value. For example, if the maximum photometric value is below the threshold value, the temporary exposure value is set so that the exposure amount increases. Exposure compensation is performed, and when the maximum photometric value exceeds the threshold value, the temporary exposure value is subjected to exposure compensation so that the exposure amount is reduced. That is, the temporary exposure value (Ev) is corrected based on the threshold value. Thereby, in particular, when the maximum photometric value is below the threshold value, the exposure correction is performed so that the exposure amount increases, so that underexposure can be improved. Therefore, as shown in FIG. 6, the CPU 33 calculates the temporary exposure value as 14 Ev (point A in the figure), and at the same time adds 12.0 Ev to +12.0 Ev (point A ′ in the figure). ) As a threshold value. In this embodiment, the threshold value is a value that is higher by +2.0 Ev than the temporarily calculated value. However, any value may be used as long as it is an upper limit value that allows gradation reproduction.

After calculating the threshold value, the CPU 33 calculates a difference amount between the threshold value 16.0Ev and the maximum photometric value 15.0Ev (indicated by hatching in FIG. 4). The difference amount based on the threshold is the following formula:
Difference amount (Ev) = 15.0 Ev-16.0 Ev = -1.0 Ev
More demanded. In this case, since the maximum photometric value is 1.0 Ev smaller than the threshold value, the temporary exposure value is corrected so that the captured image is not underexposed. Also, for example, when the maximum photometric value is 1.0 Ev larger than the threshold value, the temporary exposure value is corrected so that the captured image is not overexposed.

  When the difference amount is calculated, the CPU 33 calculates the exposure correction amount based on the difference amount, for example, using a correction amount calculation membership function associating the difference amount and the exposure correction amount as shown in FIG. To do. This membership function for calculating the correction amount is obtained in advance from experiments so that the temporary exposure value can be corrected to an appropriate exposure value in accordance with the difference amount. The function data (function formula) of this function is It is stored in the aforementioned EEPROM 49. In this embodiment, the exposure correction amount −0.6 Ev (point B ′ in the figure) is calculated from the difference amount −1.0 Ev (point B in the figure) based on the membership function for calculating the correction amount. In addition to using the membership function described in FIG. 7, the exposure correction amount may be calculated using, for example, an arithmetic expression that associates the difference amount with the exposure correction amount, a data table, or the like.

When the exposure correction amount is calculated, the CPU 33 adds the exposure correction value −0.6 Ev to the temporary exposure value 14.0 EV. As a result, the proper exposure value (Ev) is
14.0Ev-0.6Ev = 13.6Ev
It becomes. Therefore, for example, when the aperture value (Av value) is fixed at 3.0 (the shooting sensitivity is fixed at the same standard), the shutter speed (Tv) is
13.4Ev-3.0Av = 10.4Tv
Thus, the shutter speed is slower than 11.0 Tv (= 14.0 Ev−3.0 Av) before correction. Based on this result, the CPU 33 controls the TG 37 to control the shutter speed of the electronic shutter of the CCD 36. When the aperture value and the shooting sensitivity are not fixed, the CPU 33 adjusts the shutter speed of the electronic shutter of the CCD 36, the shooting sensitivity, and the aperture 30 according to the calculated appropriate exposure value. As a result, a bright photographed image can be obtained because the amount of exposure increases.

  Next, the procedure of AE adjustment of the digital camera 10 of the present invention having the above configuration will be described with reference to the flowcharts of FIGS. First, when a shooting mode (automatic mode in this description) is selected by the mode changeover switch 26, subject light incident through the imaging lens 13 and the diaphragm 30 is photoelectrically converted by the CCD 36, sampled by the CDS 38, and converted into RGB data. The data is output to the AMP 39 (see FIG. 3).

  The image data output from the CDS 38 is amplified by the AMP 39 and converted into digital data by the A / D converter 40. The digitally converted image data is stored in the SDRAM 44 via the image input controller 41 and displayed on the LCD 15 as a through image. Next, when the release button 19 is half-pressed by the photographer, the AE detection circuit 52 and the CPU 33 start the AE adjustment shown in FIG.

  The AE detection circuit 52 divides the photographing screen 55 into 8 × 8 = 64 areas as shown in FIG. 4, for example. The AE detection circuit 52 integrates the luminance components of the R, G, and B signals for each area based on the image data stored in the SDRAM 44, and performs photometry for each area based on the integrated value. The value is calculated and stored in the SDRAM 44. The CPU 33 substitutes the photometric value of each area stored in the SDRAM 44 and the weighting coefficient of the weighting pattern 56 stored in advance in the SDRAM 44 into the above-described provisional exposure value calculation formula stored in the EEPROM 49, A weighted temporary exposure value is calculated.

  When the temporary exposure value is calculated, the CPU 33 starts exposure correction of the temporary exposure value as shown in FIG. First, the CPU 33 calculates a threshold value obtained by adding +2.0 Ev to the calculated temporary exposure value, and calculates a difference amount between the threshold value and the maximum photometric value. Next, the CPU 33 calculates the exposure correction amount using the correction amount calculation membership function stored in the EEPROM 49 based on the calculated difference amount.

  As shown in FIG. 8, when the exposure correction amount is calculated, the CPU 33 calculates an appropriate exposure value by performing exposure correction on the temporary exposure value based on the correction amount calculation result. Then, the CPU 33 adjusts the shutter speed of the electronic shutter of the CCD 36, the diaphragm 30, and the like according to the calculated appropriate exposure value. Since the appropriate exposure value is weighted by the division photometry method, an appropriate exposure can be obtained even when the photographer intentionally increases the proportion of the desired subject in the shooting screen. Further, since the exposure correction amount is calculated using the maximum photometric value, it is possible to reduce the view angle dependency in which the exposure changes due to the difference in the view angle. Furthermore, the threshold value is calculated from the temporary exposure value, and the temporary exposure value is corrected according to the difference between the maximum photometric value and the threshold value. However, underexposure can be suppressed. Further, unlike the digital cameras described in Patent Document 1 and Patent Document 2, no complicated calculation is required for calculating the appropriate exposure value, so that the AE adjustment time can be shortened.

  In this embodiment, the amount of difference between the maximum photometric value and the threshold value is calculated, and the exposure correction amount is calculated using the membership function shown in FIG. 7 based on the calculation result. However, for example, when the difference amount is a positive number (plus), exposure correction is performed so that the captured image becomes dark. At this time, if the temporary exposure value calculated by weighting is large, if exposure correction is performed so as to be dark, there is a possibility that overcorrection is not performed and proper exposure is not obtained. Accordingly, a second embodiment of the present invention that can prevent overcorrection will be described below.

  The digital camera of the second embodiment is basically the same as the digital camera 10 of the first embodiment of the present invention except that the method for calculating the exposure correction amount is different. Therefore, in the following description, the description will be made using the digital camera 10 of the first embodiment, and the description other than calculating the exposure correction amount will be omitted.

  In the second embodiment, overcorrection is prevented by varying the amount of correction when performing exposure correction according to the brightness of the subject, that is, the size of the temporary exposure value calculated by weighting. Therefore, for example, as shown in FIGS. 10A and 10B, the correction upper limit in which the temporary exposure value is associated with the correction upper limit value α (Ev) and the correction lower limit value β (Ev) of the exposure correction amount. Values and a correction lower limit value calculating membership function are stored in the EEPROM 49 in advance.

  As shown in (a), in this embodiment, the correction upper limit value α is constant at L (Ev) until the magnitude of the temporary exposure value exceeds C (Ev), and from C (Ev) to D ( Ev) It decreases linearly in the range of [D> C], and when it exceeds D (Ev), it becomes 0 (Ev). Further, as shown in (b), the correction lower limit β is constant at 0 (Ev) when the magnitude of the temporary exposure value is equal to or less than E (Ev), and from E (Ev) to F (Ev) [F> E] increases linearly within a range, and when F (Ev) is exceeded, it becomes constant at M (Ev). Here, L, C, D, and M, E, and F are values obtained in advance by experiments or the like so that an optimal captured image can be obtained. These parameters are stored in an upper limit setting table 60 and a lower limit setting table 61 stored in the SDRAM 44 as shown in FIG. The correction upper limit value α and the correction lower limit value β may be calculated using, for example, an arbitrary arithmetic expression, a data table, or the like other than using the membership function described in FIG.

  At the time of exposure correction, the CPU 33 reads out each parameter from the setting tables 60 and 61, and creates a correction upper limit value and a correction lower limit value membership function shown in FIGS. Then, based on the calculated temporary exposure value, a correction upper limit value α and a correction lower limit value β are determined. Next, the CPU 33 creates a correction amount calculation membership function as shown in FIG. 12 based on the determined correction upper and lower limit values α and β. The exposure upper limit value α and the correction lower limit value β are determined for the exposure correction amount of the membership function for exposure correction. Therefore, the temporary exposure value is equal to or higher than the correction upper limit value α or lower than the correction lower limit value β. No correction is performed. For example, when the temporary exposure value is greater than or equal to D (Ev), the upper limit value α of exposure correction is 0 Ev. Therefore, even if the difference amount becomes a positive number, exposure correction is performed so that the captured image becomes dark. Absent. Conversely, when the temporary exposure value is equal to or less than E (Ev), the lower limit value β of the exposure correction is 0 Ev. Therefore, the exposure correction that makes the captured image brighter even if the difference amount becomes a negative number. Not done. As a result, unnecessary exposure correction (overcorrection) can be prevented.

  In addition, as described above, the digital camera 10 of the present embodiment has a plurality of shooting modes such as the auto mode, the manual mode, and the scene position mode. Therefore, if the exposure is corrected, there is a possibility that the exposure intended by the photographer may not be achieved. In addition, since the scene position mode is a specific shooting condition such as landscape, night view, sports, or a specific scene-specific shooting mode, each appropriate exposure value is often determined in advance. Therefore, exposure correction is not necessary in the scene position mode.

  Therefore, a third embodiment of the present invention in which exposure correction is not executed in a specific shooting mode such as a manual mode or a scene position mode will be described below. The digital camera of the third embodiment also has the same configuration as that of the digital camera 10 of the first embodiment of the present invention, and will be described using the digital camera 10 of the first embodiment in the following description.

  As described above, in the digital camera 10, the shooting mode selection menu (not shown) is displayed on the LCD 15, and the shooting mode selection menu (not shown) is displayed using the operation keys 27 (see FIG. 3). Each shooting mode is switched by selecting a desired shooting mode. Therefore, as shown in FIG. 13, when the divided photometry value is calculated and then the weighted temporary exposure value is calculated, if the shooting mode is the auto mode, the CPU 33 performs the above-described procedure. When the exposure compensation is performed according to (see FIG. 9) and the shooting mode is set to the manual mode or the scene position mode, the exposure value set by the photographer is set in advance without performing exposure compensation on the temporary exposure value. The shutter speed of the electronic shutter of the CCD 36, the aperture 30 and the like are adjusted in accordance with the set exposure value. As a result, it is possible to prevent the exposure value intended by the photographer from being corrected. Note that the shooting mode in which exposure correction is not performed is not limited to the manual mode and the scene position mode, and includes a mode in which the photographer sets the exposure value or a mode in which the exposure value is set in advance.

  When the manual mode or the scene position mode is selected, the temporary exposure value is not calculated according to the flowchart shown in FIG. 13, but the exposure value set by the photographer as it is or the preset exposure value. Accordingly, the shutter speed of the electronic shutter of the CCD 36, the diaphragm 30, etc. may be adjusted.

  In this embodiment, the digital camera provided with the CCD 36 has been described as an example. However, the present invention is not limited to the digital camera, and the digital video camera provided with the AE adjustment function and the camera are provided. The present invention can be applied to various imaging devices such as mobile phones and photo cameras. In addition, the photometry means for performing the division photometry is not limited to the CCD, and in particular, when the present invention is applied to a photographic camera, a photometry sensor may be used.

It is a front perspective view of the digital camera which implemented this invention. It is a rear perspective view of a digital camera. It is explanatory drawing which showed the electrical structure of the digital camera. It is explanatory drawing which showed the example of calculation of the photometric value for every area by the division | segmentation photometry method of a digital camera. It is explanatory drawing which showed the example of registration of each weighting coefficient of the weighting pattern of a digital camera. It is explanatory drawing for demonstrating the procedure which calculates a threshold value. It is explanatory drawing which showed the membership function for exposure correction amount calculation. It is a flowchart for demonstrating the procedure of AE adjustment of a digital camera. It is a flowchart for demonstrating the procedure of exposure correction amount calculation in AE adjustment. (A) is an explanatory view showing a membership function for calculating an upper limit value of an exposure correction amount, and (b) is a membership function for calculating a lower limit value of the exposure correction amount. It is explanatory drawing which showed the example of the table for parameter registration of the correction upper limit value and the correction lower limit value membership function. It is explanatory drawing which showed the membership function for exposure correction amount calculation which defined the upper limit and lower limit of the correction amount. It is a flowchart for demonstrating the procedure of AE adjustment which did not perform exposure correction | amendment by specific imaging | photography mode.

Explanation of symbols

10 Digital Camera 13 Shooting Lens 19 Release Button 30 Aperture 33 CPU
36 CCD
44 SDRAM
49 EEPROM
52 AE detection circuit

Claims (12)

Divided photometry means for dividing the imaging screen into a plurality of areas and measuring the brightness of each area;
Exposure value calculating means for calculating a first exposure value by weighting the photometric value for each area; a second exposure value that is larger than the first exposure value by a predetermined value; and a photometric for each area A difference amount detecting means for obtaining a difference amount from the maximum value among the values;
Correction value storage means for storing correction values in association with each difference value obtained by the difference amount detection means;
Exposure value correction means for correcting the first exposure value with the correction value obtained from the correction value storage means in correspondence with the difference amount obtained by the difference amount detection means;
An image pickup apparatus comprising: an exposure control unit that controls at least one of an aperture and an exposure time so that the exposure value corrected by the exposure value correction unit is obtained.   When the maximum value among the photometric values is smaller than the second exposure value, the corrected exposure value is smaller than the first exposure value, and the maximum value is smaller than the first exposure value. The correction value is associated with the value of the difference amount so that the corrected exposure value is larger than the first exposure value. Imaging device.   The predetermined value is set so that the second exposure value is an upper limit value capable of reproducing the gradation of the captured image when imaging is performed with the first exposure value. The imaging apparatus according to claim 1, wherein the imaging apparatus is characterized. For each value of the calculated first exposure value, an upper limit value of the correction value that does not overcorrect the first exposure value is associated in advance and stored in the correction value storage means.
When the correction value obtained when the first exposure value is calculated exceeds the upper limit value corresponding to the first exposure value, the exposure value correction unit uses the upper limit value to calculate the first exposure value. 4. The imaging apparatus according to claim 1, wherein the first exposure value is corrected. For each value of the calculated first exposure value, a lower limit value of the correction value that does not overcorrect the first exposure value is associated in advance and stored in the correction value storage means.
When the correction value obtained when the first exposure value is calculated is lower than the lower limit value corresponding to the first exposure value, the exposure value correction unit uses the lower limit value to The imaging apparatus according to claim 1, wherein the first exposure value is corrected. A plurality of shooting modes including a normal shooting mode, a manual shooting mode in which a user manually sets an exposure value, a specific shooting mode for shooting under a specific condition or shooting of a specific subject,
6. The exposure value correcting unit does not correct the first exposure value when the shooting mode is switched to at least the manual shooting mode or the specific shooting mode. The imaging device described. The imaging screen is divided into a plurality of areas, the luminance for each area is metered, the first exposure value is calculated by weighting the metering value for each area,
While obtaining the difference amount between the second exposure value that is larger than the first exposure value by a predetermined value and the maximum value among the photometric values for each area,
A correction value is associated in advance for each difference value,
The first exposure value is corrected with the correction value corresponding to the obtained difference amount, and at least one of the aperture and the exposure time is controlled so as to obtain the corrected exposure value. An exposure control method for an imaging apparatus.   When the maximum value among the photometric values is smaller than the second exposure value, the corrected exposure value is smaller than the first exposure value, and the maximum value is smaller than the first exposure value. The correction value is associated with the value of the difference amount so that the corrected exposure value is larger than the first exposure value. An exposure control method for an imaging apparatus.   The predetermined value is set so that the second exposure value becomes an upper limit value capable of reproducing the gradation of the captured image when imaging is performed with the first exposure value. 9. The exposure control method for an imaging apparatus according to claim 7, wherein the exposure control method is used. For each calculated value of the first exposure value, an upper limit value of the correction value that does not overcorrect the first exposure value is associated in advance,
When the correction value obtained when the first exposure value is calculated exceeds the upper limit value corresponding to the first exposure value, the first exposure value is corrected using the upper limit value. 10. The exposure control method for an image pickup apparatus according to claim 7, wherein the exposure control method is used. For each value of the calculated first exposure value, a lower limit value of the correction value that does not overcorrect the first exposure value is associated in advance.
When the correction value obtained when the first exposure value is calculated is lower than the lower limit value corresponding to the first exposure value, the first exposure value is corrected using the lower limit value. 11. The exposure control method for an image pickup apparatus according to claim 7, wherein the exposure control method is used.   A plurality of shooting modes including a normal shooting mode, a manual shooting mode in which a user manually sets an exposure value, a specific shooting mode for shooting under a specific condition or shooting of a specific subject, and the shooting mode includes: 12. The exposure control method for an imaging apparatus according to claim 7, wherein the first exposure value is not corrected at least when the manual shooting mode or the specific shooting mode is switched.

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