JP4323105B2 - Imaging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an imaging apparatus, and more particularly to an imaging apparatus having an exposure control function.
[0002]
[Prior art]
Conventionally, automatic exposure control (AE) has been known for both silver halide cameras and digital cameras, and in average photometry, the subject standard reflection coefficient (about 18%) is generally used as the exposure control target value. It was.
[0003]
The subject standard reflection coefficient means the logarithmic midpoint of the subject range for a general subject. This is numerically the logarithmic midpoint of the white level (assuming a reflectance of about 98% for a subject with the highest diffuse reflectance) and the black level (assuming a reflectance of about 3.3% for the lowest subject). This corresponds to a certain 18%. Based on this knowledge, in the photographic technology field, there is a situation that 18% is adopted as the standard reflectance value representing the subject or the reflectance of the standard reflector for evaluation.
[0004]
[Problems to be solved by the invention]
However, in a digital camera using an image sensor such as a CCD, the exposure control target value in AE is always a value obtained by multiplying the maximum input level of the γ characteristic by the subject standard reflection coefficient, regardless of the γ characteristic used. It has been. This is shown in FIG.
[0005]
FIG. 7 shows a γ characteristic of 0.45 that is used in a digital camera as a standard. The .gamma. Characteristic itself of 0.45 is a gradation conversion characteristic determined so that the total .gamma. Characteristic becomes linear when the characteristics on the display device side are taken into consideration. In FIG. 7, the knee characteristic for white compression is shown. A γ characteristic having no is used. This defines a γ characteristic of 0.45 with reference to the maximum output range (example value 255) without expecting a white margin. In this case, the exposure control target value is set to a value (example value 46) obtained by multiplying the maximum input range (example value 255) by the subject standard reflection coefficient 18%, and the AE control is performed so that the photometric value becomes 46. By doing so, the entire luminance range of the main subject can be within the range of the maximum input range corresponding to the maximum output range.
[0006]
However, if the exposure control target value is set based only on the maximum output range in this way, depending on the tone conversion characteristics used, the luminance range of the main subject cannot be kept within a good reproduction range, which causes image quality degradation. It was. In particular, when the tone conversion characteristic has a knee characteristic for white compression, setting the exposure control target value as described above makes it easy to compress the gradation of the high-intensity part of the main subject. The image quality will be deteriorated. This is shown in FIG.
[0007]
FIG. 8 shows a γ characteristic having a white reference level (example value 213) set with a white margin for the maximum output range (example value 255) as a knee point. Here, the γ characteristic up to the knee point is set to 0.45, and the knee characteristic is given to the higher luminance side than the input level 213 corresponding to the knee point. When the imaging range of the CCD is sufficiently large, such tone conversion characteristics having knee characteristics for white compression are suitable characteristics in that the dynamic range on the high luminance side of the imaging signal can be expanded. In FIG. 8, the maximum input level is 511, which is twice that of FIG. 7, but this is an example value assuming that the imaging range of the CCD is sufficiently large, and the numerical value itself has meaning. is not.
[0008]
By the way, when the conventional method of setting the exposure control target value based on only the output maximum range is applied to the γ characteristic of FIG. 8 as it is, the exposure control target value is the input maximum range (255) corresponding to the output maximum range (255). Here, the value (example value 92) obtained by multiplying the example value 511) by the subject standard reflection coefficient 18% is set. In this case, since the low luminance side and the high luminance side of the main subject are distributed around the input level of 92, the total γ characteristic is limited to the range up to the input level 213 and diffused. A considerable portion on the high luminance side of the main subject having a reflectance lower than 98% (in this example, a portion having a reflectance of 18 × 213/92 = 41.7% or more) is included in the compression target. Become.
[0009]
Since the white margin from the white reference level to the maximum output range is to be used as a reproduction area of highlight parts such as metal surfaces and light emitters that exceed the diffuse reflectance of about 98%, The compression on the high luminance side related to the main subject as described above becomes a factor of image quality deterioration. Furthermore, this means that the conventional exposure control target value setting method sufficiently reproduces a metal surface or a light emitter that exceeds about 98% even if the CCD imaging range is expanded. It means you can't.
[0010]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an imaging apparatus capable of obtaining a high-quality image by optimizing the exposure control target value. is there.
[0011]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention provides an image sensor, an exposure control unit that controls exposure in the image sensor, and a gradation that can generate image signals having different gradation characteristics based on the image sensor output signal. Conversion means, wherein the exposure control means analyzes an image signal input to the gradation conversion means to calculate a photometric value for exposure control, and matches the photometric value with a predetermined exposure control target value The exposure control target value used at the time of photographing is white based on the maximum output level in the generation signal area of the gradation conversion characteristic used by the gradation converting means. The white reference level set with allowance is set to a value obtained by multiplying the white reference-corresponding input value corresponding to the gradation conversion characteristic in the input signal range by the subject standard reflection coefficient. The features.
[0012]
In this imaging apparatus, not only the maximum output level is used as a reference, but a white reference level set with a white margin based on the maximum output level is set as a reference, and an input signal area corresponding to the white reference level is set. A value obtained by multiplying the input value corresponding to the white reference value by the subject standard reflection coefficient is used as the exposure control target value. Therefore, since the entire luminance range of the main subject can be within the range of the input value corresponding to the white reference instead of the maximum input range, the high luminance portion of the main subject can be reproduced well. Furthermore, since the margin of white can be used as a reproduction area for metal surfaces and light emitters that exceed the white level of the main subject, it is possible to obtain images with a wide dynamic range as the imaging range of the image sensor increases. It becomes.
[0013]
The present invention also includes an image sensor, exposure control means for controlling exposure in the image sensor, and gradation conversion means capable of generating image signals having different gradation characteristics based on the image sensor output signal. The exposure control unit analyzes the image signal input to the gradation conversion unit to calculate a photometric value for exposure control, and performs exposure control to match the photometric value with a predetermined exposure control target value. The exposure control target value used at the time of photographing is a value corresponding to the knee conversion characteristic used by the gradation conversion means in the input signal area with respect to the gradation conversion characteristic. The white reference corresponding input value is set to a value obtained by multiplying the standard reflection coefficient of the subject.
[0014]
In this way, when using tone conversion characteristics with a clear knee point, the exposure control target is based on the knee point instead of the white reference level set with the white margin based on the maximum output level. The value can be determined. Even in this case, the entire luminance range of the main subject can be within the range of the white reference corresponding input value corresponding to the knee point instead of the maximum input range, and the same effect as in the above case can be obtained.
[0015]
The present invention also provides gradation conversion on the image sensor output signal by selectively using an image sensor, exposure control means for controlling exposure in the image sensor, and a plurality of gradation conversion characteristics. Gradation control means for performing, wherein the exposure control means analyzes an image signal input to the gradation conversion means to calculate a photometric value for exposure control, and uses the photometric value as a predetermined exposure control target. The exposure control target value is set so that the luminance range distribution relating to the main subject in the image sensor output signal is white by gradation conversion by the gradation conversion means. When multiple tone conversion characteristics with different input levels corresponding to the white reference level set with the white margin based on the maximum output level are used so that they fall within the range that is not subject to compression Characterized in that it is variably set in accordance with these gradation conversion characteristic.
[0016]
Furthermore, the present invention is directed to the image sensor output signal by selectively using an image sensor, exposure control means for controlling exposure in the image sensor, and a plurality of gradation conversion characteristics having different knee points. Gradation conversion means for performing gradation conversion, and the exposure control means calculates a photometric value for exposure control by analyzing an image signal input to the gradation conversion means, and calculates the photometric value. The exposure control target value is configured to perform exposure control that matches a predetermined exposure control target value. The exposure control target value is determined by the gradation conversion unit according to the distribution of the luminance range related to the main subject in the image sensor output signal. When multiple tone conversion characteristics with different knee points are used so that they fall within the range that is not subject to white compression in the tone conversion, they should be variably set according to these tone conversion characteristics. And features.
[0017]
As described above, when a plurality of gradation conversion characteristics having different input levels corresponding to the white reference level set with the white margin based on the maximum output level are used, or a plurality of levels having different knee points are used. When tone conversion characteristics are used, the exposure control target value should be variably set according to the gradation conversion characteristics used so that the distribution of the luminance range for the main subject is within the range that is not subject to white compression. Thus, it is possible to always keep the luminance range of the main subject within the optimum reproduction range regardless of the gradation conversion characteristics to be used.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of an electronic camera according to an embodiment of the present invention.
[0019]
In the figure, 101 is an imaging lens system composed of various lenses, 102 is a lens driving mechanism for driving the lens system 101, 103 is an exposure control mechanism for controlling the aperture and shutter device of the lens system 101, and 104 is low-pass and red. An outer cut filter 105 is a CCD color image sensor for photoelectrically converting a subject image, 106 is a CCD driver for driving the image sensor 105, 107 is a preprocess circuit including an A / D converter, and 108 is A digital process circuit for performing various kinds of digital processing such as γ conversion, 109 is a card interface, 110 is a memory card, and 111 is an LCD image display system.
[0020]
Also, 112 in the figure is a system controller (CPU) 112 for overall control of each part, 113 is an operation switch system composed of various SWs, 114 is an operation display system for displaying operation states and mode states, 115 Is a lens driver for controlling the lens driving mechanism 102, 116 is a strobe as a light emitting means, 117 is an exposure control driver for controlling the exposure control mechanism 103 and the strobe 116, and 118 is for storing various setting information and the like. A non-volatile memory (EEPROM) is shown.
[0021]
In the electronic camera according to the present embodiment, the system controller 112 performs overall control, and controls exposure (charge accumulation) and signals by controlling the driving of the CCD image sensor 105 by the exposure control mechanism 103 and the CCD driver 106. Is read into the digital process circuit 108 via the preprocess circuit 107, and subjected to various signal processing such as gradation conversion in the γ correction circuit 108a in the digital process circuit 108, and then the card interface 109. Are recorded in the memory card 110 via In the gradation conversion by the γ correction circuit 108a, a plurality of types of γ characteristics can be selectively used under the control of the system controller 112. This is because, for example, the optimum gradation setting can be selected according to the user's preference and the situation of the scene.
[0022]
The basic configuration so far is the same as that of a conventional general electronic camera. In this embodiment, in addition to this, an exposure control target value for automatic exposure control (AE) is set in accordance with the γ characteristic to be used. A function for optimization is provided. In order to achieve this, the system controller 112 analyzes the imaging signal output from the CCD 105 before being input to the γ correction circuit 108a and calculates a photometric value for AE, and the photometric value is exposed. An AE control unit 112b for performing exposure control so as to match the control target value and an exposure control target value setting unit 112c for setting the exposure control target value are provided. The exposure control target value setting unit 112c determines the exposure control target value based on the white reference level set with a white margin from the maximum output level, not the maximum output level in the generation signal range of the γ correction circuit 108a. To do.
[0023]
In the camera of the present embodiment, an imaging operation exactly the same as that of a normal camera is started by a shutter trigger operation by the user. That is, first, photometry information is acquired and exposure control is performed based on the preliminary trigger (first stage of the two-stage trigger switch). Metering is performed by average metering (including center-weighted average metering), and exposure control is performed so that the metering value (signal level average value of the imaging signal) is equal to the exposure control target value. In exposure control, calculation processing for calculating an optimal exposure level based on a photometric value and an exposure target value, or feedback control for increasing / decreasing the exposure level by a signal corresponding to a difference between the photometric value and the exposure target value is used.
[0024]
As described above, when exposure control is performed so that the average signal level of the imaging signal is equal to the exposure control target value, the output value obtained in the case of a flat (no pattern) subject corresponds to the exposure control target value. . It is well known that such a flat subject can be used as a representative or substitute when assuming a main subject under normal exposure conditions.
[0025]
Next, the relationship between the γ characteristic used in the γ correction circuit 108a and the exposure control target value will be described.
[0026]
FIG. 2 shows γ characteristics with a white reference level (example value 213) set with a white margin for the maximum output range (example value 255) as a knee point. According to NTSC, which is a standard for television video signals, the white reference level is preferably a value obtained by multiplying the maximum output range by 100/120. (To be more precise, in the NTSC standard itself, the gradation management region (that is, the gradation characteristic main part) is up to a so-called 100% (100IRE) level corresponding to a voltage value of 0.714 V, Although not defined, there is a fact that in many actual image systems targeting NTSC signals, this over 100% region is estimated to be about 20% and used as a margin region (white margin). The JEIDA SIS RIF standard, which is the only camera signal standard that defines the correspondence between the NTSC signal voltage level and the digital signal level, also expects a white margin of 20% and defines an 8-bit digital value of 100% level as 213. Can also be supported.)
In FIG. 2, the linear characteristic of γ = 1 is set up to the knee point corresponding to the white reference level, and the knee characteristic is given to the higher luminance side than the input level 213 corresponding to the knee point. The gradation of the high luminance portion from the input level 213 to the maximum input level 511 is compressed.
[0027]
When the γ characteristic as shown in FIG. 2 is used, the exposure control target value is not the maximum output level 255 in the generated signal area, but the white reference level 213 set with a white margin from the maximum output level. That is, it is determined based on the knee point. That is, 38, which is a value obtained by multiplying the white reference corresponding input value 213 which is the value of the input signal range corresponding to the knee point by the subject standard reflection coefficient 18%, is set as the exposure control target value. In this case, since the luminance range of the main subject (diffuse reflectance of about 3.3% to about 98%) is distributed within the range up to the white reference corresponding input value 213, the high luminance portion of the main subject is also good. Can be reproduced. Furthermore, since the white margin portion can be used as a reproduction area of a metal surface or a light emitter that exceeds the white level of the main subject, an image with a wide dynamic range can be obtained as the imaging range of the imaging element 105 is expanded. It becomes possible.
[0028]
According to the study of the present inventor, in actual photographing, it is better to adopt a logarithmic midpoint value of 20% as the subject standard reflection coefficient when the numerical value of the minimum diffuse reflectance is assumed to be 4%. In this sense, about 18 to 20% of the set value of the exposure control point can be set target value (target value) for obtaining the best result. That is, 18 to 20% is given as a numerical example of the standard reflection coefficient in the present invention. In the example of FIG. 2, the exposure control target value is 38 to 43.
[0029]
FIG. 3 shows a γ characteristic of 0.45 with reference to the output y = 213 up to a white reference level (example value 213) set in consideration of white margin with respect to the maximum output range (example value 255). A knee characteristic is given to the higher luminance side than the input level 213 (white reference corresponding input value) corresponding to the knee point. The gradation of the high luminance portion from the input level 213 to the maximum input level 511 is compressed.
[0030]
In this case, 38 to 43, which is a value obtained by multiplying the white reference corresponding input value 213 that is the value of the input signal range corresponding to the knee point by the subject standard reflection coefficient of 18% to 20%, is set as the exposure control target value. . The exposure control target value is the same as that in FIG. 2 because the input level corresponding to the knee point is 213 in both of the gradation conversion characteristics in FIGS.
[0031]
FIG. 4 shows a γ characteristic of 0.45 with reference to the output y = 255 up to a white reference level (example value 213) set with a white margin for the maximum output range (example value 255). The knee characteristic is given to the higher luminance side than the input level 172 (white reference corresponding input value) corresponding to the point. The gradation of the high luminance portion from the input level 172 to the input maximum level 511 is compressed.
[0032]
In this case, 31 to 34, which is a value obtained by multiplying the white reference corresponding input value 172 that is the value of the input signal range corresponding to the knee point by the subject standard reflection coefficient of 18% to 20%, is set as the exposure control target value. . The luminance range of the main subject (diffuse reflectance of about 3.3% to about 98%) is distributed within the range up to the input value 172 corresponding to the white reference.
[0033]
As described above, in this embodiment, since an optimal exposure control target value is set for each γ characteristic to be used, when a plurality of γ characteristics having different knee points are selectively used, the luminance range of the main subject is selected. The exposure control target value is changed in accordance with the γ characteristic to be used so that the whole is within the range up to the knee point.
[0034]
In many cases, the knee point is not clear depending on the γ characteristic. In this case, the white reference level corresponds to the knee point.
[0035]
Next, the operation of this embodiment will be described with reference to the flowchart of FIG.
[0036]
First, average photometry (including center-weighted average photometry) and photometry values are calculated by a preliminary trigger (steps S101 and S102). The photometry is performed by analyzing (calculating) the information taken into the digital process 108 by the photometry unit 112a in the system controller 112, and based on the photometry result, a photometric value (signal level average value of the imaging signal) is calculated. The Next, the AE control unit 112b in the system controller 112 performs exposure control by setting exposure conditions so that the photometric value becomes equal to the exposure control target value (steps S103 and S104). However, the exposure control target value at this time is set by the exposure control target value setting unit 112c in the system controller 112 based on the knee point of the γ characteristic used in the γ correction circuit 108a as described above. Therefore, by setting the exposure condition so that the photometric value becomes equal to the exposure control target value, the exposure is such that the luminance range of the main subject is within the range up to the knee point of the γ characteristic used in the γ correction circuit 108a. Control will be performed.
[0037]
The exposure control is performed by the AE control unit 112b in the system controller 112 driving the exposure control mechanism 103 or the strobe 115 via the exposure control driver 117. When the element shutter function is used as the shutter, the CCD driver 106 is also used (charge accumulation time control).
[0038]
Subsequently, when a main trigger (second stage) operation is performed, main imaging is performed under proper exposure conditions, and main exposure and signal readout of the CCD image sensor 105 are performed (step S105). The captured image signal captured by the digital process circuit 108 is subjected to gradation conversion processing by the γ correction circuit 108a (step S106), and then recorded on the memory card 110.
[0039]
FIG. 6 shows the procedure of the variable setting operation of the exposure control target value performed when the γ characteristic to be used is switched.
[0040]
For example, when the γ characteristic to be used is switched by a user's switch operation or the like (step S201), the exposure control target value setting unit 112c corresponds to the white reference level (knee point) from the new γ characteristic of the switching destination. The white reference corresponding input value is checked (step S202). Then, the exposure control target value setting unit 112c sets a new exposure control target value based on the white reference corresponding input value and the subject standard reflection coefficient (step S203).
[0041]
As described above, according to the present embodiment, by providing a mechanism for variably setting the exposure control target value in accordance with the γ characteristic to be used, the luminance range of the main subject can be always kept in a good reproduction range.
[0042]
In addition, this invention is not limited to embodiment mentioned above. In the above-described embodiment, the digital still camera has been described as an example, but the present invention can be similarly applied to a digital movie.
[0043]
Further, the above embodiments include inventions at various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, even if some constituent requirements are deleted from all the constituent requirements shown in the embodiment, the problem described in the column of the problem to be solved by the invention can be solved, and the effect described in the column of the effect of the invention Can be obtained as an invention.
[0044]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain a high-quality image by optimizing the exposure control target value.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an electronic camera according to an embodiment of the present invention.
FIG. 2 is a view showing a relationship between gradation characteristics and exposure control target values used in the electronic camera of the embodiment.
FIG. 3 is a second diagram showing a relationship between gradation characteristics and exposure control target values used in the electronic camera of the embodiment.
FIG. 4 is a third diagram showing the relationship between gradation characteristics and exposure control target values used in the electronic camera of the embodiment.
FIG. 5 is a flowchart showing the operation of the electronic camera of the embodiment.
FIG. 6 is an exemplary flowchart for explaining an exposure control target value variable setting operation in the electronic camera of the embodiment;
FIG. 7 is a first diagram showing a relationship between a conventional gradation conversion characteristic and an exposure control target value.
FIG. 8 is a second diagram showing a relationship between a conventional gradation conversion characteristic and an exposure control target value.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 101 ... Lens system 102 ... Lens drive mechanism 103 ... Exposure control mechanism 104 ... Filter system 105 ... CCD color image sensor 106 ... CCD driver 107 ... Pre-process part 108 ... Digital process part 109 ... Card interface 110 ... Memory card 111 ... LCD image Display system 112 ... System controller (CPU)
112a ... metering unit 112b ... AE control unit 112c ... exposure control target value setting unit

Claims (4)

An image sensor, exposure control means for controlling exposure in the image sensor, and gradation conversion means capable of generating image signals having different gradation characteristics based on the image sensor output signal,
The exposure control unit analyzes an image signal input to the gradation conversion unit to calculate a photometric value for exposure control, and performs exposure control to match the photometric value with a predetermined exposure control target value. Configured,
The exposure control target value used at the time of photographing is a white reference level set with a white margin based on the maximum output level in the generation signal area of the gradation conversion characteristic used by the gradation conversion means. An imaging apparatus, wherein a conversion reference characteristic is set to a value obtained by multiplying an input value corresponding to a white reference, which is a value corresponding to the input signal range, by a subject standard reflection coefficient. An image sensor, exposure control means for controlling exposure in the image sensor, and gradation conversion means capable of generating image signals having different gradation characteristics based on the image sensor output signal,
The exposure control unit analyzes an image signal input to the gradation conversion unit to calculate a photometric value for exposure control, and performs exposure control to match the photometric value with a predetermined exposure control target value. Configured,
The exposure control target value used at the time of shooting is a white reference corresponding input value that is a value corresponding to the knee conversion characteristic knee point of the gradation conversion characteristic used in the input signal range with respect to the gradation conversion characteristic. An imaging device, wherein the imaging device is set to a value multiplied by the subject standard reflection coefficient. An image sensor, exposure control means for controlling exposure in the image sensor, and gradation conversion means for performing gradation conversion on the image sensor output signal by selectively using a plurality of gradation conversion characteristics. Comprising
The exposure control unit analyzes an image signal input to the gradation conversion unit to calculate a photometric value for exposure control, and performs exposure control to match the photometric value with a predetermined exposure control target value. The exposure control target value is configured so that the distribution of the luminance range related to the main subject in the image sensor output signal is within a range that is not subject to white compression by gradation conversion by the gradation conversion means. When a plurality of gradation conversion characteristics having different input levels corresponding to the white reference level set with the white margin based on the maximum output level are used, the gradation conversion characteristics are variably set according to the gradation conversion characteristics. An imaging device characterized by that. By selectively using an image sensor, an exposure control means for controlling exposure in the image sensor, and a plurality of tone conversion characteristics having different knee points, the level at which tone conversion is performed on the output signal of the image sensor. Key conversion means,
The exposure control unit analyzes an image signal input to the gradation conversion unit to calculate a photometric value for exposure control, and performs exposure control to match the photometric value with a predetermined exposure control target value. The exposure control target value is configured so that the distribution of the luminance range related to the main subject in the image sensor output signal is within a range that is not subject to white compression by gradation conversion by the gradation conversion means. An imaging apparatus characterized in that when a plurality of gradation conversion characteristics having different knee points are used, the gradation conversion characteristics are variably set according to the gradation conversion characteristics.

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