JPH09275527A - Digital still camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an image not defective for gradation reproduction even in the case of an object with a large luminance region. SOLUTION: An object with a prescribed luminance or over is picked up with plural charge coupled devices (CCDs) (CCD-A4, CCD-B5) by using an exposure control means 91 to change the exposure time for the CCD-A41 and the exposure time for the CCD-B5. An image data arithmetic means 92 is used to shift levels of the signals obtained from both the CCDs and to synthesize the signal into image data of one frame.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a digital still camera for recording an image by using a plurality of charge coupled devices (CCD) for image capturing, and particularly for a subject having a large luminance region, loss of gradation in a high luminance region and The present invention relates to a device that suppresses a feeling of roughness due to noise mixing in a low luminance region.

[0002]

2. Description of the Related Art A digital still camera using an area CCD (hereinafter simply referred to as CCD) as an image pickup device is on the market. Also, some digital video cameras use a plurality of CCDs as an image pickup device.

It is a characteristic of digital still cameras and the like that the amount of exposure given to a subject is one type in one shooting.

[0004]

However, the quality of image data generated by these digital still cameras is often insufficient when hard copy output is assumed. This is due to the human characteristic that a still image is much easier to recognize than a moving image like a video, and a hard copy is much easier to recognize a loss of the image than a monitor screen. Image data generated by a digital camera has insufficient gradation reproduction, especially for subjects with a large luminance range.
There is a tendency that gradation loss occurs in the high-luminance region, and a rough feeling due to noise mixing in the low-luminance region is mixed, and the subjective image quality on the final hard copy is impaired.

The inadequate gradation reproduction is due to the low dynamic range of the CCD as an image sensor used in digital still cameras and the like. FIG.
FIG. 4 is a photoelectric characteristic diagram of CCD. As shown in FIG. 4, in the high-luminance region, a portion where the amount of charge accumulated in the CCD is saturated occurs, so that a portion where bright and dark cannot be detected occurs. Further, in the low luminance region, there is a portion in which the amount of charge accumulated in the CCD is only at the noise level, so that here also there is a portion in which bright and dark are not detected.

The present invention solves the above-mentioned problems.
It is an object of the present invention to provide a digital still camera that generates image data for a subject having a large luminance range, in which the loss of gradation in the high luminance region and the roughness caused by noise mixing in the low luminance region are suppressed.

[0007]

A digital still camera of the present invention for recording a subject image by using the output of a CCD for image capturing includes at least two CCDs, an image data calculating means, and an exposure control means. The exposure amount control means provides different exposure times to the first CCD and the second CCD in one shooting when the brightness is equal to or more than a predetermined subject brightness range, and the image data calculation means It is characterized in that the signals obtained from the first and second CCDs are respectively level-shifted and then combined into one frame of image data.

[0008]

1 is a system block diagram of a digital camera showing an embodiment of the present invention. FIG. 2 is a diagram showing a filter array of CCDs used in the embodiment of the present invention, and FIG. 3 is a diagram showing a positional relationship between two different CCDs.

After passing through the lens 1 and the diaphragm 2, the subject image is divided by the semi-transparent mirror 3 and is divided into two different CCD-A's.
4, image on CCD-B5.

The CCD used in the present invention is an all-pixel independent reading system, and each pixel has an RG as shown in FIG.
The B primary color checkered filter is applied. Furthermore, CCD
As shown in FIG. 3, the mutual positional relationship between -A4 and CCD-B5 is one pixel in the horizontal or vertical direction, or
The arrangement is preferably shifted by an odd number of pixels.
By arranging in this way, the G component having a large amount of lightness information can be obtained in all the pixels, and it becomes possible to generate image data with excellent resolution.

Here, the photometric element 20 or CCD-
The brightness of the subject is measured by the A4 or CCD-B5, and the exposure amount (aperture, exposure time) is determined by the exposure amount control means 91 included in the CPU 9. Subsequently, the exposure amount control means 91 causes the CCD-A4, the CCD
The electronic shutter speed of -B5 is set, the time is controlled by the timing generator 11, and the subject is exposed. By light exposure, photoelectric conversion is performed in each light receiving element (photodiode (not shown)) on CCD-A4 and CCD-B5. After that, the charges stored in the CCD-A4 and CCD-B5 are converted into digital data via the amplifier 6 and the A / D converter 7, and stored in the frame memory. Further, the digital data is calculated by the image data calculation means 92 included in the CPU 9 so as to be image data as one frame. The generated image data is recorded in the flash memory 10 in a compressed or non-compressed state.

In the present invention, depending on the size of the brightness range of the subject,
Two types of exposure methods are performed.

Here, the size of the subject brightness range is determined by
A manual switch (not shown) may be used by the photographer to select the size of the subject brightness range, or the CPU may automatically determine based on the photometric information from the photometric element or CCD to select the size of the subject brightness range. May be included in the CPU 9. Further, regarding the size of the subject brightness range, it is desirable to adopt 5 EV as a threshold value in consideration of the dynamic range of the CCD. Where EV is E
xpose value.

First, the operation when the subject brightness range is not large will be described. In this case, the brightness distribution of the subject falls within the middle brightness range of FIG. Therefore, even if the recording is performed with the average exposure amount measured by the external photometric device or the CCD, the gradation reproduction failure does not occur.

FIG. 5 is a time chart showing the exposure time of the CCD-A4 and CCD-B5 for a subject having a large luminance range. In this embodiment, the exposure time is controlled by the electronic shutter of the CCD. Here, the electronic shutter is to apply a reset pulse for initializing the charge amount of the light receiving element in the CCD and a charge transfer pulse for sending the charge accumulated in the light receiving element to the vertical register.

First, at time t1, CCD-A4, CCD-
A reset pulse for initializing the charge amount of the light receiving element is sent to B5. At this point, both CCDs are ready to store light. Subsequently, at time t2 after the exposure time Tex calculated by the CPU 9 in advance, a charge transfer pulse for sending the charges stored in the light receiving elements of both CCDs to the vertical registers of both CCDs is sent. Electric charges corresponding to the exposure for the time Tex are generated in both CCDs.
It is sent to the A / D converter via the vertical register and the horizontal register.

FIG. 6 is a diagram showing the relationship between the subject brightness and the output level after AD conversion. The digital signals of both CCDs stored in the frame memory are operated by the CPU 9 which includes an operation means for generating image data of one frame so as to be image data of one frame. Since a plurality of CCDs are used, the final image data is generated using a larger number of original signals than when a single CCD is used, so that high quality image data can be obtained.

Next, the operation when the subject brightness range is large will be described. In this case, the brightness distribution of the subject extends from the low brightness region to the high brightness region of FIG. Similar to the case where the brightness range is not large as described above, when recording is performed as it is with the average exposure amount measured by the external photometric device or CCD, a part of the low brightness region and a part of the high brightness region are detected.
The range of the effective recording luminance range is exceeded and a failure occurs in gradation reproduction.

Therefore, the CCD-A is used as follows.
4. The exposure amount of CCD-B5 is given to be close to the above average exposure amount. Here, the difference in exposure amount between the two CCDs may be calculated by the CPU 9 based on the photometric value of the CCD, or may be arbitrarily determined by the photographer.

FIG. 7 shows that in a subject having a large luminance range,
It is a time chart which shows the exposure time of CCD-A4 and CCD-B5.

First, at time t1, CCD-A4, C-C
A reset pulse for initializing the charge amount of the light receiving element is sent to both CD-B5. At this point, CCD-A
4 and CCD-B5 are also in a state in which light can be stored electrically. Subsequently, at time t2 after the exposure time Tex / A of the CCD-A4 calculated by the CPU in advance, a charge transfer pulse for sending the charge accumulated in the light receiving element of the CCD-A4 to the vertical register of the CCD is sent. . Where Tex / A = T
ex / ave−ΔTex, where Tex / ave is the exposure time calculated from the average exposure amount, and ΔTex is 1 / the exposure time difference calculated from the exposure amount difference between both CCDs.
2. In CCD-A4, electric charges corresponding to the exposure of time Tex / A are generated, and these are sent to the A / D converter via the vertical register and horizontal register of the CCD. Then, at time t4 = t2 + 2ΔTex, the CCD
-A charge transfer pulse for sending the charge stored in the light receiving element of B5 to the vertical register of the CCD is sent, and the time Tex
A charge corresponding to the exposure of / B = Tex / ave + ΔTex is sent to the A / D converter via the vertical register and the horizontal register of the CCD. As described above, the exposure time can be changed between the CCDs by a predetermined amount.

FIG. 8 is a diagram showing the exposure amount of both CCDs of the present invention and the photoelectric characteristics of the CCDs alone for a subject having a large luminance range. As described above, by changing the exposure time between both CCDs, an exposure amount difference of 2ΔE is provided between both CCDs. In the CCD-A4, since a small exposure amount is given by a short exposure time, the accumulated charge amount of the CCD does not reach the saturated charge amount in the high brightness area of the subject. As a result, it becomes possible to record the light-dark information in the high-luminance region to the medium-luminance region of the subject. Similarly CCD-B
In No. 5, since a larger exposure amount is given by a longer exposure time, the amount of charge accumulated in the CCD exceeds the noise level in the low brightness area of the subject. As a result, it becomes possible to record the light-dark information in the high-luminance region to the medium-luminance region of the subject.

As described above, CCD-A4 and CCD-B5
The information recorded in (1) is level-shifted as follows by the CPU 9 including the arithmetic means, and then combined to form one piece of frame information.

FIG. 9 is a diagram showing the relationship between subject brightness and output level in each of the CCD-A4 and CCD-B5.
CCD-A4 and CCD-B5 are Δ for standard exposure
Since the output level y is changed around E minutes, each output level y is corrected based on the following relational expression.

CCD-A4 0 ≦ y <Hi-γΔE y = y + γΔE Hi-γΔE ≦ y ≦ Max y = y + γΔE (Max-y) / (γΔE + Max-Hi) CCD-B5 0 ≦ y < Lo + γΔE y = y Lo / (γΔE + Lo) Lo + γΔE ≦ y ≦ Max y = y −γΔE where Max: maximum level value, Hi: highlight level value, Lo: shadow level value, γ: brightness and It is a gradient between output levels, and an appropriate value is set. However, when the output level is 0 to 255, it is desirable that Hi is set to 200 to 230 and Lo is set to 30 to 5. ΔE is 0.
It is desirable to set between 5EV and 1EV.
An exposure amount difference of 1 EV to 2 EV is provided between them.

According to the above relational expressions, CCD-A4, CCD
By combining the output levels of -B5, the relationship between the subject brightness and the output level shown by the broken line in the center of FIG.
Image data of the frame is obtained.

Here, the output level information is substantially lost in the high-luminance area of the CCD-B5 and in the low-luminance area of the CCD-A4. Regarding these areas, the output level is determined by interpolation based on the output level of the CCD, which has substantial information. In general, a high-luminance region and a low-luminance region rarely include high spatial frequency information, and therefore will not be broken as image data.

Further, in the above calculation, the γ characteristic in the middle luminance portion is the same as that in the case of a subject having a narrow luminance range where it is not necessary to change the exposure between different CCDs. For this reason, there is little difference in gradation and softness in gradation reproduction depending on the size of the subject brightness range, and the photographer is not confused.

Furthermore, since a plurality of CCDs are used,
More source signals are available than if a single CCD were used. Therefore, the quality of the final image data is high regardless of the size of the brightness range of the subject.

In the present invention, the case where the number of CCDs is two has been described. However, even when using a larger number of CCDs, it is effective to give different exposure amounts to the respective CCDs. It is possible to expand the dynamic range.

[0031]

As described above, according to the digital still camera of the present invention, even if a CCD having a low dynamic range is used, a gradation loss and a low gradation in a high luminance region and a low tone can be obtained for a subject having a large luminance region. It is possible to generate image data that can withstand hard copy output and that suppresses the roughness caused by noise mixing in the luminance range.

[Brief description of drawings]

FIG. 1 is a system block diagram of a digital still camera showing an embodiment of the present invention.

FIG. 2 is a diagram showing a filter arrangement of a CCD.

FIG. 3 is a diagram showing a positional relationship between two CCDs.

FIG. 4 is a photoelectric characteristic diagram of a CCD.

FIG. 5 shows a CCD-for an object whose luminance range is not large.
It is a time chart which shows the exposure time in A4 and CCD-B5.

FIG. 6 is a diagram showing the relationship between the subject brightness and the output level after AD conversion for a subject whose brightness range is not large.

FIG. 7 shows a CCD-A for a subject having a large luminance range.
4 is a time chart showing the exposure time of CCD-B5.

FIG. 8 is a CCD-of the present invention for a subject having a large luminance range.
It is a figure which shows the exposure amount of A4, CCD-B5, and the photoelectric characteristic of CCD.

FIG. 9 is a CCD-A4, C for a subject with a large luminance range.
It is a relationship diagram between the subject brightness and the output level after AD conversion in each of CD-B5.

[Explanation of symbols]

 1: Lens 2: Aperture 3: Semi-transparent mirror 4: CCD-A 5: CCD-B 6: CPU 7: A / D converter 8: Frame memory 9: CPU 91: Exposure amount control means 92: Image data calculation means 10: Flash memory 11: Timing generator 20: Photometric element

Claims (4)

[Claims] 1. A digital still camera for recording a subject image using the output of a CCD for image capturing, comprising at least two CCDs, an image data calculation means, and an exposure amount control means, and a predetermined subject brightness range. In the case of the above, the exposure amount control means may control the first CC in one shooting.
D and a second CCD are provided with different exposure times, and the image data calculation means is configured to provide the first and second CCs.
After level shifting each of the signals obtained from D,
A digital still camera characterized by being combined with one frame of image data. 2. A digital still camera for recording a subject image using the output of a CCD for image capturing, wherein at least two CCDs and charge amount signals stored in the first and second CCDs are converted into digital signals. A / D to convert
A converter, an image data calculation means for converting a digital signal obtained from the first and second CCDs into one frame of image data, and an exposure amount control means for controlling the electronic shutter of the first and second CCDs. The exposure amount control means applies a reset pulse for initializing the charge amount to the first and second CCDs at the same time when the brightness is above a predetermined subject brightness range, and then stored in the first CCD. A charge transfer pulse for transferring a charge, and a charge transfer pulse for transferring a charge accumulated in the second CCD after a predetermined time has passed after the charge transfer pulse is applied; 1st and 2nd CC
A digital still camera, characterized in that a digital signal of D is level-shifted and then combined into one frame of image data. 3. The digital still camera according to claim 1, wherein the exposure amount control means gives an exposure amount difference of 1 EV to 2 EV to the first and second CCDs. 4. The digital still camera according to claim 1, wherein a case where the subject brightness range is 5 EV or more is a case where the subject brightness range is a predetermined subject brightness range or more.