JP4158767B2 - Imaging device - Google Patents

Description

  The present invention relates to an imaging apparatus using a solid-state imaging device such as a CCD image sensor, and relates to an imaging apparatus having an automatic focusing position (AF) control function.

  A conventional imaging device photoelectrically converts a subject image incident through a lens using a solid-state imaging device such as a CCD (Charge Coupled Device), and amplifies the obtained video signal by an analog processing unit. A live view display (preview display) is displayed on a screen such as a liquid crystal display (LCD) in order to update the image every 20-second vertical synchronization signal (VD) cycle and confirm the state of the subject before actually capturing the image. ) At a frame rate of 20 frames / second.

In order to reduce the noise (which causes the focus position detection accuracy to decrease) and increase the accuracy of AF control, when the subject is low brightness, the video signal amplification factor is kept low and the frame rate is lowered. AF control is performed. (For example, see Patent Document 1)
Also, until AF control, the “high-speed mode” is set as a “high-speed mode”, and the n-line pixel signals (m> n: m and n are both natural numbers) are added and read for every m lines in the vertical direction to increase the frame rate and preview display. And then press the shutter button to change the readout mode of the solid-state imaging device to the “high image quality mode” and to change to the so-called “all pixel readout mode” in which all pixels are read out one line at a time in the vertical direction. Taking a still image. (For example, see Patent Document 2)
JP 2000-50147 A (page 2-8, FIG. 11) Japanese Patent Laid-Open No. 10-136244 (page 3-9, FIG. 1)

  However, in Patent Document 1, since the frame rate is lowered when the subject has low brightness, the moving image of the preview is not displayed smoothly when the subject is moving, and an unnatural impression is given, and the shutter button is pressed. It is difficult to understand when to press. In addition, since the captured image is blurred by the amount of time until AF focusing, the in-focus position may be lost due to a lack of a contour portion of the subject that is originally a high-frequency component when integrating video signals during AF control. There is a drawback that the accuracy of determining is reduced.

  Further, in the above-mentioned Patent Document 2, since the illuminance is high and the brightness of the subject is high when shooting outdoors on a sunny day, the amplification factor (gain) of the analog signal processing unit is low, and the noise component is not amplified. When shooting in places with low illuminance, such as indoors, the brightness of the subject is low, and when taking a still image, simply taking an image in all-pixel readout mode increases the amplification factor, and noise components are also amplified, resulting in high image quality. There is a disadvantage that it cannot be recorded.

  The present invention has been made to solve the above-described problems. Even when the subject has low brightness during AF control, the moving image is displayed smoothly on the preview screen, and the timing of pressing the shutter button is also easily understood. In the case of recording an image, an imaging apparatus capable of high-quality recording with a short time to AF focusing and reduced noise components is provided.

  An imaging apparatus according to the present invention includes a lens that can move on an optical axis, an imaging unit that forms an object image incident through the lens on a solid-state imaging device, and converts an analog signal into a digital signal; An amplifying unit for controlling an amplification factor; and a focusing position control unit for detecting a focusing position. When an average luminance signal level before capturing a still image is smaller than a predetermined value, the focusing position is After the detection, the charge accumulation time when capturing the still image is set longer than the charge accumulation time before capturing the still image, and the amplification factor when capturing the still image is captured. Decrease the amplification factor before.

  According to the present invention, a smooth moving image display is performed even when the subject has low brightness during AF control, the time to focus position determination in AF control is shortened, and noise is reduced when recording an image. It is possible to provide an image pickup apparatus that can perform high-quality image recording.

  The solid-state imaging device according to the present embodiment is an interline CCD image sensor having 1 million pixels or more and corresponding to an arrangement structure such as a honeycomb arrangement or a Bayer arrangement. “All-pixel readout” method for reading out pixel signals, pixel signals for one line for every two lines are mixed and read out by a vertical CCD transfer unit, or two horizontal pixels are mixed in a line memory and horizontal This corresponds to the “pixel mixture readout” mode in which the data is read out to the CCD transfer unit. However, the image sensor is not limited to a CCD, and a CMOS image sensor or other type of image sensor may be used.

  In addition, the image pickup apparatus is provided with a two-stage push-in shutter button (not shown). When the AF control mode is set, AF control is performed by half-pressing the shutter button, and the shutter button is fully pressed. By doing so, an operation is performed until an image is captured and compressed and recorded. Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment 1 FIG.
FIG. 1 is a diagram showing a basic configuration of an imaging apparatus according to the present invention. As shown in FIG. 1, a subject image incident through a lens 1 is photoelectrically converted by a solid-state imaging device 2 such as a CCD. The lens 1 can be moved on the optical axis (not shown) by a control signal output from the focus driving unit 9 to an AF motor 24 that is an electric means for driving the lens 1.

  The output signal of the subject image photoelectrically converted by the solid-state imaging device 2 is subjected to correlated double sampling processing (CDS) and automatic amplification processing (AGC) in the analog signal processing unit 3. A horizontal transfer pulse and a vertical transfer pulse of the solid-state imaging device 2 and a CDS sampling pulse of the analog signal processing unit 3 are supplied by a timing generator 8. The horizontal drive pulse and vertical drive pulse of the solid-state imaging device 2 output from the timing generator 8, the CDS sampling pulse of the analog signal processing unit 3, the output signal level in the predetermined image area and the charge accumulation time during AF control The generation timing of the electronic shutter pulse for controlling the electronic shutter function is defined by the control signal from the CPU 7.

  The output signal of the analog signal processing unit 3 is converted into a digital signal separated into three colors of R (Red), G (Green), and B (Blue) by the A / D conversion unit 4. , R, G, B digital signals are converted into color difference signals (YCbCr). The signal processing unit 5 performs white balance correction, γ (gamma) correction, contour enhancement correction, and the like, and converts them into a video signal. The video signal output from the signal processing unit 5 is displayed on the LCD 6.

  In general, when a photographer shoots a still image, the photographer determines the composition while looking at a subject displayed as a moving image on the LCD 6. The video signal recorded at the time of shooting is compressed by the image recording unit 10 and then recorded in the memory 25 in the main body or an external recording medium (not shown). For example, a JPEG (Joint Photographic Group) format is used as the image compression format, but other formats may be used.

  The ROM 11 stores data necessary for the control of the CPU 7, and stores, for example, an LUT (Look Up Table) that is a set value of an amplification factor with respect to an average luminance signal level in a predetermined image area during AF control. The average luminance signal level referred to here is the total number of pixels constituting one screen after the digital signal (8 bits) obtained by converting the output signal from the solid-state imaging device into a digital signal by the A / D converter is integrated for one screen. The value divided by.

  The CPU 7 functions as a unit that performs overall control of the imaging apparatus and performs various calculations. When the CPU 7 converts the amplification factor to the analog signal processing unit 3 and converts the digital signal into a video signal by the signal processing unit 5, the CPU 7 generates the automatic exposure control (AE) / automatic white balance control (AWB) and the timing generator 8. Drive pulse control of the solid-state imaging device 2 and control signal output from the focus drive unit 9 to the AF motor 24 are performed. Data reading from the ROM 11, reading of video signals in the image recording unit 10, and writing control are performed.

  Next, the amplification factor with respect to the output signal level in the predetermined image area during AF control will be described. FIG. 2A shows the amplification factor with respect to the output signal level of the subject image photoelectrically converted by the solid-state imaging device in the predetermined image area in the conventional AF control, and FIG. 2B shows the gain in the conventional AF control. The charge accumulation time with respect to the output signal level of the subject image photoelectrically converted by the solid-state imaging device in the predetermined image region is shown.

  As shown in FIGS. 2A and 2B, the values δ and ε of the output signal level have a relationship of 0 <δ <ε <255. ε is a threshold value of the electronic shutter function. When the output signal level becomes ε or more, the electronic shutter function becomes effective and the charge accumulation time is shortened. The amplification factor with respect to the output signal level is the lowest when the output signal level is ε or more. Charge accumulation time when the output signal level is ε 1/30 seconds (described as 1/30 seconds, but actually slightly shorter than 1/30 seconds. Other charge accumulation times described in this specification are also It is read as being slightly shorter than the described time.) Charge accumulation is not performed over a time longer than the initial state (one frame (between the rising edge and the rising edge of the vertical synchronization signal)), and the solid-state imaging device 2 As the output signal level decreases, the amplification factor is increased. When the output signal level falls below a certain value δ, the charge accumulation time is reduced to 1/15 seconds while the amplification factor remains constant. The effect of noise is not increased because the amplification factor is not increased.

  On the other hand, FIG. 2C shows the amplification factor with respect to the output signal level of the subject image photoelectrically converted by the solid-state imaging device in the predetermined image area during the AF control of the present embodiment, and FIG. The charge accumulation time with respect to the output signal level of the subject image photoelectrically converted by the solid-state imaging device in the predetermined image area during the AF control of the embodiment is shown. When the output signal level is near 0, the amplification factor is maximum (in this embodiment, the maximum amplification factor is set to 40 dB), and when the output signal level is ε, the amplification factor is the lowest (in this embodiment, the lowest). The amplification factor is set to 18 dB). If the initial state of the charge accumulation time is 1/20 second, the charge accumulation time is always left at 1/20 second regardless of the value of the average luminance signal level (FIG. 2 (d)).

  Next, the focus position determination method in the AF control will be described in detail with reference to FIGS. As shown in FIG. 3, for each frame, for example, the video signal 20 in an AF target area (not shown) defined as horizontal M pixels × vertical N pixels (M and N are arbitrary natural numbers) in the center of the image is a high-pass filter. By passing through (HPF) 21, only (spatial) high-frequency components pass. For the video signal, for example, the G signal among the R, G, and B signals is used. The signal waveform 22 after passing through the high-pass filter 21 at this time has a larger integrated region area (integrated value) 23 as the high-frequency component increases. The integrated value 23 is passed to the CPU 7.

  FIG. 4 is a diagram showing the relationship between the high frequency component and the lens position. As shown in FIG. 4, in the “mountain climbing servo system”, the CPU 7 explained in FIG. 1 controls the focus driving unit 9 to move the lens and update the integrated value of the high frequency component, and finally the maximum value. The lens position where the integrated value is obtained is determined as the in-focus position.

  More specifically, it is assumed that five-step lens positions A, B, C, D, and E are prepared in advance. First, an integrated value of high-frequency components when the lens position is A is calculated, and then an integrated value when the lens position is B is calculated. When the integrated value when the lens position is B is larger than the integrated value when the lens position is A, the integrated value when the lens position is C is calculated. In the case of FIG. 4, since the integrated value when the lens position is C is larger than the integrated value when the lens position is B, the lens position is moved to the D position and the integrated value is calculated in the same manner. In the case of FIG. 4, the integrated value when the lens position is D is smaller than the integrated value when the lens position is C. Therefore, when the lens position is C, the integrated value is determined to be the maximum and the focus position is determined. Determine.

  The moving range of the lens position (number of AF steps) can be arbitrarily set, but the number of calculations for obtaining the maximum integrated value of the high-frequency component increases as the number of steps increases. Therefore, when it is necessary to place more importance on the AF speed than the AF accuracy, for example, the number of steps may be reduced.

  Next, a process until a still image is recorded will be described with reference to a flowchart of FIG. First, when the camera power is turned on to enable shooting, the subject displayed on the lens on the LCD 6 is previewed as a moving image (step S1). The charge accumulation time is fixed at 1/20 second (step S2), and the CPU 7 moves to a comparison between the average luminance signal level of the video signal and a predetermined signal level determined in advance (step S3).

  When the average luminance signal level is higher than the predetermined signal level in a state where the subject is outdoors on a clear sky (step S3: YES), the CPU 7 increases the shutter speed (shortens the charge accumulation time). Is controlled to approach a predetermined signal level. In step S3, when the average luminance signal level of the video signal does not reach the predetermined signal level (step S3: NO), the subject has low luminance. That is, it is determined that the image is in a dark place, and the analog signal processing unit 3 increases the gain of the video signal (step S10). At this time, the charge accumulation time remains fixed at 1/20 second, and the amplification factor is increased to a maximum of 40 dB (100 times).

  At the same time that the shutter button is half-pressed in step S4, AF control is started (step S5), and the lens position is moved back and forth on the optical axis so that the integrated value of the high-frequency component in the AF target area in the image is maximized. The position is determined as a focus position (step S6).

  When the shutter button is fully pressed (step S7), it is determined whether or not the value of the amplification factor in the analog signal processing unit 3 is smaller than a certain value a (step S8). The amplification factor a can be set arbitrarily. For example, when the amplification factor a is set to 20 dB, the amplification factor is not high when the value of the amplification factor in the analog signal processing unit 3 is less than 20 dB. That is, it is determined that the luminance of the subject is high (step S8: YES), all pixels are read out in the same charge accumulation time as that during AF control, and the image is compressed and recorded (step S9).

  If the amplification factor a is a high value of 20 dB or more, for example, a setting value corresponding to the maximum amplification factor of 40 dB, it is determined that the luminance of the subject is low (step 8: NO), and the charge accumulation time is increased to Pixel reading is performed (step S11), and the amplification factor is lowered (step S12). Then, the image is compressed and recorded in a state where it can be regarded as the signal level of the same video as before the image capturing (step 9).

  FIG. 6 is a diagram showing the relationship between the vertical synchronization signal (VD) cycle, the charge readout pulse, the charge accumulation time, and the amplification factor. The charge readout pulse is a pulse signal for reading out signal charges accumulated by photoelectric conversion in the solid-state imaging device 2 to a vertical transfer CCD unit (not shown). When the charge accumulation time is 1/5 second, the average luminance signal level (corresponding to step S11 in FIG. 5) is four times the average luminance signal level when the charge accumulation time is 1/20 second. The amplification factor is when the charge accumulation time is 1/20 second (as described in paragraph 0030, it is actually a time shorter than 1/20 second, so the drawing also shows a time shorter than 1/20 second). 1/4 of that is enough.

  Therefore, although the image has a high amplification factor at the time of preview display and the noise component is also amplified, the recorded image has a reduced amplification factor (corresponding to step S12 in FIG. 5) and shortens the time until the focus position is determined in the AF control. High-quality recording with less noise components is possible.

  Next, a timing chart showing the operation of the imaging apparatus when the luminance of the subject is low will be described with reference to FIG. As shown in FIG. 7, the charge accumulation time during preview is 1/20 second, and AF control is started at the rising edge of the vertical synchronizing signal (VD) that comes next after the shutter button is half-pressed. The period from the start to the end of AF control is determined by the product of the number of steps of the lens movement position in AF control and the charge accumulation time. The time required for AF control is that the charge accumulation time of the solid-state imaging device during AF control is within one frame (between the rising and falling edges of the vertical synchronization signal), and high frequency components in the AF target area are integrated for each frame. Therefore, the charge accumulation time is shorter than that in the case of several frames.

  After the in-focus position is detected, it is possible to fully press the shutter button by displaying on the LCD 6 a notification that the in-focus state has been detected or by making a sound. At the rising edge of the vertical synchronizing signal that comes next after the shutter button is fully pressed, switching to all pixel readout with a charge accumulation time of 1/5 second is started. By switching the charge accumulation time from 1/20 second to 1/5 second, the output signal level of each pixel is quadrupled. Therefore, even if the amplification factor is reduced from α to 1 / 4α, the image is recorded before and after the charge accumulation time is changed. The average luminance signal level is the same.

  As described above, it is possible to provide an image pickup apparatus that can reduce the noise component amplified by the gain at the time of AF control by lowering the gain at the time of imaging, and enables high-quality recording. Become.

  Note that a moving image is displayed on the LCD 6 until the AF control is finished and imaging is started. Further, the display on the LCD 6 is a still image when the solid-state imaging device 2 is operated by all-pixel readout. After all pixel readout at the time of imaging is completed and the video signal processing is performed in the signal processing unit 5, when the captured image is compressed and stored in the product main body memory 25 or an externally connected memory (not shown). Displays a message indicating that the image is being saved on the LCD 6. When the saving of the image is completed, the image displayed on the LCD 6 is returned to the original preview display (moving image). The time taken to store the image is changed depending on the captured image size and the compression rate. In addition, since there are various combinations of the readout mode and the gain value as described above with reference to FIG. 7, other combinations will be described in the second to fifth embodiments in the following embodiments.

Embodiment 2. FIG.
In FIG. 7 described in the first embodiment, the case where the pixel signal readout mode of the solid-state imaging device is always set to the all-pixel readout mode has been described. However, in the second embodiment, the pixel signal readout mode is described with reference to FIG. A case where the pixel mixture readout mode is set except when imaging is described.

As shown in FIG. 8, at the time of pixel mixture reading in the second embodiment, a method is adopted in which two pixels are mixed in the horizontal direction and one line is thinned out in the vertical direction, which is twice that in the all pixel reading mode. The horizontal drive pulse and vertical drive pulse generation timings for the solid-state imaging device 2 are adjusted so that the frame rate becomes (the rising interval of the vertical synchronization signal is ½).
Therefore, since the accumulation time at the time of pixel mixture reading can be set to 1/40 seconds and a high frame rate of 40 frames / second can be set, a moving image can be displayed more smoothly.

Embodiment 3 FIG.
In FIG. 7 described in the first embodiment, the mode is always all-pixel reading, and the case where the amplification factor is smaller when the display on the LCD 6 is a still image than when the display is other than the still image is described. In FIG. 8 described in FIG. 8, it has been described that all pixel readout is performed when the display on the LCD 6 is a still image, and pixel mixed readout is performed when the display is not a still image. In the third embodiment, a case will be described in which pixel mixed readout is performed from the start of AF control to the start of imaging, and all pixel readout is performed except from the start of AF control to the start of imaging, with reference to FIG.

  As shown in FIG. 9, in the third embodiment, the charge accumulation time until AF control is started is 1/5 second and the amplification factor is 1 / 4α times. Until this occurs, the charge accumulation time is set to 1/40 seconds and the amplification factor is multiplied by α. At this time, since the frame rate is 40 frames / second, the time until the in-focus position is determined in the AF control can be shortened.

  After the AF control is completed, the in-focus position is confirmed and the whole pixel readout is switched to the charge accumulation time 1/5 second. The charge accumulation time from the start of AF control to the start of imaging is 1/8 of the other times, but the mode from the start of AF control to the start of imaging is pixel mixture readout. Therefore, the output signal level from the start of AF control to the start of imaging is substantially ¼ that at other times. If the amplification factor from the start of AF control to the start of imaging is four times that at other times, the average luminance signal level is always the same. (In FIG. 9, the other time is reduced to ¼α from the amplification factor α from the start of AF control to the start of imaging).

  Thus, the noise component amplified by the amplification factor at the time of AF control can be reduced by lowering the amplification factor.

Embodiment 4
In the fourth embodiment, as in the first embodiment, the mode is always all-pixel readout. However, when the display on the LCD 6 is a moving image, the relationship between the frame and the charge accumulation time is not always the same. As shown in FIG. 10, the charge accumulation time from the start of AF control to the start of imaging is performed every frame, but the charge accumulation time other than from the start of AF control to the start of imaging is 4 frames. I go every time.

The charge accumulation time until AF control starts is 1/5 second, but the charge accumulation time is reduced to 1/20 second from the start of AF control until imaging is started, and the charge accumulation time is reduced to 1/5 during still image imaging. Switch to seconds. By switching the charge accumulation time from 1/20 second to 1/5 second, the charge accumulation time becomes four times that during AF control. Therefore, since the average luminance signal level is quadrupled, the average luminance signal level is the same before and after the charge accumulation time is changed even if the amplification factor is reduced to ¼ (in FIG. 10, the amplification factor value is changed from α to ¼α). Lowered).
Accordingly, it is possible to shorten the time until the in-focus position is determined in the AF control.

Embodiment 5
In the fifth embodiment, as in the second embodiment, the mode is all pixel readout only during imaging and mixed readout except during imaging, but the case where the relationship between the frame and the charge accumulation time is not always the same will be described.

  As shown in FIG. 11, the charge accumulation time is performed every 4 frames except when the AF control is started until the imaging is started. However, the charge accumulation time is 1 frame until the imaging is started after the AF control is started. I go every time. The charge accumulation time until the AF control is started is 1/10 second, and the AF control is started at the rising edge of the vertical synchronizing signal (VD) that comes next after the shutter button is half-pressed, and the charge accumulation time is 1/40. Although the mode is changed to the second, the mode remains the pixel mixed readout.

  After the in-focus position is detected, for example, a display informing that the in-focus state has been displayed on the LCD 6 or a sound is output to notify the user that the shutter button can be fully pressed. AF control ends at the rising edge of the vertical synchronizing signal that comes next after the shutter button is fully pressed, the charge accumulation time is 1/5 second, and the mode is switched to all pixel readout. In the mixed pixel readout mode, the charge accumulation time is halved until the AF control is started compared to the all pixel readout mode, but two pixels are mixed for each pixel from the start of the AF control to the start of imaging. The output signal level is four times, and the amplification factor may be ¼ that when reading out all pixels.

  Therefore, the AF control is started, and the amplification factor from the start of the AF control to the start of imaging is 4 times that before the start of the AF control. However, by reducing the charge accumulation time to 1/40 seconds, the moving image is smoothed. In addition to being able to display, it is possible to shorten the time until the in-focus position is determined in the AF control.

  It is obvious that the charge accumulation time and the amplification factor used in Embodiments 1 to 5 of the present imaging device do not stick to these values and can be arbitrarily determined.

  Further, in Embodiments 1 to 5 of the imaging apparatus, the two-stage push-in shutter button is used. However, a button for starting AF control and a shutter button for imaging may be prepared separately.

1 is a block diagram of an imaging apparatus according to the present invention. It is a figure which shows the amplification factor control method concerning this invention. 4 is a flowchart showing AF control according to the present invention. It is a figure which shows the relationship between the high frequency component concerning this invention, and a focus position. 3 is a flowchart showing the operation of the imaging apparatus according to the present invention. It is a figure which shows the relationship between the frame rate of the imaging device concerning this invention, and electric charge accumulation time. 3 is a timing chart illustrating the operation of the imaging apparatus according to the present invention. 3 is a timing chart illustrating the operation of the imaging apparatus according to the present invention. 3 is a timing chart illustrating the operation of the imaging apparatus according to the present invention. 3 is a timing chart illustrating the operation of the imaging apparatus according to the present invention. 3 is a timing chart illustrating the operation of the imaging apparatus according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Lens, 2 Solid-state image sensor (CCD), 3 Analog signal processing part, 4 A / D conversion part, 5 Signal processing part, 6 LCD, 7 CPU, 8 Timing generator, 9 Focus drive part, 10 Image recording part, 11 ROM, 20 video signal, 21 high-pass filter (HPF), 22 signal waveform through HPF, 23 integration area, 24 AF motor, 25 memory

Claims (3)

A lens whose focal position can be adjusted in the direction of the optical axis, an imaging unit that forms an object image incident through the lens on a solid-state imaging device and converts an analog signal into a digital signal, and an amplification unit that controls an amplification factor And an in-focus position detecting means for detecting the in-focus position,
During the in-focus position detection by the in-focus position detection means, pixel mixture readout for mixing a plurality of pixels into one pixel is performed, the amplification factor of the amplifying means is set to m times during imaging, and the charge accumulation time is 1 during imaging. An imaging device characterized by being shorter than / m times . The image pickup apparatus according to claim 1, wherein in the pixel mixture reading, a part of the plurality of lines is read out. A lens whose focal position can be adjusted in the direction of the optical axis, an imaging unit that forms an object image incident through the lens on a solid-state imaging device and converts an analog signal into a digital signal, and an amplification unit that controls an amplification factor And an in-focus position detecting means for detecting the in-focus position,
While the in-focus position is detected by the in-focus position detecting means, pixel mixing readout is performed by mixing two pixels in the horizontal direction into one pixel and thinning out one of the two lines in the vertical direction, and imaging the amplification factor of the amplifying means. An image pickup apparatus characterized in that the charge accumulation time is 1/2 m times that at the time of imaging .

Images