JP2005091456A - Digital camera - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a digital camera taking an excellent color picture in focus by restraining the lowering of automatic focusing performance when luminance is low. <P>SOLUTION: When a release button 8 is half-depressed, the digital camera 1 tries automatic focusing based on the contrast of a subject image temporarily picked up by an imaging device 2e. If focusing is not realized, whether the measured value of the luminance of the subject by a luminance detection part 11 is smaller than a fixed value previously set is examined. If the measured value of the luminance of the subject is smaller than the fixed value previously set, an OLPF driving part 9d is actuated to make an OLPF 2d retreat from an optical path. Then, the automatic focusing is tried again. If the focusing is implemented, the cutoff frequency of the low-pass filter of a YC separation processing circuit in a signal processing part 4 is lowered, and then photographing is performed when the button 8 is fully depressed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a digital camera having an automatic focus adjustment function for adjusting a focus position based on a high frequency component of an image signal output from an image sensor.

  Many digital cameras have a contrast type automatic focus adjustment function that automatically adjusts the focus position of a photographing lens based on a high-frequency component of an image signal obtained from a solid-state imaging device such as a CCD (Charge Coupled Device). The contrast type automatic focus adjustment function is degraded when the subject brightness at the time of shooting is low. This is because the level of the evaluation value (see FIG. 5) for determining the in-focus lens position is lowered overall when the luminance is low and is strongly influenced by noise, and the peak of the evaluation value can be detected. This is because it becomes difficult.

  As a conventional technique for avoiding this type of problem, there is a technique in which an infrared cut filter is retracted from an optical path at low luminance (low illuminance) and automatic focus adjustment is performed in a state where the amount of received light is increased (see Patent Document 1). Also known is a high-resolution mode in which an optical low-pass filter that limits the spatial frequency of subject light incident on the image sensor is retracted from the optical path of the subject light to perform automatic focus adjustment and photographing (see Patent Document 2). ).

JP-A-5-292388

JP-A-7-123421

  However, in the technique described in Patent Document 1, the infrared cut filter is retracted from the optical path when it is determined that the brightness is low, regardless of whether or not automatic focus adjustment is possible. I can't do it. Even in the technique described in Patent Document 2, a color image cannot be taken in the high resolution mode.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a digital camera capable of taking a good color image in focus while suppressing a decrease in automatic focusing performance at low luminance. And

  The digital camera of the present invention is a digital camera having an automatic focus adjustment function for adjusting a focus position based on a high-frequency component of an image signal output from an image sensor, the luminance detection means for detecting the luminance of a subject, Optical low-pass filter moving means for selectively moving an optical low-pass filter for limiting the spatial frequency of incident light to the image sensor to a position retracted from the optical path of the incident light, and the automatic focus adjustment function When the in-focus position cannot be determined and the brightness detection value by the brightness detection means is smaller than a predetermined value, the optical low-pass filter is retracted from the optical path by the optical low-pass filter moving means. And a control means for controlling the focus position to be adjusted again by the automatic focus adjustment function.

  With this configuration, when the in-focus position cannot be determined by the automatic focus adjustment function and the luminance detection value by the luminance detecting means is smaller than a certain value, the optical low-pass filter is retracted from the optical path and incident on the image sensor. Since the focus adjustment is performed again by the automatic focus adjustment function after the amount of light to be increased is increased, it is possible to suppress a decrease in the automatic focus adjustment performance at the time of low luminance and to capture a good color image in focus.

  The digital camera of the present invention further includes a low-pass filter having a variable cut-off frequency for attenuating a high-frequency component of the color difference signal obtained from the image signal, and the control means removes the optical low-pass filter from the optical path. The cutoff frequency of the low-pass filter after being retracted is controlled to be lower than the cutoff frequency of the low-pass filter before retracting the optical low-pass filter.

  With this configuration, it is possible to capture a better color image by making the influence of a false signal less noticeable when the optical low-pass filter is retracted from the optical path.

  According to the present invention, it is possible to provide a digital camera that can suppress a decrease in automatic focusing performance at low luminance and can capture a good color image in focus.

The best mode for carrying out the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing a schematic configuration of a digital camera for explaining the best mode for carrying out the present invention.
The digital camera 1 includes an imaging system 2, an A / D conversion unit 3, a signal processing unit 4, a buffer memory 5, an image recording unit 6, an image display unit 7, a release button 8, and an imaging system drive unit. 9, EEPROM 10, luminance detection unit 11, and control unit 12.

  The imaging system 2 includes a photographing lens 2a, a diaphragm 2b, a shutter 2c, an optical low-pass filter (hereinafter referred to as OLPF) 2d, and an imaging element 2e.

  The photographing lens 2a forms an optical image of the subject on the light receiving surface of the image sensor 2e through the aperture 2b, the shutter 2c, and the OLPF 2d. The diaphragm 2b adjusts the amount of light reaching the light receiving surface of the image sensor 2e. The shutter 2b operates when the release button 8 is pressed, and exposes the light receiving surface of the image sensor 2e at a set shutter speed. The OLPF 2d limits the spatial frequency of light incident on the image sensor 2e. The image sensor 2 e converts the received optical image of the subject into an image signal and outputs it to the A / D converter 3.

  The A / D converter 3 converts the input analog image signal into a digital image signal and outputs it to the signal processor 4.

  The signal processing unit 4 performs predetermined digital signal processing such as YC separation processing on the input digital image signal. FIG. 2 is a diagram showing a schematic configuration of a YC separation processing circuit in the signal processing unit 4.

  As shown in FIG. 2, the digital image signal from the A / D conversion unit 3 is subjected to RGB interpolation by the RGB interpolation unit 41 and then converted to a YC signal by the RGB → YC conversion unit 42. The luminance signal Y is output with the luminance automatically controlled by an APC (Automatic Picture Control) 43. The high-frequency components of the color difference signals Cr and Cb are attenuated by the low-pass filter 44 and are output via the color difference matrix 45. The low-pass filter 44 can change the cutoff frequency under the control of the control unit 12. The YC signal output from the signal processing unit 4 is temporarily stored in the buffer memory 5, compressed into data of a predetermined format, and then recorded in the image recording unit 6.

  The image display unit 7 functions as a finder for displaying an image being picked up by the image pickup device 2e, a function as a display device for reproduction that displays an image recorded in the image recording unit 6, and the state of the camera and photographing. And a function for displaying various messages related to. A liquid crystal device or the like is used for the image display unit 7.

  The release button 8 is an operator for instructing the digital camera 1 to start photographing.

  The imaging system driving unit 9 includes a lens driving unit 9a, an aperture driving unit 9b, a shutter driving unit 9c, an OLPF driving unit 9d, and an imaging element driving unit 9e. Are driven.

  The lens driving unit 9a drives the photographing lens 2a. The aperture driver 9b drives the aperture 2b. The shutter drive unit 9c drives the shutter 2c. The OLPF driving unit 9d drives the OLPF 2d. The OLPF driving unit 9d is a driving unit for selectively moving the OLPF 2d to a position on the optical path of incident light to the image sensor 2e or a position retracted from the optical path. The image sensor driving unit 9e drives the image sensor 2e.

  The EEPROM 10 stores programs and data for realizing various functions of the digital camera 1.

  When the release button 8 is half-pressed, the luminance detection unit 11 detects the luminance of the subject and notifies the control unit 12 of the detected luminance value.

  The control unit 12 performs overall control of the entire digital camera 1 by executing a program stored in the EEPROM 10.

  When the release button 8 is half-pressed, the control unit 12 controls the imaging system driving unit 9 to move the photographing lens 2a, and extracts and integrates high-frequency components of the image signal obtained from the imaging element 2e, It has an automatic focus adjustment function that determines the position of the photographic lens 2a that maximizes the AF evaluation value, which is the integration result, as the in-focus position, and controls the focal position of the photographic lens 2a based on the determined in-focus position. When the automatic focus adjustment function is executed, the OLPF 2d may be retracted from the optical path depending on the photographing conditions.

  When the release button 8 is pressed down (fully pressed), the control unit 12 issues an instruction to the imaging system driving unit 9 and outputs an imaging signal from the imaging device 2e, the signal processing unit 4, the image recording unit 6, and the like. Let the capture occur.

  The control unit 12 determines that the AF evaluation value for determining the in-focus position by the automatic focus adjustment function is smaller than a predetermined value and the maximum value cannot be detected. For this reason, the in-focus position is determined. If the luminance detection value by the luminance detection unit 11 cannot be determined and is smaller than a predetermined value, the optical low pass filter 2d is retracted from the optical path by the OLPF driving unit 9d, and then again by the automatic focus adjustment function. Control to adjust the focal position. At this time, the control unit 12 controls the cutoff frequency of the low-pass filter 44 after the optical low-pass filter 2d is retracted from the optical path to be lower than the cutoff frequency of the low-pass filter 44 before the optical low-pass filter 2d is retracted. Even if the AF evaluation value for determining the focus position by the automatic focus adjustment function is larger than a predetermined value, the control unit 12 cannot determine the focus position unless the maximum value can be detected. You may judge.

Next, the operation of the digital camera 1 configured as described above will be described according to the flow of FIG.
FIG. 3 is a diagram showing an operation flow of the digital camera for explaining the best mode for carrying out the present invention.
When the release button 8 is half-pressed, the digital camera 1 moves the photographing lens 2a, extracts and integrates the high-frequency component of the image signal obtained from the image sensor 2e, and the maximum AF evaluation value that is the integration result Is detected (S1). If the maximum AF evaluation value can be detected and the focus position of the photographic lens 2a can be determined (S2: Yes), the lens driver 9a performs automatic focus adjustment based on the determined focus position, and the release button Shooting is performed when 8 is pressed deeply (S10).

  On the other hand, if it is determined that the AF evaluation value is smaller than the predetermined value and the maximum value cannot be detected and the in-focus position of the photographic lens 2a cannot be determined (S2: No), subject brightness by the brightness detection unit 11 is determined. Whether the measured value is smaller than a predetermined value is checked (S3). If the measured value of the subject brightness is smaller than a predetermined value (S3: Yes), a warning message is displayed in the finder image of the image display unit 7 to notify the user that the OLPF 2d is to be retracted from the optical path. After that (S4), the OLPF driving section 9d is operated to retract the OLPF 2d from the optical path (S5). Then, the photographing lens 2a is moved again, high frequency components of the image signal obtained from the image sensor 2e are extracted and integrated, and the maximum value of the AF evaluation value as the integration result is detected (S6). If the maximum AF evaluation value can be detected and the in-focus position of the photographic lens 2a can be determined (S7: Yes), the cutoff frequency of the LPF 44 of the YC separation processing circuit and the OLPF 2d through the optical path as shown in FIG. (S8), a message to cancel the warning is displayed on the image display unit 7 (S9). Then, photographing is performed when the release button 8 is deeply pressed (S10).

  As described above, according to the present embodiment, once the maximum value of the AF evaluation value is detected by the control unit 12, the maximum value cannot be detected, the focal position cannot be determined, and the brightness is low. In the case where the OLPF 2d is retracted from the optical path and the amount of light incident on the image sensor 2e is increased, the AF evaluation value is detected again to determine the in-focus position. Even under low-luminance shooting conditions in which the maximum evaluation value cannot be detected and focus cannot be achieved, it is possible to capture a good color image in focus by performing automatic focus adjustment.

  Further, according to the present embodiment, even when the brightness is low, if the control unit 12 can detect the maximum AF evaluation value and determine the in-focus position, the OLPF 2d is retracted from the optical path and photographed. Therefore, it is possible to perform processing that flexibly corresponds to shooting conditions.

  Further, according to the present embodiment, a false signal is likely to be generated when the OLPF 2d is retracted from the optical path. However, when the OLPF 2d is retracted from the optical path, the cutoff frequency of the LPF 44 of the YC separation processing circuit is Since it is made lower than that before the retreat, the influence of the false signal can be made inconspicuous. A false image is conspicuous in a color image taken by retracting the OLPF 2d from the optical path, but a good color image can be obtained by making the cutoff frequency of the LPF 44 of the YC separation processing circuit low to make the false color inconspicuous. Can do.

  Further, according to the present embodiment, when the OLPF 2d is withdrawn from the optical path, the user is notified by a warning message, so that the user takes a picture while the OLPF 2d is withdrawn, It is possible to prevent an image partially covered with the OLPF 2d from being taken. Instead of displaying a warning message or displaying a warning message, the shutter may not be released while the OLPF 2d is moving.

The figure which shows schematic structure of the digital camera for demonstrating the best form for implementing this invention The figure which shows schematic structure of the YC isolation | separation processing circuit in the signal processing part of the digital camera for demonstrating the best form for implementing this invention The figure which shows the operation | movement flow of the digital camera for demonstrating the best form for implementing this invention The figure which shows the characteristic of the low-pass filter of the YC isolation | separation processing circuit in the signal processing part of the digital camera for demonstrating the best form for implementing this invention. The figure which shows the example of the evaluation value used for contrast system automatic focus adjustment, and a search range.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Digital camera 2 Imaging system 2a Shooting lens 2d Optical low-pass filter 2e Image pick-up element 4 Signal processing part 9 Imaging system drive part 9a Lens drive part 9d Optical low-pass filter drive part 11 Luminance detection part 12 Control part

Claims (3)

A digital camera having an automatic focus adjustment function for adjusting a focus position based on a high-frequency component of an image signal output from an image sensor,
Luminance detection means for detecting the luminance of the subject;
An optical low-pass filter moving means for selectively moving an optical low-pass filter for limiting a spatial frequency of incident light to the image pickup device to an optical path of the incident light and a position retracted from the optical path;
When the in-focus position cannot be determined by the automatic focus adjustment function and the luminance detection value by the luminance detection unit is smaller than a predetermined value, the optical low-pass filter is moved by the optical low-pass filter moving unit. A digital camera comprising control means for controlling the focus position to be adjusted again by the automatic focus adjustment function after being retracted from the optical path. The digital camera according to claim 1,
The control means is such that an evaluation value for determining the in-focus position is not more than a predetermined value, the in-focus position cannot be determined by the automatic focus adjustment function, and a brightness detection value by the brightness detection means is preset. If the optical low-pass filter is retracted from the optical path by the optical low-pass filter moving means, the focus position is adjusted again by the automatic focus adjustment function. Is a digital camera. The digital camera according to claim 1 or 2,
A low-pass filter having a variable cutoff frequency for attenuating a high-frequency component of a color difference signal obtained from the image signal;
The control means controls the cutoff frequency of the low-pass filter after the optical low-pass filter is retracted from the optical path to be lower than the cutoff frequency of the low-pass filter before the optical low-pass filter is retracted. Digital camera.

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