JP2002196220A - Electronic camera and automatic focusing control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electronic camera and an automatic focusing(AF) control method capable of realizing accurate focusing on a subject where high- luminance dot light sources are scattered such as a night scene. SOLUTION: The average brightness of the subject is detected by AE processing. When the brightness is darker than that of a specified value, AF searching operation is performed at an appropriate exposure level based on AE, and operation to detect a focusing position showing the peak of a focus evaluated value is performed, and further, when the focusing position is not detected by the AF searching operation, the AF searching operation is performed by reducing exposure than appropriate exposure, whereby the saturation of an image pickup signal by the dot light source is avoided. Thus, based on the result of the AF searching operation performed several times, the focusing position is more accurately detected and a lens is moved to the obtained focusing position.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic camera, and more particularly, to detecting high frequency components of an image based on an output signal from an image sensor, and performing autofocus (AF) control using the detection result. Regarding the technology to be performed.

[0002]

2. Description of the Related Art In an electronic camera such as a digital still camera or a video camera, an AF system which detects a high-frequency component of a photographed image and sets a focus position at which a peak of an integrated value of a high-frequency signal is a focus position as an actual subject Since the feedback control is performed based on the signal obtained from the above, focusing control can be performed with high accuracy. Prior arts related to the AF method include JP-A-2000-13666 and JP-A-7-287162.

[0003]

The AF method using the high-frequency component of an image cannot detect an edge component of a high-frequency component for a subject having no contrast in principle.
It has the property that it is difficult to focus. Also,
For subjects with high brightness point light sources such as night scenes,
There is a disadvantage that it is difficult to obtain accurate focusing. This is because, when the autofocus operation is performed at a normal appropriate exposure level, the image signal output of the point light source portion is saturated, and the high-frequency component does not change even if the position of the focus lens changes.

The present invention has been made in view of such circumstances, and applies an electronic camera and an AF control method that can accurately focus on an object that cannot be focused by the conventional AF method. The purpose is to do.

[0005]

In order to achieve the above object, an electronic camera according to the present invention comprises: a photographing lens; an image pickup means for converting an optical image incident through the photographing lens into an electric signal; Lens driving means for driving a photographing lens to perform focus adjustment; photometric means for detecting the brightness of a subject; and exposure control means for performing exposure control so as to obtain an appropriate exposure according to a photometric result of the photometric means. Calculating means for calculating a focus evaluation value based on a high-frequency component of an image signal obtained from the imaging means; and moving the lens by the lens driving means, based on the focus evaluation values obtained at a plurality of lens positions. A focus position detecting means for detecting a focus position; and an exposure control section for obtaining the proper exposure when the brightness of the subject detected by the photometric means is lower than a specified value. The detection operation by the in-focus position detection means is executed, and the detection operation by the in-focus position detection means is executed under the exposure control that makes the exposure amount lower than the appropriate exposure. Control means for obtaining a focus position based on the result, and controlling the lens driving means so as to move the taking lens to the obtained focus position.

According to the present invention, the average brightness of a subject is detected by photometric processing, and exposure control is performed. When the brightness of the subject is brighter than a predetermined value, an AF search operation is performed under an appropriate exposure level obtained by exposure control, and a focus evaluation value is calculated for each lens position. Then, the focus position indicating the peak of the focus evaluation value is detected, and the photographing lens is moved to the focus position.
On the other hand, if the brightness of the subject is lower than the specified value, it may not be possible to detect the focus position indicating the peak of the focus evaluation value when the AF search operation is performed under an appropriate exposure level. . Therefore, AF with appropriate exposure level
In addition to the search operation, an AF search operation with an exposure amount lower than the proper exposure is performed, and a more accurate focus position is detected based on the results of these multiple AF search operations. Thus, it is possible to correctly focus on a subject such as a night scene where high-luminance point light sources are scattered.

[0007] The subject image adjusted in focus by the AF control is taken in from the imaging means in response to an instruction to start photographing. The image signal output from the imaging unit is processed by a predetermined signal processing unit and then recorded on a recording medium by the recording unit.

[0008] According to another aspect of the present invention, the control means detects an operation of the in-focus position detecting means based on an image signal obtained from the imaging means under the exposure control for obtaining the proper exposure. After executing the above, it is determined whether or not the in-focus position has been detected by the detection operation.If the in-focus position cannot be detected, the exposure control is performed under an exposure amount that is lower than the appropriate exposure. The detection operation by the in-focus position detecting means is performed based on an image signal obtained from an imaging means.

Further, according to the auto focus control method of the present invention, the brightness of a subject is detected, and when the brightness is lower than a specified value, the lens is moved under exposure control for obtaining an appropriate exposure; A focus evaluation value is calculated based on a high-frequency component of an image signal obtained from the imaging unit at a plurality of lens positions, and an operation of detecting a focus position indicating a peak of the focus evaluation value is performed. The lens is moved under the exposure control that results in a low exposure amount, a focus evaluation value is calculated based on a high-frequency component of an image signal obtained from the imaging unit at a plurality of lens positions, and a peak of the focus evaluation value is calculated. An operation of detecting the indicated focus position is performed, a focus position is obtained based on a result of the plurality of detection operations, and the lens is moved to the obtained focus position.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of an electronic camera and an autofocus control method according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a block diagram of a digital camera 10 according to an embodiment of the present invention. The taking lens 12 is 1
A single focal length (fixed focal length) lens composed of one or more lenses may be used, or a variable focal length lens such as a zoom lens or a telephoto / wide-angle bifocal switchable lens may be used. The light passing through the photographing lens 12 is adjusted in its light amount by an aperture 14 and then adjusted by a CCD image sensor (hereinafter referred to as C).
It is called CD. ) 16. Photo sensors are arranged in a plane on the light receiving surface of the CCD 16.
The subject image formed on the light receiving surface of No. 6 is converted by each photosensor into signal charges of an amount corresponding to the amount of incident light. The signal charges thus accumulated are sequentially read out as voltage signals (image signals) corresponding to the signal charges based on the pulse given from the CCD driver 18.

The image signal output from the CCD 16 is sent to an analog processing section 20. The analog processing unit 20
The analog processing section 20 includes a sampling hold circuit, a color separation circuit, a gain adjustment circuit, and the like. The analog processing section 20 performs correlated double sampling (CDS) processing and color separation processing for each of R, G, and B color signals. Is adjusted (pre-white balance processing).

The signal output from the analog processing section 20 is converted into a digital signal by an A / D converter 22 and stored in a memory 24. The timing generator (TG) 26 includes a CCD driver 18, an analog processing unit 20, and an A / D converter 22 in accordance with a command from the CPU 30.
, And a timing signal is provided to synchronize the respective circuits.

The data stored in the memory 24 is transmitted to the bus 3
2 to the signal processing unit 34. Signal processing unit 34
Is a digital signal processor (D) including a luminance / color difference signal generation circuit, a gamma correction circuit, a sharpness correction circuit, a contrast correction circuit, a white balance correction circuit, and the like.
SP), which processes an image signal in accordance with a command from the CPU 30.

The image data input to the signal processing unit 34 includes a luminance signal (Y signal) and a color difference signal (Cr, Cb signal).
After being converted to a predetermined processing such as gamma correction, it is stored in the memory 24.

The image data stored in the memory 24 is C
It is read out according to the command of the PU 30 and is displayed on the display memory 36.
Sent to The data stored in the display memory 36 is
After being converted into a signal of a predetermined system for display (for example, a color composite video signal of the NTSC system), it is output to the display unit 40 via the D / A converter 38. As the display unit 40, a liquid crystal display, a TV monitor, or the like can be used.
Thus, the image content of the image data is output on the screen of the display unit 40 by the monitor.

The image data in the memory 24 is periodically rewritten by the image signal output from the CCD 16, and the image signal generated from the image data is supplied to the display unit 40, so that the image captured by the CCD 16 can be changed in real time. It is displayed on the display unit 40. The photographer uses the display unit 4
The photographing angle of view can be confirmed by an image projected on the image No. 0 or an optical finder (not shown).

Release switch 5 included in operation unit 50
By pressing the button 2, a recording start instruction signal is issued, and in response to the reception of the instruction signal, the capture of image data for recording is started. When the mode for compressing and recording the image data is selected, the CPU 30
0, the compression / expansion circuit 60
The image data on the memory 24 is compressed according to JPEG or another predetermined format. The compressed image data is recorded on the memory card 64 via the card interface 62. If a mode for recording uncompressed image data (non-compression mode) is selected, the compression / expansion circuit 60
, The image data is recorded on the memory card 64 without being compressed.

In the digital camera 10 of this embodiment, for example, smart media (So
lid-State Floppy Disk Card) is applied. The form of the recording medium is not limited to this, and may be a PC card, a compact flash (registered trademark), a magnetic disk, an optical disk, a magneto-optical disk, a memory stick, or the like.
Various media that can be read and written according to a method of electronic, magnetic, or optical, or a combination thereof can be used. Signal processing means and interfaces corresponding to the medium used are applied. A configuration may be adopted in which a plurality of media can be mounted irrespective of different types or the same type of recording media.

In the reproduction mode, the image data read from the memory card 64 is expanded by the compression / expansion circuit 60 and output to the display unit 40 via the display memory 36 and the D / A converter 38.

The CPU 30 is a control section for controlling the circuits of the camera system. The CPU 30 has a ROM 6
6 and a storage means such as a RAM 68. The ROM 66 stores programs to be processed by the CPU 30 and various data necessary for control. The RAM 68 is used as a work area when the CPU 30 performs various arithmetic processing and the like. .
The CPU 30 controls the operation of the corresponding circuit based on an input signal received from the operation unit 50 and
Control of display at 0, automatic focus (AF) control, automatic exposure (AE) control, and the like are performed.

The operation unit 50 is a block including various instruction input means such as a release switch 52, a zoom switch 54, a macro switch 56, and a mode dial 58. The operation unit 50 includes a push button switch, a dial,
The mode is not limited to a lever-type switch, a slide-type knob, and the like, and a mode in which a desired item is selected by a cursor, a pointer, a touch panel, or the like from a setting menu or a selection item displayed on the screen of the display unit 40 is also available. The operation unit 50 may be provided in the camera body, or may be configured such that part or all of the operation unit 50 is separated from the camera body as a remote control transmitter.

The CPU 30 performs various calculations such as a focus evaluation value calculation and an AE calculation based on an image signal output from the CCD 16, and controls a lens driving unit 70 including a focus motor based on the calculation results. The lens is moved to the in-focus position, and the aperture drive unit 72 including the iris motor is controlled to set the aperture to an appropriate aperture.
The charge accumulation time of D16 is controlled.

The signal output from the A / D converter 22 is
It is added to the AE operation unit 74 and the AF operation unit 76. AE
The arithmetic unit 74 integrates the R, G, and B signals, and provides the integrated value to the CPU 30. The CPU 30 includes an AE operation unit 74
The average brightness (subject brightness) of the subject is detected based on the integrated value input from
v value). When the release switch 52 is half-pressed, the above-described AE photometry operation is repeated a plurality of times to obtain an accurate exposure value. An aperture value and a shutter speed at the time of shooting are determined according to the obtained exposure value and a predetermined program chart. Then, when the release switch 52 is fully pressed, the aperture driving unit 72 and the electronic shutter are controlled to obtain an optimal exposure amount.

As shown in FIG. 2, the AF calculation section 76 includes a high-pass filter 80, an absolute value processing section 82, an AF area extraction section 84, and an accumulation section 86. The high-pass filter 80 allows only the high-frequency component of the G signal output from the A / D converter 22 to pass. Absolute value processing unit 82
Takes the absolute value of the output signal from the high-pass filter 80, and the AF area extraction unit 84 sets the focus target area (A) set in advance in the screen (for example, the center of the screen).
It is called F area. The process of cutting out the signal in parentheses) is performed. The integrating unit 86 integrates the absolute value data in the AF area, and provides the obtained integrated value (corresponding to the focus evaluation value) to the CPU 30.

The CPU 30 performs an AF calculation at a plurality of AF detection points while moving the photographing lens 12, and integrates the focus evaluation values calculated at each AF detection point to obtain
The position at which the focus evaluation value becomes maximum is determined as the focus position. Then, the lens driving unit 70 is controlled so as to move the taking lens 12 to the obtained focusing position.

Next, the AE and AF operations of the digital camera 10 configured as described above will be described. FIG.
5 is a flowchart illustrating a procedure of AE and AF control.
When the release switch 52 is half-pressed and the process starts (step S300), the CCD 16 is driven to capture an image signal for photometry (step S31).
0). The input image signal is subjected to arithmetic processing in the AE arithmetic unit 74 (step S312), and the brightness of the subject is detected.

The CPU 30 determines whether or not the brightness of the subject detected by the AE operation is darker than a predetermined value (step S314). If the subject is brighter than the specified value (NO determination), step S3
Proceed to 16. In step S316, step S312
Exposure control is performed according to the exposure value obtained by the AE calculation so that an appropriate exposure amount is obtained. Then, an AF calculation operation is performed under proper exposure control (step S318),
A focus evaluation value is calculated at a plurality of AF detection points. AF
As a result of the calculation operation, it is determined whether or not the in-focus position at which the focus evaluation value is maximum has been detected (step S3).
20).

For example, when a subject having no contrast, such as a white wall, is photographed, the in-focus position cannot be detected, and the determination in step S320 is NO, and the determination in step S320 is NO.
Proceed to 322. In step S322, the lens is moved to a predetermined focus position (for example, a focus position at which a subject at a distance of about 2 m is focused) as a countermeasure when a peak value of the focus evaluation value cannot be detected.

On the other hand, if the peak value of the focus evaluation value can be detected in step S320 (when YES is determined), the flow advances to step S332 to move the lens to a focus position corresponding to the peak position. Step S32
2, or when the lens position is determined in step S332, the present control is terminated (step S340), and input of a full-press operation of the release switch 52 is waited. Then, in response to the input of the full-press operation of the release switch 52, the image for recording (capturing) is executed.
The obtained image is recorded on the memory card 64.

If it is determined in step S314 that the subject is darker than the specified value (when YES is determined), step S32 is executed.
Proceed to 6. In step S326, exposure control is performed according to the exposure value obtained by the AE calculation in step S312 so that an appropriate exposure amount is obtained. Then, an AF operation is performed under proper exposure control (step S328), and a focus evaluation value is calculated at a plurality of AF detection points.

As a result of the AF operation, it is determined whether or not the in-focus position at which the focus evaluation value is maximum has been detected (step S330). In this determination, if the in-focus position cannot be detected (NO determination), step S
Branch to 334. In step S334, exposure control is performed to further reduce the exposure amount as compared with the case of proper exposure (step S334). Thereafter, the process returns to step S326, where A
Execute the F operation. By performing AF reading with the exposure level lowered, saturation of an image such as a point light source scattered in a night scene can be avoided, and high-frequency components of an image signal can be detected.

If the in-focus position can be detected in step S330 (when the determination is YES), the process proceeds to step S332,
The lens is moved to the obtained in-focus position, and the AF control ends (step S340).

According to the AF control method according to the present embodiment,
Focusing can also be performed on a subject such as a night view, for which focusing was conventionally difficult.

In the above embodiment, when calculating the focus evaluation value, the high frequency component was extracted from the G signal of the image signal.
A mode in which a high-frequency component is detected from a luminance signal (Y signal) obtained after the C processing is also possible.

A modification of the above-described embodiment will be described. When a variable focal length lens such as a zoom lens is applied as the photographing lens 12, the depth of field varies depending on the focal length. It is preferable to change the number of points. For example, the number of AF detection points is set to be larger as the depth of field becomes smaller, such as 7 to 8 points on the wide side and 16 points on the tele side. In addition, the AF search range and AF in the normal shooting mode and the macro (short distance) shooting mode are set.
A mode in which the number of detection points is changed is also preferable. For example, in the normal shooting mode, a subject within a range of about 50 cm to infinity can be focused, and in the macro shooting mode, a close-up shooting of about 9 cm to 50 cm can be performed. In this case, when the macro shooting mode is selected by the macro switch 56, the AF search range is shifted to the short distance side, and the AF detection point is set to 20 points or more.

FIG. 4 is a schematic diagram showing a variation of the AF search operation. In the method shown in FIG. 7A, the first AF search operation is performed at an appropriate exposure level according to the result of AE. At this time, while moving the lens along the AF search range in a fixed direction from the closest side to the infinity side,
AF calculation is performed at a plurality of AF detection points. as a result,
If the in-focus position cannot be detected, the exposure level is lowered and the AF search operation is performed again. Even during the second AF search operation, the AF search range moves from the closest side to the infinity side. The AF calculation is performed at a plurality of AF detection points by moving the lens along a certain direction. Before starting the second AF search operation, the lens
It is necessary to return to the search starting point. Note that the lens moving direction in the AF search operation may be a direction moving from the infinity side to the closest side.

The method shown in FIG. 4B is the second AF
The search operation is performed by moving the lens from the infinity side to the close side, and the return operation of the lens is omitted as compared with the method shown in FIG. Can be.

The method shown in FIG. 4C uses the AF calculation based on the first exposure level and the second AF detection at one AF detection point.
And the AF calculation based on the exposure level. At each AF detection point, AF reading is performed a plurality of times (for example, twice) while changing the exposure level, and then the lens is moved to the next AF detection point. According to this method, it is possible to acquire a focus evaluation curve with different exposure levels by moving the AF search range only once.

The applicable range of the present invention is not limited to a digital camera for recording a still image, but can be applied to a digital camera or a video camera capable of recording a moving image.

[0041]

As described above, according to the present invention, in addition to performing a normal AF search operation with an appropriate exposure level at low illuminance where there is a high probability that a high-luminance point light source is present, Since the AF search operation is performed with the exposure amount lower than the exposure, the saturation of the imaging signal by the point light source is avoided, and even the subject which cannot be focused by the conventional AF method is accurately focused. Can be.

[Brief description of the drawings]

FIG. 1 is a block diagram of a digital camera according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of an AF calculation unit shown in FIG.

FIG. 3 is a flowchart showing a procedure of AE and AF control in the digital camera of the present embodiment.

FIG. 4 is a schematic view showing a variation of an AF search operation.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Digital camera (electronic camera), 12 ... Photographing lens, 16 ... CCD (imaging means, photometric means), 30 ... CP
U (exposure control means, focus position detection means, control means), 7
0: lens drive unit (lens drive unit), 74: AE calculation unit (exposure control unit), 76: AF calculation unit (calculation unit)

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 5/232 G02B 7/11 D // H04N 101: 00 KG03B 3/00 A (72) Inventor Fuji Shigeo Tsukasa 3--11-46 Izumi, Asaka-shi, Saitama Prefecture Fuji Photo Shashin Film Co., Ltd. 2H002 DB02 DB06 DB19 JA07 2H011 BA31 DA01 2H051 BA47 BA68 CE14 DA22 DA30 DC10 EA09 EA11 EB03 2H054 AA01 5C022 AA11 AB04 AB22 AB66 AC32 AC42 AC69 AC74

Claims (3)

[Claims] An imaging unit configured to convert an optical image incident through the imaging lens into an electric signal; a lens driving unit configured to drive the imaging lens to perform focus adjustment; Photometric means for detecting, an exposure control means for performing exposure control so as to obtain an appropriate exposure according to a photometric result of the photometric means, and a focus evaluation value is calculated based on a high-frequency component of an image signal obtained from the imaging means. Calculating means for moving the lens by the lens driving means, and a focus position detecting means for detecting a focus position based on the focus evaluation values acquired at a plurality of lens positions; When the brightness of the subject is lower than a specified value, the detection operation by the in-focus position detecting means is executed under the exposure control for obtaining the appropriate exposure, and the Run the detection operation by the focus position detecting means under a low exposure amount become exposure control than exposure,
The in-focus position is obtained based on the results of these multiple detection operations,
An electronic camera comprising: a control unit that controls the lens driving unit so as to move the photographing lens to the obtained focus position. 2. The control unit executes a detection operation by the in-focus position detection unit based on an image signal obtained from the imaging unit under the exposure control that obtains the proper exposure, and then performs the detection operation. It is determined whether or not the in-focus position has been detected, and if the in-focus position has not been detected, the image signal obtained from the imaging unit under the exposure control under which the exposure amount becomes lower than the appropriate exposure is determined. The electronic camera according to claim 1, wherein a detection operation is performed by the focus position detection unit based on the detection result. 3. When the brightness of a subject is detected, and when the brightness is lower than a predetermined value, the lens is moved under exposure control for obtaining an appropriate exposure, and a plurality of lens positions are obtained from the imaging means. A focus evaluation value is calculated based on a high frequency component of the image signal to be detected, and an operation of detecting a focus position indicating a peak of the focus evaluation value is performed, and under the exposure control that the exposure amount becomes lower than the appropriate exposure. The lens is moved at a plurality of lens positions, a focus evaluation value is calculated based on a high-frequency component of an image signal obtained from the imaging unit, and an operation of detecting a focus position indicating a peak of the focus evaluation value is performed. An autofocus control method comprising: obtaining an in-focus position based on the results of the plurality of detection operations; and moving the lens to the obtained in-focus position.