JP2004038114A - Auto-focus camera - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform auto-focus control by deriving out-of-focus from deviation of a subject image which is brought about by moving a diaphragm. <P>SOLUTION: An aperture diaphragm 11 is provided with aperture blades 16 and 17. Spur gear parts 16a and 17a are formed in lower parts of aperture blades 16 and 17 and are engaged with small gears 20 and 21 provided on driving shafts of iris motors 18 and 19 and are freely moved in the horizontal direction. Aperture diameter control and shift movement of the aperture diaphragm 11 are possible. When the aperture diaphragm 11 is shifted between two points symmetrical with respect to an optical axis, a subject image formed on a CCD imaging device 12 is deviated. An extent of this deviation is derived by image analysis, and an extent of out-of-focus is calculated by the extent of image deviation. A moving direction and an extent of movement of a focus lens 10 are calculated in accordance with the extent of out-of-focus, and the focus lens 10 is driven up to an in-focus position. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an autofocus camera that determines an in-focus state using an output signal from an image sensor.
[0002]
[Prior art]
As an autofocus camera that automatically performs focusing, an active rangefinder that projects infrared rays and a passive rangefinder that uses subject light are known. In either case, the focus position of the focus lens is specified from the subject distance determined by triangulation, and focusing is performed. In recent years, digital still cameras and video cameras that incorporate a CCD image sensor, evaluate the contrast value of the captured subject image for each frame, and perform autofocus control to obtain the sharpest contrast are also widely known. ing.
[0003]
Focusing based on the contrast of the image has an advantage that the increase in cost can be suppressed as compared with a built-in distance meter because the imaging device itself is used as a detection device. On the other hand, when the photographed image is a low-contrast image with a narrow brightness distribution, it takes time to evaluate the contrast of the image, which is also referred to as hill-climbing control, and has a drawback of missing a photo opportunity.
[0004]
Therefore, Japanese Patent Application Laid-Open No. 2001-281530 describes a digital still camera that uses both a focusing method using a contrast method and a focusing method using a phase difference detection method. In the phase difference detection method, it is possible to detect the phase difference from the photoelectric signal of the subject light captured on the optical path symmetric with respect to the optical axis, and to instantly determine the defocus amount to specify the in-focus position. As a result, in the digital still camera, after performing rough focusing by phase difference detection, fine focusing can be performed by contrast evaluation, and the focus speed is improved without reducing accuracy.
[0005]
[Problems to be solved by the invention]
However, components that use the above-described conventional phase difference detection method often use components that require high mounting accuracy, such as a line sensor that detects a shift of subject light and an optical system in the vicinity thereof. In addition, a sensor group drive circuit, a signal processing circuit, and the like provided separately from the image pickup device are required, resulting in a large increase in manufacturing cost.
[0006]
The present invention has been made in consideration of the above problems, and an object of the present invention is to provide an autofocus camera that can detect the size of a focus shift using an image sensor and can reduce the cost. .
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the autofocus device of the present invention moves the aperture stop arranged on the subject optical path between two distance measuring positions in a plane perpendicular to the optical axis, and at the time of out of focus. The image sensor detects a shift of the subject image caused by the diaphragm parallax. By comparing the distance measurement images captured at each distance measurement position, the size of the image shift calculated is converted into the focus shift size, and the focus position of the focus lens is specified. Simultaneously with the determination of the in-focus position, the amount of movement of the focus lens to the in-focus position is calculated, and autofocus control is performed.
[0008]
According to the second aspect of the present invention, it is possible to detect image shift even when the subject brightness is low. In order to detect a shift of the subject image caused by parallax of the aperture movement in an out-of-focus state, it is necessary to narrow down the subject light as much as possible and increase the depth of field to obtain a clear image. Therefore, the luminance of the image is corrected so that the positional deviation detection and photographing can be performed while maintaining a high F value.
[0009]
The invention according to claim 3 uses focus control by a contrast method in combination. When out-of-focus, which cannot be detected due to the opening of the diaphragm, is switched from focus control based on image position detection to contrast-based focus control.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1, a digital camera 1 incorporating the present invention is provided with a main control unit 2, an imaging data processing unit 3, a display control unit 4, a recording control unit 5, and an operation unit 6. The main control unit 2 outputs a control signal to each unit while performing arithmetic processing, and manages the operation sequence of the digital camera 1. The imaging data processing unit 3 performs various processes on the analog imaging data output from the imaging unit 7. The display control unit 4 converts the imaging data and recorded data output from the imaging data processing unit 3 into display data, and causes a liquid crystal monitor (not shown) to display and output a captured image. The recording control unit 5 performs control for recording a captured moving image or still image on a video tape or a memory card. The operation unit 6 outputs an operation signal generated by an operation of a recording button, a still image shooting button or the like (not shown) to the main control unit 2.
[0011]
The imaging unit 7 includes a zoom lens 9, a focus lens 10, an aperture stop 11, and a CCD imaging device 12. The zoom lens 9 is driven in the optical axis direction by a zoom lens driving unit 13 having a zoom motor, and optically zooms the subject image formed on the CCD image sensor 12. The focus lens 10 is driven in the optical axis direction by a focus lens driving unit 14 having a focus motor, and forms an image of subject light on the CCD image sensor 9. The iris driving unit 15 includes an iris motor, and adjusts the aperture diameter of the aperture stop 11 and shift-drives the aperture stop 11.
[0012]
In FIG. 2, the aperture stop 11 is composed of two stop blades 16 and 17. Flat teeth 16a and 17a are formed below the diaphragm blades 16 and 17, respectively. The diaphragm blades 16 and 17 are driven by iris motors 18 and 19. On the drive shafts of the iris motors 18 and 19, small gears 20 and 21 that mesh with the spur tooth portions 16a and 17a are provided. By driving the rack and pinion, the diaphragm blades 16 and 17 move freely in the left and right directions, and the aperture diameter of the aperture diaphragm 11 and the shift movement of the aperture diaphragm 11 are performed.
[0013]
In FIG. 3, the aperture stop 11 shift-drives between two distance measuring positions P1 and P2 that are symmetric with respect to the optical axis. As shown in FIG. 3 (a), when the focus lens 10 is in the position for focusing light from the object point O1 on the imaging plane P _I, consider when the aperture diaphragm 11 is moved to the position P1. Principal ray R1 from the object point O1 is ideally reaches the imaging plane P _I at point S1 of the optical axis A1. Principal rays R2 object point O2 in the distance from the object point O1 intersects with the optical axis A1 in front of the image plane P _I, reaches the point S2 of the imaging plane P _I. The main ray R3 object point O3 near than the object point O1, without intersecting the optical axis A1, reaches the point S3 in the imaging plane P _I. When the aperture stop 11 is moved to the position P2, as shown in FIG. 3 (b), the principal rays R1, R2, R3 S1 is on the imaging plane _{P I,} S2 ', S3' reach.
[0014]
4 (a), the light from the object point O1 is imaged on the imaging surface P _I, illustrates this as focus. In FIG. 4 (b), shows how the light from the object point O2 is imaged before the imaging plane P _I as a front focus state. The image shift G2 in the front pin state is equal to the distance between the intersection S2 and the intersection S2 ′. Further, as shown as a rear focus state in FIG. 4 (c), the light from the object point O3 is imaged behind the imaging plane P _I. The image shift G2 when it is in front focus state positive, the image shift G3 when in the rear focus state by leaving predetermined negative deviation amount, the distance from the imaging plane P _I to the imaging surface of the front positive distance to the imaging plane at the back of the imaging surface P _I is a negative distance, seen the movement direction of the focus lens 10.
[0015]
If the distance D from the aperture stop 11 to the imaging surface P _I is smaller enough than the object distance, the triangle can be approximated to similar relationship when connecting the opening center of the aperture stop 11, and a principal ray on the imaging surface P _I Is formed. And the shift amount of movement M of the reciprocating of the aperture stop 11, because the similarity ratio and an image shift amount G is calculated, the distance to the imaging plane is calculated from the image plane P _I by the the similarity ratio and the distance D. If the distance to the imaging surface P _I and the image plane are determined, it is possible to determine the movement amount of the focus lens 10.
[0016]
In FIG. 5, the imaging data processing unit 3 includes a luminance correction circuit 25, an image processing circuit 26, a focus evaluation circuit 27, and an imaging data output circuit 28. The brightness correction circuit 25 is provided with a gain controller 31 and a pixel mixture processing circuit 32.
[0017]
The gain controller 31 obtains the average level of the luminance signal from the analog imaging data output from the CCD imaging device 12 and measures the subject luminance. The gain controller 31 adjusts the gain for each imaging frame so that the average level of the luminance signal after amplification is always a constant level, and normal processing for amplifying the luminance signal to the standard level and up to a quarter of the standard level. Switching is controlled by special processing to be amplified.
[0018]
The pixel mixing circuit 32 receives imaging data amplified by the gain controller 31 to a level that is a quarter of the standard level. In the pixel mixing circuit 37, the luminance signals of four square pixels adjacent in the image are added and replaced with the luminance signal of one pixel. The imaging data input to the pixel mixing circuit 32 has a number of pixels that is a quarter of that at the time of imaging, and the contrast is improved four times.
[0019]
The image processing circuit 26 is provided with an image quality correction circuit 33 that performs white balance adjustment and γ adjustment on the luminance-corrected imaging data, and an A / D converter 34 that digitally converts the analog imaging data. The digitally converted image data is output to the focus evaluation circuit 27 and the image data output circuit 28, respectively.
[0020]
The focus evaluation circuit 27 includes an image comparison circuit 35, a focus calculation circuit 36, and a contrast detection circuit 37. The image comparison circuit 35 compares the standard imaging data and the reference imaging data captured after that for each frame, and calculates the size of the image shift between the two in units of pixels.
[0021]
The focus calculation circuit 36 obtains the image shift in the left-right direction due to the movement of the diaphragm from the size of the image shift determined by the image comparison circuit 35, and converts this to the size of the focus shift. Further, the distance between the current position of the focus lens 10 and the in-focus position is determined from the magnitude of this focus shift, and the drive amount of the focus motor required to move the focus lens 10 to the in-focus position is calculated. The contrast calculation circuit 37 analyzes the contrast distribution of the image from the input image data and calculates a contrast evaluation value.
[0022]
The imaging data output circuit 28 is provided with an image blur correction circuit 38. The image blur correction circuit 38 performs an image blur determination process based on the magnitude of the image shift determined by the image comparison circuit 35, and a two-dimensional image shift due to shift of the aperture stop 11 and a two-dimensional subject image due to camera shake. Is corrected, and the position of the image is corrected. In the image comparison circuit 35, an image shift between a plurality of temporally continuous frames is determined, and the image shift determined between the frames is subjected to image blur determination. Image blur correction is performed.
[0023]
Next, the operation of the present invention will be described with reference to FIG. When driving of the CCD image pickup device 12 is started, an exposure control signal is sent from the main control unit 2 to the iris drive unit 15 to drive the diaphragm blades 16 and 17, respectively, and the aperture stop 11 is set to the aperture diameter for distance measurement. Is done. Here, the subject controller brightness is measured by the gain controller 31, and distance measurement is started when a proper exposure is obtained. When the subject brightness is high, the aperture diameter of the aperture stop 11 is further reduced. On the other hand, when the subject brightness is low, the aperture stop 11 is not driven and exposure correction is performed according to the set state.
[0024]
When luminance correction by gain adjustment is set, the luminance signal of the imaging data input to the gain controller 31 is automatically amplified to a standard level. When luminance correction by pixel mixing is set, the luminance signal of the imaged data is automatically amplified to the quarter level of the standard level by the gain controller 31, and then luminance addition processing is performed in the pixel mixing circuit 32. Become standard level. When the exposure control by opening the aperture is set, the aperture stop 11 is driven to open, and the aperture diameter becomes larger than the aperture diameter for distance measurement.
[0025]
When a proper exposure is obtained, or when brightness correction by gain correction or pixel mixture is set, ranging operation is started. A distance measurement start signal is output from the main control unit 2 to the iris driving unit 15, and the iris driving unit 15 drives the iris motors 18 and 19 to drive the diaphragm blades 16 and 17 in the same direction. The aperture stop 11 first starts a right shift and moves to the ranging position P1. The image comparison circuit 35 determines the deviation of the subject image that occurs during the movement of the aperture stop 11 and sends it to the image blur correction circuit 38. The imaging data output circuit 28 outputs the imaging data subjected to the image blur correction to the display control unit 4 and the recording control unit 5, respectively. When the aperture stop 11 reaches the distance measurement position P1, the main controller 2 sends a distance measurement image capture signal to the image quality correction circuit 26, and the captured image data is stored in the focus evaluation circuit 27 as the first distance measurement image.
[0026]
When the capture of the first distance measurement image is completed, the main control unit 2 sends a shift switching signal to the iris driving unit 15. The iris driving unit 15 switches driving of the iris motors 18 and 19 to start the left shift of the aperture stop 11. While the aperture stop 11 is moving, image blur correction is performed as in the right shift. When the aperture stop 11 reaches the distance measurement position P 2, the main control unit 2 outputs a distance measurement image capturing signal, and the focus evaluation circuit 27 captures the distance measurement image. When the second range-finding image is captured, the aperture stop 11 returns to the original position centered on the optical axis A1.
[0027]
The image comparison circuit 35 compares the distance measurement images captured when the aperture stop 11 is right-shifted and left-shifted, and determines the size of the image misregistration. Position shift information of the distance measurement image is sent from the image comparison circuit 35 to the focus calculation circuit 36. The focus calculation circuit 36 calculates the size of the focus shift from the position shift size of the ranging image and the movement amount of the aperture stop 11. Further, the distance required to move the focus lens 10 to the in-focus position is determined from the size of the focus shift. The focus calculation circuit 36 sends lens drive amount information specifying the moving direction and moving distance of the focus lens 10 to the main control unit 2.
[0028]
The main control unit 2 sends a focus control signal to the focus lens driving unit 14 based on the lens driving amount information. The focus lens driving unit 14 drives the focus motor according to the moving direction and moving distance of the focus lens 10 determined by the focus calculation circuit 36. The focus lens 10 moves to the in-focus position along the optical axis direction, and the imaged subject light is imaged on the CCD image sensor 12.
[0029]
Next, when the subject has low brightness and the aperture is set to open, the main controller 2 outputs an aperture open signal to the iris drive unit 15. The diaphragm blades 16 and 17 are driven to open according to the subject brightness, and the focus lens 10 is adjusted and driven in the optical axis direction. The contrast calculation circuit 35 analyzes the contrast change of the image accompanying the adjustment driving of the focus lens 10 and calculates a contrast evaluation value obtained by taking the difference between the maximum luminance level and the minimum luminance level of the measurement symmetrical pixel. The main control unit 2 stops driving the focus lens 10 at a position where the contrast evaluation value is maximized, and completes focusing.
[0030]
In the above-described embodiment, the aperture stop 11 is configured to focus at the center of the shooting range by shifting the aperture stop between two points that are symmetric with respect to the optical axis. However, in the present invention, the focus spot can be freely set. It is possible to change. For example, when the aperture stop is reciprocated in the left and right directions, if the shift movement amount in one direction is increased and the shift movement amount in the other direction is decreased, the focus spot can be shifted to the left and right from the center of the shooting range. .
[0031]
In addition to a mechanism for reciprocating the aperture stop to the left and right, a mechanism for reciprocating the aperture stop up and down can be provided to provide a multi-area autofocus function that can set the focus spot at any position in the shooting range. . As shown in FIG. 7, the aperture stop 41 and the iris motors 42 and 43 are held by the stop drive frame 44. A spur tooth portion 44 a is formed on the side surface of the diaphragm drive frame 44 and meshes with a small gear 46 provided on the drive shaft of the motor 45. A left-right shift and a vertical shift of the aperture stop 41 are independently performed, and the focus spot can be switched to an arbitrary position on the imaging surface, that is, an arbitrary position in the imaging range.
[0032]
【The invention's effect】
As described above, according to the present invention, it is possible to determine the focus shift based on the shift of the subject image caused by the movement of the aperture stop when out of focus, and to perform autofocus control using only the image sensor as the detection element. be able to. For this reason, it is not necessary to separately provide a high-performance sensor or the like, and an increase in cost can be suppressed.
[0033]
In addition, by performing luminance correction when the exposure is insufficient, the aperture diameter can be kept small in order to detect a shift in the subject image, and there is no problem in detecting the focus shift. In addition, contrast-type focus control can be used in combination, and the aperture can be opened when the exposure is insufficient to support focusing in various shooting situations.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an electrical configuration of a digital camera incorporating the present invention.
FIG. 2 is a perspective view of an aperture stop.
FIG. 3 is an explanatory diagram illustrating image misalignment caused by shift movement of an aperture stop.
FIG. 4 is an explanatory diagram illustrating image misalignment that occurs in each focus state.
FIG. 5 is a circuit block diagram of an imaging signal processing unit.
FIG. 6 is a flowchart showing a flow of operation of the digital camera.
FIG. 7 is a perspective view showing another embodiment of a diaphragm moving mechanism.
[Explanation of symbols]
3 Imaging signal processing unit 7 Imaging unit 10 Focus lens 11 Aperture stop 12 CCD imaging device 14 Focus lens driving unit 15 Iris driving unit 16, 17 Aperture blades 16 a, 17 a Spiral teeth 18, 19 Iris motor 20, 21 Small gear 25 Brightness Correction circuit 27 Focus evaluation circuit 35 Image comparison circuit 36 Focus calculation circuit 37 Contrast calculation circuit

Claims (3)

Imaging means for imaging subject light and outputting imaging data;
A focus lens that moves on the lens optical axis and forms an image of the subject light on the imaging surface at a focus position corresponding to the subject distance;
An aperture stop that adjusts the amount of subject light reaching the imaging surface;
A diaphragm moving means for moving the aperture diaphragm between at least two distance measuring positions on a plane perpendicular to the lens optical axis;
Comparison operation means for acquiring the respective distance measurement images by capturing the imaging data when the aperture stop is moved to each distance measurement position, and comparing the distance measurement images to determine the size of the image position shift;
A focus calculation unit that calculates the size of the focus shift from the size of the position shift of the image, specifies a focus position, and calculates a lens driving amount;
An autofocus camera, comprising: lens driving means for driving the focus lens to a focus position in accordance with a lens driving amount. The aperture stop is configured to be able to switch an aperture diameter, and when the aperture moving unit is operated, the aperture stop is prohibited from being opened and the brightness level of the imaging data is corrected. Item 1. The autofocus camera according to item 1. The aperture moving means is prohibited to operate, the aperture stop is opened, the contrast lens is driven while the focus lens is driven, and the focus lens is driven toward the in-focus position while referring to the contrast change. The autofocus camera according to claim 1.

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