Patent Application
20120268613
1. Field of the Invention
The present invention relates to image capturing apparatuses and control methods thereof, and more particularly relates to image capturing apparatuses equipped with a shake correction function and configured to perform focus control by a phase difference detection method based on an image signal obtained from an imaging device and control methods thereof.
2. Description of the Related Art
Recently live-view photography in which a photograph is taken while viewing a subject image formed on imaging pixels in real-time at the time of capturing still or moving images is becoming popular.
For live-view photography, autofocus control by a contrast method has been widely used in which a subject image is photoelectrically converted at the imaging pixels, and a focus state of the subject image is determined by detecting a contrast change of the subject image with reference to a focus change of a photographing optical system. In the contrast method, however, since a focus state is detected while performing focus control processing, a focus lens cannot be driven quickly.
To cope with this, an image capturing apparatus capable of performing focus control by a phase difference detection method is proposed, in which a plurality of photoelectric conversion devices are formed in each pixel of an imaging device, and a signal is read out from each photoelectric conversion device to obtain a pair of image data pieces having a phase difference therebetween (see Japanese Patent Laid-Open No. 2004-191629, for example). Such an image capturing apparatus can achieve quick focus lens driving.
In such focus detection processing by the phase difference detection method, concurrently with focus detection, an image received by the imaging device can be live-view displayed in real time or moving images can be photographed. Meanwhile, for image recording in real time, an aperture value of the photographing optical system has to be changed with the brightness of a subject image. For this reason, the diaphragm cannot be always held in a full-open state for focus detection.
Then, the following problem occurs. As the effective F-number of the photographing optical system is increased (darkened) by a stop-down operation during photographing, the exit pupil range of the photographing optical system is made smaller. As a result, vignetting occurs in some cases in light captured for focus detection. Then electric signals (hereinafter called “image signals”) captured at focus detection pixels varies, and accordingly a relationship of the phase shift variation to the focus shift variation (base-line length) varies and an error occurs in correlation of image signals. Additionally, when focus detection is performed using image signals from focus detection pixels placed at an image height at a periphery of the image area of the photographing optical system having a tendency of decreased aperture efficiency, photo vignetting occurs.
To cope with these problems, Japanese Patent Laid-Open No. 2004-191629, for example, proposes a method of acquiring photo vignetting information on the photographing optical system during focus detection and correcting image signals for focus detection.
Meanwhile, along with the recent tendency to seek more compact image capturing apparatuses and optical systems of higher power, shake of the image capturing apparatuses are becoming a major cause of degradation of the quality of captured images. Focusing on this, various shake correction functions have been proposed to correct the blur of a captured image due to the shake of the apparatus. As one example of conventional shake correction functions incorporated in image capturing apparatuses, a correction method using both of an optical image stabilization method and an electronic image stabilization method is available (see Japanese Patent No. 2803072, for example).
Firstly the optical image stabilization functions to detect shake of the image capturing apparatus and drive an optical system for image stabilization to cancel the detected shake so that subject light incident on the imaging device is located always at the same position on the imaging area. Next the electronic image stabilization functions to find blur between images so as to detect the remaining shake that cannot be corrected with the optical image stabilization method and move a reading area of the image so as to cancel the found blur between images, thus correcting the remaining shake of low frequencies.
In the optical image stabilization method, however, a correction lens group of the optical system is decentered, and such a decentered correction lens group further changes photo vignetting and adversely affects the focus detection accuracy.
The present invention has been made in consideration of the above situation, and the present invention provides an image capturing apparatus enabling high focus detection accuracy even when the image capturing apparatus uses a photographing optical system provided with the aforementioned image blur correction mechanism.
According to the present invention, provided is an image capturing apparatus, comprising: an imaging device configured to receive light fluxes passing through different areas of an exit pupil of an optical system independently and output an image signal obtained therefrom; a calculation unit configured to calculate a correction value for the image signal corresponding to each of the different areas of the exit pupil to correct an influence from vignetting on the light fluxes due to shift of a shake correction unit in response to activation of the shake correction unit which corrects an image blur corresponding to a shake amount exerted on the image capturing apparatus; a correction unit configured to correct the image signal corresponding to each of the different areas of the exit pupil using the correction value calculated by the calculation unit; and a focus control unit configured to perform focus control on a basis of a phase difference between the image signals corresponding to the different areas of the exit pupil, the image signals subjected to correction by the correction unit.
According to the present invention, further provided is a method for controlling an image capturing apparatus, comprising: a reading step of, from an imaging device configured to receive light fluxes passing through different areas of an exit pupil of an optical system independently and output an image signal obtained therefrom, reading out the image signal corresponding to each the different areas of the exit pupil; a calculation step of calculating a correction value for the image signal corresponding to each of the different areas of the exit pupil read in the reading step to correct an influence from vignetting on the light fluxes due to shift of a shake correction unit in response to activation of the shake correction unit which corrects an image blur corresponding to a shake amount exerted on the image capturing apparatus; a correction step of correcting the image signal corresponding to each of the different areas of the exit pupil using the correction value calculated in the calculation step; and a focus control step of performing focus control on a basis of a phase difference between the image signals of corresponding to different areas of the exit pupil, the image signals subjected to correction in the correction step.
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the description, serve to explain the principles of the invention.
Exemplary embodiments of the present invention will be described in detail in accordance with the accompanying drawings.
An optical lowpass filter 105 is an optical device to alleviate a false color and moiré of a captured image. Reference numeral 106 denotes an imaging device including a two-dimensional CMOS sensor and a peripheral circuit thereof. The imaging device 106 may be a two-dimensional single-panel color sensor with an on-chip primary-colored mosaic filter in Bayer layout formed thereon including photo-receiving pixels that are square arranged M pixels by N pixels respectively in the horizontal and vertical directions, for example. The structure of pixels making up of the imaging device 106 is described later in detail, with reference to
A zoom actuator 111 rotary-moves a tubular cam not illustrated manually or with an actuator, thus driving the first lens group 101 to the third lens group 104 in the optical axis direction for zoom operation. More specifically, the first lens group 101 to the third lens group 104 are driven so as to change intervals therebetween and change a focal length to implement a zoom function. A diaphragm actuator 112 controls the aperture diameter of the shutter 102 doubling as the diaphragm to control the photography light amount and controls an exposure time during still-image photography. A correction lens actuator 113 eccentric-shifts the second lens group 103, for example, in the direction orthogonal to the optical axis to correct image blur of a subject image formed at the imaging device 106. Normally the correction lens actuator 113 shifts the second lens group 103 biaxially at right angles to deal with a change of the image blur direction with reference to the imaging device 106 on the basis of the synthesized shift amount (eccentricity amount) and such a shift direction. A focus actuator 114 drives the third lens group 104 back and forth in the optical axis direction for focus control.
A wireless communication unit 115 includes an antenna and a signal processing circuit to communicate with a server computer via a network such as the Internet. A shake detection sensor 116 includes an angular velocity sensor such as a gyro sensor, for example, that detects shake exerted to the apparatus by shake of a hand or a body, for example, as a shake signal and outputs the shake signal.
Reference numeral 121 denotes a CPU in the camera for various controlling of the camera main body, including an operation unit, a ROM, a RAM, an A/D converter, a D/A converter, a communication interface circuit and the like. The CPU 121 drives various circuits of the camera in accordance with a predetermined program stored in the ROM to execute a series of operations such as AF, photography, image processing and recording.
A communication control circuit 122 functions to transmit a captured image to a server computer or receive an image and various types of information from the server computer via the communication unit 115. A shake amount calculation unit 123 calculates a shake amount on the basis of the shake signal from the shake detection sensor 116. An imaging device driving circuit 124 controls an imaging operation of the imaging device 106 and A/D converts an acquired image signal and transmits the same to the CPU 121. An image processing circuit 125 performs processing of images acquired at the imaging device 106 such as γ-conversion, color interpolation, JPEG compression, and so on.
A focus driving circuit 126 drives and controls the focus actuator 114 on the basis of a focus detection result to drive the third lens group 104 back and forth in the optical axis direction for focus control. A correction lens driving circuit 127 drives and controls the correction lens actuator 113 in accordance with a driving amount found by the CPU 121 on the basis of the shake amount calculated by the shake amount calculation unit 123 to eccentric-shift the second lens group 103. A diaphragm driving circuit 128 drives and controls the diaphragm actuator 112 to control the aperture of the shutter 102 doubling as the diaphragm. A zoom driving circuit 129 drives the zoom actuator 111 in accordance with a zoom operation by a photographer.
A display 131 such as a LCD displays information on a photography mode of the camera, a preview image before photography, an image for checking after photography, a focus-state display image during focus detection and the like. A control switch group 132 includes a power-supply switch, a release (photography trigger) switch, a zoom operation switch, a photography mode selection switch and the like. A removable flash memory 133 records photographed images.
In
Further, in
Further, among a group of pixels having a shape similar to a pixel 301, the waveform of an image (image A) obtained from photoelectric conversion units 302 aligned in the X direction and the waveform of an image (image B) obtained from photoelectric conversion units 303 aligned in the X direction can be used to detect a focus state of a subject having a brightness difference (edge) in the X direction.
Then, an image is recorded by adding photoelectric conversion signals obtained from the photoelectric conversion units 302 to 305 for each pixel (i.e., adding photoelectric conversion signals from the photoelectric conversion units 302 and 303 and adding photoelectric conversion signals from the photoelectric conversion units 304 and 305). Thereby, a photoelectric conversion signal for each pixel can be obtained.
More specifically, in
Further, photoelectric conversion signals obtained from photoelectric conversion units 401 and 403 are added, and photoelectric conversion signals obtained from photoelectric conversion units 402 and 404 are added. On the basis of waveforms of the thus obtained pair of photoelectric conversion signals (image A, image B), a focus state of a subject having a brightness difference (edge) in the X direction can be detected.
These two adding methods for focus detection may be performed by dividing a pixel group on the imaging device 106 into blocks and changing the adding methods for each block. For instance, the adding methods may be changed alternately like a checkered pattern arrangement, whereby a pixel layout structure equivalent to
The adding method may be switched depending on the photographing state or for all pixels on a time-series basis. In that case, photoelectric signals in a dense state can be obtained from the focus detection pixels in the Y direction or the X direction. As a result, a problem that a focus state of a subject having thin lines cannot be detected in the vicinity of focusing, which occurs for loose photoelectric conversion signals obtained from the focus detection pixels, can be avoided.
An image can be recorded by adding photoelectric conversion signals obtained from the photoelectric conversion units 401 to 404 for each pixel to obtain a photoelectric conversion signal for each pixel.
The following describes focus detection processing by a phase difference detection method when shake correction is performed using the second lens group 103 in the camera including the imaging device 106 illustrated in
For the focus detection processing by a phase difference detection method using a photoelectric conversion signal obtained from the imaging device 106 having the structure as illustrated in
Referring next to
In
When the imaging device 106 has the structure of
EP0 in
Herein, when the pupils of the pixels for image A and image B are vignetted as illustrated in
H in
Herein, the correction curve H is represented, for example, as the following polynomial function, from which the correction value K is derived for the coordinate X of the waveform BI1 to be corrected:
K(X)=1+a·X+b·X2+c·X3+ . . . .
In this function, coefficients a, b, c, . . . may be changed in accordance with the eccentricity amount of the second lens group 103.
Note here that, the aforementioned example describes the correction in the X direction as one-dimensional direction only. However, correction in the Y direction of the second lens group 103 (depth direction in
These correction values (or the coefficients a, b, c, . . . ) may be stored in a memory (not illustrated) in the CPU 121 or a nonvolatile memory (not illustrated) in the camera. Then, the eccentricity amount (position) of the second lens group 103 is found on the basis of a driving amount of the correction lens driving circuit 127, and a correction value corresponding to the found eccentricity amount is read out for correction. Alternatively, the position of the second lens group 103 may be detected by a sensor not illustrated, and a correction value corresponding to the detected position may be read out.
As described above, an output signal of the imaging device 106 is corrected in accordance with the eccentricity amount (position) of the second lens group 103, whereby focus detection accuracy can be improved.
In the case of focus detection pixels having the shapes as illustrated in
The above describes the case where correction processing is performed to one of a pair of output signals to be subjected to correlation operation which requires correction. However, correction processing may be performed for both of the output signals as needed.
Referring next to the flowchart of
In response to a switch-on operation of a power supply of a camera by a photographer, the CPU 121 checks operations of various actuators and the imaging device in the camera and initializes the memory and execution programs, while executing a photography preparation operation. Then, in response to the manipulation of a shutter button by the photographer, the second lens group 103 in the photographing optical system starts image blur correction processing together with eccentric-shifting, and focus detection processing is concurrently started.
At Step S101, charge accumulation is started in which light beams incident on the focus detection pixels are converted into an image signal for autofocus control. At Step S102, at the timing when the charge accumulation is started at Step S101, an eccentricity amount (in this example, X and Y coordinate positions with reference to the optical axis position as the origin point of the coordinate) of the second lens group 103 performing image blur correction is stored. At Step S103, determination is made as to whether the charge accumulation started at Step S101 is finished or not, and when it is finished, the procedure proceeds to Step S104.
At Step S104, at the timing when the charge accumulation is finished, the eccentricity amount of the second lens group 103 is stored similarly to Step S102. At Step S105, on the basis of the eccentricity amounts of the second lens group 103 stored at Steps S102 and S104, an average X, Y coordinate position of the correction lens group in the charge accumulation time to obtain image signal for focus detection is calculated. Then, at Step S106, on the basis of the calculation result at Step S105, a correction value corresponding thereto is selected from a memory (not illustrated) that stores the aforementioned correction values.
Then, at Step S107, interpolation processing is performed so as to change a signal waveform of a pair of image signals for AF obtained from the focus detection pixels where the charge accumulation is finished at Step S103 into a signal waveform for correlation operation, and then correction processing is performed using the correction value selected at Step S106.
Next at Step S108, correlation operation is performed to the corrected pair of image signals to calculate an image shift amount, and on the basis of a relationship between the obtained image shift amount and a base-line length found beforehand, conversion into a defocus amount is performed. Herein, the conversion into a defocus amount may be performed using a method described in Japanese Patent Laid-Open No. 2004-191629 or other well-known methods. At Step S109, determination is made as to whether in-focus state is achieved or not on the basis of the obtained defocus amount, and when it is determined as focused, the focus detection processing ends.
On the other hand, when it is determined at Step S109 as not-focused, at Step S110 a focus driving amount toward the in-focus state is found on the basis the defocus amount calculated at Step S108. Then, after the third lens group 104 is driven via the focus driving circuit 126 and the focus actuator 114, the procedure returns to S101 to repeat the aforementioned processing.
The above describes the correction processing for focus state detection while exemplifying the case where the second lens group 103 as a correction lens group of the photographing optical system is eccentric-shifted. The present invention is not limited to this example, and even in the case of shake correction by eccentric-shifting the imaging device 106, correction values corresponding to the eccentricity amounts may be stored in advance similarly, and a correction value may be selected in accordance with the eccentricity amount during shake correction for correction of an image signal.
As described above, according to the present embodiment, an image capturing apparatus provided with an image blur correction mechanism by an optical image stabilization method and configured to perform focus detection on the basis of a signal from an imaging device can achieve high focus detection accuracy.
The present invention is applicable to single-lens reflex cameras and compact digital cameras using an imaging device having focus detection pixels in a phase difference detection method as well as image capturing apparatuses such as a video camera.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2011-095280, filed on Apr. 21, 2011 which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-095280 | Apr 2011 | JP | national |
