Imaging apparatus and control method

Abstract

In an imaging apparatus, wherein when performing auto focus control while continuously moving a focusing lens by using an image sensor that has a lag in timing of electric charge accumulation, a focusing accuracy can be improved. To this end, the imaging apparatus includes the rolling shutter function, an imaging unit configured to convert an object image into an image signal, and a setting unit. When a difference exceeds a predetermined amount in the barycentric position of the optical system between an upper line and an a lower line of the af area due to the rolling shutter function, the setting unit imposes a limitation on the af area to reduce the size of a desired af area to an af area where the difference in the barycentric position becomes a predetermined amount.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a focus adjustment technique. More specifically, obtaining an optimum in-focus position when focus adjustment is performed based on an image signal from an image sensor.

2. Description of the Related Art

Conventionally, as an imaging control method for a CMOS image sensor, there is an electronic shutter control method called the rolling shutter. In a shooting operation with the rolling shutter, a plurality of pixels arranged two-dimensionally are sequentially scanned in small units, such as row by row, to read electric charge from the pixels and reset the pixels. After the lapse of a predetermined period of time (charge accumulation time or exposure time) that starts immediately after the reset of the pixels, the pixels are scanned again in the same way as the pixels were reset, so that the electric charge is read from the pixels, and the read charge (image signal) is output. When a moving image is captured, the above operation is performed in one-frame periods.

When shooting a photo with the rolling shutter described above, a time difference occurs between image capturing timing of the first row of pixels and of the last row of pixels. Therefore, in an image of one frame, the scenes having the time difference between the top row and the bottom row coexist.

Meanwhile, a method of an auto focus operation is well known, which moves the focusing lens to a plurality of positions and captures images and determines an in-focus position based on a series of AF evaluation values obtained by an arithmetic operation of an image signal. This method is divided into two types: one type stops the focusing lens after the lens has been moved, and then captures an image, and another type captures images while moving the lens continuously. A problem with the former type is that it takes time because before you take a photo and you have to wait for the lens to stop. Therefore, the latter is advantageous for speed-up of auto focusing.

In the latter type, it is necessary to determine a lens position corresponding to auto focus (AF) evaluation value. As a concrete example, a method is well known which obtains a barycentric position of the center line in an AF area by performing a correction calculation based on a drive time of the optical system, a size of the AF area on the image sensor, and an exposure time (Japanese Patent Application Laid-Open No. 2007-267278, for example).

However, if the latter method is adopted, the image sensor, which shows a difference in timing for charge accumulation in different AF areas, produces a negative effect as follows.

When a contrast difference is large between the upper and lower portions of the image in the AF area, the following problem arises. FIG. 12A illustrates a case where an AF evaluation value and a barycentric position of the AF area in a single AF area are calculated. FIG. 12B illustrates a case where in order to calculate an AF evaluation value and a barycentric position around the area where intrinsically there ought to be a contrast, the inside of the AF area is divided into sections and an AF evaluation value and a barycentric position are calculated for each section.

When AF evaluation values are calculated while moving the lens continuously, as shown in FIG. 12C, as usual, there are differences in the correspondence relation between the AF evaluation values and the barycentric positions obtained in the single AF area, and the AF evaluation values and the barycentric positions calculated in the vicinity of the area where there is a contrast. This is attributable to differences in the position of the lines used for calculation of the barycentric positions and also to the influence of the rolling shutter, which are difficult to avoid, in principle.

In other words, a time lag becomes large between an intrinsic in-focus position and an in-focus position indicated by actual AF evaluation values, depending on the size of the AF area and the contrast of the image in the AF area, a degree of time lag in charge accumulation due to the rolling shutter, or a moving speed of the lens when the lens is moved continuously. This leads to a worsening of the focusing accuracy of auto focus.

SUMMARY OF THE INVENTION

An embodiment of the present invention is directed to improving a focusing accuracy when auto focus control is implemented while the focusing lens is moved continuously by using an image sensor which shows a time lag in charge accumulation timing between different positions in an AF area.

According to an aspect of the present invention, an imaging apparatus includes an imaging unit having a plurality of pixels arranged two-dimensionally and configured to obtain an image of an object by sequentially scanning a plurality of pixels to read out an image signal from each pixel while moving the position of a focusing lens; a detection unit to detect an object area in an image obtained by the imaging unit; a setting unit to set an AF area to be corresponding to the object area detected by the detection unit; and a focusing control unit configured to obtain in-focus states of a plurality of images in the AF area set by the setting unit, the plurality of images being captured by the imaging unit, and based on information about the in-focus states, perform focusing control of the focusing lens.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a block diagram illustrating a schematic functional structure of a digital camera according to an exemplary embodiment of the present invention.

FIG. 2 illustrates a time lag of a barycentric position in the AF area due to an influence of the rolling shutter when the focusing lens is moved continuously.

FIG. 3 is a flowchart illustrating a basic operation flow of the digital camera according to an exemplary embodiment.

FIG. 4 is a flowchart illustrating an AF operation before pressing SW1 of the digital camera according to an exemplary embodiment.

FIG. 5 is a flowchart illustrating an AF operation after pressing SW1 of the digital camera according to an exemplary embodiment.

FIG. 6 is a flowchart illustrating a scanning operation of the digital camera according to an exemplary embodiment.

FIG. 7 is a flowchart illustrating an AF area control process of the digital camera according to an exemplary embodiment.

FIGS. 8A through 8D are diagrams illustrating the relation among an image capturing posture, a big face detection result, and setting of the AF area according to an exemplary embodiment.

FIG. 9 is a flowchart illustrating the AF area limiting process of the digital camera according to an exemplary embodiment.

FIG. 10 is a diagram illustrating the relation among the image capturing posture, the big face detection result, and setting of the AF area according to an exemplary embodiment.

FIG. 11 is a diagram illustrating an allowance calculation method when the AF area is limited according to an exemplary embodiment.

FIGS. 12A through 12C are diagrams illustrating the relation between an apparent in-focus position and an intrinsic in-focus position based on the relation between the states of contrast in the AF area and the barycentric positions.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram of a digital camera as an example of an imaging apparatus including an auto focus (AF) function.

A camera according to an exemplary embodiment, which is provided with an optical system 1, including a focusing lens 2, photo-electrically converts, by an image sensor, light of a focused image from the optical system 1 into an image signal (pixel signal) and outputs the signal. The image sensor 3 used here is a CMOS sensor, for example, which has a plurality of pixels, each including a photoelectric conversion element, arranged in a matrix form (in a two-dimensional array of rows and columns). The image sensor is driven by a rolling shutter method.

In an image capturing operation by the rolling shutter, a plurality of pixels arranged two-dimensionally are scanned in small units, for example, row by row to read electric charge from each pixel and reset each pixel Immediately after the rest, the light from an object is received and the light undergoes photoelectric conversion, and after the lapse of a predetermined time (charge accumulation time or exposure time), the plurality of pixels is scanned again in row units in the same manner as when they were reset, thereby reading a charge from each pixel, and the read charge (image signal) is output.

When capturing a moving image, the operation described above is performed in one-frame periods. As illustrated in FIG. 2, in this case, images are captured while moving the lens continuously. When a barycentric position is found in the center line of the AF area, differences in image capture timing occur according to different positions of the area in the screen due to the influence of the rolling shutter. This is because the barycentric positions of the center line, an upper line and a lower line of the AF area are at different positions, PosC, PosA, and PosB (FIG. 2).

In this embodiment, it is assumed that the rolling shutter function has the effects in the vertical (landscape) direction of the camera when the camera is in the normal position (reference position). An output image signal is processed by a correlated double sampling (CDS) circuit and a pre-processing circuit, which includes a nonlinear amplifier circuit to process the image signal before the image signal is sent for A/D conversion. Then, the image signal is converted into a digital form by the A/D converter 5, then passes through a memory controller 6, and the signal is stored in a memory 7, and converted into image data in an optional format by a signal processing circuit (not illustrated), and recorded in a storage medium.

A focusing operation is controlled by a control unit 11. The control unit 11 drives a focusing lens 2 via a focusing lens drive circuit 12 to move it to a plurality of lens positions to capture images. The control unit 11 instructs an AF evaluation calculation circuit 13 to calculate a signal (AF evaluation value) representing an in-focus state according to image contrast using an image signal in the AF area captured by the image sensor 3. Next, based on a calculated AF evaluation value, an in-focus position determination circuit 14 determines an in-focus position, which is a lens position where the AF evaluation value is at a maximum. After that, the control unit 11 controls to drive the focusing lens 2 to this position. In this manner, focusing control is completed. When SW1 (9) including an operation member is operated, the above-described focusing operation is performed. When SW2 (10) including an operation member is operated, actual photographing is carried out.

Using an image signal obtained by the image sensor 3, face object detection unit 15, an animal or a car, other than a human face, may also be used as an object. Moreover, an object can be detected by inputting and specifying a position in the image screen via an external input device, or by detecting the line of sight of the photographer, who is looking into the finder.

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 modifications, equivalent structures, and functions.

This application claims priority from Japanese Patent Application No. 2008-236309 filed Sep. 16, 2008, which is hereby incorporated by reference herein in its entirety.

Claims

1. An imaging apparatus comprising:

an imaging unit having a plurality of pixels arranged two-dimensionally in a row direction and a column direction and configured to obtain an image of an object by sequentially scanning a perform charge accumulation of the plurality of pixels to read out an image signal at a different timing between each row or each column while moving the position of a focusing lens;

a an object detection unit to detect an object area in an image obtained by the imaging unit; and

a control unit communicatively-coupled to a memory, wherein the memory stores a program executed by the control unit to,

a setting unit to set an af area corresponding to the object area detected by the object detection unit;, and

a focusing control unit configured to obtain in-focus states of by a plurality of images in the af area set by the setting control unit, the plurality of images being captured by the imaging unit, and based on information about the in-focus states, to perform focusing control of the focusing lens,

wherein when in a case that the object area is larger than a predetermined size, the setting control unit sets an af area which is smaller than an af area which corresponds to the object area by decreasing only the size of the object area length in the row direction or the length in the column direction in which, when there is a difference occurs in charge readout accumulation timing.

2. The imaging apparatus according to claim 1, wherein the object detection unit detects the object by extracting face information from the image, and the setting control unit sets the af area according to the face information.

3. The imaging apparatus according to claim 1, wherein the setting control unit switches over depending on whether to set a the smaller af area than an the af area corresponding to the object area according to at least one of a state and a mode of an af operation.

4. The imaging apparatus according to claim 3 1, further comprising an operation member,

wherein in a mode of placing more priority on tracking of the object than in a case of shooting a still image, the setting unit does not set a smaller af area than an af area corresponding to the object area even if the object area is larger than the predetermined size the control unit sets the smaller af area than the af area corresponding to the object area in a case where the object area is larger than the predetermined size after the operation member is operated, and sets the af area corresponding to the object area in a case where the object area is larger than the predetermined size before the operation member is operated.

5. An imaging apparatus comprising:

an imaging unit having a plurality of pixels arranged two-dimensionally and configured to obtain an image of an object by sequentially scanning a perform charge accumulation of the plurality of pixels to read out an image signal at a different timing between each row or each column while moving the position of a focusing lens;

a an object detection unit to detect an object area, eyes of the object and a face orientation of the object in an image obtained by the imaging unit; and

a control unit communicatively-coupled to a memory, wherein the memory stores a program executed by the control unit to,

a setting unit to set an af area corresponding to the object area detected by the object detection unit;, and

a focusing control unit configured to obtain in-focus states of by a plurality of images in the af area set by the setting control unit, the plurality of images being captured by the imaging unit, and based on information about the in-focus states, to perform focusing control of the focusing lens,

wherein when in a case that the object area is larger than a predetermined size, the setting control unit sets an af area which is smaller than an af area which corresponds to the object area, and

wherein when in a case that the setting control unit sets an the af area smaller than an the af area corresponding to the object area, if the face orientation detected by the object detection unit is the same as the direction in which a difference occurs in the charge readout accumulation timing, the setting control unit sets an the af area so as to include both of eyes detected by the object detection unit, and if those two directions differ from each other, the setting control unit sets an the af area so as to include one eye detected by the object detection unit.

6. The imaging apparatus according to claim 5, wherein the setting control unit switches over depending on whether to set a the smaller af area than an the af area corresponding to the object area according to at least one of a state and a mode of an af operation.

7. The imaging apparatus according to claim 6, wherein in an the af operation, when a still image is captured, the setting control unit sets a the smaller af area than an the af area corresponding to an area of the object area.

8. The imaging apparatus according to claim 5, further comprising a calculation unit configured to calculate barycentric position of the focusing lens corresponding to the af area,

wherein the focusing control unit performs focusing control by moving the focusing lens to a barycentric position at which the in-focus state in the af area is at a maximum.

9. A control method for an imaging apparatus having an imaging unit including a plurality of pixels arranged two-dimensionally and configured to capture an image of an object by sequentially scanning a perform charge accumulation of the plurality of pixels to read out an image signal from each pixel at a different timing between each row or each column while moving the position of a focusing lens, and configured to perform focusing control of the imaging apparatus by taking a plurality of images of an object while moving a position of a focusing lens, and obtaining in-focus states in the af areas set in the plurality of images, the method comprising:

detecting the object areas in the images, eyes of the object and a face orientation of the object; and

setting af areas associated with the object areas,

wherein when in a case that an object area is larger than a predetermined size, a smaller af area than the af area corresponding to the object area is set, and

wherein when a the set af area is smaller than the af area corresponding to the object area, if a the face orientation detected is the same as the direction in which a difference occurs in a charge readout accumulation timing, an the af area is set so as to include both eyes detected, and if those two directions differ from each other, the an af area is set so as to include at least one eye detected.

10. The imaging apparatus according to claim 1, wherein the imaging unit is a CMOS sensor.

11. The imaging apparatus according to claim 5, further comprising an operation member, wherein the control unit sets the smaller af area than the af area corresponding to the object area in a case where the object area is larger than the predetermined size after the operation member is operated, and sets the af area corresponding to the object area in a case where the object area is larger than the predetermined size before the operation member is operated.

12. An imaging apparatus comprising:

an imaging unit having a plurality of pixels arranged two-dimensionally, wherein charge accumulation timing of the plurality of pixels differs between each row or each column;

an object detection unit to detect an object area of an image acquired by the imaging unit; and

a control unit communicatively-coupled to a memory, wherein the memory stores a program executed by the control unit to,

set an af area of a size of a first value corresponding to a size of the detected object area or an af area of a size of a second value, the size of the second value is smaller than the size of the first value only in a direction that a difference occurs in timing of charge accumulation of the plurality of pixels, and

detect a focus state of the af area of the image acquired by the imaging unit while moving the focus lens, and control positions of the focus lens based on the focus state,

wherein the control unit sets the af area of the size of the second value in a case where the size of the object area is larger than a predetermined size.

13. The imaging apparatus according to claim 12, wherein the set the af area of the size of the first value is an area corresponding to a profile of a face.

14. The imaging apparatus according to claim 12, wherein the object detection unit detects the object area by extracting face information of the image.

15. The imaging apparatus according to claim 14, wherein the object detection unit detects eyes of the object and a direction of the face of the object and, in a case where the size of the object area is larger than the predetermined size, the control unit sets the af area of the size of the second value so that both eyes are included when direction of the face and the direction which the difference occurs in timing of charge accumulation of the plurality of pixels is the same.

16. The imaging apparatus according to claim 14, wherein the object detection unit detects eyes of the object and a direction of the face of the object and, in a case where the size of the object area is larger than the predetermined size, the control unit sets the af area of the size of the second value so that one of left eye and right eye is included when direction of the face and the direction which the difference occurs in timing of charge accumulation of the plurality of pixels is different.

17. The imaging apparatus according to claim 12, wherein the control unit changes whether to set the af area of the size of the second value or not according to at least one of a state and a mode of af operation.

18. The imaging apparatus according to claim 17, wherein the control unit sets the af area of the size of the second value in a mode of imaging still image.

19. The imaging apparatus according to claim 12, further comprising an operation member,

wherein the control unit sets the af area of the size of the second value in a case where the size of the object area is larger than the predetermined size after the operation member is operated, and sets the af area of the size of the first value in a case where the size of the object area is larger than the predetermined size before the operation member is operated.

20. The imaging apparatus according to claim 12,

wherein the control executes the program stored in the memory to calculate position of the focus lens for corresponding to the focus state of af area of the image, and

wherein the control unit moves the focus lens to a focus position where focus state for corresponding to focus state of af, are of the image in a peak state.

21. A control method for an imaging apparatus having an imaging unit including a plurality of pixels arranged two-dimensionally and timing of charge accumulation of the plurality of pixels differs between each rows or each columns, the method comprising:

detecting an object area of an image acquired by the imaging unit;

setting an af area of a size of a first value corresponding to size of the detected object area or an af area of a size of a second value, the size of the second value is smaller than the size of the first value only in a direction that a difference occurs in timing of charge accumulation of the plurality of pixels; and

detecting a focus state of the af area of the image acquired by the imaging unit while moving the focus lens, and controlling positions of the focus lens based on the focus state,

sets the af area of the size of the second value in a case where the size of the object area is larger than a predetermined size.

22. A control method for an imaging apparatus having an imaging unit including a plurality of pixels arranged two-dimensionally and configured to perform charge accumulation of the plurality of pixels at a different timing between each row or each column while moving the position of a focusing lens, and configured to perform focusing control of the imaging apparatus by taking a plurality of images of an object while moving a position of a focusing lens, and obtaining in-focus states in the af areas set in the plurality of images, the method comprising:

detecting the object areas in the images;

setting af areas associated with the object areas; and

detecting a focus state of the af area of the image acquired by the imaging unit while moving the focus lens, and controlling positions of the focus lens based on the focus state,

wherein in a case that the object area is larger than a predetermined size, an af area is set so as to be smaller than an af area which corresponds to the object area by decreasing only the length in the row direction or in the column direction in one of which a difference occurs in charge accumulation timing.