JP4574559B2 - Imaging device, focus position detection device, and focus position detection method - Google Patents

Description

  The present invention relates to a focus adjustment technique performed based on a video signal obtained from an image sensor.

  For autofocus devices (focusing devices) used in digital cameras and digital video cameras, a signal for evaluating the in-focus state is created using the video signal obtained from the image sensor, and the focus position is determined using that signal. There is something to decide. Such a method does not require a sensor dedicated to autofocus, and thus has the advantages of low cost, no parallax, and high accuracy. However, since it is necessary to drive the focus lens within a predetermined distance range and create evaluation signals sequentially, the focus lens is driven over a relatively wide range, such as when the focal length of the lens is long or the open F value is small. In this case, there is a disadvantage that it takes a long time to focus.

For this problem, for example, a method described in Patent Document 1 is known. According to the present invention, the subject distance range is searched in a relatively rough step to identify the maximum value of the evaluation value, and the search is performed again in a fine step in the vicinity of the maximum value to determine the in-focus position. According to this method, when the search range is sufficiently wide, the total number of data to be acquired can be reduced and the focusing time can be reduced as compared with the case where the search is performed only with fine steps.
Japanese Patent No. 2708904

  However, in the above method, since the search is always performed in a fine step after searching in a rough step, it is necessary to reciprocate the focus lens many times, and the time until focusing (focusing time) is long. May be.

  As a result of intensive studies to solve the above problems, the present inventor has come up with various aspects of the invention described below.

Based on the image signal obtained while moving the focus lens, a focus evaluation value indicating a focus state of the focus lens is acquired at a first interval or a second interval narrower than the first interval, A focus position detection method for obtaining a focus position based on a focus evaluation value acquired at a first interval or a second interval, the focus position corresponding to a focus lens position obtained at the first interval. Based on the distribution state of the evaluation value, when the width of the portion where the distribution of the focus evaluation value is in the shape of a mountain is equal to or smaller than a predetermined value, there are a plurality of local maximum values of the focus evaluation value. When the value interval is equal to or smaller than a predetermined value, and the change between the focus evaluation value at which the maximum value is obtained and the focus evaluation value acquired at the focus lens position adjacent to the focus lens position at which the maximum value is obtained. When the amount is below a predetermined value In the case of either Re, the second control to acquire an in-focus state at intervals, the case of controlling so as not to acquire the in-focus state in the second interval, if obtained in the first interval When the focus position is obtained based on the focus state and control is performed so as to acquire the focus state at the second interval, the focus position is obtained based on the focus state obtained at the second interval. It is characterized by.
Further, based on the image signal obtained while moving the focus lens, a focus evaluation value indicating a focus state of the focus lens is acquired at a first interval or a second interval narrower than the first interval. A focus position detection method for obtaining a focus position based on a focus evaluation value acquired at the first interval or the second interval,
When it is determined that the subject is rich in high-frequency components based on the distribution state of the focus evaluation value corresponding to the focus lens position obtained at the first interval, there are a plurality of peaks of the focus evaluation value. When it is determined that it is impossible to distinguish whether the focus evaluation value is fluctuating due to noise or when it is determined that a plurality of local maximum values overlap. When the control is performed so as to acquire the in-focus state at the second interval and the in-focus state is not acquired at the second interval, the in-focus state obtained at the first interval is obtained. When the focus position is obtained based on the focus state and the focus state is acquired at the second interval, the focus position is obtained based on the focus state obtained at the second interval. To do.

  ADVANTAGE OF THE INVENTION According to this invention, the focusing technique which can shorten a focusing time can be provided, ensuring a focusing precision.

  The best mode for carrying out the present invention is as shown in Example 1 and Example 2 below.

  FIG. 1 is a diagram illustrating an outline of a digital camera including a focusing device according to an embodiment of the present invention.

  The digital camera according to the present embodiment is provided with an optical system 1 and a focus lens 2, and the image sensor 3 photoelectrically converts light imaged by these. The analog signal output from the image pickup device 3 is processed by a pre-processing circuit 4 including a CDS circuit for removing output noise and a non-linear amplification circuit performed before A / D conversion, and then the A / D converter 5. Digitized by The digital signal is stored in the memory 7 via the memory controller 6, converted into image data by a signal processing circuit (not shown), and recorded on the recording medium 8.

  The focus operation (focusing operation) for focusing the focus lens 2 on the main subject is mainly controlled by the control unit 11. The focus lens drive circuit 12 is configured to drive the focus lens 2 at two speeds (high speed and low speed) here.

  Here, an outline of the focus operation in the present application will be described. First, the focus lens drive circuit 12 drives the focus lens 2 at high speed, and the AF evaluation value calculation unit 14 calculates a signal (AF evaluation value) corresponding to the contrast of the image using an image signal captured at a predetermined period. Ask.

  This calculation is performed for one or more AF frames set by the AF frame setting unit 13.

  Here, calculation of a signal (AF evaluation value) corresponding to the contrast of the captured image is performed as follows. First, a horizontal band-pass filter that transmits a predetermined high-frequency signal is applied to each line of image data output from the image sensor 3 corresponding to the AF frame. Next, the one having the largest bandpass output signal (absolute value) is selected for each line. Then, the selected signal is integrated in the vertical direction. Accordingly, a signal having a large contrast in the horizontal direction is detected and integrated in the vertical direction, so that the S / N ratio of the signal is improved. In addition, by performing such calculation, it is possible to obtain a signal that has the largest value in the in-focus state and decreases in the defocus state. Therefore, if photographing is performed at a position where such a maximum value of the signal is obtained, an image in a focused state on the subject can be obtained.

  When the AF evaluation value is obtained, the fine search determination unit 15 determines whether or not a low-speed focusing operation is necessary using the AF evaluation value. In addition, when driving at a low speed, an AF evaluation value is obtained using an image signal captured at the same cycle as that at the time of the high speed driving described above. Therefore, the position interval of the focus lens 2 for acquiring the AF evaluation value is shorter than that at the time of high speed driving.

  When the operation for detecting the in-focus state at low speed is not required, the in-focus position determination unit 16 determines the in-focus position, and then drives the focus lens to that position. On the other hand, when an operation for detecting a focused state at a low speed is necessary, the focus lens 2 is driven at a low speed by the focus lens driving circuit 12. At that time, after calculating the AF evaluation value as described above, the in-focus position is determined. Then, the focus lens is driven to this in-focus position. The switches 9 and 10 are switches for switching the operation mode. For example, when the switch 9 is operated, the operation shifts to an operation for detecting the in-focus state, and when the switch 10 is operated, the operation shifts to an operation for capturing an image and recording an image.

  Next, the focusing operation will be described in more detail with reference to the flowchart shown in FIG.

  First, in step S1, the focus lens 2 is driven at high speed, and an AF evaluation value is obtained by calculation of the AF evaluation value calculation unit 14. Using the obtained AF evaluation value, it is determined in step S2 whether a low-speed search is necessary. Details of the case where it is necessary to acquire the AF evaluation value by driving the focus lens 2 at low speed will be described later.

  In step S2, it is determined whether it is necessary to acquire an AF evaluation value (search operation) associated with low-speed driving of the focus lens 2. In step S3, based on the determination result in step S2, the process proceeds to step S4 when the AF evaluation value acquisition (search operation) accompanying the low-speed driving of the focus lens 2 is necessary, and to step S6 when it is not necessary.

  When the process proceeds to step S4, the focus lens 2 is driven at a low speed near the temporary maximum value obtained by high speed driving to obtain an AF evaluation value. At step S5, the focus lens 2 near the temporary maximum value is driven at low speed. As a result of the acquisition of the AF evaluation value accompanying this, the in-focus position is determined based on the obtained AF evaluation value. On the other hand, when the process proceeds to step S6, the in-focus position is determined based on the AF evaluation value obtained as a result of obtaining the AF evaluation value accompanying the high-speed driving of the focus lens 2.

  Then, after the focus position is determined in step S5 or S6, the focus lens 2 is driven to the focus position in step S7, thereby obtaining a focused state.

  As an example of the case where it is necessary to acquire the AF evaluation value accompanying the low-speed driving of the focus lens 2, there are the following three cases, for example.

  The first example is a case where an object such as a thin line contains a lot of high frequency components. In general, when the focus lens is shifted from the in-focus position, the defocus characteristic attenuates faster as the subject image contains more high-frequency components. For example, A shown in FIG. 6 is a case where the subject image contains a lot of high-frequency components, while B shows a case where the subject image does not contain many high-frequency components. Therefore, the peaks of the AF evaluation values are steep, and when the focus lens 2 is driven at high speed, the AF evaluation values may be acquired at a rough interval that is greater than the required sampling interval. For example, in the case shown in FIG. 3, an accurate in-focus position cannot be obtained. That is, only one point of the AF evaluation value is acquired from the portion having the shape of the mountain, and the in-focus position varies depending on from which portion of the mountain the data is obtained. In FIG. 3, ■ indicates points where sampling was performed and AF evaluation values were acquired.

  Whether or not an in-focus position can be specified by acquiring an AF evaluation value associated with high-speed driving of the focus lens 2 in such a scene can be determined by, for example, obtaining a width of a peak of the AF evaluation value. That is, first, i = 1, 2,..., N is the position of the data obtained by acquiring the AF evaluation value associated with high-speed driving of the focus lens 2, and the AF evaluation value at position i is V (i). Define. Then, the position of the maximum value (i = Imax) is obtained, and further “L (i) = | V (i) −V (i + 1) |” is obtained in the order of i = Imax, Imax + 1, Imax + 2,. . Then, the number of positions i until the inequality “L (i)> predetermined value” is not satisfied is counted. Similarly, the number of positions i is counted in the order of i = Imax-1, Imax-2,.

  When such processing is performed, in the example shown in FIG. 3, the inequality “L (i)> predetermined value” is not satisfied when i = Imax + 1 and i = Imax−2. Therefore, the total number of positions i is two, and the peak width of the AF evaluation value signal with respect to the position of the focus lens 2 can be determined to be two.

  Then, a predetermined value of the peak width in the subject image including a lot of high frequency components as described above is determined in advance, and the peak width obtained as described above is compared with the predetermined value. As a result of the comparison, if the difference is equal to or smaller than a predetermined value, it may be determined that the subject image is narrow and has a high frequency component. For example, in the example shown in FIG. 3, if the predetermined value of the mountain width is set to 2 in advance, it is determined that it is necessary to acquire the AF evaluation value accompanying the low-speed driving of the focus lens 2. .

  When the difference between the maximum value Vmax and the minimum value Vmin of the AF evaluation value is less than or equal to a predetermined value, the peak of the AF evaluation value is steep and the subject image contains a lot of high-frequency components. As an alternative, it may be determined that it is necessary to acquire an AF evaluation value associated with low-speed driving of the focus lens 2. This is because if the AF evaluation value is acquired at high speed when the mountain is steep, the entire steep mountain may not be detected.

  The second example is a case where there are a plurality of subjects in the AF area and the subject distances are different. An AF area is an area for acquiring a signal for an AF evaluation value among captured image data, and there are one or two or more for a captured image. The photographer can visually recognize the positional relationship of the AF area on the captured image by displaying an area corresponding to the AF area as an AF frame on the viewfinder (or rear monitor).

  When there are a plurality of subjects in such an AF area and the subject distances are different, there are a plurality of peaks of the AF evaluation value for the position of the focus lens 2. As shown in FIG. 4, when the peaks of the AF evaluation values with respect to the position of the focus lens 2 overlap, the focus position cannot be determined by acquiring the AF evaluation values associated with high-speed driving of the focus lens 2.

  That is, as shown in FIG. 4, it is difficult to clearly distinguish whether there are a plurality of peaks of AF evaluation values for the position of the focus lens 2 or whether the AF evaluation values fluctuate due to noise. . Therefore, it is necessary to acquire an AF evaluation value accompanying the low-speed driving of the focus lens 2.

  As a method for determining whether or not it is necessary to acquire an AF evaluation value associated with low-speed driving of the focus lens 2 when there are a plurality of subjects in the AF area, for example, there is the following method. That is, when there are a plurality of maximum AF evaluation values for the position of the focus lens 2 and the interval is equal to or less than a predetermined value, the AF evaluation value associated with the low-speed driving of the focus lens 2 is acquired. May be determined to be necessary. For example, when the predetermined value of the interval is set to 2 in advance, the AF evaluation value for the position of the focus lens 2 as shown in FIG. 4 is obtained. When such an AF evaluation value is obtained, since the interval between the maximum values is 2, it is determined that it is necessary to acquire the AF evaluation value accompanying the low-speed driving of the focus lens 2. Further, when there are a plurality of local maximum values and the number is equal to or greater than a predetermined value, it may be determined that it is necessary to acquire an AF evaluation value associated with low-speed driving of the focus lens 2. .

  In consideration of the case where the focus lens 2 or the subject has a maximum value due to minute fluctuations, for example, a signal has been generated informing that the AF evaluation value fluctuation is minute or camera shake has occurred. In some cases, the maximum value may not be considered. Alternatively, only the maximum value of the AF evaluation value with respect to the acquired position of the focus lens 2 that is a predetermined ratio or more with respect to the maximum value may be regarded as the maximum value as a determination target.

  When a plurality of object distances shown in FIG. 4, in this case, two subject distances are closer, an AF evaluation value as shown in FIG. 5 may be obtained. This is the third example. In such a case, although the plurality of subjects are present at different subject distances, the maximum value of the acquired AF evaluation value becomes one, and thus cannot be determined by the above method.

  In such a case, it can be determined as follows, for example. That is, among the plurality of AF evaluation values obtained by high-speed driving of the focus lens 2, the maximum value and the decrease rate of the AF evaluation value adjacent to the maximum value of the AF evaluation value are a single peak. In comparison, the reduction is noted.

  This is done by using the maximum AF evaluation value V (Imax) and the adjacent V (Imax-1) and V (Imax + 1), and V (Imax) / V (Imax-1) and V (Imax) / Calculate V (Imax + 1). Then, when at least one of them is equal to or less than a predetermined value set in advance, that is, when the change in the AF evaluation value is moderate, it is determined that a plurality of peaks overlap, and the focus lens 2 is driven at low speed. It is determined that it is necessary to acquire an AF evaluation value associated with.

  According to such an embodiment of the present invention, it is possible to shorten the focusing time while ensuring sufficient accuracy. This is because, for a subject that can be focused by acquiring an AF evaluation value accompanying high-speed driving of the focus lens, the in-focus position is determined without acquiring an AF evaluation value accompanying low-speed driving of the focus lens.

  In addition, even if AF evaluation values associated with low-speed driving of the focus lens cannot be obtained, an appropriate focus position cannot be determined and sufficient focus accuracy cannot be obtained. Can do. This is because the AF evaluation value accompanying the driving of the focus lens in a fine step is acquired.

  In the above-described embodiment, the AF evaluation value acquisition period is fixed, and the focus lens 2 is driven at high speed or low speed, so that the acquisition interval of the AF evaluation value with respect to the position of the focus lens 2 is increased or reduced. I did. However, the method of changing the acquisition interval is not limited to changing the driving speed of the focus lens 2. If the driving speed of the focus lens 2 is constant and the AF evaluation value acquisition cycle is lengthened, the acquisition interval of the AF evaluation value with respect to the position of the focus lens 2 can be increased, and the acquisition cycle can be shortened. The acquisition interval can be made fine. Further, when acquiring the AF evaluation value, the AF evaluation value may be acquired while the focus lens is stopped or moved.

It is a figure which shows the outline of the digital camera provided with the focusing apparatus which concerns on embodiment of this invention. It is a flowchart which shows operation | movement of the focusing apparatus which concerns on embodiment of this invention. It is a figure which shows the 1st example of distribution of AF evaluation value which requires the search by a fine step. It is a figure which shows the 2nd example of distribution of AF evaluation value which requires the search by a fine step. It is a figure which shows the 3rd example of distribution of AF evaluation value which requires the search by a fine step. It is a figure which shows the example of distribution of AF evaluation value of the image which contains many high frequency components, and the image which does not contain many high frequency components.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical system 2 Focus lens 3 Image pick-up element 4 Pre-processing circuit 5 A / D converter 6 Memory controller 7 Memory 8 Recording medium 9, 10 Switch 11 Control part 12 Focus lens drive circuit 13 Distance measurement frame setting part 14 AF evaluation value Calculation unit 15 Fine search determination unit 16 In-focus position determination unit

Claims (8)

Acquisition means for acquiring a focus evaluation value indicating a focus state of the focus lens based on an image signal obtained while moving the focus lens;
Control means for controlling the acquisition means to acquire the focus evaluation value at a first interval or a second interval that is narrower than the first interval;
Focusing position detection means for obtaining a focusing position based on the focusing evaluation value acquired at the first interval or the second interval;
Based on the focus evaluation value distribution state corresponding to the focus lens position obtained at the first interval, the control means determines in advance the width of the portion where the focus evaluation value distribution forms a mountain shape. When the predetermined value is less than or equal to the predetermined value, there are a plurality of local maximum values of the focus evaluation value, and when the interval between the local maximum values is equal to or less than the predetermined value, and The focus state is acquired at the second interval when the amount of change from the focus evaluation value acquired at the focus lens position adjacent to the obtained focus lens position is equal to or less than a predetermined value. Control to
The focus position detection unit obtains a focus position based on the focus state obtained at the first interval when the control unit performs control so as not to acquire the focus state at the second interval. When the control is performed such that the in-focus state is acquired at the second interval, the in-focus position is obtained based on the in-focus state obtained at the second interval .   When acquiring the focus evaluation value at the second interval, the control unit controls the acquisition unit to acquire a focus evaluation value for a narrower movement range within the preset movement range. The in-focus position detecting device according to claim 1, wherein The control means causes the acquisition means to acquire the in-focus state at a fixed period, and changes the driving speed of the focus lens, thereby changing the first interval and the second interval, and The in-focus position detection apparatus according to claim 1 or 2 , wherein a driving speed when acquiring the in-focus state at an interval is faster than a driving speed at the second interval. The control unit drives the focus lens at a constant driving speed, and changes the first interval and the second interval by changing a period for acquiring the in-focus state by the acquisition unit, and The in-focus position detection apparatus according to claim 1 or 2 , wherein a period when the in-focus state is acquired at the first interval is longer than that at the second interval. Acquisition means for acquiring a focus evaluation value indicating a focus state of the focus lens based on an image signal obtained while moving the focus lens;
Control means for controlling the acquisition means to acquire the focus evaluation value at a first interval or a second interval that is narrower than the first interval;
Focusing position detection means for obtaining a focusing position based on the focusing evaluation value acquired at the first interval or the second interval;
Said control means, based on the distribution state of the focus evaluation value corresponding to the focus lens position obtained by the first interval, when it is determined that the subject including many high frequency components, focusing evaluation value If it is determined that it is not possible to distinguish whether there are multiple peaks or whether the focus evaluation value fluctuates due to noise , and at least if it is determined that multiple local maximum values overlap In either case, control is performed to obtain the in-focus state at the second interval ,
The focus position detection unit obtains a focus position based on the focus state obtained at the first interval when the control unit performs control so as not to acquire the focus state at the second interval. When the control is performed such that the in-focus state is acquired at the second interval, the in-focus position is obtained based on the in-focus state obtained at the second interval . An image sensor for outputting the image signal;
An in-focus position detection device according to any one of claims 1 to 5 ,
An imaging apparatus comprising: an imaging control unit that controls to perform imaging at an in-focus position determined by the in-focus position detecting device. Based on the image signal obtained while moving the focus lens, a focus evaluation value indicating a focus state of the focus lens is acquired at a first interval or a second interval narrower than the first interval, A focus position detection method for obtaining a focus position based on a focus evaluation value acquired at a first interval or a second interval,
Based on the distribution state of the focus evaluation value corresponding to the focus lens position obtained at the first interval, the width of the portion where the distribution of the focus evaluation value forms a mountain shape is equal to or less than a predetermined value. In the case where there are a plurality of local maximum values of the focus evaluation value and the interval between the local maximum values is equal to or smaller than a predetermined value, and the focus evaluation value at which the local maximum value is obtained and the focus lens position at which the local maximum value is obtained In any case where the amount of change from the focus evaluation value acquired at the focus lens position adjacent to is equal to or less than a predetermined value, the focus state is controlled to be acquired at the second interval ;
When control is performed so that the in-focus state is not acquired at the second interval, the in-focus position is obtained based on the in-focus state obtained at the first interval, and the in-focus state is acquired at the second interval. When the control is performed, the focus position detection method is characterized in that the focus position is obtained based on the focus state obtained at the second interval . Based on the image signal obtained while moving the focus lens, a focus evaluation value indicating a focus state of the focus lens is acquired at a first interval or a second interval narrower than the first interval, A focus position detection method for obtaining a focus position based on a focus evaluation value acquired at a first interval or a second interval,
When it is determined that the subject is rich in high-frequency components based on the distribution state of the focus evaluation value corresponding to the focus lens position obtained at the first interval, there are a plurality of peaks of the focus evaluation value. When it is determined that it is impossible to distinguish whether the focus evaluation value is fluctuating due to noise or when it is determined that a plurality of local maximum values overlap. When the control is performed so as to acquire the in-focus state at the second interval and the in-focus state is not acquired at the second interval, the in-focus state obtained at the first interval is obtained. When the focus position is obtained based on the focus state and the focus state is acquired at the second interval, the focus position is obtained based on the focus state obtained at the second interval. To detect the in-focus position.

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