JP5896767B2 - Imaging apparatus and focus control method - Google Patents

Description

  The present invention relates to an imaging apparatus such as a digital still camera or a video camera having a focus control function.

  For autofocus (hereinafter referred to as AF) of an imaging apparatus, an evaluation value (hereinafter referred to as AF evaluation value) generated using a high-frequency component of a luminance signal obtained from the output of an image sensor such as a CCD sensor is maximized. There is a method of moving the focus lens to a position. Such an AF method is referred to as a contrast detection method AF.

  The following scan method AF is known as one of the contrast detection methods AF. In the scan method AF, while moving the focus lens, AF evaluation values generated in a focus detection area (hereinafter referred to as an AF frame) set in the image sensor are stored at a plurality of focus lens positions. Then, the focus lens is moved to the position where the maximum one of the stored AF evaluation values is stored.

  As the AF evaluation value, a signal value (TES value) on the high frequency side corresponding to the contour of a subject in a video called TES among luminance signals is generally used. A plurality of pixels are present in the horizontal direction in the AF frame, and a plurality of lines in which the plurality of pixels are present are arranged in the vertical direction. A high-pass filter (HPF) process is performed on the output of each pixel in the horizontal direction to extract a high frequency component from the output. Then, the absolute value of the extracted high-frequency component is taken, and the maximum value obtained for each line is integrated to obtain the TES value.

  The sum of all pixel output values before the HPF processing in the AF frame is referred to as YI. The YI represents the luminance information of the AF frame reflecting the result of the AE process. The maximum value of YI before HPF processing is called YP. Further, the difference between the maximum value and the minimum value among the pixel output values before the HPF process for each pixel line in the AF frame is obtained, and the maximum value is called MM. When the contrast in the AF frame is high or the size of the AF frame is large, the AF evaluation value becomes large. For this reason, a focus lens position (hereinafter referred to as a peak position) at which the maximum value as the maximum value of the AF evaluation value is obtained is set as an in-focus position where the subject is assumed to be in focus.

  However, when the illuminance and contrast of the subject are low, the S / N ratio of the luminance signal deteriorates due to the influence of noise. In this case, there is a problem that the accuracy is lowered, for example, the AF evaluation value varies randomly, and as a result, the focusing accuracy is lowered.

  As a method for maintaining the focusing accuracy even when the S / N ratio of the luminance signal deteriorates, Patent Document 1 discloses the following method. In this method, the subject brightness is determined based on the evaluation value using YP, and the contrast is determined based on the evaluation value using MM. Then, the frequency characteristic of the filter used for calculating the AF evaluation value is controlled according to the subject brightness and contrast.

Japanese Patent Laid-Open No. 08-265631

  However, in the contrast determination by the MM used in the method disclosed in Patent Document 1, the MM increases due to the entry and exit of the subject with respect to noise and the imaging range even though the subject is a low contrast, and erroneously determined as high contrast. May end up. Moreover, even if the frequency characteristics of the filter used for calculating the AF evaluation value are changed, the focused state may not be obtained.

  The present invention provides an imaging apparatus capable of ensuring high focusing performance without being greatly affected by noise or the like.

An imaging apparatus according to one aspect of the present invention uses a filter from an image sensor that photoelectrically converts a subject image formed by a photographing optical system and a video signal generated by using an output signal of the image sensor. Generated by the evaluation value generating means for extracting a frequency component and generating an evaluation value indicating the contrast state of the image from the extracted frequency component, and for each time the focus element included in the photographing optical system is moved by a predetermined amount Control means for controlling the movement of the focus element to the in-focus position using the evaluated value. The control means performs a first determination process for determining whether or not the subject is a low-contrast subject using the output signal of the image sensor, and determines that the subject is a low-contrast subject by the first determination process. Secondly, it is determined whether or not the evaluation value is a low contrast evaluation value from a change in the evaluation value generated for each filter using a plurality of filters for extracting frequency components in different frequency ranges. The specific filter used for generating the evaluation value determined not to be the low contrast evaluation value by the second determination process is selected from the plurality of filters, and the specific filter is used. When a third determination process is performed to determine whether or not the change in the specific evaluation value that is the generated evaluation value satisfies a predetermined condition, and the change in the specific evaluation value satisfies the predetermined condition, Evaluation value and determining the focus position of the position where the maximum.

  The focus control method according to another aspect of the present invention is used in an imaging apparatus having an imaging element that photoelectrically converts a subject image formed by a photographing optical system. The focus control method uses a filter to extract a specific frequency component from a video signal generated using an output signal of an image sensor, and generates an evaluation value indicating the contrast state of the video from the extracted frequency component. An evaluation value generating step, and a control step for controlling movement of the focus element to the in-focus position using the evaluation value generated by the evaluation value generating step each time the focus element included in the photographing optical system is moved by a predetermined amount; Have In the control step, when a first determination process is performed to determine whether or not the subject is a low-contrast subject using the output signal of the image sensor, and it is determined by the first determination process that the subject is a low-contrast subject, A second method for determining whether or not the evaluation value is a low-contrast evaluation value from a change in the evaluation value generated for each filter using a plurality of filters for extracting frequency components in different frequency ranges A determination process is performed, and a specific filter used to generate an evaluation value that is determined not to be a low contrast evaluation value by the second determination process is selected from the plurality of filters, and is generated using the specific filter. A third determination process is performed to determine whether or not the change in the specific evaluation value that is the evaluation value satisfies the predetermined condition, and the change in the specific evaluation value satisfies the predetermined condition該特 reputation worth and determining as an in-focus position the position where the maximum.

  According to the present invention, it is possible to select a filter (specific filter) appropriate for the contrast state of the subject, and to ensure high focusing performance without being greatly affected by noise or the like.

1 is a block diagram illustrating a configuration of an imaging apparatus that is Embodiment 1 of the present invention. 3 is a flowchart illustrating AF processing in the imaging apparatus according to the first exemplary embodiment. FIG. 6 is a diagram illustrating an AF frame setting method according to the first embodiment. 3 is a flowchart illustrating filter setting processing according to the first embodiment. The flowchart explaining the 2nd low contrast determination process in the said filter setting process. The figure explaining the said 2nd low contrast determination process. 10 is a flowchart for explaining a third low contrast determination process in the filter setting process. 9 is a flowchart illustrating AF processing in an imaging apparatus that is Embodiment 2 of the present invention. FIG. 10 is a diagram illustrating a rescan setting method according to the second embodiment.

  Embodiments of the present invention will be described below with reference to the drawings.

<Configuration of imaging device>
FIG. 1 shows the configuration of an imaging apparatus such as a digital still camera or a video camera that is Embodiment 1 of the present invention. Reference numeral 101 denotes a zoom lens that moves in the optical axis direction to change the focal length of the photographing optical system (perform zooming). Reference numeral 102 denotes an aperture shutter as a light amount control unit that controls the amount of light passing through the photographing optical system. Reference numeral 103 denotes an AE processing unit that controls the operation of the aperture shutter 102. Reference numeral 104 denotes a focus lens (focus element) that moves in the optical axis direction and focuses the photographing optical system on the subject. Reference numeral 105 denotes an AF processing unit that controls the movement (position) of the focus lens 104. The zoom lens 101, the aperture shutter 102, the focus lens 104, and a lens (not shown) constitute a photographing optical system.

  Reference numeral 106 denotes an image sensor that photoelectrically converts a subject image formed by the photographing optical system. Reference numeral 107 includes a CDS circuit that removes noise from the analog output of the image sensor 106 and a circuit that performs non-linear amplification before A / D conversion on the analog output, and converts the analog output after non-linear amplification into a digital signal. D conversion unit. An image processing unit 108 performs various image processing on the digital signal from the A / D conversion unit 107 to generate a video signal.

  Reference numeral 109 denotes a system control unit (hereinafter referred to as a CPU) as control means for controlling the operation of the imaging apparatus such as a shooting sequence. Reference numeral 110 denotes a shooting mode switch. In this embodiment, the face detection mode can be turned ON / OFF by the user's operation of the shooting mode switch 110. A shooting preparation switch (SW1) 111 is operated by the user to instruct the start of shooting preparation operations such as AF and AE. Reference numeral 112 denotes a shooting switch (SW2) operated by the user to instruct generation of a recording video or recording still image and start of shooting including the recording after the shooting preparation operation.

  Reference numeral 113 denotes a face detection module which detects a human face from the video signal generated by the image processing unit 108 and sends information (position, size and detection reliability) regarding the detected face to the CPU 109.

  Reference numeral 114 denotes an operation display unit, which is an operation display unit that displays an image obtained from the video signal generated by the image processing unit 108, information for assisting operation and information indicating the state of the imaging device, and an AF frame described later. is there.

The AF processing unit 105 high-passes high-frequency components (predetermined frequency components) in one or more AF frames of the video signal generated by the image processing unit 108, that is, generated from the output signal of the image sensor 106. Extraction is performed using a filter (HPF: hereinafter simply referred to as a filter). Then, arithmetic processing such as cumulative addition of high-frequency components extracted in each AF frame is performed, and AF evaluation is a high-frequency signal (TES) corresponding to the contour of the subject included in the image in each AF frame. Generate a value signal. An AF evaluation value (also referred to as a contrast evaluation value or a focus evaluation value) that is a value of an AF evaluation value signal (TES value) indicates a contrast state of an image, in other words, a focus state of a photographing optical system. As described above, the AF processing unit 105 extracts a predetermined high-frequency component from the video signal generated using the image sensor 106 in the process of performing the AF process, and generates an evaluation value from the high-frequency component. Function as.
<AF processing>
Next, an AF process (focus control method) for performing focus control in the imaging apparatus of the present embodiment will be described using the flowchart of FIG. The AF processing is performed by the CPU 109 and the AF processing unit 105 (hereinafter collectively referred to as the CPU 109) according to the computer program.

  In step S201, the CPU 109 sets an AF frame according to the shooting mode designated by the user through the operation of the shooting mode switch 110. For example, when the face detection mode is set to ON and a face is detected by the face detection module 113, AF frames 301 to 307 are set around the face and body as shown in FIG. If no face is detected, a plurality of AF frames 308 to 316 are set as shown in FIG. If the face detection mode is set to OFF, one AF frame 317 is set at the center of the screen as shown in FIG.

  Next, in step S202 (evaluation value generation step), the CPU 109 performs an AF scan in each AF frame set in step S201. In AF scan, the focus lens 104 is moved by a predetermined amount in a scan range from a scan start position (for example, infinity end) to a scan end position (for example, closest end), and AF evaluation is performed each time the predetermined amount is moved. This is an operation of acquiring and storing a value. Thereby, a plurality of AF evaluation values are acquired (stored) within the scan range.

  Next, in step S203, the CPU 109 performs first low contrast determination (first determination processing) and second low contrast determination (second determination processing). Then, a filter (HPF) used by the AF processing unit 105 is set (selected) for each AF frame in accordance with the determination result. Detailed processing in step S203 will be described later with reference to FIGS. Further, the processing from step S203 to step S211 described later corresponds to a control step.

  Next, in step S204, the CPU 109 obtains the peak position of the AF evaluation value by calculation using the filter set in step S203. The peak position is the position of the focus lens 104 at which the maximum AF evaluation value is obtained among a plurality of AF evaluation values acquired (stored) within the scan range by AF scanning.

  Next, in step S205, the CPU 109 performs third low contrast determination (third determination processing). Then, in-focus determination (determination of whether or not a good in-focus state is obtained) is performed at the peak position obtained in each AF frame according to the result of the third low contrast determination. Detailed processing in step S205 will be described later with reference to FIG.

  Next, in step S206, the CPU 109 selects (determines) the AF frame that has been determined to obtain a good in-focus state in step S205 as the in-focus frame.

  In step S207, the CPU 109 determines whether or not the focusing frame can be selected in step S206. If the focusing frame can be selected, the process proceeds to step S208. If the focusing frame cannot be selected, step S210 is performed. Proceed to each.

  In step S208, the CPU 109 moves the focus lens 104 to the peak position (focus position) where the AF evaluation value is maximum in the focus frame selected in step S206. Then, the process proceeds to step S209.

  In step S209, the CPU 109 displays a focusing frame on the operation display unit 114 so as to be superimposed on the video. Then, the AF process ends. For example, when the AF frame including a face (face frame) among the plurality of AF frames is the in-focus frame, the face frame is displayed and the other AF frames are not displayed. At this time, it is preferable that the in-focus frame is displayed in a color that is easily recognized intuitively by the user such as green.

  On the other hand, in step S210, the CPU 109 moves the focus lens 104 to a position (fixed point) corresponding to a distance that generally has a high object existence probability. Then, the process proceeds to step S211.

In step S <b> 211, the CPU 109 displays a non-focused frame on the operation display unit 114. Then, the AF process ends. An out-of-focus frame is a frame that indicates a region where a subject is present in a video or a predetermined region, and a focus frame is used to easily recognize that a user is not in focus. Are preferably displayed in different colors (for example, yellow).
<Filter setting process>
Next, the filter setting process performed in step S203 of FIG. 2 will be described using the flowchart of FIG. This processing is also performed by the CPU 109 according to the computer program.

  In step S401, the CPU 109 compares the gain set by the AE processing unit 103 during AF scanning with a predetermined value (predetermined gain). If the gain is less than or equal to the predetermined gain, the process proceeds to step S402, and if greater than the predetermined gain, the process proceeds to step S406. If the gain is larger than the predetermined gain, the S / N ratio deteriorates and the accuracy of the AF evaluation value decreases, and it is correctly determined whether or not the subject in steps S402 and S403 described later is a low-contrast subject (low-contrast subject). Judgment may not be possible. From the following step S402, processing is sequentially performed for each set AF frame.

  In step S402, the CPU 109 determines whether or not an evaluation value generated based on the MM (hereinafter referred to as MM evaluation value) is smaller than a predetermined value (predetermined MM). This determination is referred to as a first low contrast determination. The MM performs the HPF process for each horizontal pixel line in the AF frame in the image sensor 106, that is, the maximum value and the minimum value of the luminance signal value that is the pixel output value (output signal of the image sensor 106) before passing through the filter. Is the largest of the differences. If the subject in the AF frame is a low-contrast subject with low contrast, the MM value (that is, the MM evaluation value) becomes small. Therefore, if the MM evaluation value is smaller than the predetermined MM, it is determined that the subject is a low contrast subject, and the process proceeds to step S403. If the MM evaluation value is greater than the predetermined MM, the process proceeds to step S407.

  Note that if the subject in the AF frame is a low-contrast subject, the AF evaluation value also decreases. Therefore, the first low contrast determination may be performed by determining that the subject is a low contrast subject when the maximum AF evaluation value obtained by moving the focus lens 104 is smaller than a predetermined value. The use of the AF evaluation value in this way is also included in determining whether or not the subject is a low-contrast subject using the output signal of the image sensor 106.

  In step S403, the CPU 109 generates an AF evaluation value using a filter (first filter) having a frequency characteristic (cut-off frequency) corresponding to a predetermined high-frequency component in the video signal. Also, an AF evaluation value is generated using a filter (second filter) having a frequency characteristic (cutoff frequency) corresponding to a component in a frequency range lower than the predetermined high frequency range. Then, a second low contrast determination is performed on these AF evaluation values. The second low contrast determination will be described later with reference to FIGS.

  In the following description, a filter having a frequency characteristic corresponding to a component in a predetermined high frequency range is referred to as a high pass filter, and a filter having a frequency characteristic corresponding to a component lower than the predetermined high frequency range is referred to as a low pass filter. These filters correspond to a plurality of filters for extracting frequency components in different frequency ranges from the video signal. The high-pass filter is a filter used for a normal subject in the imaging apparatus of the present embodiment. An AF evaluation value obtained using a high-pass filter is referred to as a high-pass AF evaluation value, and an AF evaluation value obtained using a low-pass filter is referred to as a low-pass AF evaluation value. After the second low contrast determination, the CPU 109 proceeds to step S404.

  In step S404, if the determination result of the high frequency AF evaluation value in the second low contrast determination is NG and the determination result of the low frequency AF evaluation value is OK, the CPU 109 proceeds to step S405. The determination result of the high-frequency AF evaluation value is NG. The AF evaluation value generated using the high-pass filter corresponds to the low contrast evaluation value, and even when this is used, a good focus state (high focus accuracy) This means that there is a high possibility that The determination result of the low-pass AF evaluation value is OK. The AF evaluation value generated using the low-pass filter does not correspond to the low-contrast evaluation value, and if this is used, a good in-focus state can be obtained. It means that the nature is high. That is, when the process proceeds to step S405, it is determined that the focusing performance is determined to be higher when the low-frequency AF evaluation value is used than when the high-frequency AF evaluation value is used.

  On the other hand, if the determination result of the high-frequency AF evaluation value in the second low contrast determination is OK, the CPU 109 proceeds to step S406. If the determination result of the low-frequency AF evaluation value is NG, although not shown, it is preferable not to use the AF frame for the AF processing.

In step S <b> 405, the CPU 109 selects a low-pass filter as a specific filter used in the AF processing unit 105. That is, the low-pass filter used to generate the AF evaluation value determined not to be the low contrast evaluation value is selected. When the determination result of the low-frequency AF evaluation value is NG,
Thereafter, the process proceeds to step S407.

  On the other hand, in step S <b> 406, the CPU 109 selects a high-pass filter as a specific filter used in the AF processing unit 105. That is, the high-pass filter used to generate the AF evaluation value determined not to be the low contrast evaluation value is selected.

  Thereafter, the process proceeds to step S407.

  In step S407, the CPU 109 stores the filter selected in step S405 or step S406.

  Next, in step S408, the CPU 109 checks whether or not there is an AF frame that has not been subjected to the processing of steps S402 to S407. If there is, the process returns to step S402, and if not, this processing ends.

In this embodiment, first, whether or not the subject is a low contrast subject is determined by the first low contrast determination. Then, when it is determined by the first low contrast determination that the subject is a low contrast subject, the following filters are selected according to the result of the second low contrast determination. That is, an AF evaluation value with higher accuracy from a normal filter for a high frequency range (high pass filter) and a filter for a lower frequency range than normal (low pass filter) corresponding to the deterioration of the S / N ratio. Select a filter (specific filter) that provides
<Second low contrast determination>
Next, the second low contrast determination performed in step S403 in FIG. 4 will be described with reference to FIGS.

  Except for special cases such as near-far competition, the AF evaluation value takes the focus lens position on the horizontal axis and the AF evaluation value on the vertical axis. Therefore, the difference between the maximum value and the minimum value of the AF evaluation value, and the portion where the AF evaluation value changes with an inclination that makes a difference equal to or greater than a predetermined value (predetermined change amount: Slope Thr) between two points (inclined portion). The shape of the mountain is determined from the length and the slope of the inclined portion. Thereby, it can be determined whether or not the AF evaluation value is a low contrast evaluation value. The low-contrast evaluation value is an AF evaluation value with little change (not having a mountain shape) that can be obtained when the subject is a low-contrast subject. This determination is the second low contrast determination.

  Here, for the sake of clarity, the expression of determining the shape of the AF evaluation value is used. However, in actuality, how the AF evaluation values generated at a plurality of scan points described later change for each filter. (That is, a change in AF evaluation value). Then, from the change in the AF evaluation value, it is determined whether or not the AF evaluation value is a low contrast evaluation value.

  The length L and the gradient SL / L of the inclined portion described above will be described with reference to FIG. SL indicates the height of the inclined portion. Points that are recognized to be inclined from the top of the mountain (point A) toward both sides are D and E points, and the width of points D and E is the width of the mountain, that is, the length L of the inclined portion. To do. The range in which the inclination is recognized to be continued is a range in which a scan point in which the AF evaluation value falls from the point A by a predetermined change amount (Slope Thr) or more continues. The scan point is a point at which the AF evaluation value is acquired while the focus lens 104 is continuously moved from the scan start position to the scan end position. The sum (SL1 + SL2) of the difference SL1 between the AF evaluation values at the points A and D and the difference SL2 between the AF evaluation values at the points A and E is SL.

  Next, the second low contrast determination process will be described with reference to the flowchart of FIG. The following processing is performed on the normal high frequency AF evaluation value and low frequency AF evaluation value.

  In step S501, the CPU 109 obtains the maximum value (Max) and the minimum value (Min) of the AF evaluation value.

  Next, in step S502, the CPU 109 obtains the length L of the inclined portion for judging the shape of the mountain of the AF evaluation value.

  Next, in step S503, the CPU 109 determines whether or not the length L of the inclined portion is equal to or greater than a predetermined value (predetermined length), and the difference between the maximum and minimum AF evaluation values is equal to or greater than a predetermined difference. If so, the process proceeds to step S504, and if not, the process proceeds to step S505.

  In step S504, the CPU 109 determines that the obtained AF evaluation value has a mountain shape corresponding to a subject having normal contrast and is not a low contrast evaluation value (the subject is not a low contrast subject). The determination result is OK. Then, this process ends.

On the other hand, in step S505, the CPU 209 determines that the obtained AF evaluation value is a low contrast evaluation value (subject is a low contrast subject) that does not have the mountain shape as described above, and sets the determination result to NG. . Then, this process ends.
<Third low contrast determination>
Next, the third low contrast determination process performed in step S205 of FIG. 2 will be described using the flowchart of FIG. In the third low contrast determination, the shape of the AF evaluation value is confirmed in more detail than in the second low contrast determination described above, and the final focus is determined. In other words, it is determined whether or not the change in the AF evaluation value (specific evaluation value) generated using the specific filter described above satisfies a predetermined condition. When the predetermined condition is satisfied, the peak position in the focusing frame selected in step S206 in FIG. 2 can be finally determined as the focusing position.

  In step S701, the CPU 109 obtains the maximum value (Max) and the minimum value (Min) of the AF evaluation value. Here, the scan point having the maximum AF evaluation value is the peak position obtained in step S206.

  Next, in step S702, the CPU 109 obtains Max-Min, which is the difference between the maximum value and the minimum value of the AF evaluation value.

  Next, in step S703, the CPU 109 obtains the length L and the height SL of the inclined portion for judging the shape of the mountain of the AF evaluation value from the scan point and the AF evaluation value.

  Next, in step S704, the CPU 109 determines whether or not the shape of the AF evaluation value is a shape that monotonously increases toward the closest side (stop on the near side). The CPU 109 determines that the near-side climbing is stopped when the following two conditions are satisfied. One condition is that the scan point at which the AF evaluation value becomes the maximum value is the closest end (closest scan end) in the scan range. Another condition is that the difference between the AF evaluation value at the closest scanning end and the AF evaluation value at the scanning point on the infinity side by one point is not less than a predetermined value (predetermined change amount). . If the CPU 109 determines that it is a near-side climb stop, the process proceeds to step S709; otherwise, the process proceeds to step S705.

  In step S705, the CPU 109 determines whether or not the shape of the AF evaluation value is a shape that monotonously increases toward the infinity side (far-side climbing stop). The CPU 109 determines that the far side climb is stopped when the following two conditions are satisfied. One condition is that the scan point at which the AF evaluation value is the maximum is the end on the infinity side (infinity side scan end) in the scan range. Another condition is that the difference between the AF evaluation value at the infinity side scan end and the AF evaluation value at the closest scan point by one point is not less than a predetermined value (predetermined change amount). . The CPU 109 proceeds to step S708 when determining that it is the infinity side climb stop and proceeds to step S706 otherwise.

  In step S706, the CPU 109 determines that the length L of the inclined portion is equal to or greater than the predetermined length, the gradient (average value) SL / L of the inclined portion is equal to or greater than the predetermined value (predetermined gradient), and the maximum AF evaluation value (Max). ) And the minimum value (Min) is determined whether or not the predetermined condition that the difference is equal to or larger than the predetermined difference is satisfied. If so, the CPU 109 proceeds to step S707, otherwise proceeds to step S708.

  In step S707, since the obtained AF evaluation value has a mountain shape, the CPU 109 sets the determination result to OK (focusing), assuming that the subject is a subject having a good focusable contrast. Then, this process ends.

  In step S708, the CPU 109 determines that the obtained AF evaluation value does not have a mountain shape and the subject is a low-contrast subject that cannot be focused well. ). Then, this process ends.

  In step S709, since the obtained AF evaluation value does not have a mountain shape, but has a near-end climbing shape, there is a possibility that a peak position may exist on the closer side. The determination result is NO (not determined). Then, this process ends.

  As described above, in this embodiment, as described above, when the first low contrast determination determines that the subject is a low contrast subject, the AF is performed according to the result of the second low contrast determination. An appropriate filter is selected (set) for the shape of the evaluation value. Then, final focus determination (focus confirmation) is performed by performing third low contrast determination on each AF frame for which the peak position is calculated using the selected filter. By performing these multi-stage low contrast determinations, it is possible to set an appropriate filter according to the contrast state of the subject, and to improve the focusing performance of the imaging apparatus without being greatly affected by noise.

  In the present embodiment, the case where the specific filter is selected from the two filters of the high-pass filter and the low-pass filter by the second low contrast determination has been described. However, the specific filter can be selected from a larger number of filters. It may be.

  Next, a second embodiment of the present invention will be described. Since the configuration of the imaging apparatus of the present embodiment is the same as the configuration described in the first embodiment (FIG. 1), the same reference numerals as those in FIG. Instead of

  The flowchart in FIG. 8 shows AF processing performed by the imaging apparatus of the present embodiment. This AF process corresponds to a modification of the AF process described in the first embodiment with reference to FIG.

  In step S801, the CPU 109 sets an AF frame according to the shooting mode designated by the user through the operation of the shooting mode switch 110.

  In step S802, the CPU 109 performs AF scanning in each AF frame set in step S801.

  Next, in step S803, the CPU 109 performs a first low contrast determination and a second low contrast determination. Then, a filter (HPF) used by the AF processing unit 105 is set (selected) for each AF frame in accordance with the determination result. The processing at this step is the same as the processing described in the first embodiment with reference to FIGS.

  Next, in step S804, the CPU 109 obtains the peak position of the AF evaluation value by calculation using the filter set in step S803.

  Next, in step S805, the CPU 109 performs a third low contrast determination. Then, in-focus determination at the peak position obtained in each AF frame is performed according to the result of the third low contrast determination. The processing at this step is the same as the processing described in the first embodiment with reference to FIG.

  In step S806, the CPU 109 selects the AF frame that has been determined in step S805 that a good focus state can be obtained as the focus frame.

  In step S807, the CPU 109 determines whether or not the in-focus frame can be selected in step S806. If the in-focus frame can be selected, the process proceeds to step S811, and if the in-focus frame cannot be selected, step S808. Proceed to each.

  In step S808, the CPU 109 determines whether rescanning has been performed. If it has not been performed yet, the process proceeds to step S809. If rescanning has already been performed, the process proceeds to step S813.

  In step S809, the CPU 109 performs rescan settings. Here, the rescan setting will be described with reference to FIG.

  FIG. 9A shows the shape of the entire peak of the AF evaluation value that can be estimated from the AF evaluation value obtained in the first AF scan performed in step S802. Based on the result of this first AF scan, FIG. Configure scan settings.

  In FIG. 9B, the AF evaluation value when the rescan is performed with the predetermined rescan settings (scan width and scan point) is applied to the shape of the AF evaluation value in FIG. 9A. It is a prediction. In the predetermined rescan setting, the number of scan points is the same as that in the first AF scan setting, but the scan width is changed to be narrower. The white squares in the graph of FIG. 9B are prediction results of AF evaluation values at each scan point in the rescan.

  In the prediction result of FIG. 9B, the difference between the AF evaluation values corresponding to the slope of the AF evaluation value near the peak position is smaller than the predetermined change amount (Slope Thr), and the difference between the maximum value and the minimum value of the AF evaluation value. Is smaller than a predetermined difference (defMax-Min). For this reason, even if rescanning is performed with a predetermined setting, it is predicted that the in-focus state is determined by the third low contrast determination.

  As described above, when it is predicted that the in-focus determination will be made again by the third low contrast determination in the current rescan setting, the in-focus determination is made by the third low contrast determination. Change the settings for rescanning (so that the predetermined conditions are met). In other words, when the AF evaluation value (specific evaluation value) generated by the first AF scan does not satisfy the predetermined condition in the third low contrast determination, the AF evaluation is performed according to the first AF evaluation value. Change the settings for regenerating values (rescan settings). Then, the AF evaluation value (specific evaluation value) is regenerated using the changed setting for rescanning.

  In this embodiment, for example, as shown in FIG. 9C, the width of the AF scan (scan range) is widened to change the setting to have more scan points. Thereby, the focusing performance by rescanning can be guaranteed.

  As another rescan setting, as shown in FIG. 9 (d), the aperture value of the aperture 102 is set to an open value, and the depth of field of the photographing optical system is reduced, thereby increasing the AF evaluation value peak. Can be made more steep so as to satisfy a predetermined condition in the third low contrast determination.

  Furthermore, as another rescan setting, as shown in FIG. 9E, the drive mode of the image sensor 106 is a mode in which the number of lines in the vertical direction (that is, the number of pixels for generating an AF evaluation value) is larger. It is also possible to rescan by switching to. Thereby, the AF evaluation value becomes high, and it is possible to satisfy a predetermined condition in the third low contrast determination.

  In step S810, the CPU 109 performs a rescan (second AF scan) for regenerating (re-acquiring) the AF evaluation value according to the setting in step S809. Then, the process returns to step S803.

  In step S811, the CPU 109 moves the focus lens 104 to the peak position (focus position) in the focus frame selected in step S806. Then, the process proceeds to step S812.

  In step S812, the CPU 109 displays an in-focus frame on the operation display unit 114 so as to be superimposed on the video. Then, the AF operation ends.

  On the other hand, in step S813, the CPU 109 moves the focus lens 104 to a fixed point. Then, the process proceeds to step S814.

  In step S814, the CPU 109 displays an out-of-focus frame on the operation display unit 114. Then, the AF operation ends.

  According to the present embodiment, the rescan setting is determined based on the AF evaluation value obtained in the first AF scan. For this reason, even when the in-focus state cannot be obtained only by changing the filter settings (filter frequency characteristics) in the first AF scan, it is possible to improve the in-focus performance by re-scanning.

  Each embodiment described above is only a representative example, and various modifications and changes can be made to each embodiment in carrying out the present invention.

  An imaging apparatus such as a digital still camera or a video camera with high AF performance can be provided.

104 Focus lens 106 Image sensor 105 AF processing unit 109 System control unit (CPU)

Claims (8)

An image sensor that photoelectrically converts a subject image formed by the photographing optical system;
An evaluation value generating means for extracting a specific frequency component from a video signal generated using the output signal of the imaging device using a filter and generating an evaluation value indicating a contrast state of the video from the extracted frequency component; ,
Control means for controlling movement of the focus element to the in-focus position using the evaluation value generated by the evaluation value generation means each time the focus element included in the photographing optical system is moved by a predetermined amount. And
The control means includes
Performing a first determination process for determining whether or not the subject is a low-contrast subject using the output signal of the image sensor;
When the first determination process determines that the subject is the low-contrast subject, the evaluation generated for each filter using a plurality of filters for extracting the frequency components in different frequency ranges. A second determination process is performed to determine whether the evaluation value is a low contrast evaluation value from a change in value,
From the plurality of filters, select a specific filter used for generating the evaluation value determined not to be the low contrast evaluation value by the second determination process,
Performing a third determination process for determining whether a change in the specific evaluation value, which is the evaluation value generated using the specific filter, satisfies a predetermined condition;
When the change in the specific evaluation value satisfies the predetermined condition, an image pickup apparatus that determines the position where the specific evaluation value is maximized as the focus position.   When the change of the specific evaluation value does not satisfy the predetermined condition in the third determination process, the control unit changes a setting for regenerating the evaluation value according to the specific evaluation value. The imaging apparatus according to claim 1, wherein the specific evaluation value is regenerated in the changed setting.   The first determination process determines whether or not the subject is the low-contrast subject based on whether or not a difference between a maximum value and a minimum value of an output signal of the image sensor is smaller than a predetermined difference. The imaging apparatus according to claim 1 or 2.   The said 1st determination process determines whether the said to-be-photographed object is the said low-contrast object by whether the maximum value of the said evaluation value is smaller than predetermined value, The said 1st determination process is characterized by the above-mentioned. Imaging device.   In the second determination process, the evaluation value is reduced from the difference between the maximum value and the minimum value in the evaluation value generated for each filter, and the length and gradient of the portion where the evaluation value is changing. The imaging apparatus according to claim 1, wherein it is determined whether or not the contrast evaluation value.   The third determination process includes: an average value of a length and a gradient of a portion where the specific evaluation value is changed; a difference between a maximum value and a minimum value of the specific evaluation value; and the focus element that maximizes the specific evaluation value The imaging apparatus according to claim 1, wherein it is determined whether or not a position satisfies a predetermined condition.   The control means includes a range for moving the focus element, a number of positions of the focus element that generates the evaluation value, an aperture value of the photographing optical system, and a change in the setting for regenerating the evaluation value. The imaging apparatus according to claim 2, wherein any one of the number of pixels of the imaging element for generating an evaluation value is changed. A focus control method for an image pickup apparatus having an image pickup element for photoelectrically converting a subject image formed by a photographing optical system,
An evaluation value generating step of extracting a specific frequency component from the video signal generated using the output signal of the image sensor using a filter and generating an evaluation value indicating a contrast state of the video from the extracted frequency component; ,
A control step for controlling the movement of the focus element to the in-focus position using the evaluation value generated by the evaluation value generation step each time the focus element included in the photographing optical system is moved by a predetermined amount. And
In the control step,
Performing a first determination process for determining whether or not the subject is a low-contrast subject using the output signal of the image sensor;
When the first determination process determines that the subject is the low-contrast subject, the evaluation generated for each filter using a plurality of filters for extracting the frequency components in different frequency ranges. A second determination process is performed to determine whether the evaluation value is a low contrast evaluation value from a change in value,
From the plurality of filters, select a specific filter used for generating the evaluation value determined not to be the low contrast evaluation value by the second determination process,
Performing a third determination process for determining whether a change in the specific evaluation value, which is the evaluation value generated using the specific filter, satisfies a predetermined condition;
When the change of the specific evaluation value satisfies the predetermined condition, a position at which the specific evaluation value is maximized is determined as the focus position.