JP2006011035A - Automatic focusing device, imaging apparatus, and focusing position detecting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an automatic focusing device capable of obtaining a correct focal point by correcting a lens position obtained by a contrast detection system, and to provide an imaging apparatus and a focal point detecting method. <P>SOLUTION: The automatic focusing device comprises: optical lenses 101 including a focusing lens 101a for forming an imaging in a prescribed position; a lens drive means 110 for moving the focusing lens 101a; an imaging device 102 for photoelectrically converting the imaging formed by the optical lenses 101, and outputting an image signal; and an electronic shutter 103 for adjusting charge accumulation time of the imaging device 102. The automatic focusing device detects the high frequency component of the image signal while moving the focusing lens 101a, and finds a focusing position at which the high frequency component is maximum. Further, the automatic focusing device calculates an amount of correction of the detected focusing position based upon the charge accumulation time of the imaging device 102. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an automatic focusing apparatus and a focusing position detection method for determining a focusing position of a lens by detecting contrast of an image signal.

  In an automatic focusing device that performs focusing in a digital video camera or the like, an active method using a distance measuring means using infrared rays or a passive method using an imaging signal photographed through a lens has been used. Passive systems having features such as miniaturization and macro photography have come to dominate.

  As one of the passive methods, there is a contrast detection method for obtaining a focal point by moving a lens position so that a high-frequency component in an image signal obtained by forming an image on an image sensor with a lens, that is, a contrast is maximized. . This contrast detection method does not require distance measuring means and has a simple configuration, so it is suitable for cost reduction and miniaturization, and is widely used as a control method in an automatic focusing device.

In the contrast detection method, how to accurately focus at high speed and focus is an issue in performance competition. For this reason, conventionally, a technique for speeding up the focusing operation by predicting the in-focus point that is the maximum point of the high-frequency component based on the characteristics of the high-frequency component of the imaging signal has been proposed (for example, Patent Document 1). Thru 3).
JP-A-62-203485 Japanese Patent Laid-Open No. 62-208012 Japanese Patent No. 2912649

  On the other hand, in digital video cameras and digital still cameras equipped with such an automatic focusing device, the size of the image sensor and the lens are shortened in order to further reduce the size. One way to reduce the size is to use the electronic shutter function of the image sensor instead of the aperture mechanism that has been used for exposure control in the past, with the aim of simplifying the lens structure and reducing the size of the lens. Some control. This method has an advantage that it is easy to manufacture a small camera block because the diaphragm mechanism is unnecessary or simple.

  However, performing exposure control using an electronic shutter has caused problems that have not been considered in the past. Specifically, when the focusing lens is moved to detect the peak position (maximum point) of the high-frequency component of the subject image, the time difference between the exposure time of the subject image on the image sensor and the detection time of the high-frequency component is Further, since the time difference changes depending on the shutter speed, the peak position of the high-frequency component detected during the lens movement is detected by deviating from the correct focal point, and the accuracy of focusing is lowered.

  Such a phenomenon occurs also in a CCD sensor or a CMOS sensor, for example, regardless of the type of the image sensor. Furthermore, since the exposure method for obtaining the shutter speed differs depending on the type of the image sensor, it is necessary to separately consider the correction of the focus shift.

  The present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide an automatic focusing apparatus capable of correcting the lens position obtained by the contrast detection method and obtaining an accurate focal point, and It is to provide a focus position detection method.

  An automatic focusing device according to the present invention includes an optical lens system including a focusing lens for adjusting an imaging imaging position, a driving unit that moves the focusing lens, and an image formed by the optical lens system. An image sensor that photoelectrically converts the captured image and outputs an image signal; an electronic shutter unit that adjusts a charge accumulation time of the image sensor; and a high-frequency component of the image signal is detected while moving the focusing lens A second means for calculating a correction amount for the in-focus position detected by the first means on the basis of a first in-focus position at which the high-frequency component is maximized and a charge accumulation time of the image sensor; It has a means. Thereby, since the focusing lens can be moved to the correct focusing position, an accurate focusing point can be obtained.

  In addition to the charge accumulation time, the second means is based on the moving speed information of the focusing lens and the type information of the image sensor, and is used for the in-focus position detected by the first means. The correction amount may be calculated. Thereby, the correction amount according to the type of the image sensor can be calculated.

  In the case where the image sensor is a CMOS image sensor, the second means further includes the second frame based on an image frame gate time required for operating a high frequency component extraction range on the image signal. It is desirable to calculate a correction amount for the in-focus position detected by means 1. Thereby, the influence of the fluctuation of the image frame gate time can be corrected, and the focal point can be obtained more accurately.

  Further, the second means may calculate a correction amount for the focus lens position detected by the first means, further based on a charge readout cycle from the image sensor. Thereby, even when the charge readout cycle, that is, the frame rate changes, the focal point can be obtained accurately.

  In addition, an imaging apparatus according to the present invention includes the above-described automatic focusing device.

  Furthermore, the focus position detection method according to the present invention detects a high frequency component from an image signal of a subject imaged on an image sensor through an optical lens, and moves the position of the optical lens so that the detected high frequency component is maximized. A focus position detection method for performing focusing, wherein a first step of detecting a high-frequency component of the image signal while moving the optical lens and detecting a focus position at which the high-frequency component is maximized; Based on the charge accumulation time of the image sensor, a second step of calculating a correction amount for the focus position detected by the first means, and a focus position corrected by the correction amount calculated by the second step. And a third step of moving the optical lens.

  According to the present invention, it is possible to provide an automatic focusing apparatus, an imaging apparatus, and a focusing position detection method capable of correcting the lens position obtained by the contrast detection method and obtaining an accurate focusing point.

Embodiment 1 of the Invention
FIG. 1 is a configuration diagram of an automatic focusing apparatus according to an embodiment of the present invention. The lens 101 is an optical lens for forming an image of a subject on the image sensor 102. A lens driving unit 110 controlled by a focusing control circuit 107 described later moves the position of the focusing lens 101a included in the lens 101 in the optical axis direction to perform focusing. The image sensor 102 is a sensor that converts an optical signal incident through the lens 101 into an electrical signal, and is, for example, a CCD sensor or a CMOS sensor.

  The electronic shutter 103 is means for giving a timing signal for determining the charge accumulation time, that is, the shutter speed, to the image sensor 102. Specifically, the charge accumulation time is electronically controlled by providing a timing signal for resetting the charge accumulated in the image sensor and a timing signal for reading the charge accumulated in the image sensor 102.

  The signal processing circuit 104 converts an image signal obtained by the image sensor 102 into a digital image signal. Reference numeral 104 a denotes an image signal output terminal from the image processing circuit 104. The automatic focusing circuit 105 performs a focusing operation by moving the position of the focusing lens 101a included in the lens 101 so that the high-frequency component of the image signal is extracted to increase the contrast.

  As shown in FIG. 1, the automatic focusing circuit 105 includes a high-pass filter 106, a detection circuit 107, a focusing control circuit 108, and a correction amount calculating means 109. Here, the high-pass filter 106 is a filter that extracts a high-frequency component from the image signal obtained by the image sensor 102, and is a high-pass filter or a high-pass band-pass filter. The detection circuit 107 is a circuit that detects a high-frequency component obtained through the high-pass filter 106, integrates the detected signal, and calculates a high-frequency component amount. The obtained integrated value (high-frequency component amount) is automatically focused. It outputs to the focusing control circuit 108 as a signal for use (hereinafter referred to as an AF signal).

  The focus control circuit 108 is a circuit that executes an automatic focus operation by moving the position of the focus lens 101a by the lens driving means 109 so that the AF signal becomes maximum. The lens driving means 109 is a stepping motor that moves the lens position.

  The correction amount calculating unit 109 performs a calculation described later based on a signal S1 indicating the shutter speed that the electronic shutter 103 instructs the image sensor 102 and a signal S2 indicating the lens moving speed output from the focusing control circuit 108. Then, the lens position correction amount for the in-focus lens position obtained by the focus control circuit 108 is calculated. The calculated lens position correction amount is input to the focus control circuit 108. The focusing control circuit 108 instructs the lens driving unit 110 to move the focusing lens 101a to a position reflecting the lens position correction amount.

  There is a delay time from obtaining an image in the image sensor 102 to obtaining an AF signal. Since the focusing lens 101a is moved during this delay time, in order to obtain an accurate focusing point, the lens movement during the delay time is obtained after obtaining the lens position that becomes the focusing point from the AF signal. The amount needs to be corrected.

  Next, a method for calculating the lens position correction amount in the correction amount calculation unit 109 will be described with reference to FIGS. FIG. 2 is a diagram showing the charge accumulation / readout timing when the image sensor 102 is a CCD image sensor. FIG. 3 is a diagram showing the charge accumulation / reading timing in the same manner as in FIG. 2 in the case where the image sensor 102 is a CMOS image sensor. FIG. 4 is a graph showing the relationship between the lens position deviation from the correct focal point and the shutter speed.

  First, an example in which the image sensor 102 is an interline transfer type CCD image sensor will be described with reference to FIG. In FIG. 2, (a) a reset pulse instructs the timing of discharging the charge accumulated in the CCD image sensor. The reset pulse is input from the electronic shutter 103 to the image sensor 102 (CCD image sensor in the example of FIG. 2).

  The (b) transfer pulse in FIG. 2 indicates the timing for transferring the electric charge accumulated in the photodiode in the CCD image sensor to the adjacent vertical CCD. The period from the reset pulse to the transfer pulse is the “charge accumulation time (Sd in the figure)”. The charge accumulation time Sd has the same meaning as the shutter speed.

  The (c) AF signal capture timing in FIG. 2 indicates a timing at which a high frequency component is extracted from an image signal obtained by the image sensor 102 (CCD image sensor) and obtained as an AF signal. Here, the time from when charge accumulation in the CCD image sensor is completed by the transfer pulse until the AF signal is obtained is referred to as “read delay time (Dd in the figure)”.

  The shutter speed (d) in FIG. 2 schematically shows the charge accumulation time from the reset pulse to the transfer pulse.

From the above, in the case of FIG. 2, the delay time Vdd until the AF signal is obtained from the middle point of the charge accumulation time indicated by the black triangle is
Vdd = Sd / 2 + Dd (1)
It becomes. Thus, it can be seen that the delay time Vdd when the image sensor 102 is a CCD image sensor is determined by the charge accumulation time Sd and the read delay time Dd. Therefore, the delay time Vdd changes when the charge accumulation time Sd, that is, the shutter speed changes.

  Next, a focus error caused by the delay time Vdd and a method for correcting the error when performing the focusing operation by the contrast detection method will be described with reference to FIG. 4 indicates the position of the lens 101, specifically, the position of the focusing lens 101a included in the lens 101. The vertical axis represents the amount of high-frequency components in the image signal that is an AF signal. Vf1 in the drawing represents the relationship between the lens position and the high frequency component when it is assumed that the position of the focusing lens 101a does not move until the charge accumulation of the image sensor 102 is completed and the high frequency component is obtained as the AF signal. It is the shown graph. The lens position L1 at which the high-frequency component is maximum (max) at Vf1 is an accurate focal point.

  On the other hand, in the actual focusing operation, the time when the position of the focusing lens 101a is moved and the AF signal is obtained before the charge accumulation of the image sensor 102 is completed and the high frequency component is obtained as the AF signal. If the lens positions are taken together, the lens position will be shifted. Vf2 in FIG. 4 indicates a case where the lens position is shifted. In Vf2, the deviation between the lens position L2 at which the high-frequency component is maximum and the accurate focus position L1 is caused by the delay time Vdd shown in the above equation (1). For this reason, the longer the charge accumulation time Sd, that is, the slower the shutter speed, the greater the deviation of the lens position from the correct focal point. Vf3 in FIG. 4 shows an example in which the shutter speed is slower than Vf2, and in this case, the lens position shift further increases.

  Therefore, in order to correct the lens position to the accurate focus position L1, it is only necessary to obtain the magnitude of the deviation of the lens position from the focus position L1 and correct the lens position by that amount.

  In the present invention, the correction amount calculating means 109 obtains the amount of movement of the focusing lens 101a during the delay time Vdd from the delay time Vdd expressed by the equation (1) and the moving speed of the focusing lens 101a. The focus control circuit 108 corrects the in-focus lens position based on the movement amount calculated by the correction amount calculation unit 109, so that an accurate in-focus point can be obtained even when the shutter speed changes.

  Next, an example in which the image sensor 102 is a CMOS image sensor will be described with reference to FIG. In FIG. 3, (a) a reset pulse indicates the timing for discharging the charge accumulated in the CMOS image sensor. The reset pulse is input from the electronic shutter 103 to the image sensor 102 (a CMOS image sensor in the example of FIG. 3). Note that the position of the reset pulse in FIG. 3 indicates, as an example, the start position of an image frame gate, which will be described later, that is, the timing at which charge accumulation is started for the pixel that is scanned first in the image frame gate.

  The (b) vertical selection pulse in FIG. 3 indicates the timing for reading out the charges accumulated in the photodiode. In FIG. 3, the position of the vertical selection pulse indicates the timing at which the head position of the image frame gate is scanned in the same manner as the reset pulse. The period from the reset pulse to the vertical selection pulse is the “charge accumulation time (Sm in the figure)”.

  The (c) image frame gate in FIG. 3 is set to narrow down the target range of the AF signal on the image signal. That is, among the time required for scanning for one frame shown by the sensor output in FIG. 3D, the image range scanned within the time of the image frame gate in FIG. 3C becomes the target range of the AF signal. In the CMOS image sensor, when reading the electric charges by sequentially scanning the inside of the image frame gate, a shift in the accumulation time occurs for each scanning line. Therefore, a finite time is required until the scanning inside the image frame gate is completed. This time is the image frame gate time Gm in the figure.

  The (e) AF signal capture timing in FIG. 3 indicates the timing at which a high-frequency component is extracted from the image signal obtained by the image sensor 102 (CMOS image sensor) and obtained as an AF signal. Here, Dm is a “read delay time” from when the image frame gate is closed until an AF signal is obtained, and corresponds to Dd in the case of the CCD image sensor of FIG.

  The (f) shutter speed in FIG. 3 schematically shows the charge accumulation time from the reset pulse to the vertical selection pulse. Further, the image frame gate time Gm is a time required for scanning the entire image frame gate that designates the image signal range that is the target of the AF signal. In the CMOS image sensor, when the electric charges are read out by sequentially operating in the vertical direction, an accumulation time shift occurs for each scanning line, so that an image frame gate time Gm is generated.

In the case of a CMOS image sensor, the delay time Vdm corresponding to Vdd described in FIG.
Vdm = (Sm + Gm) / 2 + Dm (2)
It is represented by Thus, it can be seen that the delay time Vdm when the image sensor 102 is a CMOS image sensor is determined by the charge accumulation time Sd, the image frame gate time Gm, and the readout delay time Dm. Therefore, in the case of a CMOS image sensor, not only the charge accumulation time Sm but also the AF signal range, that is, the focusing range on the image signal, changes the image frame gate time Gm, resulting in a delay. The time Vdm will also change.

  In the present invention, the correction amount calculating means 109 obtains the amount of movement of the focusing lens 101a during the delay time Vdm from the delay time Vdm expressed by the equation (2) and the moving speed of the focusing lens 101a. The in-focus control circuit 108 corrects the in-focus lens position based on the movement amount calculated by the correction amount calculating unit 109, whereby an accurate in-focus point can be obtained.

  Note that not only the shutter speed but also the time corresponding to the delay time Vdd or Vdm fluctuates due to the change in the charge accumulation time even when the frame rate when reading an image from the image sensor 102 changes.

  For example, when displaying a moving image for use in determining a camera angle on a liquid crystal monitor such as a digital still camera, unlike the case of a digital video camera, it is not necessary to reproduce an image with a constant field period. If the image is dark, the frame rate is reduced in order to increase the charge accumulation time. A case where the frame rate changes in this way will be described with reference to FIG.

  The pulse waveforms in FIGS. 5A to 5E indicate vertical selection pulses. Except (d) in the figure, the pulse interval corresponds to the charge accumulation time in the image sensor. (D) corresponds to the case where the discharge by the reset pulse is performed and the charge accumulation time is short. Also, the trapezoid shown below the pulse waveform schematically shows the charge accumulation time for the image sensor, similar to that of FIG.

  FIG. 5A shows an example of moving image accumulation / readout in a digital video camera. In the case of the NTSC system, images are output at a period of 60 fields per second by interlace scanning. An image of one frame is obtained with two fields, but the resolution of the moving image is increased by increasing the readout cycle.

  On the other hand, a digital still camera normally performs progressive scanning and often reads out one frame image at a time. FIG. 5B shows this case, and when the number of pixels such as VGA is relatively small, about 30 frames / second is common. This image is displayed on a liquid crystal monitor of the camera and the camera angle is operated.

  As described above, with a digital still camera, it is not necessary to pay much attention to moving images. Therefore, when the illuminance of the subject decreases and the imaging signal level decreases, the frame rate is reduced and charge storage is performed. Increasing time is done. For example, FIG. 5C shows a case where the frame rate is reduced to 15 frames / second. In the figure, even when the frame rate is changed, the read rate is constant. Of course, the read rate may be changed according to the change of the frame rate. FIG. 5D shows a case where the charge accumulation time is shortened by a reset pulse from the electronic shutter 103. FIG. 5E shows a case where the frame rate is further reduced to 7.5 frames / second. Is shown.

  The black triangles in FIG. 5 indicate the intermediate points of the respective charge accumulation times, and the white triangles indicate the AF signal capture timing. As described above, when the charge accumulation time is changed by changing the frame rate, the midpoint of the charge accumulation time is also changed. As a result, the delay time from the middle point of the charge accumulation time described above with reference to FIGS. 2 and 3 until the signal is taken in as an AF signal varies depending on the frame rate. However, even in this case, the lens can be moved to an accurate in-focus position by calculating the delay time Vdm or Vdd described above and correcting the lens position. Can do.

Embodiment 2 of the Invention
FIG. 6 is a configuration diagram of the automatic focusing device according to the embodiment of the present invention. Parts having the same functions as those of the configuration according to the first embodiment of the invention shown in FIG. 1 are denoted by the same reference numerals as those in FIG. In FIG. 6, the correction amount calculation means 209 includes frame rate information in addition to the signal S1 indicating the shutter speed that the electronic shutter 103 instructs the image sensor 102 and the signal S2 indicating the lens moving speed output from the focus control circuit 108. The lens position correction amount for the in-focus lens position obtained by the in-focus control circuit 108 is calculated based on the input of the signal S3 indicating the signal S and the signal S4 indicating the image sensor type. Since the lens position correction amount calculation method is the same as that described in the first embodiment of the invention, detailed description thereof is omitted, but the charge accumulation time Sd or Sm is calculated based on the input of frame rate information. What is necessary is just to calculate, and what is necessary is just to select said Formula (1) or (2) based on the input of image sensor classification.

  The calculated lens position correction amount is transmitted to the focusing control circuit 108 as in the automatic focusing device described in the first embodiment of the invention, and the focusing lens 101a is moved to a position reflecting the lens position correction amount. Therefore, the focus control circuit 108 instructs the lens driving unit 110 to do so.

  In the configuration shown in FIG. 6, the correction amount calculation unit 109 obtains the shutter speed, frame rate, and image sensor type from the electronic shutter 103, and obtains the lens movement speed from the focus control circuit 108. If the other part having the information, for example, the signal processing circuit 103 has such information, it may be obtained from the signal processing circuit 103. The present invention corrects the lens position based on these pieces of information, and it goes without saying that the source of these pieces of information is not limited.

Embodiment 3 of the Invention
A configuration of the imaging apparatus according to the present embodiment, which includes the automatic focusing apparatus described in the first embodiment of the invention, will be described with reference to FIG. It should be noted that the imaging apparatus provided with the automatic focusing device according to the present invention can be in various forms. The configuration of FIG. 7 shows an example.

  A CPU 601 in the figure is a controller that controls the entire imaging apparatus. Specifically, normal operations such as shooting, storage of image data in the recording medium 607 via the recording medium IF (interface) 606, acceptance of an operation by the operator via the operation switch 604, and display on the liquid crystal monitor 605, etc. In addition to the control of the processing performed by the imaging apparatus, the control according to the present invention such as the above-described focusing control, correction amount calculation, and driving of the focusing lens 101a is performed. The control in the CPU 601 may be performed by loading a control program stored in the ROM 602 into the RAM 603 and executing it. The RAM 603 is also used as a work area when the CPU 601 performs the above-described control.

  7 has the same function as the automatic focusing apparatus according to the first embodiment of the invention shown in FIG. Further, the principle and method for obtaining an accurate in-focus position by correcting the lens position are the same as those described in the first embodiment of the present invention, and therefore description thereof is omitted.

It is a block diagram of the automatic focusing apparatus concerning Embodiment 1 of invention. It is a timing chart for demonstrating the operation | movement which acquires a high frequency component from the image obtained with the image pick-up element. It is a timing chart for demonstrating the operation | movement which acquires a high frequency component from the image obtained with the image pick-up element. It is a figure which shows the relationship between the shift | offset | difference of a lens position, and shutter speed. It is a figure which shows the change of the charge accumulation time by the difference in a frame rate. It is a block diagram of the automatic focusing apparatus concerning Embodiment 2 of invention. It is a block diagram of the imaging device concerning Embodiment 3 of invention.

Explanation of symbols

101 lens, 101a focusing lens, 102 imaging device, 103 electronic shutter, 104 signal processing circuit, 105 automatic focusing circuit, 108 focusing control circuit, 109 correction amount calculating means, 110 lens driving means

Claims (6)

An optical lens system including a focusing lens for adjusting the imaging position of the imaging;
Driving means for moving the focusing lens;
An image sensor that photoelectrically converts an image formed by the optical lens system and outputs an image signal; and
Electronic shutter means for adjusting the charge accumulation time of the image sensor;
Detecting a high frequency component of the image signal while moving the focusing lens, and detecting a focusing position where the high frequency component is maximized;
Second means for calculating a correction amount for the in-focus position detected by the first means based on the charge accumulation time of the image sensor;
An automatic focusing device.   The second means is a correction amount for the in-focus position detected by the first means based on the moving speed information of the focusing lens and the type information of the image sensor in addition to the charge accumulation time. The automatic focusing device according to claim 1, wherein The image sensor is a CMOS image sensor,
The second means further calculates a correction amount for the in-focus position detected by the first means based on an image frame gate time necessary for operating a high-frequency component extraction range on the image signal. The automatic focusing device according to claim 1 or 2.   The automatic means according to any one of claims 1 to 3, wherein the second means calculates a correction amount for the in-focus lens position detected by the first means based further on a charge readout period from the image sensor. Focusing device.   An imaging apparatus comprising the automatic focusing device according to claim 1. A focus position detection method for detecting a high frequency component from an image signal of a subject imaged on an image sensor through an optical lens, and moving the position of the optical lens so that the detected high frequency component is maximized. ,
Detecting a high frequency component of the image signal while moving the optical lens, and detecting a focusing position where the high frequency component is maximized;
A second step of calculating a correction amount for the in-focus position detected by the first means based on the charge accumulation time of the image sensor;
A third step of moving the optical lens to the in-focus position corrected by the correction amount calculated in the second step;
In-focus position detection method.

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