JP5454546B2 - Autofocus device - Google Patents

Description

  The present invention relates to an autofocus device and a camera provided in a camera or the like, and aims to improve focusing accuracy.

  A camera that performs contrast AF (autofocus) control is known. In the contrast method, a subject is imaged by an image sensor such as a CCD to obtain an image signal, a component in a predetermined spatial frequency band is extracted from the signal in a predetermined AF area, and the absolute value is integrated to obtain a focus evaluation value. Is calculated. This focus evaluation value is an amount corresponding to the contrast of the focus detection area, and the higher the contrast, the higher the value. Since the contrast of the image becomes higher as the focusing lens is closer to the in-focus position, the lens position where the focus evaluation value reaches a peak can be determined as the in-focus position. By driving, it is possible to focus on the subject in the AF area with high accuracy.

  A camera is known in which the above-described contrast AF is performed with the aperture fully open (see, for example, Patent Document 1). This is to obtain the focus evaluation value calculation image data in a state where the depth of field is the shallowest (aperture open state), thereby facilitating detection of the peak position and thus improving the focus accuracy.

JP 2004-280048 A

  However, in practice, due to the spherical aberration of the lens, the best image plane position does not match when the aperture is opened and when the aperture is reduced, and the best image plane position changes due to the aperture stop. For this reason, AF based on the data when the aperture is fully open is effective for shooting when the aperture is fully open or when the aperture is small. However, when shooting with a large aperture, a focus error occurs due to the shift of the best image plane.

An autofocus device according to a first aspect of the present invention receives a light beam that has passed through a photographing lens and a diaphragm and detects a focus adjustment state, and performs focus adjustment control based on a detection result of the focus detection unit. A control unit; and a determination unit that determines a focus detection aperture value that is set when detecting the focus adjustment state based on a shooting aperture value, wherein the determination unit has a first luminance value as a luminance of the subject. wherein when photographing aperture is smaller than a predetermined value, the photographing aperture is enough to determine the focus detection aperture said as focus detection aperture value increases significantly, the luminance of the subject is in the first luminance value in wherein when shooting aperture value is larger than the predetermined value in the case, the photographing aperture even increases the focus detection aperture value become not the focus detecting diaphragm as large And determining the.

  According to the present invention, it is possible to minimize the influence of the movement of the best image plane position due to the narrowing down, and to suppress a decrease in the reliability of focus detection due to insufficient light quantity, thereby realizing highly accurate autofocus control.

The control block diagram of the digital camera in one Embodiment of this invention. The figure explaining the movement of the best image plane position by narrowing down. The flowchart which shows the procedure of AF control. The diagram used when calculating | requiring AF aperture value. The flowchart which shows the control procedure in other embodiment.

An embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a functional block diagram of a digital still camera according to this embodiment. The photographing optical system 30 includes a diaphragm 31 and a focusing lens 32, and the transmitted light flux is guided to the image pickup device 2 such as a CCD. The photoelectric conversion output of the image sensor 2 is input as an image signal to the analog signal processing unit 3, where processing such as correlated double sampling processing (CDS processing) is performed. The image signal processed by the analog signal processing unit 3 is converted into a digital signal by the A / D converter 4 and then temporarily stored in the buffer memory 12.

  The data stored in the buffer memory 12 is read out to the digital signal processing unit 5 where various image processing such as contour compensation and gamma correction is performed. The data after the digital processing is stored again in the buffer memory 12 and then recorded in the external storage medium 15 such as a memory card via the recording / reproducing signal processing unit 14.

  The display device 8 includes a liquid crystal monitor capable of displaying an image. When displaying an image, the image data stored in the buffer memory 12 is read out to the monitor processing unit 6, and the image resized for display is converted into an analog video signal by the D / A converter 7, and the analog video signal is used. It is displayed on the display device 8.

  The focus motor 10 adjusts the focus by driving the focusing lens 32 of the photographing optical system 30 in accordance with an instruction from the CPU 9. The exposure control unit 11 performs exposure control by driving a diaphragm 31 and a shutter (not shown) in accordance with an instruction from the CPU 9.

  The operation member 13 is provided separately from a power switch for turning the camera on and off, a half-push switch that is turned on by half-pressing the release button, a full-push switch that is turned on by full-pressing the release button, and a release button. And a plurality of switches that are linked to other buttons. The CPU 9 detects the presence / absence of operation of each button according to the state of these switches.

The AF control in this embodiment will be described.
An AF area is set in advance in one or a plurality of locations on the shooting screen, and contrast-type AF control is performed based on image signals in the AF area. The contrast method pays attention to the fact that there is a correlation between the degree of blur of an image and the contrast, and performs focusing using the fact that the contrast of the image is maximized when focused. Since the magnitude of the contrast can be evaluated by the magnitude of the high-frequency component of the image signal, the CPU 9 extracts a high-frequency component in a predetermined band from the image signal obtained by imaging, and calculates the absolute value of the extracted high-frequency component for each AF area. And this is used as the focus evaluation value.

The focus evaluation value is an amount that changes in accordance with the contrast of the image, in other words, the focus adjustment state of the focusing lens 32, and becomes the maximum value (peak value) when the contrast becomes maximum after focusing. Therefore, by obtaining a lens position where the focus evaluation value reaches a peak and bringing the focusing lens 32 to that position, it is possible to focus on the subject in the AF area. Hereinafter, an operation for obtaining a lens position where the focus evaluation value reaches a peak is referred to as an AF search.

  As an AF search method, for example, a mountain climbing method is known. In the hill-climbing method, a focus evaluation value is calculated for each predetermined sampling pitch while moving the focusing lens 32, and the focus evaluation value is compared with the same evaluation value (memory value) at the previous sampling each time the calculation is performed. If the current evaluation value is higher than the previous evaluation value, the peak is approaching, so sampling is continued while moving the focusing lens 32 in the same direction. If the current evaluation value is smaller than the previous evaluation value, the distance from the peak means that the sampling is continued while moving the focusing lens 32 in the opposite direction. By repeating this operation, the focusing lens 32 can finally be brought to the peak position (the peak of the mountain).

  The above-described contrast AF is basically performed based on the image signal when the aperture is opened. This is because it is easier to use the focus evaluation value when the depth of field is shallower to grasp the focus mountain. However, AF control with the aperture fully open has the following drawbacks.

  FIG. 2 is a diagram for explaining the movement of the best image plane by narrowing down. This is an example in which a lens with an open aperture value of F1.4 is used, and the best image plane position when the aperture is open is 0. The movement of the best image plane occurs because the image forming position is shifted between the light passing through the center of the lens and the light passing through the periphery due to the spherical aberration of the lens. As the aperture 31 is reduced (the aperture value is increased), the peripheral light quantity is cut, so that the best image plane moves to the paraxial image plane side. For this reason, even if high-precision contrast AF is performed using the image signal when the aperture is opened, if the image is subsequently reduced to the imaging aperture value shown in the figure, a focus shift occurs by a1. End up.

  In order to prevent such out-of-focus, it is necessary to make the aperture value (hereinafter referred to as AF aperture value) when obtaining image data used in contrast AF coincide with the photographing aperture value. For example, when the photographing aperture value is set to F5.6, the focus evaluation value is calculated from the image data captured with the aperture stopped down to F5.6, and the contrast AF is performed based on the calculated focus evaluation value, thereby preventing the above-described defocusing. .

  However, as the amount of narrowing increases, the amount of light guided to the image sensor 2 decreases, and as the accumulation time of the image sensor 2 increases, the value of the image signal decreases and the ratio of noise components increases (the S / N ratio increases). Getting worse). These problems become more prominent as the subject brightness is darker. In contrast AF, imaging (accumulation) is repeatedly performed as described above. Therefore, if the accumulation time for one time is increased, the AF speed is significantly reduced. In addition, the deterioration of the S / N ratio causes a decrease in the reliability of the focus evaluation value, leading to a deterioration in AF accuracy. For this reason, when the aperture value is large (when the amount of aperture is large) or when the subject is dark, the AF aperture value is not forced to match the aperture value, but rather than the aperture value. It is desirable to make it small to some extent (the value on the open side).

  However, if the AF aperture value is too small, the difference from the shooting aperture value increases and the above-described focus shift occurs. For this reason, the amount of opening of the diaphragm 31 in AF should be kept to the minimum necessary.

  Therefore, in this embodiment, the AF aperture value is not always constant, but the AF aperture value is determined based on the shooting aperture value and the brightness of the subject. Details will be described below.

FIG. 3 is a flowchart showing a processing procedure during photographing including AF control.
When the operation of the AF-ON button is confirmed in step S1, the CPU 9 performs imaging with the aperture opened in step S2, and performs AE calculation based on the obtained image signal. An exposure value (shooting aperture value and shutter speed) for obtaining proper exposure is obtained by AE calculation. In addition, when the photographing aperture value is set in advance by the photographer, it is obtained only at the time of photographing shutter. The AE calculation is also performed when both the photographing aperture value and the shutter time are set by the photographer.

In step S3, subject brightness is obtained based on a signal corresponding to the AF area among the image signals. In step S4, the AF aperture value is obtained from the diagram of FIG. 4 based on the photographing aperture value and the brightness of the AF area. That is,
(1) If the brightness of the AF area is equal to or greater than the predetermined value b1, it is determined that the deterioration of the S / N ratio and the decrease in the AF speed are within the allowable range even when the aperture value is reduced to the minimum aperture value, and AF aperture value = shooting aperture value To do.
(2) When the brightness of the AF area is less than the predetermined value b1 and is not less than b2 (b1> b2), for example, if the shooting aperture value is F16 or less, AF aperture value = shooting aperture value, but the shooting aperture value is F16. If it exceeds A, it is determined that the deterioration of the S / N ratio or the decrease in the AF speed exceeds the allowable range, and the AF aperture value is limited to F16.
(3) When the brightness of the AF area is lower than the predetermined value b2, for example, if the shooting aperture value is F8 or less, AF aperture value = shooting aperture value, and if the shooting aperture value exceeds F8, the S / N ratio It is determined that the deterioration or the decrease in AF speed exceeds the allowable range, and the AF aperture value is limited to F8.

  FIG. 2 shows an example in which the AF aperture value is limited to F8. In this case, the amount of deviation of the best image plane during AF and shooting is a2, which is smaller than a1 (when AF aperture value = open aperture value).

  FIG. 4 is merely an example, and the maximum aperture value and the minimum aperture value differ depending on the lens. Further, the threshold values b1 and b2 of the brightness, the limit value of the AF aperture value, and the like vary depending on how much the allowable range is set. Further, the image plane movement characteristics vary depending on the lens, and a high-performance lens in which spherical aberration is corrected has a small amount of image plane movement due to narrowing. Therefore, it is desirable to determine the threshold values b1 and b2 and the limit value for each lens. In that case, information relating to the amount of movement of the image plane is stored in the lens memory.

  When the AF aperture value is determined, the aperture 31 is narrowed down to the AF aperture value in step S5. Needless to say, no narrowing is performed when the AF aperture value is an open value. After narrowing down, the contrast AF is performed in step S6 to bring the focusing lens 32 to the in-focus position.

  In the loop of steps S7 and S8, the release button is fully pressed or half-pressed is released, and the process is terminated when the half-press is released. When the full pressing operation is performed, exposure control and imaging are performed in step S9. At this time, if the shooting aperture value is not equal to the AF aperture value, the aperture is reduced to the shooting aperture value, and then imaging is performed. Next, the obtained image data is recorded in the external storage medium 15 in step S10, and the process is terminated.

  As described above, in this embodiment, the AF aperture value is basically equal to the shooting aperture value, and the AF aperture value is set only when the deterioration of the S / N ratio and the decrease in the AF speed due to insufficient light quantity cannot be ignored. The value was set to the open side rather than the value. Also, when the AF aperture value is set to the open side value, the aperture value is not set to the maximum value, but the difference between the two aperture values is kept to the minimum necessary (for example, a2 in FIG. 2). Therefore, compared with a conventional camera in which the AF aperture value is unconditionally set to the full aperture value, it is possible to reduce the focus shift due to the movement of the best image plane. In addition, a decrease in AF reliability and a decrease in AF speed due to a deterioration in the S / N ratio can be minimized.

FIG. 5 shows another embodiment.
In this method, instead of using the diagram of FIG. 4, the subject brightness is obtained after actually narrowing down to the photographing aperture value, and the AF aperture value is determined based on the brightness value. Steps similar to those in FIG. 3 are denoted by the same step numbers.

  In FIG. 5, after the shooting aperture value is determined in step S2, the aperture 31 is narrowed down to the shooting aperture value (step S21). Imaging is performed in this state, and the brightness of the AF area is obtained from the obtained image signal (step S22). The luminance in this case reflects both the actual luminance of the subject and a decrease in the light amount due to narrowing down. Therefore, the AF aperture value can be obtained from the brightness (step S23). For example, if the luminance is greater than or equal to a predetermined value c, AF aperture value = shooting aperture value, and if it is less than the predetermined value c, the largest aperture value that is greater than or equal to c is the AF aperture value. In the latter case, the AF aperture value is smaller than the shooting aperture value, and the difference between both aperture values is minimized. Therefore, the same effect as the previous embodiment can be obtained.

  In addition, when the AF aperture value and the photographing aperture value do not match, a more accurate AF result can be obtained by correcting the focus lens position by the amount of image plane movement corresponding to the difference. In this case, the lens-specific image plane movement characteristics stored in the lens memory described above are read out, the image plane movement amount corresponding to the difference between the two aperture values is obtained from the characteristics, and the movement amount is converted into the lens movement amount. The correction value may be used. In the case of a compact camera in which lenses cannot be exchanged, the same control can be performed if the above characteristics are stored in a memory in the camera.

  The contrast AF has been described above, but the present invention can also be applied to other AF methods.

2 Image sensor 9 CPU
DESCRIPTION OF SYMBOLS 10 Focus motor 11 Exposure control part 31 Aperture 32 Focusing lens

Claims (4)

A focus detection unit that detects a focus adjustment state by receiving a light beam that has passed through a photographing lens and an aperture;
A control unit that performs focus adjustment control based on a detection result of the focus detection unit;
A determination unit that determines a focus detection aperture value that is set when detecting the focus adjustment state based on a shooting aperture value;
In the case where the luminance of the subject is the first luminance value and the photographic aperture value is smaller than a predetermined value, the determining unit determines the focus detection aperture value so that the focus detection aperture value increases as the photographic aperture value increases. When the brightness of the subject is the first brightness value and the shooting aperture value is greater than the predetermined value, the focus detection aperture value does not increase even if the shooting aperture value increases. An autofocus device characterized by determining a focus detection aperture value. The autofocus device according to claim 1,
A luminance detection unit for detecting the luminance of the subject;
The determination unit is configured to increase the predetermined value when the luminance is high than when the luminance is low. The autofocus device according to claim 1 or 2,
The determining unit increases the focus detection aperture value in proportion to the imaging aperture value when the imaging aperture value is smaller than a predetermined value, and the focus detection aperture value when the imaging aperture value is greater than the predetermined value. An autofocus device characterized by setting a constant value. The autofocus device according to claim 1,
Including a luminance detector that detects the luminance of the subject by detecting the amount of light that has passed through the aperture in a state where the aperture is set to the shooting aperture value;
The focus detection unit detects a focus adjustment state by a contrast method,
The auto-focus device, wherein the determination unit determines the focus detection aperture value based on the luminance detected by the luminance detection unit .

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