JP2810403B2 - Automatic focusing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic focusing device suitable for use in a video camera or the like.

[Prior Art] Conventionally, in a video camera or the like, focusing is performed by detecting the sharpness of a subject image on an imaging screen from a video signal obtained from an imaging element and driving an optical system so that the sharpness is maximized. Is known. This method is basically a bandpass filter (hereinafter abbreviated as BPF).
Alternatively, the intensity of a high-frequency component of a video signal extracted by a differentiating circuit or the like is used as an evaluation value of the sharpness of an image, and the sharpness of two images having different imaging states obtained by driving the optical system is compared. Is determined, and the optical system is stopped at the position where the sharpness is maximized.

The evaluation value of the sharpness is obtained when a general subject is photographed,
FIG. 5 shows a mountain-like change in accordance with the amount of lens extension, and the point A at the top of this mountain is the focal point.

In this method, assuming that the extended state of the lens is initially point B, a series of steps such as starting climbing the mountain at the start of the focusing operation, passing point A, confirming that the lens has passed the top of the mountain, and then returning to point A again. Perform the operation. The driving speed of the optical system in these operations is generally largely blurred in the region (c) of FIG.
Faster is preferred. In order to accurately stop at the focus position without causing hunting in the area (a) near the focus, it is preferable that the driving speed is relatively slow. In the region (b) between these, an intermediate speed is preferable in consideration of both the focusing speed and the focusing accuracy.

As shown in FIG. 5, as a means for determining these drive speeds, as shown in FIG. 5, when the focus evaluation value level such as the amount of the high-frequency component is used, the shape of the peak of the curve and the value of the peak are determined according to the luminance. This is not preferable because it changes depending on the subject. Accordingly, a method of providing a plurality of BPFs having different bandwidths as indicated by FA, FB, and FC in FIG. 7A and determining the drive speed by comparison as shown in FIG. 6 in accordance with their respective outputs. Can be considered. When the subject image formed by the optical system is far away from the imaging surface, it is in a so-called large blur state, and the frequency component of the video signal has a small high frequency range and is low as shown by the characteristic a in FIG. 7 (b). Only the components of the region are detected. Accordingly, only the output of the BPF having a low pass band increases as indicated by FA in FIG. As the subject image approaches the in-focus state, the frequency components included in the video signal also include the high frequency side as shown by b and c in FIG. 2B, and are indicated by FB and FC in FIG. As described above, the output of the BPF having the pass band of the middle band and the high band also has a large value. FIG. 6 shows the relationship between the output of the BPF and the driving speed of the optical system.

That is, the frequency characteristics shown in FIG. 7 (b) are detected according to the respective output levels of the BPF FA, FB and FC, and based on this, the peaks (a) and (b) of the characteristic curve in FIG. ,
In (c), it is determined in which region the imaging lens is located, and the lens driving speed is controlled to be switched between low speed, medium speed, and high speed in accordance with each of the above regions. As a result, the lens drive speed is increased at a position far from the focal point to shorten the time until focusing, and the lens drive speed is reduced as the focal point is approached. A malfunction such as stopping at a position other than the above can be prevented, and a quick and highly accurate focusing device can be provided.

[Problems to be Solved by the Invention] However, in the above-described conventional example, since the components of each frequency band including the value of the high-frequency component included in the video signal change depending on the brightness and pattern of the subject, stable detection and speed are achieved. Control cannot be performed, and depending on the subject, the drive speed control is insufficient and the time until focusing (hereinafter referred to as the focusing speed) may be long, or the focusing accuracy may be deteriorated.

Further, since the characteristics of the lens drive amount and the change of the degree of focus change depending on the focal length and the aperture value, it is difficult to always perform an accurate and stable focus adjustment operation regardless of the subject.

[Means for Solving the Problems] The present invention has been made to solve the above problems, and the feature of the invention described in claim (1) is that an image pickup surface is formed by a photographing lens. An imaging unit that converts the formed subject image into a video signal and outputs the video signal, a unit that detects a high-frequency component included in the video signal, and the video signal that corresponds to a sharpness of an edge of the subject image from the video signal. A means for detecting a sharpness component having a steeper characteristic than a high-frequency component, and a driving means for performing a hill-climbing control by changing a relative position between the photographing lens and the imaging surface so that the level of the high-frequency component is maximized. And comparing the level of the sharpness component with a predetermined threshold value, so that the drive speed of the drive means is reduced stepwise as the level of the sharpness component increases, and the threshold is reduced. Control means for controlling the switching point of the drive speed with respect to the in-focus position to be constant irrespective of the depth of field by changing the value in accordance with the depth of field. In the matching device.

Further, the invention according to claim 2 of the present application is characterized in that an imaging means for converting a subject image formed on an imaging surface by an imaging lens into a video signal and outputting the video signal, Means for detecting the sharpness to be obtained as a focus evaluation value, driving means for performing hill-climbing control by changing the relative position between the photographing lens and the imaging surface so that the level of the focus evaluation value is maximized, and By comparing the level of the evaluation value with a predetermined threshold value, the drive speed of the driving unit is gradually reduced as the level of the focus evaluation value increases, and the threshold value is set to a depth of field. Control means for controlling the position of the drive speed switching point with respect to the in-focus position to be constant regardless of the depth of field. Located in.

[Operation] It is possible to obtain a constant focusing speed and a constant focusing accuracy regardless of the situation and type of the subject. Further, by detecting the aperture value and the focal length of the optical system and changing the set value of the blur width detection output for changing the drive speed, a constant focusing speed can be obtained regardless of the state of the aperture or the state of zooming. And focusing accuracy can be obtained.

[Embodiment] An embodiment of an automatic focusing apparatus according to the present invention will be described below in detail with reference to the drawings.

In FIG. 1, 1 is an optical system including a photographing lens, 2 is an image sensor such as a CCD, 3 is a preamplifier, 4 is gamma correction, blanking processing, and addition of a synchronization signal to an image signal output from the image sensor 2. And a process circuit 5 for outputting a standardized television signal by performing processing such as a band-pass filter (5) for extracting a frequency component of a predetermined band used for focus detection in the image pickup signal output from the preamplifier 3. BPF) and 6 are detection circuits for averaging the amount of high frequency components, and 7 is an edge width detection circuit for detecting sharpness from the width of the edge portion of the subject image. The width of the edge portion is a so-called blur width indicating the degree of out-of-focus, and becomes smaller as the focus approaches, and the sharpness becomes higher.
The reciprocal of the edge width is defined as sharpness. The sharpness based on the edge width is not affected by the pattern and contrast of the subject, as described later.

Reference numeral 8 denotes a logic control unit that controls the entire apparatus.
The moving direction, moving speed, moving amount, and the like of the photographing lens are controlled based on the output of the edge width detecting circuit 7. Reference numeral 9 denotes a motor drive circuit for controlling the drive of the photographing lens 1 drive motor 10 based on a drive control signal output from the logic control unit 8, reference numeral 11 denotes an aperture, reference numeral 12 denotes an aperture encoder for detecting an aperture value of the aperture 11, reference numeral 13 Is a zoom encoder for detecting the focal length of the zoom lens of the taking lens optical system.

Hereinafter, the operation of this circuit will be described. The subject image formed on the imaging surface by the imaging lens 1 is converted into an electric signal by the imaging device 2 and output as an imaging signal.
This signal is amplified to an appropriate level by the preamplifier 3 and supplied to the process circuit 5, where it is converted into a standardized video signal such as NTSC and output. On the other hand, only predetermined high-frequency components are extracted by the band-pass filter 5 from the imaging signal that has not been subjected to gamma correction or the like of the preamplifier 3, and the peaks in the entire screen or a limited area for distance measurement are detected by the detection circuit 6. A value or an integrated value is obtained and output to the logic control unit 8. Further, the output of the preamplifier 3 is also output to an edge width detection circuit 7, which calculates the edge width of the subject image on the imaging surface of the image sensor 2, and outputs the calculated edge width to the logic control unit 8. In the logic control unit 8, the detection circuit 6
Based on the sharpness information obtained from the edge width detection circuit 7, the change in the sharpness information with time when the photographing lens optical system is driven can be used to determine the driving direction of the optical system, and whether the optical system is stopped or restarted. Do. Further, the drive speed of the optical system is determined based on the sharpness information obtained from the edge width detection circuit 7.

The logic control unit controls the motor drive circuit 9 so as to drive the photographing lens in a direction in which the output level of the detection circuit 6, that is, the amount of the high-frequency component in the video signal increases, and drives the photographing lens to a focal point. I do.

Here, the edge width detection circuit 7 will be described. The edge width detection output indicates the sharpness of the subject image independent of the contrast and brightness of the subject, the pattern of the subject image, and the like. A specific example of such a method is described in, for example,
It is also shown in 62-103616. In this method, only the sharpness of a subject image is accurately evaluated regardless of the contrast of the subject by detecting the width of an edge portion of the subject image. Here, the configuration of the edge width detection circuit will be described with reference to FIGS.

FIG. 3A shows an imaging screen in which the subject 100 is imaged. FIG. 3B shows, for example, a change in the luminance of the video signal on the straight line 1 on this screen.
become that way. The vertical axis indicates the luminance level, and the horizontal axis indicates the position on the screen. Assuming that the subject 100 is in focus,
The luminance level in the video signal of the portion of the subject 100 is high, and the luminance level of the background portion is low. The same applies to the high frequency component in the luminance signal. Now pay attention to the edge of the subject image,
It is assumed that the width of the edge portion is ΔX, and the luminance difference corresponding to the edge width ΔX is ΔI.

The width ΔX of the edge portion becomes smaller as the focal point is approached, increases as the focal point becomes out of focus, and takes a minimum value at the focal point. The ΔX is determined by the diameter of the confusion circle of the optical system, the resolving power of the image sensor, and the bandwidth of the image signal processing system, but is independent of the focusing and defocusing of the latter two optical systems. The former circle of confusion changes according to the in-focus state and the out-of-focus state. However, it is not affected by the situation and brightness of the subject.
Therefore, by detecting the edge width ΔX and comparing the edge width with the minimum circle of confusion, it is possible to accurately determine in-focus or out-of-focus.

FIG. 4 is a block diagram showing the internal configuration of the edge width detection circuit 7.

In the drawing, 71 is a differentiating circuit for differentiating the video signal output from the preamplifier 3 to obtain dl / dx, 72 is an absolute value circuit for obtaining its absolute value | dl / dx |, and 73 is a luminance difference エIn a circuit for obtaining I, ΔI can be obtained by integrating dl / dx in the x direction in a minute section.

74 is a delay circuit for delaying | dl / dx | by the delay time based on the integration for calculating ΔI and adjusting the operation timing, and 75 is the ΔI operation of the output | dl / dx | of the delay circuit 74. Divide by the output △ I of the circuit 73 (dl / dx) / △ I = 1 / △ x
Is a division circuit that obtains the edge width Δx in the form of a reciprocal by performing the following operation.

Since the edge width detection circuit 7 outputs the edge width Δx in the form of a reciprocal, the closer the value is, the closer to the focal point.

The logic control circuit 8 drives the photographing lens 1 in a direction in which the output level of the detection circuit 6 increases to perform a focus matching operation, and performs a value of sharpness 1 / △ x based on an output of the edge width detection circuit 7. Then, the motor drive circuit is controlled based on the control signal to control the lens drive speed. That is, in FIG. 2A, based on the sharpness 1 / △ x based on the edge width,
The threshold values t ₁ and t ₂ are set, and it is determined whether the lens position is in the area near the focal point (a) or outside the area (b) or (c). Switch to.

Thus, an accurate, quick, and highly accurate focus adjustment operation can be performed without being affected by the pattern, luminance, and the like of the subject.

Next, lens drive speed control according to the depth of field of the taking lens optical system will be described in detail. In general, an optical system used for a video camera or the like has an unfocused amount of a lens, that is, a deviation amount from a focus position on a distance ring, when an aperture value and a focal length are constant, a blur width on an imaging surface, that is, confusion. It is uniquely determined by the diameter of the circle. Accordingly, as shown in FIG. 2A, the position of the lens optical system with respect to the focal point is detected in accordance with the values t ₁ and t ₂ of the output (reciprocal of the edge width) of the edge width detection circuit 7 and the driving speed of the lens is determined. Is changed, the drive speed can be controlled only by the amount of defocus of the distance ring of the lens, independent of the subject. In FIG. 2, the edge width detection output is expressed as an evaluation value of the sharpness of the subject image, and therefore the direction in which the edge width decreases, that is, the direction in which the sharpness of the subject, which is the reciprocal of the edge width, increases. ".

The above description is based on the premise that the aperture value and the focal length of the optical system are constant. However, the curve shape of the edge width detection output shown in FIG. When the aperture value and the focal length, that is, the depth of field change, the shape changes as shown in FIGS. Generally, most of the optical systems of video cameras are provided with an aperture mechanism, and many of them are provided with a so-called zoom lens that can change the focal length. In a video camera equipped with these functions, if the driving speed of the optical system is changed uniformly, the lens driving amount with respect to the out-of-focus amount differs depending on the shape of the mountain in FIG. It is necessary to change the threshold value setting of the speed switching of the edge width detection output to be controlled in accordance with the shape of the mountain.

For example, when the focal length becomes longer or the aperture is opened with respect to the focal length and the aperture value when the mountain shape shown in FIG. 2A is exhibited, even if the lens is slightly moved,
Since the change in the degree of focus (sharpness) is the first, and the shape of the mountain is overloaded and steep as shown in FIG. 3B, the amount of change in sharpness (edge width) with respect to the lens drive amount is small. Threshold
By changing them as t ₁ ′ and t ₂ ′, the area in the vicinity of the focal point (a), the area outside the focus direction (b), and the area outside the focus (c) can be obtained. The motor drive speed can be kept constant regardless of the depth of field.

Conversely, when the focal length is shortened or the aperture is narrowed down and the depth of field is deepened, the shape of the curve becomes sharp and gentle as shown in FIG. The change in edge width is small. Therefore, the threshold value of the sharpness is also changed to t ₁ ′, t ₂ ″, so that the motor driving speed, that is, the lens driving speed, in each of the regions (a), (b) and (c) with respect to the focal point, Can be switched.

In FIG. 1, the aperture value of the photographic lens optical system is detected by the aperture encoder 12 and the focal length is detected by the zoom encoder 13, and the information is supplied to the logical control unit 8, and based on the information, FIG. 2 (a) is a characteristic curve showing a change in lens movement amount and sharpness (edge width).
(B) and (C) are calculated, and based on the result, a sharpness threshold for switching the lens driving speed between high, medium, and low is obtained, and the speed of the lens driving motor is calculated. Control.

As described above, the logic control unit 8 determines the drive direction, stop, restart, and drive speed of the optical system as described above, outputs the determination result to the motor drive circuit 9, and outputs the result to the motor drive circuit 9. Is the lens drive motor according to this information
Drive 10

According to the above-mentioned embodiment, FIGS. 2 (a), (b),
As shown in (C), the curve has been described in three types, but it is not necessary to classify the curve into three types. If the curve is further divided, more accurate lens control can be performed.

Switching of the speed is also performed in the areas (a), (b),
In contrast to (c), switching is performed in three stages of low speed, medium speed, and high speed. However, the number of switching stages is not limited to three, and it is easy to set more switching stages. it can.

[Effect of the Invention] As described above, the sharpness is obtained from the detection output of the information on the edge of the subject image, the degree of blurring of the subject image on the imaging surface of the imaging device is obtained, and the driving speed of the optical system is determined according to this value. Is changed, the automatic focusing operation can satisfy both the focusing speed and the focusing accuracy irrespective of the situation and type of the subject.

Further, by detecting the aperture value and the focal length of the optical system, and by changing the set value of the blur width detection circuit that changes the driving speed of the optical system based on the information, the aperture value and the focal length of the optical system are affected. Instead, an automatic focusing operation that satisfies both the focusing speed and the focusing accuracy can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an automatic focusing apparatus according to the present invention, FIG. 2 is a characteristic diagram for explaining an operation of the present invention, and FIG. 3 is a diagram for explaining an edge width detecting operation in the present invention. FIG. 4, FIG. 4 is a block diagram showing a configuration of an edge width detecting circuit in the present invention, FIG. 5 is a diagram for explaining general focus characteristics, and FIG. 6 is a diagram showing a focus adjustment speed control operation before the present invention. FIG. 7 is a diagram for explaining the relationship between the characteristics of a filter for detecting a focus signal and its output.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Masamichi Toyama 770 Shimonoge, Takatsu-ku, Kawasaki-shi, Kanagawa Prefecture Inside the Tamagawa Office of Canon Inc. In-house (56) References JP-A-62-103616 (JP, A) JP-A-63-127217 (JP, A) JP-A-64-8771 (JP, A) (58) Fields investigated (Int. ⁶ , DB name) H04N 5/232

Claims (2)

(57) [Claims] An imaging unit configured to convert a subject image formed on an imaging surface by an imaging lens into a video signal and to output the video signal; a unit configured to detect a high-frequency component included in the video signal; Means for detecting a sharpness component having a sharper characteristic than the high-frequency component according to the sharpness of the edge of the subject image; and a relative position between the photographing lens and the imaging surface such that the level of the high-frequency component is maximized. A driving unit that performs hill-climbing control by changing a position; and comparing the level of the sharpness component with a predetermined threshold value, thereby increasing the driving speed of the driving unit as the level of the sharpness component increases. And by changing the threshold value according to the depth of field, regardless of the depth of field, the switching point of the drive speed with respect to the focal point position An automatic focusing device, comprising: control means for controlling the position to be constant. 2. An imaging means for converting a subject image formed on an imaging surface by an imaging lens into a video signal and outputting the video signal; a means for detecting sharpness included in the video signal as a focus evaluation value; Driving means for performing a hill-climbing control by changing a relative position between the photographing lens and the imaging surface so that the level of the focus evaluation value is maximized; and comparing the level of the focus evaluation value with a predetermined threshold value. As the level of the focus evaluation value increases, the driving speed of the driving unit is gradually reduced, and the threshold value is changed according to the depth of field. Control means for controlling the position of the drive speed switching point relative to the in-focus position to be constant.