JP2832026B2 - 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 band pass 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. 7 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. In general, the driving speed of the optical system in these operations is 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. Further, in the area (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.

Therefore, a method is conceivable in which a plurality of BPFs having different bandwidths are provided, the focus state is determined according to the output of each of the BPFs, and the drive speed according to the focus state is selected. That is, 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 components of the video signal have few high frequencies and only low frequency components are detected. Therefore, only the output of the BPF having a low pass band increases. As the subject image approaches the in-focus state, the frequency component included in the video signal also includes the high-frequency side, and the output of the BPF having the middle and high-pass bands becomes a large value. Based on this, it is determined which of the peaks (a), (b), and (c) of the characteristic curve in FIG. 7 the photographing lens is located in, and the lens driving speed is set in accordance with each of the above-mentioned regions. Switching control is performed at constant speed, medium speed, and high speed. As a result, the lens driving speed is increased at a position far from the focal point to shorten the time until focusing, and the lens driving speed is reduced as the focal point is approached. A malfunction such as stopping at a position other than the above can be prevented.

[Problems to be Solved by the Invention] However, in the above-described conventional example, in each case, 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. For this reason, stable detection and speed control cannot be performed, and depending on the subject, the drive speed control is insufficient, so that the time until focusing becomes longer or the focusing accuracy is 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-mentioned problems, and the feature of the present invention is that a subject image formed on an imaging surface by a photographic lens optical system is formed. Imaging means for converting into a video signal and outputting the same; detection means for detecting a sharpness signal that changes according to a focus state from the video signal; high-luminance detection means for detecting a high-luminance portion in the imaging surface; By changing the relative position between the imaging lens optical system and the imaging surface in a direction in which the detection output of the detection means increases, and comparing the speed of the change with the level of the sharpness signal with a predetermined threshold value When the driving means for switching to a plurality of stages and the high luminance detecting means detects that a high luminance object is included, the level change of the sharpness signal due to the high luminance object is detected. There is provided an automatic focusing apparatus including a control unit for changing the threshold value in a direction for correcting a shift in timing of switching the speed of the driving unit.

[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 output from the BPF 5, and 7 is an edge width detection circuit for detecting sharpness from the width of the edge of the subject image. The width of this edge portion is a so-called blur width indicating the degree of out-of-focus,
Since the value becomes smaller and the sharpness becomes higher as the focal point approaches, in this embodiment, the sharpness is defined as the reciprocal of the edge width. The sharpness based on the edge width is not affected by the pattern and contrast of the subject, as described later.

8 a voltage level of the video signal output from the preamplifier 3 as compared to the reference voltage level V _ref which is pre Ji because setting the reference voltage level V _ref following time low level, the reference level V _ref
At this time, a high-level signal is output. Then, a portion having a signal level equal to or higher than the reference level _Vref is determined to be a high-luminance subject. Details will be described later.

Reference numeral 9 denotes a logic control unit that controls the entire apparatus.
Based on the output of the edge width detection circuit 7, the moving direction, moving speed, moving amount, etc. of the photographing lens are controlled. Reference numeral 10 denotes a motor drive circuit for driving and controlling the motor 11 for driving the photographing lens 1 based on a drive control signal output from the logic control unit 9, reference numeral 12 denotes an aperture, reference numeral 13 denotes an aperture encoder for detecting an aperture value of the aperture 12, reference numeral 14 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 a predetermined high-frequency component is extracted by the hand-pass filter 5 from the imaging signal which has not been subjected to the gamma correction or the like of the preamplifier 3, and the detection circuit 6 detects a peak in the entire screen or in a limited area for distance measurement. A value or an integrated value is obtained and output to the logic control unit 9. Further, the output of the preamplifier 3 is also output to the edge width detection circuit 7, which calculates the edge width of the subject image on the imaging surface of the imaging device 2, and outputs it to the logic control unit 9. In the logic control unit 9, 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 imaging lens optical system is driven can be used to determine the driving direction of the optical system, and whether the optical system has been 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 10 so as to drive the photographing lens in the 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. .

Here, the edge width detection circuit 7 will be described. The detection output of the edge width indicates the sharpness of the subject image without depending on 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 the subject image is accurately evaluated by detecting the width of the edge portion of the subject image regardless of the contrast of the subject. Here, regarding the configuration of the edge width detection circuit,
This will be described with reference to FIGS.

FIG. 4 (a) shows an imaging screen in which the subject 100 is imaged. FIG. 4 (b) shows, for example, the luminance change 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 of the luminance signal. Now, paying attention to the edge portion of the subject image, let the width of the edge portion be 、 X, and let the luminance difference corresponding to the edge width △ X be △ I.

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 the 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 of the subject and the luminance. Therefore, by detecting the edge width ΔX and comparing the detected edge width with the minimum circle of confusion, it is possible to accurately determine in-focus and out-of-focus.

FIG. 5 is a block diagram showing the internal configuration of the edge width detection circuit 7.

In the figure, 71 is a differentiating circuit for differentiating the video signal output from the preamplifier 3 to obtain d1 / dx, 72 is an absolute value circuit for obtaining its absolute value | d1 / dx |, and 73 is an edge part luminance difference △.
In a circuit for calculating I, △ I is obtained by dividing d1 / dx by x
It can be obtained by integrating in the direction.

74 is a delay circuit for delaying | d1 / dx | by the delay time based on the integral for calculating ΔI and adjusting the operation timing, and 75 is an output of the delay circuit 74 | d1 / dx | Divide by the output △ I of the circuit 73 (d1 / dx) / △ I = 1 / △ x
Is a division circuit that obtains the edge width △ x in the form of a reciprocal by performing the above operation.

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

Now, 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 the focus matching operation, and at the same time, the value of the sharpness 1 / △ x by the 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, the high luminance detecting operation will be described. In FIG. 1, the video signal output from the preamplifier 3 is supplied to the voltage comparison circuit 8 and compared with the reference voltage _Vref as described above.

FIG. 3 is for explaining this high luminance detecting operation. As shown in FIG. 3 (a), consider a case where a high-luminance subject A such as another light source of a subject that is originally desired to be focused exists in the imaging plane. When a high-luminance subject is present in the imaging screen, the output of the edge width detection circuit 7 is shown in FIG.
As shown in FIG.

That is, in the video signal output from the image sensor 2, the third
FIG. 3B shows a waveform representing a video signal level corresponding to the line B in FIG. That is, the higher the luminance in the imaging plane, the higher the signal level. In particular, the portion A having extremely high luminance due to the presence of the light source and the like, as shown by C in FIG. Exceeds its capacity, and is in a so-called saturated state. Therefore, the waveform has a shape in which the upper portion is clipped, which is the same as long as the luminance is sufficiently high even if the subject image is blurred. Such a clipped waveform has a high frequency component, the output of the edge width detection circuit 7 has a high value over the entire lens movement range, and the height difference between the peaks is small. Therefore, the characteristic curve when a high-luminance subject is photographed has a gentle mountain shape as described above.

The output of the voltage comparison circuit 8 shown in FIG.
As shown in the _figure , only a portion of the signal level exceeding the reference voltage level _Vref has a high level pulse signal waveform. Then, as shown in FIG. 3 (b), the reference voltage level V _ref
Is set to the voltage immediately before the video signal is saturated, the output of the voltage comparison circuit 8 becomes "high level" only when a high-luminance subject exists as shown in FIG. 3 (c).

The logic control unit 9 determines whether a high-luminance subject exists in the imaging screen based on the high-luminance information,
When a high-luminance subject is present, as shown in FIG. 2 (b), the characteristic becomes a mountain shape which is gentler than the normal focusing characteristic curve of FIG. 2 (a). The setting value of the edge width detection output for switching
From t ₁ and t ₂ , the value is changed to a value suitable for a mountain with small undulations, such as t ₁ ′ and t ₂ ′ shown in FIG. 2B. Such a set value
If the driving speed of the optical system is changed according to t ₁ ′ and t ₂ ′,
Even in the case where a high-brightness subject is present, a focus matching operation in which the motor speed can be reliably switched in each area with respect to the focal point is performed, similarly to the case of the normal subject shown in FIG.

The logic controller 9 selects the drive speed of the optical system based on the blur width detection output set as described above according to the set value,
This information is output to the motor drive circuit 10. The motor drive circuit 10 actually supplies power to the motor 11 based on this information, and drives the optical system 1 at a selected speed.

Next, lens drive speed control according to the depth of field of the taking lens optical system will be described in detail.

 That is, the focusing characteristic changes according to the depth of field.

The drive speed control in accordance with the depth of field is performed simultaneously with the drive speed control in accordance with the presence or absence of the high-luminance subject, but will be described individually for convenience of explanation. 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.

The curve shape of the edge width detection output shown in FIG. 6A, that is, the shape of the so-called "mountain" changes as the aperture value and the focal length, that is, the depth of field of the flying vehicle change, as shown in FIGS. I do. 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 according to the shape of the mountain.

For example, when the focal length is longer or the aperture value is larger than the focal length and aperture value when the mountain shape shown in FIG. 6A is exhibited, or when the aperture is opened, the depth of field becomes shallower, Even if the lens is slightly moved, the degree of focusing (sharpness) changes greatly and the shape of the mountain becomes sharp and steep as shown in FIG. 4B, so that the sharpness (the reciprocal of the edge width) with respect to the lens driving amount is increased. By changing the threshold value of the change amount in the range t ₁ ″, t ₂ ″, the area near the focal point (a), the area outside the in-focus direction (b), Each motor drive speed in the outer region (c) can be 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. Edge width) becomes smaller. Therefore, the threshold value of the sharpness is also changed as t ₁ and t ₂ , and the motor drive speed, that is, the lens drive speed is reliably switched in each of the regions (a), (b) and (c) with respect to the focal point. be able to.

In FIG. 1, the aperture value of the imaging lens optical system is detected by the aperture encoder 13 and the focal length is detected by the zoom encoder 14, and the information is supplied to the logical control unit 9. Based on the information, FIG. 6 (a) shows a characteristic curve showing a change in lens movement amount and sharpness (edge width).
(B) and (c) are calculated, and a sharpness threshold value for switching the lens drive speed between high, medium, and low is determined according to the result, and the speed of the men's drive motor is determined. Control.

As described above, the logic control unit 9 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 10, and outputs the result to the motor drive circuit 10. Is the lens drive motor according to this information
Drive 11

According to the above embodiment, FIGS. 6 (a), (b),
As shown in (c), three types of curves have been described. However, it is not necessary to classify the curves into three types. If the curves are 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 the number of switching stages can be changed as necessary.

Further, in the above-described embodiment, when a high-brightness subject is present in the lower surface of the imaging, the set value of the edge width detection output that is a threshold for changing the driving speed of the optical system is changed. The control of the drive speed of the optical system is not limited to this method. For example, in FIGS. 2 and 6, without changing the threshold values t ₁ and t ₂ of the edge width detection output,
The motor speed itself can be changed by the threshold values t ₁ and t ₂ .

In some cases, the setting of the threshold values t ₁ and t ₂ and the motor speed can be simultaneously changed.

[Effects of the Invention] As described above, the drive speed of the optical system is controlled in accordance with the degree of blurring of the subject image on the imaging surface of the imaging device, which is determined from the sharpness detected from the video signal, and the detection information of the high-luminance subject. By performing the change, the automatic focusing operation can satisfy both the focusing speed and the focusing accuracy regardless of the situation and the type of the subject, even if the focusing characteristic is changed due to the high brightness subject.

Furthermore, at the same time, the aperture value and the focal length of the optical system are detected,
By changing the set value for changing the drive system speed of the optical system in parallel with this information, the focus speed and the automatic focusing that satisfies both the focus speeds are not affected by the aperture value and the focal length of the optical system. A focus operation 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 a drive control operation of an optical system according to a high-luminance subject of the present invention, and FIG. FIG. 4 is a diagram for explaining an operation of detecting a luminance subject image, FIG. 4 is a diagram for explaining an edge width detecting operation in the present invention, FIG. 5 is a block diagram showing a configuration of an edge width detecting circuit in the present invention, FIG. 6 is a diagram for explaining a drive control operation of the optical system according to the depth of field, and FIG. 7 is a diagram for explaining general focus characteristics.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masamichi Toyama 770 Shimonoge, Takatsu-ku, Kawasaki-shi, Kanagawa Prefecture Inside the Tamagawa Office of Canon Inc. (56) References JP-A-63-20972 (JP, A) JP-A-63-151180 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04N 5/232

Claims (2)

(57) [Claims] 1. An image pickup means for converting a subject image formed on an image pickup surface by a photographing lens optical system into a video signal and outputting the video signal, and a detection means for detecting a sharpness signal that changes according to a focus state from the video signal. And high-luminance detecting means for detecting a high-luminance portion in the imaging surface; and changing a relative position between the imaging lens optical system and the imaging surface in a direction in which a detection output of the detection signal increases; Driving means for switching the speed of the sharpness signal to a plurality of levels by comparing the level of the sharpness signal with a predetermined threshold value; and when the high brightness detection means detects that a high brightness subject is included, The threshold value is changed in such a direction as to correct a shift in timing for switching the speed of the driving means caused by a level change of the sharpness signal due to a high-brightness subject. An automatic focusing device comprising: control means; 2. The control device according to claim 1, wherein the control unit is configured to detect a shift in a timing of switching the speed of the driving unit caused by a change in a characteristic of the sharpness signal that changes according to a depth of field. An automatic focusing device configured to perform correction by changing the focal length.