JPH0531350B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic focusing device for a video camera.

Traditionally, automatic focusing devices for video cameras detect the definition of the screen using high-frequency components in the video signal, and then control the rotation of the lens focusing ring (hereinafter referred to as helicoid) to maximize the definition. So-called mountain climbing control is known. This method is described in NHK Technical Research Report, Volume 17, 1963.
Volume 1, Volume 86, page 21, is described in detail as ``Automatic focus adjustment of television cameras using mountain-climbing servo system'' by Ishida et al. Below, this system will be briefly explained using Figures 1 and 2. explain.

FIG. 1 is a block diagram showing the configuration of an automatic focusing device (hereinafter referred to as AF device) using a hill climbing method. 1 is the lens system, 2 is the camera circuit,
3 is a video signal output terminal, 4 is a high-pass filter, 5 is a detector, 6 is a differential hold circuit, 7 is a motor drive circuit, and 8 is a motor for rotating the helicoid of the lens system 1.

The operation of the configuration shown in FIG. 1 will be explained below using the characteristic diagram shown in FIG. 2.

Optical information incident on the lens system 1 from a subject (not shown) is converted into an electrical signal by the camera circuit 2, and outputted to the terminal 3 as a video signal. Only the high-frequency components of the video signal are extracted by the high-pass filter 4, detected by the detector 5, and then appear at the terminal 51. The voltage corresponding to the high-frequency component of the video signal (hereinafter referred to as focal voltage) corresponds to the definition of the captured image, so as shown in FIG.
The focal voltage appearing in is maximum when the helicoid position (denoted as A) of the lens system 1 matches the distance to the subject, and decreases as the helicoid position of the lens system 1 deviates from the above-mentioned position A. If the helicoid position is controlled by some means so that the focal voltage supplied to the terminal 51 is maximized,
Focusing will be performed automatically. The differential hold circuit 6 is connected to a terminal 51 as shown in FIG.
Sample and hold the focal voltage appearing at fixed time intervals, and generate a positive voltage if the focal voltage is increasing over time, and a negative voltage if the focal voltage is decreasing over time. If the output voltage of the differential hold circuit 6 appearing at the terminal 61 is positive, the motor drive circuit 7 rotates the motor 8 in the forward direction, for example, to move the helicoid position in the forward direction. If so, the motor 8 is reversed to move the helicoid position in the opposite direction.

The output voltage of the differential hold circuit 6 shown in FIG. 2 was shown when the helicoid was rotated from a close position to infinity, but it is easy to see that the same applies when the helicoid is rotated from an infinity position to a close position. be understood.

In this way, the control system that controls the helicoid position of the motor 8 uses the output voltage of the differential hold circuit 6 to climb the mountain created by the focal voltage of the mountain climbing circuit 9, and finally reaches the top of this mountain with a small Focusing is automatically performed by vibrating in increments until a steady state is reached.

The above is an automatic focusing device for a video camera using the mountain climbing method.

Although the above-mentioned device has sufficient performance as an automatic focusing device, it has the following drawbacks.

The first drawback is due to the characteristics of the lens system 1, and the lens system that has a front lens rotation type focusing mechanism, which is currently mainly used,
In other words, in a system that focuses by moving the position of an objective lens placed close to the subject, when the helicoid position is moved for focusing, the focal length of the lens system 1 changes as well as the focusing distance of the lens system 1. The distance itself will change slightly. In other words, in the mountain climbing method, the helicoid position oscillates and stays at the top of the mountain by feeding back the increase and decrease of the focus signal, so even during focusing, the helicoid position oscillates with the center of gravity at the top of the mountain, and the lens system The focal length of No. 1 also oscillates, albeit slightly, due to the reasons mentioned above. A change in the focal length of the lens system 1 directly means a change in the angle of view of the lens system 1, that is, a change in the magnification of the captured image. This means that the size of the image is oscillating. This fluctuation in image size is caused by the steep slope of the mountain caused by the focus signal when shooting when the depth of field of the lens system 1 is shallow, such as when the aperture value is small and the zoom magnification is high. Therefore, the amplitude of vibration is also small,
Although it is hardly noticeable, when the depth of field is deep, such as when the aperture value of lens system 1 is large and the zoom magnification is low, the slope of the mountain caused by the focus signal becomes very broad, which causes vibration of the helicoid. The amplitude of will increase, and the size of the captured image will change by more than a few percent in accordance with the vibration of the helicoid position.

The second drawback is that power consumption is large, although it is not as fatal as the first drawback described above. In other words, this method requires the helicoid to be constantly rotating, so the motor 8 that drives the helicoid always consumes power, making it suitable for cameras where power supply is a problem, such as portable video cameras. That's a big drawback.

The purpose of the present invention is to eliminate the above-mentioned conventional drawbacks,
To provide an automatic focusing device that does not cause noticeable vibration in the size of a photographed movie image during focusing.

The drawback of conventional autofocusing devices using the mountain climbing method is that they always control the helicoid. Therefore, when the depth of field is large and the subject is virtually in focus no matter where the helicoid is located, the shape of the mountain becomes very broad, imitating the vibrations of a large helicoid.

In the present invention, the depth of field of the lens at that point is calculated using the aperture value and focal length (zoom magnification) of the lens. Calculate the stop position (hereinafter referred to as the Harken point) of the camera, use the mountain climbing method only to find which of these stop positions is the in-focus position, and once focus is established. Once this is done, the closed loop of the mountain climb is opened, and at the same time the helicoid is stopped at a predetermined position (Harken Point) closest to the mountain top. When the camera goes out of focus for some reason, the mountain climbing closed loop is closed again, and the mountain climbing system focusing function is restarted.

Hereinafter, one embodiment of the present invention will be described in detail with reference to FIGS. 3 and 4.

FIG. 3 is a block diagram showing the structure of an embodiment of an automatic focusing device according to the present invention.

1, 2, 8, 9, 51, and 61 are the same lens system, camera circuit, motor, mountain climbing circuit, and terminal to which the focusing voltage is output, respectively, as used in the conventional automatic focusing circuit explained using FIG. ,
This is a terminal to which the output voltage of the differential hold circuit is output. 11 to 14 are focal voltage, focal length information of lens system 1, and lens system 1, respectively.
A/A for converting the aperture value information and the helicoid position information of lens system 1 into digital values
It is a D converter. 15 is a Harken point calculation circuit, 16 is a Harken point memory, 17,1
9, 21 and 23 are comparison circuits, 18 is a stop Harken point memory, 20 is a Harken point number memory, 22 is a focal voltage memory, 24 and 25 are summation circuits, 26 is a counter, 27 is a first helicoid position memory, and 28 is a 2nd helicoid position memory, 29
30 is an average value circuit, 30 is a motor drive circuit, and 31 is an electronic switch. Note that the motor drive circuit 30 is C
If a signal is applied to the input line on the D side, it operates in the same way as the conventional motor drive circuit 7 shown in FIG. 1, but if a signal is applied to the input line on the D side, it has the function of stopping the motor 8.

Before explaining the automatic focusing operation in the present invention, the Harken point will be explained.

The Harken point is a predetermined stopping position of the helicoid that can cover from close range to infinity (hereinafter referred to as ∞) considering the depth of field of the lens system 1 at that point.
It can be calculated using formula (1).

Ri=f ² /ρF・1/(2i−1) where i is a positive integer...Equation (1) Here, Ri is the i-th Harken point counting from the infinite ∞ side, and ρ is the tolerance of the captured image circle of confusion diameter,
f is the focal length of the lens 1, and F is the aperture value of the lens 1.

In addition, the required number N of Harken points is given by Rmin, which is the closest shooting distance of lens 1.
Since the Nth Harken point should be equal to or smaller than Rmin, it can be calculated using the following equation (2).

N≧1/2・f ² /ρF・1/Rmin ...Formula (2) For example, as lens 1, focal length f=50mm,
Using the aperture value F = 1.8, the close distance Rmin = 1 m, and the diameter of the permissible circle of confusion as ρ = 40 μm, the number of Harken points N is 18 from equation (2), and the number of Harken points N from equation (1) is The position Ri is R ₁ = 34.72m, R ₂ =
11.57m, R ₃ = 6.94m, ...R ₁₈ = 0.99m.
In other words, in this case, no matter what distance the subject is, the helicoid position is set to Harken Point R ₁ ~
By selecting the most suitable one among _R18 , it is possible to take pictures with a predetermined focus. This is the explanation of Harken Point.

In the present invention, a conventional mountain climbing method is used to find the optimal Harken point. The operation will be explained below.

Information on the aperture value F and information on the focal length f of the lens system 1 are A/D converted by A/D converters 11 and 12, respectively, to digital codes, and both are input to the Harken point calculation circuit 15. The Harken point calculation circuit 15 calculates the Harken point number N and Harken point positions R1 to RN based on the above-mentioned equations (1) and (2), stores the Harken point number in the Harken point number memory 20, Each Harken point position is stored in the Harken point memory 16.

The operation of the configuration shown in FIG. 3 will be explained in the following three steps.

(First step) Find the Harken point position where the helicoid of the lens system 1 should stop.

(Second stage) Control the helicoid to the Harken point position determined in the first stage and stop.

(Third stage) It is detected whether or not there is a need to perform a focusing operation again, and the focusing operation is started again.

First, the first stage will be explained. The first stage is divided into the following two substages.

(Step 1.1) Calculate the number of Harken points and the position of each Harken point from the focal length f and aperture value F of the lens system 1 using equations (1) and (2) above,
Remember.

(Step 1.2) The focusing helicoid position is determined using the same mountain climbing method as before, and then the optimal Harken point position is determined.

In step 1.1, information on the focal length f of the lens 1 and information on the aperture value F are sent to the A/D converter 1.
1 and 12, it is converted into a digital value and input to the Harken point calculation circuit 15. The Harken point calculation circuit 15 calculates the Harken point number N and each Harken point position R ₁ to
Rn is calculated and stored in the Harken point number memory 20 and the Harken point memory 16, respectively.

In the 1.2 stage, the electronic switch 31 is on the A side, so the focusing operation is performed in almost the same way as the conventional mountain climbing method explained using FIG. 3rd time) is different in that the electronic switch 31 is set to the B side and the mountain climbing loop is opened.

This operation will be explained with reference to FIG. Figure a
indicates the peak of the focal voltage, the starting point of the helicoid is h ₁ , and the new helicoid position is h ₂ . In this state, for example, the helicoid at position _h1 is
This occurs when it moves up to h ₂ . Now, at the start of the mountain climbing operation, if the motor 8 starts moving the helicoid in the direction of climbing the mountain as shown in FIG _. It reaches position h ₂ and moves in the opposite direction from position h ₃ , which is a little too far past the in-focus position h ₂ due to the inversion of the output signal of the terminal 61, passes the in-focus position h ₂ again and returns to position h ₄ again. to move in the opposite direction. If the mountain-climbing loop is left as is, the helicoid will continue to oscillate between positions _h3 and _h4 , as in the conventional case. However, the fact that the helicoid position reached around the focal point position h ₂ means that
It is known by detecting the reversal of times. Therefore, in the configuration shown in FIG. 3, the counter 26 calculates the reverse signal which is the output signal of the terminal 61, and the first
The information on the helicoid position _h3 at the time of the second reversal is stored in the first helicoid position memory 27, and the information on the helicoid position _h4 at the second reversal is stored in the second helicoid position memory 28. At the same time, the electronic switch 31 is set to the B side via the sum circuit 25 to open the mountain climbing loop.

Also, at the start of the mountain climbing operation, the motor 8
When the helicoid starts moving down the mountain as shown in Figure c, by the time it reaches the focal point position _h2 , the helicoid trajectory will change from position _h1 to position _h5 to position _h2 until it overlaps with the trajectory shown in figure b. It is necessary for the motor 8 to reverse once in the direction. In this case, the determination circuit 32
However, at the start of the mountain climbing operation, the output voltage of the terminal 61 decreases, and knowing that the mountain is descending, resets the contents of the counter 26 after the first reversal, and starts the second and third reversals. In order to prepare for
In the case of figure c, as in the case of figure 4b, the second
The third reversal brings the helicoid position to the in-focus position _h2
It is detected that the mountain has reached the vicinity of , and the mountain climbing loop is opened.

Now, at the same time that the content of the counter 26 becomes value 2 and the mountain climbing loop is opened, the content of the Harken point memory 16 is sequentially input to the comparison circuit 17, and the content of the first helicoid position memory 27 and the second helicoid position are input. The contents of the memory 28 are compared with a value averaged by an average value circuit 29, and the contents of the Harken point memory 16 closest to this average value are stored in the stop Harken point memory 18.

Next, the comparison circuit 19 compares the B of the electronic switch 31.
side, the motor drive circuit 30 controls the helicoid position of the lens system 1 via the motor 8, and inputs the contents of the stop Harken point memory 18 and the A/
The information on the helicoid position of the lens system 1, which is the output of the D converter 14, is matched, and when the two match, a signal is supplied to the motor drive circuit 30 through the D-side input line, and the second stage is completed. At this time, Harken point number memory 20, focal voltage memory 22
The information stored in the comparator circuit 19 is erased by the output signal of the comparator circuit 19.

Next, we will discuss the third stage, a case where refocusing becomes necessary due to a change in the distance between the subject and the camera or a change in the aperture value F of the lens system 1 and the focal length f.

When the aperture value F or focal length f of the lens system 1 changes, the content of the Harken point number memory 20 input to the comparison circuit 21 and the new Harken point number input from the Harken point calculation circuit 15 are the same. There is no need to refocus. However, when these differ, the comparator circuit 21 performs the focusing operation again, so the sum circuits 24 and 2
5, the electronic switch 31 is connected to the A side, and the determination circuit 32 resets the counter 26, starting the focusing operation from the first stage.

Furthermore, if the distance between the subject and the camera changes, the focal voltage that is the output of the A/D converter 13 input to the comparison circuit 23 and the output voltage of the focal voltage memory 22 will differ, so the difference between the two will be determined in advance. When the value exceeds the set value, the comparison circuit 23 switches the electronic switch 3 via the summation circuits 24 and 25 to perform the focusing operation again.
1 is connected to the A side, the counter 26 is reset by the determination circuit 32, and the focusing operation is started from the first stage.

In any of the above cases, after the motor 8 rotates, the initial value of the helicoid position is not the position _h1 as explained using FIG. 4, but a different value. A new focal point can be reached through a process similar to that described.

Note that in the above explanation, we did not explain the case where the helicoid becomes close or at infinity during focusing operation, but in this case, the motor 8 must be reversed once and then the operation can be resumed. continue.

As explained above in detail using FIGS. 3 and 4, if the automatic focusing device according to the present invention is used, after focusing, changes in the aperture value of the lens, changes in focal length, movement of the subject, movement of the camera, etc. Since the helicoid can be completely stopped until it is necessary to perform a focusing operation again, it is possible to take a good image without the vibration of the shooting angle caused by the vibration of the helicoid that is dependent on the conventional mountain climbing method.

It was explained that if the distance between the subject and the lens changes after focusing, the refocusing operation will start immediately if the focal voltage memory and the focal voltage at that point are different, but according to experiments, both The difference is about 10%~20
It has been found that it is preferable to perform the refocusing operation only after % has occurred.

In the description of the embodiments of the present invention, the number of Harken points was set to be the minimum number to cover the depth of field without gaps, and the position of each Harken point was selected to correspond to the number of Harken points. The number of Harken points used in the present invention may be any number as long as it is greater than or equal to the above minimum number, and in this case, the Harken point positions only need to be such that the depth of field covered by each adjacent Harken point overlaps with each other. Needless to say.

In addition, the Harken point calculation circuit 15 in FIG.
In the special case where the number of Harken points calculated by is 1, the helicoid may be unconditionally moved to that Harken point without performing any mountain climbing motion.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional mountain climbing type automatic focusing device, FIG. 2 is a waveform diagram for explaining the operation of the configuration shown in FIG. 1, and FIG. 3 is an example of an automatic focusing device according to the present invention. A block diagram showing the embodiment, FIG. 4 is a waveform diagram for explaining the operation of the configuration of FIG. 3. 1: Lens system, 2: Camera circuit (photoelectric conversion means), 7: Motor drive circuit, 8: Motor,
9: Mountain climbing circuit (focal voltage detection means), 11~
14: A/D converter, 15: Harken point calculation circuit, 16: Harken point memory, 17:
Comparison circuit, 18: Stop Harken point memory,
19: comparison circuit, 20: Harken point number memory, 21: comparison circuit, 22: focal voltage memory (storage means), 23: comparison circuit, 24, 25: sum circuit,
26: Counter (focus state detection means), 27: First helicoid position memory, 28: Second helicoid position memory, 29: Average circuit, 30: Motor drive circuit, 31: Electronic switch (switch means), 3
2: Judgment circuit, 8: Motor.

Claims (1)

[Claims] 1. Imaging means for converting light from an object into a video signal; a lens provided between the object and the imaging means and movable in the optical axis direction; A motor for moving the lens in the direction, and a high frequency component of the video signal output from the imaging means is extracted, and the high frequency component is maximized at a focusing lens position in the middle of the moving range of the lens, and the lens moves away from this focusing lens position. A feedback device comprising: a focal voltage generating means for generating a focal voltage having a characteristic of decreasing according to the following; and a motor driving means for controlling the rotation of the motor so that the lens moves in a direction that increases the focal voltage. When performing an automatic focusing operation using a loop, a focusing position detecting means detects that the lens has reached the focusing lens position, and when the detection by the focusing position detecting means is performed, the feedback is returned. a lens setting means for opening a loop to stop the movement of the lens; and sequentially storing imaging information determined from aperture value information and focal length information and a focal voltage while the lens is stopped; a storage means, and compares the values stored in the storage means with the imaging information and the focal voltage that are sequentially input, and when the difference in at least one exceeds a predetermined value, the feed and a comparison means for generating an output signal that closes the back loop, thereby preventing vibration of the lens near the position of the focusing lens due to the automatic focusing operation by the feedback loop, and An automatic focusing device characterized in that an automatic focusing operation is restarted in response to a change in state.