JPH0715648A - Auto-focus video camera - Google Patents

Abstract

PURPOSE:To suppress the overrunning of a lens at the positions near the in-focus point by increasing the driving speed of a focus motor, increasing the displaced variable of a lens per unit time and increasing the variance value of the focus evaluation value to suppress the malfunctions when the depth of field is large due to the large aperture value and then by reducing the driving speed of the motor and reducing the displaced variable of the lens per unit time when the depth of field is small due to the small aperture value. CONSTITUTION:A focus evaluation value generating circuit 5 outputs the high band component value of the image pickup video signal as the focus evaluation value. Then a focus motor 3 shifts the lens 1 to a position where the maximum focus point evaluation value is secured. In such an in-focus operation, the driving speed of the motor 3 is varied in response to the aperture value of an aperture mechanism 21 which controls the quantity of light made incident on an image pickup element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus device for a video camera which automatically adjusts a focus by using an image pickup video signal obtained from an image pickup device.

[0002]

2. Description of the Related Art In an autofocus device for a video camera, a method of using a picked-up image signal obtained from an image pickup device itself for evaluating a focus control state is essentially free of parallax and has a depth of field. There are many advantages such as accurate focusing even when the subject is shallow or at a distance. Moreover, a special sensor for autofocus is unnecessary, and the mechanism is extremely simple.

JP-A-63-215268 (H04
N5 / 232) discloses an example of the autofocus device as described above. The essence of this prior art will be described below with reference to FIGS. 2 and 3.

FIG. 2 is a circuit block diagram of the entire autofocus device. An image formed by the incident light formed on the image pickup element by the lens 1 becomes an image pickup video signal by the image pickup circuit 4 including the image pickup element and is input to the focus evaluation value generation circuit 5.

The focus evaluation value generating circuit 5 is shown in FIG.
It is configured as shown in. The luminance signal in the picked-up video signal passes through the high-pass filter (HPF) 5c to separate only the high-frequency component, and the amplitude is detected by the detection circuit 5d at the next stage. The detected output is sequentially converted into a digital value while being sampled by the A / D conversion circuit 5e, and the gate circuit 5f
Then, only the information in the focus area in the center of the screen is extracted and integrated by the integrating circuit 5g for each field, that is, A / A in the focus area obtained in one field period.
Digital integration is performed to add all the outputs of the D conversion circuit 5e, and this integrated value is output as the focus evaluation value of the current field.

At this time, the vertical sync signal and the horizontal sync signal separated from the picked-up video signal by the sync separation circuit 5a are input to the gate control circuit 5b in order to set the focus area. The gate control circuit 5b sets a rectangular focus area in the central portion of the screen based on the vertical synchronizing signal, the horizontal synchronizing signal, and the fixed oscillator output, and passes the digital value of the high frequency component of the luminance signal only in this area. Is supplied to the gate circuit 5f.

The focus evaluation value generation circuit 5 configured as described above always outputs the focus evaluation value for one field, and each circuit in the subsequent stage starts the focusing operation using this focus evaluation value.

Immediately after starting the focusing operation, the first focus evaluation value is held in the maximum value memory 6 and the initial value memory 7. After that, the focus motor control circuit 10 rotates the focus motor 3 in a predetermined direction to rotate the focus ring 2 that supports the lens 1, and displaces the lens 1 in the optical axis direction. Monitor the output.

The second comparator 9 compares the focus evaluation value obtained after driving the focus motor with the initial evaluation value stored in the initial value memory 7, and outputs the magnitude.

The focus motor control circuit 10 rotates the focus motor 3 in the first direction until the second comparator 9 outputs a large or small output, and the current focus evaluation value is larger than the initial evaluation value. If it is judged that the current evaluation value is smaller than the initial evaluation value, the rotation direction of the focus motor is reversed and the moving direction of the lens is changed. Is reversed and the output of the first comparator 8 is monitored.

The first comparator 8 compares the maximum focus evaluation value stored so far in the maximum value memory 6 with the current or latest evaluation value, and the current focus evaluation value is stored in the maximum value memory 6. Larger than content (first mode), preset first
The two comparison signals S1 and S2 of the second mode that are reduced to the threshold value M or more of are output. Here, the maximum value memory 6 updates the value based on the output of the first comparator 8 when the current evaluation value is larger than the content of the maximum value memory 6, and always updates the focus evaluation value up to the present time. The maximum value is retained.

A lens position memory 13 stores a lens position signal output from the motor-one detection circuit 30 and indicating the position (lens position) of the lens 1 in the optical axis direction. Here, the lens position signal is calculated by the motor position detection circuit 30 by detecting the rotation amount of the focus motor 3. Like the maximum value memory 6, the lens position memory 13 is updated so as to always hold the lens position when the maximum evaluation value is reached based on the output S1 of the first comparator 8.

The focus motor control circuit 10 monitors the output of the first comparator 8 while rotating the focus motor 3 in the direction determined based on the output of the second comparator 9, and the focus evaluation is performed as shown in FIG. At the same time as instructing the second mode in which the value is smaller than the preset first threshold value M as compared with the maximum evaluation value, the focus motor 3 is reversed.

By the reverse rotation of the focus motor 3, the moving direction of the lens 1 is changed, for example, from a direction approaching the image pickup device to a direction away from the image pickup device, or vice versa.

After this reverse rotation, the contents of the lens position memory 13 and the current lens position signal are compared by the third comparator 14, and when they match, that is, the lens 1 returns to the position where the focus evaluation value is maximum. When this happens, the focus motor control circuit 10 functions to stop the focus motor 3. At the same time, the focus motor control circuit 10 outputs a lens stop signal LS.

Reference numeral 11 denotes a fourth focus holding value at the same time as the lens stop signal LS is issued after the focusing operation by the focus motor control circuit 10 is finished.
This is a memory, and the content held in the fourth memory 11 is compared with the latest focus evaluation value after the focusing operation by the fourth comparator 12 in the subsequent stage, and the latest focus evaluation value is compared with the content of the fourth memory 11. When it becomes smaller than the preset second threshold value, it is determined that the subject has changed, and the subject change signal is output. When the focus motor control circuit 10 receives this signal, the focus motor control circuit 10 performs the focusing operation again to follow the change of the subject.

[0017]

The autofocus system is excellent in focusing accuracy and adaptability to a wide range of subjects, but has the following drawbacks. That is, when there is no change in the subject, the in-focus state can be obtained without any problem with the above-described focusing operation, but in actual shooting, the subject may move back and forth, left and right, or the shape of the subject itself may change. , The focus evaluation value changes due to factors other than the movement of the focus lens.

When the change in the subject is large, the direction of the in-focus point is erroneously recognized, or the first comparator enters the second mode before the in-focus point is reached, and the focus motor is reversed. May cause malfunction of.

On the other hand, when the diaphragm position (F value) of the diaphragm mechanism for adjusting the amount of light incident on the image sensor changes, the relationship between the lens position and the focus evaluation value changes as shown in FIG. This Figure 5
When the diaphragm of the diaphragm mechanism opens (small F value) and the amount of incident light is not blocked, the focus evaluation value changes as a curve indicated by a solid line, and the diaphragm is changed during shooting of the same subject under the same environment. When it is closed (large F value), the lens position range in which a certain level of focus evaluation value can be obtained is widened, that is, the depth of field is deeper, as compared with the case of a solid line as shown by a broken line.

When the diaphragm is closed and the depth of field is deep,
As is clear from FIG. 5, the change curve of the focus evaluation value becomes gentle and the ratio of the focus evaluation value change to the lens displacement becomes small, so that the above-mentioned malfunction is likely to occur.

In order to prevent such malfunction, it is effective to keep the focus evaluation value variation due to the lens displacement sufficiently large with respect to the focus evaluation value variation due to the subject change. For this purpose, the drive speed of the focus motor is increased. Can be dealt with by increasing the lens displacement and increasing the displacement amount of the lens per unit time.

By the way, in the hill climbing focusing operation as described above, the arrival at the in-focus point is detected by the decrease of the focus evaluation value, and further the focus evaluation value is obtained from the time when the light is incident on the image sensor until the focus evaluation value is obtained. Since there is a time delay until the integration in the generation circuit ends, it is unavoidable that the focal point is overshooted in principle. Therefore, if the motor drive speed is constantly increased as a measure against subject changes, the amount of lens displacement per field period required to update the latest focus evaluation value becomes large, and as described above, the focal point is exceeded. When detecting a drop in the threshold M and rotating the motor in the reverse direction, there is a high possibility that this detection will be performed at a position beyond the overshoot amount from the focal point that is the minimum necessary for detecting the drop in the threshold M, and this extra It is inevitable that the screen blurring caused by overshooting will become large.

[0023]

According to the present invention, there is provided an image pickup means for forming an image pickup video signal from incident light which is formed on an image pickup element through a lens, and a high frequency component amount of the image pickup video signal is used as a focus evaluation value. The focus evaluation value generating means for outputting and the relative lens position, which is the relative position of the lens with respect to the image sensor,
Lens position changing means for changing the focus evaluation value to a maximum value, a diaphragm mechanism for adjusting the amount of light incident on the image sensor, and a lens position changing speed by the lens position changing means according to the diaphragm amount of the diaphragm mechanism. In particular, a diaphragm detector for detecting the position of the diaphragm blades constituting the diaphragm mechanism or an F value detector for detecting the F value of the diaphragm is provided, and the driving speed of the lens is determined by the aperture position detection output. It is characterized in that it is determined in proportion to the reciprocal or the output of the F-number detector.

[0024]

The present invention has the above-mentioned configuration,
When the diaphragm closes and the depth of field becomes deep, the displacement speed of the lens can be increased to increase the fluctuation amount of the focus evaluation value with respect to the displacement amount of the lens per unit time, and malfunctions can be suppressed. .

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the entire circuit block of the apparatus of this embodiment, and the same parts as those in FIG.

Reference numeral 20 denotes an aperture position detector for detecting the aperture position of the mechanical aperture mechanism 21 arranged in the optical image pickup system, that is, the aperture amount, and specifically, it is composed of a hall sensor 22 and a processing circuit 23. This output is supplied to the focus motor control circuit 110 and used for controlling the drive speed of the focus motor 3.

The specific structures of the diaphragm position detector 20 and the diaphragm mechanism 21 are shown in FIG. The diaphragm mechanism 21 includes a movable blade 24a having a V-shaped notch 24a on the upper side and a movable blade 25 having a V-shaped notch 25a on the lower side.
The respective arm portions 24b and 25b are pivotally supported on the rotor 29 by the support shafts 26 and 27, and the rotor 29 is further pivotally supported on the camera body by the support shaft 40 so that the arm portions 24b and 25b are disposed in the lens barrel of the camera. Therefore, a rectangular aperture opening 28 is formed by both the notches 24a and 25a. By locating the aperture 28 on the optical axis, incident light is transmitted through the aperture opening 28 and the image pickup element. Will be reachable.

When the rotor 29 is rotated counterclockwise,
The movable blade 25 moves downward, and conversely the movable blade 24 moves upward, the area of the aperture opening 28 increases, the aperture amount decreases, and the aperture is gradually opened to increase the incident light amount. become. When the rotor 29 is rotated clockwise, the movable blade 25 moves upward and the movable blade 24 moves downward to reduce the area of the aperture opening 28, increase the aperture amount, and gradually increase the aperture. When closed, the amount of incident light is suppressed and reduced.

A magnet 41 is adhered to the arm portion 25b of the movable blade 25. The Hall sensor 22 is fixed to the camera chassis at a position near the magnet 41, and the rotor 29 is attached.
When the rotation amount changes and the aperture amount changes, the distance between the hall sensor 22 and the magnet 41 changes, which changes the output level of the hall sensor 22. That is, the hall sensor output level rises as the magnet 41 approaches and the magnetic force from the magnet 41 increases.

Assuming that the vertical component of the distance between the Hall sensor 22 and the magnet 41 is X and the output level of the sensor 22 is V as shown in FIG. 6, V = V0-a * X. still,
V0 is the output of the hall sensor 22 when the magnet 41 is closest to the hall sensor 22 at X = 0, and a is a constant.

The area S of the aperture opening 28 is proportional to the square of the distance X, and S = b × X2. Here, b is a constant. Further, since the F value is inversely proportional to the square root of the area S of the opening 28, when the F value is indicated by F, F = c / S1 / 2. Here, c is a constant.

On the other hand, since the depth of field is proportional to the F value,
The depth of field H = dF (d is a constant), and from these four equations, V = V0-A / H (where A = a.c.d / b1 /
2)

The output of this hall sensor 22 is a processing circuit 23.
Will be properly normalized. That is, the voltage V0 is subtracted from the Hall sensor output level, and the subtracted value is multiplied by -1 to obtain the normalized output Y = A / H. It can be seen that the normalized output Y of the Hall sensor 22 is inversely proportional to the depth of field.

The focus motor control circuit 110 is shown in FIG.
The same hill climbing focusing operation as in the conventional example is executed based on the outputs of the first, second, third and fourth comparators 8, 9, 14, 12 as in the control circuit 10 of FIG. Also, the function of controlling the drive speed of the focus motor 3 in response to the Hall sensor output is added. That is, after subtracting a predetermined offset amount m from the normalized output Y and further multiplying by a predetermined constant n, the reciprocal of the multiplication result is output to the focus motor drive circuit 50 as a speed control signal SC. When this speed control signal SC is expressed by an equation, SC = 1 / {n × (Y−
m)}.

The constants a, b, c, d, and
Both m and n are optimum values set in advance by experiments.

As a result of the above, in the situation where the diaphragm mechanism 21 is closed from the open state and the depth of field becomes deeper (H becomes larger) as the diaphragm amount becomes larger, the normalized output Y
Becomes smaller, and the signal level of the speed control signal SC increases accordingly. On the contrary, in a situation where the diaphragm mechanism 21 opens in the opening direction and the depth of field becomes shallower (H becomes smaller) as the diaphragm amount becomes smaller, the normalized output Y becomes larger, and accordingly, the speed control signal SC. Will reduce the signal level.

The focus motor drive circuit 50, in the same manner as the focus drive circuit 31 of FIG. 2, receives an instruction from the focus motor control circuit 110 based on the outputs of the first, second and third comparators 8, 9 and 14. The focus motor 3 is driven, stopped, and rotated in the reverse direction. In addition to these controls, the drive speed when the motor 3 is driven is changed according to the level of the speed control signal SC. Here, the driving speed changes in proportion to the signal level of the speed control signal SC, and the higher the signal level of the speed control signal SC, the higher the driving speed, and as a result, the driving is performed in a situation where the diaphragm amount is small and the depth of field is shallow. The speed becomes relatively low, and the amount of overshoot from the in-focus point can be suppressed. Further, in a situation where the diaphragm amount is large and the depth of field is deep, it is possible to suppress the malfunction due to the change of the subject by increasing the speed and keeping a large ratio of the fluctuation of the focus evaluation value due to the displacement of the focus lens 1.

It is needless to say that the circuit operation of each circuit block diagram of the above-described embodiment can be processed by software by a microprocessor. Further, in the above-described embodiment, the focusing operation is performed by moving the lens 1 forward and backward in the optical axis direction, but the same effect can be obtained by fixing the lens 1 and moving the image pickup element itself forward and backward in the optical axis direction. Needless to say. In this case, a piezoelectric element may be used in addition to the motor to move the image pickup element forward and backward, and the control signal from the focus motor control circuit 110 may be supplied to these motors or piezoelectric elements.

[0039]

As described above, according to the present invention, the drive speed of the focus motor is changed according to the aperture amount, and the drive speed of the focus motor is increased when the depth of field is deep due to the large aperture amount. , The amount of lens displacement per unit time is increased, the amount of fluctuation of the focus evaluation value is increased to suppress malfunctions, and conversely, the small aperture amount reduces the drive speed when the depth of field is shallow. Then, the displacement amount of the lens per unit time can be reduced, and the overshoot amount of the lens in the vicinity of the focal point can be suppressed.

[Brief description of drawings]

FIG. 1 is a circuit block diagram of an embodiment of the present invention.

FIG. 2 is a circuit block diagram of a conventional example.

FIG. 3 is a circuit block diagram of a main part of a conventional example.

FIG. 4 is a diagram showing a relationship between a lens position and a focus evaluation value associated with a focusing operation.

FIG. 5 is a diagram showing a relationship between a lens position and a focus evaluation value due to a difference in aperture amount.

FIG. 6 is a diagram illustrating a diaphragm mechanism according to an embodiment of the present invention.

[Explanation of symbols]

 1 Lens 4 Imaging Circuit 5 Focus Evaluation Value Generation Circuit 3 Focus Motor 21 Aperture Mechanism

Claims (1)

[Claims] 1. An image pickup means for creating an image pickup video signal from incident light imaged on an image pickup element through a lens, and a focus evaluation value generation means for outputting a high frequency component amount of the image pickup video signal as a focus evaluation value. A relative position changing unit that changes a relative position of one of the lens and the image pickup device to the other, to a position where the focus evaluation value has a maximum value, and a diaphragm that adjusts an amount of incident light to the image pickup device. An autofocus video camera, wherein a mechanism and a speed of changing the relative position by the relative position changing unit are variable according to a diaphragm amount of the diaphragm mechanism.