JPH0728391B2 - Autofocus video camera - Google Patents

Description

The present invention relates to an autofocus system for a video camera that automatically adjusts the focus based on a video signal obtained from an image sensor.

(B) Conventional technology In an autofocus device for a video camera, the method of using the image signal itself from the image sensor for evaluating the focus control state has essentially no parallax and has a depth of field. It has many advantages such as being able to focus accurately with a shallow subject or a distant subject. Moreover, a special sensor for autofocus is unnecessary, and the mechanism is extremely simple.

The present applicant has previously proposed an example of the above-described autofocus device as Japanese Patent Application No. 62-164382.

The essence of the prior application technique will be described below with reference to FIGS. 2 to 7. FIG. 2 is an overall circuit block diagram. In this figure, the image formed by the lens (1) is
An image pickup circuit (4) having an image pickup element forms an image pickup video signal, and the luminance signal therein is input to the gate circuit (5c) and the sync separation circuit (5a).

The vertical sync signal and the horizontal sync signal separated from the luminance signal by the sync separation circuit (5a) are input to a gate control circuit (5b) for setting a sampling area. The gate control circuit (5b) sets a rectangular sampling area in the center of the screen based on vertical and horizontal sync signals and fixed oscillator output, and opens and closes the gate to allow passage of luminance signals only within this sampling area. The signal is supplied to the gate circuit (5c).

Thus, only the luminance signal in the sampling area set at the center of the screen by the gate circuit (5c) is input to the first and second filter circuits (15) and (16) in the subsequent stage. Both of the filter circuits (15) and (16) are high-pass filters (HPF) that extract only high-frequency components of the luminance signal, but the first filter circuit (15) is higher than the second filter circuit (16). Has a cutoff frequency. Specifically, the cutoff frequency of the first filter circuit (15) is 500 KHz,
That of the second filter circuit (16) is set to 100 KHz. The outputs of the filter circuits (15) and (16) are input to the switch circuit (17) controlled by the focus motor control circuit (100) and are alternately input to the integrating circuit (18).

The integrating circuit (18) is a detection circuit (18a), A as shown in FIG.
A / D conversion circuit (18b) and an integrator (18c) are connected in series. The output of the switch circuit (17) is amplitude-detected by the detection circuit (18a), and this detection output is A / D converted to 1 The focus evaluation value of the current field can be obtained by integrating the A / D conversion values over the field.

As a result, the first filter circuit (15) and the switch circuit (1
7), the first focus evaluation value generating circuit is configured by the integrating circuit (18), and the second filter circuit (16) and the switch circuit (1
7), the integrating circuit (18) constitutes a second focus evaluation value generating circuit.

The cutoff frequency of the first focus evaluation value generation circuit is the second
Since it is higher than that of the second focus evaluation value generation circuit, the output of the first focus evaluation value is higher than that of the second focus evaluation value generation circuit as shown in FIG. The shape of is sharp. The output of the integrating circuit (18) is the switch circuit (1
Input in 9). The switch circuit (19) is switched for each field in synchronization with the switch circuit (17), and the output of the first focus evaluation value generation circuit is supplied to the first focus detection circuit (20)
The output of the focus evaluation value generation circuit is alternately input to the second focus detection circuit (21) for each field.

Specifically, the first and second focus detection circuits (20) and (21) have the configuration shown in FIG. Immediately after the start of the autofocus operation, the first focus evaluation value is held in the maximum value memory (20a) and the initial value memory (20b). afterwards,
The focus motor control circuit (10) rotates the focus motor (3) in a predetermined direction to rotate the focus ring (2) supporting the light receiving lens (1),
The light receiving lens (1) is displaced in the axial direction, and the second comparator (20
d) Monitor the output. The second comparator (20d) compares the focus evaluation value after driving the focus motor with the initial evaluation value stored in the initial value memory (20b), and outputs the magnitude. The focus motor control circuit (10) rotates the focus motor (3) in the first direction until the second comparator (20d) outputs a large or small output, and the current focus evaluation value is higher than the initial evaluation value. When the output that the value is large is output, the rotation direction is maintained as it is, and when the current evaluation value is smaller than the initial evaluation value, the rotation direction of the focus motor (3) is reversed and the first rotation is performed. Monitor the output of the comparator (20c).

The first comparator (20c) compares the current maximum focus evaluation value held in the maximum value memory (20a) with the current evaluation value, and the current focus evaluation value is stored in the maximum value memory (20a). Two comparison signals (S1) and (S2) that are larger than the contents (first mode) and have decreased below the preset first threshold (R1) (second mode) are output. Here, the maximum value memory (20
a) is based on the output of the first comparator (20c), if the current evaluation value is larger than the content of the maximum value memory (20a), the value is updated, and the focus evaluation value up to the present is always Holds the maximum value of.

Reference numeral (30) is a motor position detection circuit for detecting the motor position of the focus motor (3), and specifically, an FG pulse output according to the rotation of the focus motor (3) (for example, 100 pulses are generated in one rotation). Is an UP / DOWN counter that counts, and when the command is issued to rotate the focus motor (3) in the direction that moves the lens from the near point side to the far point side, add the FC pulse, If the command is issued to rotate in the opposite direction, the FG pulse is subtracted.
Therefore, the count value of this counter itself becomes the motor position and is output as a motor position signal.

(20e) is a focus motor (3) that rotationally drives a focus ring (2) that supports the focus lens (1).
It is a motor position memory that stores the motor position by receiving the motor position signal that indicates the rotational position of, and has the maximum evaluation value based on the output of the first comparator (20c) as with the maximum value memory (20a). In this case, the motor position is updated so as to always hold the position. The focus ring (2) supports the light receiving lens (1), and the rotation of the ring itself causes the light receiving lens (1) to move back and forth in the optical axis direction. Therefore, the above-mentioned motor position is the optical axis of the light receiving lens (1). It corresponds substantially to the lens position in the direction.

The second focus detection circuit (21) is the first focus detection circuit (20).
It has exactly the same configuration as.

The focus motor control circuit (10) rotates the focus motor (3) in the direction determined based on the second comparator output of the first or second focus detection circuit (20) (21), The output of the comparator is monitored, and the focus motor (3) is reversed at the same time when the second mode in which the focus evaluation value is smaller than the first evaluation threshold value (R1) set in advance as compared with the maximum evaluation value is instructed.

Due to the reverse rotation of the focus motor (3), the moving direction of the light receiving lens (1) changes, for example, from a direction approaching the image sensor to a direction away from the image sensor, or vice versa.

After this reverse rotation, the contents of the motor position memory (20e) and the current motor position signal are compared by the third comparator (20f) and when they match, that is, the focus ring (2) has the maximum focus evaluation value. The focus motor control circuit (10) functions to stop the focus motor (3) when returning to the position. At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS). Incidentally, FIG. 6 is a diagram showing the relationship between the lens position and the focus evaluation value associated with the above-described focusing operation, and the point (P) shows the initial position of the lens (1).

The second focus detection circuit (21) also has the same configuration as the first focus detection circuit (20), and outputs (21x) (21y) (21
z) corresponds to the output (20x) of the third comparator (20f), the output (20y) of the first comparator, and the output (20z) of the second comparator, respectively.

The switch circuits (17) and (19) controlled by the focus motor control circuit (10) output the outputs of the first and second filter circuits (15) and (16) for each field by the integrating circuit (18). After being integrated, the switching is performed so as to be supplied to the first and second focus detection circuits (20) and (21). Therefore the first
The first focus evaluation value for one field from the first focus evaluation value generating circuit configured by the filter circuit (15), the switch circuit (17) and the integrating circuit (18) is the switch circuit (19).
Is input to the first focus detection circuit (20), and then the switch circuits (17) and (19) are switched to the second filter circuit (16), the switch circuit (17), and the integrating circuit (18). The second focus evaluation value for the next one field from the configured second focus evaluation value generation circuit is input to the second focus detection circuit (21) via the switch circuit (19) and corresponds to this. The output (20x) (20y) (20z) and the output (21x) (21y) (21z) are supplied to the focus motor control circuit (10) for each field.

By the way, the focus motor control circuit (10) is always supplied with the outputs of the first and second focus detection circuits (20) and (21) as described above, and the first focus evaluation value is shown by the alternate long and short dash line in FIG. When the value is larger than the reference value (Q) shown in, the first peak is steep.
Assuming that the focusing operation based on the focus evaluation value is optimum, the focus motor (3) is controlled using the output of the first focus detection circuit (20), and the first focus evaluation value is the reference value (Q ) Is smaller than the first focus evaluation value, the first focus evaluation value is too small and is likely to be affected by noise or the like. Therefore, the focusing operation is executed based on the second focus evaluation value whose level is higher than the first focus evaluation value. Therefore, in this case, the focus motor (3) is controlled by using the output of the second focus detection circuit (21). Here, the determination as to whether the first focus evaluation value is larger or smaller than the reference value (Q) is executed by the fifth comparator (13). The reference value (Q) is stored in the reference value memory (14) in advance.

(11) is a fourth memory that holds the second focus evaluation value at that time at the same time when the focusing operation by the focus motor control circuit (10) ends and the lens stop signal (LS) is issued. , The content held in the fourth memory (11) is compared with the current focus evaluation value by the fourth comparator (12) in the subsequent stage, and the current focus evaluation value is preset in comparison with the content of the fourth memory (11). When it becomes smaller than the determined second threshold value (R2), it is determined that the subject has changed, and a subject change signal is output. Upon receiving this signal, the focus motor control circuit (10) performs the autofocus operation again and follows the change of the subject.

When the focusing operation is restarted, the focus motor control circuit (10) performs the focusing operation while switching the switch circuits (17) and (19) for each field as described above.

The arithmetic circuit (22) inputs the first and second focus evaluation values from the switch circuit (19) and always calculates the ratio (relative ratio) of the latest second focus evaluation value to the latest first focus evaluation value. .
The relationship between the relative ratio (first focus evaluation value / second focus evaluation value) and the degree of blurring of the subject (movement amount or shift amount from the lens position at the time of focusing) is shown in a graph in FIG. It becomes such a monotonically decreasing characteristic curve.

The state quantity having the relative ratio is a function value capable of expressing the in-focus state (degree of blurring) of the subject similarly to the focus evaluation value, and is a kind of normalized state quantity because it is expressed as a ratio. , It has the property of being less susceptible to the environment in which the subject is placed. For example, when the illuminance of the subject changes, the absolute value of the focus evaluation value changes, but the relative ratio does not change significantly. Usually, since the above-mentioned property is independent of the type of subject, it is possible to use this relative ratio as a parameter of the degree of blur.

Since the relative ratio as described above is the state quantity that represents the degree of blurring, the third threshold (R) preset in the threshold holding circuit (24) is used.
3) It is determined by a sixth comparator (relative ratio comparing means) (23) whether it is smaller or larger than (specifically, 0.5 in the characteristic diagram of FIG. 7). The result of this determination is input to the focus motor control circuit (10) to control the rotation speed of the focus motor (3) during the focusing operation. That is, if the relative ratio during the focusing operation is smaller than the third threshold value (R3), the focus motor control circuit (10) instructs the normal speed mode by the control signal (NSP), and the focus motor (3) is normal. Rotate at speed. If the relative ratio is larger than the third threshold value (R3), the focus motor control circuit (1
0) indicates the low speed mode by the control signal (LSP), and the focus motor (3) rotates at a speed lower than the normal speed.

Therefore, when the relative ratio is smaller than the third threshold value (R3) and is out of focus, the lens (1) moves forward and backward at a high speed, and the relative ratio is larger than the third threshold value (R3) and near the in-focus point. When the change in the focus evaluation value with respect to the lens movement is small, the lens (1) can move back and forth at a low speed, and the focus motor (3) by the output (S2) of the first comparator (20c).
It is possible to suppress the overrun at the time of reverse rotation of.

By the way, in the above-mentioned prior art, the lens cannot accurately return to the maximum point of the focus evaluation value for the following three reasons.

One is that it takes a certain time until the focus evaluation value for a certain lens position is output. For example, as for the focus evaluation value at a certain lens position A, the focus evaluation value is output only after an optical image is picked up by the image sensor for one field period at the lens position A and read out as a video signal in the next one field. That is, a certain lens position A
The focus evaluation value of is output with a delay of two fields. That is, when the focus motor (3) is moving,
The focus ring (2) has already passed the maximum value when the maximum focus evaluation value is output, and the focus motor (3) is returned to the motor position when the maximum focus evaluation value is output. Then, the focus ring (2) will be insufficiently returned.

The second is due to the backlash (backlash) of the drive system from the focus motor (3) to the focus ring (2). Even if the focus motor (3) is rotated in the reverse direction, only a certain amount after the reverse rotation is applied to the focus ring. This is because the movement is not transmitted to (2).

The third is due to the inertia of the focus ring (2), and when a reverse rotation command is issued to the focus motor (3) when the focus evaluation value drops from the maximum evaluation value by the first threshold value (R1), In synchronization, the counter, which is a motor position memory, immediately switches from a state in which the FG pulse is added to a state in which it is subtracted, or vice versa. However, the focus motor (3) and the focus ring (2) themselves are switched. Since the motor rotates in the same direction for some time due to the inertial force that it has, the actual reverse rotation is delayed from the time when the calculation state of the motor position memory is changed, and the motor position stored in the motor position memory is It cannot be said that the focus position is accurate.

Due to the above factors, even if the motor position is returned to the motor position in which the maximum evaluation value is recorded, the focus ring (2) cannot be returned to the position showing the maximum evaluation value.

In order to solve such a drawback, the applicant further discloses in Japanese Patent Application No. 62-49513 that after returning the motor to the position stored in the motor position memory, a constant value for the motor return correction is given. It proposes a method to stop the motor after just overshooting.

Next, this prior application technique will be described with reference to FIG. When the lens passes the in-focus position and the focus evaluation value drops from the maximum evaluation value by the first threshold value (R1), the focus motor (3) is rotated in reverse, and the contents of the motor position memory (20e) and the current focus The operation of returning the focus ring to the position where the maximum focus evaluation value is stored while comparing the ring position signal with the third comparator (20f) is the same as in the prior art. However, at this time, the third comparator (20
The output of the motor return correction amount holding circuit (20g) is input to f), and the third comparator (20f) is connected to the motor position memory (20g).
After the contents of e) are compared with the current motor position and they match, the motor position is the motor return correction amount holding circuit (20g).
The motor position coincidence signal is output to the focus motor control circuit (10) only after an excessive amount of the motor return correction amount of is moved. When this motor position coincidence signal is input, the focus motor control circuit (10) stops the focus motor (3) even if the lens (1) returns to the position where the focus evaluation value is substantially the maximum evaluation value. And outputs the lens stop signal (LS) at the same time.

(C) Problems to be Solved by the Invention In the conventional autofocus control circuit, when the point where the maximum evaluation value is obtained is exceeded and the reverse rotation is performed, the lens is returned to the position where the original maximum evaluation value is obtained due to the above three factors. There were cases where it was not. In order to improve this, after returning the motor to the motor position that seems to have recorded the maximum evaluation value,
There is also a method of stopping the motor after the motor return correction amount has exceeded a certain value, but like the above-mentioned prior art,
When the moving speed of the focus lens is switched according to the relative ratio level of the two types of focus evaluation values, the characteristics of the relative ratio are slightly different depending on the target subject in reality. As a result, a difference also occurs in the inertial force of the focus ring, which is the third factor. Therefore, it is difficult to apply the correction amount set in advance as a constant to many subjects and perform accurate autofocus operation for all of them.

(D) Means for Solving the Problem The present invention provides a period during which the focus motor rotates in the normal speed mode and a period during which the focus motor rotates in the low speed mode until the focus evaluation value decreases by the first threshold value or more. Have a means of storing, and from their comparison results,
An accurate autofocus operation is performed by appropriately modifying the motor return correction amount.

(E) Operation Since the present invention is configured as described above, when the lens is returned to the position where the focus evaluation value is the maximum evaluation value, the return amount does not become excessive or insufficient depending on the subject to be measured, and the lens can be accurately adjusted. It becomes possible to return to the focus.

(F) Embodiment One embodiment of the present invention will be described below with reference to the drawings. The same parts as those of the conventional example are designated by the same reference numerals, and the description thereof will be omitted.

FIG. 1 is an overall circuit block diagram of this embodiment, in which the output of the switch circuit (19) is alternately input to the first and second focus detection circuits (120) (121) for each field. To be done. That is, the output of the first focus evaluation value generation circuit, which includes the first filter circuit (15), the switch circuit (17), and the integration circuit (18), is output to the second focus detection circuit (120) and the second filter circuit (16). ), A switch circuit (17), and an integrating circuit (18), the output of the second focus evaluation value generating circuit is input to the second focus detecting circuit (121).

As shown in FIG. 9, the first focus detection circuit (120) is similar to the configuration of the first focus detection circuit (20) shown in FIG. Return correction amount holding circuit (120h), adder (120k), subtractor (120
m), a first speed mode counter (120i), and a second speed mode counter (120j).

The control pulse (DN) is supplied to the first or second focus detection circuit (12
0) When the focus motor control circuit (100) sets the focus motor (3) to the normal speed mode after the rotation direction of the focus motor (3) is determined by the second comparator output of (121), One is input to the second focus detection circuits (120) and (121) for each field, and similarly, one control pulse (DL) is input for each field when the focus motor (3) is set to the low speed speed mode.

Therefore, for example, when the first focus evaluation value is larger than the reference value (Q), the output of the second comparator (20d) is issued, the rotation direction is determined, the focus motor (3) is in a driving state, and the focus is adjusted. Output (S) from the first comparator (20c)
2) is issued to enter the second mode, that is, when the normal speed mode is executed for n fields and the low speed mode is executed for m fields before the motor reverses, the first speed mode counter (120i)
Has a count value of n, the second speed mode counter (120
The count value of j) is m. In this state, when the first comparator (20c) instructs the second mode and the focus motor (3) is rotated in the reverse direction, the initial correction amount (do) stored in the motor return correction amount memory (120g) in advance. Is added to the count value (n) of the first speed mode counter (120i) and a small amount (Δd), and the added value (do
From + Δd ・ n) to the second speed mode counter (120j)
The value obtained by integrating the count value (m) with the minute amount (Δd) is subtracted, and the subtracted value {do + Δd · (n−m)} is used as the corrected motor return correction amount holding circuit (12).
Hold for 0h). Therefore, this correction amount becomes large when the normal speed mode of the focus motor continues for a long time, and conversely becomes small when the mode is switched to the low speed speed mode at an early stage. The initial correction amount (do) is set in advance experimentally in consideration of the first and second factors described in the conventional example.

After the focus motor reversely rotates, the contents of the motor position memory (20e) and the current motor position are compared with each other by the third comparator (2
0f), the two match, and when the motor position has returned an extra amount by the correction amount of the motor return correction amount holding circuit (120h), a motor position match signal is output to the focus motor control circuit (100). .

The focus motor control circuit (100) functions to stop the focus motor (3) when the focus motor (3) returns to the position where the focus evaluation value becomes the maximum evaluation value when receiving the motor position coincidence signal. , At the same time, the lens stop signal (LS) is output.

Thereafter, each time the focusing operation is completed, the motor return correction amount is adjusted as described above, and the motor return correction amount is always corrected to the optimum value.

Therefore, even if the speed history of the focus motor changes for each subject and a difference occurs in the inertial force of the focus ring, an accurate focusing operation can always be realized. The second focus detection circuit (12
The configuration of 1) also has exactly the same configuration as the first focus detection circuit (120) described above, and when the first focus evaluation value is smaller than the reference value (Q), this second focus detection Focusing operation is performed based on the output of the circuit (121), but the control pulse (DN) (D
Both L) are input to both focus detection circuits, and the initial correction amount (do) is also set to the same value, so the calculation of the return correction amount is exactly the same in both focus detection circuits (120) (121). Done,
Even if the first focus evaluation value becomes large and the basis of the focusing operation is switched from the output of the second focus detection circuit (121) to the output of the first focus detection circuit (120), no problem occurs.

In addition, in the present embodiment, a value obtained by integrating the outputs of the second filter (16) is used as the second focus evaluation value, but instead of this, integration of the outputs of the first and second filters (15) (16) is used. It is also possible to use the sum of the values.

Further, it goes without saying that the operation of the circuit block diagram of FIG. 1 can be easily processed by software using a microcomputer.

(G) Effect of the Invention As described above, according to the present invention, when the drive speed of the focus motor is switched in multiple stages in the process of detecting the position where the focus evaluation value is maximum, the speed history varies depending on the subject. In addition, even when the influence of the inertia of the focus ring causes a difference, an accurate focusing operation can be realized without causing an excess or deficiency in the returning amount when returning the lens to the in-focus point.

[Brief description of drawings]

1 and 9 are circuit block diagrams of an embodiment of the present invention. FIGS. 2, 3, 5, and 8 are circuit block diagrams of a conventional example, FIGS. 4 and 6 are diagrams showing the relationship between the lens position and the focus evaluation value, and FIG. 7 is the degree of blurring. It is a figure which shows the relationship of a relative ratio. (15) ... first filter (first focus evaluation value detection means),
(16) ... second filter (second focus evaluation value detection means),
(23) …… Sixth comparator (relative ratio comparison means), (100)…
Focus motor control circuit (3) Focus motor, (20e) Motor position memory

Claims (1)

[Claims] 1. A first focus evaluation value detecting means for outputting a first high frequency component level of a video signal obtained from an image pickup device as a first focus evaluation value, and the first high frequency component level of the video signal. A second focus evaluation value detection means for outputting a second focus evaluation value including a second high-frequency component level including a low-frequency component, and a ratio of the first focus evaluation value to the second focus evaluation value. Relative ratio calculating means for calculating as a relative ratio, relative ratio comparing means for comparing the relative ratio with a reference value, focus motor for moving the lens forward and backward in the optical axis direction, and control obtained from at least one of the both focus evaluation values A motor position memory for storing the rotational position of the focus motor as a motor position when the evaluation value for use becomes the maximum evaluation value, and the drive speed of the focus motor is changed by the output of the comparison means. When the lens goes over the position where the control evaluation value becomes the maximum evaluation value, and the focus motor is rotated in the reverse direction when the lens value is reduced to the first threshold value or more, and the motor position matches the position stored in the motor position memory, And a focus motor control unit that stops the focus motor when the amount of return correction is changed, and the amount of return correction is changed according to the drive time at each drive speed until the focus motor rotates in the reverse direction. And an autofocus video camera.