JP3157356B2 - Auto focus video camera - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic focusing apparatus for a video camera which performs automatic focus adjustment using an image signal obtained from an image sensor.

[0002]

2. Description of the Related Art In an autofocus apparatus for a video camera, a method of using an image pickup video signal itself obtained from an image pickup element to evaluate a focus control state is essentially free from parallax, and has a large depth of field. There are many excellent points, such as the ability to focus accurately with respect to shallow and distant subjects. In addition, no special sensor for autofocus is required, and the mechanism is extremely simple.

[0003] JP-A-63-215268 (H04)
N5 / 232) discloses an example of the autofocus device as described above. Hereinafter, the gist of the prior art will be described with reference to FIGS.

FIG. 2 is an overall circuit block diagram of the autofocus device. An image formed by the incident light formed on the image sensor by the lens 1 is converted into an image signal by the image sensor 4 including the image sensor, and the luminance signal in the image signal is input to the focus evaluation value generator 5.

[0005] The focus evaluation value generating circuit 5 is, for example, shown in FIG.
It is configured as shown in FIG. The luminance signal in the captured video signal passes through a high-pass filter (HPF) 5c to separate only high-frequency components, and is subjected to amplitude detection by a detection circuit 5d at the next stage. The detection output is sequentially converted into a digital value while being sampled by an A / D conversion circuit 5e, and is converted into a gate circuit 5f.
, Only the information in the focus area at the center of the screen is extracted and integrated by the integration circuit 5g for each field, that is, A / A in the focus area obtained in one field period.
Digital integration for adding all the outputs of the D conversion circuit 5e is performed, and this integrated value is output as the focus evaluation value of the corresponding field.

At this time, the vertical synchronizing signal and the horizontal synchronizing signal separated from the picked-up video signal by the sync separation circuit 5a are input to the gate control circuit 5b for setting a focus area. The gate control circuit 5b sets a rectangular focus area in the center 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 generating circuit 5 configured as described above always outputs a focus evaluation value for one field, and each subsequent circuit starts a focusing operation using this focus evaluation value. Here, the focus evaluation value is stored in the reference value memory 15 from the focus evaluation value generation circuit 5 while the lens 1 is kept stopped prior to the start of the focusing operation. The new focus evaluation value and the focus evaluation value one field before stored in the reference value memory 15 are level-compared by the fifth comparator 16 at the subsequent stage, and the above-described fluctuation of the focus evaluation values of two consecutive fields, ie, both It is detected whether the absolute value of the difference between the evaluation values is smaller than a third threshold value J set in advance. Here, the third threshold value is an evaluation value variation amount that is generated when the possibility of malfunction is high even when the focusing operation is performed because the panning of the camera itself or the change of the subject is severe, and based on an experiment in advance. Is set.

When the comparison operation is completed, the contents of the reference value memory 15 are updated with the new focus evaluation value.
Thus, in the comparison operation in the fifth comparator 16, the absolute value of the difference between the focus evaluation values of the two most recent fields and the third threshold value J is always compared. The comparison result of the fifth comparator 16 is supplied to the subsequent start control circuit 17 together with the latest focus evaluation value.

The start control circuit 17 executes the focusing operation as described above when the comparison output indicating that the change of the focus evaluation value exceeds the third threshold value J is input from the fifth comparator 16, and the start control circuit 17 calculates the focus evaluation value. When it is determined that the fluctuation is too large and a malfunction is likely to occur, a delay signal DS for delaying the start time of a focusing operation described later is output to the focus motor control circuit 10 at the subsequent stage.

The start control circuit 17 also performs an evaluation value monitoring operation for monitoring whether or not the absolute value of the latest focus evaluation value is 0, and when the focus evaluation value is 0, As in the case described above, it is assumed that the subject has a remarkably low contrast like a solid white wall, it is difficult to obtain a focus evaluation value serving as a basis for performing a focusing operation, and a malfunction is likely to occur when the focusing operation is performed. To output the delay signal DS.

Further, the start control circuit 17 controls the delay signal DS
Is output, the monitoring operation of the evaluation value fluctuation and the absolute value of the evaluation value is continued, and while the fluctuation of the evaluation value exceeding the third threshold value J or the absolute value of the focus evaluation value continues to be 0, this delay signal Continue to output DS. However, the start waiting time also has an upper limit, and if the delay signal continues to be output for a preset upper limit time T after the delay signal is generated, the output of the delay signal is performed after the upper limit time is reached. Is prevented, and the possibility of a malfunction remains, but the focusing operation is forcibly performed because it is considered that no change is found in the situation and further waiting is not preferable.

Normally, the focus motor control circuit 10 starts the drive control of the focus motor drive circuit 31 so that the lens 1 is displaced immediately after the power supply to the camera section of the video camera is turned on and the focusing operation described later is started. However, while the delay signal DS is being input, the focusing operation is prohibited,
The drive control for the focus motor drive circuit 31 is not performed, and during this time, the focus motor 3 remains stopped. However, when the delay signal DS is continuously output and the upper limit time T elapses, the delay signal DS is no longer emitted, and thus the synchronizing operation is started assuming that the start waiting limit has been exceeded.

Next, the focusing operation will be described. Immediately after the start of the focusing operation, the first focus evaluation value is stored in the maximum value memory 6.
Is stored in the initial value memory 7. Thereafter, the focus motor control circuit 10 rotates the focus motor 3 in a predetermined direction, rotates the focus ring 2 supporting the lens 1, displaces the lens 1 in the optical axis direction, and causes the second comparator 9 to rotate. 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 of the comparison.

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. Is output, the rotation direction is kept as it is, and if it is determined that the current evaluation value is smaller than the initial evaluation value, the rotation direction of the focus motor is reversed and the movement direction of the lens is changed. And the output of the first comparator 8 is monitored.

The first comparator 8 compares the current maximum focus evaluation value held in the maximum value memory 6 with the current, that is, the latest evaluation value, and the current focus evaluation value is stored in the maximum value memory 6. Larger than the content (first mode), the preset first
(The second mode), the two comparison signals S1 and S2, which are reduced to the threshold M or more. 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. The maximum value is kept.

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

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 focus evaluation as shown in FIG. At the same time that the second mode in which the value is smaller than the preset first threshold value M as compared with the maximum evaluation value is instructed, the focus motor 3 is rotated in the reverse direction.

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

After this reversal, 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 becomes the maximum. Then, the focus motor control circuit 10 functions so as 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 point in which the focusing operation by the focus motor control circuit 10 is completed, the lens stop signal LS is issued, and the focus evaluation value at that time is held.
The content stored in the fourth memory 11 is compared with the latest focus evaluation value after the focusing operation is completed by the fourth comparator 12 at the subsequent stage, and the latest focus evaluation value is compared with the content of the fourth memory 11. When it becomes smaller than or equal to the second threshold value set in advance, 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 focusing operation again and follows the change of the subject.

[0022]

The autofocus system is excellent in focusing accuracy and adaptability to a wide range of subjects, but has the following drawbacks. That is, the upper limit time T of the start waiting time set by the start control circuit.
If is set to be short, the contrast will be extremely low like a solid wall, and if a subject that cannot extract the high frequency component from the luminance signal of the captured video signal fills the entire imaging screen during panning, the screen will While the camera is occupied, it reaches the upper limit time and the focusing operation starts,
Malfunctions often occur.

On the other hand, if the upper limit time T is increased, the possibility of the above-described malfunction is reduced. However, when the camera is redirected to a subject at a great distance, the blur amount becomes large and the focus evaluation value becomes 0, and the operation starts. Waiting time becomes longer and response becomes worse.

[0024]

According to the present invention, information on the focal length is obtained from a focal length detecting section for detecting the focal length of the image pickup system, and the upper limit time of the waiting time for starting the focusing operation is determined based on this focal length. The upper limit time is lengthened when the focal length is long, and the upper limit time is shortened when the focal length is short.

[0025]

The present invention has the above-described structure,
When the focal length is long, the angle of view is narrow, and as a result, the screen is more likely to be covered by a subject having no high-frequency component such as a plain wall, and the covering time is longer. At this time, by increasing the upper limit time, it is possible to prevent an erroneous focusing operation from being performed.

When the focal length is short, the screen is rarely covered by a subject having no high-frequency component. Even if such a situation occurs during panning, the time is short. Therefore, if the upper limit time is shortened at this time, the responsiveness when the focus evaluation value becomes 0 due to blurring can be improved.

[0027]

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 of FIG.

The image formed by the lens 1 is converted into an image signal by an image pickup circuit 4 including an image pickup element, and a luminance signal is supplied to a focus evaluation value generation circuit 5.
As in FIG. 2, one field of the high frequency component of the luminance signal in the focus area is sequentially output as a focus evaluation value.

The focal length detector 40 is connected to a zoom mechanism (not shown) having a well-known zoom lens mounted on the present video camera, and a focal length detector 40 is provided according to the zoom range from the wide angle to the telephoto of the zoom mechanism. The focal length data indicating the focal length Z of the zoom lens is output.

The focal length data is supplied to an upper limit time setting circuit 41, and is used for setting the upper limit time T. That is, the upper limit time T, which is the maximum value of the duration of the output of the delay signal DS in the start control circuit 47, is not a fixed value as in the conventional example of FIG. 2, but is variable according to the focal length Z. As a method for determining the upper limit time T, a proportional constant a
, And adding the offset value b to this, T =
It is calculated as a × Z + b. Here, the proportional constant a and the offset value b are set in advance so as to be optimum values through experiments.

According to the above formula, when the focal length Z is long, the upper limit time T becomes longer and the maximum waiting time of the focusing operation becomes longer. Conversely, when the focal length Z is shorter, the upper limit time T becomes shorter. The maximum waiting time will be set to be short.

The upper limit time T determined by the above formula is input to the start control circuit 47. The start control circuit 47 includes the fifth comparator 16 as in the conventional example.
According to the comparison result in the above, when it is determined that the change of the focus evaluation value in the lens stopped state exceeds the third threshold value J, or when the absolute value of the focus evaluation value is 0, the delay signal DS is output. 2 has the same function as the start control circuit 17, but differs in that the upper limit time T is made variable by the upper limit time setting circuit 41.

The delay signal DS has a focus evaluation value variation of the third
As long as at least one of the state exceeding the threshold value J and the state where the absolute value of the focus evaluation value is 0 is continued, the output is continuously performed until the upper limit time T elapses after the start of the output. After the elapse of the time T, the output is forcibly blocked even if any of the above states is continued.

The focus motor control circuit 10 issues various commands for driving, stopping and reversing to the focus motor drive circuit 31 based on the outputs of the first, second and third comparators 8, 9 and 14, as in FIG. Then, a focusing operation for gradually displacing the lens 1 to a focusing position at which the focus evaluation value becomes the maximum value is performed, and this focusing operation is performed in a standby state while the delay signal DS is input. However, in a shooting state in which the delay signal DS is generated, the focusing operation is set to be the maximum when the power to the camera unit is turned on or when a command to restart the focusing operation due to the subject change from the fourth comparator 12 is input. Upper limit time T
Only the focusing operation is in a state of waiting for start.

Next, with reference to FIG. 5, the start of the focusing operation at the time of actual photographing will be described. FIG. 5 shows a state in which a person Q and a tree R are located in front of the camera, and a plain white wall P is arranged in the background of both.

When panning from the right-side person Q in focus to the left-side tree R once, a long focal length A1
If the person Q is photographed at the angle of view and panning is performed to the position A2, there is time to cover a background wall having no high-frequency component in the luminance signal during the panning. Along with this panning, the fourth comparator 12 detects that the subject has changed, and issues a command to restart the focusing operation to the focus motor control circuit 10, but the background wall covers the screen and the focusing When the focus evaluation value as the basis of the operation becomes 0, the delay signal DS is output from the start control circuit 47 for a relatively long time, and the focusing operation is in a standby state for a relatively long time. In a situation where it is impossible to determine the direction of the hill-climbing of the focusing operation because the value is 0, the focusing operation is automatically made to wait for a long time to wait for the generation of the focus evaluation value.

Also, at the angle of view B1 having a short focal length, the person Q
And if panning is performed to the position of B2, either one of the person Q and the tree R is included in the screen during the panning, so there is no period during which the focus evaluation value becomes 0, Or very short. Therefore, the focusing operation may be started immediately when the subject change is confirmed by the panning and the fourth comparator 12 issues a command to restart the focusing operation. Under such circumstances, since the upper limit time T is set shorter, the start of the focusing operation is earlier than in the case of the angle of view A1, and high responsiveness is obtained when the focus evaluation value becomes 0 due to out-of-focus. Can be demonstrated.

It is needless to say that the circuit operation of each circuit block diagram of the above embodiment can be processed by software by a microprocessor. In the above-described embodiment, the focusing operation is performed by moving the lens 1 in the optical axis direction. However, the same effect can be obtained by fixing the lens 1 and moving the image sensor itself in the optical axis direction. Needless to say. In this case, a piezoelectric element may be used instead of the motor to move the image pickup element, and a control signal from the focus motor control circuit 10 may be supplied to the motor or the piezoelectric element.

[0039]

As described above, according to the present invention, the maximum time for waiting for the focusing operation to be varied depending on the focal length, thereby preventing the malfunction of the focusing operation and the focusing operation when the focus evaluation value itself is extremely small. Responsiveness can be improved at the same time.

[Brief description of the drawings]

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

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

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

FIG. 4 is a diagram illustrating a low-level removal operation.

FIG. 5 is a diagram illustrating a relationship between a lens position and a focus evaluation value associated with a focusing operation.

[Explanation of symbols]

 Reference Signs List 1 lens 4 imaging circuit 5 focus evaluation value generation circuit 3 focus motor 40 focal length detector 47 start control circuit 41 upper limit time setting circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/222-5/257 G02B 7/36

Claims (2)

(57) [Claims] An imaging means for creating an image signal from incident light formed on an image sensor through a lens in an image pickup system, and a focus for outputting a high-frequency component amount of the image signal as a focus evaluation value. Evaluation value generating means; position changing means for executing a focusing operation for changing a relative position of one of the lens and the image sensor with respect to the other so that a focus evaluation value becomes a maximum value; A first instructing means for issuing a first focusing instruction for instructing a first focusing operation with the first focusing operation, and after the focusing operation is completed once and the relative position is fixed, the focus evaluation value is not less than a threshold width. A second instructing unit that monitors whether a change occurs and issues a second focusing instruction for instructing a restart of the focusing operation when the change occurs, and a focus that detects a focal length of the imaging system. Distance detection means and focus evaluation just before the start of focusing operation Level comparing means for detecting whether or not the focus reaches a predetermined level; and focusing from the time when the first focusing instruction or the second focusing instruction is received until the focus evaluation value reaches the predetermined level by the comparing means. Start control means for delaying the start of the operation, time determination means for determining whether a delay time associated with the delay reaches a predetermined time, and time change means for changing the predetermined time according to the focal length. An auto-focus video camera, wherein when the delay time reaches the predetermined time, the delay is immediately released to start a focusing operation. 2. The system according to claim 1, wherein the predetermined time is longer when the focal length is long than when the focal length is short.
Autofocus video camera as described.