JPH0591390A - Automatic focusing device for video camera - Google Patents

Abstract

(57) [Summary] [Purpose] By changing the responsiveness of the focus matching operation to changes in the shooting distance according to the shooting conditions, it is possible to perform shooting with a stable focus position even in telephoto shooting. [Configuration] Two comparison circuits 21 and 22 for comparing the focus evaluation value of the current field from the focus evaluation value generation circuit 12 with the focus evaluation value of the previous field stored in the memory 13 are provided. In the second comparison circuits 21 and 22, the evaluation value reference reduction rate and the reference dead time, which serve as the reference when the focus matching is restarted, are set by changing the value for each circuit. Then, the comparison circuits 21 and 22 are switched by the mode setting switch 24 and the mode changeover switch 23 to change the values of the evaluation value reference decrease rate and the reference dead time. This changes the frequency of refocusing restarts when the shooting distance is constantly changing. That is, the responsiveness of the focus matching operation can be changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic focusing device incorporated in a video camera for automatically focusing, that is, focusing on an object.

[0002]

2. Description of the Related Art In a video camera, especially a home video camera, an automatic focusing device is incorporated to automate focusing in order to simplify a photographing operation.

As a method of automatically performing focus matching, a high-frequency component is extracted from a luminance signal in a video signal to obtain a focus evaluation value, and focus adjustment is performed so that the focus evaluation value becomes maximum. The method called is widely used. This hill-climbing method uses the characteristic that the high-frequency component of the luminance signal generally becomes maximum when focusing, and it is possible to perform accurate focus matching without parallax to the image being shot. However, there is an advantage that the sensor for exclusive use of focus adjustment is not required and the structure of the automatic focus adjustment device is simple.

FIG. 3 is a block diagram showing a conventional example of this hill-climbing type automatic focus adjusting device incorporated in a video camera.

As shown in the figure, light from a subject incident on a zoom lens 1 and a focus lens 2 mounted on a video camera is imaged on an image pickup device 3 and converted into an electric signal. The electric signal is processed by the process circuit 4 to be a video signal, and the luminance signal in the video signal is sent from the process circuit 4 to the automatic focus matching device 11 (hereinafter simply referred to as the matching device 11).

The matching device 11 is provided with a focus evaluation value generation circuit 12, a memory 13, a comparison circuit 14, a control circuit 15, a drive circuit 16 and a focus motor 17, and the focus evaluation value generation circuit 12 has the process circuit described above. The luminance signal from 4 is input.

The focus evaluation value generating circuit 12 extracts a high frequency component from the input luminance signal to generate a focus evaluation value. First, the gate circuit 12a sets the input luminance signal to the center of the screen. Only the luminance signal of the portion corresponding to the focused area is passed. Then, from the passing luminance signal, a bandpass filter (BPF) 12
The high frequency component is extracted at b, and the high frequency component is detected by the detection circuit 12c. Next, the detection circuit 12c
The detection output of is converted into a digital value by the A / D conversion circuit 12d, and the digital value is integrated by the integration circuit 12e for each field of the video signal to obtain the focus evaluation value of each field.

The memory 13 stores the focus evaluation value of each field successively generated by the focus evaluation value generating circuit 12 while updating and stores the maximum focus evaluation value at the time of focusing.

The comparison circuit 14 compares the focus evaluation value of the current field from the focus evaluation value generation circuit 12 with the focus evaluation value of the previous field stored in the memory 13. Further, the comparison circuit 14 is preset with an evaluation value reference decrease rate and a reference dead time, which are references when reactivating focus adjustment as described later.

The control circuit 15 controls the drive circuit 16 based on the comparison result of the comparison circuit 14, and the drive circuit 16 drives the focus motor 17 according to the control of the control circuit 15 and the focus lens 2 Is moved in the optical axis direction to change its focal position. Further, the drive circuit 16 drives the zoom motor 19 according to a command from the zoom switch 18 to move the zoom lens 1 in the optical axis direction to change the focal length thereof.

FIG. 4 is a diagram showing changes in the focus evaluation value with respect to the focus position of the focus lens 2 when a certain subject is photographed by the video camera incorporating the hill climbing matching device 11 as described above. . The focus matching operation of the matching device 11 will be described with reference to this drawing.

Now, assuming that the focus position at the start of shooting is the position La closer to the subject than the focus position LM, the focus evaluation value generation circuit 12 determines the focus evaluation value Fa at the focus position La. It is generated and stored in the memory 13. Subsequently, when the control circuit 15 moves the focus lens 2 to change the focus position thereof to the near point side or the infinity side, the focus evaluation value generation circuit 12 generates the focus evaluation value of the current field for comparison. The circuit 14 compares the focus evaluation value of the current field with the focus evaluation value Fa of the previous field stored in the memory 13. As a result of the comparison, the focus evaluation value of the current field is the focus evaluation value Fa of the previous field.
If it is larger, the control circuit 15 moves the focus lens 2 in the same direction to change its focus position in the same direction,
If it is smaller, the focus lens 2 is moved in the opposite direction to change the focus position in the opposite direction.

By repeating this operation for each field, the control circuit 15 returns the focus position of the focus lens 2 from the position Lb, which has passed the focus position LM, to the focus position LM. In addition, the memory 13 stores the focus position LM.
The maximum focus evaluation value FM at is stored.

By the way, if the focus matching operation as described above is continued, if the distance to the subject is constantly changing, the focus position always changes, and the image becomes unsteady. Further, even when a person or an object crosses the front of the subject quickly, the focal position changes unnecessarily.

Therefore, the control circuit 15 fixes the focus position of the focus lens 2 to the in-focus position LM, and the subsequent reduction rate of the focus evaluation value exceeds the evaluation value reference reduction rate preset in the comparison circuit 14. , And the dead time, which is the duration of the state in which the evaluation value reference decrease rate is exceeded,
The focus adjustment is restarted when the reference dead time set in advance is exceeded. That is, every time the focus position is fixed to the in-focus position LM, the evaluation value reference reduction rate k (0 <k
The threshold value FS of the focus evaluation value is set by multiplying by <1), and when the focus evaluation value is lower than the threshold value FS continuously longer than the reference dead time, focus adjustment is performed again.

The values of the evaluation value reference reduction rate and the reference dead time in focus matching determine the responsiveness of the focus matching operation with respect to changes in the shooting distance. In addition, it is appropriately set so that the focus matching is restarted at the minimum necessary frequency within the range where the image of the subject is not greatly blurred.

[0017]

However, the zoom lens 1
When the telephoto shooting is performed by extending the focal length of, the depth of field becomes shallow, and thus the focus evaluation value easily changes.
Therefore, like the conventional matching device 11, when the values of the evaluation value reference decrease rate and the reference dead time are constant, focus adjustment is frequently performed and the focus position is frequently adjusted when a subject having a large amount of movement is telephoto-photographed. Becomes unstable.

When the values of the evaluation value reference reduction rate and the reference dead time are set in accordance with telephoto photography with a shallow depth of field,
The focus matching is limited even in shooting at other focal lengths, and it becomes impossible to perform the focus matching with high responsiveness.

The present invention has been made to solve the problem that the focal position becomes unstable in the telephoto shooting, and the response of the focus matching operation to the change of the shooting distance is changed according to the shooting situation. It is an object of the present invention to provide an automatic focusing device for a video camera capable of achieving the following.

[0020]

In order to achieve the above object, in the automatic focusing apparatus according to the present invention, the values of the evaluation value reference reduction rate and the reference dead time, which are the reference when restarting the focus adjustment, are set. The means to change is provided.

[0021]

By changing the values of the evaluation value reference decrease rate and the reference dead time, the frequency of restarting focus adjustment when the distance to the subject is constantly changing is changed. That is, it is possible to change the responsiveness of the focus matching operation with respect to the change in the shooting distance.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an automatic focusing device according to the present invention incorporated in a video camera.
Incidentally, in FIG. 1, constituent elements that are not different from the conventional example shown in FIG.

The feature of the automatic focus matching device 10 (hereinafter simply referred to as the matching device 10) according to the present invention is that the values of the evaluation value reference reduction rate and the reference dead time, which are the references when reactivating the focus adjustment, are changed. In this embodiment, the first and second comparison circuits 2 are used as the variable means.
1, 22, a mode changeover switch 23, and a mode setting switch 24 are provided.

The first and second comparison circuits 21 and 22 compare the focus evaluation value of the current field from the focus evaluation value generating circuit 12 with the focus evaluation value of the previous field stored in the memory 13. In each of the comparison circuits 21 and 22, the evaluation value reference reduction rate and the reference dead time, which serve as the reference when the focus matching is restarted, are preset. Moreover, the values of the evaluation value reference decrease rate and the reference dead time are calculated by the comparison circuit 2
1 and 22, the evaluation value reference decrease rate k1 of the first comparison circuit 21 is larger than the evaluation value reference decrease rate k2 of the second comparison circuit 22 (1>k1>k2> 0), and the first comparison circuit 2
The reference dead time T1 of 1 is set shorter than the reference dead time T2 of the second comparison circuit 22 (T1 <T2). That is, the evaluation value reference decrease rate and the reference dead time are changed depending on which of the first and second comparison circuits 21 and 22 is used, and thereby the responsiveness of the focus matching operation is determined by using the first comparison circuit 21. It is possible to change to two modes, that is, a mode 1 for performing the operation and a mode 2 for using the second comparing circuit 22.

The mode changeover switch 23 selectively outputs the focus evaluation value of the current field from the focus evaluation value generation circuit 12 to the first and second comparison circuits 21 and 22 to set the mode 1 And mode 2 are switched, and the switching is controlled by the control circuit 15.

The mode setting switch 24 commands the control circuit 15 to switch between the modes 1 and 2 by the mode changeover switch 23, whereby the modes 1 and 2 are switched.
Is to be set.

Further, in this embodiment, the focal length is detected from the position of the zoom lens 1 so that the two modes 1 and 2 can be automatically changed according to the focal length of the zoom lens 1. A focal length sensor 25 is provided. The detection data of the focal length sensor 25 is sent to the control circuit 15. Other configurations are similar to those of the conventional example.

The circuits of the matching device 10, the memory 13 and the mode changeover switch 23 are actually integrally formed as a microcomputer.

Next, the focus matching operation in the two modes 1 and 2 of the matching device 10 will be described with reference to FIG. The description of the conventional example will be referred to for the basic focus matching operation of the matching device 10 by the hill climbing method.

FIG. 2 is a diagram showing an example of the change over time of the focus evaluation value. Focusing is performed on a certain subject to bring it into focus, and then the subject moves and the photographing distance changes. It shows that the focus evaluation value from the focus evaluation value generation circuit 12 has dropped from the maximum focus evaluation value FM at the time of focusing.

As shown in the figure, each mode 1, which is calculated by multiplying the maximum focus evaluation value FM by the evaluation value reference decrease rates k1 and k2,
The focus evaluation value thresholds FS1 and FS2 in 2 are higher in the mode 1 threshold value FS1 due to the relationship between the respective evaluation value reference reduction rates k1 and k2 described above.

Now, the in-focus evaluation value from the in-focus evaluation value generation circuit 12 after focusing is the threshold value FS of the mode 1 at time t1.
1 and then at time t2 the threshold value FS for mode 2
It is assumed that the threshold value FS2 is lower than 2 and further lower than the threshold value FS2 of mode 2 at time t3 thereafter. In this case, the dead time until time t3 is Δt13 in mode 1 and Δt23 in mode 2, and the dead time Δt1 in mode 1 is
3 is longer than the dead time Δt23 in mode 2.

In mode 1, the focus matching is restarted when the dead time Δt13 exceeds the reference dead time T1, and in mode 2, the focus matching is restarted when the dead time Δt23 exceeds the reference dead time T2. Will be done. At this time, as described above, the dead time Δt13 in mode 1 is set longer than the dead time Δt23 in mode 2, and the reference dead time T1 in mode 1 is set shorter than the reference dead time T2 in mode 2. Therefore, the frequency of restarting focus adjustment when the shooting distance is constantly changing is higher in mode 1 than in mode 2. That is, the responsiveness of the focus matching operation to the change in the shooting distance is high in the mode 1 and high in the mode 2
Can be lowered at.

For example, if the dead time Δt13 in mode 1 exceeds the reference dead time T1 at time t3 and the dead time Δt23 in mode 2 does not exceed the reference dead time T2, focus adjustment is re-established in mode 1. Start-up is done, mode 2
Then, the focus position remains fixed without restarting.

As described above, according to the matching apparatus 10 having the above-described configuration, the evaluation value reference decrease rate k1 in each mode 1 and 2 is
The values of k2 and the reference dead times T1 and T2 are set appropriately,
By selecting one of the modes 1 and 2 with the mode setting switch 24, the responsiveness of the focus matching operation with respect to the change in the shooting distance can be made more appropriate according to the shooting situation. Therefore, when performing telephoto photography with a shallow depth of field, it is possible to perform photography with a stable focus position by setting mode 2 to reduce the responsiveness of the focus matching operation.

Further, the mode setting switch 24 is used to set the mode 2
Is selected, and when the focal length sensor 25 detects that the zoom lens 1 is on the telephoto side, the control circuit 15
If the mode 2 is set to the mode 2, the mode 2 can be automatically set during the telephoto shooting unless the mode 1 is set by the mode setting switch 24.

[0038]

As described above, according to the automatic focus adjusting device for a video camera of the present invention, the responsiveness of the focus adjusting operation with respect to the change of the photographing distance can be changed according to the photographing situation. Even in the telephoto shooting, it is possible to shoot with a stable focus position, and it is possible to create an image that is easier to see.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an automatic focusing device according to the present invention.

FIG. 2 is a diagram showing an example of a change over time of a focus evaluation value.

FIG. 3 is a block diagram of a conventional automatic focusing device.

FIG. 4 is a diagram showing a change in a focus evaluation value with respect to a focus position, for explaining a hill-climbing automatic focus matching operation.

[Explanation of symbols]

 10 Automatic Focus Matching Device 12 Focus Evaluation Value Generation Circuit 13 Memory 15 Control Circuit 16 Drive Circuit 17 Focus Motor 21 First Comparison Circuit 22 Second Comparison Circuit 23 Mode Change Switch 24 Mode Setting Switch 25 Focal Length Sensor

Claims (1)

[Claims] 1. A high-frequency component is extracted from a luminance signal in a video signal of a video camera as a focus evaluation value, and focus adjustment is performed so that the focus evaluation value is maximized to fix the focus position. The rate of decrease of the focus evaluation value exceeds a preset evaluation value reference reduction rate, and when the dead time in a state of exceeding the evaluation value reference reduction rate exceeds a preset reference dead time. The automatic focus adjusting device for a video camera for restarting focus matching, further comprising means for changing the values of the evaluation value reference decrease rate and the reference dead time.