JPS61281774A - Automatic focusing circuit - Google Patents

Abstract

PURPOSE:To prevent a malfunction of an automatic focusing operation by reducing a weight of a focus evaluation value in a peripheral section of a sampling area in comparison with a central section. CONSTITUTION:A luminance signal obtained by an image pickup element 1 is fed to a gate circuit 2 and a synchronizing separator circuit 12. A vertical synchronous signal VD and a horizontal synchronous signal HD separated by the circuit 12 are supplied to a gate control circuit 13 for controlling an opening and a closing of the gate circuit 2. A voltage gain control amplifier 3 constitutes a restricting means together with a function generating circuit 18 and amplifies the luminance signal passing through the circuit 2. An amplifying gain is small in positions of a front end and a back end of a gain control signal of a trapezoidal wave and large in a central position. The amplified luminance signal is integrated in an integration circuit 6, and a focus evaluation value of the present field is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION ←) Industrial Application Field The present invention relates to an improvement in an autofocus circuit employed in a video camera.

(b) Conventional technology In the autofocus device of a video camera.

There is one method that uses the captured image signal of the video camera itself to evaluate the focus control state.
It has many advantages, such as being able to focus accurately even when the depth of field is shallow or for distant subjects. In such an autofocus device, a portion of one screen (for example, a rectangular area near the center of the screen) is used for evaluating one normal focus, and this is called a focus area.

This is to accurately focus on the subject of greatest interest among subjects such as backgrounds at different distances when photographing with a video camera.

As an example of the above-mentioned autofocus device,
There is one disclosed in Japanese Patent No.-107685.

This conventional technology performs autofocus control using changes in the level of the luminance signal in the video signal captured by the video camera, and also divides the focus area into sections so that it can be freely selected. It was designed to focus on the

(c) Problems to be solved by the invention When the autofocus operation is performed using the above-mentioned conventional technique, @! If the focus evaluation value changes due to factors other than (in-focus degree ← in-focus degree), malfunction may occur. In other words, the focus evaluation value changes due to camera premovement or focus ring movement.

This occurs due to factors other than the degree of focus, even though the subject itself has not changed. This mainly occurs due to a change in the range of the subject within the focus area, and even if the focus area is subdivided as in the prior art, a similar phenomenon may occur for each individual area.

In general, in the video signal P-value autofocus method, parts of the subject with large differences in brightness greatly contribute to the focus evaluation value.

For this reason, if a subject has a small difference in brightness at the center of the focus area and a large difference in brightness at the edges of the focus area, if the part with the large difference in brightness falls within the focus area due to changes in camera play or angle of view, then The evaluation value increases regardless of the degree of focus, and as the portion with a large difference in brightness moves out of the focus area, the evaluation value decreases. In particular, with zoom lenses commonly attached to video cameras, the angle of view changes by moving the focus ring, so even when both the camera and subject are stationary,
Simply moving the focus ring by 1 for focusing changes the subject that enters the focus area, causing the evaluation value to change due to factors other than the in-focus point, causing malfunctions.

B) Means for Solving the Problems The present invention is characterized by comprising a suppressing means for suppressing the amount of change in the captured video signal in one peripheral part of the sampling area compared to the central part.

(E) Function Since the present invention is constructed as described above, weighting of the -C liver value is performed in the central part and the peripheral part in the sampling area.

(f) Example An embodiment of the present invention will be described below with reference to one drawing.

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

The luminance signal (A) in the imaged video signal obtained by the image sensor (1) is sent to the gate circuit (2) and the synchronous separation circuit 1z.
sent to. Luminance signal (a) at the synchronization separation circuit qz
Vertical synchronization signal (VD), horizontal synchronization signal (
HD) is supplied to a gate control circuit u3 that controls opening and closing of the gate circuit (2) in order to set the sampling area.

Gate control circuit α3 is vertical area control circuit 1) 4
1, a horizontal area control circuit (151), an oscillator (J8゜0) (le, and an AND gate αη), and the vertical synchronization signal (VD) in FIG. 2 (&) is connected to the reset input of the vertical area control circuit a4. , and the horizontal synchronizing signal (HD) in FIG. 2) serves as a reset input for the vertical area control circuit (count input 141 and horizontal area control circuit 1) 9, and also serves as the reset input for the oscillator αe in FIG. 2 (0). This is the input to the small output horizontal area control circuit α90.

The vertical area control circuit I counts horizontal synchronizing signals (HD), and when the count value is within a preset range, outputs an output to define the vertical range of the chimpling area.

The horizontal area control circuit T:19 counts the output of one oscillator αG, and when the count value is within a preset range, outputs an output to define the horizontal range of the sampling area.

This vertical/horizontal area control circuit (141α9 output is.

It is supplied to the gate circuit (2) via the AND gate αnt as a gate control signal as shown in FIGS. 2 and 3).

That is, this signal acts as a sampling timing pulse. The gate circuit (2) allows the luminance signal (7) to pass during the output period of this gate control signal, and only the luminance signal in the sampling area is supplied to subsequent circuits.

U is a weighting function voltage generation circuit that creates a gain control signal for a voltage gain control amplifier (V, C, A) (3), which will be described later, and is composed of a monomulti (not shown) and an integrating circuit (not shown). After creating a square wave with a pulse width narrower than the gate control signal using a monomulti, it is passed through an integrating circuit, and as shown in Figure 3 (13), the base is the pulse width of the gate control signal. generates a trapezoidal wave equal to .

A voltage gain control amplifier (VOA) (3) constitutes a suppressing means together with the weighting function voltage generating circuit, and amplifies the luminance signal that has completely passed through the gate circuit (2) in accordance with the gain control signal. Its amplification gain is proportional to the level of the gain control signal. In other words, the amplification gain is small at the front and rear slope positions of the gain control signal, which is a trapezoidal wave, and becomes large at the center position, where the level is completely H. As shown in Figure 1g3 (f), the input luminance signal is (g )
It will be like this. Here, the slope position of the gain control signal is the front and rear ends of the sampling timing pulse.

That is, it corresponds to both ends of the sampling area in the horizontal direction, and the H level position corresponds to the center of the sampling area. Therefore, the luminance signal is only amplified to a smaller extent at the periphery of the sampling area than at the center.

The luminance signal amplified in this way passes through the HPF (41) and only the high frequency components are separated.
The amplitude detection is performed in step 5), and the detected output of step 6 is integrated for each field in an integrating circuit (detection means) (6), and the amount of change in the luminance signal is accumulated to obtain the focus measurement value of the current field. The integrating circuit (6) may be a normal integrating circuit using a resistor and a capacitor or a digital integrating circuit. The amount of change in the brightness signal obtained in this way is: It is suppressed in the peripheral part of the sampling area compared to the central part, and the value in the central part of the focus meter value has more weight as an evaluation value.

The output of the integrating circuit (6) is delayed by one field in the delay circuit (7), the levels of the integrated output and the delayed output are compared in the comparing circuit (8), and the comparison output is sent to the delay circuit (7),
The signal is input to a focus motor control circuit (9) which constitutes a control means together with a comparison circuit (8). The focus motor control circuit (9) controls the focus motor which is constantly rotating in one direction when the comparison output changes from an increasing tendency to a decreasing tendency.
Stop the decoy and set the 7 orcus ring [1D] to the focal point position.

Note that in this embodiment, a voltage gain control amplifier (VOA) (3)
is inserted between the gate circuit (2) and the HPF (41), but it is inserted between the gate circuit (2) (or immediately after the integrator circuit (6)
The same effect can be obtained by inserting it immediately before.

Further, in this embodiment, weighting is performed only in the horizontal direction of the sampling area, and the weighting is uniform in the vertical direction. When using the horizontal resolution of the Y signal to evaluate the focus as in this embodiment, weighting only in the horizontal direction is sufficient, but when using the two-dimensional resolution from the video signal to evaluate the focus.

At the same time, weighting in the vertical direction is also required. This is the third
This can be achieved by lowering the height of the upper side of the trapezoidal wave of the direct function voltage in (8) in the figure at the vertical end.

(G) Effects of the Invention As described above, according to the present invention, by making the weight of the focus meter value at the peripheral part of the sampling area lighter than that at the central part, the angle of view can be changed by camera pre-movement or rotation of the focus ring. It is possible to reduce the change in the focal point value that is caused by the change and is not dependent on the degree of focus, and f-) it is possible to prevent malfunctions in the focus operation.

[Brief explanation of the drawing]

1 to 3 relate to one embodiment of the present invention, in which FIG. 1 is a circuit block diagram, and FIGS. 2 and 3 are timing charts. (3)...Voltage gain control term ranger (inhibition means), +
61... Integration circuit (detection means), (7)... Delay circuit (control means). (8)... Comparison circuit (control means), (91...
Focus motor control circuit (control means), 08--Weighting function voltage generation circuit (suppression means).

Claims (1)

[Claims] (1) In an autofocus circuit comprising a detection means for accumulating the amount of change in a captured video signal in a sampling area and detecting an evaluation value, and a control means for controlling a focus motor based on the evaluation value, An autofocus circuit characterized by comprising a suppression means for suppressing the amount of change in the peripheral part of the sampling area compared to the central part.