JPS60212071A - Automatic focusing device of video camera - Google Patents

Abstract

PURPOSE:To prevent malfunction at automatic focusing by correcting deficiency of dynamic range of a detecting circuit to a degree without hindrance in the range of normal operation. CONSTITUTION:If a focus voltage is lowered, the dynamic range of a detection circuit 4 surpassed it and the detection circuit 4 cannot sometimes detect the voltage. In order to prevent it, the 2nd threshold level L2 is set to a level slightly higher than the minimum detecting level of the detecting circuit 4 by a threshold value circuit 11, and if the focus voltage is lower than the 2nd threshold value level L2, a discriminating circuit 12 switches a switch circuit 14 so as to increase the focus voltage thereby increasing the amplification factor of an amplifier circuit 10. Thus, the focus voltage is kept in the dynamic range of the detection circuit 4 at all times regardless of the state of an object O, thereby preventing malfunction of the automatic focus circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an autofocus device for a video camera that converts an image obtained using an image pickup tube or a solid-state image sensor into an electrical signal, and measures distance using the converted electrical signal. It is something.

In the autofocus of a conventional video camera, a circuit that converts an image into a video signal and then performs distance measurement using this video signal is shown in FIG. That is, an image of the object O is formed on the imaging device 2 by the focusing lens l, and a video signal output from the imaging device 2 is sent to the bypass filter 3 and the detection circuit 4. The motor control circuit 5 drives the motor 6 based on the signal from the detection circuit 4 to perform focus adjustment using the focusing lens 1.

However, since the video signal input to the detection circuit 4 covers a wider range than the dynamic range of the detection circuit 4 as shown in FIG. 2, the focal voltage is higher than the saturation level S or lower than the minimum detectable level. If a low focus voltage is input to the detection circuit 4, the autofocus circuit shown in FIG. 1 may malfunction.

An object of the present invention is to provide an autofocus device for a video camera that eliminates the above-mentioned drawbacks, corrects the normally used signal range with a simple structure, and can perform accurate distance measurement. In an autofocus circuit that performs distance measurement using a video signal, an amplifier circuit is inserted before a signal detection circuit for the video signal so that the video signal falls within a predetermined range according to the magnitude of the video signal from the detection circuit. Furthermore, the present invention is characterized in that it is configured to control the amplification factor of the amplifier circuit.

The present invention will be explained in detail based on the embodiment shown in FIG. 3 and below.

In FIG. 3, the same symbols as in FIG. 1 indicate the same circuits. Here, the video signal obtained by the imaging device 2 is input to the bypass filter 3 after passing through the amplifier circuit 10.

Further, the output of the detection circuit 4 is output to the motor control circuit 5 and is also input to the dark value circuit 11. Then, the output of the threshold circuit 11 is fed back through the discrimination circuit 12, the counting circuit 13, and the switch circuit 14 so as to change the amplification factor of the amplifier circuit IO again.

It is now assumed that a focal voltage as shown in FIG. 4 is input to the detection circuit 4. At this time, the helicoid of the focusing lens 1 rotates from the out-of-focus state to the in-focus position A, but it is thought that the saturation level S of the detection circuit 4 will be exceeded at this helicoid position. It will be done. Therefore, a transmitting section of the first threshold level L1 is provided in the dark value circuit 11, which is slightly lower than the saturation level S. When a focal voltage exceeding this first threshold level Ll is input, the threshold circuit 11 and the discrimination circuit 1
2 senses this state and reduces the amplification factor of the amplification circuit 10 by the switch circuit 14 to keep the focal voltage lower than the saturation level S of the amplification circuit 10. The counting circuit 13 controls the switch circuit 14 when the amplification factor of the amplifier circuit 10 is in three or more stages.

Furthermore, if the focal voltage decreases for some reason, it may fall below the dynamic range of the detection circuit 4 and cannot be detected by the detection circuit 4. For such a case, a second dark value level L2 is set by the threshold circuit 11 to a level slightly higher than the lowest detection level of the four-wave detection circuit 4, and the focal voltage is set to a level slightly higher than the lowest detection level of the four-wave detection circuit 4. When the focus voltage is increased, the discrimination circuit 12 switches the switch circuit 14 to increase the amplification factor of the amplifier circuit 10.

In this way, regardless of the situation of the subject 0, the focal voltage can always be kept within the dynamic range of the detection circuit 4, and malfunctions of the autofocus circuit can be prevented.

Note that there is no problem with the connection position of the switch circuit 14 and the amplifier circuit 1O as long as it is between the imaging device 2 and the detection circuit 4, and the positional relationship with the bypass filter 3 does not matter. Further, the amplification factor of the amplifier circuit 10 is determined as Vo=K m Vi, where the input voltage is Vi and the output voltage is Vo. Here, when O<K<1, that is, this amplifier circuit has no attenuation. This also includes cases where the

Furthermore, the number of amplification/attenuation stages is not particularly limited.

Lotto Trout 11B1. +1-chi1=+Tame-tapushilGosuzogi Elementary 7-2's autofocus device can correct the lack of dynamic range of the detection circuit to the extent that it does not cause any problems during normal use, so it can prevent malfunctions during autofocus. It can be prevented.

[Brief explanation of the drawing]

Fig. 1 is a block circuit diagram of a conventional autofocus device, Fig. 2 is a diagram showing the relationship between the output of the detection circuit and the lens position, and Fig. 3 and subsequent figures show an embodiment of the autofocus device for a video camera according to the present invention. 3 is a block circuit configuration diagram thereof, and FIG. 4 is a diagram showing the relationship between the output characteristics of the detection circuit and the threshold level. Symbol l is a focusing lens, 2 is an imaging device, 3 is a bypass filter, 4 is a detection circuit, 5 is a motor control circuit, 6 is a motor, 1o is an amplifier circuit, 11 is a threshold circuit, 12
1 is a discrimination circuit, 13 is a counting circuit, and 14 is a switch circuit. Patent applicant Canon Co., Ltd.

Claims (1)

[Claims] 1. In an autofocus circuit that performs distance measurement using a video signal, an amplifier circuit is inserted in front of a signal detection circuit for the video signal, and the video signal is adjusted according to the magnitude of the video signal from the detection circuit. An autofocus device for a video camera, characterized in that the amplification factor of the amplifier circuit is controlled so that the signal falls within a predetermined range. 2. The autofocus device for a video camera according to claim 1, wherein the signal detection circuit is a detection circuit.