JPH01137886A - Automatic focus adjusting device - Google Patents

Abstract

PURPOSE:To attain the focus adjusting action of a highly accurate and stable response speed at any time by changing the passing band of a band pass filter with the output of an F value (an opening ratio) detecting means and the output of a focal distance detecting means. CONSTITUTION:The outputs of an F value detecting means 18 to detect the F value of an image pickup lens and a focal distance detecting means 19 to detect a focal distance are inputted to a selector means 20. Based on an inputted signal, the selector means 20 changes the passing band frequencies of a band pass filter 14. That is, by changing the coefficient of the band pass filter with respect to the change of the opening ratio of the image pickup lens and the focal distance, the passing band is changed, and the shape of a focal voltage curved line can be held to be approximately constant, and the high accuracy of the focus adjusting action and the stabilization of the response can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an automatic focusing device used in a video camera.

Conventional technology Automatic focusing devices used in video cameras include:
Various methods have been proposed and put into practical use. Among these, a method using an output signal from an image sensor of a video camera is known as so-called hill-climbing control, in which an optical focus adjustment mechanism is controlled to adjust the focus so that the high frequency component of the video signal is maximized. (For example, "Automatic focus adjustment of a television camera using a mountain-climbing servo system" by Ishida et al., NHK Technical Research Report, Vol. 17, No. 1, p. 21).

Hereinafter, an example of the above-mentioned conventional automatic focusing device will be described with reference to the drawings.

FIG. 3 is a block diagram showing the configuration of a conventional automatic focus adjustment device. FIGS. 4(a) and 4(b) are waveform diagrams of main parts of the output of a conventional automatic focusing device.

In FIG. 3, ■ is an imaging lens, 2 is an image sensor and camera circuit, 3 is a high-pass filter, 4 is a detector, 5 is a differential hold circuit, 6 is a servo amplifier, 7 is a motor drive circuit, and 8
is a motor that rotates the distance ring of the imaging lens 1.

The operation of the conventional embodiment configured as described above will be explained using the main waveform diagram of FIG. 4.

A subject image formed on the image sensor 2 via the image pickup lens 1 is converted into an electrical signal, which is transmitted from the camera circuit 2 to the high-pass filter 3.
is input. High frequency components of the input video signal are extracted by a high-pass filter 3 and detected by a detector 4. The output of the detector 4 has a value (hereinafter referred to as focal voltage) proportional to the amount of high frequency components in the video signal. The focal voltage signal input to the differential hold circuit 5 is held at regular time intervals, a difference is taken, and a signal indicating the amount of temporal displacement of the focal voltage is output.

In Fig. 4(a), the horizontal axis shows the distance ring position of the imaging lens 1, and the horizontal axis shows the output of the detector 4, and when the distance ring of the imaging lens 1 is rotated from the closest position A to the infinite position B. This shows the change in the output value of the wave detector 4.

(Hereinafter, referred to as a focal voltage curve.) When the imaging lens 1 is in a focused state, the focal voltage shows the maximum value, and as it deviates from the focused state, the focal voltage value decreases and the focal voltage curve as a whole is mountain-shaped. The shape will be .

FIG. 4(b) shows the change in the output value of the differential hold circuit 5, and shows the shape obtained by differentiating the focal voltage curve on the time axis.

In order to bring the imaging lens l into focus, as shown in Fig. 4(b)
When the output value of the differential hold circuit 5 is positive, it means that the focal voltage curve is climbing, so the distance ring is further rotated in that direction, and when the temporal displacement of the focal voltage becomes zero, the distance The motor may be driven to stop the ring. When the output value of the differential hold circuit 5 is negative, it means that the focus voltage curve is falling, so the direction of rotation of the distance ring is reversed. Therefore, if the output of the differential hold circuit 5 is amplified by the servo amplifier 6 and then input to the motor drive circuit 7, and the distance ring is rotated by the motor 8, the focus adjustment operation can be performed.

Problems to be Solved by the Invention However, in the above configuration, the F of the imaging lens
There is a problem in that the shape of the focal voltage curve changes due to changes in value or focal length, making it impossible to perform a stable focusing operation.

First, the effects of changes in F number and focal length on the shape of the focal voltage curve will be explained.

The depth of focus d of the imaging lens 1 can be expressed by the following equation.

d=FXδ ・・・・・・・・・ (1) F: Aperture ratio δ: Permissible circle of confusion diameter Depth of focus d is the amount of defocus within the range where the amount of deterioration in image definition cannot be observed. , is proportional to the F value of the imaging lens. On the other hand, the focal voltage curve represents the relationship between the definition of the screen and the amount of defocus, so the slope of the focal voltage curve is proportional to the f-number, and the slope becomes gentler as the f-number increases. It will be done.

Also, the change i1s in the imaging position with respect to the movement Na of the front lens group in the optical axis direction based on the rotation of the distance ring of the imaging lens 1
The relationship is approximately s=k −a ・(f / f t) ” ---(2
) f: Focal length of the imaging lens fL: Focal length of the imaging lens at the telephoto end: Since it can be expressed as a proportionality constant, the slope of the focal voltage curve changes with the square of the focal length of the imaging lens, and the focal length The shorter the distance, the looser it becomes.

In this way, the slope of the focal voltage curve changes greatly with respect to the F value and changes in the F value.

If the shape of the focus voltage curve changes significantly, focus adjustment accuracy cannot be ensured sufficiently, and the response speed of the focus adjustment operation becomes slow. That is, when the aperture diameter of the lens becomes smaller or the focal length becomes shorter, the change in the focal voltage curve becomes gentler and the differential output becomes smaller. Therefore, detection sensitivity decreases. Furthermore, since the differential output is amplified and input to the motor drive circuit, if the differential output is small, the response speed will be slow.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an automatic focus adjustment device that compensates for changes in the focus voltage curve and enables stable focus adjustment operations at all times.

Means for Solving the Problems An automatic focus adjustment device according to the present invention includes an analog-to-digital converter to which a video signal from an image sensor is input, and a differentiation circuit for digitally differentiating the output of the analog-to-digital converter. , a gate circuit that samples the output signal of the differential circuit on the time axis, a bandpass filter that passes only signals in a specific frequency band among the output signals of the gate circuit, and the bandpass filter. an absolute value circuit that takes the absolute value of the output of the detector, a detection circuit that detects the peak value between one field of the output of the absolute value circuit, and a focusing lens of the imaging lens so that the output of the detection circuit is maximized. A control unit that outputs a driving signal, a drive circuit that drives a focusing lens of an imaging lens according to the output of the control unit, and an F-number detection unit that detects an aperture ratio of the imaging lens and a focal length. This configuration includes a focal length detection means and a selection means for changing the passband of the bandpass filter based on the output of the F-number detection means or the focal length detection means.

Function: With the above-described configuration, the automatic focus adjustment device of the present invention changes the passband and changes the shape of the focal voltage curve by changing the coefficient of the bandpass filter in response to changes in the aperture ratio and focal length of the imaging lens. It can be kept almost constant, and it is possible to achieve high precision of the focus adjustment operation and stabilization of the response. In addition, since the video signal is converted to digital and processed,
This provides a configuration suitable for changing the passband of a bandpass filter.

Embodiment An embodiment of an automatic focusing device according to the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an automatic focus adjustment device according to an embodiment of the present invention. In FIG. 1, 1 is the t-most image lens, 10
1 is an image pickup device and a camera circuit, which converts the subject image formed by the image pickup lens 1 into an electrical signal and outputs a video signal. The output of the image sensor and camera circuit 10 is analog.
The signal is converted into a digital signal by the digital converter 11 and input to the digital signal processing section 21 . 12 is a differentiating circuit that removes the DC component of the video signal. Among the waveforms generated when the DC component is removed by the differentiating circuit 12, the high frequency components included at the beginning and end of the effective scanning period are not the high frequency components included in the subject, so they are sampled on the time axis. Only the center portion of the horizontal scanning period is extracted by the gate circuit 13, and unnecessary portions are removed. Gate circuit 1
A desired frequency component of the signal outputted from 3 is extracted by a bandpass filter 14, the absolute value is taken by an absolute value circuit 15, and then detected by a detection circuit 16. The detection circuit 16 is configured to detect the peak value during the -field period. The output of the detection circuit 16 has a value proportional to the amount of high frequency components of the video signal, as in the conventional example. Therefore, the output of the detection circuit 16 is taken into the control section 17. The control unit 17 is composed of, for example, a microcomputer, and calculates differences between fields of input data. When the output value of the difference data is positive, it means that it is climbing the focal voltage curve, so the focusing lens is further driven in that direction, and when the difference data becomes less than a certain value, the focusing lens is stopped. Then, a signal for driving the motor is output to the motor drive circuit 7. Furthermore, when the output value of the difference data is negative, by reversing the driving direction of the focusing lens, the focusing lens of the imaging lens 1 can be driven so that the output of the detection circuit 16 is maximized, and the focus is adjusted. Can perform movements.

Further, F value detection means 18 detects the F value of the imaging lens.
The output of the focal length detection means 19 for detecting the focal length is input to the selection means 20. The selection means 20 changes the passband frequency of the bandpass filter 14 based on the input signal.

The bandpass filter 14 is configured as shown in FIG. 2, for example.

FIG. 2 is a block diagram of a bandpass filter, which constitutes a biquadratic digital filter.

Here, 22 is an input terminal into which the output data of the gate circuit 13 is input, 23, 24, 25 is an adder, and 26.27 is l
The delay element of the clock, 2B, 29.30.31 is a multiplier that multiplies by a predetermined coefficient, and 33.34.
Reference numerals 35 and 36 denote coefficient switching means for switching coefficients according to the output of the selection means 20. 32 is an input terminal to which the output of the selection means 20 is input. 37 is a signal output terminal. As is well known, the characteristics of a digital filter can be determined by coefficients. Therefore, the passband frequency of the bandpass filter 14 can be easily changed by the coefficient switching means 33 to 36 that change the coefficients of the digital filter according to the output of the selection means 20.

As already explained, the slope of the focal voltage curve is considered to be approximately proportional to the F value. Furthermore, an increase in the F value means that, when considering the lens as a low-pass filter, its cutoff frequency extends toward the high frequency side. Therefore, if the passband of the bandpass filter 14 is shifted to the higher frequency side, the slope of the focal voltage curve becomes steeper. Therefore, when the F value becomes large according to the output of the F value detection means 18, if the selection means 20 changes the coefficient of the bandpass filter 14 and shifts the pass band to the high frequency side, the focal voltage The slope of the curve can be kept almost constant. If the slope of the focus voltage curve is constant, the difference data output from the control unit 17 will also be constant regardless of the F value, and stable focus adjustment operation will always be possible.

Similarly, the slope of the focal voltage curve changes with the square of the focal length of the imaging lens, and becomes gentler as the focal length becomes shorter. When the focal length becomes shorter in accordance with the output of the focal length detection means 19, the selection means 20 changes the coefficient of the bandpass filter 4 to shift the passband to the higher frequency side, thereby changing the slope of the focal voltage curve ρ. It can be kept almost constant.

In the embodiment, the passband frequency was changed by changing the coefficients of the bandpass filter, but it is also possible to change the configuration of the bandpass filter.

Effects of the Invention As described above, according to the automatic focus adjustment device of the present invention, the focus voltage curve can be adjusted by changing the passband of the bandpass filter using the output of the F-number detection means and the output of the focal length detection means. Since the tilt can be kept almost constant, the excellent effect of always being able to perform focus adjustment operations with high precision and stable response speed can be achieved. Additionally, when configuring a bandpass filter for analog signals, the number of components increases depending on the number of passbands required, but in the present invention, video signals are converted to digital signals. Since the processing is performed as follows, conversion of the passband of the bandpass filter can be realized only by changing its coefficients, and by implementing it into an LSI, a compact and highly accurate automatic focusing device can be realized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an automatic focus adjustment device according to an embodiment of the present invention, FIG. 2 is a block diagram of a bandpass filter according to an embodiment of the present invention, and FIG. 3 is a block diagram of an automatic focus adjustment device according to a conventional example. FIG. 4 is a block diagram of the main part of the output of the automatic focusing device in the conventional example. 1... image lens, 2, 10... image sensor and camera circuit, 7... motor drive circuit,
8...Motor, 11...Analog-digital converter, 12...Differential circuit, 13...
... Gate circuit, 14 ... Bandpass filter, 15
...Absolute value circuit, 16...Detector, 1
7... Control unit, 18... F value detection means, 19... Focal length detection means, 20...
- Selection means, 21...Digital signal processing section, 3
3゜34, 35.36... Coefficient switching means. Name of agent: Patent attorney Toshio Nakao (1 person) Figure 2 Figure 3 Figure 4

Claims (3)

[Claims] (1) Analog where the video signal from the image sensor is input
a differential circuit that digitally differentiates the outputs of the digital converter and the analog-to-digital converter; a gate circuit that samples the output signals of the differential circuit on the time axis; A bandpass filter that passes only signals in a specific frequency band, an absolute value circuit that takes the absolute value of the output of the bandpass filter, and a peak value between one field of the output of the absolute value circuit that is detected. a detection circuit;
a control unit that outputs a signal for driving a focusing lens of an imaging lens so that the output of the detection circuit is maximized; and a drive circuit that drives the focusing lens of the imaging lens in accordance with the output from the control unit. An automatic focus adjustment device comprising: (2) F-number detection means for detecting the aperture ratio of the imaging lens;
It is characterized by comprising a focal length detection means for detecting a focal length, and a selection means for changing the passband of the bandpass filter based on the output of the F-number detection means or the output of the focal length detection means. An automatic focus adjustment device according to claim (1). (3) The bandpass filter is configured to include coefficient switching means for switching the coefficients of the bandpass filter according to the output of the selection means. Automatic focus adjustment device.