JPH0360580A - Automatic focus camera - Google Patents

Abstract

PURPOSE:To suppress fluctuation caused in a focus evaluation value being a basis of focusing due to the flicker effect of a discharge light whose frequency is 50Hz by integrating a low frequency signal component of a video signal of the same field as a video signal acquiring the focus evaluation value and applying the automatic focusing depending on the increase/decrease in a normalized focus evaluation value resulting from the division of the focus evaluation value by the integrated component. CONSTITUTION:A gate circuit 30c extracts only a luminance signal corresponding to a range of a focus area in time division similarly to the case with a gate circuit 5c, the result is inputted to an LPF 30d, only the low frequency component is separated and inputted to a poststage A/D conversion circuit 30f, integrated digitally over one field period at an integration device 30g and the result is outputted for each field as a brightness evaluation value K representing an average brightness level at a relevant field. The obtained focus evaluation S and brightness evaluation value K are inputted to an arithmetic circuit 31, the focus evaluation value S is divided by the brightness evaluation value K at the current field to calculate J=S/K thereby obtaining the normalized focus evaluation value.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to an auto-7 focus device for a camera that automatically aligns the focus based on a 4 degree signal in a captured video signal obtained from an image sensor.

(b) Conventional technology In the autofocus device of a camera, the method of using the high-frequency component of the video signal itself from the imaging probe to evaluate focus control is essentially free of variation and has a limited depth of field. It has many advantages, such as being able to focus accurately even when photographing shallow or distant objects. Furthermore, there is no need for a special sensor for autofocus, and the mechanism is extremely simple.

Japanese Unexamined Patent Publication No. 1983-125910 (GO2B77・'1
1) discloses an example of the above-mentioned so-called mountain-climbing auto 7-o-ka-otsu system. Here, regarding this conventional technology,
The outline will be explained using FIGS. 2 and 3. FIG. 2 is an overall circuit block diagram of the conventional technology. In this figure, an image formed by a lens (1) is turned into a video signal by an image pickup circuit (4) including an image pickup element, and a focus evaluation value generation circuit ( 5). The focus evaluation value generation circuit (5) is configured as shown in FIG. A vertical synchronization signal (VO) and a horizontal synchronization signal (HD) separated from the video signal by a synchronization separation circuit (5a) are input to a gate control circuit (5b) in order to set sampling areas as seven orcus areas. Gate control circuit (5b)
Now, we set a rectangular sampling area in the center of the screen based on the vertical synchronizing signal (VD), the horizontal synchronizing signal (HD), and the fixed oscillator output that drives the image sensor, and calculate the luminance signal only within this sampling area. A gate opening/closing signal allowing passage is supplied to the gate circuit (5c).

Only the luminance signals corresponding to the seven orcus areas are filtered by the gate circuit (5c) and passed through the high-pass filter (H,
P, F) (5d), only high-frequency components are separated, and the amplitude is detected in the next stage detection circuit (5e). This detection output is converted into a digital value by an A/D conversion circuit (5f) at a predetermined sampling period, and is sequentially input to an integrator (5g).

Specifically, this integrator (5g) consists of an adder that adds A/D conversion data and latch data of a subsequent launch circuit, and a latch circuit that latches this added value and is reset every field. It is a so-called digital integrator,
The sum of all A7/D conversion data for one field period is output as a focus evaluation value. Therefore, the focus evaluation value generation circuit extracts the luminance signals within the seven orcus areas in a time-division manner, digitally integrates this high frequency component over one field period, and uses this integrated value as the focus evaluation value for the current field. It will be output. Immediately after the auto 7 orcus operation starts, the initial focus evaluation value is stored in the maximum value memory (6
) and are held in the initial value memory (7). After that, the focus mode control circuit (10) is a focus motor (focus control means) that moves the lens (T) back and forth in the optical axis direction.
(3) in a predetermined direction and the second comparator (9
) monitor the output. A second comparison number (9) compares the focus evaluation value after driving the seven orcus motors with the distance evaluation value held in the initial value memory (7), and outputs the magnitude thereof.

The focus motor control circuit (10) includes a second comparator (9).
) rotates the focus motor (3) in the initial direction until it outputs an output that is large or small, and outputs that the current focus evaluation value is larger than the initial evaluation value by a preset variation range. If the current evaluation value is smaller than the initial evaluation value and the above variation range, the focus motor (3) is output. The output of the first comparator (8) is monitored by reversing the direction of rotation of the comparator (8).

The first comparison 'f5 (8) compares the maximum focus evaluation value up to now held in the maximum value memory (6) with the current focus evaluation value, and the current focus evaluation value is stored in the maximum value memory (6). (first mode), the preset first
In the second mode, two types of comparison signals (PI) (P2) are output in order to reduce the value to a threshold value or more. Here the maximum value memory (
6) is based on the output of the first comparator (8), and if the current focus evaluation value is larger than the content of the maximum value memory (6), the value is updated, and the current focus evaluation value is always updated. The maximum value of is maintained.

(13) is a focus ring (
2) is a motor operation memory that stores the focus ring position upon receiving the focus ring position signal that indicates the position of the first comparator (8) as well as the maximum value memory (6).
Based on the output of , the focus ring position is updated so as to always maintain the focus ring position when the maximum evaluation value is reached. Here, it is a well-known technique that the focus ring (2) is rotated twice by the focus motor (3), and the lens (1) moves forward and backward in the optical axis direction in accordance with this rotation. Note that the focus ring position signal is output by a potential meter that detects the focus ring position, but the focus ring position signal is output by a potential meter that detects the focus ring position.
It is also possible to use 3) as a steno-knobbing motor and use the amount of steps of this motor in the periapsis or direction.

The focus motor control circuit (10) includes a second comparator (9).
) 7 orcus motors (
3), the output of the first comparator (8) is monitored, and in order to prevent malfunctions due to noise in the evaluation value, the current evaluation value is set to the maximum evaluation value at the output of the first comparator (8). On the other hand, the second mode in which the second mode is smaller than the preset first threshold value (Δy) is instructed (reaches Q in FIG. 4), and at the same time the 7 orcus motor (3) is reversed. After this reversal, the contents of the motor position memory (13) and the current focus ring position signal are compared in the third comparator (14), and when they match, that is, the 7 orcus ring (2) has a focus evaluation value. The focus motor control circuit (10) functions to stop the 7 orcus motor (3) when it returns to the maximum position (P). At the same time, the focus mask control circuit (10) outputs a lens stop signal (LS).

(11) indicates that the auto 7 orcus operation by the focus morph control circuit (10) has been completed and the lens stop signal (LS
) is issued and at the same time the focus evaluation value at that time is held, and the fourth comparison 5 (12)
The content held in this fourth memory (11) is compared with the current focus evaluation value, and if the value becomes larger than the second threshold for restarting, focus morking control is performed as it is assumed that the subject has changed. A subject change signal is output to the circuit (10). When the focus motor control circuit (10) receives this signal, it restarts the autofocus operation to follow changes in the subject.

(c) Problems to be Solved by the Invention The method of the prior art described above has extremely high followability and high focusing accuracy, but the illuminance of the subject changes at a constant frequency that is different from the frequency of one field of the video signal. If it is, the change in illuminance may cause malfunction. This situation will be explained in some detail below.

For example, in an NTSC video camera, the frequency of the l field is 60 Hz, but such malfunctions occur when this camera is used under illumination of a discharge lamp such as a fluorescent lamp that is lit at 50H2. Since the brightness of a discharge lamp lit at 50 Hz changes at a frequency of 100 Hz, the illuminance of the subject also changes at a frequency of 100 Hz.

Since one field of the video signal has a frequency of 60 Hz, a ripple of 20 Hz, which is the beat frequency, occurs.

That is, when a predetermined subject is photographed under illumination with a constant illuminance, the average brightness level for each field is normally maintained at a constant value as shown in FIG.
Flicker occurs under the illumination of a 0Hz discharge lamp, and the fifth
As shown in figure (b), (\11) → (M2
)→013)→(Ml)→... and the brightness level changes. By the way, similar to the focus evaluation value average brightness level in the prior art, it has a characteristic that it changes in proportion to the amount of light incident on the image sensor, even if the distance to the object does not change and the object itself does not change. Figure 5 (b>
If flicker occurs as shown in the figure, the focus evaluation value will change even though the same subject distance is maintained and the same subject is photographed, and the focus evaluation value affected by such flicker will change. If a focusing operation is performed as in the conventional technique based on the above, the peak of the focusing position may be incorrectly detected, or the focusing operation may be incorrect due to a change in the maximum value of the focus evaluation value, even though there is no change in the subject. There is a habit of rebooting.

(d) Means for Solving the ProblemsThe frequency spectrum intensity of a video signal that has not been subjected to camera process processing such as r correction is proportional to the intensity of light input to the image sensor, and the spectral distribution at that time is constant. Therefore, in the present invention, at the same time as obtaining a focus evaluation value, a luminance integrated value is obtained by integrating the low-frequency signal components of the video signal of the same field as the video signal from which the focus evaluation value was obtained, and the focus is evaluated using this luminance integrated value. By dividing the value, a normalized focus evaluation value is obtained by normalizing the focus evaluation value with respect to the average luminance signal, and an auto 7 orcus operation is performed by increasing or decreasing this normalized focus evaluation value.

(E) Function Since the present invention is configured as described above, fluctuations caused in the focus evaluation value, which is the basis of focusing operation, due to the influence of flicker caused by the 50 Hz discharge lamp can be suppressed.

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

FIG. 1 is an overall circuit block diagram of the device of this embodiment.

In FIG. 1, the same parts as in FIG. 2 of the conventional example are given the same reference numerals, and their explanation will be omitted.

The luminance signal in the captured video signal from the imaging circuit (4) is input to the focus evaluation value generation circuit (5) (first focus evaluation value detection means), and is detected within the focus area at the center of the screen as in the conventional example. The digital integral value of the high-frequency component of the luminance signal over l field period is the focus evaluation fi in the current field.
It is output as t(S) (first focus evaluation value). Also,
vi degree multiplied signal in the captured video signal, brightness evaluation value generation circuit (
30) It is also input to C brightness evaluation value detection means).

As shown in FIG. 6, the brightness evaluation value generation circuit (30) includes a gate circuit (30c), an LPF (low pass filter) (3
0d), A/D conversion circuit (30f) and product Ii (
30g).

The opening/closing of the gate circuit (30c) is controlled by the gate opening/closing signal from the gate control circuit (5b) in the focus evaluation value generation circuit (5), and corresponds to the range of 7 orcus areas just like the gate circuit (5C). Only the luminance signal is extracted in a time-division manner and input to L P F (30d), and only its low frequency component is separated and sent to the subsequent A/D conversion circuit (30d).
1) is input.

This LPF output is converted to A/D in the A/D conversion circuit (30f).
A/D conversion is performed in exactly the same synchronization as the sampling period of the /D conversion circuit (5f), and the A/D conversion values are sent to the integrator (
The integrated value for one field of the low-frequency component of the luminance signal within this focus area is digitally integrated over the field period at 30g), and the integrated value for one field indicates the average luminance level in the corresponding field. K) is output for each field. In addition, gate circuits (5c) (30c),
A/D conversion circuit (5f) (30f), integrator (5g) (
30g) have exactly the same configuration. In addition, the function of the gate circuit (30c) is also used by the gate circuit (5C), and the output of the gate circuit (5c) is L P F
(30d), the gate circuit (30d)
It goes without saying that 0c) can be omitted.

The focus evaluation value (S) and brightness evaluation value (K) thus obtained from the circle generation circuits (5) and (30) are then sent to the calculation circuit (31).
is input.

The arithmetic circuit (31) is a division circuit, which converts the focus evaluation value (S) in the current field into the brightness evaluation (i(K)
) to calculate J=S, /K.

Here, if the brightness evaluation value and focus evaluation value when the amount of incident light to the image sensor is Fo are respectively = and So, then the brightness and focus evaluation values when the amount of incident light changes to Fl (K) (S) Since the average brightness level changes in proportion to the amount of incident light, we get K ''< F +, / Fo ) X K
It becomes a. Furthermore, the ratio of the change in the amount of incident light at this time (F5.
, / F O) is L, then K = I-x Ka
It can be expressed as In addition, since the high-frequency component of the brightness signal usually changes in proportion to the amount of incident light (L), the focus evaluation value (S
) is S ” L x S.

It can be expressed as

Therefore, the above division result (J) is J = (A/K) X S = l/ (LXKO)l X (LxSo)-A
S O / K O 1), the dependence of the change in the amount of light incident on the image sensor on the ratio (L) can be removed. That is, the division result (J) is the focus evaluation value (S) normalized by the average brightness level. This division result (J) is supplied as a normalized focus evaluation value to each subsequent circuit in place of the focus evaluation value of the prior art.
1st. 2nd comparator cs) (9> The following circuit performs exactly the same operation as shown in Fig. 2 based on the normalized focus evaluation value (J), and reaches the position where the normalized focus evaluation value takes the maximum value. The lens (1) stops, and after this stop, the normalized focus evaluation value is changed to the second one.
The focus motor (3) is activated when a fluctuation exceeding the threshold value occurs.
A series of auto7 orcus operations are executed to restart the auto7 orcus operation.

In addition, (32) is γ in the imaged video signal from the imaging circuit (4).
This is a camera process circuit that performs the necessary processing for the video signal of the correction lamp.

In addition, in this embodiment, the focus ring (2) is rotated by the focus motor (3), and the lens (1) is moved forward and backward in the optical axis direction, thereby changing the relative position of the lens (1) toward the image sensor. 7 orcus control is performed, but the lens (
1> is fixed, the image sensor is supported by the piezoelectric element, and the focus motor control circuit (10) is connected to the 7 orcus motor (
Even if the control signal supplied to 3) is supplied to the piezoelectric element and the image sensor itself moves forward and backward in the optical axis direction, it is possible to perform focus control by changing the relative position of the lens (1) with respect to the image sensor. It goes without saying that there is.

Furthermore, when deriving the brightness evaluation value which is the average brightness level, the low frequency component was extracted using L P F (30d) in Fig. 6, but the gate circuit ( 30c) Direct detection circuit for output (30e)
Needless to say, it may be supplied to

Incidentally, in this embodiment, a series of circuit operations from the arithmetic circuit (31) to the focus motor control circuit (10) can be processed by software using a microcomputer.

(g) Effects of the Invention As described above, according to the present invention, it is possible to prevent malfunctions in focusing operations due to flicker even under illumination such as a 50 Hz discharge lamp.

[Brief explanation of drawings]

Fig. 1 is an overall circuit block diagram of an embodiment of the present invention, Fig. 6 is a circuit block diagram of the same main part, Figs. 2 and 3 are circuit block diagrams of a conventional example, and Fig. 4 is a mountain climbing auto 7. FIG. 5, which is a diagram explaining the principle of the Orcus method, is a diagram illustrating changes in brightness level due to flicker. (5)...Focus evaluation value generation circuit (first focus evaluation value detection means), (30)...Brightness evaluation value generation circuit (brightness evaluation value detection means), (31)...Arithmetic circuit, (3 )・・
・7 Orcus motor () Orcus control means).

Claims (1)

[Claims] (1) A first method that detects the high-frequency component level of the imaging luminance signal obtained from the image sensor as a first focus evaluation value at predetermined intervals.
focus evaluation value detection means; brightness evaluation value detection means for detecting the brightness level of the imaging brightness signal as a brightness evaluation value at each of the predetermined periods; An autofocus camera comprising: arithmetic means for outputting an evaluation value; and a focus control means for performing focus control so that the second focus evaluation value becomes a maximum value.