JPH03258171A - Automatic focus device - Google Patents

Abstract

PURPOSE:To facilitate the focusing onto a desired object by controlling a selecting operation of a focus area in response to a luminance difference in one area among plural areas set in a pattern. CONSTITUTION:A luminance signal in a video signal from an image pickup circuit 4 is inputted to a focus evaluation generating circuit 50 to form evaluation values Va, Vb of large and small areas set in the pattern. Each evaluation value is separated (51, 52) into evaluation values V, V1' and V2, V2' for each two fields and their variances V1, V2, and relative ratio variances r1, r2 are formed and they are fed to a focus motor control circuit 100. The circuit 100 executes the climb-up focusing and monitor of an object change and decides the direction of the focus based on each data in the area by means of the fuzzy inference and selects the area.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to an autofocus device used in a video camera.

(b) Conventional technology In the autofocus device of a video camera, the method of using the video signal itself from the image sensor to evaluate the focus control state 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 shooting shallow or distant objects. Moreover, there is no need for a special sensor for autofocus, and the mechanism is extremely simple.

JP-A-63-21.5268 (HO4N 5/
232) discloses an example of the autofocus device as described above.

The gist of this prior art will be explained below with reference to FIGS. 2 and 3. FIG. 2 is a circuit block diagram of the entire autofocus circuit related to the prior art. lens(
The image formed by step 1) is converted into a video signal by an image pickup circuit (4) including an image pickup element, and a luminance signal therein is input to a focus evaluation value generation circuit (5).

This focus evaluation value generating circuit (5) is configured, for example, as shown in FIG.

The luminance signal is passed through a high pass filter (HP F) (5c)
The high-frequency components are separated and sent to the next stage detection circuit (
5d), the amplitude is detected. This detection output is converted into a digital value by the A/D conversion circuit (5e), only the signal of the focus area set at the center of the screen is extracted by the gate circuit (5f), and then the signal of the focus area set at the center of the screen is extracted, and the signal is sent to the integration circuit (5g). It is integrated for each field, and the focus evaluation value of the current field is changed. At this time, the vertical and horizontal synchronization signals separated from the luminance signal by the synchronization separation circuit (5a) are input to the gate control circuit (5b) in order to set the focus area. The gate control circuit (5b) sets a rectangular focus area in the center of the screen based on vertical and horizontal synchronization signals and a fixed oscillator output, and generates a gate opening/closing signal that allows the luminance signal to pass only within this area. is supplied to the gate circuit (5c).

The focus evaluation value generation circuit (5) configured as described above is as follows:
The focus evaluation value for one field is always output.

Immediately after the start of the focusing operation, the first focus evaluation value is held in the maximum value memory (6) and the initial value memory (7).

After that, the focus motor control circuit (1o) rotates the focus motor (3), which is a stepping motor, in a predetermined direction to rotate the focus ring (2) that supports the light receiving lens (1). Light receiving lens (1
) in the optical axis direction and monitor the output of the second comparator (9). The second comparator (9) compares the focus evaluation value after driving the seven orcus motors with the initial 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).
) until the current focus evaluation value is larger than the initial evaluation value. If the current evaluation value is determined to be smaller than the initial evaluation value, the rotation direction of the focus motor is reversed, and from then on, the rotation direction of the focus motor is maintained as it is. Comparator (8)
Monitor output.

The first comparator (8) compares the current maximum focus evaluation value held in the maximum value memory (6) with the current evaluation value, and determines whether the current focus evaluation value is the maximum value memory (6). There are two types of comparison signals (Sl
) (S2) is output. Here, the maximum value memory (6) is updated based on the output of the first comparator (8), and if the current evaluation value is larger than the contents of the maximum value memory (6), the value is updated and always current. The maximum focus evaluation value up to this point is retained.

(13) is a focus ring (
This is a position memory that instructs the position of 2), receives a focus ring position signal detected by the motor position detection circuit (30), and stores the focus ring position as a lens position, and is similar to the maximum value memory (6). , the first comparator (8
) The focus ring position is updated so as to always maintain the focus ring position when the maximum evaluation value is reached based on the output.

Note that the motor position detection circuit (30) specifically includes:
It consists of an up/down counter that is reset at the start of the focusing operation and counts up or down the step amount of the 7 orcus motor (3), which is a stepping motor, as positive toward the near point and negative toward the far point. The ring position signal becomes this count value.

The focus motor control circuit (10) includes a second comparator (9).
) focus motor in the direction determined based on the output (
3), monitor the output of the first comparator (8), and compare the focus evaluation value with the maximum evaluation value to the preset first comparator (8).
The focus motor (3) is reversed at the same time as the second mode, in which the value is smaller than the threshold value (M) of , is instructed.

By reversing the focus motor (3), the direction of movement of the light-receiving lens (1) changes, for example, from a direction approaching the image sensor to a direction away from it, or vice versa, from a direction away from it to a direction toward it.

After this reversal, the contents of the position memory (13) and the current ring position are compared in the third comparator (14), and when they match, that is, the 7 orcus ring (2), that is, the lens (1) is in focus evaluation. The focus motor control circuit (10) functions to stop the focus motor (3) when the value returns to the maximum position. At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS).

The change in lens position in the so-called mountain climbing focusing operation described above is the fourth
As shown in the figure.

(11) is the fourth point where the focus evaluation value at that point is held at the same time that the focusing operation by the focus motor control circuit (10) is completed and the lens stop signal (LS) is issued.
It is a memory, and the fourth comparison in the latter part! In (12), the content held in the fourth memory (11) is compared with the current focus evaluation value, and the current focus evaluation value is greater than or equal to the preset second threshold value compared to the content of the fourth memory (11). When the value becomes smaller than , it is determined that the subject has changed, and a subject change signal is output. When the focus motor control circuit (10) receives this signal, it restarts the hill-climbing focusing operation to follow changes in the subject.

Although this autofocus system has excellent focusing accuracy and adaptability to a wide range of subjects, it has the following drawbacks.

If the region in which the high-frequency components in the screen are integrated, that is, the focus area, is set large, the desired subject may not be in focus due to the influence of the background or the like. On the other hand, if you make it smaller, there will often be no objects with contrast within the area, resulting in unstable operation.

In order to prevent this, a technique for preparing a plurality of areas and selecting one of the areas as a focus area according to the shooting situation is disclosed in Japanese Patent Application Laid-Open No. 01-284181 (HO4N
5/232).

This has two areas, large and small, and if the focus evaluation value in the small area is above a certain value, the small area is used, and if it is less than a certain value, it is assumed that there is no subject to focus on, and the large area is used. The area is set as the focus target area.

(c) Problems to be Solved by the Invention When selecting a focus area based on the size of the focus evaluation value as in the prior art described above, there is a situation where there is no subject to focus on;
It is difficult to distinguish between states where the focus evaluation value is low because the subject is present but largely blurred, and the main subject in the center is not in focus, but the periphery is likely to be in focus. This will result in a screen where the subject is not in focus.

(d) Means for Solving the Problems The present invention provides a selection means for performing a selection operation of selecting one of a plurality of areas set within a screen as a focus area, and a high-frequency range of a luminance signal within the focus area. A focus evaluation value detection means for extracting the amount of the component as a focus evaluation value, and a focus control means for performing a focusing operation by driving an imaging system to a position where the focus evaluation value is maximum, The selection operation is controlled according to the brightness difference of one area in the screen, and more specifically, the selection operation is controlled according to the brightness difference of one area in the screen. a selection means for selecting one of the second regions larger in area than the second region as a focus area; a focus evaluation value detection means for extracting the amount of high-frequency components of a luminance signal within the focus area as a focus evaluation value; a focus control unit that performs a focusing operation by driving an imaging system to a position where the value is maximum;
The present invention is characterized in that when the brightness difference is large, the selection means preferentially selects the first area as the focus area. Further, it is characterized in that fuzzy inference is used to determine the magnitude of the difference in brightness at this time.

(E) Function The present invention has the groove bottom as described above, so when the difference in brightness within the area is large, it is determined that some object exists within the area, and this area is prioritized as the focus area. This makes it easier to focus on the desired subject.

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

FIG. 1 is a circuit block diagram of this embodiment.

Incidentally, in FIG. 1, the same parts as in FIGS. 2 and 3 of the prior art are given the same reference numerals, and explanations thereof will be omitted.

The luminance signal in the video signal output from the imaging circuit (4) is input to the focus evaluation value generation circuit (50).

As shown in FIG. 5, the focus evaluation value generation circuit (50) uses two types of HP F (50) (50c) with different cutoff frequencies, 200 KHz and 600 KHz, and these HP
A switching circuit (5h) that selects and outputs the F(50) (50c) output for each field, a detection circuit (5d) that detects the amplitude of the output of the switching circuit (5h), and an A/D converter that converts this detection output into a digital value. Based on the D converter (5e), the synchronization separation circuit (5a) that separates the synchronization signal from the luminance signal, the separated synchronization signal, and the fixed oscillator output, it is set at the center of the screen as shown in Figure 6. A gate circuit (5
f), and a second gate opening/closing signal that allows passage of only the A/D converted value of the luminance signal portion corresponding to the second area (B) which includes the first area (A) and has a larger area than this area. The gate control circuit (50b) supplies the gate circuit (50f) to the gate circuit (50f), and the digital values of the high-frequency components of the luminance signals in each area output from the gate circuit (5f) are added over one field to obtain the result. digital integration is performed, and this integrated value is output as the first and second focus evaluation values (Va) and (Vb) for each field, respectively, and
Integration circuit (5g) (50
g).

Here, the switching circuit (5h) selects HP F (5c) (50c) for each field at the output of the synchronous separation circuit (5a).
To select one of the outputs alternately, HP F (5c)
In the field where is selected, the integration circuit (5g) calculates the HP F (5c) of the luminance signal in the first area (A).
7) The digital integral value (■1) for one field of the high-frequency component of 200 KHz or more, which is the off frequency, is the first
It is output as a focus evaluation value (Va), and at the same time it is output as an integration circuit (
50g), the luminance signal in the second area (B) is the same <2
A digital integral value (m') of one field of high-frequency components of 00 KHz or higher is output as a second focus evaluation value (vb). Also, in the next field, the first focus evaluation value (V
a) is the HPF (50
c) digital integral value (V2) of one field of high-frequency components of 600 KHz or higher, which is the cutoff frequency;
The second focus evaluation value (vb) becomes the digital integral value (V2') of the same high-frequency component of <600 KHz or higher, and the same process is repeated thereafter.

First and second focus evaluation values (Va) obtained in this way
(Vb) are input to separation circuits (51) and (52), respectively.

The separation circuits (51) and (52) both perform switching control generated based on a vertical synchronization signal generated from the synchronization separation circuit (5a) in order to switch the switching operation in the switching circuit (5h) for each field. The separation timing is controlled by the signal (SC), and the separation circuit (51) uses the first focus evaluation value (Va).
is separated into integral values m)(Vl') using each HPF, and each is output as an evaluation value m)(Vl'). Similarly, the separation circuit (52) separates the second focus evaluation value (Vb) into integral values (V2) (V2') using each HPF, and outputs each as an evaluation value (V2) (V2'). . Therefore, the evaluation values m)m')(V2)(V2') are all updated every two fields.

Next, the evaluation value m) (V2) is the initial value memory (7) (
57), subtraction circuit (70) (80), memory (24) (
58), relative ratio calculation circuits (25), (59), and switching circuit (20). On the other hand, evaluation value m') (V2')
are input to relative ratio calculation circuits (25) and (59), respectively.

The initial value memory (7) (57) stores an evaluation value (Vl) at the time when the focus motor (3) first starts rotating in a predetermined initial direction in order to start the focusing operation.
(V2), and while the motor (3) rotates in the initial direction and the lens (1) is displaced, the evaluation value m) is obtained when two fields have passed after the motor is started.
(V2) is updated, the subsequent subtraction circuit (70) (
80), the initial value memory (7) (
The value obtained by subtracting the contents of 57) is the evaluation value (Vl)
(V2) is output to the focus motor control circuit (100) as the amount of change (AVI) (ΔV2).

The memory (24,) (58) holds the evaluation value m) (V2) obtained after starting the motor (3) for two fields and inputs it to the subsequent relative ratio calculation circuit (25) (59). It is.

The relative ratio calculation circuits (25) and (59) are divided into a divider (61), a memory (62), and a subtracter (63) as shown in FIG. Value fVl')(V
2°) is updated, the ratio of the latest evaluation value (Vl) (V2) held in the memory (24) (58), V
l'/Vl and V2'/V2 are calculated as relative ratios (rl) (r2).

Here, the relative ratio (rl) is determined by using HP F (5c) and the integral value (vl) for one field of time, and HP F
(50c), and the relationship between both integral values and the focus ring position (lens position) when the subject is the same is as shown in FIG. In other words, the integral value at HP F (50c) with a high cutoff frequency becomes a steep mountain, and the integral value at HP with a low cutoff frequency (50c) becomes a steep mountain.
The integral value at F (5c) becomes a gentle peak. Therefore, the relationship between this relative ratio and the degree of blur of the subject (the amount of movement or deviation from the lens position at the time of focus) is shown in a graph.
This results in a monotonically decreasing characteristic curve as shown in FIG.

This is because the relative ratio state quantity is a function value that can express the in-focus state (degree of blur) of the subject in the same way as the focus evaluation value, and because it is expressed as a ratio, it is a kind of normalized state quantity. , and has the property of being less affected by the environment in which the subject is placed. For example, when the illuminance of the subject changes, the absolute value of the focus evaluation value changes, but the relative ratio does not change significantly. Normally, the above properties are independent of the type of subject, so this relative ratio can be used as a parameter for the degree of blur. When this monotonically decreasing characteristic curve in Fig. 8 is made to correspond to the lens position, that is, the focus ring position, it forms a substantially straight line from the in-focus position as the apex to the near point and point (1) side, as shown by the one-dot chain line in Fig. 9. A changing characteristic diagram is obtained.

The relative ratio (rl) (r2) obtained by the divider (61) is input to a memory (62) and a subtracter (63). The memory (62) functions to hold the input relative ratio for two fields, delay it, and supply it to the subtracter (63).
The subtracter (63) subtracts the previous relative ratio held in the memory (62), that is, two fields ago, from the latest relative ratio obtained from the divider (61), and converts this subtracted value into the relative ratio ( rl) b2) are output to the focus motor control circuit (100) as changes (Δr1) (Δr2). still,
It goes without saying that the amount of change (Δr1) (Δr2) can be a negative value if the initial rotational direction of the motor is opposite to the focusing direction.

The switching circuit (20) is a focus motor control circuit (10).
) to select the area to be used in the focusing operation and set the bifocal evaluation value m)(
V2).

The focus evaluation value selected by the switching circuit (20) is supplied to the maximum value memory (6) and the first comparator (8), and similarly to the conventional example, a signal for executing hill climbing control is sent to the focus lens. Supplied to the control circuit (100). Incidentally, the position memory (13) and the third comparator (14) operate exactly the same as in the conventional example shown in FIG.

However, the focus evaluation value obtained from the switching circuit (20) is updated every two fields, and the comparison operation of the first comparator (8) for hill climbing operation is updated every two fields. will be done.

In addition, the focus evaluation value selected by the switching circuit (20) is
The signal is also supplied to a fourth memory (11) and a fourth comparator (12), and monitors changes in the subject as in the conventional example.

The zoom position detection unit (64) generates a signal indicating the current focal length (Z) of the lens according to the wide-angle to telephoto zoom range of the zoom mechanism having a well-known zoom lens attached to this video camera. Output to the control circuit (100).

As shown in FIG. 11, the average brightness detection circuit (65) is composed of a detection circuit (65a), an A/D conversion circuit (65b), and an integration circuit (65c), and detects a brightness signal corresponding to the entire screen. The detection circuit (65a) performs amplitude detection, and the detected output is sequentially A/D converted by the A/D conversion circuit (65b), and then the integration circuit (65C) performs A/D conversion for one field.
All the converted data is integrated, digitally integrated, and the control circuit (
100).

The contrast detection circuit (66) is located at the bottom of the tank as shown in FIG. 13, and detects the contrast in the horizontal direction within the first area (A). Here, the operation of this contrast detection circuit (66) will be explained.

First, in order to detect contrast, the first area (A) is divided into 16 small areas (Nij) (++
j = 1 to 4), and a separation circuit (67) separates the luminance signal for each small area, and 16 signals are prepared for each small area.
digital integrators (Kij) (i, j=1~4
).

Each of the digital integrators has the same tank bottom as the average brightness detection circuit (65) shown in FIG. ij). These integrals 1 (
Fij) is input to the Max-Min calculation circuit (Li) (i=1 to 4) with a set of four small areas arranged in the horizontal direction. That is, the integral M (Fll) (j = 1 to 4) is sent to the Max-Min calculation circuit (Ll), and the integral value (F *+) (Fitton (F 41) is
It is input to the x-Min calculation circuit (L1) (L*) (L,).

The Max-Min calculation circuit (L i) selects the maximum value and minimum value among the four integral values of the small areas arranged in the horizontal direction,
(Maximum value - Minimum value) is subtracted, and this subtracted value (G
i) is output to the maximum value detection circuit (68) at the subsequent stage.

The maximum value detection circuit (68) subtracts 1 it (Gi) for 4 lines.
It functions to extract the maximum value among , and outputs this maximum value as contrast (ΔEl). Therefore, in the end, the contrast (ΔEl) corresponds to the digital value of the luminance difference itself in the row having the largest horizontal luminance difference in the first area (A) for one field.

The focus motor control circuit (100) includes a first comparator (
8) and the third comparator (14,), the mountain climbing focusing operation is performed as in the conventional example, and the fourth comparator (12,)
) Based on the output, the monitoring operation for changes in the subject is executed in the same way as in the conventional example. In addition, the outputs from the subtraction circuits (70) (80), that is, the amount of change (Δ■1) (ΔV2) from the initial value of the evaluation value (Vl) (V2), the relative ratio calculation circuit (25) (
59), that is, the evaluation fin) for the first area (A), the changes (Δr1) (Δr2) from the initial values of the relative ratios (rl) and (r2) of each area, and the contrast detection circuit (66) The direction of the in-focus point is determined by fuzzy inference based on five types of data of the brightness contrast (ΔEl) in the first area (A), and
The evaluation value (vl) output from the separation circuit (51), the output from the average brightness detection circuit (65), that is, the average brightness of the entire screen (IR5), and the output from the zoom position detection circuit (64), that is, the current Focal length (Z) and contrast (△E
Area selection fuzzy inference is performed based on the four types of data (1).

Next, the processing for direction control and area selection using the above-mentioned fuzzy inference will be described in detail.

First, the direction determination process is executed according to the flowchart shown in FIG.
△rl) and contrast (△El) are input variables, take a value from O to l as the conclusion part, and when it is large, the direction of the in-focus point is the current traveling direction of the lens, and when it is small, it is the opposite direction. di).

Here, the rule is [Rule (1)] rifΔV1 is large and Δr1 is large then
d+=1. OJ [Rule (2) Corif △v1 is large and △r1 is not large the
n d*=o, 7J [Rule (3) CorifVl is small and △v2 is small and △E1
is set to small then ds=0.24.

Next, each of the above rules will be explained.

[Rule (1)] is defined by membership functions as shown in FIGS. 11(a) and (b). FIG. 14(a) shows
Indicates the membership value of condition (1) of rule (1) that “Δv1 is large”, and represents the input variable change amount (ΔV
l), and the amount of change (ΔV
As l) increases, the membership value (u, ,
) is a function that includes a monotonically increasing straight line that increases, and from this function, membership [(U
,,) can be found.

Figure 14(b) shows the rule that △r1 is large J (1
) is the membership value of condition (2), and is a membership function for the input variable change (Δrl), which is a monotonous function in which the membership value (u,,) increases as the change (Δrl) increases. This is a function including an increasing straight line, and membership M(u,=) according to the change (Δrl) is determined from this function.

Rule (1) takes into consideration the case where both the focus evaluation value and the relative ratio in the first area (A,) increase, and in this case, the focused point is in the current moving direction of the lens (1). There is a high possibility that there is, so select the first area (A).
The conclusion part (dl) is set to d,=1.

[Rule (2)] is defined by membership functions as shown in FIGS. 11(a) and (b). Figure 15(a) shows “
Indicates the membership value of condition (1) of rule (2) that △Vl is large j, and the amount of change (△Vl
), from which the membership value (u, , ) can be found.

FIG. 15(b) shows the membership value of condition (2) of rule (2) that “Δr1 is not large”, and as the change (Δrl) increases, the membership value (U
, ) is a function that includes a monotonically decreasing straight line that decreases, and from this function, the membership value (
u,,) can be found.

Rule (2) is for the case where the fluctuation trends of the focus evaluation value and relative ratio are different, and the in-focus point may be in the opposite direction to the current lens movement direction, so the conclusion part (d,) is better than d. The value d is set to be slightly smaller, d=0.7.

[Rule (3)] is shown in Figure 11 (a), (b), and (c).
) is defined by a membership function such as 16th
Figure (a) shows the membership value of condition (1) of rule (3) that "rVl is small", and the monotonically decreasing straight line in which the membership value (U,,) decreases as the evaluation M (■1) increases The membership value (u, , ) corresponding to the evaluation value (■1) is determined from this function.

Figure 16(b) shows the ``Δr2 is small'' rule (3).
) is the membership value of condition (2), and the amount of change (Δ
As V2) increases, the membership value Cu,
, ) is a function that includes a monotonically decreasing straight line that decreases, and the membership value (
U,) is found.

Figure 16 (C) shows the rule (3) that “△E1 is small”.
) shows the membership value of condition (3), and as the contrast (△El) increases, the membership value (
uss) is a function that includes a monotonically decreasing straight line that decreases,
From this function, the membership value (u, . . .) corresponding to the contrast (ΔEl) is determined.

Rule (3) considers the case where the focus evaluation value is low in the first area (A) and no change can be extracted, and in this case, if the contrast of the first area (A) is small, the focus evaluation value is low in the first area (A).
Assuming that there is no subject to be focused on, the direction is determined by the change in the focus evaluation value in the second area (B), and the amount of change in the focus evaluation value in the second area (B) is negative. In this case, the conclusion part (d3) is set as low as d3 = 0.2 in order to make it easier to reverse the lens movement direction, assuming that the in-focus point is likely to be in the opposite direction to the current lens movement direction. has been done.

A direction determination process in which a direction determination parameter (D) is calculated from each of the above-mentioned rules and a direction is finally determined from this parameter will be described using the flowchart in FIG. 120.

Each membership value (
U. (U+).For example, rule (1)
According to Fig. 14, since u++ > u + t, the degree of validity (Ul) is U+ = u + t, and for rule (2), from Fig. 15, u , , > u ts, so the degree of validity (Ul) is becomes Us=utt, and further the rule (
Regarding 3), from Fig. 15, u sl< u ss<
u 1! Therefore, the degree of validity (U,) is U 2 = u
It will be 31.

Based on the degree of establishment (U,) of each rule obtained in this way, S
At T E P (102), a parameter (D) for direction determination is calculated using the following equation. This formula (1) means that each conclusion part is weighted and averaged based on the degree of establishment of each rule.

In S T E P (103), the obtained parameter (
D) determines the direction, and specifically, if D≧0.5, the focus position is in the current lens movement direction, so a control command is issued to control the drive of the motor (3) to maintain the current direction. If D<0.5, the focus position is in the opposite direction to the current direction, so a control command to immediately reverse the motor (3) is issued to the focus morph drive circuit (31).

In this way, the direction of the in-focus point is determined with high precision by considering the five factors, and the motor (3)
The aforementioned hill-climbing focusing operation is performed while rotating the lens and moving the lens.

Next, the area selection process is executed according to the flowchart shown in FIG. 21, and the fuzzy inference used at this time is
The focus evaluation value (vl), contrast (△El), focal length (Z), and average brightness (IR5) are input variables, and the conclusion part takes a value between 0 and 1, and when it is small, the first area (
A), a membership value (a,) is set which means that the second area (B) is used when it is large.

Here, the rules are: [Rule (4)] rif (Vl) is large then al: o, 0" [Rule (5)] rif (Vl) is medium and (IR3) is small then at = o, 8" [Rule (6)] rif (Vl) is small and (ΔEl) is large and ancl (Z) is not small then as=0.3.1 Next, each of the above rules will be explained.

[Rule (4)] is defined by a membership function as shown in Fig. 17, and this Fig. 17 shows the membership value of condition (1) of rule (4) that "■1 is large",
It is a membership function for the focus evaluation value (vl), and it is a function that includes a monotonically increasing straight line in which the membership value (u,,) increases as the evaluation value (■1) increases, and from this function, the evaluation 1f (Vl ) is determined according to the membership value (u,,).

Rule (4) is that when the focus evaluation value (Vl) is large, the conclusion part (al) is a+= in order to give priority to the area (A) assuming that the subject exists in the first area (A).
Set to 0.0.

[Rule (5) is defined by membership functions as shown in FIGS. 18(a) and (b). Figure 18(a) shows r
Condition (1) of rule (5) that “(Vl) is medium”
It is a mountain-shaped function in which the membership value is the maximum when the evaluation value (Vl) is a predetermined value, and from this function, the membership value (u
,,) can be found.

FIG. 18(b) shows the membership value of condition (2) of rule (5) that "r(IR5) is small", and as the average luminance (IR5) increases, the membership value 1f increases.
This is a function that includes a monotonically decreasing straight line in which f(uis) decreases, and from this function, the membership value (u, , ) according to the average luminance (IR3) is determined.

Rule (5) is the focus evaluation value (v
l) is not very large and the average brightness of the screen is dark.In this case, the S/N ratio of the video signal is poor and the reliability of the focus evaluation value is low, so in order to capture more information, focus The conclusion part (a4) is set to a*=0.8 so that the second area (B) is likely to be given priority as the area.

[Rule (6)] is shown in Figure 11 (a), (b), and (c).
) is defined by a membership function such as
) has a small r(Vl)” is the condition of rule (6) (
It is a function that shows the membership value of 1) and includes a monotonically decreasing straight line in which the membership value decreases as the evaluation value (■1) increases, and from this function, the membership value ( U,) is found.

Figure 19(b) shows the membership value of condition (2) of rule (6) that "(ΔEl) is large", and the membership value (U, ) increases as the contrast (ΔEl) increases. This is a function including a monotonically increasing straight line, and a membership value (U, ) corresponding to the contrast (ΔEl) is determined from this function.

Figure 19 (C) shows the membership value of article f't-(3) of rule (6) that "(2) is not small",
It is a function including a monotonically increasing straight line in which the membership value (u, . . .) increases as the focal length (Z) increases, and the membership value (uo) corresponding to the focal length (Z) can be found from this function.

Rule (6) considers the case where the focus evaluation value of the first area (A) is small but the contrast within the first area (A) is high, and the case where the focal length is short and the depth of field is deep. The conclusion part (a6) is set to a6=0.3 so that the first area (A) is relatively likely to be given priority as it is assumed that some object exists, although it is blurry.

An area selection process in which an area selection parameter (Y) is calculated from each of the above-mentioned rules and an area is finally selected based on this parameter will be described using the flowchart of FIG. 21.

STEP (200)! :l: Condition (j) (i
= 4 to 6, J = 1 or 2 or 3) is determined, then S T
In E P (201), the minimum membership value for each rule is calculated as the degree of establishment (U i ) of each rule.

For example, regarding rule (4), the degree of establishment (U4> is U).

= u41, and for rule (5), from Fig. 18, u , , < u st, so the degree of establishment (U,) is U, = L1g+, and further, for rule (6), from Fig. 19, u Since at< u ax< u is, the degree of establishment (U6) is U.

=u, 1.

Based on the degree of establishment (U,) of each rule obtained in this way, S
At T E P (202), a parameter (Y) for area selection is calculated using the following equation. This formula means that each conclusion part is weighted and averaged based on the degree of establishment of each rule. STE P (2
In 03), select Enoa according to the obtained parameter (Y), specifically, if Y≧0.5, select the second area CB) as the focus area, and if Y<0,5, select the first area CB).
Area (A) is selected.

In this way, a highly accurate area selection is made taking into account the four factors, and focus evaluation is performed by the switching circuit (20) in order to execute the hill climbing focusing operation with the corresponding focus evaluation value. A value selection is made.

Note that this area switching process is also executed during a focusing operation and an operation of monitoring changes in the subject after reaching the in-focus point. In addition, immediately after the area is actually switched, the switching circuit (20)
In order to suppress malfunctions caused by these fluctuations, the first and fourth comparators (8) and (12) are The comparison operation is designed so that the comparison result is output as valid only when the same comparison result is obtained three times in a row.

Further, the area division, rules, etc. are not limited to those in this embodiment. Further, it goes without saying that the operation of this embodiment can be executed by software using a microcomputer.

(g) Effects of the Invention As described above, according to the present invention, it is possible to distinguish between a state in which there is no subject in the focus area and a state in which a largely blurred subject exists.

Furthermore, by using fuzzy inference that includes other factors such as the brightness difference in this judgment, it becomes possible to switch the focus area with higher accuracy.

[Brief explanation of drawings]

1, 4 to 21 relate to one embodiment of the present invention, in which FIG. 1 is an overall circuit block diagram, FIG. 4 is an explanatory diagram of mountain climbing focusing operation, and FIGS. 5 and 10. , Figures 11 and 13 are main circuit block diagrams, Figure 6 is an explanatory diagram of area setting,
Figure 7 is a relationship diagram between lens position and focus evaluation value, Figure 8 is a relationship diagram between relative ratio and degree of blur, Figure 9 is a relationship diagram between lens position, focus evaluation value and relative ratio, and Figure 12 is a relationship diagram between lens position and focus evaluation value and relative ratio. An explanatory diagram of area division, Figures 14, 15, 16, 17, 18, and 19 are diagrams showing membership functions,
0 and 21 are flowcharts. Further, FIGS. 2 and 3 are circuit block diagrams of conventional examples. (1) Lens, (50) Focus evaluation value generation circuit, (20) Switching circuit, (100) Focus motor control circuit, (4) Imaging circuit.

Claims (3)

[Claims] (1) A selection means that performs a selection operation to select one of a plurality of areas set in the screen as a focus area, and a focus evaluation that extracts the amount of high-frequency components of a luminance signal within the focus area as a focus evaluation value. a value detection means; a focus control means for performing a focusing operation by driving an imaging system to a position where the focus evaluation value is maximum; An autofocus device that controls the selection operation. (2) selection means for selecting either a first area set at the center of the screen or a second area including the first area and having a larger area than the first area as a focus area; and brightness within the focus area. a focus evaluation value detection means for extracting the amount of high-frequency components of a signal as a focus evaluation value; a focus control means for performing a focusing operation by driving an imaging system to a position where the focus evaluation value is maximum; and the first area. An autofocus device characterized in that the first area is selected as a focus area preferentially by the selection means when the brightness difference is large. (3) The autofocus device according to claim 1, wherein fuzzy inference is used to determine the magnitude of the brightness difference.