JPH01293772A - Image pickup device - Google Patents

Abstract

PURPOSE:To adjust an exposure in which the deterioration in an S/N is minimized by holding the gain of an AGC circuit constant until a diaphragm mechanism becomes an opened state. CONSTITUTION:An optical diaphragm mechanism 6 to control the exposure is driven by an iris motor 7 and fixed the gain of an AGC amplifier 301 to amplify an image pickup video signal in a constant state while diaphragm quantity is adjusted by the motor 7. When it is difficult to obtain an appropriate exposure only by adjusting incident quantity, the amplifier 301 is operated, and the gain is made into a variable one. Namely, the gain is increased or decreased according to the height of the level of the image pickup video signal to be inputted and amplified until the output becomes a constant level. Thus, for an ordinary object, the incident light quantity can be made appropriate, and a high S/N can be maintained by controlling the mechanism 6, and a sufficient image pickup signal level can be secured at the amplifier 301 even by sacrificing the S/N of only an object having low luminance.

Description

Detailed Description of the Invention (a) Industrial Application Field The present invention is directed to an imaging device such as a video camera that enables exposure adjustment using a mechanical aperture mechanism and an AGC circuit that amplifies F@ and @ video signals. Regarding.

(B) Conventional Technology In the auto-iris device of a video camera, control of the imaging level (exposure) using aperture, gain, etc. is a very important issue along with focus detection.

Conventionally, an automatic exposure adjustment mechanism has been used that detects the average brightness level, peak value, etc. of the image capture screen, and controls the aperture based on this.

For example, JP-A-62-110369 (H04N
5/243>, an example of this automatic exposure adjustment mechanism is disclosed. This conventional technology controls the amount of light incident on the imaging device by driving a mechanical optical aperture mechanism so that the brightness level of the entire imaging screen matches a reference level, and also controls the amount of light incident on the imaging device. The gain of the gain control circuit through which the imaged video signal passes is set so that the central area is given priority over the peripheral area using the ratio of brightness levels, and the influence of abnormal brightness areas on light sources, etc. in the peripheral area is reduced. In other words, a mechanical aperture mechanism and a gain control circuit are used together for exposure adjustment.

However, in this case, if the amplification to maintain the appropriate level is performed by gain control using a circuit, the S/N ratio will inevitably deteriorate compared to the case where the amount of incident light is increased by an optical diaphragm. Adjustment alone does not provide a sufficient imaging signal level for a low-luminance subject even when the aperture is wide open.

(c) Problems to be Solved by the Invention According to the prior art, exposure adjustment is always performed by using both an optical diaphragm mechanism and a gain control circuit. Even when the exposure Ii4 adjustment can be performed optically by the mechanism without any electrical processing of the captured video signal, the gain control circuit operates and the S/N ratio deteriorates.

(d) Means for Solving the Problems In the present invention, the brightness of the subject is extremely low and the optical diaphragm mechanism alone cannot adequately cope with the problem. The present invention is characterized in that electrical exposure adjustment is performed with a variable gain by a gain control circuit only when the state is such that the incidence of light is allowed.

(E) Since the present invention is configured as described above, for normal subjects, the amount of incident light is controlled by the optical diaphragm mechanism to maintain a high S/N ratio, and for subjects with low brightness, it is possible to maintain a high S/N. only for the body
Even at the expense of S/N, the 1-gain control circuit ensures a sufficient imaging signal level.

(f) Examples Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

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

(1) is the video camera section, and the focus lens (2
), a focus morph (4) that drives a focus ring (3) that supports the lens and moves it forward and backward in the optical axis direction, an optical diaphragm mechanism (6) that controls exposure, and an iris motor ( 7) and an imaging circuit (8) having a solid-state imaging device (CCD) that converts subject light into a captured image 18.

The luminance signal in the captured image No. 8 obtained by the imaging circuit (8) is passed through a bypass filter (HPF) (9), a low bass filter <LPF) (11), and a sync separation circuit (
12).

Vertical synchronization signal No. 18 (VD) and horizontal synchronization signal (HD) separated from the luminance signal by the synchronization separation circuit (12) are supplied to the switching control circuit (13) in order to set a sampling area.

The switching control circuit (13) outputs a vertical/horizontal synchronizing signal (VD
) (HD) and COD, a rectangular sampling area (A1) is created in the center of the screen as shown in Figure 2, and this area (
A selection signal ( S2) is the subsequent selection circuit (15
), and is also output to HPF (9) output and L P F
(11) A switching signal (Sl) for selecting an output is supplied to the switching circuit (14).

The switching circuit (14) receives the switching signal (Sl) and continues to select the HPF (9) output for a predetermined period, for example, between 32 fields, and keeps the output low for one field every 32 fields.
P F (11) Select the output, and repeat this switching operation at the same cycle thereafter.

Based on the selection signal (S2), the selection circuit (15) converts the output of the HPF or LPF selected by the switching circuit (14) into the integration circuit (16) (1) according to the sampling area.
7)... Selectively output to <21). That is, each filter output regarding the first sampling area (A1) is sent to the integrating circuit (16), each filter output regarding the second sampling area (A2) is sent to the integrating circuit (17), and the following third sampling area (A1) is sent to the integrating circuit (16).
to 6th sampling area (A3) (44>(A5)
The filter outputs related to (A6) are each integrated circuit (1
8) (19) (20) (21).

The integration circuit (16) includes an A/D converter (22) and an adder (2
3), consists of a memory circuit (24), and the A/D converter (22) sequentially A/D converts each filter output that passes through the selection circuit (15) and sends it to the adder (23). Output.

The adder (23) constitutes a digital integrator together with the A/D converter (22) at the front stage and the memory circuit (24) at the rear stage, and the output from the memory circuit (24) and the output from the A/D converter (22) and send the addition result back to the memory circuit (24).
supply to. The memory circuit (24) is rehit for each field and holds one field of the output of the adder (23), that is, the digitally converted value of the level of the luminance signal that has passed through the filter, for the first sampling area (A1). become.

Regarding the integration circuits (17), (18)...(21),
It has exactly the same configuration as the integrating circuit (16), and the memory circuit built into each integrating circuit stores the level 1 of the luminance signal that has passed through the filter selected in the current field for each sampling area. The integral value for the field will be retained. The integral value of each of these memory circuits is calculated from the memory 11 circuit (25
) can be summarized and memorized.

Specifically, the cutoff frequency of HPF (9) is 200
It allows passage of the band from KHz to 2.4MHz, and LPF (
The cutoff frequency of 11) is set to allow passage of the band from 0 to 2.4 MI'lz. In addition, 2゜4M
Hz is an extremely high frequency that is not directly related to the luminance signal, and its value has little meaning, and HPF (9) is 20
It is sufficient if the band above 0KHz can be extracted, and LPF〈11
) may be substantially omitted, and the imaging luminance signal may be directly supplied to the switching circuit (14) when L P F (11) is selected.

Therefore, the high-frequency or low-frequency components of the luminance signal that have passed through either HPF (9) or LPF (11) can be digitally integrated for one field, and the current field component can be integrated for each sampling area. will be stored in the memory circuit (25) as an evaluation value.

Here, among the integral values stored in the memory circuit (25), the integral value of the low frequency component when L P F (11) is selected is normalized to the exposure evaluation value for exposure control. Also, when the HPF (9) is selected, the integral value of the high frequency component is processed by the subsequent microcomputer (26) as the first focus evaluation for focus control.

These evaluation values are processed by software by the microcomputer (26), and based on the processing results, a command is issued to the focus motor control circuit (27) to control the focus camera motor (
4) to advance and retreat the focus lens (2),
It executes the O1-focus operation so that the focus evaluation value is maximized, and also issues a command to the iris motor control circuit (28) to drive the iris motor (7) and operate the optical diaphragm mechanism (6). ℃, automatic exposure adjustment is possible so that the exposure evaluation value becomes a predetermined value.

Next, referring to the flowchart in Figure 3, the microcontroller (2
The main routine of 6) autofocus operation and auto iris operation will be explained.

When the video camera enters the operating state, the microcomputer 26 executes the main routine shown in FIG.

First, at S T E P (30), the memory circuit (25
), the integral value for one field in each sampling area in the current field is read into the microcomputer (26).

Next, the counter (A E-CNT) provided for performing autofocus operation and auto iris operation in a time-sharing manner is decremented, that is, 1 is subtracted to 5 TEP (32))
, to determine whether the counter value is O or not S T E
P (33)), if the auto iris operation is not O, the auto 1-cas operation is executed, and the auto iris operation is executed only when the counter value is 0. Auto iris operation is H
Based on the focus evaluation value, which is the integral value of the pF(9) output,
A for holding the focus lens (2) in the focus position
This is done by executing the F line (35).

During the AF routine execution, when HPF (9) is selected,
The integral value (DATA (1) > (DATA (2)) of the first and second sampling areas (At) (A2) is calculated as the focus evaluation value (X (+)) (X (-)) of each area in the current field. )
First, specify the first sampling area (A1) as the focus area, drive the focus motor (4) to displace the focus lens (2), and obtain the focus evaluation value X in the first sampling area (A1). r + ), every time the focus evaluation value (Continuing the rotation in step 4), detect the top of the mountain, that is, the position where the maximum evaluation value is obtained, and when this position is reached, set this as the focusing position and stop the focus morph (4). Fix it to complete the focusing operation.

In addition, even though the lens position changed from the far point to the near point when detecting the top of the mountain, a clear top of the mountain could not be detected in the focus evaluation value X (,) in the first sampling area (A1).
Focus evaluation value X(,
) is the maximum evaluation value of the first sampling area (A1
) is larger than the maximum evaluation value in (1) per unit area of the area, the second sampling area (A2) is designated as the focus area, and from then on, the focus evaluation value The lens position that obtains the maximum evaluation value is the focal position.

and hold this lens position to complete the focusing operation.

Furthermore, even after the AF scene reaches the top of the mountain and the lens is temporarily fixed at this position and the focusing operation is completed, changes in the focus evaluation value are monitored, and if the focus evaluation value changes significantly, In this case, an object change monitoring operation is performed in which the object moves and leaves the focus area and the focusing operation is restarted from the beginning. In this monitoring operation, when the focusing operation is completed with the first sampling area (A1) as the focus area, the monitoring operation is first performed on this first sampling area (A1), and the focus evaluation of the first sampling area (A1) is temporarily performed. When a large change occurs in the value X, 1., it is further determined whether or not there is a change in the focus evaluation value However, if there is no significant change in this focus evaluation value Assuming that the focus area is simply moved horizontally to a position away from the first sampling area (A1) in
A2) and continues the surrogate operation.

When the above-mentioned AF period ends, the counter <AE-C
NT) content is 1i! i! ! The count is complete and the counter value is 0.
It is determined whether or not (S T E P (36))
, O, switch j# from the microcomputer (26).
A control signal is issued to the I control circuit (13), and in response to this, a switching signal is sent to the switching circuit (14) to select LPF (11). (51) was published, L P F (11
) is selected <S T E P (37)). In this way, L P F (11) is selected and the CPU waits for the evaluation value obtained by this selection to be read.

On the other hand, when auto iris operation is selected in S T E P (33'), the AE routine (38) which is the basis of auto iris operation is executed, and then the counter (
5TEP (39)) to return AE-CNT) to the initial state
, select the filter to HPF (9) (STEP (4)
0)), waits for the integration of the evaluation value of the next field, where: The initial state of the recounter (AE-CNT) is to calculate the exposure evaluation value based on the luminance signal that has passed through the LPF (11) every 32 fields. This refers to the state in which the initial value "32" is set for calculation.

Next, the auto iris operation will be explained according to a flowchart. At 5TEP (38) of the main routine, the counter (
When the curvature of the auto-iris (AE-CNT) reaches zero, that is, 32 fields have elapsed since the start of the focusing operation, the auto-iris (AE) routine is executed as shown in FIG. First, the first to sixth sampling areas (At) read in STEP (30) of the main routine
(A2)...In (A6), the integral value DATA(
1), DATA(2),...DATA<6) are normalized by the area of each area, that is, the area of the first to sixth sampling areas (Al)(A2)...(A6) The integral value per position area obtained by dividing by SMI ) (5M2) ... (5M6) is used as the exposure evaluation value Z (1), z (, ... Z (,) for each area. 5T
Calculated using EP (200), and then the average value for all areas, immediately. )) as the average exposure evaluation value < ZA)
Calculated using E P (201).

Next, a target evaluation value (Zl) that represents the brightness level of this screen and is to be controlled is determined. First, in the autofocus operation described above, the first sampling area (A1
), and determines whether the exposure evaluation value Z (1) of this first sampling area (A1) is within a certain predetermined tolerance range with respect to the average exposure evaluation value (ZA),
If both exposure evaluation values fall within the pair (a), S T E
If judged in P (202), S T E
P (203), this exposure evaluation value is 23. ) is the target evaluation value (zT), and in the autofocus operation described in Mari 11.OG, if the larger second sampling area (A2) is specified as the focus area, S T
When it is determined in E P (204), it is determined whether the exposure evaluation value Z (f) of the second sampling area (A2) is within a predetermined range with respect to the average exposure evaluation value (ZA). When it is determined in S T E P (205), this exposure evaluation value Z (13 is set as the target evaluation value (z7) in 5TEP (206)).

When it is determined that the is not satisfied, or when the I LOG waste area is designated as the first sampling area (A1>), the exposure evaluation value Z (+3N-1 to 6) of each area is
Among them, a predetermined value Ca) for the average exposure evaluation value (ZA)
The average of the following is the target evaluation value (Zl)
Calculated using T E P (207). In addition, if the exposure evaluation values in all areas are not within the specified range, STE
When it is determined in S T E P (290), the exposure evaluation value Z (1) of the first sampling area (A1) is set as the target evaluation value (2 shots), and further, in S T E P (208), the exposure evaluation value Z The maximum value of +++(f-1 to f-6) is set as ZmaX, and the minimum value is set as Zmin, and these values are set as values necessary for exposure determination.

In S T E P (202) < 205) (207), each exposure evaluation value is within a preset tolerance range with respect to the average exposure evaluation value (ZA), or the value is significantly different from the average exposure evaluation value (ZA). There is no problem in simply using the ratio between the two, but in this example, logarithmic compression was performed taking into consideration that the dynamic range of the ratio between the two is extremely wide. It is compared with the predetermined value (a) in (h).

As described above, when executing the auto iris operation among the exposure evaluation values of a plurality of sampling areas, the target evaluation fa, which is the exposure evaluation value of the area used, is the exposure evaluation value z( of the first sampling area (A1)). 1) is prioritized, and there are extremely high brightness areas such as light sources and extremely low brightness areas such as deep green areas, that is, abnormal brightness areas, in this first sampling area (A1), and the ratio with the average evaluation value (ZA) is When the logarithmic compression value of is not within the range of the predetermined value (a), if the focus area is the second sampling area (A2), priority is given to the exposure evaluation value of this area (2°). However, in order to eliminate the influence of the exposure evaluation value Z Ll ) of the first sampling area (A1) where the abnormal brightness part exists, (Z(1
)X(5M2) Z(1)X(SMI))/(5M2
-SMI) (where SMI and 5M2 are the areas of the first and second sampling areas (At)<A2), respectively) as the target evaluation value (Zl). The value calculated using the above formula is the second sampling area. It is the exposure evaluation value (2,,) of the area (A2) excluding the first sampling area (A1), and if there is an abnormal brightness part in this second sampling area (A2), The average exposure evaluation value (direct) of the area where no brightness part exists is set as the target evaluation value, and the area corresponding to this is used for the auto iris operation.

With each setting as described above, the aperture is first determined based on the flowchart of FIG. S T E
In P (210), first, the logarithm L of the ratio of the target evaluation value (ZT), the target evaluation value (zT), and the minimum value (Z m1n) is derived. This light/dark discrimination value (D> is the target evaluation value (Zl
) is a parameter that determines whether the main subject is relatively bright or dark in the screen, and the main subject is bright and the object evaluation value (2?) is relatively thick, and the brightness discrimination value (D)
becomes larger. Conversely, when the target evaluation value (z7) is relatively small, the former term becomes small, the latter term becomes large, and the brightness discrimination value (D) becomes small.

The reason why the logarithm of the ratio of the evaluation values is used when calculating the brightness/darkness discrimination value (D) is that human vision normally recognizes brightness because the brightness of the actual subject is exponential. For example, we focus on the fact that when the brightness level increases from 2x to 4x to 8x, the visual brightness changes linearly.

(b) When it is determined that it is within the range of (b>0), that is, lnl<b holds, it is assumed that the brightness in the screen is intermediate, and the target value for controlling the target evaluation value (Za) is set. The L limit (ZU) and the lower limit (2,) are set to (V) (v), respectively, in 5TEP (213), and when the discriminant value (D) is determined to be +5 or more, the upper limit (Z8) is determined as relatively bright. 〉、
Set the lower limit (ZL) at S T E P (214) respectively (
U) (U). Furthermore, when the discriminant value (D) is determined to be less than -b, it is considered relatively dark and the upper limit (ZU) is set.
, the lower limit (ZL) is determined as (W>(W)) in S T E P (215).Here, the relationships U>V>W and u>v>w are set in advance for these upper and lower limits, respectively. By providing this, a target range corresponding to the relative brightness within the screen of the target evaluation value (21) can be obtained.

Note that the above-mentioned predetermined value (b) is a limit value when the brightness level of the main subject can be visually recognized as being significantly brighter or significantly darker than the brightness level of the entire screen,
It has been experimentally determined in advance.

Next +, : S T E P (216) (217), the target evaluation value (ZT) and the target value upper and lower limits (ZLI), (
ZL), and if Z,>ZT>ZL holds, it is determined that proper exposure has been obtained, and the aisle smoke (7) that drives the optical diaphragm mechanism (6) is maintained in a stopped state.
Maintain the current aperture and set the target evaluation value (zl) to the upper limit (ZU
), it is considered overexposure, and S T E
At P (219), the aperture mechanism drives the iris motor (7) in the direction of closing the aperture amount by one step, and conversely, if the target evaluation value (ZT> is smaller than the lower limit (Z, ), S is determined to be underexposure. Set the aperture amount to 1 at T E P (218).
The iris motor (7) is driven in the step opening direction.

Note that the iris motor (7) is constituted by a stepping motor.

While adjusting the aperture amount by this iris motor (7), the S
The gain of the AGC amplifier (301) that amplifies the imaged video signal at T E P (222) is fixed at a constant value (the gain may be -0) (this state is called the AGC operation OFF state), and the incident If it becomes difficult to obtain proper exposure by simply adjusting the light intensity, that is, if the subject is extremely low brightness and
While repeating TEP (218), it is determined in STEP (220) that the aperture mechanism is in the open state, and if proper exposure cannot be obtained even after reaching this state, STE At P (221), connect the AGC amplifier (
301), its amplification gain is made variable, and the gain is increased or decreased depending on the level of the input imaged video signal, and the output is amplified until it reaches a constant level (this state is referred to as the AGC operation state N). state).

The open state of the optical diaphragm mechanism (6) can be detected by monitoring the total rotation amount (total number of steps) of the iris motor (7) or by mechanically detecting the operation of the optical diaphragm mechanism (6) itself. It is possible.

As mentioned above, specific examples in which exposure adjustment is performed by subtly changing the upper and lower limits of the target value according to the magnitude of the brightness discrimination value (D>) are shown in FIGS. 7 to 9. In the figure, the target evaluation value (21) is obtained from the first sampling area (A1) which is the focus area, the main subject exists in this first sampling area (A1), and the brightness level of the main subject is the target evaluation value (21). Corresponds to Z7).

In the figure, the horizontal axis is the actual brightness level of all subjects including the main subject and background without exposure adjustment, and the brightness area of the entire screen is indicated by (L) (indicated by an arrow →). The actual brightness level is shown in parentheses. The vertical axis is the brightness level of the captured video signal after passing through the diaphragm mechanism (6) and the AGC amplifier (301) and exposure adjustment, which is the permissible level that allows human vision to recognize the image as being of good quality. The appropriate exposure range (M) is indicated by an arrow.

Here, FIG. 7 shows that the relationship IDI<b holds between the brightness discrimination value (D>) and the predetermined value (b), and the target evaluation value (Zl),
In other words, the actual brightness level () of the main subject is located approximately at the center of the brightness area (L) of the entire screen, and the main subject has a relatively intermediate brightness. D<
Figure 9 shows a case where the relationship D>+b is strong, the main subject's actual brightness level () is at a slightly lower position in the area (L>, and the main subject is relatively dark). holds true, the actual brightness level of the main subject is at a slightly higher position in region (L), and the main subject is relatively bright.

In each figure, (P) is a type of conventional example, which is the brightness of the captured video signal after exposure adjustment for all subjects when a method of performing exposure adjustment based only on the average brightness level of the entire screen is adopted. The actual brightness level of all objects and the brightness level of the imaged video signal at this time have a relationship shown by a straight line (p>.The average value (A
By matching V>(midpoint of P) to the optimum value (m>, which is the midpoint of the proper exposure range (M)), the proper exposure range (M) can be positioned approximately at the center for the entire screen. However, as shown in Figures 8 and 9, if the actual brightness level () of the main subject is relatively low or high relative to the actual brightness area (L) of the entire screen, The imaging brightness level of the subject is (tl), and the appropriate exposure range (
M>, the main subject will be underexposed.

In addition, (Q) is a type of conventional example, and when a method is adopted in which the imaging brightness level of the main subject or the imaging brightness level of the area containing this main subject is matched to the optimal value (m), That is, this is the brightness level region of the imaged video signal of the entire screen, and the actual brightness level of all objects and the imaged brightness t·bell at this time have a relationship shown by a straight line (q).

Using this method, the main subject can be properly exposed, but in the case of Figures 8 and 9, other areas such as the background are significantly out of the appropriate exposure range, resulting in dim or white areas. It becomes a screen.

(R) is the brightness level region of the imaged video signal for all subjects in the method of this embodiment, and the actual brightness level of the entire screen is directly different from the brightness level of the imaged video signal after exposure adjustment! The relationship shown in (r) is obtained. As will be explained later, this straight line (
By shifting r) in the vertical direction, the target value is slightly changed.

In the flowchart of FIG. 5, if the absolute value of the light/dark discrimination value (D> is within the range of the predetermined value (b) and the brightness is intermediate to the entire screen, the upper and lower limits of the target value are set ( V
)(v), the target evaluation value (21) that includes the main subject is the upper limit (V)(v)
The aperture mechanism operates so that the image brightness level (t3) of the main subject is located in the area (Q) as shown in Figure 7.
Similarly, the image pickup brightness level region (R) of the entire screen by all the objects matches the optimum value (m), and the appropriate exposure range (M) is located approximately at the center, so that appropriate exposure adjustment can be performed.

When the relationship D<-b holds between the brightness discrimination value (D) and the predetermined value (b), and the brightness level of the main subject is recognized to be relatively dark, the target value of the object evaluation value is The lower limits (ZL, ) (ZL) are changed to (W) (w) smaller than (V) (v), respectively, and the aperture mechanism is activated.
By positioning the target evaluation value (z7) within the upper and lower limits (zU) (ZL), the straight line (r) in FIG. ), and by matching the imaging brightness level area (R> of the entire screen as much as possible with the appropriate exposure range (M)), the main subject can be properly exposed with sufficient visual quality. In addition, a good screen can be obtained without other screens being significantly outside the appropriate exposure range (<M).

Furthermore, D〉+b between the brightness discrimination value (D) and the predetermined value (b)
When the relationship holds true and the brightness level of the main subject is recognized as relatively bright, the target evaluation value fi!
(7) By changing the upper and lower limits (zu) (Zt) to (U) and (u) which are larger than (V) and (V), respectively, and operating the aperture mechanism, the straight line (r) in Fig. 8 can be changed. Avoid shifting upward to adjust the imaging brightness level of the main subject to the appropriate exposure range (M)
By aligning the imaging brightness level area (R) of the entire screen with the appropriate exposure range (M) as much as possible, the main subject can be properly exposed with sufficient visual quality. In addition, a good screen can be obtained without the other screens being significantly out of the appropriate exposure range (M).

Next, the determination of the gamma value will be explained based on the flowchart of FIG. First, I! at S T E P (230)! ! The contrast (△) of irMi is derived as the ratio of the maximum f + fi (Zmax) and the minimum value (Z m1n) among the exposure evaluation values, and the preset reduction interval number r (△)
The calculation is performed to change the correction gamma (7) to the optimal value by substituting the contrast (△) into
+ b *= a & L OG (△) ten.

(However, ao and b are constants, and the target correction gamma (7) is derived in S T E P (231) using the formula a, <0, b, <O).

Here, in S T E P (232), the subject is extremely low brightness C1.
If it is determined in (221) that the gain of the AGC amplifier (301) is not fixed at a constant value and that normal AGC operation is ON, the correction gamma (7 ) by a predetermined amount (d,) and compressing the screen image contrast, the No. 18 level of a low-luminance subject is substantially raised.

In addition, -C target evaluation value (
21) The light/dark discrimination value (D) for (usually the exposure evaluation value of the prioritized focus area) falls within the range of the predetermined value (b), that is, D<-b'j" or D>+b. Sometimes, the correction gamma (7
) by a predetermined amount (dI), and the screen weight last is set to IE$1. For example, if D<-b holds and the brightness level of the main subject, that is, the target evaluation value, is significantly lower than the brightness level of the entire screen, the optical diaphragm mechanism (
6), the exposure is adjusted so that the brightness level of the main subject is located near the lower limit of the appropriate exposure range (M>) as shown in <R) in Figure 8, and the brightness area of the entire screen is within the appropriate exposure range. Efforts were made to position (M) approximately in the center to obtain appropriate exposure for both the main subject and the entire screen, but as shown in (R), the high brightness area after adjustment is 1 exposure. (r,
) remains outside the proper exposure range (M). At this time, if the screen contrast is compressed by reducing the correction gamma (7) by a predetermined amount (d8) as described above, the straight line (r) will change to the curved line (r ), the brightness area of the entire screen changes from (R) to (R'), and the brightness area of the entire screen changes while keeping the brightness level of the main subject near the lower limit of the appropriate exposure range (M). It becomes possible to substantially match the appropriate exposure range (M), and the exposure by the optical aperture mechanism (6), ti1t! Proper exposure is achieved by further correcting l.

Furthermore, even if D>+b holds true and the brightness level of the main subject is relatively higher than the brightness level of the entire screen, the low brightness after exposure adjustment will be reduced as shown in (R) in Figure 9. The region (r,) remains outside the proper exposure range (M>, and even at this time, the correction gamma (7) is set to a predetermined value i(a).
When the screen = T fun last is compressed by decreasing it by m), the straight line (r) changes like a curve (rl), and the luminance area of the entire screen changes from (R) to (R ”), it is possible to position the brightness level of the main subject close to the upper limit of the appropriate exposure range (M), while making the brightness area of the entire screen approximately match the appropriate exposure range (M>), and the optical diaphragm mechanism ( The exposure adjustment made in step 6) is further corrected to achieve proper exposure, thereby eliminating overexposure in high-brightness areas and underexposure in low-brightness areas.

In addition, the luminance areas (L) (R) and the straight line (
r) are the same, and the luminance region (
L) (R) and straight line (r) are the same. The correction gamma (7) value determined in this way is sent to the gamma correction circuit (302).
If the correction gamma (7 degrees) differs significantly from the previous correction gamma (7 degrees), the correction will be completed in -degrees, resulting in an unsightly screen, so the correction gamma will gradually change. It is necessary to do so.

Therefore, in S T E P < 236), the correction gamma (7) in the current field is compared with the correction gamma (7°) detected last time, that is, detected 32 fields ago, and the correction gamma (7°) in the current field is compared. If the correction gamma is larger than S T E P (241), the correction gamma is increased by one step (d7), and if the previous correction gamma (7°) is larger, S T E P (2
In step 42), the correction gamma is decreased by one step (d7). Here, the one step value (d7) is set by dividing the difference between the maximum value and the minimum value that the correction gamma (7) can take into equal parts (n: natural number). 7) will change in n stages.

When switching the correction gamma (7), in order to make a large change in one direction from the previous correction gamma (7°), it is effective to change it continuously every 32 fields, but this and the previous When the correction gammas are close to each other, the brightness level of the screen will change slightly due to camera shake, etc., and the correction gun 7 will follow this and vibrate up and down each time it is switched, resulting in complicated switching. . Therefore, in order to prevent this complicated switching, the previous correction gamma change direction and the current change direction are compared according to the state of the flag (SX) in S T E P (237) < 238), and if the same If it is 5T
Jump over EP (239) (240) and S T E P
(241) (242), and if they are different, the difference between the correction gamma (7°) (7) between the previous and current field is 17°-
71 provides hysteresis by changing the correction gamma only when the correction gamma is greater than or equal to a predetermined value (C>) for which correction is deemed essential.

A control signal corresponding to the correction gamma determined in this way is input to a gamma correction circuit (302) connected after the AGC amplifier (301), and based on this, it is amplified according to the input level of the imaged video signal. The ratio is changed to perform optimal gamma correction, and appropriate brightness can be obtained across the screen even for subjects with high screen contrast. The captured video signal that has undergone gamma correction in the gamma correction circuit (302) can be displayed on a CRT (not shown),
It can be recorded on a VTR (not shown).

(G) Effects of the Invention As described in (H), according to the present invention, by prioritizing the optical diaphragm mechanism over the AGC circuit, exposure can be achieved while minimizing S/N deterioration! Li adjustment becomes possible.

[Brief explanation of the drawing]

The drawings all relate to one embodiment of the present invention, and FIG. 1 is an overall circuit block diagram, FIG. 2 is an explanatory diagram of area division, FIG. 3 is a flowchart of the main routine, and FIG. 4 is an AE seal.
Fig. 5 is a broach route for determining the aperture amount, Fig. 6 is a flowchart for seven-prong correction correction, Figs. 7, 8, and 9 are characteristic diagrams related to exposure u4t, Fig. 10
11 is a characteristic diagram related to 7 corrections, and FIG. 12 is a diagram illustrating the movement of the main subject. (26)...Microcomputer (level detection means), (6)...Optical aperture mechanism, (301)...AG
C amplifier < A G C circuit).

Claims (1)

[Claims] (1) Level detection means for detecting the brightness level of a captured video signal obtained from an image sensor; an optical diaphragm mechanism that controls the amount of light incident on the image sensor so that the brightness level becomes a target level; and the brightness level and an AGC circuit that amplifies the captured video signal by controlling the gain so that it reaches a target level, and the gain of the AGC circuit is held constant until the aperture mechanism is in an open state. imaging device.