JP2971477B2 - Exposure control method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure control method capable of effectively adjusting an exposure amount of a video signal electronically captured and input.

[Related Art] In recent years, electronic cameras that electronically image a subject using a solid-state imaging device such as a CCD have been remarkably popularized, and in particular, electronic still cameras have recently come into practical use. Exposure control is one of important control techniques when electronically imaging a subject with this kind of electronic camera. This exposure control is for optimizing the luminance level of a video signal input as an image. Generally, aperture control is performed on an incident light amount using an aperture mechanism incorporated in an imaging lens system, and a solid-state image sensor is used. The control is performed by controlling the accumulation time of the signal charge according to the amount of incident light, ie, by controlling the shutter speed using a so-called element shutter.

In some cases, exposure control is performed only by the above-described aperture control while the shutter speed is kept constant. In other cases, shutter speed control is performed using a mechanical shutter mechanism separately from the above-described element shutter. In any case, exposure control for a video signal electronically imaged and input is very important in optimizing the luminance level.

By the way, conventionally, generally, the luminance (brightness) of a subject is measured using a dedicated photometric sensor, and the exposure amount is controlled according to the photometric value. More specifically, the brightness of a subject (photographing target) is obtained by making use of techniques such as average photometry, center-weighted average photometry, and spot photometry, and exposure control is performed according to the photometric value.

However, it is extremely difficult to properly use these photometric techniques according to the purpose of photographing or the object to be photographed, and it is also extremely difficult for a general user to properly use these photometric techniques. Therefore, in a general electronic camera, exposure control is performed by detecting the brightness of a subject by average photometry.

[Problems to be Solved by the Invention] However, there are various types of subjects to be photographed, and in many cases, the above-described exposure control based on the average photometry cannot obtain a video signal of an appropriate luminance level. For example, when the background portion is very bright compared to the person to be photographed, or when a high-luminance body such as the sun is included in the imaging screen, exposure control is performed according to brightness information that is averagely obtained. In some cases, the brightness level of the main subject may be too low (underexposure). Conversely, if the exposure control is performed according to the above-described averaged brightness information for shooting where the background is very dark, such as a subject on the stage, the brightness level of the main subject may become too high. (Overexposure).

In view of the above, various studies have conventionally been made on how to cope with this kind of photographing condition, but there has been a problem that the control mechanism is unnecessarily complicated.

The present invention has been made in view of such circumstances,
An object of the present invention is to provide a highly practical exposure control device capable of effectively performing appropriate exposure control for various photographing conditions.

Means for Solving the Problems According to the present invention, the luminance level of a video signal at a plurality of discretely set sampling points in a screen is determined, and a first level which is set in advance as an excessively high level range is determined. The number of sampling points included in the level area and the number of sampling points included in the second level area previously set as an excessively low level area are determined, and the first level area, the second level area, Is divided into a third level area on the relatively high side and a fourth level area on the relatively low side, and the number of sampling points included in the third level area and the fourth And the number of sampling points included in the first level area is determined such that the number of sampling points included in the first level area is equal to the number of sampling points included in the second level area. Controls the exposure so as to reduce the difference between these numbers. Next, the sum of the number of sampling points included in the first level area and the number of sampling points included in the second level area is determined by a predetermined value. When the value is not reached, the number of sampling points included in the third level area and the number of sampling points included in the fourth level area become equal to each other, or the difference between these numbers decreases. Is an exposure control method for controlling the exposure amount as described above.

In this case, weighting for exposure control is performed by changing the positions of a plurality of sampling points set in the screen. .

[Operation] According to the present invention, the luminance level of the video signal at each of the plurality of sampling points discretely set in the screen is determined, and the first level previously set as an excessively high level range is determined. The number of sampling points included in the area and the number of sampling points included in the second level area set in advance as an excessively low level area are obtained, and the first level area, the second level area, Between the number of sampling points included in the third level area on the relatively high side in the luminance level range
And the number of sampling points included in the first level area is first determined so that the number of sampling points included in the first level area and the number of sampling points included in the second level area are equal to each other. Or the exposure amount is controlled so that the difference between these numbers is reduced, and then the number of sampling points included in the first level area and the number of sampling points included in the second level area are controlled. When the sum does not reach the predetermined value, the number of sampling points included in the third level area and the number of sampling points included in the fourth level area become equal to each other, or a difference between these numbers is obtained. Is controlled so as to reduce the exposure. Thus, by making the ratio of the high luminance side and the low luminance side of the excessive luminance level portion which deviates from the proper exposure with the high contrast equal or substantially equal, the luminance level of the main subject is equal to the first level region and the second level region. The exposure level is included in the appropriate exposure area sandwiched between the areas, and even for an image with a low contrast, the exposure level can be effectively and appropriately set.

Hereinafter, an exposure control method according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram showing an exposure control system of an electronic camera configured by applying the embodiment method.

In FIG. 1, reference numeral 1 denotes an optical lens system incorporating an aperture mechanism 2 for controlling an aperture, and a subject image is formed on the imaging surface of a CCD 3 which is a solid-state imaging device via the optical lens system 1. A half mirror 4 for guiding the subject image to the viewfinder system and an optical filter 5 for removing unnecessary light components from the subject image are inserted in the imaging optical system. The optical lens system 1 is controlled to move forward and backward in the direction of the optical axis by an actuator mechanism 6 so as to adjust the focal position.

Thus, the CCD 3 is driven under the control of the synchronization signal generator (SSG) 7 and accumulates signal charges corresponding to the incident light amount of the subject image for a time corresponding to the set shutter speed. The screen is line-scanned and sequentially read out and output as a video signal of the stored signal charges. The video signal read from the CCD 3 in this manner is given to the signal processing unit 8 and subjected to, for example, γ (gamma) correction, color separation processing, etc., and then via the recording unit 9 to the still video floppy disk ( SVF) and video tapes. When the video signal is monitored and output to a TV receiver or the like, the signal processing unit 8 includes, for example,
Performs processing such as conversion to NTSC signal format.

Here, the exposure control method according to the present invention is characterized in that the video signal output from the CCD 3 is converted to a low-pass reactor (LP).
F) The signal is extracted in parallel to the signal processing unit 8 via 10 and the video signal is used to perform exposure control for electronic imaging of the subject image by the CCD 3 as follows. is there.

That is, the video signal output from the CCD 3 is subjected to a furnace wave via the LPF 10 and its luminance component is obtained. The luminance component of this video signal is input in parallel to each of the three comparators 11, 12, and 13, and the luminance level is determined. These comparators 11, 12, and 13 compare the comparison reference levels Th1, Th2, and Th3 set by the reference power supplies 14, 15, and 16 with the levels of the luminance signal, respectively, to determine the luminance signal level. The comparison reference levels Th1, Th2, and Th3 satisfy the relationship of Th1>Th2> Th3 with respect to the dynamic range of the video signal (the range of levels of the video signal from the black level to the white level) as shown in FIG. Levels are set.

Here, the comparison reference level Th1 defines a first level range H in which the luminance level of the video signal is excessively high, and is set in advance to correspond to a so-called overexposed luminance level limit. Also, reference level Th3
Defines a second level range L in which the luminance level of the video signal is excessively low, and is set in advance to correspond to a so-called underexposure luminance level limit. The comparison reference level Th2 is in a range suitable for the luminance level, but the luminance level is relatively high.
The level is previously set as a level for determining whether the pixel belongs to the MH or a relatively low luminance level range ML.

Here, the comparators 11 and 12 emit a signal of “H” level when the level of the luminance signal is higher than the comparison reference levels Th1 and Th2. An “H” level signal is issued when the signal is lower than Th3. Counters 14, 15, and 16 provided in correspondence with these comparators 11, 12, and 13 output the "H" level signals generated by the comparators 11, 12, and 13, respectively, to the synchronous signal generator. Under the control of 7, the data is fetched in accordance with a predetermined sampling block, and the number of sampling points that take an "H" level value is counted.

The sampling points in the screen set by the synchronizing signal generator 7 are, for example, for each horizontal scanning line determined every predetermined number as shown in FIG. The sampling points are set discretely in the screen. The plurality of sampling points discretely set in this manner may be set at equal intervals (average) within one screen with a predetermined cycle, but the sampling intervals in a specific screen area may be finely adjusted. Alternatively, it may be determined by giving a predetermined weight by making the sampling interval of another area coarse or the like.

Thus, the first counter 14 counts the number of the sampling points belonging to a level area (exposed area) H having an excessively high luminance whose luminance level exceeds the comparison reference level Th1, and the third counter 16 Counts the number of sampling points belonging to a level region (underexposed region) L in which the luminance level of the video signal is excessively low. Further, the second counter 15 counts the number of sampling points exceeding the comparison reference level Th2 in order to obtain the number of sampling points belonging to a relatively high luminance level range.

Note that the number of sampling points counted by the second counter 15 includes the number of sampling points belonging to the high luminance level range H counted by the first counter 14 described above. Therefore, when only the number of sampling points belonging to the relatively high luminance level range MH in the appropriate luminance level range is obtained, the first counter 14 counts the number of sampling points counted by the second counter 15 as described later. What is necessary is just to subtract the number of sampling points performed. Alternatively, the comparator 11,1
The output of 2 may be gate-controlled, and only when the output of the comparator 11 is at the "L" level, the "H" level output of the comparator 12 is sampled by the counter 15 and counted. By controlling the counting operation of the second counter 15 in this way, it is possible to count only the sampling points belonging to the relatively high luminance level region MH by the counter 15.

Although the counting operation is not particularly performed in this embodiment, the number of sampling points belonging to the relatively low luminance level ML described above can be counted independently of the counters 14, 15, and 16 described above.

By the level determination by the comparators 11, 12, and 13 as shown in FIG. 2, the luminance level of the video signal is divided into the luminance level areas H, Th, Th2, and Th3, as shown in FIG. It is determined which of MH, ML, and L belongs. Then, the counters 14, 15, 16 count the number of sampling points belonging to the respective brightness level regions H, MH, L.

The CPU 17 determines the number of sampling points belonging to each of the brightness level areas H, MH, and L counted by the counters 14, 15, and 16 as described above, and determines the exposure amount of the subject by the CCD 3 as follows. This is to control the optimization. That is, CPU1
7 basically compares the number of sampling points belonging to the high luminance level range H counted by the counter 14 with the number of sampling points belonging to the low luminance level range L counted by the counter 16, and their count values are equal. So that the difference between the count values is small.
The exposure amount by CCD3 is variably controlled. The variable control of the exposure amount in the CCD 3 is performed by controlling the accumulation time of the signal charges corresponding to the subject light under the control of the synchronization signal generator 7, that is, variably controlling the shutter speed by the so-called element shutter. This is done by:

For the video signal obtained after such exposure amount adjustment, the above-described determination of the luminance level is similarly performed, and under the control of the CPU 17, the number of sampling points belonging to the above-described high luminance level range H and The variable control of the exposure amount by the CCD 3 is repeatedly performed in a feedback manner so that the number of sampling points belonging to the low luminance level range L becomes equal or the difference between the count values becomes small.

According to such an exposure amount control, for example, when an object including a very high brightness portion such as the sun is photographed in the background, the brightness of the entire image is affected by the local high brightness portion. It is possible to effectively prevent the level from lowering. Incidentally, for example, when an image having a luminance distribution as shown in FIG. 3 is captured and input, a histogram of the image area with respect to the light intensity is as shown in FIG. 4, and when the average luminance level of the entire image is obtained, It often happens that the average luminance level itself falls within an excessively high luminance level range. In this case, if the exposure amount is adjusted in accordance with the average luminance level, the luminance level of the main subject portion, which occupies a large area, becomes very low.

In this regard, as described above, the number of sampling points included in the high luminance level range H deviating from the appropriate luminance level range and the number of sampling points included in the low luminance level range L deviating from the appropriate luminance level range are equalized. If the exposure control is performed in such a manner, the area of the part where the luminance becomes excessively high outside the appropriate exposure range (luminance level) is equal to the area of the part where the luminance becomes excessively low, and most of the area in the image is adjusted to the appropriate luminance. It is possible to stay within the level range.
As a result, the brightness level of the main subject is set to the appropriate brightness level range, and an image with a stable exposure amount can be obtained.

In other words, by making the number of samplings on the high luminance side and the number of samplings on the low luminance side of the image part outside the appropriate luminance level range equal, without being pulled by the presence of extreme bright or dark parts, By setting the appropriate exposure value, it is possible to set the imaging luminance level for the main subject in an appropriate luminance level range. As a result, although the brightness level of an extremely bright or dark portion is further increased or decreased, the entire image becomes an image in which most of the image area including the main subject has an appropriate exposure level range, Here, an image at an appropriate exposure level can be obtained.

In the above description, the exposure amount is controlled so that the number of sampling points included in the high luminance level range H and the number of sampling points included in the low luminance level range L are equalized or the difference therebetween is reduced. However, if the sum is equal to or less than a certain value, the number of sampling points on the relatively high luminance level side and the number of sampling points on the relatively low luminance level side of the sampling points included in the appropriate exposure level range are described above. The above-mentioned exposure control is switched in a stepwise manner so that the difference between them becomes equal or the difference becomes small.

That is, there is a case where it is difficult to properly set the exposure amount only by controlling the exposure amount based on the number of sampling points included in the level region H and the level region L described above. In some cases, the luminance level of the main subject portion in the appropriate exposure level range is biased. In other words, when the contrast of the entire screen is low, all the sampling points are between Th1 to Th3 described above, and exposure control cannot be performed only by the above-described control method.

Therefore, the above-described exposure control is pre-processed, and after a certain exposure control value is obtained, or when it is difficult to set an appropriate exposure value only by the above-described exposure control,
Further, the exposure amount is finely adjusted according to the number of sampling points counted by the second counter 15 described above.

Specifically, the CPU 17 obtains the number of sampling points included in the relatively high-luminance level range MH, and subtracts the number of samplings included in each of the level ranges H, L, and MH from the entire sampling number to obtain a relatively low number. Bright level range
Obtain the number of sampling points included in ML. Then, the exposure amount in the CCD 3 described above is adjusted so that the number of sampling points included in each of the level ranges MH and ML is equal or the difference is reduced.

By performing such two-stage exposure adjustment,
Even if the image has low contrast, the exposure level can be effectively and appropriately set.

FIG. 6 shows the flow of the above-described exposure control.
5 shows a specific flow of a procedure for adjusting the exposure amount in the CCD 3 according to the luminance level based on the video signal captured by the CCD.

In the processing procedure shown in FIG. 6, an up-down counter is used as the second counter 15 in order to directly obtain the difference between the number of sampling points included in each of the level ranges MH and ML. When the first comparator 11 outputs the "L" level, the "H" level output of the second comparator 12 is up-counted by the up / down counter in accordance with a predetermined sampling clock. 3
When the comparator 13 outputs the "L" level, the "L" level output of the second comparator 12 is counted down according to a predetermined sampling clock. That is, when the luminance level at each sampling point is included in the high-luminance-side level range MH, the up-down counter counts this up, and the luminance level at the sampling point is included in the low-luminance-side level range ML. Sometimes, the up / down counter counts down. As a result, the difference between the number of sampling points included in each of the level ranges MH and ML can be directly obtained by the up / down counter.

However, in the processing procedure shown in FIG.
The subject is imaged and input by the CCD 3 under predetermined exposure control (step a). Then, the video signal of the captured subject image is read out from the CCD 3 (step b), and the luminance level of the video signal is set in advance using the comparators 11, 12, and 13 and the counters 14, 15, and 16 described above. It is checked which level range it belongs to, and the number of sampling points in each level range is counted (step c). The information of the level range thus counted is input to the CPU 17, and first, a determination parameter for exposure control is obtained by a predetermined arithmetic processing (step d). Specifically, the number of sampling points included in the excessively high luminance level range H counted by the first counter 14 and the number of sampling points in the excessively low luminance level range L counted by the third counter 16 are calculated. Is obtained.
At the same time, a difference B between these sampling points is obtained. The difference B is obtained by subtracting the number of sampling points in the low luminance level range L from the number of sampling points included in the high luminance level range H. Then, the count value of the second counter (up / down counter) 15 is set in the level range MH, M.
It is obtained as a difference C between the number of sampling points in L.

Thereafter, according to these determination parameters A, B, and C, a determination process for exposure control is executed as follows. That is, first, it is determined whether or not the determination parameter A is larger than a predetermined determination threshold α (step e). With this determination, it is determined whether or not there are many pixel regions (sampling points) that are out of the appropriate luminance level range.

If there are many pixel areas (the number of sampling points) outside the appropriate luminance level range, it is next determined whether or not the absolute value of the determination parameter B exceeds a predetermined determination threshold β (step f). . This determination is based on whether the difference between the number of sampling points included in the excessively high luminance level range H and the number of sampling points included in the excessively low luminance level range L is large, and whether or not there is a large bias in the luminance balance. To check. By this determination processing,
When it is determined that the difference between the sampling points is large,
It is determined whether the difference takes a positive value (step g).

According to such a determination result, if the number of sampling points included in the high-luminance level range H is extremely large, the exposure amount for the subject is determined to be too large and the exposure amount is narrowed down (step h). This narrowing down is performed by, for example, doubling the shutter speed and reducing the exposure amount by half.

If the determination in step g is NO and it is determined that the number of sampling points included in the low-luminance level range L is very large, the exposure amount for the subject is determined to be too small and the exposure amount is increased (step j).
The exposure amount is increased by, for example, halving the shutter speed and doubling the exposure amount.

If the difference between the number of sampling points included in the high-luminance level range H and the number of sampling points included in the low-luminance level range L is smaller than a predetermined threshold β, the process proceeds to the next process with the exposure amount being unchanged. Step i).

The above processing procedure is repeatedly executed while changing the exposure amount by the CCD3.

On the other hand, when the number of pixel regions (the number of sampling points) that are out of the appropriate luminance level range described above is small, the absolute value of the determination parameter C is compared with a predetermined determination threshold γ,
It is checked whether or not there is a bias in the brightness level of the image within the appropriate exposure level range (step k). If there is no bias, the exposure value is left as it is (step i),
If there is a bias, it is checked whether or not the determination parameter C is positive.
Alternatively, it is checked whether the image is biased toward the low luminance side (step 1).

If it is biased toward the high luminance side, the exposure amount is reduced (step h), and if it is biased toward the low luminance side, the exposure amount is increased (step j).

By repeating such a processing procedure, the CC
The exposure level for the video signal captured by D3 is optimized.

Thus, according to the processing procedure of the exposure amount control, it becomes possible to optimize the exposure amount in a feedback manner according to the luminance level of the video signal. Moreover, instead of performing exposure control based on the average luminance level of a video signal as in the past, the amount of exposure is controlled by focusing on a luminance level that is excessively deviated from an appropriate exposure level.
Even when a region having an extremely high brightness level is included in the captured image, an appropriate exposure level can be set without being influenced by the region. Therefore, it is possible to obtain a captured image with good gradation and good image quality by effectively adjusting the exposure level to the main subject.

In the above-described embodiment, since the luminance signal level of the output video signal of the CCD 3 is directly determined, the signal processing circuit 8
It is possible to optimize the luminance level of the signal before being subjected to the γ correction. In other words, since the exposure amount of the CCD3 is controlled by using the luminance signal itself of the wide dynamic range of the output video signal of the CCD3 before the gamma correction is performed and the dynamic range is compressed, the imaging characteristics of the CCD3 are sufficiently improved. Exposure amount control utilized effectively can be performed. As a result, it is possible to apply an appropriate correction to the luminance level range compressed by the γ correction, and obtain an image with good gradation characteristics in which the luminance information in the area is reflected in the captured image by simple control. Becomes

Note that the present invention is not limited to the above-described embodiment. For example, the above-described first threshold value Th1 and third threshold value Th
Although 3 may be determined as the expression limit value of the luminance level by the γ correction, it is needless to say that it may be determined according to the specifications of the camera. The second threshold value Th2 may be variably set according to the type of image to be imaged. Further, the above-mentioned luminance level range MH, M
After omitting the determination process in L and performing control for an excessive luminance level, it is also possible to perform exposure control by obtaining the average of the luminance levels in the luminance level ranges MH and ML. Further, as described above, it is of course possible to weight image information to be subjected to exposure control by changing the sampling points in the screen. This weighting can be realized by changing the horizontal scanning lines to be extracted and changing the number of sampling points in each horizontal scanning line. In addition, the number of samples of the luminance level to be sampled in the screen, the position thereof, and the like may be determined according to the specifications of the apparatus. This exposure control is applicable not only to an electronic movie camera but also to an electronic still camera. When the present invention is applied to an electronic still camera, exposure control may be performed in accordance with data at the time of pre-metering, and then main exposure may be performed. In addition, the present invention can be variously modified and implemented without departing from the gist thereof.

[Effects of the Invention] As described above, according to the present invention, even if an image portion having an extremely high luminance level exists locally, the exposure value is not deviated due to the luminance level. Exposure can be easily and effectively adjusted, and even for images with low contrast, the exposure level can be set effectively and appropriately. By changing the position and applying weight to the exposure control, it is possible to obtain appropriate exposure control with emphasis on the main subject, and because the exposure control is performed using the video signal output of the imaging system as it is, Practically significant effects can be obtained, such as setting an appropriate exposure amount very simply and with high accuracy.

[Brief description of the drawings]

FIG. 1 shows an exposure control method according to an embodiment of the present invention. FIG. 1 is a schematic diagram of a main part of an electronic camera configured by applying the embodiment method, and FIG. FIG. 3 is a diagram schematically illustrating a concept of setting a luminance level range, FIG. 3 is a diagram illustrating an example of a captured image including an extreme luminance level portion, and FIG. 4 is a histogram of an image area with respect to light intensity of the image illustrated in FIG. FIG. 5 is a diagram showing an example of sampling points for an image for performing exposure control, and FIG. 6 is a diagram showing a flow of a processing procedure of exposure control according to the embodiment method. is there. 1 ... optical lens system, 2 ... aperture mechanism, 3 ... CC
D, 4 Half mirror, 5 Optical filter, 6
Actuator mechanism 7, Synchronous signal generator 8, Signal processing unit 9, Recording unit 10, Low-pass reactor (LPF) 1,
1,12,13 ... Comparator, 14,15,16 ... Counter, 17 ... CP
U, Th1, Th2, Th3: comparison reference level, H: excessively high luminance level range (first level range), L: excessively low luminance level range (second level range), MH ... Relatively high luminance level range (third level range), ML... Relatively low luminance level range (fourth level range).

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 5/222-5/257 G03B 7/00-7/28

Claims (2)

(57) [Claims] 1. A method for determining a luminance level of a video signal at a plurality of sampling points discretely set in a screen, and determining a luminance level of a sampling point included in a first level range previously set as an excessively high level range. The number and the number of sampling points included in a second level range that has been set in advance as an excessively low level range are determined, and a luminance level range sandwiched between the first level range and the second level range Is divided into a relatively higher third level area and a relatively lower fourth level area, and the number of sampling points included in the third level area and the number of sampling points included in the fourth level area are included. And the number of sampling points included in the first level range,
The exposure amount is controlled so that the number of sampling points included in the second level area becomes equal to each other or the difference between these numbers is reduced. When the sum of the number of sampling points included and the number of sampling points included in the second level area does not reach a predetermined value, the number of sampling points included in the third level area and the fourth level An exposure control method, wherein the exposure amount is controlled so that the number of sampling points included in the area becomes equal to each other or a difference between these numbers is reduced. 2. The exposure control method according to claim 1, wherein the exposure control is weighted by changing the positions of the plurality of sampling points set in the screen.