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Digital Camera Patent Abstract
A digital camera includes: an image sensor that captures an image
of a subject and outputs an image signal; and a control device that
engages the image sensor to capture an image at an exposure value
having been set, makes a decision as to whether or not an overflow
or an underflow deviating from a dynamic range of the image sensor
manifests, calculates an exposure correction quantity with which
the exposure value having been set is to be corrected in order to
reduce the overflow or the underflow based upon results of the decision
and engages the image sensor to capture a new image at an exposure
value having been corrected in correspondence to the exposure correction
quantity.
Digital Camera Patent Claims
1. A digital camera comprising: an image sensor that captures an
image of a subject and outputs an image signal; and a control device
that engages the image sensor to capture an image at an exposure
value having been set, makes a decision as to whether or not an
overflow or an underflow deviating from a dynamic range of the image
sensor manifests, calculates an exposure correction quantity with
which the exposure value having been set is to be corrected in order
to reduce the overflow or the underflow based upon results of the
decision and engages the image sensor to capture a new image at
an exposure value having been corrected in correspondence to the
exposure correction quantity.
2. A digital camera according to claim 1, wherein: the control
device makes a decision as to whether or not an overflow or an underflow
manifests and calculates the exposure correction quantity based
upon the results of the decision by using an image signal of an
image captured before an operation with an image-capturing button,
and engages the image sensor to capture a new image at the corrected
exposure value in response to a shutter release operation.
3. A digital camera according to claim 2, wherein: the control
device makes a decision as to whether or not an overflow or an underflow
manifests and calculates the exposure correction quantity based
upon the results of the decision in response to a photographing
preparation start operation performed prior to the shutter release
operation.
4. A digital camera according to claim 3, wherein: if the shutter
release operation is performed immediately following the photographing
preparation start operation, the control device engages the image
sensor to capture an image at an initial exposure value having been
set without making a decision as to whether or not an overflow or
an underflow manifests and calculating the exposure correction quantity
based upon the results of the decision.
5. A digital camera according to claim 1, wherein: the control
device ascertains an overflow frequency count and an underflow frequency
count, determines a direction for exposure correction based upon
the overflow frequency count and the underflow frequency count having
been ascertained and determines the exposure correction quantity
by taking into consideration the direction.
6. A digital camera according to claim 5, wherein: the control
device sets a plurality of exposure correction quantities as candidates
in correspondence to the direction for the exposure correction having
been determined, corrects the exposure value having been set individually
in correspondence to each of the plurality of exposure correction
quantities having been set as the candidates, engages the image
sensor to capture a plurality of images each at one of a plurality
of exposure values resulting from correction, selects an optimal
exposure correction quantity among the candidates by analyzing a
plurality of image signals each corresponding to one of the plurality
of images having been captured and engages the image sensor to capture
a new image at an exposure value corrected in correspondence to
the optimal exposure correction quantity.
7. A digital camera according to claim 6, wherein: the control
device engages the image sensor to capture the plurality of images
in succession, stores an image signal each time an image is captured,
and analyzes the plurality of image signals having been stored and
selects the optimal exposure correction quantity after the plurality
of images have been captured.
8. A digital camera according to claim 1, wherein: the control
device ascertains a brightness frequency distribution in the image
signal based upon the image signal and makes a decision based upon
the brightness frequency distribution having been ascertained as
to whether or not an overflow or an underflow deviating from the
dynamic range of the image sensor manifests.
9. A digital camera according to claim 2, wherein: the control
device records into a recording medium image data based upon a signal
output from the image sensor in response to the shutter release
operation.
10. A digital camera according to claim 2, wherein: the shutter
release operation is performed by fully pressing down on a shutter
release button.
11. A digital camera according to claim 3, wherein: the photographing
preparation start operation is performed by pressing a shutter release
button halfway down.
12. A digital camera according to claim 1, wherein: the image sensor
is constituted with a plurality of pixels and outputs a plurality
of signals each corresponding to one of the pixels as the image
signal.
13. A digital camera according to claim 1, wherein: the overflow
is a signal that causes white clipping in an image and the underflow
is a signal that causes black clipping in an image.
14. A digital camera comprising: an image sensor that is constituted
with a plurality of pixels, captures an image of a subject and outputs
a plurality of signals each corresponding to one of the pixels;
and a control device that controls an image-capturing operation
at the image sensor, wherein: the control device controls the image
sensor so as to capture a subject image at an exposure value having
been set, counts a number of signals indicating values exceeding
a predetermined upper limit value and a number of signals indicating
values under a predetermined lower limit value among the plurality
of signals output by capturing an image at the exposure value having
been set, corrects the exposure value having been set based upon
the number of signals indicating values exceeding the predetermined
upper limit value and the number of signals indicating values under
the predetermined lower limit value having been counted and controls
the image sensor so as to capture a subject image at the corrected
exposure value.
15. A digital camera according to claim 14, wherein: the control
device corrects the exposure value having been set toward an under-exposure
side if signals with values exceeding the predetermined upper limit
alone have been counted or the number of signals indicating values
exceeding the predetermined upper limit value is greater than the
number of signals indicating values under the predetermined lower
limit value, and corrects the exposure value having been set toward
an over-exposure side if signals with values under the predetermined
lower limit value alone have been counted or the number of signals
indicating values under the predetermined lower limit value is greater
than the number of signals indicating values exceeding the predetermined
upper limit value.
16. A digital camera according to claim 14, wherein: the control
device determines a direction for exposure correction with regard
to the exposure value having been set based upon the number of signals
indicating values exceeding the predetermined upper limit value
and the number of signals indicating values under the predetermined
lower limit value having been counted, selects a plurality of correction
values along the direction for exposure correction having been determined,
corrects the exposure value having been set individually by using
each of the plurality of correction values having been selected,
controls the image sensor so as to capture subject images each at
one of a plurality of corrected exposure values, determines an optimal
corrected exposure value among the plurality of corrected exposure
values by analyzing a plurality of signal sets each output as an
image is captured at one of the plurality of corrected exposure
values, and controls the image sensor so as to capture a subject
image at the optimal corrected exposure value having been determined.
17. A digital camera according to claim 16, wherein: the control
device counts signals indicating values exceeding the predetermined
upper limit value and signals indicating values under the predetermined
lower limit value among the plurality of signals output as an image
is captured in correspondence to each of the plurality of signal
sets, and determines the optimal corrected exposure value corresponding
to a signal set with a smallest number of signals indicating values
exceeding the predetermined upper limit value or a smallest number
of signals indicating values under the predetermined lower limit
value.
18. A digital camera according to claim 14, further comprising:
a recording device that records into a recording medium image data
based upon a plurality of signals output by the image sensor, wherein:
the control device records into the recording medium image data
based upon a plurality of signals output by capturing a subject
image at the corrected exposure value without recording into the
recording medium image data based upon a plurality of signals output
as the image is captured at the exposure value having been set.
19. A digital camera according to claim 16, further comprising:
a recording device that records into a recording medium image data
based upon a plurality of signals output by the image sensor, wherein:
the control device records into the recording medium image data
based upon a plurality of signals output as a subject image is captured
at the optimal corrected exposure value having been determined without
recording into the recording medium image data based upon a plurality
of signals output by capturing an image at the exposure value having
been set or a plurality of sets of image data corresponding to the
plurality of signal sets each output as an image is captured at
one of the plurality of corrected exposure values.
Digital Camera Patent Description
[0001] The disclosures of the following priority application are
herein incorporated by reference:
Japanese Patent Application No. 2004-343539 filed Nov. 29, 2004
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a digital camera capable
of producing images less likely to include white areas or black
areas attributable to over-exposure or under-exposure.
[0004] 2. Description of Related Art
[0005] In a digital still camera, a subject light flux having been
transmitted through a photographic lens is received at an image
sensor such as a CCD, the received light flux undergoes photoelectric
conversion, an image is obtained based upon the output resulting
from the photoelectric conversion, and the image signal then undergoes
various corrections and is finally recorded into an image recording
medium such as a memory card. The digital still camera may include
a liquid crystal monitor at which images can be displayed, and in
such a digital still camera, the monitor can be used as a viewfinder
by sequentially updating the display at the liquid crystal monitor
with continuously captured images. In addition, the digital still
camera may be capable of displaying at the monitor a graph indicating
the brightness frequency distribution in an image, i.e., a histogram
(see, for instance, Japanese Laid Open Patent Publication No. H7-38801).
SUMMARY OF THE INVENTION
[0006] Before capturing an image, exposure values (an aperture
value and/or a shutter speed) is set in the camera so as to photograph
the main subject with the optimal exposure. However, when photographing
a scene in which there is a significant difference between the brightness
of the main subject and the brightness of the background, the brightness
in the background area may deviate from the dynamic range of the
image sensor, resulting in the appearance of white areas due to
over-exposure or black areas due to under-exposure. While the appearance
of such areas can be reduced by the photographer by performing exposure
correction, the photographer needs to have a great deal of experience
to be able to select the optimal exposure correction quantity. While
the photographer is able to set the exposure correction quantity
by checking the histogram on a camera capable of providing real-time
display of the histogram, such an exposure correction operation
is complicated and novice users are likely to find it unsatisfactory.
[0007] According to the 1st aspect of the invention, a digital
camera comprises: an image sensor that captures an image of a subject
and outputs an image signal; and a control device that engages the
image sensor to capture an image at an exposure value having been
set, makes a decision as to whether or not an overflow or an underflow
deviating from a dynamic range of the image sensor manifests, calculates
an exposure correction quantity with which the exposure value having
been set is to be corrected in order to reduce the overflow or the
underflow based upon results of the decision and engages the image
sensor to capture a new image at an exposure value having been corrected
in correspondence to the exposure correction quantity.
[0008] According to the 2nd aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the control
device makes a decision as to whether or not an overflow or an underflow
manifests and calculates the exposure correction quantity based
upon the results of the decision by using an image signal of an
image captured before an operation with an image-capturing button,
and engages the image sensor to capture a new image at the corrected
exposure value in response to a shutter release operation.
[0009] According to the 3rd aspect of the invention, in the digital
camera according to the 2nd aspect, it is preferred that the control
device makes a decision as to whether or not an overflow or an underflow
manifests and calculates the exposure correction quantity based
upon the results of the decision in response to a photographing
preparation start operation performed prior to the shutter release
operation.
[0010] According to the 4th aspect of the invention, in the digital
camera according to the 3rd aspect, it is preferred that if the
shutter release operation is performed immediately following the
photographing preparation start operation, the control device engages
the image sensor to capture an image at an initial exposure value
having been set without making a decision as to whether or not an
overflow or an underflow manifests and calculating the exposure
correction quantity based upon the results of the decision.
[0011] According to the 5th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the control
device ascertains an overflow frequency count and an underflow frequency
count, determines a direction for exposure correction based upon
the overflow frequency count and the underflow frequency count having
been ascertained and determines the exposure correction quantity
by taking into consideration the direction.
[0012] According to the 6th aspect of the invention, in the digital
camera according to the 5th aspect, it is preferred that the control
device sets a plurality of exposure correction quantities as candidates
in correspondence to the direction for the exposure correction having
been determined, corrects the exposure value having been set individually
in correspondence to each of the plurality of exposure correction
quantities having been set as the candidates, engages the image
sensor to capture a plurality of images each at one of a plurality
of exposure values resulting from correction, selects an optimal
exposure correction quantity among the candidates by analyzing a
plurality of image signals each corresponding to one of the plurality
of images having been captured and engages the image sensor to capture
a new image at an exposure value corrected in correspondence to
the optimal exposure correction quantity.
[0013] According to the 7th aspect of the invention, in the digital
camera according to the 6th aspect, it is preferred that the control
device engages the image sensor to capture the plurality of images
in succession, stores an image signal each time an image is captured,
and analyzes the plurality of image signals having been stored and
selects the optimal exposure correction quantity after the plurality
of images have been captured.
[0014] According to the 8th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the control
device ascertains a brightness frequency distribution in the image
signal based upon the image signal and makes a decision based upon
the brightness frequency distribution having been ascertained as
to whether or not an overflow or an underflow deviating from the
dynamic range of the image sensor manifests.
[0015] According to the 9th aspect of the invention, in the digital
camera according to the 2nd aspect, it is preferred that the control
device records into a recording medium image data based upon a signal
output from the image sensor in response to the shutter release
operation.
[0016] According to the 10th aspect of the invention, in the digital
camera according to the 2nd aspect, it is preferred that the shutter
release operation is performed by fully pressing down on a shutter
release button.
[0017] According to the 11th aspect of the invention, in the digital
camera according to the 3rd aspect, it is preferred that the photographing
preparation start operation is performed by pressing a shutter release
button halfway down.
[0018] According to the 12th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the image
sensor is constituted with a plurality of pixels and outputs a plurality
of signals each corresponding to one of the pixels as the image
signal.
[0019] According to the 13th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the overflow
is a signal that causes white clipping in an image and the underflow
is a signal that causes black clipping in an image.
[0020] According to the 14th aspect of the invention, a digital
camera comprises: an image sensor that is constituted with a plurality
of pixels, captures an image of a subject and outputs a plurality
of signals each corresponding to one of the pixels; and a control
device that controls an image-capturing operation at the image sensor.
And the control device controls the image sensor so as to capture
a subject image at an exposure value having been set, counts a number
of signals indicating values exceeding a predetermined upper limit
value and a number of signals indicating values under a predetermined
lower limit value among the plurality of signals output by capturing
an image at the exposure value having been set, corrects the exposure
value having been set based upon the number of signals indicating
values exceeding the predetermined upper limit value and the number
of signals indicating values under the predetermined lower limit
value having been counted and controls the image sensor so as to
capture a subject image at the corrected exposure value.
[0021] According to the 15th aspect of the invention, in the digital
camera according to the 14th aspect, it is preferred that the control
device corrects the exposure value having been set toward an under-exposure
side if signals with values exceeding the predetermined upper limit
alone have been counted or the number of signals indicating values
exceeding the predetermined upper limit value is greater than the
number of signals indicating values under the predetermined lower
limit value, and corrects the exposure value having been set toward
an over-exposure side if signals with values under the predetermined
lower limit value alone have been counted or the number of signals
indicating values under the predetermined lower limit value is greater
than the number of signals indicating values exceeding the predetermined
upper limit value.
[0022] According to the 16th aspect of the invention, in the digital
camera according to the 14th aspect, it is preferred that the control
device determines a direction for exposure correction with regard
to the exposure value having been set based upon the number of signals
indicating values exceeding the predetermined upper limit value
and the number of signals indicating values under the predetermined
lower limit value having been counted, selects a plurality of correction
values along the direction for exposure correction having been determined,
corrects the exposure value having been set individually by using
each of the plurality of correction values having been selected,
controls the image sensor so as to capture subject images each at
one of a plurality of corrected exposure values, determines an optimal
corrected exposure value among the plurality of corrected exposure
values by analyzing a plurality of signal sets each output as an
image is captured at one of the plurality of corrected exposure
values, and controls the image sensor so as to capture a subject
image at the optimal corrected exposure value having been determined.
[0023] According to the 17th aspect of the invention, in the digital
camera according to the 16th aspect, it is preferred that the control
device counts signals indicating values exceeding the predetermined
upper limit value and signals indicating values under the predetermined
lower limit value among the plurality of signals output as an image
is captured in correspondence to each of the plurality of signal
sets, and determines the optimal corrected exposure value corresponding
to a signal set with a smallest number of signals indicating values
exceeding the predetermined upper limit value or a smallest number
of signals indicating values under the predetermined lower limit
value.
[0024] According to the 18th aspect of the invention, in the digital
camera according to the 14th aspect, it is preferred that: there
is further provided a recording device that records into a recording
medium image data based upon a plurality of signals output by the
image sensor; and the control device records into the recording
medium image data based upon a plurality of signals output by capturing
a subject image at the corrected exposure value without recording
into the recording medium image data based upon a plurality of signals
output as the image is captured at the exposure value having been
set.
[0025] According to the 19th aspect of the invention, in the digital
camera according to the 16th aspect, it is preferred that: there
is further provided a recording device that records into a recording
medium image data based upon a plurality of signals output by the
image sensor; and the control device records into the recording
medium image data based upon a plurality of signals output as a
subject image is captured at the optimal corrected exposure value
having been determined without recording into the recording medium
image data based upon a plurality of signals output by capturing
an image at the exposure value having been set or a plurality of
sets of image data corresponding to the plurality of signal sets
each output as an image is captured at one of the plurality of corrected
exposure values.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a control block diagram related to the camera
achieved in an embodiment of the present invention;
[0027] FIG. 2 presents a basic flowchart of camera operations;
[0028] FIG. 3 presents a detailed flowchart of the exposure value
determination processing, which includes automatic exposure correction
control;
[0029] FIG. 4 presents a flowchart in continuation from FIG. 3;
[0030] FIG. 5 is a time chart indicating the timing with which
individual operations are executed when determining the exposure
correction quantity;
[0031] FIG. 6 is a time chart representing an example of the automatic
exposure correction processing in which each image is analyzed as
it is captured; and
[0032] FIG. 7 is a time chart representing another example of the
automatic exposure correction processing, in which images are analyzed
after all the images are captured.
DESCRIPTION OF PREFERRED EMBODIMENT
[0033] In reference to FIGS. 1 to 7, an embodiment of the present
invention is explained.
[0034] FIG. 1 is a block diagram of the digital still camera according
to the present invention. An image is formed at the light receiving
surface of an image sensor (an image-capturing element or device)
2 constituted with a plurality of pixels, such as a CCD, with a
subject light flux having been transmitted through a photographic
lens 1. The image sensor 2 captures the image and outputs an image
signal (electrical signal) constituted with a plurality of pixel
signals each corresponding to one of the plurality of pixels and
indicating the intensity of the light in the subject image having
been formed. The image signal, constituted with the plurality of
pixel signals, may be otherwise referred to as a signal set. The
image signal is converted to a digital signal at an A/D converter
3, the digital signal then undergoes various types of image processing
at a signal processing circuit 4 and thus, image data are generated.
The image data are temporarily stored into a buffer memory 5 and
are recorded into a memory card MC via a recording circuit 6. The
image data may be compressed in a predetermined compression format
at an image compression circuit 7 as necessary before they are recorded.
The individual circuits described above are controlled by a CPU
8.
[0035] The photographic field is photometered by a photometering
circuit 9 and the results of the photometering operation are input
to the CPU 8 before an image is captured. The CPU 8 executes an
exposure calculation based upon the photometering results, the ISO
sensitivity level and the like and thus determines exposure values
(an aperture value and a shutter speed) that will allow the subject
to be photographed with the optimal exposure. Once the exposure
values are determined, the aperture (not shown) is adjusted based
upon the aperture value and the image is captured over an exposure
period the length of which corresponds to the shutter speed. At
a liquid crystal monitor 10, which includes a liquid crystal screen,
a liquid crystal drive unit, a backlight, a backlight control unit
and the like, an image is displayed based upon image data or other
types of information are displayed at the screen thereof.
[0036] A halfway press switch SW1 that comes on in response to
a halfway press operation (photographing preparation start operation)
of a shutter release button, a shutter release switch SW2 that comes
on in response to a full press operation (a shutter release operation)
of the shutter release button, other operation members 11 and a
memory 12 constituted with a ROM, a RAM and the like for storing
information needed for executing control are connected to the CPU
8. The full press operation (shutter release operation) of the shutter
release button, which constitutes an instruction for an actual image-capturing
(photographing) operation and also constitutes an instruction for
recording the image data obtained through the image-capturing operation
into the memory card MC or the like, may be otherwise referred to
as an image-capturing instruction operation or an image data recording
instruction operation. Likewise, the shutter release button may
be referred to as an image-capturing instruction button or an image
data recording instruction button.
[0037] As the power to the camera structured as described above
is turned on, the photometering operation, the exposure calculation
and the image-capturing operation described above are repeatedly
executed and images each obtained through a single cycle of operations
are successively displayed at a liquid crystal monitor 10. The display
mode in which images successively captured are sequentially displayed,
thereby constantly updating the display is referred to as a through
image display mode. In this mode, the photographer is able to decide
on the image composition by checking the through images. It is to
be noted that through images are not recorded into the memory card
MC.
[0038] The automatic exposure correction control, which is the
feature that characterizes the embodiment, is now explained. As
the shutter release button is pressed halfway down (SW1 ON) in the
state described above, the CPU 8 determines an exposure correction
quantity based upon which the extent of the appearance of white
areas due to over-exposure or black areas due to under-exposure
is to be reduced while continuously displaying through images. More
specifically, it obtains a through image brightness (or luminance)
frequency distribution (histogram) and, based upon the distribution,
it determines whether or not an overflow or an underflow manifests,
i.e., whether or not there are any pixel signals deviating from
the dynamic range of the image sensor 2 toward the over-exposure
side or the under-exposure side. Since an overflow will result in
the appearance of white areas (white-clipping or white saturation)
and an underflow will result in the appearance of black areas (black-clipping
or black saturation), the exposure will need to be corrected toward
the under-exposure side in order to reduce the appearance of white
areas if an overflow manifests, whereas the exposure will need to
be corrected toward the over-exposure side in order to reduce the
appearance of black areas if an underflow manifests.
[0039] Accordingly, if an overflow or an underflow is detected,
an image is captured a plurality of times by correcting the initial
exposure value to varying extents, brightness frequency distributions
are obtained each based upon a given set of image-capturing results
(each image) and the exposure correction quantity having been set
for the image with the least overflow or underflow frequency count
is extracted in the embodiment. If there is a plurality of images
with the least overflow frequency count, the exposure correction
quantity with the smallest absolute value is extracted, so as to
stay as close as possible to the initial exposure value while reducing
the extent of the appearance of white areas due to over-exposure
or black areas due to under-exposure. The initial exposure value
is corrected based upon the exposure correction quantity thus extracted
and the exposure value is reset. Subsequently, in response to a
shutter release operation (SW2 ON), an image-capturing operation
is executed at the adjusted exposure value and the resulting image
data are recorded into the memory card MC.
[0040] Through the automatic exposure correction control described
above, an image manifesting a lower extent of appearance of white
areas due to over-exposure or black areas due to under-exposure
is ultimately recorded. In addition, since the camera automatically
decides whether to execute white area due to over-exposure reduction
processing or the black area due to under-exposure reduction processing
and obtains the optimal exposure correction quantity for either
the white area due to over-exposure reduction or black area due
to under-exposure reduction, even a novice photographer is likely
to be able to produce an optimally exposed photograph simply by
performing a photographing operation without having to concern himself
with exposure. Furthermore, since the processing for determining
the exposure correction quantity is executed during the halfway
press operation, a time lag to elapse between the shutter release
operation and the image recording operation is exactly the same
as that elapsing during a regular photographing operation, thereby
assuring stress-free handling of the camera for the photographer.
[0041] As an alternative, a plurality of images may be captured
at varying exposure values in response to a shutter release operation
and an image manifesting the least extent of appearance of white
areas due to over-exposure or black areas due to under-exposure
among the plurality of images may be selected and recorded. However,
this alternative is not ideal since the time lag to elapse between
the shutter release operation and the recording completion is bound
to be protracted. In this case, since the photographer cannot take
the next picture immediately and thus, he may miss a good photo
opportunity.
[0042] In reference to FIGS. 2 to 4, each showing a processing
procedure for executing the control described above in software,
the automatic exposure correction operations are explained in further
detail.
[0043] FIG. 2 shows the basic camera operations executed based
upon a program. As the power to the camera is turned on, the program
is started up by the CPU 8, and in step S001, power ON processing
is executed. As the power ON processing is executed, through-image-capturing,
executed over 1/30 sec time intervals, starts. In step S002, the
liquid crystal monitor 10 is turned on and in step S003, through
image display at the liquid crystal monitor 10 starts.
[0044] In step S101, AE processing, constituted with the photometering
operation and the exposure calculation explained earlier, is executed
and the exposure values are determined. In step S102, AF processing
is executed and the photographic lens 1 is driven to the focus match
position based upon the results of focal point detection executed
by a range finding device (not shown). The processing in steps S101
and S102 is executed repeatedly until it is decided that a halfway
press operation has been performed in step S103 or until power OFF
is indicated in step S104. Each set of the AF/AE results is reflected
in the corresponding through image on the through image display
having been initially started in step S003. If it is verified that
power OFF has been indicated, the liquid crystal monitor 10 is turned
off in step S201 and power OFF processing is executed in step S202
before the processing sequence ends.
[0045] If, on the other hand, it is decided in step S103 that a
halfway press operation has been performed (SW1 ON), an AF lock
is set in step S301 to hold the photographic lens in the focus matched
position, and then, exposure determination processing is executed
in step S302. The exposure determination processing executed at
this time, which includes the automatic setting processing for automatically
setting the exposure correction quantity explained earlier, is to
be described in detail later.
[0046] Following step S302, the operation follows through the loop
made up with steps S401 and 402 to wait for a shutter release operation
or a halfway press operation clearance. If it is decided that the
halfway press operation has been cleared (SW1 OFF), the operation
returns to step S101 and the processing described above is repeatedly
executed. If, on the other hand, it is decided that a shutter release
operation has been performed (SW2 ON), photographing processing
is executed in step S501. This photographing processing includes
aperture control executed based upon the aperture value having been
determined in step S302 and an image-capturing operation executed
based upon the shutter speed having been determined in step S302.
In step S502, the through image display is cleared, and instead,
the image having been obtained in step S501 is brought up on display
at the liquid crystal monitor 10. Then, in step S503, the image
data are recorded into the memory card MC.
[0047] In step S504, the operation waits for the shutter release
operation to be cleared and, once the shutter release operation
is cleared, it waits for the halfway press operation to be cleared
in step S505. Once the halfway press operation is cleared, the monitor
display is reset to the through image display mode in step S506
before the operation returns to step S101.
[0048] Next, the exposure determination processing executed in
step S302 is explained in detail in reference to FIGS. 3 and 4.
[0049] In step S1001, the photometering operation and the exposure
calculation are executed and the aperture value and the shutter
speed are thus determined through the arithmetic operation. These
values are stored as the exposure value settings (the aperture value
setting and the shutter speed setting). In step S1002, the aperture
is controlled based upon the aperture value setting. In this state,
an image is captured at the shutter speed having been set, and the
resulting image is displayed as a through image.
[0050] In step S1002A, a decision is made as to whether or not
the camera is currently set in an automatic exposure correction
mode. If it is decided that the camera is not currently set in the
automatic exposure correction mode, which can be selected/cleared
via a specific operation member or via a menu setting screen, the
operation proceeds to step S1401 in FIG. 4 to effect an AE lock.
In this case, the automatic exposure correction is not executed
and the exposure value settings (the values calculated in step S1001)
are fixed as the final exposure values. If, on the other hand, it
is decided that the camera is currently set in the automatic exposure
correction mode, a decision is made in step S1003 as to whether
or not a shutter release operation has been performed (SW2 ON/OFF),
and the operation proceeds to step S1004 to ascertain the brightness
frequency distribution in the through image currently on display
if it is decided that no shutter release operation has been performed.
Then, based upon the brightness frequency distribution, an overflow
frequency count and an underflow frequency count are determined.
[0051] In steps S1005A through S1005D, overflow/underflow decision-making
processing is executed based upon the brightness frequency distribution
ascertained in step S1004. If it is decided in step S1005A that
neither an overflow nor an underflow has manifested the operation
proceeds to step S1401 to effect an AE lock, since there is no need
for exposure correction. In this case, too, the exposure value settings
are fixed as the final exposure value.
[0052] If an overflow alone is detected, a white area reduction
exposure setting mode is selected in step S1101, whereas if an underflow
alone is detected, a black area reduction exposure setting mode
is selected in step S1201. If both an overflow and an underflow
are detected, their frequency counts are compared. The operation
proceeds to step S1101 if the overflow count is equal to or greater
than the underflow count, but the operation proceeds to step S1201
otherwise.
[0053] Namely, through the processing executed in steps S1005A
through S1005D, the direction of the correction is determined, i.e.,
a decision is made whether to correct the exposure toward the over-exposure
side or toward the under-exposure side.
[0054] In the processing, if an overflow and an underflow manifest
to substantially equal extents, the correction priority is given
to the reduction of white areas due to over-exposure, since human
visual perception characteristics are such that we tend to find
white areas due to over-exposure more visually disturbing than black
areas due to under-exposure. For this reason, even when the underflow
frequency count is greater than the overflow frequency count, the
operation may proceed to execute the white area reduction exposure
setting mode processing if the overflow frequency count is equal
to or greater than a predetermined value. In addition, overflow/underflow
manifesting to slight extents may be regarded as no overflow/underflow
if they are within allowable ranges.
[0055] Once the white area reduction exposure setting mode is set,
exposure correction quantities A through E are set in step S1102.
The exposure correction quantities A through E are set in stages
relative to the exposure value setting so that A=+0.5:B=0:C=-0.5:D=-1.0:E=-1.5.
Since the object is to reduce the appearance of white areas due
to over-exposure, the correction range is mainly set over the under-exposure
side. In this case, the exposure correction quantities C, D and
E alone may be set. If, on the other hand, the black area reduction
exposure setting mode is selected, exposure correction quantities
A through E are set in a similar manner in step S1202. The exposure
correction quantities A through E are set so that A =+1.5:B=+1.0:C=+0.5:D=0:E=-0.5
in this mode. Since the object is to reduce the appearance of black
areas due to under-exposure, the correction range is mainly set
over the over-exposure side. In this case, the exposure correction
quantities A, B and C alone may be set.
[0056] Subsequently, through the processing executed in steps S1301
through S1316, the exposure correction values A through E are sequentially
selected, an image is captured at the exposure value corrected in
correspondence to each selected correction quantity and the resulting
image is displayed as a through image. The "V synchronization
(vertical synchronization)" indicates the length of time (=
1/30 sec) of time intervals over which the through images are captured.
The exposure correction is executed by adjusting the shutter speed
in correspondence to the values A through E while the aperture value
remains fixed at the aperture value setting. It is to be noted that
the exposure correction may instead be executed by adjusting the
aperture value.
[0057] FIG. 5 is a time chart of the operations executed through
the processing in steps S1301 through S1316. As the first exposure
correction quantity A is set, a through image is exposed (captured)
at the exposure value corrected based upon the exposure correction
quantity A with the subsequent V synchronous timing, and in synchronization
with this through image exposure, the next exposure correction quantity
B is set. With the next V synchronous timing, the image obtained
through the through image exposure (corresponding to the correction
quantity A) is read, and in synchronization with the image read,
a through image is exposed (in correspondence to the correction
quantity B) and the exposure correction quantity C is set. Then,
with the next V synchronous timing, the through image having been
read is displayed (corresponding to the correction quantity A),
the image (corresponding to the correction quantity B) is read,
a through image (corresponding to the correction quantity C) is
exposed and the exposure correction quantity D is set. Thus, the
operations are executed with the V synchronous timing until the
last through image (corresponding to the correction quantity E)
is displayed, and the brightness frequency distributions are extracted
in correspondence to the individual through images (in correspondence
to the individual correction quantities A through E).
[0058] In step S1317, the five brightness frequency distributions
are analyzed and the optimal exposure correction quantity is determined.
Namely, if the white area reduction exposure setting mode is currently
set, the exposure correction quantity corresponding to the image
with the least overflow frequency count among the exposure correction
quantities A through E is extracted. If, on the other hand, the
black area reduction exposure setting mode is currently set, the
exposure correction quantity corresponding to the image with the
least underflow frequency count among the exposure correction quantities
A through E is extracted. If there is a plurality of images with
the least overflow/underflow frequency count, the exposure correction
value with the smallest absolute value should be extracted so as
to capture an image at an exposure value closer to the exposure
value setting. The exposure correction quantity thus extracted is
the optimal exposure correction quantity and the exposure value
corrected based upon the optimal exposure correction quantity is
the final exposure value. In step S1318, a through image captured
at the final exposure value is displayed, and in step S1401, the
final exposure value is held through the AE lock before the operation
makes a return.
[0059] The processing described above is executed after a negative
decision is made in step S1003. If an affirmative decision is made
in step S1003, the operation directly proceeds to step S1401. An
affirmative decision made in step S1003 indicates that a shutter
release operation has been performed in immediate succession to
a halfway press operation, i.e., the halfway press operation has
directly shifted into a full press operation without holding the
halfway press state. In such a case, it is judged that the photographer
wishes to take a photograph immediately and, accordingly, the operation
skips the processing in step S1004 and subsequent steps even if
the automatic exposure correction mode is currently set. Instead,
an AE lock is set to fix the exposure value settings as the final
exposure values in preparation for a photographing operation. As
a result, the likelihood of catching a good photo opportunity is
increased.
[0060] FIGS. 6 and 7 are time charts of two different methods of
automatic exposure correction processing.
[0061] In the method shown in FIG. 6, the brightness frequency
distribution is analyzed each time an image is captured, and the
individual operations, i.e., photographing (image-capturing), image
read, image processing, display and analysis, are each executed
with V synchronous timing. The numerals attached to the letter V
each indicate the number of operations having been executed at the
corresponding time point. In this method, the next image is captured
after the preceding image has been analyzed, and thus, 20V (V= 1/30sec)=667
ms elapses before analyzing of the fifth image is completed, i.e.
before the optimal exposure correction value is determined. However,
since images having already been captured do not need to be saved,
the buffer memory does not need to have a large capacity.
[0062] Higher-speed processing is achieved through the method shown
in FIG. 7, by capturing images successively, storing the individual
images as they are captured and obtaining and analyzing their brightness
frequency distributions after all the images are stored. In this
case, the buffer memory needs to have a large enough capacity to
save at least five images. However, since each image can be captured
without having to wait for the analysis of the preceding image to
be completed, the optimal-exposure correction value can be determined
in 8V=267 ms. Thus, the automatic exposure correction control is
executed too fast to be noticed by the photographer.
[0063] It is to be noted that while five images are captured to
determine the optimal exposure correction quantity in the example
explained above, the image-capturing operation does not need to
be executed five times. In addition, the processing may be simplified
by skipping the image-capturing operations and univocally determining
the exposure correction quantity in correspondence to the overflow
(or underflow) frequency count. For instance, if the overflow (or
underflow) frequency count is less than a predetermined value, the
exposure value setting may be used without correction, the exposure
value setting may be corrected by -1 stage (or +1 stage) if the
frequency count is equal to or greater than a predetermined value
within a specific range, and the exposure value setting may be corrected
by -2 stage (or +2 stage) if the frequency count is beyond the range.
Through this method, too, a certain extent of correction effect
is achieved. Furthermore, it is desirable to set certain limits
with regard to the optimal exposure correction quantity. If the
exposure value setting is corrected by an excessive extent, the
areas to be correctly exposed (in particular the main subject) may
become greatly under-exposed or greatly over-exposed while the appearance
of white areas due to over-exposure or black areas due to under-exposure
is reduced.
[0064] In addition, while the exposure correction quantity is selected
in response to a halfway press operation, they may be obtained in
response to a shutter release operation instead. While the shutter
release time lag may become protracted in the latter case, the appearance
of white areas due to over-exposure or black areas due to under-exposure
can be effectively eliminated without the photographer being particularly
aware of the process.
[0065] An explanation is given above in reference to the embodiment
on an example in which the brightness frequency distribution is
ascertained and a decision is made as to whether or not an overflow
or an underflow manifests, i.e., whether or not there are pixel
signals deviating from the dynamic range of the image sensor 2 toward
the over-exposure side or the under-exposure side. However, it is
not strictly necessary to ascertain the brightness frequency distribution.
A decision as to whether or not an overflow or an underflow manifests
may be made simply by judging whether or not the image signal corresponding
to each pixel indicates a value equal to or greater than a predetermined
over-exposure side threshold value (an upper limit value) or judging
whether or not the value is equal to or less than a predetermined
under-exposure side threshold value (a lower limit value) Then,
the overflow frequency count can be determined by counting the image
signals with values equal to or greater than the predetermined over-exposure
side threshold value (upper limit value) and the underflow frequency
count can be determined by counting image signals with values equal
to or less than the predetermined under-exposure side threshold
value (lower limit value).
[0066] While an explanation is given above in reference to the
embodiment on an example in which the present invention is adopted
in a digital still camera, the present invention is not limited
to this example. The present invention may instead be adopted in
conjunction with images captured on a camera mounted at a portable
telephone. In addition, the present invention may be adopted in
conjunction with still images photographed with a video camera or
the like. Namely, the present invention may be adopted in all types
of apparatuses in which a subject image is captured at an image
sensor by setting a specific exposure value. Such cameras and apparatuses
that invariably handle digital image signals may be regarded as
digital cameras.
[0067] The above described embodiment is an example and various
modifications can be made without departing from the spirit and
scope of the invention.
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