U.S. patent application number 11/285186 was filed with the patent office on 2006-08-24 for digital camera.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Norikazu Yokonuma.
Application Number | 20060187315 11/285186 |
Document ID | / |
Family ID | 36635108 |
Filed Date | 2006-08-24 |
United States Patent
Application |
20060187315 |
Kind Code |
A1 |
Yokonuma; Norikazu |
August 24, 2006 |
Digital camera
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.
Inventors: |
Yokonuma; Norikazu; (Tokyo,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
Nikon Corporation
Tokyo
JP
|
Family ID: |
36635108 |
Appl. No.: |
11/285186 |
Filed: |
November 23, 2005 |
Current U.S.
Class: |
348/229.1 ;
348/333.02; 348/E5.035 |
Current CPC
Class: |
H04N 2101/00 20130101;
H04N 5/2351 20130101 |
Class at
Publication: |
348/229.1 ;
348/333.02 |
International
Class: |
H04N 5/235 20060101
H04N005/235; H04N 5/222 20060101 H04N005/222 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2004 |
JP |
2004-343539 |
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.
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.
* * * * *