Digital Camera Patent Abstract
An exposure controller of a digital camera, using an image pick-up
device, includes a photometering sensor, and a control device. The
control device calculates a first exposure time in accordance with
a photometering value obtained via the photometering sensor, and
performs a pre-exposure in which a sensitive surface of the image
pick-up device is exposed at a second exposure time shorter than
the first exposure time. Thereafter, the control device performs
a main exposure in which the sensitive surface of the image pick-up
device is exposed at a third exposure time obtained by changing
the value of one of the first exposure time and the second exposure
time in accordance with a picture signal output from the image pick-up
device, the picture signal being output when the image pick-up device
is exposed at the second exposure time by the pre-exposure. Digital Camera Patent Claims
What is claimed is:
1. An exposure controller of a digital camera, comprising: a photometering
sensor; and a control device; wherein said control device calculates
a first exposure time in accordance with a photometering value obtained
via said photometering sensor; said control device calculates a
second exposure time shorter than said first exposure time in the
case where said first exposure time is longer than a reference time
duration; said control device performs a pre-exposure in which a
sensitive surface of an image pick-up device of said digital camera
is exposed at said second exposure time to calculate a brightness
value in accordance with a picture signal which is output from said
image pick-up device at said pre-exposure; and said control device
calculates a third exposure time which is to be used at a main exposure,
in which said sensitive surface of said image pick-up device is
exposed to obtain a picture signal which is to be stored in a memory,
by changing the value of one of said first exposure time and said
second exposure time in accordance with said calculated brightness
value.
2. The exposure controller of a digital camera according to claim
1, wherein said reference time duration is a predetermined flash
synchronization speed.
3. The exposure controller of a digital camera according to claim
1, wherein said exposure time is equal to said first exposure time
divided by the Nth power of two (2.sup.N).
4. The exposure controller of a digital camera according to claim
2, wherein said control device calculates said third exposure time
in accordance with an average brightness value of all the pixels
of said image pick-up device which are obtained by said pre-exposure.
5. The exposure controller of a digital camera according to claim
4, wherein said third exposure time is calculated by multiplying
said second exposure time by an exposure compensation factor; wherein
said exposure compensation factor is calculated by dividing a predetermined
value by said average brightness value, and multiplying the result
thereof by the Nth power of two (2.sup.N).
6. The exposure controller of a digital camera according to claim
1, wherein said reference time duration is a longest exposure time
which can just prevent an image formed on said sensitive surface
of said image pick-up device from becoming blurry, caused by hand
movement.
7. The exposure controller of a digital camera according to claim
1, wherein said digital camera is an SLR digital camera.
Digital Camera Patent Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure controller of a digital
camera, more specifically, to improving the pre-exposure operation
in a digital camera which is performed to calculate an optimum exposure
time (shutter speed) at which the main exposure is performed.
2. Description of the Related Art
Conventional digital cameras are provided with a CCD image sensor
(image pick-up device) on which the image is captured. Since the
dynamic range of the CCD image sensor is generally narrow, the shutter
speed (charge accumulating time/exposure time) has to be precisely
determined to capture a clear image. In order to determine the shutter
speed precisely, a method of performing a pre-exposure before the
main exposure is known in the art. Namely, an exposure calculation
is performed to determine an optimum shutter speed in accordance
with a picture signal obtained from the CCD image sensor by the
pre-exposure. Note that in the main exposure, the image captured
on the CCD image sensor is stored in a recording medium such as
a memory.
In the case where the shutter speed for the pre-exposure is relatively
slow due to lack of quantity of light from an object to be photographed,
the time lag between the pre-exposure and the main exposure becomes
large. As a result, electric power consumption necessary to keep
holding each of the quick-return mirror, the shutter, and the diaphragm
of the camera at a predetermined position increases, especially
in a single-lens reflex electronic camera, which is not preferable.
SUMMARY OF THE INVENTION
The primary object of the present invention is to provide an exposure
controller which makes it possible to reduce the time necessary
for performing the pre-exposure before the main exposure to thereby
reduce the time lag between the pre-exposure and the main exposure.
To achieve the object mentioned above, according to an aspect of
the present invention, an exposure controller of a digital camera
is provided, using an image pick-up device on which an image of
an object to be photographed is captured, the exposure controller
including a photometering sensor; and a control device. The control
device calculates a first exposure time in accordance with a photometering
value obtained via the photometering sensor, and performs a pre-exposure
in which a sensitive surface of the image pick-up device is exposed
at a second exposure time shorter than the first exposure time.
Thereafter, the control device performs a main exposure in which
the sensitive surface of the image pick-up device is exposed at
a third exposure time obtained by changing the value of one of the
first exposure time and the second exposure time in accordance with
a picture signal output from the image pick-up device, the picture
signal being output when the image pick-up device is exposed at
the second exposure time by the pre-exposure.
Preferably, the digital camera is an SLR digital camera.
According to another aspect of the present invention, an exposure
controller of a digital camera is provided, including a photometering
sensor, and a control device. The control device performs an exposure
operation to calculate a first exposure time in accordance with
a photometering value obtained via the photometering sensor. The
control device calculates a second exposure time shorter than the
first exposure time in the case where the first exposure time is
longer than a reference time duration. The control device performs
a pre-exposure in which a sensitive surface of an image pick-up
device of the digital camera is exposed at the second exposure time
to calculate a brightness value in accordance with a picture signal
which is output from the image pick-up device at the pre-exposure.
The control device calculates a third exposure time which is to
be used at a main exposure, in which the sensitive surface of the
image pick-up device is exposed to obtain a picture signal which
is to be stored in a memory, by changing the value of one of the
first exposure time and the second exposure time in accordance with
the calculated brightness value.
The reference time duration can be a predetermined flash synchronization
speed (exposure time).
In an embodiment, the second exposure time is equal to the first
exposure time divided by the Nth power of two (2.sup.N)
In an embodiment, the control device calculates the third exposure
time in accordance with an average of brightness values of all pixels
of the image pick-up device which are obtained by the pre-exposure.
In an embodiment, the third exposure time is calculated by multiplying
the second exposure time by an exposure compensation factor, wherein
the exposure compensation factor is calculated by dividing a predetermined
value by the average brightness value, and multiplying the result
thereof by the Nth power of two (2.sup.N).
In an embodiment, the reference time duration is a slowest exposure
time which can just prevent an image formed on the sensitive surface
of the image pick-up device from becoming blurry, caused by hand
movement.
Preferably, the digital camera is an SLR digital camera.
According to another aspect of the present invention, an SLR digital
camera includes a photometering sensor for measuring a object brightness,
a CCD image sensor, a focal plane shutter positioned in front of
the CCD image sensor, and a control device. The control device calculates
a first exposure time in accordance with a photometering value obtained
by the photometering sensor, and drives the focal plane shutter
at a second exposure time shorter than the first exposure time.
Furthermore, the control device subsequently drives the focal plane
shutter at a third exposure time obtained by changing the value
of one of the first exposure time and the second exposure time in
accordance with a picture signal output from the CCD image sensor,
the picture signal being output when the CCD image sensor is exposed
at the second exposure time via the control device.
The present disclosure relates to subject matter contained in Japanese
Patent Application No. 11-203249 (filed on Jul. 16, 1999) which
is expressly incorporated herein by reference in its entirety.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described below in detail with reference
to the accompanying drawings in which:
FIG. 1 is a block diagram of an embodiment of a digital camera
having an exposure controller according to the present invention;
FIG. 2 is a flow chart showing the first half of a photographic
operation control routine performed in the digital camera shown
in FIG. 1; and
FIG. 3 is a flow chart showing the latter half of the photographic
operation control routine.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A digital camera whose essential elements are shown in the form
of blocks in FIG. 1 is an embodiment of an SLR (Single-Lens Reflex)
digital camera having an interchangeable lens 11 and a camera body
(not shown) to which the interchangeable lens 11 is attached.
The interchangeable lens 11, attached to the camera body, is electrically
connected to electrical circuits provided within the camera body
via a set of electrical contacts 12 and a corresponding set of electrical
contacts 13, which are formed on, e.g., each of a lens mount (lens
mount ring) and a body mount (body mount ring) provided on the rear
end of the interchangeable lens 11 and the front of the camera body,
respectively.
The interchangeable lens 11 is provided therein with a front lens
group 14 and a rear lens group 15. The interchangeable lens 11 is
provided between the front lens group 14 and the rear lens group
15 with a diaphragm (iris diaphragm) 16. A lens control circuit
17 provided in the interchangeable lens 11 controls each of the
front and rear lens groups 14 and 15 to move along an optical axis
of the interchangeable lens 11 to adjust the focal point so as to
bring a object into focus. The lens control circuit 17 operates
in accordance with a control signal transmitted from a system controller
(control device) 31 provided in the camera body via the electrical
contacts 12. The diaphragm 16 operates to adjust the aperture size
thereof in accordance with a control signal transmitted from a diaphragm
control circuit 32 provided in the camera body via the electrical
contacts 13. The diaphragm control circuit 32 is connected to the
system controller 31 to be controlled thereby.
The camera body (a mirror box 30) is provided therein behind the
front and rear lens groups 14 and 15 on the optical axis thereof
with a quick-return mirror 21. The quick-return mirror 21 is rotatable
between an inclined initial position (shown in FIG. 1) and a raised
or retracted position (horizontal position). The camera body is
provided above the quick-return mirror 21 with a focusing screen
22. The camera body is provided above the focusing screen 22 with
a pentagonal prism (erecting prism) 23. The camera body is provided
behind the pentagonal prism 23 with an eyepiece 24.
The camera body is provided behind the quick-return mirror 21 with
a focal plane shutter 25. The camera body is provided behind the
focal plane shutter 25 with a filter 26 for filtering infrared rays
and an optical low-pass filter 27, in this order from the focal
plane shutter 25. The camera body is provided behind the optical
low-pass filter 27 with a CCD image sensor (image pick-up device)
33. Accordingly, the quick-return mirror 21, the focal plane shutter
25, the filter 26, the optical low-pass filter 27 and the CCD image
sensor 33 are arranged in this order from the object side (i.e.,
from left to right as viewed in FIG. 1) on the optical axis O of
the interchangeable lens 11.
The quick-return mirror 21 is driven to rotate by a mirror drive
circuit 34 connected to the system controller 31, while the focal
plane shutter 25 is driven to open and shut by a shutter control
circuit 35 connected to the system controller 31. The mirror drive
circuit 34 and the shutter control circuit 35 are each controlled
by the system controller 31.
The quick-return mirror 21 is normally positioned in an inclined
initial position shown in FIG. 1. The quick-return mirror 21 in
the initial position reflects the incident light, which emerges
from the rear end of the rear lens group 15 to enter the quick-return
mirror 21, upwardly towards the pentagonal prism 23. At this time
the focal plane shutter 25 is closed to close a light path which
leads light to the sensitive surface of the CCD image sensor 33.
On the other hand, when a picture is taken (when a photographic
operation is undertaken), the quick-return mirror 21 is driven to
swing up to the raised position thereof by the mirror drive circuit
34 to retreat from an optical path between the rear lens group 15
and the focal plane shutter 25. Immediately after the quick-return
mirror 21 is completely raised, the focal plane shutter 25 is driven
to open by the shutter control circuit 35, so that the light which
enters into the camera body via the interchangeable lens 11 is incident
on the sensitive surface of the CCD image sensor 33 via the filter
26 and the optical low-pass filter 27. Therefore, an image is formed
on the sensitive surface of the CCD image sensors 33 via the front
and rear lens groups 14 and 15, so that the CCD image sensor 33
converts light incident on the sensitive surface thereof into an
electrical signal which corresponds to the image formed on the sensitive
surface of the CCD image sensor 33.
A pulse-signal pattern generator (PPG) 36 is connected to the system
controller 31. The system controller 31 controls the pulse-signal
pattern generator 36 to generate various types of pulse signals.
In accordance with these pulse signals, a CCD drive circuit 37,
an A/D converter 38 and a picture signal processing circuit 39 are
controlled. The operation of the CCD image sensor 33 is controlled
by the CCD drive circuit 37. A picture signal read out of the CCD
image sensor 33 is converted into a corresponding digital signal
by the A/D converter 38. The picture signal processing circuit 39
receives the digital signal from the A/D converter 38 to give a
predetermined image processing to the digital signal. A memory 40
(e.g., a flash memory), having a storing capacity that is a plurality
of times the capacity necessary for storing a picture signal of
a single picture frame (a single image), is connected to the picture
signal processing circuit 39.
A monitor interface 41, a card interface 42 and a PC interface
43 are connected to the picture signal processing circuit 39. The
interfaces 41, 42 and 43 are controlled by the system controller
31. An LCD backlight 45 and an LCD panel 46 are connected to the
monitor interface 41 via an LCD drive circuit 44. The LCD drive
circuit 44 is controlled in accordance with the picture signal read
out from the memory 40 so that an image represented by the picture
signal is indicated on the LCD panel 46. A card connector 47 and
a PC connector 48 are connected to the card interface 42 and the
PC interface 43, respectively. Removable IC memory card (e.g., a
flash memory card) can be connected to the card connector 47. A
personal computer can be detachably connected to the PC connector
48.
An AF sensor 51 and a photometering sensor 52 are connected to
the system controller 31. The AF sensor 51 is a conventional type
of AF sensor (e.g., a phase-difference detection type of AF sensor)
for detecting an amount of defocus. The photometering sensor 52
measures the object brightness to determine an aperture size of
the diaphragm 16 (i.e., an aperture value Av) and a charge accumulating
time (a exposure time) for the CCD image sensor 33 which are used
for the pre-exposure.
A set of operation switches 54 and a state indicating device 55
are connected to the system controller 31. The set of operation
switches 54 include a photometering switch and a release switch
(both not shown). The photometering switch is turned ON to make
the photometering sensor operate immediately after a release button
(not shown) of the digital camera is depressed half way down. Immediately
after the release button is fully depressed, the release switch
is turned ON the quick-return mirror 21 moved up, the focal plane
shutter 25 operates (opens and shuts) at the set exposure time therefor,
and thereafter the quick-return mirror 21 is moved down again. Consequently,
the sensitive surface of the CCD image sensor 33 is exposed, so
that it outputs a picture signal corresponding to the image formed
on the sensitive surface of the CCD image sensor 33. The state indicating
device 55 is provided with an LCD panel (not shown) which indicates
various photographic information on the digital camera. The camera
body of the digital camera is provided thereon with a hot shoe 56
to which an external flash can be attached. The camera body of the
digital camera is provided therein with a flash control circuit
57 which controls the flash attached to the hot shoe 56. The flash
control circuit 57 is controlled by the system controller 31.
FIGS. 2 and 3 show a flow chart showing a photographic operation
control routine performed in the present embodiment of the SLR digital
camera. In the following descriptions of the flow chart shown in
FIGS. 2 and 3, in the pre-exposure operation and the main exposure
operation, the quick-return mirror 21 is moved up, and the focal
plane shutter 25 opens and closes at the set exposure time thereof.
Accordingly, the sensitive surface of the CCD image sensor 33 is
exposed, and thereafter, the quick-return mirror 21 is moved down.
Firstly it is determined at step S101 whether the photometering
switch is ON. The operation at step S101 is repeatedly performed
as long as the photometering switch is OFF. If the photometering
switch is turned ON, an exposure calculation is performed at step
S102. In the exposure calculation, the aperture size of the iris
diaphragm 16 and a first exposure time are calculated in accordance
with the photometering value obtained via the photometering sensor
52.
Subsequently, at step S103 it is determined whether the first exposure
time calculated at step S102 is longer than a predetermined flash
synchronization speed (e.g., 1/100 second). If it is determined
that the first exposure time is longer than the flash synchronization
speed, control proceeds to step S104 at which a parameter N is set
at 3. If it is determined that the first exposure time is equal
to or shorter than the flash synchronization speed, control proceeds
to step S105 at which the parameter N is set at 0 (zero).
After the operation at step S104 or S105, control proceeds to step
S106. At step S106 a second exposure time (a pre-exposure time)
which is equal to the first exposure time divided by the Nth power
of two (2.sup.N) is calculated (wherein "N" represents
the aforementioned parameter set at step S104 or step S105). Therefore,
if the first exposure time is 1 second on condition that the flash
synchronization speed is 1/100 second, the operation at step S104
is performed, so that the second exposure time is determined to
be 1/8 second. If the first exposure time is 1/125 second on condition
that the flash synchronization speed (exposure time) is 1/100 second,
the operation at step S105 is performed, so that the second exposure
time is determined to be 1/125 second (i.e. the first exposure time
is same as the second exposure time).
Thereafter, it is determined at step S107 whether the release switch
is ON. If it is determined that the release switch is not ON, control
returns to step S101. If it is determined ON, control proceeds to
step S108. At step S108 the pre-exposure is performed at the second
exposure time to make the sensitive surface of the CCD image sensor
33 exposed. Subsequently, at step S109, the picture signal obtained
by the pre-exposure is read out of the CCD image sensor 33. The
CCD image sensor 33 is made up of thousands of individual photodiodes
(also known as picture elements or pixels) arranged in a two-dimensional
matrix (e.g., 600.times.400 matrix). A picture signal output from
the CCD image sensor 33 is made up of pixel data obtained by each
pixel of the CCD image sensor 33.
After the operation at step S109, control proceeds to step S110.
At step S110, in the picture signal processing circuit 39, firstly
the brightness value of each pixel of the CCD image sensor 33 is
calculated in accordance with the pixel data obtained by the pixel
of the CCD image sensor 33 and subsequently the average of all the
calculated brightness values is multiplied by 2.sup.N after the
average is calculated. For instance, in the case where the first
exposure time and the second exposure time are 1 second and 1/8
second, respectively, the pre-exposure is performed at an exposure
time which is an eighth of the main exposure, i.e., is divided by
the third power of two (2.sup.3). Accordingly, at step S110 the
average of all the brightness values is multiplied by the third
power of two (2.sup.3) to calculate the average of all the brightness
values when the pre-exposure is performed at the first exposure
time (1 second).
At step S111 an exposure compensation factor K is calculated by
dividing a predetermined appropriate value (predetermined value)
by the average of all the brightness values obtained at step S110.
In the case where each data such as pixel data is processed to be
expressed by an eight binary digit, since the maximum luminance
is 255, the predetermined appropriate value is, e.g., 120, which
is roughly half of the maximum luminance. Accordingly, if the average
of all the brightness values is 100, the exposure compensation factor
K is equal to 120 divided by 100, namely, 1.2 (K=120/100=1.2). At
step S112 a third exposure time is calculated by multiplying the
first exposure time by the exposure compensation factor K. For instance,
if the first exposure time and the exposure compensation factor
K are 1 second and 1.2, respectively, the third exposure time is
1.2 seconds.
At step S113, the main exposure is performed at the third exposure
time to make the sensitive surface of the CCD image sensor 33 exposed.
At step S114, the picture signal obtained at the main exposure is
read out of the CCD image sensor 33. At step S115, image processing
operations such as an interpolation operation, a color adjusting
operation and a gamma control operation are performed in accordance
with all the pixel data of the picture signal read out of the CCD
image sensor 33 at step S114. These image processing operations
are conventional ones known by those skilled in the art, so that
the details are not herein discussed. At step S116 image data obtained
through the image processing operations performed at step S115 are
stored in the memory 40. This completes the photographic operation
control routine shown in FIGS. 2 and 3.
As can be understood from the foregoing, in the present embodiment
of the digital camera, if the first exposure time, which is calculated
in accordance with the photometering value, is longer than a reference
time duration (the flash synchronization speed in this particular
embodiment), the second exposure time that is shorter than the first
exposure time is set to perform the pre-exposure at the second exposure
time. Therefore, the time necessary for performing the pre-exposure
can be shortened, which makes it possible to reduce the time lag
between the pre-exposure and the main exposure. At the same time,
electric power consumption necessary to keep holding each of the
quick-return mirror, the shutter and the diaphragm of the digital
camera at a predetermined position can be reduced, which prolongs
the life of the battery.
As a reference time duration with which the first exposure time
is compared, a slowest exposure time (e.g., 1/60 second) which can
just prevent the image from becoming blurry which is caused by hand
movement can be used instead of the flash synchronization speed.
Although the aforementioned parameter N is set at 3 or 0 at step
S104 or step S105, respectively, the parameter N can be set at any
other value which is determined in accordance with the reference
time duration so that the calculated second exposure time for the
pre-exposure falls within an allowable range. In the illustrated
embodiment, although the exposure compensation factor K can be calculated
by dividing the predetermined appropriate value by the average of
all the brightness values for an entire image, the exposure compensation
factor K can be calculated by dividing the predetermined appropriate
value by the average of the brightness values for an entire image
except for peripheral edge thereof.
In the illustrated embodiment, although the third exposure time
is calculated by changing the value of the first exposure time using
the exposure compensation factor K (specifically, by multiplying
the first exposure time by the exposure compensation factor K),
the third exposure time can be calculated by changing the value
of the second exposure time. In this case, the exposure compensation
factor K is replaced by another exposure compensation factor which
is equal to the exposure compensation factor K multiplied by the
Nth power of two (K2.sup.N).
As can be understood from the foregoing, according to an exposure
controller of a digital camera to which the present invention is
applied, the time necessary for performing the pre-exposure can
be shortened, which makes it possible to reduce the time lag between
the pre-exposure and the main exposure.
Furthermore, in the drawings, the quick-return mirror 21 in the
pre-exposure and main exposure is shown in the up and down positions,
respectively; however, when the quick-return mirror 21 is moved
to the up position during the pre-exposure, the quick-return mirror
21 can be maintained in the up position until the main exposure
has completed.
In the drawings, the first, second, and third exposure times are
the opening and shutting drive speeds of the focal plane shutter;
however, the first, second and third exposure times can be set as
electronic shutters speeds for the CCD 33, wherein the time for
accumulating electric charge is electronically controlled.
Obvious changes may be made in the specific embodiment of the present
invention described herein, such modifications being within the
spirit and scope of the invention claimed. It is indicated that
all matter contained herein is illustrative and does not limit the
scope of the present invention.
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