U.S. patent application number 11/366713 was filed with the patent office on 2006-07-06 for imaging apparatus in which exposure control is performed to suppress changes in sensitivity due to changes in gradation mode.
This patent application is currently assigned to OLYMPUS OPTICAL CO., LTD.. Invention is credited to Keiichi Mori, Hideaki Yoshida.
Application Number | 20060146164 11/366713 |
Document ID | / |
Family ID | 27343763 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060146164 |
Kind Code |
A1 |
Mori; Keiichi ; et
al. |
July 6, 2006 |
Imaging apparatus in which exposure control is performed to
suppress changes in sensitivity due to changes in gradation
mode
Abstract
A digital camera that can change gradation properties is
provided which includes a CCD image pickup element for
photographing a subject, an exposure control mechanism for
controlling an exposure on the image pickup element, a digital
processing circuit including a gradation converting circuit for
generating image signals having different gradation properties
(gamma properties) based on the output signal from the image pickup
element, and a system controller including a selecting circuit for
selecting one of the different gradation properties, which is used
in the gradation converting circuit. In the digital camera, to
suppress change of sensitivity (output level) due to a change of
the gradation property, the exposure control mechanism changes a
control target value to control the exposure in accordance with the
selected gamma value so that the output level is maintained at a
constant in the average exposure.
Inventors: |
Mori; Keiichi;
(Hachioji-shi, JP) ; Yoshida; Hideaki;
(Hachioji-shi, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 Fifth Avenue
16TH Floor
NEW YORK
NY
10001-7708
US
|
Assignee: |
OLYMPUS OPTICAL CO., LTD.
Tokyo
JP
|
Family ID: |
27343763 |
Appl. No.: |
11/366713 |
Filed: |
March 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09883833 |
Jun 18, 2001 |
|
|
|
11366713 |
Mar 2, 2006 |
|
|
|
Current U.S.
Class: |
348/311 ;
348/E5.036; 348/E5.074 |
Current CPC
Class: |
H04N 5/2352 20130101;
H04N 1/407 20130101; H04N 5/202 20130101 |
Class at
Publication: |
348/311 |
International
Class: |
H04N 5/335 20060101
H04N005/335 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 19, 2000 |
JP |
2000-183310 |
Jun 22, 2000 |
JP |
2000-188030 |
Jun 22, 2000 |
JP |
2000-188031 |
Claims
1. An imaging apparatus, comprising: image pickup device, on which
an image of a subject is formed, and which includes a plurality of
pixel elements in which charge is generated that are arranged in a
matrix array and an interline type charge transfer path; driving
means for driving the image pickup device to readout the pixels as
an image signal from the image pickup device, and for setting an
addition mode in which the pixels are added and the added pixels
are readout as the image signal; an analogue to digital converter
for quantizing the image signal to output a quantized image signal;
and quantizing level setting means for setting a maximum quantizing
level in the analogue to digital converter, which is changed in
accordance with a number of the added pixels in the addition
mode.
2. The imaging apparatus according to claim 1, further comprising
controlling means for controlling an exposure of the image on the
image pickup device, wherein the controlling means sets a target
value of the exposure, and the quantizing level setting means sets
the maximum quantizing level in accordance of the target value of
the exposure as well as the number of the added pixels in the
addition mode.
3. The imaging apparatus according to claim 1, wherein the
interline type charge transfer path includes a plurality of
vertical transfer lines which are arranged along the array of the
pixel elements in a vertical direction, and a horizontal transfer
line arranged at one side of the vertical transfer lines in a
lateral direction; wherein the driving means causes the image
pickup device to add adjacent pixels generated from the vertical
transfer lines in the horizontal transfer line; and wherein the
imaging apparatus further comprises an amplifier connected to the
horizontal transfer line for amplifying the image signal, and the
adjacent pixels transferred from the horizontal transfer line are
added in the amplifier.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a Divisional application of U.S.
application Ser. No. 09/883,833, filed Jun. 18, 2001, which is
based upon and claims the benefit of priority from the prior
Japanese Patent Applications No. 2000-183310, filed Jun. 19, 2000,
No. 2000-188030, filed Jun. 22, 2000, and No. 2000-188031, filed
Jun. 22, 2000, the entire contents of which are incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an imaging apparatus in
which a gradation (gamma: .gamma.) property (gradation property
curve) can be changed, and particularly to an imaging apparatus in
which the gradation property can be changed and exposure control is
improved.
[0004] Moreover, the present invention relates to an imaging
apparatus in which an image pickup device such as CCD is
incorporated, and particularly to an imaging apparatus in which
sensitivity is enhanced by addition of pixels.
[0005] 2. Description of the Related Art
[0006] In recent years, an imaging apparatus has intensively been
developed in which a still picture or an image of a photographic
subject is taken by an image pickup device such as CCD and
converted to an image signal. In such known imaging apparatus, a
gradation property (output signal level property corresponding to
photographic subject luminance) can be changed. Concretely, for
example, in a broadcasting TV camera or a so-called industrial
camera (ITV), the gradation property is set to one of a standard
set mode (y=0.45) and a linear property set mode (.gamma.=1) by
so-called "gamma change".
[0007] Here, a .gamma. (gamma) value is an index. Dissociation from
a linear input/output property is noted as well known, and a
relation between input x and output y is approximately represented
by an equation y=a.times.x.gamma.+b (a, b are constants).
Therefore, when .gamma.=1, the input and output are in a
proportional relation. Additionally, since b corresponds to a
pedestal or an offset, it is often taken into consideration
independently of the .gamma. value.
[0008] A value .gamma.=0.45 is a standard system value in
broadcasting, and selected as a value for obtaining a general high
quality (visually satisfactory image quality for a purpose of
appreciation) in consideration of the gradation property of a
reproduction system. On the other hand, .gamma.=1 is selected as a
value for removing an error factor attributed to a gradation
conversion circuit for a purpose of measurement or in camera
adjustment.
[0009] An automatic exposure control is often used for the camera
described above. Generally, a control object of the automatic
control is an output from the image pickup device, that is, a
linear signal before inputted to the gradation conversion circuit
usually called ".gamma. conversion circuit". According to the
control method, optimum exposure control directly corresponding to
a dynamic range of the image pickup device can be realized.
[0010] In recent years, an electronic still camera which is one
type of the electronic imaging apparatus described above and which
is capable of recording a still picture has come into wide use in
the name of a "digital camera", and there has been a need for a
change of the gradation property in a meaning different from that
of a broadcasting TV camera or an industrial camera. That is, there
has been a demand for a digital camera in which an optimum
gradation can selectively be set in accordance with user's taste or
scene situation in order to obtain a high-quality picture
comparable to a picture on a silver-halide or silver-salt film.
[0011] On the other hand, for the digital camera intended to take
the picture corresponding to the silver-halide picture, a natural
requirement is that a person skillful in taking the silver-halide
picture can satisfactorily use the digital camera. Therefore,
specifications and operability (so-called handiness) equivalent to
those of the silver-halide picture are required in background. To
this end, for example, an attempt to indicate "sensitivity" of the
digital camera in so-called "ISO indication" similarly as a
conventional silver-halide film has been carried out.
[0012] One attempt is described in "Television Society Technical
Report/Yoshida: Study on Digital Camera Sensitivity (Speed)
Indicating Method: ITE Technical Report Vol. 20, No. 58, PP. 85 to
90, CE'96-25 (November, 1996)". In a measuring method used in the
sensitivity indicating method, the sensitivity is defined with "an
exposure amount which gives a predetermined value (proposed value
106.5/255) defined in an intermediate range (satisfactory gradation
reproduction range) of a digital value recorded by the digital
camera". With the camera having the same "sensitivity" obtained by
the measuring method, the same digital signal output is obtained
during photography with the predetermined exposure amount for a
measurement reference.
[0013] Additionally, another discussion is necessary for judging
whether this proposal can be said to correspond to the so-called
"ISO indication". However, the "sensitivity" itself proposed in the
document is "an index indicating the exposure amount necessary for
obtaining an image with the same brightness (output level)", and is
therefore significant. Moreover, in the following description, the
"sensitivity" indicates such sensitivity as described in the
document (the predetermined value is indefinite) unless otherwise
specified.
[0014] When the gradation property is changed in accordance with
the user's taste and scene situation and the gradation is selected
as described above, even with a constant gain of an imaging system,
the sensitivity changes with the gradation property change.
Therefore, a problem is that the obtained output level changes.
[0015] This respect will be described in more detail. Since the
gradation property is changed, it is originally impossible to
obtain an equal output level over a whole luminance distribution of
the photographic subject. However, considering from an average or
typical portion of the luminance distribution of the photographic
subject, in other words, considering only from a main photographic
subject, it is preferable that the output level for the subject
should not change even with the changed gradation property.
Additionally, in the conventional digital camera, since these are
not considered, the output level disadvantageously changes largely
as described above.
[0016] Moreover, concerning sensitivity enhancement, even when the
imaging apparatus is a dynamic imaging apparatus or a still imaging
apparatus, adjacent pixel information of the image pickup device is
added, for example, signals of four pixels in total of two pixels
in a vertical direction and two pixels in a horizontal direction
are added, then resolution is deteriorated, but the sensitivity can
be enhanced.
[0017] Known examples of an adding method include an outer digital
adding system, device inside analog adding system, and the like.
The outer digital adding system comprises: reading a pixel signal
from the image pickup device for each pixel in a usual method,
subsequently subjecting the signal to A/D conversion, and adding
the pixel signals in a digital system. On the other hand, in the
device inside analog adding system, transfer drive of a CCD image
pickup device is devised in the image pickup device, and charges
are added in a transfer path. As a result of comparison of the
systems, the analog adding system is superior in that a frame rate
can also be enhanced.
[0018] Additionally, when this technique is actually applied to the
electronic camera, the sensitivity can be enhanced, but a noise
increases with the sensitivity enhancement. Beyond simple
resolution deterioration because of a decrease of the number of
pixels, image quality deterioration further occurs.
[0019] In this respect, for example, it is also described in the
conventional publication that fourfold-sensitivity can be obtained
in addition of four pixels. However, this cannot be said to be
correct in consideration about the noise. Because it is known that
noise improvement effect in the pixel addition, that is, SN
enhancement is proportional to a square root of an added number of
the pixels in respect to a statistic property of random noise. In
the addition of four pixels, only double SN (i.e., 6 dB) can be
expected. Therefore, during imaging while the image quality, that
is, the noise level is kept, the imaging with double sensitivity,
that is, 1/2 of the standard exposure amount is a limitation.
Supposing that the image is taken with a 1/4 exposure amount and
four pixels are added in order to obtain the fourfold-sensitivity,
a signal component S is 1/4.times.4=once, noise N is 1.times.
{square root over (4)}=twice, and SN is deteriorated by 6 dB.
[0020] Then, addition of four pixels and exposure with the 1/2
exposure amount are considered. In an analog adding system in the
digital camera, there is a problem of saturation in an A/D
converter. That is, if a maximum output level of a horizontal
transfer path or an image pickup device output amplifier as a
position for executing the addition is not limited, it is necessary
to handle a charge amount for 1/2.times.4=2 pixels, that is, double
output voltage in the addition of four pixels with the 1/2 exposure
amount. Therefore, when the conventional usual A/D converter
corresponding to the voltage for one pixel device is used as it is,
an A/D input voltage exceeds A/D quantized maximum voltage, and the
signal is clipped.
[0021] In this case, in order to prevent the clip during the
quantization in the A/D converter, it is necessary to set a maximum
quantization level (Dmax) of A/D to be twice the conventional
level. With this setting, the problem of saturation can securely be
avoided. However, in a usual non-addition time, the maximum value
of CCD output signal is Dmax/2, and a larger digital bit is wasted.
In other words, a relative quantized error disadvantageously
becomes double from an ideal state.
[0022] Additionally, to avoid the problem, the A/D converter
quantized error itself may be reduced. In other words, the A/D
converter with a larger number of bits than an originally necessary
number of bits is necessary, and cost increase is unfavorably
caused.
[0023] In the conventional art, the addition of pixels is carried
out in this manner in order to enhance the sensitivity of the
imaging apparatus. However, in the analog adding system, since the
A/D input voltage exceeds the A/D quantized maximum voltage, the
signal is clipped. There is a problem that the image quality is
deteriorated.
BRIEF SUMMARY OF THE INVENTION
[0024] An object of the present invention is to provide an imaging
apparatus which can hold sensitivity and output level to be
constant even when a gradation property is changed.
[0025] Moreover, another object of the present invention is to
provide an imaging apparatus in which imaging with an enhanced
sensitivity can be carried-out by addition of pixel information in
an analog adding system, and an image quality can be prevented from
being deteriorated by clip in an A/D converter.
[0026] According to the present invention, there is provided an
imaging apparatus for photographing a photographic subject,
comprising:
[0027] an image pickup device in which light is received from the
photographic subject, a photographic subject image is formed, and
the image is converted to an original image signal;
[0028] designating means for designating one of a first gradation
mode and a second gradation mode;
[0029] converting means for converting the original image signal
from the image pickup device to an output image signal in
accordance with the designation of one mode by the designating
means, the output image signal having a first gradation in the
designation of the first gradation mode, and a second gradation in
the designation of the second gradation mode; and
[0030] adjusting means for adjusting a level of the original image
signal inputted to the converting means in accordance with the
designation of one mode by the designating means, and keeping an
average level of the output image signal outputted from the
converting means at a substantially constant level even in the
designation of the first and second gradation modes.
[0031] Moreover, according to the present invention, there is
provided an imaging apparatus for photographing a photographic
subject, comprising:
[0032] an image pickup device for receiving light rays transferred
from the subject, a photographic subject image being formed on the
image pickup device, and the image being converted to an original
image signal;
[0033] designating means for designating one of first and second
gradation modes;
[0034] converting means for converting the original image signal
from the image pickup device to an output image signal in
accordance with the designated mode, the output image signal having
a first gradation in accordance with a first gradation curve in the
designation of the first gradation mode, and a second gradation in
accordance with a second gradation curve in the designation of the
second gradation mode; and
[0035] adjusting means for adjusting a level of the original image
signal inputted to the converting means in accordance with the
designated mode, and maintaining an average level of the output
image signal outputted from the converting means at a substantially
constant level;
[0036] wherein the first and second gradation property curves
intersect each other at a certain target value, and the target
value substantially corresponds to the average output level of the
output image signal.
[0037] Furthermore, according to the present invention, there is
provided an imaging method of photographing a subject,
comprising:
[0038] an image pickup step of receiving light rays transmitted
from the photographic subject, a photographic subject image being
formed, and the image being converted to an original image
signal;
[0039] designating step of designating one of a first gradation
mode and a second gradation mode;
[0040] converting step of converting the original image signal to
an output image signal in accordance with the designated mode, the
output image signal having a first gradation in the designation of
the first gradation mode, and a second gradation in the designation
of the second gradation mode; and
[0041] adjusting step of adjusting a level of the original image
signal in the converting step in accordance with the designated
mode, and maintaining an average level of the output image signal
in the converting step at a substantially constant level even in
the designation of the first and second gradation modes.
[0042] Yet further, according to the present invention, there is
provided an imaging method for photographing a photographic
subject, comprising:
[0043] an imaging step of receiving light rays from the
photographic subject, forming a photographic subject image, and
converting the image to an original image signal;
[0044] a designating step of designating one of a first gradation
mode and a second gradation mode;
[0045] a converting step of converting the original image signal to
an output image signal in accordance with the designation of one
mode in the designating step, the output image signal having a
first gradation in accordance with a first gradation curve in the
designation of the first gradation mode, and a second gradation in
accordance with a third gradation curve in the designation of the
second gradation mode; and
[0046] an adjusting step of adjusting a level of the original image
signal inputted to the converting step in accordance with the
designation of one mode by the designating step, and maintaining an
average level of the output image signal outputted from the
converting step at a substantially constant level;
[0047] wherein the first and second gradation property curves
intersect each other at a certain target value, and the target
value substantially corresponds to the average output level of the
output image signal.
[0048] Yet furthermore, according to the present invention, there
is provided an imaging method for photographing a photographic
subject, comprising:
[0049] an imaging step of receiving light rays from the
photographic subject, forming a photographic subject image, and
converting the image to an original image signal;
[0050] a designating step of designating one of a first, second and
third gradation modes; and
[0051] a converting step of converting the original image signal to
an output image signal in accordance with the designation of one
mode in the designating step, the output image signal having a
first gradation in accordance with a first gradation curve in the
designation of the first gradation mode, a second gradation in
accordance with a second gradation curve in the designation of the
second gradation mode, and a third gradation in accordance with a
third gradation curve in the designation of the third gradation
mode;
[0052] wherein the first, second and third gradation property
curves intersect one another at a substantially same point.
[0053] As described above, in some of the cameras in which a
plurality of gradation properties (gamma) can be changed, when
exposure control of a predetermined target value is carried out in
a linear system (on a gamma input side), sensitivity (output signal
level) changes by the gamma change.
[0054] To solve the problem, in the imaging apparatus having a
plurality of gamma properties, exposure control is carried out in
such a manner the output level for average exposure becomes the
same even with the gamma change. Concretely; the exposure is
controlled and a plurality of control target values are changed in
accordance with the gamma change on a gamma input side.
Alternatively, the exposure is controlled and the same control
target value is used on a gamma output side. Thereby, even when the
gamma is changed, the output level does not change with respect to
average exposure. That is, even when the gradation property is
changed, the output level can be kept to be constant.
[0055] According to the present invention, there is also provided
an imaging apparatus for photographing a photographic subject,
comprising:
[0056] an image pickup device for receiving light rays transferred
from the subject, a photographic subject image being formed on the
image pickup device, and the image being converted to an original
image signal;
[0057] designating means for designating one of first, second and
third gradation modes; and
[0058] converting means for converting the original image signal
from the image pickup device to an output image signal in
accordance with the designated mode, the output image signal having
a first gradation in accordance with a first gradation curve in the
designation of the first gradation mode, a second gradation in
accordance with a second gradation curve in the designation of the
second gradation mode, and a third gradation in accordance with a
third gradation curve in the designation of the third gradation
mode;
[0059] wherein the first, second and third gradation property
curves intersect one another at a substantially same point.
[0060] Here, preferred embodiments of the present invention are as
follows.
[0061] (1) The intersection of the property curves is set to
correspond to 18 to 20% of a maximum signal level in a value on an
input side of the gradation converting property.
[0062] (2) At least one of the gradation converting properties is
constituted to have a knee property in which a knee point is set in
a region with a signal value larger than that of the intersection
of the property curves.
[0063] As described above, the camera which can change a plurality
of gradation properties (gamma) is known. However, when the
exposure control is performed at the predetermined target value in
the linear system (gamma input side), the sensitivity (output
signal level) changes by the gamma change.
[0064] To solve the problem, according to the present invention, in
an imaging apparatus having a plurality of gamma properties, the
exposure control target value (recommended average exposure level)
is the same on the gamma input side, and a plurality of gamma
curves cross one another at the point. When there are three or more
properties, all the properties have a common cross point. In this
constitution, even when the gamma property is changed, the output
level in the cross point is the same. Even when the gradation
property is changed in the vicinity of the cross point, sensitivity
and output level can be kept to be constant.
[0065] Moreover, the cross point is a point corresponding to 18 to
20% (.+-.1/3 EV) of an input full scale. This value is in the
vicinity of a logarithmic midpoint of a photographic subject range,
and an region with a maximum possibility of correspondence to a
main photographic subject is in the vicinity of the logarithmic
midpoint of the photographic subject range. Therefore, the value
can be the to be a best selection. Furthermore, the knee property
is disposed so that the range is ensured with the intersecting of
the properties. Thereby, a photographic subject reproduction range
can be prevented from being narrowed with a large gamma.
[0066] According to the present invention, there is provided an
imaging apparatus, comprising: image pickup device including a
plurality of pixel elements arranged in a matrix arrays and a
charge transfer path of interline type, a image of the subject
being formed on the image pickup device and change being generated
in the pixel elements; driving means for driving the image pickup
device to readout the pixels as a image signal from the image
pickup device, the driving means setting a addition mode in which
the pixels are added and the added pixels are readout as the image
signal; analogue to digital converter for quantizing the image
signal to output a quantized image signal; and quantizing level
setting means for setting a maximum quantizing level in the
analogue to digital converter, which is changed in accordance with
the addition number of the pixels.
[0067] Here, preferred embodiments of the present invention are as
follows.
[0068] (1) The constitution comprises exposure control means for
controlling an exposure amount with respect to the image pickup
device, and the quantized level setting means variably sets the
quantized maximum level in accordance with the added number of
adding/reading, and the exposure amount target value set by the
exposure control means.
[0069] (2) The interline type charge transfer path comprises a
plurality of vertical transfer paths disposed adjacent to the
pixels arranged in a matrix in a column direction, and a horizontal
transfer path disposed adjacent to an end of these vertical
transfer paths in a row direction. The driving means adds the
signals corresponding to the pixels adjacent in the vertical
direction in the horizontal transfer path, and adds the signals
corresponding to the pixels adjacent in the horizontal direction in
an output amplifier connected to an output end of the horizontal
transfer path.
[0070] (3) The number of pixels added by the driving means is the
same in the horizontal and vertical directions.
[0071] (4) The image pickup device is a CCD image pickup
device.
[0072] According to the present invention, the quantized level
setting means variably sets the quantized maximum level in the A/D
converting means in accordance with the added number of
adding/reading by the driving means. Thereby, a phenomenon in which
the A/D input voltage exceeds the A/D quantized maximum voltage
with the signal addition can be suppressed. Therefore, the added
signal can be prevented beforehand from being clipped by the A/D
converting means. Therefore, photography with enhanced sensitivity
is enabled by addition of the pixel information of the analog
adding system without deteriorating the image quality by the clip
in the A/D converting means.
[0073] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0074] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0075] FIG. 1 is a block diagram schematically showing a circuit of
a digital camera according to one embodiment of the present
invention.
[0076] FIG. 2 is a graph of a gradation property curve utilized in
a gradation converter shown in FIG. 1, showing a relation between a
gradation property and an exposure target value.
[0077] FIG. 3 is a flowchart showing a setting procedure in which a
.gamma. value in the digital camera shown in FIG. 1 is selectively
set.
[0078] FIG. 4 is a flowchart showing an exposure target setting
procedure in which the .gamma. value in the digital camera shown in
FIG. 1 is selectively set.
[0079] FIG. 5 is a graph of the gradation property curve utilized
similarly in the gradation converter shown in FIG. 1, showing the
relation between the gradation property and the exposure target
value.
[0080] FIG. 6 is a plan view schematically showing a basic
structure of a CCD image pickup device shown in FIG. 1.
[0081] FIG. 7 is a block diagram schematically showing a
preprocessing circuit shown in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0082] A detail of a digital camera according to an embodiment of
the present invention will be described hereinafter with reference
to the drawings.
[0083] FIG. 1 is a block diagram showing a circuit configuration of
the digital camera according to one embodiment of the present
invention. In FIG. 1, reference numeral 101 denotes a photography
lens system constituted of various lenses for photographing a
photographic subject, 102 denotes a lens driving mechanism for
driving the lens system 101 in accordance with the photographic
subject, and the lens driving mechanism 102 drives the photography
lens system 101 to focus the photography lens system 101 on the
photographic subject. Light rays reflected from the photographic
subject are directed to CCD unit 105 via the lens system 101 and a
stop included in an exposure control mechanism 103. In the exposure
control mechanism 103, the stop is adjusted, the light rays passed
through the stop is controlled and an exposure amount is
controlled. The light transmitted through the stop is transmitted
through a filter system 100 including a low pass filter and
infrared cutting filter and a mechanical shutter 104, and is
incident upon the CCD color pickup unit 105. A photographic subject
image is formed on imaging arrays of the CCD color pickup unit 105.
The pickup unit 105 is driven by a CCD driver 106, and the
photographic subject image on the imaging array is converted to an
image signal. The image signal is processed in a preprocessing
circuit 107 which includes a gain control amplifier 107A which can
change an amplification factor, i.e., a gain of the image signal
and an A/D converter for converting the amplified image signal to a
digital image signal. The digital image signal is processed by a
digital processing circuit 108 for performing various digital
constitutions such as a color signal generation processing and
matrix conversion processing, and stored in a memory card 110 such
as CF via a card interface 109. Moreover, the image is displayed in
an LCD image display system 111 based on the image signal from the
digital processing circuit 108.
[0084] Moreover, reference numeral 112 in FIG. 1 denotes a system
controller (CPU) for generally controlling respective components
shown in FIG. 1. Numeral 113 denotes an operation switch system
constituted of various switches including a mode setting SW 113A
for setting gradation (gamma) property, i.e., actually setting a
standard mode, a soft mode or a hard mode. Numeral 114 denotes an
operation display system for displaying a digital camera operation
state, mode state, and the like, and 115 denotes a lens driver for
controlling the lens driving mechanism 102. Numeral 116 denotes a
strobe which emits photography light rays during photography, 117
denotes an exposure control driver for controlling the exposure
control mechanism 103 and strobe 116, and 118 denotes a nonvolatile
memory (EEPROM) for storing various setting information, and the
like.
[0085] In the digital camera of the present embodiment, the system
controller 112 generally controls the respective components.
Particularly, the system controller 112 controls the driving of the
CCD color pickup unit 105 by the exposure control mechanism 103 and
CCD driver 106 to control exposure (charge accumulation). The
system controller 112 similarly controls the reading of the image
signal from the CCD color pickup unit 105, and the digital
processing circuit 108 takes the image signal via the preprocessing
circuit 107. Furthermore, under the control of the system
controller 112, the image signal is subjected to various signal
processings and subsequently recorded in the memory card 110 via
the card interface 109. Additionally, examples of the CCD color
pickup unit 105 include an interline progressive (successive)
scanning unit with a vertical overflow drain structure.
[0086] Moreover, the digital processing circuit 108 shown in FIG. 1
includes a gradation converter 108A which can generate image
signals having different gradation properties with respect to the
same image signal. When the mode setting SW 113A in the operation
switches 113 sets the gradation property (.gamma. property), i.e.,
one of the standard, soft and hard modes, a gradation designation
signal of the set mode is supplied to the system controller 112.
Then, the system controller 112 designates the gradation to be
converted, that is, .gamma. value with respect to the gradation
converter 108A of the digital processing circuit 108. Moreover,
with the designation of the gradation by the system controller 112,
similarly the gain control amplifier 107A of the preprocessing
circuit 107 sets the gain in accordance with the designated
gradation in response to a command of the system controller 112.
Furthermore, the system controller 112 transmits information of the
designated gradation to the exposure control driver 117, and the
exposure control driver 117 controls exposure in accordance with
the selected gradation property.
[0087] FIG. 2 is a graph of a gradation property and exposure
target value which can be controlled in the digital camera shown in
FIG. 1. To simplify description, in the graph of FIG. 2, eight bit
data is assumed for input and output digital values. In FIG. 2, a
solid line is a graph showing a property of a gradation
(.gamma.=0.45) of the standard mode, a broken line is a graph
showing the gradation property in a mode of .gamma.=0.7, and a
dashed line is a graph showing the property of the gradation in a
mode of .gamma.=1 (the output digital value is in a linear relation
with respect to the input digital value). Additionally, in the
digital camera shown in FIG. 1, the mode setting switch 113A can
change the mode to other two modes (.gamma.=0.7 and .gamma.=1) from
default "standard", and even the .gamma. value can be selected in
the modes.
[0088] The standard mode (.gamma.=0.45) is actually a recommended
property of JEIDA DCF specification as a digital camera standard
specification: y=1.099.times.x.sup.0.45-0.099. (Additionally,
x=input/255, y=output/255, this equation is applied to
x.gtoreq.0.018, and y=4.5.times.x is applied in x<0.018.)
Moreover, .gamma.=0.7 corresponds to property of y=x.sup.0.7, and
.gamma.=1 corresponds to property of y=x.
[0089] In the camera shown in FIG. 1, a brightness or luminous of
the photographic subject is measured in center selective average
photometry, and exposure is controlled by calculation or feedback
control so that a photometric value (signal level average value) is
equal to an exposure target value. Therefore, an output value
obtained from a flat photographic subject (with no pattern)
corresponds to the exposure control target value. When the main
photographic subject in a usual exposure condition is assumed, the
flat photographic subject can be represented or substituted as well
known.
[0090] Luminance information about the photographic subject taken
by the digital processing circuit 108 is analyzed or calculated by
the system controller 112 so that the photographic subject
luminance is measured. The system controller 112 drives the
exposure control mechanism 103 or the strobe 116 via the exposure
control driver 117 based on an analysis result so that the exposure
is controlled. When a device shutter function is used as the
shutter, the CCD driver 106 can control/drive CCD to realize the
function.
[0091] As shown in FIG. 2, an exposure control point 46 in the
standard (.gamma.=0.45) corresponds to 18% of a maximum input
digital value 255. The input digital value 46 substantially
corresponds to a logarithmic midpoint of an input range in the
standard .gamma.. When an ideal de-gamma (a total linear value) is
assumed in the reproduction system, the input digital value 46
actually corresponds to the logarithmic midpoint of a display range
of a reproduction system. From FIG. 2, an output digital value 104
corresponds to the input digital value 46.
[0092] Additionally, for the gradation property, generally during
photographing of the main photographic subject, a region having a
highest possibility of correspondence to the photographic subject
luminance is in the vicinity of the logarithmic midpoint of the
photographic subject range. The region numerically corresponds to
18% as the logarithmic midpoint between white and black levels.
Here, it is assumed that the white level is a reflectance of about
98% of the photographic subject having a highest diffusion
reflectance and that the black level is a reflectance of about 3.3%
of the photographic subject having a lowest diffusion reflectance.
Based on this finding, 18% has been used as the numeric value of a
standard reflectance representing the photographic subject or a
reflectance of a standard reflector for evaluation in a
photographic technical field.
[0093] Additionally, according to the present inventor's study, in
the actual photography, when a numeric value 4% of the lowest
reflectance is assumed and a logarithmic midpoint value 20% is
used, a good result is produced in many cases. In this meaning, an
exposure control point value of about 18 to 20% can be set as a
target value (aimed value) for obtaining the best result.
[0094] Here, the photographic subject range has been considered in
the above. However, when ideal gamma conversion (i.e., total
linear) for the imaging/display system is assumed, the above
indicates a similar meaning even in a final light output gradation
(display range) in an "output apparatus (display apparatus)". In
actual, for a visual property of observation of an output image,
the "logarithmic midpoint" is preferable on the assumption of a
logarithmic recognition property for a stimulus strength governed
by Weber's law or Fechner's law.
[0095] In this meaning, for the exposure control target value, the
value (18 to 20% as described above) on a conversion input side
(photographic subject proportional signal) in a standard gradation
converting property (total linear property is assumed) should be a
standard. However, when the property is changed to an arbitrary
converting property, the corresponding value on a conversion output
side (post-gamma signal) in the standard property is rather
preferably used as the standard.
[0096] Moreover, in the real camera, there are various error
factors, and there is a certain degree of freedom of a tolerance
limit in product evaluation. Therefore, set values substantially in
a range of .+-.1/3 EV with respect to the aforementioned set target
value (i.e., 18%-1/3EV=14.3% or more, 20%+1/3EV=25.2% or less) are
included in a numeric value limited range of the present invention.
Additionally, .+-.1/3EV is a numeric value frequently used as a
standard tolerance error range in the technical field, for example,
in standards such as JIS and ISO. Additionally, to describe that by
way of precaution, it is apparent that with an error of .+-.0.1 EV
or less, the value can be regarded as the same without any
condition in almost all cases of the field.
[0097] By the aforementioned control, the main photographic subject
(the photographic subject represented by the photometric value) can
be reproduced as an image whose luminance is distributed in
substantially the same output luminance range as that of the
photography in the standard gradation property, that is, in the
vicinity of the logarithmic midpoint of the display range in the
output apparatus (display apparatus).
[0098] Moreover, when the mode is changed, the exposure control
target value is changed to respective values 71, 104 in accordance
with the changed mode. That is, for a target value s with
.gamma.=0.7, 71 is obtained as a solution of
(105/255)=(s/255).sup.0.7. With .gamma.=1, since y=x, an output
corresponding value is still 104. In this case, the exposure
control target value does not correspond to the logarithmic
midpoint of the input range, but the output level of the main
photographic subject (the photographic subject represented by the
photometric value) does not change.
[0099] As described above, according to the present invention, a
plurality of control target values for the gamma value (the gamma
output value is the same) are preset, exposure is controlled on the
gamma input side in accordance with the gamma value during change
of the gamma property, and a plurality of control target values are
changed. Thereby, the average level of the image signal can be kept
to be substantially constant. Therefore, the exposure can be
controlled so that the output level is the same for average
exposure even with the changed gamma, and usefulness of the control
is remarkably high.
[0100] The change of the .gamma. property will be described in
further detail with reference to FIGS. 3 and 4. In the descriptions
below, the amplified value of the gain control amplifier 107A is
assumed to be a fixed value, unless otherwise stated.
[0101] FIGS. 3 and 4 are flowcharts illustrating the setting
control of the .gamma. property, which is executed prior to actual
imaging. The flowcharts also illustrate the setting control of a
corresponding exposure target value. These setting control
operations are described as sub-flows which branch off from the
main flow (the control flow of the operation of the entire camera)
and which are executed as need arises (the sub-flows corresponds to
sub-routines of a program).
[0102] Let it be assumed that the user designates .gamma. by
operating the mode setting switch 113A before the execution of
these sub-flows, and that the setting parameter designated then
(one of .gamma.A, .gamma.B and .gamma.C) is stored in a memory (not
shown). If, intentionally, the user does not designate .gamma.,
default setting .gamma.C is selected. The default setting in the
present camera is standard .gamma. (=0.45) described above with
reference to FIG. 2.
[0103] The setting of .gamma. is executed in the manner shown in
FIG. 3. First, the system controller 112 reads out a .gamma.
setting parameter from the memory. When the check in step S2
indicates that .gamma.=.gamma.A, the flow advances to step S5. In
this step, the conversion property of the gradation converter 108A
of the digital processing circuit 108 is set in accordance with
.gamma.A. (The conversion property is so determined as to satisfy
the equation .gamma.=1 in FIG. 2.) When the check in step S2 does
not indicate that .gamma.=.gamma.A, the flow advances to step S3.
If the check in this step S3 indicates that the equation
.gamma.=.gamma.B is satisfied, the flow advances to step S6. In
this step, the conversion property of the gradation converter 108A
of the digital processing circuit 108 is set in accordance with
.gamma.B. (The conversion property is so determined as to satisfy
the equation .gamma.=0.7 in FIG. 2.) If the check in step S3 does
not indicate that the equation .gamma.=.gamma.B is satisfied, the
flow advances to step S4. In this step S4, the conversion property
of the gradation converter 108A of the digital processing circuit
108 is set in accordance with .gamma.C. (The conversion property is
so determined as to satisfy the equation .gamma.=0.45 in FIG. 2.)
In any case, step S7 is executed to terminate the sub-flow, and
predetermined steps of the main flow are executed again.
[0104] After the value of .gamma. is determined, the processing for
setting the exposure target value shown in FIG. 4 is started. When
the check in step S12 determines that .gamma.=1, the system
controller 112 executes step S15, wherein it sets the exposure
target value (in input digital values) to "104." When the check in
step S12 does not determine that .gamma.=1, the next step S13 is
executed to see if y=0.7. If this is the case, step S16 is executed
to set the exposure target value (in input digital values) to "71."
If step S13 does not determine that .gamma.=0.7, step S14 is
executed to set the exposure target value (in input digital values)
to "46." Each of these target values corresponds to the output
digital value 104 in the gradation property determined then. In any
case, step S17 is executed to terminate the sub-flow, and
predetermined steps of the main flow are executed again.
[0105] In the embodiment described above, the gain of the gain
control amplifier 107A of the preprocessing circuit 107 is kept
fixed. It should be noted that the exposure control method is not
limited to the method in which the amount of charge in an ordinary
type of element is controlled. That is, the gain control amplifier
107A may be employed to vary the circuit gain of the linear system.
This variable circuit gain method may be used singly or in
combination with the method described above.
[0106] When any one of the input digital values 104, 71, 46 is set
(as a target value) in the manner described above, the system
controller 112 shown in FIG. 1 sets the exposure control driver 117
so that exposure control is carried out using the input digital
value set by the exposure control driver 117 as a control target.
Moreover, the system controller 112 sets the digital processing
circuit 108 so that the gradation converter 108A of the digital
processing circuit 108 performs a processing of converting the
input digital image signal to the output digital image signal by
use of the gradation set by the user with the mode setting switch
113A as described above, that is, .gamma..
[0107] When the photography is started, the exposure control
mechanism 103 is controlled in accordance with the set gradation
property and the photographic subject is photographed by the CCD
105 with the proper exposure based on the exposure target value.
The image signal from the CCD 105 is supplied to the preprocessing
circuit 107 and is then subjected to a predetermined .gamma.
conversion processing by the gradation converter 108A of the
digital processing circuit 108, converted to the image signal such
that the digital output value as the target is the center output
value, and stored in the memory card 110. Moreover, the signal is
supplied to the LCD 111, and displayed in the LCD 111.
[0108] As described above, even when the user changes the gradation
property by a series of processing and performs the photography in
accordance with user's taste, the exposure target value of the
input digital value is changed, and an output center point is kept
at a constant reference point. In this state, the output
corresponding to the photographic subject image having the
gradation property according to the user's taste is outputted via
the digital processing circuit 108.
[0109] The present invention is not limited to the aforementioned
embodiment. In the embodiment, the exposure is controlled on the
gamma input side. However, it is also possible to fix the target
value to 104 and control the exposure on the gamma output side. In
this case, control of output property can be simplified.
Additionally, an influence of the gamma value sometimes occurs in
average calculation of the photographic subject.
[0110] FIG. 5 is a graph showing the control system of the
gradation property and exposure target value according to another
embodiment of the present invention. In FIG. 5, to simplify the
description, the eight bit data is assumed for both the input and
output digital data. In FIG. 5, a solid line is a graph of a
standard gradation (.gamma.=0.45), a broken line is a graph for
.gamma.=0.7 (with the knee), and a dashed line is a graph for
.gamma.=1 (with the knee). Similarly as the aforementioned
embodiment, in the digital camera of the present embodiment, the
mode setting switch 113A can change the mode to other two modes
(.gamma.=0.7, .gamma.=1) from the default "standard".
[0111] As described above, the standard .gamma.=0.45 is actually
the recommended property of JEIDA DCF specification as the digital
camera standard specification: y=1.099.times.x.sup.0.45-0.099.
(Additionally, x=input/255, y=output/255, the equation is applied
to x.gtoreq.0.018, and y=4.5.times.x is applied in x<0.018.)
[0112] In the present embodiment, the gradation converter 108A
shown in FIG. 1 has a property of converting the input image signal
to the output image signal with the .gamma. converting property
shown in FIG. 5. For the y property shown in FIG. 5, each .gamma.
property graph shown in FIG. 2 is multiplied by a coefficient so
that the graphs of .gamma.=1, 0.7 and 0.45 pass through a certain
exposure control point (input digital value 46 and output digital
value 104). In other words, the .gamma. property graphs shown in
FIG. 2 are multiplied by the respective coefficients so that the
graphs of .gamma.=1, 0.7 and 0.45 intersect one another at the
certain exposure control point. y=2.2609.times.x
y=1.3525.times.x.sup.0.7
[0113] When the .gamma. graph shown in FIG. 2 is simply multiplied
by the coefficient, the photographic subject reproduction range is
narrowed. Therefore, for the region y>0.75 (output value digital
value of 192 or more) whose signal value is sufficiently larger
than that of the vicinity of the intersection corresponding to the
main photographic subject gradation region, so-called knee property
is disposed. The property has a high contrast with respect to the
main photographic subject (i.e., the region in the vicinity of the
intersection), but is devised so as to secure the reproduction
range by compressing only the high level region.
[0114] The exposure is controlled on the input side of the gamma
converter 108A, that is, in a circuit portion referred to as
so-called linear system, so that the control target value
corresponds to the digital value 46 at all times. Even when the
mode is changed during this exposure control, the sensitivity does
not change, a sensitivity measurement reference point is kept at
104, and the center level of the gradation in the output level of
the main photographic subject is not changed.
[0115] As in the embodiment described above, the system shown in
FIG. 1 performs the exposure control by causing the system
controller 112 to set the parameter of the exposure control driver
117. In the meantime, the control target value is kept at a fixed
digital value 46 and is not varied when the value of .gamma. is
switched from one to another. The setting of the value of .gamma.
is performed in a similar manner to that shown in FIG. 3. It should
be noted that the property corresponding to .gamma.A is ".gamma.=1"
shown in FIG. 5, and the property corresponding to .gamma.B is
".gamma.=0.7" shown in the same Figure.
[0116] Additionally, the intersections of three property curves
completely agree with one another in this example. However,
needless to say, with the "substantial agreement", the object can
sufficiently be achieved. That is, for example, the intersections
of three properties are handled in the above example. When one
intersection is generated for the two property curves, three
intersections in total are generated. If these three points are
sufficiently close to one another, an effect similar to that of the
"agreement" can be fulfilled.
[0117] Therefore, "a common property curve intersection (the point
at which one input value is converted to the same output value
irrespective of the selected property)" in the present invention is
"common" including the aforementioned case, and naturally means
"the same". Moreover, in this case, for judgment of substantial
agreement, similarly as the aforementioned embodiment, the range of
.+-.1/3 EV (-20.6%, +26.0% in terms of relative %) can
substantially be a standard. (Alternatively, the range of .+-.0.1
EV or less is of course further preferable.)
[0118] As described above, when the intersection of the property
curve agrees with a measurement point of "sensitivity", the
sensitivity in the measurement and display can be kept to be
unchanged. When only this viewpoint is noted, it is not essential
to match the sensitivity target value with this point.
[0119] That is, when the exposure control target value deviates
from the property intersection, the output level fluctuates, but
the "sensitivity" at least in the measurement and display is kept
to be constant regardless of the fluctuation. It can be said that
this produces an effect different from/independent of "the setting
of the output level by measurement of the exposure target
value".
[0120] Moreover, when the action of keeping the output level to be
constant is noted, the exposure control target value is set in the
vicinity of the property intersection. Then, if the setting has a
slight deviation, the output level fluctuation is small in the
vicinity of the intersection, and can be in a substantially
negligible range.
[0121] Furthermore, when the property intersection is set to the
sensitivity measurement reference point, and the exposure target
value further agrees with the intersection as in the aforementioned
embodiment, the unchanged sensitivity is fully and remarkably
effectively compatible with the unchanged output level.
[0122] Additionally, the present invention is not limited to the
aforementioned embodiment. In the embodiment, the digital camera
has an automatic exposure function, but this is not limited, and a
camera having a manual exposure may be used. In this case, the
photographic subject exposure amount and output level for the
common intersection of a plurality of property curves are presented
to a camera user (photographer) as a "recommended average exposure
amount" and "recommended average exposure level". Then, the
photographer can use, for example, a single-unit photometer, or an
outer strobe with an independent dimmer function attached thereto
to perform satisfactory photography. In this case, it is
effectively unnecessary to change the setting of the photometer or
the strobe by the gradation property.
[0123] Moreover, the use of the progressive CCD image pickup device
has been described in the embodiment, but a signal reading system
is not limited to this, and an interlace type device may be used.
Furthermore, various solid image pickup devices other than CCD may
be used. Furthermore, the present invention is not necessarily
limited to the digital camera, and can be applied to a movie
camera. Additionally, the present invention can variously be
modified and implemented in a range which does not depart from the
scope of the present invention.
[0124] In the aforementioned embodiment, the sensitivity of CCD 105
shown in FIG. 1 can be enhanced and the output of the image signal
from the CCD 105 can be increased in an addition photography mode
in order to increase the input digital value shown in FIG. 1.
[0125] In addition, in respect of utilizing pixel signals from the
CCD 105, there is known a technique of so called "addition of
pixel", in which pixel signals output from the adjacent pixels are
added to improve a sensitivity or SN ratio of CCD 105. This
technique can be applied, as "addition photographing mode" to the
above embodiment so that the sensitivity of the CCD 105 is
improved. Thus, if the "addition photographing mode" can be set in
the digital camera described above, a level of the image signal
from the CCD 105 can be increased, which corresponds to the input
digital value shown in FIGS. 2 and 3.
[0126] Setting of the addition photography mode will be described
with reference to FIGS. 1, 6 and 7. Additionally, the addition mode
is set with the mode setting switch 113A shown in FIG. 1.
[0127] In the CCD 105 shown in FIG. 1, as shown in FIG. 6, an
interline (IT) type progressive (successive scanning) driving
system is used which is constituted of photodiodes 201 arranged in
a matrix, a plurality of vertical line CCDs 202, one horizontal
line CCD 203, and an output amplifier 204. Here, in order to
simplify the description, it is assumed that the CCD 105 is a
monochromatic device.
[0128] Moreover, as shown in FIG. 7, the image signal outputted
from the image pickup device 105 and inputted to the preprocessing
circuit 107 is inputted to a correlated double sampling circuit
(CDS circuit) 301 in order to obtain a difference between a reset
level and a signal level. In the correlated double sampling circuit
(CDS circuit) 301, a reset noise is removed from the image signal
and the signal is inputted to an OB clamp circuit (OBCLP circuit)
302. In the OB clamp circuit (OBCLP circuit) 302, a part of the
image signal is clamped based on the signal (OB standard level)
from an optical black (OB) pixel. The image signal outputted from
the OB clamp circuit (OBCLP circuit) 302 is inputted to an A/D
converter 303. Moreover, the image signal is converted to a digital
signal in a predetermined sampling frequency for driving the CCD in
the A/D converter 303.
[0129] Here, in the A/D converter 303, a minimum input level (input
0 reference) is equal to a reference voltage VrefZ on a - side and
0V, a maximum input level (i.e., maximum quantized level) is equal
to a reference voltage VrefP on a + side, and this reference
voltage VrefP can be varied. When the reference voltage VrefP is
changed, the maximum quantized level can be varied. Therefore, for
example, when VrefP is doubled, the maximum quantized level is
doubled. Additionally, the maximum input level does not have to be
equal to the reference voltage VrefP. The circuit including the A/D
converter 303 may be constituted such that the maximum quantized
level can be varied by changing the reference voltage VrefP.
[0130] The photography mode includes a usual photography mode and
addition photography mode set with the mode setting switch 113A.
The mode setting switch 113A can change the mode to the addition
photography mode from the usual photography mode or vice versa. In
the addition photography mode, the system controller 112 sets the
CCD driver 106 to the driving system for the addition photography
mode, and the CCD driver 106 drives the CCD 105 so that the CCD 105
adds a plurality of image signals in the device and reads the
signal as follows.
[0131] (1) A VCCD driving pulse for n pixels (n transfer units) is
outputted in each horizontal blanking period. Concretely, n=2 is
set, and two pixels of a vertical direction are added in a
horizontal transfer path.
[0132] (2) During horizontal transfer, m pixels are added by a
diffusion section of a floating diffusion amplifier (FDA) disposed
in an output section of the horizontal transfer path by m pixels
addition driving. That is, after each reset pulse is applied, an
HCCD driving pulse for m pixels (m transfer units) is outputted
during a charge transfer period. Concretely, m=2 is set, and
vertically added two pixels, that is four pixels of a pixel portion
are added.
[0133] The horizontal vertical 2.times.2 pixels are added in the
aforementioned method. Moreover, the exposure amount target value
is controlled to be 1/2 of the value of the usual photography mode.
Furthermore, a size of the reference voltage VrefP in the A/D
converter 303 is set to be twice the size in the usual photography
mode.
[0134] In this case, when four pixels are added with 1/2 of the
exposure amount, twice the usual image signals (1/2.times.4=2) are
outputted from the image pickup device. Here, since VrefP of the
A/D converter 303 is set to be double the voltage of the usual
mode, the image signal is A/D converted without being clipped (more
accurately at the same clip level as the usual level) by the A/D
converter 303. That is, even when four pixels are added, the added
pixel signal can be prevented from being clipped by the A/D
converter 303, and image quality deterioration can be
prevented.
[0135] Moreover, the exposure amount is 1/2 (1/N) and the number of
pixels to be added is 4 in the above description, but other
arbitrary settings may be used. For example, when the exposure
control target value is the same as the usual value (once), the
signal level becomes fourfold, and VrefP is therefore set to be
fourfold as compared with the usual photography mode. The relation
is shown in the following Table 1. TABLE-US-00001 TABLE 1 Addition
of Exposure target 4 pixels level VrefP N 1 1 Y 1/2 2 Y 1 4
[0136] Moreover, the relation for the addition (addition of two
pixels) only of the vertical direction may be set as shown in the
following Table 2. TABLE-US-00002 TABLE 2 Addition of Exposure
target 2 pixels level VrefP N 1 1 Y 1/2 1 Y 1 2
[0137] Also in the addition of two pixels only of the vertical
direction, (digital) addition averaging operation of horizontal two
pixels (digital) is performed in the beginning of a digital signal
processing after the AD conversion, and the addition of four pixels
can comprehensively be realized.
[0138] Here, any addition is regarded as the comprehensive four
pixels addition. A difference between two types of imaging will
supplementally be described for two exposure target levels (1/2 and
1) during the pixel addition in Table 1 or 2. The target level of
1/2 corresponds to the imaging with the enhanced double sensitivity
while an SN and saturation level equal to those of a non-addition
time are secured as described above. Moreover, the target level of
1 corresponds to the imaging with the enhanced double SN (6 dB) and
at the same sensitivity and saturation levels as those of the
non-addition time.
[0139] As described above, according to the present embodiment, the
reference voltage VrefP in the A/D converter is varied in
accordance with the number of added pixels in the addition
photography mode, the quantized maximum level is variably set, and
the pixels are added by the analog addition system. Even in this
case, since the A/D input voltage exceeds the A/D quantized maximum
voltage, a problem of clipped signal can be solved. Therefore, the
image quality deterioration because of the clip in the A/D
converter is prevented, while the photography with the enhanced
sensitivity by the pixel information addition of the analog
addition system can be realized.
[0140] Additionally, the present invention is not limited to the
aforementioned embodiment. The CCD is used as the image pickup
device in the embodiment, but the device is not limited to the CCD,
and charge transfer devices (CTD) including BBD, CID, and the like
can also be applied. Furthermore, the number of added pixels is not
limited to four or two, and can appropriately be changed in
accordance with the specification.
[0141] Moreover, "the quantized maximum level in the A/D converting
means for quantizing the image signal read from the image pickup
device" is referred to by noting a relative quantized level with
respect to the image signal outputted from the image pickup device.
Therefore, means for variably setting the level may be other than
means for variably setting the reference voltage of the A/D
converter according to the embodiment. Arbitrary means may be used,
such as a constitution in which an amplifier or attenuator is
disposed before the image signal outputted from the image pickup
device is inputted to the A/D converter, and an amplification or
attenuation factor is variably set.
[0142] Furthermore, an example of monochromatic imaging apparatus
has been described in the embodiment, but the present invention can
also be applied to a color imaging apparatus. Furthermore, needless
to say, the present invention can be applied not only to the
digital still camera but also to arbitrary imaging apparatuses
including a movie camera.
[0143] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *