Digital Camera Patent Abstract
A digital camera includes: an image-capturing device which captures
an image of a photographic subject which passes through a photographic
lens, and outputs image data; a gain adjustment section which performs
gain adjustment by multiplying the image data output from the image-capturing
device by a predetermined gain; a detection section which detects
an optical axis of the photographic lens upon the image-capturing
device; and a gain adjustment coefficient calculation section which
calculates a gain adjustment coefficient according to distance between
a position of a part of the image on the image-capturing device
and the optical axis, and the gain adjustment section performs gain
adjustment of image data corresponding to the position of the part
of the image on the image-capturing device using the gain adjustment
coefficient. Digital Camera Patent Claims
What is claimed is:
1. A digital camera comprising: an image-capturing device which
captures an image of a photographic subject which passes through
a photographic lens, and outputs image data; a gain adjustment section
which performs gain adjustment by multiplying the image data output
from said image-capturing device by a predetermined gain; a detection
section which detects an optical axis of said photographic lens
upon said image-capturing device based upon the image data output
by the image-capturing device; and a gain adjustment coefficient
calculation section which calculates a gain adjustment coefficient
according to a distance between a position of a part of the image
on said image-capturing device and said optical axis, wherein said
gain adjustment section performs the gain adjustment of the image
data corresponding to the position of the part of the image on said
image-capturing device using said gain adjustment coefficient.
2. A digital camera according to claim 1, wherein: the detection
section detects the optical axis by detecting a point which has
a maximum value among image data obtained from a photographic subject
which has brightness that is constant over an entire photographic
region.
3. A digital camera according to claim 1, further comprising: a
color filter provided upon said image-capturing device, wherein
said gain adjustment coefficient calculation section calculates
a gain adjustment coefficient for each color for the image data
which have been captured via said color filter.
4. A digital camera according to claim 1, wherein: said gain adjustment
coefficient is a coefficient which compensates for reduction of
light intensity based upon a cosine fourth power law for said photographic
lens.
5. A digital camera according to claim 1, wherein: said image-capturing
device comprises a plurality of pixels; an on chip lens which increases
light gathering rate is formed upon each of said plurality of pixels;
and said gain adjustment coefficient is a coefficient which compensates
for reduction of transmitted light intensity due to said on chip
lens.
6. A digital camera according to claim 1, wherein: said image-capturing
device comprises a plurality of pixels; an on chip lens which increases
light gathering rate is formed upon each of said plurality of pixels;
and said gain adjustment coefficient is a coefficient which compensates
for reduction of light intensity based upon a cosine fourth power
law for said photographic lens, and for reduction of transmitted
light intensity due to said on chip lens.
7. A digital camera comprising: an image-capturing device which
captures an image of a photographic subject which passes through
a photographic lens, and outputs image data, a gain adjustment section
which performs gain adjustment by multiplying the image data output
from said image-capturing device by a predetermined gain; a detection
section which detects an optical axis of said photographic lens
upon said image-capturing device based upon the image data output
by the image-capturing device; and a gain adjustment coefficient
calculation section which calculates a gain adjustment coefficient
according to a distance between a position of a part of the image
on said image-capturing device and said optical axis, and lens photographic
conditions of said photographic lens including at least one of pupil
position and aperture value, wherein said gain adjustment section
performs the gain adjustment of the image data corresponding to
the position of the part of the image on said image-capturing device
using said gain adjustment coefficient.
8. A digital camera according to claim 7, wherein: the detection
section detects the optical axis by detecting a point which has
a maximum value among image data obtained from a photographic subject
which has brightness that is constant over an entire photographic
region.
9. A digital camera according to claim 7, further comprising: a
color filter provided upon said image-capturing device, wherein
said gain adjustment coefficient calculation section calculates
a gain adjustment coefficient for each color for the image data
which have been captured via said color filter.
10. A recording medium on which a program for image signal processing
is recorded, said program comprising instructions defining: a detection
process in which an optical axis of a photographic lens located
on an image-capturing device is detected based upon image data obtained
by the image-capturing device; a gain adjustment coefficient calculation
process in which a gain adjustment coefficient is calculated according
to a distance between an image position on the image-capturing device
and the optical axis; and a gain adjustment process in which gain
adjustment of the image data is performed corresponding to the image
position on the image-capturing device using said gain adjustment
coefficient.
11. A recording medium according to claim 10, wherein in said gain
adjustment coefficient calculation process, the gain adjustment
coefficient is calculated according to the distance between the
image position on the image-capturing device and the optical axis,
and lens photographic conditions of the photographic lens including
at least one of pupil position and aperture value.
12. A data signal transmitted via a communication line, the data
signal comprising a program for image signal processing, said program
comprising instructions defining: a detection process in which an
optical axis of a photographic lens located on an image-capturing
device is detected based upon image data obtained by the image-capturing
device; a gain adjustment coefficient calculation process in which
a gain adjustment coefficient is calculated according to a distance
between an image position on the image-capturing device and the
optical axis; and a gain adjustment process in which gain adjustment
of the image data is performed corresponding to the image position
on the image-capturing device using said gain adjustment coefficient.
13. A data signal according to claim 12, wherein: in said gain
adjustment coefficient calculation process, the gain adjustment
coefficient is calculated according to the distance between the
image position on the image-capturing device and the optical axis,
and lens photographic conditions of the photographic lens including
at least one of pupil position and aperture value.
14. An image signal processing apparatus that processes image data
output by an image-capturing device, on which a plurality of pixels
are arranged in two dimensions, and which captures an image of a
photographic subject which has passed through a photographic lens,
comprising: a detection section that detects a pixel corresponding
to an optical axis of the photographic lens based upon the image
data output by the image-capturing device; a gain adjustment coefficient
calculation section that calculates a coefficient for adjusting
a gain for each of said pixels according to a distance between the
pixel corresponding to a position of the optical axis of the photographic
lens and each of said pixels; and a gain adjustment section which
adjusts a gain for the image data of each of said pixels by multiplying
a predetermined gain by said coefficient.
15. An image signal processing apparatus according to claim 14,
wherein: the image data output by the image-capturing device includes
data related to lens photographic conditions of the photographic
lens including at least one of pupil position and aperture value;
and said gain adjustment coefficient calculation section calculates
the coefficient for adjusting the gain for each of said pixels according
to the distance between the pixel corresponding to the position
of the optical axis and each of said pixels, and said data related
to lens photographic conditions of the photographic lens including
at least one of pupil position and aperture value.
Digital Camera Patent Description
INCORPORATION BY REFERENCE
The disclosure of the following priority application is herein
incorporated by reference: Japanese Patent Application No. 11-206140
filed Jul. 21, 1999.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital camera and to an image
signal processing apparatus in which an image of a photographic
subject is stored as electronically compressed image data.
2. Description of the Related Art
A known type of digital camera comprises an image-capturing device
such as a CCD which captures an image of a photographic subject
via a photographic lens and outputs image data, and an image processing
circuit which performs image processing, such as for example white
balance adjustment or gamma correction, by adjusting the gain for
the image data output from the image-capturing device. In the image
processing circuit image processing is performed by calculating
parameters such as the R-gain and/or the B-gain for the white balance
adjustment, or the gradation curve for the gamma correction or the
like, according to a previously determined algorithm, based upon
the image data output from the image-capturing device.
With the above described digital camera, a white cap is fitted
upon the photographic lens or an image is captured of a photographic
subject which has uniform white brightness, a white balance adjustment
coefficient is calculated based upon the image data output from
a predetermined region of the CCD, and white balance adjustment
is performed for the image data from all of the regions of the CCD
using this adjustment coefficient which has been calculated. Generally
the light gathering rate for the CCD is reduced, since a large number
of pixels (picture elements) are gathered in a specified area in
order to increase the number of pixels, and therefore the area of
the CCD for one pixel becomes small. In this connection, in order
to increase the light gathering rate of the CCD, on chip micro lenses
are formed upon the CCD.
With this type of CCD, the strength of the R, G, and B components
of the light which is received by the photodiodes via the on chip
lenses changes according to the angle of the light which is incident
upon the on chip lenses. Accordingly, with a digital camera which
photoelectrically converts light which passes through a photographic
lens and is directed to a CCD, when the angle of incidence of the
light upon the on chip lenses on the CCD changes due to change of
the position of the pupil of the photographic lens, change of the
aperture value, or change of the relative positions of the optical
axis of the photographic lens and the CCD, it is easy for the strengths
of the R, G, and B components of the light received by the photodiodes
to change and for the white balance color adjustment to become improperly
adjusted, and there is a danger that color loss and color fogged
images may occur. In particular, a problem can easily occur with
a digital camera of the high picture quality type in which the number
of pixels can exceed 200,000.
SUMMARY OF THE INVENTION
The objective of the present invention is to propose a digital
camera which is capable of sufficiently suppressing the phenomena
of color loss and color fog by correcting changes in the output
values of image data of the CCD generated by changes of the relative
positions of the optical axis of the photographic lens and an image-capturing
device such as a CCD or the like, and changes of the position of
the pupil and the aperture value of the photographic lens, and to
propose an image signal processing apparatus which is capable of
this type of image processing.
In order to attain the above objective, a digital camera according
to the present invention comprises: an image-capturing device which
captures an image of a photographic subject which passes through
a photographic lens, and outputs image data; a gain adjustment section
which performs gain adjustment by multiplying the image data output
from the image-capturing device by a predetermined gain; a detection
section which detects an optical axis of the photographic lens upon
the image-capturing device; and a gain adjustment coefficient calculation
section which calculates a gain adjustment coefficient according
to distance between a position of a part of the image on the image-capturing
device and the optical axis, and the gain adjustment section performs
gain adjustment of image data corresponding to the position of the
part of the image on the image-capturing device using the gain adjustment
coefficient.
In this digital camera, it is preferred that the gain adjustment
coefficient is a function of distance between the position of the
part of the image and the optical axis.
Also, it is preferred that a color filter provided upon the image-capturing
device is further provided and the gain adjustment coefficient calculation
section calculates a gain adjustment coefficient for each color
for image data which have been captured via the color filter.
Also, it is preferred that the gain adjustment coefficient is a
coefficient which compensates for reduction of light intensity based
upon a cosine fourth power law for the photographic lens.
Also, it is preferred that the image-capturing device comprises
a plurality of pixels; an on chip lens which increases light gathering
rate is formed upon each of the plurality of pixels; and the gain
adjustment coefficient is a coefficient which compensates for reduction
of transmitted light intensity due to the on chip lens.
Also, it is preferred that the image-capturing device comprises
a plurality of pixels; anon chip lens which increases light gathering
rate is formed upon each of the plurality of pixels; and the gain
adjustment coefficient is a coefficient which compensates for reduction
of light intensity based upon a cosine fourth power law for the
photographic lens, and for reduction of transmitted light intensity
due to the on chip lens.
Another digital camera according to the present invention comprises:
an image-capturing device which captures an image of a photographic
subject which passes through a photographic lens, and outputs image
data; a gain adjustment section which performs gain adjustment by
multiplying the image data output from the image-capturing device
by a predetermined gain; a detection section which detects an optical
axis of the photographic lens upon the image-capturing device; and
a gain adjustment coefficient calculation section which calculates
a gain adjustment coefficient according to distance between a position
of a part of the image on the image-capturing device and the optical
axis, and lens photographic conditions of the photographic lens
including at least one of pupil position and aperture value, and
the gain adjustment section performs gain adjustment of image data
corresponding to the position of the part of the image on the image-capturing
device using the gain adjustment coefficient.
In this digital camera, it is preferred that the gain adjustment
coefficient is a function of distance between the position of the
part of the image and the optical axis.
Also, it is preferred that a color filter provided upon the image-capturing
device is further provided, and the gain adjustment coefficient
calculation section calculates a gain adjustment coefficient for
each color for image data which have been captured via the color
filter.
On a recording medium according to the present invention a program
for image signal processing is recorded. The program comprises:
a detection process in which an optical axis of a photographic lens
located on an image-capturing device is detected in image data obtained
by the image-capturing device; a gain adjustment coefficient calculation
process in which a gain adjustment coefficient is calculated according
to distance between an image position on the image-capturing device
and the optical axis; and a gain adjustment process in which gain
adjustment of image data corresponding to the image position on
the image-capturing device using the gain adjustment coefficient.
In this recording medium, it is preferred that in the gain adjustment
coefficient calculation process, a gain adjustment coefficient is
calculated according to distance between an image position on the
image-capturing device and the optical axis, and lens photographic
conditions of the photographic lens including at least one of pupil
position and aperture value.
A data signal according to the present invention transmitted via
a communication line comprises a program for image signal processing.
The program comprises: a detection process in which an optical axis
of a photographic lens located on an image-capturing device is detected
in image data obtained by the image-capturing device; a gain adjustment
coefficient calculation process in which a gain adjustment coefficient
is calculated according to distance between an image position on
the image-capturing device and the optical axis; and a gain adjustment
process in which gain adjustment of image data corresponding to
the image position on the image-capturing device using the gain
adjustment coefficient.
In this data signal, it is preferred that in the gain adjustment
coefficient calculation process, a gain adjustment coefficient is
calculated according to distance between an image position on the
image-capturing device and the optical axis, and lens photographic
conditions of the photographic lens including at least one of pupil
position and aperture value.
An image signal processing apparatus according to the present invention
that processes image data output by an image-capturing device, on
which a plurality of pixels are arranged in two dimensions, and
which captures an image of a photographic subject which has passed
through a photographic lens, comprises: a gain adjustment coefficient
calculation section that calculates a coefficient for adjusting
a gain for each of the pixels according to distance between a pixel
corresponding to a position of an optical axis of the photographic
lens and each of the pixels; and a gain adjustment section which
adjusts a gain for image data of each of the pixels by multiplying
a predetermined gain by the coefficient.
In this image signal processing apparatus, it is preferred that
the image data output by the image-capturing device includes data
related to lens photographic conditions of the photographic lens
including at least one of pupil position and aperture value; and
the gain adjustment coefficient calculation section calculates a
coefficient for adjusting a gain for each of the pixels according
to distance between the pixel corresponding to the position of the
optical axis and each of the pixels, and the data related to lens
photographic conditions of the photographic lens including at least
one of pupil position and aperture value.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a figure showing the structure of one embodiment of a
single lens reflex digital still camera.
FIG. 2 is a block diagram showing one embodiment of a signal processing
system of a single lens reflex digital still camera.
FIG. 3 is a block diagram for explanation of a circuit which performs
line processing in the signal processing system shown in FIG. 2.
FIG. 4 is a figure showing an arrangement of color filters provided
in a CCD.
FIG. 5 is a figure for explanation of a cosine fourth power law
for a photographic lens.
FIG. 6 is a figure showing an X-Y coordinate system giving the
positions of the pixels on the CCD for the optical system shown
in FIG. 5.
FIG. 7 is a figure showing changes of signal value output from
the CCD with respect to image height.
FIGS. 8A and 8B are figures showing the situation when parallel
light incident upon the photographic lens falls upon the CCD via
an aperture.
FIG. 9 is a figure showing changes of signal value output from
the CCD with respect to aperture value of an interchangeable zoom
lens.
FIG. 10A is a figure showing the ratios between a signal value
output from a central portion of the CCD and a signal value output
from its peripheral portions, and the position of an exit pupil
of the interchangeable zoom lens.
FIG. 10B is a figure showing a numerical array of FIG. 10A.
FIG. 11 is a figure for explanation of a focal point detection
device.
FIG. 12 is a flow chart showing presetting processing.
FIG. 13 is a flow chart showing a program initiated by a half press
switch.
FIG. 14 is a figure showing the structure of processing by a personal
computer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following an embodiment of the present invention will be
described with reference to the drawings. As shown in FIG. 1, the
single lens reflex digital still camera according to this embodiment
comprises a camera main body 70, a viewfinder device 80 detachably
fitted to the camera main body 70, and an interchangeable zoom lens
90 detachably fitted to the camera main body 70 which incorporates
a lens 91 and an aperture 92. Light from a photographic subject
passes through the interchangeable zoom lens 90 and enters the camera
main body 70, is directed to a viewfinder device 80 by a quick return
mirror 71 which before shutter release is in the position shown
by the dotted lines, and, as well as being focused upon a viewfinder
matte 81, is also focused upon a focal point detection device 36.
The image of the photographic subject upon the viewfinder matte
81 is also conducted to an eyepiece lens 83 by a pentaprism 82.
Further, before shutter release, the image of the photographic subject
is incident via a prism 84 and a focusing lens 85 upon a white balance
sensor 86 which detects the color temperature of the image of the
photographic subject. After shutter release the quick return mirror
71 rotates to its position shown by solid lines, and the light from
the photographic subject is focused via a shutter 72 upon an image-capturing
device 73. A white cap 93 is fitted to the interchangeable zoom
lens 90 when setting a white balance adjustment value. Since the
white cap 93 is used for illuminating uniform diffused light into
the camera main body 70, it is not used during normal photography.
FIG. 2 is a block diagram showing an embodiment of a circuit of
a digital camera according to the present invention. A half press
signal and a full press signal are respectively input to a CPU 21
from a half press switch 22 and a full press switch 23 which operate
together with a shutter release button. When the half press switch
22 is actuated and the half press signal is input, upon a command
from the CPU 21 a focal point detection device 36 detects the focal
point adjustment state of the interchangeable zoom lens 90, and
drives the lens 91 to a focusing position so as to focus light from
the photographic subject which is incident into the interchangeable
zoom lens 90 into an image upon the image-capturing device 73. Further,
a CCD 26 of the image-capturing device 73 is driven and controlled
via a timing generator 24 and a driver 25. And the operating timing
of an analog processing circuit 27 and an A/D conversion circuit
28 are controlled by the timing generator 24.
When the full press switch 23 is operated to be ON after the half
press switch 22 is operated to be ON, the quick return mirror 71
rotates upwards, light from the photographic subject from the interchangeable
zoom lens 90 is focused to an image upon the light receiving surface
of the CCD 26, and signal electrical charges are accumulated upon
the CCD 26 according to the brightness of the image of the photographic
subject. The signal electrical charges accumulated upon the CCD
26 are swept out or discharged by a driver 25, and are input to
an analog signal processing circuit 27 which comprises an AGC circuit
and a CDS circuit and the like. After analog processing such as
gain control, noise removal and the like has been executed upon
the analog image signal by the analog signal processing circuit
27, it is converted into a digital signal by the A/D conversion
circuit 28. The signal which has been converted into digital form
is conducted to an image processing circuit 29 which, for example,
may be constituted by an ASIC, and therein image preprocessing such
as white balance adjustment, outline compensation, and gamma correction
is performed.
Further, format processing (image postprocessing) for JPEG compression
is performed upon the image data which has been subjected to image
preprocessing, and this image data is then temporarily stored in
a buffer memory 30.
The image data written into the buffer memory 30 is processed into
image data for display by a display image generation circuit 31,
and is displayed upon an external monitor 32 such as an LCD or the
like as the result of photography. Further, the image data written
into the buffer memory 30 is subjected to data compression according
to the JPEG method by a compression circuit 33 at a specified ratio,
and is recorded upon a recording medium (PC card) 34 such as a flash
memory or the like.
FIG. 3 is a block diagram showing the details of the image processing
circuit 29 for a digital camera which operates as described above.
FIG. 3 is a line processing circuit 100 which processes the signals
on each line of the image data from the CCD 26, and performs the
above described image preprocessing. This line processing circuit
100 is for performing various types of signal processing described
hereinafter upon 12 bit R, G, and B signals which are output from
the A/D conversion circuit 28, and comprises a digital clamp circuit
101, a gain setting circuit 102, a gain adjustment circuit 103,
a black level circuit 104, and a gamma correction circuit 105.
The 12 bit R, G, and B signals which are output from the A/D conversion
circuit 28 are input to the digital clamp circuit 101 in the order
of the points in each line as to the output of the CCD 26, after
the data from pixels which are faulty (whose addresses are specified
in advance and are stored in a register) has been compensated. The
digital clamp circuit 101, in the order of the points in each line
as to the output of the CCD 26, subtracts from each pixel data on
that line the weighted average of a plurality of pixel data which
are used as optical black.
The gain setting circuit 102 sets the gain for adjustment of the
pixel data of each of the colors R, G, and B. For gain adjustment
according to this embodiment, beforehand the white cap 93 is fitted
onto the zoom lens 90, and diffused light is captured so as to constitute
a white standard, and, based upon this image data, presetting processing
is performed so as to calculate the gain for adjustment of the pixel
data for each color. In the presetting processing, while changing
the aperture value and the focal distance of the interchangeable
zoom lens 90, the gain for adjustment is calculated, and the calculated
gain for adjustment is stored. The gain setting circuit 102 reads
out an adjustment value of gain, which was calculated and stored
in presetting processing, corresponding to the focal distance and
the aperture value of the interchangeable zoom lens 90, and sets
it in a gain adjustment circuit 103.
Presetting Processing
Presetting processing is performed by fitting the white cap 93
which is to constitute the white standard to the interchangeable
zoom lens 90 to calculate an adjustment value of gain for compensating
for white balance adjustment and for reduction of received light
intensity in the circumference of the image-capturing surface of
the CCD 26. The white balance detection circuit 35 of FIG. 2 includes
a white balance sensor 35A (the white balance sensor 86 of FIG.
1) which is a color temperature sensor, an A/D conversion circuit
35B which converts the analog signal from the white balance sensor
35A into a digital signal, and a CPU 35C which produces a white
balance adjustment signal based upon the digital color temperature
signal. The white balance sensor 35A may, for example, be made up
from a plurality of photoelectric conversion elements which are
respectively sensitive to red color R, blue color B, and green color
G, and it is illuminated by the light image of the entire photographic
field. The CPU 35C obtains a gain adjustment value for the R signal
and a gain adjustment value for the B signal in the order explained
below, based upon the outputs of the plurality of photoelectric
conversion elements.
In order to capture an image with the diffused light which is to
become the white standard, the white cap 93 is fitted to the interchangeable
zoom lens 90, and, from among the image data of the entire region
of the photographic field which has been captured, image data is
extracted, for example, for a 512.times.512 region centered upon
the area for which the focal point detection device 36 performs
focal point detection, and a gain adjustment value R-gain for the
R signal and a gain adjustment value B-gain for the B signal are
calculated according to the following Equations (1) and (2). At
this time, if for example color filters are arranged upon the 512.times.512
pixel region as shown in FIG. 4, the average values of the R, G,
and B signals are calculated with Equations (3) through (5), and,
as shown in Equations (1) and (2), the gain adjustment values R-gain
and B-gain are calculated from the ratio of the average value G-ave
of the G signal and the average value R-ave of the R signal, and
the ratio of the average value G-ave of the G signal and the average
value B-ave of the B signal.
These gain adjustment values R-gain and B-gain which have been
calculated are temporarily stored in a register of the CPU 21 as
preset white balance adjustment values, along with the focal point
distance and the aperture value of the interchangeable zoom lens
90 which are input from a lens information input section 38. This
type of average value method results in determining the average
value of the gradation of each R, G, and B signal of the image data,
and the adjustment result for white balance (overall white balance)
is made satisfactory experimentally. That is to say, the gain adjustment
values R-gain and B-gain are calculated so as to bring the average
color temperature of the screen close to white, and white balance
adjustment is performed based upon these calculated gain adjustment
values R-gain and B-gain.
The lens information which is input from the lens information input
section 38 relates to the focal point distance (position of the
pupil) of the interchangeable zoom lens 90, and this focal point
distance of the lens is changed by a signal from a zoom switch not
shown in the figures, and the focal point distance of the lens is
recognized by a signal from a zoom encoder housed within the lens.
Next, compensation is performed of the light intensity incident
upon the CCD 26, based upon the image data for the entire photographic
field for the light diffused by the white cap 93 which has been
captured. Generally, with a photographic lens, reduction of the
peripheral light intensity occurs according to a so called cosine
fourth power law. Referring to FIG. 5 for explanation of the cosine
fourth power law of a photographic lens, an image 99 of a photographic
subject 9 positioned on the optical axis of a lens 91 is focused
upon the CCD 26. The point 9a on the photographic subject 9 is focused
to the point 99a on the CCD 26, and the point 9b on the photographic
subject 9, which is incident at an angle .theta. with respect to
the optical axis of the lens 91, is focused as the point 99b on
the CCD 26. If the brightness of the points 9a and 9b on the photographic
subject 9 is the same, the intensity of the incident light at the
point 99b on the CCD 26 is reduced to be proportional to cos.sup.4.theta.
with respect to the intensity of the incident light at the point
99a. Since the brightness of the entire region of the photographic
field is made constant by the operation of the white cap 93 which
is fitted to the interchangeable zoom lens 90, in the image data
of the image which have been captured, the value output from the
pixel on the CCD 26 corresponding to the optical axis of the interchangeable
zoom lens 90 constitutes the maximum value, and the output value
for a pixel whose pixel position on the CCD 26 is offset from the
optical axis is reduced in proportion to the above described cosine
fourth power law.
If the light intensity upon the CCD 26 on the optical axis of the
lens 90 is termed L.sub.0, then the light intensity L upon the CCD
26 is given by the following Equation (6). In the optical system
of FIG. 5, when the pixel position on the CCD 26 is expressed in
the coordinates XY shown in FIG. 6, if the origin O which is the
central point of the CCD 26 is taken as the optical axis, then the
image height his given by Equation (7), and the angle .theta. is
given by Equation (8).
Since the angle of incidence I of the light upon the CCD 26 changes
as the image height h becomes higher, in case that the transmission
loss of the on chip lenses formed on the CCD 26 changes according
to the angle of incidence I, the level of the signal output from
the CCD 26 changes, since the transmitted light intensity on the
CCD 26 is reduced by the on chip lenses in addition to the above
described cosine fourth power law. The reduction Lm of light intensity
due to the on chip lenses on the CCD 26 is, for example, given by
the following Equations (9) through (11).
where d1 and d2 are predetermined values of image height, and Ka
through Ki are coefficients.
FIG. 7 is a graph showing the relationship between distance (image
height) h from the optical axis and the signal for the G color component
output from the pixel at that position. Since the reduction of peripheral
light intensity on the CCD 26 which occurs as the image height h
becomes greater is expressed by the product of Equation (6) and
Equations (9) through (11), the above described preset white balance
adjustment value is multiplied by the reciprocal of the product
of the above Equations (6) and (9) through (11) when setting gain
adjustment values for the signals which are output from each of
the pixels on the CCD 26. In other words, the image height h is
calculated from the coordinates of the position of the pixel which
has output the signal and the coordinates of the position of the
pixel which corresponds to the optical axis of the lens 91, and
a gain adjustment value is determined by obtaining the reduction
of the peripheral light intensity is corresponding to this image
height h as shown in FIG. 7 and by taking its reciprocal. That is,
the gain adjustment value for compensation for the reduction of
peripheral light intensity is small for pixel data close to the
optical axis, and becomes greater for pixel data further from the
optical axis.
The adjustment value for gain, which has been determined as the
product of the reciprocal of the product of the above Equations
(6) and (9) through (11) and the preset white balance adjustment
value, is stored in a register of the CPU 21, along with the focal
point distance (the position of the exit pupil) and the aperture
value of the interchangeable zoom lens 90. It should be understood
that the above Equations (6) and (9) through (11) are calculated
for each of the R, G, and B color components output from the pixels
of the CCD 26.
The above described preset white balance and reduction of peripheral
light intensity change in the following manner according to the
aperture value of the adjustable zoom lens 90 and its focal point
distance (exit pupil position). FIGS. 8A and 8B are figures showing
the situation when parallel light incident upon the interchangeable
zoom lens 90 falls via the aperture 92 upon the CCD 26. As shown
in FIG. 8A, with a typical zoom lens 90, the exit pupil Pa becomes
smaller as the setting value of the aperture 92 becomes larger and
it is closed down, and as the opening angle Aa of the lens 91 becomes
narrower. Since at this time the angle Ia of incidence of the light
incident upon the CCD 26 hardly changes, the changes in the levels
of the R, G, and B color signals output from the CCD 26 are small.
However, as shown in FIG. 8B, when the setting value of the aperture
92 approaches fully open, the exit pupil Pb becomes larger, and
the opening angle Ab becomes larger. And the angle of incidence
Ib of the light falling upon the CCD 26 changes according to widening
of the opening angle Ab. When the transmission loss of the on chip
lenses formed on the CCD 26 has a wavelength dependence corresponding
to the angle of incidence, or when the characteristics of a color
filter formed on the CCD 26 as shown in FIG. 4 vary according to
the angle of incidence, the levels of the R, G, and B color signals
output from the CCD 26 change individually.
FIG. 9 is a graph of sensitivity dependence on aperture value showing
an example of change of the value of the G color component signal
output from the CCD 26 with respect to the aperture value of the
interchangeable zoom lens 90 fitted to the camera main body 70.
This data was obtained by measuring the output of the CCD 26 while
adjusting the intensity of the light incident upon the interchangeable
zoom lens 90, i.e. the brightness of the light source, so that the
brightness at the mounting position of the CCD 26 remained constant
even though the aperture value of the interchangeable zoom lens
90 changed, and shows the output of the CCD 26 corresponding to,
for example, a region taking the area for which focus detection
is performed as a center. Since the vertical axis of the FIG. 9
graph takes as standard the value output from the CCD 26 when the
aperture value=F8, it will be understood from the graph of FIG.
9 that, due to the above described reasons, the signal value diminishes
when the aperture 92 of the interchangeable zoom lens 90 is put
to the fully open side.
Further, FIG. 10A is a graph showing an example of change of the
signal value output from the CCD 26 with respect to the position
of the exit pupil of the interchangeable zoom lens 90 fitted to
the camera main body 70, and FIG. 10B shows all the numerical values
therein together. This data was obtained by measuring the output
of the CCD 26 while adjusting the intensity of the light incident
upon the interchangeable zoom lens 90, i.e. the brightness of the
light source, so as to keep constant the brightness level at the
position at which the CCD 26 was mounted even if the position of
the pupil of the interchangeable zoom lens 90 changed, and, for
example, shows the output of the CCD 26 corresponding to the region
centered upon the area for which focus detection is performed. And
the vertical axis in the graph of FIG. 10A shows the ratio between
the value of the signal output from the peripheral portion of the
CCD 26 and the value of the signal output from the central portion
of the CCD 26. According to FIGS. 10A and 10B it will be understood
that the position of the exit pupil changes as the focal point distance
of the interchangeable zoom lens 90 changes, and the shorter the
position of the exit pupil becomes, the lower does the value of
the signal output from the peripheral portion of the CCD 26 become.
This is due to the same reason as stated above.
As explained above, the adjustment value for the gain set by multiplying
together the reciprocal of the product of the above Equations (6)
and (9) through (11) and the preset white balance adjustment value
of the above Equations (1) through (5) needs to be changed according
to the focal point distance and the aperture value of the interchangeable
zoom lens 90. In this embodiment, by way of example, three conditions
are set for the focal point distance of the interchangeable zoom
lens 90: a wide side, a tele side, and an intermediate therebetween.
Further, by way of example, two conditions are set for the aperture
value of the interchangeable zoom lens 90: a fully opened side and
a stopped down side. Adjustment values for the gain which have been
calculated for these six combinations of conditions are stored in
registers of the CPU 21 along with the focal point distances and
the aperture values of the interchangeable zoom lens 90.
It should be understood that the above exemplary explanation related
to the case of the optical axis of the lens 91 being in agreement
with the origin O which was the center of the CCD 26, but, in the
case that photography is performed with the positional relationship
between the lens 91 and the center of the CCD 26 having undergone
slippage, so called camera movements, for example when the optical
axis is positioned at the coordinate (x.sub.1, y.sub.1), it will
be sufficient to substitute (x-x.sub.1) for x and (y-y.sub.1) for
y in the above Equations (7) through (8).
Moreover, when the gain setting circuit 102 sets the gain adjustment
value upon the gain adjustment circuit 103, at the same time compensation
is also performed with a predetermined compensation value prepared
in advance in order to compensate for unevenness among CCDs 26 one
of which is used. In these processes of compensation, the unevenness
of sensitivity of the G signal is compensated, and the unevenness
of sensitivity ratio to the R signal or the B signal is compensated.
Accordingly, the gain adjustment values become: for the R component,
Equation (1).times.{the reciprocal of the product of Equation (6)
and any one of Equations (9) through (11) }.times.(the unevenness
compensation of sensitivity ratio to the G signal); and, for the
B component, Equation (2).times.{the reciprocal of the product of
Equation (6) and any one of Equations (9) through (11)}.times.(the
unevenness compensation of sensitivity ratio to the G signal).
Referring to FIG. 3, when by doing this the gain is set by the
gain setting circuit 102, the gain adjustment circuit 103 multiplies
the R and B signals by R-gain and B-gain, which are the gain adjustment
values set by the gain setting circuit 102. The black level circuit
104 adds to the R, G, and B signals the values determined in advance
and stored in the registers of the CPU 21 in the order of the points
in each line as to the output of the CCD 26. The gamma correction
circuit 105 performs gamma correction in the order of the points
in each line as to the output of the CCD 26 by using a gradation
lookup table.
Next, referring to FIG. 11, the structure of the focal point detection
device 36 and the principle of focus detection operation by this
focal point detection device 36 will be explained. The focal point
detection device 36 comprises an infrared light cut filter 700,
a visual field mask 900, a field lens 300, an aperture mask 400,
refocusing lenses 501 and 502, and image sensors 310 etc. A region
800 is the exit pupil of the lens 91 (FIG. 1). Further, regions
801 and 802 are regions upon the region 800 where images of aperture
portions 401 and 402 which are cut through the aperture mask 400
are reverse projected by the field lens 300. It should be understood
that it does not matter whether the position of the infrared light
cut filter 700 is on the left side or on the right side of the visual
field mask 900. Ray bundles which are incident via the regions 801
and 802, after being focused to a point upon a screen 600 equivalent
to the image-capturing device, are focused upon image sensor arrays
310a and 310b via the infrared light cut filter 700, the visual
field mask 900, the field lens 300, the aperture portions 401 and
402, and the refocusing lenses 501 and 502.
The pair of images of the photographic subject focused upon these
image sensor arrays 310a and 310b are mutually brought closer together
in a so called pre-focus state in which the lens 91 focuses a clear
image of the photographic subject more before the screen 600 equivalent
to the image-capturing device (towards the photographic subject),
while by contrast they are mutually farther separated in a so called
after-focus state in which it focuses a clear image of the photographic
subject more after the screen 600 equivalent to the image-capturing
device. And when the images of the photographic subject which are
focused upon the image sensor arrays 310a and 310b are at a predetermined
interval apart, a clear image of the photographic subject is positioned
upon the screen 600 equivalent to the image-capturing device. Accordingly
the focal point adjustment state, in other words in which direction
and by just how far the position at which a clear image is formed
by the lens 91 is separated from the screen 600 equivalent to the
image-capturing device, i.e. the amount by which it deviates, is
determined by photoelectrically converting this pair of images of
the photographic subject into electrical signals by the image sensor
arrays 310a and 310b, and by calculation processing these signals
and obtaining the relative distance between the pair of images of
the photographic subject. Referring to FIG. 11, the focus detection
region corresponds to the superposed portion in the vicinity of
the screen 600 equivalent to the image-capturing device to which
the image sensor arrays 310a and 310b are reverse projected by the
refocusing lenses 501 and 502. By doing this, the focal point is
detected for a focal point detection region within the above described
photographic screen.
When the focal point detection device 36 has detected the focal
point of the focal point detection region, the lens 91 is driven
to a focusing position according to this focal point detection data.
It should be understood that, if the focal point detection region
is multiple, in the above described presetting processing the gain
adjustment values R-gain and B-gain are calculated using 512.times.512
image data centered upon a region selected by the focal point detection
device 36.
The operation of the digital still camera constructed in this manner
will now be explained. FIG. 12 is a flow chart showing the presetting
processing, while FIG. 13 is a flowchart showing a program which
is started by the half press switch. In a step S10 of FIG. 12, when
the full press switch 23 is operated in the state with the white
cap 93 fitted to the interchangeable zoom lens 90, control proceeds
to a step S11 and the CCD 26 accumulates received light signals.
After the accumulation is completed, the accumulated electrical
charges of all the pixels are read out. In a step S12, after the
read out image data has been processed by the analog signal processing
circuit 27, it is converted into digital image data by the A/D conversion
circuit 28, and is stored in the buffer memory 30 via the image
processing circuit 29. In a step S13, the data from the image data
stored in the buffer memory 30 for which the signal value (brightness)
is greatest is detected, and the pixel position (x.sub.1, y.sub.1)
which has output this data is read out. This pixel position (x.sub.1,
y.sub.1) is taken as the position of the optical axis. It should
be understood that, if the center position of the CCD 26 agrees
with the optical axis, it is possible to take the pixel position
at the center of the CCD 26 as the position of the optical axis,
and the step S13 may also be curtailed.
In a step S14, while taking the pixel position (x.sub.1, y.sub.1)
as the optical axis, the image height h is obtained for all pixel
positions, and the reciprocal of the product of the above Equation
(6) and Equations (9) through (11) is calculated. After the preset
white balance adjustment values R-gain and B-gain have been calculated
in a step S15 by the above Equations (1) through (5), control proceeds
to a step S16 in which the product of the above described reciprocal
and the preset white balance adjustment values R-gain and B-gain
is calculated as the adjustment value of the gain. In a step S17,
the adjustment value of the gain is stored in a register of the
CPU 21 as the gain adjustment value, along with the focal point
distance and the aperture value of the interchangeable zoom lens
90. In a step S18, by changing the focal point distance and the
aperture value of the interchangeable zoom lens 90, it is determined
whether or not processing has been performed for all of the above
described six conditions, and in the case of a negative decision
(N) a new condition is set and control is transferred to the step
S11. On the other hand, in the case of a positive decision (P) in
the step S18, the presetting processing of FIG. 12 is concluded.
By the above processing the gain is stored for the six conditions
of focal point distance and aperture value.
Referring to FIG. 13, when the half press switch 22 is operated,
in a step S20 the brightness of the photographic subject is measured
and the exposure is calculated by the photometric device 37, and
the focal point adjustment state of the focal point detection region
is detected by the focal point detection device 36. In a step S21
the adjustment value of the gain which is stored in a register of
the CPU 21 is read out as described above, based upon the focal
point distance of the interchangeable zoom lens 90 output from the
lens information input section 38 and upon the aperture value obtained
by exposure calculation. If no adjustment value of the gain corresponding
to the above described focal point distance and aperture value is
stored in any register of the CPU 21, a pair of adjustment values
of the gain calculated in conditions close to the above described
focal point distance and aperture value are read out from among
the stored adjustment values of the gain, and an adjustment value
of the gain is obtained by interpolation processing from these two
adjustment values of the gain.
If in a step S22 it is determined that the full press switch 23
has been operated, then the quick return mirror springs upwards
and the photographic sequence is started. In a step S23, each pixel
of the CCD 26 accumulates a received light signal, and after accumulation
is completed the accumulated electrical charges in all of the pixels
are read out in order. In a step S24, after the read out image data
has been processed by the analog signal processing circuit 27, it
is converted into digital image data by the A/D conversion circuit
28 and is input to the image processing circuit 29. Next control
passes to a step S25, in which gain adjustment according to the
gain adjustment value described above, .gamma. gradation adjustment
correction, conversion to JPEG format, and the like are performed
by the image processing circuit 29. When image processing has been
completed control passes to a step S26, in which the image data
after image processing is temporarily stored in the buffer memory
30. In a step S27, the image data is read out from the buffer memory
30 and is compressed by the JPEG compression circuit 33. In a step
S28 the compressed data is stored in the PC card 34.
The characteristics of this embodiment will now be summarized.
(1) Since image capture is performed with the white cap 93 fitted
to the interchangeable zoom lens 90 and the optical axis of the
interchangeable zoom lens 90 is detected from the pixel position
upon the CCD 26 for which the pixel output value is the greatest,
and it is arranged that, for all the pixel data output from the
CCD 26, the adjustment values of the gain are compensated according
to image height from the optical axis, therefore it becomes possible
to suppress color loss and color fog caused by the cosine fourth
power law of the lens 91 and reduction of the light intensity transmitted
by the on chip lenses formed upon the CCD 26, and a high quality
image is obtained. (2) It is arranged that the adjustment values
of the gain compensated according to image height h from the optical
axis are stored along with the focal point distance and the aperture
value of the interchangeable zoom lens 90 when these adjustment
values were determined. And, since the adjustment values of the
gain according to the focal point distance and the aperture value
of the interchangeable zoom lens 90 fitted at the time of photography
are read out, and gain adjustment is performed according to the
read out adjustment values of the gain, therefore the occurrence
of image color loss and color fog is prevented and a high quality
image is obtained, even if the light receiving sensitivity of the
CCD 26 changes according to focal point distance and aperture value.
In the above explanation, each of the R, G, and B color components
output from the CCD 26 was calculated individually in the Equations
(6) and (9) through (11) which give the reduction of the peripheral
light intensity, and the gain adjustment coefficients for the R
signal and the B signal were obtained, but it would also be possible
to perform the calculation based only upon the signal values for,
for example, the G color component output from the CCD 26, and to
calculate simply for the R and B colors by multiplying by specified
coefficients to the G color.
Further, in the above explanation, by way of example it was explained
that the gains for the R signal and for the B signal were adjusted
by taking the G signal as a standard. In other words, as an example,
it was explained that, in gain adjustment for the R signal and for
the B signal, Equation (1) or Equation (2), and coefficients based
upon Equation (6) and anyone of Equations (9) through (11) were
multiplied together. However, it would also be acceptable to perform
gain adjustment by multiplying together the G signal and coefficients
based upon Equation (6) and any one of Equations (9) through (11),
as well.
Further, it would also be acceptable to reduce the number of conditions
under which presetting processing was performed, within the range
in which no influence is exerted upon the intensity of the light
received by the CCD 26 and the white balance. In the above explanation,
with regard to the conditions of aperture value and focal point
distance, presetting processing is performed with two aperture values
which are for a fully opened side (minimum aperture) and a stopped
down side (maximum aperture), and with three focal point distances
which are for a wide side, a tele side, and an intermediate between
wide and tele. As a simple method, for example, it will be acceptable
to perform presetting processing with one point at which the aperture
value=F8 and the focal point distance is 75 mm, and to calculate
the adjustment value for the gain for other setting conditions by
multiplying the adjustment value of the gain calculated according
to the above condition by specified coefficients.
It will also be acceptable to make it selectable whether or not
to perform this compensation processing of the gain adjustment value
for the cosine fourth power law as explained above and for the reduction
in transmitted light intensity due to the on chip lenses formed
on the CCD 26.
The above Equations (9)through (11) are separated according to
the image height h, but a compatible equation for all image heights
h like Equation (12) is also acceptable.
where K1 through K4 are coefficients, and h is the image height.
Although the above explanation has been made in terms of a digital
still camera, it is also possible to store the line processing circuit
100 in the form of software as an image processing program upon
a recording medium such as a CD-ROM or a floppy disk or the like,
and to use it when processing an image with a personal computer.
In this case, the image data which have been captured by the CCD
26 and digitized are stored upon a high capacity recording medium
for image data, and when this recording medium is set into the personal
computer and the image data have been read in, line processing as
described before is performed by the above described image processing
program. For example, in FIG. 3, the output data from the black
level circuit 104 can be stored as raw data upon the PC card 34,
and this PC card 34 can be set into the personal computer so that
image processing of the raw data can be performed.
FIG. 14 is a figure showing the processing organization by the
above described personal computer. A personal computer 201 comprises
a CD-ROM drive device (not shown in the figure), and a CD-ROM on
which the above described image processing program is stored is
loaded and the program is installed. A PC card 203 upon which the
image data is stored is loaded, and it becomes possible to perform
processing identical to the above described processing upon the
personal computer 201 as well. That is to say, the personal computer
201 works as an image signal processing apparatus. It should be
understood that it would also be acceptable for the personal computer
201 and the digital camera 204 to be connected together by a cable,
and for the image data to be transmitted. Or the digital camera
204 might also be a CCD camera housed in the personal computer 201.
Further, as far as the recording medium upon which the program is
provided is concerned, any recording medium such as floppy disk
or DVD or the like may be used.
Yet further, it is also possible for the above described image
processing program for a personal computer to be provided via a
transmission medium such as a communication line or the like such
as the internet or the like. That is to say, it is also possible
to convert the program into a data signal which is carried by the
transmission medium, and thus to transmit it as a signal. The personal
computer 201 has a capability of connection to the communication
line 205. The computer 206 is a server computer which provides an
image processing program, and the image processing program is stored
thereon. The communication line 205 is the internet, a communication
line for personal computer communication, or a dedicated communication
line or the like. The communication line 205 also includes a telephone
line or a wireless telephone line for a portable telephone or the
like, etc.
As described above, when performing image processing on a personal
computer, in the case that the image data which has been stored
on the above described recording medium for image data has already
been subjected to white balance adjustment, a program is written
so as only to perform compensation processing of the gain adjustment
value for the cosine fourth power law and for reduction of transmitted
light intensity by the on chip lenses formed upon the CCD 26. In
this case, the focal point distance and the aperture value of the
interchangeable zoom lens 90 are also stored together on the recording
medium for the above described image data, and, when performing
image processing on the above described personal computer, are used
as information for performing compensation processing of the gain
adjustment value. On the other hand, in the case that the image
data which has been stored on the above described recording medium
for image data has not been subjected to white balance adjustment,
a program is written so as to perform white balance adjustment processing
and compensation processing of the gain adjustment value for the
cosine fourth power law and for reduction of transmitted light intensity
by the on chip lenses formed upon the CCD 26. In this case, captured
data from the CCD 26 and color temperature information for the photographic
subject detected by the white balance sensor 35A or an adjustment
value for the gain calculated based upon this color temperature
information, and moreover the focal point distance and the aperture
value of the interchangeable zoom lens 90 as well, are stored together
on the recording medium for the image data, and compensation processing
is performed for the adjustment value of the gain based upon these
data items.
Although in the above explanation the explanation has been made
in terms of a single lens reflex digital still camera, the present
invention can also be applied to a digital still camera in which
the lens cannot be changed over, or to a digital video camera which
can take a moving image.
Although in the above explanation the explanation has been made
in terms of the case of on chip lenses being formed upon the CCD
26, the present invention can also be applied to the case in which
no on chip lenses are formed upon the CCD 26, or to the case in
which, instead of on chip lenses, another optical member, for example
an optical filter, is provided. If no on chip lenses are formed,
only the above Equation (6) is used, and the above Equations (9)
through (11) are not used. Further, in the case that another optical
member is used, an Equation (corresponding to the above Equations
(9) through (11) ) relating to this optical member should be prepared
representing the reduction of light intensity due to the image height
h and to this optical member, and it will be possible to calculate
the reduction of the peripheral light intensity upon the CCD 26
by taking the product with the above Equation (6). Although the
explanation has been made, by way of example, in terms of a CCD
as the image-capturing device, there are of course acceptable such
devices which are not CCDs.
It should be understood that, although in the above explanation
the optical axis upon the CCD 26 was detected by fitting the white
cap 93 to the interchangeable zoom lens 90 and performing image
capture, it would also be acceptable to arrange for the optical
axis upon the CCD 26 to be detected by capturing the image of a
uniform photographic subject for which the brightness upon the photographic
field was uniform, for example a white wall or the like, without
using the white cap 93. Further, there is no limitation to white,
and a different color could be employed. |