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Digital Camera Patent Abstract
A digital camera includes: an image-capturing unit that receives
light transmitted through a photographic optical system and outputs
image signals; a lens drive unit that moves a focus lens constituting
the photographic optical system along an optical axis; an evaluation
value calculation unit that calculates a focal point evaluation
value that changes in correspondence to a position assumed by the
focus lens based upon the image signals; a focal adjustment unit
that executes an AF search by repeatedly engaging the image-capturing
unit and the evaluation value calculation unit in operation as the
focus lens is made to move along the optical axis by the lens drive
unit, detecting a focus match position based upon resulting focal
point evaluation values and driving the focus lens to the focus
match position; a face recognition unit that recognizes a face of
a person in an image by analyzing the image signals; a distance
calculation unit that determines an approximate distance to the
face based upon the size of the recognized face; and a lens position
determining unit that determines a focus lens position to be set
as a start point of the AF search by the focal adjustment unit based
upon the approximate distance to the face having been determined
by the distance calculation unit.
Digital Camera Patent Claims
1. A digital camera comprising: an image-capturing unit that receives
light transmitted through a photographic optical system and outputs
image signals; a lens drive unit that moves a focus lens constituting
the photographic optical system along an optical axis; an evaluation
value calculation unit that calculates a focal point evaluation
value that changes in correspondence to a position assumed by the
focus lens based upon the image signals; a focal adjustment unit
that executes an AF search by repeatedly engaging the image-capturing
unit and the evaluation value calculation unit in operation as the
focus lens is made to move along the optical axis by the lens drive
unit, detecting a focus match position based upon resulting focal
point evaluation values and driving the focus lens to the focus
match position; a face recognition unit that recognizes a face of
a person in an image by analyzing the image signals; a distance
calculation unit that determines an approximate distance to the
face based upon the size of the recognized face; and a lens position
determining unit that determines a focus lens position to be set
as a start point of the AF search by the focal adjustment unit based
upon the approximate distance to the face having been determined
by the distance calculation unit.
2. A digital camera according to claim 1, wherein: the lens position
determining unit determines the focus lens position to be set as
the start point of the AF search based upon the distance having
been determined by the distance calculation unit and a zoom condition
of the photographic optical system.
3. A digital camera according to claim 1, wherein: the lens position
determining unit determines the focus lens position to be set as
the start point so that a range over which the lens moves during
the AF search increases when the size of the recognized face is
smaller.
4. A digital camera according to claim 1, wherein: the lens position
determining unit selects a face with a largest size if a plurality
of faces are recognized in a single image by the face recognition
unit and determines the focus lens position to be set as the start
point based upon the approximate distance corresponding to the largest
size face.
5. A digital camera according to claim 1, wherein: the lens position
determining unit also determines a focus lens position to be set
as an end point of the AF search.
6. A digital camera according to claim 1, wherein: the distance
calculation unit includes a table correlating the face size with
the approximate distance to the face and ascertains the approximate
distance to the face by referencing the table.
7. A digital camera according to claim 1, wherein: the start point
of the AF search determined by the lens position determining unit
corresponds to a position offset from a position corresponding to
an actual distance to the face that is estimated by a predetermined
extent along a direction opposite from a direction of the AF search.
8. A digital camera according to claim 1, wherein: the evaluation
value calculation unit calculates the focal point evaluation value
by designating an area corresponding to the face of the person recognized
by the face recognition unit as an AF area.
9. A digital camera according to claim 1, wherein: the evaluation
value calculation unit splits a photographic image plane into a
predetermined plurality of areas and calculates the focal point
evaluation value by designating an area among the plurality of areas
that contain the face of the person recognized by the face recognition
unit as a selected AF area.
10. A digital camera according to claim 1, wherein: the focal adjustment
unit starts the AF search by adopting a so-called hill-climbing
method from the AF search start point determined by the lens position
determining unit.
11. A digital camera according to claim 1, wherein: the focal adjustment
unit starts and AF search scan by adopting a scanning method from
the AF search start point determined by the lens position determining
unit.
12. A digital camera according to claim 11, wherein: the focal
adjustment unit scans over a predetermined distance range from the
AF search start point determined by the lens position determining
unit.
13. A digital camera according to claim 1, wherein: the lens position
determining unit sets the AF search start point further toward an
infinity side when the size of the recognized face is smaller.
Digital Camera Patent Description
INCORPORATION BY REFERENCE
[0001] The disclosures of the following priority application and
publications are herein incorporated by reference [0002] Japanese
Patent Application No. 2005-010591 filed Jan. 18, 2005 [0003] Japanese
Laid Open Patent Publication No. 2001-215403 [0004] Japanese Laid
Open Patent Publication No. H08-063597 [0005] Japanese Laid Open
Patent Publication No. H09-251534 [0006] Japanese Laid Open Patent
Publication No. H10-232934
BACKGROUND OF THE INVENTION
[0007] 1. Field of the Invention
[0008] The present invention relates to a digital camera that executes
AF (autofocus) controlled by adopting a contrast method so as to
achieve a faster focus match when photographing a person.
[0009] 2. Description of related art
[0010] In the contrast AF control, image signals are obtained by
capturing an image of a subject with an image sensor such as a CCD,
extracting a component in a predetermined spatial frequency band
in the signals contained within a predetermined AF area and a focal
point evaluation value is calculated by integrating the absolute
values. The focal point evaluation value indicates a quantity corresponding
to the contrast in the focal point detection area, and the value
increases as the contrast level rises. Since the contrast of the
image becomes higher as the focus lens assumes a position closer
to the focus match position, the lens position at which the focal
point evaluation value peaks (hereafter referred to as the peak
position) is determined, the peak position is judged to be the focus
match position and the focus lens is driven to this focus match
position.
[0011] When detecting the peak position (AF search) in the contrast
method, the so-called hill-climbing control is executed. The hill-climbing
control is executed by calculating the focal point evaluation value
over a predetermined sampling pitch while driving the focus lens
along the optical axis. The direction along which the lens is to
be driven to move is determined by comparing the current focal point
evaluation value with the preceding evaluation value and data are
continuously sampled along the direction in which the evaluation
value increases until the peak position (the top of the hill) is
detected (see, for instance, Japanese Laid Open Patent Publication
No. 2003-315665). Methods alternative to the hill-climbing control
include the full range scanning method whereby the focus lens is
driven from the infinity side to the close-up side or vice versa,
the focal point evaluation value is calculated and stored each time
the focus lens has moved over a distance matching a predetermined
sampling pitch, and the plurality of focal point evaluation values
having been stored are evaluated to determine the peak position
upon completing the scan (see, for instance, Japanese Laid Open
Patent Publication No. 2003-307669).
[0012] There are cameras adopting the contrast method described
above, which achieve an improvement in the focus matching accuracy
when photographing a person by detecting skin color in the image
and selecting an AF area based upon the detection results (see,
for instance, Japanese Laid Open Patent Publication No. 2004-037733).
There are also digital cameras that detect the face of a person,
i.e., the subject in an image, detect the person's eyes based upon
the information corresponding to the detected face and execute AF
control so as to achieve focus on the detected eyes (see, for instance,
Japanese Laid Open Patent Publication No. 2001-215403).
[0013] The face of a person in an image may be extracted by extracting
a face candidate area that fits a face shape and determining a face
area within the extracted area, instead of through the skin color
detection (see, for instance, Japanese Laid Open Patent Publication
No. H8-063597). As a further alternative, templates each representing
one of a plurality of face shapes may be prepared in advance, a
correlation between a template and an image may be calculated and
a face may be extracted based upon the correlational value having
been calculated. Japanese Laid Open Patent Publication No. H9-251534
and Japanese Laid Open Patent Publication No. H10-232934 also disclose
technologies related to face recognition and eye detection.
SUMMARY OF THE INVENTION
[0014] While highly accurate focus matching results are normally
achieved through the contrast AF control described above, it takes
a greater length of time to detect the focus match position because
of the nature of the control method, compared to, for instance,
the phase difference detection method. Accordingly, a function that
allows focus to be achieved quickly on the face of a person being
photographed under control executed by adopting the contrast method
is eagerly awaited.
[0015] According to the 1st aspect of the invention, a digital-camera
comprises: an image-capturing unit that receives light transmitted
through a photographic optical system and outputs image signals;
a lens drive unit that moves a focus lens constituting the photographic
optical system along an optical axis; an evaluation value calculation
unit that calculates a focal point evaluation value that changes
in correspondence to a position assumed by the focus lens based
upon the image signals; a focal adjustment unit that executes an
AF search by repeatedly engaging the image-capturing unit and the
evaluation value calculation unit in operation as the focus lens
is made to move along the optical axis by the lens drive unit, detecting
a focus match position based upon resulting focal point evaluation
values and driving the focus lens to the focus match position; a
face recognition unit that recognizes a face of a person in an image
by analyzing the image signals; a distance calculation unit that
determines an approximate distance to the face based upon the size
of the recognized face; and a lens position determining unit that
determines a focus lens position to be set as a start point of the
AF search by the focal adjustment unit based upon the approximate
distance to the face having been determined by the distance calculation
unit.
[0016] According to the 2nd aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the lens
position determining unit determines the focus lens position to
be set as the start point of the AF search based upon the distance
having been determined by the distance calculation unit and a zoom
condition of the photographic optical system.
[0017] According to the 3rd aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the lens
position determining unit determines the focus lens position to
be set as the start point so that a range over which the lens moves
during the AF search increases when the size of the recognized face
is smaller.
[0018] According to the 4th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the lens
position determining unit selects a face with a largest size if
a plurality of faces are recognized in a single image by the face
recognition unit and determines the focus lens position to be set
as the start point based upon the approximate distance corresponding
to the largest size face.
[0019] According to the 5th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the lens
position determining unit also determines a focus lens position
to be set as an end point of the AF search.
[0020] According to the 6th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the distance
calculation unit includes a table correlating the face size with
the approximate distance to the face and ascertains the approximate
distance to the face by referencing the table.
[0021] According to the 7th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the start
point of the AF search determined by the lens position determining
unit corresponds to a position offset from a position corresponding
to an actual distance to the face that is estimated by a predetermined
extent along a direction opposite from a direction of the AF search.
[0022] According to the 8th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the evaluation
value calculation unit calculates the focal point evaluation value
by designating an area corresponding to the face of the person recognized
by the face recognition unit as an AF area.
[0023] According to the 9th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the evaluation
value calculation unit splits a photographic image plane into a
predetermined plurality of areas and calculates the focal point
evaluation value by designating an area among the plurality of areas
that contain the face of the person recognized by the face recognition
unit as a selected AF area.
[0024] According to the 10th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the focal
adjustment unit starts the AF search by adopting a so-called hill-climbing
method from the AF search start point determined by the lens position
determining unit.
[0025] According to the 11th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the focal
adjustment unit starts and AF search scan by adopting a scanning
method from the AF search start point determined by the lens position
determining unit.
[0026] According to the 12th aspect of the invention, in the digital
camera according to the 11st aspect, it is preferred that the focal
adjustment unit scans over a predetermined distance range from the
AF search start point determined by the lens position determining
unit.
[0027] According to the 13th aspect of the invention, in the digital
camera according to the 1st aspect, it is preferred that the lens
position determining unit sets the AF search start point further
toward an infinity side when the size of the recognized face is
smaller.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a control block diagram of the digital camera
achieved in an embodiment of the present invention;
[0029] FIG. 2 presents an example of an image containing a face
area mark displayed at the EVF;
[0030] FIG. 3 presents an example of a relationship that may be
achieved between the lens position (distance) and the focal point
evaluation value;
[0031] FIG. 4 shows a table that correlates the distance representing
the AF search start point with specific combinations of the face
area size and the zoom position; and
[0032] FIG. 5 presents a flowchart of the AF control processing
executed in the embodiment.
DESCRIPTION OF PREFERRED EMBODIMENT
[0033] An embodiment of the present invention is now explained
in reference to FIGS. 1 through 5.
[0034] FIG. 1 is a functional block diagram of the digital still
camera achieved in the embodiment. Reference numeral 1 indicates
a photographic optical system that includes a focus lens, and an
image is formed at a light receiving surface of an image sensor
(an image-capturing element) 2 with a subject light flux having
been transmitted through the photographic optical system 1. The
image sensor 2 is a photoelectric conversion element that outputs
electrical signals indicating the level of light intensity in the
subject image formed on the light receiving surface. The image sensor
may be a solid-state image sensor such as a CCD type sensor or a
MOS type sensor. The image signals output from the solid-state image
sensor 2 are input to an analog signal processing unit 3 where they
undergo processing such as correlated double sampling processing
(CDS processing).
[0035] The image signals having undergone the processing at the
analog signal processing unit 3 are then converted to digital signals
at an A/D converter 4 and the digital signals are stored into a
buffer memory 12 on a temporary basis. The buffer memory 12 is a
frame memory capable of storing image-capturing data corresponding
to a plurality of photographic frames. The data having been stored
in the buffer memory 12 are read out into a digital signal processing
unit 5 where they undergo various types of image processing including
edge compensation and gamma correction. The digital signal processing
unit 5 includes various signal processing circuits such as a gain
control circuit, a luminance signal generation circuit and a color
difference signal generation circuit, which are all controlled in
conformance to instructions provided by a CPU 9.
[0036] The digitized data are stored back into the buffer memory
12 and are then recorded into an external storage medium 15 such
as a memory card via a recording/reproduction signal processing
unit 14. Before the image data are recorded into the storage medium
15, they may be compressed in a predetermined compression format
(e.g., the JPEG format) as necessary. The recording/reproduction
signal processing unit 14 executes data compression when image data
are to be recorded into the external storage medium 15 and it also
executes data decompression processing when the compressed image
data are to be read from the storage medium 15. The recording/reproduction
signal processing unit 14 includes an interface that enables data
communication with the storage medium 15.
[0037] An EVF (electronic viewfinder) 8 is a liquid crystal finder
device at which subject images captured successively are sequentially
displayed at the liquid crystal screen thereof, with each image
updating of the preceding image (through image display). The photographer
is able to decide on the image composition by checking the through
image. The through image is displayed at the EVF 8 based upon analog
image signals obtained by reading image data stored in the buffer
memory 12 into an EVF processing unit 6 and converting the image,
now resized for display, to analog image signals via a D/A converter
7.
[0038] The CPU 9 includes arithmetic operation units such as an
AE arithmetic operation unit, an AWB arithmetic operation unit and
an AF arithmetic operation unit. The AE arithmetic operation unit
executes automatic exposure calculation to determine the optimal
exposure with which the subject should be photographed based upon
the image signals provided by the A/D converter 4. The AWB arithmetic
operation unit sets white balance adjustment gains based upon the
image signals (signals corresponding to the R, G and B color components)
provided by the A/D converter 4. The AF arithmetic operation unit
executes AF calculation by using the image signals provided by the
A/D converter 4. Based upon the results of the calculation executed
by the AF arithmetic operation unit, the CPU 9 drives the focus
lens constituting part of the photographic optical system 1 via
a focus motor 10. The AF arithmetic operation will be explained
later in detail. The focus motor 10 may be, for instance, a stepping
motor.
[0039] A face recognition arithmetic operation unit 11 recognizes
a person's face in the image by adopting a method of the known art
such as that in the Description of Related Art, and once a face
is successfully recognized, it inputs to the CPU 9 the coordinates
indicating the position and the size of an area (rectangular area)
containing the recognized face. The image that undergoes the face
recognition is an image prepared for the through image display.
Namely, the CPU 9 recognizes the face of a person, i.e., the subject,
and obtains the coordinates of the face area corresponding to the
size of the face by executing image processing on the image data
for the through image display. As the coordinates are input thereto
from the face recognition arithmetic operation unit 11, the CPU
9 creates an image by superimposing a face area mark over the image
for the through image display based upon the coordinates and displays
the image thus created at the EVF 8, as shown in FIG. 2. MK in the
figure indicates the face area mark.
[0040] A plurality of operation members 13 are connected to the
CPU 9. The operation members 13 include a power switch through which
power to the camera is turned on/off, a halfway press switch which
enters an ON state in response to a halfway press operation of a
shutter release button, a full press switch that enters an ON state
in response to a full press operation of the shutter release button
and a mode switch operated to select various modes.
[0041] Next, the AF arithmetic operation (AF control) executed
in the embodiment is described in detail.
[0042] An AF area is set in advance at a given location or at a
plurality of locations within the photographic plane, and contrast
AF control is executed based upon image signals obtained in the
AF area. In the embodiment, a face area is designated as an AF area
in a face recognition AF mode, as detailed later.
[0043] By taking advantage of the correlation between the extent
of blur to manifest in an image and the contrast in the image, focal
adjustment is executed under contrast control based upon the contrast
in the image, which peaks when the focal point is optimally adjusted.
The contrast level can be evaluated in correspondence to the level
of the high-frequency component in the image signal. Accordingly,
the AF arithmetic operation unit of the CPU 9 includes a digital
filter that extracts a high-frequency component in a predetermined
band in each image signal obtained by capturing an image. The absolute
values of the high-frequency component extracted with the digital
filter are integrated in each AF area, and the integral value is
used as a focal point evaluation value.
[0044] The focal point evaluation value indicates the contrast
in the image, i.e., a quantity that changes in correspondence to
the state of focal adjustment at the focus lens, and assumes the
largest value (peak value) when a focus match is achieved and the
contrast is at the highest level. Accordingly, a focus match can
be achieved at the subject within the AF area by determining the
lens position at which the focal point evaluation value peaks and
driving the focus lens to this position. In the following explanation,
the operation executed to determine the lens position at which the
focal point evaluation value peaks is referred to as an AF search.
[0045] The AF search may be executed by adopting the hill-climbing
method or the full range scanning method. In the hill-climbing method,
the focal point evaluation value is calculated over a predetermined
sampling pitch while moving the focus lens and each focal point
evaluation value having been calculated is compared with the evaluation
value (stored value) calculated at the preceding sampling point.
If the current evaluation value is greater than the preceding evaluation
value, the evaluation value is yet to peak and accordingly, data
are continuously sampled by moving the focus lens along the same
direction. If the current evaluation value is smaller than the preceding
evaluation value, the evaluation value has already peaked and is
moving further away from the peak. Accordingly, data are continuously
sampled by moving the focus lens along the opposite direction. By
repeatedly executing this operation, the focus lens can ultimately
be set to the peak position (the top of the hill).
[0046] In the full range scanning method, the focus lens is driven
to scan along a given direction, i.e., from the infinity side through
the close-up side, the focal point evaluation value is determined
as the focus lens being driven moves by a distance matching a predetermined
sampling pitch, and the focal point evaluation value thus determined
is stored. Once the scanning drive is completed, the plurality of
focal point evaluation values having been stored are analyzed, the
peak position is determined and the focus lens is moved to the peak
position designated as the focus match position.
[0047] FIG. 3 presents an example of a relationship that may be
observed between the focus lens position and the focal point evaluation
value. The relationship in this example reflects the photographing
conditions shown in FIG. 2, in which a person standing to the front
of a background object (mountain) is photographed. While the focal
point evaluation value peaks at two positions, one corresponding
to a distance L0 over which the person is present and the other
corresponding to a distance L1 over which the background object
is present, the peak that should be detected corresponds to L0.
[0048] The greatest disadvantage of the contrast method is that
it takes a long time to achieve the focus match. In other words,
it is necessary to select a small sampling pitch to assure a high
level of peak position detection accuracy in the AF search, and
as a result, the focal point evaluation value needs to be calculated
many times, requiring a great length of time before the peak position
is detected. In the case of the full range scanning method, the
full search range must be set from the infinity end through the
close-up end, and thus, a long time elapses before the search is
completed. While a focus match can be achieved more quickly without
lowering the focus matching accuracy by reducing the search range,
it is necessary to ascertain an approximate distance to the main
subject, i.e., the person, prior to the search to reduce the search
range and there is no way of ascertaining the distance in advance
in the related art.
[0049] In the embodiment, an approximate distance to the main subject
is ascertained based upon the face recognition results so as to
reduce the search range by setting an AF search start point based
upon the approximate distance. The following is a detailed explanation
of the embodiment.
[0050] As explained earlier, the size of the face area (hereafter
referred to as the face area size) within the image is indicated
by the face recognition results. Provided that the zoom condition
of the photographic optical system 1 can be ascertained, an approximate
distance to the main subject, i.e., the person's face, can be estimated
based upon the face area size and the zoom condition. Once the approximate
distance becomes known, it becomes possible to reduce the AF search
range.
[0051] Accordingly, a table correlating the approximate distance
to the face with the face area size and the zoom position, such
as that shown in FIG. 4, is prepared in advance and the table is
stored in memory inside the camera. The face area size is indicated
by the length (the number of pixels) of either the longitudinal
side or the lateral side of the rectangular face area.
[0052] It is to be noted that different face area size values may
be calculated through different methods that may be adopted to determine
the face area for a given subject present over a given distance.
However, there should not arise any problems as long as a table
such as that in FIG. 4 is prepared by adopting a specific face recognition
method and the approximate distance to a face corresponding to the
size of the face area ascertained through the specific face recognition
method is determined in advance through testing or calculation.
[0053] Since the approximate distance in the table is used to set
the AF search start point subsequently, it must assume a value that
allows a peak to be detected in correspondence to the face without
fail when the AF search is started from this distance. Since it
is assumed that the search is executed by moving the focus lens
toward the close-up side in this example, the approximate distance
is set so that the search is started at a position slightly on the
infinity side relative to the estimated face position. In other
words, the AF search start point corresponds to a position offset
from the position corresponding to the actual distance to the face
that is estimated by a predetermined extent along the direction
opposite from the AF search direction.
[0054] Since the size of the face varies from individual to individual
and there is a significant difference in the face size between adults
and children, it is necessary to set the approximate distance by
anticipating this difference and allowing for a safe margin. It
is also necessary to take into consideration any tendency that may
exist in the particular face recognition method, e.g., the detected
face size tends to be smaller or larger than the actual face. In
addition, the reliability with which the distance to the face is
estimated tends to be lower when the face area size is small, and
this tendency becomes more pronounced toward the wide-angle side.
Accordingly, the approximate distance is set by taking this point
into consideration as well, so that when the face area size is smaller,
the search is started further away from the estimated face position
toward the telephoto side. Under such circumstances, the search
range is bound to become greater as the face area size becomes smaller,
but the likelihood of achieving a focus match at the face increases.
[0055] It is to be noted that while only three settings, wide,
midrange and telephoto are indicated in correspondence to the zoom
position in the table, the zoom position may assume finer settings.
[0056] FIG. 5 presents a flowchart of the AF control procedure
executed by the CPU 9.
[0057] Before starting up this program, the CPU 9 starts an image-capturing
operation, AE processing, white balance processing, and through
image display processing, as the power is turned on. These processing
operations are repeatedly executed and the AE processing results
and the white balance processing results are reflected in the corresponding
through image on display. At the same time, face recognition is
executed, and if a face is recognized, the face area mark MK is
displayed in the through image. The photographer decides on the
image composition by checking the through image at the EVF 8 and
once he decides on the composition, he presses the shutter release
button halfway down.
[0058] In response to the halfway press operation, the CPU 9 calls
up the program shown in FIG. 5, in conformance to which part of
photographing preparation processing is executed. In step S1, a
decision is made as to whether a regular AF mode or the face recognition
AF mode is set. The photographer is able to select in advance either
mode by performing a specific operation. If it is decided that the
regular AF mode is currently set, an AF search of the related art
is executed without using the face recognition results. Namely,
the AF search start point is set at, for instance, an infinity end
L1 (see FIG. 3) in step S3, and an AF search is executed in step
S6. The scanning method may be either the hill-climbing method or
the full range scanning method. In either case, with the start point
set at the infinity end L1, the search is executed by moving the
focus lens toward the close-up side. The search range assumed at
this time is indicated as A1 in FIG. 3. Once the peak position is
ascertained, the focus lens is fixed at the peak position. It is
to be noted that the search may instead be executed by moving the
focus lens from the close-up side toward the infinity side.
[0059] If, on the other hand, it is decided that the face recognition
AF mode is currently set, the face recognition results available
at the current time point are input from the face recognition arithmetic
operation unit 11 and a decision is made as to whether or not a
face has been recognized in step S2. If no person's face is contained
in the image, no face is recognized and accordingly, a negative
decision is made in step S2. In addition, if the recognition ends
in failure for any reason even though a face is contained in the
image, a negative decision is made in step S2. After making a negative
decision in step S2, the operation proceeds to step S3 to execute
the regular AF control.
[0060] If, on the other hand, a face has been recognized, the face
area size can be ascertained and in this case, the approximate distance
L to the face is ascertained in step S4 by referencing the table
shown in FIG. 4 based upon the face area size and the corresponding
zoom position. It is to be noted that the zoom position is determined
to be the telephoto position, a midrange position or the wide-angle
position in correspondence to the focal length of the photographic
lens. In step S5, an AF search is started at the start point set
in correspondence to the distance Ls (see FIG. 3) ascertained from
the table. Namely, the focus lens is first driven to the lens position
corresponding to the distance Ls and then, the AF search is executed
by moving the focus lens from the start point toward the close-up
side (step S6). The AF area set for the AF search is the face area,
and the search is executed by calculating the focal point evaluation
value by using the signals from the face area among the image signals.
Once the peak position is ascertained, the focus lens is fixed at
the detected peak position, before the operation makes a return.
[0061] As FIG. 3 clearly indicates, the peak at L0 can be detected
more quickly by starting the search at the position corresponding
to the distance Ls based upon face recognition results, compared
to the length of time required to detect the peak by starting the
search at the infinity end Li and thus, the length of time required
to achieve a focus match is greatly reduced. The peak detection
is speeded up regardless of whether the search is executed through
the hill-climbing method or the scanning method (adopting the concept
of the full range scanning method without actually scanning over
the entire range).
[0062] If the hill-climbing method is adopted, the search ends
as the focus lens is moved back by a slight extent once the focus
lens moves past the first peak position (the peak corresponding
to L0). If the scanning method is adopted, it is desirable to set
the search end point Le at a point further toward the close-up side
by a predetermined distance relative to the search start point Ls.
In this case, the search is executed over the range A2, which is
much smaller than A1. In addition, the range is greatly reduced
compared with the range over which a scan is executed from the search
start point Ls through the close-up side. Namely, the scan is executed
so as to end at the search end point Le instead of from the search
start point Ls through the close-up side.
[0063] It is to be noted that while the approximate distance to
the face is determined by using the table shown in FIG. 4 in the
example described above, the distance may instead be determined
by substituting specific values for parameters (the face area size
and the focal length) in a predetermined arithmetic expression.
In addition, while the face area of a face recognized in an image
is designated as the AF area, the process of selecting a threshold
value or the like to be used to decide whether or not a focus match
is achieved is bound to become more complicated if the size of the
AF area changes constantly. Accordingly, the image plane may be
divided in advance into a plurality of areas of a given size (e.g.,
the image plane may be split into four equal areas or six equal
areas) and one of the split areas containing the face area may be
designated as the AF area. In this case, the AF area greater than
the face area, often containing relatively high contrast areas such
as a portion around the neck or the hair, is set, which is likely
to contribute to an improvement in the focus matching accuracy achieved
through the AF search.
[0064] If the image of the face area is significantly blurred,
the face recognition may not be successful. Accordingly, prior to
the face recognition, the focus lens may be driven to a position
of the hyper-focal length determined in correspondence to the focal
length and the aperture value to raise the likelihood of the face
area becoming contained in the photographic depth of field and thus
increase the reliability of the face recognition.
[0065] Also, a plurality of faces may be recognized through the
face recognition. In such a case, priority should be given to the
subject present on the close-up side so that the face with the greatest
face area size, i.e., the face present at the closest position,
is selected and the approximate distance to be used to set the search
start point is determined based upon the greatest face area size.
[0066] As described above, a person's face in an image is recognized
by analyzing image signals and if a face is recognized, the approximate
distance to the face is ascertained based upon the size of the recognized
face. The focus lens position at which the AF search is to start
is then determined based upon the distance thus ascertained. As
a result, the range of the AF search executed when photographing
a person can be reduced and the focus lens can be focused on the
person quickly.
[0067] The above described embodiment is an example and various
modifications can be made without departing from the spirit and
scope of the invention. |