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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automatic focusing device for a camera
which is able to detect a focusing condition of an objective lens of a
camera by receiving light beams of an object image having passed through
the object lens and to drive the objective lens to a focus position
thereof automatically according to data obtained by said detection.
2. Description of the Prior Art
There has been known a focus condition detecting device in which two images
are formed by refocusing light bundles of an object image having passed
through a first and second areas of an objective lens being symmetric with
respect to the optical axis of the objective lens, a relative distance
between these two images is calculated and a defocus amount of the focus
position detected from a predetermined focus position and a direction of
the defocus are determined based on the relative distance calculated.
A typical optical system for such a focus condition detecting device as
mentioned above is shown in FIG. 17.
In this system, a condenser lens 6 is arranged on a predetermined focal
plane 4 or on a plane positioned behind the focal plane and two refocusing
lenses 8 and 10 are arranged rearward of the condenser lens 6. There are
arranged two image sensors 12 and 14 on each of focal planes of two
refocusing lens. Each of the image sensors 12 and 14 is comprised of a CCD
(change coupled device) image sensor having a plurality of light sensing
elements.
As shown in FIG. 17 schematically, when an image (A) of an object is
focused forward of the predetermined focal plane 4, two images a and a'
are refocused on the image sensors 12 and 14 so as to approach to each
other with respect to the optical axis 18 of the objective lens. On the
contrary to the above, when focused rearward of the predetermined focal
plane 4, two images b, b' are refocused apart from each other. If an image
is focused just on the predetermined focal plane 4, a distance between two
points corresponding to one to one of two images refocused on two image
sensors 12 and 14 becomes a specific value which is determined by the
composition of the optical system of the focus condition detecting device.
Accordingly, a focus condition of the objective lens can be determined
from the distance between corresponding two points of the refocused
images.
In an automatic focusing device of a camera including the focus condition
detecting device as mentioned above, a control circuit including at least
one micro computer controls the integration of light by the CCD image
sensors to have the CCD image sensors generate image signals corresponding
to the intensity distributions of the object images formed on the CCD
image sensors by the refocusing lenses, respectively. Thereafter, the
control circuit controls calculation of the focus condition (the amount of
defocus) based on the image signals from the CCD image sensors, driving
the objective lens according to the amount of defocus calculated, stopping
the objective lens at a focus position thereof and a shutter release (when
the shutter release button is pushed) sequentially according to control
programs stored in the micro computer. The automatic focusing device
repeats the automatic focus adjusting control towards a focus position
successively and due to repeated controls, the objective lens is attained
to an exact focus position finally.
Meanwhile, in the automatic focusing device as mentioned above, if an
object is moving toward or going away from the camera, an exact focus
position can not be attained by moving the objective lens to a focus
position according to a defocus amount obtained by one focus condition
detecting operation since the object is moving during said operation. FIG.
18 shows a behavior of the automatic focusing device diagrammatically when
an object is moving relative to the camera.
In the graph of FIG. 18, horizontal axis represents time and the vertical
axis represents an amount of defocus on a film surface.
A continuous curve 1 shows a variation of a defocus amount being taken with
respect to the film surface in the case that an object is moving toward
the camera and no focus adjustment is carried out. A noncontinuously bent
line m is a line obtained by plotting defocus amounts caused when the
automatic focusing operation is repeated.
The time t.sub.0 indicates a middle timing during first integration by each
of the CCD image sensors. A defocus amount at that time is defined as
D.sub.0. During the time from t.sub.0 to t.sub.1, a remaining integration
and calculation for obtaining a defocus amount are executed. During the
time from t.sub.1 to t.sub.2, the objective lens is driven and then, the
objective lens is stopped and the next integration is started during the
time from t.sub.2 to t.sub.3. Further, the next calculation is executed
during the time from t.sub.3 to t.sub.4.
As shown in FIG. 18, while the defocus cmount increases to the position A
due to the movement of the object, the objective lens is driven to move
the object image only to a position O. Therefore, a defocus with an amount
represented by a difference (D.sub.0 -D.sub.1) results. Next integration
is done to obtain a defocus amount (D.sub.2 -D.sub.1) (at the timing
t.sub.3), and driving of the objective lens is completed at the timing
t.sub.5. However, an image of object by objective lens has been already
moved to a position B, and, therefore, a defocus amount (D.sub.3 -D.sub.2)
is caused. This defocus amount (D.sub.3 -D.sub.2) is enlarged when
compared with the preceding defocus amount (D.sub.2 -D.sub.1).
Similarly to the above, the defocus amount increases as indicated at the
positions C (D.sub.5 -D.sub.4) and D (D.sub.7 -D.sub.6) respestively,
though the automatic focusing operation is repeatedly carried out. Thus it
becomes impossible to attain an exact focus position. Such a delay in the
automatic focusing controls as is caused by a moving object becomes more
serious in the case that an interchangeable lens having a long focal
length such as a telescopic lens is on the camera body mounted.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an automatic focusing
device being able to minimize a defocus amount by appropriate correction
of a defocus amount detected by focus detection even when the object is
moving rapidly relative to the camera.
To this end, according to the present invention, there is provided an
automatic focusing device for a camera having a focus adjustable objective
lens for forming an image of an object, comprising:
focus detection means for repeatedly detecting defocus amount of the image
formed by the objective lens from a predetermined focal plane of the
objective lens to generate a defocus signal corresponding to the detected
defocus amount upon each detecting operation;
object movement detection means for detecting movement of the object based
on the defocus signals generated upon the latest detecting operation and a
former detecting operation of the focus detection means to generate a
movement signal corresponding to the amount of movement of the object;
drive means for driving the objective lens for focus adjustment;
correction means for generating a corrected defocus signal based on the
defocus signal generated upon the latest detecting operation by the focus
detection means and the movement signal regarding to the object; and
drive control means for causing the drive means to drive the objective lens
in accordance with the corrected defocus signal, the corrected defocus
signal corresponded to a defocus amount of the image from the
predetermined focal plane at a given moment after the latest detecting
operation by the focus detection means.
With the above construction, the amount of defocus at a given moment after
the latest detecting operation of the focus detecting means can be
estimated based on the movement signal which is taken in for generation of
the corrected defocus signal. Thus, the lens drive control based on the
corrected defocus signal assures automatic focusing of the objective lens
with respect to the object at the given moment, so that the automatic
focusing in pursuit of the movement of the object is achieved.
The above and other objects and features of the present invention will
become more apparent from the following description of preferred
embodiments of the present invention taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIGS. 1(a) and 1(b) show a flow chart of a main routine program for
automatic focusing control according to the present invention;
FIGS. 2(a) and 2(b) are a flow chart for showing contents of step #1 in the
flow chart shown in FIG. 1(a);
FIG. 3 is an explanative plan view showing a CCD image sensor used in an
automatic focusing device according to the present invention;
FIG. 4 is a table for showing the composition of the CCD image sensor;
FIGS. 5(a) and 5(b) are a flow chart showing contents of steps #8 to #17 of
FIG. 1(b) concretely;
FIGS. 6 and 7 are time charts showing automatic focusing operations in the
single picture-taking mode and continuous picture-taking mode,
respectively,
FIG. 8 is a flow-chart for showing a method of automatic focusing operation
according to the present invention;
FIG. 9 is a flow chart for showing an interrupt routine to be executed when
the shutter release button is operated;
FIG. 10 is a graph for showing repeated automatic focusing operations
according to the present invention,
FIG. 11 is a flow chart showing another method of automatic focusing
operation according to the present invention;
FIG. 12 is a flow chart showing one more method of automatic focusing
operation according to the present invention;
FIGS. 13(a) and 13(b) are a flow chart of a main routine to be executed
when an object is going away from the camera;
FIG. 14 is a block-diagram of a camera in which the automatic focusing
device according to the present invention is installed;
FIG. 15 is a block-diagram of an automatic focusing control circuit
according to the present invention;
FIG. 16 is an explanative view for showing compositions of an optical
system for a focus-condition detecting device;
FIG. 17 is an explanative view for showing the principle of focus condition
detection; and
FIG. 18 is a graph showing delay in the conventional automatic focusing
device caused by a moving object;
DESCRIPTION OF PREFERRED EMBODIMENTS
[I] Composition of system for Automatic Focusing Device.
FIG. 14 shows a system for an automatic focusing device of a camera.
As shown in FIG. 14, the camera is comprised of a camera body (BD) and an
interchangeable lens (LZ). The interchangeable lens (LZ) is mounted to the
camera body (BD) and is connected thereto via cluch means (106), (107)
mechanically.
When the interchangeable lens (LZ) is mounted on the camera body (BD), a
circuit for lens data provided therein is electrically connected to a
circuit in the camera body via joint terminals (JL1) to (JL5) and (JB1) to
(JB5). In this system, a light image of an object having been passed
through the objective lens (LZ) passes through a central half-mirror
portion provided on a reflex mirror (108) so as to be reflected by a
sub-mirror 109 and, then is received by a CCD image sensor (FLM) housed in
a focus detecting module (AFM). An interface circuit (112) is provided for
driving the CCD image sensor (FLM) for reading data obtained by the CCD
image sensor and for transmitting said data to an automatic focusing
controller (113). The auto-focusing controller (113) calculates an amount
of defocus .vertline..DELTA.L.vertline. which is defined as a shift amount
of an image of an object from a focus position and a direction of defocus
which is defined as the direction of the shift of the object image from
the focus position.
A motor driving circuit (114) is provided to drive a motor (MO1) for moving
a focusing lens (FL) of the interchangeable lens (LZ) according to data
outputted from the auto-focusing controller. The driving force of the
motor (MO1) is transmitted to a transmission mechanism (105) of the
interchangeable lens via a slipping mechanism (SLP), driving mechanism
(LDR) and the clutch means (107) and (106). The slipping mechanism (SLP)
is provided for protecting the motor (MO1) by slipping when a torque
larger then a predetermined value is loaded thereon.
The transmission mechanism (105) drives the focusing optical system (FL) in
the direction of the optical axis in order for focusing. An encoder (ENC)
for monitoring a driving amount of the motor (MO1) is connected to the
driving mechanism (LDR) in the camera body. The number of pulses outputted
by the encoder (ENC) is proportional to a driving amount of the motor
(MO1) for driving the focusing lens of the interchangeable lens. Assuming
that the number of rotations of the motor (MO1) is "NH", the number of
pulses from the encoder (ENC) is "N", the resolution of the encoder is
".rho. (1/rot.)", a deceleration ratio of the transmission mechanism
including mechanical elements from the drive shaft of the motor to the
support shaft for the encoder is ".mu.P", a deceleration ratio of the
transmission mechanism including mechanical elements from the drive shaft
of the motor (MO1) to the clutch means (107) of the camera body side is
".mu.B", a deceleration ratio of the transmission mechanism including
mechanical elements from the clutch means (106) of the lens side to the
lens assembly of the interchangeable lens is ".mu.L", a lead of a helicoid
off a focus adjusting member (102) is "LH (mm/rot)" and a moving amount of
the focusing lens (FL) is ".DELTA.d(mm)", there are given equations as
follows.
N=.rho...mu.P.NM
.DELTA.d=NM..mu.B..mu.L.LH
Namely,
.DELTA.d=N..mu.B..mu.L.LH/(.rho...mu.P) (1)
Further, the ratio of .DELTA.d to a shift amount .DELTA.L(mm) of a focal
plane caused by the movement (.DELTA.d) of the focusing lens (FL) is
defined as follows.
Kop=.DELTA.d/.DELTA.L (2)
From equations (1) and (2), there is obtained an equation as follows.
N=Kop..DELTA.L..rho...mu.P/(.mu.B..mu.L.LH) (3)
Assuming following equations,
KL=Kop/(.mu.L.LH) (4)
KB=.rho...mu.P/.mu.B (5)
there is introduced an equation as follows.
N=KB.KL..DELTA.L (6)
In the equation (6), .DELTA.L is obtained from the defocus amount
.vertline..DELTA.L.vertline. and the direction thereof.
The date KB in the equation (5) is a constant determined according to the
deceleration ratio .mu.B of the camera body side and is stored in a
control circuit (111) in the camera body.
Connections between the reading circuit (LDC) and the lens data circuit
(LEC) are as follows. From the reading circuit (LDC) to the lens data
circuit (LEC), electric power is supplied via connecting terminals (JB1)
and (JL1), synchronizing clock pulses are applied via connecting terminals
(JB2) and (JL2) and a starting signal for reading lens data is applied via
connecting terminals (JB3) and (JPL3). From the lens data circuit (LEC) to
the reading circuit (LDC), lens data KL are outputted via connecting
terminals (JB4) and (JL4) serially. Remaining connecting terminals (JB5)
and (JL5) are used as common ground terminals.
The lens data circuit (LEC), when the starting signal is entered thereinto
via terminals (JB3) and (JL3), outputs data KL to the reading circuit
(LDC) serially in a manner synchronized with clock pulses being applied
via terminals (JB2) and (JL2). The reading circuit (LDC) reads serial data
KL in a manner synchronized with same clock pulses as mentioned above and
convert serical data to parallel data. The camera controller (111)
calculates (KL.KB=K) according to data KL from the lens data circuit (LEC)
and data KB stored therein. The auto-focusing controller (113) calculates
a defocus amount .vertline..DELTA.L.vertline. with use of data about an
object image sent from the inter-face circuit (112) and the number N of
pulses to be detected by the encoder (ENC) according to the equation
N=K..vertline..DELTA.L.vertline. with use of the defocus amount
.vertline..DELTA.L.vertline. calculated and data K sent from the camera
controller (111). The auto-focusing circuit (113) drives the motor (MO1)
through the motor driving circuit (114) in a direction determined
according to the direction of defocus having been sought with use of data
of an object image. When pulses have been outputted from the encoder (ENC)
by a number equal to the number N having been calculated by the
auto-focusing controller (113), the motor (MO1) is stopped since it is
considered that the focusing lens of the interchangeable lens has been
moved to a focus position by the amount .DELTA.d having been sought.
Although the value of K was sought from the data KB stored in the camera
body and the data KL stored in the interchangeable lens in the embodiment
just mentioned above, the calculating method is not limited thereto.
For instance, in the case that a common interchangeable lens can be mounted
to anyone of camera bodies having a specific value of KB respectively, the
lens data circuit (LEC) housed in the interchangeable lens (LZ) is so
designed as to output data K1 (K1=KL.KB1) determined by the specific value
KB of the specific camera bodies but varying with the set focal length of
the interchangeable lens. Meanwhile, on the side of the specific camera
body, the calculation of KL.KB is omitted and, inplace of that, data K1
are entered into the auto-focusing controller (113) from the reading
circuit (LDC). In the case that another camera body having a value KB2
being different from the KB1 value is used, the camera controller (111) is
so designed as to store data of KB2/KB1 and to obtain a value of K2
(K2=KL.KB2) by calculating following equation.
K2=K1.KB2/KB1=KL.KB2
Especially in the case of zoom lenses in which a focusing lens assembly is
arranged forward of a zooming lens assembly, a value of KL or K or
individual zoom lenses varies in a very wide range since a value of Kop is
given by a following equation.
Kop=(f1/f).sup.2 (7)
Wherein f1 is the focal length of the focusing lens assembly and f is the
focal length of the zooming lens. In such a case as mentioned above, data
KL or K to be memorized in the lens data circuit (LEC) is divided into
data representing the exponent thereof and data representing significant
digit thereof (for instance, if data is comprised of eight bits, top four
bits are used for representing the exponent and bottom four bits are used
for representing the significant digit). In this case, the reading circuit
(LDC) in the camera body reads data of the exponent and data of the
significant digit respectively and outputs data KL or K to the camera
controller (111) after converting to a normal figure due to the exponent
and the significant degit having been read therein. According to such a
format as mentioned above, it becomes possible to treat with data KL or K
even in the case that it varies in a very wide range. It is to be noted
that almost all functions regarding to the auto-focusing system are
carried out by at least one micro-computer although the system is shown as
a block diagram in FIG. 14 in order to be able to understand these
functions and operations more easily.
FIG. 15 shows a block diagram of a focus condition detecting control
circuit. A control circuit (31) constituted by a micro-computer starts a
focus condition detecting operation when a shutter release button (not
shown) is pushed down by a half stroke thereof while a switch for the
focus condition detecting mode is turned ON.
At first, an integration clear pulse signal is outputted from the control
circuit 31 to a CCD image sensor provided in a photo-electric transducer
circuit (20) acting as a first and second arrays of micro photo-electric
transforming elements. Due to this signal, all of elements of the CCD
image sensor are reset to initial states and an output AGCOS of an
illumination monitoring circuit (not shown) housed in the CCD image sensor
is set up to the level of the voltage of the power source. At the same
time, the control circuit (31) outputs a permission signal SHEN of "High"
level for permitting to generate a shift pulse. As soon as the integration
clear signal ICGS disappears, integration of photocurrent is started in
every element of the CCD image sensor. At the same time, while the output
AGCOS of the illumination monitoring circuit in the photo-electric
transducer circuit begins to drop with a velocity corresponding to the
illumination of an object, a reference signal DOS generated by a reference
signal generating circuit (not shoen) provided in the photo-electric
transducer circuit (20) is kept to a constant reference level. A gain
control circuit (32) compares the output AGCOS with the reference signal
DOS and controls a gain of a differential amplifier (26) of a gain
variable type according to an amount of drop of the output AGCOS relative
to the reference level DOS within a predetermined time interval (for
instance, it is set to 100 m sec upon the focus condition detecting
operation). The gain control circuit (32) outputs a signal TINT of "High"
level as soon as it detects that AGCOS signal have dropped to a level
equal to or lower than a predetermined level against the reference level
DOS within the predetermined time interval starting from the disappearance
of the integration clear signal ICGS. The signal TINT is input to a shift
pulse generating circuit (34) via a AND gate (AN) and an OR gate (OR) and
the shift pulse generating circuit (34) outputs a shift pulse SH in
response thereto. When the shift pulse SH is input to the photo-electric
transducer (20), the integration operation of photo-current by each light
sensing element of the CCD image sensor is stopped and, then, charges
accumulated in each light sensing element and correcponding to integrated
values of the photo-current are transmitted parallel to cells in a shift
register provided in the CCD image sensor so as to correspond one to one
to the light sensing elements of the CCD image sensor.
Further, a transmission pulse generating circuit (36) outputs two sensor
driving pulses .PHI.1 and .PHI.2 having phases different from each other
by 180.degree. in a manner synchronized with clock pulses from the control
circuit (31). The CCD image sensor in the photo-electric transducer
circuit (20) outputs signals OS forming image signals respectively by
discharging a charge of each cell of the CCD shift register serially in
the order of alignment of elements. This OS signal has a higher voltage as
an intensity of incident light to a corresponding element is weaker. A
subtraction circuit (22) subtracts OS signal from DOS signal and outputs
the difference (DOS-OS) as a picture element signal.
On the contrary to the above, if the predetermined time interval has
elapsed been passed without outputting TINT signal after the disappearance
of ICGS signal, the control circuit (31) outputs an instruction signal SHN
for generating a shift pulse of "High" level. Therefore, in this case, the
shift pulse generating circuit (34) generates a shift pulse in response to
this instruction signal SHM.
Further, the control circuit (31) outputs a sample-hold signal S/H when
element signals from seventh to tenth element are outputted. This area of
the CCD image sensor corresponding to these elements is covered with an
alumium mask, so that these elements integrate only dark currents inherent
to the CCD image sensor. Namely, these picture elements are shuted from
the incident light. A peak hold circuit (24), when the sample hold signal
S/H is applied thereto, holds a difference between the reference signal
DOS and one of output signals from the seventh to tenth elements covered
with the aluminum mask. Thereafter, the difference VP and element signal
DOS' are input to the gain variable amplifier (26). That gain variable
amplifier (26) amplifies a difference (VP-DOS') between VP and DOS' with a
gain controlled by the gain control circuit (32). The amplified signal
DOS" is converted from analogue data to digital data by an A/D converter
(28) and digital data are applied to the control circuit (31) as picture
element signal data. Though the A/D conversion by the A/D converter is
done in a unit of 8 bits, data are transmitted to the control circuit in
each lump of top four bits and bottom four bits.
The control circuit (31) stores these picture element signal data in an
internal memory thereof and, when all of element signal data have been
stored therein, processes those data according to programs set therein to
calculate a defocus amount and a direction of defocus, to display these
data on a display (38) and to drive a lens driving device (40) according
to the defocus amount and the direction thereof in order for auto-focusing
adjustment of the interchangeable lens.
[II] Automatic Focusing Method
<II-1> Main flow for Automatic Focusing
FIGS. 1(a) and 1(b) show a flow chart of a main routine program for
automatic focusing.
When the main routine is started, CCD image sensor is driven to integrate
photo currents by it's light sensing elements and stores data regarding an
object image at step #1. At step #2, the control circuit (31) reads
picture element data from CCD image sensor while converting them to
digital data. At step #3, a defocus amount is calculated according to
picture element data obtained. An example of calculation method will be
stated below.
At step #4, it is decided whether a focus condition can be detected or not.
If an image of an object is out of focus too much to detect a focus
condition or contrast of an object image is too low to ensure the focus
detection, the process proceeds to step #5. Steps from #5 to #7 are
provided for processing in the case of low contrast. In this case, the
focusing lens of the interchangeable lens is driven to scan the whole
range thereof in order to seek for a range having a relatively high
contrast by repeating detections of focus condition (this processing is
referred to "low-contrast scan". If any range showing a high contrast has
not been found during the low contrast scan, the process proceeds to step
#7 to indicate that the focus condition detection is impossible by
flushing the display.
If it is decided at step #4 that the focus condition detection is possible,
a driving amount for the interchangeable lens is calculated at step #8
from the defocus amount obtained at step #3. It is decided at step #9
whether the lens has been stopped or not. If the lens has been stopped,
the process proceeds to step #10 to decide whether the lens is in focus or
out of focus. If it is in focus, the process proceeds to step #11 to
indicate that it is in focus and then, is returned to step #1. If it is
out of focus, the process proceeds to step #12 in order to decide whether
the direction of defocus at the present time is inverted from that of the
last time or not. If it is inverted, the process proceeds to step #13 in
order to remove possible back-rushes of the driving mechanism which might
cause errors upon driving the lens in the inverted direction.
If not inverted, the process proceeds to step #14 to decide whether a
follow-up correction which will be stated below in detail is needed or
not. At step #15, it is decided whether condition and/or timing for the
follow up correction to be done are satisfied or not. If conditions are
satisfied, the driving amount of the lens is corrected or adjusted. The
correction of the driving amount will be stated below in connection with
the follow-up correction mode.
If the lens is moving, the process proceeds from step #9 to #21 to
calculate an amount of movement overshoot of the lens from the dumping of
image data until a completion of the focus condition detecting calculation
(See, for instance Japanese patent laid open publication No. 78823/1981),
At step #22, the driving amount of the lens is corrected due to the amount
of movement of the lens calculated.
Although the driving amount is corrected by the movement of the lens at
step #22, it is also possible to correct the driving amount by not only
the movement of the lens but also data regarding to a movement of an
object. At step #23, the direction of driving lens obtained at the present
time is compared with that obtained at the last time. If it is decided
that the direction is inverted, the process proceeds to step #24 in order
to stop the lens and then, returned to step #1. A reason why the lens is
made stopped is that credibility in a focus condition detection would be
lowered when it were done during movement of the lens. If a driving
direction of the lens is not inverted, the process proceeds to the step
#17. At step #17, it is decided whether the defocus amount having been
calculated is near to zero or not, that is, whether the focusing lens is
in a near focus zone with a given width or not. If it is near to zero, the
process proceeds to step #19 to set a predetermined low speed for driving
the lens. If it is not near to zero, the process proceeds to step #18 to
set a predetermined high speed for driving the lens. At step #20, a
driving operation of the lens is started. If the lens has been already
started to drive, the driving is continued. Then, the process is returned
to step #1 to repeat the next calculation of a defocus amount.
<II-2> Calculation of Defocus Amount
FIG. 2 shows contents of the calculation of a defocus amount to be done at
step #3 of FIG. 1.
Since the principle of the calculation of a defocus amount is disclosed
precisely in Japanese patent laid open publication No. 126517/1985 or No.
4914/1986., concrete processings thereabout will be stated hereinafter.
Before starting the explanation of the flow chart, a composition of the CCD
image sensor will be stated in order for better understanding thereabout.
As shown in FIG. 3, the CCD image sensor has a lot of light sensing
elements aligned linearly and the central portion thereof is formed as a
separating zone. On one side of the separating zone, a standard portion L
is defined so as to have forty picture elements from l.sub.1 to l.sub.40
and a reference portion R is defined so as to have forty eight picture
elements from r.sub.1 to r.sub.48 on the other side.
In the standard portion L, there are defined first to third blocks I to III
being overlapped with each other. These first to third blocks I to III
comprise picture elements from l.sub.1 to l.sub.20, from l.sub.11 to
l.sub.30 and from l.sub.21 to l.sub.40, respectively. A calculation for
calculating correlation degrees is executed at first with use of the
second block II. If any effective minimum value is not found out by the
correlation calculation with respect to the second block, a next
correlation calculation is executed in the order of the first and third
blocks. As shown in Table and FIG. 4, each shift amount of the outputs of
the picture elements belonging to each block of the standard portion (L)
with respect to those of the picture elements belonging to the reference
portion (R) is obtained so as to overlap partically with each other.
TABLE
__________________________________________________________________________
Area of Pic Left Most Elem.
Dect. Area for
Element
Diff. Data
for Corr. Calc.
Image Dist. Error(Max)
__________________________________________________________________________
First
Block (I)
l.sub.1 .about.l.sub.20
ls.sub.1 .about.ls.sub.16
.gamma.5(rs 5)
-4.about.14 pitch
Stand.
Second
Port.
Block (II)
l.sub.11 .about.l.sub.30
ls.sub.11 .about.ls.sub.26
r.sub.15 (rs 15)
-8.about.8 pitch
(L) Third
Block (III)
l.sub.21 .about.l.sub.40
ls.sub.21 .about.ls.sub.36
r.sub.25 (rs 25)
-14.about.4 pitch
Ref.
Port.
All .gamma.1.about..gamma.48
.gamma.s.sub.1 .about.rs.sub.44
(R)
__________________________________________________________________________
Next, a calculation method of defocus amount will be descripted according
to the flow chart shown in FIG. 2.
At steps #25 and #26, a preliminary processing about object image data is
done. As shown in blocks of steps #25 and #26, picture element
differential data lSk and rSk are made by subtracting every fourth picture
element data, respectively (lSk=lk-lk+.sub.4 ; wherein k is one of
integers from 1 to 36, rSk=rk-rk+.sub.4 ; wherein k is one of integers
from 1 to 44). This data-processing is done for the purpose of filter
effect of a kind due to a low-pass filter and is effective to remove
possible errors in the focus condition detection which might be caused by
an unbalance between two images which is induced due to manufacturing
errors in the optical system for detecting a focus condition.
At step #27, a first correlation calculation between the standard portion L
and the reference portion R is executed with use of the second block II of
the standard portion. The range in which a defocus amount from the focus
position can be sought due to the first correlation calculation is a range
defined between (-8)th pitch and (+8)th pitch of picture element when the
focus position as a null position, defined as shown in FIG. 4. In other
words, the range is a range defined between the sixth picture element and
the twenty second picture element of the reference portion R (rSk+l;
l=6-22).
At step #28, minimum correlation function H.sub.2 (lM.sub.2) which shows
the highest correlation is sought among correlation functions from H.sub.2
(6) to H.sub.2 (22). It is decided at step #29 whether the correlation
calculation having been done has an credibility enough for calculating a
defocus amount therefrom. If decided "YES" at step #29, the process
proceeds to step #30 to execute such an interpolation calculation as shown
in the block of step #30. According to this interpolation calculation, the
maximum correlation position XM.sub.2 is sought. At step #31, a shift
amount P from the regular focus position is sought with use of the maximum
correlation position XM.sub.2 having been sought with a high precision and
a defocus amount DF is calculated from the shift amount P sought at step
#32.
If it is decided at step #28 that the correlation calculation having been
done not have credibility, the process proceeds to step #33 in order to
execute a second correlation calculation with use of the first block I.
The range of the first block I is defined from (-4)th pitch to (+14)th
pitch, namely from zeroth position to 18th position in the reference
portion (See. FIG. 4). Similarly to the correlation calculation with use
of the seiond block I, a maximum correlation position lM.sub.1 is sought
at step #34 and, at step #35, it is decided whether the reliability or
credibility of the second correlation calculation is enough for
calculating a defocus amount or not.
When decided "YES", the process proceeds to step #36 to execute an
interpolation calculation similarly to that of step #30. However, it is to
be noted that lM.sub.2, XM.sub.2 and H.sub.2 contained in the equations of
step #30 are replaced to lM.sub.1, xM.sub.1, and H.sub.1, respectively.
Then, a shift amount P from the regular focus position is sought from the
maximum correlation position and a defocus amount DF is calculated at step
#32.
When decided "NO" at step #35, the process proceeds to step #38 to execute
a third correlation calculation with use of the third block III. The range
of third block III is defined from (-14)th pitch to (+4)th pitch, namely
from 10th position to 28th position in the reference portion R, as shown
in FIG. 4. Similarly to the correlation calculation with use of the first
and second blocks I and II, a maximum correlation position XM.sub.3, a
shift amount P and a defocus amount DF are sought from step #38 to #42,
respectively.
If it is decided that the correlation calculation with use of the third
block III has not reliability or credibility sufficient for calculation a
defocus amount, the process proceeds to step #43 in order to set a flag
indicating the impossibility of the focus detecting calculation at step
#43 and, then, returns to step #3 of FIG. 1. This flag is read at step #4
of the main routine of FIG. 1. If the flag has been set, the low contrast
scan is started at step #5.
<II-3> Follow-up Mode
FIG. 5 is a flow chart showing proceedings to be executed during the stop | | |
