Automatic focusing device

This invention relates to an automatic focusing device, particular capable of foreseeing the position of an object after a predetermined future time based on the results of a plurality of past focusing operations, thus enabling the focusing to the object. An automatic focusing device is provided which executes calculations for the abovementioned foreseeing after the execution of a predetermined number of focusing operations. However, if a focused state is identified in a focusing operation in the course of the execution of the predetermined number of focusing operations, the device is adapted to disregard the first-mentioned focusing operation as if it had not been conducted and to exclude the first-mentioned focusing operation from the counting of the predetermined number of focusing operations.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an automatic focusing device adapted for 
use in a camera or the like. 
2. Related Background Art 
Most of the automatic focusing methods conventionally employed in the 
single-lens reflex cameras achieve focusing to an object by repeating a 
cycle of focus state detection (signal input from a sensor and calculation 
of focus state) and lens drive. The amount of lens movement in each cycle 
is based on the amount of defocus determined by the focus state detection 
in said cycle, and it is anticipated that the amount of defocus is brought 
to zero at the end of the lens drive. 
The focus state detection and the lens movement naturally require a certain 
amount of time. However, in the case of a stationary object, the amount of 
defocus does not change unless the lens is moved, so that the amount of 
defocus to be cancelled at the end of lens movement is equal to the amount 
of defocus detected at the focus state detection, and correct focusing can 
therefore be achieved. 
On the other hand, for a fast moving object, the amount of defocus varies 
in the course of focus state detection and lens movement, so that the 
amount of defocus to be cancelled may be significantly different from the 
detected amount of defocus. As a result, the focusing to the object may 
not be achieved at the end of lens movement. 
Automatic focusing devices for avoiding such a drawback have been proposed 
in the Japanese Laid-Open Patents Sho 62-125311, Sho 62-139512, Sho 
62-139511 and Sho 62-269936. 
The automatic focusing devices disclosed in the above-mentioned patents 
are, in summary, to correct the amount of lens movement by foreseeing the 
change of the amount of defocus resulting from the movement of the object, 
in consideration of the change in the amount of defocus detected in said 
cycles and of the interval of said cycles, and are expected to improve the 
accuracy of focusing at the end of lens movement. 
However, such an automatic focusing method involving correction may result 
in the following drawback when the object is stopped. 
Even when the object is stopped, the result of focus state detection by 
detecting means is not constant but shows a certain fluctuation due, for 
example, to the influence of noises in said detecting means, and such 
fluctuating results may be misunderstood to indicate that the object is 
moving. Thus, the correction in such an automatic focusing method may lead 
to an improper focusing. 
FIG. 9 is a chart showing the conventional method of correcting the amount 
of lens movement, indicating the position d of the image plane of the 
object in the ordinate, as a function of time in the abscissa. 
A solid line f(t) indicates the image plane position of the object, while a 
broken line l(t) indicates the image plane position of the lens. 
More detailedly, the line f(t) indicates the position, at time t, of the 
image plane of an object axially approaching to the camera when the 
focusing optical system of the photographing lens is focused at an 
infinite object distance, while the line l(t) indicates the position of 
the image plane of said object at the focusing state at time t of the 
focusing optical system. Each section [t.sub.i, t.sub.i '] indicates a 
focus state detecting operation, and each section [t.sub.i ', t.sub.i+1 ] 
indicates a lens driving operation. 
Consequently, a so-called defocus amount is represented by the difference 
between f(t) and l(t) at the same time t, along the ordinate d. DFi is the 
defocus amount detected at a time t.sub.i ; DLi is the amount of lens 
movement determined from the focus state detection at a time t.sub.i-1 and 
represented by the change in the image plane position; and TMi is the 
interval in time of the focusing operations. 
The conventional example shown in FIG. 9 is based on an assumption, for the 
corrective calculation, that the image plane position of the object varies 
according to a second-order function. More specifically, it is assumed, at 
a time t.sub.3, that the image plane position at a time t.sub.4 is 
foreseeable if three image plane positions (t.sub.1, f.sub.1), (t.sub.2, 
f.sub.2) and (t.sub.3 f.sub.3) at past and present are known. 
In practice, however, the camera cannot detect the image plane positions 
f.sub.1, f.sub.2, f.sub.3 but the defocus amounts DF1, DF2, DF3 and the 
lens drive amounts DL1, DL2 represented in the amount of image plane 
movement. The future time t.sub.4 is unfixed and varies according to the 
change in the accumulating time of the charge accumulating sensor by the 
luminance of the object, but, for the purpose of simplicity in determining 
f.sub.4, t.sub.4 is defined as being known from t.sub.4 -t.sub.3 =t.sub.3 
-t.sub.2. 
Under the assumptions explained above, the lens drive amount in the section 
from t.sub.3 ' to t.sub.4, based on the result of the focus state 
detection at t.sub.3 is calculated according to the following relations: 
EQU a.t.sup.2 +b.t+c=f(t) (1) 
EQU a.t.sup.2.sub.1 +b.t.sub.1 +c=f(t.sub.1) (2) 
EQU a.t.sup.2.sub.2 +b.t.sub.2 +c=f(t.sub.2) (2') 
EQU a.t.sup.2.sub.3 +b.t.sub.3 +c=f(t.sub.3) (2") 
Taking the point l.sub.1 in FIG. 9 as the original point; 
EQU f.sub.1 =DF1 (3) 
EQU f.sub.2 =DF2+DL1 (3') 
EQU f.sub.3 =DF3+DL2+DL1 (3") 
EQU t.sub.1 =0 (4) 
EQU t.sub.2 =TM1 (4') 
EQU t.sub.3 =TM1+TM2 (4") 
The coefficients a, b and c are determined by substituting the equations 
(3), (3'), (3"), (4), (4'), and (4") into the equations (2), (2') and 
(2"): 
##EQU1## 
Consequently the lens drive amount represented in the amount of image plane 
movement DL3 at time t.sub.4 is given by: 
##EQU2## 
Thus DL3 is determined from the equation (8) on the aforementioned 
assumption that TM3=TM2. 
Thereafter the lens drive amount at a time t.sub.i can be obtained in a 
similar manner, as indicated below, from the defocus amounts DFi-2, DFi-1, 
DFi in three past detections, the lens drive amounts DL1-2, DL1-1 in two 
past lens drives, and the two past time intervals TMi-2, TMi-1: 
##EQU3## 
Thus a proper focusing is obtained at the end of the lens driving operation 
even for a moving object, by determining the defocus amount DLi for the 
lens movement from the detected defocus DFi according to the equations 
(9), (10) and (11). 
In the correcting method explained above, there are required data of at 
least two focusing operations in the past, in order to extrapolate the 
image plane position by a second-order function. However in the first two 
cycles of focusing, in which such data are not yet available, the lens is 
driven based on the detected defocus itself as shown in FIG. 9. In these 
cycles the correction is not applied by the correction means explained 
above. The actual corrective calculation is started from the third lens 
driving operation, and the effect of correction appears from the time 
t.sub.4, as shown in FIG. 9. 
FIG. 10 shows a case of a stopped object, in which the lens is driven with 
a correction erroneously determined by misunderstanding the noises as a 
movement of the object. As in FIG. 9, the abscissa indicates time t, while 
the ordinate indicates the image plane position d of the object. However, 
a unit in the ordinate of FIG. 9 is in a magnified scale. A solid line 
f(t) indicates the image plane position of the object, while a folded line 
l(t) indicates the image plane position of the lens. Broken lines indicate 
the depth of focus of the optical system, by a value F .delta./2 on either 
side of the detected focus position, wherein F is the fully open F-number 
of the lens and .delta. is the size of a minimum aberration circle. Stated 
differently, photographing in a focused state is possible if the folded 
line l(t) is positioned inside an area defined by the broken lines. 
In FIG. 10, the correction by the correction means is not applied from the 
cycle f.sub.1 to the defocus detection in f.sub.3. In the illustrated 
example, the lens is not driven in the cycle f.sub.1 since the defocus 
detected at (t.sub.1, f.sub.1) is within the depth of focus. However, the 
lens is driven at the next cycle because the defocus detected at (t.sub.2, 
f.sub.2) exceeds the depth of focus for some reason such as noise. At the 
next lens drive in response to the detection at (t.sub.3 f.sub.3), the 
abovementioned correction is added to obtain a result l.sub.4. However, 
even if the detection (t.sub.4, f.sub.4) provides a substantially correct 
result for the stopped object, a correction is added to the next lens 
drive to reach a result l.sub.5 by a misunderstanding that the object has 
moved. Consequently, said corrections bring the lens into defocus states 
at l.sub.4 and l.sub.5. 
SUMMARY OF THE INVENTION 
An aspect of the present invention is to provide an automatic focusing 
device for effecting a corrective calculation for determining the lens 
drive amount for focusing to the object after a predetermined time, based 
on the data of past plural focusing operations, which is capable of 
disregarding a focusing operation if a focused state is identified in said 
operation in the course of plural focusing operations for obtaining said 
data of past plural focusing operations. 
Another aspect of the present invention is to provide, under the 
above-mentioned object, an automatic focusing device which, in the course 
of focusing operations of N times for obtaining said data, if a focused 
state is identified in the M-th focusing operation (M.ltoreq.N), regards 
the next focusing operation as the M-th operation, and thereafter 
continues the focusing operations. 
Still another aspect of the present invention is to provide, under the 
above-mentioned objects, an automatic focusing device capable of 
cancelling the M-th focusing operation identified as a focused state and 
utilizing the data of a next focusing operation regarded as M-th 
operation, as the m-th data. 
Still another aspect of the present invention is to provide, under the 
above-mentioned objects, an automatic focusing device which, in the course 
of said focusing operation and after the identification of the focused 
state, increases the range of identification of the focused state in the 
succeeding focusing operations. 
Still other objects of the present invention will become fully apparent 
from the following description of embodiments which is to be taken in 
conjunction with the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
At first the working principle of an embodiment of the present invention 
will be explained with reference to FIG. 8, and in comparison with FIG. 
10. 
FIG. 8 shows the control of said embodiment of the present invention, based 
on the same results of focus state detection as in FIG. 10. As in FIG. 10, 
FIG. 8 shows the image plane position d of the object in the ordinate, as 
a function of time t in the abscissa. A solid line f(t) indicates the 
image plane position of the object, while a straight line l(t) indicates 
the image plane position of the lens, and broken lines indicate the depth 
of focus used in identifying the focused state. 
The focus state detection at (t.sub.1, f.sub.1) in FIG. 8 is conducted with 
a range of focused state the same as the depth of focus in FIG. 10 and the 
lens is not driven. Therefore, at the next focus state detection (t.sub.2, 
f.sub.2), the range of focused state is enlarged, about 4 times in the 
illustrated case. Because of the thus enlarged range of focused state, the 
focus state detection at (t.sub.2, f.sub.2) also identifies a focused 
state, so that the next focus state detection at (t.sub.3 f.sub.3) is 
conducted without lens driving. In the conventional method, the corrective 
calculation is conducted on the past data including the result of said 
focus state detection, and a correction is added to the lens drive based 
on the result of the focus state detection at (t.sub.3 f.sub.3). In the 
present embodiment, however, the correction is added after the focus state 
detection at (t.sub.3 f.sub.3), Also, the range of focused state remains 
in the enlarged state because the result of a preceding focus state 
detection indicates the focused state, so that the focus state detection 
at (t.sub.3,F.sub.3) also indicates a focused state and the lens is 
therefore not driven. Then, the next focus state detection at 
(t.sub.4,f.sub.4) is conducted under the same conditions as at (t.sub.3 3) 
and identifies a focused state, so that the lens is not driven. 
Consequently, there is not conducted a lens drive involving unnecessary 
correction such as l.sub.4 or l.sub.5 in FIG. 10, and the lens maintains a 
constant position for a stopped object, thereby always enabling 
photographing in the focused state. 
In summary, in the conventional lens driving method of adding a certain 
correction to the detected defocus amount for focusing to a moving object, 
in the course of accumulation of data necessary for calculation of said 
correction, namely in a state in which the correction is not applied by 
the correction means (focus state detecting period without the function of 
the correction means), the range of focused state is enlarged if a focused 
state is identified in the preceding focus state detection, and if the 
present and preceding focus state detections both identify a focused state 
while the correction is not applied, the correction is not applied even 
after the next focus state detection. This method eliminates unnecessary 
lens drive for a stopped object, thereby always enabling photographing in 
the focused state. 
In the following there will be explained the structure of the device and 
the control sequence thereof for realizing the above-mentioned method, 
with reference to FIGS. 1 to 7. 
FIG. 1 is a circuit diagram of a camera equipped with a device embodying 
the present invention. 
A process circuit PRS of the camera is composed of a one-chip microcomputer 
provided therein with a central processing unit (CPU), a ROM, a RAM and an 
A/D converter, and controls various functions of the camera such as 
automatic exposure control, automatic focusing, film winding and film 
rewinding according to sequence programs stored in the ROM. For this 
purpose the process circuit PRS controls the peripheral circuits of the 
camera and a control unit in the lens by communicating therewith by means 
of communication signals SO, SI, SCLK and communication selection signals 
CLCM, CSDR, CDDR. 
The signal SO is the data signal released from the process circuit PRS, the 
signal SI is the data signal entered thereto, and the signal SCLK is the 
clock signal for the signals SO, SI. 
A lens communication buffer circuit LCM supplies electric power to a lens 
power supply terminal VL while the camera is in function, and serves as a 
communication buffer between the camera and the lens when the selection 
signal CLCM from the process circuit PRS is at a high (H) level state. 
When the process circuit PRS shifts said signal to the H-level and sends 
predetermined data as the signal SO in synchronization with the clock 
signal SCLK, the lens communication buffer circuit LCM sends the buffered 
signals LCK, SO through camera-lens communication contacts, respectively 
corresponding to said signals SCLK, SO. At the same time, it sends a 
buffered signal of the signal DCL from the lens as the data signal SI, 
which is received by the process circuit PRS in synchronization with the 
clock signal SCLK. 
A sensor drive circuit SDR, for focus state detecting line sensors composed 
for example of CCD's, is selected when the signal CSDR is at the H-level 
and is controlled by the process circuit PRS through the signals SO, SI 
and SCLK mentioned above. A clock signal CK is used for generating CCD 
driving clock signals .phi.1, .phi.2, and a signal INTEND is used for 
informing the process circuit PRS of the completion of an accumulating 
operation. 
An output signal OS of the line sensors SNS is a time-sequential image 
signal synchronized with the clock signals .phi.1, .phi.2, and, after 
amplification in an amplifier in the drive circuit SDR, is supplied as an 
image signal AOS to the process circuit PRS. The process circuit PRS 
receives said image signal AOS through an analog input port, converts it 
into a digital signal by the internal A/D converter in synchronization 
with the clock signal CK, and stores said digital signal in succession at 
predetermined addresses of the RAM. 
Another output signal SAGC of the line sensors SNS, from an automatic gain 
control sensor therein, is supplied to the drive circuit SDR for 
accumulation time control of said sensors SNS. 
A photosensor SPC for exposure control, receiving the light from the object 
signal SSPC which is supplied to an analog input port of the process 
circuit PRS is used, after A/D conversion, for automatic exposure control 
according to a predetermined program. 
A switch detection/display circuit DDR is selected when the signal CDDR is 
at the H-level, and is controlled by the process circuit PRS through the 
signals SO, SI and SCLK. It serves to switch the display on a display unit 
DSP of the camera according to the data supplied from the process circuit 
PRS, and to inform the process circuit PRS of the on/off state of various 
operating members of the camera by communication procedures. 
Switches SW1, SW2 are linked with an unrepresented shutter release button. 
The switch SW1 is closed by the depression of said button over a first 
stroke, and the switch SW2 is subsequently closed by the depression of 
said button over a second stroke. The process circuit PRS executes light 
metering and automatic focusing in response to the closing of the switch 
SW1, and exposure control and film winding in response to the closing of 
the switch SW2. 
Said switch SW2 is connected to an interruption input port of a 
microcomputer constituting the process circuit PRS, whereby an 
interruption program is immediately started by the closing of the switch 
SW2, even if a program in response to the closing of the switch SW1 is 
under execution. 
A film feeding motor MTR1 and a motor MTR2 for mirror movement and shutter 
charging are driven in forward or reverse direction respectively by drive 
circuits MDR1, MDR2, according to motor control signals M1F, M1R, M2F, M2R 
supplied from the process circuit PRS. 
Magnets MG1, MG2 for respectively releasing the leading and trailing 
curtains of the shutter are respectively energized by signals SMG1, SMG2 
supplied through amplifying transistors TR1, TR2, from the process circuit 
PRS. 
The switch detection/display circuit DDR, motor drive circuits MDR1, MDR2 
and the method of shutter control will not be explained further as they 
are not directly related to the present invention. 
The signal DCL supplied to a lens process circuit LPRS in synchronization 
with the clock signal LCK represents command data sent from the camera to 
the lens FLNS, which executes predetermined operations in response to said 
commands. The lens process circuit LPRS analyzes said commands according 
to a predetermined procedure, thereby performing focusing and diaphragm 
control, and releases the functional states of the lens (state of focusing 
optical system and of diaphragm control) and various parameters 
(fully-open F-number, focal length, coefficient of movement of the 
focusing optical system to defocus amount etc.) as an output signal DLC. 
In the present embodiment there is employed a zoom lens. In response to a 
focusing command from the camera, a focusing motor LMTR is driven by 
signals LMF, LMR according to the amount and direction of lens drive 
supplied simultaneously, thereby axially moving the optical system. The 
amount of movement is monitored by a pulse signal SENCF of an encoder 
circuit ENCF, countered by a counter in the lens process circuit LPRS, 
and, upon completion of predetermined movement, the lens process circuit 
LPRS shifts the signals LMF, LMR to the L-level, thereby braking the motor 
LMTR. 
Therefore, once a focusing command is sent from the camera, the process 
circuit PRS need not be involved in the lens drive until its completion. 
Also, in the case of a request from the camera, the content of said 
counter can be transmitted to the camera. 
When a diaphragm control command is sent from the camera, a stepping motor 
DMTR known for diaphragm control is driven according to the number of 
stops transmitted simultaneously. Since the stepping motor is capable of 
open control, there is not required an encoder for monitoring the 
operation thereof. 
An encoder circuit ENC2 is attached to the zoom optical system, and the 
lens process circuit LPRS detects the zoom position by receiving a signal 
SENCZ from said encoder circuit ENCZ. The lens process circuit LPRS stores 
therein lens parameters corresponding to various zoom positions, and sends 
the parameters corresponding to the current zoom position to the camera, 
in response to a request therefrom. 
In the following there will be explained the function of the 
above-mentioned camera, with reference to the flow charts shown in FIG. 2 
and the ensuing drawings. 
When an unrepresented power switch is turned on, the process circuit PRS is 
powered and starts the execution of a sequence program stored in the ROM. 
FIG. 2 is a flow chart showing the entire flow of the above-mentioned 
program. 
When the program execution is started in a step (001), a step (002) detects 
the state of the switch SW1 to be closed by the depression of the shutter 
release button over the first stroke. If the switch SW1 is off, the 
sequence proceeds to a step (003) for effecting the initialization by 
clearing all the flags and variables for control set in the RAM. 
The above-mentioned steps (002) and (003) are repeated until the switch SW1 
is turned on or the power switch is turned off. In response to the closing 
of the switch SW1, the sequence proceeds from the steps (002) to (004). 
The step (004) executes a "photometry" subroutine for exposure control. The 
process circuit PRS receives the output signal SSPC of the photosensor SPC 
shown in FIG. 1 through the analog input port, effects the A/D conversion 
of said signal, calculates the optimum control values of shutter speed and 
diaphragm from said digital photometry value, and stores said control 
values in predetermined addresses of the RAM. At the shutter releasing 
operation, the shutter and the diaphragm are controlled based on these 
values. 
Then, a step (005) executes an "image signal input" subroutine, shown in 
FIG. 3, for entering an image signal from the focus state detecting line 
sensors SNS, as will be described later. 
Then, a step (006) executes a "focus state detection" subroutine, for 
calculating the defocus amount DEF of the photographing lens according to 
the entered image signal. The specific method of calculation is disclosed 
for example in the Japanese Patent Application Sho 61-160824 and will not 
therefore be explained further. 
A next step (007) executes a "foreseeing calculation" subroutine, shown in 
FIG. 5, for correcting the lens drive amount according to the foregoing 
equations (9), (10) and (11). 
Then, a next step (008) executes a "lens drive" subroutine, shown in FIG. 
7, for moving the lens according to the defocus amount DL corrected in the 
step (007). 
After the completion of lens drive, the sequence returns to the step (002), 
and the steps (004) to (008) are repeatedly executed until the switch SW1 
is turned off, thereby executing proper focusing even to a moving object. 
The switch SW2, to be closed by the depression of the shutter release 
button over the second stroke, is connected to the interruption input port 
of the process circuit PRS, whereby, as explained before, the shutter 
releasing sequence is immediately started by interruption regardless of 
the step under execution when the switch SW2 is turned on, but the shutter 
releasing operation is not directly related to the present invention and 
will not, therefore, be explained further. 
In the following there will be explained the "image signal input" 
subroutine shown in FIG. 3. 
The "image signal input" is executed at the start of each focusing cycle. 
When said subroutine is called in a step (101), a step (102) stores the 
timer value TIMER of a self-running timer of the process circuit PRS in a 
memory area TN of the RAM, thereby recording the start time of the 
focusing operation. 
A next step (103) renews the time intervals TM1, TM2 corresponding to 
TMi-2, TMi-1 in the foregoing equations (9), (10) and (11). Prior to the 
execution of the step (103), the areas TM1, TM2 store the time intervals 
TMi-2, TMi-1 used in the preceding focusing cycle, and the area TN1 stores 
the start time of the preceding focusing cycle. 
Consequently, TM2 indicates the interval from the focusing cycle 
immediately before the last one to the last cycle, while TN-TN1 indicates 
the interval from the last focusing cycle to the current one, and these 
values are stored in the memory areas TM1, TM2 of the RAM, corresponding 
to TMi-2 and TMi-1 in the equations (9), (10) and (11). Also, the area TN1 
stores the start time of the current cycle TN for the next focusing cycle. 
Then a next step (104) causes the line sensors SNS to start the charge 
accumulation. More detailedly, the process circuit PRS sends an 
accumulation start command to the sensor drive circuit SDR, which in 
response shifts a clear signal CLR, for the photoelectric converting 
elements of the line sensors SNS, to the L-level, thereby starting the 
charge accumulation. 
Then, a step (105) stores the current time, by storing the value of the 
self-running time in a variable area T1. 
A next step (106) disciminates whether the accumulation is completed, by 
detecting the state of the input port INTEND of the process circuit PRS. 
Simultaneously with the start of accumulation, the sensor drive circuit 
SDR shifts the signal INTEND to the L-level, then monitors a signal SAGC 
from the line sensors SNS and, when said signal SAGC reaches a 
predetermined level, shifts the signal INTEND to the H-level and 
simultaneously a charge transfer signal SH to the H-level for a 
predetermined period, thereby transferring the charges of the 
photoelectric converting unit to the CCD unit. 
The sequence proceeds from the step (106) to a step (110) if the INTEND 
port is at the H-level indicating the completion of accumulation, or to a 
step (107) if said port is at the L-level indicating that the accumulation 
is not yet complete. 
The step (107) subtracts the time T1 stored in the step (105) from the 
value TIMER of the self-running timer, and stores the obtained difference 
as a variable TE. Consequently, the area TE stores the so-called 
accumulation time, namely the time from the start of accumulation to each 
respective time. 
A next step (108) compares the variable TE with a constant NAXINT, and, if 
the former is smaller than the latter, the sequence returns to the step 
(106) to await the completion of charge accumulation. When the former 
becomes equal to or larger than the latter, the sequence proceeds to a 
step (109) to forcedly terminate the charge accumulation. Such a forced 
termination is conducted by sending an accumulation terminating command 
from the process circuit PRS to the drive circuit SDR. 
In response to the accumulation terminating command from the process 
circuit PRS, the sensor drive circuit SDR shifts the charge transfer 
signal SH the H-level for a predetermined period, thereby transferring the 
charges accumulated in the photoelectric conversion unit to the CCD unit. 
The charge accumulation of the sensor is completed by the sequence up to 
the step (109). 
A step (110) executes A/D conversion of the signal AOS, obtained by 
amplifying the image signal OS of the line sensors SNS by the sensor drive 
circuit SDR, and storage of a thus obtained digital signal into the RAM. 
More detailedly, in synchronization with the clock signal CK from the 
process circuit PRS, the sensor drive circuit SDR provides a control 
circuit SSCNT of the line sensors SNS with CCD drive clock signals .phi.1, 
.phi.2, which drive the CCD unit of the sensors SNS to release the charges 
thereof as timesequential image signal OS. Said signal is amplified by an 
amplifier in the drive circuit SDR, and supplied, as the image signal AOS, 
to the analog input port of the process circuit PRS. The process circuit 
PRS effects A/D conversion in synchronization with the clock signal CK 
released by the process circuit PRS itself, and stores the digital image 
signal, obtained by said A/D conversion, in succession in predetermined 
addresses of the RAM. 
After the image signal is entered in the aboveexplained fashion, the image 
signal input subroutine is terminated in a step (111). 
FIG. 4 is a flow chart of the "lens drive" subroutine. 
When this subroutine is started, a step (202) discriminates whether a 
second distance measurement has been completed. According to the present 
invention, as explained before, the corrective calculation is conducted 
after three distance measurements, and the correction is made by said 
calculation on the defocus detected by the distance measurement. 
Consequently, said step is to confirm whether the current state is 
immediately before the addition of the corrective value. A counter CON is 
subjected to an increment at each distance measurement in a step (306') in 
FIG. 5, and the sequence proceeds to a step (203) only when CON=2. The 
step (203) discriminates whether the preceding, or first, distance 
measurement is identified as an in-focus state, by an in-focus flag to be 
set in a step (210) in the preceding distance measurement. If the in-focus 
state was identified in the preceding cycle, a step (204) enlarges the 
in-focus range. This operation corresponds to the foregoing expression 
that "the in-focus range is enlarged if the focused state is identified in 
the preceding cycle, in a period in which correction is not applied by the 
correction means". 
If the step (202) identifies that the current state is not after a second 
distance measurement, namely either after the first one or after the third 
or ensuing one, a step (205) adopts the ordinary in-focus range, which in 
the examples shown in FIGS. 8 and 10 is selected as F .delta./2, wherein F 
is the fully-open F-number of the lens, and .delta. is the size of the 
minimum aberration circle. This is to effect the focusing with better 
precision after the first, third or ensuing cycle. The same applies even 
after the second distance measurement if the in-focus state was not 
identified in the preceding cycle. Only when the steps (202) and (203) 
identify that the second cycle has been completed and that the in-focus 
state was identified in the preceding cycle, the step (204) enlarges the 
in-focus range, for example to four times the usual range, or 2F .delta. 
in the case shown in FIG. 8. 
A step (206) identifies the focus state, by comparing the absolute value 
.vertline.DL.vertline. of the defocus DL with the current in-focus range. 
If the step (206) identifies an in-focus state, a step (207) sets the 
defocus DL equal to 0. A next step (208) discriminates whether the current 
lens drive is the second time, namely after the second distance 
measurement. If this is the second drive (CON=2), a step (209) sets the 
apparatus at a state after the first cycle (CON=1). Stated differently, 
the object is identified as being stopped, and the current distance 
measurement is regarded as the first one. In the ordinary situation, the 
next distance measurement is the third one, and the past data including 
the result thereof are used in a foreseeing calculation to correct the 
lens drive. In the present case, however, the object is identified as 
being stopped, so that the lens drive with correction is not started 
immediately after the next distance measurement but at least after two 
additional distance measurements. Thus, the result of the preceding, or 
first, distance measurement is discarded. Stated otherwise, for a stopped 
object, the state is controlled by the distance measurement of number so 
as not to start the foreseeing calculation until the object starts to 
move. This operation corresponds to the foreseeing expression that "if the 
preceding and current focus state detections both identify an in-focus 
state while the correction is not applied by the correction means, the 
correction is not applied after the next focus state detection". 
A next step (210) identifies that the current distance measurement 
indicates the in-focus state, since the lens drive is not required. In 
this case the sequence proceeds to a step (215) to terminate the "lens 
drive" subroutine. 
If the step (206) does not identify the in-focus state, a step (211) 
receives two data parameters "s" and "PTH" by communication with the lens. 
The data parameter "s", is the "coefficient of the amount of the movement 
of the image plane to the amount of movement of the focusing optical 
system", or the amount of movement of the image plane of the photographing 
lens, when it is axially moved by a unit distance. For an entirely movable 
single lens, s=1 because the entire photographing lens constitutes the 
focusing optical system, of which movement corresponds to the movement of 
the image plane. In the case of a zoom lens, the value "s" varies 
according to the position of the zooming optical system. 
"PTH" is the amount of movement of the focusing optical system LNS per an 
output pulse of an encoder circuit ENCF linked with the axial movement of 
said optical system. 
Consequently, so-called lens drive amount FP, namely the amount of movement 
of the focusing optical system converted into the number of output pulses 
of the encoder circuit, is given by the following equation, based on the 
defocus amount DL (to be determined in a step (308) or (312) in FIG. 5), 
and the above-mentioned parameters "s" and "PTH": 
EQU FP=DL.s/PTH 
A step (212) executes the calculation of this equation. 
A step (213) instructs the lens to drive the focusing optical system, by 
sending the amount FP determined in the step (212) to said lens. 
A next step (214) discriminates, through communication with the lens, 
whether the lens drive of the amount FP instructed in the step (212) has 
been completed, and, if completed, the sequence proceeds to a step (215) 
to terminate the "lens drive" subroutine. 
FIG. 5 is a flow chart of the "foreseeing calculation" subroutine. 
In the present embodiment, the calculations of the correcting equations 
(9), (10) and (11) are conducted by replacing the defocus amount therein 
with the amount of lens movement. 
Said replacement is made by: 
EQU DFi=DEF.s (12) 
wherein DEF is the latest detected defocus, and s is the lens coefficient 
explained above. After the replacement according to equation (12), 
progressive corrections are made according to the equations (9), (10) and 
(11) to obtain a corrected lens drive amount DLi. 
Steps (302), (303) effect renewal of data for the present corrective 
calculation, because the equations (9), (10) and (11) are in progressive 
forms, employing the data of past plural cycles. The step (302) renews the 
data of the detected defocus amount converted into the lens drive amount, 
while the step (303) renews the corrected defocus amount converted into 
the lens drive amount. 
A next step (304) stores the value of TM2 into TM3 corresponding to the 
time interval TMi from the present focusing cycle to the next one. As 
already explained in relation to the equation (11), the time interval TM3 
from the present focusing cycle to the next one is assumed equal to the 
interval TM2 from the preceding cycle to the present one. 
A step (305) receives the lens coefficient "s" from the lens, and a next 
step (306) converts the defocus, according to the equation (12), into the 
lens drive amount. Since the equations (9), (10) and (11) are in 
progressive form, the calculation of the equation (12) on the defocus 
detected in the present cycle allows conversion of all the defocus amounts 
into the lens drive amounts. 
Then, a step (306') adds one to the content of the counter CON. 
A next step (307) discriminates whether data for a foreseeing calculation 
are all ready, namely whether the correction is to be actually added or 
not. If the data of the past two focusing cycles and of the present 
focusing cycle are not yet available (CON&lt;3), the sequence proceeds to a 
step (308) for taking the latest defocus amount DEF as the defocus amount 
DL for the lens drive, and then to a step (313) for terminating the 
"foreseeing calculation" subroutine. 
If the data for the foreseeing calculation are ready (CON.gtoreq.3), the 
sequence proceeds to steps (309), (310) and (311) respectively for 
effecting the calculations of the equations (9), (10) and (11), thereby 
obtaining a lens drive amount DLS converted from the defocus amount for 
lens drive. 
Then, a step (312) calculates: 
EQU DL=DLS/s 
thereby obtaining the defocus amount DL for use in the "lens drive" 
subroutine. Thereafter, a step (313) terminates the "foreseeing 
calculation" subroutine. 
The control sequence in the flow of FIG. 4 can be summarized as follows. 
At the first lens drive after the first distance measurement, since CON=1, 
there are executed the step (205) and then either the steps (211)-(214) or 
(207)-(210). At the lens drive after the second distance measurement, 
since CON=2, there are executed the step (203) and then either the step 
(204) or (205). If the second distance measurement indicates the in-focus 
state, the steps (207), (208), (209) and (210) are executed to shift the 
counter CON to "1". Thus, even after the third distance measurement, a 
state CON=2 is obtained instead of CON=3, whereby the lens drive is 
conducted in the same manner as after the second distance measurement. 
Consequently, as long as the second distance measurement continues to 
identify the in-focus state (as long as the cycle in the state CON=2 
continues to identify the in-focus state), there is not executed the lens 
drive according to the foreseeing calculation (steps (309) to (312) in 
FIG. 5). 
On the other hand, if the second distance measurement (in the state CON=2) 
does not identify the in-focus state, a state CON=3 is reached after the 
next distance measurement, whereby the steps (309)-(312) in FIG. 5 are 
executed to drive the lens according to the foreseeing calculation. 
In the foregoing embodiment, the change in the image plane position 
resulting from the movement of the object is approximated by a 
second-order function, but the present invention is evidently also 
applicable to first-order functions or higher-order functions, or 
functions of other suitable forms. 
Also, in the foregoing embodiment, if the second focus state detection (in 
a state CON=2) identifies an in-focus state while the correction is not 
applied by the correction means, the device is retained in a correction 
free state after the first detection cycle, but it is also possible to 
retain the device in a state after the second detection cycle. Stated 
differently, in the foregoing embodiment, the discrimination is made after 
the second distance measurement, and the correction applying state is 
entered when necessary data become available. On the other hand, in the 
latter case, the discrimination is made after three distance measurements, 
and the correction applying state is not necessarily entered even if the 
data are available. 
In the following, this modified embodiment will be explained, with emphasis 
on the differences from the foregoing embodiment. 
FIG. 6 is a flow chart of the "lens drive" subroutine, corresponding to 
FIG. 4. 
In said subroutine, a step (402) discriminates whether the current state is 
after the first distance measurement. If not (CON.noteq.1), the sequence 
proceeds to a step (403) for confirming whether the correction is to be 
added this time (by confirming the state of a correction flag to be 
described later in relation to FIG. 7). If the correction is not to be 
added this time, the sequence proceeds to a step (404) to discriminate 
whether the in-focus state was identified in the preceding cycle. If 
identified, a step (405) enlarges the in-focus range. On the other hand, 
if the step (402) identifies a state after the first distance measurement 
(CON=1), or if the step (403) identifies that the correction is to be 
added this time, or if the step (404) identifies that the in-focus state 
was not identified in the preceding cycle, the sequence proceeds to a step 
(406) for adopting the normal in-focus range. 
Then, a step (407) executes the focus state detection with the in-focus 
range determined in the step (405) or (406). 
If the step (407) identifies the in-focus state, the sequence proceeds to a 
step (408) to set the defocus amount DL as 0. Then, a step (409) 
discriminates whether the next lens drive is the third drive, namely after 
the third distance measurement (CON=3). If it is the third lens drive 
(CON=3), a step (410) sets the device at a state after the second lens 
drive (CON=2). Stated differently, the object is identified as being 
stopped, and the present distance measurement is regarded as the second 
one. In the ordinary situation, the present distance measurement is the 
third one, and the past data including the result of the third measurement 
are used in the foreseeing calculation to correct the lens drive. In the 
present case, however, the object is identified as being stopped, so that 
the lens drive with correction is not started immediately after the 
present distance measurement but at least after one additional distance 
measurement. A step (411) identifies that the in-focus state was 
identified in the present distance measurement since the lens drive was 
not needed. In this case the sequence proceeds to a step (416) to 
terminate the "lens drive" subroutine. 
If the step (407) does not identify the in-focus state, the sequence 
proceeds to a step (412). The subsequent steps (412) to (415) are the same 
as those of steps (212) to (215) in the "lens drive" subroutine of the 
foregoing embodiment. 
FIG. 7 is a flow chart of the "foreseeing calculation" subroutine 
corresponding to FIG. 5. 
At the start of said subroutine, steps (502) to (507) are the same as those 
of steps (302)-(307) in the "foreseeing calculation" subroutine of the 
foregoing embodiment. 
When the step (507) confirms that the data for foreseeing are available 
sufficient in quantity (CON&gt;3), a step (508) discriminates whether the 
correction was actually applied in the preceding cycle, by confirming 
whether the correction flag, set in a step (511), is equal to "1". If the 
correction was not applied in the preceding cycle (correction flag=0), 
namely if the data have become available this time, a step (509) compares 
the absolute value .vertline.DEF.vertline. of the defocus DEF detected in 
the distance measurement with the in-focus range selected in this state. 
If the former is larger than the latter, there is discriminated the 
necessity of applying the correction, starting from the present cycle. The 
in-focus range is determined in the "lens drive" subroutine explained 
before. However, in the first cycle, the "foreseeing calculation" 
subroutine is executed at first without any change in the in-focus range, 
so that there is employed the normal in-focus range. 
If the step (509) identifies that .vertline.DEF.vertline. is not larger 
than the in-focus range, or if the step (507) identifies that the data for 
foreseeing are not yet available, the correction is identified as being 
unnecessary for the present cycle. Therefore, the sequence proceeds to a 
step (510) for taking the latest defocus amount DEF as the amount of lens 
movement DL, and then to the step (516) for terminating the "foreseeing 
calculation" subroutine. 
If the step (508) confirms the application of a correction in the preceding 
cycle (correction flag=1), or if the step (509) identifies that 
.vertline.DEF.vertline. is larger than the in-focus range, the application 
of correction is identified as being necessary for the present cycle. 
Thus, a step (511) sets the application of the correction (correction 
flag=1), and subsequent steps (512) to (516) are executed in the same 
manner as those steps (309) to (313) in the "foreseeing calculation" 
subroutine of the foregoing embodiment. 
In this embodiment, if the preceding focus state detection identifies the 
in-focus state while the correction is not actually applied, the in-focus 
range to be used in the focus state detection of the present cycle is 
enlarged. Also, if the preceding and present focus state detections both 
identify the in-focus state while the correction is not actually applied, 
the state of the device is so controlled that the correction is not 
applied immediately after the next focus state detection. Such a control 
method avoids the lens driving operations involving unnecessary 
corrections even in the presence of certain fluctuations in the focus 
state detection for a stopped object, thereby maintaining a stable lens 
state and always enabling a photographing of the stopped object in a focus 
state.