Automatic focusing system for video camera

This invention relates to automatic focusing systems for video cameras and, more particularly, to an automatic focusing system, which is adopted to effect a focusing operation such as to render maximum the output of a detector detecting a high frequency component of a video signal.

DESCRIPTION OF THE PRIOR ART 
Automatic focusing systems for video cameras of this kind are disclosed in 
Japanese Pat. No. 5265/64 entitled "Automatic Focusing System for 
Television Camera" and also in NHK Technical Research Report, 1965, Vol. 
17, No. 1, p. 21 under the title of "Automatic Focusing System for 
Television Camera Based upon Climbing Servo System". These prior art 
disclosures teach techniques of passing the video signal from a video 
camera through a high-pass filter to obtain a high frequency component, 
sampling a voltage proportional to the fineness or extent of freedom from 
confusion of the television image (hereinafter referred to as focus 
voltage) by detecting the extracted high frequency component and bringing 
a focusing ring (hereinafter referred to as helicoid) to a position, at 
which the focus voltage is maximum, i.e., a position of focus, by taking 
the difference between two consequtive sampled values and controlling the 
rotation of the helicoid by a reversible motor such that the difference is 
always positive. 
These systems, however, have the following drawbacks. 
The first drawback is that with a zoom lens system based upon an objective 
lens moving system extensively used for ordinary video cameras and having 
a focusing mechanism the movement of the helicoid position that is caused 
for the purpose of focusing results in a slight change of the focal 
distance of the lens system itself together with the movement of the 
objective lens. 
Since in the above focusing systems the helicoid is moved to the position 
corresponding to the maximum focus voltage by feeding back the increase or 
decrease of the sampled value of the focus voltage, even during the 
focusing operation the helicoid is continuously oscillated back and forth 
about the position corresponding to the maximum focus voltage. This 
oscillation causes changes of the focal distance of the lens system. Since 
the changes of the focal distance of the lens system directly mean 
corresponding changes of the angle of view of the lens system, i.e., the 
magnification factor of the television image, with the above systems this 
magnification factor is always oscillating. This oscillation of the 
magnification factor of the television image is hardly recognizable in 
case when the reduction factor is low and the zoom factor is high, in 
which case the depth of field of the lens system is small so that the hill 
of the focus voltage is sharp and the amplitude of the oscillation is 
small. However, in case when the reduction factor of the lens system is 
high and the zoom factor is low, the depth of field is large so that the 
hill of the focus voltage is very broad, the amplitude of the oscillation 
of the helicoid is so large as to cause changes of the magnification 
factor of the television image by several percent or more in 
correspondence to the oscillation of the helicoid position. In this case, 
very unstable picture reproduction results. Particularly, when the depth 
of the field is very large so that focus on the televised sceen is 
obtained with the helicoid in whatever position, the shape of the focus 
voltage hill is very broad to cause very large oscillations of the 
helicoid, so that the afore-mentioned drawback is extremely pronounced. 
The second drawback is that the above systems consume high power, although 
this drawback is not so critical as the first-mentioned drawback. In the 
above systems the helicoid is always caused to rock back and forth about 
the position of focus, and a motor for driving the helicoid is always 
consuming power. This is a serious drawback in case of a video camera 
having problems in the power supply such as the portable video camera. 
SUMMARY OF THE INVENTION 
The primary object of the invention is to provide an automatic focusing 
system for a video camera, which eliminates the oscillation of the 
magnification factor of the television image accompanying the focusing 
operation, and also with which the power consumption accompanying the 
focusing operation is reduced. 
According to the invention, the prevailing depth of field of the lens is 
calculated from the reduction factor and focal distance (zoom factor) of 
the lens, the number of focusing points that can cover a focal distance 
range from the closest proximity distance to the camera to an infinite 
distance and stop positions of the helicoid (hereinafter referred to as 
preset focusing positions) corresponding to the respective focusing points 
are calculated according to the calculated depth of field, a closed loop 
control system based upon the focus voltage is used only for finding out 
one of the preset focusing positions closest to a goal focusing position 
of the helicoid, and once an in-focus state is obtained the helicoid is 
held at the preset focusing position that is closest to the apex of a hill 
of the focus voltage curve.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, designated at 1 is a lens system of a television 
camera, and at 2 a television camera circuit including a video signal 
generator for converting a two-dimensional optical image formed by the 
lens system into a corresponding serial electric signal. The video signal 
is produced from the camera circuit and appears at an output terminal 3. 
It is also coupled through a high-pass filter 4 to obtain a high frequency 
component corresponding to the fineness of the image, and this high 
frequency component is detected in a detector 5 to obtain a focus voltage 
proportional to the magnitude of the high frequency component signal. The 
focus voltage is dependent upon the fineness of the television image, and 
it is maximum when the television image is in focus. As shown in FIG. 2, 
in case when the televised scene is at a distance A (in m) from the 
camera, the focus voltage is maximum when the helicoid is at a position of 
a graduation corresponding to A (in m) and is reduced as the helicoid 
departs from that position. 
It will be understood from FIG. 2 that automatic focusing can be obtained 
through the control of the helicoid position such as to maintain the 
maximum focus voltage. 
A sample and hold circuit 6 samples the focus voltage appearing from the 
detector 5 at a constant time interval and holds each sampled value, and 
also it produces a positive voltage when the focus voltage is increasing 
with the lapse of time and produces a negative voltage when the focus 
voltage is decreasing with the lapse of time. The high-pass filter 4, 
detector 5 and sample and hold circuit 6 constitute a focus voltage 
circuit 9. The output voltage of the sample and hold circuit 6 is coupled 
through a switching circuit 31 to a motor drive circuit 7. The motor drive 
circuit 7 functions such that when the input voltage to it is zero or 
positive, it continually causes the rotation of a motor 8 in the same 
direction as before for moving the helicoid in the corresponding 
direction, while when the input voltage becomes negative it reverses the 
direction of rotation of the motor 8 for moving the helicoid in the 
opposite direction. 
While the sample hold circuit output voltage shown in FIG. 2 corresponds to 
the case of rotating the helicoid away from the position thereof 
corresponding to a focus point in the close proximity of the camera to a 
position corresponding to an infinitely remote focus point, it will be 
readily understood that the output voltage is as shown by a dashed curve 
in case when the helicoid is rotated from the infinitely remote focus 
point toward the close proximity focus point. 
With the above arrangement, automatic focusing can be obtained as the 
helicoid position control circuit causes the helicoid to be driven by the 
motor 8 in a direction of climbing the hill of the focus voltage curve and 
eventually oscillate with a small amplitude about a position corresponding 
to the apex of the hill according to the output voltage of the sample and 
hold circuit 6. With this circuit construction alone, however, the 
afore-mentioned drawbacks are not eliminated. 
The control circuit 10 calculates from signals representing the helicoid 
position, focal distance and reduction factor of the lens system 1 the 
number of preset focusing positions of the helicoid that cover a focus 
range from a focus point in the close proximity of the camera to an 
infinity remote focus point and the corresponding distance values of these 
preset focusing positions according to the prevailing value of the depth 
of field of the lens system, and memorizes the calculated data. It also 
detects from the output voltage of the sample hold circuit 6 the goal 
focusing position of the helicoid, reads out a preset focusing position 
closest to the goal focusing position, and switches the input terminal of 
the switching circuit 31 on the side of the sample and hold circuit 6 to 
that on the side of its own for producing a voltage corresponding to the 
difference between the actual helicoid position and the read-out focusing 
position. The helicoid in the lens system 1 is controlled through the 
motor drive circuit 7 and motor 8 such that the output voltage of the 
control circuit 10 is reduced to zero, and it is held at the read-out 
preset focusing position. The control circuit 10 has a further function of 
switching the input terminal of the switching circuit 31 on the side of 
the sample and hold circuit 6 to that on the side of its own when it 
detects from the focus voltage that the distance between the televized 
scene and camera is changed beyond the focus range or when the number of 
preset focusing positions is changed with a change of the focal distance 
or a change of the reduction factor due to a zoom action. 
The afore-mentioned focusing positions will now be discussed. They are set 
such as to cover a focus range from a focus point in the close proximity 
to the camera to an infinitely remote focus point at the prevailing value 
of the depth of field of the lens system. The value of the i-th focusing 
position from that corresponding to the infinitely remote focus point is 
given as 
##EQU1## 
where i is a positive integer, .rho. is the diameter of the permissible 
circle of confusion of the television image, f is the focal distance of 
the lens system 1, and F is the reduction factor of the lens system 1. 
The inverse of the value R.sub.i of the i-th preset focusing position is, 
from equation (1), 
##EQU2## 
and it will thus be seen that the preset focusing positions can be set at 
a uniform interval. The inverse of the value of the focusing position 
corresponding to the limit focus point that is thought as the infinitely 
remote focus point is 
EQU (1)/(R.sub.1)=(.rho.F)/(f.sup.2). 
This means that when the focus is set to a point at the distance of 
1/R.sub.1 (in m.sup.-1), the image of a scene at an infinite distance, 
i.e., at a distance ranging from 0 to 2.rho.F/f.sup.2 (in m.sup.-1) is in 
focus. Thus, it is possible to have an image of a scene at a distance 
within a range from the infinite distance to the closest proximity 
distance in focus by setting the number N of the focusing positions to be 
##EQU3## 
where R.sub.min (in m) is the closest proximity distance of the scene 
capable of televization. In other words, the number N of the preset 
focusing positions may be 
EQU N.gtoreq.(1/2).multidot.(f.sup.2 /.rho.F).multidot.(1/R.sub.min)(2) 
For example, when a lens system with a focal distance of f=50 mm, a 
reduction factor of F=1.8 and a closest proximity distance of R.sub.min =1 
m is used as the lens system 1 and the diameter of the permissible circle 
of confusion is .rho.=40 .mu.m, the number N of the preset focusing 
positions is 18 as calculated from equation (2), and the values R.sub.i of 
the preset focusing positions are R.sub.1 =34.72 m, R.sub.2 =11.57 m, 
R.sub.3 =6.94 m, . . . , R.sub.18 =0.99 m. In this case, with whatever 
distance of the scene a television image in a predetermined focus can be 
obtained by selecting the best one of the preset focusing positions 
R.sub.1 to R.sub.18, to which the helicoid is controlled. 
FIG. 3 shows a construction, which is used for finding out the best preset 
focusing position in the afore-mentioned prior art closed loop system 
based upon the focus voltage. The operation of this construction will now 
be described. The reduction factor F and focal distance f of the lens 
system 1 are digitally converted by respective analog-to-digital (A/D) 
converters 11 and 12 into respective digital signals, which are coupled to 
a preset focusing position calculating circuit 15. In the circuit 15 the 
number N of the preset focusing positions and the values R.sub.1 to 
R.sub.N thereof are calculated on the basis of equations (1) and (2). The 
data about the number N of the preset focusing positions thus calculated 
is coupled to and memorized in a preset focusing position number memory 
20, while the data about the calculated values of the individual preset 
focusing positions are coupled to and memorized in a preset focusing 
position value memory 16. 
The operation of the construction of FIG. 3 consists of three stages, 
namely a first stage of determining a preset focusing position, at which 
the helicoid of the lens system 1 is to be held, a second stage of 
bringing the helicoid to and holding it at the preset focusing position 
determined in the first stage, and a third stage of checking if it is 
necessary to make the focusing operation afresh and start the focusing 
operation again if it is necessary to do so. 
The first stage will now be described. 
This stage consists of two sub-stages. In the first sub-stage, the number 
of preset focusing positions and the values thereof are calculated from 
the focal distance f and reduction factor F of the lens system 1 using 
equations (1) and (2), and the resultant data are memorized. 
In the second sub-stage, the goal focusing position of the helicoid is 
determined by substantially the same closed loop system based upon the 
focus voltage as the prior art system, and then a preset focusing position 
closest to the goal focusing position is found out. 
In the first sub-stage, the focal distance f and reduction factor F of the 
lens system 1 are converted through the respective A/D converters 11 and 
12 to digital values, which are coupled to the preset focusing position 
calculating circuit 15. In the circuit 15, the number N of the preset 
focusing positions and the values R.sub.1 to R.sub.N thereof are 
calculated using equations (1) and (2), and the resultant data are 
memorized in the preset focusing position number memory 20 and preset 
focusing position value memory 16 respectively. 
In the second sub-stage, substantially the same focusing operation as in 
the prior art closed loop control system is made when a switching circuit 
31 is in its state of connection to A side, but after the completion of 
two or three rotations of the motor 8 the switch circuit 31 is switched to 
B side to open the closed loop control system in order that the helicoid 
will not be continually oscillated after it is brought to the focusing 
position. 
This operation will now be explained with reference to FIG. 4. In the 
Figure, a hill of the focus voltage is shown in (a). Designated at h.sub.1 
is the position of start of movement of the helicoid, and at h.sub.2 a new 
focusing position thereof. This situation results, for instance, when a 
televized scene, which has initially been at a position h.sub.1, is moved 
to position h.sub.2. In case when the helicoid starts to be moved by the 
motor 8 in the climbing direction as shown in FIG. (b) in FIG. 4, at the 
time of the climbing focusing operation, it reaches the focusing position 
h.sub.2 once and is then reversed at a position h.sub.3 with the inversion 
of the output signal from the output terminal 61 of the sample and hold 
circuit 6. Then, it passes the focusing position h.sub.2 again and is then 
reversed again at a position h.sub.4. If the climbing focusing loop is 
continually held in the closed state, the helicoid infinitely continues to 
oscillate between the positions h.sub.3 and h.sub.4 as in the prior art. 
The fact that the helicoid has reached the focusing position can be known 
by detecting the second reversal of the helicoid. Thus, in the 
construction of FIG. 3 only the instant of inversion of the output signal 
at the terminal 61 from positive to negative is detected by a counter 26. 
At the time of the first reversal of the helicoid, the position h.sub.3 
thereof is memorized in a first helicoid position memory 27. At the time 
of the second reversal of the helicoid, the position h.sub.4 is memorized 
in a second helicoid position memory 28, while at the same time the 
switching circuit 31 is switched to B side to open the climbing focusing 
loop. 
In case when the helicoid starts to be moved by the motor 8 in the 
direction of ascending the hill as shown in (c) in FIG. 4, it has to be 
reversed once so that its orbit overlaps that shown in (b) in FIG. 4 
before it reaches the focusing position, after starting from its position 
h.sub.1 it has to be reversed at a position h.sub.5 to proceed toward the 
position h.sub.2. At the time of the reversal at the position h.sub.1 the 
output signal form the terminal 61 is inverted from negative to positive, 
so that the reversal at this time is not detected by the counter 26. 
Subsequently, the counter 26 detects the second reversal at the position 
h.sub.3 and the third reversal at the position h.sub.4 similar to the case 
shown in (b) in FIG. 4. At the time of the start of driving of the motor 8 
a reset circuit 32 resets the counter 26 according to a signal from the 
motor drive circuit 7, while at the same time stopping the counting 
operation of the counter to prevent malfunction in the initial stage of 
the driving. 
When the content of the counter 26 reaches "2" so that the climbing 
focusing loop is opened, the comparing operation of the comparator circuit 
17 is started, and the content of the memory 16 is successively coupled to 
the comparator circuit 17 and compared the output data of an average value 
circuit 29, which takes the average of the contents of the first and 
second helicoid position memories 27 and 28, whereby a preset focusing 
position closest to the average value is memorized in a stop position 
memory 18. 
A comparator circuit 19 compares the preset focusing position data 
memorized in the memory 18 and a digital value obtained from an A/D 
converter, which converts a signal representing the prevailing position of 
the helicoid in the lens system 1 into the digital signal, and produces a 
positive or negative signal depending upon the difference between its two 
inputs. This output signal is coupled through the B side of the switching 
circuit 31 and motor drive circuit 7 to the motor 8, so that the helicoid 
in the lens system 1 is moved toward the preset focusing position 
memorized in the memory 18. When the output signal of the circuit 19 is 
reduced to zero, that is, when the detected helicoid position comes into 
coincidence with the preset focusing position memorized in the memory 18, 
the comparator circuit 19 supplies a stop signal S to the motor drive 
circuit 7 to stop the motor, while at the same time the prevailing focus 
voltage at the terminal 51 is converted by an A/D converter 13 into a 
digital value which is memorized in a focus voltage memory 22. 
In the third stage of the operation, whether or not it is necessary to make 
the focusing operation afresh is checked, and the focusing operation is 
caused again when the distance between the televized scene and camera is 
changed or when the reduction factor F or focal distance f of the lens 
system 1 is changed. When only the reduction factor F or focal distance f 
of the lens system 1 is changed, the preset focusing position calculating 
circuit 15 calculates the number of preset focusing positions afresh, and 
the calculated data are coupled to a comparator circuit 21. The comparator 
circuit 21 compares the new preset focusing position number data with the 
content of the memory 20. If the two input data are identical, no focusing 
operation is caused afresh. If the two are different from each other, the 
comparator circuit 21 delivers a signal which is coupled through an OR 
circuit 24 to the switching circuit 31 to switch the circuit 31 to the A 
side for causing the focusing operation afresh. Also, after a 
predetermined delay time the preset focusing position number data from the 
circuit 15 is memorized in the memory 20, and the focusing operation is 
started from the first stage. 
When the distance between the televized scene and camera is changed, the 
focus voltage provided from the A/D converter 13 and the output voltage of 
the memory 22, these voltages being coupled to the comparator circuit 23, 
become different from each other. Thus, the comparator circuit 23 produces 
a signal coupled through the OR circuit 24 to the switching circuit 31 to 
switch the circuit 31 to the A side for causing the focusing operation to 
be effected afresh from the first stage. 
In either of the above cases, once the driving of the motor 8 is started 
the helicoid is moved from its initial position to the new focusing 
position through the same course as described earlier in connection with 
FIG. 4. 
Further details of the circuit shown in FIG. 3 are not described since this 
circuit can be readily realized by using digital circuit techniques and 
can also be realized by a microcomputor program. 
When the closest proximity or infinity graduation position is reached by 
the helicoid, the motor 9 is reversed without any condition for continuing 
the focusing operation. 
As has been described in detail in the foregoing in connection with FIGS. 3 
and 4, with the automatic focusing system according to the invention once 
the in-focus state is obtained the helicoid is completely stopped and held 
stationary until it becomes necessary to make the focusing operation again 
due to a change of the reduction factor or focal distance of the lens 
system or movement of the televized scene or camera, so that it is 
possible to obtain excellent picture reproduction free from the 
oscillations of the angle of view that may be caused with oscillations of 
the helicoid controlled by the prior art closed loop control system. Also, 
the motor need not always be rotated, so that the power consumption can be 
widely reduced. 
While it has been described that the focusing operation is effected again 
immediately when the memorized focus voltage and prevailing focus voltage 
become different from each other in case when the distance between the 
televized scene and camera is changed after the previous focusing 
operation, experiments have proved that it is preferable to cause the 
focusing operation afresh after a difference of 10 to 20% is produced 
between the two voltages. 
Further, while in the above embodiment the number of preset focusing 
positions is made to be the least number required for covering the whole 
depth of field and also the preset focusing positions are set at a uniform 
interval, according to the invention it is possible to provide any number 
of preset focusing positions greater than the least number, and these 
preset focusing positions may be arranged in any desired spacing so long 
as the depth of field covered by each preset focusing positions overlap 
that covered by the adjacent one. 
Further, in case when the preset focusing position number calculated by the 
preset focusing position calculating circuit 15 shown in FIG. 3 is "1", 
i.e., when the focal distance of the lens system 1 is small or when the 
televized scene is very bright, the reduction factor F may be very large. 
In such a case, the focus voltage characteristic shown in FIG. 2 is very 
broad, and with the prior art closed loop control system the amplitude of 
the oscillation of the helicoid is very large, so that the afore-mentioned 
drawbacks due to the oscillation are extremely pronounced. According to 
the invention, the helicoid is fixed to the single focusing position in 
such a case, so that satisfactory results are obtainable.