Automatic focus control system for video camera with improved position detecting apparatus

An automatic focus control system for a video camera includes a first circuit which provides a detection signal having higher frequency components of a video signal provided by the video camera. The level of the detection signal varies according to a relative distance between the lens system of the video camera and the light receiving plane of the image pickup device of the video camera. The focus control system also includes a second circuit which detects an extreme value of the amplitude of the detection signal, and fixes the relative distance between the lens system and the light receiving plane of the image pickup device by stopping the actuation of the focusing mechanism of the video camera.

BACKGROUND OF THE INVENTION 
This invention relates to a focus control apparatus for automatically 
controlling the focus adjustment of an optical system in a television 
camera, etc. 
Where an image is picked up through a television camera, etc., the focal 
depth or depth of field must be properly controlled at all times to obtain 
a clear-cut image. When in particular the image of an object in a 
relatively dark, near distance is picked up, since the depth of focus is 
shallow, the focus control must be made each time the distance between the 
object and the camera greatly varies. Unless, in this case, focus control 
is properly made according to the variation of the distance between the 
object and the camera, the resolution of a reproduced image is lowered, 
resulting in the image being out-of-focus. It is very difficult and 
cumbersome to effect focus control of the camera according to each 
variation of the object distance. A camera having manual focus control 
requires much skill on the part of the operator and involves a complicated 
arrangement for the enhancement of the focussing operation. 
SUMMARY OF THE INVENTION 
It is accordingly the object of this invention to provide a focus control 
apparatus which can automatically effect focus control according to a 
distance between an object to be picked up and a camera. 
This invention pays attention to the fact that higher frequency components 
are decreased in a video signal corresponding to a lower-resolution image 
not correctly in focus and that the higher frequency components are 
increased in the video signal as the resolution of an image in focus in 
enhanced. In the focus control apparatus according to this invention the 
higher frequency components of the video signal are extracted and focus 
control is automatically fixed to a position where the high frequency 
component takes a substantially extreme value. An automatic focus control 
operation can be initiated using, for example, an exclusive start switch 
for standby mode-automatic mode changeover. The start switch may be 
connected in interlock with a picture-recording start button of the 
camera. Although the extreme value of the high frequency components of the 
video signal is usually a maximal value, it is possible to utilize a 
minimal value. This is because, if a signal including the high frequency 
components is phase-inverted or polarity-reversed, the maximal value 
becomes a minimal value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before proceeding with the description of the embodiments of the invention, 
it will expressly be understood that like reference symbols are used to 
designate like portions throughout the drawings for simplicity of 
illustration and that the components designated by like reference symbols 
may easily be replaced with each other or one another with minor change 
thereof by a skilled person in the art. 
FIG. 1 shows a mechanism in a television camera body equipped with a focus 
control apparatus. A lens system 10 is fixed in place in the camera body. 
The light receiving surface, i.e. the target surface 14.sub.1, of an image 
sensor element 14, such as CCD, vidicon, is arranged on an optical axis 12 
of the lens system 10. A yoke assembly 16 is disposed on the outer 
peripheral surface of the element 14 and the assembly 16 is supported at 
its forward end by a slide support 18 and at its rear end by a slide 
support 20. A guide bar 22 is inserted through bores 18.sub.1 and 20.sub.1 
in the supports 18 and 20. Similarly, a guide bar 24 is inserted through 
bores 18.sub.2 and 20.sub.2 in the supports 18 and 20. The guide bars 22 
and 24 are fixed to the camera body parallel to the optical axis 12. An 
internal thread is provided on a screw feed portion 18.sub.3 of the 
support 18 and a feed screw or a worm gear 26 is inserted through the 
internal thread of the support 18. A shaft on which the worm gear 26 is 
formed is coupled to a shaft of a motor 32 through gears 28 and 30. The 
shaft of the motor 32 is coupled to a shaft of a potentiometer 36 through 
gears 30 and 34. 
As a motor 32 used herein, for example, a DC reversible motor is suitable. 
The image sensor element 14 is moved forward and backword, in the 
directions of arrows A, B and parallel to the optical axis 12, according 
to the rotation direction and rotation amount of the motor 32. This 
movement is such a relative movement that the center of the target surface 
14.sub.1 is aligned with the optical axis 12 and that a relative angle of 
the lens surface of the lens system 10 to the target surface 14.sub.1 is 
maintained in a predetermined relation. The component elements 18 to 32 
constitute a moving means for varying a relative distance of the lens 
system 10 to the light receiving surface 14.sub.1 such that an object is 
focused on the light receiving surface 14.sub.1 of the image sensor means 
14. The rotation direction and rotation amount of the motor 32 correspond 
to the rotation direction and rotation amount of the potentiometer 36. 
From the potentiometer 36 it is possible to obtain information, i.e. a 
position signal E10, relating to the relative position of the target 
surface 14.sub.1 to the lens system 10. In this embodiment, the full 
stroke of the target surface 14.sub.1 corresponds to, for example, 10 
rotations of the potentiometer 36. 
FIG. 2 shows an electric circuit system constituting a focus control 
apparatus together with the mechanism as shown in FIG. 1. The position 
signal E10 is applied to a negative input of a subtracter 38. A position 
designation signal E12 is outputted from an amplifier 40 and supplied to a 
positive input of the subtracter 38. An error signal E14 corresponding to 
a difference of the signals E12 and E10 are outputted from the subtracter 
38. The signal E14 is inputted to a current amplifier 42 through a 
resistor R10. The amplifier 42 has a current capacity great enough to 
drive the motor 32 and supplies a drive current IF corresponding to the 
signal E14 to the motor 32 through a resistor R12. As set out in 
connection with FIG. 1, the movement of the image sensor element 14 by the 
rotation of the motor 32 or the relative position of the target surface 
14.sub.1 to the lens system 10 is detected by the potentiometer 36. The 
motor 32 and potentiometer 36 are mechanically connected by the gears 30 
and 34. The component elements 36, 38, 42 and 32 form a servo loop to 
which a control target is given by the signal E12. 
The above-mentioned control target or the position designation signal E12 
is determined by a position signal E12s or E12a which is inputted to the 
amplifier 40. The signal E12s is derived from the slider of a variable 
resistor 46 through a first contact of a switch 44.sub.1. The signal E12a 
is derived from the slider of a variable resistor 48 through a second 
contact of the switch 44.sub.1. A positive voltage +V.sub.s and negative 
voltage -V.sub.s are supplied to the corresponding terminals of the 
variable resistors 46 and 48 and the corresponding terminals of the 
potentiometer 36. The switch 44.sub.1 is in interlock with the 
later-described switch 44.sub.2 and constitutes a changeover switch 44 of 
a two-circuit two-gang type. The switch 44 is used to effect changeover 
between the standby mode (the first contact) and the automatic mode (the 
second contact). When the automatic focus control operation starts, the 
switch 44 is switched from the first contact side to the second contact 
side. Optical information is focused on the target surface 14.sub.1 of the 
image sensor element 14 and converted to image information E20 and 
inputted to a preamplifier 50. A signal E22 amplified at the preamplifier 
50 is processed at a video signal processor 52 and converted to a video 
signal (composite video signal) E24. The signal E24 is sent to, for 
example, a monitor TV and VTR. The arrangement of the processor 52 is 
generally known and a detail of the processor is therefore omitted. 
The video signal E24 is inputted to a highpass filter (HPF) 54 and the 
frequency response characteristic of HPF 54 can be relatively freely 
determined. For example, it is possible to use an active filter of a 
cutoff frequency of 500 kHz to 1 MHz. HPF 54 delivers a first signal E26 
corresponding to the higher frequency components of the signal E24. The 
signal E26 is inputted into a detector 56. The detector 56 produces a 
second signal E28, as shown in FIG. 3(a), having an envelope corresponding 
to the amplitude of the signal E26. The time constant of detection of the 
detector 56 is set to an optimal value according to the trackability of a 
servo loop including the motor 32. In this embodiment, the time constant 
of detection is about 0.1 second. The signal E28 is inputted into a 
sample/hold circuit 58. The sample/hold circuit 58 samples and holds the 
level of the signal E28 at a predetermined ratio. The predetermined ratio 
is determined according to the cycle of an oscillation output E30 of an 
oscillator 60. The cycle has a relation to the accuracy of the focus 
control operation, but it is tentatively 0.1 second (corresponding to 10 
Hz) in this embodiment. If the cycle is too short, an unstable operation 
is involved due to the irregular rotation of the motor 32, etc. This is 
the reason why the cycle cannot be made excessively short. The output E30 
is inputted into a sampling pulse generator 62 where it is converted to a 
sampling pulse E32, as shown in FIG. 3(c), which is in synchronism with 
the output E30. The second signal E28 is supplied to the sample/hold 
circuit 58 where it is sampled and held in the generation timing of the 
pulse E32. By so doing, a detection signal E34 as shown in FIG. 3(b) is 
outputted from the sample/hold circuit 58. The resolution of the object 
whose image is focused on the target surface 14.sub.1 varies according to 
the variation of the relative distance of the target surface 14.sub.1 to 
the lens system 10. If the image is not correctly in focus, the image on 
the target surface 14.sub.1 is blurred. For this reason, the higher 
frequency components of a frequency spectrum of the image information E20 
are relatively low in their level, as compared with the level of their 
lower frequency components. When the image of the object is correctly in 
focus, the image focused on the target surface 14.sub.1 is made clear in 
its detailed contrast. The level of the higher frequency components of the 
image information E20 becomes greater with an increase in the accuracy of 
focusing. The greatness of the level is more prominently manifested, as 
the frequency components become nearer to the higher frequency side in a 
frequency range including the image signal. In consequence, the high 
frequency components of the video signal E24 also vary according to the 
extent of image focusing. The component elements 54 to 62 as shown in FIG. 
2 constitutes a first means 64 for producing the detection signal E34 
whose level varies according to a variation of the relative distance of 
the target surface 14.sub.1 (or the light receiving surface) to the lens 
system 10. The variation of the relative distance is indicated on the 
abscissa (the time base) as shown in FIG. 3. 
The detection signal E34 is inputted into a differentiation circuit 66. 
From the circuit 66 a third signal E36 is outputted according to the 
points of change in level of the signal E34 as shown in FIG. 3(d), the 
magnitude and direction of the change of the signal E36 corresponding to 
those of the signal E34. The circuit 66 constitutes a differentiation 
circuit which provides the third signal E36 representative of a rate of 
change in level of the detection signal E34. The signal E36 is inputted 
into a level sensor 68. The sensor 68 has a predetermined threshold level 
L.sub.TH as shown in FIG. 3(d) and, when the level of the signal E36 is 
less than a level L.sub.TH, a fourth signal E38 as shown in FIG. 3(e) is 
outputted. That is, the level sensor 68 constitutes an identification 
means for providing the fourth signal E38 when the third signal E36 
corresponds to the predetermined threshold level L.sub.TH. 
The fourth signal E38 is applied to the clock input terminal CK of J-K 
flip-flop 70. The inverted output terminal Q of the flip-flop 70 is 
connected to the J and K input terminals. The clear terminal CLR of the 
flip-flop 70 is connected to the positive voltage +V.sub.s, corresponding 
to a logic level "1", through a resistor R14. When the clear terminal CLR 
of the flip-flop 70 is grounded through a first contact of the switch 
44.sub.2, the flip-flop 70 is not clocked and the output terminal Q of the 
flip-flop 70 is at the logic level "0". When the switch 44.sub.2 is 
switched to the second contact side and the clear state is released, the 
flip-flop 70 can be clocked by the signal E38. Then, a fifth signal E40 of 
a logic level "1" as shown at time t18 et. seq. in FIG. 3(f) is outputted 
from the output terminal Q of the flip-flop 70. The signal E40 is applied 
to the base of an NPN transistor 72 through a resistor R16. The emitter of 
the transistor 72 is grounded and the collector of the transistor 72 is 
connected to the input circuit of the current amplifier 42. When the 
flip-flop 70 is clocked and the signal E40 is at a logic "1", the 
transistor 72 is turned ON. The input circuit of the amplifier 42 is 
grounded through a collector-to-emitter path of the transistor 72. In this 
case, the input level of the amplifier 42 becomes zero and, as indicated 
in time t18 et. seq. in FIG. 3, the drive current IF of the motor 32 
becomes zero. As a result, the rotation of the motor 32 is stopped. That 
is, the flip-flop 70 and transistor 72 constitute a stopping means for 
stopping the change of the relative movement of the image sensor element 
to the lens system by the movement means (18 to 32) based on the fourth 
signal E38. 
As shown in FIGS. 3(a) to 3(g), the point at which the transistor 72 is 
turned ON by the logic "1" of the fifth signal E40 and the drive current 
IF to the motor 32 becomes zero corresponds to a point at which the 
detection signal E34 becomes substantially maximal. This point is 
indicated by P2 on the second signal E28 in FIG. 3(a). A true maximal 
point on the signal E28 or E34 is indicated by P1 at time t14. When the 
cycle of the sampling pulse E32 is sufficiently short, it can be 
considered that P1.apprxeq.P2. That is, the resolution of the image at the 
point P2 is substantially equal to that of the image at the point P1. The 
point P2 can be regarded as a point at which the detection signal E34 
takes a substantially extreme value. The point P2 is a point at which the 
image is correctly in focus and thus a better resolution is obtained (A 
best point is indicated by P1 and an arrangement for finding that point 
will be set out later.) That is, the component elements 66 to 72 
constitute a second means 74 for fixing the relative distance of the 
target surface 14.sub.1 to the lens system 10 when the detection signal 
E34 takes a substantially extreme value. Such fixing is effected by making 
the drive current IF of the motor 32 zero in FIG. 2. 
The fifth signal E40 is inputted into a one-shot monostable multivibrator 
(MMV) 76. MMV 76 is triggered by a logic level variation ("0".fwdarw."1") 
of the signal E40. The output signal E42 of MMV 76 is inputted into the 
current amplifier 78. The current amplifier 78 amplifies the signal E42 
and delivers a reverse-drive current IR corresponding to the level of the 
signal E42. Before MMV 76 is triggered, the output potential E44 of the 
amplifier 78 becomes higher. A diode 80 is connected between the current 
amplifier 78 and the motor 32 and reverse-biased such that I.sub.R =0. If, 
on the other hand, the signal E40 becomes a logic "1", MMV 76 is triggered 
and the output potential E44 of the amplifier 78 is lowered by a 
predetermined time period TR which is determined according to the time 
constant of MMV 76. By so doing, the diode 80 is forward-biased and, as 
shown in FIG. 3(h), during the time period TR the reverse-drive current IR 
flows through the motor 32. The term TR and magnitude of the current IR 
may be experimentally determined so as to bring the relative distance of 
the image sensor element to the lens system at IF (current)=0 back to the 
relative distance corresponding to the point P1. The component elements 76 
to 80 constitute a third means 82 which, when the change of the relative 
distance mentioned is in the first direction (for example, in the 
direction of A in FIG. 1) and the detection signal E34 exceeds the extreme 
value by a predetermined amount, causes a variation (corresponding to Lx 
in FIG. 3(a)) of the relative distance corresponding to the predetermined 
amount to be imparted to the second direction (the direction B in FIG. 1). 
The focus control apparatus as shown in FIGS. 1 and 2 is operated as 
follows: 
When the changeover switch 44 is in the first contact side, the position 
designation signal E12 which is the control target of the servo loop 
varies according to the variable resistor 46 and the flip-flop 70 is 
cleared. In this case, the transistor 72 is rendered OFF and MMV 76 is in 
a ready state for triggering. By adjusting the variable resistor 46 the 
target surface 14.sub.1 is moved to a position nearest to the lens system 
10 and in this way the relative distance is made minimal. At this time, 
the signal E12s corresponds to the standby state and the focal distance of 
the lens system 10 is made maximal (in a state in which the object at 
infinity is brought to a focus). The signal E12a extracted from the 
variable resistor 48 is used to the automatic control and the variable 
resistor 48 is controlled such that the focal distance of the lens system 
10 is minimal (in a state that the nearest object is brought to a focus). 
That is, the component elements 40, 44, 46 and 48 constitute a fourth 
means 84, whereby before focus control is commenced the relative distance 
of the image sensor element to the lens system is set at a position 
corresponding to the terminal position of the second direction as 
indicated by the arrow B in FIG. 1 and, when the focus control is started 
by switching the switch 44 from the first contact side to the second 
contact side, the direction of a variation or change of the relative 
distance mentioned is switched over to the first direction as indicated by 
the arrow A in FIG. 1. Before time t10 in FIG. 3 it is shown that the 
changeover switch 44 is in the standby state i.e. on the first contact 
side. Here, consider the case where a distance between the object and the 
camera corresponds to a position intermediate between the maximal and 
minimal distances between which focal control is permitted. Consider also 
that at time t10 the switch 44 is switched over to the second contact side 
in interlock with the "shot" start button of a camera not shown. In this 
case, the target surface 14.sub.1 is moved in the direction as indicated 
by the arrow A and the image of the object which is focused on the target 
surface 14.sub.1 becomes gradually clearer. As mentioned earlier, 
therefore, the higher frequency components of the video signal E24 become 
greater in their level and, as shown in FIG. 3(a), the signal E28 becomes 
gradually greater. As shown in FIG. 3(b) the level of the detection signal 
E34 is increased in a step-like fashion. Such a step-like variation is in 
synchronism with the sampling pulse E32 as shown in FIG. 3(c). 
The level of the signal E34 undergoes a step-like variation, corresponding 
to the amplitude variation of the signal E28, in a timing in which the 
pulse E32 is generated. The step-like variation, if differentiated, offers 
the signal E36. At time t14, the signal E34 takes a maximal value and, at 
the following time, is decreased in its level. At time t16, the signal E36 
goes from a positive to a negative level. Since at this time the level of 
the signal E36 does not exceed the threshold level L.sub.TH, the signal 
E38 does not vary. When at time t18, a negative-level signal E36 exceeding 
the threshold level L.sub.TH is produced, the fourth signal E38 as shown 
in FIG. 3(e) is produced. Then, the flip-flop 70 is clocked, causing the 
signal E40 to become a logic "1" level as shown in FIG. 3(f). At this 
time, the J and K terminals of the flip-flop 70 become the logic "0" 
levels, holding the clocked state. 
At the rise of the signal E40 at time t18 the transistor 72 is turned ON 
and MMV 76 is triggered. When the transistor 72 is turned ON, the drive 
current IF for moving the target surface 14.sub.1 in the direction of the 
arrow A in FIG. 1 becomes zero. While MMV 76 is being triggered i.e. 
during the time period TR from t18 to t20 in FIG. 3(h), the reverse-drive 
current IR is supplied to the motor 32 in place of the current IF. By so 
doing, the target surface 14.sub.1 is returned, during the time period TR, 
in the direction of the arrow B by an amount corresponding to the current 
IR and the period TR, and stopped. The time constant of MMV 76 and 
magnitude of the current IR are experimentally decided so that the stopped 
position at this time is corresponding to the point P1 in FIG. 3. In this 
embodiment, the time constant of MMV 76 is determined to be about 0.2 
second. Although the parameters TR and IR are properly varied dependent 
upon the inertia of the mechanical system as shown in FIG. 1, the cycle of 
the pulse E32 and so on, it is not necessary to sequentially change the 
parameters TR and IR, once determined, in the same television camera. 
FIGS. 4A and 4B show the case where the potentiometer 36 is replaced by a 
photoelectric converter. In FIG. 4A, a disk 158 with a screen 160 is 
disposed on the same axis as that of the gear 34 and rotated together with 
the gear 34. The screen 160 crosses an optical path of the 
photointerrupter 162. The screen 160 includes a wing portion 160.sub.1 
which varies in a manner to describe, for example, an involute curve. When 
the screen 160 is rotated by the rotation of the gear 34, an amount of 
light shut off by the interrupter 162 with respect to the screen 160 
varies. 
FIG. 4B is a cross-sectional view as taken along line B--B in FIG. 4A. The 
photointerrupter 162 includes an LED as a light source and a 
phototransistor as a light sensor. A flow of a flux of light from LED to 
the phototransistor is partially shut off by the screen 160. The extent of 
the shutting off of the light varies according to the rotation position of 
the gear 34 i.e. the relative position of the screen 160 to the 
photointerrupter 162. In consequence, the collector current E10 (position 
signal) of the phototransistor varies according to a distance between the 
lens system 10 and the assembly 16. 
FIG. 5 shows another example in which the potentiometer 36 is replaced by 
the photointerrupter. In this example, a light-transmissive screen 160A 
for varying an average value of an amount of light transmitted is used in 
place of the involute screen 160. The light is passed at the white 
portions of the screen 160A, but not at the black portions of the screen 
160A. Light is difficult to transmit at the crowded portion 160A.sub.1 of 
the screen 160A (an average amount of light transmitted is small) and easy 
to transmit at the noncrowded portion 160A.sub.2 of the screen 160A (an 
average amount of light transmitted is great). The position signal E10 is 
outputted from the interrupter 162 according to the rotation position of 
the gear 34. 
This invention is not restricted in any way to the embodiment as disclosed 
in the specification and drawings. This invention can be varied in a 
variety of ways without departing from the spirit and scope of this 
invention. For example, HPF 54 may be replaced by a band pass filter 
having a band width of about 1 MHz to 4 MHz. If the inertia of the 
mechanical system of FIG. 1 and the nonuniformity of the transmission 
characteristic are small and the cycle of the sampling pulse E32 is 
sufficiently short, the third means 82 may be practically omitted when a 
stable operation is obtained. In this case, the point P2 in FIG. 3(a) 
practically corresponds to the point P1. Although in the embodiment the 
component elements 54 to 62 have been explained as the first means 64, the 
component elements 54 and 56 may be regarded as the first means. While in 
this embodiment the fixing of the relative distance of the target surface 
14.sub.1 to the lens system 10 utilizes the ON operation of the transistor 
72, any arrangement may be adopted so long as it permits the motor to be 
stopped. An analog gate such as an FET may be connected between the 
subtracter 38 and the amplifier 42. It is to be noted that the focus 
control apparatus of this invention can be used in connection with a focus 
control apparatus of Japanese Patent Application No. 55-58399 and/or an 
iris servo apparatus of Japanese Patent Application No. 55-56867, whose 
assignee is the same with that of this application.