Contrast detecting apparatus for controlling an automatic focusing operation of an imaging apparatus

A contrast detecting apparatus includes first extracting means for extracting a signal component of a first frequency band from a luminance signal obtained by imaging a subject, second extracting means for extracting a signal component of a second frequency band lower than the first frequency band from the luminance signal obtained by imaging the subject, and determining means for determining a state of contrast of the subject in accordance with extracted outputs of the first and second extracting means. Therefore, when the contrast detecting apparatus is used, it can be exactly and automatically determined whether the contrast of the subject exists or not in accordance with a luminance signal obtained from the subject. In addition, when the contrast detecting apparatus is applied to the automatic focusing apparatus, it is possible to control the drive of lens or information supplying means to users in accordance with whether the contrast of the subject exists or not, and as a result when the subject having no contrast is imaged, the lens is stopped to be driven so that function and operability of the automatic focusing apparatus can be improved.

FIELD OF THE INVENTION 
The present invention relates to a contrast detecting apparatus which 
detects a state of contrast of a subject in accordance with a video signal 
obtained by imaging the subject and, more particularly, to a contrast 
detecting apparatus for controlling automatic focusing operation of an 
imaging apparatus. 
DESCRIPTION OF THE BACKGROUND ART 
An imaging apparatus such as video camera comprises an automatic focusing 
apparatus for automatically focusing on a subject. Some of the above 
automatic focusing apparatuses focus on the subject by using a signal 
obtained from an imaging device. Such automatic focusing method using the 
video signal has many advantages, for example correct focusing is 
performed in spite of a depth of field or the like or its structure is 
very simple because a sensor especially for automatic focusing is not 
necessary. A hill-climbing servo mechanism which is an example of that 
automatic focusing method is described in "T.V. Camera Automatic Focal 
Point Adjustment by Hill-climbing Servo Mechanism", Vol. 17, No. 1 (Whole 
Number 86), pp.26, 1965, in NHK Technical Report. 
FIG. 8 is a schematic block diagram showing a conventional automatic 
focusing apparatus using the hill-climbing servo mechanism, which is 
comprised in a video camera. 
Referring to FIG. 8, a camera part 1 comprises a focus lens which is moved 
by operation of the automatic focusing apparatus, a zoom lens for changing 
an imaging angle and other series of lenses. The focus lens and the zoom 
lens move in the right and left directions in the FIG. 8 embodiment in 
accordance with rotary movement of a focus ring 16 provided at an outer 
periphery of the camera part I. The focus ring 16 is driven to rotate by a 
focus motor 28. An imaging circuit 2 is provided behind the camera part 1. 
The imaging circuit 2 comprises an imaging device. 
When the subject is imaged, the subject focuses into an image on the 
imaging device in the imaging circuit 1 by the series of lenses comprising 
the focus lens in the camera part 1. This image of the subject is 
converted to a video signal comprising a luminance signal having 
horizontal/vertical synchronizing signal by the imaging circuit 2 
comprising the imaging device. This luminance signal is input to a gate 
circuit 3 and a synchronization isolating circuit 5. The synchronization 
isolating circuit 5 isolates vertical and horizontal synchronizing signals 
from the luminance signals. The isolated horizontal/vertical synchronizing 
signals are input to a gate control circuit 6 for setting a relatively 
small prescribed region in the image screen as a luminance signal 
extracting region (sampling area) for focusing operation. 
The gate control circuit 6 comprises a fixed oscillator oscillating with a 
constant frequency and applies a gate switching signal GC1 which passes 
only the luminance signal obtained from the above prescribed region to the 
gate circuit 3 so that the prescribed region may be set as the sampling 
area in accordance with the vertical synchronizing signal, the horizontal 
synchronizing signal and an output of the fixed oscillator. 
Therefore, the luminance signal corresponding to the sampling area set by 
the gate control circuit 6 is only applied to an HPF (high-pass filter) 7 
provided behind the gate circuit 3. Therefore, a high frequency component 
is extracted from the luminance signal corresponding to the sampling area 
which passed through the gate circuit 3 by the HPF 7. Then, amplitude 
detection of the extracted high frequency component is performed by a 
detector 8 at next stage. The output from the detector 8, that is, the 
level of the high frequency component is integrated by an integration 
circuit every field and applied to an A/D converter 10. The A/D converter 
10 converts the output of the integration circuit 9 to a digital value. 
The digital value is applied to a maximum value memory 11, comparators 12 
and 14 and a memory 13 as a value indicating a focused state of the 
optical system against the subject (a focal point evaluating value). 
FIG. 9 is a view showing the relation between the thus obtained focal point 
evaluating value and a position of the focus lens. In FIG. 9, the abscissa 
shows a position of the focus ring 16 which indirectly indicates the 
position of the focus lens and the ordinate shows the focal point 
evaluating value. For example, in a case where the subject is at a 
distance of 2m from the camera part 1, the focal point evaluating value 
indicates the maximum value when the focus lens moved by the focus ring 16 
is focused on the subject at a distance of 2m from the camera part 1 as 
shown in FIG. 9. More specifically, the focal point evaluating value 
varies while forming a conical shape with the position of the focus lens 
focused on the subject as its center. 
The maximum value memory 11 stores the focal point evaluating value applied 
from the A/D converter 10 in the first place as the maximum evaluating 
value until a comparison signal S1 to be described later is applied from 
the comparator 12 and also inputs the maximum evaluating value to the 
comparator 12 in response to an input of the focal point evaluating value 
from the A/D converter 10. 
The comparator 12 compares the value applied from the maximum value memory 
11, that is, the previous maximum focal point evaluating value with the 
present focal point evaluating value applied from the A/D comparator 10. 
Then, the comparator 12 outputs a comparison signal S1 when the present 
focal point evaluating value is bigger than the value stored in the 
maximum value memory 11 (first mode) and outputs a comparison signal $2 
when the present focal point evaluating value is smaller than the value 
stored in the maximum memory 11 by a predetermined second threshold value 
or more (second mode). The comparison signal S1 is applied to the maximum 
value memory 11 and a motor position memory 17 and the comparison signal 
$2 is applied to the focus motor control circuit 15. 
As described above, the focal point evaluating value is the maximum when 
the focus lens is focused. Therefore, as the focus lens approaches the 
focused position from a nonfocused position, the focal point evaluating 
value is increased. Therefore, the first mode shows the state ,where the 
focus lens moves toward the focused position and the second mode shows the 
state where the focus lens passed through the focused position and goes 
away from the focused position. 
The maximum memory 11 restores the newest focal point evaluating value 
applied from the maximum focal point evaluating value A/D converter 10 in 
response to the output S1 of the comparator 12, that is, in the first mode 
in which the focus lens has not yet reached the focused position. 
Therefore, the maximum value of the focal point evaluating values up to 
the present is stored as the maximum value evaluating value in the maximum 
value memory 11. 
On the other hand, the focus motor control circuit 15 drives the focus 
motor 28 so as to move the focus lens backward or forward at the same time 
when the camera starts to image the subject. Then, the focus motor control 
circuit 15 reverses the rotating direction of the focus motor 28 in the 
second mode in which the focus lens passed through the focused position in 
response to the comparison signal S2 output from the comparators 12 and 
also in a third mode in which the moving direction of the focus lens is 
not appropriate (gone away from the focused position) in the initial stage 
in response to a control signal S3 from a comparator 14 to be described 
later. Thus, the focus lens changes its moving direction from a direction 
in which it approaches the imaging device to a direction in which it goes 
away from it or other way around, with the result that the focus lens 
starts to move toward the focused position again. 
Referring to FIG. 9, for example, in a case where the subject which is at a 
distance of 2m from the camera part 1 is imaged, when the focus lens 
starts to move from a position P which is at a distance of 10m from the 
camera part 1 and is focused on the subject in a direction (a) in which 
the focus lens approaches the focused position, the focal point evaluating 
value starts to monotonously increase until the focus lens reaches the 
focused position Q. Therefore, the comparison signal S1 is output from the 
comparator 12 and then the contents of the maximum value memory 11 and the 
motor position memory 17 are continuously renewed until the focus lens 
reaches the focused position. At this time, the focus motor is controlled 
by the focus motor control circuit 15 to successively move the focus ring 
16 so that the focus lens approaches the focused position. Thus, the focus 
lens reaches the focused position and passes through this. In this case, 
if a first threshold value is a reverse reference value with a width shown 
by A in FIG. 9, when the focus ring 16 reaches a position R1 shown in FIG. 
9 after the focus lens passed through the focused position, the comparison 
signal S2 indicating the second mode is output from the comparator 12. In 
response to this the focus motor 28 is controlled by the focus motor 
control circuit 15 and then moves the focus ring 16 in the reverse 
direction, whereby the focus lens starts to move toward the focused 
position again. 
Meanwhile, the motor position memory 17 is provided to store a focus lens 
position X which is the nearest to the focused position until the present 
time. The motor position memory 17 receives an output from a focus ring 
position detecting sensor 29 connected to the focus ring 16. The focus 
ring position detecting sensor 29 detects the position of the focus ring 
16 to indirectly detect the position of the focus lens and then outputs a 
focus lens position signal indicating the position of the focus lens. 
Then, the motor position memory 17 renews its contents, that is, the focus 
lens position X so that the present focus lens position signal from the 
focus lens position detecting circuit 29 is stored therein in response to 
the comparison signal S1 output from the comparator 12. More specifically, 
in the first mode the contents of the motor position memory 17 always 
corresponds to the present focus lens position. 
However, in the second mode, since the comparison signal S1 is not applied 
to the motor position memory 17, the motor position memory 17 continues to 
hold the contents stored when the first mode is changed to the second 
mode, that is, holds the focus ring position signal indicating the focus 
lens position (focused position) in which the focal point evaluating value 
is the maximum. More specifically, the focus lens position corresponding 
to the focal evaluating value which is stored in the maximum value memory 
11 as the maximum evaluating value is always stored in the motor position 
memory 17. 
Meanwhile, a comparator 18 compares the contents of the motor position 
memory 17 with the focus ring position signal from the focus ring position 
detecting sensor 29 and when they coincide with each other, it outputs a 
prescribed control signal CL to the focus motor control circuit 15. The 
focus motor control circuit 15 stops the focus motor 28 in response to 
this control signal CL. Accordingly, the movement of the focus ring 16, 
that is, the movement of focus lens is stopped. At this time, the contents 
of the motor position memory 17 in the second mode is the signal 
indicating the focus ring position in which the focal point evaluating 
value is the maximum. Therefore, in the second mode, the focus motor 28 
starts to rotate in the reverse direction in response to the output signal 
S2 from the comparator 12 and when the focus lens reaches the focused 
position again, the comparator 18 operates as described above and then the 
focus motor stops and the focus lens stops at the focused position. 
Thus, basic automatic focusing operation of this apparatus in which the 
prescribed region in the imaging screen is set as a focus detecting region 
is completed. As can be seen from the above, a focal point evaluating 
value forming part 30 for finding the focal point evaluating value in the 
focus detecting region comprises the HPF 7, the detector 8, the 
integration circuit 9 and the A/D converter 10, a focal point evaluating 
value change detecting part 31 for detecting a change of the focal point 
evaluating value in accordance with movement of the focus lens comprises 
the maximum value memory 11 and the comparator 12, and a focused position 
detecting part 32 for detecting that the focus lens reaches a wrong 
focused position after the focus motor 28 rotates in the reverse direction 
comprises the motor position memory 17 and the comparator 18. In addition, 
a cut-off frequency of the HPF 7 is generally selected from 200 kHz to 800 
kHz or more. 
Meanwhile, the focus motor control circuit 15 stops the focus motor 28 and 
outputs a lens stopping signal LS to the memory 13 at the same time in 
response to the control signal CL from the comparator 18. 
The memory 13 keeps the focal point evaluating value input from the A/D 
converter 10 in response to the lens stopping signal LS output when the 
automatic focusing operation is completed, that is, the focal point 
evaluating value when the subject is in focus until the next lens stopping 
signal is applied and then outputs it to the comparator 14 of the next 
stage. Thus, the present focal point evaluating value is applied from the 
A/D converter 10 to the comparator 14. After the automatic focusing 
operation is completed, the comparator 14 compares the output of the 
memory 13, that is, the focal point evaluating value when the subject is 
in focus with the present focal point evaluating value. Then, when the 
difference between the present focal point evaluating value and the 
contents of the memory 13 is a prescribed second threshold value or less, 
the comparator 14 outputs a subject change signal S4 indicating that the 
subject is changed to the focus motor control circuit 15. The focus motor 
control circuit 15 drives the focus motor 28 in either direction and 
starts the above series of focusing operation again in response to the 
subject change signal S4. As a result, the automatic focusing operation is 
performed following the change of the subject. Thus, a subject change 
detecting part 33 for detecting the change of the subject comprises the 
memory 13 and the comparator 14. 
At the same time, the subject change detecting part 33 has the function of 
determining whether the rotating direction of the focus motor 28 is 
appropriate or not just after the automatic focusing operation starts, and 
correcting it to the right direction. More specifically, the memory 13 
stores not only the focal point evaluating value when the automatic 
focusing operation is completed but also the focal point evaluating value 
applied from the A/D converter 10 when the automatic focusing operation 
starts. Then, the comparator 14 outputs the control signal S3 to the focus 
motor control circuit 15 when the focal point evaluating value applied 
from the A/D converter 10 just after the automatic focusing operation 
starts is less than the focal point evaluating value (referred to as an 
initial value hereinafter) stored in the memory 13 when the automatic 
focusing operation starts (when the present lens moving direction is not 
directed to the focused position). The focus motor control circuit 15 
reverses the focus motor 28 in response to the control signal S3 from the 
comparator 14. At the same time, the control signal S3 is input to the 
comparator 18 and inactivates comparison operation of the comparator 14. 
The comparator 14 continuously outputs the control signal S3 until the 
present focal point evaluating value becomes more than the initial value 
stored in the memory 13. As a result, the control signal CL is not output 
from the comparator 18 until the present focal point evaluating value 
becomes more than the focal point evaluating value when the automatic 
focusing operation starts and the focus motor 28 continuously moves in the 
direction opposite to the first driving direction. Therefore, even if the 
moving direction of the focus lens when the automatic focusing operation 
starts is not appropriate, the focus lens is surely moved in a direction 
in which the focal point evaluating value becomes the maximum. 
Referring to FIG. 9, for example, when the subject at a distance of 2m from 
the camera part 1 is imaged, if the focus lens starts to move from a 
position P which is at a distance of 10m from the camera part 1 and is 
focused on the subject in a direction in which it goes away from the 
focused position (shown by (b) in FIG. 9), the focal point evaluating 
value is decreased as the focus ring 16 moves. At this time, if a second 
threshold value is a reverse reference value with a width shown by B in 
FIG. 9, when the focus ring 16 reaches a position R2 in FIG. 9 after the 
automatic focusing operation starts, the control signal S3 is output from 
the comparator 14. Accordingly, the focus motor 28 starts to move the 
focus ring 16 in the reverse direction. Then, since the operation of the 
comparator 14 is not activated until the focus ring 16 passes through the 
position P in FIG. 9, the focus ring 16 returns the focus lens to the 
position when the automatic focusing operation starts and then 
continuously moves it toward the focused position. Thereafter, when the 
focus ring 16 moves slightly beyond a focused position Q, it returns to 
the focused position and then stops there by the above operation of the 
motor position memory 17 and the comparator 18 or the like. 
As described above, the focal point evaluating value forming part 30, the 
focal point evaluating value change detecting part 31, the focused 
position detecting part 32 and the subject change detecting part 33 
control the focus motor control circuit 15, whereby the automatic focusing 
operation utilizing hill-climbing servo mechanism is performed. 
In addition, not the whole screen but the prescribed region at the center 
of the screen is generally the object of focusing (this region is referred 
to as an on-focus detecting region) in order to focus into the image 
projected in the center of the screen. 
FIG. 10 is a view showing a structure of an automatic focusing apparatus 
shown in which a zoom motor 35 and a zoom switch 36 for changing an 
imaging angle are further provided in the automatic focusing apparatus 
shown in FIG. 8. 
The zoom lens is driven by the zoom motor 35 and moves in the camera part 1 
in parallel with an optical axis. Thus, a focal distance of the lens 
system in the camera part 1 varies, whereby the imaging angle varies. The 
zoom motor 35 moves the zoom lens in response to a key input from the 
external switch (zoom switch) 36 which is provided so that the user can 
vary the imaging angle in accordance with necessity. More specifically, 
when the zoom switch 36 is pushed, the zoom motor 35 operates to move the 
zoom lens in a direction in which the imaging angle is increased (wide 
angle side) or in a direction in which it is decreased (telephoto side). 
Therefore, while the zoom switch 36 is pushed, the image taken by the 
imaging circuit 2 is expanding or reducing. 
As described above, the conventional automatic focusing apparatus using the 
hill-climbing servo mechanism performs focusing operation by determining 
whether the subject is in focus or out of focus in accordance with an 
added value with an amplitude of a prescribed high frequency component 
comprised in a luminance signal for one field obtained by imaging the 
subject. 
However, a frequency band of the obtained luminance signal or its level 
varies with brightness or contrast of the subject. Therefore, in some 
cases there is no frequency band to be extracted in the obtained luminance 
signal or the level thereof is very low even if it exists according to the 
condition of the subject. Thus, in some cases the prescribed high 
frequency component can not be obtained or the level thereof is very low 
even if it is obtained according to the subject. In these cases, the focal 
point evaluating value can not be obtained or the value is not correct 
even if it is obtained. Therefore, it is not possible to focus on such 
subject in accordance with the prescribed high frequency component in view 
of its principle. 
Meanwhile, since the conventional automatic focusing apparatus using the 
hill-climbing servo mechanism has no function of determining a spectrum of 
a video signal of the subject at all, the above series of focusing 
operation is performed regardless of the difference in spectrum of the 
video signal of the subject. More specifically, the conventional automatic 
focusing apparatus performs focusing operation both on a subject in which 
the luminance signal comprising no prescribed high frequency component is 
obtained and on a subject in which the prescribed high frequency component 
is sufficiently obtained. Therefore, according to the conventional 
automatic focusing apparatus using the hill-climbing servo mechanism, when 
the subject in which the luminance signal comprising no prescribed high 
frequency component is obtained is imaged, the focus lens wastefully 
continues to operate while the focus lens position in which the focal 
point evaluating value is the maximum is not found, that is, while the 
image is out of focus. In addition, although the prescribed high frequency 
component for finding the focal point evaluating value is derived from the 
luminance signal obtained from a subject having clear contrast, it can 
hardly be derived from the luminance signal obtained from a subject having 
very small contrast such as a remote mountain or the sky, or a subject 
having no contrast such as a wall or a ceiling. Therefore, the above 
problems are generated when a subject having small contrast is imaged. 
As described above, since the conventional automatic focusing apparatus has 
no function of determining the spectrum of the image signal of the 
subject, the existence of the contrast of the subject can not be detected, 
so that it does not comprise means for showing the user that the subject 
imaged at that time can not be in focus from the viewpoint of its 
principle, that is, that the subject has no contrast. Therefore, even if 
the above phenomenon occurred, the user can not find its cause and means 
for changing the subject to that having clear contrast or the like so as 
to prevent the focus lens from wastefully being moved. As a result, when 
the subject having no contrast is imaged, the image is out of focus for a 
long time. 
In order to solve the above problems caused by unnecessary drive of the 
focus lens which can not bring into focus, the contrast detecting 
apparatus capable of detecting whether the contrast of the subject exists 
or not is necessary. 
SUMMARY OF THE INVENTION 
The present invention was made to solve the above problems and it is an 
object of the present invention to provide a contrast detecting apparatus 
which can be easily applied to an automatic focusing apparatus. 
Other objects and advantages of the present invention will become apparent 
from the detailed description Given hereinafter; it should be understood, 
however, that the detailed description and specific embodiment are given 
by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description. 
According to a contrast detecting apparatus of the present invention, the 
apparatus comprises first extracting means for extracting a signal 
component of a first frequency band from a luminance signal obtained by 
imaging the subject, second extracting means for extracting a signal 
component of a second frequency band lower than the first frequency band 
and determining means for determining the state of contrast of the subject 
in accordance with the output of the first extracting means and the output 
of the second extracting means. 
According to another aspect of the contrast detecting apparatus of the 
present invention, it is used for controlling automatic focusing operation 
of an imaging apparatus which forces a state of an image in a first region 
to be in "focus" by changing the state of an optical system in accordance 
with a luminance signal obtained from the prescribed first region. The 
contrast detecting apparatus comprises first setting means for setting the 
first region in an imaging screen, second setting means for setting a 
second region in the imaging screen, first region component extracting 
means for extracting a signal component corresponding to the first region 
set by the first setting means and having a predetermined band, from the 
luminance signal obtained by imaging the subject, second region component 
extracting means for extracting a signal component corresponding to the 
second region set by the second setting means and having a second band 
lower than the predetermined band, from the luminance signal obtained by 
imaging the subject, first detecting means for detecting whether the 
contrast of the subject exists in the first region or not in accordance 
with the output of the first region component extracting means and the 
output of the second region component extracting means, and means for 
activating automatic focusing operation of the imaging apparatus in 
response to the detected output indicating "contrast exists" from the 
first detecting means. 
According to a preferred embodiment of the present invention, the contrast 
detecting apparatus further comprises second detecting means for detecting 
whether the contrast of the subject exists or not in the second region 
which is set larger than the first region in response to the output 
indicating "no contrast exists" from the first detecting means, expanding 
means for expanding the first region by a prescribed area in response to 
the output indicating "contrast exists" from the second detecting means, 
expanded region component extracting means for extracting a signal 
component corresponding to the region expanded by the expanding means and 
having the prescribed band, from the luminance signal, third detecting 
means for detecting whether the contrast exists in a part corresponding to 
the expanded region of the subject or not in accordance with the output of 
the expanded region component extracting means and the output of the 
second region component extracting means, and means for prohibiting 
automatic focusing operation of the imaging apparatus in response to the 
output indicating "no contrast exists" from the third detecting means. 
According to another preferred embodiment of the present invention, the 
contrast detecting apparatus is used in the imaging apparatus having a 
structure capable of changing an imaging angle and further comprises 
imaging angle change detecting means for detecting that the imaging angle 
is changing or not and means for inactivating operation of the first and 
second detecting means in response to the output from the imaging angle 
change detecting means. 
According to a further preferred embodiment of the present invention, the 
contrast detecting apparatus is used in the imaging apparatus further 
comprising means for displaying whether the contrast of the subject exists 
or not to the outside in accordance with the detected output of the 
detecting means for detecting existence of the contrast. 
The contrast detecting apparatus in accordance with the present invention 
extracts two signal components having different frequency band widths from 
the luminance signal obtained by imaging the subject and determines the 
state of the contrast of the subject in accordance with the extracted 
signal components. Therefore, when the video signal obtained by imaging 
the subject by the imaging apparatus or the like is input to the contrast 
detecting apparatus, it can be detected whether the contrast of the imaged 
subject exists or not. 
In addition, according to another aspect, the contrast detecting apparatus 
of the present invention sets first and second regions which are focus 
detecting regions of the imaging apparatus in the imaging screen and makes 
the imaging apparatus perform the automatic focusing operation when the 
contrast exists in the first region. According to the preferred embodiment 
of the present invention, the contrast detecting apparatus sets the second 
region larger than the first region and resets the first region by 
expanding the first region when the contrast does not exist in the first 
region but exists in the second region. Therefore, the imaging apparatus 
confirms that the contrast exists in the first region and then starts the 
automatic focusing operation. Thus, when the subject having contrast in a 
region outside the first region is imaged, the first region is changed to 
the region in which contrast exists and the imaging apparatus detects a 
focused position using the above region as the focus detecting region. 
Furthermore, according to the preferred embodiment of the present 
invention, the contrast detecting apparatus prohibits the automatic 
focusing operation of the imaging apparatus when the signal component 
corresponding to the region expanded by the expanding means and having a 
prescribed band for focusing operation of the imaging apparatus is 
extracted from the luminance signal obtained by imaging the subject and 
the existence of the contrast in a part corresponding to the expanded 
region of the subject is detected by the second detecting means in 
accordance with the extracted signal component and the extracted output of 
the second region component extracting means and the third detecting means 
determines that "no contrast exists" in a case where there is not 
existence. Therefore, in a case where the contrast detecting apparatus in 
accordance with the present invention is applied to the imaging apparatus, 
when the subject having no contrast in the expanded region is imaged, the 
automatic focusing operation is automatically inactivated. As a result, 
even if contrast exists in some parts of the imaging screen but the first 
region has no contrast, the apparatus does not immediately determine that 
"no contrast exists" in the subject and then the automatic focusing 
operation is prevented from being inactivated. Furthermore, when there is 
no contrast in the imaging screen, the automatic focusing operation of the 
imaging apparatus is prohibited by the detected output of the third 
detecting means. 
Furthermore, according to another preferred embodiment of the present 
invention, the contrast detecting apparatus of the present invention is 
applied to the imaging apparatus having a structure capable of changing 
the imaging angle, in which a change of the imaging angle is detected and 
the first and second detecting means are inactivated. Therefore, in this 
case, while the imaging angle of the imaging apparatus is changing, the 
first and second detecting operation is inactivated, so that wrong 
contrast determination to the focus detecting region due to a change of 
the imaging screen caused by the change of the imaging angle is not made. 
Therefore, the imaging apparatus detects the focused position in 
accordance with a video signal obtained by imaging the subject using, for 
example the hill-climbing servo mechanism under a preferred condition, 
with the result that more accurate focusing operation can be implemented. 
In addition, according to a further preferred embodiment, the contrast 
detecting apparatus is applied to the imaging apparatus comprising means 
for displaying whether the contrast of the subject exists or not to the 
outside. In this case, the user can notice that the subject has no 
contrast. Therefore, the subject capable of automatically being in focus 
is selected by the user.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic block diagram showing an automatic focusing apparatus 
in accordance with an embodiment of the present invention, which is used 
in a video camera. The automatic focusing apparatus in accordance with the 
present invention is different from a conventional one in that it 
comprises a contrast detecting part 27 which determines whether contrast 
of a subject exists or not and an alarm displaying part 34 which alarms a 
user whether the contrast of the subject exists or not. 
Prior to a description of overall operation of the automatic focusing 
apparatus, a description will be given of a principle of how it is 
determined whether the contrast of the subject exists or not in reference 
to FIGS. 5 and 7. 
FIG. 5 are views describing operation of the automatic focusing apparatus 
shown in FIG. 1 and also a difference in spectrum of a luminance signal 
due to the existence of the contrast of the subject. FIG. 7 are views 
describing a difference between a luminance signal of the subject capable 
of being in focus by a hill-climbing servo mechanism when the subject is 
in focus and that when it is out of focus. 
FIG. 7(A) is a view showing an example of a subject capable of being in 
focus by the hill-climbing servo mechanism, FIGS. 7(B) and (C) are 
schematic views showing waveforms of luminance signals obtained from the 
subject shown in FIG. 7(A) when it is in focus and when it is out of 
focus, respectively and FIG. 7(D) is a view showing frequency 
characteristics of the luminance signals shown in FIGS. 7(B) and (C), for 
example when an imaging device has a size of 2/3 inch. 
For example, when the subject has vertical stripes of black and white as 
shown in FIG. 7(A), the luminance signal obtained from that shows a 
rectangular waveform in which a high level part and a low level part which 
correspond to white and black of the subject, respectively are alternately 
arranged when it is in focus as shown in FIG. 7(B). In this case, the 
contrast of the subject is clearly displayed. In practice a higher 
harmonic component with fine amplitude is superimposed on the high level 
part and the low level part around its average level. Therefore, the 
luminance signal when the subject is in focus comprises signal components 
ranging from a relatively low frequency region corresponding to the 
rectangular wave indicating the contrast of the subject to a signal 
component in a considerable high frequency region corresponding to the 
higher harmonic component superimposed on the rectangular wave. However, 
since the amplitude of the higher harmonic component is considerably small 
as compared with the amplitude of the rectangular wave, response of the 
signal component in the high frequency region of the luminance signal of 
the subject having clear contrast is small. 
Meanwhile, as shown in FIG. 7(C), in the luminance signal obtained from the 
subject when it is out of focus, the higher harmonic component 
superimposed on the level parts corresponding to white and black of the 
subject is decreased as compared with the luminance signal shown in FIG. 
7(B) and a difference in level between the white part and the black part 
of the subject is also decreased, so that a sine wave having amplitude 
smaller than that of the rectangular wave is formed. Therefore, the 
luminance signal when it is out of focus comprises the signal component in 
a relatively low frequency region showing the contrast of the subject with 
amplitude smaller than that when it is in focus and it hardly comprises a 
signal component in a high frequency region corresponding to the higher 
harmonic component. 
The difference in waveform of the luminance signals when it is in focus and 
when it is out of focus is represented by the difference of the frequency 
characteristics shown in FIG. 7(D). Referring to FIG. 7(D), the frequency 
characteristic of the luminance signal when the subject is in focus is 
represented by a curve a and that when it is out of focus is represented 
by a curve b. In FIG. 7, the abscissa shows a video signal frequency f 
(MHz) or a spatial frequency N (pair/mm) and the ordinate shows a response 
G (dB). These curves of frequency characteristics are found in accordance 
with the Bessel function J.sub.1 with a minimum scattering circle .delta. 
(which is decreased when the subject is in focus and increased when it is 
out of focus) of reflected light from the subject and the video signal 
frequency f (or the spatial frequency N) as variables. More specifically, 
the relation between the spatial frequency N and the response G is 
represented as follows, that is, G=J.sub.1 (.pi. N.sigma.)/.pi.N.sigma. 
using the Bessel function. 
As can be seen from the curves a and b, the frequency characteristic of the 
luminance signal of the subject having strong contrast when it is in focus 
and that when it is out of focus are considerably different in a higher 
frequency region but almost the same in a low frequency region of 500 kHz 
or less. More specifically, the luminance signal of the subject having 
strong contrast obtained when it is in focus or out of focus comprises the 
signal component of 500 kHz or less. Therefore, in General, a frequency 
band extracted from the luminance signal to find a focal point evaluating 
value is set in a relatively higher frequency region of 200 kHz to 800 kHz 
or more. Thus, the focal point evaluating value obtained from the 
luminance signal when the subject is in focus shown in FIG. 7(B) is higher 
than that obtained from the luminance signal when it is out of focus shown 
in FIG. 7(C). As a result, it is possible to focus on the subject having 
strong contrast by a conventional automatic focusing apparatus using a 
hill-climbing servo mechanism. 
Similar to the luminance signal corresponding to a black or white part of 
the image of FIG. 7(a) shown in FIGS. 7(B) and (C), the luminance signal 
obtained from the subject having small contrast or the subject having 
extremely fine patterns comprises a higher harmonic component with small 
amplitude superimposed around its average level. The frequency of the 
higher harmonic component is approximately several MHz. 
As can be seen from the above, the luminance signal obtained from the 
subject with small contrast or the subject having extremely fine patterns, 
that is, the subject in which its luminance signal does not comprise the 
high frequency region component to be extracted in order to obtain the 
focal point evaluating value, comprises a component of a band of several 
MHz when the subject is in focus or when it is out of focus and does not 
comprise a component of a band of 500 kHz or less. Meanwhile, the 
luminance signal obtained from the subject having the strong contrast 
comprises the component of a band of 500 kHz or less when the subject is 
in focus or when it is out of focus. Thus, signal components whose band 
widths largely differ can be easily separated, for example the component 
of the band of 500 kHz and the component of the band of several MHz can be 
easily separated. Then, by using the difference in spectrum between the 
luminance signal of the subject incapable of being in focus and the 
luminance signal of the subject capable of being in focus, it is possible 
to determine whether the subject belongs to the former or latter. 
According to this embodiment, in order to make this determination, a first 
LPF (low-pass filter) having a cutoff frequency of several MHz, that is, 
the first LPF which passes the component of the band of several MHz or 
less and a second LPF having a cut-off frequency of 100 kHz are used. 
Then, the luminance signals corresponding to regions having different 
areas in an imaging screen are input to the first and second LPF's. 
FIG. 5(A) is a view showing an example of a subject having no contrast over 
the whole imaging screen and FIGS. 5(B) and (C) are views showing output 
signal waveforms of the first and second LPF's, respectively to which the 
luminance signal of the subject shown in FIG. 5(A) is input. Similarly, 
FIG. 5(D) is a view showing an example of a subject having clear contrast 
all over the imaging screen and FIGS. 5(E) and (F) are views showing 
output signal waveforms of the first and second LPF's, respectively to 
which the luminance signal of the subject shown in FIG. 5(D) is input. 
Also, FIG. 5(G) is a view showing an example of a subject comprising a 
region having no contrast and a region having clear contrast over the 
imaging screen and FIGS. 5(H) and (I) are views showing output signal 
waveforms of the first and second LPF's, respectively to which the 
luminance signal of the subject shown in FIG. 5(G) is input. Only FIGS. 
5(A) to (F) are referred to in the following description. In addition, in 
the waveforms, the abscissa shows a horizontal scanning direction. 
For example, when a uniformly white subject shown in FIG. 5(A) is imaged, 
the luminance signal corresponding to a small region E1 in this subject 
comprises a higher harmonic component with small amplitude of several MHz 
around its average luminance level. Therefore, the higher harmonic 
component appears almost as it is in an output signal waveform of the 
first LPF receiving such luminance signal as an input. More specifically, 
as shown in FIG. 5(B), in the output signal waveform of the first LPF the 
higher harmonic component with small amplitude is superimposed on a signal 
of an average luminance level of the subject shown in FIG. 5(A). On the 
other hand, the second LPF receiving the luminance signal obtained from a 
large region E2 in the subject shown in FIG. 5(A) removes the component of 
the band of several MHz comprised in the input luminance signal in 
accordance with a cut-off frequency. Therefore, as shown in FIG. 5(C), the 
output signal of the second LPF is a signal with a constant level which is 
equal to an average level V1 of the output signal with the first LPF. 
Therefore, when subtraction is made on the output signals of the first and 
second LPF's in the region E1, the signal obtained from this subtraction 
comprises only the higher harmonic component with small amplitude 
comprised in the output signal of the first LPF. 
For example, when the subject having vertical stripes of black and white 
shown in FIG. 5(D) is imaged, in the waveform of the luminance signal 
obtained from the region E1 in the subject the higher harmonic component 
(several MHz) with small amplitude is superimposed on the average 
luminance level corresponding to black and white when it is in focus as 
described above. Therefore, the output signal waveform of the first LPF 
receiving such luminance signal is almost the same as the waveform of the 
input luminance signal (referring to FIG. 5(E)). On the other hand, since 
the cut-off frequency of the second LPF is several 100 kHz, the second LPF 
levels the input signal component of an order of several 100 kHz or more. 
In this case, the level of the luminance signal of the subject shown in 
FIG. 5(D) alternately repeats the white level and the black level with a 
frequency of several 100 kHz in accordance with a width of the vertical 
stripe. Therefore, the output signal waveform of the second LPF receiving 
the luminance signal obtained from the region E2 in the subject shown in 
FIG. 5(D) is a waveform in which the luminance level of the white part and 
the luminance level of the black level of the subject are leveled to some 
degree (referring to FIG. 5(F)). Thus, the level of the output signal of 
the second LPF reflects the vertical stripes of the subject and it is 
fluctuated around an average level V2 of the output signal of the first 
LPF, but which is small as compared with the that of the second LPF. 
Therefore, in the region E1, the signal level obtained by performing 
subtraction on the output signals of the first and second LPF's is higher 
than that when there is no contrast in the subject (referring to FIG. 
5(A)). 
As can be seen from the above, existence of the contrast of the subject can 
be detected by the level difference of the output signals of two LPF's 
having different cut-off frequencies. Therefore, according to this 
embodiment, it is determined that the present subject is capable of being 
in focus by the hill-climbing servo mechanism or not by determining 
whether the contrast of the subject exists or not in accordance with the 
above principle. 
Next, operation of the automatic focusing apparatus shown in FIG. 1 will be 
described in detail hereinafter on the basis of operation of the contrast 
detecting part 27. 
Referring to FIG. 1, the automatic focusing apparatus comprises all 
function parts comprised in a conventional automatic focusing apparatus 
shown in FIG. 8, the contrast detecting part 27 and the alarm displaying 
part 34. In this automatic focusing apparatus, a focal point evaluating 
value forming part 30, a focal point evaluating value change detecting 
part 31, a focused position detecting part 32 and a subject change 
detecting part 33 perform the same operation as in the conventional one, 
which control a focus motor control circuit 15, whereby automatic focusing 
by the hill-climbing servo mechanism is implemented. A first gate circuit 
3, a first A/D converter 10, a maximum value memory 11, a second 
comparator 12, a second memory 13, a third comparator 14 and a fourth 
comparator 18 in this automatic focusing apparatus correspond to the gate 
circuit 3, the A/D converter 10, the maximum value memory 11, the 
comparator 
the memory 13, the comparator 14 and the comparator 18 in the conventional 
automatic focusing apparatus as shown in FIG. 8, respectively, and 
operation of all of the function parts in the conventional automatic 
focusing apparatus is the same as described in BACKGROUND OF THE 
INVENTION. 
However, in this automatic focusing apparatus, the luminance signal from an 
imaging circuit 2 is applied to not only the first gate circuit 3 which 
applies an input signal to the focal point evaluating value forming part 
30 and a synchronous isolation circuit 5 but also a second gate circuit 4. 
The second gate circuit 4 is provided to set a region larger than a focus 
detecting region set by the first gate circuit 3 in an image screen as a 
sampling area for detecting the contrast in the contrast detecting part 
27. Then, a gate control circuit 6 applies a gate switching signal GC 1 
passing only the luminance signal obtained from the sampling area E1 to 
the first gate circuit 3 and a gate switching signal GC 2 passing only the 
luminance signal obtained from the sampling area E2 to the second gate 
circuit 4 so that the regions E1 and E2 in FIGS. 5(A) to (I) may be set as 
a sampling area for detecting the focused position and a sampling area for 
detecting the contrast, respectively, in accordance with a vertical 
synchronous signal, a horizontal synchronous signal and a fixed oscillator 
output. Thus, the luminance signal corresponding to the sampling area E1 
is only applied to the first LPF 19 and the focal point evaluating value 
forming part 30 through the first gate circuit 3 and the luminance signal 
corresponding to the sampling area E2 is only applied to the second LPF 20 
through the second gate circuit 4. 
Next, operation of the contrast detecting part 27 and the alarm displaying 
part 34 will be described in detail hereinafter. 
The contrast detecting part 27 comprises the first gate circuit 3, the 
second gate circuit 4, a first LPF 19 passing a component of a band of 
several MHz or less from the luminance signal of the small sampling area 
E1 extracted by the first gate circuit 3, a second LPF 20 passing a 
component of a band of several 100 kHz or less which is lower than that of 
the first LPF 19 from the luminance signal of the large sampling area E2 
extracted by the second gate circuit 4, a first comparator 21 comparing 
the output signal level of the first LPF 19 with that of the second LPF 
20, a second A/D converter 22 converting the output of the first 
comparator 21 to digital data and a first memory 23 storing the digital 
data converted by the second A/D converter 22. 
The contrast detecting part 27 further comprises a judgment circuit 24 
which determines whether the contrast of the present subject exists or not 
in accordance with contents of the first memory 23 and the gate control 
signal GC1 from the gate control circuit 6 and then controls the gate 
control circuit 6, the focus motor control circuit 15 and the alarm 
displaying part 34 in accordance with the result of above determination. 
Operation of the contrast detecting part 27 will be described hereinafter 
in reference to FIGS. 2, 5, 6 and 7. FIG. 2 is a flow chart of operation 
of the judgment circuit 24. FIGS. 6(A) and (B) show output signal 
waveforms of the first LPF 19, the second LPF 2 and the first comparator 
21 when the subject having no contrast is imaged. FIGS. 6(C) and (D) show 
output signal waveforms of the first LPF 19, the second LPF 20 and the 
first comparator 21 when the subject having contrast is imaged under the 
unfocused state. 
For example, when the uniformly white subject shown in FIG. 5(A) is imaged, 
the luminance signals input to the first LPF 19 and the second LPF 20 
comprise a higher harmonic component with small amplitude around the 
average luminance level of the subject, whether it is in focus or not. 
Therefore, as described above, the first LPF 19 having a high cut-off 
frequency outputs a signal with a waveform shown by FIG. 5(B) and a solid 
line in FIG. 6(A). On the other hand, the second LPF 20 having a low 
cut-off frequency cuts the above higher harmonic component and outputs a 
signal with a constant level which is equal to the average luminance 
level, shown by FIG. 5(C) and a broken line in FIG. 6(A). 
Meanwhile, for example when the subject with stripes of black and white 
shown in FIG. 5(D) is imaged, the luminance signals input to the first LPF 
19 and the second LPF 20 when the subject is in focus show waveform shown 
in FIG. 7(B). More specifically, the luminance signal in this case 
comprises a fundamental wave component having a frequency corresponding to 
a luminance change (contrast) of the subject in which the difference 
between the average luminance level of the white part and that of the 
black part is its amplitude and a higher harmonic component with small 
amplitude around the average luminance levels of the white part and the 
black part. At this time, the first LPF 19 passes both the fundamental 
wave component and the higher harmonic component and outputs a signal 
having a waveform shown in FIG. 5(E). On the other hand, the second LPF 20 
cuts the higher harmonic component, passes the fundamental wave component 
and outputs a signal having a waveform shown in FIG. 5(F). 
Meanwhile, the luminance signals input to the first LPF 19 and the second 
LPF 20 when the subject is out of focus comprises the fundamental wave 
component but does not comprise the higher harmonic component comprised in 
the luminance signal when it is in focus, so that they show a waveform 
shown in FIG. 7(C). However, the amplitude of the fundamental wave 
component in this case is smaller than that when the subject is in focus. 
Therefore, the first LPF 19 passes this fundamental wave component and 
outputs a signal having a waveform shown by a solid line in FIG. 6(C). The 
second LPF 20 outputs a signal formed by leveling the fundamental wave 
component (shown by a broken line in FIG. 6(D)). 
As can be clear in the above example, when the subject having small 
contrast all over is imaged, the output signal of the first LPF 19 
comprises a higher harmonic component having small level fluctuation shown 
by a solid line in FIG. 6(A) and the higher harmonic component of the 
output signal of the first LPF 19 is leveled as shown by a broken line in 
FIG. 6(A) in the output signal waveform of the second LPF 20. Therefore, a 
difference in level between the output signals of the first LPF 19 and the 
second LPF 20 is small in any part in the sampling area E1. 
Meanwhile, when the subject having strong contrast all over is imaged, 
since the fundamental wave component of the luminance signal representing 
the contrast of the subject appears in the output signal of the first LPF 
19, the output signal of the first LPF 19 has large level fluctuation. On 
the other hand, the output signal of the second LPF 20 shows the average 
luminance level of the subject in the sampling area E2. Therefore, the 
level difference between the output signals of the first LPF 19 and the 
second LPF 20 is increased in a luminance change part in the sampling area 
E2. 
The output signals of the first LPF 19 and the second LPF 20 are input to 
the first comparator 21. The first comparator 21 performs subtraction 
between the output signals of the first LPF 19 and the second LPF 20 and 
outputs a voltage whose level is in proportion to the level difference of 
these signals. Therefore, when the subject having small contrast is 
imaged, the output signal of the first comparator 21 is small as shown in 
FIG. 6(B) because it reflects the small amplitude of the higher harmonic 
component comprised in the output of the first LPF 19. On the other hand, 
when the subject having large contrast is imaged, the output signal of the 
first comparator 21 is large as shown in FIG. 6(D) because it reflects the 
amplitude of the fundamental wave component, which represents the contrast 
of the subject and comprised in the output of the first LPF 19. 
The output signal level of the first comparator 21 is converted to a 
digital signal by the second A/D converter 22 and then once stored in the 
first memory 23. 
Thus, data indicating whether the contrast of the subject exists in the 
sampling area E1 and in the region outside thereof (a region included in 
the sampling area E2 but not included in the sampling area E1) or not is 
stored in the first memory 23. 
Operation of the judgment circuit 24 will be described hereinafter in 
reference to FIG. 2. The judgment circuit 24 reads the data stored in the 
first memory 23 (operation step D1). Then, the judgment circuit 24 
determines whether that data, that is, the output signal levels of the 
first comparator 21 in the sampling area E1 and its outside region exceed 
a predetermined judgment reference level V.sub.r or not. This operation of 
the judgment circuit 24 will be described in detail hereinafter. 
The judgment circuit 24 takes out the output signal level of the first 
comparator 21 either in the sampling area E1 or its outside region of the 
read data in accordance with the gate control signal GC1 from the gate 
control circuit 6. 
The gate control signal GC1 is a signal for setting the sampling area E1. 
Therefore, the data which is stored in the first memory while the gate 
control signal GC1 which turns the first gate circuit 3 ON is output from 
the gate control circuit 6, is at the output signal level of the first 
comparator 21 in the sampling area E1. On the contrary, the data which is 
stored in the first memory while the gate control signal GC1 which turns 
the first gate circuit 3 OFF is output from the gate control circuit 6, is 
at the output signal level of the first comparator 21 in the region 
outside the sampling area E1. 
Then, when the gate control signal GC1 turns the first gate circuit 3 ON, 
the judgment circuit 24 takes out data M1 in the sampling area E1 from the 
read data (operation step D2). Then, the judgment circuit 24 determines 
whether the output signal level of the first comparator 21 in the sampling 
area E1 is larger than the predetermined judgment reference level V.sub.r 
or not (operation step D4). When this result is "YES", the judgment 
circuit 24 determines that the present subject has contrast (operation 
step D6) and then applies an automatic focusing start instruction signal 
D.sub.s which activates the above function of automatic focusing operation 
to the focus motor control circuit 15. 
The focus motor control circuit 15 starts the above series of operation for 
automatic focusing operation in response to the start instruction signal 
D.sub.s. Therefore, the subject with contrast can be in focus by the 
normal hill-climbing servo mechanism. 
Meanwhile, in a case where the result at the operation step D4 in FIG. 2 is 
"NO", the judgment circuit 24 takes out data M1 in the region outside the 
sampling area E1 from the data read at the operation step D1 when the gate 
control signal GC1 turns the first gate circuit 3 OFF (operation step D3). 
Then, the judgment circuit 24 determines whether that taken out data, that 
is, the output signal level of the first comparator 21 in the region 
outside the sampling area E1 is larger than the predetermined judgment 
reference level V.sub.r ' or not (operation step D5). When the result is 
"NO", that is, when the results of determination of the data M1 in the 
sampling area E1 and the data M1 in the region outside the sampling area 
E1 are the same, the judgment circuit 24 determines that the present 
subject has no contrast (operation step D8) and outputs an automatic focus 
prohibiting signal D.sub.s to the focus motor control circuit 15 and the 
alarm displaying part 34 (operation step D9). 
In this case, the judgment reference level V.sub.r is set so as to be 
larger than the output signal level of the first comparator 21 when the 
subject having small contrast is imaged but smaller than that when the 
subject having strong contrast is imaged as shown in FIG. 6(B). 
Similarly, the judgment reference level V.sub.r ' can be set at such a 
value which can determine whether the contrast of the subject exists in 
the region outside the sampling area E1 or not by comparison with the 
output signal level of the first comparator 21 in the region outside the 
sampling area E1. However, since the first gate circuit 3 is OFF and the 
second gate circuit 4 is ON in the region outside the sampling area E1, 
the output signal can not be obtained from the first LPF 19. Therefore, 
the output signal of the first comparator 21 is the output signal of the 
second LPF 20, that is, the signal which indicates a change of the average 
luminance level in the large sampling area E2. Therefore, in this case, 
the Judgment reference level V.sub.r ' other than the predetermined 
judgment reference level V.sub.r has to be a reference value for 
determining whether the contrast of the subject exists in the region 
outside the sampling area E1 or not. In this case, the reference level 
V.sub.r ' is set so that it is determined that contrast exists when the 
output signal level of the first comparator 21 in the region outside the 
sampling area E1 exceeds the reference level V.sub.r ' and that the 
contrast does not exist when it is equal to the reference level V.sub.r ' 
or less. Therefore, the judgment circuit 24 outputs the automatic focus 
prohibiting signal D.sub.s when the contrast of the subject is small in 
the sampling area E1 and its outside region, that is, when the imaged 
subject has no contrast over a large range of the imaging screen. 
The focus motor control circuit 15 inactivates the focus motor 28 in 
response to the automatic focus prohibiting signal D.sub.s from the 
judgment circuit 24. 
The alarm displaying part 34 comprises an LED (light emitting diode) 26 and 
an LED driving circuit 25. The LED driving circuit 25 drives the LED 26 in 
response to the automatic focus prohibiting signal D.sub.s. Therefore, 
when the imaged subject has not contrast over a large range of the imaging 
screen, that is, when it is not possible to focus on the subject by the 
hill-climbing servo mechanism, the focus lens is not driven and then the 
LED 26 emits light. The LED 26 is attached on the outside of the apparatus 
and notifies the user that the subject incapable of being in focus is 
imaged. 
Thus, according to this embodiment of the present invention, when the 
subject incapable of being in focus in view of its principle is selected 
as the subject to be imaged, the focus lens is not automatically driven 
and also alarm is displayed. Then, when the user notices the alarm display 
and knows the subject is not appropriate, he can immediately change the 
subject. Therefore, when the subject incapable of being in focus is 
imaged, the focus lens is not wastefully driven and then the image is not 
out of focus for a long time as in the prior art. 
In addition, according to this embodiment of the present invention, when 
the result of the judgment circuit 24 in the sampling area E1 is different 
from that in the region outside the sampling area E1, that is, when the 
result at the operation step D5 is "YES", the judgment circuit 24 outputs 
a sampling region correcting signal G.sub.E to the gate control circuit 6 
(operation step D10). The gate control circuit 6 corrects the gate control 
signal GC1 to be applied to the first gate circuit 3 so as to expand the 
sampling area E1 by a predetermined area which is set by the first gate 
circuit 3 and change the focus detecting region E1 for automatic focusing 
to, for example a region E3 shown in FIG. 5(G) in response to the 
correcting signal G.sub.E. Thereafter, the judgment circuit 24 performs a 
series of operation of the operation steps D1 to D9 again. As a result, a 
signal which indicates either prohibition or start of the automatic 
focusing operation is applied to the focus motor control circuit 15. 
More specifically, when the second result at the operation step D4 is 
"YES", the judgment circuit 24 determines that the contrast exists and 
then outputs the signal G.sub.S which indicates a start of the automatic 
focusing operation. The fact that the second result at the operation step 
D4 is "YES" means that the present subject has no contrast in the sampling 
area E1 but has contrast in the sampling area E3. Therefore, in this case, 
focusing operation is performed in accordance with a focal point 
evaluating value in the focus detecting region E3 having contrast. 
Therefore, even if the target subject having contrast exists a little 
outside the sampling area E1, the subject is surely in focus. 
The subject determined by the judgment circuit 24 as described above is 
shown in FIG. 5(G). The subject shown in FIG. 5(G) is white and has no 
contrast in the central part but has stripes of black and white in the 
outside part. Therefore, if this subject is imaged when it is in focus, 
the output signal of the first LPF 19 in the sampling area E1 shows the 
same waveform as that when the subject has no contrast all over is imaged 
(referring to FIG. 5(B)) as shown in FIG. 5(H). However, the output signal 
of the second LPF 20 in the sampling area E2 is at a constant level 
showing the average luminance of the sampling area E1 in a part 
corresponding to the sampling area E1 and has the fundamental wave 
component of the black and white stripes, which is a contrast component of 
the subject in a part corresponding to the region outside the sampling 
area E1 as shown in FIG. 5(I). Of course, when the subject is out of 
focus, although this fundamental wave component is lower than that when it 
is in focus, it is also sufficiently left. Therefore, it is possible to 
determine whether the contrast exists at that part or not from the output 
signal level of the second LPF 20 in the region outside the sampling area 
E1. On the contrary, when the second result at the operation step D4 is 
"NO", the judgment circuit 24 outputs the signal G.sub.S which indicates 
prohibition of the automatic focusing operation. The fact that the second 
result at the operation step D4 is "NO" means that the subject in the 
increased focus detecting region has also no contrast. Therefore, in this 
case, the lens is prohibited from being driven so that the lens is not 
wastefully driven. 
FIG. 3 is a view showing an automatic focusing apparatus in accordance with 
another embodiment of the present invention, which comprises all function 
parts comprised in the conventional automatic focusing apparatus shown in 
FIG. 10, a contrast detecting part 27, an alarm displaying part 34 and a 
zoom switch detecting circuit 37. According to this embodiment, the zoom 
switch detecting circuit 37 detects that a zoom switch 36 is pushed, that 
is, detects that an imaging angle almost starts to change and then outputs 
a determination prohibiting signal D.sub.Z for prohibiting a series of 
contrast determination operation to be described later to the judgment 
circuit 24. 
Next, operation of the judgment circuit 24 will be described in reference 
to FIG. 4. The judgment circuit 24 determines whether the zoom switch 36 
is pushed or not from a fact whether the determination prohibiting signal 
D.sub.Z is output from the zoom detecting circuit 37 or not (operation 
step D1). When the determination prohibiting signal D.sub.Z is not output 
from the zoom switch detecting circuit 37, the judgment circuit 24 
determines that the imaging angle is fixed and then reads data stored in 
the first memory 23 first (operation step D2). Then, the judgment circuit 
24 determines whether the data, that is, the output signal levels of the 
first comparator 21 in the sampling area E1 and in the region outside the 
sampling area E1 exceed the predetermined Judgment reference level V.sub.r 
or not. Since operation steps D2 to D11 of the judgment circuit 24 
hereinafter are the same as the above operation steps D1 to D11, 
description thereof will be omitted. 
While the zoom switch detecting circuit 37 is pushed, the imaging angle is 
changing every moment, so that the image taken by the imaging circuit 2 is 
on the expansion or reduction. Therefore, since the states of the contrast 
in the sampling area E1 and the sampling area E2 of the focus detecting 
regions are also changing for that period, it is not appropriate to 
exactly determine whether the contrast of the subject exists or not at 
that time. Then, when the determination prohibiting signal D.sub.Z is 
output from the zoom switch detecting circuit 37, the judgment circuit 24 
determines that the imaging angle almost starts to change at the operation 
step D1 and then will not perform contrast determination operation after 
the operation step D2 and determines whether the zoom switch 36 is 
continuously pushed or not. Therefore, according to this embodiment, the 
contrast determination is not made until the determination prohibiting 
signal D.sub.Z stops to be output from the zoom switch detecting circuit 
37, that is, until the imaging angle is fixed, so that automatic focusing 
operation is not also performed until then. Then, when the imaging angle 
is fixed, the contrast determination operation of the judgment circuit 24 
automatically starts. 
Thus, according to this embodiment of the present invention, only when the 
imaging angle is fixed, the contrast determination is made, so that 
correct contrast determination can be made to the subject. 
According to the above embodiment of the present invention, although the 
focus detecting region is expanded only once, the focus detecting region 
can be expanded several times in accordance with necessity and then 
finally it can be determined whether the contrast of the subject exists or 
not. 
In addition, the above operation of the contrast detecting part 27 can be 
performed in a software manner using a microcomputer or the like. 
In addition, although the center of the sampling area is set at the center 
of the range of image in the above embodiment, it is needless to say that 
the same effect can be obtained even if the center of the sampling area is 
not set at the center thereof. 
Although the LED is used as alarming means for notifying the user that the 
subject having no contrast is imaged in the above embodiments, the user 
can be alarmed by characters or signs in a view finder attached to a video 
camera or the like or by sound. 
Although an alarm to the user and prohibition of the drive of the lens are 
generated at the same time when the subject having no contrast is imaged 
in this embodiment of the present invention, only either one of them can 
be generated when the subject having no contrast is imaged. 
In addition, although the present invention is used for controlling 
operation of the automatic focusing apparatus of the video camera in 
accordance with the above embodiment, the contrast detecting apparatus of 
the present invention can be used in another field such as an apparatus 
for detecting movement of the subject in the imaging screen. 
As described above, according to the present invention, it is exactly and 
automatically determined whether the contrast of the subject exists or not 
in accordance with a luminance signal obtained from the subject. 
Therefore, when the contrast detecting apparatus in accordance with the 
present invention is applied to the automatic focusing apparatus, it is 
possible to control the drive of the lens or information supplying means 
to users in accordance with fact whether the contrast of the subject 
exists or not. As a result, when the subject having no contrast is imaged, 
the lens is not unnecessarily driven so that function and operability of 
the automatic focusing apparatus can be improved. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.