Focus detection using an image signal extracted before digital signal processing

In an image pickup apparatus having an electronic zooming function for enlarging and interpolating an image by electronic image processing of an image pickup signal output from an image pickup device, a focus is detected by extracting a predetermined focal signal varying depending upon a focal state from the image pickup signal before the image pickup signal undergoes enlargement/interpolation processing by electronic zooming.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an image pickup apparatus having an electronic 
zooming function for electronically increasing the magnification of an 
image of an object. 
2. Description of the Related Art 
Recently, video apparatuses such as video cameras and integrated 
camera/video recorders have been remarkably improved and the development 
of this kind of video apparatus having various functions and improved 
operability has been promoted. For example, automatic focusing mechanisms 
are presently provided as standard equipment, and electronic zooming 
functions for electronically increasing the image magnification by 
processing an image signal are now being introduced to realize 
large-magnification zooming without using a large optical zooming lens. 
The automatic focusing method mainly used in conventional automatic 
focusing systems is a method in which a signal formed of high-frequency 
components, edge components or the like and representing the sharpness is 
extracted from an image pickup signal to determine a focal state. This 
method is characterized in that an object can be focused no matter what 
the distance to the object, and that there is no need to provide a special 
optical system and a sensor for detecting the focus. 
However, with respect to the construction of conventional image pickup 
apparatuses having an electronic zooming device and an automatic focusing 
system using an image pickup signal, a problem described below is 
encountered. 
That is, an electronic zooming apparatus conventionally used is arranged to 
electronically increase the magnification of an image of an object by 
intermittently reading out an image pickup signal from an image pickup 
device at a transfer rate determined by the magnification of the object 
image, by forming an interpolation signal from the read-out image pickup 
signal, and inserting the interpolation signal in the read-out image 
pickup signal. In an image pickup apparatus constructed by incorporating 
an automatic focusing system with this zooming apparatus, an image pickup 
signal processed to electronically increase the magnification by such 
interpolation/insertion is used as a signal for determining 
focused/non-focused states in an automatic focusing circuit. 
Accordingly, a signal which is interpolated but thinned-out in a horizontal 
direction is used to determine focused/non-focused states, and a 
particular frequency component or edge component extracted from such a 
signal is different from the corresponding frequency component or edge 
component extracted from the original image pickup signal. There is 
therefore a problem of occurrence of a deterioration in the automatic 
focusing accuracy or a malfunction. 
SUMMARY OF THE INVENTION 
In view of the above-described problem, an object of the present invention 
is to provide an image pickup apparatus which is capable of operating 
stably and reliably and which does not cause any deterioration in the 
accuracy of an automatic focusing system for detecting a focal state from 
an image pickup signal in a video camera having the image pickup 
apparatus, the automatic focusing system and an electronic zooming 
apparatus. 
Another object of the present invention is to provide an image pickup 
apparatus arranged to prevent influence upon the automatic focusing 
operation even when electronic zooming is performed. 
To achieve these objects, according to one aspect of the present invention, 
there is provided an image pickup apparatus comprising enlargement means 
for electronically increasing the magnification of an image of an object 
by intermittently reading out an image pickup signal from an image pickup 
device at a transfer rate according to the magnification of the object 
image, by forming an interpolation signal from the read-out image pickup 
signal and by inserting the interpolation signal in the image pickup 
signal, extraction means for extracting a particular signal component 
varying depending upon a focal state from the image pickup signal 
intermittently read out from the image pickup device but not yet 
interpolated, determination means for determining focused/non-focused 
states on the basis of the signal component extracted by the extraction 
means, and focusing control means for controlling focusing on the basis of 
a result of the determination of focused/non-focused states made by the 
determination means. 
The extraction means extracts, as the signal used for the focusing control 
effected by the determination means and the focusing control means, a 
particular frequency component or an edge component from the image pickup 
signal intermittently read out from the image pickup device but not yet 
interpolated, i.e., the original image pickup signal which has been 
intermittently read out in the vertical direction but which has not 
undergone the intermittent-reading/insertion/interpolation processing in 
the horizontal direction, when an electronic zooming control is performed. 
The determination means and the focusing control means are thereby enabled 
to perform focusing stably and accurately by preventing a deterioration in 
accuracy and a malfunction even during the electronic zooming control. 
A further object of the present invention is to provide an automatic 
focusing apparatus capable of operating accurately and stably irrespective 
of the field angle. 
These and other objects, features and advantages of the present invention 
will become apparent from the following detailed description of preferred 
embodiments of the present invention taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An image pickup apparatus in accordance with the present invention will be 
described below with respect to embodiments thereof with reference to the 
accompanying drawings. An electronic zooming function will first be 
described with reference to FIGS. 2 through 4. 
FIG. 2 is a block diagram schematically showing the construction of an 
image pickup apparatus having an electronic zooming function. The image 
pickup apparatus has a scanning timing pulse generation means 1 for 
controlling scanning timing of an image pickup device 2, the image pickup 
device 2, formed of a charge coupled device (CCD) or the like, a 
correlative double sampling circuit 3, an automatic gain control circuit 
4, an A/D converter 5 for converting an image pickup signal into a digital 
signal, a camera signal processing circuit 6, an image memory circuit 7, 
and a D/A converter 8. FIGS. 3(A), 3(B), 3(C) and 3(D) constitute a 
waveform diagram of scanning timing of the image pickup device 2, showing 
vertical transfer pulses FIG. 3(A), horizontal transfer pulses FIG. 3(B), 
horizontal scanning pulses FIG. 3(C), and an image pickup device output 
signal FIG. 3(D). Vertical transfer pulses FIG. 3(A), horizontal transfer 
pulses FIG. 3(B), horizontal scanning pulses FIG. 3(C) are generated by 
the scanning timing pulse generation means 1. FIGS. 4(a) and 4(b) show 
images of an object in picture frames when zooming is performed. FIG. 4(a) 
schematically shows an object image on a light receiving plane of the 
image pickup device 2. A region of the image to be enlarged is surrounded 
by a smaller frame, while the rest of the entire image area, i.e., an 
unnecessary portion of the image is represented by a hatched area. FIG. 
4(b) shows an enlarged object image on a monitor screen 15. 
Only an image region on the image pickup device light receiving plane 14 to 
be enlarged, such as that shown in FIG. 4(a), is set as a scanned region 
by a method described below. As shown in FIG. 3(A), with respect to the 
region other than the enlarged image region, vertical transfer pulses 3(A) 
for driving a vertical CCD (not shown) in the image pickup device 2 are 
initially supplied at a higher rate to rapidly sweep off an unnecessary 
charge corresponding to upper and lower portions of the hatched area of 
the image pickup device light receiving plane shown in FIG. 4(a). With 
respect to the enlarged image region, if, for example, the image 
magnification is to be increased to 2, the image pickup device output 
signal FIG. 3(D) is read out by generating one vertical transfer pulse 
FIG. 3(A) in the period of supplying two horizontal scanning pulses FIG. 
3(C). That is, in a case where the image magnification is increased to 2, 
3, . . . on the basis of a control signal from the scanning timing pulse 
generation means 1 in order to intermittently read out, in the vertical 
direction, signals in the horizontal scanning direction in accordance with 
the transfer rate corresponding to the magnification, the image pickup 
device output signal FIG. 3(D) is read out from the image pickup device 2 
by being thinned out one with respect to two, three, . . . scanning lines. 
The image pickup device output signal FIG. 3(D) intermittently read out in 
the vertical direction in this manner is processed by the correlative 
sampling circuit 3 to remove noise components therein, is controlled by 
the automatic gain control circuit 4 so that the amplitude gain is 
automatically maintained at a constant level, and is supplied to the A/D 
converter 5. The A/D converter 5 converts the image pickup device output 
signal FIG. 3(D) into a digital signal which undergoes color signal 
processing and gamma correction and the like in the signal processing 
circuit 6 and is thereafter supplied to the memory circuit 7. The memory 
circuit 7 calculates an average value of the image pickup device output 
signal FIG. 3(D) corresponding to each adjacent pair of scanning lines, 
and inserts the average value between the two scanning lines, thereby 
interpolating the signal in the vertical direction. Also, the memory 
circuit 7 cuts off the part of the image pickup signal corresponding to 
unnecessary regions at the left and right sides of the light receiving 
plane 14 of the image pickup device 2 by inhibiting reading or writing of 
digital data of the image pickup signal corresponding to the unnecessary 
regions, and effects interpolation in the horizontal direction by 
intermittently reading the image pickup signal corresponding to the 
enlarged image region in accordance with the transfer rate corresponding 
to the magnification and by calculating and inserting an average value of 
two digital data items of the read-out image pickup signal adjacent in the 
horizontal direction. 
The digital image pickup signal processed for enlargement by interpolation 
in the vertical and horizontal directions is input to the D/A converter 8 
to be reconverted into an analog signal, thereby forming the desired image 
on the monitor screen 15. 
The electronic zooming operation is thus performed. The conventional 
apparatuses have been arranged in such a manner that the image pickup 
signal obtained by the above-described enlargement processing and the 
reconversion into the analog form is supplied to an automatic focusing 
circuit (not shown) to extract particular components, such as components 
of particular frequencies, edge components or the like, which vary 
depending upon a focal state, and focused/non-focused states are 
determined by the magnitude of these components. Also, a well-known 
automatic focusing system called a mountaineering servo has widely been 
used for automatic focusing of conventional apparatuses. Therefore, the 
problem of a deterioration in accuracy, malfunction and the like is 
encountered since focus detection is performed on the basis of the signal 
after the enlargement and interpolation processing, as described above. 
The present invention has been achieved to solve this problem, and the 
specific arrangement of the present invention is as illustrated in FIG. 1. 
The arrangement shown in FIG. 1 ensures that, even though both an 
electronic zooming function and an automatic focusing function are 
provided, the operation can always be performed in optimal conditions 
without reducing the effect of each function. 
The same components as those described above with reference to the block 
diagram of FIG. 2 are indicated by the same reference characters in FIG. 
1, and the description for them will not be repeated. 
That is, the image pickup apparatus has an automatic focusing system formed 
of an edge extraction circuit 9, a peak hold circuit 10, a microcomputer 
11 for an overall control of the apparatus, a focusing lens driver 12, and 
a focusing lens 13, as well as the scanning timing pulse generation means 
1, the image pickup device 2, the correlative double sampling circuit 3, 
the automatic gain control circuit 4, the A/D converter 5, the signal 
processing circuit 6, the memory circuit 7 and the D/A converter 8 shown 
in FIG. 2. 
The edge extraction circuit 9 serves for detecting edge components for 
determining a focal state from the image pickup signal, and is connected 
to the automatic gain control circuit 4. The edge extraction circuit 9 
receives a signal from the automatic gain control circuit 4, i.e., the 
original image pickup signal which has been intermittently read from the 
image pickup device 2 in the vertical direction on the basis of pulses 
from the scanning timing pulse generations means 1 (and has been processed 
by automatic gain control (AGC)) but which is not yet been processed by 
intermittent-reading/insertion/interpolation processing in the horizontal 
direction of the memory circuit 7. 
The peak hold circuit 10 for detecting a peak value of the edge components, 
i.e., the maximum of the sharpness, is connected to an output terminal of 
the edge extraction circuit 9, and the microcomputer 11 is connected to an 
output terminal of the peak hold circuit 10. A control line of the 
microcomputer 11 is connected to the focusing lens driver 12. The focusing 
lens 13 is controlled and moved along the optical axis on the basis of a 
drive signal from the focusing lens driver 12. The microcomputer 11 
controls the overall operation of the image pickup device and has control 
lines (control bus) connected to all the components of the apparatus. In 
FIG. 1, however, such control lines are not illustrated while only the 
control line to the focusing lens driver 12 is illustrated. The 
microcomputer 11 performs a mountaineering control to maximize the 
sharpness by driving the focusing lens in a direction such that the output 
from the peak hold circuit 10 is increased. 
The focusing operation under an electronic zooming control, peculiar to 
this embodiment will now be described below. The electronic zooming 
control is the same as that of the conventional apparatus, and only the 
focusing operation will be described. 
The original image pickup signal, which has been intermittently read from 
the image pickup device 2 in the vertical direction but which has not 
undergone the intermittent-reading/insertion/interpolation processing in 
the horizontal direction, is supplied from the automatic gain control 
circuit 4 to the edge extraction circuit 9. The edge extraction circuit 9 
then extracts a rising-edge component and a falling-edge component from 
the supplied original image pickup signal. The extracted rising-edge and 
falling-edge components are supplied to the peak hold circuit 10. The peak 
hold circuit 10 holds peak values of the rising-edge component and the 
falling-edge component in the vertical direction or both in the vertical 
and horizontal directions. The held peak values are supplied to the 
microcomputer 11. 
The microcomputer 11 compares the supplied peak values of the rising-edge 
and falling-edge components to determine a focused state or a non-focused 
state (a fore focus state or a rear focus state). If it determines that 
the peak values of the rising-edge and falling-edge components are not 
equal to each other and that the lens is not focused, it forms a focusing 
lens control signal (for setting the direction of rotation of an 
unillustrated focusing lens driving motor, starting the rotation and 
stopping the rotation) on the basis of the difference between the peak 
values of the rising-edge and falling-edge components, and supplies this 
signal to the focusing lens driver 12. By the drive signal from the 
focusing lens driver 12, the focusing lens 13 is controlled and moved 
along the optical axis to be focused. 
As described above, the automatic focusing (AF) control is performed using 
an AF control signal which is formed on the basis of the original image 
pickup signal which has been intermittently read out in the vertical 
direction but which has not undergone the 
intermittent-reading/insertion/interpolation processing in the horizontal 
direction, when the electronic zooming control is effected. It is thereby 
possible to achieve stable accurate focusing free from deterioration in 
accuracy and a malfunction even during the electronic zooming control. 
The present invention is not limited to the above-described embodiment. For 
example, the determination of focused/non-focused states may be performed 
by extracting particular frequency components from the signal output from 
the automatic gain control circuit 4 instead of using an edge signal 
extracted from the edge extraction circuit 9. Also, the determination of 
focused/non-focused states may be performed by adding a signal other than 
the edge component signal and particular frequency components to the 
information for the determination. Further, a method may be adopted in 
which a signal formed by processing the output signal from the automatic 
gain control circuit 4 by gamma correction or kneeing is input to the edge 
extraction circuit 9. 
In the above-described embodiment, as described above in detail, the 
automatic focusing (AF) control is performed during the electronic zooming 
control by using an AF control signal which is formed on the basis of the 
original image pickup signal which has been intermittently read out in the 
vertical direction but which has not undergone the 
intermittent-reading/insertion/interpolation processing in the horizontal 
direction, thereby achieving stable accurate focusing free from a 
deterioration in accuracy and a malfunction even during the electronic 
zooming control. 
A second embodiment of the present invention will next be described. 
While in the first embodiment the automatic focusing control is performed 
by the electronic zooming function and by using the image pickup signal 
before processing for enlargement, interpolation and the like to improve 
the accuracy of automatic focusing and to prevent a malfunction, the 
second embodiment described below is arranged to improve the accuracy of 
automatic focusing upon an imaged object by performing automatic focusing 
in accordance with the actual field angle after processing for enlargement 
and the like. 
That is, in the first embodiment, the automatic focusing control is 
performed by using the image pickup signal before electronic zoom 
processing to improve the focus detection accuracy. From the view point of 
the field angle, however, a problem described below must be discussed. If 
an image is enlarged by the electronic zooming function and where the 
field angle of the enlarged portion is smaller than that of the original 
picture, and if a region for focus detection, i.e., a distance measuring 
region is larger than or deviates from the region defined by the field 
angle from which the image is enlarged, there is a risk of a deterioration 
in the focus detection accuracy, because image information other than 
information on the image portion within the area defined by the 
actually-framed field angle is mixed in. 
This problem will be described with reference to FIGS. 4(a), 4(b) and FIG. 
5. If a distance measuring region is set in an area larger or different 
from the area corresponding to the field angle from which an image is to 
be enlarged, as in the case of enlargement from the central image portion 
other than the hatched portion shown in FIG. 4(a) to the image shown in 
FIG. 4(b), it includes a region other than the region corresponding to the 
field angle at which the image actually displayed on a monitor display or 
recorded is framed. In such a condition, the automatic focusing accuracy 
may be deteriorated. Referring to FIG. 5 for more detail, a line 5a 
indicates an entire picture frame, a line 5b indicates a region 
corresponding to a field angle at which an image is cut out to be enlarged 
by electronic zooming, and lines 5c and 5c' indicate distance measuring 
regions. As long as a distance measuring region is within the region 
corresponding to the field angle at which the image is cut out to be 
enlarged, as indicated at 5c, accurate focus detection in accordance with 
the image actually displayed can be achieved. However, if a distance 
measuring region is larger than or deviates from the region 5b 
corresponding to the field angle at which the image is cut out to be 
enlarged, as indicated at 5c', information other than information on the 
displayed image is used for focus detection, resulting in a reduction in 
the focus detection accuracy. 
In the second embodiment, therefore, a distance measuring region is set so 
that the size and the position thereof are variable, and the size and the 
position of the distance measuring region are controlled in accordance 
with the electronic zooming operation so that the distance measuring 
region is always within the area defined by the field angle for actual 
enlargement processing or the like, whereby focusing can be performed 
accurately with respect to the region within the actual picture frame. 
FIG. 6 is a block diagram of an arrangement achieving this effect. 
Components identical or corresponding to those shown in FIG. 1 are 
indicated by the same reference characters and the description for them 
will not be repeated. In FIG. 6, the same components as those omitted in 
FIG. 1, i.e., a monitor 17, a video recorder 19, a memory control circuit 
16 and an operating section 18, and other components are illustrated as 
well as those illustrated in FIG. 1. 
Referring to FIG. 6, an edge signal output from the edge detection circuit 
9 is supplied to the gate circuit 14 for setting a distance measuring 
region. Only a part of the edge signal corresponding to a distance 
measuring region set in the picture frame (e.g., the distance measuring 
region indicated by line 5c in FIG. 5) is extracted in the gate circuit 14 
and is supplied to the peak hold circuit 10. The operation is thereafter 
performed in the same manner as the first embodiment. That is, an output 
from the peak hold circuit 10 is supplied to the microcomputer 11 to 
control the focusing lens driver 12 so that its level is maximized, 
thereby controlling the focusing lens 13 in a mountaineering control 
manner. 
A gate control circuit 15 controls the gate circuit 14 to control the 
position and the size of the distance measuring region (indicated by line 
5c in FIG. 5) in the picture frame. The gate control circuit 15 controls 
the gate circuit 14 in an on-off manner to enable only a signal 
representing a desired region in the picture frame to pass. The gate 
control circuit 15 is controlled by a field angle setting signal 
(described below) which is output from the microcomputer 11 to determine 
the field angle at which an image is cut out from the picture frame. 
The memory control circuit 16 controls the A/D converter 5, the signal 
processing circuit 6, the memory circuit 7 and the D/A converter 8 by the 
field angle setting signal output from the microcomputer 11 to perform 
electronic zooming based on cutting out a desired region in the picture 
frame and performing enlargement/interpolation processing. 
That is, if a field angle at which an image in the picture frame is to be 
cut out and enlarged by electronic zooming is designated through the 
operating section 18, the microcomputer 11 outputs, on the basis of the 
designation, a field angle setting signal to the scanning timing pulse 
generator 1 and controls driving thereof to read out an image pickup 
signal corresponding to the cutting-out field angle from the image pickup 
device 2. A signal corresponding to the region set in the picture frame is 
thereby obtained. 
The field angle setting signal is also supplied to the memory control 
circuit 16 to control the process of converting the signal of the region 
set in the picture frame from the analog form into the digital form, 
performing predetermined signal processing of the converted signal, 
controlling the writing rate and writing addresses to store the processed 
signal in the memory 7, controlling the reading rate and reading addresses 
at the time of reading out data in the memory and simultaneously 
performing processing for enlargement of the image, average-value 
interpolation and the like. 
The field angle setting signal is also supplied to the gate control circuit 
15. The gate circuit 14 is thereby controlled to limit the size and the 
position of the distance measuring region 5c so that the distance 
measuring region 5c is always positioned within the region 5b of the field 
angle at which image is cut out by electronic zooming. 
It is thereby possible to perform focusing always accurately with respect 
to the set field angle by preventing occurrence of a situation where the 
distance measuring region includes, as shown in FIG. 5, image information 
other than the information limited by the field angle at which the image 
is cut out by electronic zooming. 
The gate control circuit 15 is simultaneously controlled by a control 
signal 11a which is output from the microcomputer 11. The control signal 
11a serves for a control such that the size of the distance measuring 
region is changed according to, for example, the depth of field which 
changes depending upon the focal length of the lens and the aperture and 
other factors. 
It may be understood that the field angle setting signal limits the control 
signal 11a so that the distance measuring region is always set within the 
area of the field angle set by electronic zooming. 
While the present invention has been described with respect to what 
presently are considered to be the preferred embodiments, it is to be 
understood that the invention is not limited to the disclosed embodiments. 
To the contrary, the present invention is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims. The scope of the following claims is to be 
accorded the broadest interpretation so as to encompass all such 
modifications and equivalent structures and functions.