Auto focus apparatus

An apparatus for generating an auto-focus measurement value for use in performing an auto-focus operation on the basis of contrast components in a video signal. The apparatus includes a line peak circuit which sequentially receives lines of the video signal and generates therefrom a sequence of maximum values of the contrast components of each of the lines. An averaging circuit calculates a rolling average of the line maximum values to form average values. A peak hold circuit outputs a maximum value of the average values as the auto-focus measurement value. The resulting auto-focus measurement value is not greatly affected either by noise or by moving objects within the image represented by the video signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an auto-focus apparatus of the type in which an 
auto-focus measurement value is obtained from contrast components of a 
video signal. 
2. Description of the Prior Art 
In a conventional auto-focus apparatus that is part of an image processing 
system, an auto-focus operation is carried out by adjusting a focusing 
lens so that an auto-focus measurement value obtained from contrast 
components of a video signal is maximized. This is based on the concept 
that proper focus is achieved when the high frequency components of the 
video signal are at a maximum. More particularly, as shown in FIG. 1, the 
direction and speed of adjustment are controlled on the basis of the sign 
and magnitude of a change .DELTA.y in the auto-focus measurement value 
which results when the focusing lens is adjusted by a very small distance 
.DELTA.x. 
FIG. 2 shows a conventional circuit for generating an auto-focus 
measurement value. As shown in FIG. 2, a digital video signal Y is 
received at an input terminal 1 and is band-limited by a high-pass filter 
(HPF) 2. The band-limited signal output from HPF 2 is converted into an 
absolute value signal representative of contrast components of the video 
signal by an absolute value (ABS) circuit 3. The signal output from the 
ABS circuit 3 is supplied to a detecting unit 4 which generates an 
auto-focus measurement value EST on the basis of the absolute value signal 
according to one of various conventional approaches which will be 
described below. The resulting auto-focus measurement value EST is then 
output from an output terminal 5. 
In describing the ways in which the auto-focus measurement value EST may be 
obtained according to the above-mentioned conventional approaches, 
S.sub.1,1 - - - S.sub.1,k2 ; S.sub.2,1 - - - S.sub.2,k2 ; . . . ; 
S.sub.k1,1 - - - S.sub.k1,k2 will represent absolute value (contrast 
component) signals output from ABS circuit 3 in response to a frame 
(hereinafter referred to as the "range finder frame") that is part of a 
field or frame of the video signal from which the auto-focus measurement 
value EST is to be obtained. 
A conventional approach known as an "integration system" will first be 
described. According to this system, as shown in the following equation 
(1), all of the signals S.sub.1,1 through S.sub.k1,k2 are added in the 
detecting unit 4 and the resulting sum is provided as the auto-focus 
measurement value EST (see FIG. 3A). 
##EQU1## 
Next a so-called horizontal line (H line) peak hold system will be 
described. According to this system, as shown in the following equation 
(2), there is generated a maximum value of each line of signals (S.sub.1,1 
- - - S.sub.1,k2), (S.sub.2,1 - - - S.sub.2,k2), . . . , (S.sub.k1,1 - - - 
S.sub.k1,k2). The line maximum values are summed and the result is 
provided as the auto-focus measurement value EST (see FIG. 3B). 
##EQU2## 
A third conventional system, known as the vertical peak (V peak) hold 
system will next be described. According to this system, as shown in the 
following equation (3), the maximum of all of the signals S.sub.1,1 - - - 
S.sub.k1,k2 is detected by detecting unit 4 and provided as the auto-focus 
measurement value EST (see FIG. 3C). 
##EQU3## 
All of these conventional systems suffer from disadvantages. An auto-focus 
measurement value obtained according to the integration or H line peak 
hold system is not greatly affected by noise but tends to be affected by a 
moving object within the scene represented by the video signal. 
Conversely, an auto-focus measurement value obtained according to the V 
peak hold system is not greatly affected by moving objects within the 
scene of the video signal, but tends to be affected by noise. 
Turning to another aspect of conventional approaches for generating an 
auto-focus measurement value, reference is again made to the curve shown 
in FIG. 1, which represents changes in the auto-focus measurement value in 
response to changes in position of the focusing lens. In order to perform 
a satisfactory auto-focus operation, the base of this curve must be 
inclined. For that purpose, HPF 2 (FIG. 2) is required to have a low 
cut-off frequency so that its output signal contains as many components as 
possible other than DC. 
However, the lower the cut-off frequency of HPF 2, the longer the time 
period during which a signal change at a certain point in time continues 
to affect the output of HPF 2. FIGS. 4A and 4B respectively show the step 
response and the frequency characteristic of embodiments of HPF 2 
depending on variations in its transfer characteristic (1-D)/(1-kD). It 
will be seen that as the parameter k is increased, the cut-off frequency 
is decreased, but the period of response to a signal step increases. 
Other problems with conventional auto-focus systems arise when an edge of 
an object is present near the left side of the range finder frame, or when 
the leading edge after a horizontal blanking interval (i.e. the black 
level) affects the output of HPF 2. FIG. 5A shows the input video signal Y 
and FIG. 5B shows the corresponding output signal from HPF 2. As seen from 
FIGS. 5A and 5B, the output signal of the HPF 2 during the period 
corresponding to the range finder frame is affected by the leading edge 
EDGE which follows the horizontal blanking period. 
Another disadvantage of conventional auto-focus systems will be explained 
with reference to the flow chart shown in FIG. 6, which illustrates a 
process according to the prior art for determining whether an auto-focus 
operation should be performed. 
It should be understood that the automatic focus operations described 
herein are conducted with respect to real-time moving images, not still 
pictures. Accordingly, when a change occurs in the auto-focus measurement 
value, it is necessary to distinguish between two different cases: (1) An 
object within the scene has moved but the scene as a whole is unchanged; 
and (2) the entire scene has changed. In the second case, a complete auto 
focus operation must be performed; in the first case only a fine 
adjustment should be made, for otherwise the picture would become 
unstable. 
At the beginning of the routine shown in FIG. 6, it is determined, at step 
31, whether or not the auto-focus measurement value has changed from the 
value for the preceding field. If so, then the routine proceeds to step 32 
in which a field count t is set to an initial value t0 (for example, 
t0=20). 
Following step 32 is step 33, at which it is determined whether the 
auto-focus measurement value for the next field has changed. If not, the 
routine returns to step 31. Otherwise, following step 33 is step 34, at 
which the field counter t is decremented. After step 34 is step 35, at 
which it is determined whether the field counter t has been decremented to 
0. If not, then the routine returns to step 33. Otherwise, i.e. if the 
auto-focus measurement value has changed for a consecutive number of 
fields equal to t0, then it is determined that an auto-focus operation 
should be performed (step 36). 
It is a disadvantage of this prior art decision making process that the 
start of the auto-focus operation does not occur for a relatively long 
time, e.g., 20 fields, after a change of scene. However, if the number of 
fields t0 is reduced, then it often occurs that motion of an object within 
the scene is mistaken for a change of scene, resulting in such problems as 
an unstable picture. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved auto-focus apparatus in which the aforesaid shortcomings and 
disadvantages of prior art systems can be eliminated. 
More specifically, it is an object of the present invention to provide an 
auto-focus apparatus in which the auto-focus measurement value is not 
greatly affected either by noise or by moving objects within the scene 
represented by the video signal. 
It is another object of the present invention to provide an auto-focus 
apparatus in which the auto-focus measurement value is not affected by the 
spurious leading edge following a horizontal blanking period. 
It is a further object of the present invention to provide an auto-focus 
apparatus in which it is determined rapidly and reliably when an 
auto-focus operation should be performed. 
In accordance with an aspect of the present invention, an apparatus for 
generating an auto-focus measurement value to be used in performing an 
auto-focus operation on the basis of contrast components of a video signal 
includes a line peak circuit for sequentially receiving lines of the video 
signal and generating respective maximum values of contrast components of 
the received lines of the video signal. The apparatus also includes an 
averaging circuit for sequentially averaging the maximum values generated 
by the line peak circuit over predetermined groups of the lines of the 
video signal to form a plurality of average values and a peak hold circuit 
for outputting as the auto-focus measurement value a maximum value of the 
plurality of average values formed by the averaging circuit. 
With the apparatus according to this aspect of the present invention, 
because the auto-focus measurement value is obtained in a manner similar 
to the V peak hold system, a moving object within the scene represented by 
the video signal does not greatly affect the resulting auto-focus 
measurement value. Moreover, since the respective line maximum values are 
sequentially averaged over predetermined groups of the lines, and the 
maximum value of the resulting average values is used as the auto-focus 
measurement value, the influence of noise on that value is reduced. 
According to another aspect of the invention, an apparatus for generating 
an auto-focus measurement value to be used in performing an auto-focus 
operation on the basis of contrast components in a video signal includes a 
line peak circuit for sequentially receiving lines of the video signal and 
generating respective maximum values of contrast components of the 
received lines of the video signal. The apparatus also includes a low-pass 
filter for receiving and low-pass filtering the maximum values generated 
by the line peak circuit to produce a filtered output signal and a peak 
hold circuit for outputting as the auto-focus measurement value a maximum 
value of the filtered output signal produced by the low-pass filter. 
Further, according to the latter aspect of the invention the low-pass 
filtering of the line maximum values reduces the effect of noise upon the 
resulting auto-focus measurement value, while at the same time the system 
remains similar to the V peak hold system so that the measurement value is 
not greatly affected by a moving object within the scene. 
According to still another aspect of the invention, an apparatus for 
performing an automatic focus operation on the basis of contrast 
components of a video signal includes a circuit for detecting maximum 
values of contrast components of respective lines of the video signal, a 
circuit for averaging the detected maximum values corresponding to the 
lines of the video signal to form a first auto-focus measurement value, a 
circuit for detecting a maximum value of contrast components with respect 
to a field of the video signal and for outputting said detected maximum 
value for said field as a second auto-focus measurement value, and a 
circuit for actuating an auto-focus operation in response to a change in 
both the first and second auto-focus measurement values. 
According to further aspects of the invention, the apparatus also includes 
a circuit for replacing a horizontal period of the video signal with a 
signal that has a level that is substantially equal to a starting portion 
of a line within a range finder frame of the video signal and a circuit 
for replacing a vertical blanking period of the video signal with a signal 
that has a level that is substantially equal to a starting portion of a 
line within a range finder frame of the video signal. 
Because the decision on whether to begin an auto-focus operation is based 
on changes in two auto-focus measurement values, one of which is the 
average of line maximum values and the other of which is a maximum field 
value, the decision can be made more accurately and rapidly. 
Moreover, because the synchronizing portion of the video signal is replaced 
with a signal having a level that is substantially equal to the starting 
portion of the video signal, a high pass filter with a lower cut-off 
frequency can be used so as to widen the frequency band, while reducing 
the effect of the leading edge at the end of the horizontal blanking 
period and also reducing the effect of the vertical blanking period. 
The above, and other objects, features and advantages of the present 
invention will be apparent from the following detailed description thereof 
which is to be read in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will now be described with reference 
to the drawings. In each of the embodiments, there is provided an 
auto-focus apparatus which generates an auto-focus measurement value on 
the basis of contrast components of a video signal. 
FIG. 7 is a block diagram of an auto-focus measurement value generating 
circuit according to a first embodiment of the invention. In FIG. 7, 
elements corresponding to those of FIG. 2 are marked with the same 
reference numerals and therefore need not be described in detail. 
In the embodiment of FIG. 7, the detecting unit 4' that is connected to the 
output of the ABS circuit 3 includes a horizontal line peak hold circuit 
4a, an averaging circuit 4b and a vertical peak hold circuit 4c. 
ABS circuit 3 generates lines of signals (S.sub.1,1 - - - S.sub.1,k2) 
(S.sub.2,1 - - - S.sub.2,k2), . . . , (S.sub.k1,1 - - - S.sub.k1,k2) in 
response to the portion of the video signal within the range finder frame. 
In response to these lines of signals, the horizontal line peak hold 
circuit 4a generates maximum values SA.sub.1, SA.sub.2, . . . , SA.sub.k1 
(see FIGS. 8A, 8B) according to the following equation (4): 
##EQU4## 
The averaging circuit 4b provides sequential rolling averages of the 
maximum values SA.sub.1, SA.sub.2, . . . , SA.sub.k1 output from the 
horizontal line peak hold circuit 4a, with groups of n lines being 
averaged to produce a plurality of average values SB.sub.1, SB.sub.2, . . 
. , SB.sub.(k1+1-n), as shown in FIG. 8C. The number of lines n in each 
group that is averaged may be four or five, for example. The formulas by 
which the rolling average values SB.sub.1, SB.sub.2, . . . , 
SB.sub.(k1+1-n) are obtained are shown in the following equation (5): 
##EQU5## 
(Although not shown in equation (5) it will be recognized that a factor 
such as 1/n may be applied to each of the values SB.sub.1, SB.sub.2, . . . 
, SB.sub.(k1+1-n).) 
The peak hold circuit 4c generates a maximum value SC (FIG. 8D) on the 
basis of the rolling average values SB.sub.1, SB.sub.2, . . . 
SB.sub.(k1+1-n) provided by the averaging circuit 4b. The maximum value SC 
is obtained in accordance with the following equation (6): 
EQU SC=max(SB.sub.1, SB.sub.2, . . . , SB.sub.(k1+1-n)) (6) 
According to the embodiment of FIG. 7, the maximum value SC provided by the 
peak hold circuit 4c is output from detecting unit 4' and provided at the 
output terminal 5 as the auto-focus measurement value EST. The auto-focus 
measurement value EST generated by the embodiment of FIG. 7 is inherently 
similar to the value provided in the V peak hold system described above 
with reference to FIG. 3C and equation (3). Accordingly, the auto-focus 
measurement value generated by the circuit of FIG. 7 is not greatly 
affected by motion of an object within the scene represented by the video 
signal. 
Moreover, averaging circuit 4b, as noted above, provides rolling averages 
SB.sub.1, SB.sub.2, . . . , SB.sub.(k1+1-n) on the basis of the line 
maximum values SA.sub.1, SA.sub.2, . . . , SA.sub.k1, and the auto-focus 
measurement value EST is established as the maximum value SC of the 
average values SB.sub.1, SB.sub.2, . . . , SB.sub.(k1+1-n). Because of the 
averaging performed at the averaging circuit 4b in the embodiment of FIG. 
7, the influence of noise upon the auto-focus measurement value EST is 
reduced as compared with the V peak hold system. In other words, if the 
maximum value of a given line is large, and the maximum values of the 
immediately preceding and succeeding lines are relatively small, then it 
is likely that the large line maximum is the result of noise. Because of 
the averaging carried out in the circuit of FIG. 7, the effect of that 
noise is reduced. 
Although the averaging circuit 4b as described above provides an n line 
rolling average shifted one line at a time, the present invention is not 
limited to such averaging. For example, the n line rolling average may be 
shifted by two lines or more. As another alternative, it is possible for 
averaging circuit 4b to average groups of lines that do not overlap. 
FIG. 9 is a block diagram of an auto-focus measurement value generating 
circuit in accordance with a second embodiment of the present invention. 
In FIG. 9, elements corresponding to those of FIG. 2 are again marked with 
the same reference numerals and need not be described in detail. 
In the embodiment of FIG. 9, the detecting circuit 4" that is connected to 
the output of the ABS circuit 3 includes a horizontal line peak hold 
circuit 4a and a processing circuit 4d. The horizontal line peak hold 
circuit 4a is the same as that described in connection with the embodiment 
of FIG. 7, so that, as before, the lines of signals (S.sub.1,1 - - - 
S.sub.1,k), (S.sub.2,1 - - - S.sub.2,k2), . . . , (S.sub.k1,1 - - - 
S.sub.k1,2) provided by ABS circuit 3 on the basis of the contrast 
components in a range finder frame of a field of the video signal are 
provided to the horizontal line peak hold circuit 4a, which generates 
therefrom line maximum values SA.sub.1, SA.sub.2, . . . , SA.sub.k1 (FIGS. 
8A, 8B). 
The processing circuit 4d performs low-pass filtering on the maximum values 
SA.sub.1, SA.sub.2, . . . , SA.sub.k1. The maximum value SD of the 
resulting filtered line maximum signals is output by the detecting unit 4" 
and provided at output terminal 5 as the auto-focus measurement value EST. 
FIG. 10 is a block diagram showing details of the processing circuit 4d. As 
shown in FIG. 10, the maximum values SA.sub.1, SA.sub.2, . . . , SA.sub.k1 
provided by horizontal line peak hold circuit 4a are sequentially provided 
via an input terminal 41 to a comparator (CMP) 42 as a comparison signal a 
at intervals of one horizontal line period. An output signal b from the 
comparator 42 is provided to a low-pass filter (LPF) 43 which provides 
low-pass filtering in the vertical direction. An output signal c from the 
low-pass filter 43 is provided to a peak hold circuit 44. 
An output signal d from the peak hold circuit 44 is provided as the 
auto-focus measurement value EST via an output terminal 45 and also is 
provided to a 1 H (one horizontal period) delay circuit 46. The delayed 
output signal e from the 1 H delay circuit 6 is provided to comparator 42 
as a reference signal f and also is supplied to the peak hold circuit 44. 
In operation, the comparator 42 compares signals a and f and outputs the 
larger of the two. In other words, the output b of comparator 42 is as 
follows: b=max(a,f). The peak hold circuit 44 compares the signals c and e 
and outputs the larger so that its output signal d=max(c,e). The signal d 
is held in the peak hold circuit 44 as the auto-focus measurement value 
EST when all of the line maximum values SA.sub.1, SA.sub.2, . . . , 
SA.sub.k1 have been supplied to input terminal 41. 
In the auto-focus measurement value generating circuit of FIG. 9, as 
described above, the auto-focus measurement value EST is inherently 
similar to the value provided by the conventional V peak hold system 
described previously with respect to FIG. 3C and equation (3). Therefore, 
the auto-focus measurement value EST provided by the circuit of FIG. 9 is 
not greatly affected by a moving object within the scene represented by 
the video signal. Moreover, the effect of noise upon the auto-focus 
measurement value is reduced because of the low-pass filtering that is 
applied to the line maximum values SA.sub.1, SA.sub.2, . . . , SA.sub.k1 
before the field maximum value SD is obtained. In other words, if the 
maximum value of a given line is large while the respective maximum values 
of the immediately preceding and succeeding lines are relatively small, 
then it is likely that the relatively large line maximum is the result of 
noise. The low-pass filtering carried out in the embodiment of FIG. 9 
reduces the effect of such noise upon the auto-focus measurement value. 
FIG. 11 is a block diagram of an auto-focus measurement value generating 
circuit according to a third embodiment of the invention. In FIG. 11, 
elements corresponding to those of FIG. 2 are marked with the same 
reference numerals and therefore need not be described in detail. 
As shown in FIG. 11, the video signal Y received at the input terminal 1 is 
supplied directly to a fixed contact 6a of a change-over switch 6 and is 
also supplied to a sample and hold (S/H) circuit 7. A sampling pulse Ps is 
applied to the sample and hold circuit 7 at a time that coincides with the 
start of the portion of each line that is in a range finder frame 8 (FIG. 
12). In response to the sampling pulse Ps, the video signal is sampled and 
a pixel signal level Ds is held in sample and hold circuit 7, where the 
pixel signal level Ds corresponds to the first pixel of the portion of the 
line within the range finder frame 8. 
An output signal from the sample and hold circuit 7 is supplied to a fixed 
contact 6b of the change-over switch 6. The position of change-over switch 
6 is controlled by a signal applied thereto from a control circuit which 
is not shown. The position of change-over switch 6 is controlled in such a 
manner that the fixed contact 6a is connected to an output terminal of 
switch 6 at times when the video signal Y represents the inside of the 
range finder frame 8, and the output signal of sample and hold circuit 7 
is connected via fixed contact 6b to the output terminal of switch 6 at 
other times. The provision of control signals in this manner to switch 6 
via the control circuit which is not shown is well within the capabilities 
of those having ordinary skill in the art, so that there is no need to 
show or further describe the control circuit. As shown in FIG. 11, the 
output of switch 6 is provided as an input to HPF 2. The balance of the 
auto-focus measurement value generating circuit of FIG. 11 may be, for 
example, the same as the corresponding portions of the circuit of FIG. 2. 
Alternatively, the detecting unit 4 of FIG. 11 may be like the detecting 
unit 4' of FIG. 7 or the detecting unit 4" of FIG. 9. As still another 
alternative, the two detecting units as described below in connection with 
FIG. 15 may take the place of the detecting unit 4 of FIG. 11. 
It will be appreciated that the portions of the video signal Y within range 
finder frame 8 are provided to HPF 2. On the other hand, at times when the 
video signal Y represents portions of the field outside of range finder 
frame 8, the pixel signal level Ds that had been sampled and held in 
sample and hold circuit 7 is provided to HPF2 in place of the video 
signal. Referring to FIG. 12, the shaded portions thereof represent times 
at which the pixel signal level Ds is provided to HPF 2 instead of the 
video signal Y. Because of line correlation, the level of the pixel signal 
Ds which represents the first pixel within the frame 8 of the preceding 
line is substantially equal to the level of the first pixel of the present 
line. It will be recognized that the signal level Ds replaces the 
horizontal blanking interval of a line of the video signal. Referring to 
FIG. 12A, shaded portions W thereof represent times at which a signal 
level Ds' is provided to HPF2 instead of the video signal Y. The signal 
level Ds' is substantially equal to the pixel signal level at the start of 
the first line within the range finder frame. 
Those having skill in the art will recognize that sample and hold circuit 7 
may receive a separate pulse Ps' to cause the level Ds' to be sampled and 
held, and that circuit 7 may include circuitry for separately and 
simultaneously holding the levels Ds and Ds' and for providing a selected 
one of the levels to the fixed contact 6b of switch 6 according to a 
control signal from the control circuit referred to above. 
In the embodiment of FIG. 11, an edge in the form of the shaded portion of 
FIG. 13 is detected by HPF 2, but an edge of an object that is outside of 
the range finder frame 8, or the leading edge resulting from the end of 
the horizontal blanking period, will not be detected at HPF 2 and so will 
not affect the auto-focus measurement value EST output from the circuit of 
FIG. 11. As a result, the cut-off frequency of HPF2 can be reduced and a 
satisfactory auto-focus operation can be performed. 
In addition, in the embodiment of FIG. 11 an edge that runs in the vertical 
direction, as shown by the shaded portion of FIG. 14, is also properly 
detected. 
With replacement of the portion of the video signal Y that is outside of 
the range finder frame 8 with a stored value from within the range finder 
frame 8, a satisfactory auto-focus operation can be carried out even when 
the image represented by video signal Y is an object that is difficult to 
pick up, such as horizontal stripes or the like. 
It should be noted that according to the embodiment of FIG. 11 as described 
up to this point, all of the video signal Y outside of the range finder 
frame 8 is replaced by the signal held in the sample and hold circuit 7. 
In other words, all of the vertical blanking intervals as well as the 
horizontal blanking intervals are replaced, as well as peripheral portions 
of the picture itself. However, the present invention is not limited 
thereto. For example, the control of the switch 6 and the application of 
the sampling pulse Ps can be carried out so that only the horizontal 
blanking interval is replaced by the signal output from the sample and 
hold circuit 7, thereby eliminating the undesirable effect upon the 
auto-focus measurement value by the leading edge that occurs at the end of 
the horizontal blanking interval. As another alternative, a delay element 
can be provided before or after the sample and hold circuit 7 so that the 
replacement signal provided instead of the portions outside of the range 
finder frame is at the level of a pixel from a prior field or frame. 
FIG. 15 is a flow chart of a procedure for determining, in accordance with 
the present invention, when an auto-focus operation should be performed. 
For the purposes of the procedure illustrated in FIG. 15, it should be 
understood that two auto-focus measurement values, ESTa and ESTb, are 
generated. For example, the first value ESTa may be generated as a sum or 
average of line maximum values according to the conventional H line peak 
hold system as previously described, and value ESTb may be generated 
according to the conventional V peak hold system as previously described. 
Referring now to FIG. 15, at the beginning of the routine it is determined, 
at step 51, whether or not the auto-focus measurement value ESTa has 
changed. If not, the routine cycles back to the beginning. Otherwise, step 
52 follows, at which it is determined whether the auto-focus measurement 
value ESTb has also changed. Again, if not, the routine returns to the 
beginning, i.e. to step 51. However, if both of the measurement values 
have changed, then step 53 follows, at which the field count t is set to 
an initial value to. 
Following step 53 is step 54, at which it is determined for the next field 
of video signal whether the first auto-focus measurement value ESTa has 
changed. If not, again the routine returns to step 51. Otherwise, step 55 
follows step 54. At step 55 it is determined whether the second auto-focus 
measurement value ESTb has changed. Again, if not, the routine cycles back 
to the beginning and step 51. However, if both of the values have changed, 
then step 56 follows, at which the count t is decremented. After step 56 
comes step 57, at which it is determined whether the count has been 
decremented to zero. If not, the routine loops back to step 54. However, 
if at step 57 the count t is found to have reached 0, then it is 
recognized that both of the auto-focus measurement values ESTa and ESTb 
have changed over a period of to fields so that an auto-focus operation 
(step 58) is performed. 
Summarizing the routine of FIG. 15, a determination as to whether to 
perform an auto-focus operation is made on the basis of two auto-focus 
measurement values, one of which is provided according to an H line peak 
hold system and the other being provided on the basis of a V peak hold 
system. An auto-focus operation is performed when both are found to have 
changed over a period of t0 fields. Using this procedure, it can be 
determined with relative reliability whether an auto-focus operation 
should be performed. 
A circuit in which the routine of FIG. 15 is carried out may be similar to 
the conventional circuit of FIG. 2 except for the following modifications: 
instead of a single detecting unit 4, two detecting units are provided, 
connected so as to receive in parallel the signals S.sub.1,1 - - - 
S.sub.k1,k2 output from ABS circuit 3. One of the detecting units 
generates the first auto-focus measurement value ESTa according to the 
conventional H line peak hold system and the other detecting unit 
generates the second auto-focus measurement value ESTb according to the 
conventional V peak hold system. The measurement values ESTa and ESTb are 
provided via respective output terminals to a conventional control circuit 
such as a microprocessor (not shown) suitably programmed to carry out the 
routine of FIG. 15. 
Alternatively at least one of the detecting circuits may generate an 
auto-focus measurement value according to the embodiments of FIG. 7 or 9. 
Considering first the auto-focus measurement value ESTb (V peak hold 
system), it should be noted that if an object is located at the limit of 
the range finder frame, and moves in and out of the range finder frame, 
the value ESTb is subject to fluctuation so that the movement of the 
object can be misinterpreted as a change of the entire scene. However, 
such a situation has little effect on the first auto-focus measurement 
value ESTa, so that it can be correctly determined that the scene has not 
changed. 
On the other hand, with respect to the auto-focus measurement value ESTa (H 
line peak hold system), movement of an object in the longitudinal 
direction can cause an erroneous determination that the scene has changed. 
However, such a moving object does not affect the auto-focus measurement 
value ESTb, so that again there is a correct determination that the scene 
has not changed. 
Because of the increased reliability provided by using the two different 
auto-focus measurement values, the determination period of to fields can 
be made relatively short so that an auto-focus operation can be performed 
more promptly, but without causing an unstable picture. 
Having described preferred embodiments of the invention with reference to 
the accompanying drawings, it is to be understood that the invention is 
not limited to those precise embodiments and that various changes and 
modifications may be effected therein by one skilled in the art without 
departing from the spirit or scope of the invention as defined in the 
appended claims.