Focus detection apparatus

A focus detection apparatus has a pair of photoelectric element arrays, a plurality of filters having different MTF characteristics, and an operation unit. A filter is selected from the plurality of filters in accordance with an object or a focusing state. The selected filter filters the data from the photoelectric element arrays, and the operation unit calculates a focus detection signal based on the filtered data.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for photoelectrically 
detecting an optical image. More particularly, the present invention 
relates to a measuring apparatus for photoelectrically detecting an 
optical image of an object to be measured and detecting a focusing state 
of an imaging optical system or a distance to the object to be measured. 
2. Description of the Prior Art 
Conventional focus detection apparatus for single lens reflex cameras are 
roughly classified into relative displacement detection systems and 
sharpness detection systems. In a relative displacement detection system, 
two images of substantially the same portion of an object to be 
photographed are formed on a pair of photoelectric element array, in each 
of which a number of photoelectric elements are arrayed, by light rays 
passed through different regions of a pupil of a phototaking lens. 
Photoelectric outputs from the photoelectric element arrays are used for 
operations to detect a relative displacement of the optical images on the 
photoelectric element arrays. Focus detection is performed based on the 
detection result obtained. On the other hand, in a sharpness detection 
system, images of an object to be photographed formed by a phototaking 
lens are directed onto pair of photoelectric element arrays which are 
arranged before and after a predetermined focal plane of the phototaking 
lens. Photoelectric outputs from the photoelectric element arrays are used 
for performing operations to detect sharpness of the image of the object 
and to thereby perform focus detection. The operations performed on the 
photoelectric outputs involves the spatial frequency components in a given 
spatial frequency band of the image of the object. Low spatial frequency 
components remain in the image of the object, that is, in the 
photoelectric electric outputs, even if the phototaking lens is 
considerably spaced apart from the in-focus position and the object image 
is blurred accordingly. Therefore, using only the low-order spatial 
frequency components for the operations, a front-focus or a rear-focus (a 
front-focus or a rear-focus state indicates a state wherein an object 
image is formed in front of or behind the predetermined focal plane) can 
be discriminated even in the case wherein a defocus amount (the defocus 
amount is the amount of displacement between the predetermined focal plane 
of an imaging optical system and an object image along the direction of 
the optical axis) is large. However, focus detection using low spatial 
frequency components is adversely affected by various electrical or 
optical errors, and the in-focus position itself cannot be determined with 
high accuracy. On the other hand, if the spatial frequency components to 
be subjected to operations are relatively high, the detection result is 
less adversely affected by such errors and a correct defocus amount can be 
obtained if the object image is located near the predetermined focal 
plane. That is, high-precision focus detection can be performed. However, 
when the phototaking lens is considerably spaced apart from the in-focus 
position, high spatial frequency components of an object image decreases 
significantly. Then, precision of focus detection is significantly 
lowered, and discrimination between a front-focus and a rear-focus cannot 
even be performed in some cases. The relative displacement detection 
system has another disadvantage in that if the operations are performed 
using only high spatial frequency components, an in-focus signal (to be 
referred to as a pseudo in-focus signal hereinafter) may be generated even 
if the phototaking lens is at some position distant from the true in-focus 
position. If the operations are performed using photoelectric outputs in 
which high spatial frequency components are not separated from low spatial 
frequency components, the disadvantages involved in operations using 
either one of the high- and low-order spatial frequency components are 
increased and the advantages obtained in operations using the other of the 
high and low spatial frequency components are impaired. 
More particularly, focus detection is roughly classified into (1) correct 
calculation of a defocus amount when a phototaking lens is near a in-focus 
position, and (2) approximate calculation of a defocus amount or 
discrimination between a front-focus and a rear-focus when the phototaking 
lens is significantly spaced apart from the in-focus position. Ideal focus 
detection can be performed using mainly high spatial frequency components 
for the former (1) and using low spatial frequency components for the 
latter (2). This also applies to a sharpness detection system. There are 
cases wherein an object itself contains a large amount of high spatial 
frequency components and only a small amount of low spatial frequency 
components, and vice versa. It has been impossible to perform 
high-precision detection for all of such various types of objects. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a measuring apparatus 
which is capable of measuring with high precision a focusing state or a 
distance to an object to be measured from a phototaking lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of a focus detection apparatus of the present 
invention will now be described with reference to the accompanying 
drawings. 
Referring to FIG. 1 showing a focus detection optical system, a small lens 
array 2 consisting of a number of small lenses 2a, 2b, . . . , 2n which 
are linearly aligned is arranged on the predetermined focal plane of an 
imaging optical system 1 such as a phototaking lens to be subjected to 
focus detection. The small lens array 2 is at the flat side of a 
plane-convex lens 3. First and second object images are formed on the 
small lens array 2 by light rays which have passed through regions 1A and 
1B, respectively, on the pupil of the imaging optical system 1. The 
displacement amount between the object images in a direction perpendicular 
to the optical axis corresponds to the defocus amount of the imaging 
optical system 1. This displacement amount may also be interpreted to 
represent a distance from the detection apparatus to the object when the 
position of the imaging optical system is considered. A photoelectric unit 
4 for photoelectrically detecting the displacement between the two images 
is arranged immediately behind the small lens array 2 and has a pair of 
photoelectric element groups (a1, b1), (a2, b2), . . . , (an, bn), which 
are respectively opposed to the small lenses 2a to 2n. Of the pair of 
arrays, the upper, first photoelectric element array a1, a2, . . . , an 
subjects the first object image to photoelectric conversion, and the 
lower, second photoelectric element array b1, b2, . . . , bn subjects the 
second object image to photoelectric conversion. When photoelectric 
outputs from the photoelectric elements a1 to an and b1 to bn are 
respectively designated by a1 to an and b1 to bn, the photoelectric device 
4 produces these photoelectric outputs in the order of a1, b1, a2, b2, . . 
. , an and bn. Such a focus detection system is disclosed in detail in 
U.S. Pat. No. 4,185,191. Note that photoelectric outputs used herein 
include outputs which are obtained by linear amplification or logarithmic 
amplification of the outputs from the photoelectric elements. As shown in 
FIG. 2, the MTF (Modulation Transfer Function) characteristics which are 
determined by the aperture shapes of the small lenses 2a to 2n of the 
small lens array 2 have high values within a frequency range between zero 
frequency and the Nyquist frequency 1/2Po given as a function of a lens 
array pitch Po. When it is assumed that the first and second object images 
contain sufficient amounts of the low spatial frequency components to the 
high spatial frequency components near the Nyquist frequency, it is seen 
from the MTF characteristics as described above that the photoelectric 
outputs contain low to high spatial frequency components. 
Referring to FIG. 3, the input terminals of a first filter means 5 and a 
second filter means 6 are connected to the output terminal of the 
photoelectric device 4. The first and second filter means 5 and 6 filter 
the photoelectric outputs a1, b1, a2, b2, . . . , an, bn which are 
time-serially supplied from the photoelectric device 4, to time-serially 
produce A1, B1, A2, B2, . . . , An, Bn. FIGS. 4A and 5A show the MTF 
characteristics of the first and second filter means 5 and 6, 
respectively. As may be apparent from these figures, the MTF of the first 
filter means 5 is sufficiently high at a low frequency near zero, 
continuously decreased, and reaches substantially zero at the Nyquist 
frequency 1/2Po. The MTF of the second filter means 6 is sufficiently high 
near a frequency 1/4Po which is half the Nyquist frequency, continuously 
decreases toward low and high frequency sides, and reaches substantially 
zero at zero frequency (D.C. component) and the Nyquist frequency 1/2Po, 
respectively. Accordingly, the first filter means 5 serves to suppress the 
high spatial frequency components in contrast to the low spatial frequency 
components and passes a large amount of the low spatial frequency 
components. Conversely, the second filter means 6 suppresses the low 
spatial frequency components in contrast to the high spatial frequency 
components and passes a large amount of the high spatial frequency 
components. A filter selecting means 7 selects one of the outputs from the 
first and second filter means 5 and 6 and supplies the selected input to a 
memory means 9 through an A/D converter 8. The memory means 9 has a 
capacity sufficient to store all the output from the A/D converter 8 which 
corresponds to the photoelectric outputs a1, b1, a2, b2, . . . , an, bn. 
In accordance with filtered outputs A1 to An and B1 to Bn stored in the 
memory means 9, an operation means 10 calculates a focus detection signal 
Zi representing a relative displacement between the first and second 
object images on the first and second photoelectric element arrays, and 
also an information amount signal Di representing the certainty of 
information used for operations. The signal Zi is proportional to a 
defocus amount. The sign of the signal Zi represents a front-focus or a 
rear-focus, and the absolute value of the signal Zi represents the 
absolute value of the defocus amount. When the signal Di is above a 
predetermined value Do, it means that the corresponding focus detection 
signal Zi is reliable. An example of a configuration of such an operation 
means is disclosed in U.S. Pat. No. 4,333,007. The focus detection signal 
Zi and the information amount signal Di are supplied to a control means 
11. The control means 11 compares the absolute value of the focus 
detection signal Zi with a predetermined value Zo. If the former is larger 
than the latter, the control means 11 supplies a first filter means 
selection signal for selecting the first filter means 5 to the filter 
selecting means 7. When the former is smaller than the latter, the control 
means 11 supplies a second filter means selection signal for selecting the 
second filter means 6 to the filter selecting means 7. 
The mode of operation of the system described above will now be described 
below. 
When the imaging optical system 1 forms the first and second object images 
on the small lens array 2, the first photoelectric element array a1, . . . 
, an converts an illuminance distribution pattern of the first object 
image into photoelectric outputs a1 to an, and the second photoelectric 
element array b1, . . . , bn converts the second object image into 
photoelectric outputs b1, . . . , bn. The photoelectric device 4 
alternately and time-serially produces the photoelectric outputs from the 
first and second photoelectric element arrays in the order of a1, b1, a2, 
b2, . . . , an, bn. Such a series of photoelectric outputs is repeatedly 
produced at a predetermined time interval. When the filter selecting means 
7 selects the second filter means 6, the series of photoelectric outputs 
a1, b1, . . . , an, bn is filtered by the second filter means 6 to be 
converted into the filtered outputs A1, B1, . . . , An, Bn which are 
stored in the memory means 9 through the A/D converter 8. On the basis of 
the stored contents of the memory means 9, the operation means 10 
calculates a focus detection signal Zi and an information amount signal 
Di. Hereinafter, signals Zi; Di when the first and second filter means are 
selected, respectively, are represented by Zi(1); Di(1), and Zi(2); Di(2). 
The control means 11 first discriminates if the signal Di(2) has reached 
the predetermined value Do and if the absolute value .vertline.Zi(2) 
.vertline. of the focus detection signal is larger than a predetermined 
value Zo. When the signal Di(2) is larger than or equal to Do, that is, 
Di(2).gtoreq.Do and when .vertline.Zi(2).vertline..ltoreq.Zo, the 
information is accurate. Then, based on the signal Zi(2), the control 
means 11 controls a display unit 12 and a driving unit 13 and continues to 
produce a second filter means selection signal. Then, the display unit 12 
displays the focusing state, and the driving unit 13 drives the imaging 
optical system 1 toward the in-focus position. If the signals Zi(2) and 
Di(2) do not satisfy the above conditions, the accuracy of the information 
is low; the first and second object images are significantly blurred, the 
amount of high spatial frequency components is small, and the amount of 
low spatial frequency components is large. Accordingly, the display/drive 
is not performed in accordance with the current information, and the 
control means 11 supplies a first filter selection signal to the filter 
selecting means 7. When the filter selecting means 7 accordingly selects 
the first filter means 5, a series of subsequently produced photoelectric 
outputs a1, b1, . . . , an, bn, is filtered by the first filter means 5, 
and the operation means 10 calculates a focus detection signal Zi(1) and 
an information amount signal Di(1) in the same manner as described above. 
If Di(1) .gtoreq.Do, the control means 11 controls the display unit 12 and 
the driving unit 13 in accordance with the signal Zi(1). When the first 
filter means 5 is selected, which of the first and second filter means is 
to be selected next may be determined in accordance with the drive method 
of the imaging optical system. A case of intermittent driving will be 
described with reference to a case wherein the imaging optical system 1 is 
stopped to perform operations for detecting the image displacement as 
described above, the imaging optical system is driven to move for a 
distance corresponding to the defocus amount calculated in accordance with 
these operations, operations for detecting the image displacement are 
performed again after the imaging optical system has been stopped, and the 
imaging optical system is driven again. In this case, if the effective 
signal Zi(1) is obtained after selection of the first filter means 5, the 
imaging optical system must have been driven in accordance with the 
operation results in accordance with this signal and the imaging optical 
system must have been driven near the in-focus position when the image 
displacement is detected next. Accordingly, it is suitable to select the 
second filter means 6 next. For this reason, in the case of intermittent 
driving of the imaging optical system, when the first filter means 5 is 
selected at one time, it is suitable to select the second filter means 6 
next irrespective of the values of the operation results Zi(1) and Di(1). 
On the other hand, if driving of the imaging optical system and focus 
detection are performed simultaneously, when conditions Di(1).gtoreq.Do 
and .vertline.Zi(1).vertline.&gt;Zo are satisfied, the first filter means 
continues to be selected next. If, on the other hand, these conditions are 
not satisfied, the second filter means is selected. More specifically, 
when the conditions Di(1).gtoreq.Do and 
.vertline.Zi(1).vertline..ltoreq.Zo are satisfied, the imaging optical 
system is near the in-focus position and the images are expected to 
include a large amount of high spatial frequency components. Therefore, it 
is better to select the second filter means. When the condition Di(1)&lt;Do 
is satisfied, depending upon the type of an object, the object may not 
contain low spatial frequency components and may only contain high spatial 
frequency components. Then, even if the result obtained with the first 
filter means indicates deficiency of information [i.e., Di(1)&lt;Do], it may 
be possible to perform detection [Di(2) .ltoreq.Do] if the second filter 
means is used. For this reason, it is suitable to switch to the second 
filter means in this case. In this manner, the control means 11 controls 
the filter selecting means 7 to select one of the first and second filter 
means 5 and 6. 
Accordingly, when the imaging optical system is near the in-focus position, 
the defocus amount is small, and the high spatial frequency components in 
the first and second object images are large, the second filter means 
which suppresses the low spatial frequency components is automatically 
selected. Then, high precision focus detection can be performed based on 
the high spatial frequency components without being adversely affected by 
the low spatial frequency components when the optical system is near the 
in-focus position. On the other hand, if the imaging optical system 1 is 
significantly spaced apart from the in-focus position, the high spatial 
frequency components are small, and the low spatial frequency components 
are large, the first filter means 5 which passes the low spatial frequency 
components is automatically selected and focus detection is performed 
based on the low spatial frequency components. 
The position of the filter selecting means 7 is not limited to that shown 
in the figure. For example, the filter selecting means 7 can also be 
inserted between the output terminal of the photoelectric unit 4 and the 
input terminals of the first and second filter means 5 and 6. 
Alternatively, the filter selecting means 7 can also be inserted after the 
operation means 10. In the latter case, a series of photoelectric outputs 
is processed by the first and second filter means 5 and 6. Outputs from 
the first and second filter means 5 and 6 are processed by the operation 
means 10 to calculate the focus detection signal Zi(1) based on the output 
from the first filter means 5 and the focus detection signal Zi(2) based 
on the output from the second filter means 6. Thereafter, the filter 
selecting means 7 selects one of the signals Zi(1) and Zi(2). In this 
manner, according to the present invention, the selection between the 
first and second filter means includes cases wherein one of the outputs 
from the first and second filter means which is to be supplied to the 
operation means 10 is selected, and one of the outputs from the operation 
means 10 which is based on the output from the first filter means and the 
output from the operation means 10 which is based on the output from the 
second filter means is selected. 
The configurations of the first and second filters 5 and 6 will now be 
described with reference to FIGS. 4B and 5B. 
Referring to FIG. 4B, delay circuits D1, D2 and D3 corresponding to one 
pixel are series-connected, and an adder T1 adds outputs from the delay 
circuits D1, D2 and D3. When the time-serial photoelectric outputs a1, b1, 
a2, b2, a3, b3, . . . , are sequentially supplied from the photoelectric 
device 4 to the delay circuits D1 to D3, the adder T1 produces sums 
(a1+a2), (b1+b2), (a2+a3), . . . . 
In the configuration as shown in FIG. 5B, differences (a1-a3), (b1-b3), . . 
. , of the outputs from first and fifth delay circuits D1 and D5 among 
series-connected delay circuits D1 to D5 corresponding to one pixel are 
calculated by a subtracter S1. When the outputs from the second 
photoelectric element array are read out after all the outputs from the 
first photoelectric element array are read out, that is, if the order of 
the time-serial photoelectric outputs from the photoelectric unit 4 is a1, 
a2, . . . , an, b1, b2, . . . , bn, the configurations of the first and 
second filter means 5 and 6 are as shown in FIGS. 4C and 5C, respectively. 
It is to be noted that instead of using filter means 5 and 6 which are 
constituted as hardware, a microcomputer or the like may be used to 
calculate (a1+a2), (a1-a3), . . . . 
As may be seen from a comparison of FIG. 4A showing the MTF characteristics 
of the first filter means 5 and FIG. 2 showing the MTF characteristics of 
the original signal, the first filter means 5 serves to suppress the high 
spatial frequency components. However, since the basic function of the 
first filter means 5 is to pass at least the low spatial frequency 
components, a filter having an infinite frequency passband may be used as 
the filter means 5. More specifically, the photoelectric outputs may be 
directly connected to the A/D converter 8 through the filter selecting 
means 7. In this case, the filter means 5 practically effects no filtering 
processing. However, in this specification, based upon the assumption that 
the first filter means has an MTF frequency band which is different from 
that of the second filter means, an imaginary filter means which effects 
no filtering processing is also included in the concept of a filter means. 
A case will now be described wherein the first filter means is an imaginary 
filter means as described above and the filtering processing of the second 
filter means is performed within the operation means 10. In this case, as 
described above, the photoelectric outputs are directly supplied to the 
A/D converter 8. The memory means 9 stores the data having information 
about the spatial frequency band as shown in FIG. 2. The operation means 
10 performs operations based on the data stored in the memory means 9 to 
calculate the focus detection signal Zi(1) and the information amount 
signal Di(1). If the condition Zi(1) &lt;Zo is satisfied, the operation means 
10 calculates every other differential data A1=a1-a3, . . . , 
Ai=ai-a(i+2), . . . , B1=b1-b3, . . . Bj=bj-b(j+2), . . . to thereby 
produce the filtered outputs A1, A2, . . . , A(n-1), B1, B2, . . . , 
B(n-1). These filtered outputs are subjected to operations again to 
calculate the focus detection signal Zi(2) and the information amount 
signal Di(2). When Zi(1).gtoreq.Zo, the control signal 11 controls the 
display unit 12 and the driving unit 13 based on the focus detection 
signal Zi(1). On the other hand, when Zi(1)&lt;Zo, the control means 11 
controls the display unit 12 and the driving unit 13 based on the focus 
detection signal Zi(2). For the next series of photoelectric outputs, the 
operation means 10 calculates the signals Zi(1) and Di(1) first in the 
manner as described above. When the signal Zi(1) is smaller than the 
predetermined value Zo, the filtering processing is performed to extract 
the high spatial frequency components, and the signals Zi(2) and Di(2) are 
calculated. 
As may be seen from the above description, various methods of selecting the 
filter means can be adopted. For example, as in the embodiment shown in 
FIG. 3, one of the first and second filter means is selected for each 
series of photoelectric outputs in accordance with the magnitudes of the 
signals Zi and Di. Alternatively, the second filter means can always be 
selected after the first filter means is selected. Still alternatively, as 
in the last case described above, the first filter means (the first filter 
means is an imaginary filter in the above case) is selected for each 
series of photoelectric outputs. The signal Zi(1) obtained with the thus 
selected first filter means determines whether or not the second filter 
means is to be selected. Irrespective of which one of these methods is 
selected, if the imaging optical system is near the in-focus position and 
a sufficient amount of high spatial frequency components is present, the 
display and driving units must be driven in accordance with the focus 
detection signal Zi(2) which is obtained with the filtered outputs from 
the second filter means. 
The MTF characteristics of the first and second filter means 5 and 6 are 
preferably selected such that the center frequency of the passband of the 
second filter means is set to be higher than that of the first filter 
means. The MTF characteristics of the first and second filter means 5 and 
6 which satisfy this condition may be those of the first embodiment shown 
in FIGS. 4A and 5A or those shown in FIGS. 6A to 6C. The MTF 
characteristics indicated by the solid curve in FIG. 6A are set to 
suppress high spatial frequency components and to pass relatively low 
spatial frequency components. The MTF characteristics indicated by the 
alternate long and short dashed line are set to pass high spatial 
frequency components. In this manner, since the first filter means 
sufficiently suppresses the high spatial frequency components, even if the 
object contains a relatively large amount of high spatial frequency 
components as compared to that of low spatial frequency components, a 
pseudo in-focus signal may not be generated when the first filter means is 
selected. The MTF characteristics of the first and second filter means 
shown in FIG. 6B resemble those shown in FIGS. 4A and 5A and differ 
therefrom in that the high spatial frequency components are further 
suppressed as compared to FIG. 4A. Then, the generation of a pseudo 
in-focus signal can be prevented as in the case of FIG. 6A. In this case, 
when the imaging optical system is near the in-focus position, the second 
filter means having the MTF characteristics as shown in FIG. 6B is used so 
as to eliminate the low spatial frequency components and to perform focus 
detection based on a signal mainly consisting of the high spatial 
frequency components. Accordingly, the focus detection precision is high. 
When the defocus amount is relatively large, the first filter means having 
the MTF characteristics as shown in FIG. 6B is used to eliminate the high 
spatial frequency components which may otherwise result in generation of a 
pseudo in-focus signal. Thus, defocus amount is detected based only on the 
low spatial frequency components. For this reason, even if the imaging 
optical system is significantly spaced apart from the in-focus position, a 
pseudo in-focus signal may not be generated and a front- or rear-focus 
state may be detected. In the MTF characteristics of the first filter 
means shown in FIG. 6C, of the low spatial frequency components, a zero 
frequency component, that is, a D.C. component is considerably suppressed 
and a peak value appears at a frequency slightly higher than the zero 
frequency. The MTF characteristics of the second filter means shown in 
FIG. 6C are set to have a peak value at a higher frequency than the first 
filter means. When both the first and second filter means have the 
function of suppressing the D.C. component, the following advantage is 
obtained. In some cases, there are differences between the sensitivity of 
the first photoelectric element array elements a1, . . . , an, the D.C. 
component of the photoelectric output therefrom, and those of the second 
photoelectric element array elements b1, . . . , bn. Such differences can 
lead to a degradation in the focus detection precision. However, when the 
D.C. component is suppressed, such an adverse effect of the differences 
can be eliminated. Such differences originate from the differences in the 
sensitivities of the arrays or differences in the temperature drift 
amounts of the arrays when the first and second photoelectric element 
arrays are formed on separate chips. 
In the above description, the passband of the MTF characteristics mainly 
involves the high spatial frequencies or involves the low spatial 
frequencies. However, the upper limit of the effective band is the Nyquist 
frequency determined by the sampling pitch of the data to be subjected to 
operations. Accordingly, the high or low spatial frequency components 
described above must fall within the frequency range below the Nyquist 
frequency. 
The second embodiment of the present invention will now be described. 
Referring to FIG. 7, a field lens 15 is arranged near the predetermined 
focal plane of an imaging optical system such as a phototaking lens. The 
field lens 15 has a rectangular light-transmitting region 15a at its 
center and a light-shielding region surrounding it. A substantially 
rectangular transparent block 16 consists of a material having a high 
refractive index such as glass or plastic. The first lens 15 is adhered on 
one end face 16a of the block 16. A pair of concave mirrors 17 and 18 are 
inclined in opposite directions and are arranged at the other end face 16b 
of the block 16. A pair of mirrors 19 and 20 are inclined at an angle of 
about 45.degree. and are disposed at a certain distance in the portion of 
the block 16 which is between the one and the other ends 16a and 16b. A 
photoelectric unit 21 is arranged below the transparent block 16. 
Photoelectric element arrays 22 and 23 are formed in the photoelectric 
unit 21 below the mirrors 19 and 20, respectively. 
Light rays which have passed through the imaging optical system 1 pass 
through the light-transmitting region 15a of the field lens 15, enter into 
the block 16, and become incident on the concave mirrors 17 and 18 through 
a gap between the mirrors 19 and 20. The concave mirror 17 reflects the 
incident light toward the mirror 19, while the concave mirror 18 reflects 
the incident light toward the mirror 20. The light reaches the 
photoelectric element arrays 22 and 23 through the mirrors 19 and 20, 
respectively. In this manner, a pair of object images for the object are 
formed on the arrays 22 and 23, respectively. 
Circuitry for processing the photoelectric outputs from the photoelectric 
device 21 will now be described with reference to FIG. 8. 
Referring to FIG. 8, the photoelectric device 21 comprises a CCD image 
sensor which includes the first photoelectric element array 22, the second 
photoelectric element array 23, a transfer gate 24, and shift registers 25 
and 26. The photoelectric device 21 may also have other parts such as 
amplifiers for linearly or logarithmically amplifying the photoelectric 
outputs. It is to be noted that the photoelectric device 21 may 
alternatively comprise a MOS image sensor or the like. Photoelectric 
elements a1, . . . , an constituting the first photoelectric element array 
22 are arrayed in close succession at a pitch Po. The second photoelectric 
element array 23 has the same configuration. Photoelectric outputs a1, . . 
. , an from the first photoelectric element array 22 and photoelectric 
outputs b1, . . . , bn from the second photoelectric element array 23 are 
alternately produced as in the case of the first embodiment. A series of 
photoelectric outputs a1, b1, a2, b2, . . . , an, bn is repeatedly 
produced at a predetermined period. The first and second photoelectric 
element arrays 22 and 23 as described above have the MTF characteristics 
as shown in FIG. 9A. The input terminals of first and second filter means 
27 and 28 are connected to the output terminal of the photoelectric device 
21. The first filter means 27 has the MTF characteristics as shown in FIG. 
9B wherein the low spatial frequency components are passed while the high 
spatial frequency components near the frequency 1/8Po are sufficiently 
suppressed. Meanwhile, the second filter means 28 has the MTF 
characteristics as shown in FIG. 9C wherein the low spatial frequency 
components are passed while the high spatial frequency components near a 
frequency 1/4Po are sufficiently suppressed. Thus, the second filter means 
28 is determined to pass spatial frequency components higher than those 
passed by the first filter means 27. A filter selecting means 29 used in 
the second embodiment is of the same configuration as that used in the 
first embodiment; it selects one of the outputs from the first and second 
filter means 27 and 28 and supplies the selected output to a sample & hold 
means 30. The sample & hold means 30 comprises a first sample & hold 
circuit 30A and a second sample & hold circuit 30B connected in series 
with each other. An A/D converter 31 performs the A/D conversion of the 
output from the sample & hold circuit 30B. A memory 32 and an operation 
means 33 are of the same configuration as those of the first embodiment 
described above. A discrimination means 34 receives a focus detection 
signal Zi and an information amount signal Di from the operation means 33. 
If an information amount signal Di(1) or Di(2) which is obtained when the 
first or second filter means 27 or 28 is selected is smaller than a 
predetermined value Do(1) or Do(2), respectively, the discrimination means 
34 selectively produces, at an output terminal 34a, first and second 
filter selection signals irrespective of the focus detection signal Zi. 
More specifically, if the filter selecting means 29 is currently selecting 
the first filter means 27, the discrimination means 34 produces a second 
filter selection signal (H level signal) for selecting the second filter 
means 28. On the other hand, if the filter selecting means 29 is currently 
selecting the second filter means 28, the discrimination means 34 produces 
a first filter selection signal (L level signal) for selecting the first 
filter means 27. If the information amount signal Di(1) or Di(2) is equal 
to or larger than a corresponding predetermined value Do(1) or Do(2), 
respectively, and the absolute value of the focus detection signal Zi(1) 
or Zi(2) is larger than the corresponding predetermined value Zo(1) or 
Zo(2), respectively, the discrimination means 34 produces the first filter 
selection signal at an output terminal 34a. When the absolute value of the 
focus detection signal Zi(1) or Zi(2) is smaller than the corresponding 
predetermined value Zo(1) or Zo(2), respectively, the discrimination means 
34 produces the second filter selection signal at the output terminal 34a. 
When the information amount signal Di(1) or Di(2) is larger than the 
corresponding predetermined value Do(1) or Do(2), it produces a memory 
update signal at the output terminal 34b. In response to the memory update 
signal, a memory 35 stores the current focus detection signal Zi. In 
accordance with the focus detection signal Zi stored in the memory 35, a 
display unit 36 displays the focusing state and a driving unit 37 drives 
the imaging optical system 1 toward the in-focus position. A sampling 
pulse generating circuit 38 is connected to the output terminal 34a of the 
discrimination means 34 to supply a sampling pulse for starting sample & 
hold of the sample & hold circuits 30A and 30B. The sampling pulse period 
or sampling pitch changes in accordance with the output from the 
discrimination means 34. When the discrimination means 34 produces a first 
filter selection signal, the sampling period is larger than (twice in this 
embodiment) that of a case wherein the discrimination means 34 produces a 
second filter selection signal. Upon reception of a start signal or an H 
level signal from an output terminal 39a of a first counter 39, the 
sampling pulse generating circuit 39 starts generating sampling pulses. In 
response to an end signal or an H level signal from an output terminal 40a 
of a second counter 40, the sampling pulse generating circuit 38 stops 
generating the sampling pulses. The first counter 39 is a presettable 
counter; a preset value is set in the first counter 39 from a setting 
circuit 41 through a gate means 42. The first counter 39 down-counts the 
pulses from an AND gate 43. When the count of the first counter 39 becomes 
zero, it produces a start signal or an H level signal. The second counter 
40 is also a presettable counter; a preset value is set in the second 
counter 40 from the setting circuit 41 through a gate means 44. The second 
counter 40 down-counts the sampling pulses supplied to the sample & hold 
circuit 30B. When the count of the second counter 40 reaches zero, the 
counter 40 supplies an end signal or an H level signal. In the setting 
circuit 40, there are preset the first preset value for the first counter 
and the first preset value for the second counter which are used when the 
first filter means 27 is selected, and the second preset value for the 
first counter and the second preset value for the second counter which are 
used when the second filter means 28 is selected. The first-counter first 
preset value and the second-counter first preset value are produced at 
output terminals 41a and 41c, respectively. The first-counter second 
preset value and the second-counter second preset value are produced from 
output terminals 41b and 41d, respectively. In this embodiment, the first 
preset value which appears at the output terminal 41a is set to be smaller 
than the second preset value which appears at the output terminal 41b. The 
first preset value which appears at the output terminal 41c is set to be 
equal to the second preset value which appears at the output terminal 41d. 
An input terminal 45 receives an H level signal from a sequence controller 
(not shown) in synchronism with start of transfer of a series of 
photoelectric outputs a1, b1, . . . , an, bn from the photoelectric unit 
21. This signal is reset at L level at a suitable timing and has a 
duration from the end of sample & hold of the data to setting of the next 
preset values in the presettable counters 39 and 40. An input terminal 46 
receives a clock synchronous with a transfer clock for the series of the 
photoelectric outputs described above. 
The mode of operation will now be described. 
Assume that the discrimination means 34 produces an L level signal as first 
filter selection signal at the output terminal 34a. In response to this 
selection signal, the filter selecting means 29 selects the first filter 
means 27. The gate means 42 and 44 respectively pass the first-counter 
preset value and the second-counter preset value from the output terminals 
41a and 41c, respectively, of the setting circuit 41 to set these preset 
values in the respective counters 39 and 40. Thereafter, in response to a 
signal from the sequence controller (not shown), a series of photoelectric 
outputs a1, b1, a2, b2, . . . , an, bn is read out from the photoelectric 
unit 21. Of these photoelectric outputs, the photoelectric outputs a1, a2, 
. . . , an from the first photoelectric element array 22 are shown in FIG. 
10A. The series of photoelectric outputs a1, b1, . . . , an, bn is 
filtered by the first filter means 27 and converted into filtered outputs 
A1, B1, . . . , An, Bn as shown in FIG. 11A. Of the filtered outputs A1, 
B1, An, Bn, those associated with the first photoelectric element array 
(which will be described as the a series hereinafter) A1, . . . , An are 
shown in FIG. 10B. It is apparent from a comparison of FIGS. 10A and 10B 
that good suppression of the high spatial frequency components is obtained 
by the first filter means 27. Meanwhile since an H level signal is 
supplied to the input terminal 45 synchronously with readout of the 
photoelectric outputs from the photoelectric unit 21, transfer clocks from 
the input terminal 46 pass through an AND gate 43. The first counter 39 
down-counts the transfer clocks from the preset value therein. When the 
number of input clocks reaches the preset value (when the count reaches 
zero), the first counter 39 produces an H level signal as a start signal. 
The start signal is supplied to the sampling pulse generating circuit 38, 
and is inverted and supplied to the AND gate 43 to close its gate. In 
response to the start signal, the sampling pulse generating circuit 38 
generates first and second sampling pulses SP1 and SP2 as shown in FIGS. 
11B and 11C, respectively, to the first and second sample & hold circuits 
30A and 30B, respectively. In response to the pulse SP1, the first sample 
& hold circuit 30A samples outputs A4, B4, A8, B8, A12, B12, . . . from 
the filtered outputs A1, B1, . . . , An, Bn. The first sample & hold 
circuit 30A holds the a series outputs A4, A8, . . . for a short period of 
time and b series outputs B4, B8, . . . associated with the second 
photoelectric element array for a long period of time, as shown by a range 
indicated by arrows in FIG. 11B. In order to render the hold times of the 
two series of outputs equal to each other, the second sample & hold 
circuit 30B samples and holds the output from the first sample & hold 
circuit 30A in response to the second sampling pulse SP2 as shown in FIG. 
11C. The second counter 40 counts the second sampling pulses SP2. When the 
number of the second sampling pulses SP2 received reaches the first preset 
value, the second counter 40 supplies an end signal to stop generation of 
the first and second sampling pulses SP1 and SP2. 
Referring to FIG. 10B, of the a series filtered outputs A1, . . . , An, 
sampled, filtered outputs A4, A8, A12, . . . are indicated by marks Ms 
thereunder. As may be seen from this figure, a distribution range l2 (to 
be referred to as a sampling region hereinafter) of the sampled, filtered 
outputs shares most of the range of the filtered outputs A1 . . . , An. 
The A/D converter 31 performs A/D conversion of the output from the second 
sample & hold circuit 30B and supplies the digital signals obtained to the 
memory 32. The second sample & hold circuit 30B is incorporated herein for 
the following reason. If the output from the second sample & hold circuit 
30A is directly A/D converted, the holding time by the first sample & hold 
circuit 30A of the outputs B4, B8, . . . is shorter than that of the 
outputs A4, A8, . . . . Accordingly, an expensive high-speed A/D converter 
must be used as the converter 31 such that the A/D conversion can be 
completed within the shorter period of time of the two holding times. Even 
if such a high-speed A/D converter is used, the effect of high-speed 
conversion is not important in the case of A/D conversion of the outputs 
B4, B8, . . . which are held for a long period of time. However, this 
problem is resolved when the second sample & hold circuit 30B is used. As 
shown in FIGS. 11D and 11E, the sampling period obtained when the second 
filter means is selected is smaller than that obtained when the first 
filter means is selected. In other words, the holding time of the second 
sample & hold circuit 30B when the second filter means is selected is 
shorter than that of the circuit 30B when the first filter means is 
selected. Accordingly, time required for A/D conversion by the A/D 
converter 31 is determined by the holding time obtained when the second 
filter means is selected. This means that the holding time of the second 
sample & hold circuit 30B becomes longer than the A/D conversion time when 
the first filter means is selected. In order to solve this problem, the 
frequency of the transfer clocks for the photoelectric outputs obtained 
when the first filter means is selected can be selected to be larger than 
that obtained when the second filter means is selected. The holding time 
of the second sample & hold circuit 30B is thus the same when either of 
the first and second filter means is selected. 
The operation means 33 performs operations based on the filtered outputs 
stored in the memory 32 and produces a focus detection signal Zi(1) and an 
information amount signal Di(1). The discrimination means 34 compares the 
signals Zi(1) and Di(1) with the corresponding predetermined values Zo(1) 
and Do(1). (a) When Di(1) is equal to or larger than Do(1) 
In this case, the discrimination means 34 generates a memory update signal 
at the output terminal 34b so that the current focus detection signal 
Zi(1) is stored in the memory 35. The display unit 36 and the driving unit 
37 perform the display operation and the driving operation of the imaging 
optical system 1 based on the stored signal Zi(1). When the signal Zi(1) 
is equal to or larger than the predetermined value Zo(1), the 
discrimination means 34 continues to produce a first filter selection 
signal. Accordingly, when the photoelectric device 21 produces the series 
of photoelectric outputs a1, b1, . . . , an, bn, the overall circuitry 
performs the operation as described above. 
When the signal Zi(1) is smaller than the predetermined value Zo(1), the 
discrimination means 34 produces an H level signal as a second filter 
selection signal at the output terminal 34a. In response to the second 
filter selection signal, the filter selecting means 29 selects the second 
filter means 29. The gate means 42 and 44 transmit the first- and 
second-counter second preset values from the setting circuit 41 to the 
first and second counters 39 and 40, respectively. Thereafter, the series 
of photoelectric outputs a1, b1, . . . , an, bn read out from the 
photoelectric device 21 is filtered by the second filter means 28 and 
converted into the outputs A1, B1, . . . , An, Bn. FIG. 10C shows the a 
series filtered outputs A1, A2, . . . , An. It is seen from a comparison 
of FIGS. 10C and 10B that the waveform shown in FIG. 10C is less smooth, 
i.e., the second filter means 28 passes a larger amount of high spatial 
frequency components than the first filter means 27. Meanwhile, in 
response to readout of the photoelectric outputs, the first counter 39 
counts the transfer clocks from the AND gate 43. When the count of the 
first counter 39 reaches the second preset value, the counter 39 produces 
a start signal. Since the first-counter second preset value when the 
second filter means is selected is set to be larger than the first-counter 
first preset value when the first filter means is selected, the time when 
the start signal is generated is lagged from that obtained when the first 
filter means selected. In response to the start signal, the sampling pulse 
generating circuit 38 genreates the first and second sampling pulses SP3 
and SP4 as shown in FIGS. 11D and 11E, respectively. In accordance with 
the second filter selection signal supplied from the discrimination means 
34, the periods of the sampling pulses SP3 and SP4 are selected to be 
shorter than (1/2 times in this embodiment) those of the sampling pulses 
SP1 and SP2 which are used when the first filter means is selected. As 
shown in FIGS. 11D and 11E, the first and second sample & hold circuits 
30A and 30B sample the filtered outputs A1, B1, . . . , An, Bn at the 
period which is 1/2 times that when the first filter means is selected. 
The circuits 30A and 30B then produce outputs A8, B8, A10, B10, A12, B12, 
. . . . The second counter 40 counts the second sampling pulses SP4. When 
the count of the pulses SP4 reaches the second preset value, the second 
counter 40 generates an end signal to stop the generation of the sampling 
pulses SP3 and SP4. Since the second-counter second preset value is set 
equal to the second-counter first preset value obtained when the first 
filter means is selected, the number of sampled, filtered outputs A8, B8, 
A10, B10, . . . is equal to that obtained when the first filter means is 
selected. 
Of the filtered outputs which are sampled in this manner, those which are 
associated with the first photoelectric element array are indicated with 
marks Ms in FIG. 10C. In this embodiment, the sampling period and sampling 
number when the first filter means is selected are twice or the same as 
those when the second filter means is selected respectively. Accordingly, 
a sampling region l1 shown in FIG. 10B is twice the sampling region l2 
shown in FIG. 10C. Note that the sampling numbers when the first and 
second filter means are selected respectively need not be the same. In the 
graphs shown in FIGS. 11D and 11E, the sampling start timings are shown to 
be earlier than the actual timings for the sake of ease in drafting these 
graphs. 
The sampled outputs are supplied to the operation means 33 through the A/D 
converter 31 and the memory 32. Since the filtered outputs contain a 
larger amount of high spatial frequency components than that obtained when 
the first filter means is selected, the focus detection signal Zi(2) has a 
higher precision at proximity of the in-focus position. When the 
conditions of Di(2).gtoreq.Do(2) and 
.vertline.Zi(2).vertline..ltoreq.Zo(2) are satisfied, the discrimination 
means 34 continues to produce a second filter selection signal at the 
output terminal 34a and produces a memory update signal at the output 
terminal 34b. Then, the current focus detection signal Zi(2) is stored in 
the memory 35. The display and imaging optical system driving are 
performed in accordance with the focus detection signal thus stored in the 
memory 35. However, when the conditions Di(2).gtoreq.Do(2) and 
.vertline.Zi(2).vertline.&gt;Zo(2) are satisfied, the discrimination means 34 
produces a first filter selection signal. 
(b) When Di(1) or Di(2) is smaller than Do(1) or Do(2), respectively 
In this case, independently of the focus detection signal Zi, the 
discrimination means 34 produces at the output terminal 34a a second 
filter selection signal if the first filter means is currently selected, 
and a first filter selection signal if the second filter means is 
currently selected. Then, the filter selecting means 29 selects the 
corresponding filter means. Since the focus detection signal Zi obtained 
when the signal Di is smaller than Do has a very low precision, the 
discrimination means 34 does not produce a memory update signal in this 
case. Accordingly, this signal Zi is not used for display and imaging 
optical system driving operations. Selection of the filter means 
independently of the signal Zi is performed for the following reason. For 
example, if an object does not contain a substantial amount of low spatial 
frequency components but contains a large amount of high spatial frequency 
components, or vice versa, necessary information can be obtained if the 
second filter means 28 or the first filter means 27 is selected. 
In this embodiment, when the first filter means is selected, in other 
words, when the relative displacement between the first and second object 
images on the first and second photoelectric element arrays is great, the 
sampling region l1 is selected to be wide as shown in FIG. 10B. On the 
other hand, when the second filter means is selected, in other words, the 
displacement as described above is small, the sampling region l2 is set to 
be narrow as shown in FIG. 10C. This is effective for precise focus 
detection. To explain it in more detail, if the sampling region is wide, 
the displacement of the object images can be detected even if the object 
images are relatively deviated from each other significantly. This means 
that the defocus amount can be detected even if the photo-taking lens is 
significantly spaced apart from the in-focus position. On the other hand, 
if the sampling region is wide, objects at different distances or an 
object of a great depth can enter into the sampling region. Focus 
detection when the defocus amount is great need only be discrimination 
between a front- or rear-focus state or rough determination of the defocus 
amount; the absolute value of the defocus amount need not be determined 
precisely. For this reason, an object of a relatively great depth or the 
like can enter the sampling region in this case of great defocus amount. 
However, if the defocus amount is small and its absolute value must be 
determined precisely, the presence of an object of a great depth or the 
like can lead to a big error in focus detection. Therefore, focus 
detection precision can be improved when the second filter means is used, 
and when the sampling region is narrowed so that a possibility of an 
object of a great depth entering into the sampling region is reduced to 
the minimum. 
In general, even if the sampling region is widened, the sampling period 
need not be increased. For example, the sampling region l1 associated with 
the output shown in FIG. 10B can be smaller than the sampling pitch 4Po; 
sampling can be performed at a pitch Po or 2Po. However, when the sampling 
pitch is reduced to Po or 2Po, the number of sampled data becomes four 
times or twice. This leads to a larger capacity of the memory 32 and a 
larger amount of operations to be performed by the operation means 32, 
which is not preferable. Accordingly, it is extremely advantageous to keep 
the sampling number the same even if the sampling region is changed as in 
this embodiment. 
Thus, when the first filter means which passes only the low spatial 
frequency components is selected, the sampling period is set to be 4Po. 
However, when the second filter means which passes the high spatial 
frequency components is selected, the sampling pitch can be reduced to 
2Po. This is very advantageous for effective use of information. The 
relationship between the sampling pitch and the filter MTF characteristics 
will now be described in detail. Since the sampling pitch is 4Po when the 
first filter means is selected, the corresponding Nyquist frequency is 
1/8Po. Since the frequency components above the frequency 1/8Po can lead 
to an erroneous operation as per sampling theorem, such components are 
preferably eliminated. As shown in FIG. 9B, the MTF characteristics of the 
first filter means are set such that the components around and above the 
Nyquist frequency 1/8Po are sufficiently suppressed and the components 
below this frequency are passed. Accordingly, the components passed 
through the first filter means can be effectively utilized. However, if 
the sampling pitch is set to be Po when the first filter means is 
selected, the Nyquist frequency becomes 1/2Po so that the spatial 
frequency components below this frequency can also be used for focus 
detection. However, as shown in FIG. 9B, the components above the 
frequency 1/8Po are eliminated by the first filter means. Accordingly, 
even if the sampling pitch is Po and the sampling number is four times 
that when the sampling pitch is 4Po, the amount of the spatial frequency 
components available for operation remains the same and an increase in the 
sampling number as described above does not provide any better result. As 
may be seen from the above description, the sampling pitch is preferably 
set such that the Nyquist frequency which is determined by this sampling 
pitch is near the limit defining the MTF frequency band of the filter 
means. 
A transversal filter as an example of the filter means 27 and 28 shown in 
FIG. 8 will now be described. 
FIG. 12A shows a transversal filter which sequentially receives data or 
photoelectric outputs a1, b1, a2, b2, . . . , an, bn. Delay circuits D1 to 
Dm corresponding to one pixel are connected in series with each other. 
Multipliers W1 to Ws are respectively connected to the delay circuits D1, 
D3, D5, . . . , Dm through amplifiers Am, respectively. These multipliers 
W1 to Ws multiply the inputs with corresponding weighting coefficients W1 
to Ws. The weighting coefficients may be positive or negative values. An 
adder T2 adds the outputs from the respective multipliers. A series of 
data a1, b1, a2, b2, . . . , an, bn from the photoelectric device 21 are 
supplied to this filter. When the first data a1 reaches the delay circuit 
D1, the adder T2 produces a1.W1+a2.W2 +, . . . , +am.Wm as the filtered 
outputs. When the second data b1 reaches the delay circuit D1, the adder 
T2 produces b1.W1+b2.W2+, . . . ,+bm.Wm. Likewise, as the data is 
sequentially shifted, the adder T2 alternately produces the a and b series 
filtered outputs. 
FIG. 12B shows a transversal filter which sequentially receives the data in 
the order of a1, a2, . . . , an, b1, b2, . . . , bn. 
Filtering wherein date of a series of data are multiplied with 
corresponding weighting coefficients and the obtained products are summed 
will be referred to as weighting/adding, and a filter for performing such 
weighting/additing will be referred to as a weighting/adding filter. 
The weighting coefficients W1 to Ws for providing predetermined MTF 
characteristics may be determined in various manners and are not limited 
to a single combination. Some examples will be described below. 
In order to realize a filter means having the MTF characteristics as shown 
in FIG. 9C, Ws is selected to be W5, and the weighting coefficients W1 to 
W5 are selected such that the relative magnitudes are as shown in FIG. 
13A. As an example, the weighting coefficients may be determined such that 
W1=0.28, W2=0.76, W3=1, W4=0.76, and W5=0.28. Similarly, in order to 
implement a filter means having the MTF characteristics as shown in FIG. 
9B, Ws is determined to be W9, and the weighting coefficients W1 to W9 are 
determined as shown in FIG. 13B. As an example, the weighting coefficients 
may be determined such that W1=0.28, W2=0.52, W3=0.76, W4=0.94, W5=1, 
W6=0.94, W7=0.76, W8=0.52, and W9=0.28. The weighting coefficients as 
shown in FIGS. 13C and 13D may be used to obtain the MTF characteristics 
as shown in FIG. 9D, the weighting coefficients as shown in FIGS. 13E and 
13F may be used to obtain the MTF characteristics indicated by the dotted 
curve e1 and the solid curve e2 shown in FIG. 9E, and the weighting 
coefficients as shown in FIG. 13G may be used to obtain the MTF 
characteristics as shown in FIG. 9F. 
A combination of the first and second filter means can be obtained by 
suitably combining the MTF characteristics as shown in FIGS. 13A to 13G. 
FIG. 14 shows the configuration of the discrimination means 34 shown in 
FIG. 8. 
Referring to FIG. 14A, first and second memories 340 and 341 supply 
predetermined preset values Do(1) and Do(2), and Zo(1) and Zo(2) to 
comparators 344 and 345 through gate means 342 and 343, respectively. The 
comparator 344 compares one of the outputs Do(1) and Do(2) from the memory 
340 which is selected by the gate means 342 with an information amount 
signal Di from the operation means 33. Similarly, the comparator 345 
compares one of the outputs Zo(1) and Zo(2) from the memory 341 which is 
selected by the gate means 343 with a focus detection signal Zi. A gate 
means 346 receives an output .alpha. from the comparator 344, an output 
.beta. from the comparator 345, and an output .gamma. from the 
discrimination means 34. The configuration of the gate means 346 is shown 
in FIG. 14B. In response to a clock pulse from a terminal 348 after the 
generation of the outputs .alpha. and .beta., a D-type flip-flop 347 
stores an output .delta. from the gate means 346. An updated output from 
the flip-flop 347 is used as an output from the discrimination means 34. 
The operation of the discrimination means 34 is shown in Table below. 
TABLE 
______________________________________ 
.alpha. H H L L H H L L 
.beta. H L H L H L H L 
.gamma. H H H H L L L L 
.delta. L H L L L H H H 
______________________________________ 
.alpha. = L for Di(1) &lt; Do(1) or Di(2) &lt; Do(2) 
.alpha. = H for Di(1) .gtoreq. Do(1) or Di(2) .gtoreq. Do(2) 
.beta. = L for .vertline.Zi(1).vertline. &lt; Zo(1) or 
.vertline.Zi(2).vertline. &lt; Zo(2) 
.beta. = H for other cases 
In the description of the second embodiment, a plurality of filter means is 
switched, the sampling region l1 or l2 is switched, and the sampling pitch 
is switched in accordance with the magnitude of the defocus amount Zi. 
However, a quite good effect may be obtained even if the sampling region 
and the sampling pitch alone are switched in accordance with the defocus 
amount Zi. For example, a filter having the MTF characteristics as shown 
in FIG. 9C is used as the filter means. When the optical system is near 
the in-focus position, operations are performed to calculate the defocus 
amount based on the data which is sampled within the sampling region l2 at 
the sampling pitch 2Po as shown in FIG. 10C. However, when the optical 
system is not near the in-focus position, although the same filter means 
is used, the operations are performed based on the data which is sampled 
within the sampling region l1 at the sampling pitch 4Po as shown in FIG. 
10B. In this case, as compared with a case wherein the MTF characteristics 
of the filter means are switched to those shown in FIG. 9B, some 
components of frequencies higher than the Nyquist frequency are extracted. 
For this reason, an erroneous operation tends to be caused, and the 
possibility of generation of a pseudo in-focus signal is increased due to 
the presence of the high spatial frequency components. However, a quite 
good effect is obtained since blurring of the optical images deceases high 
spatial frequency components significantly when the defocus amount is 
great. When a single filter is used, it is not limited to one which has 
the MTF characteristics as shown in FIG. 9C but may be other filters such 
as one having the characteristics indicated by the dotted curve e2 shown 
in FIG. 9E. 
In both of the first and second embodiments, one of the focus detection 
signals based on the output from the first filter means and the output 
from the second filter means is selected in accordance with the defocus 
amount. A third embodiment of the present invention will now be described 
wherein the respective focus detection signals are combined in a 
predetermined relationship instead of using one selected output. 
Referring to FIG. 15, a series of photoelectric outputs a1, b1, a2, b2, . . 
. , an, bn are supplied to a first filter means 52 through a delay means 
51 and to a second filter means 53 directly. The photoelectric device 50 
is of the same configuration as that used in the first and second 
embodiments described above. The first and second filter means 52 and 53 
are also of the same configuration as those used in the first and second 
embodiments. The delay time of the delay means 51 is set such that after 
the filtered output from the second filter means 53 is completely supplied 
to a sample & hold means 54, the filtered output from the first filter 
means 52 is supplied to the means 54. It is to be noted that the delay 
means 51 can alternatively be arranged at the side of the second filter 
means 53. The sample & hold means 54, and subsequent A/D converter 55, 
memory 56, and operation means 57 are of the same configurations as those 
used in the second embodiment. The operation means 57 first performs the 
operations based on the filtered output from the second filter means 53 to 
produce signals Di(2) and Zi(2) and then performs the operations based on 
the filtered output from the first filter means 52 to produce signals 
Di(1) and Zi(1). A memory 58 stores all of the signals Di(1), Di(2), Zi(1) 
and Zi(2) from the operation means 57. A combining means 59 receives the 
signals from the memory 58 and produces an output Z which is obtained by 
combining the signals Zi(1) and Zi(2) in a predetermined relationship. 
More specifically, the combining means 59 performs a calculation 
Z=(1-.alpha.)Zi(1)+.alpha.Zi(2) where a weighting coefficient .alpha. is a 
number falling within a range of 0 to 1 and is determined in accordance 
with the magnitudes of the signals Zi(1), Zi(2), Di(1), and Di(2). The 
value of the coefficient .alpha. is determined in the following manner. 
When the signal Zi(1) or Zi(2) is small, i.e., when the imaging optical 
system is near the in-focus position, the coefficient .alpha. is set to be 
1 or a value close to 1 such that the signal Zi(2) based on the output 
from the second filter means 53 is enhanced. On the other hand, when the 
signal Zi(1) or Zi(2) is sufficiently large, the coefficient .alpha. is 
set to be zero or a value close to zero so that the signal Zi(1) based on 
the output from the first filter means 52 is enhanced. When the signal 
Di(2) is very small while the optical system is near the in-focus 
position, the signal Zi(2) has only a low precision. Accordingly, if the 
signal Di(1) is large, the weighting coefficient for the signal Zi(1) is 
increased. On the other hand, when the signal Di(1) is very small and the 
signal Di(2) is large even if the optical system is not near the in-focus 
position, the weighting coefficient for the signal Zi(2) is increased. 
A memory 60 stores the output Z obtained when at least one of the signals 
Di(1) and Di(2) has exceeded the predetermined value Do(1) or Do(2), 
respectively. The display and imaging optical system driving operations 
are performed in accordance with the output from the memory 60, as in the 
case of the first and second embodiments described above. A sample & hold 
means 61 is of the same configuration as that shown in FIG. 8. 
The weighting coefficient series of a weighting/adding filter as shown in 
FIGS. 13E, 13F and 13G is disadvantageous in that the number of individual 
weighting coefficients becomes so large that the weighting/adding filter 
becomes complex in structure or weighting/adding processing requires a 
long time. This will be described with reference to a fourth embodiment of 
the present invention below. 
Referring to FIG. 16, a photoelectric device 61 is of the same 
configuration as the photoelectric device 21 shown in FIG. 8, and has a 
pair of photoelectric element arrays 62 and 63. The array 62 produces an a 
series of data a1, a2, . . . , an, and the array 63 produces a b series of 
data b1, b2, . . . , bn. FIG. 17A shows the MTF characteristics of the 
arrays 62 and 63. 
The a series primary data ai and the b series primary data bi from the 
photoelectric device 61 are logarithmically amplified at 64 and are then 
supplied to a forward positioned filter means 65. The forward positioned 
filter means 65 comprises a hardware item, for example, a transversal 
filter as shown in FIGS. 12A or 12B. Weighting coefficients W1 to W5 of 
the prefilter means are, for example, 0.28, 0.76, 1, 0.76, and 0.28, 
respectively, as shown in FIG. 17B-2. The MTF characteristics of a 
weighting/adding filter having this weighting coefficient series (0.28, 
0.76, 1, 0.76, 0.28) are as shown in FIG. 17B-1. According to these MTF 
characteristics, the D.C. and low frequency components are sufficiently 
extracted and the frequency components above the frequency 1/4Po are 
eliminated. In other words, the spatial frequency components above the 
Nyquist frequency f.sub.N =1/2P determined by a sampling pitch P =2Po are 
eliminated. 
The filtered data from the forward positioned filter means 65 is sampled 
and held at a sampling period or pitch P =2Po by a sample & hold means 66. 
In this manner, the a and b series primary data ai and bi of the spatial 
pitch Po are converted into a and b series secondary data Ai and Bi having 
the spatial pitch 2Po through the forward positioned filter means and the 
sample & hold means. Since the secondary data Ai and Bi have the pitch 
2Po, the number of the secondary data is one half of the primary data ai 
and bi, respectively. 
The secondary data Ai and Bi are subjected to A/D conversion by an A/D 
converter 67 and supplied to a microcomputer 68 to be stored in a memory 
69 therein. The CPU (Central Processing Unit) of the microcomputer 68 has 
functions of filtering, displacement operation and so on. In order to 
visually present these functions, imaginary blocks are included in the 
block 68. Thus, the block 68 includes a rearward positioned filter section 
70 for performing weighting/adding filtering and a displacement operation 
part 71 for performing displacement operations. The rearward positioned 
filter section 70 subjects the secondary data Ai and Bi in the memory 69 
to filtering/adding processing using the weighting coefficient series 
(-0.25, 1, -0.25) having the spatial pitch 4Po shown in FIG. 17C-3 or the 
weighting coefficient series (-0.5, 1, -0.5) having the spatial pitch 4Po 
shown in FIG. 17D-3 so as to provide tertiary data. More specifically, 
since the secondary data Ai and Bi have the spatial pitch P =2Po, the 
rearward positioned filter section 70 multiplies the five consecutive 
secondary data (FIG. 17C-3 or 17D-3) with the weighting coefficient series 
(-0.25, 0, 1, 0, -0.25) or (-0,5, 0, 1, 0, -0.5). 
The tertiary data has been thus subjected to the weighting/adding filtering 
processing by both the forward positioned filter means 65 and the rearward 
positioned filter section 70. FIG. 17C-1 shows the synthetic MTF 
characteristics of the two filters having the weighting coefficient series 
shown in FIGS. 17B-2 and 17C-3, and FIG. 17D-1 similarly shows the 
synthetic MTF characteristics of the filters having the weighting 
coefficient series shown in FIGS. 17B-2 and 17D-3. 
The displacement operation part 71 calculates the relative displacement 
between the object images on the arrays 62 and 63 from the a and b series 
secondary data or the tertiary data. The calculated displacement is 
supplied to a display unit 72 for displaying the focusing state and to a 
focusing unit 73 for performing focusing of the phototaking lens. 
Before describing the operation of the circuitry, the relationship between 
the spatial frequency components and the operation results will be 
described. 
In general, when the data having the sampling pitch P is subjected to 
operations to detect a displacement, the spatial frequency components 
above the Nyquist frequency f.sub.N =1/2P determined by the sampling pitch 
P may result in an erratic detection. However, the spatial frequency 
component around a frequency f.sub.N /2 which is half the Nyquist 
frequency is extremely effective for providing high-precision operation 
results. Precision is lowered as the frequency decreases from f.sub.N /2. 
The mode of operation of the circuitry shown in FIG. 16 will now be 
described below. 
The primary data ai and bi having the spatial pitch Po and time-serially 
produced from the photoelectric device 61 are sequentially passed through 
a non-linearizing means 64 and the forward positioned filter means 65 and 
are then sampled by the sample & hold means 66 at the pitch P=2Po to be 
converted into the secondary data Ai and Bi having the spatial pitch 2Po. 
The secondary data Ai and Bi are then subjected to A/D conversion and are 
supplied to the microcomputer 68. As shown in FIG. 18, the microcomputer 
68 stores the input secondary data Ai and Bi in a memory region or area 
(1) in step [1]. In step [2], the secondary data Ai and Bi stored are 
subjected to operations to calculate a value Lm associated with the 
displacement of the object images. The secondary data Ai and Bi have been 
subjected to the filtering processing by the forward positioned filter 
means 65 having the MTF characteristics as shown in FIG. 17B-1 and the 
non-desirable spatial frequency component above the Nyquist frequency 
f.sub.N =1/4Po has been eliminated. Furthermore, since the secondary data 
Ai and Bi may contain a D.C. component, even if the displacement is 
relatively large and the object images are blurred, a displacement 
detection can be performed with a satisfactory precision. 
In step [4], it is discriminated if the operation result Lm indicates that 
the photographic lens is near the focal plane, that is, if the value Lm is 
below a predetermined value. If NO in step [4], the flow immediately 
advances to step [8] wherein the defocus amount is calculated in 
accordance with the operation result Lm. On the other hand, if YES in step 
[4], there is a great possibility that the object images contain high 
spatial frequency components near the frequency f.sub.N /2. In this case, 
a higher detection precision is obtained if the operations are performed 
based on the information which contains only high spatial frequency. Thus, 
the flow advances to step [5] in order to perform the displacement 
operation based on the high spatial frequency components. In step [5], the 
secondary data Ai and Bi are subjected to the weighting/adding processing 
using the weighting coefficient series shown in FIG. 17C-3 or 17D-3 to 
calculate the tertiary data which is stored in a memory region (2). Since 
the tertiary data has been subjected to the filtering having the MTF 
characteristics shown in FIG. 17C-1 or 17D-1, it is expected to contain a 
high spatial frequency of f.sub.N /2 (=1/8Po or so) and no longer contain 
the D.C. component. The memory region (2) is selected in step [6], and the 
displacement operation is performed based on the tertiary data from the 
memory region (2) in step [7]. In step [8], the operation result Lm is 
converted into the defocus amount. 
If the photographic lens is significantly deviated from the in-focus 
position, high-precision displacement detection is performed based on the 
secondary data which contains relatively large amount of the low spatial 
frequency components. On the other hand, if the phototaking lens is near 
the in-focus position, the high-precision displacement detection is 
performed based on the tertiary data which contains relatively large 
amount of high-order spatial frequency components. 
The display unit 72 and the focusing unit 73 perform the display and 
focusing operations based on the defocus amount calculated in the manner 
described above. 
In the above embodiment, the sampling pitch P of the sample & hold means 66 
is determined to be 2Po. However, in general, the pitch P can be nPo 
(where n is a natural number). In this case, the MTF characteristics of 
the forward positioned filter means are so selected to eliminate the 
frequency component above the Nyquist frequency 1/2nPo is determined by 
the sampling pitch nPo. Logarithmic conversion performed by the 
non-linearizing means 64 has an effect of reducing the displacement 
detection error caused by vignetting. Accordingly, where the data is to be 
inputted to a weighting/adding filter for removing the D.C. component, the 
means 64 may be inserted anywhere before the D.C. component eliminating 
weighting/adding filter. 
The memory region (2) may be replaced by the memory region (1). If the 
memory capacity has some margin to allow use of separate regions for the 
memory regions (1) and (2) and if the A/D conversion requires a 
considerable period of time and the CPU capacity has some margin, the 
filtering processing of step [5] may be performed simultaneously with the 
storage operation of step [1]. 
The characteristics of the forward positioned filter means and the rearward 
positioned filter means will be described. 
The synthetic or composite MTF characteristics of the forward positioned 
filter having the weighting coefficient series shown in FIG. 17B-2 and the 
rearward positioned filter having the weighting coefficient series shown 
in FIG. 17C-3 or 17D-3 are as shown in FIG. 17C-1 or 17D-1. When the MTF 
characteristics shown in FIG. 17C-1 or 17D-1 are to be obtained with a 
single weighting/adding filter, the weighting coefficient series of the 
single weighting/adding filter becomes as shown in FIG. 17C-2 or 17D-2. 
The total number of the weighting coefficients used for the forward 
positioned filter and rearward positioned filter is 8, and the number of 
weighting coefficients of the single filter as described above is 13. 
Similarly, a combined filter comprising a series connection of three 
weighting/adding filters respectively having the weighting coefficient 
series shown in FIGS. 17B-2, 17E-3 and 17F-3 has the synthetic MTF 
characteristic as shown in FIG. 17F-1. These MTF characteristics can also 
be obtained with a single weighting/adding filter having the weighting 
coefficient series as shown in FIG. 17F-2. The total number of weighting 
coefficients of the combined filter is 11, and the number of weighting 
coefficients of the single filter is 25. The synthetic MTF characteristics 
of the combined filter having the weighting coefficient series shown in 
FIGS. 17E-3 and 17F-3 are the same as those of the single filter having 
the weighting coefficient series shown in FIG. 17F-4. The total number of 
the weighting coefficients of the combined filter is 6, and that of the 
single filter is 9. 
In this manner, the MTF characteristics of a single weighting/adding filter 
can be generally obtained with a combined filter comprising a series 
connection of a plurality of weighting/adding filters. In this case, the 
total number of weighting coefficients of the plurality of 
weighting/adding filters is significantly smaller than that of the single 
weighting/adding filter. Such a decrease in the number of weighting 
coefficients can lead to a simplified hardware construction when the 
filter comprises hardware. If weighting/adding is performed by software, 
the processing time can be shortened. 
As will be described with reference to a subsequent embodiment, a plurality 
of relatively simple weighting coefficient series may be used for the 
rearward positioned filter and one of such series may be suitably selected 
to be combined with the forward positioned filter, thereby easily 
providing a plurality of combined filters. 
When the forward positioned filter has the MTF characteristics to remove 
the spatial frequency component above the Nyquist frequency determined by 
the spatial pitch of the data used for displacement operation, as in the 
case of the fourth embodiment, the following advantages are obtained. 
Since the output from the forward positioned filter is free from any 
component which might adversely affect the displacement operation, the 
output from the rearward positioned filter as well as that from the 
forward positioned filter may be used for calculating the displacement of 
the object images. 
If the forward positioned filter comprises hardware and a plurality of 
filters are prepared as the rearward positioned filter, and the filtering 
processing by these filters is performed by software, the output from the 
forward positioned filter can be stored in the memory and the data read 
out from the memory can be used sequentially for filtering processing by 
the plurality of filters. 
In the above description, the displacement operation is performed in the 
same manner as that disclosed in U.S. Pat. No. 4,333,007. This 
displacement operation has the following disadvantages. According to this 
displacement operation, correlation amount C(L) is calculated by shifting 
the a and b series data by a shift amount L in units of the pitch P. A 
calculation V(L) =C(L-1)-C(L+1) is performed based on these values. The 
function V(L) is plotted for each shift amount L in FIGS. 19A, 19B and 
19C. FIGS. 19A, 19B and 19C respectively correspond to the displacements 
of zero, 0.5P and 4.5P, and the displacements are indicated by thick 
arrows. Adjacent plotted points are connected by line segments, and the 
slopes of the line segments are calculated. The maximum slope is 
determined, and a point at which the line segment having this maximum 
slope crosses the axis of abscissa is determined. This point defines the 
shift amount which provides the maximum correlation amount, i.e., the 
displacement. 
However, this method is subject to a problem. That is, when the number of 
shift operations is small, erroneous detection can occur. For example, 
when the shift number L is 7 (-3, -2, -1, 0, 1, 2, 3), the line segment 
having the maximum slope within this shift region l3 is a line passing 
L=0. Accordingly, even if the displacement of the images is actually 4.5P, 
the displacement is erroneously detected to be zero. 
An operation method which is free from this problem will now be described 
with reference to FIG. 20. 
Referring to FIG. 20, it is assumed that the memory 69 stores N a series 
data Al to AN and N b series data Bl to BN. A correlation operating part 
71A of the displacement operation part 71 shifts the a series data 
relative to the b series data by a predetermined number of data L and 
calculates the following correlation amount C(L): 
##EQU1## 
where the initial term q and the final term r may be set to be various 
values but preferably be set to be as follows: 
##EQU2## 
herein a mark [] denotes Gauss's notation and value S is a constant 
independent of the shift amount L. Such determination of the terms q and r 
enables accurate comparison between C(L-1) and C(L) or C(L) and C(L+1) 
which will be described hereinafter. 
A memory 71B stores three correlation amounts C(L-1)=C.sub.-1, C(L)=C.sub.0 
and C(L+1)=C.sub.1 when the data shift amount L is respectively L-1, L and 
L+1. A discriminating part 71C compares the magnitudes of C.sub.-1 and 
C.sub.0 and C.sub.1 and determine if the conditions C.sub.-1 
.gtoreq.C.sub.0 and C.sub.1 &gt;C.sub.0 are satisfied. The discriminating 
part 71C serves to determine if a correlation function F (indicated by the 
broken curve) obtained by interpolating discrete correlation amounts C(L) 
plotted as shown in FIG. 21A has a minimum value between C.sub.-1 and 
C.sub.1. When the above conditions are satisfied, the minimum value can 
possibly be present between C.sub.-1 and C.sub.1. 
When the above conditions are satisfied, a first interpolating part 71D 
interpolates the minimum value C.sub.ext from the correlation amounts 
C.sub.-1, C.sub.0 and C.sub.1 stored in the memory 71B in accordance with 
the following relations: 
EQU DL=0.5.times.(C.sub.-1 -C.sub.1) (1) 
EQU E=MAX{C.sub.1 -C.sub.0, C.sub.-1 -C.sub.0 } (2) 
EQU C.sub.ext =C.sub.0 -.vertline.DL.vertline. (3) 
where MAX {C.sub.a, C.sub.b } indicates a larger one of C.sub.a and C.sub.b 
is selected. 
This interpolation method will now be described with reference to FIG. 21B. 
In the case shown in FIG. 21B, C.sub.0 &lt;C.sub.-1 &lt;C.sub.1. Of these three 
correlation amounts, the maximum value C.sub.1 and the minimum value 
C.sub.0 are connected by a line l.sub.1, and a line l.sub.-1 having a 
slope of the same absolute value as that of the line l.sub.1 but of the 
opposite sign is drawn to pass the intermediate value C.sub.-1. The 
intersection of the two lines l.sub.1 and l.sub.-1 provides the minimum 
value C.sub.ext of the correlation function F. When the distance between 
C.sub.0 and C.sub.ext along the axis C(L) is designated by DL, it is given 
by the equation (1) above. Accordingly, the minimum value C.sub.ext is 
given to be C.sub.0 -.vertline.DL.vertline.. Since the magnitude of the 
correlation amount C(L) changes largely depending upon the pattern of the 
object images, the minimum value is normalized such that it may not be 
dependent on the pattern of the object images. Using the term E in 
equation (2) above as a normalizing factor, the value obtained by dividing 
the term C.sub.0 -.vertline.CL.vertline. by E is given as the normalized 
minimum value C.sub.ext. If the minimum value is normalized so that it is 
not dependent on the object images, the minimum value of the correlation 
function F, that is, a correlation amount Fm which gives the maximum 
correlation is a value close to zero and is independent of the object 
images, as shown in FIG. 21A, and another minimum value Fe is quite large 
as compared to Fm. 
A memory 71E temporarily stores outputs DL, E a C.sub.ext operated by the 
interpolating part 71D. 
A discriminating part 71F compares the minimum value C.sub.ext stored in 
the memory 71E with a reference value C.sub.ref. When the former is 
smaller than the latter, the discriminating part 71F produces a comparison 
output. The reference value C.sub.ref is determined to be a value 
intermediate between the correlation amount Fm which gives the maximum 
correlation and the other minimum value Fe. Accordingly, the 
discriminating part 71F discriminates if the minimum value Fe interpolated 
by the interpolating part 71D corresponds to the correlation amount Fm 
which gives the maximum correlation. 
When the minimum value Fe is determined to correspond to the correlation 
amount Fm which gives the maximum correlation in accordance with the 
comparison output from the discriminating part 71F, a second interpolating 
part 71G interpolates a shift amount Lm corresponding to the correlation 
amount Fm in accordance with the equation Lm=L+DL/E. 
A memory 71H stores the shift amount Lm which provides the maximum 
correlation which is supplied from the second interpolating part 71G. The 
second interpolating part 71G converts the shift amount Lm into a defocus 
amount Z in accordance with the equation Z=k.multidot.Lm and stores the Z 
in the memory 71H, where k is the coefficient for converting the shift 
amount Lm into the defocus amount Z. 
The algorithm for performing the above operations will now be described 
with reference to the flow chart shown in FIG. 22. 
Referring to FIG. 22, in step [11], the microcomputer sets the shift amount 
L to zero and sets an information amount parameter Infom to be described 
later to zero. In step [12], correlation amounts C(L-1), C(L) and C(L+1) 
for the shift amounts of L-1, L and L+1 are calculated in accordance with 
the equation 
##EQU3## 
and are stored in memories C.sub.-1, C.sub.0 and C.sub.1 In step [13], if 
MAX {C.sub.1, C.sub.0, C.sub.-1 }&gt;C.sub.th, the Infom is set to be 1. In 
step [14], it is discriminated if the conditions C.sub.1 &gt;C.sub.0 and 
C.sub.-1 &gt;C.sub.0 are satisfied. If the above conditions are satisfied, 
the values DL, E and C.sub.ext are calculated in accordance with the 
contents of the memories C.sub.-1, C.sub.0 and C.sub.1 in step [15]. In 
step [16], the normalized minimum value C.sub.ext is compared with the 
reference value C.sub.ref, and it is determined if the condition C.sub.ext 
&lt;C.sub.ref is satisfied. If this condition is satisfied, in step [17], an 
interpolation shift amount Lm =(L +DL/E) is calculated and stored, and at 
the same time, a parameter Correl is set to 1 so as to indicate that the 
shift amount Lm falls within the maximum shift amount range. 
In step [18], the parameter E is compared with a predetermined threshold 
value Eth. This comparison is performed for the following reason. In 
general, if the data Ai and Bi does not contain a sufficient amount of the 
effective spatial frequency components, then the parameter E decreases, 
and reliability of the operation results in steps [15] and [17] is 
lowered. Accordingly, the reliability of the operations in steps [15] and 
[17] can be determined from the magnitude of the parameter E. It should be 
noted that the parameter E serves a similar function to the information 
amount signal Di in the first embodiment. 
When E &gt;Eth, it is determined that the displacement operations are reliable 
and the parameter Infom is set to 1 in step [19]. However, if the 
condition E &gt;Eth is not satisfied, the parameter Infom is set to 0 in step 
[20]. 
Meanwhile, if NO is obtained in step [14] or [16], the sign of the value L 
is inverted in step [21]. Since the shift amount L is currently set to be 
zero, the value L is also zero. In step [22], it is discriminated if the 
shift amount L satisfies the condition L.gtoreq.0. If YES in step [22], 
the value L is incremented by 1 in step [23]. In the case under 
discussion, since L=0 and the condition of step [22] is satisfied, L=1 is 
set through step [23]. In step [24], the updated shift amount L is 
compared with a predetermined value lf. The predetermined value lf is an 
integer which determines the maximum shift amount. If NO in step [22] or 
step [24], the flow returns to step [12]. After a return to step [12] is 
made and the flow advances to step [21] again, since L=1, the sign of the 
value L is inverted to provide L=-1. Then, the flow returns to step [12] 
again through step [22]. In this manner, until the maximum correlation is 
obtained for lf=5, the shift amount L is sequentially incremented from 0 
in the order of 0, 1, -1, 2, -2, 3, -3, 4, -4, and so on to finally 
determine the shift amount which gives the maximum correlation. If the 
shift amount L reaches the predetermined value lf without providing the 
maximum correlation, the parameter Correl is set to 0 in step [25] so as 
to indicate that the maximum correlation cannot be obtained even if the 
data is shifted to the maximum shift amount. The routine is then ended. 
Step [13] is for determining if the discrimination for Correl=0 is 
performed with a sufficient amount of information. 
In the above correlation operation, the correlation amounts C(L) and C(L+1) 
for L=0 are equal to the correlation amounts CL-1) and C(L) for L=1. The 
correlation amounts CL-1) and C(L) for L=0 are equal to the correlation 
amounts C(L) and C(L+1) for L=-1. Similarly, C(L) and C(L+1) for L=1 are 
equal to CL-1) and C(L) for L=2. CL-1) and C(L) for L=-1 are equal to C(L) 
and C(L+1) for L=2. Despite this, in the flow chart shown in FIG. 22, 
three correlation amounts C(L-1), C(L) and C(L+1) for each L are all 
calculated. Since the calculation of each C(L) takes a considerable period 
of time, this means an increase in the operation time. Accordingly, in an 
actual program except for L=0, the previous correlation amounts are stored 
in a memory, and the amount to be calculated and updated alone is 
calculated. In this case, step [13] is replaced with a step for 
determining if an updated correlation amount C(L') is greater than Cth 
every time the correlation amount is changed. 
According to the algorithm of displacement operation, L=0 is initially set, 
and the value L is sequentially incremented in the order of 1, -1, 2, -2, 
and so on. When a minimum value which satisfies the predetermined 
conditions is found, it can be determined to provide a maximum correlation 
even if the operation for the entire range of -lf.ltorsim.L.ltorsim.lf has 
not finished. In practice, during automatic focusing driving, the true 
focal point is frequently near L=0. Accordingly, generally only several 
operations in accordance with the equation (1) must be performed for the 
corresponding number of shift amounts, thus realizing a significant 
decrease in the processing time. 
Focus using the displacement operation algorithm as described above will 
now be described with reference to FIG. 23. 
The flow before obtaining the secondary data Ai and Bi having the spatial 
pitch 2Po from the a and b series primary data ai and bi having the 
spatial pitch Po is the same as that described with reference to the 
fourth embodiment. 
The A/D converted secondary data Ai and Bi are stored in a memory region 
(1) in Step [31]. In step [32], weighting/adding filtering processing 
using the weighting coefficient series shown in FIG. 17C-3 or 17D-3 is 
performed to prepare the tertiary data free from the D.C. component and 
the tertiary data is stored in a memory region (2). In step [33], the 
memory region (2) is selected, and the maximum shift amount lf is set to 
lf2 (where lf2 is 2 or 3). In step [34], the displacement operation 
algorithm processing shown in FIG. 22 is performed for the high spatial 
frequency components near f.sub.N /2 within range of the maximum shift 
amount. In step [35], it is discriminated if the maximum correlation is 
present within the maximum shift amount lf2 (Correl=1) and the reliability 
of the operation is sufficient (Infom=1). If YES in step [35], the shift 
amount Lm is converted into the defocus amount in step [36]. 
If NO in step [35], the memory region (1) is selected in step [37] and the 
maximum shift amount lf is set to lf1 (where lf1&gt;lf2 and lf1=N/2). In step 
[38], the displacement operation algorithm processing as shown in FIG. 22 
is performed based on the spatial frequency component containing a 
sufficient amount of D.C. component. Step [36] is executed in accordance 
with the processing result. 
In this embodiment, the displacement operation is first performed based on 
the data from which the D.C. component is eliminated. If the result 
obtained is not satisfactory, the displacement operation is performed 
based on the data which contains the D.C. component. This order is 
opposite to that adopted in FIG. 18. If the automatic focusing control of 
the photo-taking lens is continuously performed, the photo-taking lens is 
almost always near the in-focus position. Accordingly, it is most frequent 
that YES is obtained in step [35], thereby significantly reducing the 
processing time. 
FIG. 24 shows another flow chart, and steps [41], [42], and [43] of FIG. 24 
remain the same as the steps [31], [37], and [38] shown in FIG. 23. It is 
first discriminated in step [44] whether the interpolated shift amount Lm 
calculated based on the data containing a sufficient amount of the D.C. 
component represents that the photographic lens is near the in-focus 
position. If NO is obtained in step [44], the corresponding shift amount 
is converted into the defocus amount in step [50]. However, if YES in step 
[44], the operation results Lm.sup.(1) and E.sup.(1) obtained in step [43] 
are stored. Steps [46], [47] and [48] in FIG. 24 are the same as the steps 
[32], [33] and [34] in FIG. 8. In these steps, the values Lm.sup.(2) and 
E.sup.(2) are calculated based on the data which is free from the D.C. 
component. In step [49], one of the shift amounts Lm.sup.(1) and 
Lm.sup.(2) obtained by interpolation of the parameters E.sup.(1) and 
E.sup.(2) is selected. If the object contains only the spatial frequency 
component near the D.C. component, the reliability of the operation result 
Lm.sup.(2) obtained in step [48] is very low and the corresponding 
parameter E is also very small. Accordingly, if E.sup.(2) is below the 
threshold value Eth and E.sup.(1) is sufficiently great, Lm.sup.(1) is 
selected. Otherwise, Lm.sup.(2) is selected. In step [50], the defocus 
amount is calculated in accordance with the value Lm. 
In the above description, the microcomputer performs only processing by a 
single rearward positioned filter. However, a case will now be described 
wherein the microcomputer performs processing by a plurality of rearward 
positioned filters. 
Referring to FIG. 16, in order to clearly represent that the microcomputer 
68 has a plurality of rearward positioned filters, a jth rearward 
positioned filter 70j is represented by the broken line block outside the 
rearward positioned filter section 70. 
Referring to FIG. 25, steps [51] to [55] are the same as the steps [41] to 
[45] in FIG. 24. In step [55], operation results Lm.sup.(1) and E.sup.(1) 
obtained based on the data which contains the D.C. component are stored. 
In step [56], a parameter j is set to be 2. In step [57], the 
weighting/adding filtering processing having the weighting coefficient 
series shown in FIG. 17D-3 is performed on the data stored in the memory 
region (1). According to this filtering processing, the filter has the MTF 
characteristics wherein a peak frequency f.sub.2 is obtained near 1/2 the 
Nyquist frequency f.sub.N =1/2P determined by the pitch P=2Po as shown in 
FIG. 17D-1, and the operation result data is stored in the memory region 
(2). In step [58], the memory region (2) is selected, 2 is set as the 
maximum shift amount lf=lf2, and the threshold value Eth=Eth.sub.2 is set. 
Under these conditions, the algorithm processing as shown in FIG. 22 is 
performed in step [59]. Then, if it is determined in step [60] that 
Infom=1, and Correl=1, the corresponding interpolated shift amount Lm is 
used to calculate the defocus amount in step [65]. 
If NO in step [60], the corresponding operation results Lm.sup.(2) and 
E.sup.(2) are stored in step [61]. Subsequently, the parameter j is 
incremented by 1 to set j=3 in step [62]. In step [63], it is 
discriminated if j is 5. Since j=3 currently, the flow goes to step [57]. 
In step [57], the filtering is performed such that the peak frequency f3 
for j=3 is near 1/2 the peak frequency f.sub.2 as shown in FIG. 17F-1 and 
the D.C. component is sufficiently elixinated, and the filtered data is 
stored in a memory region (3). This filtering is perforxed by sequentially 
subjecting the data of the memory region (1) to the weighting/adding 
processing shown in FIG. 17E-3 and that shown in FIG. 17F-3. 
The filtering processing for J=3 may be performed by only weighting/adding 
processing shown in FIG. 17F-3. That is, the weighting/adding processing 
shown in FIG. 17E-3 is not required for j=3. This is because the 
weighting/adding processing in FIG. 17F-3 serves to suppress the 
components between frequencies 1/8Po to 1/4Po and insufficient reliability 
of the operation, namely, Inform=0 for j=2 ascertains that the filtered 
data in the memory region (1) does not include said range of frequency 
components. 
The processing in the subsequent steps [58] to [62]is the same as that for 
j=2. In step [62], the parameter j becomes 4. The weighting/adding filter 
in step [57] for j=4 has the MTF characteristics wherein a peak frequency 
f4 is near 1/4 the peak frequency f.sub.2 and the D.C. component is 
sufficiently eliminated. The subsequent steps are the same as those for 
j=3. Although the filter characteristics are sequentially changed in 
accordance with the incremented value of j in the above case, the maximum 
shift amount lfj may remain constant independently of the value of j. When 
the parameter j reaches 5, the flow goes to step [64] wherein a value of a 
highest reliability is selected from the interpolated shift amounts 
Lm.sup.(1), Lm.sup.(2), Lm.sup.(3) and Lm.sup.(4), based on the values of 
the data E.sup.(1), E.sup.(2), E.sup.(3) and E.sup.(4). If none of these 
amounts is reliable, selection is not made and the flow goes to step [65]. 
In the example described above, a maximum of three filtering processings is 
performed. However, the number of filtering processings to be performed 
may be changed as needed. Furthermore, as the parameter j increases, a 
filter for step [57] is selected wherein the MTF peak frequency 
sequentially shifts to the lower frequency side and the D.C. component is 
eliminated. Accordingly, even if the spatial frequency of the object 
contains a large amount of a specific frequency components, high-precision 
detection can be performed by one of the filtering processings as 
described above. 
In the embodiments described above, the data having the sampling pitch 
P=2Po and stored in the memory region (1) is directly used for calculating 
the defocus amount when the photo-taking lens is not near the in-focus 
position. However, if the photo-taking lens is not near the in-focus 
position, the maximum shift amount lf is preferably set to be about N/2. 
However, when the number N of the a and b series data is 50, the operation 
time becomes quite long. If the defocus amount is to be calculated when 
the photo-taking lens is not near the focal plane, the calculation 
precision of the defocus amount is not very high. If the object images are 
blurred, the displacement operation can use data of relatively rough 
sampling pitch, so that the operation time can be shortened. An example of 
such a case will now be described with reference to FIG. 26. 
The secondary data having the sampling pitch P and after A/D conversion is 
stored in a memory region (1) (step [71]). Filtering is performed to 
eliminate, from the data in the memory region (1), the spatial frequency 
component above the Nyquist frequency when the sampling pitch is 
multiplied with n, and the data sampled at the sampling pitch n.times.p is 
stored in a memory region (1') (step [72]). In general, it is preferable 
to perform filtering processing wherein the component above the Nyquist 
frequency is eliminated simultaneously when the sampling pitch is changed. 
However, if the defocus amount is large, the image is blurred. In such a 
case, the images do not contain the frequency component above the Nyquist 
frequency of the sampling pitch which is under discussion. Accordingly, 
the above-mentioned filtering processing can be omitted and sampling can 
be performed at the sampling pitch n.times.p. 
Next, the memory region (1') is selected, and the maximum shift amount 
lf=lf1.congruent.N'/2 is set. Note that the sampling number N' in this 
case is about 1/n of the sampling number N of the memory region (1) (step 
[73]). The obtained shift amount Lm is corrected by multiplication with n, 
that is, Lm.times.n.fwdarw.Lm (step [75]). Subsequently, the defocus 
amount is calculated (step [76]). 
Reference will now be made to the flow chart shown in FIGS. 27A-27C, which 
are assembled as in FIG. 27. The secondary data filtered by a hardware 
filter having the characteristics shown in FIG. 17B-2 is stored in a 
memory region (1) (step [81]). In step [82], the data processed by the 
filter shown in FIG. 17D-3 is stored in a memory region (2). 
The memory region (2) is selected, the maximum shift amount lf is set to be 
for example lf =lf2 =3, and the threshold value of the information amount 
is set to be Eth (step [83]). Based on this, the algorithm processing for 
calculating the shift amount shown in FIG. 22 (step [84]) is performed. If 
the information amount is sufficient (Infom=1) and the maximum correlation 
point is within the range of the maximum shift amount (Correl=1) (step 
[85]), the defocus amount is calculated from the obtained shift amount Lm 
(step [86]). If NO in step [85], the photo-taking lens is not near the 
in-focus position or even if the photo-taking lens is near the in-focus 
position the images contain only the frequency component lower than the 
spatial frequency f2=1/8Po. The subsequent operation can be performed 
using the sampling pitch of 2Po so as to shorten the overall processing 
time if the number of newly sampled data with pitch 2Po is enough. If the 
subsequent sampling pitch is designated by P', we have P'=2P=4Po. 
More specifically, the filtering processing as shown in FIG. 17E-3 is 
performed (step [87]). In other words, weighting with the weighting 
coefficients of (0.5, 1, 0.5) is performed for the successive three terms 
of the data stored in the memory region (1) and new data is stored in a 
memory region (3) for the data of each P'=2P. The filter characteristics 
for this case are as shown in FIG. 17E-1 wherein substantially no spatial 
frequency component above the Nyquist frequency f.sub.N =1/2P'=1/8Po 
associated with the sampling pitch P' is present. If it is determined in 
step [88] that the information amount obtained as a result of the 
algorithm processing in step [84] is not sufficient (Infom =1), it is 
determined that the photo-taking lens still has the possibility to be near 
the in-focus position and the flow goes to a loop for changing the filter 
which eliminates DC component. First, in step [89], the parameter j=4 is 
set as the initial value of the loop. In step [90], the data stored in the 
memory region (3) and having the sampling pitch P'=2P=4Po is subjected to 
filtering processing with the weighting coefficient series having the 
spatial pitch 8Po as shown in FIG. 17F-3, and the data having the sampling 
pitch P' is stored in a memory region (4). The data stored in the memory 
region (4) is thus data obtained by a filter having characteristics to 
eliminate the D.C. component and to mainly extract the spatial frequency 
components near f4=1/4P'=1/16Po, as shown in FIG. 17F-1. The memory region 
(4) is then selected, the maximum shift amount lf=lf4=3 is set, and the 
information amount threshold value Eth=Eth.sub.4 is set (step [91]). 
The algorithm processing as shown in FIG. 22 is performed (step [92]). If 
the information amount is sufficient (Infom=1; step [93]) and the maximum 
correlation point falls within the range of the maximum shift amount 
(Correl=1; step [96]), a displacement X is calculated by X=Lm.times.2P 
using the corresponding value Lm and the sampling pitch 2P (step [100]). 
If NO in step [93], the parameter j is incremented by 1 to set j=5. In this 
case, the flow returns to step [90] through step [95]. In step [90] 
filtering processing is performed wherein the D.C. component is eliminated 
from the memory region (3) and the frequency band has the center frequency 
of a spatial frequency f5 (f5&lt;f4), and the obtained data is stored in a 
data memory region (5). Here, filtering processing is performed for 
f5=(1/2)f4=1/8P'=1/32Po. In this case, depending upon the filter used, the 
sampling pitch may be rendered more rough. However, if the sampling pitch 
is rendered too rough, the number of data available is decreased and the 
precision is degraded. Accordingly, it is not preferable to decrease the 
data number N below about 20 to 25. 
Steps [91] and [92] are performed in the manner as described above. If the 
information amount is sufficient (step [93]) and the maximum correlation 
point falls within a correlation detection range (step [96]), the image 
displacement is calculated in step [100]. 
If NO in step [93], j=6 is set in step [94]. Then, the condition of step 
[95] is satisfied, and the flow goes to step [97]. If YES in step [88] or 
step [96], the flow goes to step [97]. At this time, it is apparent that 
the photo-taking lens is not near the in-focus position or the object 
contains only a spatial frequency component very close to the D.C. 
component. 
For this reason, the data having the pitch P'=2P and containing the D.C. 
component stored in the memory region (3) is selected. The maximum data 
amount lf is set to be lf3=N'/2 (N'=N/2), and Eth.sub.4 is set as the 
threshold value (step [97]). The algorithm processing as shown in FIG. 22 
is performed (step [98]). If Infom =1 and Correl=1 (step [99]), the image 
displacement X=Lm.times.2P is calculated based on the corresponding Lm and 
the pitch P'=2P (step [100]). The defocus amount is calculated in step 
[100] in accordance with the result obtained in step [100]. If NO in step 
[99], detection cannot be performed and the flow ends. 
In this embodiment, the a and b series primary data ai and bi having the 
spatial pitch Po are passed through the forward positioned filter means 65 
shown in FIG. 16, sampled at the sampling pitch P=2Po, and stored in the 
memory region (1) of the microcomputer 68. The data stored in the memory 
region (1) is sampled at the sampling pitch P'=2P=4Po, and the sampled 
data is stored in a memory region (3). When the number of the a and b 
series primary data is respectively assumed to be 100, the a and b series 
data stored in the memory regions (1) and (3) respectively is about 50 and 
about 25. In this manner, since the number of data to be processed is 
determined in accordance with the various conditions, high-speed operation 
can be performed utilizing limited information. 
FIG. 28 shows the fifth embodiment of the present invention. 
Referring to FIG. 28, a photoelectric device 61, a non-linearizing means 
64, a forward positioned filter means 65, a sample & hold means 66, an A/D 
converter 67, a microcomputer 68, a display part 72 and a focusing part 73 
are of the same configuration as those shown in FIG. 1. A switching means 
74 is interposed between the forward positioned filter means 65 and the 
sample & hold means 66. The switching means 74 serves to select one of the 
outputs from the forward positioned filter means 65 and the 
non-linearizing means 64 and supplies the selected output to the sample & 
hold means 66. A mode setting means 75 is connected to the microcomputer 
68 and can be set in one of first and second modes externally. In 
accordance with the set mode, the mode setting means 75 controls the 
switching means 74, the sample & hold means 66 and the A/D converter 67 
through the microcomputer 68 in the following manner. When the switching 
means 74 is set in the first mode, it supplies the output (about 100 a and 
b series data respectively) from the forward positioned filter means 65 to 
the sample & hold means 66. The sample & hold means 66 samples the input 
data at the sampling pitch P=2Po and produces about 50 a and b series 
data, respectively. The A/D converter 67 performs A/D conversion of the 
data at the timing of the pitch 2Po. When the switching means 74 is set in 
the second mode, it supplies the 100 a and b series data respectively from 
the non-linearizing means 64 to the sample & hold means 66. Then the 
sample & hold means 66 samples at the sampling pitch P=Po the primary data 
ai and bi which respectively correspond to 50 elements at the center of 
the arrays 62 and 63, respectively. The sample & hold means 66 produces 
the 50 data for the a and b series data, and the A/D converter 67 performs 
the A/D conversion of the input data at the timing of the pitch Po. The 
relationships between the sampling pitch and the sampling region in the 
first mode with those in the second mode resemble those shown in FIGS. 10B 
and 10C. 
Irrespective of the mode of the switching means 74, the microcomputer 68 
processes the data received from the A/D converter 67 in accordance with 
one of the procedures of the first to fourth embodiments. 
When the switching means 74 is set in the second mode, the primary data is 
supplied to the microcomputer 68 without the intermediary of the forward 
positioned filter means 65. Accordingly, the data in the second mode may 
contain an undesirable spatial frequency above the Nyquist frequency. 
However, if the pitch Po is very small, for example, about 50.mu., such a 
problem can be eliminated for the following reason. When the pitch is as 
small as 50.mu., the spatial frequency components above the Nyquist 
frequency can be sufficiently removed from the primary data ai and bi due 
to the aberration of the focusing detection optical system and other 
reasons. Accordingly, even if the primary data is not passed through the 
forward positioned filter means 65 in the second mode, the detection 
precision may not be inadvertently degraded. 
In the embodiments described above, the weighting/adding processing and the 
displacement operation in the microcomputer are performed separately from 
each other. However, these two processings may be performed in a combined 
manner. For example, A.sub.i -B.sub.i+L is calculated for a plurality of 
values of i and L, respectively, and the results are stored in a memory. 
The memory result can be used for the weighting/adding filtering and then 
the displacement operation may be performed thereafter. Furthermore, the 
weighting/adding processing and the displacement operation may be 
completely integrated as in the following equation: 
##EQU4## 
In this case, the weighting coefficient series was (-0.5, 1, -0.5). 
There has been described combined filter means of forward and rearward 
filter means connected in series with each other to provide various MTF 
characteristics in FIGS. 16 and 28. FIG. 29 shows another combined filter 
means in which a pair of weighting/adding filters are connected in 
parallel with each other. 
Referring to FIG. 29, primary data a1, b1, a2, b2, . . . , are sequentially 
supplied to an input terminal 80a of a combined filter means 80. As shown 
in FIG. 29, the combined filter means 80 comprises two filters 80A and 80B 
and a mixing means 80C. The two filters 80A and 80B have different MTF 
characteristics. The mixing means 80C multiplies an output V1 from the 
filter 80A with a coefficient .alpha. and an output V2 from the filter 80B 
with a coefficient .beta., and adds the products. That is, the mixing 
means 80C calculates V1.times..alpha.+V2.times..beta., and produces the 
obtained sum from an output terminal 80b. A setting means 81 sets the 
values of the coefficients .alpha. and .beta. in accordance with a command 
from the operator or information from a signal of the phototaking lens. 
The function of the filter means 80 will now be described. 
Assume that the respective filters 80A and 80B have the weighting 
coefficient series of (0, 0, 0.16, 0.465, 0.86, 1, 0.86, 0.465, 0.16, 0, 
0) and (0.18, 0.32, 0.44, 0.335, 0.14, 0.0, 0.14, 0.335, 0.44, 0.32, 
0.18), as shown in FIGS. 30A and 30B, respectively. When it is assumed 
that the values of the coefficients .alpha. and .beta. are 1 and -0.6, 
respectively, the synthetic weighting coefficient series becomes as shown 
in FIG. 31A-1. The synthetic MTF characteristics, that is, the MTF 
characteristics of the filter means 80 become as shown in FIG. 31A-2 
wherein Ao=0.75 Ap where MTF at zero frequency is Ao and MTF at peak 
frequency is Ap and there is a peak between the D.C. component and the 
frequency f.sub.N /2. If it is assumed that .alpha.=1 and .beta.=-1, the 
synthetic weighting coefficient is as shown in FIG. 31B-1 and the 
synthetic MTF characteristics are as shown in FIG. 32B-2 wherein 
Ao.apprxeq.0.4 Ap. If it is assumed that .alpha.=1 and .beta.=-1.4, the 
synthetic weighting coefficient and the synthetic MTF characteristics are 
as shown in FIGS. 31C-1 and 31C-2 wherein Ao=0. When it is assumed that 
.alpha.=1 and .beta.=0, the synthetic weighting coefficient and the 
synthetic MTF characteristics are as shown in FIGS. 31D-1 and 31D-2 
wherein Ao=Ap. In this manner, desired MTF characteristics wherein Ao=0 to 
Ao can be obtained by suitably selecting the values of the coefficients 
.alpha. and .beta.. 
In the above description, the spatial frequency component which is above 
the Nyquist frequency f.sub.N =1/2P determined by the sampling pitch P and 
which is contained in the data used for the displacement operation is 
described as not desirable for the displacement operation, and the spatial 
frequency components at about half the Nyquist frequency are assumed to be 
very effective for high-precision displacement operation. The relationship 
between the displacement operation precision and the spatial frequency 
components will now be described. 
FIGS. 32A-1 to 32F-1 show the states wherein periodic lattice images 
(hatched) having a spatial frequency of 3/4Po are moved in the direction 
indicated by arrows on photoelectric element arrays a1, a2, a3, a4, and a5 
having a pitch Po. FIGS. 32A-2 to 32F-2 respectively show photoelectric 
outputs a1 to a5 corresponding to FIGS. 32A-1 to 32F-1, respectively. 
FIGS. 33A-1 to 33F-1 and FIGS. 33A-2 to 33F-2, FIGS. 4A-1 to 34F-1 and 
FIGS. 34A-2 to 34F-2, and FIGS. 5A-1 to 35F-1 and FIGS. 35A-2 to 35F-2 
respectively correspond to periodic lattice images having spatial 
frequencies of 1/2Po, 3/8Po and 1/4Po, and are similar to FIGS. 32A-1 to 
32F-1 and FIGS. 32A-2 to 32F-2. 
As shown in FIGS. 32A-1 to 32F-1 and FIGS. 2A-2 to 32F-2, for a lattice 
image having a spatial frequency 3/4Po 1.5 times the Nyquist frequency 
1/2Po, the direction of movement of the image and the direction of the 
resultant movement of the photoelectric output pattern are opposite to 
each other. In general, in a lattice image having a spatial frequency 
falling within the range between the Nyquist frequency f.sub.N and the 
frequency 2f.sub.N, the direction of movement of the image is opposite to 
the phase change of the photoelectric output pattern. Referring to FIGS. 
33A-1 to 33F-1 and FIGS. 33A-2 to 33F-2, in a lattice image having a 
spatial frequency 1/2Po which is equal to the Nyquist frequency, upon 
movement of the image only the amplitude of the photoelectric output 
pattern changes and the phase of the photoelectric output pattern does not 
change. Referring to FIGS. 34A-1 to 34F-1 and FIGS. 34A-2 to 34F-2, in a 
lattice image having a spatial frequency 3/8Po which is 3/4 of the Nyquist 
frequency, upon movement of the image in the direction indicated by the 
arrow, the phase of the photoelectric output pattern changes but such a 
change in the phase is not smooth. Referring to FIGS. 35A-1 to 35F-1 and 
FIGS. 35A-2 to 35F-2, in a lattice image having a spatial frequency 1/4Po 
which is 1/2 Nyquist frequency, upon movement of the image in the 
direction indicated by the arrow, the phase of the photoelectric output 
pattern also changes in the same direction and smoothly. 
A conventional light-receiving array (a generic term including, for 
example, the small lens array 2 shown in FIG. 1 or the photoelectric 
element arrays 22 and 23 shown in FIG. 8) has the characteristics as shown 
in FIG. 17A wherein the MTF gradually decreases from the D.C. component 
and becomes zero near a frequency 1/Po. This array therefore extracts an 
undesirable spatial frequency component above the Nyquist frequency 1/2Po 
determined by the pitch Po. In view of this, the characteristics of a 
weighting/adding filter which can satisfactorily remove the component 
above the Nyquist frequency from the photoelectric output data from such a 
light-receiving array will be described below. 
FIG. 36A shows the MTF characteristics of a filter having the weighting 
coefficient series as shown in FIG. 36B. These characteristics are 
symmetrical with respect to the frequency 1/2Po; the MTF gradually 
decreases from the D.C. component, becomes very small around the frequency 
of 1/4Po, remains sufficiently small up to a frequency of about 3/4Po, and 
then increases. In this manner, since the MTF of the filter is 
sufficiently sxall within the range of 1/4Po to 3/4Po denoted by lo and 
the MTF of the photoelectric array is sufficiently small at a frequency of 
about 1/Po or higher, the synthetic MTF of the array and the filter as 
shown in FIG. 36B is obtained from which the frequency components above 
the Nyquist frequency 1/2Po deterxined by the pitch Po and the Nyquist 
frequency 1/4Po determined by the pitch 2Po are removed. 
The MTF characteristics of the filter having the weighting coefficient 
series as shown in FIG. 37B are as shown in FIG. 37A wherein the frequency 
1/2Po is the center frequency and MTF suppressing region lo is 3/8Po to 
5/8Po. Although the MTF suppressing region lo of this filter means is 
narrower than that of the filter shown in FIG. 36A, it is capable of 
suppressing to a substantially satisfactory degree the high spatial 
frequency components above the frequency 1/2Po together with the MTF 
characteristics of the light-receiving array. In the filter means as shown 
in FIGS. 38A and 38B, since the MTF suppressing region lo is as wide as 
1/8Po to 7/8Po, if the Nyquist frequency is determined to be 1/2Po, the 
effective frequency components of 1/8Po to 1/2Po are suppressed too much. 
Accordingly, in this case, the sampling pitch is suitably selected so that 
the Nyquist frequency becomes 1/4Po or 1/8Po. 
The filter means according to the present invention which eliminate 
frequency components above Nyquist frequency must satisfy the following 
conditions. That is, the number of weighting coefficients which are not 
zero must exceed 4, the MTF characteristics must be sufficiently small 
within a frequency range lo of at least 3/8Po to 5/8Po, and the MTF 
gradually increases from the lower limit of this range toward the lower 
frequencies. If the number of weighting coefficients is less than 4, it is 
difficult to atain the frequency range lo as described above. When the 
frequency range lo is narrower than that as described above, suppression 
of the frequency components above a frequency 1/2Po by the MTF 
characteristics of the light-receiving array becomes extremely difficult. 
When n of the sampling pitch nPo is 2 or more, the frequency range lo must 
be 3/4nPo to (1/Po-3/4nPo) or more and is preferably 1/2nPo to 
(1/Po-1/2nPo) or more. The lower limit 3/4nPo of the frequency range lo is 
equivalent to 3/2 times the corresponding Nyquist frequency 1/2nPo. When 
the lower limit is higher than the value 3/4nPo, the adverse effect of the 
component from the Nyquist frequency to the lower limit becomes 
non-negligible. 
In the above description, the conditions for the MTF characteristics of the 
filter means in the frequency range above the Nyquist frequency determined 
by the sampling pitch are explained. Conditions for the MTF 
characteristics of the filter means in the frequency range below the 
Nyquist frequency will now be described. 
FIG. 40A shows a graph wherein the relative displacement between the object 
images on a pair of light-receiving arrays is plotted along the axis of 
abscissa, and the displacement detected by the focus detection apparatus 
is plotted along the axis of ordinate. A solid curve (A) corresponds to an 
ideal case wherein the image displacement and the detected displacement 
coincide with each other. An alternate long and short dashed curve (B) and 
a broken curve (C) correspond to cases of conventional focus detection 
apparatus. The curves (B) and (C) intersect with the curve (A) at integer 
multiples of the sampling pitch P. Accordingly, if the image displacement 
is equal to an integer multiple of the sampling pitch P, it may be 
correctly detected. However, if the image displacement is not equal to an 
integer multiple of the sampling pitch P, it cannot be correctly detected 
and an error is caused. Such an error is caused when the frequency 
component used for displacement detection/operation contains a component 
higher than the Nyquist frequency or even when only the frequency 
component below the Nyquist frequency is used. This is attributed to the 
following reason. At a frequency near the Nyquist frequency f.sub.N within 
the frequency range f.sub.N /2 to f.sub.N, the phase change in the 
photoelectric output upon a movement of the images is not smooth as shown 
in FIGS. 34A-1 to 34F-2. For this reason, when the frequency component 
near the Nyquist frequency f.sub.N within the frequency range f.sub.N /2 
to f.sub.N is used for focus detection, an increased error is generated 
for an image displacement which is not equal to an integer multiple of the 
sampling pitch as shown in FIG. 40A. The non-smoothness of the phase 
change in the photoelectric output pattern within the frequency range 
f.sub.N /2 to f.sub.N is more significant with a higher frequency within 
this frequency range. Accordingly, the MTF characteristics of the filter 
means below the Nyquist frequency are preferably as shown in FIG. 41A 
wherein the MTF is sufficiently small near the Nyquist frequency f.sub.N, 
gradually increases with a decrease in the frequency, and is sufficiently 
large at a frequency smaller than the frequency f.sub.N /2. Although the 
MTF must take a sufficiently large value when the frequency is about 
f.sub.N /2, it preferably takes a sufficiently large value within the 
frequency from about f.sub.N /2 to f.sub.N /4 considering effective use of 
information. The solid curve shown in FIG. 41B shows a case wherein the 
MTF is sufficiently large at a frequency of f.sub.N /4. In the two MTF 
characteristic curves described above, the point at which the MTF starts 
to increase gradually, that is, the leading point is near the Nyquist 
frequency. However, such a leading point may be shifted toward a higher 
frequency or to a lower frequency. When such a shift to a lower frequency 
is great, the frequency component falling within the frequency range of 
f.sub.N /2 to f.sub.N which results in non-smooth phase change in the 
photoelectric output pattern can be eliminated in a more efficient manner. 
However, at the same time, this also results in the problem of elimination 
of the effective data as well. Considering the elimination of the 
frequency component which results in non-smooth phase change and the 
elimination of the effective information, the lower limit of the frequency 
at which the MTF rises is preferably about f.sub.N /2 as indicated by the 
alternate long and short dashed curve. According to the MTF 
characteristics indicated by the alternate long and short dashed curve, 
the MTF is sufficiently small in a frequency range below about f.sub.N /2, 
gradually increases from about f.sub.N /2, and is sufficiently large near 
f.sub.N /4. Accordingly, the frequency component within a frequency range 
which causes non-smooth phase change in the photoelectric output pattern 
is mostly eliminated. 
A filter means having the MTF characteristics as shown in FIG. 41A can be 
obtained using the weighting coefficients as shown in FIG. 39B and 
sampling pitch P=2Po, f.sub.N is 1/4Po. The MTF characteristics indicated 
by the alternate long and short dashed curve in FIG. 41B correspond to 
those shown in FIG. 39A wherein f.sub.N is 1/2Po. 
When a photo-taking lens is replaced with another, and when part of the 
images on the pair of photoelectric element arrays causes an vignetting or 
the amplification factor of the arrays is non-uniform, a curve 
representing a detected image displacement does not pass the origin of the 
coordinate system and erratic focus detection is caused, as shown in FIG. 
40B. In order to eliminate such an error, the component near a zero 
frequency must be eliminated. 
A weighting/adding filter is described below wherein zero spatial frequency 
component is eliminated, the component higher than the Nyquist frequency 
is also eliminated, the MTF peak value falls within a range of f.sub.N /4 
to f.sub.N /2, and the MTF at a frequency of (3/4)f.sub.N is less than 
half the MTF peak. 
FIG. 42B shows the weighting coefficient series (-0.5, 1, -0.5) for the 
input data having the pitch P, that is, the series (-0.5, 0, 1, 0, -0.5) 
of the pitch P. FIG. 42A shows the MTF characteristics of a filter having 
such a series, wherein the MTF peak value Pk is at f.sub.N /2 (=1/4P) and 
the MTF value at the frequency (3/4)f.sub.N is half the peak value Pk. 
FIGS. 43A to 45A and FIGS. 43B to 45B show a filter wherein the number of 
weighting coefficients is 4 or more, the MTF peak value is obtained at a 
frequency lower than the frequency f.sub.N /2, and the MTF within a wide 
range near the Nyquist frequency is sufficiently small. 
In FIGS. 42B to 45B, the number of weighting coefficients must be 3 or 
more. However, when the number of weighting coefficients is small, the 
configuration of the filter can be simplified accordingly. In view of 
this, the conditions to be satisfied when the number of weighting 
coefficients is 3 are described below. The first condition is that 
weighting coefficients W1, W2 and W3 must be multiplied with every other 
input data, that is, the pitch of the weighting coefficients W1, W2 and W3 
must be twice pitch P of the input data. 
The first condition must be satisfied if the input data contains a 
component near the Nyquist frequency. Accordingly, if the input data is 
free from such a component near the Nyquist frequency, the pitch of the 
weighting coefficient series can be P. 
The second condition is that the sum of the three weighting coefficients 
W1, W2 and W3 is substantially zero. 
The third condition is that the weighting coefficients W1 and W3 have the 
same sign, and the coefficient W2 and the coefficients W1 and W3 have 
opposite signs. 
It will be apparent that the coefficient series shown in FIG. 39B satisfies 
these three conditions. 
In order to render zero the MTF at the D.C. component and the Nyquist 
frequency, the sum must be definitely zero. However, if the MTF must be 
substantially zero, the sum need only be substantially zero. 
In this manner, when the component at a frequency of zero is completely 
eliminated, information required for focus detection is substantially 
eliminated for images containing mainly low spatial frequency components, 
so that the focus detection is disabled or detection precision is 
extremely lowered. 
In order to solve this problem, the filter must have the MTF 
characteristics wherein the MTF Ao at the zero frequency is 0.1 to 0.8 
times the MTF peak Ap. Preferably, the MTF Ao at the zero frequency is 0.2 
to 0.7 times the Ap. Such MTF characteristics, strictly speaking, are not 
those of a single filter but are synthetic MTF characteristics of the 
filter and those of the light-receiving array. 
The configuration of the filter having such MTF characteristics will now be 
described. 
Referring to FIG. 46A, a dotted curve Xo shows the MTF characteristics 
which are determined by the shape of the light-receiving array. The MTF 
characteristics represented by a broken curve b1 are synthetic MTF 
characteristics for the weighting coefficient series (-0.1, 1, -0.1) for 
the pitch P shown in FIG. 46B and those of the light-receiving array. 
According to these synthetic MTF characteristics, the MTF at the zero 
frequency is 0.74 times the MTF peak value. When the weighting coefficient 
series as shown in FIG. 46C is used, the synthetic MTF characteristics as 
indicated by a solid curve C1 in FIG. 46A are obtained. FIGS. 47A, 7B and 
47C, FIGS. 48A, 48B and 48C, and FIGS. 49A, 49B and 49C show cases wherein 
5, 7 and 11 weighting coefficients are used, respectively. 
The weighting coefficient series is preferably symmetrical with respect to 
a central coefficient so that the filtering effect may not be dependent on 
the illuminance distribution of the images, as shown in FIGS. 43B, 43C, 
45B, 45C and 46B. 
For these reasons and the problems of symmetry, a filter which is capable 
of eliminating the data having the sampling pitch P and the D.C. component 
and of effectively extracting high spatial frequency components of up to 
f.sub.N /2=1/4P preferably has the weighting coefficient series shown in 
FIGS. 46B, 46C, 48B and 48C. 
The conditions for three symmetrical weighting coefficients W1, W2 and W3 
to satisfy the MTF chracteristics as described above; .circle.1 
0.1Ap&lt;Ao&lt;0.8Ap and .circle.2 0.2Ap&lt;Ao&lt;0.7Ap will be described below. 
When it is assumed that W2=1 and W1=W3=x (negative value), the D.C. 
component extraction efficiency Ao and the peak extraction efficiency Ap' 
of the filter hold the following relationship: 
EQU Ao={(1+2x)/(1-2x)}Ap' 
The maximum extraction efficiency Ap of the synthetic MTF characteristics 
is obtained as Ap=.gamma..multidot.Ap' (where .gamma. is a value 
determined by the shape of the light-receiving array). The efficiency Ap 
and the maximum extraction efficiency Ap hold the following relation: 
EQU Ao={(1+2x)/(1-2x)}(1/.gamma.)Ap 
When this is substituted in the relation .circle.2 above, we have: 
EQU 0.1&lt;{(1+2x)/(1-2x)}(1/.gamma.)&lt;0.8 
x can then be obtained as: 
EQU -1/2{(10+.gamma.)}&lt;x&lt;-(1/2){(10-8.gamma.)/(10+8.gamma.)} 
The value of .gamma. based on the MTF characteristics and determined by the 
shape of the light-receiving array generally satisfies the relation: 
EQU 1.tbd..gamma..tbd.0.9 
Therefore, we have: 
EQU -0.41&lt;x&lt;-0.06 for .gamma.=1 
EQU -0.42&lt;x&lt;-0.08 for .gamma.=0.9 
When Ap is substituted in the relation .circle. above to solve it for x, 
we have: 
EQU -(1/2){(10-2.gamma.)/(10+2.gamma.)}.ltoreq.x.ltoreq.-(1/2). 
Therefore, 
EQU -0.33.ltoreq.x.ltoreq.-0.09 for .gamma.=1 
EQU -0.35.ltoreq.x.ltoreq.-0.11 for .gamma.=0.9