Focus detecting apparatus

An apparatus for measuring the distance between an extremum or object and the apparatus comprises a scanning system for scanning the entire distance range to be measured, the scanning system detecting objects at different distances from the apparatus within the range and producing in a time series an output which assumes an extremal value upon the detection, apparatus for producing a distance signal representing the scanned distance in a time series, a distance selecting device extraneously operable to select a desired distance range within the entire distance range to be measured, extremal value detecting apparatus for detecting the extremal value of the output of the scanning system corresponding to the selected distance range by the output of the distance selecting device, and apparatus for providing an output representing the distance between an object within the selected distance range and the apparatus from the output of the extremal value detecting apparatus and the distance signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a range finding apparatus, and more particularly 
to an automatic focusing apparatus. 
2. Description of the Prior Art 
In cameras, for example, it is known to move a focus position detecting 
lens in the direction of the optic axis thereof, to detect a position 
having the highest contrast from the contrast of an object image formed by 
the light passed through said lens as the focused position of an image 
forming lens (which is called a picture-taking lens in case of cameras), 
and to move the image forming lens to the detected position to provide a 
properly focused image on a film. 
However, in such cameras, where an intended object is to be photographed 
through a wire netting or where an object beyond a clump of trees is to be 
photographed therethrough, the focus is adjusted to an object having a 
higher contrast and thus, it could not be assured whether or not the focus 
could be adjusted to the intended object. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a focused position 
detecting apparatus which can reliably detect the focused position of the 
image forming lens to be focused to a desired object even when several 
objects lie before and behind the object to be photographed. 
The invention will become fully apparent from the following detailed 
description thereof taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Some embodiments of the present invention will hereinafter be described by 
reference to the accompanying drawings. Description of the embodiments 
will be made by reference to an automatic focusing device in a camera or 
the like for detecting the focused position of an image forming lens whose 
focus may be adjusted to a desired object and for coinciding the position 
of the image forming lens with the focused position. 
Here, of the objects to which the lens may be focused, the nearest and the 
remotest (farthest) objects from the camera will be referred to as the 
nearest and the remotest objects, and the distances thereto will be 
referred to as the nearest and the remotest distances, and the distance 
range between the nearest and the remotest objects will be referred to as 
the focusable distance range. 
In FIG. 1, a picture-taking or objective lens 11 in a camera to be 
automatically focused is movable in the direction of the optic axis 
between the infinity position D.sub.o and the nearest distance position 
D.sub.e and when the lens 11 is in the position D.sub.o, the image of the 
object at infinity, namely, the remotest object, is properly focused or in 
focus to a film surface F, and when the lens is in the position D.sub.e, 
the image of the nearest object is properly focused or in focus to the 
film surface F. 
A reproduction unit 1 includes an auxiliary lens 1a, a photoelectric 
element 1b and a driving device 1c. The auxiliary lens 1a may form the 
image of substantially the same object as the picture-taking lens 11. The 
photoelectric element 1b may be moved at a uniform velocity in the 
direction of the optic axis of the lens 1a between the infinity position 
P.sub.o indicated by solid line and the nearest position P.sub.e indicated 
by dotted line, by the driving device 1c. The auxiliary lens 1a may form 
the image of the infinity object on the photoelectric element 1b when at 
the position P.sub.o and may form the image of the nearest object on the 
photoelectric element 1b when at the position P.sub.e. Thus, when the 
photoelectric element 1b is moved from the infinity position P.sub.o to 
the nearest position P.sub.e, the images of several objects lying within 
the focusable distance range of the picture-taking lens 11 may be 
successively formed on the photoelectric element 1 b. The photoelectric 
element 1b produces an output I.sub.s corresponding to the contrast of 
the image formed thereon and therefore, when it starts from the position 
P.sub.o at a point of time t.sub.o and reaches the position P.sub.e at a 
point of time t.sub.e, the output I.sub.s of this element assumes, for 
example, the waveform as shown in FIG. 2(a). The extremum or extremal 
values f.sub.1, f.sub.2 and f.sub.3 of this waveform represent a remote 
object, an intermediate object and a near object, respectively. Since this 
output I.sub.s represents the contrast of the object on the photoelectric 
element 1b, it will hereinafter be referred to as the contrast signal. 
The waveform of this contrast signal I.sub.s is similar to the contrast of 
the image on the film F when the picture-taking lens 11 is moved from the 
infinity position D.sub.o to the nearest distance position D.sub.e and 
therefore, the auxiliary lens 1a and the photoelectric element 1b 
reproduce the variation in contrast of the image on the film resulting 
from the aforementioned movement of the picture-taking lens. Such contrast 
signal may also be provided by moving the auxiliary lens 1a instead of the 
photoelectric element 1b in the direction of the optic axis. 
The output I.sub.s of the photoelectric element 1b is applied through an 
amplifier 2 to a gate circuit 3. A distance range selecting unit 4 sets a 
desired distance range within the focusable distance range by extraneous 
operation such that the present automatic focusing device effects the 
focusing operation only for the objects within said desired distance 
range. When the distance range selecting unit 4 selects a distance range, 
for example, from the infinite to the intermediate position between the 
objects f.sub.2 and f.sub.3, it produces a gate signal for opening the 
gate circuit 3 from a point of time t.sub.o to a point of time t.sub.y as 
shown in FIG. 2(b). Thus, the gate circuit 3 passes therethrough the 
contrast signal I.sub.s of the photoelectric element 1b from the point of 
time t.sub.o to the point of time t.sub.y. A peak detector 5 detects the 
maximum value of the so passed contrast signal I.sub.s and produces an 
output as shown in FIG. 2(c). A comparator 6 compares the output of the 
gate circuit 3 with the output of the peak detector 5 and produces a low 
or "L" level output only when the output of the peak detector 5 is not 
increased, as shown in FIG. 2(d). Accordingly, a point of time t2 whereat 
the output of the comparator 6 finally changes from "H" level to "L" level 
is the point of time whereat the contrast signal I.sub.s becomes maximum 
in the selected range of t.sub.o -t.sub.y. 
A pulse generator 8 generates a pulse upon starting of movement of the 
photoelectric element 1b. Of course, the driving device 1c may be operated 
by the pulse output from the pulse generator 8. A last-in/first-out shift 
register 7 writes or stores the output of the comparator 6 with the pulse 
generated by the pulse generator as the shift pulse from the point of time 
t.sub.o whereat the movement of the photoelectric element 1b is started 
till the point of time t.sub.e whereat the movement of the photoelectric 
element 1b is terminated, and memorizes the point of time t.sub.2. The 
time t.sub.2 so memorized represents the position of the photoelectric 
element 1b when the object f.sub.2 is imaged on the element 1b and 
therefore, the distance between the object f.sub.2 to be focused and the 
camera may be recognized from the time t.sub.2. The time t.sub.2 is 
converted into a distance and indicated by a circuit which will 
hereinafter be described. A detector 16 including a limit switch detects 
that the photoelectric element 1b has reached the position P.sub.e at the 
point of time t.sub.e, and produces a "H" level output to change over the 
shift register 7 into read-out condition. Thereby, in accordance with the 
shift pulse from the pulse generator 8, the shift register 7 effects 
read-out in the opposite direction from the writing. Namely, as shown in 
FIG. 3(a), the information at the point of time t.sub.e to the information 
at the point of time t.sub.o are read out successively. While the "L" 
level output of the shift register 7 regarding the points of time t.sub.e 
-t.sub.2 is being supplied to an AND gate 17a, the Q output terminal of a 
flip-flop 17b is at "H" level and as already noted, the output of the 
detector device 16 is at "H" level and therefore, an AND gate 17c is 
opened so that the clock pulses from the pulse generator 8 are applied 
through the AND gate 17c to a counter 17d which counts the clock pulses. 
The information read out from the shift register 7 next to the information 
of the point of time t.sub.2 is at "H" level as shown in FIG. 3(a), 
whereby the Q output of the flip-flop 17b is inverted into "L" level to 
close the AND gate 17c. Accordingly, thereafter, the pulse is not supplied 
to the counter 17d. Of course, even if the "H" level information is again 
read out during the read-out, the AND gate 17a is closed by the Q output 
of "L" level so that the AND gate 17c is not opened. In this manner, the 
time t.sub.e -t.sub.2 is counted by the counter 17d and it is converted 
into a distance by a conversion circuit 17e and indicated by an indicator 
17f. The distance so indicated represents the distance to the object 
f.sub.2 to be focused within the distance range selected by the distance 
range selecting unit 4. 
Description will now be made of automatically focusing the picture-taking 
lens to the object f.sub.2 by the output of the shift register 7. A 
potentiometer 10 produces a voltage 10s corresponding to the position of 
the picture-taking lens 11 prior to the focusing operation, as shown in 
FIG. 2(e). A step-wave generator 9 is responsive to the pulse from the 
pulse generator 8 to produce a step voltage 9s which is increased with the 
lapse of time from the starting point of time t.sub.o of the photoelectric 
element 1b, as shown in FIG. 2(e). A comparator 12, as shown in FIG. 2(f), 
produces a "H" level output when the step voltage 9s of the generator 9 is 
smaller than the output 10s of the potentiometer 10, and produces a "L" 
level output when the step voltage 9s is greater than the output 10s. The 
output of the comparator 12 is written into a last-in/first-out shift 
register 13 by the shift pulse from the pulse generator 8 in synchronism 
with the shift register 7. Thus, the inverting point of time t.sub.x of 
the output of the comparator 12 represents the position of the 
picture-taking lens 11 and accordingly, the position of this lens 11 is 
memorized in the shift register 13 as the inverting point of time t.sub.x. 
This register 13 reads out in the opposite direction from the writing in 
synchronism with the read-out of the shift register 7 by the "H" level 
output of a detector device 16. A gate circuit 14 receives the read-out 
output signals of the shift registers 7 and 13 shown in FIGS. 3(a) and 
(b), respectively, and puts out the difference between the first rising 
points of time t.sub.x and t.sub.2 of the respective signals as the pulse 
number from the pulse generator 8, as shown in FIG. 3(c). This gate 
circuit 14 puts out the information representing the sign of the 
difference between the points of time t.sub.x and t.sub.2, as will 
hereinafter be described in detail. A driver unit 15 including a pulse 
motor or the like moves the picture-taking lens 11 by an amount 
corresponding to the pulse number from the gate circuit 14 in a direction 
corresponding to the information representing said sign therefrom to focus 
the lens 11 to the object f.sub.2. 
A null method type indicator 18 indicates the difference between the object 
distance corresponding to the position of the picture-taking lens 11 prior 
to the automatically focusing and the object distance to be focused within 
the selected distance range detected by the range finding system 1-7, in 
accordance with the output of the gate circuit 14. 
Reference is now made to FIG. 4 to describe a specific example of the 
distance range selecting unit 4. A potentiometer 4a produces a voltage 
output corresponding to the distance selected within the focusable 
distance range of the picture-taking lens 11. A comparator 4b compares the 
output of the potentiometer 4a with the voltage output 9s of the step-wave 
generator 9. A flip-flop 4c produces a "H" level output at an l output 
terminal when the output of the potentiometer 4a is greater than the 
voltage output of the step-wave generator 9, and produces a "L" level 
output when the former is smaller than the latter. An m output terminal of 
the flip-flop 4c assumes the opposite polarity. Accordingly, the output 
relation between the potentiometer 4a and the step-wave generator 9 and 
the l output terminal of the flip-flop 4c is similar to the output 
relation between the potentiometer 10 and the step-wave generator 9 and 
the comparator 4b shown in FIGS. 2(e) and (f). A terminal 4d is used to 
determine whether the selected distance range is from infinity t.sub.o to 
the aforementioned selected distance t.sub.y as shown in FIG. 2(b) or from 
the nearest distance t.sub.e to the aforementioned selected distance 
t.sub.y as shown in FIG. 6(b), and when a "H" level input is supplied to 
this terminal 4d, the output of the l terminal of the flip-flop 4c shown 
in FIG. 2(b) appears at the output terminal of this selecting unit 4 
through an AND gate 4e and an OR gate 4f. On the other hand, when a "L" 
level input is supplied to the terminal 4d, an AND gate 4h is opened by an 
inverter 4g so that the output of the m terminal of the flip-flop 4c shown 
in FIG. 6(b) appears at the output terminal of the selecting unit 4 
through the gate 4h and an OR gate 4f. A terminal 4i is used to select the 
entire focusable distance range and an "H" level input may be supplied for 
this purpose. 
A specific example of the gate circuit 14 is shown in FIG. 5. Flip-flops 
14a and 14b receives as input the outputs of the shift registers 7 and 13, 
respectively. If the rising t.sub.x is supplied earlier than the rising 
t.sub.2 to the flip-flop 14b as shown in FIGS. 3(a) and (b), an AND gate 
14d puts out clock pulses from the pulse generator 8 for a time 
corresponding to the difference (t.sub.x -t.sub.2) and in the converse 
case, an AND gate 14c puts out clock pulses. The outputs of these AND 
gates 14c and 14d are supplied to the normal rotation input terminal and 
the reverse rotation input terminal of the pulse motor of the driver means 
16, respectively. 
The operation of the first embodiment hitherto described may be summarized 
as follows. First, the photoelectric element 1b of the reproduction until 
1 is moved at a uniform velocity from the infinity position P.sub.o to the 
nearest position P.sub.e by the driving system 1c to prepare the contrast 
signal as shown in FIG. 2(a). Simultaneously with the starting of the 
movement of the photoelectric element, the clock pulse generator 8 is 
operated and the step-wave generator 9 puts out a step voltage 9s which 
increases with time as shown in FIG. 2(e). The distance range selecting 
unit 4 compares this step voltage with a voltage of the potentiometer 4a 
(FIG. 4) corresponding to the limit of the desired distance range 
determined in accordance with the intention and transmits the gate signal 
to the gate circuit 3 by a predetermined time (t.sub.o -t.sub.y) 
corresponding to the desired selected distance range, as shown in FIG. 
2(b). The gate circuit 3 transmits the contrast signal to the peak 
detector 5 in accordance with the gate signal for the predetermined time 
(t.sub.o -t.sub.y) corresponding to the desired distance range. The 
contrast signal so selected is transmitted to the shift register 7 through 
the peak detector 5 and the comparator 6. The shift register 7 is operated 
by the shift pulse from the aforementioned clock pulse generator 8 and 
memorizes the point of time t2 whereat the contrast signal corresponding 
to the desired distance range assumes its maximum value. On the other 
hand, the output of the step-wave generator 9 which is operated with the 
start of the movement of the photoelectric element 1b is transmitted to 
the comparator 12 and the position whereat the picture-taking lens 11 is 
placed is detected by the comparison between the output voltage of the 
step-wave generator 9 and the voltage 10s corresponding to the position of 
the picture-taking lens 11. The shift register 13 also memorizes the point 
of time t.sub.x whereat the position of the picture-taking lens 11 is 
detected, by the shift pulse from the aforementioned clock pulse generator 
8. The shift registers 7 and 13 are operated at a time and memorize the 
respective points of time. When the photoelectric element completes its 
entire stroke P.sub.o -P.sub.e, read-out in the opposite direction is 
effected from the shift registers 7 and 13 in synchronism therewith. The 
gate circuit 14, as shown in FIG. 3(c), puts out clock pulses from the 
outputs of the shift register 7 and 13 for a time corresponding to the 
difference between the point of time t2 whereat the maximum contrast is 
detected and the point of time t.sub.x whereat the position of the 
picture-taking lens 11 is detected. The driver means 15 receives this 
output to focus the picture-taking lens 11 to the object f2 having the 
highest contrast within the selected range. 
Of course, when a distance from infinity to the intermediate distance 
between the objects f.sub.1 and f.sub.2 is selected by the distance range 
selecting unit 4 as indicated by dotted line in FIG. 2(b), the distance 
from the camera to the object f.sub.1 for which the contrast signal 
assumes its extremal value within this range is indicated by the indicator 
17f in a manner similar to that described above, and the picture-taking 
lens 11 is focused to the object f.sub.1. 
Also, when a distance from the nearest distance to the intermediate 
distance between the objects f.sub.1 and f.sub.2 is selected as shown in 
FIG. 6(b), the output I.sub.s of the photoelectric element 1b becomes as 
shown in FIG. 6(a) in the same manner as already noted, and the outputs of 
the peak detector 5 and the comparator 6 become as shown in FIG. 6(c) and 
(d), respectively, and it is also possible to focus the picture-taking 
lens 11 to the object f.sub.2 having the highest contrast within this 
selected distance range. 
The first embodiment so far described can obtain a satisfactory result in 
most cases. However, where the contrast signal I.sub.s at the point of 
time t.sub.y representing the limit of the selected distance range is 
greater than the extremal value f.sub.1 of the contrast signal within the 
selected distance range, as shown in FIG. 7, the peak detector 5 does not 
detect the point of time t.sub.1 for this value f.sub.1 but the point of 
time t.sub.y. Thus, the desired object f.sub.1 is not in focus. This also 
holds true with the case of FIG. 8. In this manner, the first embodiment 
suffers from a disadvantage that where the selected distance range is not 
appropriate, the picture-taking lens 11 cannot be focused on the object 
within that range. 
Description will now be made of a second embodiment which overcomes such 
disadvantage. In this embodiment, the elements functionally similar to 
those in the first embodiment are given similar reference characters and 
need not be described. Description will first be made by taking as an 
example the case where the distance range selecting unit 4 selects a 
distance range for which the automatic focusing cannot take place in the 
first embodiment, as shown in FIG. 10(c) and in the same manner as in FIG. 
8(c). In FIG. 9, a differential coefficient discriminator circuit 21 
receives the output of an amplifier 2 which is varied with time and, as 
shown in FIG. 10(b), produces a "H" level signal when the differential 
coefficient of this output is positive, and produces an "L" level signal 
when the differential coefficient is negative. A specific example of the 
differential coefficient discriminator circuit 21 is shown in FIG. 12. An 
AND gate circuit 22 opens a gate circuit 3 when this signal and the 
aforementioned signal from the distance range selecting unit 4 are at "H" 
level, and thereby permits the output of the amplifier 2 to be transmitted 
to a peak detector 5. Consequently, the output of the gate circuit 3 
becomes as shown in FIG. 10(d). The outputs of the peak detector 5 and 
comparator 6 are provided in the same manner as that described in 
connection with the first embodiment and become as shown in FIGS. 10(e) 
and (f), respectively. 
An AND gate circuit 23 applies the pulse of the clock pulse generator 8 to 
the CK terminal of a D-type flip-flop 24 for the range t.sub.y -t.sub.e 
selected by the distance range selecting unit 4. The output of the 
comparator 6 shown in FIG. 10(f) is applied to the D terminal of the 
D-type flip-flop 24. After the point of time t.sub.y, clock pulses is 
supplied to the CK terminal of the flip-flop 24 through the AND gate 23, 
so that the output of the flip-flop circuit 24 becomes similar to the 
output of the comparator 6 (FIG. 10(f)), as shown in FIG. 10(g), and a 
point of time t.sub.5 whereat the output of the flip-flop circuit 24 
finally changes from "H" level to "L" level is coincident with a point of 
time whereat the contrast assumes its extremal value f.sub.3 within the 
selected distance range. More strictly, the output of the D-type flip-flop 
24 is delayed by a period of the clock pulse at maximum with respect to 
the output of the comparator 6, but the period of the clock pulse is 
sufficiently small so that the two may be regarded as being substantially 
coincident. If the point of time t.sub.5 is read out from the shift 
register 7 in the same manner as that described in connection with the 
previous embodiment, it will be possible to detect the distance of the 
object f.sub.3. Thus, the function of the differential coefficient 
discriminator circuit 21 and the AND gate circuit 22 is to realize that 
even the maximum value of the contrast signal within the selected distance 
range shown in FIG. 10(c) may have a negative differential coefficient and 
that if not an extremal value, the peak detector 5 may not detect this 
maximum value. 
Also, as shown in FIGS. 11(a) and (c) wherein the contrast signal at the 
limit t.sub.y of the selected distance range is the maximum value of the 
contrast signal within this selected distance range and not an extremal 
value but the differential coefficient thereat is positive, there may be 
provided the outputs of the differential coefficient discriminator circuit 
21, the gate circuit 3, the peak detector 5 and the comparator 6, as shown 
in FIGS. 11(b), (d), (e) and (f), respectively. Since the clock pulses 
from the pulse generator 8 is supplied to the D-type flip-flop circuit 24 
through the AND gate 23 during the time (t.sub.o -t.sub.y), the flip-flop 
24 produces an output of the same waveform as the output waveform of the 
comparator 6 shown in FIG. 11(f) for the time (t.sub.o -t.sub.y) as shown 
in FIG. 11(g), but after the point of time t.sub.y, namely, outside the 
selected distance range, the supply of the clock pulse to the flip-flop 24 
is cut off, so that the output of the flip-flop 24 holds the output level, 
i.e. the "H" level even after the point of time t.sub.y, as shown in FIG. 
11(g). The point of time t.sub.1 whereat the output of the flip-flop 24 
finally changes from its "H" level to its "L" level represents the 
extremal value f.sub.1 of the contrast within the selected distance range 
of the distance range selecting unit 4. When the point of time t.sub.1 
whereat the contrast becomes extremal is read out from the shift register 
7, it will be possible to detect the distance of the desired object 
f.sub.1 within the selected distance range. Thus, the function of the AND 
gate 23 and the D-type flip-flop circuit 24 is to realize that even the 
maximum value of the contrast signal within the selected distance range 
may have a positive differential coefficient and that if not an extremal 
value, this maximum value may not be detected. 
In the second embodiment, the detection of the maximum value other than the 
extremal value of the contrast signal within the selected distance range 
may be thus avoided by the set of the differential coefficient 
discriminator circuit 21 and the AND gate 22 and the set of the AND gate 
23 and the D-type flip-flop 24. 
Another D-type flip-flop circuit 25 is provided to detect the position of 
the picture-taking lens 11 in synchronism with the aforementioned 
flip-flop circuit 24, and it compensates for the delay of the output 
signal of the flip-flop circuit 24 with respect to the detection signal of 
the position of the picture-taking lens. The shift register 13 receives 
the output of this flip-flop circuit 25. In the other points, the 
construction of the second embodiment is similar to that of the first 
embodiment and need not be described. 
In FIG. 13 which shows a third embodiment of the present invention, the 
reproduction unit 1 includes an auxiliary lens 1.sub.a moved in the 
direction of the optic axis by a biasing member or the like, and a fixed 
photoelectric element 1.sub.b. The auxiliary lens 1.sub.a is moved between 
the infinity position d.sub.o and the nearest position d.sub.e and at the 
position d.sub.o, it forms on the photoelectric element 1.sub.b the image 
of the remotest object, namely, the infinity object, with respect to the 
unshown picture-taking lens to be automatically focused and at the 
position d.sub.e, it forms on the photoelectric element 1.sub.b the image 
of the nearest object with respect to the picture-taking lens. The 
photoelectric element, as shown in FIG. 14(a), puts out the variation in 
contrast of the image resulting from the variation in position of the 
picture-taking lens as an electrical signal I.sub.s (contrast signal) 
which is varied by the displacement of the position of the auxiliary lens 
1.sub.a. A lens position detector device 31 receives the signal from the 
auxiliary lens 1.sub.a of the reproduction unit 1 and converts the 
position of this lens into a voltage and puts out the same. The output 
voltage of the detector device 31 is so set that it is linearly decreased 
as the auxiliary lens 1.sub.a is moved from the infinity position d.sub.o 
to the nearest position d.sub.e, as shown in FIG. 14(b). 
The distance range selecting unit 4 prepares a gate signal to the gate 
circuit 3 in accordance with the distance range selected by extraneous 
operation. The specific construction of this selecting unit 4 is entirely 
identical to the aforementioned one with the exception that the output of 
the position detector device 31, instead of the output of the step-wave 
generator 9, is applied to one input terminal of the comparator 4b of FIG. 
4. By this, the potentiometer 4a of FIG. 4 is so set that the output 
thereof assumes the level as indicated by dot-and-dash line in FIG. 14(b), 
and a "L" level output is applied to the terminal 4d of FIG. 4, thereby 
providing a gate signal as shown in FIG. 14(c). 
The amplifier 2, the gate circuit 3, the peak detector 5 and the comparator 
6 are similar in construction to those in the previously described 
embodiments and produce the outputs as shown in FIGS. 14(d), (e) and (f) 
in accordance with the variation in position of the auxiliary lens 
1.sub.a. A sample hold circuit 32 receives the signals from the lens 
position detector device 31 and the comparator 6 and, when the output of 
the comparator 6 is at "H" level, it puts outs the voltage of the detector 
device 31 and, when the output of the comparator 6 assumes "L" level, it 
holds and puts out the then voltage from the detector device 31. The 
output voltage of the sample hold circuit 32 shown in FIG. 14(g) 
represents the distance to the object lying within the selected distance 
range. 
Another lens position detector device 33 converts into a voltage the 
position of the picture-taking lens which starts moving after the 
auxiliary lens 1.sub.a has moved over its entire stroke d.sub.o -d.sub.e. 
The lens position detector devices 21 and 33 are designed so as to put out 
the same voltage when the respective lenses are focused to a common 
object. That is, the output of the detector device 33 is varied in the 
same manner as the output of the detector device 31 when the 
picture-taking lens is moved from the infinity position to the nearest 
position. 
A comparator 34 compares the output of the sample hold circuit 32 with the 
output of the lens position detector device 33 and, when the latter is 
greater than the former, it supplies power to an electromagnet 35 and when 
the latter is smaller than the former, it cuts off the power supply. 
Therefore, when the auxiliary lens 1.sub.a has completed its movement but 
the picture-taking lens is resting at its infinity position, the output 
voltage of the lens position detector device 33 is greater than the output 
voltage of the sample hold circuit 32 so that the electromagnet 35 is 
energized to attract and hold a pawl 36. Thereafter, when the 
picture-taking lens is moved by unshown means and the lens has reached the 
position d.sub.2 with the output of the lens position detector device 33 
decreased as shown in FIG. 14(h), the output of the detector device 33 
becomes equal to the output of the sample hold circuit 32 so that the 
current flowing to the electromagnet 35 is cut away to release the pawl 
36. Then, the picture-taking lens is restrained at its position d.sub.2 by 
the pawl 36 to thereby enable the picture-taking lens to be focused to an 
object within the selected distance range. 
In the above-described embodiments, the reproduction unit 1 scans the 
entire focusable distance range of the picture-taking lens and generates 
an output relating to the distance of the object lying within that range 
and thus, the reproduction unit is not restricted to the present 
embodiment but may be applied to any range finder device and for example, 
the finder device disclosed in U.S. Pat. Nos. 4,002,899 and 4,059,757.