U.S. patent number 3,798,662 [Application Number 05/284,351] was granted by the patent office on 1974-03-19 for digital exposure time determining system for cameras.
This patent grant is currently assigned to Asahi Kogaku Kogyo Kabushiki Kaisha. Invention is credited to Shinichi Suzuki.
United States Patent |
3,798,662 |
Suzuki |
March 19, 1974 |
DIGITAL EXPOSURE TIME DETERMINING SYSTEM FOR CAMERAS
Abstract
An electrical computer circuit for determining exposure time
according to a digital system. A series of timing circuits are
provided for respectively providing different exposure times
according to which of the timing circuits is selected for
operation. A particular one of the timing circuits is selected for
operation as a result of comparing by suitable comparing circuits
an electrical quantity corresponding to exposure time with a
plurality of reference electrical quantities, a series of selecting
circuits being provided to select from that one of the comparing
circuits whose reference electrical quantity is closest to the
electrical quantity corresponding to exposure time a signal to be
transmitted to a corresponding timing circuit.
Inventors: |
Suzuki; Shinichi (Fukuoka,
JA) |
Assignee: |
Asahi Kogaku Kogyo Kabushiki
Kaisha (Tokyo-to, JA)
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Family
ID: |
27987615 |
Appl.
No.: |
05/284,351 |
Filed: |
August 28, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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259214 |
Jun 2, 1972 |
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Foreign Application Priority Data
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Sep 7, 1971 [JA] |
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46-81201 |
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Current U.S.
Class: |
396/254;
396/287 |
Current CPC
Class: |
G03B
7/093 (20130101); G03B 7/083 (20130101) |
Current International
Class: |
G03B
7/093 (20060101); G03B 7/083 (20060101); G03B
7/08 (20060101); G03B 7/091 (20060101); G03b
007/08 () |
Field of
Search: |
;95/1CT ;307/215
;324/99D,14R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Peters, Jr.; Joseph F.
Assistant Examiner: Gellner; Michael L.
Attorney, Agent or Firm: Steinberg & Blake
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation in part of copending application
Ser. No. 259,214, filed June 2, 1972 and entitled SYSTEM FOR
INDICATING THE MAGNITUDE OF AN ELECTRICAL QUANTITY.
Claims
What is claimed is:
1. In a camera, light-responsive means for responding to the
factors of object brightness, exposure aperture, and film speed to
determine therefrom an electrical output quantity corresponding to
exposure time, a series of sequentially arranged reference circuit
means for respectively providing D.C. reference electrical
quantities which differ in magnitude one from the next by a given
increment, a series of comparing circuit means respectively
connected electrically with said series of reference circuit means
and arranged in the same sequence as the latter for receiving the
reference electrical quantities therefrom, all of said comparing
circuit means being electrically connected with said
light-responsive circuit means for receiving said output quantity
therefrom and for comparing said output quantity with all of said
D.C. reference electrical quantities, said comparing circuit means
producing a first type of output for all said D.C. reference
electrical quantities equal to or less than said output quantity
and a second type of output for all said D.C. reference electrical
quantities greater than said output quantity, a series of selecting
circuit means sequentially arranged in the same way as said series
of comparing circuit means and respectively connected electrically
therewith for respectively receiving inputs therefrom, said series
of selecting circuit means being responsive to a transition between
said first type of output and said second type of output for
providing an output signal at only that one of the selecting
circuit means which receives an input from that one of said
comparing circuit means which receives a reference electrical
quantity closest to said output quantity, a series of timing
circuit means sequentially arranged in the same way as said series
of selecting circuit means and electrically connected respectively
therewith for receiving said output signal at that one of said
timing circuit means which is arranged in the sequence thereof at a
position corresponding to the position of that one of said
selecting circuit means in the sequence of the latter which
provides said output signal, said plurality of timing circuit means
respectively providing a sequential series of timing signals which
differ one from the next by a given increment with the timing
signals provided by said series of timing circuit means
respectively corresponding sequentially to the sequential series of
reference electrical quantities provided by said plurality of
reference circuit means so that a timing signal is provided only by
that one of said timing circuit means which determines a timing
interval corresponding to said output quantity provided by said
light-responsive circuit means, and exposure-terminating circuit
means electrically connected with all of said timing circuit means
to be actuated only by that one of said timing circuit means which
receives said output signal for terminating an exposure after an
interval corresponding to the output quantity provided by said
light-responsive circuit means.
2. The combination of claim 1 and wherein a plurality of
sequentially arranged indicating circuit means are respectively
connected electrically to said plurality of selecting circuit means
in the same sequence as the latter for indicating only at one of
said indicating circuit means the exposure time which will be
provided.
3. The combination of claim 2 and wherein said plurality of
indicating circuit means respectively include a plurality of
luminescent diodes one of which is illuminated by said output
signal with said luminous diodes being arranged in a sequence which
will indicate the exposure time according to the sequential
location of that one of said luminous diodes which becomes
illuminated.
4. The combination of claim 1 and wherein said light-responsive
circuit means includes a photosensitive means for receiving an
internal signal object brightness from light which has already
passed through an objective of the camera, and a plurality of
memory circuit means sequentially arranged with respect to each
other in the same way as said sequentially arranged series of
selecting circuit means and said sequentially arranged series of
timing circuit means, said sequentially arranged series of memory
circuit means being respectively connected electrically between
said series of selecting circuit means and said series of timing
circuit means for retaining the output signal for transmission to
one of said timing circuit means.
5. The combination of claim 4 and wherein a series of sequentially
arranged indicating circuit means are respectively connected
electrically to said series of sequentially arranged memory circuit
means for receiving through the latter a signal corresponding to
the selected exposure time which is selected by one of said
selecting circuit means and for providing an indication of the
selected exposure time.
6. The combination of claim 1 and wherein said series of timing
circuit means respectively include a series of resistors the
magnitude of which corresponds to the timing signals, a series of
switching transistors respectively connected electrically with said
resistors and said series of selecting circuit means, and a timing
capacitor electrically connected with all of said resistors to be
charged according to the magnitude of that one of said resistors
which receives said output signal.
7. The combination of claim 1 and wherein each of the selecting
circuit means includes an AND gate and a NOT gate, the AND gate
receiving as one input the output from its corresponding one of the
comparing circuit means, the NOT gate receiving as its input the
output from the next successive comparing circuit means, the output
from the NOT gate serving as a second input to the AND gate and the
output from the AND gate serving as the output from the selecting
circuit means.
Description
BACKGROUND OF THE INVENTION
The present invention relates to cameras.
More particularly, the present invention relates to computer
systems for automatically computing the exposure time in a
camera.
It is conventional with camera computers of this type to provide
analog memory operations by way of a capacitor. Thus, in accordance
with the photographic factors such as object brightness, selected
aperture size, and film speed, the required exposure time is
computed and an electrical magnitude corresponding to the required
exposure time is stored temporarily at a capacitor, particularly in
the case where the intensity of the light is measured internally
after the light has traveled through the objective of the camera,
as is conventional with certain types of single lens reflex
cameras.
With these conventional structures according to which the memory
operations are carried out by a capacitor, a low degree of accuracy
has been encountered due to current leakage from the capacitor when
the storing or memorizing operations are extended over a fairly
long period of time. As a result capacitors of large capacity are
required in order to achieve the required accuracy. The capacitors
which are used for such a purpose therefore have a large size and
the time required for the completion of the memory operations is
undesirably extended, so that the shutter cannot be actuated as
frequently as desired and the usefulness of the camera, including
the automatically controlled shutter thereof, is decreased.
SUMMARY OF THE INVENTION
It is accordingly a primary object of the present invention to
provide for a camera an exposure time computing system which will
avoid these drawbacks.
In particular, it is an object of the invention to provide a system
which does not require the storage or memory capacitors which are
conventionally used to store information in the form of an
electrical quantity which is transmitted to the structure which
determines when the shutter will close to terminate an
exposure.
Moreover, it is an object of the present invention to provide a
structure of this type which does not require the use of meters
which are conventionally used and are another source of faulty
operation in conventional cameras.
In particular, it is an object of the present invention to provide
a computing system capable of operating almost instantaneously so
that the time lag involved in the operation of the system itself is
reduced to a minimum.
Furthermore, it is an object of the present invention to provide a
system which will operate in a digital manner so that the use of
meters or the like as is conventional with analog systems can be
avoided.
Furthermore, it is an object of the present invention to provide
for a system of the above type an indicating arrangement capable of
indicating the exposure time to the operator in a highly convenient
manner.
In particular, it is an object of the present invention to provide
an electrical computing system of the above type which is
exceedingly simple and reliable in operation and which at the same
time is capable of being accommodated in a relatively small space
so that the camera size is not undesirably increased by reason of
the structure of the present invention.
In accordance with the invention a light-responsive circuit means
is provided for producing an output quantity corresponding to
exposure time in accordance with parameters such as the brightness
of the light at the object which is to be photographed, the
selected exposure aperture size, and the speed of the film which is
exposed in the camera.
A plurality of reference circuit means are provided for providing a
series of reference electrical quantities which differ one from the
next by a given increment. A plurality of comparing circuit means
are respectively connected with the reference circuit means for
receiving the reference electrical quantities therefrom, and all of
these comparing circuit means are connected electrically with the
light-responsive circuit means for receiving therefrom the output
quantity corresponding to the exposure time. This output quantity
is compared at the several comparing circuit means with the
different reference electrical quantities received thereby
respectively from the plurality of reference circuit means, and a
plurality of selecting circuit means are electrically connected
with the series of comparing circuit means for determining which
one of the latter receives a reference electrical quantity closest
to the output quantity which corresponds to exposure time. This one
selected comparing circuit means provides a single output at the
corresponding one of the selecting circuit means, and this single
output is transmitted to one of the series of timing circuit means.
The series of timing circuit means respectively are capable of
providing different exposure times which differ one from the next
by a given increment, and in accordance with the sequential
arrangement and interconnection of the several series of circuits
the single output from one of the selecting circuit means will
trigger a particular one of the timing circuit means which will
provide an exposure time corresponding to the electrical quantity
determined by the light-responsive circuit means. This one timing
circuit means will actuate an exposure terminating circuit means
which terminates the exposure of the film after an exposure
interval corresponding to the time interval determined by the one
timing circuit means which responds to the one signal which is
transmitted from one of the plurality of selecting circuit
means.
BRIEF DESCRIPTION OF DRAWINGS
The invention is illustrated by way of example in the accompanying
drawings which form part of the present application and in
which:
FIG. 1 is a schematic block diagram illustrating the entire system
of the present invention;
FIG. 2 is a circuit illustrating the light-responsive circuit means
of the invention which determines the output electrical quantity
corresponding to exposure time;
FIG. 3 is a schematic representation of a plurality of reference
circuit means which provides reference electrical quantities;
FIG. 4 is a wiring diagram of a comparing circuit;
FIG. 5 is a diagram of some of the selecting circuits;
FIG. 6 is a diagrammatic illustration of a memory circuit means
used when light is measured internally after passing through the
camera objective;
FIG. 7 is a schematic representation of an indicator means for
indicating the exposure time;
FIG. 8 is a diagrammatic representation of the timing circuit
means; and
FIG. 9 is a wiring diagram of the exposure terminating circuit
means.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the block diagram of FIG. 1, the light which enters
through the objective is measured by a photosensitive
light-receiving means 1 while the diaphragm is fully open, and this
photosensitive means converts the light electrically into the
corresponding electrical quantity in the form of a voltage or
current. The signal from the photosensitive means 1 is transmitted
to a logarithmic compression section 2 composed, for example, of
diodes which convert the incident light, the intensity of which
varies according to a geometric progression of two, into a linear
electrical signal which is transmitted to a computing amplifying
section 3. Computation is carried out in accordance with a
photographic formula by use of three different input signals such
as the light input signal, the film speed signal, and an F signal
according to the selected exposure aperture. The latter two signals
are developed in the form of an F.sup.. ASA signal developing
section 4 and also transmitted to the computing amplifying section
3.
Of course, all of these operations are carried out according to the
Additive System of Photographic Exposure and are almost equal to
the first half steps of well-known prior art electrical shutter
controlling systems in which this sequence of operational steps
such as light measurement, logarithmic compression, computation,
logarithmic expansion, and timing are performed in the latter
order. In such a controlling system, an analog exposure time is
obtained by use of the output signal from this computation.
According to the present invention, however, the output signal from
the light-responsive means which includes the components 1-4 is
initially quantized by comparing the output quantity with a
reference quantity in the form, for example, of a sampling voltage.
The quantized signal is then used for selectively operating the
required timing circuit so as to develop in this way the exposure
time.
With the particular advantage of the invention described below and
shown in the drawings there are eleven different exposure times
corresponding to 1/1000 sec, 1/500 sec . . . , 1/2 sec. 1/1 sec. A
threshold or sampling voltage forms a reference electrical
quantity, and a series of these reference electrical quantities are
obtained from a series of reference circuits 6 which are arranged
in a suitable sequence, eleven such circuits being provided in the
illustrated example. Thus, the plurality of reference circuit means
6 operate to develop eleven different electrical quantities in the
form of reference voltages which are sequentially arranged so as to
differ by equal intervals from one to the next. Each of these
voltage levels is applied to a corresponding comparing circuit in
the form of a plurality of comparing means 5 which form a series of
comparing circuit means electrically connected respectively to and
in a corresponding sequence to the series of reference circuit
means 6. The output quantity from the computing section 3 of the
light-responsive circuit means is in the form of an electrical
quantity corresponding to the required exposure time, and this
output is applied to all of the comparing circuit means 5 while the
latter receive the different reference electrical quantities so
that the one output from the light-responsive circuit means is
compared with the several reference quantities.
Assuming that the output quantity from light-responsive circuit
means, which is proportional to the APEX number T.sub.V of the
exposure time, is represented by V.sub.T, then V.sub.T is compared
with eleven different reference or threshold voltages V.sub.1,
V.sub.2 . . . V.sub.11. The several comparing circuit means 5 are
constructed in such a way that those which receive reference
voltages less than V.sub.T will provide a logic output 1, while the
remaining plurality of comparing circuit means receive reference
voltages greater than V.sub.T and will develop a logic output O. In
this way the output quantity V.sub.T of the light-responsive
circuit means 1-4 is quantized by the use of eleven different
reference electrical quantities in the form of the sampling
voltages derived from the several reference circuit means 6.
FIG. 2 illustrates the circuitry of the light-responsive means 1-4.
Thus, referring to FIG. 2 it will be seen that the light-responsive
means is formed by the CdS component forming a cadmium sulfide
photoconductive element and responding to the light intensity for
providing a corresponding electrical quantity. The exposure
aperture size and the film speed are introduced by way of the
variable resistor R.sub.F.ASA so that in this way the diaphragm
setting and film speed are introduced into the circuitry. The
photosensitive means formed by the CdS element electrically
converts the incident light into an electrical voltage which varies
according to a geometrical progression of two. This varying voltage
is applied to a diode group D.sub.1 so as to be logarithmically
compressed thereby. The voltage output from the variable resistor
R.sub.F.ASA, which varies according to a geometrical progression of
two also, is compressed logarithmically by a second diode group
D.sub.2. These logarithmically compressed voltages from the diode
groups D.sub.1 and D.sub.2 are applied simultaneously to a
differential amplifying computations circuit so as to attain the
output quantity V.sub.T, at the terminal 13, corresponding to the
required exposure time.
FIG. 3 illustrates a simple series of reference circuit means used
to develop the several threshold voltages. Referring to FIG. 3,
there are connected in series a plurality of resistors which have
equal resistances. The battery 15 forms the voltage source
connected in series with the series connected resistors 14. The
plurality of terminals 5-1, 5-2, . . . 5-11 are adapted to apply a
constant biasing input to the several comparing circuit means,
respectively.
FIG. 4 illustrates one of the comparing circuit means by way of
example. Thus, the series of comparing circuit means are each
composed of circuitry as illustrated in FIG. 4. Each comparing
circuit has an input terminal 17 and the several input terminals 17
of the several comparing circuit means 5 are respectively connected
electrically with the several output terminals 5-1, 5-2 . . . 5-11
of the sequential series of reference circuit means 6. On the other
hand, the eleven comparing circuit means 5 respectively have second
input terminals 16 all of which are connected electrically to the
output terminal 13 of the light-responsive circuit means which is
illustrated in FIG. 2. Thus, all of the input terminals 16 are
connected to a common input terminal 13. Depending upon whether
V.sub.T is larger or smaller than V.sub.X, which represents the
reference voltage derived from each of the reference circuit means
6, either a logic output 1 or a logic output 0 is developed at the
several output terminals 18 of the eleven comparing circuit
means.
The sequential series of comparing means 5 are respectively
connected electrically with a sequential series of selecting
circuit means 7. The series of selective circuit means 7 shown in
FIG. 1 operate to select from the several comparing circuits which
provide the logic output 1 a specific comparing circuit which is
biased by the highest reference voltage less than the threshold
voltage. The several selecting circuit means which respond only to
the several comparing circuit means which provide the logic output
1 select from the latter group only that one comparing circuit
which receives a reference quantity in the form of a threshold
voltage which is closest to the computed output quantity
V.sub.T.
FIG. 5 illustrates the construction of the plurality of selecting
circuit means 7. Each individual selecting circuit means includes a
combination of a NOT gate 19 and an AND gate 20. The plurality of
selecting circuit means 7 are connected electrically to the outputs
18 of the plurality of comparing circuit means 5. As will be seen
from FIG. 5, the NOT gate of each selecting circuit means 7
receives the output from the terminal 18 of a corresponding
comparing circuit means 5, while the AND gate 20 receives the
output from the terminal 18 of the next comparing circuit means 5,
and in this way that one of the selecting circuit means which is
situated at the transition between the plurality of comparing
circuits which receive reference voltages less than the voltage
V.sub.T and the group which receive reference voltages greater than
the voltage V.sub.T is detected so that a corresponding selection
is made. Thus, only one selecting circuit of the plurality of
selecting circuit means 7 will develop a logic output 1, while all
of the other selecting circuits develop a logic output 0. As a
result one subsequent circuit will receive the logic output 1 to be
excited thereby.
In the above example it is assumed that the photosensitive means
CdS is an internal photosensitive means situated in the interior of
the camera to receive light after it has traveled through the
objective of the camera. With a camera of this type it is necessary
to provide a plurality of memory circuit means 8 so that even after
the quick-return mirror of the camera has been swung up by
depressing the shutter-release button and the leading curtain has
started to run so as to start the exposure and terminate the
receiving of light by the photosensitive means, the memory circuit
means operates in such a way that it will retain the computed
output quantity V.sub.T corresponding to the light value which is
measured immediately before release of the shutter. Also, the logic
output 1 obtained from one of the selecting circuit means by
quantizing with respect to the computation output quantity V.sub.T
is retained at least until the trailing shutter curtain starts to
run down so as to terminate the exposure.
FIG. 6 illustrates details of the memory circuit means. The memory
circuit illustrated in FIG. 6 includes a combination of memory
switch 21, a NOT gate 22, and a NAND gate 23. During light
measuring operations when the shutter is cocked, the memory switch
21 is maintained in an open state so that logic output 1 from the
selecting circuit means can be transmitted at any time to a series
of indicating circuit means 10 one of which is energized to
indicate the exposure time. Depending upon the variation of the
output quantity V.sub.T, the selecting circuit which develops the
logic output also changes with the result that there is a variation
in the indication of exposure time at the plurality of indicating
circuit means 10.
When the shutter is released to make an exposure, the memory switch
21 closes simultaneously and the series of memory circuit means 8
operate in such a way that only that one of the memory circuits
which receives the logic input 1 from one of the selecting circuit
means transfers the logic output 1 to a subsequent stage formed by
the plurality of timing circuit means 9 referred to below. In this
way each memory circuit means 8 maintains the same state as at the
time immediately before interruption of the light input, even
though there has been an interruption in the light input to the
photosensitive means CdS. At the same time, it is possible through
the memory circuit means to transmit a signal to one of a series of
indicating circuit means 10 for indicating at the latter the
exposure time while only one of the timing circuit means 9 is
selected for operation, each of the timing circuit means 9 being
composed, for example, of a capacitor and resistor, although
actually there is one capacitor used in common with the plurality
of resistors of the several timing circuits, as will be apparent
from the description below.
Referring to FIG. 7, a simple construction for each of the several
indicating circuit means 10 is illustrated therein. Thus, the
output from the eleven memory circuits 8 are capable of being
applied respectively to the corresponding base of eleven
transistors of the several indicating circuit means which
respectively include lamps L.sub.1, L.sub.2 . . . L.sub.11 which
are correspondingly illuminated, depending upon which one of the
indicating circuit means receives the signal. Inasmuch as only one
memory circuit means among the entire series operates, the
particular indicating circuit means is electrically active
therewith, only one of the eleven indicating lamps is turned on.
When the memory switch 21 is in its open condition, one of the
lamps L.sub.1, L.sub.2, . . . L.sub.11 is selectively illuminated
depending upon the variation in the light input, and accordingly, a
suitable exposure time can be recognized by observation of the
indicating lamps. Thus, these lamps will be arranged in a series
according to which the position of the illuminated lamp in the
series will be indicative of the exposure time.
FIG. 8 illustrates the structure of the plurality of timing circuit
means 9. The several timing circuit means 9 respectively include
the resistors R.sub.1, R.sub.2 . . . R.sub.11 all connected
respectively at one of their ends to eleven switching transistors,
at the collector terminals of the latter, as illustrated in FIG. 8.
All of the resistors are connected at their other ends to a common
timing capacitor C so as to form an RC integrating circuit
therewith.
The timing switch 24, which bridges the capacitor C and which is
opened at precisely the same instant that the leading curtain runs
to start the exposure, by depression of the shutter-releasing
button, and the RC integrating circuit composed of one of the
resistors and the capacitor C starts the charging of the capacitor
C. The several magnitudes of the resistors R.sub.1, R.sub.2 . . .
R.sub.11 are selected with the magnitude of the capacitor C in such
a way that the time period, starting from the initiation of the
charging operation for the capacitor C and terminating at a time
when the terminal voltage therof reaches a predetermined value
becomes a suitable exposure time, and the charging of the capacitor
C of this predetermined value results in deenergizing of an
electromagnet 25 (FIG. 9) which retains the trailing curtain in its
cocked position ready to run down and terminate the exposure upon
deenergizing the electromagnet 25. Thus, the electromagnet 25
releases the trailing curtain to terminate the exposure when the
capacitor C is charged to a predetermined extent, and thus the
circuitry of FIG. 9 forms the exposure-terminating circuit means 11
shown in FIG. 1. As may be seen from FIG. 9 the terminal voltage of
the capacitor C is applied to a Schmidt circuit as illustrated in
FIG. 9 to trigger the deenergizing of the electromagnet 25 when the
charge on the capacitor C at FIG. 8 reaches a given magnitude.
It is thus apparent that with the structure of the invention the
exposure time is digitally computed in that only one of a series of
discreet predetermined exposure times will be provided in
accordance with which of the resistors R.sub.1 . . . R.sub.11
receives the one signal which is transmitted through one of the
memory circuit means 8 from one of the selecting circuit means 7.
The sequentially arranged timing circuit means 9 are respectively
connected electrically with the sequentially arranged memory
circuit means 8 which in turn are sequentially connected with the
series of selecting circuit means 7 in such a way that only that
one output signal which is delivered by one of the selecting
circuit means 7, as referred to above, will be transmitted so as to
trigger in this way only one of the resistors of the several timing
circuit means to bring about a corresponding predetermined exposure
time which of course is very close to the exposure time called for
by the output quantity V.sub.T provided by the light-responsive
circuit means shown in FIG. 2 and formed by the components 1-4 of
FIG. 1.
Although the invention has been described above in connection with
a particular embodiment where one of eleven different magnitudes of
exposure time is selected, it is possible to provide a more finely
graded suitable exposure time by increasing the number of
gradations or in other words the number of discreet circuits which
determine the exposure time in a digital manner. The increase in
the number of gradations of course leads to an increase in the
number of circuit components. However, due to the modern
development in the field of integrated circuit techniques, it is an
extremely simple matter to form such circuits as single chips, so
that the size of the entire circuitry can be maintained extremely
small.
Many advantages are acheived with the present invention. Thus,
according to the present invention the photographic parameters are
first computed photographically and then quantized, so that there
is no analog memory operation by means of a capacitor or the like
as is required with previously known techniques using a
through-the-lens type of single lens reflex camera. With these
conventional operations, the accuracy of the memorizing phase by
means of a capacitor has been extremely low due to the current
leakage from the capacitor when the memorizing operation is
extended over a fairly long period of time. As a result, capacitors
must have a large capacity in order to achieve the required
accuracy. Therefore, the capacitors which are used become large and
the time required for completing the memorizing operations is
extended so that the shutter cannot be released very frequently,
and the usefulness of the camera including the automatically
controlled shutter thereof is decreased.
According to the present invention, however, the memorizing
operation requires no time duration whatsoever. This is of
extremely great advantage to the operation of the automatically
controlled shutter. In addition, the exposure time is indicated by
a digital indicator rather than a meter as is required with the
prior art devices, so that any mechanical disorders which are
conventionally encountered with meters are completely avoided with
the present invention. By using luminescent diodes rather than
conventional indicating lamps, the indicator means of the invention
will have an almost limitless operating life. Thus, the exposure
time controlling circuit including the indicator means constructed
in accordance with the present invention is highly reliable and
will not encounter any operation failures.
In addition, it is possible to provide the principles of the
present invention not only to TTL (through-the-lens) single lens
reflex cameras where the light is measured after traveling through
the objective but also to cameras where the light is measured
externally by a photosensitive means which is situated adjacent the
objective to receive light at the exterior of the latter rather
than light which passes through the objective. With a construction
of this latter type it is possible to omit the plurality of memory
circuit means 8 shown in FIG. 1, so that in this case the plurality
of indicating circuit means 10 are connected directly to the
several selecting circuit means 7, respectively, and the plurality
of timing circuit means 9 are connected directly to the plurality
of selecting circuit means 7.
* * * * *