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Claims  |
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What is claimed is:
1. A method of automatically controlling exposure in a photographic camera
with a lens equipped with an automatically adjusting diaphragm, said
method comprising the steps of:
including in said camera a control device for setting a working aperture of
said lens in response to a control signal;
providing a measuring signal proportional to a luminance of an object, a
sensitivity of a film and an initial aperture of the diaphragm;
providing a reference signal by combining said measuring signal and a
signal proportional to a shutter speed limit value;
electromechanically varying a signal proportional to said measuring signal
in accordance with the actual diaphragm aperture;
comparing continuously the electromechanically varying signal proportional
to the actual diaphragm aperture with said reference signal; and
providing said control signal when said signals in said comparing step are
equal.
2. The method of claim 1 wherein said providing a reference signal step
includes the step of combining said measuring signal and a signal
proportional to a shutter speed limit value in a one-to-one ratio.
3. An exposure control device for a photographic camera comprising:
a lens equipped with an automatically adjusting diaphragm, said diaphragm
adjustable from an initial aperture to stop at a working aperture setting
in response to a control setting;
means for generating a measuring signal proportional to a luminance of an
object, a sensitivity of a film and a diaphragm aperture of the lens;
storing means for storing said measuring signal at said initial aperture;
means for generating a reference signal indicative of a selected shutter
speed;
means for electromechanically varying a signal proportional to said
measuring signal in accordance with the actual diaphragm aperture; and
comparator means, responsive to said reference and said varying signals,
for providing a control signal to stop said automatically adjusting
diaphragm at said working aperture.
4. An exposure control device for a photographic camera comprising:
a lens equipped with an automatically adjusting diaphragm, said diaphragm
adjustable from an initial aperture to stop at a working aperture setting
in response to a control signal;
means for generating a measuring signal proportional to a luminance of an
object, a sensitivity of a film and an initial diaphragm aperture of the
lens;
storing means for storing said measuring signal at said initial aperture;
means for generating at least one signal indicative of a selected shutter
speed including means generating a shutter speed limit signal indicative
of the slowest shutter speed available in the camera;
means, responsive to said storing means, for providing a variation signal
of said stored measuring signal, said variation signal proportional to
movement of said diaphragm from an initial aperture to said working
aperture;
means for combining said stored measuring signal and said shutter speed
limit signal in a predetermined ratio and for providing a reference signal
indicative of said combination; and
comparator means, responsive to said reference and said variation signals,
for providing a control signal to stop said automatically adjusting
diaphragm at said working aperture.
5. An exposure control device according to claim 4, wherein said means for
providing a variation signal comprises a potentiometer having a slide
mechanically interconnected with said adjusting diaphragm.
6. An exposure control device according to claim 4, wherein said means for
combining comprises a voltage divider connected between said stored
measuring signal and said shutter speed limit signal, said voltage divider
comprises at least two resistances connected in series and having a
resistance ratio of 1:1.
7. An exposure control device according to claim 4, wherein said means for
combining comprises a voltage divider connected between said stored
measuring signal and said shutter speed limit signal, said voltage divider
comprises at least two resistances connected in series and having a
resistance ratio not equal to 1:1.
8. An exposure control device according to claim 4, wherein said means for
combining comprises a first voltage divider connecting said stored
measuring signal and said shutter speed limit signal, said divider having
a predetermined resistance ratio, said combination signal provided at a
terminal on said divider, said means for combining including means for
changing said resistance ratio in response to said stored measuring
signal.
9. An exposure control device according to claim 8, wherein said voltage
divider comprises at least two portions, a first portion connecting said
measuring signal and said terminal, and a second portion connecting said
shutter speed limit signal and said terminal, said means for changing said
resistance ratio comprises means for placing at least one further
resistance in parallel with one of said portions of said divider thereby
changing said resistance ratio.
10. An exposure control device according to claim 9, wherein said means for
placing comprises a transistor and a resistance connected with said
transistor's collector/emitter forming a series circuit, said series
circuit connected in parallel with one of said portions, said means for
placing further including a comparator whose output is connected to a base
of said transistor, said comparator having two inputs, one of which is
connected to said at least one signal indicative of said selected shutter
speed and the other of which is connected to said variation signal, said
comparator affecting conduction through said transistor and thus the
resistance ratio when said comparator input reaches a predetermined ratio.
11. An exposure control device according to claim 9, wherein:
said means generating at least one signal indicative of a selected shutter
speed comprises a second voltage divider, one end of which is grounded and
another end of which is connected to a variable voltage supply, said
second voltage divider including at least two intermediate terminals
disposed at different points on the divider between ground and said
voltage supply;
said means for placing comprises two separate circuits, each circuit
comprising a transistor and a resistance connected with said transistor's
collector/emitter forming a series circuit, each of said series circuits
connected in parallel with a respective one of said portions of said first
divider; and
said means for placing further including two comparators each having
outputs connected to respective bases of transistors, said comparators
each having two inputs, one input of each comparator connected to said
variation signal and the other input of each comparator connected to a
respective one of said two intermediate terminals.
12. An exposure control device according to one of claims 4-11, wherein
there is included switch means for controllably disconnecting said
combination signal from said one comparator means input and simultaneously
connecting said one comparator means input to said at least one signal
indicative of a selected shutter speed.
13. A method of automatically controlling exposure in a photographic camera
with a lens equipped with an automatically adjusting diaphragm, said
method comprising the steps of:
including in said camera a control device for setting a working aperture of
said lens in response to a control signal;
providing a measuring signal proportional to a luminance of an object, a
sensitivity of a film and an initial aperture of the diaphragm;
sensing a shutter speed of said camera;
providing a reference signal by combining said measuring signal and a
signal proportional to a shutter speed limit value in a ratio and
adjusting said ratio in response to said sensing step;
providing a variation of said measuring signal in proportion to the
automatic adjustment of said diaphragm aperture;
comparing said variation of said measuring signal with said reference
signal; and
providing said control signal when said signals in said comparing step are
equal. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates generally to photographic equipment and
particularly to automatic exposure control systems in a camera.
Automatic exposure control is known in a photographic camera with
exchangeable lenses where each lens is equipped with an automatically
adjusting diaphragm. A control device determines the magnitude of the
working aperture by comparing a signal potential which is proportional to
a shutter timing value and serves as the reference with a signal potential
which is proportional to the luminance of the scene, the sensitivity of
the film and the existing diaphragm aperture of the taking lens used. The
control device passes the signal resulting from said comparison to a
timing unit. Processes and devices of this type are intended to control
the exposure in a photographic camera so that following the selection of a
shutter speed value the diaphragm of the lens being used adjusts
automatically as a function of the scene luminance and film sensitivity to
an aperture yielding the correct exposure.
In West German Offenlegungsschrift No. 24 33 894 equivalent to U.S. Pat.
No. 3,964,073 an exposure control device for a photographic camera is
disclosed wherein an exposure measuring circuit initially produces a
signal proportional to the luminance of the scene, film sensitivity and
the initial aperture of the lens used. The signal is compared with the
signal indicative of the shutter speed value preselected on the camera.
The difference between the two potentials is an indication of the setting
of the lens diaphragm aperture necessary to produce a correct exposure.
Due to the movement of the lens diaphragm, released by the triggering of
the camera, the signal at the output of the exposure measuring circuit
varies continuously. When the potentials of the signals of the exposure
measuring circuit and the shutter speed value are equal, a comparator
activates a magnet and stops the closing of the diaphragm at the working
aperture.
Faults inherent in the diaphragm arresting device caused by certain time
constants as there are the dropping off of a magnet, the falling in of
latches etc, for instance operate to permit the closing of the diaphragm
beyond the set value intended and are detrimental to the exact shutter
speed desired for exposure of the film.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to eliminate the
deficiencies caused by time constant delays.
It is a further object of the present invention to provide an automatic
programming device with time priority for an automatic exposure control
camera.
It is a still further object of the present invention to provide a
controllably adjusting, automatic system for obtaining the proper exposure
value for film in an automatic camera.
The above and other objects are attained by the combination of apparatus
for and the method steps of: providing a control device for setting the
aperture of the lens in response to a comparison signal; providing a
measuring signal proportional to the luminance of the object, the
sensitivity of the film and the initial diaphragm aperture of the lens;
providing a reference signal by combining the measuring signal and a
signal proportional to the shutter speed limit value; providing a
variation of the measuring signal in proportion to the automatic
adjustment of the diaphragm aperture; comparing the variation of the
measuring signal with the reference signal; and providing a comparison
signal when the variation of the measuring signal is equal to the
reference signal.
In one preferred embodiment, the reference signal is generated by combining
the measuring signal and a signal proportional to the shutter speed limit
value in a 1:1 ratio. In a further preferred embodiment of the present
invention, a voltage divider is utilized to provide the reference signal
but the ratio of the measuring signal to the shutter speed limit signal is
controllably varied depending upon the selected shutter speed.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and the intended advantages
thereof will be more clearly understood by reference to the following
drawings, wherein:
FIG. 1 is an electrical schematic of a prior art exposure control device;
FIG. 2 is an electrical schematic of an exposure control device in
accordance with the present invention;
FIG. 3 is a graph plotting aperture settings versus shutter speed settings
for different combining ratios; and
FIG. 4 is an electrical schematic of a further embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Generally the present invention is an automatic exposure control for a
photographic camera with a lens which is equipped with an automatically
adjusting diaphragm. A control device determines the magnitude of the
working aperture by comparing a reference signal potential proportional to
a shutter speed value with a measured signal proportional to the scene
luminance, film sensitivity and the initial diaphragm aperture of the lens
used. The control device then passes the signal resulting from said
comparison to the diaphragm adjusting mechanism. However, in the present
invention: a measuring signal which is proportional to the scene
luminance, film sensitivity and initial aperture of the diaphragm is
stored; a a variation of the measured signal resulting from the automatic
adjustment of the diaphragm is generated electromechanically; and the
reference signal is generated by the combination of the measured signal
stored and a signal proportional to a limiting shutter speed value.
In a preferred embodiment, the reference signal may be formed as a function
of a resistance ratio and changes in priority of aperture and shutter
speed may be controlled by varying the resistance ratio. To achieve this
function, a storage means is inserted following the light measuring
device, which stores the measured signal proportional to the scene
luminance, film sensitivity and initial diaphragm aperture upon the
release of the camera. A disconnecting switch is provided to disconnect
the storage means following the arrival of the output signal generated by
the light measuring device from said light measuring device. An electric
structural element is provided to form an electric signal proportional to
the actual diaphragm aperture. In one embodiment, the electrical
structural element is a potentiometer having a slide mechanically
interconnected with the continuously varying diaphragm. Further, the
voltage divider may consist of resistances connected in series in a
resistance ratio of 1:1. If another ratio of the resistances is preferred
for the generation of the reference voltage, the resistances connected in
series may assume a resistance ratio not equal to 1:1. A shutter speed
voltage generator is connected to a voltage divider and provides a signal
that is proportional to a limiting value of the shutter speed. The voltage
divider combines the shutter speed limit signal and the signal stored in
the storage means.
It may be desirable that upon the variation of the exposure value (E.sub.V)
for different conditions the aperture and shutter speed should not change
in an equivalent manner but with differential priorities. For example, to
attain a range of shutter speeds safe with respect to blurring as soon as
possible, means capable of varying the reference signal may be included
between the shutter speed voltage generator and the voltage divider, in
order to vary the action priority between the diaphragm and timing. For
this reason, comparators and transistors may be provided as circuit
elements. It is proposed as a further embodiment of the invention to
provide a switch in the exposure control device capable of cutting out the
generation of the reference signal, so that the photographic camera may
also be operated with the automatic diaphragm device as known from the
above-cited reference.
Referring now to the drawings wherein like reference characters designate
like parts throughout the several views, FIG. 1 shows a known exposure
control device with an automatic diaphragm adjustment. It consists of a
light measuring unit 1, which includes a photoelectric receiver system 2,
an adder stage 3 and a film sensitivity transmitter 4. Also in the control
device is a shutter speed value transmitter 5, a comparator stage 6 and a
holding magnet 7 which is mechanically connected (as shown by the dashed
lines) with the automatically and continuously self-adjusting diaphragm 8
of an exchangeable lens (not shown).
As indicated by the arrows 9, the reflected light of the object to be
photographed impacts the photoelectric receiver system 2 through the
diaphragm 8 at its initial position, of the light measuring device 1. This
generates a measuring signal proportional to the object luminance
(B.sub.V) and the film sensitivity (S.sub.V) less the magnitude of the
initial diaphragm aperture (A.sub.Vo) of the lens being used. This signal
is conducted to the input 6a of the comparator stage 6. At the other input
6b a reference signal (T.sub.V) is applied, said reference signal being
proportional to a desired shutter speed value and emitted by the shutter
speed value transmitter 5 which is supplied with a voltage V. The
difference between the two signals applied to the comparator indicates the
extent to which the diaphragm 8 must close to provide the correct
exposure.
The determination of the working aperture of the diaphragm 8 is initiated
by the release of the camera, not shown. When released, the diaphragm
heretofore held in its initial aperture setting, begins to close. With the
diaphragm, which is now changing continuously, the signal at the output of
the light measuring device also varies continuously. This signal is
compared continuously in the comparator stage 6 with the reference signal
generated in the shutter speed value transmitter 5. When equality between
the two signals is attained, the magnet 7 connected to the comparator
stage 6 in series is excited by a control signal and the further closing
of diaphragm is prevented by means of the active interaction between the
magnet and the diaphragm. At this point, the actual shutter speed is
determined by the Time Forming Unit (not shown) which is also connected to
the output of the light measuring unit 1.
The stopping of the diaphragm 8 may be effected with a greater or lesser
accuracy, if for example, the diaphragm exceeds its set value because of
certain time constants as there are the dropping off of the magnet, the
falling in of latches, etc. inherent in the device. These faults, however,
do not create errors of exposure, because the output signal of the light
measuring device 1 and not the signal (T.sub.V) generated by the shutter
speed value transmitter 5 is used to determine the actual shutter speed.
If, for example, the diaphragm 8 has narrowed excessively, said signal
will be correspondingly smaller and the resulting exposure time will be
correspondingly longer. However, in most cases this effect is undesirable.
An exposure control device that lacks this deficiency and further comprises
a programmed automatic device is shown in FIG. 2. It is to be noted that
all of the structural elements performing the functions indicated in FIG.
1 are identified by the same reference symbols.
The exposure control device shown in FIG. 2 also comprises a means for
generating a measuring signal proportional to the luminance of the camera
object, the film sensitivity and the initial lens aperture such as a light
measuring device 1; a means for generating at least one signal indicative
of a selected shutter speed such as shutter speed value transmitter 5; a
comparator means such as comparator stage 6 and a magnet 7 in active
interaction with the diaphragm 8 of the lens used. The exposure device
further comprises storage means, for example, a storage capacitor 10 that
is connected with the light measuring unit 1 by means of a disconnecting
switch 11. A means for providing a variation signal of the measuring
signal, in one embodiment, a potentiometer 12 having a slide 12a which is
connected both mechanically to the diaphragm 8 of the lens being used and
electrically to the comparator stage 6. A means for combining the stored
measuring signal with the shutter speed value signal may comprise a
voltage divider consisting of the resistances 13 and 14 connected between
the stored measuring signal and the shutter speed value transmitter 5.
Further, between the comparator stage 6 and the speed value transmitter 5,
there is a selector switch 15 switchable to one of the contacts B
(automatic diaphragm unit) or P (automatic program unit).
The exposure control device connected in this manner operates as follows:
The light measuring device 1 provides an output signal proportional to the
APEX value B.sub.V +S.sub.V -A.sub.vo. When the camera is triggered, this
signal is initially transferred to the storage capacitor 10 connected to
the light measuring device and then the light measuring device 1 is
disconnected from the capacitor 10 by means of the disconnecting switch
11. High impedance amplifier 16 prevents the charge on capacitor 10 from
being bled off through potentiometer 12 and maintains at its output, a
voltage equal to the charge on the capacitor. When the selector switch 15
is switched to the contact B (automatic diaphragm as shown in dotted
lines) the shutter speed value transmitter 5 provides a reference signal
(T.sub.V) at the input 6b of the comparator stage 6, which is proportional
to a manually selected shutter speed value.
The circuit with the switch in position B operates as in FIG. 1 and the
difference between the two voltages indicates the extent to which the
diaphragm must close in order that the exposure will be correct for a
desired shutter speed which is then set by the Time Forming Unit (not
shown) connected to input 6a.
In order to automatically set the working aperture of the diaphragm 8, the
latter is mechanically coupled with the slide 12a of the potentiometer 12,
said slide being positioned at the upper end 12b of the potentiometer 12
if the diaphragm is open. When the triggering of the camera automatically
begins closing the diaphragm 8, the slide 12a of the potentiometer 12 is
simultaneously started. This provides a signal at 12a which is
proportional to the prevailing aperture of the diaphragm. If the signals
at the inputs 6a, 6b of the comparator are equal, the magnet 7 is
energized and further closing of the diaphragm and movement of the slide
12a is stopped. The voltage at the potentiometer 12a, indicative of the
luminance of the object, the film sensitivity, the initial diaphragm
aperture and the extent to which the aperture has been closed, is passed
for further processing in a known manner to a Time Forming Unit, not
shown.
When the selector switch 15 is on the contact P (program automation, as
shown), the reference signal (T.sub.V) will be generated automatically.
For this purpose, a signal is taken from the shutter speed value
transmitter 5 proportional to a shutter speed limit value t.sub.go and
connected with the voltage output of amplifier 16 by means of the
resistances 13 and 14 forming a voltage divider. The combination signal
resulting from this connection is passed to the input 6b of the comparator
stage 6. Herein, the measuring signal produced by the light measuring unit
1 and proportional to the APEX values B.sub.V +S.sub.V -A.sub.vo
corresponds to a shutter speed value T.sub.Vo which, with an open
diaphragm, would lead to a correct exposure. The signal is compared with a
signal taken from the shutter speed value transmitter at a terminal 5a
which is proportional to the shutter speed limit value t.sub.go (for
example 1/15 s with the diaphragm open).
Between the shutter speed values of T.sub.Vo and t.sub.go there are
numerous different shutter speed/aperture combinations which will provide
the correct exposure value. When the resistances 13, 14 forming the
voltage divider are in a ratio of 1:1, a voltage will be established
during the above-described combination of the two signals at the terminal
P corresponding to the arithmetic average of the two signals for T.sub.Vo
and t.sub.go. This reference is conducted to the comparator stage at the
input 6b, where it is compared with the variation signal, which varies in
keeping with the aperture of the diaphragm 8. When the voltages are equal,
as described hereinabove, the closing of the diaphragm is terminated by
means of the energization of the holding magnet 7. The voltage present at
slide 12a of the potentiometer 12 is thus also proportional to the
arithmetic average of the voltages for t.sub.go and T.sub.Vo. It is
conducted to the Time Forming Unit (not shown) where it is converted into
a shutter speed corresponding to the geometric mean value of the speeds
T.sub.Vo and t.sub.go.
For example, a shutter speed limit may be t.sub.go =1/15s which corresponds
to U.sub.t.sbsb.go =550 mV and a shutter speed value of T.sub.vo =1/1000 s
which corresponds to U.sub.T.sbsb.vo =658 mV. Here use of U.sub.t.sbsb.go
is the limiting voltage at 5a corresponding to a voltage indicative of the
aperture necessary for proper exposure at the slowest shutter speed and
U.sub.T.sbsb.vo is the voltage indicative of the shutter speed necessary
for proper exposure with a fully open aperture. This yields for
U=(658 mV+550 mV)/2=604 mV
and
t=.sqroot.1/15.multidot.1/1000 s.sup.2 =1/125 s.
With a ratio of 18 mV per change in shutter speed settings or diaphragm
aperture values, respectively, (at room temperature), as obtained with the
use of semiconductor paths in logarithmic and delogarithmic circuits,
respectively, it is seen that with a variation of 604 mV-550 mV=54 mV or
3.times.18 mV, time must also vary by 3 values, i.e. from 1/15 s to 1/125
s. If this average time is fed to the comparator stage 6 as the reference
voltage (switch 15 in position P), it will cause the magnet 7 to arrest
the closing of the diaphragm 8 at the average geometric diaphragm
aperture--i.e. the working aperture which is between the maximum aperture
and the aperture necessary for proper exposure at the slowest shutter
speed.
While the output signal of the light measuring unit 1 is less than or equal
to the shutter speed limit voltage value (U.sub.T.sbsb.vo
.ltoreq.U.sub.t.sbsb.go) the diaphragm will not move upon release of the
camera, because the comparator stage 6 will immediately energize the
magnet 7. The shutter speed indicated will be the slowest available which,
as noted earlier, corresponds to the voltage of U.sub.T.sbsb.vo. If,
however, the voltage from the light measuring unit exceeds the shutter
speed limit voltage given (U.sub.T.sbsb.vo >U.sub.t.sbsb.go), then the
difference of this voltage (at a ratio of the voltage dividing resistances
13 and 14 of 1:1) is converted by one half into a closing of the aperture
and shortening of the shutter speed, as shown by the following example:
U.sub.T.sbsb.vo =622 mV represents t=1/250 s
U.sub.t.sbsb.go =550 mV represents t=1/15 s,
then, a voltage of
U.sub.T =(U.sub.T.sbsb.vo =U.sub.t.sbsb.go)/2=(622+550)/2=586 mV
is applied to the inlet 6b of the comparator stage 6. A diaphragm aperture
proportional to this voltage will be set following the release of the
camera. The potentiometer slide 12a scanning the closing of the diaphragm
8 will show a voltage change of 622 mV-586 mV=36 mV=2.times.18 mV, i.e.
the diaphragm will close by two value units from its maximum aperture.
Because the same voltage is also conducted by means of the slide 12a of
the potentiometer 12 to the Time Forming Unit, not shown, the 586 mV
voltage is 36 mV higher than the 550 mV of U.sub.t.sbsb.go and thus the
time forming unit will provide a shutter speed of 2(i.e. 36 mV/18 mV=2)
increments faster (i.e., 1/60 s) than the slowest shutter speed (1/15 s).
With a ratio of 1:1 between the resistances 13 and 14, the automatic
programming has a characteristic curve as shown by "a" in FIG. 3,
originating at the speed limit value of 1/15 s and progressing as a
straight line with a slope of 45.degree.. In this process, as described
hereinabove, the aperture of the diaphragm and shutter speed vary at an
entirely equal rate with the variations in the exposure values EV.
Because there are many shutter speed and aperture combinations which will
provide the same exposure, lines of constant exposure value (EV) have been
provided on FIG. 3. For obvious reasons, it is often desirable to change
the aperture and the shutter speed differently, for example to emerge more
rapidly from the range of lower shutter speeds. An example of the possible
configuration of such a characteristic curve is shown by the curve "b" in
FIG. 3. According to the curve, beginning with the limiting shutter speed
value 1/15 s, the larger aperture settings for a given exposure value are
chosen. In this manner, the range of slow shutter speed is left more
rapidly than in curve "a". Following the attainment of a shutter speed
which will prevent blurring, the reduction of the diaphragm aperture is
preferentially effected, which may be desirable in order to increase the
depth of field of the camera. In the last section of the curve, the
diaphragm aperture and the shutter speed again vary equally, but at a
smaller aperture and slower shutter speed for a given exposure value than
curve "a".
In order to obtain the line segments of curve b, the ratio of the
resistances 13, 14 for each segment of the curve must be of different, but
definite, values. This condition is satisfied by the embodiment of the
present invention presented in FIG. 4, wherein all structural elements
having similar functions as in FIGS. 1 and 2, are identified with the same
reference symbols and wherein for the sake of clarity the light measuring
unit 1 and the diaphragm 8 are no longer shown. For this purpose, a means
for varying the resistance ratio is provided such as a resistance 17
connected in parallel with resistance 13 and a resistance 18 in parallel
with the resistance 14, either of which may be controllably connected and
disconnected as required. The connection and disconnection is effected by
means of the comparators 19 and 20, respectively. The signal produced in
the light measuring unit 1 and stored in the storage capacitor 10 is
conducted to one set of inputs 19a and 20a. The other inputs 19b and 20b,
respectively, are connected to intermediate terminals 5b and 5c,
respectively of the shutter speed value transmitter 5, from which a
voltage, proportional to the shutter speed value at which the resistances
17 and 18, respectively, are to be connected or disconnected, may be taken
off. The outputs 19c and 20c, respectively, of the comparators 19 and 20,
are connected so as to base bias switching transistors 21, 23 into
conduction adding resistances 17, 18 in parallel with resistances 13, 14,
respectively, as desired.
The following sequence takes place for the characteristic curve "b" shown
in FIG. 3. At exposure values of less than 9.5, the resistance ratio
between resistances 13 and 14 is 1:2. Thus, as the luminance level of the
camera's object increases (presuming the film sensitivity and initial
aperture remain the same), the automatic programming mode will reach a
shutter speed of 1/125 s more quickly than in curve "a". This is because
an increase in the voltage U.sub.T.sbsb.vo (the stored measuring signal)
the shutter speed will increase by two values for every one value that the
diaphragm closes.
As U.sub.T.sbsb.vo is increasing (indicating a brighter camera object) a
point will be reached in which U.sub.T.sbsb.vo is greater than
U.sub.t.sbsb.g1 which may be adjustable by movement of the intermediate
terminal along the shutter speed value transmitter 5. When equality is
reached between the two inputs to comparator 20, the comparator will
provide an output 20c through current limiting resistor 24 to base bias
transistor 23 into conduction. When transistor 23 conducts, resistance 18
is effectively applied in parallel with resistance 14 and thus the
resistance ratio would be:
##EQU1##
The combined resistance of the resistances 14 and 18 in parallel by the
above formula is less than resistance 14 alone and in the embodiment shown
which provides the line segment of curve "b" between shutter speeds of
1/125 and 1/250 s, would be equal to 0.28. Thus, during this portion of
the graph (when transistor 23 is in conduction), for an increase in
voltage U.sub.t.sbsb.vo, time will increase by 0.28 jumps when the
diaphragm increases by 1 jump or as can be seen, the diaphragm increases
by 3.5 jumps when the shutter speed increases by only 1 jump. This permits
the smaller diaphragm settings to be more quickly reached as
U.sub.T.sbsb.vo is increased in order to provide better depth of field for
a given lens.
As can be seen by looking at the last segment of curve "b" (shutter speeds
greater than 1/250 s) when U.sub.T.sbsb.vo reaches a sufficiently high
level such that it equals U.sub.t.sbsb.g2 (the input 19b to comparator 19)
an output will be provided at 19c of comparator 19 which through current
limiting resistor 22 base biases transistor 21 into conduction placing
resistor 17 in parallel with resistance 13. At this point then the
resistance ratio will be as follows:
##EQU2##
Because the ratio is 1:1, a single step in shutter speed will be matched
by a single step in aperture as the exposure value number is increased.
As can be seen in FIG. 4, the intermediate terminals 5b and 5c can be
adjusted for the voltage at which the comparator provides the necessary
output to bias its respective transistor into conduction. Accordingly, the
point at which curve "b" changes slope can be controllably adjusted. By
varying the initial resistance ratio between resistors 13 and 14 as well
as changing resistance values 17 and 18, and varying combination of
desirable slopes can be automatically programmed into the camera.
Additionally, numerous other comparator/transistor/resistance stages could
be added such that there are more than three line segments utilized in
making up curve "b".
Although the invention has been described relative to specific embodiments
thereof, it is not so limited and many modifications and variations
thereof will be readily apparent to those skilled in the art in light of
the above teachings. It is, therefore, to be understood that, within the
scope of the appended claims, the invention may be practiced otherwise
than as specifically described.
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
* * * * *
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