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The present invention relates to an exposure meter for measuring light
reflected from an objective scene to be photographed and giving the
photographer an indication permitting selection of the requisite time of
exposure of the scene and automatically setting the exposure time when
automatic control is required. More particularly the invention relates to
an improved exposure meter which gives the photographer an immediate and
easily appreciable indication of the average brightness and of the
brightness of the brightest and the darkest portions of an objective
scene.
The amount of light which is allowed to reach the film in a camera, depends
on the intensity of the light and the length of time the light is allowed
to fall on the film, this latter factor being controllable by varying the
lens aperture and the shutter speed. There have been proposed many means
and methods for determining the intensity of light falling on or reflected
from an objective scene, referred to for simplicity below as the
brightness of the objective scene. The simplest type of conventional means
is an electric photometer which is directed towards the objective scene
and has an indicator that moves to different settings depending on average
brightness of the objective scene. A main disadvantage of this known means
is that the indicator needle often fails to settle in a fixed position and
it is difficult to obtain an exact reading. Another disadvantage is that
since the brightness recorded is the average brightness there is often a
failure to achieve correct exposure of the main portion of the objective
scene. For example, in the same lighting conditions a portion of a
photograph carrying the image of a person will be under-exposed if the
background is white and over-exposed if the background is black. This
problem can be avoided by moving the photometer close to the main portion
of the objective scene, to determine its brightness. However, in this case
the overall brightness of the objective scene is not known and must
therefore be determined by separate measurement, in addition to which it
is not always possible to bring the photometer close to the main portion
of the objective scene.
Alternatively, it has been proposed to measure the brightness of the
brightest and the darkest portions of the objective scene in order to
determine the range of brightness and then to set the exposure conditions
in accordance with the characteristics of the film employed. This
procedure theoretically gives the best results, but in practice it is
frequently found difficult to determine rapidly which are brightest and
darkest portions of the objective scene and what their relative importance
is, and this method is unsuitable when the photographer wishes to take a
photograph quickly.
Other proposed methods are to measure the darkest portion of the objective
scene and determine the minimum exposure time required, or measure the
brightest portion and set the exposure in reference to this portion. Both
these methods have the disadvantage that if there is considerable contrast
in the objective scene there is over-exposure or under-exposure in
portions of the photograph produced.
It is accordingly a principal object of the invention to provide an
exposure meter which automatically gives an indication representative of
the brightness range and the average brightness of an objective scene to
be photographed.
It is another object of the invention to provide an exposure meter which
produces easily viewable information on the brightness of an objective
scene.
It is a further object of the invention to provide an exposure meter which
automatically indicates whether the brightness range of an objective scene
exceeds the latitude of the film employed.
It is yet another object of the invention to provide an exposure meter
which stores and displays the values of the brightness of different
portions of an objective scene measured by spot metering elements.
In accomplishing these and other objects, there is provided, according to
the present invention, an exposure meter which comprises an array of
photoelectric elements which are disposed to face an objective scene to be
photographed.
Each separate photoelectric element produces an electric current in
response to incident light as an output signal. This is converted to a
value indicative of the required exposure time for given values of lens
aperture and sensitivity of the film employed. This is supplied to a means
which calculates the highest value of output and the lowest value of
output, that is, which determines what exposure times are required by the
brightness of the brightest and the darkest portions of the objective
scene. These values are displayed by a dot display, which is suitably
provided in a viewfinder window or screen, together with a median value.
This median value is calculated on the basis of latitude of the film
employed and the abovenoted maximum and minimum output values, and is also
supplied to an exposure time control circuit.
According to another embodiment of the invention, the value of the
brightness of different portions of an objective scene selected by the
photographer is recorded and displayed upon actuation of an external
switch means by the photographer.
A better understanding of the present invention may be had from the
following full description of several preferred embodiments thereof when
read with reference to the attached drawings, in which like numbers refer
to like parts, and
FIG. 1 is a block diagram of an exposure meter according to a first
embodiment of the invention;
FIG. 2 shows examples of the layout of a photometering block and a
brightness display;
FIG. 3 is a schematic circuit diagram of a photometering block and an
operational circuit for calculation of brightness information;
FIG. 4 is a schematic circuit diagram illustrating the principles of action
of the operational circuit of FIG. 3;
FIG. 5 is a block diagram of a circuit for detecting and calculating
maximum, minimum, and median values of the output of the operational
circuit of FIG. 3;
FIG. 6 is a block diagram of a circuit and means for display of the output
of the circuit of FIG. 5;
FIG. 7 is a block diagram of a modification of the first embodiment of the
invention including a digital display giving information relative to film
latitude;
FIG. 8 is a block diagram of the maximum value detection circuit employed
in the means of FIG. 7;
FIG. 9 is a block diagram of another embodiment of the invention;
FIG. 10 is the plane view of an example of a display employed in the means
of FIG. 9;
FIG. 11 is a perspective view of a camera incorporating the means of FIG.
9; and
FIG. 12 is a schematic circuit diagram of a portion of the means of FIG. 9.
Referring initially to FIGS. 1 and 2, successive output signals from
photometering block 1 which indicate the intensity of light incident on
elements of block 1 are supplied via output line 1b to operational circuit
2 in response to successive input signals to photometering block 1 from
scan control circuit 1a of conventional type. Photometering block 1
consists of an array of photodiodes or similar means for production of
electrical signals which is suitably, but not essentially, mounted on a
camera not shown in a fixed disposition relative to the focal plane of the
film employed in the camera. In the specific example shown in FIG. 2,
block 1 consists of a 5.times.5 square array of 25 photodiodes P1 to P25,
although, of course, different numbers or other types of photo-elements,
or other forms of array may be employed. Light from the objective scene is
incident on the entirety of the photometering block 1, but the output from
block 1 at any given moment is the output signal from only one of the
photodiodes P1 to P25, which supply output signals in their numbered
order. Thus, if different portions of the objective have different degrees
of brightness, the output signals from photometering block 1 supplied as
input to operational circuit 2 vary accordingly.
Operational circuit 2 also receives externally set input indicating the
sensitivity of the film employed and the lens aperture, and for given
values of film sensitivity and lens aperture produces an output which is
proportional to the logarithmn of the value of input supplied thereto by
photometering block 1, i.e., which is in effect proportional to the
logarithmn of the value of intensity of light incident on successive
photodiodes P1 to P25. Operational circuit 2 indicates the requisite
exposure time for correct exposure of the different portions of the
objective scene whose brightness is detected by the different photodiodes
P1 to P25. Output from operational circuit 2 is supplied via line 2a to a
maximum value detection circuit 3 and separately to a minimum value
detection circuit 4. Maximum value detection circuit 3 produces an output
which indicates of the the maximum value of input received from
operational circuit 2. This output is supplied to decoder 6 and also to
median value calculation circuit 5. Minimum value detection 4 produces an
output which indicates of the lowest value of input received from
operational circuit 2. This output is supplied to decoder 7 and also to
median value calculation circuit 5. The output of median value calculation
circuit 5 is supplied to exposure time control circuit P and to decoder 8.
Designating the output of maximum value detection circuit 3 as Emax, and
that of minimum value detection circuit 4 as Emin, the median value
calculation circuit 5 produces an output Emid which is calculated by the
formula
Emid=(Emax-Emin)k+Emin . . . (1)
k being a constant in the range 0.ltoreq.k.ltoreq.1 which is externally set
in order to adjust the value of input supplied to exposure time control
circuit P, k being settable at any one of the five values 0, 0.25, 0.5,
0.75, and 1, for example. From Eq. (1) it is seen that (Emid=Emin) when
k=0, (Emid=(Emax+Emid)/2) when k=0.5, and (Emid=Emax) when k=1. As an
example of the output Emid, if, for given values of film sensitivity and
lens aperture, the brightness of the brightest part of the objective scene
is such as to require an exposure time of 1/250 second (Emin), and the
brightness of the darkest portion of the objective is such as to require
an exposure time of 1/15 second (Emax) output Emid supplied to exposure
time control circuit P causes circuit P to make the exposure time 1/60
second.
Decoders 6, 7, and 8 all have the same basic construction and change the
inputs received from circuits 3, 4, and 5 respectively to a form suitable
for actuating display 10. The output from the decoders 6, 7, and 8 may be
supplied to display 10 via a common OR gate assembly 9. The output from
decoder 8 is always supplied to OR gate assembly 9. The supply of output
from decoders 6 and 7 to assembly 9 is dependent on closure of ganged,
normally open switches SW1 and SW2, respectively.
Display 10 is a dot display which is suitably constituted by photoemissive
diodes, liquid crystal elements, or similar display elements L1 to L15
which are arranged in a single line, as shown in FIG. 2. Display 10 may be
provided in the viewfinder system of the camera, where it is immediately
viewable by the photographer, and the display elements L1 to L15 may have
provided beside them numerals indicating the exposure times they
represent, as in the example in the drawing. For display of the abovenoted
examples of outputs of circuits 3, 4, and 5, Emax=1/15, Emin=1/250,
Emid=1/60, presuming switches SW1 and SW2 are closed, display elements L6,
L8 and L10 light up, giving a simple, easily appreciable indication of the
range of exposure times necessary for correct exposure of different parts
of the objective scene. Since the photographer knows that the shortest and
longest exposure times are required for correct exposure of the brightest
and darkest portions respectively of the objective scene, he or she may
adjust the lens aperture if for example the brightest portion of the
objective is the portion for which it is required to effect optimum
exposure, and it is observed that there is an extremely wide range between
Emax and Emin, with the result that Emid, which is also the control input
supplied to exposure time control circuit P of FIG. 1, is very different
from Emin, for correct exposure of the brightest portion. More simply,
exposure adjustment may be made by altering the value of k set by circuit
23 of FIG. 5, in order to alter the control input supplied to exposure
time control circuit P. Note that necessary information is displayed in a
very straight forward manner in display 10, and even if display elements
L1 to L15 have no indication beside them the photographer may immediately
assess the range of values from Emax to Emin, and make any adjustments
that may be necessary.
Elements of the means of the invention will now be described with reference
to FIGS. 3 through 6.
In FIG. 3, which shows details of photometering block 1, scan control
circuit 1a, and operational circuit 2, the cathodes of all the photodiodes
P1 to P25 are connected to a common voltage source. The anodes of
photodiodes P1, P2, . . . P25 are connected individually through field
effect transistors (FET) T1a, T2a, . . . T25a to the collector of NPN
transistor LT1 and input of amplifier A1, and through FETs T1b, T2b, . . .
T25b to the collector of NPN transistor LT2 and input of amplifier A2. The
emitter of transistor LT1 is connected to ground, and amplifier A1 is
provided between the base and collector of transistor LT1 and constitutes
together therewith a negative feedback amplifier circuit. Amplifier A2 and
transistor LT2 are similarly connected. Output terminals 12-1, 12-2, . . .
12-25 of a shift register 12 constituting the scan control circuit 1a
connect directly to the gates of FETs T1a, T2a, . . . T25a and to the
gates of FETs T1b, T2b, . . . T25b through inverters NOT1, NOT2, . . .
NOT25. Upon actuation of an external start means not shown an output
signal is produced at each of the output terminals 12-1, 12-2, . . . 12-25
of shift register 12 in turn. Thus, at any given moment only one of the
FETs T1a to T25a can conduct and therefore an output from only one of the
photodiodes P1 to P25, the one that is connected to the currently
conductive FET 1a, 2a . . . 25a, can be supplied to the transistor LT1 and
amplifier A1 circuit. This circuit therefore receives a succession of
photocurrent signals from succeeding photodiodes P1, P2, . . . P25 as a
result of successive outputs from shift register 12. On the other hand,
during the action of shift register 12, because of the provision of
inverters NOT1-NOT25, all FETs T1b-T25b conduct except the one that is
connected to the shift register terminal 12-1, 12-2, . . . 12-25 at which
an output signal is currently being produced, and so the output from all
the photodiodes P1 to P25 except one is supplied to the transistor LT2 and
amplifier A2 circuit.
Provision of amplifier A1 between the base and the collector of transistor
LT1 keeps the collector voltage level of transistor LT1 more or less
constant, with the result that there appears between base and emitter of
transistor LT1, i.e., at output terminal 1b of FIG. 3, a voltage output
which is proportional to the logarithmn of the collector current of
transistor LT1. Thus, series-out signals that are proportional to the
logarithmn of the intensity of light indicated on successive photodiodes
P1, P2, . . . P25 appear at output terminal 1b.
By a similar action the transistor LT2 and amplifier A2 circuit produces at
output terminal 1c an output which is effectively proportional to the
logarithmn of total output of photometering block 1.
Output signals from terminal 1b are supplied to the non-inverting input
terminal of operational amplifier A3, which constitutes a part of
operational circuit 2 as indicated by the dashed-line enclosure of FIG. 3.
The inverting input terminal of operational amplifier A3 is connected to
wiper W1 of potentiometer PM1 in parallel to potentiometer PM2, and the
output terminal is connected to the upper junction a1 of potentiometers
PM1 and PM2. The lower junction of potentiometers PM1 and PM2 is connected
to constant current circuit I. Wiper W1 of potentiometer PM1 is moved to
different settings selected in reference to the sensitivity of film
employed by an external setting means not shown and may be calibrated in
ASA numbers for example. Wiper W2 of potentiometer PM2 is connected to
output terminal 2a and is movable by external means not shown to the
different settings selected in reference to lens aperture. Operational
circuit 2 produces at output terminal 2a output signals which are equal to
the potential between wiper W1 and the upper junction a1 of potentiometers
PM1 and PM2 minus the potential between wiper W2 and junction a1 and is
indicative of required exposure time. The action of operational circuit 2
is essentially the same as that of a conventionally known circuit for
calculating exposure time with reference to APEX indices. The principles
of this action are described briefly with reference to a circuit 2' of
FIG. 4.
In FIG. 4, the junction P2' of photodiode D2 and diode D1 which are in
series with one another and with first voltage source E1 is connected to
wiper W3 of potentiometer PM3 which is in parallel to potentiometer PM4.
The negative terminal of first voltage source E1 is connected to the
cathode of diode D1. Potentiometers PM3 and PM4 are also in parallel to
and have an upper junction P3' connecting to the positive terminal of a
second voltage source E2. Potential VB across opposite terminals of diode
D1 varies in accordance with intensity of light incident on photodiode
PD2. Wipers W3 and W4 are set to different settings corresponding to
different values of the film sensitivity and the lens aperture,
respectively. For any settings of wipers W3 and W4, a voltage Vs appears
between wiper W3 and the upper junction P3' of potentiometers PM3 and PM4,
and a voltage VA between junction P3' and wiper W4. With this circuit
there appears between ground, or a line at a reference voltage, and the
output terminal P4' connected to wiper W4 a voltage VT such that
VT=VB+VS-VA
This is equivalent to the known equation for required exposure time Tv
using APEX indices
Tv=Bv+Sv-Av
Bv being the luminance index representing the brightness of the scene, Sv
the film sensitivity, and Av an index indicating the lens aperture. An
equivalent output is obtained if the negative terminal of the second
voltage source E2 is connected to upper junction P3' of potentiometers PM3
and PM4. In this case there is a voltage VS' and VA' between junction P3'
and wipers W3 and W4 respectively, and output VT is
VT=VB-VS'+VA'.
The basic circuit of FIG. 4 is commonly employed to supply output VT to a
shutter control means which controls shutter speed in accordance with the
value of VT. In the circuit of FIG. 3, presuming different portions of the
objective scene to be photographed have different degrees of brightness,
the output at terminal 2a is indicative of the required exposure time but
varies in value in succeeding output signals.
Referring now to FIG. 5, maximum value detection circuit 3 and minimum
value detection circuit 4 which receive as inputs the output signals from
terminal 2a are shown in greater detail. Maximum value detection circuit 3
comprises comparator 15 which receives input from terminal 2a and which,
when conditions specified below are satisfied, supplies an output on line
15c to AND gate 16. AND gate 16 three input terminals, and also receives
an input of clock pulses from clock generator G and an input from start
control circuit S, which supplies a continuous input along line 16a to AND
gate 16 upon actuation of an external means not shown such as the scan
control circuit 1a. When AND gate 16 receives inputs on lines 15c and 16a,
clock signals are passed to and increment the content of count-up counter
13. For each clock pulse received by counter 13, a signal indicating the
new content of counter is supplied to memory 17, which stores the content,
and to digital-analog (D - A) converter 14, which converts the input
signal into an analog output signal which is supplied along line 15b to
comparator 15. Thus signals from D - A converter 14 to comparator 15
gradually increase in value as the content of count-up counter 13
increases. At the start of the actuation of maximum value detection
circuit 3, a reset signal from a source actuated by start circuit S is
supplied to and clears count-up counter 13, and at this time the level of
the output from D - A converter 14 is made the lowest level of output
produced by converter 14.
Comparator 15 compares the inputs from terminal 2a of operational circuit 2
with the input from D - A converter 14, and, whenever an input signal from
terminal 2a is greater than the input from D - A converter 14, supplies a
signal along line 15c to AND gate 16, whereupon a further clock pulse is
passed through AND gate 16, and the content of counter 13 and memory 17 is
incremented, and the value of the output from D - A converter 14 is
increased accordingly. Thus, presuming that the brightness of at least
some portions of the objective scene is below a certain level, when all
the photodiodes P1 to P25 of photometering block 1 have been scanned the
content of counter 13 and memory 17 is a numerical value representative of
the exposure Emax required for correct exposure of the darkest portion of
the objective.
Minimum value detection circuit 4, which has basically the same
construction as and functions in a manner analogous to that of maximum
value detection circuit 3. It comprises a count-down counter 18, which
supplies an input to D - A converter 19 and memory 22 for storing the
content of counter 18. Counter 18 receives a reset input at the same time
as count-up counter 13 of circuit 3, and receives a count input from AND
gate 21 when AND gate 21 receives input clock pulses from clock pulse
generator G, an input from start circuit S, and receives an input from
comparator 20. Comparator 20 receives an input from terminal 2a of
operational circuit 2 and from D - A converter 19, level of which is
determined by the input supplied thereto from counter 18. Upon resetting
of count-down counter 18, the content of counter 18 is made its maximum
value and input supplied thereby to D - A converter 19 is such that level
of input from D - A converter 19 to comparator 20 is the highest
produceable by D - A converter 19. Comparator 20 compares the input from
terminal 2a with the input from D - A converter 19, and when the former is
smaller than the latter supplies an input to AND gate 21, which thereupon
produces an output which decrements the content of count-down counter 18.
Thus, after scanning of photodiodes P1-P25 counter 18 and memory 22
contain values indicative of exposure time required for correct exposure
of the brightest portions of the objective.
After completion of scanning of photodiodes P1 to P25, the contents of
memory 17 and 22 are supplied to decoders 6 and 7, respectively, and to
calculation circuit 24. Calculation circuit 24 together with k
specification circuit 23, constituted as an encoder, for example,
constitutes the abovementioned calculation circuit 5, for calculation of
the value Emid by the abovementioned Eq. (1). Suitably, k is normally set
at 0.5, in order to make Emid the mean of Emax and Emin, but may be set to
other values in order to adjust the exposure time, as noted above. The
constant k may be selected with reference to the latitude of film
employed, as noted above, or k may be selected with reference to the
contrast and brightness of the objective scene as a whole. The output from
Emid calculation circuit 24 is supplied to exposure time control circuit
P, and to decoder 8.
Referring now to FIG. 6, decoders 6,7, and 8 are shown in more detail. Each
decoder 6, 7, and 8 has four input terminals for receipt of a binary-coded
input. These terminals permit input of a number up to decimal "16" to each
decoder, but the decoders can only deal with values up to decimal "15",
since an extra "1" is always included in the input, in order to avoid
output from the decoders in response to a "0000" input, which could be
meaningless.
An input is supplied directly to decoder 8 from Emid calculation circuit 5,
the input to decoder 6 must pass through two-input AND gates 1-4, and the
input to decoder 7 must pass through two-terminal AND gates 5-8. There is
one AND gate for each input terminal of decoders 6 and 7, and one input of
each AND gate 1-8 is supplied by line 3a. The AND gates 1-8 correspond to
the ganged switches SW1 and SW2 of FIG. 1, and are actuated by an external
means not shown. In other words, when an input is supplied along line 3a,
an input may be supplied from memories 17 and 22 to decoders 6 and 7,
respectively.
Decoder 6, has output terminals 6-1, 6-2, . . . 6-15 which may supply an
input signal to OR gates 1, 2, . . . 15, respectively. Decoders 7 and 8
have the same construction as decoder 6, and have output terminals 7-1,
7-2, . . . 7-15 and 8-1, 8-2, . . . 8-15, respectively, each of which
terminals may supply an input signal to the correspondingly numbered OR
gates 1, 2, . . . 15. Thus, each OR gate 1-15 may receive an input signal
from one output terminal of each of the decoders 6, 7, and 8.
The output from OR gates 1, 2, . . . 15 may supply an actuating input
signal to photoemissive elements L1, L2, . . . L15, respectively, which
constitute the display 10 shown in plane view in FIG. 2. With this
arrangement, therefore, simultaneous display of Emax, Emin, and Emid
values can be effected in display 10, as noted earlier, the Emid value
being shown upon actuation of start circuit S of FIG. 5, and Emax and Emin
values being shown when required by actuation of an external means by the
photographer.
In this manner the invention permits a very rapid and easy assessment of
the required exposure conditions, but at the same time making possible a
highly accurate display of this assessment, and permitting reading with
much greater precision than is normally possible with other photometering
means employing an indicator needle.
Various modifications to the abovedescribed means are of course possible
without departure from the principles of the invention. For example,
display of all values Emax, Emin, and Emid may be effected automatically,
instead of the display of Emax and Emin values being dependent on external
switch action.
Also, display 10 may be used for direct indication of the brightness levels
rather than the exposure times. To do this, the output of the amplifier A1
and transistor LT1 circuit of FIG. 3 is supplied directly to the maximum
value detection circuit 3 and the minimum value detection circuit 4,
instead of being passed through operational circuit 2. In this case, the
exposure time may be calculated by an operational circuit such as circuit
2 which also receives input from the amplifier A2 and transistor LT2
circuit, or the brightness information for calculating exposure time may
be supplied from output terminal 1c (FIG. 1) of the amplifier A2 and
transistor LT2 circuit. As noted earlier, the amplifier A2 and transistor
LT2 circuit receives input from all the photodiodes except one at any
given time, and so produces an output indicating the of overall brightness
of the objective scene. Alternatively, the means of the invention may be
provided with a switch means which need not be a separate means, but could
be the switch means for actuating the start circuit S of FIG. 5, for
example, and which causes the exposure time to be calculated on the basis
of the output from the amplifier A1 and transistor LT1 circuit and a
display to be given in display 10 for photographing objective scenes for
which the exposure time is difficult to judge, and which causes the
exposure time to be calculated on the basis of the output of the amplifier
A2 and transistor LT2 circuit when the photographer judges that special
information on the objective scene brightness is unnecessary.
The means of the invention may of course also be employed for display of
the required lens aperture for particular set values of exposure time. In
this case, wiper W2 of potentiometer PM2 in FIG. 3 is set to different
positions in correspondence to the different exposure times.
Referring now to FIG. 7 which illustrates a modification of the above
described means, the maximum value detection circuit 3' and minimum value
detection circuit 4' are supplied inputs from the operational circuit 2
through a digital-analog converter 25. The digital number values Emax and
Emin are supplied through suitable decoding means to display unit 29 to
provide a dot display 30. The Emid value, which is calculated in the same
manner as described above by calculation circuit 24, is supplied to adder
27 and to subtractor 28, each of which also receives an input from encoder
26. The value of the output from encoder 26 depends on an externally set
input to encoder 26 indicating the latitude L of the film employed. The
units of film latitude L encoded by encoder 26 are suitable units employed
to represent steps in APEX indices, i.e., 5Ev for each unit change of
value of film latitude, and the output from encoder 26 to adder 27 and 28
represents the value 1/2L. The sum and the difference of the film latitude
information supplied by encoder 26 and the Emin value supplied by
calculation circuit 24 are determined by adder 27 and subtractor 28,
respectively, which supply signals indicating of the results determined
through suitable decoding means to display unit 31 to provide a dot
display 32 in which the upper value is equal to Emid+1/2L and the lower
value is Emid-1/2L, i.e., the range indicated by dot display 32
corresponds to the film latitude and is centred on the value of Emid. Dot
display 32 is provided alongside dot display 31, thus permitting a
photographer to determine at a glance whether or not the brightness range
of the objective scene exceeds the latitude of the film employed.
FIG. 8 illustrates as an alternative construction of maximum value
detection circuit 3". A register circuit 33 which is constituted as a
latch circuit, receives a signal from terminal 2a through A - D converter
25', and supplies a signal to a conventionally known comparator circuit.
The content of register 33 is shifted in a single transfer and is compared
with the content of a latch circuit 35 by a comparator 34. When the
content of latch circuit 35 is equal to or greater than the content of
register 33, comparator 34 supplies a "1" signal along line 34a to latch
circuit 35. The content of register 33 is read when a "1" signal is
supplied by comparator 34, and so after completion of the scanning of
photodiodes P1 to P25, the content of latch circuit 35 indicates the
highest value received by register 33. This value is supplied to the next
stage after completion of the scanning. The circuit configuration of FIG.
8 may be employed as a minimum value detection circuit if an inverter is
included on line 34a.
Referring now to FIGS. 9 through 12, there is shown another embodiment of
the invention in which the display indicates the brightness of portions of
the objective scene selected by the photographer.
In FIG. 9, image-wise light from the objective scene to be photographed is
passed through objective lens 201 to half-mirror 202, and passes through
half-mirror 202 to aperture 206 to impinge on photodiode 207 and is also
reflected by half-mirror 202 onto mirror 203 which directs the light to
eye-lens 204 of a viewfinder system through which the objective scene may
be viewed by photographer 205. Aperture 206 permits spot viewing of the
objective scene, i.e., viewing of successive very small portions of the
objective scene, with the angle of view being limited to 1.degree., for
example. The input terminals of operational amplifier 208 are connected
across opposite terminals of photodiode 207 and diode 209 is provided
connecting the output terminal of amplifier 208 to the inverting input
terminal thereof, to constitute a known circuit for production of output
signals proportional to the logarithm of the intensity of light incident
on photodiode 207. These output signals are supplied to an analog-digital
(A - D) converter 210 and converted to digital output signals. These
digital output signals are supplied through normally open switch 211 to
contact terminal 217, through normally open switch 212 to memory 214 whose
output is supplied to contact terminal 218 and calculation block 222, and
through normally open switch 213 to memory 215 whose output is supplied to
contact terminal 219 and calculation block 222. Contact terminals 217,
218, and 219 are successively and alternately connected by periodic switch
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