Exposure control device for a camera

A camera exposure control device having TTL light measuring circuitry capable of spot metering and also averaged metering is disclosed, the spot metering being used for measurement with the camera directed to a main object prior to the camera release operation, and the averaged metering being used for measurement with the camera positioned for making a desired composition including the main object. The difference between the logarithm of the spot metering output and that of the averaged metering output with the aperture at its initial size is stored prior to the aperture stopping-down motion. Exposure is actually controlled by the averaged metering output, which is obtained after the initiation of the aperture stopping-down motion in response to camera release operation with the stored difference between the aforementioned outputs added thereto, to thereby effectively achieve an exposure control equivalent to a control based on a spot metering output which would be obtained after the initiation of the aperture stopping-down motion.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to an exposure control device for a 
camera, and more particularly to an exposure control device for a camera 
capable of controlling camera exposure by means of light measuring output 
obtained and stored prior to an actual exposure. Such an exposure control 
is necessitated in case that an optimum exposure control is desired with 
respect to a main object located at a particular point within an area to 
be photographed, e.g., at a corner of the rectangular scene area to be 
photographed. 
2. Description of the Prior Art 
In case that, for example, one desires to photograph a composition with the 
main object located at a corner of the rectangular scene area to be 
photographed, exposure control is not generally optimum with respect to 
the main object since an automatic exposure control camera is so designed 
to control exposure in response to an averaging or a center-weighted light 
metering output determined subsequent to the camera release operation with 
respect to the whole area to be photographed. 
For the purpose of achieving an optimum exposure control with respect to 
the main object as in the above case, prior art cameras are provided with 
a manually operable switch for storing a light measuring output prior to 
the camera release operation. This is disclosed, for example, in U.S. Pat. 
No. 3,756,131. The method of photography using such a camera comprises: a 
first step to operate the manually operable switch with the camera close 
to the main object so that substantially the whole acceptance angle 
effective for light measuring may be occupied by the main object, and the 
value of the stored light measuring output may thereby be determined with 
respect to the main object only; and a second step where the camera is 
returned to a position for making the desired composition and the camera 
release operation is actually carried out so that the exposure control may 
be determined by the stored light measuring output. 
The above method, however, is only possible in case of a camera which is 
designed to control exposure in response to a TTL light measuring output 
obtained with the aperture at its initial aperture size, e.g., a fully 
open aperture size. 
Moreover, there is a camera of the type in which the exposure is controlled 
by a TTL light measuring output obtained after the initiation of the 
aperture stopping-down motion from its fully open aperture size. In this 
type of camera, the above mentioned method is not applicable since any 
reliable light measuring output is not obtainable until the aperture 
stopping-down motion actually begins subsequent to the actual camera 
release operation, thereby making the above mentioned first step 
impossible. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a camera of a type, in 
which the exposure is controlled by a TTL light measuring output obtained 
after the initiation of aperture stopping-down motion subsequent to the 
actual camera release operation, with a means for enabling an optimum 
exposure control with respect to a main object located at a particular 
point within a scene area to be photographed. 
The present invention is characterized in that the exposure is controlled 
by a light measuring output obtainable after the initiation of the 
aperture stopping-down motion subsequent to the actual camera release 
operation combined with a light measuring output stored prior to the 
actual camera release operation with the effective light measuring 
acceptance angle substantially occupied by the main object. 
Specifically, according to an embodiment of the present invention, a light 
measuring output with the effective light measuring acceptance angle 
substantially occupied by the main object is first stored prior to the 
actual camera release operation. The camera is then positioned to make a 
desired composition, in which the main object is located in a small 
portion of the effective light measuring acceptance angle, and the camera 
release operation is actually performed. Upon camera release operation, 
the difference between the logarithm of the stored light measuring output 
and that of a light measuring output with the camera positioned to make a 
desired composition is stored prior to the initiation of aperture 
stopping-down motion, and the aperture stopping-down motion follows 
thereafter. Exposure is controlled in accordance with the logarithm of a 
light measuring output, which is obtained after the initiation of the 
aperture stopping-down motion, with the stored logarithmic difference 
added thereto to make the exposure control optimum with respect to the 
main object. The reason why the exposure is optimum with respect to the 
main object is that the logarithmic difference with the aperture at its 
initial size is equal to the logarithmic difference with the aperture 
stopped-down to any size. Thus, by means of the addition of the 
logarithmic difference stored with the aperture at its initial size, the 
light measuring output obtained after the aperture stopping-down motion 
with the light measuring acceptance angle not occupied by the main object 
is succesfully converted into a light measuring output that would be 
obtained with the effective light measuring acceptance angle occupied by 
the main object. 
Further, according to the invention, it is recommended that a spot metering 
device, in addition to an averaging metering device, is utilized for the 
purpose of obtaining the light measuring output to be stored prior to the 
camera release operation. The spot metering device may make it unnecessary 
for the camera to be close to the main object for storing the light 
measuring output, since the narrow light measuring acceptance angle of the 
spot metering device can easily be occupied by the main object if the 
camera is caused to slightly deviate from the composition making position, 
in which, for example, the main object is at the corner of the object 
field, so as to be directly aimed at the main object.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, which shows a block diagram of a first preferred 
embodiment according to the present invention, light measuring circuit 1 
is designed for averaged light measuring, with photodiode PD1 disposed in 
a position where it obtains an output in response to the averaged scene 
brightness of a field to be photographed. Light measuring circuit 2 is 
provided for spot light measuring, with photodiode PD2 disposed in a 
position where it obtains an output in response to the brightness of a 
comparatively small given section of a field to be photographed, e.g. the 
central section of the field to be photographed. Light measuring circuits 
1 and 2 produce voltage signals proportional to the logarithm of the 
brightness of scene light incident respectively on photodiodes PD1 and PD2 
through diaphragm aperture D. First memory circuit 3 stores the output 
produced by light measuring circuit 2 for spot light measuring. Switch S2 
is manually operated and usually connected to contact b and changes to 
contact a, while a button provided on the camera body is kept depressed 
when photographing by spot light measuring. Switch S6 is also manually 
operated for changing photography modes, and it is closed for 
photographing by averaged light measuring and is opened for photographing 
by spot light measuring. FIG. 1 shows that switch S6 is opened for 
photographing by spot light measuring. 
With the above mentioned construction, the camera is first held so that a 
main object for spot light measuring is in the spot light measuring range, 
and switch S2 is changed to contact a. This causes the result of spot 
light measuring to be stored in first memory circuit 3 as an input, and is 
retained therein even when switch S2 is thereafter returned to its 
original position at contact b (since switch S6 is opened). Then, with the 
camera held for making a desired composition in which the main object is 
at a particular location deviating from the spot light measuring range and 
the shutter release button depressed, the outputs of light measuring 
circuit 1 and first memory circuit 3 immediately before diaphragm aperture 
D is stopped-down from its fully open aperture size, are applied to 
subtraction circuit 4, the subtraction result or a difference between the 
outputs is stored in second memory circuit 5. The signal stored in second 
memory circuit 5 can be written as the following formula, assuming that 
the output of averaged light measuring circuit 1 for a fully open aperture 
is Vao, and that the output of spot light measuring circuit 2 for a 
similarly open aperture is Vso: 
EQU Vso-Vao 
Switch S7 is opened during the shutter button depression operation 
immediately before diaphragm aperture D is stopped-down to thereby fix the 
signal storage in second memory circuit 5. With the aperture D 
stopped-down afterward, output Va of averaged light measuring circuit 1 
when the aperture was stopped-down, and a stored signal in second memory 
circuit 5, are input into calculation circuit 6. Calculation circuit 6 
performs a calculation formulated as follows: 
EQU Va+(Vso-Vao) 
The above value corresponds to the spot light measuring output with the 
aperture stopped-down. Exposure control circuit 7 carries out exposure 
control in response to an output from the above mentioned calculation 
circuit 6. 
For normal photographing by averaged light measuring, switch S6 is kept 
closed. Switch S2 remains connected to contact b since it is not operated. 
This means that the two inputs to subtraction circuit 4 are both from 
light measuring circuit 1 to make the subtraction result zero. The zero 
subtraction result of subtraction circuit 4 is also stored as a storage 
signal in second memory circuit 5. Therefore, the input to calculation 
circuit 6 is only output Va of averaged light measuring circuit 1 produced 
when diaphragm aperture D is stopped-down, and output Va is input to 
exposure control circuit 7 for exposure control by averaged light 
measuring. 
The first preferred embodiment shown in FIG. 1 is applicable to a variety 
of automatic exposure control modes, as described below. 
For automatic exposure time control with a preset aperture, calculation 
circuit 6 receives from light measuring circuit 1 averaged light measuring 
output Va when diaphragm aperture D is stopped-down to a preset position, 
and exposure control circuit 7 provides a shutter speed appropriate to 
spot light measuring made for the preset aperture. With the assumption 
that averaged light measuring output Va for a fully open aperture is an 
exposure time of 1/500 second, and spot light measuring output Vso is an 
exposure of 1/2000 second, the difference in shutter speed between the two 
exposure times is two steps, and this difference is the output of second 
memory circuit 5. And when averaged light measuring output Va with 
diaphragm aperture D stopped-down to the preset position corresponds to 
1/125 second, calculation circuit 6 produces an output corresponding to 
1/500 second with two steps added to the averaged light measuring output 
Va. 
The following is an explanation of an automatic aperture control mode with 
a predetermined exposure time using the above example. The output of 1/500 
second of light measuring circuit 1 is for a fully open aperture, and this 
means that diaphragm aperture D should be stopped-down by two steps if the 
preset shutter speed is 1/125 second. The output of second memory circuit 
5 is a signal for diaphragm aperture D to be stopped-down by an additional 
two steps, and therefore calculation circuit 6 produces an output for the 
aperture to be stopped-down by four steps from the fully open aperture. In 
response to the output from calculation circuit 6, exposure control 
circuit 7 operates to stop-down diaphragm aperture D by four steps from 
the fully open aperture. It should be understood that in this mode second 
memory circuit 5 may be omitted. 
For automatic aperture control with a predetermined exposure time, there is 
another mechanism available in which an aperture stopping-down motion 
toward the minimum aperture is interrupted when the TTL light measuring 
output obtained during the aperture stopping-down motion is in a 
predetermined relation with the preset shutter speed. In this case, 
however, while the difference in output between averaged light measuring 
and spot light measuring for a fully open aperture is input to calculation 
circuit 6 from second memory circuit 5, an averaged light measuring output 
during the time that the aperture is stopped-down is transmitted to 
calculation circuit 6 from light measuring circuit 1. With the aforesaid 
example of light measuring output, the light measuring output during the 
aperture stopping-down motion is produced from calculation circuit 6, with 
the difference of the two steps added, and this output is compared with a 
signal in response to a preset shutter speed by exposure control circuit 
7. The aperture stopping-down motion is interrupted at a position where 
the aforesaid output and signal are equal to determine an optimum aperture 
size. 
In the embodiment shown in FIG. 2, a common light measuring circuit is used 
both for averaged light measuring and spot light measuring, with 
photodiodes PD1 and PD2 arranged to be individually connected to a 
reference voltage VRef. Photodiode PD1 is used for averaged light 
measuring and photodiode PD2 for spot light measuring. Switch S1 is 
usually connected to contact b, and is connected to contact a only when a 
special button provided on the camera body remains depressed. Therefore, 
switch S1 is usually set for averaged light measuring. Transistor Q1 
converts the output current of photodiode PD1 or PD2 to a logarithmically 
compressed voltage signal. High input impedance buffer amplifier A1 
receives the output of either photodiode PD1 or PD2 and potentiometer PM1 
is used to set the film sensitivity. The output of high input impedance 
buffer amplifier A1 is the summation of the scene brightness and the film 
sensitivity in accordance with the APEX notation system. Potentiometers 
PM2 and PM3 are used to make the averaged light measuring output and spot 
light measuring output equivalent. The output of high input impedance 
buffer amplifier A1 is generally expected to be the same for an object 
field having uniform brightness when either photodiode PD1 or PD2 is used. 
In fact, however, photodiode PD1 is positioned to optimize averaged light 
measuring, and photodiode PD2 is positioned to optimize spot light 
measuring, with their respective outputs being unequal even in the above 
case. This causes the output of high input impedance buffer amplifier A1 
to vary in accordance with the output of either photodiode PD1 or PD2. 
Potentiometers PM2 and PM3 are adjusted and fixed upon assembling the 
camera so that both light measuring outputs in the above case become 
equal, thereby making the two outputs equivalent. Switch S2, interlocked 
with switch S1, is usually connected to contact b and disconnected from 
slider W3 of potentiometer PM3. Slider W2 of potentiometer PM2 is always 
connected to operational amplifier A3. Constant-current circuit I1 
supplies constant current to potentiometers PM1, PM2 and PM3. 
The following description of the circuitry shown in FIG. 2 is made with 
photography by spot light measuring. Switches S1 and S2 are first 
depressed to change their respective connection to contact a. This allows 
photodiode PD2 to operate for spot light measuring with a fully open 
aperture, and spot light measuring result Vso is stored in capacitor C1 
from slider W3 of potentiometer PM3 via switch S2. In this photographing 
mode, manual switch S6 is opened beforehand. Furthermore, switches S3 and 
S4, interlocked with each other, are connected to their respective 
contacts a, as shown in the diagram. With switches S1 and S2 then released 
to return to their original positions at their respective contacts b, 
averaged light measuring with a fully open aperture is performed by 
photodiode PD1, with averaged light measuring output Vao appearing at 
slider W2 of potentiometer PM2. Output Vao is applied as a subtrahend to a 
subtraction circuit corresponding to subtraction circuit 4 of FIG. 1, 
consisting of resistors R1 through R4 and operational amplifier A3. The 
minuend of the subtraction circuit is spot light measuring output Vso with 
a fully open aperture, which output is stored in capacitor C1 and applied 
to the subtraction circuit through operational amplifier A2 designed for 
impedance conversion. The output of the subtraction circuit is Vso-Vao, 
which output is inverted by an inversion circuit, including operational 
amplifier A4 and resistors R5 and R6 (of equal resistance), into a signal 
Vao-Vso which is charged and stored in capacitor C2. With the shutter 
button depressed, switches S3 and S4 are changed to their respective 
contacts b immediately before diaphragm aperture D is stopped-down, and 
capacitor C2 retains output Vao-Vso which is again applied via operational 
amplifier A2 to the plus terminal of operational amplifier A3 as a minuend 
to the subtraction circuit. Averaged light measuring output Va, when 
diaphragm aperture D is stopped-down to a preset position, is then 
produced from slider W2 of potentiometer PM2 and is applied to the minus 
terminal of operational amplifier A3 as a subtrahend to the subtraction 
circuit. Therefore, the output of operational amplifier A3 at this time is 
(Vao-Vso)-Va which is inverted by the inversion circuit including 
operational amplifier A4 to become Va-(Vao-Vso) which is charged and 
stored in capacitor C3. Switch S5 is opened immediately before the 
swingable mirror of an SLR camera moves, and the aforesaid output at that 
time is retained by capacitor C3. Exposure time control circuit 8 operates 
in accordance with the voltage stored in capacitor C3 to close the shutter 
with a delay, which is an optimum exposure time for the main object with 
spot light measuring, from the instant of shutter opening. 
The following description is with respect to normal photography using 
averaged light measuring. In this mode, switch S6 remains closed, and 
switches S3 and S4 are at their respective contacts a. Since switch S6 is 
closed, the charge at capacitor C1 is zero and the input (minuend) at the 
plus terminal of operational amplifier A3 in the subtraction circuit is 
also zero. Switches S1 and S2 are not operated, remaining connected to 
contact b. An averaged light measuring output is produced from slider W2 
of potentiometer PM2 and is applied to the minus terminal of operational 
amplifier A3 in the subtraction circuit as a subtrahend, and because the 
minuend is only zero, the averaged light measuring output is inverted and 
applied to operational amplifier A4 of the inversion circuit, and is 
inverted again to return to its original form and charged on capacitor C3. 
The output charged on capacitor C3 varies during aperture stopping-down 
motion from the initial fully open aperture to the final preset aperture. 
However, an averaged light measuring output with the diaphragm aperture D 
stopped-down to the preset position is retained by capacitor C3 by means 
of opening of switch S5 immediately before the mirror-up motion and in 
response to that stored output of capacitor C3 exposure time is 
controlled. It is to be noted that in this mode the output of slider W2 
may be directly input to and stored by capacitor C3 without passing 
through operational amplifiers A3 and A4. 
In the embodiment disclosed in FIG. 1, a modification is possible in which 
light measuring circuit 2 for spot metering is removed from the device. In 
this case, it is apparent that a photographer has to approach close to the 
main object for storing the light measuring output so that the light 
measuring acceptance angle of the photodiode PD1 may be occupied by the 
main object and first memory circuit 3 may receive a light measuring 
output with respect to the main object only as in the case of receiving 
output from light measuring circuit 2. In the above modification, switch 
S6 has to be modified to be constantly closed and switch S2, which 
normally contacts contact b and is opened to fix the contents of first 
memory circuit 3, has to be kept open until switch S7 is opened. The other 
operations are the same as those already disclosed.