Method of controlling exposure

The exposure of a camera is controlled based on the scene brightness measured by use of a number of light measuring elements located at positions to receive light through from the scene. The light receiving area is divided into several zones. In each of the zones, at least one light measuring element is provided to measure the brightness of the scene in each zone and are used to give the maximum or minimum brightness (Bzi) in each zone. Exposure is controlled based on a weighted mean value ##EQU1## (K.sub.i+2 is a coefficient). In a preferred embodiment of the invention, another brightness (B.sub.0) such as a simple mean value of the outputs of all the light measuring elements is obtained, and the exposure is controlled based on the scene brightness B determined by the formula of ##EQU2##

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of controlling exposure for use in a 
photographic camera, and more particularly to a method of giving 
information for exposure control based on the brightness of the object to 
be photographed measured by means of a number of light measuring elements 
arranged over the whole image area of the image object. 
2. Description of the Prior Art 
In the photographic camera, it has been known to automatically control the 
exposure based on the brightness of the object to be photographed measured 
by a light measuring means incorporated in the camera. As the light 
measuring means, one or two light measuring elements are used for 
measuring the scene brightness at a position where the element or elements 
receive light from the object. Further, there have been known in the art 
two light measuring systems one of which is a system for measuring the 
average brightness of the scene and the other of which is a system for 
measuring the brightness of the central part of the scene. 
Recently, there has been proposed a new light measuring system for use in a 
photographic camera in which a number of light measuring elements are used 
for measuring the brightness of various parts of the scene and the average 
value of the maximum brightness and the minimum brightness is calculated 
based on the outputs of the number of light measuring elements. This 
system is capable of providing proper exposure even when the scene 
involves high-light parts and/or shadow parts. 
When the brightness of the scene has a large distribution as large as 5EV 
(exposure value) or more, the exposure must be controlled so that the main 
subject matter may be photographed with appropriate exposure. In the above 
described systems, however, the subject matter cannot always be 
photographed at appropriate exposure. For instance, when the subject 
matter is in a back light, namely in a bright background, the exposure of 
the main subject matter becomes under, and when the background is very 
dark like a spot-lighted subject on a stage the exposure thereof becomes 
over. 
SUMMARY OF THE INVENTION 
The object of the present invention is, therefore, to provide a method of 
controlling exposure for a camera in which the main subject matter is 
always photographed with the appropriate exposure regardless of the 
brightness distribution of the scene. 
The present invention is characterized in that the image area or light 
measuring area covering the scene to be photographed is divided into a 
number of zones in each of which at least one light measuring element is 
disposed to measure the brightness of the scene in the zone. When only one 
element is disposed in one zone, the element provides the brightness 
representing the mean brightness of the zone. When more than one element 
is disposed in one zone, the elements are used to provide the maximum or 
minimum brightness of the zone. The outputs of the light measuring 
elements are weighted and then averaged to obtain a weighted mean value of 
the brightness of the aimed scene. 
In a preferred embodiment of the present invention, the outputs of the 
whole light measuring elements are used for obtaining the simple mean 
value of the scene brightness in addition to said weighted mean value and 
the former is added to the latter to control the exposure further based on 
the brightness level of the scene. 
In accordance with the present invention, the various parts of the scene 
are weighted before calculating the average or final brightness to be used 
for controlling exposure. It is therefore possible to reduce the influence 
of not so important parts of the scene such as the sky, windows or shades 
and control the exposure based on the really important subject matter. 
In more detail, in accordance with the present invention, the scene 
brightness (B) is determined by the following formula; 
##EQU3## 
wherein Bzi is the maximum or minimum brightness in each zone, B.sub.0 is 
the average brightness of whole the scene, and K.sub.1, K.sub.2 and 
K.sub.i+2 are coefficients. The coefficients K are properly determined by 
repeated experiments by use of the light measuring elements which are 
equivalent to those incorporated in the camera. "n" is the number of the 
zones in which the scene is divided. The average brightness B.sub.0 of 
whole the scene may be the simple mean value of the outputs of all the 
light measuring elements, or a modified mean value like a weighted mean 
value of the outputs of all the zones. Further, the brightness B.sub.0 may 
be the maximum or minimum brightness of the scene. Therefore, the above 
formula means that the exposure is controlled based on the average 
brightness of the particular parts of the scene corrected by the weighted 
brightness of the whole scene or by the maximum or minimum brightness of 
the scene. In case that the brightness B.sub.0 is the maximum brightness, 
the brightness obtained from the outputs of the zones should preferably be 
based on the minimum value in the zones, and vice versa. In other words, 
when the maximum output of each zone is employed for calculating the main 
brightness based on the formula (1), the minimum brightness of the scene 
is employed as B.sub.0. In case the minimum output of each zone is 
employed for calculating the main brightness based on the formula (1), the 
maximum brightness of the scene is employed as B.sub.0. It should be noted 
that these combinations are merely preferable examples but not absolute. 
The above coefficients K.sub.1, K.sub.2, K.sub.i+2 may be changed according 
to the selected combination as mentioned above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now the present invention will be described in detail with reference to the 
accompanying drawings as described above. 
A base board 1 is provided with a number of light measuring elements 2a, 
2b, . . . 2n arranged in columns and rows. As the light measuring elements 
2a-2n can be used photodiode, photovoltaic element, phototransistor, CdS, 
CCD etc. When CCD is used, the sensitivity thereof should be controlled 
according to the brightness of the scene by changing the store time since 
the dynamic range thereof is narrow. 
The light measuring portion 1 is divided as shown in FIGS. 2 and 3. In FIG. 
2, the measuring portion 1 is divided into four horizontal zones Z1, Z2, 
Z3 and Z4. In FIG. 3, the measuring portion 1 is divided into a central 
rectangular zone Z4' and three concentric surrounding zones Z3',Z2', and 
Z1'. 
The outputs of the light measuring elements included in one zone are 
averaged to provide an average output for the zone or the outputs are 
computed to provide the maximum or minimum value for the zone. The 
representative output thus obtained for each zone is used as the 
brightness (Bzi) in the formula (1). In the formula (1), the coefficients 
K are determined as follows for example in case of zones divided as shown 
in FIG. 2. 
______________________________________ 
K.sub.1 = 1.41 
K.sub.3 = -0.02 (for zone Z1) 
K.sub.4 = -0.08 (for zone Z2) 
K.sub.5 = 0.22 (for zone Z3) 
K.sub.6 = 0.22 (for zone Z4) 
K.sub.2 = 0.72 (for average brightness) 
______________________________________ 
When the formula (1) was applied to an example employing these coefficients 
and as many as 1276 scenes were photographed by use of the exposure 
control system employing this invention, the number of improper exposures 
was only 44. 
When, on the other hand, only the average brightness B.sub.0 was used for 
controlling exposure, the number of the improper exposures was as may as 
199. Hence, the number of improper exposures can markedly reduced in 
accordance with the present invention employing the weighted brightness. 
When the example as shown in FIG. 3 is used, the coefficients are 
determined as follows for instance. 
______________________________________ 
K.sub.1 = 1.65 
K.sub.3 = 0.09 (for zone Z1') 
K.sub.4 = -0.13 (for zone Z2') 
K.sub.5 = -0.12 (for zone Z3') 
K.sub.6 = 0.23 (for zone Z4') 
K.sub.2 = 0.79 (for average brightness B.sub.0) 
______________________________________ 
With these coefficients, the improper exposures were reduced to 35. 
FIGS. 4 to 7 show various locations of the light measuring portion in a 
camera body. FIG. 4 shows an example in which the light measurement is 
conducted in parallel to the photographing system. An objective 3 is 
located in front of a light measuring portion 5 with the intervention of a 
stop 4 located therebetween. A taking lens 6 is provided separately 
therefrom in parallel thereto to focus an image on a photographic film 8 
through a stop 7. Thus, the light measuring portion 5 measures the light 
from the object (not shown) to be photographed by the camera with the 
taking lens 6. 
FIG. 5 shows another example in which a light measuring portion 5a is 
provided in a view finder. An objective 9 is provided in front of an 
eyepiece 11 with the intervention of a semi-transparent mirror 10 located 
therebetween. The semi-transparent mirror 10 reflectes a part of the light 
coming in through the objective 9 toward the light measuring portion 5a. 
FIG. 6 shows another example in which a light measuring portion 5b is 
provided in a single lens reflex camera. A part of the swing-up mirror 14 
is made transparent to transmit the light coming in from the taking lens 
16. A concave mirror 15 is located behind the mirror 14 to reflect the 
light transmitting through the mirror 14 downward toward the light 
measuring portion 5b. A stop 17 is located behind the taking lens 16. 
Thus, a part of the light coming in through the taking lens 16 is focused 
on the light measuring portion 5b and forms a small image of the object to 
be photographed thereon. 
FIG. 7 shows still another example in which light measuring portions 5c, 5d 
and 5e are located in the vicinity of a pentagonal prism 18 of a single 
lens reflex camera as shown in FIG. 6. The mirror 14 reflects the light 
coming in from the taking lens 16 upward. The light reflected upward by 
the mirror 14 enters a pentagonal prism 18 though a focusing glass 19 and 
a condenser lens 20. The light measuring portion 5c is located above the 
prism 18, 5d in front thereof and 5e therebehind. In front of the 
respective light measuring portions 5c, 5d and 5e are located focusing 
lenses 21c, 21d and 21e, respectively. 
FIG. 8 shows an example of an analog type exposure control system carrying 
out the method of this invention. The light measuring portion 5 consists 
of four zone measuring sections 5f,5g,5h,5i corresponding to the divided 
four zones Z1,Z2,Z3,Z4 or Z1',Z2',Z3',Z4'. Each zone measuring sections 
5f-5i includes at least one light measuring elements as mentioned above. 
The number of light measuring elements included in the light measuring 
portion 5 are all connected to a log-conversion circuit 30. The outputs of 
the light measuring elements are log-converted respectively. 
The log-converted outputs of the light measuring elements are sent to 
maximum brightness detecting circuits 31a-31d provided for the respective 
zones Z1-Z4, where the maximum brightness Bmax is detected for every zone. 
Further, in order to compute the average brightness B.sub.0 for the whole 
scene, the output signal of the light measuring elements are sent to the 
average brightness detecting circuit 32 and the average brightness B.sub.0 
is obtained by use of the formula of 
##EQU4## 
The average brightness detecting circuit 32 and the maximum brightness 
detecting circuits 31a-31d for computing the maximum brightness for every 
zone are connected with the brightness computing circuit 33, where the 
operation of the formula (1) is conducted to compute the scene brightness 
B. 
The brightness information or the scene brightness B obtained by the 
brightness computing circuit 33 is sent to an exposure operating circuit 
34 where an apex operation is conducted based on the information as of 
film sensitivity, aperture size or shutter speed sent from an exposure 
information setting circuit 35. 
A signal corresponding to the aperture size or shutter speed computed by 
the exposure operating circuit 34 is sent to an exposure control circuit 
36 which controls the aperture size or the shutter speed. 
FIG. 9 shows an example of a log-conversion circuit 30 which is composed of 
an operational amplifier 40 and a log-diode 41 connected in parallel 
therewith. The log-conversion circuit 30 is provided for every light 
measuring element. 
FIG. 10 shows an example of a maximum brightness detecting circuit 31a-31d. 
The outputs of the light measuring elements provided in each zone are 
connected to and inputed into voltage follower circuits 42a,42b, . . . 42n 
after log-converted. Then, after impedance-converted, the maximum 
brightness is detected by and outputed by diodes 43a-43n and a resistor 
44. 
FIG. 11 shows an example of a brightness computing circuit 33. Resistors 
45a-45c, resistor 46 and an inversion amplifier 47 constitute an inversion 
amplifying adder. The adder is provided with the maximum zone brightness 
or the average brightness B.sub.0 which has a negative coefficient K in 
the formula (1). Similarly, resistors 48a-48c, a resistor 49,50 and 
non-inversion amplifier 51 constitute an inversion amplifying adder. The 
adder is provided with the signals which has the positive coefficients K 
in the formula (1). Then, the resistance of the resistors 45a-45c, 48a-48c 
which is determined in advance according to the coefficients K serve for 
the weighted addition. 
After the outputs are summed up with the coefficients K, which means the 
outputs are added and/or subtracted according to the positive and/or 
negative coefficients K, the summed up output is inputed into an inversion 
amplifying adder consisting of resistors 52a,52b,53 and an inversion 
amplifier 54. From the inversion amplifier 54 is outputed a signal 
corresponding to the scene brightness B. 
FIG. 12 shows an example of a digital type exposure control system 
embodying the present invention. The outputs of the light measuring 
elements of the light measuring portion 5 are inputed into the 
log-conversion circuits 60a,60b . . . 60n to be log-converted. 
The outputs of the log-conversion circuits 60a-60n are inputed into 
comparators 61a-61n. The comparators 61a-61n compare the comparison 
signals from the D/A converter 62 with the log-converted signal. 
A multiplexer 63 is connected with the outputs of the comparators 61a-61n 
for selecting one of the comparators based on a multiplexer address signal 
65 from the micro-computer 64 connected therewith. After one comparator 
61a for instance is selected by the multiplexer 63 a comparison signal 66 
for count-up is sent to the D/A converter 62 to obtain an analog 
comparison signal. The comparison signal which gradually increases is 
compared with the signal from the log-conversion circuit 60a by the 
comparator 61a. When both the signals agree with each other, an agreement 
signal 67 is outputed from the comparator 61a and inputed into the 
microcomputer 64, where the comparing signal 66 is memorized as an A/D 
converted value. 
Since the address of the light measuring element is known from the 
multiplexer address signal 65 and the data are known from the comparison 
signal 66, the data are memorized in the RAM 68 at the corresponding 
address thereof. 
Then, after the multiplexer address signal 65 is incremented the output 
signal of the log-conversion circuit 60b is A/D converted similarly to 
said example and memorized in the RAM 68. 
A ROM 69 connected with the microcomputer 64 is provided with a program for 
taking up data, detecting the maximum brightness Bmax, the average 
brightness B.sub.0, the operation of the formula (1) and the operation for 
exposure control. 
By the information signal 70 from the camera side, the operation for 
exposure control is conducted. The shutter control circuit 71 serves as a 
timer as well as a buffer which receives as an input a code signal from 
the microcomputer 64 and provides an output of a shutter control signal 
72. Further, the microcomputer 64 outputs an aperture control signal. 
FIG. 13 shows the data input method for memorizing the A/D converted data 
in the RAM 68, in which the reference characters means the following 
factors: 
DM: log-converted output of the light measuring element 
DA: set value of D/A converter 
NM: address of the light measuring element 
NA: address of RAM corresponding to NM 
N: number of the light measuring elements 
DMAX: maximum value which can be set by the D/A converter 
DN: content of NA 
FIG. 14 shows another embodiment of the present invention in which a 
log-conversion circuit 60 is connected with the multiplexer 63 for 
log-compressing the output of the multiplexer 63. The log-compressed 
signal is converted to a digital signal 76 by an A/D converter 75 which is 
controlled by a control signal 74, and memorized in the RAM 68. 
In the above described embodiments, the light measuring elements provide 
the maximum brightness in each zone by use of the maximum brightness 
detecting circuit as shown in FIG. 10. In case that the minimum brightness 
is to be detected instead of the maximum brightness, a minimum brightness 
detecting circuit as shown in FIG. 15 may be used. The outputs of the 
light measuring elements are inputed into voltage follower circuits 
77a-77n and after impedance-converted thereby the outputs are inputed into 
diodes 78a-78n connected in parallel and a resistor 79. By means of the 
diodes 78a-78n and the resistor 79, the minimum voltage can be detected 
and outputed. Hence, the minimum brightness can be detected.