Light measuring device

A light measuring device in which the field of a photometric system is divided into a plurality of areas, and the outputs of these areas are computed based on the one of various formulae which is selected depending on the relative size of the subject image to the total field as the focal length of a photographic lens varies, to obtain an exposure value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a light measuring device having a multiple 
segmented photosensor with a plurality of computation formulae. 
2. Description of the Related Art 
Of the light measuring methods, the most common ones are the averaged 
overall light reading and the partial light reading. In the former, the 
various portions of the entire field of the photosensor are integrated to 
get an average of their different degrees of brightness. When the subject 
of principal interest is far dimmer than its environment, as in a back-lit 
scene, a proper exposure for the subject can be hardly obtained. The 
latter reading is able to isolate each of the portions of the scene, and, 
therefore, is suited for to determining the exposure based on the 
brightness of the subject. Because the light measuring area is of a fixed 
relative size to the entire area of the image frame, however, if the 
subject image occupies a small fraction of the photometric area, the 
determination of an exposure will be unavoidably influenced by the 
brightness of the background of the scene. Also, to desire a balanced 
exposure over the entire area of the image frame by mental summing and 
averaging technique, the partial light reading is not very useful. So, a 
camera having these two aspects of the light reading has been developed. 
But, since the choice of the better one of the two light measuring modes 
to a given scene depends on the insight and experience of the 
photographer, for the beginner, the camera became only harder to handle, 
rather troublesome, and was often mismanaged. 
Recently, there has been proposed a light measuring device having a 
multiple segmented photosensor with an evaluation circuit receptive of the 
outputs of all the segments for obtaining a brightness distribution over 
the area of the image frame and responsive to this brightness distribution 
for determining an exposure suited to the scene. But, in this case also, 
since the relative size which the subject had to the image frame was 
ignored, the brightness of the subject was, similarly to the 
above-described partial light reading, not always primarily reflected is 
the exposure. Therefore, photographers could not take the light 
measurement with emphasis on a particular section of the scene. In more 
detail, as the size of the subject such as a person changes largely 
between landscape and portrait photography which are generally 
encountered, it was impossible to set that light measuring device in the 
mode suited to attain a best result of exposure of the subject. 
Meanwhile, when the subject lies near or at the minimum distance, the 
back-lit situation of it is rarely encountered. For the subject distance 
of 1.5 to 3 meters, the possibility of encountering back-lit situations is 
high. On this account, a light measuring device capable of changing over 
between modes in response to object distance information has been 
developed as, for example, disclosed in Japanese Laid-Open Patent 
Applications No. SHO 56-102838 and No. SHO 58-12571. 
Since, in such a proposal, however, the selection of the light measuring 
modes was made dependent only on the photographic distance, for, as the 
same subject at the same distance was shot, the focal length of the 
photographic lens varied in a wide range, even if the distance was the 
minimum, the possibility of occurrence of the back lighting in that scene 
would be very high. So, the last-named device had also a drawback that the 
exposure could not be based on the illumination of the subject. 
SUMMARY OF THE INVENTION 
With the foregoing in mind, the present invention has been made, and its 
object is to provide a method of measuring light in a more sophisticated 
manner with a higher accuracy by dividing the field of a photosensor 
positioned to cover just the entire area of the image frame into a 
plurality of regions and computing the outputs of the regions to obtain an 
exposure value based on the one of various formulae which is automatically 
selected depending on the object distance and the focal length of the 
photographic lens. 
Another object is to provide a light measuring device which operates with 
selection of modes depending on the image magnification. 
Still another object is to provide a light measuring device in which the 
selection of modes is made by discriminating the size of a subject 
relative to the image frame. 
A further object is to provide a light measuring device having a multiple 
segmented photosensor with the outputs from the segments being weighted as 
respective functions of the relative size of the subject. 
Yet another object is to provide a light measuring device which selects a 
center-weighted mode in automatic response to setting of the camera of 
either an auto-focus or an AE lock exposure mode. 
A further object is to provide a light measuring device in which the field 
of the photosensor is divided in such a fashion that a number of 
concentric segments, with their centers in exact or substantial 
coincidence with the center of the area of the image frame, are surrounded 
by the remaining area which is also divided into a number of segments. 
These and other objects and features of the invention will become apparent 
from the following description of embodiments thereof by reference to the 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is next described in connection with embodiments 
thereof by reference to the drawings. In FIG. 1, a camera 1 has a 
photographic lens 2, a main mirror 3, a submirror 4 and an optical system 
5 for a photosensor 6. Light passing through the lens 2 goes through part 
of the main mirror 3 and then is reflected by the submirror 4 to the 
optical system 5 by which an image is formed on the photosensor 6. 
FIG. 2 illustrates a segmentation pattern of the photosensor 6. The 
segments S0 to S6 each measure light individually. 
FIG. 3 is an algorithm for discriminating one scene from another by the 
distance from the camera to a subject of principal interest and the focal 
length of the zoom lens 1. From these two informations, a computer first 
obtains a photographic magnification .alpha., .alpha..apprxeq.Focal 
length/Distance. In respect to the value of .alpha., the proportion of the 
size of the subject image to the entire area of the image frame is divided 
in to three, for example large, middle and small, rates. For each rate, a 
particular one of the following formulae for computing the brightness 
values EV.sub.S0 to EV.sub.S6, sensed by the above-described segments S0 
to S6 of FIG. 2 to determine an exposure EVeva, is selected automatically. 
When .alpha..apprxeq.0.07, 0.04 or 0.006, the proportion which an image of 
a person occupies in the field of view of the finder is depicted in FIGS. 
4A to 4C respectively. 
(1) A range of .alpha..gtoreq.0.04 is defined in that, as shown in FIG. 4B, 
the size of the subject is equal to or larger than that of the area of the 
circle S2. For .alpha..gtoreq.0.06, the size of the subject is defined as 
much larger, and use is made of EVeva=(3EV.sub.S0 +2EV.sub.S1 
+EV.sub.S1)/6. 
And, for 0.06&gt;.alpha..ltoreq.0.04, EVeva=(2EV.sub.S0 +EV.sub.S1)/3. 
(2) Another range of .alpha.&lt;0.007 is defined in that, as shown in FIG. 4C, 
the size of the subject is smaller than that of the area of the circle S0, 
and the average light measuring method can be employed. So, 
EVeva=(EV.sub.S0 +EV.sub.S1 +EV.sub.S2 +EV.sub.S3 +EV.sub.S4 +EV.sub.S5 
+EV.sub.S6)/7. 
(3) Still another range of 0.04&gt;.alpha..gtoreq.0.007 is defined in that the 
size of the subject falls within a middle range of proportions, and, 
letting the smallest one of the EV.sub.S0 to EV.sub.S6 be denoted by EVmin 
and the output of a central area be expressed by EVcenter=(2EV.sub.S0 
+EV.sub.S1)/3, EVeva=.beta.EVcenter+(1-.beta.) EVmin is applied, where 
.beta. is the weight coefficient whose values are listed, as an example, 
in FIG. 5. 
The chart of FIG. 5 shows, when the size of the subject is found to lie in 
the middle range according to the algorithm, how differences in the 
contrast between the subject and its background are compensated for in 
terms of the above-defined weight coefficient. As an exposure value based 
on the environment is computed by as formula: EVaround=(EV.sub.S3 
+EV.sub.S4 +EV.sub.S5 +EV.sub.S6)/4, determination of the contrast of the 
scene in the form of a value of .beta. is made by using 
(EVcenter-EVaround) and the average of EV.sub.S0 -EV.sub.S6 as variables. 
For example, with a scene having a low average brightness, when the 
background is dimmer than the subject, the value of .beta. is made larger 
with a greater emphasis on the outputs of the central segments of the 
photosensor. When the average brightness of the scene increases, the value 
of .beta. is decreased with a greater emphasis on the dimmest portion of 
the scene. 
FIG. 6 is a block diagram illustrating a practical example of an exposure 
control system in the present invention. The above-described segments S0 
to S6 are connected to logarighmic compression circuits 7a to 7g 
respectively. The logarithmic compression circuits 7a to 7g produce 
respective outputs EV.sub.S0 to EV.sub.S6 proportional to the logarithms 
of the brightnessess of the portions of the scene corresponding to the 
segments S0 to S6. Each of the logarithmic compression circuits 7a to 7g 
comprises, as shown in FIG. 9, an operational ampifier OP and a diode Di, 
the respective one of the segments (S0 to S6), or a silicon photocell SPC, 
being connected between the inputs of the operational amplifier OP, thus 
taking the known form of the amplifier for the photosensitive element. The 
outputs of the logarithmic compression circuits 7a to 7g are applied in 
such combinations to computer circuits 8e to 8e that the outputs of the 
computer circuits 8a to 8e represent the following exposure values: 
8a: (3EV.sub.S0 +2EV.sub.S1 +EV.sub.S2)/6 
8b: EVcenter=(2EV.sub.S0 +EV.sub.S1)/3 
8c: Evaround=(EV.sub.S3 +EV.sub.S4 +EV.sub.S5 +EV.sub.S6)/4 
8d: EVmin, or the smallest value of the EV.sub.S0 to EV.sub.S6 
8e: (EV.sub.S0 +EV.sub.S1 +EV.sub.S2 +EV.sub.S3 +EV.sub.S4 +EV.sub.S5 
+EV.sub.S6)/7 
A substraction circuit 9 receptive of the outputs of the computer circuits 
8b and 8c produces an output representing EVcenter-EVaround. A 
.beta.-evaluating circuit 10 is connected to the computer circuit 8e and 9 
and, based on these two input data, produces a value of the weight 
coefficient .beta. according to the chart of FIG. 5. A computer circuit 11 
is connected to the computer circuits 8b and 8d and the .beta.-evaluating 
circuit 10, and computes these input data based on the following formula: 
EQU .beta.EVcenter+(1-.beta.)EVmin 
The computer circuits 8a, 8b, 11 and 8e are connected to an exposure 
control circuit 15 through respective analog switches 14a to 14d which are 
selectively turned on, one at a time, by a decoder 13. 
Connected to the input of the decoder 13 is the output of a computation 
control circuit 12, which receives informations from a focal length 
information forming circuit fc and an object distance information forming 
circuit DC and produces an output representing a value of the photographic 
magnification .alpha.. Depending on the output of the computation control 
circuit 12, the decoder 13 controls the analog switches 14a to 14d as 
follows: 
For .alpha..gtoreq.0.06, the analog switch 14a turns on. 
For 0.06&gt;.alpha..gtoreq.0.04, 14b turns on. 
For 0.04&gt;.alpha..gtoreq.0.007, 14c turns on. 
For .alpha.&lt;0.007, 14d turns on. 
In such a manner, the set of the analog switches 14a to 14d allows one of 
the outputs of the computer circuits 8a, 8b, 8e and 11, which is selected 
by the value of the photographic magnification, to pass to the exposure 
control circuit 15 therethrough. 
This embodiment has the following advantages. In general, the subject comes 
to the center of the field of view with high probability. So the position 
of the subject can be somewhat limited to the center. Also, the size of 
the subject varies with the distance and the focal length as shown in 
FIGS. 4A to 4C. When the subject is small relative to the entire scene, 
as, for example, in landscape photography, the exposure is made not based 
on the brightness of the subject alone, but on a good balance over all 
brightnesses of the various portions of the scene, as has been described 
in paragraph (2). Therefore, the average light measuring mode is operated, 
and photographs which will be found acceptable by the photographer can be 
taken. 
For large sizes of the subject image relative to the image frame as shown 
in FIGS. 4A and 4B, the size of the light measuring area is made to change 
with the change of the size of the subject image, as has been described in 
paragraph (1). This enables a subject of high contrast in itself to be 
exposed with an emphasis on good balance over the entire area of the 
subject alone. This or center weighted partial light measuring mode is 
made to operate over that central portion of the area of the image frame 
whcih is not reduced to less than a certain size shown at FIG. 4B, for 
example, thereby giving an advantage that even when the subject image 
takes its place more or less out of the center of the area of the image 
frame, its major sector can be even inscribed into the photosensitive 
area. 
But, because of this, despite the photographer considers isolation of the 
brightness of the subject, while disregarding the brightnesses of its 
environment, the possibility of introduction of the brightness of the 
background into the brightness of the subject as the output of the central 
segments of the photosensor will increase considerably. If the background 
is dimmer than the subject, the output of the central segments will be 
little affected by the brightness of the background. On the other hand, if 
the background is brighter than the subject, the influence of the 
background on the output of the central segments becomes appreciable with 
the result being an under-exposure of the subject. So, in this embodiment, 
to solve this problem, for photographic situations where the brightness of 
the subject cannot be truely isolated from those of the environment, that 
is, the middle range of sizes of the subject between the values shown in 
FIGS. 4B and 4C, six different computation formulae are selectively 
operated with different contrasts between the subject and its background, 
as has been described in paragraph (3). When the subject is far brighter 
than the background, the center weighted partial light metering mode is 
automatically selected to operate. When the background is far brighter 
than the subject, the center weighted average light metering mode with a 
greater emphasis on the darkest portion of the subject is automatically 
selected to operate. For this, reason, against any sort of background, the 
exposure is made as suited to the subject as possible. 
Another embodiment of the invention is described by reference to FIG. 7 
where the light measuring device of the first embodiment is combined with 
an auto-focus mechanism or an AE lock mechanism. The outputs of the 
segments of the photosensor 6 are applied through the respective 
logarithmic compression circuits 7a to 7g to the signal processing circuit 
30 shown in FIG. 6 where, after the same treatments as those described in 
connection with the first embodiment, one exposure value is derived. 
Meanwhile, an additional computer circuit 16 sums up all the outputs of 
the logarithmic compression circuits 7a to 7g after having been multiplied 
by respective different weights, and averages the total sum so that a 
center weighted average light metering aspect is formed. Whether or not 
the auto-focus mechanism AF is in use is determined by a detecting circuit 
17 in the form of a switch arranged to turn on when an auto-focus start 
member is actuated. If in use, the switch 17 produces an output signal of 
high level. Whether or not the AE lock is in use is determined by a 
detecting circuit 18 in the form of a switch arranged to turn on when the 
AE lock is operated. If the AE is in lock, the switch 18 produces an 
output signal of high level. A decoder 19 comprises an OR gate having 
inputs connected to the outputs of the detecting circuits 17 and 18, and 
whose output is connected to the control input of an analog switch 20a and 
through an inverter to the control input of another analog switch 20b. 
When at least one of the signals from the two circuits 17 and 18 is high 
level, the decoder 19 turns on the analog switch 20a, leaving the other 
analog switch 20b turned off. Thereby the output of the signal processing 
circuit 30 is transmitted to the exposure control apparatus 21. When both 
switches 17 and 18 are open, the analog switch 20b is turned on and the 
other analog switch 20a is turned off, thereby the output of the computer 
circuit 16 is selected for transmission to the exposure control apparatus 
21. 
Such second embodiment has the following advantages. When in AE lock or 
before the start of auto-focusing, the photographer carries out framing 
usually with the subject at the center of the picture frame. Since, in 
this embodiment, the segmentation pattern of the photosensor 6 is so 
configured that when the subject lies at that center, as the major portion 
of the subject is inscribed just in one of the concentric round segments 
S0 to S3, a most proper exposure value can be derived, the percentage of 
the photographs taken with such usual framings in AE lock mode or AF mode 
which will be found acceptable can be remarkably increased, because the 
processing circuit 30 is operatively connected to the exposure control 
apparatus 21. 
FIG. 8 illustrates an example of variation of the chart of FIG. 5. In a 
chart of FIG. 8, as the photosensor of the same segmentation pattern as 
that shown in FIG. 2 is employed, the average of the sum of the corner 
segments, namely EVaround of FIG. 5 is replaced by another average of the 
sum of any two brightest outputs out of the outputs of the corner segments 
EV.sub.S3 to EV.sub.S6, which is here denoted by EVaround', and the value 
of the weight coefficient .beta. is varied as a function of only one 
variable, that is, EVcenter-EVaround'. Depending on the thus-obtained 
value of the weight coefficient .beta., the exposure value determined 
based on the formula EVeva=.beta.EVcenter+(1-.beta.) EVmin varies so that 
the light measuring method varies between the center weighted partial one 
and the center weighted average one with variable emphases on the lowest 
brightness. 
FIG. 10 illustrates an example of application of a microcomputer to the 
second embodiment of the invention shown in FIG. 6. Responsive to commands 
from the microcomputer MC, a switching circuit SWC successively selects 
the outputs from the logarithmic compression circuits 7a to 7g for 
application to an analog-digital (A/D) converter AD. 
FIGS. 11A to 11B illustrates program flows in accordance which the 
microcomputer performs treatments. 
The operation of the circuit of FIG. 10 is described by reference to the 
flows of FIGS. 11A and 11B. The microcomputer MC when rendered operative 
starts to operate the switching circuit SWC in such a manner that the 
outputs of the logarithmic circuits 7a to 7g are successively applied to 
the A/D converter AD. The A/D converter AD successively produces outputs 
corresponding outputs to the outputs of the logarithmic compression 
circuits 7a to 7g in digital form. These digital values are stored as the 
light values EV.sub.S0 to EV.sub.S6 in respective memory portions of the 
microcomputer MC. The subject distance information D and the focal length 
information f are used in the formula: f/D, to compute a value of the 
photographic magnification .alpha.. This value of .alpha.is then compared 
with 0.06. When .alpha..gtoreq.0.06 is found, the light values EV.sub.S0, 
EV.sub.S1 and EV.sub.S2 are selected from the stored light values in the 
aforesaid memory, and are computed based on the formula: (3EV.sub.S0 
+2EV.sub.S1 +EV.sub.S2)/6. This computed value is made the exposure value. 
For .alpha.&lt;0.06, it is then compared with 0.04. When .alpha..gtoreq.0.04, 
the light values EV.sub.S0 and EV.sub.S1 are then used to perform the 
computation: (2EV.sub.S0 +EV.sub.S1)/3. The result is made the exposure 
value. 
For .alpha.&lt;0.04, it is then compared with 0.007. When .alpha..gtoreq.0.007 
is found, which of the light values EV.sub.S0 to EV.sub.S6 is smallest is 
then sought by executing the program shown in the flowchart of FIG. 11B, 
or the EVmin is evaluated. Then, the light values EV.sub.S0 and EV.sub.S1 
are used to perform the computation: (2EV.sub.S0 +EV.sub.S1)/3=EVcenter. 
Then, the light values EV.sub.S3 to EV.sub.S6 are used to perform the 
computation: (EV.sub.S3 +EV.sub.S4 +EV.sub.S5 +EV.sub.S6)/4=EVaround. 
Then, all the light values are to perform the computation: (EV.sub.S0 
+EV.sub.S1 +EV.sub.S2 +EV.sub.S3 +EV.sub.S4 +EV.sub.S5 +EV.sub.S6)/7, or 
to obtain the overall average. Then, the values of EVcenter and EVaround 
are substracted to determine EVcenter-EVaround. Plotting the thus-obtained 
values of EVcenter-EVaround and the overall average in the graph of FIG. 
5, the computer finds out the value of .beta.. Then based on the values of 
EVcenter and EVmin and further .beta., the .beta.EVcenter+(1-.beta.) EVmin 
is performed to obtain the exposure value. 
For .alpha.&lt;0.007, all the light values are called out from the memory to 
perform the computation: (EV.sub.S0 +EV.sub.S1 +EV.sub.S2 +EV.sub.S3 
+EV.sub.S4 +EV.sub.S5 +EV.sub.S6)/7. The result is used as the exposure 
value. 
As has been described above, according to the invention, the computing 
formula of the outputs of the multiple segmented photosensor is made to 
change not by the object distance alone, but by informations obtained as 
functions of the photographic magnification or the ratio of the focal 
length of the photographic lens to the object distance, and other 
variables, thereby giving an advantage that even in photographic 
situations in which the conventional light measuring device would be 
responsive in creatiing considerable exposure error of the subject, it is 
possible to ensure a proper exposure of the subject.