Camera and method for selectively compressing scene brightness

A photographic exposure control system incorporating a multizone shutter arrangement and a photoresponsive array, with each element of the array being responsive to one of a plurality of spacially oriented areas of the photographic scene and in control of one shutter zone which transmits image-carrying rays from that scene area to a pictorially corresponding area of the film to selectively compress the range of scene brightness to within the linear range of the film thereby enhancing shadow detail. In the illustrated embodiment, the exposure system in effect controls the multi-zone shutter to expose film areas corresponding to bright scene areas, having a relative brightness falling within the linear range of the film, at one exposure factor and film areas corresponding to darker scene areas at a greater exposure factor.

BACKGROUND OF THE INVENTION 
This invention relates to a camera, and more particularly, to an exposure 
system for differentially varying the film exposure over a film frame. 
In photographing a scene having large variations in spatial brightness, and 
particularly where exposure is carried out under the control of a 
photocell, it is not unusual for details in the brightest regions to be 
washed out by over-exposure and for details in the darkest region to be 
concealed by under-exposure of the photosensitive material on the film 
frame. For a given scene, the extent of the problem depends on the 
characteristic curve, (i.e., the density-log exposure curve) of the 
photosensitive material being used. Where the brightness range in the 
scene being photographed and the photosensitive film material are such 
that many details are lost by reason of the exposure being determined by 
the average scene brightness, improved results can be achieved, for some 
scenes, by controlling exposure in accordance with a spatially weighted 
average of scene brightness. Thus, it is conventional to weight the 
photocell viewing angle to reduce the response to light from the upper 
portion of a scene, which usually will be the sky during daylight, outdoor 
photography so that the photosensitive element will be more responsive to 
the central portion of the scene where a person's face, for example, is 
located in the usual photographic setting. In such case, the central 
region, where it is desired for the detail to be the most distinct, will 
dominate the photocell's response and will result in a more pleasing 
photograph at the expense of some regional over- or under-exposure in 
peripheral portions of the photograph. The photocell response can be 
modified by optics associated with the photocell. Alternatively, the 
response can be electronically modified such as shown in U.S. Pat. No. 
3,409,378. Regardless of the technique utilized, however, any improved 
result will depend on whether the actual scene being photographed 
corresponds to the design criteria built into the photocell response. 
Improvement in the recorded detail of the photographic image has been 
achieved by focal plane shutters having a modulated aperture width for 
differentially exposing individual regions of a film frame in accordance 
with the brightness of the regions in the scene being photographed. U.S. 
Pat. No. 3,116,670, which is typical of the above-noted shutter mechanism, 
discloses a focal plane shutter having means for controlling aperture 
width to modulate the light reaching the photosensitive material during 
the transit thereover of the shutter curtain. Such means includes a 
photocell positioned to receive light passing through the camera lens and 
reflected from a narrow strip at the leading edge of the curtain slot, and 
an electromagnet for modulating the width of the slot in accordance with 
the output of the photocell. Thus, the exposure of elemental strips of the 
photosensitive material is modulated in accordance with the average 
brightness of the scene within the strip. Other examples of variable focal 
plane shutters are described in U.S. Pat. Nos. 3,479,936 and 3,442,198. 
While the above-noted shutter mechanisms may give improved results in 
scenes where the brightness gradient runs in the same direction as the 
direction of movement of the shutter, such improvements are generally 
limited when the brightness gradient in all or a part of the scene is 
parallel to the slot of the shutter curtain. Furthermore, by reason of the 
time required for a focal plane shutter to complete its traverse of a film 
frame, this approach is not suitable for a hand-held camera where maximum 
exposure time should not exceed about 1/30 of a second to prevent blurring 
due to reflex movements on the part of the photographer. 
In the above-noted cameras, the exposure of selected scene areas are 
essentially compressed so that each area of the film frame receives 
approximately the same quantity of light flux and will have the same 
average density. 
The value of the average density is dependent on the characteristic curve 
(density vs. logarithm exposure curve) of the photosensitive material. The 
predetermined amount of light that the shutter slot is permitted to 
transmit is preferably chosen to correspond to an exposure lying about 
midway in the linear portion of the characteristic curve. This choice 
maximizes the amount of detail that can be obtained, however, a photograph 
made in this manner, is not a realistic reproduction of a scene and if the 
compression is carried to the extreme, will be "flat" or "gray" in the 
sense of its being of uniform average density throughout. Since darker 
regions of a scene are expected by an observer to be much darker in a 
photograph than lighter regions in the scene, the resultant photograph 
will not in all cases be as aesthetically pleasing as is desirable. 
It is therefore an object of the present invention to provide a new and 
improved exposure control system and apparatus for optimizing an exposure 
over the film frame. 
Another object is to provide an improved exposure control system for 
selectively compressing the range of recorded scene brightness to enhance 
picture detail while retaining the relative brightness of many of the 
scene areas. 
SUMMARY OF THE INVENTION 
Briefly, the invention comprises means for sensing the brightness of a 
plurality of spacially oriented scene areas and for determining one 
exposure value in accordance with the brightest of said areas and another 
exposure value in accordance with the darkest of said areas, means for 
independently controlling the transmission of image-carrying light rays 
from each of said scene areas to corresponding film areas, and means 
responsive to said sensing means for varying said transmission means to 
expose at least one of said film areas at said one exposure value and 
another film area at said other exposure value. 
According to the present invention, exposure control apparatus is provided 
for a camera having a shutter mechanism constituted by a plurality of 
shutter elements, independently and selectively operable, to transmit 
light from a scene being photographed to corresponding elemental areas of 
photosensitive material carried by a film frame located behind the shutter 
mechanism. The exposure control apparatus includes a photosensitive device 
associated with each shutter element for determining the average 
brightness of light transmitted by the shutter element when the latter is 
operated, and means associated with each shutter element for operating the 
same in accordance with the average brightness of light transmitted 
thereby. Control means is provided for initiating operation of each of the 
shutter elements and for causing the shutter element that transmits light 
having the maximum average brightness to operate for a minimum exposure 
time interval within which this shutter element transmits a preselected 
maximum amount of light corresponding to an upper exposure limit, and 
causes the majority of the remaining shutter elements to also operate for 
only this minimum time if the amount of light transmitted by these 
elements within said minimum time interval lies between said maximum 
amount of light and a preselected minimum amount of light corresponding to 
a lower average exposure limit. For shutter elements that transmit less 
than said preselected amount of light within said time interval, the 
control means causes their operation to continue for a period greater than 
said time interval, and until the shutter element associated with the 
given operator transmits said preselected minimum amount of light or until 
a preset time limit has elapsed. 
The elemental areas exposed by the shutter elements will have an average 
density within the limits established by the minimum and maximum average 
exposure limits which, preferably, are located near the terminal ends of 
the linear portion of the characteristic curve of the photosensitive 
material being used. The average density produced by the brightest 
elemental area of a scene being photographed will be the density 
associated with the maximum exposure regardless of the high brightness 
magnitude of the brightest area; and the average density of the darkest 
elemental areas will be the density associated with the minimum exposure 
limit regardless of the low brightness magnitude of the darkest area. As a 
consequence, relative brightness will be retained for the brightest 
portions of the photograph without washout of very bright areas while 
shadow detail of dark areas are enhanced.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, reference numeral 10 designates a shutter 
mechanism according to the present invention incorporated into camera 11 
between objective lens 12 of the camera and photosensitive film frame 13 
for photographing a scene on the film frame in response to a manual input 
applied to shutter actuator 14 (e.g., manual depression of a shutter 
release button). Mechanism 10 comprises shutter A for effecting exposure 
of area B on film frame 13 under control of a light evaluating means C 
which includes a photosensitive device or array D located behind lens 18. 
The field of view of device D roughly approximating the field of view of 
film frame 13. 
Shutter mechanism A comprises a plurality of shutter elements A1-A8, each 
of which is spacially arranged to be associated with respective elemental 
areas B1-B8 of film frame area B, and each of which is selectively 
operable independently of the other elements for controlling the 
transmission of light from the scene to only that elemental area which is 
spacially oriented in a corresponding manner. The light evaluating means C 
comprises a plurality of means C1-C8 (FIG. 2) respectively associated with 
the shutter elements for independently operating the same under the 
influence of device D which comprises a plurality of photodetectors D1-D8, 
each respectively associated with one of the operating means C1-C8. 
Detectors D1-D8 are spatially arranged in the same pattern as shutter 
elements A1-A8 so that detector D1, for example, receives light from the 
same portion of the scene being photographed as elemental area B1 when 
shutter element A1 is operated. The resistance of detector D1 is 
functionally related to the brightness of light incident on area B1 when 
element A1 is operated. 
Mechanism 10 is axially spaced from film frame 13 a distance sufficient to 
defocus the image of the shutter elements so that the edges of the 
elemental areas on the film frame actually overlap thus preventing the 
edges of the shutter elements from producing sharp boundaries on the film 
frame. To simplify the drawing, however, the elemental film areas are 
shown congruent with the shutter elements with which they are associated. 
Hence, the shutter mechanism 10 is located sufficiently close to the focal 
plane such that each shutter element essentially controls the light 
passing to the film area directly behind the shutter, but spaced far 
enough from the film plane to defocus the edges of each element, when the 
element is closed. The latter spacing which provides blurring of the 
element edges, permits some spillover from one shutter zone to film areas 
adjoining its associated area, however, if the shutter mechanism 10 is 
spaced much closer to the focal plane than the lens, each element will 
still control the primary contribution to the exposure of its associated 
film area. 
Effectively, each shutter element and the drive means associated therewith 
constitutes a separate shutter or shutter zone for controlling the amount 
of light incident on the elemental film area or film zone with which the 
shutter element is associated. For this reason, only one shutter element 
and its associated drive means will be described. 
As shown in FIG. 2, each shutter element is in the form of an opaque member 
which has a first state in which the member is in an unblocking position 
with respect to the film frame for effecting transmission of light to the 
elemental area with which the member is associated, and a second state in 
which the member is in a blocking position for preventing transmission of 
light to such elemental area. Specifically, each member comprises a flap 
19 (FIG. 3) pivotally connected at 20 between a pair of bosses 21 located 
on the forward axial face of sleeve 22 having an aperture 23 of a size 
compatible with lens 12 for transmitting light from the scene being 
photographed to film frame 13. Sleeve 22 is rigidly attached to the camera 
housing 62 (FIG. 5). 
A flange 24 extends perpendicularly from flap 19 adjacent pivot 20 and is 
provided with a pair of spaced ears 25 between which one end of link 26 is 
received to define a pivotal connection 27. The other end of link 26 is 
connected by pivot pin 28 to the armature of an electromagnet of the 
operating means associated with the member to which the link is connected. 
FIG. 3 shows electromagnet E2, which is associated with shutter element A2. 
Each electromagnet comprises a coil 29 with which armature 30 is 
operatively associated. One end of armature 30 carries yoke 31 to which 
pin 28 is attached, and the other end of the armature carries a cam 
follower 33 operatively engaged with cam means 34 carried by ring 35 
pivotally mounted on the housing of the camera for rotation, with respect 
to sleeve 22, about the optical axis 36 (FIG. 1). Spring 37 is interposed 
between coil 29 and yoke 31 such that the spring, through link 26, biases 
the flap 19 to its blocking position as shown in FIG. 3. 
As shown in FIG. 6, cam means 34 is provided with a plurality of triangular 
sets of cam grooves 38, each set being associated with one of the 
electromagnets of the operating means. Each set of grooves includes an 
axially directed portion 39 connected to a transverse portion 40 which is 
also connected by an inclined portion 41 to portion 39. Cam follower 33 
operatively engages the groove 38. When the follower is located at 
position 42, the armature occupies the position shown in FIG. 3 where flap 
19 is in its blocking position. By rotating ring 35 in the direction of 
arrow X relative to sleeve 22 through a predetermined angle, follower 33 
will move along with portion 41 of the groove axially displacing the 
armature from its extended position shown in FIG. 3 to its retracted 
position shown in FIG. 4. This axial displacement moves flap 19 from its 
blocking to its unblocking position against the bias of spring 37. If coil 
29 of the electromagnet is energized just prior to the time that the 
follower arrives at position 43 in the groove, armature 30 will remain in 
the position shown in FIG. 4 against the bias of spring 37 thus holding 
the flap in its unblocking position until coil 29 is de-energized. 
When coil 29 is de-energized, spring 37 moves armature 30 from its position 
shown in FIG. 4 back to the position shown in FIG. 3 as follower 33 moves 
in portion 39 of groove 38 in the cam means 34 to position 44. Flap 19 is 
thus returned to its blocking position. By rotating ring 35 in a direction 
opposite to arrow X, follower 33 will move along portion 40 of groove 38 
causing slide 45 to be retracted against a spring bias (not shown) in the 
process until follower 33 reoccupies position 42. When this occurs, slide 
45 snaps back to the position shown in FIG. 6 to provide an edge 46 
against which the follower bears when ring 35 is once more moved in the 
direction of arrow X. 
The manner in which ring 35 is moved in the direction of arrow X and in the 
opposite direction is shown in FIG. 5. As indicated previously, ring 35 is 
rotatably mounted with respect to camera housing 62, and is provided with 
a lug 50 projecting therefrom for operative engagement with pawl 51 which 
is pivotally mounted on the camera housing for engagement with the lug 50 
by reason of the action of spring 52 acting on tail 53 of the pawl. Ring 
35 is urged in the direction of arrow X by spring 54. When shutter release 
button 55 is depressed against the action of spring 56, tail 53 of pawl 51 
pivots about pin 57 removing the pawl from lug 50 and allowing spring 54 
to rotate ring 35 sharply in the direction of arrow X. When this occurs, 
each of the armatures associated with the coils of electromagnets E1-E8 
(FIG. 7) mounted on sleeve 22 is rapidly moved from a retracted position 
shown in FIG. 3 to an extended position shown in FIG. 4, simultaneously 
moving each of the flaps from blocking to unblocking position. 
Simultaneously with the inward movement of shutter release button 55, 
initiating switch 58 is closed by engagement with the projection 59 of the 
shutter release button for the purpose of simultaneously energizing 
electromagnets E1-E8. 
Electrical portion 16 (FIG. 7) of exposure control apparatus C de-energizes 
an electromagnet after the shutter element operated thereby transmits an 
amount of light determined by device D. For example, electromagnet E1 is 
de-energized in accordance with the average brightness of the light 
transmitted by element A1 as determined by detector D1. Upon 
de-energization of electromagnet E1, element A1 is released and returns to 
its blocking position under the bias exerted by coil spring 37 operating 
on the armature of this electromagnet. 
After each of the flaps 19 is returned to its blocking position, ring 35 
can be returned to its initial position in a direction opposite to the 
direction of arrow X by applying a manual return movement to extension 60 
which is rigidly attached to the ring and extends through an opening 61 in 
the housing 62 of the camera. Manual return movement applied to the ring 
can occur until pawl 51 snaps behind lug 50 preparing the ring for 
simultaneously opening the flaps during the next cycle of operation of the 
shutter mechanism. 
The flaps 19 are designed to meet or just overlap at the center of aperture 
23 to prevent light leakage and can be loosely hinged at 27 to prevent 
hang up of one flap on the other. Further, the electrical control circuit 
may also be arranged to always ensure slightly staggered rather than 
simultaneous closing of any two flaps. 
Hence, it should be understood that the detectors D1-D8 provide means for 
sensing the brightness of spacially oriented areas of the scene and, 
stated otherwise, means for sensing the value of light flux passing to 
each area of the film. In conjunction with its integrator circuit, later 
explained with regard to FIG. 7, they provide means for sensing the total 
amount of light flux transmitted to a given film area during the exposure 
interval of the shutter zone associated with that film area. The cam means 
34 provides selectively operative means for initiating operation, that is, 
unblocking of the shutter elements while the electromagnet arrangements E 
provide selectively operative means for terminating each shutter zone 
operation, and together provide actuatable drive means for operating or 
actuating each shutter element between a non-transmissive (unblocking) 
condition and a transmissive (light blocking) condition. 
Additionally, the electrical circuit portion 16 provides means, responsive 
to the shutter drive means, for determining two different exposure 
factors, more specifically two different exposure intervals, and for 
controlling the shutter to expose some film areas at one exposure factor 
and other film areas at a second exposure factor thereby enhancing shadow 
detail of the areas exposed at the larger of the two factors. 
Referring now to FIG. 7, portion 16 of the light evaluating means C 
according to the present invention includes a plurality of trigger 
circuits T1-T8 respectively associated with electromagnets E1-E8, a gate 
71, and a threshold generator 72, made up of a reference generator 73 and 
a two-state switch 74. Only four trigger circuits are shown in FIG. 7 to 
facilitate illustration, but all are identical and only circuit T1 will be 
described in detail. 
Trigger circuit T1 comprises a two-state comparator 75 having a signal 
input 76, a reference input 77, and a single output 78 for energizing 
electromagnet E1 when the comparator is in its first state. A reference 
voltage derived from generator 72 is applied to input 77 of the 
comparator. Generator 72 produces two levels of voltage, one of which is 
designated "Ref. 1", and is higher than the other level designated "Ref. 
2". One or the other of these levels is supplied to input 77 of the 
comparator in accordance with the state of switch 74. The comparator is in 
its first state only when the voltage at input 76 is below the voltage at 
input 77. When the voltage at input 76 is above the voltage at input 77, 
the comparator 75 is in its second state wherein the electromagnet is 
de-energized. Note that higher and lower refer to the magnitude of the 
voltage, and the actual polarity (positive or negative) depends on the 
specific circuit design. 
Input 76 to the comparator is connected to node 79 of integrator 80 (which 
constitutes a photosensitive device) and comprises photodetector D1 and 
capacitor 81, which is shunted by a normally closed switch 82 that is 
opened when shutter element A1 is moved to its unblocking position. The 
time constant of integrator 80 is thus the product of the value of 
capacitor 81 and the resistance of detector D1, which is functionally 
related to the average brightness of an elemental area of the scene being 
photographed which corresponds to elemental area B1 of the film frame. 
Instead of being responsive to light from the scene directly, photometric 
device D could be used in association with an optical system wherein 
detector D1 receives light reflected from elemental area B1. In either 
event, detector D1 and capacitor 81 of the integrator establish the time 
constant of the integrator which is dependent on the average brightness of 
light transmitted by element A1 and incident on area B1. 
When switch 58 is closed, the voltage at node 79 and input 76 is 
essentially at ground while the voltage at input 77 of comparator 75 is at 
the "Ref. 1" level causing the comparator to go into its first state 
energizing electromagnet E1. If the magnetomotive force of this 
electromagnet is sufficiently strong, its armature will be retracted 
against the bias of spring 37 (FIG. 3) moving element A1 to its unblocking 
position. However, it is preferred to mechanically retract the armature, 
as operation of element A1 is initiated, by using cam means 34 in order to 
minimize power consumption from battery 83, and to design the 
electromagnet such that its pulling force is just sufficient to hold the 
armature retracted against the bias of spring 37. With the opening of 
switch 82 (which occurs when element A1 reaches its unblocking position), 
the voltage at node 79 increases exponentially toward the battery voltage 
as capacitor 81 charges with a time constant dependent on the resistance 
of detector B1 and the value of capacitor 81. Such voltage is thus a 
time-variable signal functionally related to the average brightness of 
light transmitted by shutter element A1 when the latter is operated. 
When the voltage at node 79 reaches a threshold, as defined by the voltage 
at input 77 of the comparator, the latter will react by abruptly and 
regeneratively changing to its second state in which it de-energizes 
electromagnet E1 allowing shutter element A1 to return to its blocking 
position. For a given threshold, the amount of light transmitted by 
element A1 and incident on elemental area B1 during operation of the 
element is dependent only on the average brightness of that portion of the 
scene corresponding to this elemental area. 
The output of each comparator is applied to gate 71 of control means 70. 
Gate 71 is of the type having a first state when all of the electromagnets 
in the outputs of the comparators are conducting (i.e., when all of the 
comparators are in their first state), and having a second state when any 
one or more (i.e., at least one) of the electromagnets is de-energized. 
When gate 71 is in its first state, switch 74 will be in a state that 
applies a level of "Ref. 1" to the reference inputs of each of the 
comparators of the trigger circuits. When gate 71 is in its second state, 
the state of switch 74 changes, applying the level of "Ref. 2" to the 
reference inputs of the comparators. 
If the voltage at the signal input to any comparator of the other trigger 
circuits is above "Ref.2" level when the state of comparator 75 switches, 
the state of this comparator will also switch simultaneously with the 
change of state of comparator 75. The remaining comparators, at whose 
signal inputs the voltage is below "Ref. 2" level, will remain in their 
first state after comparator 75 changes to its second state. Thus, 
energization of the electromagnets driven by the remaining comparators 
will continue. As the voltage at the signal inputs to each of the 
remaining comparators reaches "Ref. 2" level, each comparator will change 
state de-energizing its electromagnet and terminating transmission of 
light by the element operated by that electromagnet. 
Prior to further describing the overall operation of the exposure control 
system in detail, it should be noted that the terms exposure value or 
exposure factor as used in this application refer to camera exposure 
settings which for a given scene brightness will transmit a given total of 
light flux to the film and, hence, produce a given film exposure density. 
Both exposure interval and transmissivity (that is, aperture area and the 
transmissivity along the optical path) influence the actual exposure 
factor, however, where one of these is made a constant, as in the 
illustrated embodiment where only the exposure interval is varied, the 
remaining element is determinative of the selected exposure factor. 
The present system provides one exposure factor (one exposure interval in 
the illustrated embodiment) for a first set of scene areas to retain the 
relative brightness of these areas in the recorded image and a different 
exposure factor (a different exposure interval) for the remaining scene 
areas. Hence, the brightness in the recorded image of these remaining 
scene areas are varied or compressed relative to the first set to enhance 
detail while the portion of the scene comprising the first set is 
faithfully recorded in terms of relative brightness. 
This selective brightness compression of only some scene areas may be 
applied in a number of ways depending upon the camera structure and the 
desired end result. In the illustrated embodiment, a minimum exposure 
factor is determined which corresponds to those combination of camera 
parameters which will transmit a total amount of light flux from the 
brightest scene area to respectively provide a film exposure density at 
the upper density limit (designated 100 in FIG. 9) of the linear portion 
of film density versus log exposure curve and a maximum exposure factor 
which corresponds to camera parameters providing a total light flux from 
the darkest area to provide a film exposure density at the lower limit 
(designated 101 in FIG. 9) of the film density curve. Portions of the 
scene are exposed at the minimum exposure factor and others at the 
maximum. The brightest area of the scene being exposed at the minimum 
exposure factor, the darkest area at the maximum exposure factor, and many 
other scene areas exposed at one of these two factors to preserve relative 
scene brightness in the recorded image. 
Preferably, all scene areas which at the minimum exposure factor provide a 
total amount of light flux exceeding the lower density limit are 
automatically exposed at the designated maximum exposure value; the 
precise value of the latter being determined by the brightest scene area. 
Hence, the relative brightness of these scene areas one to another are 
retained in the recorded image. The remaining (dark) scene areas whose 
total light flux during the minimum exposure factor falls below the lower 
density limit are exposed at increased exposure factors, each providing a 
light flux which achieves minimum film density; the precise value of the 
maximum exposure factor being determined by the darkest area. Hence, the 
relative brightness of the dark areas one to another are not retained but, 
more importantly, the exposure density of these dark areas are increased 
relative to other areas of the scene so as to enhance shadow detail. For 
typical photographic scenes, it is expected that a majority of the scene 
areas will be sufficiently close in relative brightness so as to be 
exposed at the selected minimum exposure factor and a minority of scene 
areas (dark areas) exposed at greater exposure factors Of course, all dark 
areas could also be exposed at the maximum exposure factor. 
In the illustrated embodiment, as noted below, at least two different 
exposure values are provided by applying different exposure intervals to 
the shutter zones or segments. All film areas are first exposed for a 
minimum exposure interval (corresponding to the minimum exposure factor) 
at which the film area corresponding to the brightest scene area receives 
a sufficient amount of light flux to produce the given maximum exposure 
density. Then, only those film areas which during this minimum exposure 
interval did not receive a total amount of light flux equal to or 
exceeding that required to produce the given minimum exposure density are 
further exposed for greater exposure intervals proportional to their 
brightness with the maximum interval being determined by the darkest scene 
area (or a fixed maximum time interval where the darkest scene area would 
require an exposure time exceeding that considered suitable for hand-held 
cameras). 
The manner in which the exposure control apparatus of FIG. 7 enhances 
contrast in the photosensitive material exposed by the above procedure 
will now be described in detail in connection with FIGS. 8 and 9. 
Assuming, for purposes of illustration, that when shutter elements A1, A2, 
A7 and A8 (FIG. 7) are operated, the average brightness of light 
transmitted by these elements is I.sub.1, I.sub.2, I.sub.7, and I.sub.8 
related as follows: 
EQU I.sub.1 &gt;I.sub.2 &gt;I.sub.7 &gt;I.sub.8 
where I.sub.1 is the maximum of the average intensities with respect to all 
of the shutter elements. The time constants .tau..sub.1, .tau..sub.2, 
.tau..sub.7, .tau..sub.8 of the corresponding trigger circuits will bear 
the inverse relationship, so that the voltage developed by the integrators 
of each of these trigger circuits will resemble curves 90-93 (FIG. 8). 
Since the light transmitted by element A1 has the maximum average 
brightness and provides the smallest integrator time constant, the voltage 
of the integrator of trigger T1 represented by curve 90, will reach the 
level "Ref. 1" before the voltages of the other triggers. As indicated, 
"Ref. 1" is reached after element A1 has been operated for the time 
interval .DELTA.t.sub.1, which is termed the minimum exposure time 
interval. For a given level of "Ref. 1", it is a function of the maximum 
average brightness transmitted by any shutter element, and will thus be 
determined by the particular scene being photographed. In this sense, the 
minimum exposure time interval is not preselected. 
The amount of light transmitted by element A1 within the minimum exposure 
time interval is determined by the level of "Ref. 1" with respect to the 
battery voltage, and is preselected to correspond to a maximum exposure 
limit E.sub.max on the characteristic curve of the photosensitive material 
being used. Preferably, E.sub.max is chosen at point 100 near the upper 
extremity of the linear portion 94 of characteristic curve 95 (FIG. 9). 
The level of "Ref. 1" establishes the maximum amount of light transmitted 
by the shutter element that transmits the maximum average amount of flux; 
such maximum amount of light being termed the "predetermined maximum light 
flux". It is transmitted in the minimum exposure time by the shutter 
element transmitting light from the scene area of maximum average 
brightness and is independent of the actual brightness of scene light. 
Thus, the brightest portion of a scene always exposes the corresponding 
area of the photosensitive material to the same average density. For this 
reason, it may be said that the brightness of the lightest portion of the 
scene is compressed to the film density associated with E.sub.max. 
The "preselected minimum" amount of light transmitted by any element is 
determined by the level of "Ref. 2" which is preselected to correspond to 
a minimum exposure limit E.sub.min on the linear portion 94 of the 
characteristic curve. Preferably point 101 lies near the lower extremity 
of portion 94. 
As seen in FIG. 8, operation of all of the shutter elements continues for 
at least a minimum exposure time interval designated .DELTA.t.sub.1. 
Further operation of a given shutter element is terminated at the end of 
this time interval, if the amount of light flux the given element 
transmits lies between the preselected minimum and maximum amounts as 
established by the levels of "Ref. 1" and "Ref. 2". Shutter elements A2 
and A7 meet this criterion (and hence close at .DELTA.t.sub.1) because 
curves 91 and 92, which represent the signals generated by the integrators 
associated with these shutter elements, exceed or are equal to the level 
of "Ref. 2" after these elements have operated for the minimum exposure 
time interval. Note that curve 92 reaches the level of "Ref. 2" precisely 
at time .DELTA.t.sub.1 with the result that shutter element A7 will 
transmit the preselected minimum amount of light. The brightness of the 
light transmitted by shutter element A7 thus can provide a criterion for 
controlling exposure since any shutter element transmitting light whose 
average brightness lies in the range defined by the maximum brightness 
(which can have any value), and the brightness of light transmitted by 
shutter element A7, will be operated only for the minimum exposure time 
interval. For this reason, the brightness I.sub.7 is termed the "minimum 
light flux"; the term being used herein as defining the lower limit of a 
range whose upper limit is specified by the maximum average brightness 
which depends on the scene being photographed. In other words, the minimum 
light flux is not a fixed value, but will depend on the maximum brightness 
which, together with the level of "Ref. 1", establishes the minimum 
exposure time interval. Once this time interval is known, the minimum 
average brightness is known since it will depend on this interval and the 
level of "Ref. 2". Light of this brightness transmitted for the minimum 
exposure time interval will provide the preselected minimum amount of 
light. 
Since the transmission of light by shutter elements A2 and A7 terminates at 
the instant transmission by element A1 terminates, these shutter elements 
transmit less than the predetermined maximum amount of light flux, but 
more than the predetermined minimum amount of light flux. Resulting 
average exposure of elemental areas B2 and B7 is indicated qualitatively 
in FIG. 9 by points 96 and 97 on curve 95. Accordingly, the average 
density of these areas will lie between the levels D.sub.min associated 
with the minimum exposure limit E.sub.min, and the level D.sub.max 
associated with the maximum exposure E.sub.max. Hence, the relative 
brightness of the scene areas corresponding with film areas B1, B2 and B7 
are retained in the exposure since they are exposed at the same identical 
exposure value, i.e., in this arrangement, for the same exposure interval. 
If the average amount of light flux transmitted by a given shutter element 
during the initial time interval is less than the minimum light flux as 
defined above, operation of the given shutter element continues beyond the 
minimum exposure time. This situation is illustrated by curve 93 which has 
not reached the level of "Ref. 2" at .DELTA.t.sub.1. In other words, 
shutter element A8 will have transmitted less than the minimum amount of 
light flux when operation of the other shutter elements is terminated. 
Consequently, operation of this shutter element continues until the signal 
developed by the integrator of trigger T8 reaches the level of "Ref. 2", 
i.e., until shutter element A8 transmits the minimum amount of light flux. 
This occurs when element A8 has operated for the interval of time 
.DELTA.t.sub.8, which is the time required for curve 93 to reach "Ref. 2". 
Consequently, area B8 will be exposed to the minimum exposure limit 
E.sub.min producing on this area an average density D.sub.min. This should 
be compared with the exposure density value that would have resulted if 
operation of element A8 had been limited to the time interval 
.DELTA.t.sub.1 in which case an exposure would have produced a film 
density indicated at point 98. 
In this manner, the exposure density of the darkest area of the scene 
(corresponding to A8) is increased relative to the recorded brightness of 
other film areas thereby enhancing shadow detail. Now, under many scene 
conditions, several dark areas of the scene may continue to be exposed 
beyond the minimum exposure interval, in which case each of these would be 
exposed to that exposure interval during which their corresponding shutter 
segment will pass an amount of flux equal to the minimum exposure limit, 
with the darkest area exposed to the maximum interval. Hence, each of 
these remaining (dark) areas is exposed to minimum film density limit to 
enhance their detail, although relative brightness between these areas 
will be lost in the recorded image. The latter may be retained, however, 
by merely exposing these remaining areas at the maximum exposure interval. 
It is conceivable that a situation could arise in which the light flux 
transmitted by a shutter element is so low that a considerable time would 
be required for the exposure to reach the minimum exposure, thereby giving 
rise to the possibility of camera movement and the consequent blurring of 
the image on the elemental area associated with that shutter element. To 
preclude this, a maximum time circuit 102 is interposed between battery 83 
and the trigger circuits to limit the value of the maximum exposure factor 
to that of the maximum exposure interval. Delay 102 automatically 
interrupts circuit current a predetermined time after closure of switch 
58. 
Threshold generator 72 can take many forms. For example, it could be in the 
form of a transistor switch having a pair of serially connected bias 
resistors, the connection node being connected to reference inputs 77 of 
the comparators. When the transistor is non-conductive, the node will be 
at "Ref. 1"; and when conductive, the node will be at "Ref. 2". 
While the shutter elements in the preferred embodiment are disclosed as 
pivotal flaps, it is possible to utilize sliding flaps instead. Such 
sliding flaps could move axially parallel to axis 36 of the shutter 
mechanism which could be provided with guides that would bend the free 
ends of the flaps toward the axis to a position that blocks light 
transmission. Alternatively, the shutter elements could be in the form of 
an electro-optical shutter wherein the transmission is a function of the 
voltage applied across a pair of transparent plates. In such case, each of 
the electro-optical shutters would be independently controlled by the 
output of a photocell. This arrangement has the advantage of not only 
providing on-off transmission, but can be utilized to provide proportional 
transmission. 
It is believed that the advantages and improved results furnished by the 
apparatus of the present invention are apparent from the foregoing 
description of the several embodiments of the invention. Various changes 
and modifications may be made without departing from the spirit and scope 
of the invention as sought to be defined in the claims that follow.