IE47583B1 - An exposure control system for use in a photographic camera - Google Patents

Abstract

A camera has shutter-diaphragm blades (40, 42) openable to define a primary aperture (46-48) for passing light to the film plane and a secondary aperture (50-52) for passing light to a photodetector (58) for controlling exposure; the blades also being arranged to admit light to the photodetector before exposure for indicating whether the brightness is adequate for exposure without flash. Preferably, a visual indication is given by means of a LED, and preferably if the brightness is not adequate, operation of the camera without flash is inhibited. The passage of light to the photodetector before exposure may be allowed by a third aperture (54- 56) in the blades, or by an extension of a secondary aperture (59, Figure 7 not shown). In another form, two blades (40,42, Figures 2a to 4a not shown) are contoured to provide tabs (54', 56') which are spaced before exposure to allow light to reach the photodetector along a path, distinct from the path provided by the secondary aperture. Alternatively, an aperture controlling the light path to the photodetector may be provided in an independently operable third blade.

Description

This invention generally relates to photographic cameras and more particularly to automatic exposure control systems for use in photographic cameras.

It is well known that the light reflected from 5 a scene to he photographed at times is insufficient to take sharp, acceptably exposed pictures without using a source of artificial illumination such as a photoflash lamp or strobe to supplement the available natural light.

Determining exactly when the available natural light is insufficient to provide a proper level of scene illumination, however, is a fairly complex process which involves a consideration -of such factors as film speed, camera exposure delivery capability, and the shutter speed at which camera motion is likely to cause blurred pictures.

Those skilled in the photographic arts have recognised the problems associated with determining when there is adequate natural light and have provided well-known apparatus (light meters) for use with cameras having manual shutters hy which a photographer can measure the scene brightness and quickly correlate the relationships between the important factors and the measured scene brightness to determine if an auxiliary light source is necessary.

In the case of cameras having automatic exposure systems where the photographer has little control over shutter speed or f-number or both, and, probably does not know what iheshutter programme is for any ‘ 47583 - 3 scene brightness, those skilled in the art have provided apparatus by which the photographer is automatically alerted of the presence of a scene brightness condition which will cause exposure problems.

Examples of the latter type of apparatus are well known in the patent literature. For instance, U.S. Patent No. 3,611,892 discloses a device for afctomatically indicating an underexposure condition and setting a diaphragm aperture in response to the movement of a shutter release member which is depressed by the photographer. This device, however requires the use of a separate fixed-speed shutter which is not automatically controlled by a photocell to provide the correct exposure interval during which the diaphragm aperture is opened.

Another example is described in U.S, Patent No. 3,355,601 where the patentees describe a camera which has a built-in low light level indicator in the sense that a changeover means will operate a display when the output of a comparator is such as to indicate that flash illumination is required. Two photocells are used here which do not appear to be associated with the camera shutter.

A further example is described in U.S. Patent No. 3,928,859. Here the patentee describes .an automatic camera which generates an anticipatory signal about 3 milliseconds after the start of an exposure interval. If sufficient light is present, a flash circuit is deactivated; if not, the flash is activated. The photocell of the camera is positioned to respond to light reflected from the film. Light from the scene heing photographed is not applied to the photocell prior to the incidence of light onto the film and thus there is no pre-exposure indication to warn the phtographer of a low scene brightness condition.

In U.S. Patent No. 4,007,469 the patentees describe a camera which includes a counter-reciprocating shutter blade arrangement, two photocells, and associated circuitry which, among other things provide a visual display in the camera's viewfinder indicating the presence of a low scene brightness condition and the absence of a fully charged strobe condition.

British Patent Specifications Nos. 136S295 and 1224545 also disclose cameras having means for providing a low-light signal. Both of these specifications are concerned v/ith exposure-control arrangements having a pair of oppositely moving blades, each having a primary aperture and a secondary aperture. The overlap of the primary apertures allows the passage of light to the focal plane, when a shutter blade is open and the overlap of the secondary apertures enables light to reach a photocell which controls the exposure time. In British Patent Specification No. 1368295, when a photographic cycle is initiated the blades move from an initial maximum-aperture position towards the minimum-aperture position and are halted in response to a signal from the photocell control circuit, the shutter being then opened and a lamp illuminated to provide a shutter-open signal. If the light is insufficient, the lamp is illuminated upon partial depress25 ion of an actuating lever, before the blades start to move.

In British Patent Specification No. 1224545, the blades are in their aperture-closed position at the beginning of a photographic cycle and in.the absence of sufficient light, the blades traverse to a maximum-aperture position without operation of the shutter blade, at which position a low-level indication is given.

A photographic camera according to the present invention comprises: blade apparatus for controlling the transmission of light along an optical path to a film plane; driving means for displacing the blade apparatus between a first arrangement in which it blocks the optical path and a second arrangement in which the optical path is unblocked; and a photo47583 detector providing an electric signal which varies as a function of the amount of the scene light incident thereon? the blade apparatus comprising blade elements which during displacement of the blade apparatus from its first to its second arrangement, co-operate to define a primary aperture aligned with the said optical path and a secondary aperture admitting scene light to the photo-detector, each aperture changing from an initial value to a final value during the said displacement; means operable when an exposure interval is initiated, by the actuation of the driving means to displace the blade element from its first arrangement, for integrating the output signal of the photo-detector to establish an exposure parameter comprising exposure aperture and shutter speed by actuating the driving means when the time integral of the output signal reaches a predetermined value to displace the blade apparatus again to block the said optical path and terminate the exposure Interval, the maximum value of aperture provided by the blade apparatus during the exposure being dependent on the amount of light incident on the photodetector during the exposure; the said blade elements being so formed that in the first arrangement of the blade apparatus they define an auxiliary aperture of predetermined constant value through which scene light is allowed to reach the said photodetector; and the camera further comprising means responsive to the said signal from the photodetector to measure scene brightness with the blade apparatus in its first arrangement and to provide a discernible signal if the scene light is below a predetermined level.

In one embodiment, scene light is allowed to reach the photodetector, when the blade apparatus is in its first arrangement, through an auxiliary aperture in the blade apparatus which is aligned with the photodetector at such time and which is moved out of alignment when the blade apparatus is displaced from its first arrangement. The blade apparatus preferably comprises a pair of blades mounted for counter-reciprocating motion, each having first, second and auxiliary apertures. In the first arrangement of the blade apparatus, the first apertures are out of alignment and block the optical path and the second apertures are similarly out of alignment, while the auxiliary apertures are aligned with one another to define the said aperture of predetermined value aligned with the photo-detector.

As the blade apparatus is moved out of its first arrangement and towards its second arrangement, the auxiliary apertures move out of their overlapping relationship and become misaligned with the photodetector, and portions of corresponding ones of the first and second apertures progressively overlap one another in alignment respectively with the optical path and photodetector.

In another embodiment of the invention light is allowed to reach the photodetector, with the blade apparatus in its first arrangement, through an auxiliary aperture which is not separate from but it an extension of the secondary aperture. With the two-blade arrangement, each of the secondary apertures is provided with an extension, the extended aperture portions overlapping in alignment with the photodetector when the blade apparatus is in its first arrangement.

In a further embodiment, two distinct light paths lead to the photosensitive surface of the photodetector. One may be a direct light path to the photosensitive surface while the other has an entrance which is offset with respect to the first light path. Light entering this second light path may be guided on to the photosensitive surface of the said photodetector by means of a fibre optics bundle.

In one form of camera having the two light paths, the blade apparatus is so shaped that in its first arrangement the photodetector is exposed to scene light along the second light path but as the blade apparatus moves out of its first arrangement a blade edge thereof moves over the said entrance to the second light path and blocks the transmission of light along this path. As the blade apparatus moves towards its second arrangement the primary aperture permits the passage of light along the said optical path and the secondary aperture admits light to the photodetector along the said first light path.

In preferred embodiments of the camera means are provided for inhibiting the actuation of the drive means to inhibit exposure of the film if the scene brightness, as measured when the blade apparatus is in its first arrangement, is below the predetermined brightness level which requires the use of an auxiliary light source to illuminate the scene.

In order that the invention may be better understood, some examples of cameras embodying the invention will now be described with reference to the acccmpanying drawings, in which:Figure 1 is a perspective view of a camera in which the invention may be embodied; Figures 2, 3 and 4 are front cross-sectional views of a portion of an exposure control system in a camera embodying the invention, and show different stages in the operation of the exposure control system; Figures 2a, 3a and 4a are front cross-sectional views of a portion of another exposure control system which can be used in a camera embodying this invention, and show different stages in the operation of this other exposure control system; Figure 5 is a diagram of a circuit which can be used with either the embodiment of Figures 2, 3 or 4 or the embodiment of Figures 2a, 3a and 4a; Figure 6 is a graph illustrating how the areas of various apertures of the exposure control system of Figure 2 vary during a typical photographic cycle of the camera apparatus of Figure 1; and Figure 7 is a front cross-sectional view of a portion of an alternative embodiment for the exposure control system.

The camera 10 of Figure 1 may be of the rigid body or box type and comprises a body 12, a front cover 14 and a door 16 which interconnect to define its outward appearance and serve as a protective enclosure for housing the apparatus' interior components. The body 12, the front cover 14, and the door 16 are preferably fabricated of an opaque plastic using injection moulding techniques in order to simplify their manufacture and reduce costs.

Located in a vertical forward wall 18 of the front cover 14 is a photographic objective taking lens 20 having an optical axis OA^, therethrough. The objective taking lens 20 may be a Cooke Triplet or similar multi-element type of lens which may have its focal length changed by adjusting the axial air spacing between its optical elements. This may be accomplished in a well-known manner by rotating a bezel, such as that designated at 22, which extends through an opening in the front cover 14 and is coupled with a screw-threaded lens mount (not shown).

Located in the base of the body 12 is a well-known film cassette receiving chamber 24 that is adapted to hold a film cassette, such as that designated at 26, in position for exposure through the objective taking lens 20. The film cassette 26 is preferably of the type which includes a stacked array of selfprocessable type film units (not shown). Located in the base of the film cassette 26, underneath the stacked array of self-processable film units, is a rectaigular flat thin battery (not shown) which may be used to supply power to the various electrical components of the camera apparatus 10. An example of such a film cassette is disclosed and described in detail in U.S. Patent No. 3, 872,487. - 9 f.

The body 12 also includes a generally planar rear wall 21 slanted at a predetermined angle with respect to the film cassette 26 and the optical axis, OA^, of the objective taking lens 20. Attached to the rear wall 21 is a trapezoidal-shaped mirror 23 positioned along the optical axis, OA^, of the objective taking lens 20, intersecting it at a predetermined angle, to fold the optical axis of the objective taking lens 20 so as to establish a folded optical path between the objective taking lens 20 and a forwardmost one of the plurality of stacked self-processable film units contained in the film cassette 26. With this optical arrangement, rays from a photographic scene which emerge from the objective taking lens 20 may be reflected from the mirror 23 onto a forwardmost one of the film units of the film cassette 26. In the foregoing manner, the photographic camera apparatus 10 is provided with means for defining a film plane and an optical path for transmitting light from a scene along the optical path to expose photosensitive film located in the film plane.

Generally designated at 25 is a well-known' flash socket that is adapted to receive a linear photoflash array (not shown) which is also of a well-known type.

Such a flash socket is described in considerable detail in, for example, ϋ. S. Patent No. 3,757,643. With this arrangement an artificial light source, such as the linear flash array, may be used with the camera apparatus 10 as a source for illuminating the photographic scene in the event that the natural -10 light available is belcw a predetermined level below which it would be difficult to take sharp well-exposed pictures.

Extending rearwardly from the body 12 is an elongated hollow portion 28 of the body 12 in which may be disposed a well-known viewfinder. The viewfinder may be of the reversed Galilean type which has an elongated eye relief aperture to improve magnification. The optical components of the viewfinder may be supported and protected within the elongated hollow portion 28 in such a manner so as to permit a photographer to aim the photographic apparatus 10 so that the subject matter of the picture to be photographed may be framed within an opening 30 located in the vertical forward wall 18 of the front cover 14. Those skilled in the art will recognize that the field of view of such a viewfinder may be made to be substantially coextensive with the field of view of the photographic camera apparatus 10.

Also located in the vertical forward wall 18 is an actuator button 32 which extends through an opening in the vertical wall 18 so as to be accessible to a photographer so that he may depress the actuator button 32 to initiate a photographic cycle in a manner to be subsequently described.

Projecting through yet another opening in the vertical forward wall 18 is an optical lens system 34 which is used for collecting radiation in a selective manner from the scene to be photographed and directing the collected radiation onto a photodetector located along an optical axis OAp, and behind the optical lens system 34. Although the photodetector is not shown in Fig. 1, it is designated at 47S83 iii Figs. 2-4 and generally operates to provide an output signal whose magnitude varies in accordance with the brightness of the photographic scene as will be subsequently, described.

The camera apparatus 10 is provided with an exposure control system which, as will be seen, is structured to deliver to film units of the film cassette 26 an exposure of predetermined value, to provide to the user of the apparatus 10 a visual indication that the scene brightness is below a predetermined value which requires that the apparatus 10 he used with an artificial light source to illuminate the scene, and to inhibit exposure if the scene brightness is below the predetermined brightness and an artificial source of illumination is not operatively connected with the apparatus 10.

Referring now to Figs. 2-4, there is shown generally designated at 36 an exposure control mechanism which forms part of the exposure control system for the apparatus 10. The exposure control mechanism 36 is generally located within the housing 12 intermediate the objective taking lens 20 and the location of the film units of the film cassette 26. More particularly, the exposure mechanism 36 resides just behind the vertical forward wall section 18 of the front cover 14 where it is in a position to block and unblock the optical path of the camera apparatus 10 in a manner to be described.

As shown in Figs. 2-4, the exposure mechanism 36 comprises a base block 38 which is fixedly stationed within the housing 12 and selectively machined to support the various components of the exposure mechanism 36. -12 _ Centrally disposed within the base block 38 there is an exposure opening 44 which defines the maximum available exposure aperture for the camera apparatus 10, and which is positioned so that its center is coincident with the optical axis, OA^, of the objective taking lens 20.

Mounted on the base block 38 there is a blade mechanism which includes two overlapping opaque shutter blade elements 40 and 42 of the so-called scanning type which will be subsequently described in greater detail herein.

A pair of scene light admitting primary apertures 46 and 48 is provided respectively in the blade elements 40 and 42 to collectively define a progressive variation of effective primary aperture openings in accordance with simultaneous longitudinal and lateral displacement of one blade element with respect to the other blade element in a manner as is fully described in 0. S. Patent No. 3,942,183.

The apertures 46 and 48 are selectively shaped so as to overlap the base block aperture 44 thereby defining a gradually varying effective aperture size as a function of the position of the blade elements 40 and 42.

Each of the blades 40 and 42 is additionally configured to have corresponding photocell sweep secondary apertures shown respectively at 50 and 52.

Secondary apertures 50 and 52 may be configured in correspondence with tne snapes of the scene light admitting primary apertures 46 and 48. As is readily apparent, the secondary apertures 50 and 52 also move in correspondence with the primary apertures 46 and 48 to define a small 47S83 secondary effective aperture for admitting the passage of scene light transmitted through the optical system 34 which is located in the front cover 14.

Scene light which emerges from the optical system 34 from the scene to be photographed may pass through the opening formed by the secondary apertures 50 and 52 to impinge upon the photodetector 58, previously mentioned,when the secondary apertures 50 and 52 are aligned with the photodetector 58.

A third set of apertures 54 and 56 is provided in the blades 40 and 42, respectively. As best illustrated in Fig. 2, the third apertures 54 and 56 overlap each other to provide an aperture of predetermined value that is aligned with the photodetector 58 so as to permit passage of light from the scene to impinge upon the photodetector 58 when the blades 40 and 42 are positioned at their first terminal position as illustrated in Fig. 2.

Projecting from the base block 38 at a location spaced laterally apart from the base block aperture 44 is a pivot pin or stud 60 which pivotally and translatably engages elongated slots 62 and 64 formed in respective blade elements 40 and 42. Pin 60 may be integrally formed with the base block 38 and the blade elements 40 and 42 may be retained in engaging relationship with respect to the pin 60 by any suitable means such as peening over the outside end of the pin 60.

The opposite ends of the blade elements 40 and 42 respectively include extended portions which pivotally connect to a walking beam 66. The walking beam 66, in turn, is disposed for rotation relative to the base block 38 by pivotal connection to a projecting pivot pin or stud 68 which may also be integrally formed with the base block 38 at a location spaced laterally apart from the base block aperture 44. The walking beam 66 may be pivotally retained with respect to the pin 68 by conventional means such as an E-ring 70. Xn the preferred mode, the walking beam 66 is pivotally connected at its distal ends to the blade elements 40 and 42 by respective pin members 72 and 74 which extend laterally outward from the walking beam 66. Pin members 70 and 72 are preferably circular in crosssection and extend through respective circular openings 76 and 78 in respective blade elements 40 and 42 so as to slidably engage respective arcuate slots or tracks 80 and 82 which may be integrally formed within the base block 38. The arcuate tracks 80 and 82 cooperate to inhibit disengagement of the blade elements 40 and 42 from their respective pin members 72 and 74 during operation of the exposure mechanism 36. Thus, the walking beam 66' and blade elements 40 and 42 collectively define a blade mechanism with the means for mounting the blade mechanism for displacement including the pivot pins 72 and 74.

Drive means for displacing the blade mechanism include a tractive electromagnetic device in the form of a solenoid 84 which is employed to displace the blades 40 and 42 with respect to each other and the base block 38.

The solenoid 84 includes an internally disposed cylindrical plunger unit 86 which retracts inwardly into the body of the solenoid 84 upon energization of the solenoid winding. The solenoid plunger 86 includes an end cap 88 at the outside end thereof together with a vertical slot or groove 90 within the end cap 88 for slidably engaging a pin 92 extending outwardly from the walking beam 66. Xn this manner, the solenoid plunger 86 is affixed to the walking beam 66 so that longitudinal displacement of the plunger 86 will operate to rotate the walking beam 66 around the pivot pin 68 so as to appropriately displace the shutter blades 40 and 42. The drive means also includes a tension spring 94 having one end pinned to the walking beam 66 via a pin 96 and its other end pinned to the base block 38 via a pin 98. With this arrangement, the spring 94 continuously urges the end cap 88 against the pin 92 of the walking beam 66 thereby also continuously urging the blade elements 40 and 42 toward a terminal position as best illustrated in Fig. 4 in which the primary apertures 46 and 48 of the blade elements 40 and 42, respectively, overlap to provide the largest opening in alignment with the base block aperture 44. Thus, with the spring arrangement described here, the exposure mechanism 36 of this invention is biased to continuously urge the blade elements 40 and 42 into an open orientation.

In the present arrangement, the blades 40 and 42 are drawn from their open second terminal position as illustrated in Fig. 4 to their closed first terminal position as illustrated in Fig. 2 when the solenoid 84 is energized. Consequently, energization of the solenoid 84 prevents the blades 40 and 42 from moving towards their maximum primary aperture opening under the urging of the tension spring 94.

As shown in Figs. 2-4, latch means are provided in the camera apparatus 10 by which the blade elements 40 and 42 may be releasably retained in their position illustrated in Fig. 2 and thereafter may be released to assume their position as shown in Fig. 4. The latch means include a latch member 100 having an elongated main body portion 102 disposed for rotation about a pivot pin or shaft 104 which may also be integrally formed with the base block 38. The main body portion 102 includes an integral arm portion 106 extending outwardly therefrom into overlapping relation therewith to ultimately define an integral hook portion 108. The hook portion 108 is adapted for releasable engagement with the pin 74 which also extends laterally outward from the side of the walking beam 66 as well as inwardly into the arcuate slot 82. More specifically, the hook portion 108 defines a first edge surface 110 which engages the pin 74 so as to inhibit clockwise rotation of the walking beam 66 about the pin 68. In addition, the hook portion 108 defines a second edge surface 112 which engages the bottom of the pin 74 to inhibit counterclockwise rotation of the latch member 100 about its pivot pin 104. A latch release slot designated generally at 114 accommodates release of the walking beam 66 from the hook portion 108 in a manner to be described. The latch member 100 is resiliently biased for yieldable clockwise rotation about the pivot pin 104 by a tension spring 116, one end of which is attached to a tab section 120 which extends laterally outward from the main body portion 102 of the latch member 100. The other end of the tension spring 116 is grounded with respect to the base block 38 by a pin member 118.

Located at the other end of the main body portion 102 of the latch member 100 is a bent over tab section 121 which extends outwardly from the main body portion 102. The tab section 121 is engaged by a latch actuating member 123 which is in turn mechanically associated with a gear train shown schematically at 138 in Fig. 5 which cooperates to rotate the latch member 100 about its pivot 104 between the positions illustrated in Figs. 2-4 in a manner which is more fully described in U. S. Patent No. 4,040,072.

In the foregoing manner the photographic apparatus 10 is provided with a blade mechanism and means for mounting the blade mechanism for displacement between a first arrangement in which the blade mechanism simultaneously is in light blocking relation to the optical path of the camera apparatus 10 thereby precluding scene light from being transmitted along the optical path and defines an aperture of predetermined value that is aligned with the photodetector 58 to pass light from the scene onto the photodetector 58, and a second terminal position in which the blade mechanism is in light unblocking relation to both the optical path of the apparatus 10 and the photodetector 58. The blade mechanism as described possesses features by which it defines a varying primary aperture, aligned with the optical path, which changes from an initial value to a final value during displacement of the blade mechanism between its first and second terminal positions and defines a secondary varying aperture, aligned with the photodetector 58, which also changes from an initial value to a final value during displacement of the blade mechanism from its first to its second terminal position. As will be noted with reference to Fig. 3, the initial values of both the primary and secondary varying apertures occur at substantially the same displacement of the blade mechanism from its first terminal position as illustrated in Fig. 2. Also note with regard to Fig. 3 that the third apertures 54 and 56 are misaligned with the photodetector 58 just prior to the secondary apertures 50 and 52 overlapping to provide the secondary varying aperture aligned with photodetector 58.

The camera apparatus 10 is provided with an electronic control system designated generally at 122 in Fig. 5. The control system 122 incorporates, among other things, the exposure mechanism 36 and the photodetector 58 to provide the camera apparatus 10 with a photographic cycle during which the scene brightness is measured to determine if an artificial light source should be used and to provide an exposure interval during which an exposure of predetermined value is delivered to film located in the film cassette 26 if the scene brightness is above the predetermined value which allows pictures to be taken utilizing the natural available light as the source for illuminating the scene.

The voltage required to operate the control system 122 and its associated elements may be derived from the battery of the film cassette 26 in a well-known manner or alternatively may be derived from a separate battery located within the camera apparatus 10. Xn either case, the voltage source for operating the control system 122 is designated as the battery 130 as shown in Fig. 5.

A switch SI is provided for coupling the positive terminal of the battery 1-30 to a· power line 132 while the negative terminal of the battery 130 is coupled to a ground line 134. The switch Si is normally open and is mechanically coupled to the actuator button 32 in a wellknown manner so that, when the user of the camera apparatus 10 depresses the actuator button 32, the switch SI is closed.

The photodetector 58 is preferably a silicon photodiode having its cathode connected to the power line 132 and its anode connected in common with the collector of an NPN transistor Q1 and the base of an NPN transistor Q2. The emitter of the transistor Q1 is connected to the ground line 134. The collector of the transistor Q2 is connected to the power line 132 while its emitter is connected in common with the base of the transistor Q1 and the base of another NPN transistor Q3.

The collector of the transistor Q3 is connected to the power line 132 via a resistor 140 while its emitter is connected to ground.

Connected in this mode of operation, the photodetector 58 operates in a reversed biased manner to produce a current output which is linearly proportional to the intensity of the scene brightness as seen through the optical system 34.

Transistor Q2 operates to balance the base currents of the transistors Ql and Q3. The transistors Ql and Q3 connected in this manner are matched devices having equal base-to-emitter potentials. However, the emitter area of the transistor Q3 is chosen to be larger than the emitter area of the transistor Ql so that the emitter current, I2, of the transistor Q3 will be larger than the emitter current, 1^, of the transistor 01 in proportion to the ratio of the areas of the emitters of the transistors Q1 and Q3. The resistor 140 is preferably chosen to match the impedance characteristics of the photodiode 58 as closely as possible and to bias the collector of <23. Those skilled in the art will recognize this circuit arrangement as a well-known current multiplier. Thus, as the intensity of the scene brightness increases, the current, 1^, also increases in proportion to the intensity of the scene brightness while the current, I2, increases by a multiple of the current, 1^, also in correspondence with increasing intensity of the scene brightness.

In the foregoing manner, light detecting means, including a photodetector, are provided in the camera apparatus 10 for providing an output signal having an electrical characteristic which varies as a function of the intensity of the scene light incident on the photodetector 58.

Connected in common with the resistor 140 and the collector of the transistor Q3 is a resistor 147 which is connected to the input of a comparator 148. The comparator 148 is preferably a level detector such as a Schmidt trigger device having a high voltage output signal (logic 1) when its input exceeds a predetermined level corresponding to the predetermined scene brightness level above which it is safe to take pictures utilizing the available natural light as a source for illuminating the scene and to have a low output signal (logic 0) when its input falls below the predetermined scene brightness level.

In this manner means are provided for monitoring the output signal of the light detecting circuit and measuring the scene brightness to provide a discernible signal that has at least one characteristic which indicates that the scene brightness is below a predetermined level.

A light emitting diode (LED) 152 has its cathode conected to the output of the comparator 148 and its anode connected to the power line 132. Connected in this manner the LED 152 is arranged to turn on when the output of the comparator 148 produces a logic 0 and to turn off when the output of the comparator 148 produces a logic 1. The LED 152 is preferably positioned within the body portion 28 of the housing 12 so that when it turns on, it emits a light signal which is visible to the user of the camera apparatus 10 indicating that the scene brightness level is below the predetermined value which requires the use of an artificial light source to illuminate the scene for proper exposure.

A timing network is provided in the control system 122 for turning off the comparator 148 and simultaneously turning on an integrator circuit comprising an operational amplifier 154 having a feedback capacitor 156. The timing network comprises a resistor 144 having one end connected to the power line 132 and the other end connected in common with the positive terminal of a capacitor 146. The negative terminal of the capacitor 146 is connected to the ground line 134. Connected in common with the positive end of the capacitor 146 and one end of the resistor 144 is the input to a conventional monostable multivibrator 150 (one shot) which has two output lines, one of which is connected to the comparator 148 and the other of which is connected to the operational amplifier 154. Both outputs of the one shot 150 provide a high voltage output signal at a predetermined .time after the switch SI is closed. The predetermined time is related in part to the values of the resistor 144 and the capacitor 146 as ie well known to those skilled in the art. When the _ 22 _ outputs of the one shot 150 go high, the comparator 148 is turned off and simultaneously the operational amplifier 154 is turned on.

Also connected to the power line 132 via a 5 resistor 142 is the collector of a conventional NPN transistor Q4. The base of the transistor Q4 is connected in common with the bases of the transistors Ql and Q3 while the emitter of the transistor Q4 is connected to the ground line 134. The value of the resistor 142 is identical to the value of the resistor 140 and the transistor Q4 is in other respects identical to the transistor Q3, Therefore the emitter current labeled, of the transistor Q4 is identical to the emitter current, I2, of the transistor Q3. Connected in common with the collector of the transistor Q4 and one end of the resistor 142 is the input to the integrating circuit comprising the operational amplifier 154 and its feedback capacitor 156.

Thus when the operational amplifier 154 is turned on by the output of the one shot 150, the current, 1^, will charge the capacitor 156. The integrating capacitor 156 and operational amplifier 154 provide the input to a conventional comparator 158 which is set to provide a high voltage output (logic 1) when the voltage output from the operational amplifier 154 exceeds a value related to the correct exposure of film units located in the film cassette 26 and to provide a low voltage output signal (logic 0) if the charge on the capacitor 156 is below the reference voltage.

Also provided in the control system 122 is an exposure sequence and control circuit 126 having one terminal labeled VCC connected to the power line 132 and another terminal labeled GND connected to the ground line 134. A terminal labeled END is connected to the output of the comparator 158 via the line 162 and a terminal labeled INB is connected to the output of the comparator 148 via a line 160. The exposure sequencing control circuit 126 operates to provide logic functions which are in part responsive to receiving signals at its terminals END and INB in a manner to be subsequently described.

Additionally provided is a motor and solenoid control circuit 124 which has one terminal labeled VCC connected to the power line 132 and another terminal labeled GND connected to the ground line 134. The solenoid 84 has one lead of its winding connected to an input terminal of the motor and solenoid control circuit 124 labeled SOL while the other end of its winding is connected to the power line 132. Also connected to the motor and solenoid control circuit 124 is a well-known motor indicated schematically at 136. The motor 136 has one of its terminals connected to the power line 132 while the other of its terminals is connected to the motor and solenoid control circuit 124 via the terminal labeled MOT. The motor 136 is mechanically associated with the gear train 138 to drive the gear train 138 in a well-known manner. The purpose of the motor and solenoid control circuit 124 is to provide the power requirements to both the motor 136 and the solenoid 84 in response to receiving a motor drive signal at a terminal labeled MDR and a solenoid drive signal at a terminal labeled SDR. The terminals labeled MDR and SDR of the motor and solenoid control circuit 124 are - 24 connected to correspondingly labeled terminals on the exposure sequencing control circuit 126. The manner in which both of these circuits operate will become apparent in the description of the operation of the camera apparatus 10 to follow. However, for a more detailed description of the nature and function of the motor and solenoid control circuit 124 and the exposure sequencing control circuit 126 and the relationship to the motor 136, the gear train 138, the solenoid 84, and the latch 100, reference may be had to D.S. Patents No. 4,035,813 ad4,040,072.

Having described the construction, the-combination of elements and the arrangement of parts for the photographic camera apparatus 10, its operation will now be described with reference to Figs. 2-5 and Fig. 7.

In describing the operation of the camera apparatus , it will he assumed first that a film cassette 26 is present in the film receiving chamber 24, the camera is aimed at the scene to be photographed, and the brightness level is below a predetermined value which requires the use of an artificial light source to illuminate the scene to be photographed, and that no artificial light source, such as a linear flash array, is present in the flash socket 25.

Under these conditions, when the photographer depresses the actuator button 32 the switch Sl is closed and the positive terminal of the battery 130 is electrically coupled with the power line 132 thereby supplying power to the motor solenoid control circuit 124, the exposure sequencing control circuit 126, and the various components of the brightness detect and integrate circuit. At this point the motor 136 and the solenoid 84 are deenergized and the blade mechanism 36 is in its first terminal position as illustrated in Fig. 2 wherein the optical path of the photographic camera apparatus 10 is blocked thereby precluding scene light from reaching the film located in the cassette 26 and wherein a predetermined aperture as defined by the apertures 54 and 56 is in alignment with the photodetector 58 to permit light from the scene to impinge on the surface of the photodetector 58. The photodiode 58 produces an output current which is proportional to the intensity of the scene brightness and the transistors, Ql, Q2, Q3 and Q4, are conducting to produce the currents, 1^, and Ij, as previously described since power is also supplied to these elements when the switch Sl is closed. It will be recalled that the current, l2, is also proportional to the intensity of the scene brightness and is larger than the current 1^ by a multiple which is related to the ratio of the areas between the emitter of the transistor Q3 and the emitter of the transistor Ql.

Because power is also supplied to the timing network at the closure of the switch Sl, it begins its time out function at that time. The comparator 148, because it is also energized at the closure of the switch Sl, instantaneously ·· 26 begins to respond in correspondence with the magnitude of the scene brightness. The input voltage to the comparator is compared with a reference voltage corresponding to a scene brightness level which is the threshold value above which the camera can be used to take pictures by utilizing the available natural light and below which the camera should be used with an artificial light source to illuminate the scene. Since it is assumed that the scene brightness is below the predetermined threshold value, the output of the comparator 148 is low (logic 0) thereby biasing the LED 152 on. The exposure sequencing control circuit 126 responds to the logic 0 output of the comparator 148 to inhibit energization of the solenoid 84 thereby inhibiting exposure of the film located in the film cassette 26 because the blades 40 and 42 cannot be displaced unless the solenoid 84 is energized. The light emitted from the LED 152 is visible to the photographer in the viewfinder of the camera apparatus 10 thereby alerting him with -a visual signal that the camera apparatus 10 should be used with an artificial source of illumination.

In this manner the user of the apparatus 10 is immediately alerted that a low scene brightness condition exists and because of the inhibit feature of the control system 122 does not lose a picture because of underexposure or camera motion., The operation of the camera apparatus 10 will now be described assuming that the scene brightness is above the predetermined threshold value and no artificial source of illumination is present in the flash socket 25.

Under these conditions the intensity of the scene brightness will be sufficiently high to cause the input to the comparator 148 to be higher than its - 27 reference level thereby causing the output of the comparator 148 to go high (logic 1). As before, the timing network starts its time out function when the switch SI is closed. The high output from the comparator 148 biases the LED 152 off so that it emits no light visible to the photographer. The exposure sequencing control circuit 126 responds to the logic 1 output of the comparator 148 by signaling the motor and solenoid control circuit 124, via the terminals labeled SDR interconnecting these two circuits, to energize the solenoid 84. When the solenoid 84 is energized, its cylindrical plunger 86 retracts into its body thereby causing the walking beam 66 to rotate in a counterclockwise fashion about its pivot 68 to a position (not shown) where the pins 72 and 74 butt up against the extreme ends of their corresponding arcuate slots 80 and 82. Displacement of the walking beam 66 to the extreme ends of the arcuate slots 80 and 82 causes the lower pin 74 to disengage from the hook portion 108 of the latch 100. The latch 100 is rotated in a counterclockwise fashion about its pivot 104 under the urging of the latch actuator member 123 to assume its position as shown in Pigs. 3 and 4. At this point the sequencing control circuit 126 deenergizes the solenoid 84 via the motor solenoid control circuit 124. When the solenoid 84 is deenergized, the walking beam 66 is caused to rotate about its pivot 68 in a clockwise fashion, as illustrated in Figs. 3 and 4, under the influence of the tension spring 94 thereby causing the blade elements 40 and 42 to translate and rotate with respect to each other and the base block 38 and the photodetector 58.

Displacement of the blade elements 40 and 42 proceeds in a manner previously described to unblock the optical path of the camera apparatus 10 to expose the film located in the cassette 26.

As best illustrated in Fig. 3, the first apertures 46 and 48 in respective blades 40 and 42, and also the second apertures 50 and 52 in respective blades 40 and 42, just begin to overlap each other to define initial aperture values at a predetermined displacement of the blade elements 40 and 42 from their first terminal position as illustrated in Fig. 2.

As can be seen in Fig. 3, when this happens, the third apertures 54 and 56 of respective blade elements 40 and 42 are misaligned with the photodetector 58 so that no light can impinge on the surface of the photodetector thereby reducing the output current of the photodetector 58. In general correspondence with the blades 40 and 4.2 assuming the position shown in Fig. 3, the predetermined time interval defined by the timing network elapses and the one shot 150 produces a high output at both of its terminals to turn the comparator 148 off and the operational amplifier 154 on.

As the blades proceed toward their second terminal position (Fig. 4), the second apertures 50 and 52 overlap each other to define a secondary varying aperture that is in alignment with the photodetector as previously described thereby again causing an increased current flow through the photodiode 58. Substantially simultaneously with the increased current flow through the photodiode 58, scene light is admitted through the base block aperture 44 to expose the film located in - 29 the cassette 26. In this manner an exposure interval is initiated during which scene light is admitted through the base block aperture 44 to expose film located within the cassette 26.

Since the operational amplifier 154 is turned on just prior to the initiation of the exposure interval, it, in combination with its feedback capacitor 156, begins to integrate the output current, 1^, until the capacitor 156 charges to the reference voltage level which is set in the comparator 158. When the reference or trigger level of the comparator 158 is reached, the comparator 158 produces a high voltage output (logic 1) at the input terminal of the exposure sequencing control circuit 126 which is labeled END. When the exposure sequencing and control circuit 126 receives the logic 1 signal at its END terminal, it reenergizes the solenoid 84 via the motor and solenoid control circuit 124. When energized, the cylindrical plunger 86 of the solenoid 84 retracts to cause the walking beam 66 to rotate again in a counterclockwise fashion about its pivot 68 so as to cause the blades 40 and 42 to return to and pass their first arrangement as shown in Fig. 2 thus terminating the exposure interval. Those skilled in the art will recognize that the reference level of the comparator 158 may be made to correspond to an exposure value which is appropriate for the particular film being used, the dynamics of the exposure mechanism 36, and its various aperture values.

After the solenoid 84 is reenergized in response to the exposure sequencing control circuit 126 responding to the logic 1 output of the comparator 158, the walking beam 66 assumes a position (not shown but previously described) in which the pins 72 and 74 are against extreme ends of respective arcuate slots 80 and 82. While in this position, the motor and solenoid control circuit 124 applies power to the motor 136 which in turn operates the gear train 138 to cause the latch actuator 123 to move from its position as illustrated in Fig. 4 back to its position as shown in Fig. 2 wherein the pin 74 is in readiness to be engaged by the hook portion 108 of the latch 100. When in this position, the motor and solenoid control circuit 124 deenergizes the solenoid 84 thereby allowing the walking beam 66 to rotate in a clockwise fashion about its pivot 68 into its first arrangement (Fig. 2) so that the pin 78 is trapped by the integral hook portion 108 of the latch 100 and is retained therein in a manner previously described.

Referring now to Fig. 6, there is shown a plot of the variation in area (curve 166) of the apertures which are aligned with the photodetector 58 as defined by the second and third apertures of the blades 40 and 42 during a representative photographic cycle and the variation in the area of the aperture that is aligned with the optical path of the photographic apparatus 10 (curve 164) as defined by the first apertures 46 and 48. The areas represented by the curves 164 and 166 are expressed in normalized fashion. The time labeled t in Fig. 1 o 3 represents the initiation of the photographic cycle when the actuator button 32 is depressed by the photographer.

At the time labeled t^ the solenoid 84 is first energized to cause the walking beam 66 to displace in a counterclockwise fashion as previously described, the time labeled fcfl is the time at which the exposure interval is initiated, and the time t is the time at which the solenoid 84 is c reenergized to move the exposure mechanism 36 towards and past its first terminal position as illustrated in Fig. 2. The predetermined time as defined by the timing circuit is the time that elapses between tQ to a time which is substantially the same as t^.

Included in the flash socket 25 is a pair of switch contacts 27 and 29 (Fig. 5) spaced apart to be normally open circuited. Switch contact 27 is connected in common with the output of the comparator 148 and switch contact 29 is connected to ground.

A well-known linear flash array, such as that designated at 31, which may be used as an artificial light source to illuminate the scene includes a blade-like conducting strip 33 which contacts the switch contacts, and 29, when the flash array 31 is inserted into the flash socket 25. Insertion of the flash array 31 into the flash socket 25 thus operates to ground the output of the comparator 148. With this arrangement, the camera apparatus 10 operates as though the scene brightness were above the predetermined threshold value as described above because the signal received at the INB terminal of the exposure and sequencing control circuit 126 is at a logic 0 when the flash array 31 is inserted into the flash socket 25.

Referring now to Fig. 7 there is shown an alternative embodiment for the exposure mechanism of this invention which is designated generally at 36'. The exposure mechanism 36’ is in most respects similar to - 32 ~ the exposure mechanism 36 except that it has no third set of apertures corresponding to the apertures 54 and 56 of the exposure mechanism 36. Instead its blades 40 and 42 are provided with apertures 50' and 52' respectively which initially overlap each other to define an aperture of predetermined value 59 that is aligned with the photodetector 58 when the exposure mechanism 36' is in its first terminal position as illustrated in Fig 7, Consequently the alternate embodiment for the exposure mechanism of this invention has separate first and second apertures each of predetermined size and location which, when the exposure mechanism 36' is in its first terminal position , as illustrated in Fig. 7, corresponding ones of the first apertures are misaligned with one another to block the optical path of the camera apparatus 10 while corresponding given portions of the other apertures 50' and 52' overlap each other and are aligned with the photodetector 58 to define an aperture of predetermined value. It will be apparent to those skilled in the art that the exposure mechanism 36' may be utilized with the control system 122 previously described in much the same way as the exposure mechanism 36 was utilized therewith.

In the arrangement shown in Figures 2a, 3a and Aa, there are first and second light paths for transmitting light to the photodetector 58.

The first light path is a direct path through the lens system 34 of Figure 1. The second light path is through a further aperture in the front wall of the camera housing, immediately ahove the lens system 34. This further aperture has an optical axis 0Ag that is generally parallel with the optical axis OAp of the first light path in Figures 2a, 3a and 4a. Immediately behind the second aperture In the front wall of the camera housing is the entrance of an elongate fibre optics bundle 15, the exit end of which is positioned to direct scene light that is transmitted along the length of the fibre optics bundle on to the photosensitive surface of the photodetector 58.

Thus, in this form, the camera provides first and second light paths for transmitting the scene light to the photodetector, the two light paths having light entrances which face the scene and are offset with respect to one another.

In the arrangement shown in Figures 2a, 3a and 4a, a pair of specially configured portions or tabs 54’ and 56'are provided on the blades 40 and 42, respectively. The tabs 54’ and 56' each define an extension of a respective longitudinal edge of the blades 40 and 42 located respectively adjacent apertures 50 and 52. As best illustrated in Figure 2a, when the blades 40 and 42 are in their first arrangement the extended tabs 54' and 56'are spaced away from one another so as to he misaligned with the said further aperture and the entrance end of the fibre, optics bundle 15 and to permit transmission of light from the scene along the second light path and on to the surface of the photodetector. When -34the blades 40 and 42 are positioned as shown in Figure 3a, the tabs 54' and 56^begin to overlap each other to block off the second light path to the photodetector 58, and as the blades 40 and 42 are moved toward their second arrangment as shown in Figure 4a, t A the tabs 54'and 56’ progressively overlap each other thereby continuing tb block the second light path to the photodetector 58 as long as the tabs 54' and 56^ have overlapping portions.

Thus, in this astodinent, the primary and secondary apertures are formed as in the embodiment of Figures 2, 3 and 4. With the apertures 46 and 48 out of alignment, and the apertures 50 and 52 out of alignment, as in Figure 2a, the optical path to the film plane is blocked, as is the first light path to the photodetector. The photodetector nevertheless receives light through the second light path including the fibre optics bundle, which at this time is exposed through the gap between the tabs 54’ and 56'. The second light path is blocked as the blades move away from the position of Figure 2a, continued movement of the blades resulting in the overlapping of apertures 46 and 48 in alignment with the exposing optical axis and the overlapping of the apertures 50 and 52 in alignment with the first light path to the photodetector. With reference to Figure 3a, it will be seen that the tabs 54' and 56' are slightly overlapped, thereby blocking the second light path, just prior to the secondary apertures 50 and 52 overlapping to pass light to the photodetector along the first light path.

The circuit of Figure 5 operates in connection with the exposure control system of Figures 2a, 3a and 4a, in exactly the same manner as with the exposure control system of Figures 2, 3 and 4.

Claims (18)

1. A photographic camera comprising: blade apparatus for controlling the transmission of light along an optical path to a film plane; driving means for displacing the blade apparatus between a first arrangement in which it blocks the optical path and a second arrangement in which the optical path is unblocked; and a photodetector providing an electric signal which varies as a function of the amount of the scene light incident thereon; the blade apparatus comprising blade elements which, during displacement of the blade apparatus from its first to its second arrangement, co-operate to define a primary aperture aligned with the said optical path and a secondary aperture admitting scene light to the photodetector, each aperture changing frcm an initial value to a final value during the said displacement; means operable when an exposure interval is initiated, by the actuation of the driving means to displace the blade element from its first arrangement, for integrating the output signal of the photodetector to establish an exposure parameter comprising exposure aperture and shutter speed by actuating the driving means when the time integral of the output signal reaches a predetermined value to displace the blade apparatus again to block the said optical path and terminate the exposure interval, the maximum value of aperture provided by the blade apparatus during the exposure being dependent on the amount of light incident on the photodetector during the exposure; the said blade elements being so formed that, in the first arrangement of the blade apparatus they define an auxiliary aperture of predetermined constant value through which scene light is allowed to reach the said photodetector; and the camera further comprising means responsive to the said signal from the photo-detector to measure scene brightness with the blade apparatus in its first arrangement and to provide a discernible signal if the scene light is below a predetermined level. /

2. A photographic camera in accordance with claim 1, in which the blade apparatus defines an auxiliary aperture which is aligned with the said photodetector to pass light from the scene to the photodetector when the blade apparatus is in its first arrangement, the said auxiliary aperture being moved out of alignment with the photodetector when the blade apparatus is displaced from its said first arrangement.

3. A photographic camera in accordance with claim 1, wherein the blade apparatus comprises a pair of blades mounted for counter-reciprocating motion with respect to one another, each of the blades having separate first, second and auxiliary apertures each of predetermined size and location such that when the blade apparatus is in its first arrangement, the first apertures are out of alignment and block the said optical path and the second apertures are out of alignment and block the passage of scene light while the auxiliary apertures are aligned with one another to define the said aperture of predetermined value aligned with the photodetector, portions of corresponding ones of the said first and second apertures progressively overlapping one another in alignment respectively with the optical path and photodetector to respectively define the said primary and secondary apertures during displacement of the blade apparatus from its first to its second arrangement, and the auxiliary apertures moving out of their overlapping relationship and becoming misaligned with the photodetector when the blade apparatus is moved out of its first arrangement.

4. A camera in accordance with claim 2 or 3, wherein the blades are of generally elongate shape and the apertures of each blade are spaced apart in the direction of the longitudinal dimension of the blade. 37 -47583

5. A camera in accordance with claim 4, wherein second and auxiliary apertures of each of the blades are bilaterally symmetric with respect to a generally longitudinal axis of respective blades and are longitudinally spaced apart along the said axis so that when the blade apparatus is displaced frcm its first arrangement toward its second arrangement, each of the auxiliary apertures become misaligned with said photodetector before any portions of corresponding ones of the second apertures overlap to define the secondary aperture in alignment with the photodetector.

6. A camera in accordance with claim 1, wherein the said auxiliary aperture is an extension of the said secondary aperture.

7. A photographic camera in accordance with claim 1 wherein the blade apparatus comprises a pair of blades mounted for counter-reciprocating motion with respect to one another, each of the blades having separate first and second apertures each of predetermined size and location so that when the blade apparatus is in its first arrangement, the first apertures are out of alignment with one another and block the said optical path while predetermined portions of the second apertures overlap each other and are aligned with the photodetector to define the said aperture of predetermined value, and wherein during displacement of the blade mechanism frcm its said first arrangement to its said second arrangement portions of the first apertures progressively overlap one another in alignment with the said optical path and portions of the second apertures, other than the said predetermined portions, progressively overlap - 33 one another in alignment with the phatodetector.

8. A photographic camera in accordance with claim 1, including means for providing first and second light paths for transmitting light from the scene 5 to the photodetector, the first and second light paths having light entrances which face the scene and are offset with respect to one another, the said secondary aperture transmitting light along the said first light path, and the scene light 10 transmitted to the photodetector when the blade apparatus is in its first arrangement passing along the said second light path.

9. Apparatus in accordance with claim 8, wherein the blade apparatus comprises a pair of blades 15 mounted for counter-reciprocatixgmotion with respect to one anotherand each having first apertures of predetermined size which are out of alignment with one another aid block the optical path when the blade apparatus is in its first 20 arrangement but progressively overlap in alignment with the optical path to define the primary aperture during displacement of the b-lade apparatus from its first to its second arrangement, each blade have a second aperture of predetermined size, the 25 second apertures being out of alignment with one another and blocking the said first light path when the blade apparatus is in its first arrangement and progressively overlapping one another in alignment with the first light path to define the 30 secondary aperture during displacement of the blade apparatus from its first to its second arrangement, the blades having predetermined contours such that the second light path is unblocked when the blade apparatus is in its first arrangement but is blocked 35 when the blade apparatus moves out of its first arrangement.

10. A camera in accordance with claim 9, wherein the blades are of elongate form and lie in a plane substantially perpendicular to the axis of the first light path and wherein the apertures of each blade are spaced apart in the direction of the longitudinal dimension of the blade.

11. Photographic apparatus in accordance with Claim 8, 9 or 10, wherein the first light path is generally perpendicular to and intersects a photosensitive surface of the said photodetector and the second light path is generally parallel, at its said offset entrance, with the axis of the first light path, the apparatus further comprising means positioned behind the said offset entrance of the second light path for changing the direction of scene light entering along the second light path so that the said scene light impinges on the said surface of the photodetector.

12. A photographic camera in accordance with any one of claims 8 to 11, wherein the second light path includes an elongate fibre optics bundle for guiding light from the said offset entrance of the second light path to the said photodetector.

13. A photographic camera in accordance with any one of the preceding claims, including means for inhibiting the actuation of the said driving means to inhibit exposure of the film if the signal from the - 40 photodetector, with the blade apparatus in its first arrangement, indicates that the scene brightness is below the predetermined level.

14. A photographic camera in accordance - with claim 5 13, wherein if the signal from the photodetector, with the blade apparatus in its first arrangement, indicates that the scene brightness is above the predetermined value, the driving means is caused to displace the blade apparatus from its first 10 arrangement to initiate an exposure.

15. A photographic camera in accordance with any one of the preceding claims, further including means responsive to the said discernible signal for providing a visual indication of the relationship 15 of the scene brightness to the said predetermined value.

16. A photographic camera in accordance with any one of the preceding claims, including means responsive to the actuation of the driving means for terminating 20 measurement cf the scene brightness to provide the discernible signal and beginning integration of the output signal of the photodetector substantially at the commencement of the exposure interval.

17. A photographic camera in accordance with any 25 one of the preceding claims, comprising: electronic timing means operative to.provide a signal at a predetermined time after the actuation of the driving means; level detecting means energised at the actuation of the driving means for monitoring the output signal 30 of the photodetector, measuring the scene brightness and providing the discernible signal, and de-energised in response to the said timing signal; integrating means energised by the said timing signal to integrate the output signal of the photodetector until the time integral of the said output signal reaches a predetermined value at which time the integrating means 41 provides an output signal; and electronic logic means energised at the acutation of the driving means and thereafter responsive to the output of the integrating means to cause the driving means to displace the blade 5 apparatus into its said first arrangement.

18. A photographic camera, substantially as herein described with reference to figures 2 to 5, or figures 2a, 3a, 4a and 5, or figure 7 of the accompanying drawings.