ELECTRONIC TRIM CONTROL FOR DUAL-PHOTODETECTOR EXPOSURE CONTROL

Abstract: A photographic camera of the type having a quench-type strobe, a scanning blade shutter (18) and an exposure control system (52) having separate ambient (56) and artificial (58) light sensing devices for establishing dis­ tinct signals representative of the amount of ambient and artificial light illuminating a particular scene, is provided with a unitary, manually operable electronic trim control (68) for simultaneously varying both of these signals, to a limited degree, to compensate for varia­ tions in ambient scene lighting conditions and/or slight variations in the sensitometric characteristics of a photographic film. The electronic trim control system includes a common voltage source capable of providing a plurality of different magnitude trim voltages for regu­ lating both ambient and artificial scene light levels. The electronic trim control couples a manually selected common trim voltage corresponding to desired ambient and artificial scene light levels to shutter and strobe control circuitry for ambient and strobe light control purposes.

Description: BACKGROUND OF THE INVENTION The present invention relates to an exposure control system for photographic camera of the type employing both a scanning blade shutter and a quench-type strobe, in general, and to a manually operable trim system for electronically controlling the aperture formed by said shutter and the strobe light levels in such a camera, in particular. Automatic exposure control systems have been incorporated within various amateur cameras for simplifying their operation by minimizing the pre-exposure adjustments to be made by a camera operator. These exposure control systems function to automatically regulate the exposure parameters for a given scene. Generally, the exposure control systems are designed for use over a wide range of scene lighting conditions. Exposure control systems embodying scanning shutter blade elements which vary the exposure aperture areas with time during an exposure interval are well known as indicated by U.S. Patent No. 3,942,183. Such mechanisms generally include a pair of counter-reciprocating shutter blade elements, each having a primary aperture that traverses the optical axis of the camera during the exposure interval. The primary apertures are shaped so that upon overlying one another during counter movement of the blades, there is defined an effective exposure aperture value which increases to a maximum value in a determinate period of time. Exposure control is provided by a pair of secondary photocell apertures in respective shutter blade elements which admit scene light to a photoresponsive element in proportion to the scene light admitted to the focal plane during shutter blade movement through an exposure cycle. The output from the photoresponsive element is directed to an integration circuit which triggers upon reaching an integration level corresponding to a desired exposure value to terminate the exposure interval by returning the shutter blade elements to their initial scene light blocking position. Such exposure control systems may be utilized with a source of artificial illumination such as an electronic flash to provide artificial illumination not only during conditions of low ambient scene light, but also under conditions of high ambient scene light if it becomes desirable to provide fill-in illumination for the photographic scene as disclosed in U.S. Patent No. 4,023,187. Since a majority of amateur pictures is taken within the range of light levels to which the control system is suited, well-exposed photographs are obtained. However, the camera operater may wish to take a photograph which is lightened or darkened to his personal taste, or may wish to accommodate for unusual lighting effects such as backlighting, highlights or the like. In order to adjust the automatic exposure control system for such unusual scenes or effects, a lighten-darken trim mechanism is provided. These trim mechanisms provide the user with a manual one-half to one stop optional exposure adjustment above or below the exposure value otherwise automatically established. Ideally, such trim control should be provided with only one manually adjustable component for ambient and artificial light level control. Where a dual trim system for ambient and artifical light (flash) is provided which has more than one control element, a camera operator may become confused or overlook a requirement for providing trim control for that mode of operation which he is currently using. In the trim control mechanism disclosed in U.S. Patent No. 4,209,243, the amount of scene light admitted to the focal plane of a camera is dependent upon the amount of light passing from the scene to a photoresponsive element through a variable density filter slidably mounted on the camera body for arbitrarily varying the amount of scene light "seen" by said photoresponsive element. Normal exposure of photographic film in the camera will result when a medial density portion of the filter is positioned in front of the cell. Correspondingly, the exposure control system may be trimmed to either an underexposure mode upon movement of the slide to reduce the filter density and thus increase the light "seen" by the cell to a value above that indicated for normal exposure, or to an overexposure mode by moving the slide in the opposite direction to increase filter density and thus reduce the amount of light "seen" by the cell to a value below that which would pass the filter for normal exposure. However, because of the degree of precision required for an exposure control system, the limitations on the space available for the overall exposure control system and the importance of the trim mechanism to overall camera effectiveness, there is a need for structural simplicity, reliability and manufacturing cost effectiveness in trim mechanisms of this type. A trim control employing a slidable filter and photosensitive element combination does not adequately satisfy these requirements. The invention relates to an exposure control system in accordance with the preamble of claim 1. A hybrid exposure control system of this kind is known from EP-A-0 108 979. With determining said first, second and third selected value for firing the flash, actuating the blade assembly to its closed position and for quenching said electronic flash a hybrid exposure control system is achieved, wherein a fine control is effected considering slide errors in determining camera-to-subject range and differing reflecting amounts due to the surface characteristics of the subject. The artificial light integrator is supplied by a photocell reponsive to IR illumination whereas the ambient light integrator is supplied by a photocell having a spectral filter attenuating electromagnetic frequencies in the IR region. No means are disclosed in the specification for effecting a trim control, but as described in connection with US-A-4 209243, a trim control employing a slidable filter over the photosensive element would not adequately satisfy the requirements. US-A-4 074 287 discloses a flash and camera device integrally incorporating an automatic exposure camera which automatically adjusts the exposure depending on the brightness of an object and an electronic flash device, which is capable of controlling the light intensity of a flash discharge tube, which fires by consuming the charged energy of a main discharging capacitor. The amount of incident light to a first light sensor for controlling the exposure of the automatic exposure camera and the amount of light reflected from the object, which impinges on a second light sensor for controlling the light intensity of the electronic flash device, are changed in linked manner for simultaneously adjusting the exposure and the light intensity in accordance with the brightness of the object. For adjusting the device to the sensitivy of a film which effectively means trim adjustment of the exposure device, is effected by operation threshold adjusting means for changing the operation threshold of ambient light control and artificial light control in a linked manner. It is a primary object of the present invention to improve an exposure control system of the type as specified in the preamble of claim 1 so that a structurally simple reliable and consteffective electronic trim control is obtained. In accordance with the present invention the technical problem is solved by the features specified in the characterizing portion of claim 1. The trim mechanism therefore is effective in changing the moment of firing the electronic flash, the moment for causing the mechanical shutter to close and the moment for quenching the electronic flash. Therefore all those requirements of a trim control are satisfied which trim control means employing a slidable filter over the photosensitive element does not. In accordance with the present invention therefore the ambient light-level is measured before any shutter action to program the camera for high or low light level exposure. On the basis of these values the instant of flash firing relative to the shutter opening is determined. Subsequently the flash is quenched by a signal generated by the artificial light integrator. As well as flash firing the instant of flash quenching are varied by the operator at will by the trim switch by means of which the sensitivity of both the ambient light integrator and the artificial light integrator may be increased or reduced relative to normal setting. The electronic trim control includes a common voltage source that provides a plurality of manually selectable, different magnitude trim voltages. A voltage selector couples a selected one of said trim voltages to said shutter and quench strobe control means for the purpose of varying the said electrical signals representative of ambient and strobe light levels to thereby simultaneously adjust the amount of ambient and/or strobe scene light admitted to the focal plane of said camera to the particular level desired by a camera operator. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of an exposure control system incorporating a preferred embodiment of the electronic trim control of the present invention. Fig. 2 is a schematic diagram of a portion of the trim switch position decoder functionally shown in drawing Fig. 1. Fig. 3 is a schematic diagram of a portion of the digital multiplexer functionally shown in drawing Fig. 1. Fig. 4 is a schematic diagram of a portion of the analog multiplexer functionally shown in drawing Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS Camera 10, schematically shown in Fig. 1, includes an objective lens 12 movably mounted for focusing image-carrying light rays of an object 14 on a film plane 16 through an aperture formed by a shutter assembly 18. Shutter assembly 18, positioned between the lens 12 and the film plane 16, includes a pair of overlapping shutter blade elements of the "scanning" type, small portions of which are schematically shown at 20a and 20b. A scene light admitting primary aperture (not shown) is provided in each of said shutter blade elements to cooperatively define a progressive variation of effective aperture openings in accordance with simultaneous longitudinal and lateral displacement of one blade element with respect to the other blade element in a manner more fully described in U.S. Patent No. 3,942,183, supra. The blade element apertures are shaped to overlap the central axis of lens 12 during an opening movement of the blades, thereby defining a gradually varying effective aperture size as a function of the position of the blade elements of shutter assembly 18. Drive means that include a tractive electromagnetic device in the form of a solenoid (not shown) are provided for displacing the blade elements with respect to one another in a manner more fully described in a well-known manner. Each of the shutter blade elements additionally includes two secondary apertures (not shown in detail) with an aperture in one blade element cooperating with an aperture in another blade element to form a pair of openings 22a, 22b therethrough. These secondary apertures may be configured to track in a predetermined corresponding relationship with respect to the scene light admitting primary apertures (not shown) within shutter assembly 18. With the primary and secondary apertures being formed in the same blade elements and therefore being mechanically coupled to one another through paths 24a, 24b and 24c, they move in the same manner as the primary apertures for controlling scene light passing through openings 22a and 22b to photoresponsive elements 26 and 28, respectively. An example of scanning blade elements having primary and secondary apertures that cooperate to control the amount of scene light admitted to a single photosensitive element is shown in U.S. Patent No. 3,942,183, supra. Photographic camera 10 is provided with a sonic ranging system that includes a ranging circuit and an ultrasonic transducer (not shown) which may be actuated to transmit a burst of sonic energy 30 toward the subject 14. The transducer thereafter operates to detect an echo 32 of said burst reflected from said subject 14. The time for sonic energy to be transmitted toward and reflected from subject 14 and detected by said transducer, is a measure of camera-to-subject distance. An electrical signal representative of this time is subsequently employed to focus adjustable focus lens 12. U.S. Patent No. 4,199,246 describes such a sonic rangefinder in detail. Automatic focus control system 34, coupled to the adjustable focus lens 12 through a path 36, causes the lens 12 to focus an image of the subject 14 on the film plane 16 during an exposure interval, in response to the electrical signal from sonic ranging system 28 through a path 38. An example of an automatic focus control system functioning in this manner is described in U.S. Patent No. 4,199,244 to E. SHENK. Camera 10 is provided with an electronic flash together with means for controlling the energizing of same to provide a portion of the exposure value required to illuminate a scene to be photographed. The electronic flash apparatus comprises a main storage capacitor 40 which may be charged up to an operating voltage of about 350 volts by a conventional voltage converter circuit as shown at 42, from a battery (not shown) in the camera, which may be in the order of 6 volts. A flash tube 46 and a quench tube 48 for interrupting the flash discharge of flash tube 46 are connected in a parallel relation with respect to storage capacitor 40. Flash tube 46 may be ignited by a suitable trigger signal on path 50 from a conventional trigger circuit (not shown) within exposure control electronics module 52, and quench tube 48 may be ignited by a suitable trigger signal on path 54 from another conventional trigger circuit (not shown) that is also included within said exposure control electronics module 52. Camera 10 also includes an ambient scene light detector and integrator device 56 and an artificial scene light detector and integrator device 58. Each of these devices include a differential operational amplifier 60 or 62 and a photoresponsive element 26 or 28 connected across the input terminals of said amplifiers 60 or 62. When considered ideally, these amplifiers have infinite gain, infinite input impedance and zero output impedance. The input circuitry of amplifiers 60 and 62, however, are structured such that the apparent input impedance "seen" by photoresponsive elements 26 and 28 is substantially zero, thereby functioning in a manner that permits each photoresponsive element to operate in a current mode. Consequently, the current generated by photoresponsive elements 26 and 28 is substantially limited only by its own internal impedance. To accomplish this effect, feedback capacitors 64 and 66 are respectively connected between an input terminal and an output terminal of the operational amplifiers 60 and 62. With the feedback arrangement described, any difference in potential supplied by the photoresponsive elements 26 and 28 across the input terminals of the operational amplifiers 60 or 62 operates to cause a current of opposite polarity to be produced through feedback capacitors 64 or 66. As a consequence, feedback capacitors 64 and 66 provide substantially instantaneous feedback signals of opposite polarity which serves to counteract any differential signal voltage impressed by photoresponsive elements 26 or 28 across the input terminals of amplifiers 60 or 62. Thus, although each operational amplifier has a very high input impedance, photoresponsive elements 26 and 28, when connected in the aforementioned manner, experience only a very low input impedance to the amplifier 60 or 62. Therefore, the current of photoresponsive elements 26 or 28 is directed into feedback capacitors 64 or 66. In this manner, photoresponsive elements 26 and 28 are connected to operate in a constant current mode of operation under conditions of non-varying scene light intensity to provide a substantially linear output response at the output terminal of amplifiers 60 and 62 as described in U.S. Patent No. 3,620,143. The exposure control system of camera 10 also includes an electronic trim control for deliberately varying the amount of ambient and/or artificial scene light transmitted to the film plane 16 through the primary apertures of the scanning blade shutter (not shown) included within shutter assembly 18, as determined by the position of a manually operable lighten/darken trim control switch 68. Switch 68 may be a three position switch having mechanical detents that maintain the position of a wiper 70 in a particular selected switch position. The three positions of trim control switch 68 are nominal, (+) trim and (-) trim. In the nominal position, the magnitude of the trim voltage applied to the input of a switch position decoding module 72 on a path 74 is approximately one-half the voltage supplied to a terminal 76 from an external voltage source (not shown), said nominal switch position voltage to the module 72 being established by a voltage divider network consisting of equal magnitude resistors R₁ and R₂. In the (+) trim position of the switch 68 the full voltage at terminal 76 is applied to the input of module 72, whereas in its (-) trim position substantially no voltage is applied to the input of said module 72. The (+) and (-) trim positions of trim switch 68 increase or decrease the total amount of light admitted to film plane 16 by two-thirds of a stop, which is slightly more than the minimum change in scene light level that an observer can detect in a finished photograph. Providing a trim control device capable of supplying a range of continuously variable settings above and/or below this level would be confusing and therefore not very helpful to a camera operator. In Fig. 2, the output from the trim control switch 68 on the path 74 is applied to the input or base of transistors 78 and 80 which are initially in their off or non-conducting states. If the switch 68 is in its nominal position, the transistors 78 and 80 will be turned on, to produce a logic 1 signal at an output Q of a monostable gate 82 and at an output Q of a monostable gate 84. In this condition an AND gate 86 is satisfied and produces a logic 1 signal at its output on a path 88. If the switch 68 is in its (-) trim position, the transistor 78 will conduct and the transistor 80 will remain off, thereby producing a logic 1 signal at output Q of the monostable gate 82 and at Q the output of the monostable gate 84. In this condition an AND gate 90 is satisfied and produces a logic 1 signal at its output on a path 92. If the switch position 68 is in its (+) trim position, the transistor 78 will remain off and the transistor 80 will be in its conducting state, thereby producing a logic 1 signal at the output Q of the monostable gate 82 and at the Q output of the monostable gate 84. In this condition an AND gate 94 is satisfied and produces a logic 1 signal at its output on a path 96. The output of the switch position decoding module 72 is simultaneously applied to the inputs of a digital multiplexer 98 and an analog multiplexer 100. The details of digital multiplexer 98 are schematically illustrated in Fig. 3. As shown in Fig. 3, the outputs from the switch position decoding module 72 on the paths 88, 92 and 96 are respectively applied to one input of AND gates 102, 104 or 106. The other input of the AND gates 102, 104 and 106 in the digital multiplexer 98 is derived from digital comparators 108, 110 and 112, respectively, shown in drawing Fig. 1. Digital comparators 110, 108 and 112 have 5, 10 and 20 counts, respectively, digitally stored therein. Counts from pulse counter 114 are simultaneously applied to the inputs of said comparators 108, 110 and 112. The counts in pulse counter 114 are derived from conventional A/D converter 116 that converts analog voltage levels derived from photodetector and integrator 56 representative of a particular ambient scene light intensity to a number of pulses corresponding to said particular scene light level. An A/D converter similar to A/D converter 116 is described in U.S. Patent No. 4,384,773. There is an output (logical 1) from digital comparators 108, 110 and 112 whenever the number of counts at their respective inputs equals the number of counts stored therein. Outputs from digital comparators 108, 110 and 112 are applied to the inputs of AND gates 102, 104 and 106 within digital multiplexer 98 on path 118, 120 and 122, respectively. When any of the AND gates 102, 104 or 106 is satisfied by inputs from switch position decoding module 72 and one of the digital comparators 108, 110 or 112, there is an output on path 124 (logical 0) through NOR gate 126. As shown in Fig. 4, the outputs from the switch position decoding module 72 on paths 88, 92 and 96 are also applied to the bases of transistors 128, 130 and 132, respectively, within the analog multiplexer 100. When the exposure control system is in its energized state, one of said transistors is in a conducting state, and the other two transistors are in a non-conducting state as determined by the position of lighten/darken trim switch 68 (Fig. 1). Transistors 128, 130 and 132 are placed in their conducting states when trim switch 68 is placed in either its nominal, (-) or (+) switch positions, respectively. One of three possible conventional voltage divider networks is established by transistors 128, 130 or 132 in conjunction with resistors R₃ and R₄, R₅ or R₆ when any one of these transistors conducts. The magnitude of the output voltage from multiplexer 100 on path 134 is determined by the particular transistor that conducts and its associated voltage divider network. Constant reference voltage 136 is divided by resistors R₃ and R₄ when transistor 128 conducts, by resistors R₃ and R₅ when transistor 130 conducts and by resistors R₃ and R₆ when transistor 132 conducts. The magnitude of the voltage appearing at the output of multiplexer 100 on a path 134 with respect to system ground is equal to the product of the magnitude of constant reference voltage 136 times In operation, switch 138 (Fig. 1) is manually actuated to its closed position, thereby initiating an exposure controlled picture taking sequence. Upon initiation of the exposure control sequence and prior to the start of an exposure interval or actual shutter blade movement, a signal representative of pre-exposure ambient light generated by ambient light detector and integrator 56 is routed to exposure control electronics 52 through a path 140 indicating the ambient light level of the scene to be photographed. The exposure control system of camera 10 has high and low ambient light level modes of operation. In a high light level scene or fill flash mode, the electronic flash is fired to supplement ambient light for the purpose of eliminating shadows such as those that might otherwise be caused by subject backlighting. In a low light level mode, which is the standard flash mode, the electronic flash supplies a relatively large portion of total scene light. In a high ambient light level or fill flash mode, the electronic flash contributes approximate ly 25% of the required exposure value, whereas in a low ambient light level mode the flash contributes most or all of the required exposure value. The exposure control system fires the flash based upon subject distance information determined prior to the exposure interval or integrated ambient light information accumulated during the exposure interval. The mode of operation, high or low ambient light level, determines how the distance information is used or what percentage of total exposure is reached by the ambient integrator in order to fire the flash. The exposure control electronics 52 processes the gathered scene information, of distance and of integrated ambient light, and decides which of the two pieces of information to use for firing the flash. In both the high and low ambient modes of operation, the flash fire aperture is determined by how far into the shutter opening profile the flash fire command is given; the scanning aperture shutter 18 takes many milliseconds to translate from the initially closed position to the fully open position, and it is during this opening time that the scene distance and light level information generate the flash fire command. Once the flash is fired, the strobe must be quenched at a flash integrator threshold corresponding to a desired percentage of total exposure that the flash will contribute to the subject. The mode of operation determines the percentage of flash. After the flash is quenched, any remaining ambient light needed to complete the exposure is obtained via reaching the end of exposure threshold with the ambient integrator. The ambient percentage used complements the flash percentage, and is determined by the mode of operation. The electronic trim control recalibrates the ambient and strobe integration electronics by changing the threshold levels up to which integration of light is to take place during exposure of the film. The resulting photograph is either overexposed (lightened) or underexposed (darkened) by the predetermined amount of trim control compensation. There are three integration thresholds affected by the electronic trim compensation, in order to obtain a balanced effect of lightening or darkening of both the subject and the background. The three affected thresholds are: 1) the ambient threshold for the flash fire command when the flash is fired based on ambient information; 2) the quench threshold; and 3) the end exposure threshold, to close the shutter, when the end exposure command is based on an ambient threshold being reached. The method by which the electronic trim control recalibrates the integration threshold levels is as follows. The digital pulse train generated by ambient light detector and integrator 56 and light intensity to pulse converter 116 must accumulate in counter 114 to the magnitude of digital comparators 108, 110 or 112, depending on the position of the trim switch 68. If the flash has not yet been fired based on subject distance information, it will be fired at the appropriate digital threshold set by comparators 108, 110 or 112. Now, artificial light detector and integrater 58, being held in a reset state up until the firing of the flash, must integrate up to a magnitude established by analog comparator 146, as determined by the output of analog multiplexer 100 which is dependent on the position of trim switch 68. A strobe quench signal is generated by said comparator 146 which energizes quench tube 48 through path 54, thereby extinguishing light from electronic flash tube 46. During the 1.5 millisecond interval of the flash firing, ambient digital pulse train counter 114 is blanked out from accumulating flash induced pulse counts, as described in U.S. Patent No. 4,255,030, supra. After artificial light from the flash is extinguished, said counter 114 continues to accumulate ambient pulse train counts until the magnitude of a set of digital comparators (not shown) parallel to comparators 108, 110 and 112 is reached, depending on the position of trim switch 68. If the exposure interval has not yet been terminated by other means (not described herein), it will be terminated at this time. From the foregoing, therefore, it can be seen that the ambient and artificial light levels admitted to the focal plane 16 of the camera 10 may be changed by two-thirds of a stop above or below a particular nominal value by an electronic trim control that employs a single three position switch that can be readily manipulated by a camera 10 operator.

Claims: An exposure control system for a camera (10) having means for defining a film plane and an optical path for directing light from a scene onto photosensitive film located in the film plane (16), said exposure control system comprising: a blade shutter (18) including a blade assembly (20a, 20b) movable between closed and open positions for controlling the amount of scene light incident onto the photosensitive film; actuatable drive means for displacing said blade assembly between said open and said closed position to define an exposure interval; means (56, 114, 116) for detecting and integrating substantially only the ambient scene light in correspondence with the scene light admitted to the film plane during the exposure interval, wherein said integrated ambient scene light is converted to a digital pulse count; an electronic flash (40-48) of the quench-type to provide artificial illumination of the scene; means (52) for controlling the firing and quenching of said electronic flash; means (52, 98, 108, 110, 112) responsive to said digital pulse count reaching a first selected value for firing said electronic flash, and reaching a second selected value for actuating said blade assembly to its closed position; means (58) for detecting and integrating substantially only the artificial scene light in correspondence with the scene light admitted to the film plane during the exposure interval, wherein said integrated artificial scene light is converted to an analog voltage value; means (52, 146) responsive to said analog voltage value reaching a third selected value for quenching said electronic flash; characterized by a manually actuatable switch (68) for simultaneously increasing or simultaneously decreasing said first, second and third values. The system of claim 1, wherein said means (108, 110, 112, 52, 98) for firing said electronic flash and for actuating said blade assembly to its closed position respectively include means (108, 110, 112) for comparing a value manually set by said switch (68) against a value of said pulse count. The system of claim 2, wherein said detecting and integrating means (56, 114, 116) include an A/D converter (116) for converting the value of illumination of the ambient light integrator (56) to digital pulses. The system of claims 2 and 3, wherein said detecting and integrating means (56, 114, 116) include a pulse counter (114) for counting said digital pulses, said pulse counter (114) being connected to said comparing means (108, 110, 112) connected to a digital multiplexer (98). The system of claim 4, wherein a switch position decoding module (72) is connected to an input of said digital multiplexer (98). The system of claim 5, wherein said digital multiplexer(98) comprises a plurality of logic gates (102, 104, 106) each being responsive to a different position of the switch (68) and of said comparing means (108, 110, 112). The system of claim 1, wherein further means (100, 146) are provided for comparing a value manually set by said switch (68) against a value of illumination derived from said artificial light detecting and integrating means (58). The system of claim 7, wherein said further comparing means (100, 146) comprise an analog multiplexer (100) having a reference voltage supply (136) to be compared against the value set by said switch (68). The system of claim 8, wherein the output of said analog multiplexer (100) is connected to one input of a comparator (146) another input of which is connected to said artificial light detecting and integrating means (58). The system of claims 6 and 9, wherein the outputs of the digital (98) and analog (100) multiplexers being connected to an exposure control electronic (52) for coordinating flash firing and quenching with movement of the shutter (18).

Inventors: ANACREON, ROBERT V.
Applicants: POLAROID CORPORATION
Application year: 1986
Main classification: G03B7
IPC codes: G03B15, G03B7