Indicating device for single lens reflex camera
BACKGROUND OF THE INVENTION 
     This invention relates to exposure indicating devices for use in lens-exchangeable, single lens reflex cameras of the through-the-lens (TTL) light measurement type, which is applicable to any type of objective lens selected, and wherein, in any selected mode of automatic exposure control, the exposure factor values to be automatically controlled may be properly indicated prior to the commencement of the diaphragm aperture stopping-down operation. 
     U.S. Pat. No. 3,829,867 discloses two automatic exposure control modes, namely, the so-called shutter-speed-priority-automatic-diaphragm-control mode, and the so-called diaphragm-priority-automatic-shutter speed-control mode. In the former control mode, the diaphragm is stopped-down from an open position in association with the shutter release operation prior to the commencement of the movement of the shutter to an open position, while the intensity of light which has passed through an objective is measured and the diaphragm is in the process of being stopped-down. When a proper aperture value for a preselected shutter speed is determined as the result of the light measurement, the stopping-down operation of the diaphragm is interrupted. With the diaphragm maintained stopped-down at that aperture value, light measurement for shutter speed control is conducted, so that the shutter speed may be controlled according to the light measuring outputs. In the latter control mode the diaphragm is stopped-down to a preset aperture value in association with the shutter release operation, prior to the commencement of the movement of the shutter to an open position, and the light measurement at the aperture value to which the diaphragm has been stopped-down is conducted, so that the shutter speed is controlled according to the light measuring outputs so obtained. 
     Various attempts have been proposed for providing a camera capable of changing over the exposure control mode from the diaphragm-priority-automatic-shutter-speed-control mode to the shutter-speed-priority-automatic-diaphragm-control mode, and vice-versa, in a TTL light measuring single lens reflex camera equipped with an exposure indicating device. However, some of these cameras, particularly a camera with an exchangeable lens, fail to transmit sufficient information from the lens to the camera body to achieve a complete indication in each exposure control mode, thereby resulting in imperfect indications of the exposure factors. Stated differently, in the shutter-speed-priority-automatic-diaphragm-control mode, the difficulty arises with such a camera to indicate an f-number corresponding to an aperture value to be automatically controlled prior to the stopping-down operation. 
     Every exchangeable lens has an inherent minimum f-number. Thus, in order to compute the proper aperture value relative to a brightness value, the film sensitivity setting and a preselected shutter speed, and to indicate an f-number corresponding to the proper aperture value, the intrinsic minimum f-number of the exchangeable lens mounted on the camera, as well as other factors, need to be transmitted to the camera body. Moreover, the manufacturers of suppliers of a new type camera or lens should consider the exchangeability of a new camera or lens with an old type camera or lens which has been in the market for a long period of time to avoid supplying a new type camera body on which an old type lens cannot be mounted, or a new type lens which cannot be mounted on an old type camera. 
     The indication of an f-number corresponding to an aperture value to be controlled in the shutter-speed-priority-automatic-diaphragm-control mode poses a critical problem for camera manufacturers who have supplied a lens having only a transmitting member for transmitting information from the exchangeable lens to the camera body of the difference in the number of steps to be stopped-down from a minimum f-number to a preset f-number. 
     A countermeasure to the problem is disclosed in co-pending patent application Ser. No. 769,832 assigned to the same assignee and entitled &#34;Exchangeable lenses for use in internal light measuring type single lens reflex camera&#34;, now U.S. Pat. No. 418,726. That is, this co-pending application discloses a first and a second type of exchangeable lenses which dissolve the problem. The first type is such that when an aperture setting member is set to a specific position other than a plurality of manually aperture setting positions, a second information transmitting member on the aperture setting member transmits to a camera body information of a maximum f-number of the exchangeable lens. On the contrary thereto, the second type is such that when an aperture setting member is set to a maximum f-number setting position among a plurality of manually aperture setting positions, a second information transmitting member on the aperture setting member transmits to a camera body information of a maximum f-number of the exchangeable lens. By combining the information of the maximum f-number with information of the step-difference from a minimum to the maximum f-numbers of the exchangeable lens, transmitted by the conventional information transmitting member, information of the minimum f-number, which is requisite to the indication of f-number corresponding to an automatically controlled aperture prior to stopping-down operation of a diaphragm, is obtained within the camera body. 
     In a co-pending patent application Ser. No. 804,290 filed on June 7, 1977 and assigned to the same assignee and entitled &#34;Exposure Indicating Device for Single Lens Reflex Cameras&#34;, the present applicants have proposed an exposure indicating device for single lens reflex cameras which employ the first type exchangeable lenses. This indicating device enables, in accordance with setting of the aperture setting member to either the specific position or to any one of the manually aperture setting positions, selection of the above two exposure control modes and selection of exposure factors to be indicated at the selected exposure control mode. That is, this indicating device has an advantage that the selections can be carried out only by setting of the aperture setting member either to the specific position or any one of the manually aperture setting positions. However, such selections are impossible if the cameras employ the second type exchangeable lenses because the aperture setting member can set to only one of the manually aperture setting position in the second type exchangeable lens. 
     SUMMARY OF THE INVENTION 
     It is, accordingly an object of the present invention to provide an exposure indicating device for a single lens reflex camera which employs the second type exchangeable lenses and which is settable either to automatic aperture control mode or automatic shutter speed control mode. 
     Another object of the present invention is to provide an exposure indicating device of the above type which enables selection of an exposure factor to be indicated in accordance with selection of an exposure control mode of the camera. 
     A still another object of the present invention is to provide an exposure indicating device of the above described type in which either an aperture scale or a shutter speed scale can be observed within the field of view of a view finder in accordance with the selection of an exposure control mode of the camera. 
     The indicating device of the present invention is used in a single lens reflex camera which is selectively settable to an automatic aperture control mode or to an automatic shutter speed control mode. An exchangeable lens is mountable on the camera body and includes a diaphragm and an objective lens. The camera body includes first and second information receiving members, and indicating means having a pair of input terminals and being capable of providing an indication in accordance with the difference between signals at the pair of input terminals. The exchangeable lens is provided with aperture setting means settable to any one of a number of manually set aperture positions. First and second information transmitting members are provided on the aperture setting means in such a relationship that the first information transmitting member transmits to the first information receiving member information of the step-difference from a minimum to a set f-number of the exchangeable lens independently of the set position of the aperture setting means. The second information transmitting member transmits to the second information receiving member information of a maximum f-number of the exchangeable lens only when the aperture setting means is set to a maximum f-number setting position. 
     Additionally, circuitry is provided to generate exposure factor signals. A first signal is representative of the first information; a second signal is representative of information of the minimum f-number obtained by composing the first and second information; a third signal is representative of the selected shutter speed; a fourth signal is representative of the set film sensitivity and the object light intensity, which has passed through the objective lens and the aperture formed by the diaphragm; and a fifth signal is a given reference signal. 
     Furthermore, on the camera body is provided manually operable mode selection member and switch means coupled with the mode selection means. By that coupling, the switch means selectively sets the camera to either the automatic aperture control mode or the automatic shutter control mode and selectively transmits either the difference between the second and fourth signals and the third signal or the difference between the first and fourth signals and the fifth signal to the input terminals of the indicating means. 
     With the above structure, an f-number corresponding to the automatically controlled aperture is indicated prior to a stopping-down operation, i.e., with the aperture maintained fully open when the aperture setting means is set to the maximum f-number position and the mode selection means is operated to a position in which the switch means sets the camera to the automatic aperture control mode. 
     However, an automatically controlled shutter speed is indicated prior to a stopping-down operation when the aperture setting means is set to any one of the manually set aperture positions and the mode selection means is operated to a position in which the switch means sets the camera to the automatic shutter speed control mode. 
     In a preferred embodiment according to the present invention, the indicating device is further provided with a viewfinder, a scale member bearing thereon an f-number scale and a shutter speed scale, and scale selection means. The scale selection means is coupled with the switch means, and moves the scale member so that the f-number scale is observed within the field of view of the viewfinder when the mode selection means is set to a position in which the switch means sets the camera to the automatic aperture control mode. The scale selection means also functions so that the shutter speed scale is observed within the field of view of the viewfinder when the mode selection means is set to a position in which the switch means sets the automatic shutter speed control mode. The indicating means is disposed within the viewfinder, so that indication thereof is viewed within the field of view of the viewfinder together with either one of the scales, which shows the exposure factor to be automatically controlled at the selected exposure control mode. 
     The above advantages, objects and features of the present invention will become more apparent from the following description taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view illustrative of an embodiment of the present invention, showing an exchangeable lens and a camera body of a single lens reflex camera in confronting relation to each other; 
     FIGS. 2 and 3 are respective perspective views showing the exchangeable lens mounted on the camera body, with a diaphragm setting ring set to F:2.8 in FIG. 2; and to maximum f-number F:16 in FIG. 3; 
     FIGS. 4 and 5 are developed views of first and second information transmitting systems of the embodiment and showing plan views of the field of view of a viewfinder, when the camera is set to the automatic shutter speed control mode with the diaphragm setting ring set to F:8, and when the camera is set to the automatic aperture control mode with the diaphragm setting ring set to F:16 as in FIG. 2; 
     FIG. 6 is an exploded perspective view of essential elements of the indicating device; 
     FIG. 7 is a combined schematic and block diagram of an automatic exposure control system and an exposure indicating system for the invention; and 
     FIG. 8 is a perspective view showing arrangement of switches shown in FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Prior to the description of preferred embodiments of the present invention, the principles of operation incorporated in the present invention are as follows. 
     According to the APEX notation system, assume index symbols for use in photographic and computing formulae, as follows: Bv for a luminance value, Sv for a film speed value, Av for an aperture value, and Tv for a time value. As is well known, a proper exposure condition is given as follows: 
     
         Sv+Bv=Tv+Av 
    
     In a lens-exchangeable, automatic TTL light-measurement camera, a fully open light measuring system is adopted, in which object light which has passed through a fully open diaphragm aperture at an aperture value Avo is received by a light measuring element. In this case, an output Bvo of the light receiving element is used as a basis of automatic exposure computation, wherein Bvo=Bv-Avo. 
     Accordingly, a general type diaphragm-priority-automatic-shutter-speed exposure control device adopts either one of the following formulae which are modifications of the formula above: 
     
         Tv=Sv+(Bv-Avo)-Av+Avo=(Sv+Bvo)-Av+Avo                      (I) 
    
     
         tv=Sv+(Bv-Avo)-(Av-Avo)=(Sv+Bvo)-P                         (II) 
    
     in the exposure control type which adopts the formula (I), two signals respectively representing a preset aperture value and a fully open aperture value Avo are transmitted from the exchangeable lens to the camera body. In contradistinct thereto, in the type which adopts the formula (II), the diaphragm information, such as the difference in step (P) between a preset aperture value (a preset f-number) and a fully open aperture value (a minimum f-number), i.e. only the rotational displacement of the diaphragm setting ring of the exchangeable lens is transmitted. That is, in the exposure control type which adopts the formula (II), only a single signal is transmitted. 
     Therefore, a comparison of the above two types of diaphragm-priority-automatic-shutter speed control devices using formulas (I) and (II), respectively, indicates that in the exposure control type adopting the formula (II), only a single signal (P) is transmitted, and thus the manufacturing cost of the exchangeable lens is less. However, this consideration is applicable only to a camera of the diaphragm-priority-automatic-shutter-speed control type. 
     In contrast thereto, in designing a camera which allows shutter-speed-priority-automatic-diaphragm control, as well, and adopts only the formula (I), that formula should be modified as follows: 
     
         Av=(Sv+Bvo-Tv)+Avo                                         (III) 
    
     so that the camera can carry out automatic diaphragm control, the proper aperture value to be controlled prior to the commencement of the stopping-down operation, and indication of the f-number corresponding to the proper aperture value, in accordance with the above automatic computing formula, must be computed. A camera adopting the formula (II) cannot compute a proper aperture value for diaphragm automatic control and indicate an f-number corresponding to a proper aperture value, before the stopping-down operation commences. 
     The aforesaid objects of the present invention may be attained by a combination of an exchangeable lens capable of transmitting the difference as set forth in formula (II) and a camera, with the addition of a special concept according to the present invention, whereby a signal representing the difference in step P between a minimum f-number and a preset f-number can be applied for automatic diaphragm control, thereby providing a low cost, reasonable information transmission system for indication of an f-number. 
     The aforesaid concept is based on the discovery that: (i) prior to the commencement of the stopping-down of the diaphragm, a proper aperture value for an automatic diaphragm control mode is calculated, based on the formula Avo=Av-P which is obtained from the relationship P=Av-Avo. A corresponding f-number is indicated, by setting the diaphragm setting ring of an exchangeable lens to a specific position, without providing a member for directly transmitting a signal representing a minimum f-number of the exchangeable lens to the camera body. (ii) The specific set position of the diaphragm setting ring is computed in terms of the preset f-number, and a rotational displacement of the diaphragm settting ring is determined. (iii) The information of the values thus obtained is substituted for Av and P, and become Avc and Pc. Avo is indirectly transmitted to the camera body, so that a proper aperture value may be computed according to the following formula: 
     
         Av=(Sv+Bvo-Tv)+Avc-Pc                                      (IV) 
    
     fig. 1 is a perspective view of camera body 10, in accordance with the present invention, and exchangeable lens which components are separated and disposed in confronting relation to each other. Provided on the peripheral portion of bayonet ring 13 on camera body 10 is first receiving member 11, similar to that of the prior art camera body, which is biased to rotate counterclockwise under the action of spring 11a, as seen in FIGS. 4 through 6. First receiving member 11 is adapted to engage first transmitting member 1 projecting from diaphragm setting member (diaphragm setting ring) 5 when exchangeable lens 4 is mounted on camera body 10, diaphragm setting ring 5 being manually rotatable relative to exchangeable lens 4. first receiving member 11, through engagement with first transmitting member 1, is rotated by a difference in the number of steps to be stopped-down from a minimum f-number and a preset f-number, so that brush 11b on first receiving member 11 slides along the resistance of potentiometer 27, thereby determining a value of resistance. Briefly, information representing a difference in the number of steps to be stopped-down from a minimum f-number to a preset f-number in exchangeable lens 4 is transmitted, by a first transmitting system consisting of a first transmitting member 1 and a first receiving member 11, to camera body 10 as a resistance value on potentiometer 27. 
     A second receiving member 12 is provided on the peripheral portion of bayonet ring 13, which is biased to rotate counterclockwise under the action of spring 12a and is rotatable along the periphery of bayonet ring 13. Second receiving member 12 is engageable with second transmitting member 2 projecting from diaphragm setting member 5, thereby constituting a second transmitting system. The engagement between the two members constituting the second transmitting system is provided only when a maximum f-number graduation 3 is set to index 8 on exchangeable lens, as best seen in FIG. 4. Second transmitting member 2 transmits information of the maximum f-number (F16 in FIGS. 1-3) on exchangeable lens 4 to camera body 10. The transmission of the information of the maximum f-number is achieved by providing a distance (a space or angular difference) between maximum f-number 3 and second transmitting member 2 on diaphragm setting ring 5, as is described in previously mentioned co-pending application Ser. No. 769,832, now U.S. Pat. No. 4,118,726. Second receiving member 12 is urged by spring 12a to a terminal stop point as shown in FIG. 4 and when diaphragm setting member 5 is rotated to any of f-number graduations other than maximum f-number graduation, rests at the terminal stop point and is out of engagement with second receiving member 12. However, when diaphragm setting member 5 is rotated so that maximum f-number graduation 3 aligns with index 8, the second receiving member 12 is urged clockwise by second transmitting member 2 on exchangeable lens 4, as seen in FIGS. 2 and 5. If second receiving member 12 is urged clockwise by one step as shown in FIG. 3, a contact 26a is contacted by a brush 15 on second receiving member 12, whereby the information that a maximum f-number on exchangeable lens 4 is F16 is transmitted to camera body 10; if second receiving member 12 is moved clockwise by two steps, a contact 26b is contacted by brush 15, whereby the information that a maximum f-number of F22 on the exchangeable lens is transmitted to camera body 10, and in case of three steps of clockwise movement, a contact 26c is contacted by brush 15, whereby the information that a maximum f-number of F32 on the exchangeable lens is transmitted to camera body 10. This occurs because, for each exchangeable lens the distance from maximum f-number graduation 3 to the second transmitting member is different. That is, for each exchangeable lens the shifting distance of the second transmitter member is different. The contacts 26a, 26b and 26c are connected to corresponding resistors respectively and the information of the maximum f-number thus transmitted corresponds to voltages across the resistors as described later. Simultaneously with the above transmission, first transmitting member 1 transmits to camera body 10 the difference in steps between a minimum f-number and a maximum f-number. 
     FIG. 1 shows film sensitivity graduations 28 and index 29 therefor, shutter speed selecting dial 30 and index 31 therefor, manual and automatic exposure control modes selecting member 32 and indexes 33a, 33b 33c therefor, automatic diaphragm stopping-down member 34, and diaphragm actuating pin 35 provided on exchangeable lens 4, which cooperates with automatic diaphragm stopping-down member 34. Exposure control modes selecting member 32 should be set to index 33a (&#34;M&#34;) for manual exposure control mode, to index 33b (&#34;D&#34;) for automatic diaphragm control mode and to index 33c (&#34;S&#34;) for automatic shutter speed control mode. 
     FIG. 2 depicts an exchangeable lens barrel mounted on the camera body, in which an f-number is preferentially determined as F2.8 in the automatic shutter speed control mode. FIG. 3 shows an exchangeable lens barrel, in which maximum f-number graduation 3 is set to index 8 in the automatic diaphragm control mode. FIG. 4 is a developed view of both transmitting systems in the diaphragm-priority-automatic-shutter-speed-control mode, also showing an indicator in the viewfinder, wherein an f-number is preferentially determined as F8. FIG. 5 is a developed view of both transmitting systems in the shutter-speed-priority-automatic-diaphragm-control mode, showing the indicator in the viewfinder. FIG. 6 is an exploded, perspective view of the essential mechanical components of the transmission and indicating system of the invention. 
     Referring to FIGS. 4 and 5, a projection 32a on modes selecting member 32 is engaged with an engaging portion 34a on a sliding plate which is supported for sliding movement along an arcuate path while being guided by a pin-slot structure. A swingable intermediate lever 35 has one arm engaged with a pin 34b on sliding plate 34 and another arm 35a engaged with an end 18a of a scale-change-over plate 18 under influence of a spring 19 which urges plate 18 leftward. Plate 18 has on its left hand arm an indication plate 18b bearing thereon shutter speed graduations 18s and f-number graduations 18d. Graduations 18s and 18d are parallel to one another so that either graduations 18s or 18d can be seen through an opening 22a of a fixed mask 22 which is disposed above indicating plate 18b. That is, when modes selecting member 32 is set to index 33b as shown in FIG. 4, shutter speed graduations 18s are seen through opening 22 a. However, when modes selecting member 32 is set to index 33c as shown in FIG. 5, sliding plate 34 is moved rightward to allow intermediate lever 35 to swing counterclockwise. Thus, scale change-over plate 18 is moved leftward under influence of spring 19, so that f-number graduations 18d are seen through opening 22a. 
     More particularly, sliding plate 18 is movable between a focussing plate 24 and a pentagonal prism 25 in accordance with the automatic exposure control modes selection by means of modes selecting member 32. Focussing plate 24 and pentagonal prism 25 constitute a viewfinder together with a not shown reflecting mirror as is conventional in the art. Either graduations 18s or 18d are seen in the finder view field through the medium of a graduation regulating frame 20, a light guide 21 and opening 22a of image plane regulating mask 22. Frame 20, light guide 21 and mask 22 are placed one upon another. Light from light emitting diodes 23 opposite light guide 21 is substantially fully reflected by reflection surface 21a of light guide 21, then oriented upwards through opening 22b provided on mask 22 and eventually indicated within the finder, as shown in FIGS. 4 and 5. 
     The following is a detailed description of the transmission of diaphragm information to the camera body. Referring first to the shutter-speed-priority-automatic-diaphragm-control mode, manual and automatic exposure control modes selecting member 32 is set to index 33b. Then diaphragm setting member 5 on exchangeable lens 4 is first turned to set maximum f-number graduation 3 to index 8, so that first receiving member 11 is shifted by first transmitting member 1 a distance corresponding to a difference in the number of steps to be stopped-down from a minimum f-number to a maximum f-number on exchangeable lens 4. 
     Second receiving member 12 is shifted according to a maximum f-number for each exchangeable lens, so that one of the above described resistors is selected, thereby producing a voltage corresponding to the value of the maximum f-number. A voltage corresponding to a minimum f-number is obtained by deducting the voltage corresponding to a maximum f-number from a voltage corresponding to a difference in steps to be stopped-down which has been obtained through movement of the first transmitting member 1. 
     Continuing with the shutter-speed-priority-automatic diaphragm control mode, since the shutter speed is preferentially set, a predetermined one of light emitting diodes 23 is energized according to the difference between a voltage corresponding to the set shutter speed and a voltage obtained by deducting the aforesaid voltage commensurate with the minimum f-number from the sum of voltages obtained by adding the voltage corresponding to the set film sensitivity and to the light measuring voltage from a light measuring circuit. 
     The sum of the light measuring voltage and a voltage corresponding to the set film sensitivity, and a voltage obtained by deducting the aforesaid voltage corresponding to the number of steps to be stopped-down, obtained by first transmitting member from the sum of the voltages, are respectively fed as inputs to the separate circuitry for comparison with a voltage corresponding to the set shutter speed. Thereby, according to the difference between the voltages to be compared with one another, individual light emitting diodes provided for indicating and out-of-interlocking condition are energized. Stated differently, if the sum of the light measuring voltage and the voltage corresponding to the set film sensitivity is higher than the voltage corresponding to set shutter speed, an underexposure results at the set shutter speed even if the diaphragm to be controlled is maintained in the minimum aperture condition. This is indicated by energization of one of the light emitting diodes for indication of the out-of-interlocking condition. If the voltage obtained by deducting the aforesaid voltage, corresponding to the number of steps to be stopped-down, from the sum of the light measuring voltage and the voltage corresponding to the set film sensitivity is lower than the voltage corresponding to the set shutter speed, then overexposure results at the set shutter speed, even if the diaphragm is stopped-down to an aperture corresponding to a maximum f-number. This is indicated by energization of another light emitting diode for indication of the out-of-interlocking condition. In this connection, the provision of an indication window with an arrow-mark contour allowing passage of light from the corresponding light emitting diode for indication of the out-of-interlocking condition is preferable, so that the exposure condition, such as &#34;underexposure&#34; or &#34;overexposure&#34; is indicated according to the direction of the illuminated arrow. 
     The following is a description of the operation of the diaphragm-priority-automatic-shutter-speed-control mode. In that exposure control mode, diaphragm setting member 5 on exchangeable lens 4 is manually set to a desired f-number beforehand with mode selecting member 32 set to index 33b. 
     Where the f-number is preset, first transmitting member 1 and first receiving member 11 are maintained in engaging relation to each other, thereby establishing the first transmitting system. The engagement of the first transmission system allows the transmission of information corresponding to the difference in the number of steps to be stopped-down from a minimum f-number to a set f-number from exchange lens 4 to camera body 10. Yet, the second transmitting member 2 and second receiving a member 12 engage one another when diaphragm setting member 5 is set to the maximum f-number. However, information of the maximum f-number cannot be transmitted since a switch interrupts the information transmission upon selection of this mode, as hereinafter described in more detail. 
     A voltage corresponding to the set film sensitivity is added to a light measuring voltage, and a voltage corresponding to the difference in the number of steps to be stopped-down, which is obtained through first receiving member 11, is deducted from the voltage obtained by the above addition. According to the difference between the resultant voltage and a given reference voltage for use for automatic shutter speed control, a corresponding one of light emitting diodes 23 is energized. 
     Thus, either in the automatic shutter speed control mode, or the automatic diaphragm control mode, the indication of the shutter speed to be controlled is achieved prior to the commencement of the stopping-down operation. 
     DESCRIPTION OF FIG. 7 
     An explanation of the selection of either one of the automatic exposure control modes and of the operation of the exposure indication follows in more detail with reference to FIG. 7, which schematically illustrates an automatic exposure control system and an exposure indicating system adaptable to the embodiment shown in FIGS. 1 to 6. To avoid cluttering the drawing, a circuit which enables a manual exposure control mode when manual and automatic exposure control modes selecting member 32 is set to index 33a &#34;M,&#34; is omitted from FIG. 7 since such a circuit is well known in the art. 
     Light measuring circuit 40 includes a photosensitive element PD disposed to receive object light which has passed through an objective lens and an aperture formed by diaphragm D of the exchangeable lens, and generates a voltage proportional to the logarithm of the intensity of the light thus received. To this voltage is added a voltage representative of film sensitivity setting, which is generated by a film sensitivity setting circuit 41, so that circuit 40 generates, at its output terminal 40a, a voltage signal Vt corresponding to the intensity of the light received by photosensitive element PD and the film sensitivity setting. Light measuring circuit 40 and film sensitivity setting circuit 41 constitute a fourth signal generating means. 
     According to the APEX notation, voltage signal Vt corresponds to Tvo in the case of fully open aperture light measurement, wherein Tov=Bv+Sv-Avo, in which Avo represents a fully open aperture value, and Tvo represents a time value for the fully open aperture. Voltage signal Vt varies by a given voltage Va for a change which corresponds to one step of the APEX index. 
     One end of variable potentiometer 27, having a linear resistance characteristic, is connected to output terminal 40a, and the other end of potentiometer 27 is connected to a constant current source I 1 . Potentiometer 27 and constant current source I1 constitute a first signal generating means. A voltage shifted down from the voltage level at output terminal 40a by a voltage Vp, commensurate with the amount of the rotational displacement of diaphragm setting member 5, is derived through sliding element 11b adapted to be slid on potentiometer 27 in response to the rotation of member 5. An operational impedance transformation amplifier OP 1  generates at a voltage at junction c of the same voltage as that at sliding element 11b. 
     Resistors r 1 , r 2  and r 3  are arranged in parallel with each other and each resistor has one end connected to a constant current source I 2  and another end connected to the corresponding contact on plate 26. Contact piece 15 on second receiving member 12, which is connected to junction c, selects either of the contacts connected to resistors r 1 , r 2  and r 3 , in accordance with a maximum f-number of the exchangeable lens mounted on camera body 10. Resistors r 1 , r 2  and r 3  have such resistance that they introduce into the circuit information of a maximum f-number of the exchangeable lens mounted on camera body 10. That is, with an exchangeable lens having a maximum f-number F:16, resistor r 1  is selected to shift up the voltage level at junction c by 1Va; with an exchangeable lens having a maximum f-number F:22 resistor r 2  is selected to shift up the same voltage level by 2Va; and further, with an exchangeable lens having a maximum f-number F:32, resistor r 3  is selected to shift up the same voltage level by 3Va. Also in parallel with these resistors a short-circuit is connected to the fourth contact on plate 26, and contact 15 selects this fourth contact when diaphragm setting member 5 is set to any one of the manually set aperture positions other than the maximum f-number setting position wherein any one of the f-number graduations other than maximum f-number graduation 3 is registered with index 8. Further, in parallel with resistors r 1 , r 2  and r 3  and sliding element 15 is arranged a switch SW 2  which is interlocked with the mode selection. As shown in FIG. 8, switch SW 2  is constituted a pair of contacts, one being on plate B and the other being on an insulating plate 34c fixed to sliding plate 34. Its contact on insulating plate 34c contacts its contact on plate B when sliding plate 34 is in its left position, in which the automatic shutter speed control mode is selected, whereas its contact on insulating plate 34c on plate B when sliding plate 34 is in its right position in which the automatic aperture control mode is selected. 
     Resistors r1, r2, r3, movable switch contact 15, operational amplifier OP1, potentiometer 27 and constant current sources I 1  and I 2  constitute a second signal generating means. 
     Exposure time setting circuit 43 generates a voltage representative of the shutter speed manually set by shutter speed dial 30. Variable resistor 44, included in circuit 43, is arranged so that constant current flows therethrough. Sliding element 45 is slid on resistor 44 in response to the setting of shutter speed dial 30, and a voltage corresponding to the selected shutter speed is derived through sliding element 45. Variable resistor 44 and sliding element 45 constitute a third signal generating means. Also through terminal 46 on resistor 44 a reference voltage is derived corresponding to an exposure time of 1/125 second. Variable resistor 44 and fixed contact 46 constitute a fifth reference signal generating means. Change-over switch Sw 1  is associated with sliding plate 12 as shown in FIG. 8, so that its contact on insulating plate 34c contacts contact b on plate B, connected to terminal 46, when sliding plate 34 is in its left position and shutter speed control mode is selected. Change-over switch Sw 1  is switched so that its contact on insulating plate 34c contacts contact a on plate B, connected to sliding element 45, when sliding plate 34 is moved to the right position. 
     Indicating device 47 is adapted to energize light emitting diodes LED-3, LED-2, --and LED 7 respectively when the difference in the voltages at its input terminals 47a and 47b is -3Va, -2Va, --and 7Va, respectively. Alternatively, in place of light emitting diodes LED-3, LED-2, --and LED 7, a meter may be used, in which the position of a light emitting diode corresponding to the aforesaid voltage difference may be indicated by its pointer. Indicating device 47 using light emitting diodes LED-3, LED-2, --and LED 7 is an equivalent of a meter in principle. 
     Operational amplifiers OP 2  and OP 3  constitute differential amplifiers and have output terminals connected to light emitting diodes LEDov and LEDun, respectively. The negative and positive input terminals of operational amplifier OP 2  are connected to junction c and to sliding element 45 respectively, so as to generate a low output level when the voltage at its positive input terminal is lower than the voltage at its negative input terminal, thereby energizing light emitting diode LEDov. In contrast thereto, the negative and positive input terminals of operational amplifier OP 3  are connected to sliding element 45 and to output terminal 40a of light measuring circuit 40, respectively, so as to generate a low output level when the voltage at its negative terminal is higher than the voltage at its positive terminal, thereby energizing light emitting diode LEDun. 
     An input terminal of exposure control circuit 48 is connected to the positive terminal of memory capacitor C, which is in turn connectable to output terminal 40a of light measuring circuit 40 through memory switch Sw 4 . As is well known in the art, memory switch Sw 4  is normally closed to charge memory capacitor C to the voltage level corresponding to that at terminal 40a, and is opened to disconnect memory capacitor C from light measuring circuit 40 immediately before a mirror of the single lens reflex camera is moved up to its picture taking position from its image viewing position. Exposure time control circuit 48 is actuated when count switch Sw 5  is closed in association with the opening of the shutter and, then energizes an electromagnet Mg 2  after a lapse of time from the closing of switch Sw 5  corresponding to the voltage stored in memory capacitor C. The energization of electromagnet Mg 2  causes the shutter to close, whereby an exposure is terminated. 
     Operational amplifier OP 4  constitutes a differential amplifier and serves as an automatic aperture control circuit. Operational amplifier OP 4  has a positive and a negative input terminal respectively connected to output terminal 40a of light measuring circuit 40 and to sliding element 45; and an output terminal connected to electromagnet Mg 1  and is adapted to energize that electromagnet when the voltage at its negative input terminal becomes equal to or lower than the voltage at its positive input terminal. The energization of electromagnet Mg 1  causes a locking member (not shown) to interrupt the stopping-down operation of diaphragm D, thereby determining an aperture formed thereby. Switch Sw 3 , connected in series to electromagnet Mg 1 , is associated with sliding plate 34 as shown in FIG. 8, so that its contact on insulating plate 34c is disconnected from its contact on plate B when sliding plate 34 is in its left position and so that its contact on insulating plate 34c contacts its contact on plate B when sliding plate 34 is moved to its right position. Thus, switches Sw 1 , Sw 3  and Sw 2 , are interconnected with the selection of either one of the automatic exposure control modes by means of modes selecting member 37. 
     The operation of the circuit shown in FIG. 7 is as follows. 
     At the outset, upon selection of the automatic aperture control mode with modes selecting member 32 set to index 33c and diaphragm setting member 5 set to the maximum f-number setting position shutter speed dial 30 is set to select a desired shutter speed, so that sliding element 45 sets the resistance of resistor 44 to the value corresponding to the selected shutter speed. Additionally, switch Sw 1  is connected to contact a, switch Sw 2  is opened, and switch Sw 3  is closed. Furthermore, assume that an exchangeable lens having a minimum f-number F:1.4 and a maximum f-number F:16 is mounted on camera body 10. Then, movable contact 15 selects contact 26a connected to resistor r 1 . 
     In this case, if the voltage at input terminals 47a of indicating device 47 and the voltage generated at output terminal 40a of light measuring circuit 40 with the diaphragm aperture maintained fully open are equal to one another, one of light emitting diodes LED-3, LED-2, --and LED 7 corresponding to F:1.4 should be energized. Although there are 7 steps from F:1.4 to F:16, a voltage which is lower than that at terminal 40a by 6Va is fed to another input terminal 47b of indicating device 47 because resistor r 1  provides information of one step. That is, the difference between the voltages fed to input terminals 47a and 47b of indicating device 47 is 6 Va and thus light emitting diode LED 6 is energized to indicate f-number F:1.4 Then, when the shutter speed to be selected thereafter is changed so that the voltage fed to input terminal 47a decreases by n Va, the difference in the voltages fed to input terminals 47a and 47b becomes (b-n)Va, and thus light emitting diode LED 6-n is energized to indicate that f-number providing an optimum exposure at the selected shutter speed. 
     Alternatively, assume that an exchangeable lens having a minimum f-number F:2.8 and a maximum f-number F:22 is mounted on camera body 10. In this case, movable contact 15 selects contact 26b connected to resistor r 2 . Although there are 6 steps from F:2.8 to F:22, a voltage which is lower by 4 Va than that at terminal 40a is fed to input terminal 47b of indicating device 47 because resistor r 2  gives information of two steps. That is, the difference in the voltages fed to input terminals 47a and 47b is 4Va if the voltage fed to input terminal 47a and the voltage generated at output terminal 40a are equal to one another, and thus light emitting diode LED4 is energized to indicate f-number F:2.8. Then, when the shutter speed to be selected thereafter is changed so that the voltage fed to input terminal 47a decreases by n Va, the difference in the voltages fed to input terminals 47a and 47b becomes (4-n)Va, and thus light emitting diode LED4-n is energized to indicate that f-number providing an optimum exposure at the selected shutter speed. 
     In other cases where an exchangeable lens having a maximum and a minimum f-number different from the above cases is mounted on a camera body, the indicating device of the present invention operates in the same manner as described above. 
     The above description provides explanations of f-number indication when diaphragm D can be controlled within the possible aperture range of an exchangeable lens mounted on camera body 10. However, there may be cases where (a) the brightness of an object is so bright that an overexposure results at the selected shutter speed even when diaphragm D is stopped-down to its minimum aperture corresponding to a maximum f-number of the exchangeable lens, and (b) where the brightness of an object is so dark that an underexposure results at the selected shutter speed even when diaphragm D is maintained at its fully open aperture corresponding to a minimum f-number of the exchangeable lens. In these cases, light emitting diodes LEDov and LEDun are energized respectively with the circuit arrangement in FIG. 7. That is, in case (a), the voltage appearing at junction c becomes higher than that appearing at sliding element 45, so that operational amplifier OP 2  generates a low output level, thereby energizing light emitting diode LEDov. In case (b), the voltage appearing at output terminal 40a is lower than that appearing at sliding element 45, so that operational amplifier OP 3  generates a low output level, thereby energizing light emitting diode LEDun. 
     In the automatic aperture control mode, an exposure is controlled in the following manner. Due to a shutter release operation, diaphragm D commences to be stopped-down and the voltage generated at output terminal 40a of light measuring circuit 40 decreases as the stopping-down of diaphragm D advances. Then, if the voltage generated at output terminal 40a reaches the same level as the voltage appearing at sliding element 45, operational amplifier OP 4  generates a low output level to energize electromagnet Mg 1 , and thus the stopping-down operation of diaphragm D is interrupted to determine the aperture. As memory switch Sw 4  is still closed at this time, a voltage including the information of the aperture thus determined as well as the information of the brightness of an object and the set film sensitivity is stored by memory capacitor C. Immediately before the mirror of the camera is moved to its picture taking position, subsequent to the dtermination of the aperture, switch Sw 4  is opened, and thereafter the shutter commences to be opened. Then, count switch Sw 5  is closed to actuate exposure time control circuit 48. After a lapse of time from the closing of switch Sw 5 , corresponding to the voltage stored in memory capacitor C, exposure time control circuit 48 generates an output to energize electromagnet Mg 2 , whereby an exposure is terminated. 
     In the above case, if the object brightness is so bright that the voltage appearing at output terminal 40a is higher than that appearing at sliding element 45 before the commencement of the stopping-down of diaphragm D, electromagnet Mg 1  is energized to maintain diaphragm D at its fully open aperture. Thus, an exposure will be effected at an exposure time which is faster than the selected exposure time, if the selected exposure time cannot provide an optimum exposure at the fully open aperture. 
     Next, upon selection of the automatic shutter speed control mode with diaphragm setting member 5 set to any one of the manually set aperture positions and modes selecting member 32 set to index 33b, switch Sw 1  is connected to contact b, switch Sw 2  is closed, and switch Sw 3  is opened. Even when movable contact 15 selects either of contacts 26a, 26b or 26c circuit due to setting of diaphragm setting member 5 to the maximum f-number position, a voltage at junction c is input to input terminal 47a since switch Sw 2  is closed to short-circuit resistors r 1 , r 2  and r 3 . In this case, a reference voltage corresponding to 1/125 second is always fed to input terminal 47a of indicating device 47. Thus, when the voltage appearing at junction c and fed to another input terminal 47b of indicating device 47 corresponds to 1/125 second, the difference in the voltages fed to input terminals 47a and 47b becomes zero, so that light emitting diode LEDo is energized to indicate a shutter speed of 1/125 second. Alternatively, when the voltage corresponding to 1/250 second appears at junction c, the difference in the voltages fed to input terminals 47a and 47b is -1va, so that light emitting diode LED-1 is energized. The voltage appearing at junction c varies in accordance with the set f-number as well as object brightness and the set film sensitivity. 
     In this exposure control mode, an exposure is controlled in the following manner. 
     In this case, electromagnet Mg 1  is always kept deenergized because switch Sw 3  is opened to interrupt the power supply to the same. Thus, automatic aperture control is disabled, and instead thereof, an aperture determining means (not shown) controls diaphragm D to the set aperture. After the determination of the set aperture, a voltage including the information of the set aperture as well as the information of the object brightness and the set film sensitivity is stored in memory capacitor C. Immediately before the mirror is moved to its picture taking position, subsequent to the determination of the set aperture, switch Sw 4  is opened to disconnect memory capacitor C from light measuring circuit 40, and thereafter count switch Sw 5  is closed in synchronism with the opening of the shutter to actuate exposure time control circuit 48. Then, exposure time control circuit 48 energizes electromagnet Mg 2  after a lapse of time from the closing of switch Sw 5  corresponding to the voltage stored in memory capacitor C, whereby an exposure is terminated.