Exposure time control circuit for camera with flash light device
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an exposure control means for use in a photographic camera provided with a built-in electronic flash light device, and more particularly to an exposure time control circuit for use in a camera provided with an electronic-flash light device in which the exposure time is controlled in accordance with the scene brightness when the electronic flash light device is not ready for emitting flash light and in accordance with the condition determined for the flash light photography when the electronic flash light device is ready for emitting light. 
     2. Description of the Prior Art 
     It has been known in the art to control the exposure time in a camera provided with an electronic flash light device in accordance with the scene brightness when the electronic flash light device is not ready for emitting light and control the same in accordance with the predetermined condition for the flash light photography when the electronic flash light device is ready for emitting flash light. In more detail, as disclosed for instance in U.S. Pat. No. 4,016,575, it has been known in the art to make change-over between an automatic daylight exposure range and an automatic flash exposure range. 
     However, in the above-described prior art, it has been impossible to manually control the exposure time in addition to the above function of automatic exposure control. 
     SUMMARY OF THE INVENTION 
     It is the primary object of the present invention to provide an exposure control means for use in a camera in which the exposure time can be controlled manually and automatically not only for daylight exposure range but also for flash exposure range. 
     A more specific object of the present invention is to provide an exposure time control circuit for use in a camera provided with a built-in electronic flash light device in which the exposure time is controlled in accordance with the scene brightness when the electronic flash light is not ready for emitting light and in accordance with the predetermined condition for flash light photography when the electronic flash light is ready for emitting light, and further in accordance with manually selected conditions when so required. 
     The above objects of the present invention are accomplished by providing a manual exposure time setting means in an exposure time control circuit including a changeover means to make change-over between automatic daylight exposure and automatic flash exposure. In more detail, the exposure control circuit in accordance with the present invention comprises a light measuring circuit for measuring the scene brightness and giving an output corresponding to the scene brightness, an exposure signal generating circuit for generating an output signal indicative of the exposure time corresponding to the scene brightness based on the output of the light measuring circuit and other exposure information such as film sensitivity and the size of the stop, and a comparator which compares the output signal from the exposure signal generating circuit with a standard signal to automatically control the exposure time, wherein the improvement comprises a manually operable exposure time input means for providing the exposure signal generating circuit with a manual exposure signal output so that the exposure signal generating circuit may generate an exposure control output signal based on a manually selected condition, and a control circuit connected with said comparator for giving an output to the comparator which is based on a predetermined condition for flash photography only when the control circuit receives a flash light emission ready signal. 
     In accordance with the present invention, the exposure time is controlled according to the manually selected condition when required and according to the scene brightness when the electronic flash light device attached to the camera is not in a ready condition. When the electronic flash light device becomes ready for emitting light, the exposure is controlled according to the predetermined condition for flash light photography. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single Figure is a block diagram showing the electric circuitry of the exposure control circuit in accordance with an embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing, a light measuring circuit 2 and an arithmetic circuit 5 to which outputs from a stop size information input means 3 and a film speed information input means 4 are transmitted are contained in an automatic exposure control circuit 1. The output from the exposure control circuit 1 is given to an exposure time signal generating circuit 6 of binary code system. This exposure time signal generating circuit may comprise an analog-to-digital converter for providing a digital signal (such as a binary &#34;1&#34;) at a selected output terminal 6a, 6b, . . . 6st . . . 6e thereof as a function of the level of the output received from the exposure control circuit. Alternatively, if the output received from exposure control circuit 1 is a digital signal (such as an n-bit signal), the exposure time signal generating circuit may comprise a 1-out-of-n decoder. The output from the exposure time signal generating circuit 6 is given to a decoder 7 of decimal code system. The decimal system output of the decoder 7 is given to one input of a digital comparator 8. On the other hand, a standard signal from a clock pulse generator 9 is input into a digital counter 11 by way of a gate circuit 10. The gate circuit 10 is opened by a synchronizing signal S given in response to start of a leading shutter blind of the camera thereby passing the standard signal therethrough to the digital counter 11 which counts a decimal signal. The output of the digital counter 11 is given to the other input of the digital comparator 8, where it is compared with the output of the output from the decoder 7. When the two outputs becomes equal, the digital comparator 8 generates an output to be given to a coil 12 which releases a trailing shutter blind locking means thereby to start the trailing shutter blind. Thus, the optimum exposure time is obtained. 
     In addition to the above described structure, in this invention a manually controlled exposure time input means 13 is connected with the exposure time signal generating circuit 6 so that an output manually selected can be input into the exposure time signal generating circuit 6 when so required. In the embodiment wherein the exposure time signal generating circuit comprises an analog-to-digital converter, the output of the manually controlled exposure time input means is an analog signal whose level represents the manually selected exposure time. In the embodiment wherein exposure time signal generating circuit 6 comprises a 1-out-of-n decoder, the output of manually controlled exposure time input means 13 is an n-bit digital signal. The output of the manually operable exposure time input means 13 and the output of the exposure control circuit 1 are selectively input into the exposure time signal generating circuit 6. Therefore, the input into the comparator 8 is selected between the outputs from the automatic exposure control circuit 1 and the manually operable exposure time input means 13. The manually operable exposure time input means 13 may be comprised of a manually rotatable dial to select a proper exposure time in the form of a level of a voltage output to be given to the exposure time signal generating circuit 6 when the latter is comprised of an analog-to-digital converter. Therefore, for instance, a potentiometer is used as the manually operable exposure time input means 13. 
     Further, in the present invention, a control circuit is connected with the comparator for giving an output to the comparator 8 based on a predetermined condition for flash light photography only when the control circuit receives a flash light emission ready signal. The control circuit is shown at 24 in the drawing and is connected with the digital comparator 8 by way of the decoder 7 and a group of AND gates 25a to 25e interposed between the decoder 7 and the exposure time signal generating circuit 6. The control circuit 24 is actually an inverter which transmits an output from a transistor 23 to the group of AND circuits 25a to 25e. The control circuit 24 operates to inhibit the transmission of the signal from the manually operable exposure time input means 13 or the automatic exposure control circuit 1 to the decoder 7 when an electronic flash light device 14 is attached to the camera and the flash light device 14 is in the ready condition to emit flash light, and give a signal indicative of a predetermined exposure time desirable for the flash light photography, e.g. 1/60 sec. 
     The electronic flash light device 14 is comprised of a switch 15 connected with a power supply 16, a booster 17 connected with the power supply 16, a flash light discharge tube 18 charged by the booster 17, a main capacitor 19 connected with the flash light tube 18 to energize the tube 18, a trigger circuit 20 connected with the discharge tube 18 operated in synchronization with start of the leading shutter blind of the camera for causing the discharge tube 18 to emit light, and a neon lamp 21 connected with the trigger 20 and the main capacitor 19 for detecting the ready state of the flash light tube energizing capacitor 19 and indicating the state. Further, the electronic flash light device 14 includes an FET 22 connected with the neon lamp 21 which is made conductive by the detecting voltage of the neon lamp 21 when the main capacitor 19 is ready for energizing the flash light discharge tube 18. The FET 22 is provided with a transistor 23 which is made non-conductive or turned off when the FET is made conductive. The output of the transistor 23 is therefore made to be High level when the FET 22 is made conductive. The output of the transistor 23 is connected with said inverter 24. 
     Accordingly, the output of the inverter 24 is kept to be of High level until the electronic flash light device 14 is ready for emitting light and is turned into Low level when it is brought into the state ready for emitting light. 
     The output of the inverter 24 is input into one of the inputs of AND gates 25a, 25b, 25c, 25d and 25e the other input of which is connected with output terminals 6a, 6b, 6c, 6d and 6e of the exposure time signal generating circuit 6, respectively. The exposure time signal generating circuit 6 has, in this embodiment shown in the drawing, five outputs 6a to 6e for different exposure time control and one output 6st for a predetermined exposure time for flash light photography, e.g. 1/60 sec. The output of 6st is connected with one input of an OR gate 25st the other input of which is connected with the output of the inverter 24. Then, the output of the AND gates 25a to 25e and the OR gate 25st is connected with the input of the decoder 7. 
     With the above described structure of the exposure control circuit in accordance with the present invention, the exposure time is controlled on basis of a manually selected exposure condition or an automatically controlled exposure condition with regard to the scene brightness when the electronic flash light device attached to the camera is not ready for emitting light even when the flash light device 14 is attached to the camera, because unless the electronic flash light device 14 is ready for emitting light the output of the inverter 24 is in High level. When the flash light device attached to the camera becomes ready for emitting light, the output of the inverter 24 changes from High level to Low level and all the AND gates 25ato 25e are turned off with only the OR gate 25st is turned on. Accordingly, the exposure time is controlled to be of the predetermined length desirable for flash light photography. In this case, upon release of the shutter means in the camera, the trigger 20 is energized to discharge the flash light tube 18 and the gate 10 is opened to start counting pulses from the clock pulse generator 9. Then, when a predetermined time has lapsed since the start of the leading shutter blind, the OR gate 25st which has been open until then is closed and the digital comparator 8 operates to generate a signal to terminate the operation of the coil 12 which has been locking the trailing shutter blind. Thus, the shutter speed or exposure time of the predetermined value is effected together with the emission of the flash light. Until the electronic flash light device 14 becomes ready for emitting light or discharging the flash light tube 18, the inverter 24 prevents the OR gate 25st from operating. Accordingly, until the flash light device 14 becomes ready, the AND gates 25a to 25e are in the operative condition and the exposure time is determined according to one of the outputs from the output terminals 6a to 6e.