Abstract:
An external flash is attached to a digital camera with an internal flash and is operable selectably in two or three modes. Its control system includes a signal detecting device for detecting a signal for starting emission of light by receiving the detected signal and a single maneuverable device for being handled by a user to select one of these modes.

Description:
PRIOR APPLICATIONS 
       [0001]    This application is a divisional of and claims the benefit of co-pending U.S. patent application Ser. No. 13/349,979, filed on Jan. 13, 2012, titled “Light Emission Control Of External Flash For Digital Camera”, which is a divisional of and claims the benefit of U.S. patent application Ser. No. 11/631,724, filed on Sep. 13, 2007, titled “Light Emission Control Of External Flash For Digital Camera”, and is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to an improved control technology on a flash adapted to be externally attached to a digital camera of the type internally containing another flash. 
         [0003]    Such a camera may be used both above ground and for underwater photography but since the effects of the present invention are believed to be more significant in the case of underwater photography, a situation of underwater use will be mainly explained but the invention is not limited to such applications. 
         [0004]    When a digital camera (as well as a camera of an ordinary non-digital kind) is used for underwater photography, it is usually sealed inside a commercially available colorless transparent waterproof housing. The housing should be colorless and transparent because when the flash contained in the camera (hereinafter referred to as the internal flash of the camera) is used, the emitted light must pass through this housing to illuminate the objects to be photographed. 
         [0005]    Underwater photography usually takes place where it is much darker than above the ground and hence stronger flash light is required. If an internal flash is used for this purpose, however, the flash light may be reflected by small objects floating in the water near the lens, preventing it to obtain a clear picture. This is referred to as a marine-snow phenomenon. With the use of an internal flash, furthermore, a center part of the obtained picture tends to become whitish such that the picture as a whole appears flat. Another problem with underwater photography is that the quantity of light available by the internal flash is sometimes not sufficient and the obtained picture tends to be underexposed. 
         [0006]    Even in the case of above-ground commercial photography, the spotted light from the internal flash may be reflected from the object of photography to produce an undesirable effect. This is because the light emitting part of the flash is too close to the camera lens. 
         [0007]    In view of the problems described above, it has been known to use an external flash adapted to detect light from the internal flash and to emit light by this detected signal. This is because the light emitting part of the external flash can be disposed farther away from the camera lens and hence the aforementioned problems associated with the use of the internal flash can be obviated or at least reduced and hence more desirable images can be obtained. 
         [0008]    Among the external flashes, those with an automatic focusing function are commonly being used such that the quantity of light can be maintained uniform even if the object distance is varied. This is because operations are much simpler than the manual focusing type which requires a manual adjustment of light quantity according to the object distance. 
         [0009]    When an automatic flash is used for a proper exposure, it is necessary to match the lens opening with the flash opening. In the above, the flash opening means the quantity of light and is usually written in terms of the lens opening value for easy reading. In other words, whenever the lens opening is varied, the flash opening value must also be varied. Moreover, if it is desired to correct the exposure, the flash opening value which has been matched to the lens opening must also be changed in the positive or negative direction. This makes the operation very complicated. For a correct operation, furthermore, the sensor of the automatic flash must be correctly oriented towards the object to be photographed. 
         [0010]    The light quantity of the external flash may be controlled manually if fine adjustments of exposure are desired. This is an effective method when the distance between the camera and the target object to be photographed does not change significantly and the user has only to adjust the light quantity of the external flash once. If the single external flash is provided with both functions for automatic and manual operations, it becomes much easier to use. 
         [0011]    For use in underwater photography, all operation knobs on the external flash must be water-proof. Thus, it is desirable to reduce the number of such knobs and since there is only a limited space for providing them, the maximum number of such number is usually considered to be two (as in the example shown in  FIG. 13 ). It is therefore desirable to provide each of these knobs as many functions as possible. 
         [0012]    In what follows, it will be assumed that flashes are used for relatively short object distances of the order of 0.5 m-1.5 m. 
         [0013]      FIG. 16  shows a conventional flush control system for exampling a conventional control method for a digital camera  1  comprising a lens  5 , a CCD  4 , a control circuit  3  and a flash light emitting part  6 , used inside a water-proof housing  2  in the case of an underwater photography. The housing  2  may not be required in the case of a use above the ground. 
         [0014]    The water-proof housing  2  is provided with an external flash  11   b  of a prior art type. The light emission from the external flash  11   b  must be synchronized with the so-called X-junction of the camera  1  but most housings are not equipped with a synchronization cord terminal for connecting to the external flash  11   b . This is because if such terminal were to be provided, its connecting part would also have to be waterproofed and also because there are cameras without a terminal for the X-junction and in such a case it is not possible to make a connection. 
         [0015]      FIG. 16  shows a simple example of method not using any synchronization cord but using instead a filter  7  adapted to screen visible light from the internal flash, while allowing infrared light (inclusive of near infrared light) from the internal flash to pass through and leading it through an optical fiber  8  as light conducting means to a light detector circuit  9  such that the external flash  11   b  is caused to emit light by the signal detected by this light detector circuit (or a sensor)  9 . If the distance between the internal flash and the outer flash is sufficiently short, the optical fiber  8  may not be required to serve as light conducting means because the infrared light (inclusive of near infrared light) from the internal flash may directly reach the light detector circuit  9  through air or water. 
         [0016]    Next, the operation of the system of  FIG. 16  is explained. As the shutter lever of the camera  1  is pressed, the internal flash  6  initially undergoes a preliminary light emission. At the same time, the light from the internal flash is lead through the optical fiber  8  to the light detector circuit  9 , is thereby converted into an electrical signal and is led to a control circuit  10   b  of the external flash  11   b.    
         [0017]    The preliminary light emission is not anything that is required by the external flash  11   b . Thus, the external flash  11   b  may be adapted to ignore its occurrence and not to emit any light as a result or to emit only a small amount of light that is negligible compared to the main light emission to be explained below. 
         [0018]    About 100 milliseconds thereafter, simultaneously as the shutter is closed, the internal flash  6  is caused to undergo a main light emission. Since the aforementioned filter  7  is present, however, the light emitted as the main light emission does not illuminate the object (shown at  14 ) to be photographed but is led, like the light emitted as the preliminary light emission, to the light detector circuit  9 . As the control circuit  10   b  judges this to be the main light emission, the external flash  11   b  is activated and emitted light  13  from a reflective mirror  12  is caused to illuminate the object  14  to be photographed. As reflected light  16  from the object  14  to be photographed is received by a sensor  17 , it is thereby converted into an electrical signal. This electrical signal is integrated by an integrator circuit within the control circuit  10   b  and when the integrated value reaches a preliminarily specified value corresponding to an appropriate exposure, a signal is outputted from the control circuit  10   b  to stop the emission of light from the external flash  11   b . This is how a properly exposed photograph is intended to be obtained. 
         [0019]    In the case of a manual control, a resistor is used instead of the sensor  17  by a known method and its resistance value is varied to change the timing for stopping the light emission and thereby adjusting the light quantity for a proper exposure. 
         [0020]    There are many problems with an apparatus as described above. 
         [0021]    Firstly, as explained above, the lens opening and the flash opening must be matched for a proper exposure and this is a troublesome operation. Secondly, even after these openings are matched, the flash opening is required to be changed in the positive or negative direction and the operation is made cumbersome. Thirdly, the sensor  17  cannot receive the reflected light correctly from the object to be photographed unless the external flash is oriented to the center of the object to be photographed but the operation of setting the sensor is troublesome. Fourthly, the exposure changes when a filter or a converter lens is attached to the front of the camera lens but this change cannot be automatically corrected. Fifthly, although a zoom lens is usually attached to the camera and the image angles are different on the telephoto side and the wide-angle side, the light-receiving angle of the sensor  17  of the external flash  11   b  is fixed and hence an error may result in the measurement of the light. Sixthly, when the camera is set in a program mode, the lens opening and the shutter speed change according to the brightness of the surrounding but it is not a simple matter to adjust the external flash accordingly and it is difficult to make a proper exposure. 
       SUMMARY OF THE INVENTION 
       [0022]    It is therefore a general object of this invention to provide an improved flash control system for a digital camera such that the control can be effected easily by a user even in the underwater environment and pictures of a high quality can be obtained. 
         [0023]    A flash control system according to one embodiment of the invention may be described as being for a digital camera having an internal flash and an external flash, the external flash being adapted to emit light by a signal from a signal detecting means as the signal detecting means receives emitted light from the internal flash, the internal flash being adapted to undergo a preliminary light emission process and subsequently a main light emission process, and may be characterized as comprising light screening means for preventing light emitted from the internal flash from illuminating a target object to be photographed by the digital camera; light conducting means for conducting light from the internal flash to the external flash for causing emission of light from the external flash; preliminary emission starting means for causing the external flash to start preliminary emission of light by using as trigger a light emitting signal when the internal flash starts the preliminary light emission process; preliminary emission stopping means for causing the external flash to stop the preliminary emission of light by using end of the preliminary light emission process of the internal flash as trigger; main emission starting means for causing the external flash to start main emission of light by using as trigger another light emitting signal when the internal flash starts the main light emission process; and main emission stopping means for causing the external flash to stop the main emission of light by using end of the main light emission process of the internal flash as trigger. 
         [0024]    Another flash control system according to a second embodiment of the invention may be characterized similarly as the system according to the first embodiment described above except that the preliminary emission stopping means is adapted to cause the external flash to stop the preliminary emission of light after a preliminarily specified time has elapsed (from the start of the preliminary emission). 
         [0025]    Still another flash control system according to a third embodiment of the invention may be described as being for a digital camera having an internal flash and an external flash, the external flash being adapted to emit light by a signal from a signal detecting means as the signal detecting means receives detects light from the internal flash, the internal flash being adapted to undergo a preliminary light emission process and subsequently a main light emission process and may be characterized as comprising preliminary emission starting means for causing the external flash to start preliminary emission of light by using as trigger a light emitting signal received by the signal detecting means when the internal flash starts the preliminary light emission process; preliminary emission stopping means for causing the external flash to stop the preliminary emission of light by using end of the preliminary light emission process of the internal flash as trigger; main emission starting means for causing the external flash to start main emission of light by using as trigger another light emitting signal received by the signal detecting means when the internal flash starts the main light emission process; main emission stopping means for causing the external flash to stop the main emission of light by using end of the main light emission process of the internal flash as trigger; and exposure correcting means for varying quantity of light emitted by the external flash by the preliminary emission. 
         [0026]    A still another flash control system according to a fourth embodiment of the invention may be characterized similarly as the system according to the third embodiment described above except that the preliminary emission stopping means is adapted to cause the external flash to stop the preliminary emission of light after a preliminarily specified time has elapsed (from the start of the preliminary emission). 
         [0027]    The present invention is particularly useful when used for underwater photography, with the camera enclosed inside a waterproof housing and the external flash is attached outside such housing. 
         [0028]    Flash control systems according to the first and second embodiments may preferably be further provided with means for varying the waveform of light emitted from the external flash so as to resemble the waveform of light emitted from the internal flash, say, by reducing the duration of the preliminary emission of light from the external flash or by delaying the stopping of the preliminary emission of light from the external flash. Examples of such means include using a plurality of discharge tubes connected in parallel, using no choke coil or, if a choke coil is inserted, using no choke coil with inductance greater than 20 microhenries, using discharge tubes with arc length 20 mm or longer (with the internal diameter is 1.9-2.1 mm) and 40 mm or less (with the internal diameter 2.6 mm or less). 
         [0029]    The exposure correcting means of the flash control system according to the third or fourth embodiment may preferably include means for varying duration of time of the preliminary emission of light, input means for varying quantity of light emitted by the external flash according to an input therethrough and/or means for varying the timing of operation of the preliminary emission stopping means. 
         [0030]    The invention also relates to a flash control system for an external flash attached to a digital camera containing an internal flash wherein the external flash is adapted to be operable selectably in at least two of the group of three modes consisting of (1) what is herein referred to as an exposure correcting mode for varying quantity of light emitted by the external flash by means of an exposure correcting means, (2) an automatic mode and (3) a manual mode and may be characterized as comprising signal detecting means for detecting a signal, the external flash being adapted to start emission of light by receiving this detected signal, and a single maneuverable device for being handled by a user to select one of these at least two modes. 
         [0031]    In the above, the automatic mode and the manual mode are of the kinds which have been conventionally available and hence will not be described in detail herein except that there have not previously been any control systems that could be operable selectably both in these two modes. Control systems operable in the exposure correcting mode are also believed to be new, and the present invention further presents control systems that are operable (1) in the combination of both the exposure correcting mode and the automatic mode, (2) in the combination of both the exposure correcting mode and the manual mode, and (3) in the combination of both the automatic mode and the manual mode, as well as (4) in the combination of all of the exposure correction mode, the automatic mode and the manual mode. In summary, the present invention presents not only multi-mode control systems for an external flash but also a single device such as a knob to make the selection out of the available two or three modes. 
         [0032]    Additionally, the present invention related to a flash control system of the kind such as described above further provided with another single maneuverable device such as a knob for being handled by a user for varying quantity of light to be emitted both when the system is in the automatic mode and in the manual mode. It now goes without saying that a system operable in such plurality of modes and provided with a single knob to make a selection out of the available modes and another single knob to select the quantity of light to be emitted in any of the selected modes is extremely more convenient to the user than previously available systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  is a schematic block diagram of a flash control system embodying this invention. 
           [0034]      FIG. 2  is a variation of the embodiment shown in  FIG. 1  with the sensor of the light detector circuit attached to one end of a cord. 
           [0035]      FIGS. 3A ,  3 B,  3 C,  3 D,  3 E,  3 F,  3 G and  3 H are examples of light screening means that may be used for the purpose of this invention. 
           [0036]      FIG. 4  is a block circuit diagram of the flash control system of  FIG. 1 . 
           [0037]      FIGS. 5A and 5B , together referred to as  FIG. 5 , are waveform diagrams showing the operation of the control system of  FIG. 4 . 
           [0038]      FIGS. 6A ,  6 B and  6 C, together referred to as  FIG. 6 , are waveform diagrams showing changes in waveform of preliminary light emission that may take place. 
           [0039]      FIGS. 7A and 7B , together referred to as  FIG. 7 , waveform diagrams of light emitted respectively from the internal and external flashes. 
           [0040]      FIG. 8A  shows two discharge tubes connected in parallel and  FIG. 8B  shows a single discharge tube connected to a choke coil in series for an external flash. 
           [0041]      FIGS. 9A and 9B  are outer views and sectional views of discharge tubes with different acceptable dimensional relationships. 
           [0042]      FIG. 10  is a block circuit diagram of another flash control system embodying this invention. 
           [0043]      FIG. 11  is block circuit diagram of another flash control system embodying this invention. 
           [0044]      FIG. 12  is a waveform diagram for showing changes in waveform effected by operations on the exposure correcting means of  FIG. 11 . 
           [0045]      FIG. 13  is a view of an operating portion of an external flash of this invention for underwater photography. 
           [0046]      FIGS. 14A ,  14 B,  14 C and  14 D, together referred to as  FIG. 14 , are views of the knobs on the operating portion shown in  FIG. 13 . 
           [0047]      FIG. 15  is a block circuit diagram of still another flash control system embodying this invention. 
           [0048]      FIG. 16  is a block circuit diagram of a prior art flash control system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0049]    The invention is described next by way of examples.  FIG. 1  shows an example of flash control system using a control method embodying this invention. Components that are similar or equivalent to those shown in  FIG. 16  and explained above will be indicated by the same numerals and will not be repetitiously explained for the convenience of description. 
         [0050]    As explained above, it is necessary according to this invention to generally prevent visible light from the internal flash to illuminate the object to be photographed. There are known methods and apparatus for this purpose including the use of a filter (such as shown at  7  in  FIG. 16  and serving as a light screening means) adapted to screen visible light but to allow infrared and near infrared light to pass through, positioning it on the front surface of the internal flash. 
         [0051]    Many examples of this type have been known and there are also situations where visible light need not be screened completely. 
         [0052]    An external flash  11   a  is shown again positioned at some distance away from the camera  1  and adapted to receive signals through the aforementioned light screen means and an optical fiber  8  serving as light conducting means. As explained above, the optical fiber  8  may not be required if the internal and external flashes are sufficiently close to each other such that the infrared and near infrared light from the internal flash can easily reach the external flash  11   a  directly, using environmental air or water as the light conducting means. 
         [0053]    According to the embodiment shown in  FIG. 1 , the filter  7  serving as the light screening means for screening visible light and passing infrared and near infrared light is preliminarily pasted onto the light emitting part  6  of the camera  1  before the camera  1  is placed inside its waterproof housing  2 . This is the same also when the camera  1  is to be used above the ground. As a result, visible light is screened when emission of light takes place from the internal flash and does not leak to the exterior to cause the marine-snow phenomenon, but the infrared portion of the emitted light is allowed to pass through the filter  7  and reach the light detector circuit  9  through the optical fiber  8  to be converted to an electrical signal. As explained above, the optical fiber  8  may be dispensed with on occasions. The converted electrical signal is received by the control circuit  10   a  to control the emission of light from the external flash  11   a.    
         [0054]      FIG. 2  shows a variation of the example shown in  FIG. 1  wherein a sensor  18 , which is a part of the light detector circuit  9  is attached to one end of a cord  19 . This variation may be adopted when it is preferred not to use the optical fiber  8  as light conducting means. 
         [0055]    In the above, the position at which the filter  7  should be pasted on may be freely selected as long as an equivalent effect can be obtained.  FIG. 3A  shows an example wherein a filter  7   a  is pasted on the inner surface of the housing  2  and  FIG. 3B  shows another example wherein a filter  7   e  is on the outside surface of the housing  2 .  FIGS. 3C and 3D  are examples of a situation where the waterproof housing  2  is not transparent to visible light, showing the housing  2  itself serving as a light screening means  7   b  having a light-passing small hole  7   c  or  7   d , respectively. Symbols  7   h  in  FIGS. 3B and 7   d  in  FIG. 3D  each indicate a light-passing opening to show that an opening for this purpose need not be provided exactly in front of the light-emitting reflecting mirror of the internal flash  6 , as long as the opening serves to pass the emitted light therethrough and to reach the light detector circuit  9 . 
         [0056]      FIG. 3E  shows still another example wherein the housing  2 , whether it is of a transparent material or of a non-transparent material, is provided with a transparent window  7   f . In such a case, a filter not transmitting visible light but allowing infrared and near infrared light to pass as explained above may be pasted on the front or back surface of this window  7   f , or a light-screening fiber material or plate  7   g  with a light-passing hole may be provided. It now goes without saying that such light-screening fiber material or plate  7   g  may be positioned inside the window  7   f , and that the window  7   f  need not necessarily be exactly in front of the light-emitting reflecting mirror of the internal flash  6 , as explained above. 
         [0057]    As explained above, the light screening means according to this invention include those that may allow a limited amount of light to leak through, as long as the effect on the photographed image is not significantly affected. Thus, a large variety of light screening means may be used according to this invention, including filters that allow to pass near infrared light close to the visible range, red filters or filters serving to reduce the quantity of passing light. In the latter case, those adapted to reduce the quantity of light to less than about ¼ are effective. 
         [0058]    The light screening means may be a molded product of a plastic resin material with the same or equivalent characteristics as described above so as to be formed in any desired shapes. The housing  2  may also comprise a resin material of the same characteristics. 
         [0059]    The filter may be replaced by any light-screening object provided with a small hole.  FIG. 3F  shows such an example of a plate  7   g  with a hole on a transparent housing. Instead, a portion of light leaking through a gap may be adapted to be collected. 
         [0060]      FIG. 3G  shows a situation when a wide conversion lens or a close-up lens  7   k  is attached in front of the camera lens. Such an attachment lens usually has a larger radius and its peripheral part may serve to screen the light from the internal flash  6  and no light screening means may be additionally required. 
         [0061]    There are cameras such as some single-lens reflective cameras with the internal flash adapted to pop up, pointing downward as shown at  7   i  in  FIG. 3H  before its light emitting part faces the forward direction completely. The light emitted therefrom is obstructed by the top portion of the camera and hardly reaches the object to be photographed while a portion may be collected, say, through an opening  7   j.    
         [0062]    Next, details of the control of light emission will be explained with reference mainly to  FIGS. 1 ,  4  and  5 . Since a control of exposure by carrying out a preliminary light emission followed by a main light emission is currently most common, this type of control will be described. Commonly known portions of the control will be omitted for the convenience of description. In other words, the omission of a description does not mean the absence of the corresponding control. 
         [0063]    As a power switch  22  is closed (or switched on), power begins to be supplied from batteries  21 , and a main capacitor  25  is charged by a step-up circuit  24 . As the voltage of the main capacitor  25  reaches a specified level, a ready circuit  26  causes a display to be made to the effect that a preparation for light emission has been completed. If the shutter button of the camera is pressed thereafter, the internal flash  6  firstly undergoes a preliminary light emission. In  FIG. 5 , numerals  71  and  72  respectively shows the rise and fall of the waveform of this preliminary light emission. The light emitted from the internal flash  6  as the preliminary light emission is guided to the light detector circuit  9 , say, through the optical fiber  8  serving as light conducting means. 
         [0064]    Means for starting emission of light from the external flash  11   a  by using this preliminary light emission as its trigger is hereinafter referred to as the preliminary emission starting means and will be described next more in detail by way of an example. 
         [0065]    The light detector circuit  9  comprises a photo-transistor  32  and a load resistor  33  and received light is thereby converted to an electrical signal. This electrical signal is inputted to an emission start signal detecting circuit  45  which is adapted to detect the rise of a signal, serving to detect the rise portion of a signal detected by capacitor  40 , to AC-amplify it by transistor  41  and detecting it by transistor  43  through capacitor  42 . As transistor  44  is further used as a buffer to input an emission start signal (as an ON signal) to a trigger  27  circuit, the trigger circuit  27  is thereby activated and the emission of light is started from a discharge tube  28 . 
         [0066]    As shown in  FIG. 5A , the time of light emission from the internal flash  6  is very short (as shown at  71  and  72 ). The stopping of the light emission is effected by a falling signal  72  of light that is transmitted through the optical fiber  8  to the light detector circuit  9  and converted into an electrical signal by the same photo-transistor  32  referred to above. Although the same photo-transistor  32  is used according to the illustrated example for the detection of both the rise and the fall of light signal (or the start and the end of the light emission), two different photo-transistors may be used for the two separate purposes. 
         [0067]    Means for stopping emission of light from the external flash  11   a  by using the end of this preliminary light emission as the trigger is hereinafter referred to as the preliminary emission stopping means and will be described next more in detail by way of an example. 
         [0068]    The falling at the end of the preliminary light emission is detected by an emission stop signal detecting circuit  39 , adapted to detect the edge of a falling by means of a differential capacitor  34  and to AC-amplify its signal by transistor  36  through capacitor  35 . This AC-amplified signal is detected by transistor  38  through capacitor  37  to form an ON signal. This ON signal is inputted to an emission stop signal input circuit  30  and then to a light emission control circuit  29  to stop the emission of light from the discharge tube  28 . Numeral  23  indicates a gate voltage generating circuit that serves to supply a necessary power to the emission stop signal input circuit  30  while the flash is emitting light. 
         [0069]    The example illustrated above is not intended to limit the structure of the circuits for detecting the starting and stopping of light emission. Devices structured differently may be used as long as similar effects can be brought about. For example, circuits for digitally detecting the rising and falling, as well as IC circuits comprising an operational amplifier and a comparator, may be substituted. 
         [0070]    A standard light emission from the external flash  11   a  is illustrated in  FIG. 5B  with numerals  75  and  76  respectively indicating the timing of the start and the end of the light emission. Some delays in time can be seen. The delay at the start is because the gas inside the discharge tube is not activated instantaneously, that is, for the reason of its physical characteristic. 
         [0071]    In order to achieve the standard light exposure, a common method is to make the preliminary emission somewhat larger than by the internal flash because the total light quantity from the external flash is much greater but it is preferable to adjust the exposure by experimentation. 
         [0072]    With the assumption of hardly any delay due to other electronic circuit components, the delay may be about 20 microseconds, as shown in  FIG. 5B . Although this delay is illustrated in an exaggerated manner, it is really a negligibly small delay. Moreover, since the light from the internal flash does not reach the object to be photographed while the light from the external flash is received, there is no doubling of light from these two flashes. 
         [0073]    In order to reduce the operational delays, the electronic circuits are simplified as much as possible. Delay components such as bypass capacitors are either totally dispensed with or minimized.  FIG. 4  shows an example where no bypass capacitor is employed. 
         [0074]    The delay related to the stopping of light emission is smaller than that related to the starting of light emission. This is because a semiconductor element is used for stopping the light emission from the discharge tube. The speed of operation is fast and there is hardly any operational delay. 
         [0075]    An insulated gate bipolar transistor (IGBT) is currently used as an element for this purpose. When the IGBT is switched off, the current that flows to the discharge tube is immediately shut off but the gas inside the discharge tube remains shining. In other words, there is an afterglow to some extent, but there is hardly any operational delay due to this afterglow. 
         [0076]    The characteristics of the discharge tube and the operation time of the electric circuit may be adjusted so as to control the timing for stopping the light emission. By a fine adjustment, this timing may be adjusted so as to be as shown at  76  in  FIG. 5B  such that the flash time is nearly equal to that of the internal flash. Thus, the circuit for the camera may operate based on the same numerical data for the light emission from the internal flash without causing any trouble. 
         [0077]    Next, as light  13  emitted as the preliminary emission hits an object  14  to be photographed and its reflection  15  is passed through the lens  5  of the camera  1 , it is converted into an electrical signal by the CCD (CMOS)  4  and the amount of light for the main light emission to follow is calculated by the control circuit  3  according to the level of this electrical signal. The main light emission of the internal flash  6  starts about 100 milliseconds later as shown by numeral  73  in  FIG. 5A . Light of this main light emission is guided to the light detector circuit  9  through the optical fiber  8  as explained above for the light of the preliminary light emission. 
         [0078]    Means for starting the main light emission from the external flash by using the light of the main light emission from the internal flash as the trigger is hereinafter referred to as the main emission starting means. Its operations are the same as those of the preliminary emission starting means and the light emission is started as shown by numeral  77  in  FIG. 5B . 
         [0079]    The main light emission from the internal flash  6  is stopped thereafter when a specified amount of light has been emitted (for a desired exposure) as shown by numeral  74  in  FIG. 5A . 
         [0080]    Means for stopping the main light emission from the external flash by leading this light signal through the optical fiber  8  to the light detector circuit  9  and using the end of the main light emission from the internal flash as the trigger is hereinafter referred to as the main emission stopping means. The operations by the main emission stopping means are the same as those by the preliminary emission stopping means described above. The main light emission of the external flash is stopped as shown by numeral  78  in  FIG. 5B . Since the external flash functions approximately in the same way as the internal flash, as explained above, an optimum exposure is possible although the light from the internal flash does not reach the object to be photographed. 
         [0081]    Next, a method of and an apparatus for adjusting the flash time for the preliminary light emission from the external flash will be described. When an external flash is used, it is desirable to adjust the light quantity of its preliminary light emission to be as closely equal to that of the internal flash (when compared by a measuring instrument with the aforementioned light screening means removed) such that an exposure can be made accurate. Since the external flash is much bigger, however, the rise at the beginning of light emission tends to become gentler, as illustrated in  FIG. 6  (comprising  FIGS. 6A ,  6 B and  6 C). 
         [0082]    If the preliminary light emission by the external flash can be made as shown at  79  in  FIG. 6A  by adjusting the circuits and the discharge tube as explained above, there is no need for any adjustment. If the rise is gentler and as shown at  80   a  in  FIG. 6B  and the light emission is stopped after 20 microseconds, the quantity of emitted light will be insufficient. In view of this problem, the time for stopping the preliminary light emission may be delayed although the end of the preliminary light emission by the internal flash is used as the trigger. 
         [0083]    The circuit diagram of  FIG. 4  includes circuits for this purpose (as means for delaying stopping of the preliminary emission). With reference again to  FIG. 4 , the preliminary light emission is started as an emission start signal is outputted from the emission start signal detecting circuit  45 . Since the aforementioned delay is to be effected only for the preliminary light emission, a preliminary emission identifying circuit  47  is provided to identify a preliminary light emission by distinguishing it from a main light emission. Its function is to distinguish between an initially received light emission and a subsequently arriving light emission that is received about 100 microseconds later (between a signal  49  for starting a preliminary light emission and a signal  48  for starting a main light emission) and this is a well known kind of circuit. 
         [0084]    The signal  49  corresponding to a preliminary light emission is transmitted to a delay circuit  50  and received by the emission stop signal input circuit  30  after a specified delay such as 10 microseconds in the case of the fall shown at  80   b  in  FIG. 6B . The delay circuit may be of a known type such as comprising a capacitor and a resistor. The delay time should preferably be adjusted to an optimum value by carrying out tests. 
         [0085]    The operations thereafter are the same as described above. Since no such delay is required on the main light emission, the signal therefor is not received by the delay circuit  50 , as explained above. As a result, the main light emission is stopped without a delay. Such circuits may be formed as a digital circuit on the same operational principles or may be operated by a personal computer. In summary, the flash time by the external flash becomes longer as shown at  80   b  in  FIG. 6B  such that the quantity of emitted light becomes about the same as that from the internal flash. 
         [0086]    Another simple method of delaying the stopping time of preliminary light emission is by delaying the operation of the emission stop signal detecting circuit  39  and this can be accomplished by adding a conventional integrating type of delay circuit using a resistor and a capacitor to the emission stop signal detecting circuit  39 . In this case, the stopping of the light emission is delayed also for the main light emission since there is no preliminary emission identifying circuit  47  but it is sufficient as a simple delay circuit since the delay in the main light emission does not affect the exposure significantly. 
         [0087]    If no such delay means is provided as in the prior art technology, the rise in the preliminary light emission may be further delayed as shown by numeral  81  in  FIG. 6C  and the flash system may cease to function properly. 
         [0088]    In order to make the external flash to function properly, means for shortening its rise time is necessary. As explained above, furthermore, the exposure becomes more accurate if the waveform of the light emission from the external flash is nearly the same as that from the internal flash. Although the light from the internal flash actually does not reach the object to be photographed, the present argument is on the assumption that it does. If the waveform of the light emission from the internal flash is as shown at  89  in  FIG. 7A  and that of the light emission from the external flash is as shown at  90  in  FIG. 7B , for example, means for shortening the flash time is therefore needed. 
         [0089]    Since the internal flash must be compact, its discharge tube is short and hence its flash time at the time of its full light emission is shorter than that for the external flash. Thus, it is desirable to provide means for shortening the flash time of the external flash so as to match that of the internal flash. The functions of these two means for shortening the rise time and the flash time are correlated with each other. If the flash time is shortened, for example, the rise time also becomes shorter. 
         [0090]    These means and methods are explained next with reference to  FIGS. 8 and 9 . 
         [0091]    As explained above, internal flashes of all cameras are nearly equal and small. On the other hands, external flashes are meaningless unless they are made much more powerful than the internal flashes. In other words, discharge tubes of external flashes cannot be of the same size as those of internal flashes. If a discharge tube of about the same size were used for an external flash, the power input would be too strong and the tube would be damaged. In summary, discharge tubes much larger than those for internal flashes are required for external flashes. If a longer discharge tube is used, however, the flash time becomes longer and the rise time becomes accordingly slower. 
         [0092]    The present inventor has carried out experiments and discovered that a parallel connection of two or more discharge tubes is highly effective.  FIG. 8A  shows such an example with two discharge tubes  82  and  83  connected in parallel. In such a connection, discharge tubes longer than those intended to be used singly may be used. 
         [0093]    When a single discharge tube is used, the rise time becomes undesirably slow if a choke coil  84  is inserted in series as shown in  FIG. 8B . In one aspect of the present invention, the external flash is characterized as having no chose coil inserted thereto. If a choke coil is to be inserted, it should be less than 20 microhenries (μH). 
         [0094]    When a single discharge tube is used, it is desirable to make the arc length as short as possible but an arc length of 20 mm or longer is necessary if the inner diameter is 2.1 mm with outer diameter=3.2 mm (as shown at  85  and  87  in  FIG. 9A ) or 1.9 mm (with outer diameter=3.1 mm). If it is shorter than this limit, the input power is excessively large to cause a damage. If the inner diameter is 2.6 mm (with outer diameter=3.6 mm), an arc length of 40 mm or less is necessary (as shown at  86  and  88  in  FIG. 9B ). If the length is greater that this limit, the flash time becomes too long and the rise time becomes too slow. 
         [0095]    In summary, discharge tubes are selected such that the flash time will look as shown at  90  and at most twice that of the internal flash. 
         [0096]    If it is desired to reduce the flash time of the external flash so as to approach that of the internal flash, it may be accomplished by way of an electric circuit, say, as shown in  FIGS. 4 ,  7 A and  7 B. 
         [0097]    Internal flashes are intended to be compact. There are limits to their volume and dimensions, and their specifications are more or less the same for all cameras. Their flash time for full emission is generally about 0.75 ms, as shown in  FIG. 7A . The circuit shown in  FIG. 10  is characterized as including an emission stop signal generating circuit  31  adapted to integrate the voltage values outputted from the gate voltage generating circuit  23  and to output an emission stop signal after a specified length of time has been elapsed. This stop signal is received by the emission stop signal input circuit  30  such that the emission of light is stopped as shown by  91  (instead of the full emission  90  which would normally last for 1.5 ms) and an overexposure is prevented. 
         [0098]    The control for the left-hand half of  FIG. 7B  (from the start of light emission to numeral  91 ) is carried out in the same way as by the prior art control. If the aforementioned function for limiting the flash time is made releasable, this function may be disabled when a large light quantity is necessary, say, when the target object to be photographed is at a large distance. 
         [0099]    As explained above, the specifications of most internal flashes are more or less the same. This is because power should be saved for the occasion of the main emission and the power to be expended for the preliminary emission is minimized by all internal flashes. In other words, the flash time for the preliminary emission from the internal flash is nearly always the same. Thus, it is not necessary to detect the end of preliminary light emission from the internal flash to use it as the trigger for stopping the light emission from the external flash. In other words, the preliminary light emission from the external flash may be otherwise controlled in some other ways such as shown in  FIG. 10 . 
         [0100]    With reference next to  FIG. 10 , as the internal flash undergoes a preliminary light emission, the emission start signal detecting circuit  45  is activated as explained above and a preliminary emission start signal is outputted and inputted to a timer  53  and preliminary emission signal generating circuit  63 . In response, the timer  53  outputs a positive voltage of duration about 0.2 seconds, and transistor  55  within the preliminary emission signal generating circuit  63  becomes switched on, causing transistor  61  to be switched on after a specified length of time by an integrator circuit comprising resistor  56  and capacitor  60 , inputting a preliminary emission stop signal to the emission stop signal input circuit  30  and thereby stopping the preliminary emission. In other words, the preliminary emission stop signal from the internal flash is not being utilized. 
         [0101]    In  FIG. 10 , capacitor  54  serves to increase the switched-on time of the transistor  55 . Capacitor  52  connected to the timer  53  is for preventing the generation of a stop signal at the time of preliminary emission of the emission stop signal detecting circuit  39  by inputting a signal to transistor  51  to inhibit the circuit. Since this capacitor  52  forms a differential circuit adapted to output a signal only at the time of a rise of the timer  53 , it is not activated at the time of the main emission and hence the transistor  51  is not switched on. Thus, there is no inhibit (disablement) and the emission stop signal detecting circuit  39  is activated when the main emission of the internal flash is stopped and a signal is transmitted therefrom. 
         [0102]    At the subsequent time of the main light transmission, a delay circuit  62  comprised of transistor  59 , capacitor  58  and resistor  57  is switched on by a signal received from the timer  53  to inhibit transistor  61 . Thus, no signal is outputted from the preliminary emission signal generating circuit  63  at the time of the main emission but a stop signal is transmitted from the emission stop signal detecting circuit  39  and the normal operation is carried out. 
         [0103]    The circuit described above may be replaced by a digital circuit having the same action principle. Alternatively, a microcomputer may be used for the operation. 
         [0104]    Such a circuit is convenient when a camera of a specified kind is used because the resistor  56  or the capacitor  60  may be preliminarily adjusted such that the external flash can be made to emit light according to the light emission from the internal flash of a specified camera. Since the preliminary light emission from internal flashes for most kinds of cameras is more or less alike, as explained above, the present invention can be used for many similar cameras of many different kinds. 
         [0105]    Next,  FIG. 11  is referenced to explain another example of preliminary emission starting means of this invention. In  FIG. 11 , components that are similar or equivalent to those shown in  FIG. 4  are indicated for convenience by the same numerals and will not be described repetitiously. 
         [0106]    A photo-transistor  32  and a load resistor  33  comprise a light detector circuit, and received light is thereby converted to an electrical signal. This electrical signal is inputted to an emission start signal detecting circuit  45  which is adapted to detect the rise of a signal, serving to detect the rise portion of a signal detected by capacitor  40 , to AC-amplify it by transistor  41  and to detect it by transistor  43  through capacitor  42 . 
         [0107]    As transistor  44  is further used as a buffer to input an emission start signal (as an ON signal) to a trigger  27  circuit, the trigger circuit  27  is thereby activated and the emission of light is started from a discharge tube  28 . 
         [0108]    As shown in  FIG. 5A  and explained above with reference to  FIG. 4 , the time of light emission from the internal flash  6  is very short (as shown at  71  and  72 ). The stopping of the light emission is effected by a falling signal  72  of light that is transmitted through the optical fiber  8  to the light detector circuit  9  and converted into an electrical signal by the same photo-transistor  32  referred to above. Although the same photo-transistor  32  is used according to the illustrated example for the detection of both the rise and the fall of light signal (or the start and the end of the light emission), two different photo-transistors may be used for the two separate purposes. 
         [0109]    Means for stopping emission of light from the external flash  11   a  by using the end of this preliminary light emission as the trigger is hereinafter referred to as the preliminary emission stopping means and will be described next more in detail by way of an example. 
         [0110]    The falling at the end of the preliminary light emission is detected by an emission stop signal detecting circuit  39 , adapted to detect the edge of a falling by means of a differential capacitor  34  and to AC-amplify its signal by transistor  36  through capacitor  35 . This AC-amplified signal is detected by transistor  38  through capacitor  37  to form an ON signal. 
         [0111]    In  FIG. 11 , numeral  113  indicates a mode-selecting switch. When the mode-selecting switch  113  is selecting the so-called TTL direct control (for effecting the preliminary emission and the main emission by a signal outputted from the camera), the preliminary emission stop signal is inputted from the emission stop signal detecting circuit  39  to a preliminary emission control circuit  108  to switch on transistor  109  and a voltage from the gate voltage generating circuit  23  is inputted to an integrating circuit  120  through resistor  111  and diode  110  for preventing the reverse current flow. Capacitor  112  is for increasing the switched-on time of the transistor  109 . 
         [0112]    The integrating circuit  120  is comprised of a speed-up resistor  121  of a known kind, an integrating capacitor  122  and a discharge resistor  123  and the integration process is carried out by resistor  111  and the integrating capacitor  122 . When the voltage of the integrating capacitor  122  reaches a specified level, an emission stop signal is outputted from an emission stop signal generating circuit  124  and inputted to the emission stop signal input circuit  30 . This signal is further inputted to the light emission control circuit  29  such that the light emission is immediately stopped. 
         [0113]    The emission stop signal generating circuit  124  serves also as means for correcting exposure and is comprised of a comparator  126 , a resistor  125  for generating a comparison voltage, a resistor group  127  and a voltage-setting switch  128 . The resistor group  127  may be replaced by a variable resistor  129 . The operating voltage of the comparator  126  may be set within a range of 0.1V (by setting the switch  128  to the farthest left) to 3V (by setting the switch  128  to the farthest right) such that the preliminary emission control circuit  108  will function if the operating voltage is at the lowest set value (0.1V) and a signal will be outputted immediately from the comparator  126 , that is, the integrating circuit  120  becomes 0.1V almost immediately. 
         [0114]    In summary, this exposure correcting means is provided with a variable means (an input means) such as the voltage-setting switch  128  which may be gradually rotated to the right such that the operating point of the comparator  126  becomes gradually higher and the output timing of the signal becomes delayed. Thus, as the external flash is activated by this signal, the timing for stopping the light emission from the discharge tube  28  is varied by the exposure correcting means and the waveform of the emitted light from the discharge tube  28  changes as shown in  FIG. 12  from  131  to  132  to  133  to  134  to  135 . In other words, the value of the resistor  111  and the setting of the integrating circuit  120  are adjusted in an optimum way.  FIG. 12  is merely intended to show how the waveform will change, not the optimum waveform itself. The flash time of the waveform  131  is made short because the start of the light emission is delayed for the reason given below but the delay in the stopping of the light emission is small. 
         [0115]    Although  FIG. 11  shows an example wherein it is a resistor group of a comparator that is varied for the adjustment, this is not intended to limit the scope of the invention. Although not separately shown with an illustration, it goes without saying that many other circuit structures are possible for effecting such an adjustment. For example, the integrating circuit may be provided with a plurality of capacitors from which an adequate one is to be selected for the adjustment. As still another example, a known circuit as shown in  FIG. 10  with transistor  61  may be used instead of a comparator for making use of the switched-on characteristics of this transistor  61 . A digital circuit or a microcomputer with a similar function may further be used instead for the same purpose. 
         [0116]    Effects of the change in the waveform of light emission from the discharge tube  28  as shown in  FIG. 12  will be further explained next. In the case of a digital camera of the type which determines the exposure of the main exposure based on the preliminary emission is adapted to measure the amount of reflected light from the target object to be photographed at the time of the preliminary emission and to determine the main exposure based on this measured amount of the reflected light. Thus, if the quantity of light at the time of preliminary emission is reduced from the optimum value, the reflected light from the target object is reduced and the exposure correcting means functions so as to increase the light of quantity at the time of the main emission. Conversely, if the quantity of light at the time of preliminary emission is increased from the optimum value, the reflected light from the target object is increased and the exposure correcting means functions so as to reduce the quantity of light at the time of the main exposure. This principle may be used such that the exposure correction can be effected simply by means of an operating knob on the external flash. 
         [0117]    In this case, the timing for stopping the main light emission need not be made variable, unlike the stopping of the preliminary light emission which is made variable as explained above. When the emission stop signal is received for stopping the main light emission, a timer  101  outputs a plus signal for about 0.2 seconds and a delay-on circuit  103  serves to transmit the signal directly without causing any delay. 
         [0118]    At the moment of the preliminary light emission of the internal flash, the delay-on circuit  103  does not function because the integration circuit formed by resistor  105  and capacitor  106  prevents any base voltage from being applied to transistors  107  and  104 . Thereafter before the time of the main light emission, however, a voltage is applied to the capacitor  106  and the transistors  107  and  104  come to be switched on and the resistor  111  is shorted. Thus, a voltage is immediately generated in the integration circuit  120  at the time of the main emission and there is no time delay. 
         [0119]    The aforementioned circuit structure including the timer  101  and the delay-on circuit  103  is only a preferred embodiment. This circuit structure is not intended to limit the scope of the invention and is not intended to be indispensable. In situations where the variable time due to the means for varying the timing of the preliminary light emission is relatively small, for example, its effect may be sufficiently small. 
         [0120]    In other aspects, the main emission is similar to the preliminary emission and is stopped as shown by numeral  78  in  FIG. 5 . In summary, since the waveform of the external flash is corrected according to the waveform of the internal flash, a correct exposure is possible although the light from the internal flash does not reach the target object to be photographed. 
         [0121]      FIG. 13  shows an example of the operating portion of the external flash of this invention for underwater photography. Since the shafts of operating knobs must be adequately waterproofed while there is a limited space on the flash, it is preferable to limit the number of operating knobs to be no more than two. Thus,  FIG. 13  shows a back surface  99  of an external flash provided with two operating knobs  92  and  96 , a battery cover  97  and a ready light  98  for indicating when the battery has been fully charged. Since the available space is limited, it is preferable to provide each of the knobs  92  and  96  with as many functions as possible. Some of these functions will be explained with reference to the circuit diagram of  FIG. 11 . 
         [0122]    As explained above, if the TTL control is selected by the mode-selecting switch  113 , the voltage-setting switch  128  functions as an exposure correcting means but there are also automatic and manual control modes that can be selected. According to this invention, this is made possible by using the emission stop signal generating circuit  124  and the integrating circuit  120  in common. 
         [0123]    If the automatic (AUTO) control is selected by the mode-selecting switch  113 , the aforementioned TTL connection is interrupted but the connection is made instead to a photo-transistor  114  to form an automatic control circuit structure of a conventional type. As is well known, as reflected light of the external flash reflected from a target object to be photographed is received by the photo-transistor  114 , the received light is converted into an electrical signal. This electrical signal is integrated by the integrating circuit  120  and when the integrated value reaches a specified value representing a correct exposure, a signal is outputted from the emission stop signal generating circuit  124  and the light emission from the discharge tube  28  is stopped as explained above. If the resistor group  127  is appropriately designed and used in conjunction with the voltage-setting switch  128 , a plurality of exposures can be provided. 
         [0124]    If the manual (MANUAL) control is selected by the mode-selecting switch  113 , a resistor  117  is used instead of the photo-transistor  114 . The resistance of this resistor  117  is adjusted to an optimum value and the resistor group  127  of the emission stop signal generating circuit  124  is further varied such that the timing for stopping the light emission from the external flash is modified such that the used quantity of light is gradually changed to an optimum level. Since the variable range cannot be optimized by the resistor  117  alone, a resistor  115  and a capacitor  119  are provided as a manual control circuit  116  to correct the rise portion and a neighboring portion of the waveform of the light emission. Their values may preferably be determined experimentally. 
         [0125]    In  FIG. 11 , numeral  102  indicates a preliminary emission canceling circuit. The preliminary emission canceling circuit  102  does not function when the TTL mode is selected by the mode-selecting switch  113  but is operated when the automatic or manual mode is selected since there is no need for emission of light when the internal flash undergoes the preliminary emission. 
         [0126]    According to the example described above, a single knob is required to make a selection out of three available modes. In the example of  FIG. 13 , the voltage-setting switch  128  is connected to the knob  96  and the mode-selecting switch  113  is connected to the knob  92 . 
         [0127]      FIG. 13  shows the knob  96  when it is usable for the correction of exposure ( 93 ), changing the light quantity in the manual mode of control ( 94 ) and changing the light quantity in the automatic mode of control ( 95 ). This, however, is not intended to limit the scope of the invention. The knob  96  may be used for a limited number of selections.  FIG. 14A  shows an example wherein the knob  96  is for the exposure control ( 93 ) and the changing of light quantity in the automatic mode of control ( 95 ).  FIG. 14B  shows another example wherein the knob  96  is for the exposure control ( 93 ) and the changing of light quantity in the manual mode of control ( 94 ).  FIG. 14C  is still another example wherein the knob  96  is for changing the light quantity in the automatic and manual control ( 93  and  94 ).  FIG. 14D  is a further example wherein the knob  96  is merely for the exposure correction. 
         [0128]    The invention has been described by way of a limited number of examples with different aspects. These examples are not intended to limit the scope of the invention. Many different combinations of these many aspects of the invention are also intended to be within its scope. For example,  FIG. 15  shows still another embodiment of the invention, which is similar to the embodiment described above with reference to  FIG. 10  but different in that the integrator circuit of the preliminary emission signal generating circuit  63  includes a resistor group  156 . In other words, the circuit structure shown in  FIG. 10  is intended to control the stopping of the preliminary emission from the external flash by a proper adjustment. According to the circuit structure shown in  FIG. 15 , the preliminary emission from the external flash is stopped after a preliminarily set time. 
         [0129]    As the internal flash undergoes a preliminary light emission, the emission start signal detecting circuit  45  is activated as explained above and a preliminary emission start signal is outputted and inputted to a timer  53  and preliminary emission signal generating circuit  63 . In response, the timer  53  outputs a positive voltage of duration about 0.2 seconds. The transistor  55  within the preliminary emission signal generating circuit  63 , serving now as exposure correcting means, becomes switched on, causing transistor  61  to be switched on after a specified length of time by an integrator circuit comprising the resistor group  156  and capacitor  60 , inputting a preliminary emission stop signal to the emission stop signal input circuit  30  and thereby stopping the preliminary emission. The capacitor  54  serves to increase the switched-on time of the transistor  55 . 
         [0130]    The resistor group  156  is provided with a resistor-selecting switch  64  and functions as a variable means (input means). As the connected resistor is gradually varied by this means, the timing for stopping the preliminary emission can be varied as described above with reference to  FIG. 12 . 
         [0131]    As explained above with reference to  FIG. 10 , capacitor  52  connected to the timer  53  is for preventing the generation of a stop signal at the time of preliminary emission of the emission stop signal detecting circuit  39  by inputting a signal to transistor  51  to inhibit the circuit. Since this capacitor  52  forms a differential circuit adapted to output a signal only at the time of a rise of the timer  53 , it is not activated at the time of the main emission and hence the transistor  51  is not switched on. Thus, there is no inhibit and the emission stop signal detecting circuit  39  is activated when the main emission of the internal flash is stopped and a signal is transmitted therefrom. 
         [0132]    According to this invention, in summary, the marine snow phenomenon can be prevented by using a filter on the inner flash and the aforementioned disadvantages of the prior art technologies can be obviated because the reflected light from of the external flash is measured through the camera lens. Furthermore, the operations of the external flash are made easy while the error in measuring light can be reduced.