Abstract:
An image pickup apparatus having:a plurality of photoelectric conversion elements each generating an electric signal through photoelectric conversion; a plurality of signal storage elements for storing the electric signals generated by the photoelectric conversion elements; a plurality of gates for reading the electric signals generated by the photoelectric conversion elements and storing the read electric signals in the signal storage elements; a controller for performing a first image pickup operation by making the photoelectric conversion elements generate the electric signals, reading the generated electric signals and storing the read electric signals in the signal storage elements, thereafter performing a second image pickup operation under an image pickup condition different from the first image pickup operation by making the photoelectric conversion elements generate the electric signals, thereafter outputting the electric signals generated by the first image pickup operation and stored in the signal storage elements to an external, and thereafter reading the electric signals generated by the second image pickup operation from the photoelectric conversion elements, storing the read electric signals in the signal storage elements, and outputting the electric signals to the external; and a synthesizing unit for generating an image signal by subjecting the output electric signals generated by the first and second image pickup operations to a white clip process and synthesizing the output electric signals.

Description:
This application is based on Japanese patent application No. HEI 10-313334, filed on Nov. 4, 1998, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The present invention relates to an image pickup apparatus and more particularly to an image pickup apparatus capable of picking up an image in a wide dynamic range and a method of controlling the image pickup apparatus. 
     b) Description of the Related Art 
     A solid state image pickup device has photodiodes disposed in a two-dimensional matrix shape and can take a two-dimensional image. Each photodiode corresponds to a pixel of an image. 
     FIG. 11 is a graph showing the photoelectric conversion characteristics of photodiodes of a solid state image pickup device. The abscissa represents the amount of light incident upon a photodiode, and the ordinate represents the voltage of a signal output from the photodiode. Characteristic curves A 1 , A 2  and A 3  show the photoelectric conversion characteristics of first, second and third photodiodes of the same solid state image pickup device. 
     Each of the characteristic curves A 1  to A 3  has a linear region R 1  with a small incidence light amount and a saturated region R 2  with a large incidence light amount. In the linear region R 1 , an output voltage is proportional to an incidence light amount. In the saturated region R 1 , the output voltage corresponding to incidence light is saturated. 
     The characteristics A 1  to A 3  of the photodiodes are the same in the linear region R 1 , whereas they are different in the saturated region R 2 . In the saturated region, the levels of output voltages of the photodiodes become irregular. In order to forcibly convert an output voltage of Vw or higher into a voltage Vw, a white clip process is performed. 
     If the white clip process is performed, the linear region R 1  is only a region which can be used for photoelectric conversion. The dynamic range capable of photoelectric conversion is therefore determined basically by the width of the linear region R 1 . 
     Solid state image pickup devices are used with digital still cameras and video cameras. The dynamic range of a solid image pickup device is very narrow as compared to that of human eyes and a photographic film. A narrow dynamic range may cause white or black crushed areas in an image. 
     In order to avoid this, techniques are known by which an image is picked up two times at different exposure times and the two images are synthesized. The details of the techniques will be described with reference to FIGS. 12A to  12 C. 
     Similar to FIG. 11, the abscissa of FIGS. 12A to  12 C represents an incidence light amount and the ordinate represents an output voltage. 
     FIG. 12A is a graph showing the photoelectric conversion characteristics used by a first image pickup operation of long-time exposure. Since the exposure time is long, even if the incidence light amount per unit time is small, an output voltage is large. Therefore, the photoelectric conversion characteristics are subjected to the white clip process at a voltage Vw. 
     FIG. 12B is a graph showing the photoelectric conversion characteristics used by a second image pickup operation of short-time exposure. Since the exposure time is short, an output voltage for an incidence light amount per unit time is lower than that obtained by the characteristics (FIG. 12A) for the long-time exposure. The photoelectric conversion characteristics shown in FIG. 12B are also subjected to the white clip process. 
     FIG. 12C is a graph showing the photoelectric conversion characteristics obtained by synthesizing the first image pickup photoelectric conversion characteristics (FIG. 12A) and the second image pickup photoelectric conversion characteristics (FIG.  12 B). For example, the synthesizing method is a simple addition of the two characteristics. 
     By using the synthesized photoelectric conversion characteristics, the dynamic range of the solid state image pickup device can be broadened. Therefore, irrespective of whether the incidence light amount is large or small, all photodiodes of a solid state image pickup device can have the uniform photoelectric conversion characteristics. 
     In the synthesized photoelectric conversion characteristics, a slope in the large incidence light amount region is gentler than that in the small incidence light amount region. The characteristics with different slopes are approximately equal to the human visual sense characteristics. Therefore, any practical problem will not occur even if the synthesized characteristics are used with a solid image pickup apparatus. 
     The first image pickup operation of long-time exposure and the second image pickup operation of short-time exposure may be performed in a reverse order. 
     Next, the operation of the solid image pickup device performing the above process will be described with reference to FIGS. 13 to  17 . In FIGS. 13 to  17 , a hatched area is an area where electric charges are stored. 
     FIG. 13 is a plan view of an all-pixel read type solid state image pickup device. Signals of all pixels (photodiodes) can be read to an external at the same time as one frame image. 
     The solid image pickup device has: photodiodes  51  disposed in a two-dimensional matrix shape for photoelectric conversion; vertical charge transfer paths (VCCD)  52  for transferring electric charges in a vertical direction; a horizontal charge transfer path (HCCD)  53  for transferring electric charges in a horizontal direction; and an output amplifier  54  for outputting a voltage corresponding to electric charges to an external. 
     First, as shown in FIG. 13, an image pickup operation of long-time exposure is performed to store electric charges of a first image in the photodiodes  51 . 
     Next, as shown in FIG. 14, the electric charges of the first image stored in the photodiodes  51  are read and stored in the right side vertical charge transfer paths  52 . After this data read, the first image pickup operation of long-time exposure is terminated, and an image pickup operation of short-time exposure starts for a second image. 
     Next, as shown in FIG. 15, the electric charges of the first image on the vertical charge transfer paths  52  are transferred downward to the horizontal charge transfer path  53 . The horizontal charge transfer path  53  transfers the received electric charges from the right side to the left side to the output amplifier  54 . The output amplifier  54  outputs a voltage corresponding to the received electric charges. Namely, it outputs a signal of the first image. 
     During this period, as shown in FIG. 16, electric charges for the second image are being stored in the photodiodes  51  by the second image pickup operation of short-time exposure which started immediately after the data read operation shown in FIG.  14 . 
     Next, as shown in FIG. 17, the electric charges of the second image stored in the photodiodes  51  are read and stored in the right side vertical charge transfer paths  52 . After this data read, the second image pickup operation of short-time exposure is terminated. 
     Next, similar to FIG. 15, the electric charges of the second image in the vertical charge transfer paths  52  are transferred downward to the horizontal charge transfer path  53 . The horizontal charge transfer path  53  transfers the received electric charges from the right side to the left side to the output amplifier  54 . The output amplifier  54  outputs a voltage corresponding to the received electric charges. Namely, it outputs a signal of the second image. 
     Thereafter, the first and second images are synthesized as illustrated in FIGS. 12A to  12 C. 
     The operation of the solid state image pickup device described above does not pose any problem so long as the subject is stationary. However, if the subject is moving, the following problem occurs. Since there is a long time between the first image pickup operation of long-time exposure and the second image pickup operation of short-time exposure, the position of the subject during the first image pickup operation becomes different from that of the subject during the second image pickup operation. The synthesized image therefore has a blurred subject image. 
     This problem can be solved by shortening the exposure time of the second image pickup operation. The exposure of the second image pickup operation starts immediately after the data read operation shown in FIG.  14  and terminates immediately after the data read operation shown in FIG.  17 . During this period, charges of the first image are transferred. Therefore, the exposure time for the second image pickup operation cannot be shortened more than the charge transfer time of the first image. The charge transfer time for one frame image is about {fraction (1/60)} to {fraction (1/15)} second. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image pickup apparatus capable of picking an image of a moving subject in a broad dynamic range and with a high image quality. 
     According to one aspect of the present invention, there is provided an image pickup apparatus comprising: a plurality of photoelectric conversion elements each generating an electric signal through photoelectric conversion; a plurality of signal storage elements for storing the electric signals generated by the photoelectric conversion elements; a plurality of gates for reading the electric signals generated by the photoelectric conversion elements and storing the read electric signals in the signal storage elements; control means for performing a first image pickup operation by making the photoelectric conversion elements generate the electric signals, reading the generated electric signals and storing the read electric signals in the signal storage elements, thereafter performing a second image pickup operation under an image pickup condition different from the first image pickup operation by making the photoelectric conversion elements generate the electric signals, thereafter outputting the electric signals generated by the first image pickup operation and stored in the signal storage elements to an external, and thereafter reading the electric signals generated by the second image pickup operation from the photoelectric conversion elements, storing the read electric signals in the signal storage elements, and outputting the electric signals to the external; and synthesizing means for generating an image signal by subjecting the output electric signals generated by the first and second image pickup operations to a white clip process and synthesizing the output electric signals. 
     After the first and second image pickup operations are performed, the electric signals generated by the first and second image pickup operations are output to the external. The second image pickup time can be shortened without being limited by the transfer time of the electric signals generated by the first image pickup operation. Since the second image pickup time can be shortened, a blurred subject in images obtained by the first and second image pickup operations can be suppressed and the image signals of a high quality can be obtained. 
     Since the electric signals generated by the first and second image pickup operations are subjected to a white clip process and synthesized, the image signals can be obtained in a broad dynamic range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the structure of an image pickup apparatus according to an embodiment of the invention. 
     FIG. 2 is a cross sectional view of a solid state image pickup device. 
     FIG. 3 is a timing chart illustrating the operation of the image pickup apparatus. 
     FIGS. 4 to  10  are plan views of a solid state image pickup device illustrating first to seventh operations of the device according to the embodiment. 
     FIG. 11 is a graph showing the photoelectric conversion characteristics of photodiodes of a solid image pickup device. 
     FIGS. 12A to  12 C are graphs illustrating an image synthesizing process which broadens a dynamic range of a solid image pickup device. 
     FIGS. 13 to  17  are plan views of a solid state image pickup device illustrating first to fifth operations of the device according to conventional techniques. 
     FIGS. 18A and 18B are circuit diagrams of a MOS sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a diagram showing the structure of an image pickup apparatus (e.g., a digital camera) according to an embodiment of the invention. 
     A lens  1  focusses an image of a subject  10  on a solid state image pickup device  3  when a mechanical shutter  2  is opened. The mechanical shutter  2  is controlled to be opened and closed in response to a signal MC supplied from a camera control section  11 . While the mechanical shutter  2  is opened, light from the subject  10  reaches the solid state image pickup device  3 . While the mechanical shutter  2  is closed, light from the subject  10  is intercepted and does not reach the solid state image pickup device  3 . 
     A shutter button  13  is activated by a photographer. After the shutter button  13  is depressed, the camera control section  11  controls the mechanical shutter  2 , a CCD driver  4  and an image processing section  5 . A power source  12  supplies electric power to the image pickup apparatus to drive it. A flash lamp FL radiates illumination light in response to a signal from the camera control section  11 . 
     The CCD driver  4  controls the solid state image pickup device  3 . The solid state image pickup device  3  generates an image signal corresponding to the incidence light amount of the subject and supplies it to the image processing section  5 . The image processing section  5  has an A/D converter  6 , two frame memories  7  and  8  and an image synthesizing processor  9 . 
     The solid state image pickup device  3  has photodiodes and charge transfer paths (CCD). Photodiodes correspond to pixels and two-dimensionally disposed in vertical and horizontal directions. The photodiodes convert light applied to a light receiving area into electric charges to perform so-called photoelectric conversion. The charge transfer path transfers electric charges converted by each photodiode. The electric charges are generally converted into a voltage which is supplied to an A/D converter  6 . The A/D converter  6  converts an analog voltage supplied from the solid state image pickup device into a digital voltage. 
     When a photographer depresses the shutter button  13  to pick up one image, the solid state image pickup device  3  outputs first and second image signals of an image picked up at different exposure times. 
     Each of the memories  7  and  8  can store an image of one frame. The memory  7  stores the first image signals, and the memory  8  stores the second image signals. 
     The image synthesizing processor  9  synthesizes the first and second image signals stored in the memories  7  and  8  to form image signals in a broad dynamic range. For example, the method of synthesizing image signals is a simple addition of image signals or an addition of image signals after they are weighted by predetermined coefficients. 
     FIG. 4 is a plan view of the solid state image pickup device  3 . 
     There are two types of the solid state image pickup device, an all-pixel read type and an interlace type. The solid state image pickup device  3  of this embodiment is an all-pixel read type and can read at the same time signals of all pixels (photodiodes) as one frame image and output them to an external. An interlace type solid state image pickup device first reads pixels of odd lines as a first frame image, and then reads pixels of even lines as a second frame image. The first and second frame images are synthesized to form image signals of one frame. 
     The solid image pickup device  3  has: a plurality of photoelectric conversion elements (e.g., photodiodes)  31  disposed in a two-dimensional matrix shape for photoelectric conversion; vertical charge transfer paths (VCCD)  32  for transferring electric charges in a vertical direction; a horizontal charge transfer path (HCCD)  33  for transferring electric charges in a horizontal direction; and an output amplifier  34  for outputting a voltage corresponding to electric charges to an external. The vertical and horizontal charge transfer paths  32  and  33  are both made of charge coupled devices (CCDs). 
     The vertical charge transfer path  32  is driven by a drive signal φV, and the horizontal charge transfer path  33  is driven by a drive signal φH. 
     The all-pixel read type solid state image pickup device  3  has at least one charge transfer stage (charge transfer packet) in the vertical charge transfer path  32  in an area corresponding to each photodiode  31 . In order to provide one charge transfer stage per one photodiode  31 , it is necessary to form three or more electrodes in the vertical charge transfer path in the area corresponding to each photodiode  31  and to drive the vertical charge transfer path by three or more phases. The charge transfer stage is the unit of partitioning the charge transfer path. When a plurality of pulse signals having different phases are applied to charge transfer electrodes consecutively formed along the charge transfer path, electric charges are transferred in the minimum charge transfer area. This minimum charge transfer area is called one charge transfer stage. If an n-phase drive (n is an integer of 2 or larger) is used, the area where n consecutive charge transfer electrodes are formed is called one charge transfer stage. 
     FIG. 2 is a cross sectional view of the solid state image pickup device  3  shown in FIG.  4  and taken along line II—II. 
     A p-type well  22  is formed in a surface layer of an n-type silicon substrate (semiconductor substrate)  21 . In a surface layer of the p-type well  22 , an n-type region  23  constituting the photodiode  31  and an n-type region  24  constituting the vertical charge transfer path  32  are formed. Between the n-type region  24  and another n-type region  23 , a p + -type region  25  constituting a channel stopper region is formed. 
     A shift gate electrode  27  is formed on an insulating film (e.g., silicon oxide film)  26  over the n-type region  24  constituting the vertical charge transfer path  32 . A shift gate signal SG is applied to the shift gate electrode  27 . As the shift gate signal SG of a positive potential equal to or larger than a predetermined value is applied, electric charges accumulated in the photodiode  31  are read and stored in the vertical transfer path  32 . 
     The shift gate electrode  27  functions also as a charge transfer electrode. As a drive signal φV is applied to the charge transfer electrode  27 , the vertical charge transfer path  32  transfers the electric charges. The drive signal φV is a pulse having a ground potential as one level and a predetermined negative potential as the other level. 
     The n-type substrate  21  is applied with a substrate potential VOD. The p-type well  22  is grounded. 
     Light  28  becomes incident upon the photodiode  31 . This light applied to the photodiode  31  generates electric charges in the n-type region  23 . As electric charges are accumulated too much in the n-type region, some of electric charges in the n-type region  23  overflows to the n-type substrate  21 . This structure is called a vertical overflow drain structure. 
     As the substrate voltage VOD is raised to a predetermined value or higher, electric charges in the photodiode  31  can be drained to the n-type substrate  21  so that the photodiode  31  can be initialized. This operation is called an electronic shutter. After the electronic shutter is activated, charge accumulation in the photodiodes can be started. 
     FIG. 3 is a timing chart illustrating the operation of the image pickup apparatus (digital camera) shown in FIG.  1 . 
     The mechanical shutter signal MC controls to open and close the mechanical shutter  2  (FIG.  1 ). While this signal MC takes a high level, the mechanical shutter  2  is opened, and while the signal MC takes a low level, the mechanical shutter  2  is closed. 
     The substrate voltage VOD has two voltage levels V 1  and V 2 . At the voltage level V 2 , electric charges stored in the photodiode  31  are drained to the substrate to thereby initialize the photodiode  31 . At the voltage level V 1 , the overflow drain function is enabled. 
     At the positive potential of the shift gate signal SG, electric charges in the photodiode  31  can be read and stored in the vertical charge transfer path  32 , and at the negative potential of the charge transfer signal φV, electric charges in the vertical charge transfer path  32  can be transferred. 
     A charge transfer signal φH is used for transferring electric charges in the horizontal charge transfer path  33 . 
     A first exposure time T 1  for a first image pickup operation is a time from when a pulse V 2  is supplied as the substrate voltage VOD to when a pulse V 12  is supplied as the shift gate signal SG. A second exposure time T 2  for a second image pickup operation is a time from when the pulse V 12  is supplied as the shift gate signal SG to when the mechanical shutter signal MC takes the low level. 
     The operation of the solid state image pickup device  3  will be described with reference to FIGS. 4 to  10 . 
     First, as shown in FIG. 4, a first exposure (e.g., short-time exposure) for a first image starts to accumulate electric charges in the photodiodes  31 . 
     This operation corresponds to a timing t 1  shown in FIG.  3 . The mechanical shutter signal MC is maintained at a high level to maintain the open state of the mechanical shutter  2  (FIG.  1 ). The mechanical shutter  2  is normally open. 
     At the timing t 1 , the substrate voltage VOD is changed from the voltage V 1  (e.g., 10 V) to the voltage V 2  (e.g., 25 to 39 V) (electronic shutter function) to initialize the photodiodes  31 . Thereafter, the substrate voltage VOD takes again the voltage V 1 . Upon initialization of the photodiodes  31 , the first exposure time T 1  starts. 
     Next, as shown in FIG. 5, electric charges of the first image stored in the photodiodes  31  are read and stored in the right vertical charge transfer paths  32 . 
     This operation corresponds to a timing t 2  shown in FIG.  3 . The shift gate signal SG is changed from the voltage V 11  (e.g., 0 V) to the voltage V 12  (e.g., 15 V) to transfer the electric charges in the photodiodes  31  to the vertical charge transfer paths  32 . Thereafter, the shift gate signal SG takes again the voltage V 11 . Upon this charge transfer, the first exposure (e.g., short-time exposure) T 1  for the first image is terminated and a second exposure (e.g., long-time exposure) T 2  for a second image starts. 
     Next, as shown in FIG. 6, the mechanical shutter  2  (FIG. 1) is closed to terminate the second exposure. At this time, the electric charges of the first image are being stored in the vertical charge transfer paths  32 , whereas the electric charges of the second image are being stored in the photodiodes  31 . 
     This operation corresponds to a timing t 3  shown in FIG.  3 . The level of the mechanical shutter signal MC is changed from the high level to the low level to close the mechanical shutter  2  (FIG.  1 ). When the mechanical shutter  2  is closed, the second exposure time T 2  is terminated. 
     Next, as shown in FIG. 7, the electric charges of the first image stored in the vertical charge transfer paths  32  are transferred downward to the horizontal charge transfer path  33 . The horizontal charge transfer path  33  transfers the received electric charges to the left and supplies them to the output amplifier  34 . The output amplifier  34  outputs voltages corresponding to the received electric charge amounts. Namely, it outputs first image signals. 
     During this charge transfer, the mechanical shutter  2  is closed so that new electric charges are not generated in the photodiodes  31  and therefore smear is not generated. This smear is the phenomenon that when strong light is applied to the photodiode, electric charges in the photodiode  31  leak to the vertical charge transfer path  32  and the image quality is degraded. 
     This operation corresponds to a timing t 4  shown in FIG.  3 . As the charge transfer signal φV, a pulse having a voltage V 11  (e.g., 0 V) as one level and a voltage V 13  (e.g., −8 V) as the other level is supplied. With this pulse, the electric charges in the vertical charge transfer path  32  are transferred in the vertical direction. 
     As the charge transfer signal φH, a predetermined pulse is supplied so that the electric charges in the horizontal charge transfer path  33  are transferred in a horizontal direction. 
     As shown in FIG. 8, the first image signal transferred and output from the output amplifier  34  is subjected to the white clip process shown in FIG.  11 . With this white clip process, an output voltage of Vw or higher is converted into a voltage Vw. The image signal subjected to the white clip process is written in the frame memory  7  (FIG.  1 ). 
     Next, as shown in FIG. 9, electric charges of the second image stored in the photodiodes  31  are read and stored in the right vertical charge transfer paths  32 . 
     This operation corresponds to a timing t 5  shown in FIG.  3 . The shift gate signal SG is changed from the voltage V 11  (e.g., 0 V) to the voltage V 12  (e.g., 15 V) to transfer the electric charges in the photodiodes  31  to the vertical charge transfer paths  32 . Thereafter, the shift gate signal SG takes again the voltage V 11 . 
     Next, as shown in FIG. 10, the electric charges of the second image stored in the vertical charge transfer paths  32  are transferred downward to the horizontal charge transfer path  33 . The horizontal charge transfer path  33  transfers the received electric charges to the left and supplies them to the output amplifier  34 . The output amplifier  34  outputs voltages corresponding to the received electric charge amounts. Namely, it outputs second image signals. 
     This operation corresponds to a timing t 6  shown in FIG.  3 . Predetermined pulses are supplied as the charge transfer signal φV and φH. With these pulses, the electric charges in the vertical charge transfer path  32  and horizontal charge transfer path  33  are transferred. 
     Next, the second image signal output from the output amplifier  34  is subjected to the white clip process. The image signal subjected to the white clip process is written in the frame memory  8  (FIG.  1 ). 
     Next, as shown in FIG. 12C, the first image signals in the frame memory  7  and the second image signals in the frame memory  8  are synthesized. With this synthesis, the dynamic range of the solid state image pickup device can be broadened. 
     The second exposure time T 2  starts when the electric charges in the photodiodes  31  are read and terminates when the mechanical shutter  2  is closed, as shown in FIG.  3 . During the second exposure time T 2 , the electric charges of the first image are being stored in the vertical charge transfer paths  32  and are not output to the external. It is not necessary to set the second exposure time T 2  equal to or longer than the charge transfer time of the first image. The second exposure time T 2  can therefore be shortened. 
     Since the second exposure time T 2  can be shortened, a time between the first and second image pickup operations can be shortened and a blurred subject image between the first and second images can be suppressed. Since the blurred subject image between the first and second images can be suppressed, the quality of a synthesized image can be improved. 
     The solid state image pickup device  3  is of the all-pixel read type. Therefore, in both the first and second image pickup operations, all pixels can be read and the image signals of a high quality can be generated. 
     The method of controlling a solid state image pickup device of this embodiment may be used together with the solid state image pickup controlling method illustrated with reference to FIGS. 13 to  17 . 
     Short-time and long-time exposures of the first and second exposures T 1  and T 2  may be reversed. The advantage that the first exposure is short-time exposure and the second exposure is long-time exposure will be described first. 
     Referring to FIG. 3, the first exposure time (short-time exposure) T 1  is, for example, {fraction (1/300)} second and the second exposure time (long-time exposure) T 2  is, for example, {fraction (1/30)} second. The second exposure time T 2  is terminated when the mechanical shutter  2  is closed. A time taken for the mechanical shutter  2  to completely close after it starts closing is preferably set to {fraction (1/10)} of the exposure time T 2  or shorter. For example, if the exposure time T 2  is {fraction (1/30)} second, the operation time of the mechanical shutter  2  is required to be {fraction (1/300)} second or faster. 
     If the second exposure time T 2  is short (e.g., {fraction (1/300)} second), the operation time of the mechanical shutter  2  is required to be {fraction (1/3000)} second or faster. It is therefore necessary to use a high speed mechanical shutter. A high speed mechanical shutter is expensive so that the image pickup apparatus (digital camera) also becomes expensive. 
     By using the first exposure of short-time and the second exposure of long-time, an economical low speed mechanical shutter  2  can be used and the cost of the image pickup apparatus (digital camera) can be reduced. 
     Next, the advantage that the first exposure is long-time exposure and the second exposure is short-time exposure will be described. 
     If the first exposure is short-time exposure and the second exposure is long-time exposure, it takes a long time to output the image signal obtained by short-time exposure to the external. Since white pixel defects (white spots) of an image and dark current increase as the output time of the image signal prolongs, the image quality is degraded and the manufacture yield of solid state image pickup devices is lowered. 
     If the second exposure is short-time exposure, the time taken to output the image signal obtained by short-time exposure can be shortened. Since the white pixel defects of an image and the like can be reduced, the image quality and the manufacture yield of solid state image pickup devices can be improved. 
     Next, modifications of the embodiment will be described. In the above embodiment, although two image pickup operations are performed at different exposure times, two image pickup operations may be performed under different image pickup conditions without being limited only to the exposure time. Examples of different image pickup conditions will be described. 
     (1) Exposure time 
     In the above embodiment, the exposure time is set differently by using a combination of the mechanical shutter and electronic shutter. The exposure time may be set differently by other methods. 
     (2) Subject illuminance 
     For example, a variable light source may be used to differently set the illuminance of the first and second image pickup operations, or a flash lamp may be used to differently set the subject illuminance. 
     (3) Optical system transmittance 
     A neutral density (ND) filter attenuates light over the whole range of wavelengths. If a neutral filter  2 ′ is set at the same position as, or at the juxtaposed position with, the mechanical shutter  2  shown in FIG. 1, the transmittance of the optical system along the incidence light path to the photodiodes can be set differently. Similarly, if a liquid crystal device is set at the same position as the mechanical shutter  2  or neutral filter  2 ′, the optical system transmittance can be set differently. 
     (4) Stop 
     If the diameter of a stop of a digital camera is made large, an image incident upon a solid state image pickup device can be made bright, whereas if the diameter of the stop is made small, an image incident upon the solid state image pickup device can be made dark. The two image pickup operations can be performed by differently setting the stop diameters. 
     (5) Number of exposures 
     The two image pickup operations may be performed by differently setting the number of exposures. The number of exposures can be differently set by changing the number of times when a mechanical shutter is opened. For example, the mechanical shutter is opened once during the first image pickup operation, and it is opened three times during the second image pickup operation. The number of exposures may be changed by changing the number of times when a flash lamp is turned on. 
     Instead of a solid state image pickup device having a charge coupled device (CCD), a MOS sensor may also be used. 
     FIG. 18A shows the structure of a MOS sensor. A MOS sensor has a plurality of cells CL disposed two-dimensionally on the same substrate. Each cell CL can be accessed by first and second addresses AD 1  and AD 2 , similar to a RAM. 
     FIG. 18B shows the structure of each cell CL. Similar to a DRAM, each cell CL has a photodiode PD, a read gate MOS transistor TR 1 , a capacitor C 1 , and an output MOS transistor TR 2 . Electric charges accumulated in the photodiode PD are read when the transistor TR 1  is made conductive, and stored in the capacitor C 1 . The electric charges stored in the capacitor C 1  are output to an external by the transistor TR 2 . This capacitor C 1  corresponds to the vertical charge transfer path  32  of the solid state image pickup device shown in FIG.  4 . 
     The image pickup apparatus of this embodiment is not limited only to a digital camera, but it may be an image scanner, a line sensor, a video camera or the like. The image pickup apparatus of this embodiment is particularly suitable for picking up a still image. 
     The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It is apparent that various modifications, improvements, combinations, and the like can be made by those skilled in the art.