Abstract:
An imaging device includes an optical imaging system, a solid-state image sensor which converts an optical image of a subject formed by the optical imaging system into an electric image signal, an exposure controller which starts an exposure in response to an instruction to start shooting, an image reader which sequentially read image signals from the solid-state image sensor while the exposure controller continues the exposure, an image processor which sequentially processes the image signals read by the image reader, a display unit which displays image data output from the image processor; and an image display processor which allows the display unit to sequentially display image data processed by the image processor with a predetermined time interval.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from Japanese Patent Application No. 2009-256601, filed on Nov. 10, 2009, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging device such as a digital camera including a solid-state image sensor to acquire image data of a subject, in particular to an imaging device which performs good imaging control in a bulb mode and an imaging control method for such an imaging device. 
     2. Description of the Prior Art 
     In prior art there is a digital camera having a bulb mode in which a photo is taken in a long exposure time while a shutter is kept open (disclosed in Japanese Patent Application Publication No. 2002-27326 (Reference 1), for example). Such a digital camera has the same problem as a silver-salt camera that an operator cannot check a change in exposure amount on a display during the exposure to determine the right shooting timing with an intended exposure level. This requires for the operator to estimate an exposure time from his/her experience and check a reproduced image on a display. In a case that the image is not a successful shot, he/she has to re-take a photo. 
     In order to deal with such a problem, Japanese Patent Application Publication No. 2005-117395 (Reference 2), No. 2005-354166 (Reference 3), and No. 2009-130470 (Reference 4) disclose imaging devices which allow users to check a change in the exposure amount on a display by repetitively reading image data at a predetermined interval during exposure and adding a previously read frame and a currently read frame to update an image display. 
     Specifically, Reference 1 discloses a technique to display live preview images using a rolling shutter of a CMOS (complementary metal-oxide semiconductor) sensor. References 2 to 4 disclose imaging devices which display preview images in a bulb mode. 
     Recently, use of the CMOS sensor in an imaging device as described in Reference 1 has increased owing to a high resolution thereof, replacing a widely used CCD (charge coupled device). With use of the CCD, incident light need be blocked with a mechanical shutter during a transfer period even in the bulb mode. This leads to a problem that data of a subject is missed out during transfer periods which are repeated with a predetermined interval. For example, when capturing a moving subject such as fireworks, a captured image will be discrete trails. With use of another drive system instead of the mechanical shutter for the purpose of avoiding such a problem, a different problem as occurrence of smears arises. 
     In contrast, the rolling shutter of the CMOS image sensor can read image data with high resolution without transfer periods and it does not generate smears. Although the CMOS image sensor has an intrinsic drawback that an image of a moving subject may be distorted due to non-simultaneous exposure of the rolling shutter and transverse stripes may occur in a single image under flickering lights, these do not cause deterioration in image quality in the bulb mode in which exposure time is expected to be over 1 second. 
     Any of the above imaging devices cannot provide sufficient usability in bulb shooting and there is a demand for improving preview display in the bulb mode. 
     In the bulb shooting, exposure amount logarithmically rises relative to exposure time. Because of this, immediately after start of exposure, the exposure amount sharply rises and thereafter it gradually rises. In shooting a subject which takes about several seconds to reach an intended exposure amount, an operator needs to quickly decide the timing to complete the bulb exposure, following the sharp rise of the exposure amount while checking a change in the exposure amount on the display. On the other hand, in shooting a subject which takes over several dozen seconds to reach an intended exposure amount, an operator has to wait for the exposure amount to gradually rise to the intended amount. 
     There is one way to solve the above problem with the bulb mode that display update interval is set independently from data read interval during exposure. For instance, it is possible to reduce a change in the rising speed of the exposure amount on the display by updating image display with a longer interval when the rise in the exposure amount is sharp and updating it with a shorter interval when the rise in the exposure amount is gradual. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide an imaging device which updates an image on a display with a gradual change in the exposure amount with a constant time interval even when the exposure amount abruptly changes in reality by making an image display process independent from an image capturing process. This enables an operator to easily decide the right timing to complete the bulb exposure. The present invention also aims to provide an imaging control method for such an imaging device. 
     According to one aspect of the present invention, an imaging device comprises an optical imaging system; a solid-state image sensor which converts an optical image of a subject formed by the optical imaging system into an electric image signal; an exposure controller which starts an exposure in response to an instruction to start shooting; an image reader which sequentially reads image signals from the solid-state image sensor while the exposure controller continues the exposure; an image processor which sequentially processes the image signals read by the image reader; a display unit which displays image data output from the image processor; and an image display processor which allows the display unit to sequentially display image data processed by the image processor with a predetermined time interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, embodiments, and advantages of the present invention will become apparent from the following description, taken with reference to the accompanying drawings, in which: 
         FIG. 1  schematically shows the essential parts of an imaging device according to first to third embodiments of the present invention; 
         FIG. 2  is a flowchart for describing an imaging process in a bulb mode of the imaging device according to the first embodiment; 
         FIG. 3  is a flowchart for describing a display and storing process in the bulb mode of the imaging device according to the first embodiment; 
         FIG. 4  is a timing chart for the imaging process in the bulb mode of the imaging device according to the first embodiment; 
         FIG. 5  is a table showing specific values of time intervals in the bulb mode of the imaging device according to the first embodiment; 
         FIG. 6  shows the necessary number of image buffer memories and a change in the number of frames of image data in the bulb mode of the imaging device according to the first embodiment; 
         FIG. 7  is a graph showing a relation between an exposure and a preview display in the bulb mode of the imaging device according to the first embodiment; 
         FIG. 8  is a flowchart for describing an imaging and storing process in the bulb mode of the imaging device according to the second embodiment of the present invention; 
         FIG. 9  is a flowchart for describing a display process in the bulb mode of the imaging device according to the second embodiment; 
         FIG. 10  is a timing chart for the imaging process in the bulb mode of the imaging device according to the second embodiment; 
         FIG. 11  is a table showing specific values of time intervals in the bulb mode of the imaging device according to the second embodiment; 
         FIG. 12  is a graph showing a relation between an exposure and a preview display in the bulb mode of the imaging device according to the second embodiment: 
         FIG. 13  is a timing chart for the imaging process in the bulb mode of the imaging device according to a third embodiment; 
         FIG. 14  a table showing specific values of time intervals in the bulb mode of the imaging device according to the third embodiment; 
         FIG. 15  a graph showing a relation between an exposure and a preview display in the bulb mode of the imaging device according to the third embodiment; 
         FIG. 16  shows an example of an image display of the imaging device according to the embodiments of the present invention; and 
         FIG. 17  shows the image display of  FIG. 16  with a different exposure amount. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, a signal processing system is configured such that an image display process is independent from an image capturing process in a bulb mode in which image display is updated using the rolling shutter of the CMOS image sensor. This enables an image display with a gradual change in the exposure amount with a constant time interval even when the exposure amount abruptly changes in reality immediately after start of exposure. Accordingly, an operator is able to easily decide the right timing to complete the bulb exposure and reliably capture photo opportunity in the bulb shooting. Further, by setting a constant display update interval, an operator can easily estimate the timing at which exposure amount rises. 
     Furthermore, by use of the existing signal processing system, it is possible to update, with a constant interval, an image display with a pseudo increase in the exposure amount by repetitively amplifying a single image signal read during exposure. This makes it possible to smoothly update image display with a constant interval even when the exposure amount gradually increases, enabling an operator to decide an intended exposure level without a delay in bulb shooting. 
     Moreover, it can be configured that at start of exposure, image display is updated at a longer time interval with a gradual increase in the exposure amount irrespective of an abrupt increase in the actual exposure amount, and in a predetermined time after the start of exposure, it is updated at a shorter interval irrespective of a gradual increase in the actual exposure amount. Accordingly, this allows an operator to check an increase in the exposure at a constant time interval immediately after or several dozen seconds after the start of exposure in the bulb mode. 
     Further, the imaging device having a bulb mode preferably comprises a CMOS image sensor instead of a CCD image sensor. However, the present invention is also applicable to an imaging device including a CCD image sensor for capturing a subject such as a still life or scenery which does not include any moving objects and is not affected by the closing of the mechanical shutter during transfer periods. 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  schematically shows the essential parts of an imaging device as a digital camera according to first to third embodiments of the present invention. In  FIG. 1  the imaging device comprises an imaging unit  100 , an optical system driver  105 , an image processor  110 , a control/operation unit  120 , a memory  121  containing a program, a manipulation unit  122 , a display unit  123 , an image buffer memory  124 , a compression/decompression unit  125 , and a storage interface (hereinafter, storage I/F)  126 . The imaging unit  100  comprises an optical system  101 , a mechanical shutter  102 , an optical filter  103 , and a CMOS image sensor  104 . The CMOS image sensor  104  comprises a pixel unit  104   a , a scan unit  104   b , a sampling unit  104   c , and an AD converter (ADC)  104   d . The image processor  110  comprises a pre-processing unit  111 , an amplifying unit  112 , a post-processing unit  113 , a reducing unit  114 , a display processing unit  115 , and an adding unit  116 . The image processor  110 , control/operation unit  120 , image buffer memory  124 , compression/decompression unit  125  and storage I/F  126  are connected with each other via a bus line BL. 
     The optical system  101  is driven by the optical system driver  105  to perform focus adjustment and zooming. The mechanical shutter  102  is always open in the bulb mode according to the present invention since a shooting is made with a rolling shutter of the CMOS image sensor  104 ; however, it is closed in still image shooting at simultaneous exposure, in acquiring light-blocked image data, and at power-off. 
     In the CMOS image sensor  104 , the scan unit  104   b  driven by the control/operation unit  120  scans the pixel unit  104   a  on which light-receiving pixels are arranged in matrix, the sampling unit  104   c  samples image signals from the pixel unit  104   a , and the A/D converter  104   d  converts the image signals into digital signals for output. Outputs from the A/D converter  104   d  are processed by the image processor  110 . In bulb shooting, for example, upon press onto a not-shown shutter button of the manipulation unit  122 , the imaging device starts the bulb mode. The image signal read from the CMOS image sensor  104  is subjected to black level correction and defective pixel correction in the pre-processing unit  111  and then temporarily stored in the image buffer memory  124  via the bus line BL. 
     Next, the currently stored image signal and a previous added image based on previous image signals are read from the image buffer memory  124  and subjected to an adding processing by the adding unit  116  to create a new added image and store it in the image buffer memory  124 . The latest added image is read from the image buffer memory  124  and subjected to post-processing including any of interpolation, white balance process, gamma conversion, color balance process, and edge enhancement by the post-processing unit  113 , and stored in the image buffer memory  124  via the bus line BL. It is also sent to the following reducing unit  114 . The amplifying unit  112  is bypassed in the first embodiment while it is used in signal processing according to a later-described second embodiment. The reducing unit  114  reduces the size of image data to one adapted to specification of the display unit  123  and stores it in the image buffer memory  124 . Reduced image data is read from the image buffer memory  124  in accordance with display update timing, converted into a signal suitable for the display unit  123  by the display processing unit  115  and displayed on the display unit  123 . 
     In storing process, a single item of image data is extracted from plural items of image data post-processed by the post-processing unit  113  and stored in the image buffer memory  124 , compressed in JPEG format by the compression/decompression unit  125 , and stored in a storage medium such as an SD card via the storage I/F  126 . The control/operation unit  120  as an image reader properly controls the operations of the scan unit  104   b  of the CMOS image sensor  104 , optical system driver  105 , image processor  110  and the respective elements connected with the bus line BL according to a program in the memory  121 , using data from the manipulation unit  122  and image processor  110  as well as data provided via the bus line BL. 
     First Embodiment 
     A bulb shooting operation of the imaging device according to a first embodiment will be described with reference to  FIGS. 2-3 .  FIG. 2  is a flowchart for shooting process (step S 01  to S 14 ) in the bulb mode while  FIG. 3  is a flowchart for display and storing process (step S 21  to S 29 ). The imaging device comprises a two-stage (first and second) release switch which operates upon a half press and a full press to the shutter button (not shown) in the manipulation unit  122  and is configured to start bulb exposure upon the turning-on of the second release switch, continue the exposure during the on-state of the second release switch and complete the exposure upon turning-off of the second release switch in the bulb mode. 
       FIG. 2  shows an operation from readout of an image signal fn from the CMOS image sensor  104  to generation of a post-processed image Gn for storage and a reduced image G′n for display. In step S 01  the operation starts. In step S 02  a determination is made on whether or not turning-on of the second release switch is detected. Upon detection of the turning-on, exposure is started in step S 03 . Then, a first image signal f 1  is read from the CMOS image sensor  104 , subjected to pre-processing including black level correction and defective pixel correction in the pre-processing unit  111  to generate image data F 1  and store it in the image buffer memory  124  via the bus line BL in step S 04 . 
     In step S 05  the image data F 1  is subjected to the post-processing including interpolation, white balance process, gamma conversion, color balance process, and edge enhancement by the post-processing unit  113 . Post-processed image data G 1  is then stored in the image buffer memory  124  via the bus line BL. In step S 06  the image data G 1  is reduced in size adapted to the specification of the display unit  123  by the reducing unit  114 . The reduced image data is stored as display data G′ 1  in the image buffer memory  124  via the bus line BL. In step S 07  a determination is made on whether or not a first time interval has elapsed from the previous readout. With the first time interval elapsed, an image signal fn (n being positive integer; second and subsequent image signals) is read from the CMOS image sensor  104 , subjected to pre-processing by the pre-processing unit  111  to generate image data f′n and store it in the image buffer memory  124  via the bus line BL in step S 08 . In step S 09  the image data f′n is subjected to adding processing by the adding unit  116  and added with the previous added image data Fn−1 to be added image data Fn (Fn=f′n+Fn−1). The new added image data Fn is stored in the image buffer memory  124  via the bus line BL. 
     In step S 10  the added image data Fn is subjected to the post-processing by the post-processing unit  113  to generate image data Gn and store it in the image buffer memory  124  via the bus line BL. In step S 11  the image data Gn is subjected to the reducing processing by the reducing unit  114 . The reduced image data is stored as display image data G′n in the image buffer memory  124  via the bus line BL. Then, in step S 12  a determination is made on whether or not the turning-off of the second release switch is detected. Without the turning-off detected, the process goes back to step S 07 , stands by for elapse of the first time interval and repeats the above steps with the first time interval. By repeating the above steps, the added image data is accumulatively added to update the display image data G′n. Upon detection of turning-off of the second release switch in step S 12 , the exposure is completed in step S 13 , and the process ends in step S 14 . 
     Next, the flowchart in  FIG. 3  shows a display update operation in which display image data G′n based on accumulatively added image data are sequentially displayed on the display unit  123  with a second time interval as well as a still image storing in accordance with an exposure determined. Accordingly, readout of first image data G′ 1  is pre-set to start after the step S 06  in  FIG. 2 . 
     In step S 21  the process starts. In step S 22  first display image data G 1 ′ is read from the image buffer memory  124  via the bus line BL and converted into display data D 1  in a signal form adapted to the specification of the display unit  123  by the display processing unit  115  in step S 22 . In step S 23  the display data D 1  is displayed on the display unit  123 . Then, in step S 24  a determination is made on whether or not the second time interval has elapsed from the previous display. When the interval has elapsed, the second and subsequent display image data G′m (m being positive integer) is read from the image buffer memory  124 , and converted to display data Dm in a signal form adapted to the specification of the display unit  123  by the display processing unit  115  in step S 25 . In step S 26  the display data Dm is displayed on the display unit  123 . 
     In step S 27  a determination is made on whether or not the turning-off of the second release switch is detected. With no detection determined, the process returns to step S 24 , waiting for the elapse of the second time interval, and the above steps are repeated with the second time interval. With the turning-off of the second release switch detected, accumulative added image data corresponding to current display image data G′m is read from the image buffer memory  124 , compressed in JPEG form by the compression/decompression unit  125 , and stored in a storage medium via the storage I/F  126  in step S 28 . The process ends in step S 29 . 
       FIG. 4  is a timing chart of the operation in  FIG. 2 . In the drawing “vertical synchronous signal” and “exposure/readout in bulb mode” show operation timing of the CMOS image sensor  104 . “exposure/readout in bulb mode” shows operation timing of the rolling shutter. As seen from the drawing, the rolling shutter can intermittently read image data in continuous exposure. “image signal” shows read timing of an image signal and pre-processing timing of the pre-processing unit  111  of the image processor  110 . “adding processing” shows adding timing of the adding unit  116 . “post processing/reduction” shows post-processing timing and reduction timing of the post-processing unit  113  and the reducing unit  114 , respectively. “tRON” on a time axis corresponds to detection timing for turning-on of the second release switch, and “tROF” corresponds to detection timing for turning-off of the second release switch. At time “t 0 =0 sec”, exposure starts. Specific values of time t 1  to tn are shown in  FIG. 5 . 
       FIG. 5  shows an example of values of the first and second time intervals when the range of exposure time is 1 second (t 1 ) to 256 seconds (t 25 ), exposure amount rises by ⅓Ev and display is updated every two seconds. To increase the exposure time by ⅓Ev, a relation, tk=1.26×tk−1 need be satisfied. The first time interval Δtk in  FIG. 5  takes values calculated by Δtk=tk−(tk−1). The second time interval ΔT is fixed to 2 seconds in order to update image display every two seconds. Also, at t 16 =T 16 =32 sec., display update time catches up with the exposure time so that at t 16  and thereafter, the same exposure image is displayed on the display at plural times. By setting the image display to update at 2 second interval as above, an operator can take time to decide the right shooting timing even during Δt 2  to Δt 7  in which exposure amount quickly rises at an interval of 1 second or less. Note that there is actually a time lag between exposure start time t 0 =0 sec. and display update time T 0 =0 sec. and between exposure time tn and display of the time Tm (tn=Tm). However, it is not considered here since it is irrelevant to the features of the present invention. 
     Four kinds of image data of the image signal f′n, added image Fn, post-processed image Gn, and reduced display image G′n are stored in the image buffer memory  124  according to the flowcharts in  FIGS. 2 ,  3 . In commercialization of an imaging device, memory capacity is limited due to cost efficiency so that unnecessary data need be overwritten or deleted. A single image signal f′ and a single added signal F are enough for the signal processing according to the above flowcharts and the minimum necessary numbers of the post-processed images Gn and display images G′n depend on the first and second time intervals. In  FIG. 6  the number of buffer memories for the images Gn, G′n and contents of stored images in the timing chart of  FIG. 4  are shown, using display update time as a parameter. 
     In  FIG. 6  display image data G′m is converted into display data Dm, and the post-processed image Gm and reduced image G′m are deleted after display of the next display data Dm+1 unless the second release switch is turned off. As shown in  FIG. 6 , the minimum necessary number of frames of image data of the post-processed image G and display image G′ is 7. This is a relatively large number since the exposure image at time tn is also displayed at time Tm (tn=Tm). 
       FIG. 7  shows an exposure curve and a display line. The exposure curve represents a relation between exposure lapse time t and exposure amount Ev:
 
Δ Ev =log 10 t /log 102
 
The display line represents a relation, ΔEv=⅙*(t−2), in which image display is updated every two seconds. Note that both longitudinal and transverse axes in  FIG. 7  are a linear scale.
 
     In  FIG. 7  an added image at t 10  (8 seconds after start of shooting) is displayed on the display unit  123  at T 10  (20 seconds after start of shooting). Likewise, there is a time lag between time t 1  to t 16  for the image capturing and time T 1  and T 16  for the image display so that the display shows the exposure amount rising at a constant rate ( FIGS. 5-6 ). 
     Second Embodiment 
     The bulb mode of the imaging device according to a second embodiment will be described with reference to  FIGS. 8-9 . 
       FIG. 8  is a flowchart for shooting and storing process (step S 41  to S 53 ) in the bulb mode while  FIG. 9  is a flowchart for display process (step S 61  to S 71 ). As in the first embodiment, the two-stage release switch turns on upon a half press and a full press to the shutter button (not shown) in the manipulation unit  122  and is configured to start bulb exposure upon the turning-on of the second release switch, continues the exposure during the on-state of the second release switch and completes the exposure upon turning-off of the second release switch in the bulb mode. 
       FIG. 8  shows an operation from readout of an image signal f from the CMOS image sensor  104  to storing of a still image at an exposure decided. In step S 41  the operation starts. In step S 42  a determination is made on whether or not turning-on of the second release switch is detected. Upon detection of the second release switch&#39;s turning-on, exposure is started in step S 43 . In step S 44  a determination is made on whether or not a predetermined period has elapsed. When the period has elapsed, the image signal f is read from the CMOS image sensor  104 , subjected to pre-processing including black level processing and defective pixel correction in the pre-processing unit  111  to generate image data f′, and stored in the image buffer memory  124  via the bus line BL in step S 45 . 
     The later-described operation shown in  FIG. 9  is that the image data f′ is amplified by the amplifying unit  112  of the image processor  110  and displayed on the display unit  123  with a second time interval, so that at every display update, a displayed exposure amount appears to rise. 
     Then, in step S 46  a determination is made on whether or not turning-off of the second release switch is detected. With the turning-off detected, an exposure time ts is calculated in accordance with the exposure amount determined by the turning-off of the second release switch in step S 47 . A determination is made on whether or not the time ts has elapsed from the start of exposure in step S 48 . With the time ts elapsed, an image signal fs is read from the CMOS image sensor  104 , subjected to pre-processing including black level processing and defective pixel correction in the pre-processing unit  111  to generate image data fs′, and store it in the image buffer memory  124  via the bus line BL in step S 49 . In step S 50  the image data fs&#39; is added with the image data f′ to be image data F (Fn=fs′+f′). In step S 51  the added image data F is subjected to the post-processing by the post-processing unit  113  to be image data G and stored in the image buffer memory  124  via the bus line BL. 
     Then, the image data G is read from the image buffer memory  124 , compressed in JPEG form by the compression/decompression unit  125 , and stored in the storage medium via the storage I/F  126  in step S 52 . The operation ends in step S 53 . 
     The flowchart in  FIG. 9  shows a display operation in which image display is updated with a second time interval. In step S 61  the process starts. In step S 62  image data f′ is read from the image buffer memory  124  via the bus line BL and subjected to the first amplifying process by the amplifying unit  112  to generate image data f′ 1  (f′ 1 =(k^(j−1))×f′ where j=1) in step S 62 . Then, the image data f′ 1  is subjected to the post-processing by the post-processing unit  113  and the reduction by the reducing unit  114  and converted to a post-processed image g 1  and then a reduced image g′ 1 . Finally, the reduced image g 1  is converted to display data d 1  by the display processing unit  115  in step S 63  (f′ 1  to g 1  to g′ 1  to d 1 ). In step S 64  the display data d 1  is displayed on the display unit  123 . 
     Next, in step S 65  a determination is made on whether or not the second time interval has elapsed from the previous display. With the interval elapsed, the coefficient j is incremented by 1 (j=j+1) in step S 66 . The image data f′ is subjected to the amplifying process by the amplifying unit  112  to generate image data f′j (f′j=(k^(j−1))×f′) in step S 67 . The image data f′j is subjected to the post-processing by the post-processing unit  113  and the reduction by the reducing unit  114 , and converted to a post-processed image gj and a reduced image g′j. Further the reduced image g′j is converted to display data dj by the display processing unit  115  in step S 68  (f′j to gj to g′j to dj). The display data dj is displayed on the display unit  123  in step S 69 . 
     In step S 70  a determination is made on whether or not the turning-off the second release switch is detected. Without detection of the switch&#39;s turning-off, the process returns to step S 65  and repeats the steps above with the second time interval until the turning-off of the second release switch is detected. With detection of the switch&#39;s turning-off in step S 70 , the operation ends in step S 71 . 
     According to the operation in  FIGS. 8-9 , after elapse of the predetermined period from the start of the bulb exposure in step S 43 , the image signal f is read and stored in the image buffer memory  124  in step S 45 . This image signal is repetitively amplified by the amplifying unit  112  of the image processor  110  with the second time interval in steps S 66 -S 67  and displayed on the display unit  123  in steps S 68 -S 69 . Thus, a displayed image appears as if the exposure thereof continuously rises at every update. When the exposure amount is determined upon detection of the turning-off of the second release switch, the exposure time ts corresponding to the determined exposure is calculated in step S 48 . Then, after elapse of the exposure time ts in step S 49 , the image signal fs is read and added with the previously read image signal f′, and stored in steps S 50 -S 53 . In the amplification in steps S 62  and S 67 , “k” defines an amplification rate. At k being 1.26 (precisely, 1.259921), image data is amplified by every ⅓v for display. At k being 1.41 (precisely, 1.4142136), image data is amplified by every ½v for display. 
       FIG. 10  is a timing chart of the operation in  FIG. 8 . In the drawing “vertical synchronous signal” and “exposure/readout in bulb mode” show operation timing of the CMOS image sensor  104 . “Exposure/readout in bulb mode” shows operation timing of the rolling shutter. As seen from the drawing, the rolling shutter can intermittently read image data in continuous exposure. “image signal” shows operation timing of read of an image signal and timing of pre-processing of the pre-processing unit  111  of the image processor  110 . “image adding processing” shows timing of adding process of the adding unit  116 . “post processing/reduction” shows timing of post processing and reduction of the post-processing unit  113  and the reducing unit  114 . “tRON” on a time axis corresponds to detection timing for turning-on of the second release switch, and “tROF” corresponds to detection timing for turning-off of the second release switch. 
     At time “t 0 =0 sec”, exposure starts. “tx” on the time axis indicates elapse of the predetermined period, and at time tx the image signal f is read and subjected to the pre-processing. Since only the image signal f is read, the mage display is updated using the image signal f without the adding process. When the second release switch is turned off at tROFF, the exposure time ts is calculated as in step S 48  of  FIG. 8 , and the image signal fs is read at the exposure time ts, added, and stored. 
     Specific values of time t 1  to tn according to the second embodiment are shown in  FIG. 11 , for example.  FIG. 11  shows values of the first and second time intervals when the range of exposure time is 1 second (t 1 ) to 256 seconds (t 25 ), exposure amount rises by ⅓Ev and display is updated every two second. To increase the exposure time by ⅓Ev, a relation, tk=1.26×tk−1 need be satisfied. The first time interval Δtk in  FIG. 11  takes values calculated by Δtk=tk−(tk−1). The second time interval ΔT is fixed to 2 seconds in order to update image display at 2 second interval. According to the present embodiment, the predetermined period is set to 8 seconds (t 10 ) at which time taken for the exposure to rise by 1 Ev is longer than display update time. The predetermined period cannot be set too long since exposure image is not displayed until the predetermined period has elapsed, so that too long predetermined period may cause an operator to doubt a failure in the device. Further, when the predetermined period is too long, there is a possibility that the initial image f may be overexposed before amplified. On the other hand, when the predetermined period is too short, an error between an amplified image and an actual exposure image becomes large and the operator may not be able to acquire an intended exposure image. For this reason, instead of a fixed length of time, the predetermined period can be selectively determined by an operator&#39;s manipulation to the manipulation unit  122  ( FIG. 1 ) according to a scene to be captured. 
     In the present embodiment, it is assumed that an operator&#39;s turning-off of the second release switch is detected at display update time T 22  (32 sec.) In this case, an actual image f acquired at t 10  (8 sec.) is amplified by 4 Ev and displayed on the screen so that an actual image the operator has intended to capture is an image at exposure time t 22  (128 sec.). That is, the exposure time is 128 seconds, and an actual image is acquired with continuous exposure for a period of 96 (128−32=96) seconds from the detection of the turning-off of the second release switch. 
     Thus, for capturing a subject which needs a long exposure time of about 128 sec. (t 22 ) to 256 sec. (t 25 ), image display is started in 32 (T 22 ) to 38 (T 25 ) seconds after start of shooting. Because of this, the operator does not have to wait for a long time, keeping pressing the shutter button of the manipulation unit  122 . Note that there is actually a time lag between exposure start time t 0 =0 sec. and display update time T 0 =0 and between exposure time tn and display of the time Tm (tn=Tm). However, it is not considered here since it is irrelevant to the features of the present invention. 
       FIG. 12  shows an exposure curve and a display line. The exposure curve represents a relation between exposure lapse time t and exposure amount Ev:
 
Δ Ev =log 10 t /log 102
 
The display line represents a relation, ΔEv=⅙*(t−8)+3, in which image display is updated every two seconds.
 
     In  FIG. 12  the display line starts rising at time t 10  (predetermined period, 8 seconds from the start of exposure; Ev=3) at the intersection of the exposure curve and the display line, and at T 22  (24 seconds from the start of exposure) it is inclined from +4 Ev and intersects with 7 Ev axis. An image with exposure 7 Ev which the operator intends to capture is one acquired at t 22 , or 128 seconds after the start of exposure so that the exposure time is in this case is from t 0  to t 22 . The longitudinal and transverse axes of the graph are a linear scale. 
     As described above, exposure is continued even after the detection of turning-off of the second release switch, which causes the operator to keep waiting. It is therefore preferable to display on the display unit  123  such a message that “exposure continuing, please wait for 96 seconds”, for example. 
     In the present embodiment, the necessary number of buffer memories is 4 for 4 kinds of image data of the image signal f, amplified (added) image f′ (F), post-processed image g and reduced image g′, respectively. 
     Third Embodiment 
     The bulb mode of the imaging device according to a third embodiment will be described with reference to a timing chart in  FIG. 13 . 
     The imaging device according to the third embodiment operates the same as in the first embodiment from start of the exposure to elapse of the predetermined period, and operates the same as in the second embodiment after the elapse of the predetermined period. 
       FIG. 13  is a timing chart of the operation according to the third embodiment. At instance when the predetermined period has elapsed from the start of the exposure, the image signals f 1  to fn have been read. Using an added image Fn of the read image signals, image display is updated after the elapse of the predetermined period. After the turning-off of the second release switch, or time tROFF, operation is almost the same as that in  FIG. 10 . 
       FIG. 14  shows values of the first and second time intervals and the predetermined period when the range of exposure time is 1 second (t 1 ) to 256 seconds (t 5 ), exposure amount rises by ⅓Ev and display is updated every two second. To increase the exposure time by ⅓Ev, a relation, tk=1.26×tk−1 need be satisfied. The first time interval in  FIG. 14  are values calculated by Δtk=tk−(tk−1). The first and second time intervals ΔT are fixed to 2 seconds in order to update image display at 2 second interval. Also, at t 16 =T 16 =32 sec., display update time catches up with the exposure time so that the predetermined period is set to t 16 . The operation until t 16  is done the same as that in the first embodiment and at and after t 17 , the operation is done the same as that in the second embodiment. Accordingly, in a period from T 1  to T 16 , an image with an actual exposure is displayed while at and after T 17  an image with a pseudo exposure, which is the added image F 16  with an exposure amount at t 16  and having been subjected to the amplification, is displayed. 
     The amplification and display update is performed by ⅓Ev in  FIG. 14 . This enables an operator to take time to decide the right shooting timing even during Δt 2  to Δt 7  in which exposure amount rises at a short interval of 1 second or less, since image display is updated at a longer interval of 2 seconds. Moreover, for capturing a subject which needs a long exposure time of about 128 sec. (t 22 ) to 256 sec. (t 25 ), image display is started in 44 (T 22 ) to 50 (T 25 ) seconds after start of shooting. Because of this, the operator does not have to wait for a long time, keeping pressing the shutter button. 
     In the present embodiment, it is assumed that an operator&#39;s turning-off of the second release switch is detected at display update time T 22 . In this case, an added image F 16  (Ev=6) acquired at t 16  is amplified by 2 Ev and displayed on the screen so that an actual image the operator intends to capture is an image acquired at exposure time t 22  (128 sec.). That is, the exposure time is 128 seconds, and an actual image is acquired with continuous exposure for a period of 84 (128−44=84) seconds from the detection of the turning-off of the second release switch. Note that there is actually a time lag between exposure start time t 0 =0 sec. and display update time T 0 =0 and between exposure time tn and display of the time Tm (tn=Tm). However, it is not considered here since it is not the feature of the present invention. 
       FIG. 15  shows an exposure curve and a display line. The exposure curve represents a relation between exposure lapse time t and exposure amount Ev:
 
Δ Ev =log 10 t /log 102
 
The display line represents a relation, ΔEv=⅙*(t−2), in which image display is updated every two seconds.
 
     In  FIG. 15  the display line and the exposure curve intersect with each other at t=16 (32 sec.; T=16; Ev=5) at which the display update time catches up with the exposure time. The display line rises by 2 Ev at T 22 , 12 seconds after T 16 , and intersects with a 7 Ev axis. Note that the longitudinal and transverse axes of the graph are a linear scale. 
     Thus, display of an actual added image is updated every two seconds until T 16  (predetermined period) so that exposure rises by ⅓ EV, and at and after T 17  display of an amplified image on the basis of the added image acquired at T 16  is updated so that exposure rises by ⅓ EV. 
     When the operator&#39;s turning-off of the second release switch is detected at T 22  (44 sec.), an operator&#39;s desired image is assumed to be one with 7 Ev and exposure is continued until t 22  (128 sec.), which keeps the operator waiting. It is therefore preferable to display on the display unit  123  such a message that “exposure continuing, please wait for 84 seconds”, for example, as described in the second embodiment. 
     In the present embodiment the minimum necessary number of buffer memories  124  is 7 until T 16  as in the first embodiment and it is 4 at and after T 17  as in the second embodiment. 
     Next,  FIGS. 16-17  schematically show an image captured by the imaging device according to any one of the first to third embodiments by way of example. The image is a nightscape image of urban high-rise buildings. The image in  FIG. 16  is an example of a display image Dm−1 or dj−1 while that in  FIG. 17  is an example of a display image Dm or dj. 
     In the first embodiment the exposure amount of the display image Dm added with the image signal fm in  FIG. 17  is higher than that of the display image Dm−1 in  FIG. 16 . Likewise, in the second embodiment the exposure amount of the display image dj as the image signal amplified by k-times in  FIG. 17  is higher than that of the display image dj−1 in  FIG. 16 . 
     Further, image histograms are shown in the lower right corners of  FIGS. 16 ,  17 . The longitudinal axes thereof are the number of pixels and the transverse axes are a brightness tone level of dark, medium, bright from the left side of the drawings. It is apparent from the drawings that a peak of darkness is in the left end of the histogram in the  FIG. 16 , however, it is shifted to the right side in that in  FIG. 17 , showing an increase in brightness. 
     As described above, according to the present invention, it is made possible to display preview images with a proper and gradual change in the exposure amount and allow an operator to easily take photo opportunity to shoot an image with proper exposure. 
     Although the present invention has been described in terms of exemplary embodiments, it is not limited thereto. It should be appreciated that fluctuations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims.