Abstract:
The object of this invention is to increase the precision of an exposure value in a scan AE scheme. To achieve this object, an image is picked at a plurality of exposure values. Screen-split brightness values and exposure value differences are obtained for the respective exposure values. Two screen-split brightness values and exposure value differences are selected from the relationship between the exposure value differences. A brightness value is calculated from the selected values to decide an exposure value in actual exposure.

Description:
This is a continuation of co-pending application Ser. No. 10/098,869, entitled “IMAGE PICKUP APPARATUS WITH PRECISE EXPOSURE VALUE, EXPOSURE DECISION METHOD, PROGRAM AND STORAGE MEDIUM,” which was filed Mar. 15, 2002, the entirety of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to an image pickup apparatus, exposure decision method, program, and storage medium. 
   BACKGROUND OF THE INVENTION 
   One of means for deciding the exposure of a digital camera is a scan type exposure decision apparatus for performing photometry at predetermined exposure values and deciding optimal exposure on the basis of the photometry data. 
   According to this method, photometry is done at a plurality of exposure values, and the exposure difference value between the brightness value of a screen and a target brightness value at each exposure value is calculated. The exposure value of actual exposure is decided by using an exposure value having the smallest absolute value of the exposure difference value out of images obtained at these exposure values, and by using the exposure difference value of this exposure value. 
   However, this apparatus may not accurately calculate an exposure difference value under the influence of the saturation characteristic of a CCD or the like. 
   The method of obtaining an exposure value in the apparatus will be exemplified. 
   In  FIG. 5 , the ordinate represents the level of the exposure value. 
   Letting BvT 0  be the exposure value of an object, exposure difference values upon exposure at exposure values Bv 0  and Bv 1  are DeltaBv 0  and DeltaBv 1 . The exposure value difference upon exposure at the exposure value Bv 0  does not become DeltaBv 0 ′ because the value is clipped due to the saturation characteristic of the CCD and Bv 0  is output as a value smaller an actual value. 
   Comparing the absolute values of DeltaBv 0  and DeltaBv 1 , DeltaBv 1  is smaller. An exposure value BvT 0  is therefore attained by Bv 1 +DeltaBv 1 . 
   If accurate values of both Bv 0  and Bv 1  cannot be obtained, as shown in  FIG. 6 , neither Bv 0 +DeltaBv 0  nor Bv 1 +DeltaBv 1  may coincide with BvT 1 . 
   This phenomenon prevents accurate determination of the exposure value of an object. 
   To attain accurate exposure by this method, the number of exposure scan operations must be increased in consideration of CCD saturation and the presence of a nonlinear region. Exposure must be more finely set to minimize errors under the influence of saturation or the like. 
   This scan must be performed when the user presses the shutter. An increase in the number of scan operations directly leads to an increased shutter time lag. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in consideration of the above situation, and has as its object to provide an exposure decision apparatus capable of obtaining an accurate exposure value while minimizing the shutter time lag, an image pickup apparatus having the exposure decision apparatus, a program, and a storage medium. 
   To solve the conventional problem and achieve the above object, an image pickup apparatus according to the present invention has the following arrangement. 
   More specifically, the image pickup apparatus comprises an image pickup device that picks up an object, and a processing device having a brightness value calculation unit that obtains brightness values of an image at respective exposure values from a plurality of image data obtained by picking up an image by the image pickup device at a plurality of different exposure values, an exposure difference value calculation unit that calculates exposure difference values representing differences between the brightness values and a preset target brightness value in actual exposure, and an exposure value calculation unit that calculates an exposure value in actual exposure on the basis of at least two image data out of the plurality of image data in accordance with calculation results of the exposure difference value calculation unit. 
   An exposure decision method according to the present invention has the following steps. 
   More specifically, the exposure decision method comprises the brightness value calculation step of obtaining brightness values of an image at respective exposure values from a plurality of image data obtained by picking up an image at a plurality of different exposure values, the exposure difference value calculation step of calculating exposure difference values representing differences between the brightness values and a preset target brightness value in actual exposure, and the exposure value calculation step of calculating an exposure value in actual exposure on the basis of at least two image data out of the plurality of image data in accordance with calculation results in the exposure difference value calculation step. 
   A program according to the present invention has the following processing. 
   More specifically, a program causes a computer to execute the exposure decision method. 
   A storage medium according to the present invention has the following structure. 
   More specifically, a storage medium computer-readably stores the program. 
   Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a processing block diagram of an embodiment; 
       FIG. 2  is a block diagram of an AE calculation unit in the embodiment; 
       FIG. 3  is a view showing a weighting factor for each screen-split area in the embodiment; 
       FIG. 4  is a view for explaining an exposure decision apparatus in the embodiment; 
       FIG. 5  is a view for explaining a scan type exposure decision apparatus; and 
       FIG. 6  is a view for explaining the scan type exposure decision apparatus. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the present invention will be described below. 
     FIG. 1  is a schematic block diagram showing a digital camera according to the embodiment of the present invention. 
   Reference numeral  101  denotes an image pickup unit which includes a lens system, a stop, a shutter, an electronic shutter, an image pickup element such as a CCD, and an A/D converter. The image pickup unit  101  outputs an image projected on the CCD via the lens system as a digital signal. 
   Reference numeral  102  denotes an AF controller. When the user releases the shutter, the AF controller  102  detects the focus. 
   Reference numeral  103  denotes an image pickup controller. The image pickup controller  103  executes lens focusing control on the basis of focus data. 
   After the lens is focused, an AE controller  105  obtains an F-number and electronic shutter speed from a predetermined exposure value ControlBv 0 , and sends these values to the image pickup controller  103 . 
   The image pickup controller  103  projects an image on the CCD at the received F-number and electronic shutter speed, thus obtaining image data. 
   The image data controlled and obtained at the exposure value ControlBv 0  is sent to an AE calculation unit  104 . 
     FIG. 2  is a block diagram showing the AE calculation unit. 
   The operation of the AE calculation unit  104  will be explained. 
   From image data  201  sent from the image pickup unit  101 , an area brightness value calculation unit  202  for calculating a brightness value calculates the average brightness value (I=0,5, J=0,5) of each 6×6 area. 
   The obtained brightness value Y(I,J) of each area is used to calculate an exposure difference value DeltaBv based on equation (1) by an exposure difference value calculation unit  203  for calculating the exposure difference value DeltaBv for a target brightness value Yref: 
                 DeltaBv   =       Log   2     (       ∑     (       Y   ⁡     (     I   ,   J     )       ×     Weight   ⁡     (     I   ,   J     )         )         ∑       Weight   ⁡     (     I   ,   J     )       ×   Yref         )             (   1   )               
where Yref is the target brightness value of an image, and Weight(I,J) is the weighting factor of each area shown in  FIG. 3 .
 
   The calculated exposure difference value and Y(I,J) calculated by the area brightness value calculation unit  202  are sent to an area brightness value &amp; exposure difference value memory  204 . These values are respectively stored as an exposure difference value DeltaBv 0  and area brightness value Y 0 (I,J) upon exposure at the exposure value ControlBv 0 . 
   Similarly, the image pickup element is exposed at different exposure values ControlBv 1  and ControlBv 2  by the AE controller  105  in  FIG. 1 . An exposure difference value DeltaBv 1  of a screen and an area brightness value Y 1 (I,J) upon exposure at the exposure value ControlBv 1 , and an exposure difference value DeltaBv 2  of a screen and an area brightness value Y 2 (I,J) upon exposure at the exposure value ControlBv 2  are calculated and stored in the area brightness value &amp; exposure difference value memory  204  in  FIG. 2 . 
   These exposure values satisfy
 
ControlBv0&gt;ControlBv1&gt;ControlBv2  (2)
 
   After the exposure difference values DeltaBv and area brightness values Y(I,J) upon exposure of the image pickup element at the three exposure values ControlBv 0 , ControlBv 1 , and ControlBv 2  are obtained, an actual exposure value calculation unit  205  for calculating an exposure value calculates an exposure value Bv  206  in actual exposure, or the basis of which an object is actually picked up for recording in a storage device, by: 
   For DeltaBv 0 ≧0,
 
 Bv= Control Bv 0+Delta Bv 0  (3)
 
For DeltaBv 0 &lt;0 and DeltaBv 1 ≦0,
 
                   Ycalc   ⁡     (     I   ,   J     )       =       (         Y0   ⁡     (     I   ,   J     )       ×     ABS   ⁡     (   DeltaBv1   )       ×     2   ^     (     ControlBv0   -   ControlBv1     )         +       Y1   ⁡     (     I   ,   J     )       ×     ABS   ⁡     (   DeltaBv0   )           )       (       ABS   ⁡     (   DeltaBv0   )       +     ABS   ⁡     (   DeltaBv1   )         )               (   4   )               DeltaBvcalc   =       Log   2     (       ∑     (       Ycalc   ⁡     (     I   ,   J     )       ×     Weight   ⁡     (     I   ,   J     )         )         ∑       Weight   ⁡     (     I   ,   J     )       ×   Yref         )             (   5   )               Bv   =     ControlBv1   +   DeltaBvcalc             (   6   )               
where ABS(A) is the absolute value of A.
 
For DeltaBv 1 &lt;0 and DeltaBv 2 ≧0,
 
                   Ycalc   ⁡     (     I   ,   J     )       =       (         Y1   ⁡     (     I   ,   J     )       ×     ABS   ⁡     (   DeltaBv2   )       ×     2   ^     (     ControlBv1   -   ControlBv2     )         +       Y2   ⁡     (     I   ,   J     )       ×     ABS   ⁡     (   DeltaBv1   )           )       (       ABS   ⁡     (   DeltaBv1   )       +     ABS   ⁡     (   DeltaBv2   )         )               (   7   )               DeltaBvcalc   =       Log   2     (       ∑     (       Ycalc   ⁡     (     I   ,   J     )       ×     Weight   ⁡     (     I   ,   J     )         )         ∑       Weight   ⁡     (     I   ,   J     )       ×   Yref         )             (   8   )               Bv   =     ControlBv2   +   DeltaBvcalc             (   9   )               
For DeltaBv 2 &lt;0,
   Bv= Control Bv 2+Delta Bv 2  (10) 
   Equation (5) will be explained with reference to  FIG. 4 . 
   The ratio A:B=ABS(DeltaBv 0 ):ABS(DeltaBv 1 ) of the absolute values of DeltaBv 0  and DeltaBv 1  is calculated from the area brightness Y 0 (I,J) obtained upon exposure at Bv 0  and the area brightness Y 1 (I,J) obtained upon exposure at Bv 1 . The calculated brightness value Ycalc(I,J) is obtained from the weighted mean of the ratio A:B. The exposure value Bv in actual exposure is calculated from the calculated brightness value Ycalc(I,J). 
   2^ (ControlBv 0 −ControlBv 1 ) is a coefficient for converting the brightness value Y 0 (I,J) calculated by the exposure value ControlBv 0  into a brightness value obtained upon exposure at the exposure value ControlBv 1 . 
   The exposure value Bv in actual exposure calculated by the actual exposure value calculation unit  205  is sent to the AE controller  105  in  FIG. 1 . The AE controller  105  decides an F-number and shutter speed for actual exposure on the basis of the exposure value Bv. 
   The decided F-number and shutter speed are output to the image pickup controller  103 . The image pickup unit  101  executes actual exposure on the basis of these values, and an actual exposure image digital signal is sent to an image processor  106 . 
   The actual exposure image digital signal undergoes image processing such as chromatic processing, luminance processing, edge emphasis processing, gamma processing, and JPEG conversion processing in the image processor  106 . The resultant data is output to a data write unit  107  and written in a storage medium. 
   In this embodiment, exposure scan is done using three exposure values. The number of exposure values is not limited to three, and can be arbitrarily set to two or more as far as the shutter time lag is permitted. 
   As described above, according to the above embodiment, the image pickup apparatus comprises an image pickup device for picking up an object, and a processing device having a brightness value calculation unit for obtaining the brightness values of a screen at respective exposure values from a plurality of image data obtained by picking up an image by the image pickup device at a plurality of different exposure values, an exposure difference value calculation unit for calculating exposure difference values representing differences between the brightness values and a preset target brightness value in actual exposure, and an exposure value calculation unit for calculating an exposure value in actual exposure on the basis of at least two image data out of the plurality of image data in accordance with the calculation results of the exposure difference value calculation unit. The embodiment can provide an exposure decision apparatus capable of minimizing the number of exposure scan operations necessary to obtain an accurate exposure value and capable of minimizing the shutter time lag, and an image pickup apparatus having the exposure decision apparatus. 
   Other Embodiment 
   The object of the embodiment is also achieved when a storage medium (or recording medium) which stores software program codes for realizing the functions of the above-described embodiment is supplied to a system or apparatus and the computer (or the CPU or MPU) of the system or apparatus reads out and executes the program codes stored in the storage medium. In this case, the program codes read out from the storage medium realize the functions of the above-described embodiment, and the storage medium which stores the program codes constitutes the present invention. The functions of the above-described embodiment are realized when the computer executes the readout program codes. Also, the functions of the above-described embodiment are realized when an OS (Operating System) running on the computer performs part or all of actual processing on the basis of the instructions of the program codes. 
   Furthermore, the functions of the above-described embodiment are realized when the program codes read out from the storage medium are written in the memory of a function expansion card inserted into the computer or the memory of a function expansion unit connected to the computer, and the CPU of the function expansion card or function expansion unit performs part or all of actual processing on the basis of the instructions of the program codes. 
   The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention the following claims are made.