A conventional exposure control method of the digital camera as disclosed in the U.S. Pat. No. 5,703,644 is to divide the captured image frame into a plurality of regions; and to determine based on the luminance value whether the picture is taken indoor or outdoor using zone light metering so as to partition the target object and the background. Afterwards, the difference between a luminance of the region where the target object is present and a luminance of the remaining portion of the image frame are measured to decide whether the object stands toward or against the light source. At the same time, an appropriate luminance adjusting value (i.e. exposure compensation value) is obtained by analyzing the value of average luminance of the bright region or the dark region, and the backlight degree or the frontlight degree using Fuzzy Theory. The above method, however, may cause invalid light metering as the target object moves or as the camera rotates.
Another conventional exposure control method as disclosed in the U.S. Pat. No. 6,690,424 is to define two luminance reference levels using the value of average luminance of the entire picture (or scene), an object scene portion in the picture is separated from a background portion on the basis of the luminance reference level. The value of the average luminance and the luminance difference of the entire picture and the object scene portion are respectively found, and based on this luminance difference, the luminance adjusting value for the object scene portion is determined. This method, however, is not suitable as the object scene portion is over-exposed.
Additionally, an orientation sensor is required to detect whether the picture was taken vertically or horizontally while using the conventional matrix light metering, so as to determine which metering matrix should be implemented. An inappropriate light metering value may be generated if there is an error occurred in detection.
While using a digital camera, if there is over-exposure (i.e. signal over saturated), the detail of the target object will disappear as shown in FIG. 4a; if there is under-exposure (i.e. signal weak), the detail of the target object will not be rendered, as illustrated in FIG. 5a. 