1. An image processing method for a digital camera which photographs a target object by using predetermined exposure time, converts an optical signal into an image signal, and outputs the image signal, the image processing method comprising: measuring a black level value caused by a dark current and calibrating a black level of the image signal with reference to the measured black level value; dividing input luminance of the image signal into sections and providing different gain values to the sections, thereby performing knee correction with respect to the image signal with reference to the gain value provided to at least one of the sections comprising luminance of the image signal having the calibrated black level; receiving the image signal processed through the knee correction and restoring a dead pixel in an image signal through interpolation; performing gamma correction with respect to the image signal in which the dead pixel is restored; separating a luminance component from a chromaticity component in the image signal processed through the gamma correction and outputting the luminance component and the chromaticity component; receiving the luminance component and outputting exposure time for the black level calibration and the knee correction; and receiving a color signal and outputting a gain value for the knee correction, wherein the sections comprise luminance ranges distinguished by knee points when at least one predetermined reference luminance is employed as a knee point, and comprise gain values as the sections approximates to a white saturation time point.