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