A contrast adjustment system includes a memory and a processor. The memory stores instructions. The processor is configured to access and execute the instructions to: access an image with a plurality of pixels, wherein the pixels are corresponding to a plurality of intensity values; generate a histogram based on a distribution of the intensity values in a range from an intensity lower bound to an intensity upper bound; divide the histogram into four sub-histograms based on a median value of the intensity values; enlarge the four sub-histograms based on a predetermined parameter; remap the four sub-histograms to form a gamma curve based on a cumulative distribution function; and apply the gamma curve to pixels of another image.