The proposed algorithm for digital imaging system ensures correct exposure to the main object of scenes with large dynamic range. The image window was divided into M×N tiles. Using the contrast between the main object and the background of large dynamic scenes,the algorithm identified the main object through a certain search process,detected the lighting condition based on the search result,and then assigned different weights to the main object and the background so that the overall brightness level was made to carry more information of the main object. The experiment results show that the lighting conditions of all the scenes for evaluation are correctly detected. For the high dynamic range lighting conditions,the number of preview frames and the exposure error are less than 8.8 frames and 6.56% respectively.