There are two situations in auto focusing: one is the long-object-distance situation; the other is the long-short-object-distance situation. The curve of image clarity evaluation function under the long-short-object-distance situation usually has multiple peaks. A new method based on Weighted Blocking Discrete Cosine Transform (WBDCT) is proposed for evaluating the image clarity in this paper. In the method, images are divided into some blocks of the same size. Then, each block is given the corresponding weighted value and is performed by Discrete Cosine Transform (DCT). Finally, the clarity evaluation function is calculated. The method is tested in the long-object-distance situation and the long-short-object-distance situation, respectively. Experimental results indicate that the proposed method can effectively focus the images under the two situations.