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First, this paper analyses the imaging relations of the lens system and the properties of image clarity evaluation function under the condition where the object distances of many objects are different. Then, a center window method is employed to stand out the interesting objects. Finally, the image clarity evaluation function based on the center blocking DCT (discrete cosine transform / DCT) method is proposed for evaluating the image clarity in this paper. Experimental results indicate that the proposed method can effectively focus the images both under long-short-object-distance situation and under long-object-distance situation.