Several focus measures were studied in this paper as the measures of image clarity, in the field of multi-focus image fusion. All these focus measures are defined in the spatial domain and can be implemented in real-time fusion systems with fast response and robustness. This paper proposed a method to assess focus measures according to focus measures' capability of distinguishing focused image blocks from defocused image blocks. Experiments were conducted on several sets of images and results show that sum-modified-Laplacian (SML) can provide better performance than other focus measures, when the execution time is not included in the evaluation.