This paper presents a fusion method for multi-focus images to produce a new clear image for the scene. First we analyze the defocused image formation process and obtain a set of ill-posed equations. By imposing some constraints, they become well-posed and the fusion task is converted to a simple optimization problem. Then the optimization problem, which minimizes the gradient difference and intensity difference with respect to the objective gradient field and intensity constraints, is equivalent to the optimal fusion method for multispectral image fusion. So we construct the objective gradient field and the intensity constraints by clarity analysis and model change respectively and obtain the minimization result by the iterative optimization steps of optimal fusion. At last, the experiments convincingly demonstrated that the proposed method has better tolerance to misalignment and noises than wavelet fusion.