准确有效的调焦评价函数是采用数字图像处理技术实现自动调焦的关键环节，而调焦评价函数的好坏又直接影响着调焦系统的结构精度和运算的复杂程度。分析了灰度标准差比较法和灰度对比度比较法的优缺点，提出了经改进的灰度比较法作为调焦评价函数，该方法具有响应速度快、计算量小、信噪比高、可靠性好等特点。实验与分析结果表明，该方法对具有复杂细节和纹理特征的图像能得到较好的处理结果，而对于背景高亮度的图像则不能判断其是否聚焦。

The accuracy and efficiency of a focus on evaluation function are the key to autofocusing based on digital image processing technique. In turn, a focus on evaluation function affects structure accuracy of autofocusing system and complexity of calculation. In this paper, the relative method of the gray standard deviation and the relative method of the gray contrast are integrated. As a result, we get the relative method of the improved gray. The characteristics of the method are the fast response speed, the unsophisticated calculation, the high signal to noise ratio, the better reliability and so on. After a great deal of experiments and analysis, the obtained conclusion is that the method is suitable for the picture with complex detail and texture. The method is not suitable for the picture with high light background.