Abstract: The accurate and efficient evaluation of image definition is a critical step in automatic focusing. In the available evaluation methods, there are a variety of limitations due to different parameters used. After analyzing the advantages and disadvantages of existing methods for image clarity evaluation, a new evaluation method for image definition based on regional contrasting is proposed based on image analysis and statistics. The mathematical model is established and analyzed, and the experimental results are given. It is proved that, this evaluation method satisfies the basic tactic requirements, and has high sensitivity and reliability in automatic focusing.