Abstract: References found that the fixed step size is used to stabilize the focusing process. However, it is difficult to be used for the practical requirements of the system due to the longer focusing time and low efficiency. In addition, amongthe existing variable step size focus search algorithms, the existing variable step size adjustment focus search algorithm has quite complex calculation formula derivation process and the computation of the algorithm is very difficult. Therefore, in the actual applications which require rapid focus, the existing algorithm is rarely used. In order to realize the fast focusing, the process of the derivation of the focusing algorithm needs to be simpleand the algorithm calculation should be simple so that the algorithm can be applied to the system of fast focusing. In this paper, the variable step size focus search algorithm is studied, and a new adaptive variable step size algorithm based on image processing is proposed. The strategy of the new algorithm is: the coarse focusing is obtained by increasing the focus step when the the system consists of a straight line of the clarity evaluation function of any two points far away from the best focusing position; a fine focusing is realized by decreasing the focus step by nearby the peak of focusing curve;finally, the variable step size, fixed step size and variable step size search algorithm based on image processing are used to automatically focus the different definition evaluation functions. Experimental results show that, in this paper, the proposed variable step size search algorithm can ensure the accuracy of the focus, can achieve rapid focus for domain definition of the evaluation function, and the adaptive variable step size search algorithm based on image processing has the advantages of shorter focusing time, less computation and so on.

Key words: clarity evaluation function, adaption, variable step, search algorithm, fast focusing