The one key problem of passive auto-focus technique based on image is image definition criterion which is
estimated by auto-focus function.This paper analyzes the limitation of traditional spatial domain auto-focus
functions which are not sensitive to small changes of images,and presents a new auto-focus evaluation function
which uses the combination filters to strengthen the image medium-frequency information.In order to evaluate the
new function performance,the parameters of unimodality,accuracy,sensitivity,calculating cost are compared and
analyzed.Compared with the traditional spatial domain evaluation functions,such as Brenner function,Laplacian
function and Tenengrad function,the experimental results show that the new function has a fast calculation
speed,and has a high sensitivity.