The actual auto-focusing devices in airborne image equipment have the problem of complexity in design and unable to integrate. In order to solve this problem

the application of image based on auto-focusing method in airborne image equipment was presented. Accor-ding to that the gradient edges in normal images are more than horizontal and vertical edges

a new focus measure method (Lean operator) derived from gradient edge detection was proposed. Series of experiments were committed to have the Lean method compared with the other several kinds of auto-focusing method. According to the experiment results

the Lean operator and correlation operator were respectively chosen as the focus measure method of coarse search and fine search. Then

local full search (LFS) strategy and change step hill-climb search (CHS) strategy were proposed in allusion to the feature of focus measure function of airborne image equipment. When using a lens of 130 steps focus length and 2 steps focus depth

the LFS and CHS search strategy can respectively cut the total focus time from the full searchs 43 s to 7 s and 5～8 s. The results show that the correlation operator with the cooperation of the lean operator can locate the prime focus position accurately

the accuracy is over 95%.