Abstract: In order to overcome the deficiency of slow focusing speed of the clarity evaluation function, a fast auto focusing algorithm is proposed based on image processing. Under the condition of changing the object distance conditions only, the function expression of the target defocus by blur difference is derived according to the defocus model of optical imaging system. The expression of defocusing amount is mediated on the Z axis motor to achieve fast focusing. The experimental results show that the ectual target defocusing amount can be precisely estimated to achieve focusing quickly with two images of different degree of defocusing image.

Key words: imaging system, automatic focusing, spread parameter, the function expression