Particles exist in many areas

their characterization measurement and monitoring are of great importance.With the development of technology

digital image processing technique as a direct-viewing and non-contact measurement method has been widely used.However

in the optical imaging system

the depth of field is limited.In order to increase the measurement range

the defocus images should be processed. Defocus image of spherical particles were studied from simulation and experiment. It was proven that the gradient of image transition area and the defocus distance were related

and the difference of two maximum gradients along the radial direction was the particle diameter in image.