Dust defect is always a critical issue to degrade image qualities in kinds of imaging capturing modules, such as compact camera modules (CCMs) and digital single lens cameras (DSLRs). Dust particle may be deposited on the surface of lens or sensor in the step of assembly and even in the step of changing lens module by users. There have been lots of studies about image defects produced by dust particles on the surface of lens or sensor and the image compensation for dust defect on images, but the studies about the direct analysis of dust defects are still deficient. In this study, geometrical optics was used to analyze the image defects due to dust in the imaging capturing module. A defocused optical system with a planar light source with Lambertian emission was developed to analyze dust defect with image process and the concept of just noticeable difference (JND) of human vision. The proposed defocused optical system provides a way to help manufacturers to improve their production line efficiently.