The application of optical differentiation method in passive range estimation was discussed. The concept of optical differentiation is extended to a more general model based on the linear property of imaging system. In this model, the distribution of the images' intensity for depth recovery can be formulated as an arbitrary order linearly differential form and it gives us more selections of the optical differential form between the two processes of imaging. For the improvement of the ranging system, the influence of the optical system's parameter on the ranging precision and the longitudinal resolution is analyzed. The design and optimization of the optical masks are also briefly investigated.