The optical diffraction,defocus,and sampling effects are investigated in the high speed real-time face detection system.Through the analysis,their point spread function (PSF) and optical transfer function (OTF) are formulated to characterize the optical system in the spatial and frequency domain respectively.With the approximate priory knowledge of their PSF,two integer focus operators are proposed and their OTFs to those of Laplacian and the optimal focus operator are compared.This system utilizes a motorized focusing mechanism to acquire high resolution images and a sinc interpolation at the preprocess stage.The demonstration system reaches 15.6 fps with satisfactory results and validates the theoretical analysis.