Theoretical analysis shows that the deviation roughness w and the lateral correlation length ξ of a weak scattering object determine the different properties of the contrast of the speckles in the image plane of a 4f system. Experimentally, we have measured the data of the speckle contrast versus the radius R of the variable filtering aperture. By fitting the theoretical results to these data, we extract the parameters w and ξ of the random surfaces with Gaussian correlation. This method can determine the two parameters simultaneously and independently, and pre-calibrations are not needed.