There are two calculation formulae in the study of diffraction limited coherent light imaging. One is the formula for calculating the amplitude distribution of the image light field of single lens imaging system under specific approximate conditions. This formula can be extended to the imaging calculation of an optical system composed of multiple elements. The other is the formula which can directly calculate the amplitude and phase distribution of the image light field of an optical system. In order to analyze the practical value of these two formulae, this paper takes the imaging system composed of two lenses as an example, places aperture stops at different spatial positions, and compares the theoretical calculations and experimental measurement results of the two formulae.