- We propose a method to express the complex field amplitude (CFA) at the output of any optical system with arbitrary masks. The method provides a general equation for any first- order optical system with given parameters and arbitrary masks. The method is based on the ABCD matrices of the system components, ignoring the masks. The obtained output CFA is formulated in a way that helps recognize the influence of each mask on the output CFA by projecting its influence on the output with the aid of ABCD matrices. The method offers a more transparent approach for efficient organization and minimization of the calculation steps.