There has recently been an increased interest in software that can propagate complex fields through optical systems. This type of optical software gives the user the ability to analyze and tolerance two main categories of coherent optical systems: (1) metrology systems and (2) systems used to propagate coherent optical signals. In order to demonstrate the methodology of modeling and analyzing coherent systems, a software model of a Fizeau interferometer was constructed. The model extended from the laser source through a 256 x 256 pixel detector with 8-bit gray level resolution. The reference mirror in the model is moved to create four, 90-degree phase shifted interferograms in the form of ASCII files. These four ASCII files are then input into another software code that calculates and unwraps the phase to determine the surface error. The peak-to-valley and RMS surface errors are calculated and the surface is fit to Zernike polynomials. This Fizeau model was and used to study the effects on surface measurement accuracy of the first surface of a test lens due to unwanted reflections from the second surface of the lens. This is the type of study for which non-sequential, coherent field propagation software has some advantage relative to geometrical ray trace codes.