We analyze the two-level atom focusing in a Gaussian standing wave laser field from the perspective of both classical mechanics and wave mechanics. The effects of source imperfection such as velocity spread and beam spread on the feature width are analyzed by numerically integrating the classical equation of atomic motion. The atomic flux distributions during its propagation in a laser field are calculated based on the Monte Carlo scheme and the trajectory tracing method, and results have shown that the focus depth of atom lens for real atomic source is longer than that for perfect source. In the absence of source imperfection, the contribution of diffractive aberration originating from the wave nature of the atom to broadening of feature width is larger than that of spherical aberration. Feature separation can be reduced by changing the detuning of the standing wave laser field from blue to red halfway through the deposition time.