In this paper, we analyze and discuss the problem of lens distortion in 3-D floating images. Lens distortion impacts the viewing zone distribution of auto-stereoscopic images. To understand the relationship between lens distortion and stereoscopic effects, we set up measurement methods to evaluate them. Measured data from these methods allow us to easily determine the optimization results of the 3-D floating image system. It will help to improve the 3-D floating display system when we try to optimize its optical design and user interface.