This book demonstrates how to design an optical system using Ansys Zemax OpticStudio, a full-featured optical design program. The complete design process from lens definition to tolerancing is developed and illustrated using the program. This text is not a user's manual for Zemax OpticStudio. Rather, it starts with a single lens to demonstrate the laws of geometrical optics and illustrate basic optical errors (aberrations) with the software.Through a series of examples and exercises, one can follow each step in the design process to analyze and optimize the system to meet the required performance specifications. The text is organized to help readers (1) reproduce each step of the process, including the plots for evaluating lens performance, and (2) understand the significance of these plots in constructing a final design.