After wrapping on the eye, a soft contact lens can improve a patient's retinal image quality by correcting the patient's wavefront aberrations, including defocus, astigmatism, etc., and its optical zone size or diameter is a critical factor that affects the lens performance. With lens decentration, the lens optical zone edge could only partially cover the patient's pupil area, leading to significant wavefront correction errors. If there is a considerable lens thickness variation at the edge of the lens optical zone, the optical zone edge could also generate a substantial amount of scattering, which will also degrade the retinal image contrast. Thus, it is essential to characterize the optical zone diameter precisely. An interferometer-based imaging system can typically be used for the characterization. However, at specific soft contact lens power ranges (-3D, for example), it is impossible to identify the optical zone boundary even with an interference fringe-based imaging system. In this experiment, we used a Shack-Hartmann-based wavefront sensor to directly measure the soft contact lens optical zone in a power range of -12 to +6D. A software package is also developed to analyze the captured images and generate the optical zone diameter. The results are compared with interferometer imaging-based results and the original lens design. Our results indicated the developed method (including both the Shack-Hartmann imaging system and the software package) was able to precisely characterize the soft contact lens optical zone within the whole lens power range.