A variable-focus liquid-filled optical lens comprises a membrane and a support frame (which consists of a chamber and a liquid channel). The optical properties of different variable-focus liquid-filled optical lenses are studied to obtain the best membrane shape. The non linear finite element method is employed to shed light on the deformation of membranes under different liquid pressures. With the information thus obtained, the optical properties (such as the focal length and image diagram) of the lens are examined. The focal length of a lens with a flat membrane shape is studied by both simulation and experimental analyses, and the two results obtained are in good agreement. The deformations of membranes show that the bending moment and the boundary conditions of the membrane are two important parameters. Finally, several membranes of different shapes are examined to obtain the membrane shape with the best optical performance.