Electrically tunable lenses (ETL), also known as liquid lenses, can be focused at various distances by changing the electric signal applied on the lens. ETLs require no mechanical structures, and therefore, provide a more compact and inexpensive focus control than conventional computerized translation stages. They have been exploited in a wide range of imaging and display systems and enabled novel applications for the last several years. However, the optical fluid in the ETL is rippled after the actuation, which physically limits the response time and significantly hampers the applicability range. To alleviate this problem, we apply a sparse optimization framework that optimizes the temporal pattern of the electrical signal input to the ETL. In verification experiments, the proposed method accelerated the convergence of the focal length to the target patterns. In particular, it converged the optical power to the target at twice the speed of the simply determined input signal, and increased the quality of the captured image during multi-focal imaging.