A variable-focus liquid lens using a transparent flat cell is demonstrated. The top substrate has an aperture which is sealed with a thin elastic membrane on the outer surface and the bottom aperture is sealed on the inner surface of the bottom substrate. By applying a pressure to the outer membrane inward causes the liquid to redistribute and swell the inner membrane outward which, in turn, forms a plano-convex lens. To prove principles, a water lens with 5 mm aperture was fabricated. The resolution is better than 25 lp/mm and the response time is approximately 40 ms. Potential applications of such a lens for real-time active imaging are emphasized.