A lens assembly, which is mounted on a digital still camera (DSC), comprises a fixed aperture, and a lens unit. The lens unit has a first lens, a second lens, a third lens, a fourth lens and a fifth lens in sequence. The first lens is a biconvex lens disposed at the front side of the lens unit. The second lens is a biconcave lens. The third lens is a positive lens with having a larger curvature radius facing the objective side. The fourth lens is a negative meniscus lens. The fifth lens is a positive lens with a surface having a larger curvature radius facing the image side. The fourth lens and the fifth lens are attached together to form a compound lens. The aperture is disposed in front of the first lens.