In this paper, we introduce two convex-plano lenses for terahertz (THz) imaging applications. The first lens is determined to use an aspherical surface and compared with a similar spherical lens that has the same diameter, thickness, and back focal length. We demonstrate the superiority of aspherical surface by electromagnetic simulations and focal plane scanning experiments. The second lens has a relatively shorter focal length and has excellent focusing capability. By comparing with other two previous studies, we estimate that this lens may achieve high-resolution in THz imaging applications.