Although spherical aberration of electrostatic lenses can be corrected in principle by employing meshes, the holes in meshes give rise to local disturbances in the electric field and the performance of mesh lenses deteriorates somewhat due to this effect. The disturbed electric field around mesh holes has a lens action, so a mesh can be regarded as a set of single-aperture lenses. It is shown that the optical properties of single-aperture lenses can be approximated to high precision by means of a transfer matrix. Using this formulation, the lens effect added by the presence of holes is extracted without ambiguity from the action of the total lens field. Moreover, the condition where a single-aperture lens can be regarded as a thin lens with a certain focal length is clarified. An image of a point source is blurred by mesh holes, and the amount of blurring is explained to high precision by their first-order defocusing action as single-aperture lenses; calculations using ray tracing have confirmed this. It is shown that the blurring can be suppressed to an acceptable level in applications such as transfer optics for electron spectrometers.