One of the recent developments in harnessing wave energy is to construct a line of submerged structures parallel to the incident swell crests in order to transform the straight crests to circular crests converging to a focus. To understand the neighbourhood of the focus, we have carried out theoretical studies by accounting for diffraction and nonlinearity, both separately and jointly. Experiments have also been conducted in a large outdoor basin and are compared with the theories. These comparisons tend to favour the approximate nonlinear theory, but the efficiency of the focusing device as an energy concentrator does not appear to be significantly impaired by nonlinear effects.