A photon sieve can be composed of a large number of circular pinholes. Each circular pinhole has a contribution to the focusing. For the case of point-to-point imaging, the focusing contribution of an individual circular pinhole can be analytically given. We investigate the dependence of the focusing contribution on the order m of local ring of underlying traditional Fresnel zone plate. In particular, we find that the focusing contribution is simply inversely proportional to the order m. We also present an intuitive explanation. These results are helpful for better understanding of the focusing property of photon sieves.