If a flat wavefront illuminates a lens and we measure the curvature of the refracted wavefront, the focal length of this lens can be determined. Different alternatives to measure the convergence or divergence power of a wavefront using fringes projected by a Ronchi ruling placed in contact with the lens are described. The Talbot autoimaging phenomenon is used to optimize the contrast of the projected fringes as much as possible. The different manners in which the fringes can be formed as well as the different manners in which these fringes can be analyzed and their spatial frequencies measured, are described using Talbot interferometry with a second ruling on the plane of observation. The two rulings have a small angle between them. The curvature of the wavefront illuminating the rulings is measured by the inclination of the moire´ fringes formed by the two ruling. Using the method described here, an instrument could be constructed to measure the power of ophthalmic lenses.