Interferometry, generally two-beam interferametry, is widely used to test the quality of optical instruments. At all stages of their manufacture this method is employed. First, it is to choose the materials, glasses or crystals, to be sure that they do not show refractive index variations and that they do not present mechanical strains. Second, the interferometric testings occur when the optical components are manufactured. They concern the shape of the surfaces, possibly their roughness, the centering of the lenses, the angle measurements. Of course the materials which are used are always of the best quality and so it is of the workshop productions. Then the obtained interference patterns are always near the flat tint and it is useful to dispose of means able to increase sensitiveness of two-beam interference patterns, that will be opening on the information processing. To test optical systems, there is also a difficult problem concerning especially the high aperture objectives used in microscopy and microlithography which are diffraction limited and require quality testing during their manufacture and for their delivery. The instruments which are measuring the MTF are not at all suitable in the field of spatial frequencies transmitted by these optical systems. Knowledge of the deformations of the wavefront by means of normal interferometry require a very perfect reference system at least of same aperture. On the other hand, the differential interferometry which is able to obtain the self comparison of the wavefront for a given spatial frequency offer solutions proposed by several authors. We will consider the use of a polarization interferometer associated with a phase modulation to increase accuracy. In spite of some-non negligible experimental complications, it is effectively possible to obtain directly the components of the MTF. In so far as a limited number of informations about some discrete frequencies are interesting the user such an instrumental project could be considered. On the other hand, to satisfy the designer it seems more useful to obtain the lateral aberrations of the objective that is leading to the restitution of the true wavefront by means of integration. That is possible from the analysis of interference pattern resulting of very small shearings in two orthogonal directions.