Profiles of vertical Common Black soap films were calculated numerically, taking into account double-layer repulsion and London-van der Waals attraction. The definition of a contact angle is discussed and attention is payed to the case that no contact angle can be defined. From the profiles Fresnel diffraction patterns were calculated. It turned out that (1) they are not very sensitive for the details of the transition region, between a film and its meniscus, (2) for contact angles larger than 16′ the results of H. M. Princen and S. Frankel ( J. Colloid Interface Sci. 35, 368 (1971)) coincide with ours within 5%, (3) this optical method will not result in reliable contact angles when they are smaller than about 10′.