The finite source size effect and the wave optics in gravitational   lensing
  We investigate the finite source size effect in the context of the wave optics in the gravitational lensing. The magnification of an extended source is presented in an analytic manner for the singular isothermal sphere lens model as well as the point mass lens model with the use of the thin lens approximation. The condition that the finite source size effect becomes substantial is demonstrated. As an application, we discuss possible observational consequences of the finite source size effect on astrophysical systems. 
Norihito Matsunaga and Kazuhiro Yamamoto