Cosmic convergence and shear with extended sources
  The standard theory of weak gravitational lensing relies on the approximation
that light beams are infinitesimal. Our recent work showed that the finite size
of sources, and the associated light beams, can cause nonperturbative
corrections to the weak-lensing convergence and shear. This article thoroughly
investigates these corrections in a realistic cosmological model. The
continuous transition from infinitesimal to finite beams is understood, and
reveals that the previous results overestimated finite-beam effects due to
simplistic assumptions on the distribution of matter in the Universe. In a
KiloDegree Survey-like setting, finite-beam corrections to the cosmic shear
remain subpercent, while percent-level corrections are only reached on
subarcmin scales. This article thus demonstrates the validity of the
infinitesimal-beam approximation in the interpretation of current weak-lensing
data.
