  Applying the Sachs formalism, the optical properties encoded in the distance
modulus are studied along curves exhibiting local rotational symmetry for some
closed inhomogeneous cosmological models whose mass content is discretized by
Schwarzschild-like sources. These models may challenge the concordance model in
its use of the distance modulus data of type Ia supernovae, because they do not
violate any energy condition. This result relies only on the symmetry
properties considered, and not on the way in which the mass is discretized. The
models with different number of sources are then compared among themselves and
with a Friedmann-Lemaitre-Robertson-Walker model with the same total mass
content by introducing a compactness parameter. The analysis shows that
observational backreaction occurs because increasing the number of sources the
features of a universe with a continuous matter distribution are not recovered.
Our models are shown to exhibit a non-trivial relationship between kinematical,
dynamical and observational backreactions, the kinematical one being
asymptotically decreasing while the latter two are present. Furthermore, the
electric part of the Weyl tensor contributes to the luminosity distance by
affecting the evolution of the scale factor, while the magnetic part has an
indirect role by affecting only the evolution of the former
