  Analytic expressions for distance-redshift relations which have been
corrected for the effects of inhomogeneities in the
Friedmann-Lema\^itre-Robertson-Walker (FLRW) mass density are given in terms of
Heun functions and are used to illustrate the significance of inhomogeneities
on a determination of the mass parameter $\Omega_m$ and the cosmological
constant $\Lambda$. The values of these parameters inferred from a given set of
observations depend on the fractional amount of matter in inhomogeneities and
can significantly differ from those obtained by using the standard
magnitude-redshift ($m$-$z$) result for pure dust FLRW models. As an example a
determination of $\Omega_m$ made by applying the homogeneous distance-redshift
relation to SN 1997ap at $z=0.83$ could be as much as 50% lower than its true
value.
