  A long standing question in cosmology is whether gravitational lensing
changes the distance-redshift relation $D(z)$ or the mean flux density of
sources. Interest in this has been rekindled by recent studies in non-linear
relativistic perturbation theory that find biases in both the area of a surface
of constant redshift and in the mean distance to this surface, with a
fractional bias in both cases on the order of the mean squared convergence
$\langle \kappa^2 \rangle$. Any such area bias could alter CMB cosmology, and
the corresponding bias in mean flux density could affect supernova cosmology.
Here we show that, in an ensemble averaged sense, the perturbation to the area
of a surface of constant redshift is in reality much smaller, being on the
order of the cumulative bending angle squared, or roughly a part-in-a-million
effect. This validates the arguments of Weinberg (1976) that the mean
magnification $\mu$ of sources is unity and of Kibble \& Lieu (2005) that the
mean direction-averaged inverse magnification is unity. It also validates the
conventional treatment of lensing in analysis of CMB anisotropies. But the
existence of a scatter in magnification will cause any non-linear function of
these conserved quantities to be statistically biased. The distance $D$, for
example, is proportional to $\mu^{-1/2}$ so lensing will bias $\langle
D\rangle$ even if $\langle \mu \rangle=1$. The fractional bias in such
quantities is generally of order $\langle \kappa^2 \rangle$, which is orders of
magnitude larger than the area perturbation. Claims for large bias in area or
flux density of sources appear to have resulted from misinterpretation of such
effects: they do not represent a new non-Newtonian effect, nor do they
invalidate standard cosmological analyses.
