  We generalize the Rindler-Ishak (2007) result for the lensing deflection
angle in a SdS spacetime, to the case of a general spherically symmetric fluid
beyond the cosmological constant. We thus derive an analytic expression to
first post-Newtonian order for the lensing deflection angle in a general static
spherically symmetric metric of the form $ ds^2 = f(r)dt^{2}
-\frac{dr^{2}}{f(r)}-r^{2}(d\theta ^2 +\sin ^2 \theta d\phi ^2)$ with $f(r) = 1
- \frac{2m}{r}-\sum_{i} b_i\; r_0^{-q_i}\; \left( \frac{r_0}{r}\right)^{q_i}$
where $r_0$ is the lensing impact parameter, $b_i\ll r_0^{q_i}$, $m$ is the
mass of the lens and $q_i$ are real arbitrary constants related to the
properties of the fluid that surrounds the lens or to modified gravity. This is
a generalization of the well known Kiselev black hole metric. The approximate
analytic expression of the deflection angle is verified by an exact numerical
derivation and in special cases it reduces to results of previous studies. The
density and pressure of the spherically symmetric fluid that induces this
metric is derived in terms of the constants $b_i$. The Kiselev case of a
Schwarzschild metric perturbed by a general spherically symmetric dark fluid
(eg vacuum energy) is studied in some detail and consistency with the special
case of Rindler Ishak result is found for the case of a cosmological constant
background. Observational data of the Einstein radii from distant clusters of
galaxies lead to observational constraints on the constants $b_i$ and through
them on the density and pressure of dark fluids, field theories or modified
gravity theories that could induce this metric.
