Lambda does not Lens: Deflection of Light in the Schwarzschild-de Sitter
  Spacetime
  Debate persists as to whether the cosmological constant $\Lambda$ can
directly modify the power of a gravitational lens. With the aim of
reestablishing a consensus on this issue, I conduct a comprehensive analysis of
gravitational lensing in the Schwarzschild--de Sitter spacetime, wherein the
effects of $\Lambda$ should be most apparent. The effective lensing law is
found to be in precise agreement with the $\Lambda=0$ result:
$\alpha_\mathrm{eff} = 4m/b_\mathrm{eff}+15\pi m^2/4b_\mathrm{eff}^2
+O(m^3/b_\mathrm{eff}^3)$, where the effective bending angle
$\alpha_\mathrm{eff}$ and impact parameter $b_\mathrm{eff}$ are defined by the
angles and angular diameter distances measured by a comoving cosmological
observer. [These observers follow the timelike geodesic congruence which (i)
respects the continuous symmetries of the spacetime and (ii) approaches local
isotropy most rapidly at large distance from the lens.] The effective lensing
law can be derived using lensed or unlensed angular diameter distances,
although the inherent ambiguity of unlensed distances generates an additional
uncertainty $O(m^5/\Lambda b_\mathrm{eff}^7)$. I conclude that the cosmological
constant does not interfere with the standard gravitational lensing formalism.
