Effect of the cosmological constant on the bending of light and the cosmological lens equation

We revisit the effect of cosmological constant Lambda on the light deflection and its role in the cosmological lens equation. First, we reexamine the motion of photon in the Schwarzschild spacetime, and explicitly describe the trajectory of photon and deflection angle alpha up to the second order in G. Then the discussion is extended to the contribution of the cosmological constant Lambda in the Schwarzschild-de Sitter or Kottler spacetime. Contrary to the previous arguments, we emphasize the following points: (a) the cosmological constant Lambda does appear in the orbital equation of light, (b) nevertheless the bending angle of light alpha does not change its form even if Lambda not equal 0 since the contribution of Lambda is thoroughly absorbed into the definition of the impact parameter, and (c) the effect of Lambda is completely involved in the angular diameter distance D-A.