Do relativistic corrections affect microlensing amplification?

Studies of gravitational microlensing are usually based on the lens equation, which is valid only to first order in the gravitational constant G. However, the amplification factor of microlensing is a second-order quantity with respect to G. Despite this fact, conventional studies are still based on the lowest-order lens equation. Then the question naturally arises: Why are these conventional studies justified? We carefully study the relativistic correction to the amplification factor at O(G(2)). We show that the amplification factor for each image is corrected. However, the total amplification remains unchanged at this order.