Moving objects together

[imperialkat]

How can I move one object based on where another object is moved?

I'm trying to simulate an object in front of a lens and the image it creates. There's one user-controlled object (A) on one side of the lens and another object (B) on the other side. When the user moves A relative to the lens I need B to 1)move a distance X (not necessarily the same distance) in the opposite direction and 2) change size based on its distance from the lens.