[SOLVED] Simple Optics - being stupid?!!! 1. The problem statement, all variables and given/known data There's an object 1.5m from a white screen. What is the focal length of a lens needed to form a real and inverted image on the screen with magnification of 4.0? How far from the object should the lens be placed? 2. Relevant equations -M=S'/S 1/f=1/S'+1/S 3. The attempt at a solution My problem is the first part of the question, what to use for S and S', surely there are an infinite number of possibilities?