Let O be the object, L be the lense, I be the image, and S be a screen for projecting an image. The lens can be placed anywhere between O and S. Let f be the focal length of the lense. Prove that for an image to be clearly formed on screen S, OS > 4f must be true. A figure of a "too short" OS distance (since the image would be past the screen) would look like this: O-----L-----------S----I I am having a very hard time proving that OS>4f must be true!! Any hints?