I don't really understand still what is required for this technique. On a standard film camera, (I have a Canon Rebel Ti), approximately what length lens is required to zoom through the second lens, and what length lens should the second lens be? Do either lenses have to be prime, and what do both lenses have to have their focusing rings set to? Also, does the second lens have to be designed for your camera? I only own one lens, but my school has several that are FD mounts, and I wonder if they could work in front of my EF mount lens. I appreciate any replies.