Deducing the camera position and lens from a still image, but doing it "old school".
Though it's a good idea to survey and capture camera information on a shoot, it's not always possible
Also sometimes we are supplied with stock footage
or already shot material and we need to find out where the camera was.
programs can help us with that but before we had them, it was still possible to do so using basic geometry.
Read the Tutorial on fxguide.com