No tracking down rights-holders, no licensing, and — in theory — no cost. But a quick internet search for public domain footage will reveal a plethora of sites charging for the stuff. What gives?
Public domain, footage that holds no copyright and is free to use by the public, is music to the ears of a documentary filmmaker. But determining what footage is in public domain, and how to get it for free, can be quite a chore.
Footage generally falls into public domain because it was created by an entity not subject to copyright (like most of the US Government), or the copyright has expired. You can check the 2015 US Public Domain and Copyright Term for current laws governing when copyright expires. If you’re new to public domain film, here’s a brief introduction courtesy of Pond5: