In the United States a film goes into the Public Domain if:

The film was never registered or if the film was registered but not renewed during the 28 year cutoff.  Because copyright laws have changed over the years, we suggest to go the the Library Of congress website to read the complete information about United States Copyright Laws. website:
There are several circulars on the Library of Congress website, such as Circular 22 that will keep you current regarding the different copyright laws.

We do suggest that you always check with your legal advisors, as we cannot know or keep up with changes in each country's laws and regulations.