Another aspect, if I understand correctly, is that older copyright rules apply to images that have been printed or published in some way. Whether this applies to a negative of, say, 1945 that has never been printed until, say, yesterday may not be addressed. I expect that may be considered a new interpreted work; or more likely a moot case. The date of pressing the shutter is more important when the author is anonymous; the date of publishing is more important to a named author. But if the author is anonymous, who then claims copyright? And if you subsequently name an anonymous photographer then the published date becomes relevant.
Modern copyright specifies the copyright includes the negative and/or a digital file.
Many aspects have not been resolved as a trial case has not come up.
For example, Karsh published many photographs prior to 1949 and they did go to the common domain; yet he still lived when the copyright was extended to years after death. The result was it was possible to imagine copyright having been in the common domain and reverting to out of it. This has never been adjudicated and common domain ownership exists in practice if not in fact.