Read more here: https://www.copyright.gov/engage/visual-artists/
Chapter 300 covers copyrightability in general: https://copyright.gov/comp3/chap300/ch300-copyrightable-auth...
> 313.4(A): A work that is a mere copy of another work of authorship is not copyrightable. The Office cannot register a work that has been merely copied from another work of authorship without any additional original authorship. [...] Bridgeman Art Library, Ltd. v. Corel Corp., 36 F. Supp. 2d 191, 195 (S.D.N.Y. 1999) ("exact photographic copies of public domain works of art would not be copyrightable under United States law because they are not original").
Chapter 900 covers visual art specifically and goes into more detail on the copyrightability of photographs: https://www.copyright.gov/comp3/chap900/ch900-visual-art.pdf
> 909.3(A): [...] A photograph that is merely a "slavish copy" of a painting, drawing, or other public domain or copyrighted work is not eligible for registration. The registration specialist will refuse a claim if it is clear that the photographer merely used the camera to copy the source work without adding any creative expression to the photo. Similarly, merely scanning and digitizing existing works does not contain a sufficient amount of creativity to warrant copyright protection.
Faithful reproductions of non-copyrighted two-dimensional work are considered non-copyrightable, because nothing of artistic value is added in the process. (There is a lot of mechanical work around photos, but mechanical work doesn't enjoy copyright.)