The normal copyright term is the life of the author plus 70 years. (The term “author” includes photographers.) The copyright expires on the last day of the calendar year 70 years after the author’s death. This term is automatic. There is no need to renew the copyright during its term, and your heirs cannot renew it when it expires.
In the case of a work where the date of the author’s death is unknown, the copyright expires 95 years after its first publication or 120 years after its creation, whichever is sooner. These rules for duration of copyright also apply to works made for hire and works published anonymously or pseudonymously. 
Different durations of copyright apply to works created before January 1, 1978, including the ability to renew the copyright term. When a copyright term expires, the work enters the public domain, which means that anyone may copy it or make derivative works from it.