During the Nazi regime in Germany, he worked as a cameraman for Leni Riefenstahl; from 1939 to 1945, he was closely associated with photographing and filming activities of higher echelons of leaders of Nazi Germany, including German dictator Adolf Hitler.

[2] At the beginning of 1945, Frentz began to photograph German cities destroyed by the war in color: Berlin, Dresden, Frankfurt am Main, Freiburg, Heilbronn, Cologne, Munich, Nuremberg, Paderborn, Ulm.

His lack of fame soon enabled him to pursue a career as a cultural filmmaker and traveling lecturer without being disturbed.

Since the 1960s, various authors (including John Toland, David Irving, Gitta Sereny) have interviewed him as a contemporary witness.

The growing interest in color photography from the “Third Reich” then led to an increased use of his photographs in German and foreign print media in the 1990s.