Nolan shot Interstellar on 35 mm film in the Panavision anamorphic format and IMAX 70 mm photography. Cinematographer Hoyte van Hoytema was hired for Interstellar, as Wally Pfister, Nolan's cinematographer on all of his previous films, was making his directorial debut working on Transcendence (2014); Pfister would later retire as a cinematographer for films. More IMAX cameras were used for Interstellar than for any of Nolan's previous films. To minimize the use of computer-generated imagery (CGI), Nolan had practical locations built, such as the interior of a space shuttle. Van Hoytema retooled an IMAX camera to be hand-held for shooting interior scenes. Some of the film's sequences were shot with an IMAX camera installed in the nose cone of a Learjet. Nolan, who is known for keeping details of his productions secret, strove to ensure secrecy for Interstellar. Writing for The Wall Street Journal, Ben Fritz stated, "The famously secretive filmmaker has gone to extreme lengths to guard the script to... Interstellar, just as he did with the blockbuster Dark Knight trilogy." As one security measure, Interstellar was filmed under the name Flora's Letter, Flora being one of Nolan's four children with producer Emma Thomas.