Drew created the photos using tintype, a technique from the 1800s in which a positive image is created from chemicals on metal plates exposed to light. (Image © Ed Drew courtesy of Robert Koch Gallery, San Francisco)

The subjects had to remain completely still for six to ten seconds while Drew exposed the chemicals. (Image © Ed Drew courtesy of Robert Koch Gallery, San Francisco)

Each photo took about 40 minutes to make. (Image © Ed Drew courtesy of Robert Koch Gallery, San Francisco)

The chemicals dried fast in the heat, which made it easy to make a mistake and destroy the photo, Drew said. (Image © Ed Drew courtesy of Robert Koch Gallery, San Francisco)

This is Drew's favorite portrait, of his copilot. (Image © Ed Drew courtesy of Robert Koch Gallery, San Francisco)

Drew served as an aerial gunner on a search and rescue helicopter. (Image © Ed Drew courtesy of Robert Koch Gallery, San Francisco)

The plates were all made between combat missions. (Image © Ed Drew courtesy of Robert Koch Gallery, San Francisco)