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For the photographs of the Civil War battlefields in “Last Measure,” Ms. Mann used the very demanding 19th-century collodion wet plate process.

The collodion wet plate process, a very inconvenient form of photography which required the photographic material to be coated, sensitized, exposed and developed within the span of about 15 minutes, necessitating a portable darkroom for use in the field, was invented in 1851 by Frederick Scott Archer.

That got him thinking about the history of photography, which in turn led him to learning the wet plate process.

For someone who yearned to get away from the instant gratification of digital, Mr. Hawkey found the wet plate process was just the thing.

Here again the wet plate process, although more clumsy, demonstrated its superiority over the dry process used by other expeditions.