Alexander Gardner was one of three photographers who “documented” the American Civil War with collodion photography, a new type of photographic process that used glass negatives. Advantages of this photographic process were that the images were clear and crisp and that the negative was durable and could be used to make countless prints. A disadvantage was that the exposure time was typically several seconds long, so the photographer could not capture an instant.
(Pinkerton, who was head of Union Intelligence Services during the war, foiled an assassination attempt against Lincoln and went on to pioneer the American private detective industry.)
Because of the exposure times of collodion negatives, Civil War photographers like Gardner took images of the aftermath of battles, at times arranging and rearranging corpses within their compositions. Of course, this rather common practice calls into question the accuracy of these photographs as documents of the Civil War, but are they true in any sense?