The Outer Limits (1995) : 6x17

Andy and Vince are Civil War buffs who spend their weekends re-enacting battles from the historic conflict with others of similar persuasion. On this weekend, they're in Gettysburg for a giant recreation of the war's bloodiest battle when a photographer, Prentice asks to take their picture with his ancient tin-type camera. But when the shutter clicks, Andy and Vince find themselves transported back to the Confederate camp of Col. Angus Devine on the eve of the infamous 1863 battle. Prentice, who joins them in the past, explains that he has sent them back as a grand experiment, to see if history can in fact be changed, proving destiny is not final.