In the year’s most haunting image of black Civil War soldiers, the opening battlefield sequence in Steven Spielberg’s “Lincoln,” Confederate forces massacre many fallen former slaves.

In reality, African-American prisoners of war were killed en masse. Black troops in action endured lower wages and poorer medical care and living conditions than their white counterparts. But soldiers of both races did have surprisingly easy access to the luxury of photography.

Photographers ran government-sanctioned booths near encampments, selling souvenir portraits. The images of black personnel, from officers to gravediggers, are now on view widely in 150th-anniversary commemorations of the Emancipation Proclamation. They provide a nuanced view of African-American life at the front, even though some of the subjects can no longer be identified....