Today with complex tools that can provide automatic facial recognition, then identifying people in photos taken in the 21st century may not be a problem. But, identifying people in older photos, say from the mid-1800s, is still hit or miss.
That's not to say it can't be done, as was brought home to me in a story on NPR's Morning Edition a week ago. "Unknown No More: Identifying A Civil War Soldier" detailed the sleuthing that was involved in finding a name to go with a face. Listen to the story here.
For those with an interest in old photographs we have these two books for you to look for: Collector's Guide to Early Photographs by O. Henry Mace [770 MAC] and
Care and Identification of 19th-Century Photographic Prints by James M. Reilly [770 REI].
Photo courtesy Library of Congress.