Photography pioneers' work goes on display in Washington
18 June 2013 Last updated at 21:17 BST
In today's era of cell phone cameras and instant snap shots, it's hard to imagine a time when photography was difficult, technical and time consuming.
But in the mid-19th Century it was confined to a few pioneers. In the United States, they included the brothers Henry and Charles Meade, the subjects of a new exhibition at the National Portrait Gallery in Washington.
Jane O'Brien reports.