Gardner was an expert in the new wet-collodion-on-glass-plate process of photography and able manager of Mathew Brady's Washington, D.C., portrait studio from 1858 until 1862, when he had a falling-out with his employer. In November 1862 he formed his own company, taking with him many of Brady's best photographers, including Egbert Guy Fowx (seen nearby). Like Brady, Gardner and his corps produced a vast photographic documentation of the Civil War, a selection of which forms the basis of his landmark publication, Gardner's Photographic Sketch Book of the War (1866). Despite decades of painstaking research by dedicated historians and Civil War buffs, a large number of the era's photographs remain unidentified, including this fine portrait of four officers in the field. What is known is that the two men in the center wear forage caps featuring a round, sloping leather brim made popular by Gen. Irvin McDowell (leader of the Union troops during the first battle of Bull Run). The other men wear regulation U.S. officer's slouch hats.