Mark Johnson of the University of Georgia writes:
Matthew Brady didn't make all of the images attributed to him. He had a staff that did a lot of that work, and a lot of those images were manipulated or set-up. It had a lot to do with the equipment of the time - they were towing a wagon around, mixing emulsions, coating glass plates and then exposing them, bringing them back into the wagon and processing. The number of images they could make in a day was pretty limited, so they spent the time getting the compositions right.
You can find out more about Matthew Brady and see the collection of Civil War photos that he and his assistants took at the Library of Congress web site.