“Mathew Brady: Portraits of a Nation” by Robert Wilson (Bloomsbury, 273 pgs., $28)

Salman Rushdie, in his novel “The Ground Beneath Her Feet” (1999), described what we see in a photograph as “a moral decision taken in one-eighth of a second.”

In the early days of photography, those moral decisions took longer to process. When Mathew Brady, the Civil War-era photographer, took a portrait, the shutter remained open for 10 to 15 seconds or more, long enough for a bit of wind, or the hint of a smile, to ruin everything. His subjects often had their heads stabilized by an unseen vise.

Brady (1823-96) was America’s first great portrait photographer, Richard Avedon and Annie Leibovitz rolled into one. Those long exposure times were a gift of sorts to a country that was still young. What Brady’s images lacked in spontaneity they more than made up for in gravitas. He defined a nation’s dignified visual sensibility.

“Mathew Brady: Portraits of a Nation” is a new biography from Robert Wilson, the editor of The American Scholar. It’s a compact, straightforward, unblinking volume that has some of the attributes of its subject.

If I sometimes wanted more expressiveness from it — if I wished it were a literary document as well as a historical one — well, you can’t have everything. The book is sober history, a flinty chunk of Americana.

Almost no one smiled in Brady’s photos. Smiles are elusive, too hard then to bottle. One of the things Wilson makes plain about Brady, however, is that he himself had a terrific smile. In his presence, one observer said, you felt “the light of an Irish shower sun.”

Brady’s personal charm helped make him the favorite of presidents, generals, celebrities and royalty. He photographed Lincoln, Grant and Lee as well as Twain and Whitman and Dolley Madison and Daniel Webster. In the 1840s and 1850s, he set up a large gallery in downtown New York. He was “Brady of Broadway.”

Not a lot is known about his early life. Brady was raised in upstate New York, near Lake George. His father was an Irish immigrant. Brady made it to Manhattan about the time the daguerreotype did. He manufactured leather cases for photographic equipment before going into the photography business himself. He opened his first studio in 1844.

Anyone who has seen Ken Burns’ documentary “The Civil War” knows how beautifully he learned to pan across Brady’s photographs from that war, among the first in history to leave a detailed photographic record. The details of Brady’s war years are both funny and revealing about the age.

There’s been abiding controversy about who actually took many of the photographs attributed to Brady. Wilson wades through these issues patiently, almost photo by photo. He mostly comes to his subject’s defense. “The idea that Brady stifled his photographers, or took undue credit for work that was theirs — undue by the standards of their day, not ours — is based on supposition and not evidence, and seems wrong

Brady’s biggest photographic accomplishment might have been the familiar image he took of the defeated Gen. Robert E. Lee in Richmond shortly after the South had surrendered at Appomattox.

“It was supposed that after his defeat it would be preposterous to ask him to sit,” Brady said later in an interview. “I thought that to be the right time for the historic picture.”

Brady was right. “Who but Brady could have pulled off this photographic and journalistic coup?” Wilson asks.