A few years ago, Paul Bolcik and Erik Davis stood on East Patrick Street in Frederick, Md., looking at a Civil War historical marker. On the plaque was one of the most famous images from that conflict: the only known candid photograph of Confederate soldiers on the march.

Reproduced in countless books, it shows nearly 100 men, most with rifles resting on their right shoulders, a few looking toward the camera, their faces inscrutable.

And to think it was taken on that very street in 1862!

Except, it wasn’t. In April, after three years of painstaking research, Bolcik and Davis revealed that the picture was actually taken around the corner in 1864.

This is a big deal in the world of Civil War photography.

“It’s one of the most compelling photographs — and candid photographs — of the Civil War,” said Bob Zeller, president of the Center for Civil War Photography, whose journal published the pair’s findings.

We’re still fighting the Civil War. We wrestle with big issues: Why was the war fought? What is its legacy? And we argue over little ones, too. That’s what drew Bolcik, a landscaper from Rockville, Md., and Davis, a Frederick cartographer, to that photo.

As they stood on East Patrick Street, two things bothered them: Downtown Frederick hasn’t changed that much, but they couldn’t get details from 1862 to line up with today. Also, where were the bayonets?

“By the mid- to late war a lot of the Southern troops had tossed away their bayonets,” Bolcik said. The soldiers decided the rifle-mounted blades were just so much extra weight.

Since you can’t see any bayonets on the dozens of soldiers’ rifles in the Frederick photo, they figured it must have been taken later than 1862.

But there was a clue: At the lower right of the photo is a sign for Rosenstock’s, a dry goods and clothing store.

“I said to Erik, ‘Look, the only way we can figure out the mystery is to figure out exactly where this guy Rosenstock’s store was during the war,’” Bolcik said.

After the war, Rosenstock’s was on East Patrick Street, but after months of poring through microfilm at the C. Burr Artz Public Library in Frederick, Bolcik discovered a tiny ad that placed it on North Market Street in 1860.

Even more promising: The store was below the studio of a prominent Frederick photographer, Jacob Byerly.

Things started to fall into place.

To Bolcik and Davis, the question had always been: How had the photo been taken? It appeared to have been snapped surreptitiously from a second- or third-floor window. Photography was complicated back then. Could the equipment have been carted up there?

But if the photographer was already in place and enemy troops just happened to march by . . .

Confederate troops came through Frederick twice: once in September 1862 during Robert E. Lee’s first invasion of the North and again on July 9, 1864, on their way to the Battle of Monocacy. While Frederick residents may have been supportive of the troops early in the war — Maryland was full of Southern sympathizers — by 1864 many were tired of the conflict, especially since the Confederates had informed Frederick’s banks that the town would be torched unless they forked over $200,000 in ransom.

It’s easy to imagine the nervousness with which Byerly tripped the shutter.

“It was a complete rush job, an on-the-spot moment of opportunity,” Bolcik said. A thumbprint on the lower left of the photo may be from the harried photographer smudging chemicals on the glass negative. The edges of the photo are blurred, suggesting there wasn’t time to swap out a portrait lens for a landscape lens.

The article in the journal Battlefield Photographer, co-written by Craig Heberton IV, is loaded with the sort of minutiae that have fed the study of Civil War photographs. For years, buffs argued over the precise location on the Gettysburg, Pa., battlefield where photographer Alexander Gardner took images of dead soldiers. In 1967, William Frassanito ended the speculation by finding a distinctive split rock, a large boulder that is still where it was in 1863.

Bolcik and Davis had their own split-rock moment when they attached an iPhone to the end of a 22-foot long pole and lifted it above North Market Street, hoping to re-create the angle from Byerly’s studio — and to not be mistaken for peeping Toms as the phone swayed outside of what is now an apartment.

Everything lined up, including hatchways on the sidewalk that lead to cellars.

As for that historical marker around the corner, it was a bear to install because of the multiple utilities that run beneath it.

“It figures it would turn out to be in the wrong spot,” said John Fieseler, executive director of the Tourism Council of Frederick County.

They’re looking into correcting it.

Twitter: @johnkelly

Correction: An earlier version of this column said that Jacob Byerly’s photo studio was across the street from Rosenstock’s. It was above it.

For previous columns, visit washingtonpost.com/people/john-kelly.