The decision to drop charges against Robert Stolarik for interfering with an arrest he had been photographing in the Bronx in 2012 and instead to prosecute an arresting officer came after prosecutors scrutinized the physical evidence: Mr. Stolarik’s digital images.

On Aug. 4, 2012, Mr. Stolarik, a freelance photographer for The New York Times since 2000, was on assignment with reporters, investigating stop-and-frisk tactics in the South Bronx, when he spotted officers arresting a teenage girl at the intersection of McClellan Street and Sheridan Avenue. Identifying himself as a journalist and holding up his press card, he kept taking pictures, even as one officer tried to cover his lens with her hand.

“Throughout the whole thing, I never stopped shooting,” he said.

Mr. Stolarik said that was when a lieutenant slammed his camera into his face and he was tackled to the ground. “They knew I was a member of the media and they beat me up,” he said.

A video recorded by a Times reporter captured about a half-dozen officers on top of Mr. Stolarik before he was led away. He was held overnight and charged with obstructing governmental administration, disorderly conduct, harassment and resisting arrest.

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What Mr. Stolarik did not realize then was that a sequence of 60 photographs he took leading up to his arrest would provide crucial evidence that not only resulted in having prosecutors drop the charges against him, but also charge one of the arresting officers instead. Last month, Officer Michael Ackermann was convicted of falsifying a record to justify the arrest of Mr. Stolarik.

“Photos played a major role in the prosecution,” said Pishoy Yacoub, a Bronx assistant district attorney. “The stills, one after the other, show the action like a cartoon flip book.”

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The case was further helped by something most people never see: the image metadata, which records exposure information. According to Officer Ackermann, Mr. Stolarik repeatedly fired a flash into his eyes at close range and struck him in the face with his camera. Although the use of a flash was never detailed or inventoried in the initial arrest report, Officer Ackermann testified to its existence later in court documents, which led to his indictment in 2013 as well as charges being dropped against Mr. Stolarik.

Officer Ackermann’s lawyer, Michael A. Martinez, argued that his client confused the camera flash with a squad car’s lights and other ambient light during a hostile and chaotic arrest. But when on the stand, Officer Ackermann was shown picture after picture that contradicted his claims, including of the arrested teenager being walked away in handcuffs before Officer Ackermann first approached the camera.

Lindsay Silverman, a Nikon senior product manager, testified about the metadata in the photos, which, among information on the camera model, aperture and exposure, showed no use of a flash. The series of images showed that Mr. Stolarik complied with officers’ orders to step back. (I was interviewed as a potential witness because I was working on the late photo assignment desk when Mr. Stolarik, handcuffed in the back of a squad car, managed to get Siri to dial The Times, but I was never called to testify.)

“In my closing arguments, I told the judge to forget about witness testimony and look at the photos and compare them to the document that he signed,” Mr. Yacoub said.

The New York Police Department declined to comment on the conviction but said Officer Ackermann, who is awaiting sentencing, had been suspended. He faces up to four years in prison.

Many journalists, who have experienced strained relations with the New York Police Department through Occupy Wall Street and protests surrounding the Eric Garner case, welcomed this decision. About a dozen photojournalists and reporters lined the hallway with Mr. Stolarik and stood in silence as Officer Ackermann left the courtroom after being read the verdict in State Supreme Court.

Many of them had faced similar situations.

“Nobody gets arrested for taking pictures because there is no such charge,” said Mickey Osterreicher, who is general counsel for the National Press Photographers Association. “Photography is not a crime, so the charges we normally see are disturbing the peace, disorderly conduct, obstruction of governmental administration, loitering, trespassing and resisting arrest.”

Mr. Osterreicher, who has educated police officers across the country on First and Fourth Amendment rights, and met with the former New York Police commissioner Raymond W. Kelly regarding arrests of journalists, said that the city had good policies, but that training might be a problem.

“If police officers don’t know or don’t care what those rights are, it’s not going to make any difference as we saw in this case, and, when push comes to shove, you’re going to get arrested,” Mr. Osterreicher said.

Indeed, on Thee Rant, a message board described as “New York City cops speaking their minds,” one commenter, dominop, responded to a post about Officer Ackermann’s conviction: “Lock up everyone in or connected with the press that you can, every chance you get! Know your enemy!”

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