First of all, they shouldn’t ask.

“As a basic principle, we can’t tell you to stop recording,” says Delroy Burton, chairman of D.C.’s metropolitan police union and a 21-year veteran on the force. “If you’re standing across the street videotaping, and I’m in a public place, carrying out my public functions, [then] I’m subject to recording, and there’s nothing legally the police officer can do to stop you from recording.”

“What you don’t have a right to do is interfere,” he says. “Record from a distance, stay out of the scene, and the officer doesn’t have the right to come over and take your camera, confiscate it.”

Officers do have a right to tell you to stop interfering with their work, Burton told me, but they still aren’t allowed to destroy film.

Yet still some officers do. Last week, an amateur video appeared to show a U.S. Marshal confiscating and destroying a woman’s camera as she filmed him.

“Photography is a form of power, and people are loath to give up power, including police officers. It’s a power struggle where the citizen is protected by the law but, because it is a power struggle, sometimes that’s not enough,” says Jay Stanley, a senior policy analyst at the American Civil Liberties Union (ACLU).

Stanley wrote the ACLU’s “Know Your Rights” guide for photographers, which lays out in plain language the legal protections that are assured people filming in public. Among these: Photographers can take pictures of anything in plain view from public space—including public officials—but private land owners may set rules for photography on their property. Cops also can’t “confiscate or demand to view” audio or video without a warrant, and they can’t ever delete images.

The ACLU’s guide does caution that “police officers may legitimately order citizens to cease activities that are truly interfering with legitimate law enforcement operations.”

What if that happens, and you disagree with the officer?

“If it were me, and an officer came up and said, ‘You need to turn that camera off, sir,’ I would strive to calmly and politely yet firmly remind the officer of my rights while continuing to record the interaction, and not turn the camera off,” Stanley told me.

“In the majority of situations, an officer is just trying to intimidate you, and stop your reporting. Once you make it clear to the officer that you do know what your rights are and that you don’t intend to be intimidated, I think in the vast majority of situations, the officer will back down,” he says.

Daniel Sanchez recommended a slightly different tactic. Sanchez is an organizer for Copwatch in the Bronx, a program organized by a local advocacy group called the Justice Committee, which films officers at work in low-income neighborhoods. “We train volunteers to calmly respond, ‘I’m just exercising my constitutionally protected right to document police activity,’” he told me.

Most officers, says Sanchez, now know that bystanders have a legal right to film police. Now, instead of hearing assertions that they can’t record at all, he says that Copwatch volunteers are accused of interfering with police activity.

“What we hear is, you can’t film here, you need to back up,” he told me.

At which point, says Sanchez, the volunteer complies—by taking one step back.

“The back-up game, is what I call it. ‘I did back up, officer, I am backing up, here, I’ll take another step back,’” Sanchez says.

The goal is to perform strictly legal compliance with the officer’s actions while still asserting the right to film. Although bystanders should make that initial assertion of legality, every situation is different, Sanchez and Stanley agree.

“Every incident is really is its own unique situation, and it depends on the nature of the police officers you're dealing with,” Sanchez says. “We can give people guidelines and suggestions, but at the end of the day people need to make their own judgements.”

“If you’re dealing with a belligerent police officer, that’s a dangerous situation—police officers have a lot of power—and you need to make a judgement about the importance of what you’re doing, and about the risk you’re willing to take, versus your own sense of justice,” says Stanley.

And Stanley’s ACLU guide supplies the one question those stopped for taking photos or video may ask an officer:

If stopped for photography, the right question to ask is, ‘am I free to go?’ If the officer says no, then you are being detained, something that under the law an officer cannot do without reasonable suspicion that you have or are about to commit a crime or are in the process of doing so. Until you ask to leave, your being stopped is considered voluntary under the law and is legal.

In many situations where officers are not already being recorded, Sanchez says, filming can change officer-bystander relations.

“I’ve seen incidences where they’re verbally berating a community member and we show up on the scene and the entire scene switches,” he told me. But he also doubted that police-worn body cameras would change officer behavior.

Burton, meanwhile, says that in Washington and other high-security cities, officers already assume they’re being filmed.

“In the District of Columbia, and in places like Boston, where there are so many cameras in public places, there’s almost nowhere you can go where you’re not being recorded. So we tell our officers, that’s the way you should behave all the time—as if you’re being recorded.”