I have always been fascinated by webcams that watch for anything that moves. When I lived in a no-frills neighborhood in San Francisco a few years ago, my camera witnessed all flavors of urban crime, from amateur fireworks shows to street fights. After I moved to the suburbs, my camera became a nature documentarian of the local fauna, like the deer that devour my rosebushes the instant they bloom.

Only recently did I force myself to weigh the potential privacy costs of this seemingly innocuous surveillance gadget against the benefits I was gaining from it — and I decided to unplug my camera.

That’s because San Francisco, long a capital of progressivism and a haven for techies, is about to embark on a citywide surveillance experiment that privacy experts warn could set a dangerous precedent. It signifies an important moment in which anyone who owns a security camera, including popular devices like Amazon’s Ring and Google’s Nest Cam, should pause to reflect on some critical questions: What are we actually getting from these cameras? What are we giving away? Are the trade-offs worth it?

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Opponents of the ordinance, like the Electronic Frontier Foundation, say research has shown that cameras do little to reduce crime. A study from New York University, for one, found that cameras installed in two privately owned apartment complexes in New York City were an ineffective crime deterrent.

Matt Guariglia, a policy analyst at the foundation, which publicly protested the legislation before the city’s Board of Supervisors approved it in a 7-to-4 vote, said San Francisco’s ordinance posed threats to consumer privacy. Although the legislation requires the police to get permission from camera owners before viewing live footage, he said, the police have been able to obtain Ring recordings directly from Amazon.

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But several privacy experts warned against being complacent. Now that there is legislative language allowing the police to request live access to camera technology, the concern is that the police will put pressure on the tech companies to cooperate.

“These companies are incredibly eager to work with law enforcement and develop features they would like,” Mr. Guariglia said. “If the San Francisco Police Department came to Amazon tomorrow and said would you mind creating a ‘share live feed’ feature, it would not surprise me in the least if Amazon complied.”