CHICAGO, Illinois - April 17, 2010 - Minutes after a suburban Chicago police officer was charged with striking a motorist with his baton, prosecutors handed out copies of a video showing the beating - taken by a dashboard camera on the officer's own squad car.

In Kalifornia, after a transit cop and an unruly train passenger slammed against a wall during a struggle and shattered a station window last fall, video from a bystander's cell phone was all over the Internet before the window was fixed.

The same cell phones, surveillance cameras and other video equipment often used to assist police are also catching officers on tape, changing the nature of police work - for better and worse.

Some say cameras are exposing behavior that police have gotten away with for years. But others contend the videos, which often show a snippet of an incident, turn officers into villains simply for doing their jobs, making them targets of lawsuits and discipline from bosses buckling to public pressure.

"We tell our officers all the time you've got to assume that everything you do is going to be videotaped," said Chicago Police Superintendent Jody Weis. "Everyone has a cell phone and almost every cell phone has a camera."

Cook County State's Attorney Anita Alvarez said the video her office gave to the media on Tuesday shows police officer James Mandarino, from the Chicago suburb of Streamwood, hitting motorist Ronald Bell 15 times after a traffic stop last month.

In the video, Mandarino is seen firing a Taser at a passenger in the car and then striking Bell, who is on his knees with his hands on his head. Bell suffered a concussion and cuts that required seven stitches.

"It's a wonderful tool," Alvarez said of the video, which she says suggests that both men posed no threat to the officer.

Though police-behaving-badly videos have become popular staples of cable news shows and the Internet, Weis said he doesn't believe his officers are overly cautious out of fears they'll be videotaped - and their superiors are not advising them to be.

Quietly, though, some officers say the prospect of being videotaped makes them hesitate even if they know they should act.

"I've heard from officers who are sent to break up a fight in the street and see a group of people leaning out windows with handheld video cameras... they go slower and are less aggressive," said Tom Needham, a Chicago attorney who has represented several police officers.