Coming via Radley Balko's excellent The Agitator (your one stop shop for questionable police behavior), are a couple of excellent videos showing very divergent reactions by police to being filmed.
It's a bit of good cop/bad cop. We'll start with the bad cop first, just to give a good example of how law enforcement officers SHOULD NOT REACT to being filmed performing their public duties within a public area.
Nothing quite like ending a march against violence by putting a TV news cameraman in a choke hold is there, ladies and gentleman?
Following this, it's very important to stress that there are obviously many, many police officers out there who are the very image of professionalism.
The differences between the following video and the proceeding one are vast. In this case, the officer is in fact encountering an objectively dangerous weapon:
For the record, a Smith and Wesson .40 is substantially more dangerous than a video camera. However, this officer didn't even feel the need to raise his voice, let alone assault anyone. He identified a possible problem and treated it in a way one would hope (let's hope for unicorns while we're at it!), all officers would. Regardless of your feelings about open carry laws (for the record: I'm permitted to carry concealed and choose to do so precisely to avoid these sorts of conversations), the level of professionalism shown by this officer is undeniably admirable.