I'm UK based and It's my intention to travel to the US a couple of times a year for 3-4 weeks at a time to shoot lifestyle/business type stock.   I've already made a couple of successful trips but after a conversation I had recently I'm a bit concerned about making sure I'm not breaking any rules in the US.    When I've been through US customs before I've not been questioned so haven't had to explain myself but I would like to be up front with the border guards.