To avoid confusion, US Customs recommends that you register certain items before you leave the United States -that way you can prove that you owned an item before you left. This is particularly important for foreign-made items like camera, computer, and electronic gear.
If you are taking a single camera, it's not a big deal, but if you take a boatload of camera gear with you; several camera bodies, etc., registering your items will prevent having to possibly pay duty on them.
You can register the items at the nearest Customs and Border Protection (CBP) office or prior to leaving the US by declaring your items on a Certificate of Registration (CBP Form 4457). You can use this form on multiple trips as long as the equipment you carry is the same.
CBP locations in the US where you can have the form signed (bring your camera gear with you and their serial numbers).