[Pete] has put together a fairly simple writeup on building a wireless USB tether for your DSLR. He’s basically using a pair of USB wireless adapters and a battery pack. In his original version, he did some slight modification to the dock that powers the USB dongle, adding his own battery pack too. He notes that this whole project can be done without the effort he put into it, at a little more expense. The only improvement we think would be nice would be a better way to package it to keep it out of the way while you shoot.
