I want to get a new flash (on and off camera) for my canon rebel xsi, but i know nothing about flashes, or flash settings on my camera. Can a shoe mount flash also be used off camera or do i need to buy both? I want the off camera flash to be triggered wirelessly so what are the advantages of the different types of triggers? What is ETTL? i've done a little bit of searching but havn't found anything helpful yet.