In this episode of Digital Photography One on One, Mark Wallace [SnapFactory.com] explains Key Shifting and how your shutter speed affects exposure when using an external flash.
1) To get the best results when using a Canon 580 flash unit on a 20D, should I stick to ISO100? Would you ever change the ISO for some reason? -Nigel
2) Can you please explain Key Shifting to me in regards to outdoor portraiture? Once I establish a meter reading on my subject of say f/8, what do I have to do to lighten and darken the background?