While hanging out in Florida, we stopped over at NAPP headquarters and I did a quick segment on PhotoshoUserTV.com last week. The episode just aired yesterday, and already I got a few emails about F-stops and Shutter Speeds. So, let me add a point of clarification to the set up. My F-stop is always F 5.6 with the Quantum flash at 1/2 power.
Since the back-lighting effect is dependant on the intensity of the ambient light, I set my shutter speed at about 2 stops less than the ambient light. Said a different way, if the church exposed correctly at F5.6 @ 1/8 second (at ISO 800), I would adjust the shutter speed up to 1/30 second thereby under exposing the church by 2 stops so that the back-lighting effect would be easily apparent. This may vary a bit depending on background tonality, illumination, stain glass windows, etc. I just check the LCD screen and make adjustments till I get the desired result - pretty simply.
By the way, my segment went a bit long, so you can download the Bonus Segment right here. Remember too, that the episodes are posted for one week on PhotoshopUserTv and then move over to iTunes where you can download them at no charge.