My wife and I were asked to shoot the annual portrait for a family of about 20 people. We are reluctant to get into sourcing the light for such a large group of people and are heavily leaning toward natural light somewhere in the outdoors. Given an open-ended assignment like this, what would you do for lighting such a large group? Open shade in a park somewhere? If the weather doesn't cooperate, what would be a good backup plan? I was thinking of a pavilion, but that could be pretty dark if it's a very rainy day.

Also, from what I know, a simple fill flash would help expose faces, but what kind of power would we need for this size group? I'm thinking that a 320EX is too weak to even have an effect.