I thought I read a formula once for figuring out how long of an exposure you would need to see a certain degree of arcing in stars when taking nite shots. Does anyone know this? I want to say 10 minutes = 15 degrees. I'm interested in getting a long exposure shot on a clear sky and I really want to nail this affect. There's a lake nearby that I want to use and hopefully if it's a calm nite I'll get a nice reflection off of the lake as well. If anyone has any other advice for this type of shot please feel free.