I'm going to be headed to Utah soon to hit some national parks, and I like to try some star trails. I'm going to shooting on Provia 100F. I did some research on the net and repeatedly came across exposure suggestions of f/5.6 - f/8 for 2 hours or more. I'm just looking for some advice about exposure from someone experienced with star trails. I'll be there during a new moon. One of the places I'll be visiting is Natural Bridges which is supposed to be a very dark area. Would it be better to open up the lens to say f/4 or even 2.8 since this is such a dark area? Thanks