Hello, normally I would just go outside and figure this out for myself. But I am living in the city and there is tooooo much light pollution to get a reliable result. I am taking a very short trip to the desert, and I'd like to get some pics of the stars. My question/scenerio.(I apologize beforehand for my crap terminology/knowledge)........... Ok, so I would like to take pics of the stars with that neat "swirl" effect. I have taken a few before and was happy with the result. I was wondering if there is a certain(approx) amount of time that I can open my shutter that will give me a accurate idea of what my pic will look like after having my shutter open for an hour+. Example... can I open my shutter for 2 minutes to test my settings and would it have the same "lighting" result as if I opened it for an hour? I only want to open it for a long time for the "swirl" effect from the stars. I am sorry if the question is somewhat confusing, I have no clue what I am doing, except having fun learning this stuff. I'd just like to save a lil time and be a little bit more prepared for this trip... more than I was for my last vacation... which I missed alot of pic ops cuz I suck. I am using a Canon EOS 7D if that matters.