You can get a bit of streak with a three to five minute exposure. But fifteen to thirty minutes is better for real trails. And yeah, it’s a slow process, because if you screw up one shot, you’ve lost a half-hour or more. I bought about six Chinese batteries for my camera, and that helps, having them to swap out. A 35 minute shot + a 35 minute dark frame at 0? can drain a full battery if you’re not careful.
You’ll get more stars the more open your aperture is, but at the same time, you’ll get more ambient light with the wider apertures, which can wash out the stars. I only shoot really wide open, like f/1.8, when it’s totally dark outside with no moon. Other times, I’m at anywhere between f/4 and f/8. I mostly shoot at ISO100, but sometimes go up to 200. I use a 20D, and if you nail the exposure on the shot and don’t have to try and stretch it in RAW, noise isn’t too much of a factor. I’ve gotten pretty good at being able to judge what exposure time will suit what light, but that’s taken four years of practice.
It’s really a trial and error process more than anything else. get a good sturdy tripod, a remote release, and be prepared to spend an inordinate amount of time standing in the dark. I find music on headphones enhances the whole experience greatly. A timer remote is a great investment if you really don’t want to drive yourself crazy. You can get a cheap but totally functional one on ebay from China for under $50.
