I found 800 got noisey up around 10minutes with the 450D I had. Short shorts on clusters, like 2-3 minutes where ok at 1600. All you can do is suck it and see. With this 40D I tend to leave it on 400 for everything although I have a HA filter in it tonight shooting 20minutes @ 1600. I'm sure I'm wasting these clear skys we have
