This was quite a lucky shot to be honest. I was going to shoot the sunset afterglow and stars, I wasn't even expecting auroras that night. I had a pretty good foreground at first when I was shooting towards north, but these auroras light up in south east sky, I was running in the dark with my headlamp trying to find a good foreground
A little tip for your future captures. Open your aperture as open as it goes, focus on "endless" and I'd keep my shutterspeed to somewhere between 8s and 25s. The reason for this is to freeze the motion of the lights. From experience a start out point that worked well for me last season was f2.8, ISO800 and 15 seconds. And I guess if you want to light up the forground a little I'd try to quickly paint it with an external lightsource - one sweep to freeze that motion as well (several sweeps might cause ghosting if it's a moving foreground like a tree or a person).
These are only my experiences and isn't like a written this-is-how-it-must-be-done-guide
I just checked all of the aurora photos I have, almost every image has a grayish area in the sky. Same as the left side of the sky in this image. Exposures I used were between 30-60 seconds. I guess the long exposure creates that grayish tone when the auroras move too fast across the scene. If you use hyperfocal you will get the widest possible depth of field. With my 17-40 at f/4 and 17mm it's possible to get depth of field from 1,34 meters to infinity. You can calculate this at dofmaster.com [link] . Anyway thanks for the tip
It's not a bad idea to try hyperfocal as the auroras will speak for themselves. However you still want to freeze the lights as much as you can. And you are right, the graytone comes from .. call it overexposure. The thing is also when you use a long shutter the lights will act the same way as if you shot a moving car at night - the lights will be "dragged" through the scene. I'm sure I'll get to see alot of nice auroras from you this season, I can't wait to get started myself